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v

Abstract

Self-elevating mobile jack-up units are employed for offshore exploration and

development in shallow waters up to approximately 150 m deep. Jack-ups are designed

to relocate to a new site after the completion of operations, which requires the extraction

of the jack-up legs and their spudcan footings from the soil. Modern spudcans are

generally circular or polygonal in shape and have diameters of approximately 20 m.

The inability to extract spudcans from deep embedments in soft clay is having a

significant financial impact on offshore operations; delays of up to ten weeks have been

recorded. To mitigate this problem, this thesis focuses on 1) understanding and

describing the breakout failure mechanism of deeply embedded spudcans, 2) developing

an analytical method to estimate the spudcan extraction resistance, 3) understanding and

describing the reduction in the maximum extraction resistance through the use of a

water jetting system, and 4) understanding and describing the extraction of spudcans

under cyclic loading.

vi

The centrifuge modelling technique was used to experimentally investigate spudcan

extraction in normally consolidated kaolin clay. For the first time, embedments up to

three spudcan diameters deep were modelled. Furthermore, the experiments

systematically varied the operating period with and without the application of top and

bottom water jetting and also when investigating the effect of extraction under regular

and irregular cyclic loading.

The results demonstrate that the extraction mechanism, which is primarily a function of

the operational period, is the same in shallow and deep embedments. An analytical

method for estimating the maximum extraction resistance is developed. Though based

on an existing method, a new set of recommendations relate the factors that characterise

the change the soil shear strength and the effects of the operational load and the strength

ratio on the extraction of deeply embedded spudcans.

It is shown that the application of water jetting at the spudcan invert reduces the peak

extraction resistance, whereas the top water jetting relieves the post-breakout resistance.

The validity of a current conceptual framework for evaluating the optimal bottom jetting

flow rate for spudcan embedments of up to three diameters is also verified.

The maximum extraction resistance decreases if the combination of the mean uplift

cyclic load and the amplitude of the cycles on the spudcan, is sufficiently high to

decrease the effective stress and shear strength by remoulding of the soil. The mean

uplift and cyclic loadings are provided by pushing and pulling the leg by floating the

hull in the water and allowing the impact of small amplitude waves on the hull. Two

preliminary graphs are proposed to estimate the number of cycles that is required for

failure as a function of the buoyancy load and the regular cyclic loads with respect to

the maximum extraction resistance.

vii

Table of Contents

Abstract ............................................................................................................................ v

Table of Contents .......................................................................................................... vii

Acknowledgement .......................................................................................................... xi

Thesis format and Authorship .................................................................................... xiii

Declaration .................................................................................................................... xix

Notations ....................................................................................................................... xxi

CHAPTER 1

General Introduction ................................................................................................... 1-1

1.1 Jack-up unit ................................................................................................................... 1-1 1.2 Jack-up installation ....................................................................................................... 1-2 1.3 Jack-up operation .......................................................................................................... 1-2 1.4 Leg extraction issues ..................................................................................................... 1-3 1.5 Mitigation methods for jack-up removal ...................................................................... 1-3 1.6 Background ................................................................................................................... 1-5

1.6.1 Suction force ........................................................................................................ 1-5 1.6.2 Failure mechanism ............................................................................................... 1-5 1.6.3 Operating period ................................................................................................... 1-7 1.6.4 Prediction method for the estimation of extraction resistance ............................. 1-8

viii

1.6.5 Application of water jetting .................................................................................. 1-9 1.6.6 Cyclic loading ..................................................................................................... 1-10

1.7 Beam geotechnical centrifuge ..................................................................................... 1-10 1.8 Aim of research ........................................................................................................... 1-11 1.9 Thesis outline .............................................................................................................. 1-13 References ............................................................................................................................ 1-15

CHAPTER 2

Spudcan Extraction from Deep Embedment in Soft Clay ....................................... 2-1

2.1 Abstract ......................................................................................................................... 2-1 2.2 Introduction ................................................................................................................... 2-2 2.3 Soil preparation and characterisation ............................................................................ 2-5 2.4 Experimental programme and procedure ...................................................................... 2-6 2.5 Experimental results ...................................................................................................... 2-8

2.5.1 Installation resistance ............................................................................................ 2-8 2.5.2 Operating period ................................................................................................. 2-10 2.5.3 Spudcan extraction - increasing embedment depth and constant operating period – tests 1 to 4 ........................................................................................................................ 2-11 2.5.4 Spudcan extraction - varying operation period at an embedment ratio of 1.5 – tests 4 to 8……. ....................................................................................................................... 2-13

2.6 Conclusions ................................................................................................................. 2-17 References ............................................................................................................................ 2-18

CHAPTER 3

Predicting Spudcan Extraction Resistance in Soft Clay........................................... 3-1

3.1 Abstract ......................................................................................................................... 3-1 3.2 Introduction ................................................................................................................... 3-2 3.3 Database ........................................................................................................................ 3-3 3.4 Extraction failure mechanism ........................................................................................ 3-4 3.5 Evaluation of the reference method ............................................................................... 3-5 3.6 Updating the input parameters ....................................................................................... 3-8

3.6.1 Cavity depth, hc ..................................................................................................... 3-9 3.6.2 Unit weight of soil above the spudcan, 'top .......................................................... 3-9 3.6.3 Change in soil shear strength above the spudcan, fg,top ....................................... 3-10 3.6.4 Gain in soil shear strength underneath the spudcan, fg,base .................................. 3-11 3.6.5 Overburden pressure adjustment factor, sb ......................................................... 3-11 3.6.6 Effect of the operation load, fol ........................................................................... 3-12 3.6.7 Effect of the strength ratio on the breakout factor for base soil resistance, fsr .... 3-13 3.6.8 Net extraction load, quplift .................................................................................... 3-14

3.7 Discussion ................................................................................................................... 3-15 3.8 Conclusions ................................................................................................................. 3-15

ix

References ........................................................................................................................... 3-17 Appendix A ......................................................................................................................... 3-39

CHAPTER 4

Centrifuge Experiments to Study Extraction of a Deeply Embedded Spudcan Using Top Jetting ......................................................................................................... 4-1

4.1 Abstract ......................................................................................................................... 4-1 4.2 Introduction ................................................................................................................... 4-2 4.3 Experimental setup ....................................................................................................... 4-4

4.3.1 Facility and setup ................................................................................................. 4-4 4.3.2 Model and instrumentation................................................................................... 4-4 4.3.3 Test procedure ...................................................................................................... 4-5 4.3.4 Soil preparation and characterisation ................................................................... 4-7

4.4 Results and discussion .................................................................................................. 4-8 4.4.1 Influence of flow rate ........................................................................................... 4-9 4.4.2 Influence of commencement of jetting ............................................................... 4-13

4.5 Conclusions ................................................................................................................. 4-13 References ........................................................................................................................... 4-15

CHAPTER 5

The Effect of Water Jetting on Spudcan Extraction from Deep Embedment in Soft Clay ................................................................................................................................ 5-1

5.1 Abstract ......................................................................................................................... 5-1 5.2 Introduction ................................................................................................................... 5-2 5.3 Experimental setup ....................................................................................................... 5-5

5.3.1 Facility and setup ................................................................................................. 5-5 5.3.2 Model and instrumentation................................................................................... 5-6 5.3.3 Centrifuge scaling factor ...................................................................................... 5-7 5.3.4 Soil preparation and characterisation ................................................................... 5-8 5.3.5 Test programme and test procedure ..................................................................... 5-8

5.4 Experimental results.................................................................................................... 5-10 5.4.1 Reference case, extraction without jetting ......................................................... 5-10 5.4.2 Undrained jetted extractions – bottom jetting .................................................... 5-11 5.4.3 Application of both top and bottom jetting ........................................................ 5-15

5.5 Conclusions ................................................................................................................. 5-17 References ........................................................................................................................... 5-18

CHAPTER 6

Experimental Investigation of the Effect of Cyclic Loading on Spudcan Extraction ........................................................................................................................................ 6-1

x

6.1 Abstract ......................................................................................................................... 6-1 6.2 Introduction ................................................................................................................... 6-2 6.3 Experimental setup ........................................................................................................ 6-5

6.3.1 Facility .................................................................................................................. 6-5 6.3.2 Model and instrumentation ................................................................................... 6-6 6.3.3 Soil preparation and characterisation .................................................................... 6-6

6.4 Regular and irregular cyclic loading ............................................................................. 6-7 6.5 Experimental programme and procedure ...................................................................... 6-8

6.5.1 Monotonic tests ..................................................................................................... 6-8 6.5.2 Cyclic tests ............................................................................................................ 6-9 6.5.3 Testing procedure ................................................................................................. 6-9

6.6 Experimental results .................................................................................................... 6-10 6.6.1 Installation and operation stages ......................................................................... 6-11 6.6.2 Vertical pull vmean ................................................................................................ 6-12 6.6.3 Regular cyclic loading ........................................................................................ 6-12 6.6.4 Irregular cyclic loading ....................................................................................... 6-15

6.7 Preliminary contours of failure under cyclic loading .................................................. 6-16 6.8 Conclusions ................................................................................................................. 6-17 References ............................................................................................................................ 6-19 Appendix A .......................................................................................................................... 6-40

CHAPTER 7

Concluding Remarks ................................................................................................... 7-1

7.1 Introduction ................................................................................................................... 7-1 7.2 Main findings ................................................................................................................ 7-1

7.2.1 Specifying the breakout failure mechanism of deeply embedded spudcans ......... 7-1 7.2.2 Improving a predictive method for spudcan extraction based on data of embedment depth and vertical load history ....................................................................... 7-2 7.2.3 Understanding the effectiveness of top jetting in reducing the extraction resistance from deep embedments ...................................................................................................... 7-3 7.2.4 Determining the performance of bottom jetting to ease the extraction of deeply embedded spudcans ........................................................................................................... 7-4 7.2.5 Providing insight into the behaviour of the spudcan during extraction under cyclic loading.. ............................................................................................................................. 7-5

7.3 Recommendations for future work ................................................................................ 7-6

xi

Acknowledgement

I would like to express my sincere gratitude towards my supervisor, Winthrop Professor

Mark Cassidy, for his continual support, infinite patience, and motivational guidance.

I would also like to express my special appreciation and thanks to my other supervisors,

Professor Christophe Gaudin and Associate Professor Britta Bienen, for their

constructive suggestions, priceless advices, and invaluable encouragement.

I appreciate the assistance provided by the beam centrifuge technicians, Manuel

Palacios and Kelvin Leong, along with the workshop and electronics technicians.

The financial support provided by the Robert and Maude Gledden Postgraduate

Research Scholarships and the departmental Ad Hoc scholarships is acknowledged.

Finally, I must thank my wife, Maryam, for her love, warm encouragement, and endless

patience. I would also like to thank my parents for their concern.

xiii

Thesis format and Authorship

In accordance with regulations of the University of Western Australia, this thesis is

submitted as a series of papers. Chapters 2, 3, and 4, are papers which have been

published, while Chapter 5 has been resubmitted after review, and Chapter 6 to be

submitted. The contributions of the candidate and co-authors for the papers comprising

Chapters 2–6 are as follows:

Paper 1

The first paper is presented as Chapter 2 and is authored by the candidate, Professor

Christophe Gaudin, Winthrop Professor Mark Cassidy, and Associate Professor Britta

Bienen. The paper is published as:

xiv

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014). Spudcan

extraction from deep embedment in soft clay. Applied Ocean Research, Vol.

48, 126-136.

The candidate:

planned the experimental testing programme in consultation with Associate

Professor Britta Bienen;

prepared the drawings for the spudcan fabrication;

performed the experiments in the beam centrifuge;

analysed the data obtained from the experiments under the guidance of Professor

Christophe Gaudin;

wrote the majority of the paper in collaboration with Professor Christophe

Gaudin, Winthrop Professor Mark Cassidy, and Associate Professor Britta

Bienen.

Paper 2

The second paper is presented as Chapter 3 and is authored by the candidate, Professor

Christophe Gaudin, Winthrop Professor Mark Cassidy, and Associate Professor Britta

Bienen. The paper is published as:

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014). Predicting

spudcan extraction resistance in soft clay. Geotechnical Engineering

Journal of the SEAGS & AGSSEA, Vol. 45, No. 4, 52-61.

The candidate:

xv

gatherd 24 centrifuge test results;

checked the validity of the existing prediction method for spudcan embedment

up to 3 diameters;

proposed a set of recommendations to update and improve the prediction method

under the guidance of Professor Christophe Gaudin and Winthrop Professor

Mark Cassidy;

wrote the majority of the paper in collaboration with Professor Christophe

Gaudin, Winthrop Professor Mark Cassidy, and Associate Professor Britta

Bienen.

Paper 3

The third paper is presented as Chapter 4 and is authored by the candidate, Associate

Professor Britta Bienen, Winthrop Professor Mark Cassidy, and Professor Christophe

Gaudin. The paper is published as:

Kohan, O., Bienen, B., Cassidy, M.J., and Gaudin, C. (2013). Centrifuge

experiments to study extraction of a deeply embedded spudcan using top

jetting. Proc. 32nd International Conference on Offshore Mechanics and

Arctic Engineering (OMAE), Nantes

The candidate:


Professor Britta Bienen and Professor Christophe Gaudin;



xvi

analysed the data obtained from the experiments under the guidance of

Associate Professor Britta Bienen;

wrote the majority of the paper in collaboration with Associate Professor Britta

Bienen, Winthrop Professor Mark Cassidy, and Professor Christophe Gaudin.

Paper 4

The fourth paper is presented as Chapter 5 and is authored by the candidate, Associate

Professor Britta Bienen, Winthrop Professor Mark Cassidy, and Professor Christophe

Gaudin. The paper has been resubmitted after revising based on the Journal reviewers’s

comments:

Kohan, O., Bienen, B., Gaudin, C., and Cassidy, M.J. (2014). The effect of

water jetting on spudcan extraction from deep embedment in soft clay.

Ocean Engineering, Submitted revised version in November 2014.

The candidate:


Professor Britta Bienen and Professor Christophe Gaudin;



analysed the data obtained from the experiments under the guidance of

Associate Professor Britta Bienen;

checked the validity of the conceptual framework for estimating the required

jetting flow rate applied at the spudcan base to enable successful extraction from

soft clay soil for deeper embedments;

xvii

wrote the majority of the paper in collaboration with Associate Professor Britta

Bienen, Winthrop Professor Mark Cassidy, and Professor Christophe Gaudin.

Paper 5

The fifth paper is presented as Chapter 6 and is authored by the candidate, Winthrop

Professor Mark Cassidy, Professor Christophe Gaudin, and Associate Professor Britta

Bienen. The paper is palnned to submit as:

Kohan, O., Cassidy, M.J., Gaudin, C., and Bienen, B. (2015). Experimental

investigation of the effect of cyclic loading on spudcan extraction. To be

submitted.

The candidate:

planned the experimental testing programme in consultation with Winthrop

Professor Mark Cassidy;

performed a hydrodynamic analysis of a submerged jack-up hull based on the

three dimensional diffraction theory in the time domain by using the MOSES

software;


analysed the data obtained from the experiments under the guidance of Winthrop

Professor Mark Cassidy;

wrote the majority of the paper in collaboration with Winthrop Professor Mark

Cassidy, Professor Christophe Gaudin, and Associate Professor Britta Bienen.

xix

Declaration

I certify that, except where specific reference is made in the text to the work of others,

the contents of this thesis are original and have not been submitted to any other

university.

xx

DECLARATION FOR THESES CONTAINING PUBLISHED WORK AND/OR WORK PREPARED FOR PUBLICATION

1. This thesis does not contain work that I have published, nor work under review for publication.

Student Signature .......................................................................................

2. This thesis contains only sole-authored work, some of which has been published and/or prepared for publication under sole authorship. The bibliographical details of the work and where it appears in the thesis are outlined below.


3. This thesis contains published work and/or work prepared for publication, some of which has been co-authored. The bibliographical details of the work and where it appears in the thesis are outlined below. The student must attach to this declaration a statement for each publication that clarifies the contribution of the student to the work. This may be in the form of a description of the precise contributions of the student to the published work and/or a statement of percent contribution by the student. This statement must be signed by all authors. If signatures from all the authors cannot be obtained, the statement detailing the student’s contribution to the published work must be signed by the coordinating supervisor.

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014). Spudcan extraction from deep embedment in soft clay. Applied Ocean Research, Vol. 48, 126-136. Chapter 2. The estimated percentage contributionof the candidate is 70%.

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014). Predicting spudcan extraction resistance in soft clay. Geotechnical Engineering Journal of the SEAGS & AGSSEA, Vol. 45, No. 4, 52-61. Chapter 3. The estimated percentage contributionof the candidate is 80%.

Kohan, O., Bienen, B., Cassidy, M.J., and Gaudin, C. (2013). Centrifuge experiments to study extraction of a deeply embedded spudcan using top jetting. Proc. 32nd International Conference on Offshore Mechanics and Arctic Engineering (OMAE), Nantes. Chapter 4. The estimated percentage contributionof the candidate is 75%.

Kohan, O., Bienen, B., Gaudin, C., and Cassidy, M.J. (2014). The effect of water jetting on spudcan extraction from deep embedment in soft clay. Ocean Engineering, Submitted in November 2014. Chapter 5. The estimated percentage contributionof the candidate is 75%.

Kohan, O., Cassidy, M.J., Gaudin, C., and Bienen, B. (2015). Experimental investigation of the effect of cyclic loading on spudcan extraction. To be submitted. Chapter 6. The estimated percentage contributionof the candidate is 75%.


Coordinating Supervisor Signature ............................................................

xxi

Notations

A contact area of spudcan

cv coefficient of consolidation

dn diameter of the jetting nozzle

D diameter of spudcan

f filling ratio

fg,base gain in shear strength of soil below spudcan base due to any soil reconsolidation after spudcan installation

fg,top change in shear strength of soil above spudcan top due to soil disturbance and any soil reconsolidation after spudcan installation

fol factor of operation ratio

fsr factor of strength ratio

Gs specific gravity

g gravity acceleration

H depth

Hc cavity depth

Hs spudcan side wall (chapter 3)

xxii

Hs significant height of the wave (chapter 6)

Ht height of backfill above spudcan top surface

Ip plasticity index

JONSWAP JOint North Sea WAve Project

LL liquid limit

N centrifuge acceleration

Nball undrained bearing capacity factor of the ball penetrometer

Nc undrained bearing capacity factor

Nc,base Breakout factor for base soil resistance

Nc,top breakout factor for top soil resistance

Nf number of cycles to failure

NT-bar undrained bearing capacity factor of the T-bar

PIV Particle Image Velocimetry

PL plastic limit

pp pore pressure

psj maximum jetted suction at the spudcan invert

psnj maximum non-jetted suction at the spudcan invert

q pressure resistance

qe extraction resistance

qj maximum jetted extraction resistance

qnj maximum non-jetted extraction resistance

qp penetration resistance

qp-op operational pressure

Q net vertical load

Qbase base soil resistance

Qe extraction load

Qj flow rate

Qp penetration load

Qtop top soil resistance

xxiii

Quplift total uplift resistance

RAO Response Amplitude Operator

S shape factor

Sb adjustment factor for overburden stress at spudcan base level

sop settlement during operation

su undrained shear strength

suHc Shear strength at the cavity depth

su,top average shear strength of backfill soil above spudcan after installation

su,base shear strength at the spudcan base level after installation

Toper operation period

Tp peak wave period

Tv time factor

U degree of consolidation

V normalised penetration velocity

Vcyc amplitude of cyclic loading

Vex maximum extraction load in cyclic test

Vmean hull buoyancy load level

Vmon maximum extraction load in monotonic test

Vop operation load

Vop-cyc operational load in cyclic test

Vp penetration load

Vop-mon operational load in monotonic test

Vp-cyc maximum penetration load in cyclic test

Vp-mon maximum penetration load in monotonic test

v spudcan penetration or extraction velocity

Weff submerged weight of spudcan

reduction factor between the correctly scaled and the actual model nozzle diameter

empirical factor

b breakout depth

xxiv

f upward displacement at failure

ui,ex excess pore pressure at invert of spudcan at the breakout point

ui,ins excess pore pressure at invert of spudcan at the end of the installation

ui,op excess pore pressure at invert of spudcan at the end of the operational period

ut,ex excess pore pressure at top of spudcan at the breakout point

ut,ins excess pore pressure at top of spudcan at the end of the installation

ut,op excess pore pressure at top of spudcan at the end of the operational period

u excess pore pressure

' angle of internal friction

peak enhancement parameter

: submerged unit weight

'top unit weight of soil at top

'v effective vertical stress

1-1

CHAPTER 1

General Introduction

1.1 Jack-up unit

Self-elevating mobile units, commonly referred to as jack-ups (Figure 1.1), are

employed for offshore exploration and development purposes in shallow waters, up to

depths of 150 m. A typical unit consists of a triangular box-type buoyant hull arranged

with three or four independent open truss or columnar movable legs using a rack and

pinion system, which are capable of raising and lowering the hull over or into the sea.

Each independent, movable leg is fitted with an inverted conical footing known as a

spudcan mounted at the lower end of the leg. Spudcans are generally like circular or

polygonal shaped steel shoes with a central pointed end similar to a cleat, providing

additional stability to the rig during operations. The biggest spudcan built to date has a

diameter of 23 m and belongs to the modern jack-up platform West Elara, which has a

maximum operating water depth of 150 m. Figure 1.2 shows the evolution of spudcan

footings.


1-2

Currently, more than 400 jack-ups are deployed worldwide (plus more than 100 under

construction) at a utilisation of approximately 85% (rigzone, 2014) and perform

approximately half of the drilling activities worldwide. These units are located in

regions such as the Gulf of Mexico and West Africa, where the seabed sediments

comprise soft normally consolidated or lightly overconsolidated clay.

1.2 Jack-up installation

Jack-up rigs are moved into a location by the use of self-propulsion or towing with their

legs up and the hull floating on the water (Figure 1.3a). Upon arrival at the location, the

legs are lowered to the sea bed (Figure 1.3b) and jacked down until meeting sufficient

bearing capacity for the hull to be raised out of the water (Figure 1.3c). Then, the jack-

ups are preloaded by pumping seawater into the preload tanks to increase the self-

weight, which causes the spudcan foundations to penetrate further into the seabed

(Figure 1.3d). This proof test ensures that the spudcan foundations would not experience

a larger vertical load during operation. The full preload is held for a minimum duration

of 2 to 4 hours after the spudcan foundation penetration is completed (Young et al.,

1984). During preloading, which normally takes approximately 24 to 36 hours, the

spudcan footings can penetrate up to three spudcan diameters in soft soil (Endley et al.,

1981; Menzies and Roper, 2008). After a stable condition is achieved, the ballast tanks

are emptied before operations on the jack-up begin (Figure 1.3e).

1.3 Jack-up operation

The operation of a mobile jack-up rig in the field can take a few weeks or longer, such

as is the case for Al Morjan, a six-legged jack-up rig that is located at a position 80 km

north of Qatar in the Persian Gulf and that has been in operation since 2003. During this

period, a jack-up is subjected to environmental loads, such as those from storm

Chapter 1

1-3

conditions, whereby the vertical load may increase by up to 50% of the gravitational

load due to waves, winds, and current forces (McClelland et al., 1981).

1.4 Leg extraction issues

After operation is complete, jack-ups are manoeuvred to a new location. Therefore, the

legs and spudcan footings must be extracted from the seabed by jacking down the hull

into the water and subsequently lowering it beyond neutral draft. The maximum

allowable overdraft, typically approximately 0.3 m, although an overdraft of 1.6 m has

also been reported by Purwana et al. (2009), provides the maximum available pull-out

force used to overcome the soil resistance. However, this tensile load may not be

sufficient to extract the foundation after long operational periods, especially in softer

soils and corresponding deep embedment.

Spudcan extraction can take one or two weeks, although durations of up to ten weeks

have been reported (InSafe JIP, 2008), and is a time-consuming process with significant

financial impact because the average jack-up day rates are in the range of US$60,000 to

US$160,000 (depending on the water depth).

1.5 Mitigation methods for jack-up removal

When difficulties in leg retraction are identified, standard spudcan extraction procedures

may be supplemented with the following methods, which can be applied individually,

simultaneously or sequentially (InSafe JIP, 2011):

Water jetting through spudcan nozzles

Excavation of soil present above the spudcan

Cyclic loads


1-4

Nevertheless, these measures do not guarantee that the spudcan can be easily extracted.

Water jetting assists in spudcan retrieval by reducing the breakout force (Bienen et al.,

2009; Gaudin et al., 2011). Most modern mobile drilling rigs are equipped with a water

jetting system integrated into the spudcan. The water is supplied from pumps located on

the hull, delivered through hoses down the jack-up legs and jetted through nozzles

located on the top, upper, side, and bottom faces of the spudcan.

Figure 1.4 shows a schematic of the leg and spudcan jetting system of a jack-up rig

belonging to the Atwood Oceanics Company. Each spudcan has 12 nozzles at the

bottom face, 3 nozzles at the top face, 9 nozzles at the upper face, and 6 nozzles at the

side face. When jetting is required, deep well pumps with a flow rate of 500 m3 per hour

and a head of 60 m are used. The jetting system is rated for 3000 psi.

However, under soft clay conditions and considering the associated deep penetrations,

the application of water jetting in offshore condition is often found ineffective for

reducing the soil resistance. For example, jetting failed to free the spudcan of a

Marathon LeTourneau design, class 53 jack-up rig that was embedded at 31.6 m, which

correspond to 2.26 spudcan diameters, in soft normally consolidated clay (Menzies and

Lopez, 2011). To free the spudcan, two independent drill strings were used to remove

soil from on top of the spudcan and jet down to below the spudcan to pump additional

water.

Cyclic loads due to small-amplitude wave loading (InSafe JIP, 2011) can also aid in

spudcan removal by remoulding the surrounding soil using cyclic loading. When jetting

fails to extract a spudcan, the jack-up rig operators often hold the legs in tension and

attempts to move the legs up and down to disturb the surrounding soil while applying

water jetting (Purwana, 2006).

Chapter 1

1-5

1.6 Background

This section presents literature that is pertinent to studies of spudcan extraction in soft

clay, focusing on studies concerned with the breakout force, and describes how

knowledge about the behaviour of spudcan during extraction has been developed.

Various other studies on the extraction of spudcans in different soils (such as sand) or

the uplifting of other objects (such as circular anchors) are not discussed here.

1.6.1 Suction force

Craig and Chua (1990) discussed the importance of base suction (negative excess pore

pressure at the base of the spudcan) generated during spudcan extraction by performing

a series of centrifuge tests at an acceleration of 100g and simulating the installation and

extraction of a model spudcan 14 mm in diameter from uniform soft clay with an

undrained shear strength in the range of 12-40 kPa. The results indicated that the

magnitude of the suction was related to the compressive loading history and the

associated embedment ratio prior to extraction. However, various issues, such as the

operating period that the jack-up is installed for, were not studied.

Later, Purwana et al. (2005) performed a comprehensive study on the suction generated

beneath spudcans during extraction, which is explained later in this chapter.

1.6.2 Failure mechanism

Gaudin et al. (2011) developed a failure mechanism during initiation of undrained

spudcan extraction from an embedment of up to 1.5 spudcan diameters from

observations of a Particle Image Velocimetry (PIV) analysis of physical tests in

Purwana et al. (2006) and from a numerical analysis in Zhou et al. (2009).


1-6

In the first stage of the undrained extraction, the mechanism is a combination of reverse

end bearing at the spudcan invert resulting in a high level of suction and an uplift

mechanism of the soil above the spudcan (Figure 1.5a). Specifically, the main soil

resistance is comprised of the weight of the soil above the spudcan, the resistance along

a shear plane generated above the spudcan and the negative excess pore pressure that

develops at the spudcan base in undrained extraction.

In the second stage, the extraction resistance typically reaches a peak followed by a

dramatic reduction in resistance. The failure mechanism is then replaced by a localised

flow-around mechanism, which is also associated with an uplift mechanism of the soil

above the spudcan (Gaudin et al., 2011) (Figure 1.5b).

Hossain and Dong (2014) investigated the behaviour of spudcans during vertical

extraction through single, double, and multilayer soils at 50~200g in centrifuge. All the

test spudcans were immediately extracted from embedment of less than 1.5 spudcan

diameters, which means that the effect of operating duration was ignored. A 60-mm-in-

diameter half-spudcan model was used for a PIV analysis, and two separate full-

spudcans with a diameter of 60 and 30 mm were employed to measure the uplift

resistance. For single, double, and multilayer clays, it was concluded that the same

components as noted by Gaudin et al. (2011) constitute the extraction resistance. The

maximum extraction resistance was found at different extraction depths, depending on

the soil layer geometries, types and properties.

Chapter 1

1-7

1.6.3 Operating period

Effect of operation period on suction pressure

Purwana et al. (2005) experimentally investigated the effect of the operating period and

operating load magnitude on spudcan extraction from normally consolidated Malaysian

kaolin clay under undrained conditions.

A series of centrifuge tests at an acceleration of 100 g was performed using a model

spudcan 125 mm in diameter, which was extracted from an embedment of

approximately 1.5 spudcan diameters, using varying operating periods. The top and

bottom faces of the model spudcan were instrumented with total pressure and pore

pressure transducers. In addition, the soil sample was instrumented with a set of pore

pressure transducers installed at various locations within the soil beneath the spudcan.

Furthermore, several displacement transducers were placed on the surface to monitor

changes in pore pressure during the installation, operation and extraction of the spudcan.

The results demonstrated that the extraction resistance increases with increasing

operating period. In contrast, the jack-up operating load (i.e., the load maintained during

the operating period) has an insignificant effect on spudcan extraction compared with

the duration that a jack-up is being operated.

It is noteworthy that Purwana et al. (2005) investigated spudcan extraction from an

embedment of up to 1.5 spudcan diameters. To the author’s knowledge, the deepest

spudcan penetration reported is 78 meters in the Gulf of Mexico, which corresponds to

an embedment ratio of 5.6 (Menzies and Lopez, 2011), although this is exceptional, and

penetrations of up to a maximum of three spudcan diameters are more common

(Menzies and Roper, 2008).


1-8

Effect of operating period on changes in soil shear strength

During operation, the effective stresses of the soil underneath and at the top of a

spudcan increase due to the dissipation of the excess pore pressures generated by the

penetration process.

Purwana et al. (2009) performed a series of T-bar tests in Malaysian kaolin clay to

measure the shear strength of the remoulded soil at the top of a spudcan during

operation. The shear strength was observed to decrease to 67% of the undisturbed shear

strength immediately after spudcan installation; however, it increased by 30% (or 87%

of the undisturbed soil shear strength) after a 400-day reconsolidation period.

Similarly, the increase in shear strength underneath the spudcan after the same operating

period was shown to be 1.70 times the undisturbed strength in Purwana et al. (2009)

based on a numerical analysis.

1.6.4 Prediction method for the estimation of extraction resistance

An estimation of the spudcan extraction resistance is necessary for jack-up operators to

anticipate potential extraction issues as a part of assessing the jack-up removal process

prior to moving to a new location. If the spudcan extraction resistance is higher than the

available extraction force, mitigation actions are required to facilitate spudcan

extraction.

Two methods have been developed to estimate the maximum spudcan extraction

resistance:

Purwana et al. (2009)

Osborne et al. (2011) or InSafe JIP (2011)

Chapter 1

1-9

The method detailed in Purwana et al. (2009) is based on measurements of the total and

pore pressures at various locations on a model spudcan in centrifuge experiments as

well as based on information regarding soil failure mechanisms from Particle Image

Velocimetry analyses for embedment of up to 1.5 spudcan diameters. In this method,

the solution of Merifield et al. (2003), which was developed for the calculation of the

uplift force of circular anchor shapes, is adopted to predict the soil resistance above the

spudcan at breakout. The base resistance is based on the assumption of a reverse bearing

capacity mechanism and was derived from back-calculated centrifuge data.

In the method in Osborne et al. (2011), the breakout load comprises the shear resistance

along the vertical planes above the spudcan, the side friction along the spudcan’s side

wall, the overburden soil weight, and the base resistance for an embedment of up to 1

spudcan diameter with respect to the cavity depth (intermediate embedment). For the

deep embedment case, the failure planes tend to develop locally; therefore, the breakout

load comprises only the overburden soil weight and the base resistance.

1.6.5 Application of water jetting

The author is unaware of any published literature on the effect of top water jetting on

spudcan extraction from clay soils. However, Bienen et al. (2009) and Gaudin et al.

(2011) experimentally investigated the effectiveness of bottom jetting through

centrifuge tests. The tests conducted at an acceleration of 200 g modelled a spudcan

17.11 m (prototype) in diameter equipped with three sets of twelve jetting nozzles at the

spudcan base. The embedment depth was up to 1.46 spudcan diameters, which

corresponded to 25 m in the prototype scale. The term "filling ratio" was introduced to

relate the reduction in maximum extraction resistance, or breakout force, to the water

volume required in the jetting process. The filling ratio is defined as the ratio of the


1-10

volume of water jetted into a theoretical void left by the extracting spudcan and is

calculated as the total jetting flow rate divided by the product of the spudcan extraction

rate and spudcan invert contact area.

The reduction in extraction resistance was shown to depend on the filling ratio rather

than on the jetting pressure, and at a filling ratio of 0.7, the jetting flow rate was found

to negate the generation of negative excess pore pressure at the spudcan invert. Bienen

et al. (2009) and Gaudin et al. (2011) proposed a conceptual framework to estimate the

required bottom jetting flow rate based on the centrifuge experimental data. Note that

the conclusions are valid only if the behaviour of the surrounding soil is considered to

be undrained during the extraction process, and the proposed framework is limited to

spudcan extraction from a maximum embedment depth of 1.5 diameters.

1.6.6 Cyclic loading

InSafe JIP (2011) postulated that the cyclic excitation loads generated by small

amplitude waves remould the soil surrounding the spudcan. This may help spudcan

extraction provided that the wave loading is sufficiently rapid to avoid consolidation

and strengthening of the soil around the spudcan.

The author is unaware of any experimental or numerical investigations performed to test

this hypothesis. Moreover, the amplitude of the cycling loads has not been discussed.

1.7 Beam geotechnical centrifuge

Soil properties are dependent on stress levels, and their strength and stiffness and

consequently the deformation and failure mechanisms are affected by the effective

stress. Therefore, to perform tests at the same effective stress as in the field, centrifuge

Chapter 1

1-11

modelling was introduced to reproduce similar soil behaviour in a reduced-scale version

of a prototype.

The experiments reported in this thesis were performed in the beam geotechnical

centrifuge at the University of Western Australia (Randolph et al., 1991) (Figure 1.6).

The centrifuge facility comprises an Acutronic Model 661 geotechnical centrifuge with

a swinging platform radius of 1.8 m and a nominal working radius of 1.55 m rated at 40

g-tonnes. The platform seats standard rectangular ‘strongboxes’, which have internal

dimensions of 650 × 390 mm and are 325-mm deep, representing an up to 80-m-wide,

130-m-long, 60-m-deep prototype test bed at 200 g.

A headroom of 900 mm above the strongbox allows a two-dimensional actuator to be

mounted on the strongbox to penetrate and extract the spudcan in the underlying soil. A

load cell that measures vertical loads on the spudcan during penetration and extraction

is connected to the actuator, which is controlled by an in-house interface based on

Labview (De Catania et al., 2010).

In centrifuge modelling, the linear dimensions of the model are scaled down by a factor

of N relative to the prototype, where N is the ratio of the centrifugal acceleration to the

gravitational acceleration on the surface of the Earth. Scale factors for other parameters,

such as load, pressure, and time, can be found in Garnier et al. (2007), who created an

inventory of the scaling laws and similitude questions related to centrifuge modelling.

Examples of some quantities are listed in Table 1.1.

1.8 Aim of research

Previous findings in the field of spudcan extraction on topics such as failure

mechanisms, effectiveness of jetting, and estimation of maximum extraction are limited


1-12

to spudcan extraction from a maximum embedment depth of 1.5 diameters. Therefore, it

is required to develop and expand the current knowledge base to deeper embedment

depths, where major spudcan extraction issues occur and which have not been addressed

by existing codes or standards for jack-up rigs design and operation.

The overarching aim of this thesis is to develop an understanding of spudcan extraction

from deep embedment in normally consolidated clay through the analysis of data

obtained via physical modelling. To achieve this, this research will investigate the

following three themes:

1. Extraction of deeply embedded spudcans

2. Extraction of deeply embedded spudcans using water jetting

3. Extraction of deeply embedded spudcans under cyclic loading

The following specific aims are derived from these three themes:

Aim 1: Specifying the breakout failure mechanism of deeply embedded

spudcans.

Aim 2: Improving a prediction method for spudcan extraction based on data of

embedment depth and vertical load history.

Aim 3: Understanding the effectiveness of top jetting in reducing the extraction

resistance from a deep embedment.

Aim 4: Realising the performance of the bottom jetting application for easing the

extraction of deep embedment spudcans.

Aim 5: Providing an insight into the behaviour of the spudcan during extraction

under cyclic loading.

Chapter 1

1-13

1.9 Thesis outline

This thesis is presented as a collection of technical papers, with each chapter comprising

a different paper. Each chapter includes an introduction section in which current

practices and literature relevant to the particular topic are reviewed. Due to the style of

the thesis, these introductory sections overlap somewhat across the various chapters.

Each chapter closes with concluding remarks specific to the topic of the chapter.

Conclusions relevant to the entire thesis are presented along with recommendations for

future work in a final chapter (Chapter 7). Chapters 2–6 form the thesis body and

address the research aims as follows.

Aim 1: Specifying the breakout failure mechanism of deeply embedded spudcans

Chapter 2 presents the failure mechanism that governs the extraction of a spudcan from

an embedment of up to 3 spudcan diameters. A series of centrifuge tests was performed

to determine if a change in extraction mechanism at deeper embedment depths affects

the suction generation at the spudcan invert. The existing developed failure mechanism

is valid for an embedment ratio of up to 1.5.

Aim 2: Improving a predictive method for spudcan extraction based on data of

embedment depth and vertical load history

Chapter 3 outlines modifications to the input parameters of the existing method for the

prediction of the maximum extraction resistance based on insights obtained from an

experimental model database of 24 centrifuge tests featuring spudcan extraction from

normally consolidated clay. The validity of the improved method for spudcan

embedment of up to 3 diameters is also discussed.

Aim 3: Understanding the effectiveness of top jetting in reducing the extraction

resistance from deep embedments


1-14

Chapter 4 addresses the efficiency of top water jetting on spudcan extraction. The

results of centrifuge tests performed using the beam geotechnical centrifuge with the

objective of investigating the extraction of deeply embedded spudcans equipped with

concentric circles of nozzles on the top outer face are presented in this chapter.

Aim 4: Determining the performance of bottom jetting to ease the extraction of deeply

embedded spudcans

Chapter 5 extends the framework for estimating the required jetting flow rate applied at

the spudcan base to enable the successful extraction from soft clay soil, which has

previously been proposed for embedment depths of up to 1.5 diameters. This chapter

reports the results of centrifuge experiments on jetted spudcan extraction from depths of

up to 3 diameters in normally consolidated clay.

Aim 5: Providing insight into the behaviour of the spudcan during extraction under

cyclic loading

Chapter 6 presents the effect of the sea state on the pull-out resistance. Spudcan

responses to the wave actions taken from results of the hydrodynamic analyses of a

submerged jack-up hull based on three-dimensional diffraction theory in the time

domain were used in the centrifuge to simulate cyclic loading on spudcan extraction.

The behaviour of the spudcan during extraction from depths of 3 spudcan diameters in

normally consolidated clay under both regular and irregular cyclic loading is examined

and discussed in this chapter.

Chapter 1

1-15

References

Atwood Oceanics (2014) "Personal correspondence".

Bienen, B., Gaudin, C., and Cassidy, M.J. (2009). The influence of pull-out load on the

efficiency of jetting during spudcan extraction. Applied Ocean Research, Vol.

31, No. 3, 202-211.

Craig, W.H. and Chua, K. (1990). Extraction forces for offshore foundations under

undrained loading. ASCE J. Geotechnical. Engineering 116, No. 5, 868–884.

De Catania, S., Breen, J., Gaudin, C., and White, D.J. (2010). Development of a

multiple axis actuator control system. Proc. of the 7th Int. Conference on

Physical Modelling in Geotechnics, Zurich, Switzerland, 325-330.

Endley, S.N., Rapoport, V., Thompson, P. J. and Baglioni, V.P. (1981). Prediction of

jack-up rig footing penetration. Proc. 13th Offshore Technology Conference,

Houston, OTC 4144.

Gaudin, C., Bienen, B. and Cassidy, M.J. (2011). Investigation of the potential of

bottom water jetting to ease spudcan extraction in soft clay. Géotechnique, Vol.

61, No. 112, 1043-1054.

Garnier, J., Gaudin, C., Springman, S.M., Culligan, P.J., Goodings, D., Konig, D.,

Kutter, B., Phillips, R., Randolph, M.F., and Thorel, L. (2007). Catalogue of

scaling laws and similitude questions in centrifuge modelling. Int. Journal of

Physical Modelling in Geotechnics, Vol. 7, Issue. 3, 1–23.

Hossain, M. and Dong, X. (2014). Extraction of spudcan foundations in single and

multilayer Soils. Journal of Geotechnical and Geoenvironmental Engineering,

Vol. 140, No, 1, 170–184.

InSafe JIP (2008). Minutes of the 2nd progress meeting of the InSafe JIP. Singapore, 20

November 2008.

InSafe JIP (2011). Improved guidelines for the prediction of geotechnical performance

of spudcan foundations during installation and removal of jack-up units. Joint


1-16

Industry-funded Project. Authors: Osborne, J.J., Teh, K.L., Houlsby, G.T.,

Cassidy, M.J., Bienen, B., Leung, C.F. 28th March 2011.

Leung, C.F., Lee, F.H. and Tan, T.S. (1991). Principles and application of geotechnical

model testing. Journal of Institute of Engineers, Singapore, Vol. 31, No. 4, 39-

45.

McClelland, B., Young, A.G. and Remmes, B.D. (1981). Avoiding jack-up rig

foundation failures. Proc. Int. Symp. Geotechnical Aspects of Coastal and

Offshore Structures, Bangkok.

Menzies, D., and Lopez, C.R. (2011). Four Atypical Jack-up Rig Foundation Case

Histories. 13th International Conference, The Jack up Platform, London.

Menzies, D., and Roper, R. (2008). Comparison of Jackup rig spudcan penetration

methods in clay. Proc. 40th Offshore Technology Conference, Houston, USA,

OTC 19545.

Merifield, R.S., Lyamin, A.V., Sloan, S.W. and Yu, H.S. (2003). Three-Dimensional

Lower Bound Solutions for Stability of Plate Anchors in Clay. Journal of

Geotechnical and Geoenvironmental Engineering, Vol. 129, No. 3, 243-253.

Poulos, H. G. (1988). Marine Geotechnics. Unwin Hyman, London.

Purwana, O.A., Leung, C.F., Chow, Y.K., and Foo, K.S. (2005). Influence of base

suction on extraction of jack-up spudcans. Géotechnique, Vol. 55, No. 10, 741-

753.

Purwana, O.A., Leung, C.F., Chow, Y.K., and Foo, K.S. (2006). Breakout failure

mechanism of jackup spudcan extraction. Proc. 6th Int. Conference on Physical

Modelling in Geotechnics, Hong Kong, 667–672.

Purwana, O.A. (2006). Centrifuge model study on spudcan extraction in soft clay. PhD

Thesis, National University of Singapore, 2006.

Purwana, O.A., Quah, M., Foo, K.S., Nowak, S., and Handidjaja, P. (2009). Leg

Extraction / Pullout Resistance - Theoretical and Practical Perspectives. In.

Proc. 12th Jack up Conf., London.

Chapter 1

1-17

Randolph, M.F., Jewell, R.J., Stone, K.J.L., and Brown, T.A. (1991). Establishing a new

centrifuge facility. Proc. Int. Conference on Centrifuge Modelling, Centrifuge

91, Boulder, Colorado, 3-9

Rigzone.com, accessed 7 January 2014.

Senner, D. W. F. (1993). Analysis of long term jack-up rig foundation performance.

Offshore Site Investigation and Foundation Behaviour, Socity for underwater

technology, Vol. 28, 691-716.

Young, A.G., Remmes, B.D. and Meyer, B.J. (1984). Foundation performance of

offshore jackup drilling rigs. Journal of Geotechnical Engineering, ASCE, Vol.

110, No. 7, 841-859.

Zhou, X.X., Chow, Y.K. and Leung, C.F. (2009). Numerical modelling of extraction of

spudcans. Géotechnique, Vol. 59, No. 1, 29-39.


1-18

Table 1.1. Centrifuge scaling relations (after Leung et al., 1991)

Quantity Relationship (Model/Prototype)

Gravity N

Length 1/N

Area 1/N2

Volume 1/N3

Displacement 1/N

Stress 1

Strain 1

Force 1/N2

Velocity 1

Acceleration N

Density 1

Mass 1/N3

Energy 1/N3

Time (consolidation) 1/N2

Chapter 1

1-19

Figure 1.1. Seafox 8, an accommodation jack-up rig suited for the harsh environment of the Norwegian North Sea (after www.google.com.au, sourced 28 October 2014)

Spudcan

Leg

Hull

General Introd

Figure 1

duction

1.2. Evolutiion of spudccan geometrSenner

ry from 195r, 1993)

55 to 1982 (after Poulos

1-20

s, 1988;

Chapter 1

1-21

(a)

(b)


1-22

(c)

(d)

Chapter 1

1-23

(e)

Figure 1.3. Jack-up rig installation sequence (www.youtube.com, sourced 20 October 2014)


1-24

(a) Leg and spudcan jetting system

Chapter 1

1-25

(b) 3 nozzles at the top face of the spudcan

(c) 9 nozzles at the upper face of the spudcan


1-26

(d) 12 nozzles at the bottom face of the spudcan

Figure 1.4. Schematic of the leg and spudcan jetting system (Atwood Oceanics, 2014)

Chapter 1

1-27

(a)

(b)

Figure 1.5. Failure mechanisms during undrained extraction (after Gaudin et al., 2011)

General Introd

duction

Figuure 1.6. Thee UWA geootechnical b

eam centriffuge

1-28

2-1

CHAPTER 2

Spudcan Extraction from Deep Embedment in

Soft Clay

2.1 Abstract

After drilling is completed, spudcan footings of mobile jack-up rigs are extracted from

the seabed before the jack-up is manoeuvred to a new location. In some instances, the

extraction may prove to be difficult and time consuming, especially when the spudcans

are deeply embedded, because the pull-out capacity of the rig is less than the extraction

resistance of the spudcans. In soft soil, the extraction resistance may be significantly

augmented by the development of suction at the spudcan invert. To investigate this

phenomenon, a deeply embedded 30 mm diameter model spudcan was extracted in a

series of physical model experiments conducted at an acceleration of 200g in a

geotechnical beam centrifuge. The spudcan, instrumented with two pore pressure

transducers, one at the top and one at the bottom face, was extracted from normally

consolidated clay and under undrained conditions. Eight tests are reported exhibiting

Spudcan Extraction from Deep Embedment in Soft Clay

2-2

embedments ranging from 1.5 to 3 spudcan diameters and varying operation periods.

The excess pore pressure and maximum breakout force measured reveal insights into

the magnitude of the suction forces at the spudcan invert, which were observed to

increase with the embedment depth. No change in failure mechanism was observed

between 1.5 and 3 spudcan diameters depth.

2.2 Introduction

Self-elevated mobile jack-up units (Figure 2.1) play an important role in offshore

drilling in shallow waters, up to approximately 150 m depth. The inverted conical

footings of jack-ups, which are known as spudcans and can be in excess of 20 m in

diameter in a modern jack-up (Cassidy et al., 2009), can be penetrated in a wide range

of soil conditions. In softer soils, spudcans require large penetration before meeting

sufficient bearing capacity to withstand the jack-up’s self-weight and the expected

operational loads. Penetration of up to two or three spudcan diameters may be necessary

before reaching equilibrium during the preloading process (Endley et al., 1981; Menzies

and Roper, 2008).

When a jack-up rig is removed from a site and redeployed, its spudcans must be

extracted from the seabed. To overcome the soil resistance, the hull is floated, and

lowered beyond neutral draft. However, tolerances on the maximum allowable overdraft

within the marine operations manual restrict the maximum extraction pull to between 30

and 50% of the maximum compressive load that can be applied during installation

(Purwana et al. 2009). In soft soils for deep spudcan penetration (> 1.5 times the

spudcan diameter) and long operation periods, the buoyancy of the hull may not be

sufficient to extract the spudcan. It is reported that spudcan extraction from penetration

depths of one or two spudcan diameters can require one or two weeks, and in some

Chapter 2

2-3

extreme cases, up to ten weeks (InSafe JIP, 2008). The spudcan extraction process,

especially from soft clay, may therefore be a time-consuming process. With average

jack up day rate in the range US$60,000 to US$160,000 (depending on the water depth),

this has significant financial impact.

Figure 2.2 shows failure mechanisms during initiation of undrained spudcan extraction,

as developed by Gaudin et al. (2011) from observations from Particle Image

Velocimetry (PIV) analysis of physical tests by Purwana et al. (2006) and numerical

analysis by Zhou et al. (2009). In the first stage of the undrained extraction of the

spudcan, the main soil resistance is comprised of the weight of the soil above the

spudcan, the resistance along a shear plane generated above the spudcan and negative

excess pore pressure, namely suction, that is developed at the spudcan base in undrained

extraction. In fact, the extraction mechanism is a combination of an uplift mechanism of

the soil at the top of the spudcan and reverse end bearing at the spudcan invert due to

suction. The contribution of both theses mechanisms is influenced by the duration of the

jack-up operation. At the top of the spudcan, Purwana et al. (2009) measured via T-bar

tests a reduction of 67% of the shear strength immediately after spudcan installation,

followed by an increase of 30% (raising the shear strength to 87% of the undisturbed

undrained shear strength) after an operation period of 400 days. Similarly, the gain in

shear strength underneath the spudcan after the same operation period time was

evaluated as 1.70 times the undisturbed strength by Purwana et al. (2009) from

numerical analysis. Both outcomes imply an increase in effective stresses within the soil

underneath and at the top of the spudcan, resulting from dissipation of the excess pore

pressures generated during the penetration process, albeit at a different rate and

magnitude. The phenomena governing the changes in effective stresses in the soil are


2-4

complex and for the soil at the top, potentially include changes in total stresses due to

arching.

In a second stage, the extraction resistance typically reaches a peak followed by a

dramatic reduction in resistance. The failure mechanism is then replaced by a localised

flow around mechanism, still associated with an uplift mechanism of the soil above the

spudcan (Gaudin et al., 2011).

From the observed failure mechanism described, it may be inferred that suction forces

contribute significantly to the peak undrained extraction resistance. The importance of

base suction generated during spudcan extraction was first revealed by a series of

centrifuge tests performed at an acceleration of 100 g and simulating the installation and

extraction of spudcans from uniform soft clay with an undrained shear strength in the

range of 12-40 kPa (Craig and Chua, 1990). Results indicated that the magnitude of

suction was related to the compressive loading history and the associated embedment

ratio prior to extraction. However, issues such as the operational period that the jack-up

is installed for were not studied by and these form an important component of the

testing programme discussed in this paper.

Purwana et al. (2005) experimentally investigated the effect of operation period and

operating load magnitude level on spudcan extraction. Results demonstrated that the

extraction resistance increases with the operation period. In contrast, the level of jack-up

operating load (i.e. the load maintained during the operation period) has an insignificant

effect on spudcan extraction in comparison with the time that a jack-up is under

operation. It is noteworthy that Purwana et al. (2005) investigated spudcan extraction

from embedment up to 1.5 spudcan diameters. To the authors’ knowledge, the deepest

spudcan penetration reported is 78 meters in the Gulf of Mexico, corresponding to an

Chapter 2

2-5

embedment ratio of 5.6 (Menzies and Lopez, 2011), although this is exceptional and

penetrations up to a maximum of three spudcan diameters are more common (Menzies

and Roper, 2008).

The objective of the present study is to extend the database of Purwana et al. (2005) to

embedment up to 3 times the spudcan diameter, to notably investigate if a change of

mechanism at deeper embedment may affect the suction generation at the spudcan

invert. For this purpose, a series of centrifuge tests were performed, featuring

penetration and extraction after varying operating period of a model spudcan penetrated

at embedment ratio between 1.5 and 3.

Vertical loads and pore pressures at the top and bottom of the spudcan during the

installation, operation period and extraction of the spudcan were monitored, and the

results are reported and discussed.

2.3 Soil preparation and characterisation

Commercial Kaolin clay with characteristics provided in Table 2.1 was used to create a

soft soil sample in the beam centrifuge at the University of Western Australia (Stewart

and Randolph, 1991; Gaudin et al., 2011). The mixture of Kaolin and water at a

moisture level of twice the liquid limit formed a de-aired clay slurry, which was then

poured into a rectangular strongbox over a 15 mm thick drainage sand layer.

Thereafter, the sample was consolidated under self-weight in the centrifuge at an

acceleration of 200 g for a period of approximately five days. Over the consolidation

time, settlement of the sample was measured, and at the end, the final height of the soil

specimen was approximately 180 mm.


2-6

A 15 mm diameter miniature piezoball penetrometer (as also used by Mahmoodzadeh et

al., 2011) was used to derive the undrained shear strength profile of the sample with a

bearing capacity factor of 10.5 (Low et al., 2011; Lee et al., 2012; Lee et al., 2013). The

test was performed in flight at a rate of 1 mm/s to ensure that undrained conditions were

measured (Chung et al., 2006). The average shear strength gradient was approximately

1.1 kPa/m (Figure 2.3).

2.4 Experimental programme and procedure

A model spudcan with diameter D of 30 mm was fabricated to investigate the extraction

of deeply embedded spudcans (Figure 2.4). The spudcan was manufactured from

aluminium alloy 6061-T6 and was connected to a two-dimensional actuator via a load

cell. The model spudcan was instrumented with two pore pressure transducers (one at

the top face and one at the base) that were installed at approximately half the distance

between the centre and the edge of the spudcan. The cross-section of the pore pressure

transducers at the top and base of the spudcan is illustrated in Figure 2.5.

Eight tests were performed at an acceleration of 200 g in a beam geotechnical centrifuge

(Randolph et al., 1991). Tests one to four were designed to investigate the effect of the

embedment depth on spudcan extraction, whereas tests five to eight were performed to

investigate the effect of the duration of operation time on spudcan extraction. In the first

four tests, the spudcan installation depth was varied from 1.5 to 3 times the spudcan

diameter. In these tests, spudcan extraction occurred after two years operating load (in

prototype scale). In the remainder of the tests, the operation period varied from

immediate extraction to three years, and the spudcan embedment ratio was 1.5 D.

Details of the test programme are provided in Table 2.2.

Chapter 2

2-7

Spudcan penetration and extraction was undertaken at a penetration rate v of 0.3 mm/s,

resulting in a normalised velocity V=vD/cv greater than 30 (assuming a coefficient of

consolidation cv of 3.99 m2/y, at a stress level consistent with the spudcan embedment,

see Table 2.2). This ensured that spudcan installation and extraction occurred under

undrained conditions (Finnie and Randolph, 1994), mimicking in–situ conditions. In the

field, successful spudcan extraction may require between 6 hours and 30 hours.

Considering spudcan diameters in the range 10 to 20 m and coefficient of consolidation

in the range 0.1 to 100 m2/year, normalised extraction velocity in–situ are typically

greater than 30.

The same test procedure was used for all cases and consisted of three stages. In the first

stage, spudcan penetration was performed in-flight in displacement-control mode. The

embedment depth ranged from approximately 8.8 m to 18.1 m (prototype scale)

corresponding to an embedment ratio of 1.5 to 3, respectively. In the second stage, the

jack-up operation period was simulated by holding a constant vertical load of

approximately 85% of the maximum installation load for up to three years in prototype

scale. For operating period of 2 years and above, pore pressure measurements at the

spudcan invert indicated that at least 85% of consolidation was achieved. Finally, in the

third stage, spudcan extraction was performed at a constant rate of 0.3 mm/s.

For all stages, the vertical force on the spudcan (corresponding to the penetration

resistance, the applied load, and the extraction resistance for the three stages of testing,

respectively) and pore pressures at the top and the invert of the spudcan were

monitored.


2-8

2.5 Experimental Results

2.5.1 Installation resistance

The development of penetration resistance Qp, excess pore pressure (with respect to the

hydrostatic pressure) at the spudcan invert ui and at the spudcan top ut, are presented

in Figure 2.6, Figure 2.7, and Figure 2.8, respectively, for the installation, operation and

extraction stages.

Figure 2.9 presents the normalised net vertical load Qp/(A.su) where Qp is the net

penetration resistance measured by the load cell, A the projected area of the spudcan

and su the undisturbed shear strength at the spudcan embedment, against the normalised

embedment H/D, where H is the penetration depth and D the spudcan diameter. Note

that the spudcan embedment is defined at the lowest full diameter of the shoulder of the

spudcan. This provides insight into the net bearing capacity factors during penetration.

During installation, excess pore pressures, both at the top and the invert of the spudcan,

increase linearly with depth. Tests performed by Purwana et al. (2005) on a larger

spudcan, instrumented with both total and pore pressure transducer at the top and invert

of the spudcan, demonstrated that excess pore pressures where are consistent with the

change in total pressures during penetration, indicating no change in effective stresses

and so a fully undrained process. Based on the same assumption, the penetrating

pressure, comprising of the applied pressure qp = Qp/A and the excess pore pressure at

the top of the spudcan ut,ins, is compared to the resisting pressure ui,ins corresponding

to the excess pore pressure at the spudcan invert in Figure 2.10. Values at the end of the

installation phase are presented in Table 2.3. The agreement is reasonably good

throughout the full penetration process, confirming the observations from Purwana et al.

(2005), and demonstrating the undrained response of the soil.

Chapter 2

2-9

This result is however surprising. The phenomena governing the changes in pore

pressures at the invert and at the top of the spudcan are complex and involve changes in

both effective and total stresses. At the spudcan invert, the soil is essentially sheared so

an element of soil underneath the spudcan is expected to experience a reduction in

effective stresses, reflecting the remoulding of the soil, as well as an increase in pore

pressures. The magnitude of the reduction in effective stresses is difficult to assess and

is likely to vary along the spudcan. At the top of the spudcan, the phenomenon is even

more complex. Pore pressures at the top of the spudcan are likely induced from the

shearing of the soil (which is flowing from underneath the spudcan), but also from a

cavity expansion mechanism associated with the cylindrical leg of the spudcan, and a

reduction in total stresses due to arching and potential silo effect along the column of

soil on the top of the spudcan. Similarly to the invert of the spudcan, changes in

effective stresses are expected, although they were not observed by Purwana et al.

(2005), and are not suggested by Figure 2.10.

Indeed, accurate assessment of the contribution of the various components to the

penetration resistance is difficult as both the top and invert pore pressure measurements

are local measurements extrapolated over the entire surface. Purwana et al. (2005),

using a larger model with several pore pressure transducers, showed that the excess pore

pressures at the spudcan invert increased towards the centre of the spudcan. In addition,

the pore pressures were measured at the soil spudcan interface (rather than in the soil

body) and do not necessarily reflect changes within the soil underneath and at the top of

the spudcan.

While spudcan penetration is a complex problem, it is noteworthy that it can be

elegantly captured by only two parameters, a bearing factor Nc and the undrained shear


2-10

strength su, as demonstrated in Figure 2.9. Immediate back-flow on the top of the

spudcan was observed visually during testing. This confirms the analysis made by

Hossain et al. (2006), indicating that deep failure mechanism, characterised by

symmetrical flow-around, occurs at a relatively shallow embedment for soft soils.

Indeed, the normalised net vertical load development in Figure 2.9 exhibits a constant

value from an embedment ratio of about 0.7. Bearing factors calculated from the

experimental measurements are compared in Figure 2.9 with large deformation finite

element (LDFE) analysis in ideal Tresca soil and Tresca soil modified to account for

strain softening and strain rate effects (Hossain and Randolph, 2009). The centrifuge

results lean towards the modified numerical solution, i.e. yielding a bearing factor in the

range 9-10.4, indicating that undrained conditions are prevalent within the soil and that

significant strain softening takes place.

2.5.2 Operating period

Following penetration, 85% of the maximum penetration load (except for Test

2.0D2.0Y in which the holding load was 100% of the installation load due to a

temporary technical problem in the centrifuge) was maintained on the spudcan for

operating times ranging from 0 to 3 years prototype (see Table 2.3). This stage resulted

in the consolidation of the soil underneath (and to a reduced degree at the top of) the

spudcan and additional spudcan settlement as summarised in Table 2.3. During the

operating period, excess pore pressure at the top and bottom of the spudcan dissipated,

as shown in Figure 2.11, which presents the development of the degree of consolidation

with the time factor Tv = tcv/D2, where t is the time since the beginning of the

operational period and cv has been assumed to be the virgin coefficient of consolidation

(estimated as a function of the stress level, see Table 2.2).

Chapter 2

2-11

It is noteworthy that degrees of consolidation ranging from 85% to 90% were achieved

at the spudcan base at the end of the operation period for all tests, whereas at the top of

the spudcan, the degree of consolidation of about 40% to 60% was inferred (Figure

2.11). The degree of consolidation was calculated as the ratio of the excess pore

pressure at the end of the consolidation to that of at the beginning of the consolidation

which is then deducted from the unit. The lower degree of consolidation at the top is

best explained by a reduction of the coefficient of consolidation by potentially one order

of magnitude. Such a large reduction may be explained partially by the lower stress

level experienced by the highly remoulded soil at the top of the spudcan, but also by a

significantly higher modulus of compressibility. It is however important to recognise, as

for the installation process, that the pore pressure measurements are undertaken at one

single point and do not necessarily reflect the behaviour of the entire mass of soil at the

bottom and at the top of the spudcan.

2.5.3 Spudcan extraction - Increasing embedment depth and constant operating

period – Tests 1 to 4

As previously reported by Purwana et al. (2005), Bienen et al. (2009), and Kohan et al.

(2013), negative excess pore pressures (or suction) generated during extraction reach a

peak at the point of maximum extraction resistance, also termed breakout point. In the

present case, maximum suction was measured slightly after the breakout point, after

displacements ranging from 0.02D to 0.06D. There is no explanation for this behaviour,

except potential delay in the pore pressure measurements resulting from poor saturation

of the transducer porous stone. Accordingly, the analysis assumes that both peak suction

and peak extraction resistance occur simultaneously.


2-12

Figure 2.7 presents the development of suction with spudcan penetration. Peak suction

values are reported in Table 2.4. It is noteworthy that the excess pore pressure at the end

of the operation period is relatively similar for tests 1 to 4 (see Table 2.3). This is

expected because they all experienced the same operation period of 2 years.

Accordingly, the change in magnitude of suction force during extraction is solely

related to the spudcan embedment.

To investigate this point further, the peak suction is plotted against the initial effective

stress v0 in Figure 2.12. It is evident that the magnitude of peak suction developed at

the spudcan invert increases linearly with the initial effective stresses. Under undrained

extraction, the variation of effective stresses during shearing is identical for all

embedment depths and is related to the spacing between the normal consolidation line

and the critical state line (or an identical portion if the operation period has not allowed

full reconsolidation). Accordingly, the suction generated is the difference between the

change in effective stresses and the change in total stresses. This will increase with the

increasing change in total stresses as the embedment increases. Therefore, unless there

is a change in mechanism (and thus a change in total stress), a linearly increasing

relationship between effective stress and excess pore pressure generated, as observed in

Figure 2.12, is expected. The only factor limiting the suction developed is the cavitation

pressure. At ambient temperature, water will undergo cavitation at pressure about 80-95

kPa below the atmospheric pressure (Thorn et al., 2004). Considering the range of

suction pressure measured (see Table 2.4), with respect to the hydrostatic pressure (from

88 to 170 kPa), it is evident that cavitation cannot occur in any of the tests. It can

therefore be reasonably concluded that the extraction mechanism described for the

embedment depth of 1.5D by Gaudin et al. (2011) is also valid for embedments up to

3D.

Chapter 2

2-13

This is further demonstrated by the value of the ratio of peak suction generated at the

breakout point to extraction resistance. For the four tests considered, the ratio varies

within a narrow range of 70% to 80%, independent of the spudcan embedment. Gaudin

et al. (2011) reported values of about 70% for a spudcan embedded at 1.5 D and with a

degree of consolidation of 90% at the end of the operating period, while Purwana et al.

(2005) reported value of about 60% for spudcans with long operation periods and an

embedment ratio of 1.5D.

Additional insights are provided in Table 2.5, which compares the variation of load q

between the end of the operation period and the peak extraction resistance with the

variation of pore pressures at the top and bottom of the spudcan u = ui + ut, both

contributing to the extraction resistance. The ratio u/q is lower than 1 for all tests (but

test 8), indicating that the change in load is not entirely accounted for by the change in

pore pressures. Interestingly, the weight of the soil plug on top of the spudcan varies

between 54 and 108 kPa (increasing with depth), assuming a value of ’ equal to 6

kN/m3 and contributes essentially for the difference between q and u (although a

significant scatter is acknowledged, that may be explained by (i) the uncertainty of the

unit weight of the remoulded soil plug at the top of the spudcan, (ii) the single point

measurement of the excess pore pressures and (iii) maybe more importantly, the

contribution of the friction along the shearing planes of the soil plug). This observation

validates the extraction mechanism at peak extraction presented in Figure 2.2.

2.5.4 Spudcan extraction - varying operation period at an embedment ratio of 1.5

– Tests 4 to 8

Five tests (test numbers four to eight) were performed to investigate the effect of the

operation period on the mechanism associated with spudcan extraction. The operation


2-14

period ranged from less than one day to three years (in prototype dimensions), all for an

embedment ratio of 1.5 (see Table 2.2).

Figure 2.13 presents the comparisons of the loads developed with displacement during

installation, operation, and extraction for different operating periods. The excess pore

pressures generated during installation dissipate during the operation period and reach a

value close to the hydrostatic pressure for operation time of 2 years and longer (Table

2.4).

Longer consolidation periods result in higher extraction resistance (Figure 2.13), which

are concomitant with a higher development of suction at the spudcan invert (Figure

2.14). This is better illustrated in Figure 2.15, which presents the evolution of peak

suction at the spudcan invert and peak extraction resistance with the time factor Tv.

The comparison of the magnitude of the excess pore pressure at the beginning of

extraction and at the breakout point ui = ui,op - ui,ex in Table 2.3 and Table 2.4

shows that the negative excess pore pressure ui generated during extraction is

approximately constant between the tests and falls within a relatively narrow range of

104-107 kPa, with the noticeable exception of the test without an operation period,

where the difference is 122 kPa (this point is discussed latter in the paper). This is

illustrated in Figure 2.14, and in Figure 2.16, which presents the measured pore

pressures at the invert and at the top of the spudcan and the end of the operation period,

and at peak extraction, as a function of the time factor Tv.

Consequently, the total level of suction generated, which directly governs the magnitude

of the extraction resistance depends on the pore pressure at the end of the operating

period, This was also observed by Purwana et al. (2005), although a higher magnitude

Chapter 2

2-15

of excess pore pressure between the end of the operation period and the peak extraction

was reported (201-230 kPa), but for a different type of clay, with a higher soil strength

ratio (0.24 compared with 0.18) and a different initial strength at spudcan embedment

(~30 kPa versus ~10 kPa).

When the spudcan is extracted immediately after penetration, the suction developed

brings the absolute pore pressure at the spudcan invert to a value close to zero, such that

no active suction at the spudcan invert contributes to the extraction resistance. This

potentially indicates that a different mechanism takes place compared to the cases where

extraction is performed after a period of consolidation. Three other elements confirm

that hypothesis:

1. The load extraction curve (Figure 2.13) exhibits a smooth reduction post peak,

while a sharper reduction is observed for the tests with consolidation period.

This indicates a change in mechanism post peak for tests with a consolidation

period (as discussed in Gaudin et al., 2011), which does not occur for immediate

extraction.

2. The excess pore pressure curve at the invert (Figure 2.14) exhibits no changes

post peak, while a sharp reduction in suction is observed post peak for the tests

with a consolidation period. This indicates that a partial suction relief

mechanism occurs for the test with a consolidation period, which is not observed

for immediate extraction. This reinforces the observation from point 1.

3. The ratio of total change in load to the total change in pore pressure at the invert

and at the top of the spudcan q/u presented in Table 2.5 is 1 for immediate

extraction, while it ranges from 0.67 to 0.71 for tests with consolidation period.


2-16

The combination of these three observations demonstrates that a flow round mechanism

takes place for immediate extraction (q/u = 1), while a partial reverse end bearing

mechanism takes place for tests with a consolidation period, due to the heterogeneous

effective stress field resulting from localised consolidation. This difference in

mechanism explains the difference in change in pore pressure at the invert ui = ui,op -

ui,ex between immediate and delayed extraction (122 kPa against 104-107 kPa). The

zero active suction indicates that the soil flows from the top below the spudcan, without

being “sucked in”, which is consistent with a full flow mechanism. Interestingly, the

ratio of extraction to penetration resistance for immediate extraction is 0.64. This is

close to the same ratio for T-bar tests in normally consolidated clay ( 0.7), indicating

that immediate spudcan extraction resistance can potentially be assessed from in-situ T-

bar tests results.

As the operation period increases, the difference in excess pore pressures ui reduces to

104-107 kPa and remains constant regardless of the consolidation time This indicates

that the extraction mechanism remains identical for all non-zero consolidation times.

Comments made about the ratio u/q for tests with increasing embedment are equally

valid for tests with increasing consolidation time. The ratio u/q is in the range 0.66 –

0.71, decreasing with consolidation time, reflecting an increasing contribution of the

soil plug, most likely due to an increase of the friction along the shearing planes.

Indeed, the excess pore pressures measured at the top of the spudcan at peak extraction

remain relatively constant, around 50 kPa (see Figure 2.15), with consolidation time,

indicating a constant contribution of the weight of the soil plug. This assumes that the

pore pressures and total pressures at the top are equal during extraction, as observed by

Purwana et al. (2005).

Chapter 2

2-17

2.6 Conclusions

Centrifuge tests have been performed to investigate spudcan extraction resistance in

normally consolidated soil as a function of the initial embedment and the operation

period. Results demonstrate that the mechanism at the point of maximum extraction

resistance involves a reverse end bearing mechanism associated with plug uplift. This

mechanism is valid for initial embedment ratio up to 3 times the spudcan diameter and

when there is an operational hold of vertical load on the spudcan. For immediate

extraction, the mechanism consists of a full flow round, with a ratio of extraction to

penetration resistance similar to that measured in a T-bar test.

It was also demonstrated that the contribution of the plug uplift is constant with the

operation period. This is in contrast with the peak suction at the spudcan invert, which

increases with the operation period, so longer operation periods result in higher

extraction resistance. However, the difference in pore pressure between the end of the

operation period and the peak suction is approximately constant. Additional work is

required to link this constant value with particular mechanisms and soil characteristics

(including strength softening and hardening due to consolidation), enabling its

assessment for a wide range of spudcan geometry and soil strength.

The above conclusions are restricted to the range of the experimental centrifuge tests,

but are believed to provide relevant insights into the extraction mechanisms taking place

for a deeply embedded spudcan. Further studies are required to understand whether the

extraction mechanism is different for spudcan embedment ratios greater than 3.


2-18

References



31, No. 3, 202-211.

Cassidy, M.J., Quah, C.K., Foo, K.S. (2009). Experimental investigation of the

reinstallation of spudcan footing close to existing footprints. Journal of

Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 135, No. 4, 474-

476.

Chung, S.F., Randolph, M.F., and Schneider, J.A. (2006). Effect of penetration rate on

penetrometer in clay. Journal of Geotechnical and Geoenvironmental

Engineering, ASCE, Vol. 132, No. 9, 1188-1196.

Craig, W.H. and Chua, K. (1990). Extraction forces for offshore foundations under

undrained loading. ASCE J. Geotechnical. Engineering 116, No. 5, 868–884.



Houston, OTC 4144.

Finnie, I.M.S. and Randolph, M.F. (1994). Punch-through and liquefaction induced

failure of shallow foundations on calcareous sediments. Proc. Int. Conference on

Behaviour of Offshore Structures, Boston, USA, 217-230.



61, No. 112, 1043-1054.

Gaudin, C., Cassidy, M.J., Bienen, B., and Hossain, M.S. (2011). Recent contributions

of geotechnical centrifuge modelling to the understanding of jack-up spudcan

behaviour. Ocean Engineering, Vol. 38, No. 7, pp. 900-914.

Hossain, M.S., and Randolph, M.F. (2009). Effect of strain rate and strain softening on

the penetration resistance of spudcan foundations on clay. International Journal

of Geomechanics, ASCE, Vol. 9, No. 3, 122-132.

Chapter 2

2-19

Hossain, M.S., and Randolph, M.F. (2009). New mechanism-based design approach for

spudcan foundations on single layer clay. Journal of Geotechnical and

Geoenvironmental Engineering, ASCE, Vol. 135, No. 9, 1264-1274.

Hossain, M.S., Randolph, M.F., Hu, Y., and White, D.J. (2006). Cavity stability and

bearing capacity of spudcan foundations on clay. Proc. 13th Offshore

Technology Conference, Houston, OTC 17770.


November 2008.

Kohan, O., Bienen, B., Cassidy, M.J., and Gaudin, C. (2013). Centrifuge experiments to

study extraction of a deeply embedded spudcan using top jetting. Proc. 32nd

International Conference on Offshore Mechanics and Arctic Engineering

(OMAE), Nantes.

Lee, K.K., Cassidy, M.J., Randolph, M.F. (2012). Use of epoxy in developing miniature

ball penetrometers for application in a geotechnical centrifuge. International

Journal of Physical Modelling in Geotechnics, Vol. 12, No. 3, 119-128.

Lee, K.K., Cassidy, M.J., Randolph, M.F. (2013). Bearing capacity on sand overlying

clay soils: Experimental and finite element investigation of potential punch-

through failure, Géotechnique, Vol. 63, No. 15, 1271-1284.

Low, H.E., Randolph, M.F., Lunne, T., Andersen, K.H., and Sjursen, M.A. (2011).

Effect of soil characteristics on relative values of piezocone, T-bar and ball

penetration resistances. Géotechnique, Vol. 61, No. 8, 651-664.

Mahmoodzadeh, H., Boylan, N., Randolph, M. F., and Cassidy, M. J. (2011). The effect

of partial drainage on measurements by a piezoball penetrometer. Proc. 30th


(OMAE), Rotterdam.

Menzies, D., and Lopez, C.R. (2011). Four Atypical Jack-up Rig Foundation Case

Histories. 13th International Conference, The Jack up Platform, London.


2-20



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753.

Purwana, O.A., Leung, C.F., Chow, Y.K., and Foo, K.S. (2006). Breakout failure

mechanism of jackup spudcan extraction. Proc. 6th Int. Conference on Physical

Modelling in Geotechnics, Hong Kong, 667–672.

Purwana, O.A., Quah, M., Foo, K.S., Nowak, S., and Handidjaja, P. (2009). Leg Extraction /

Pullout Resistance - Theoretical and Practical Perspectives. In. Proc. 12th Jack up Conf.,

London.



91, Boulder, Colorado, 3-9.

Reardon, M.J. (1986). Review of the geotechnical aspects of jack-up unit operations.

Ground Engineering, 19(7):21 6.

Stewart, D.P. (1992). Lateral loading of pile bridge abutments due to embankment

construction. PhD thesis, University of Western Australia.

Stewart, D.P. and Randolph, M.F. (1991). T-bar penetration testing in soft clay. Journal

of Geotechnical Engineering, Vol. 120, No. 12, 2230-2236.

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Chapter 2

2-21

Table 2.1. Kaolin clay characteristics (after Stewart, 1992)

Liquid limit (LL) 61%

Plastic limit (PL) 27%

Plasticity index (Ip) 34%

Specific gravity (Gs) 2.6

Angle of friction () 23°

Consolidation coefficient, cv (at OCR=1 and v =112.5 kPa) 3.99 m2/year

Submerged unit weight, (at v =112.5 kPa) 6.2 kN/m3

Spud

can

Extra

ctio

n fr

om D

eep

Embe

dmen

t in

Soft

Cla

y

2-22

Tabl

e 2.

2. T

estin

g Pr

ogra

mm

e

Test

N

umbe

rTe

st N

ame1

Pe

netra

tion

Dep

th

Embe

dmen

t R

atio

'

'v

c v

Non

-Dim

ensi

onal

V

eloc

ity

Ope

ratio

n Ti

me

m

kN

/m3

kPa

m

2 /yea

rvD

/cv

Yea

r 1

3.0D

2.0Y

18

.13

2.99

6.

20

112.

42

3.99

71

.19

1.93

2 2.

5D2.

0Y

15.0

1 2.

50

6.20

93

.06

3.65

77

.68

1.94

3 2.

0D2.

0Y

11.9

1 2.

00

6.05

72

.04

3.23

87

.80

1.99

4 1.

5D2.

0Y

8.85

1.

50

6.05

53

.52

2.81

10

1.01

2.

01

5 1.

5D3.

0Y

8.87

1.

50

6.05

53

.65

2.81

10

0.89

3.

01

6 1.

5D1.

0Y

8.84

1.

50

6.05

53

.49

2.81

10

1.04

1.

01

7 1.

5D0.

5Y

8.84

1.

50

6.05

53

.49

2.81

10

1.04

0.

52

8 1.

5D0.

0Y

8.84

1.

50

6.05

53

.48

2.81

10

1.04

0.

00

1.

The

nom

encl

atur

e of

the

nam

ing

syst

em is

the

embe

dmen

t dep

th ra

tio fo

llow

ed b

y th

e op

erat

iona

l hol

ding

per

iod

befo

re e

xtra

ctio

n in

pro

toty

pe y

ears

.

Cha

pter

2

2-23

Tabl

e 2.

3. S

umm

ary

of e

xper

imen

tal r

esul

ts (i

nsta

llatio

n an

d op

erat

ion)

Test

N

umbe

r Te

st

Nam

e Pe

netra

tion

resi

stan

ce

Settl

emen

t du

ring

oper

atio

n

Nor

mal

ised

N

et V

ertic

al

Load

Exce

ss p

ore

pres

sure

at

the

spud

can

inve

rt

Exce

ss p

ore

pres

sure

at

the

spud

can

top

Pene

tratin

g pr

essu

re

Q

p (M

N)

q p=Q

p/A

(kPa

) s o

p (m

) Q

/(A.s u

)1

(-)

end

of th

e in

stal

latio

n u

i, ins

(kPa

)

end

of th

e op

erat

ing

perio

d u

i, op

(kPa

)

end

of th

e in

stal

latio

n u

t, ins

(kPa

)

end

of th

e op

erat

ing

perio

d u

t, op

(kPa

)

q p

+u t

, ins

(kPa

) 1

3.0D

2.0Y

5.

78

200.

57

0.36

10

.06

249.

11

30.2

2 67

.57

33.4

5 26

8.14

2 2.

5D2.

0Y

4.88

17

2.44

0.

22

10.4

4 22

9.70

26

.31

56.4

4 21

.81

228.

88

3 2.

0D2.

0Y

3.69

13

2.83

0.

83

10.1

4 15

6.20

26

.48

39.3

8 18

.69

172.

21

4 1.

5D2.

0Y

2.39

87

.46

0.31

8.

99

122.

60

17.6

0 35

.82

21.3

0 12

3.28

5 1.

5D3.

0Y

2.49

91

.30

0.27

9.

36

138.

12

9.22

32

.67

16.1

4 12

3.97

6 1.

5D1.

0Y

2.31

84

.52

0.26

8.

69

126.

53

30.5

1 34

.41

24.4

3 11

8.93

7 1.

5D0.

5Y

2.24

82

.29

0.25

8.

46

110.

80

53.9

3 33

.30

30.1

8 11

5.59

8 1.

5D0.

0Y

2.45

89

.93

0.00

9.

25

119.

05

119.

05

34.0

9 34

.09

124.

02

1.

The

undr

aine

d sh

ear s

treng

th s u

is c

onsi

dere

d at

the

spud

can

inst

alla

tion

dept

h.

Spud

can

Extra

ctio

n fr

om D

eep

Embe

dmen

t in

Soft

Cla

y

2-24

Tabl

e 2.

4. S

umm

ary

of e

xper

imen

tal r

esul

ts (e

xtra

ctio

n)

Test

N

umbe

r Te

st N

ame

Extra

ctio

n re

sist

ance

B

reak

-ou

t dep

thTi

me

fact

or

Exce

ss p

ore

pres

sure

at t

he

spud

can

inve

rt

Exce

ss p

ore

pres

sure

at

the

spud

can

top

Qe

(MN

) q e

=Qe/A

(k

Pa)

b

(m)

T v=c

v.t/D

2 (-

)u

i, ex

(kPa

) u

t, ex

(kPa

) 1

3.0D

2.0Y

-6

.14

-213

.46

17.9

0 0.

217

-170

.16

116.

24

2 2.

5D2.

0Y

-5.6

2 -1

99.0

6 14

.65

0.20

3 -1

36.6

6 96

.28

3 2.

0D2.

0Y

-4.1

3 -1

48.3

2 12

.14

0.18

3 -1

02.3

8 65

.72

4 1.

5D2.

0Y

-3.2

9 -1

20.7

7 8.

68

0.16

2 -8

7.96

45

.29

5 1.

5D3.

0Y

-3.4

3 -1

26.0

8 8.

61

0.24

3 -9

7.73

46

.45

6 1.

5D1.

0Y

-2.8

2 -1

03.5

3 8.

57

0.08

1 -7

4.33

44

.88

7 1.

5D0.

5Y

-2.4

2 -8

8.83

8.

60

0.04

0 -5

0.75

47

.38

8 1.

5D0.

0Y

-1.5

8 -5

8.10

8.

27

0.00

1 -2

.95

47.1

8

Cha

pter

2

2-25

Tabl

e 2.

5. D

ata

anal

ysis

Test

N

umbe

r q p

q p

-op=

0.

85q p

q e

q=

q o

p-q e

ui, o

p

ui, e

x

ui=

u

i, op-

u

i, ex

ut, o

p

ut, e

x

ut=

u

t, op-

u

t, ex

u=

ui+

u

t

u/

q

(k

Pa)

(kPa

)(k

Pa)

(kPa

)(k

Pa)

(kPa

)(k

Pa)

(kPa

)(k

Pa)

(kPa

)(k

Pa)

-

1 20

0.57

17

0.48

-2

13.4

6 38

3.94

30

.22

-170

.16

200.

38

33.4

5 11

6.24

82

.79

283.

17

0.74

2 17

2.44

14

6.57

-1

99.0

6 34

5.63

26

.31

-136

.66

162.

97

21.8

1 96

.28

74.4

7 23

7.44

0.

69

3 13

2.83

13

2.83

1 -1

48.3

2 28

1.15

26

.48

-102

.38

128.

86

18.6

9 65

.72

47.0

3 17

5.89

0.

63

4 87

.46

74.3

4 -1

20.7

7 19

5.11

17

.6

-87.

96

105.

56

21.3

0 45

.29

23.9

9 12

9.55

0.

66

5 91

.30

77.6

1 -1

26.0

8 20

3.69

9.

22

-97.

73

106.

95

16.1

4 46

.45

30.3

1 13

7.26

0.

67

6 84

.52

71.8

4 -1

03.5

3 17

5.37

30

.51

-74.

33

104.

84

24.4

3 44

.88

20.4

5 12

5.29

0.

71

7 82

.29

69.9

5 -8

8.83

15

8.78

53

.93

-50.

75

104.

68

30.1

8 47

.38

17.2

0 12

1.88

0.

77

8 89

.93

76.4

4 -5

8.10

13

4.54

11

9.05

-2

.95

122.

00

34.0

9 47

.18

13.0

9 13

5.09

1.

00

1.

The

hol

ding

load

was

100

% o

f the

inst

alla

tion

load

.


2-26

Figure 2.1. Typical jack-up and spudcan (modified after Reardon 1986)

Chapter 2

2-27

Figure 2.2. Failure mechanisms during undrained spudcan extraction (after Gaudin et al., 2011)

Spudcan Extraaction from Dee

Figure 2.

ep Embedment

.3. Centrifu

in Soft Clay

ge sample u

undrained sh

hear strengtth profile

2-28

Chapter 2

2-29

Figure 2.4. Model spudcan and load cell

Figure 2.5. Model Spudcan and location of the pore pressure transducers (dimensions in

mm)


2-30

Figure 2.6. Penetration and extraction resistances for tests with 2 years operation period (Tests 1 to 4)

Chapter 2

2-31

Figure 2.7. Excess pore pressure at the spudcan invert for tests with 2 years operation period (Tests 1 to 4)


2-32

Figure 2.8. Excess pore pressure at the top of the spudcan for tests with 2 years operation period (Tests 1 to 4)

Chapter 2

2-33

Figure 2.9. Normalised load for tests with 2 years operation period (Tests 1 to 4)


2-34

Figure 2.10. Comparison of the penetrating (qp + ut,ins) and resistance pressure (ui,ins) for test 1.5D2.0Y

Chapter 2

2-35

Figure 2.11. Degree of consolidation during operating period for tests with 2 years operation period (Tests 1 to 4)


2-36

Figure 2.12. Variation of peak excess pore pressure during extraction at the spudcan top and invert with in-situ effective stress for tests with 2 years operation period (Tests 1 to

4)

Chapter 2

2-37

Figure 2.13. Penetration and extraction resistances for tests at an embedment ratio of 1.5 (Tests 4 to 8)


2-38

Figure 2.14. Excess pore pressure at the spudcan invert for tests at an embedment ratio of 1.5 (Tests 4 to 8)

Chapter 2

2-39

Figure 2.15. Development of uplift resistance and suction pressure at the spudcan invert with operation period for tests at an embedment ratio of 1.5 (Tests 4 to 8)


2-40

Figure 2.16. Excess pore pressure development at spudcan invert and top for tests at an embedment ratio of 1.5 (Tests 4 to 8)

3-1

CHAPTER 3

Predicting Spudcan Extraction Resistance in Soft

Clay

3.1 Abstract

Jack-ups are mobile offshore structures that are frequently relocated to new operation

sites. To be relocated, the jack-up footings, known as spudcans need to be extracted

from the seabed, using essentially the buoyancy of the hull as extraction force. This

operation may be time consuming or even jeopardised if the spudcan extraction

resistance is higher than the available extraction force. The maximum extraction (or

breakout) resistance consists of suction at the spudcan base, weight of the soil above the

spudcan, and soil shear resistance above the spudcan, with the contribution of the

suction at the spudcan invert being the dominant component of the breakout resistance.

This paper reviews an existing prediction method used to estimate spudcan extraction

resistance and proposes an update of some of the input parameters based on insights

Predicting Spudcan Extraction Resistance in Soft Clay

3-2

obtained from a large database of experimental model data on two types of clays and for

spudcan embedment up to three diameters.

3.2 Introduction

Self-elevating mobile jack-up units are the most common facilities used for offshore

drilling operations in shallow waters, up to approximately 150 m depth (Figure 3.1).

Once operation is completed, the jack-up is relocated to a new operation site,

necessitating the jack-up footings, known as spudcans, to be extracted from the seabed.

Difficulties in extraction can arise if the spudcans are deeply embedded in very soft

clays. The development of high suction forces at the spudcan invert (Purwana et al.,

2005; Gaudin et al., 2011) may augment the extraction resistance beyond the extraction

force generated by the hull buoyancy, resulting in unexpected delays and additional

costs.

As a part of assessing the jack-up removal process prior to going on a new location, an

estimation of spudcan extraction resistance is therefore necessary for the jack-up

operators to anticipate potential extraction issues and develop mitigation measures to

facilitate spudcan extraction, such as water jetting for instance (Bienen et al., 2009;

Gaudin et al., 2011).

Two methods have been developed to estimate the maximum spudcan extraction

resistance. They are detailed in Purwana et al. (2009) and Osborne et al. (2011),

respectively. The method detailed in Purwana et al. (2009) is based on measurements of

total and pore pressure at various locations on a model spudcan in centrifuge

experiments for embedment up to 1.5 spudcan diameters, as well as information

regarding soil failure mechanism from Particle Image Velocimetry analysis. The

method contained in Osborne et al. (2011) is a modified version of this.

Chapter 3

3-3

The objective of this paper is (i) to check the validity of the method established by

Purwana et al. (2009) (called here after the reference method) for spudcan embedment

up to 3 diameters and (ii) presents an update of some of the input parameters, based on

insights obtained from an experimental model database of 24 centrifuge tests featuring

spudcan extraction from normally consolidated clay.

3.3 Database

The experimental database was gathered from data reported by Purwana et al. (2005),

Purwana et al. (2009), Gaudin et al. (2011), Kohan et al. (2013), Kohan et al. (2014a),

and Kohan et al. (2014b).

A total of 24 centrifuge test results were extracted and they are summarised in Table 3.1

in prototype scale. Scale factors for geometry, load, pressure, and the diffusion process

can be found at Garnier et al. (2007) who made an inventory of the scaling laws and

similitude questions relating to centrifuge modelling. Tests were conducted at 100 and

200 g, modelling spudcans of 6, 8, 12.5 and 17.1 m in diameter (30, 40, 85.56 and 125

mm in model scale).

For all tests, the test procedure consisted of three stages. In the first stage, spudcan

penetration was performed in-flight in displacement or load control, under undrained

conditions. The spudcan installation depths varied from 1 to 3 times the spudcan

diameter. In the second stage, the jack-up operation period was simulated by

maintaining a constant vertical load between 50% and 90% of the maximum installation

load for up to five years in prototype scale, achieving varying degrees of consolidation

in the soil around the spudcan. It is noteworthy that the effect of operation load is less

significant than that of operation duration (Purwana et al., 2005). Finally, in the third

stage, spudcan extraction was performed in displacement control at a rate, v, resulting in


3-4

a normalised velocity V=vD/cv greater than 30, where cv is the virgin consolidation

coefficient and D is spudcan diameter. This ensured that spudcan extraction was also

performed under undrained conditions (Finnie and Randolph, 1994). The maximum

extraction loads are reported in Table 3.1.

Centrifuge studies on spudcan extraction employed for assessment of the spudcan

extraction resistance were performed in two different soils: UWA kaolin clay and

Malaysian kaolin clay with a coefficient of consolidation cv of approximately 2.8 to 4.8

m2/year for UWA Kaolin clay and 40 m2/year for Malaysian kaolin clay at a stress level

consistent with the spudcan embedment. Soil characteristics including soil shear

strength, soil unit weight, and soil effective stress at the spudcan installation depth for

each centrifuge test are also provided in Table 3.1.

3.4 Extraction failure mechanism

The spudcan extraction failure mechanism was described in detail by Purwana et al.

(2009) and Gaudin et al. (2011) for embedment ratios up to 1.5 times the spudcan

diameter. The mechanism at peak extraction resistance is a combination of an uplift

mechanism of the soil at the top of the spudcan, and a reverse end bearing at the

spudcan invert associated with the development of negative excess pore pressure,

namely suction (Figure 3.2). The main soil resistance is comprised of the weight of the

soil above the spudcan, the resistance along a shear plane generated above the spudcan,

and the suction pressure at the spudcan base.

This has been identified by both PIV analysis (Purwana et al., 2006a) and numerical

analysis (Zhou et al., 2009) of spudcan extraction in normally consolidated clay. Kohan

et al. (2014a) demonstrated that this mechanism is also relevant for initial embedment

ratio up to 3 times the spudcan diameter.

Chapter 3

3-5

The components involved in the spudcan extraction resistance are influenced by the

duration of the jack-up operation, i.e. by the degree of dissipation of excess pore

pressures generated during installation, in the soil surrounding the spudcan. This results

in the shear strength of the soil surrounding the spudcan increasing with operation time,

and consequently, an increase in extraction resistance, as already demonstrated by

Purwana et al. (2005).

It is noted that this mechanism may not apply for spudcans that have not seen any

dissipation of excess pore pressures at immediate extraction. In this case, a reverse flow

mechanism is more likely to develop.

3.5 Evaluation of the reference method

The method proposed by Purwana et al. (2009) (reference method) is based on the

aforementioned breakout failure mechanism, identified using Particle Image

Velocimetry (PIV) analysis for undrained extraction of a 12.5 m in diameter spudcan

(prototype scale) from a depth of approximately 1.5 spudcan diameters in Malaysian

kaolin clay (Purwana, 2006b). The vertical uplift force equilibrium condition assumed

by Purwana et al. (2009) is illustrated in Figure 3.2.

The method has been presented in details in Purwana et al. (2009). It computes the

uplift resistance as the sum of a resistance at the base Qbase (which accounts for

overburden stresses), at the top Qtop and the submerged weight of the spudcan Weff.

Table 3.2 (see also Figure 3.3 and Figure 3.4) details the calculation of the first two

components and summarises the parameters used in the method. When determining the

net extraction resistance, Weff is considered as zero.


3-6

To evaluate the performance of the method, the peak extraction resistance was

calculated for each case, based on the input parameters reported in Table 3.3. Additional

assumptions were made when data were missing, as explained below:

i. To compute the top soil resistance, the height of the soil flowing back onto the

top of the spudcan, which is a function of the depth of cavity formed during deep

installation, needs to be assessed For cases where the cavity depth Hc was not

reported, the solution developed by Hossain et al. (2006) was used as expressed

below:

(1)

where

Hc = Cavity depth (m);

D = Spudcan diameter (m);

’ = Effective unit weight of soil (kN/m3);

suHc = Shear strength at the cavity depth (kPa).

ii. The gain in soil shear strength underneath the spudcan during operation time is

characterised by the parameter fg,base, which was evaluated as 1.00 and 1.70 by

Purwana et al. (2009) (from numerical analysis) for immediate extraction and

extraction after 400 days of operation, respectively. No values were reported for

intermediate operational times. To evaluate the performance of the reference

method for the entire database, presented in Table 3.1, fg,base was calculated for

intermediate operation times, by linear interpolation between the degrees of

consolidation achieved for 0 and 400 days. At 0 days operation time, it is logical

Ds

Ds

DH uHcuHcc

'.41

'.

55.0

Chapter 3

3-7

to assume that the degree of consolidation is equal to 0. For 400 days and for

Malaysian clay, the excess pore pressure dissipation during operation time was

not reported in Purwana et al. (2009). Accordingly, data reported in Purwana et

al. (2005) for test GS5 (423 days operation time) was used (Figure 3.5), leading

to a degree of consolidation of 78% at 400 days. For tests in UWA kaolin clay,

the degrees of consolidation were extracted from pore pressure measurements.

Results of the linear interpolation of fg,base are listed in Table 3.3.

iii. The change in soil shear strength at the top of the spudcan due to installation is

characterised by the parameter fg,top. Purwana et al. (2009) performed a series of

T-bar tests in Malaysian kaolin clay to measure the shear strength of the

remoulded soil at the spudcan top during the operation period. The shear

strength was measured to reduce to 67% of the undisturbed shear strength

immediately after spudcan installation, but increased by 30% (or 87% of the

undisturbed soil shear strength) after 400 days reconsolidation period. This

resulted in values of 0.67 and 0.87 recommended by Purwana et al. (2009) for 0

and 400 days of operation time, respectively. Similarly to the calculation of

fg,base, linear interpolation was conducted to assess fg,top for intermediate

consolidation times. From Figure 3.6, showing pore pressure responses at the

end of the installation and operation time with respect to the hydrostatic pressure

for test GS5, a degree of consolidation of 41% was deduced for 400 days of

operation time. Calculated values of fg,top are presented in Table 3.3.

Predictions from the reference method are compared with the measured uplift

resistances in Figure 3.7. Two observations are made:


3-8

1. The method predicts reasonably the peak extraction resistance in Malaysian clay

(which is expected as the data underpin the development of this method), with a

mean percentage error of about 9%. The performance is reduced for UWA

kaolin clay, with a mean percentage error of about 57%. This potentially

indicates that the performance of the reference method may be affected by the

nature of the clay and that a better understanding of the various parameters

associated with the soil characteristics is required.

2. The performance of the reference method is consistent for both clays for

spudcan embedment up to 3 diameters, extending the validity of the method

from embedment of 1.5 to 3 spudcan diameters. This is consistent with findings

from Kohan et al. (2014b), which demonstrated that the failure mechanism

during extraction was identical between embedment of 1.5 and 3 spudcan

diameters.

3.6 Updating the input parameters

The reference method is based on a rigorous description of the failure mechanism,

incorporating the change in strength at the base and top of the spudcan resulting from

installation and operation. They are estimated via two empirical factors fg,base and fg,top,

with limited insights into the values to adopt for intermediate operational times (i.e.

between no and full consolidation) and different type of clays. The lower performance

of the method for kaolin clay indicates that some aspects of the soil characteristics,

which are not accounted for in the method, require a closer examination. Potential

candidates include soil sensitivity, undrained bearing capacity factor, operation load,

and consolidation coefficient.

Chapter 3

3-9

The database gathered enables additional insights into each parameters involved in the

reference method, although it is noted that a range of parameter combinations can be

derived to fit individual test data (non-unique solution) and a holistic view of the fit to

the database must be taken. Accordingly, the paper proposes updated recommendations

to estimate the model parameters, notably the values of fg,top and fg,base as a function of

the operation time and the type of clay used. Two plots are suggested that enable the

assessment of gain in shear strength of soil at top and base of spudcan depending on the

operation time. Additional recommendations relate to the estimation of the cavity depth,

the unit weight of the soil on top of the spudcan, the overburden adjustment factor, and

the introduction of two additional factors to account for the effects of the strength ratio

and operation load. The updated recommendations for the input parameters are

explained in detail below and are summarised in Table 3.2.

3.6.1 Cavity depth, Hc

The top soil resistance is a function of the height of the soil flowing back onto the top of

the spudcan and of the depth of the cavity formed during installation. The solution

developed by Hossain et al. (2006) is therefore recommended to estimate the cavity

depth as explained in the previous section.

3.6.2 Unit weight of soil above the spudcan, 'top

The unit weight of the soil above the spudcan is slightly lower than that of the

undisturbed soil due to the heavy remoulding occurring during penetration. Purwana et

al. (2009) assumed the remoulded unit weight of Malaysian clay was about 92% of the

virgin soil. Without indications about the variation of unit weight with the level of

remoulding or estimated values for other types of clay, it is suggested for the updated

method to use the virgin soil unit weight for all predictions. The impact of such a


3-10

simplification on the performance of the method is limited, especially for shallow

penetrations (and limited volume of soil). It is made for ease of calculation.

3.6.3 Change in soil shear strength above the spudcan, fg,top

During installation, the soil is experiencing heavy remoulding and softening, resulting

in a reduction of the shear strength of the soil resting above the spudcan. The reduction

in shear strength is a function of the soil sensitivity St, which may vary between 2 to 2.5

for UWA kaolin clay and between 2 to 4 for the Malaysian clay. Regardless of the soil

sensitivity, the remoulded soil regains some of its shear strength during operation

through consolidation.

The evolution of the factor fg,top with operation time has been back calculated from the

experimental data presented in Table 3.1. The process results in solving one linear

equation with two unknowns, fg,top and fg,base, requiring additional assumptions on both

parameters. Accordingly, the following criteria were used to determine soil shear

strength at top (and base as explained in next section) of the spudcan:

1. The lower bound of fg,base is 1, corresponding to the value immediately after

extraction, before any consolidation occurs.

2. The upper bound of fg,top is equal to 1, corresponding to full strength recovery of

the soil above the spudcan after full reconsolidation.

The values of fg,top that were considered a best-fit of the database are plotted in Figure

3.8 against the degree of consolidation U for all tests, excluding tests S1UEnJ, S1UEnJ,

GS2 to GS4, and GS6, for which U is unknown. All points fall within in single

logarithmic curve demonstrating a loss of strength of 40% immediately after installation

and a rapid recovery to a value of about 78-85% of the initial shear strength after 10%

Chapter 3

3-11

of consolidation. This indicates that a fairly complex soil hardening process is taking

place, which may involve mechanisms other than consolidation. For prediction

purposes, it is suggested to adopt a value of fg,top of 0.6 for immediate extraction and a

value of 0.78 to 0.85, increasing linearly between 10 and 100% consolidation. If more

detailed knowledge of an offshore soil is known these limits may potentially be altered.

However, without such knowledge the suggested values are a good guide that fits the

experimental database well.

3.6.4 Gain in soil shear strength underneath the spudcan, fg,base

The soil below the spudcan consolidates under the load held during the operational

period. This results in a gain in soil shear strength, described by the factor fg,base, which

lower bound value is established at 1. Values of fg,base considered to be the best holistic

fit to the centrifuge data (and are consistent with the fg,top values of Figure 3.8) are

plotted against the degree of consolidation U in Figure 3.9. This figure covers a wide

range of degree of consolidation ensuing from different operation periods. It is evident

from Figure 3.9 that a higher degrees of consolidation results in a larger gain in soil

shear strength beneath the spudcan, with a linear fit reasonably representing the data.

Values that range from 1 to 1.8 provide a reasonable fit for the two clays. The linear

increase of strength with degree of consolidation is somewhat surprising and potentially

indicates that the gain in strength is not homogenous underneath the spudcan

3.6.5 Overburden pressure adjustment factor, Sb

The weight of the overlying soil imposes an overburden pressure at the spudcan

installation depth. Since the failure mechanism at the spudcan invert has been identified

as a reverse end bearing before changing to a localised flow around mechanism at the


3-12

peak extraction resistance, the overburden pressure is required to be calculated to

determine the net extraction resistance.

Purwana et al. (2009) considered the overburden stress as part of the base resistance,

and assumed that it was partially mobilised from embedment between 0.35 and 1.5,

before being fully mobilised for embedment ranging from 1.5 to 2 (see adjustment

factor in Table 3.2). This factor was established from back calculation of the centrifuge

data and therefore may be applicable only for Malaysian clay. In the present study, to

cover all embedment, the two types of clay, and to simplify the approach, it is assumed

that the overburden stress is fully mobilised at any spudcan embedment depth, resulting

in an adjustment factor equal to one.

3.6.6 Effect of the operation load, fol

Purwana et al. (2005) examined the effect of the operational load on the spudcan

extraction. Three tests with the operational load Vop set at 25%, 50% and 75% of the

installation load Vp and with the same penetration depth and operation period were

performed. Comparing the test results shows that the operation load does not influence

the top soil resistance, whereas base soil resistance increases by approximately 10%,

between an operation load ratio of 50% and 75% (Figure 3.10).

As the reference method was established based on the tests with an operation load ratio

of 50%, a new factor fol (Vop/Vp) with a value of 1 at the operation load ratio of 0.5 and

upper bound value of 1.2 at the operation load ratio of 1 is defined as:

fol = 1 + (2 (Vop/Vp) - 1) (2)

where

Vop = Operation load (MN);

Chapter 3

3-13

Vp = Penetration load (MN);

= 0.2 (-).

The value of =0.2 has been chosen as it best fits on increase of 20% on the base soil

resistance, as measured experimentally by Purwana et al. (2005). It should be noted that

Equation 2 has been fit to the data for 0.5 ≤ Vop/Vp ≤ 1. For Vop/Vp less than 0.5, a value

of 0.9 is recommended.

3.6.7 Effect of the strength ratio on the breakout factor for base soil resistance, fsr

Figure 3.8 and Figure 3.9 demonstrate that a consistent set of parameters could be

chosen for representing the consolidation characteristics and sensitivity of the two soils.

However, the different performance of the two clays still requires differentiation and a

parameter is required to explain the lower performance of the reference method for the

UWA Kaolin clay.

One component of the increased extraction resistance measured in the UWA tests may

be because of the difference in undrained shear strength profile; with the UWA tests

having lower increasing strength with depth compared to the Malaysian clay tests.

Usually for shallow foundations, considering their behaviour in compression, this would

lead to a lower bearing capacity factor. However, close inspection of the lower bound

bearing capacity factors of Houlsby and Martin (2003) for spudcans in an open cavity

actually shows on increase in the bearing capacity factors with decreased strength

gradient once the spudcan becomes embedded more than one diameter. As the

mechanism of the reference method has a spudcan bottom contribution as the reverse of

the Houlsby and Martin (2003) solution (i.e. uplift rather than penetration) it can be

assumed that a lower strength gradient can increase the bearing capacity factor. This is

accounted for by introducing a new factor fsr.


3-14

An approach is proposed here whereas the effect of soil strength defined as the soil

shear strength normalised by effective stress is considered. As the reference method was

developed based on the results of the centrifuge tests in Malaysian clay, an additional

factor fsr (’v / su) is defined as a function of the ratio of the effective stress normalised

by soil shear strength for any soft soils to that of Malaysian kaolin clay. The effective

stress normalised by soil shear strength for Malaysian kaolin clay is approximately 4;

therefore, after performing a holistic fit of the database, fsr can be expressed as below:

fsr =1 + ((’v / su) / 4 – 1) (3)

where

’v = soil effective stress (kN/m2);

su = soil shear strength (kN/m2);

= empirical factor = 0.4 (-).

The value of the 0.4 provides the best fit to the experimental database.

3.6.8 Net extraction load, Quplift

The net extraction load Quplift is computed in the improved method as:

Quplift = Qtop + Qbase + Weff (MN) (4)

Qtop = 0.25 D2 (Nc,top su,top fg,top) (MN) (5)

Qbase = 0.25 D2 (fsr Nc,base su,base fg,base fol –’H) (MN) (6)

To estimate the net extraction load, a program was developed in VB.NET. A practical

example for calculating the required uplift load is presented in Appendix A.

Chapter 3

3-15

3.7 Discussion

Figure 3.11 illustrates the performance of the improved method through comparison

between the predicted and experimental net extraction resistance. The method predicts

the peak extraction resistance equally well in both types of clay, with a mean difference

of about 8% (Table 3.4).

Although the method was used here to simulate test of immediate extraction (i.e. no

load hold and therefore no excess pore pressure dissipation and consolidation), and was

found to provide a resistance similar to the experiment, it is questioned if the

mechanism that the reference method if based on is appropriate for this case. It is more

likely a localised reverse flow mechanism for deep embedments and this mechanism

should be the basis of a method to predict immediate extraction.

3.8 Conclusions

A database of centrifuge tests on spudcan extraction in two different types of clay has

been gathered to assess the performance of an analytical method developed to predict

the peak extraction resistance. The method proved to predict accurately the

experimental results in Malaysian clay, but exhibited a significantly lower performance

for UWA kaolin clay with a mean difference of about 57%.

A set of recommendations is proposed to update and improve the prediction method.

The recommendations relates to the factors characterising the change in soil shear

strength at the base and on the top of the spudcan and two new factors considering the

effect of the operation load and strength ratio on spudcan extraction in clay. Additional

details predicting when flow around occurs during installation have also been


3-16

incorporated. The improved method demonstrated a higher degree of accuracy with a

mean difference reduced to 8% for both types of clay.

Chapter 3

3-17

References



31, No. 3, 202-211.

Finnie, I.M.S., and Randolph, M.F. (1994). Punch-through and liquefaction induced





61, No. 112, 1043-1054.





Hossain, M.S., Randolph, M.F., Hu, Y. and White, D.J. (2006). Cavity stability and

bearing capacity of spudcan foundations on clay. Proc. Offshore Technology

Conference, Houston, USA, OTC 17770.

Houlsby, G.T. and Martin, C.M. (2003). Undrained bearing capacity factors for conical

footings on clay. Géotechnique, Vol. 53, No. 5, 513-520.

Osborne, J.J., Teh, K.L., Houlsby, G.T., Cassidy, M.J., Bienen, B., and Leung, C.F.

(2011). InSafeJIP: Improved guidelines for the prediction of geotechnical

performance of spudcan foundations during installation and removal of jack-up

units. Joint Industry-funded Project.




(OMAE), Nantes.


3-18

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014a). Spudcan extraction from

deep embedment in soft clay. Applied Ocean Research, Vol. 48, 126-136.

Kohan, O., Bienen, B., Gaudin, C., and Cassidy, M.J. (2014b). The effect of water

jetting on spudcan extraction from deep embedment in soft clay. Ocean

Engineering, Submitted in January 2014 .

Merifield, R.S., Lyamin, A.V., Sloan, S.W. and Yu, H.S. (2003). Three-Dimensional

Lower Bound Solutions for Stability of Plate Anchors in Clay. Journal of




753.

Purwana, O.A., Leung, C.F., Chow, Y.K., and Foo, K.S. (2006a). Breakout failure

mechanism of jackup spudcan extraction. Proc. 6th Int. Conf. of Physical

Modelling in Geotechnics, ICPMG06, Hong-Kong, Ng, Zhang & Wang (eds),

667-672.

Purwana, O.A. (2006b). Centrifuge model study on spudcan extraction in soft clay. PhD

Thesis, National University of Singapore.




Purwana, O.A., Krisdani, H., Zheng, X.Y., Quah, M., and Foo, K.S. (2010). An

assessment of jack up spudcan extraction. Proc. Int. Symp. on Frontiers in

Offshore Geotechnics, Perth, Australia, 679–684.

Purwana, O.A. (2010). Personal correspondence.



Cha

pter

3

3-19

Tabl

e 3.

1. D

atab

ase

Test

na

me

Ref

eren

ce

Rat

io o

f the

ce

ntrif

ugal

ac

cele

ratio

n to

the

earth

gra

vity

Spud

can

diam

eter

Sp

udca

n de

pth

ratio

Ope

ratio

n tim

e O

pera

tion

load

le

vel o

f the

m

axim

um

inst

alla

tion

load

Bre

akou

t lo

ad

Soil

unit

wei

ght

Soil

shea

r st

reng

th a

t in

stal

latio

n de

pth

Soil

effe

ctiv

e st

ress

D

(m)

H/D

(-)

t

(day

)

Vop

/Vp

(-)

Qc

(MN

)

'

(kN

/m3 )

s u

(kPa

)

'v

(kPa

)

3.0D

2.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

3.02

73

0 85

%

-6.1

4 6.

20

1.10

H

112.

41

2.5D

2.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

2.50

73

0 85

%

-5.6

2 6.

20

1.10

H

93.0

6

2.0D

2.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.99

73

0 85

%

-4.1

3 6.

05

1.10

H

72.0

6

1.5D

2.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.48

73

0 85

%

-3.2

9 6.

05

1.10

H

53.5

4

1.5D

3.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.48

10

95

85%

-3

.43

6.05

1.

10 H

53

.66

1.5D

1.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.47

36

5 85

%

-2.8

2 6.

05

1.10

H

53.4

8

1.5D

0.5Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.47

18

3 85

%

-2.4

2 6.

05

1.10

H

53.4

8

1.5D

0.0Y

K

ohan

et a

l. (2

013b

) 20

0 6.

00

1.47

5

0%

-1.5

8 6.

05

1.10

H

53.4

8

Pred

ictin

g Sp

udca

n Ex

tract

ion

Res

ista

nce

in S

oft C

lay

3-20

Test

na

me

Ref

eren

ce

Rat

io o

f the

ce

ntrif

ugal

ac

cele

ratio

n to

the

earth

gra

vity

Spud

can

diam

eter

Sp

udca

n de

pth

ratio

Ope

ratio

n tim

e O

pera

tion

load

le

vel o

f the

m

axim

um

inst

alla

tion

load

Bre

akou

t lo

ad

Soil

unit

wei

ght

Soil

shea

r st

reng

th a

t in

stal

latio

n de

pth

Soil

effe

ctiv

e st

ress

D

(m)

H/D

(-)

t

(day

)

Vop

/Vp

(-)

Qc

(MN

)

'

(kN

/m3 )

s u

(kPa

)

'v

(kPa

)

Noj

et2

Koh

an e

t al.

(201

4)

200

8.00

3.

02

730

85%

-1

3.91

7.

50

1.04

H

180.

79

Noj

et1

Koh

an e

t al.

(201

3a)

200

8.00

2.

50

730

85%

-1

4.24

7.

50

1.08

H

182.

01

S1U

EnJ

Gau

din

et a

l. (2

011)

20

0 17

.11

1.46

16

64

90%

-8

0.52

6.

00

1.17

H

150.

00

S2U

EnJ

Gau

din

et a

l. (2

011)

20

0 17

.11

1.05

16

64

90%

-4

6.97

6.

00

1.17

H

108.

00

GS1

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

45

<1

0%

-17.

57

6.50

1.

56 H

11

7.65

GS2

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

51

53

75%

-1

9.60

6.

50

1.56

H

122.

85

GS3

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

51

126

75%

-2

4.62

6.

50

1.56

H

122.

85

GS4

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

47

244

75%

-2

7.69

6.

50

1.56

H

119.

60

GS5

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

52

423

75%

-3

1.26

6.

50

1.56

H

123.

50

Cha

pter

3

3-21

Test

na

me

Ref

eren

ce

Rat

io o

f the

ce

ntrif

ugal

ac

cele

ratio

n to

the

earth

gra

vity

Spud

can

diam

eter

Sp

udca

n de

pth

ratio

Ope

ratio

n tim

e O

pera

tion

load

le

vel o

f the

m

axim

um

inst

alla

tion

load

Bre

akou

t lo

ad

Soil

unit

wei

ght

Soil

shea

r st

reng

th a

t in

stal

latio

n de

pth

Soil

effe

ctiv

e st

ress

D

(m)

H/D

(-)

t

(day

)

Vop

/Vp

(-)

Qc

(MN

)

'

(kN

/m3 )

s u

(kPa

)

'v

(kPa

)

GS6

Pu

rwan

a et

al.

(200

5)

100

12.5

0 1.

50

843

75%

-3

6.19

6.

50

1.56

H

122.

20

D-0

1 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

21

400

50%

-2

3.57

6.

50

1 +

1.60

H

98.5

4

D-0

2 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

51

400

50%

-3

1.89

6.

50

1 +

1.60

H

122.

85

D-0

3 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

77

400

50%

-3

7.93

6.

50

1 +

1.60

H

143.

59

C-0

3 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

71

<1

0%

-20.

12

6.50

1

+ 1.

60 H

13

8.65

C-0

2 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

51

<1

0%

-19.

05

6.50

1

+ 1.

60 H

12

2.66

C-0

1 Pu

rwan

a et

al.

(200

9)

100

12.5

0 1.

45

<1

0%

-17.

39

6.50

1

+ 1.

60 H

11

7.59

Pred

ictin

g Sp

udca

n Ex

tract

ion

Res

ista

nce

in S

oft C

lay

3-22

Tabl

e 3.

2. P

aram

eter

s of r

efer

ence

met

hod

and

met

hod

of th

is st

udy

Para

met

er

Des

crip

tion

Ref

eren

ce m

etho

d U

pdat

ed M

etho

d C

omm

ents

D

Spud

can

diam

eter

(m)

H

Spud

can

inst

alla

tion

dept

h (m

)

Hc

Cav

ity d

epth

(m)

Mea

sure

d du

ring

cent

rifug

e te

st

Prov

ides

uni

vers

al m

etho

d of

all

clay

con

ditio

ns

Hs

Spud

can

side

wal

l (m

)

Ht

Hei

ght o

f bac

kfill

abo

ve sp

udca

n to

p su

rfac

e (m

) Se

e Fi

gure

3.3

' top

U

nit w

eigh

t of s

oil a

t top

(k

N/m

3 ) ' t

op =

0.92

'

' top

='

Sim

plifi

catio

n re

quire

d w

ithou

t rel

iabl

e m

etho

d to

es

timat

e th

e ch

ange

in

''

Uni

t wei

ght o

f soi

l (kN

/m3 )

Soil

prop

erty

s u,to

p A

vera

ge sh

ear s

treng

th o

f ba

ckfil

l soi

l abo

ve sp

udca

n af

ter

inst

alla

tion

(kPa

)

Ds

Ds

DHuH

cuH

cc

'.41

'.

55.0

Cha

pter

3

3-23

Para

met

er

Des

crip

tion

Ref

eren

ce m

etho

d U

pdat

ed M

etho

d C

omm

ents

s u,b

ase

Shea

r stre

ngth

at t

he sp

udca

n ba

se le

vel a

fter i

nsta

llatio

n (k

Pa)

f g,to

p C

hang

e in

shea

r stre

ngth

of s

oil

abov

e sp

udca

n to

p du

e to

soil

dist

urba

nce

and

any

soil

reco

nsol

idat

ion

afte

r spu

dcan

in

stal

latio

n (-

)

0.67

for i

mm

edia

te e

xtra

ctio

n 0.

87 fo

r 400

day

s ope

ratio

n Fi

gure

3.8

Pr

ovid

e es

timat

ion

for t

he

full

rang

e of

ope

ratio

nal

perio

ds

f g,b

ase

Gai

n in

shea

r stre

ngth

of s

oil

belo

w sp

udca

n ba

se d

ue to

any

so

il re

cons

olid

atio

n af

ter

spud

can

inst

alla

tion

(-)

1.00

for i

mm

edia

te e

xtra

ctio

n 1.

70 fo

r 400

day

s ope

ratio

n Fi

gure

3.9

Pr

ovid

e es

timat

ion

for t

he

full

rang

e of

ope

ratio

nal

perio

ds

S Sh

ape

fact

or

See

Figu

re 3

.4

Nc,

top

Bre

akou

t fac

tor f

or to

p so

il re

sist

ance

1 (-)

Nc,

base

B

reak

out f

acto

r for

bas

e so

il re

sist

ance

(-)

56.12

.)

(.

*54

89.3

1

56.12

.)

(.

2ln

56.2*

1

,,

',

,

'

top

gav

eto

pu

tto

pt

t

top

gav

eto

pu

tto

pt

t

fs

HDH

DHif

fs

HDH

SDH

if

2.52.0

1.4

,

DHN

base

c

Pred

ictin

g Sp

udca

n Ex

tract

ion

Res

ista

nce

in S

oft C

lay

3-24

Para

met

er

Des

crip

tion

Ref

eren

ce m

etho

d U

pdat

ed M

etho

d C

omm

ents

S b

Adj

ustm

ent f

acto

r for

ov

erbu

rden

stre

ss a

t spu

dcan

ba

se le

vel

1.00

for a

ll ca

ses;

th

eref

ore,

is n

ot p

art o

f th

e up

date

d m

etho

d

Sim

plifi

catio

n re

quire

d w

ithou

t rel

iabl

e m

etho

d to

es

timat

e th

e ch

ange

of S

b

Qup

lift

Tota

l upl

ift re

sist

ance

Qto

p To

p so

il re

sist

ance

Qba

se

Bas

e so

il re

sist

ance

2

Qba

se =

0.2

5

D2 (f

sr

Nc,

base

s u,b

ase f

g,ba

se f o

l –’

H)

To c

onsi

der e

ffec

t of

oper

atio

n lo

ad a

nd

stre

ngth

ratio

Wef

f Su

bmer

ged

wei

ght o

f spu

dcan

W

eff i

s ign

ored

for n

et u

plift

resi

stan

ce

f ol

Fact

or o

f ope

ratio

n ra

tio

)1

.2(2.01

pop

olVV

f

0.5

≤ V

op/V

p ≤ 1

0.25.1

0.1

5.135.0

305

.087.0

35.000.0

DHfo

rDH

for

DHDH

for

S b

)(

eff

base

top

uplif

tW

QQ

Q

top

gto

pu

top

cto

pf

sN

DQ

,,

,2

*).

(25.0

).

'..

.(

25.0,

,,

2b

base

gba

seu

base

cba

seS

Hf

sN

DQ

Cha

pter

3

3-25

1.

It sh

ould

be

note

d th

at in

cal

cula

tion

of b

reak

out f

acto

r fo

r to

p so

il re

sist

ance

, par

amet

er f

g,to

p w

as n

ot m

entio

ned

in th

e or

igin

al f

orm

ulae

pre

sent

ed b

y Pu

rwan

a et

al.

(200

9) a

nd P

urw

ana

et a

l. (2

010)

. How

ever

, the

firs

t au

thor

was

inf

orm

ed b

y th

e pe

rson

al c

orre

spon

denc

e th

at i

t is

inc

lude

d in

the

ove

rbur

den

pres

sure

term

for d

eter

min

atio

n of

top

soil

resi

stan

ce b

reak

out f

acto

r (Pu

rwan

a, 2

010)

. 2.

It

is n

oted

that

for

imm

edia

te o

r sh

ort c

onso

lidat

ion

perio

ds f

g,ba

se w

ill b

e 1

or c

lose

to 1

and

Qba

se c

an p

redi

ct a

neg

ativ

e va

lue.

Thi

s is

inap

prop

riate

and

re

flect

s tha

t the

mec

hani

sm fo

r thi

s cas

e is

not

real

istic

. Pa

ram

eter

D

escr

iptio

n R

efer

ence

met

hod

Upd

ated

Met

hod

Com

men

ts

f sr

Fact

or o

f stre

ngth

ratio

)14(4.0

1'

uv

srs

f

B

est f

it to

con

ditio

ns o

f da

taba

se

Pred

ictin

g Sp

udca

n Ex

tract

ion

Res

ista

nce

in S

oft C

lay

3-26

Tabl

e 3.

3. P

erfo

rman

ce o

f the

refe

renc

e m

etho

d

Test

na

me

Cav

ity

dept

h So

il sh

ear

stre

ngth

at

top

Soil

shea

r st

reng

th

at b

ase

Shea

r st

reng

th

gain

at

top

Shea

r st

reng

th

gain

at

base

Bea

ring

fact

or a

t to

p

Bea

ring

fact

or a

t ba

se

Top

resi

stan

ceB

ase

resi

stan

cePr

edic

ted

brea

kout

M

easu

red

brea

kout

Er

ror

- D

c (m

) s u

,top

(kN

/m2 )

s u,b

ase

(kN

/m2 )

f g,to

p

(-)

f g,b

ase

(-

) N

c,to

p (-

) N

c,ba

se

(-)

Qto

p (M

N)

Qba

se

(MN

) Q

brea

kout

(M

N)

Qc

(MN

) - (%

) 3.

0D2.

0Y

0.43

9.

16

19.9

4 0.

92

1.45

12

.56

5.20

-3

.25

-1.0

7 -4

.32

-6.1

4 -4

2.05

2.5D

2.0Y

0.

43

8.03

16

.51

0.97

1.

54

12.5

6 5.

20

-2.8

5 -1

.12

-3.9

7 -5

.62

-41.

56

2.0D

2.0Y

0.

43

6.09

13

.10

0.93

1.

47

12.5

6 5.

20

-2.1

6 -0

.79

-2.9

5 -4

.13

-39.

91

1.5D

2.0Y

0.

43

4.25

9.

74

0.87

1.

36

12.5

6 5.

18

-1.5

1 -0

.46

-1.9

7 -3

.29

-67.

33

1.5D

3.0Y

0.

43

4.50

9.

76

0.92

1.

45

12.5

6 5.

18

-1.6

0 -0

.58

-2.1

8 -3

.43

-57.

27

1.5D

1.0Y

0.

43

3.97

9.

72

0.81

1.

26

12.5

6 5.

18

-1.4

1 -0

.31

-1.7

2 -2

.82

-63.

78

1.5D

0.5Y

0.

43

3.50

9.

72

0.72

1.

08

12.5

6 5.

18

-1.2

4 -0

.07

-1.3

1 -2

.42

-85.

23

1.5D

0.0Y

0.

43

3.27

9.

72

0.67

1.

00

12.5

6 5.

18

-1.1

6 0.

05

-1.1

1 -1

.58

-42.

56

Noj

et2

0.46

11

.04

25.1

4 0.

89

1.39

12

.56

5.20

-6

.97

-0.0

6 -7

.04

-13.

91

-97.

78

Noj

et1

0.46

11

.46

26.2

1 1.

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.56

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-7

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.58

-14.

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-87.

90

S1U

EnJ

2.09

13

.29

29.2

5 0.

87

1.36

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.56

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8.38

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3.80

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2.18

-8

0.52

-5

4.32

S2U

EnJ

2.21

9.

80

21.0

6 0.

87

1.36

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.53

4.84

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6.92

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6.63

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3.55

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6.97

-7

.86

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1.

92

9.42

28

.24

0.67

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00

12.5

6 5.

16

-14.

51

-4.0

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8.60

-1

7.57

5.

56

Cha

pter

3

3-27

Test

na

me

Cav

ity

dept

h So

il sh

ear

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ngth

at

top

Soil

shea

r st

reng

th

at b

ase

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r st

reng

th

gain

at

top

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r st

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th

gain

at

base

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ring

fact

or a

t to

p

Bea

ring

fact

or a

t ba

se

Top

resi

stan

ceB

ase

resi

stan

cePr

edic

ted

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kout

M

easu

red

brea

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ror

- D

c (m

) s u

,top

(kN

/m2 )

s u,b

ase

(kN

/m2 )

f g,to

p

(-)

f g,b

ase

(-

) N

c,to

p (-

) N

c,ba

se

(-)

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p (M

N)

Qba

se

(MN

) Q

brea

kout

(M

N)

Qc

(MN

) - (%

) G

S2

1.92

10

.55

29.4

8 0.

72

1.09

12

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5.20

-1

6.27

-5

.41

-21.

68

-19.

60

9.59

GS3

1.

92

11.6

3 29

.48

0.79

1.

22

12.1

9 5.

20

-17.

41

-7.9

2 -2

5.32

-2

4.62

2.

78

GS4

1.

92

12.0

3 28

.70

0.84

1.

31

11.5

2 5.

18

-17.

00

-9.5

9 -2

6.59

-2

7.69

-4

.12

GS5

1.

92

12.8

4 29

.64

0.87

1.

36

11.5

5 5.

20

-18.

19

-10.

61

-28.

80

-31.

26

-8.5

3

GS6

1.

92

12.7

9 29

.33

0.88

1.

37

11.4

1 5.

20

-17.

91

-10.

70

-28.

61

-36.

19

-26.

48

D-0

1 2.

50

11.7

7 25

.26

0.87

1.

36

8.86

4.

97

-12.

79

-11.

92

-24.

71

-23.

57

4.61

D-0

2 2.

50

14.3

7 31

.24

0.87

1.

36

10.2

7 5.

20

-18.

12

-12.

08

-30.

20

-31.

89

-5.5

8

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3 2.

50

16.5

9 36

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0.87

1.

36

11.6

0 5.

20

-23.

62

-13.

98

-37.

60

-37.

93

-0.8

9

C-0

3 2.

50

12.3

7 35

.13

0.67

1.

00

12.5

6 5.

20

-19.

07

-5.4

0 -2

4.47

-2

0.12

17

.78

C-0

2 2.

50

11.0

5 31

.19

0.67

1.

00

12.0

6 5.

20

-16.

35

-4.8

5 -2

1.20

-1

9.05

10

.16

C-0

1 2.

50

10.6

3 29

.94

0.67

1.

00

11.6

5 5.

16

-15.

20

-5.1

9 -2

0.38

-1

7.39

14

.69

Pred

ictin

g Sp

udca

n Ex

tract

ion

Res

ista

nce

in S

oft C

lay

3-28

Tabl

e 3.

4. P

redi

ctio

ns u

sing

upd

ated

par

amet

ers

Test

na

me

Shea

r st

reng

th

gain

at

top

Shea

r st

reng

th

gain

at

base

Bea

ring

fact

or a

t to

p

Bea

ring

fact

or a

t ba

se

Nor

mal

ised

st

reng

th

Fact

or o

f st

reng

th

ratio

Fact

or o

f op

erat

ion

load

Top

resi

stan

ce

Bas

e re

sist

ance

Pr

edic

ted

brea

kout

Er

ror

- f g

,top

(-

) f g

,bas

e

(-)

Nc,

top

(-)

Nc,

base

(-

) '

v/su

(-)

f sr

(-)

f ol

(-)

Qto

p (M

N)

Qba

se

(MN

) Q

brea

kout

(M

N)

- (%)

3.0D

2.0Y

0.

84

1.75

12

.56

5.20

5.

64

1.16

1.

14

-2.8

4 -3

.63

-6.4

7 5%

2.5D

2.0Y

0.

84

1.75

12

.56

5.20

5.

64

1.16

1.

14

-2.3

3 -3

.00

-5.3

3 -5

%

2.0D

2.0Y

0.

84

1.75

12

.56

5.20

5.

50

1.15

1.

14

-1.8

2 -2

.38

-4.2

0 2%

1.5D

2.0Y

0.

84

1.75

12

.56

5.20

5.

50

1.15

1.

14

-1.3

2 -1

.76

-3.0

7 -7

%

1.5D

3.0Y

0.

86

1.84

12

.56

5.20

5.

50

1.15

1.

14

-1.3

5 -1

.93

-3.2

8 -4

%

1.5D

1.0Y

0.

81

1.66

12

.56

5.20

5.

50

1.15

1.

14

-1.2

7 -1

.59

-2.8

5 1%

1.5D

0.5Y

0.

77

1.54

12

.56

5.20

5.

50

1.15

1.

14

-1.2

1 -1

.36

-2.5

7 6%

1.5D

0.0Y

0.

60

1.00

12

.56

5.20

5.

50

1.15

1.

14

-0.9

4 -0

.35

-1.2

9 -1

8%

Noj

et2

0.83

1.

75

12.5

6 5.

20

7.21

1.

32

1.14

-6

.26

-8.1

8 -1

4.44

4%

Noj

et1

0.83

1.

75

12.5

6 5.

20

6.94

1.

29

1.14

-6

.54

-8.5

4 -1

5.09

6%

S1U

EnJ

0.88

1.

85

12.5

6 5.

17

5.13

1.

11

1.16

-3

2.79

-4

8.54

-8

1.33

1%

S2U

EnJ

0.88

1.

85

12.5

6 4.

84

5.13

1.

11

1.16

-2

2.20

-3

1.16

-5

3.36

14

%

Cha

pter

3

3-29

Test

na

me

Shea

r st

reng

th

gain

at

top

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r st

reng

th

gain

at

base

Bea

ring

fact

or a

t to

p

Bea

ring

fact

or a

t ba

se

Nor

mal

ised

st

reng

th

Fact

or o

f st

reng

th

ratio

Fact

or o

f op

erat

ion

load

Top

resi

stan

ce

Bas

e re

sist

ance

Pr

edic

ted

brea

kout

Er

ror

- f g

,top

(-

) f g

,bas

e

(-)

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top

(-)

Nc,

base

(-

) '

v/su

(-)

f sr

(-)

f ol

(-)

Qto

p (M

N)

Qba

se

(MN

) Q

brea

kout

(M

N)

- (%)

GS1

0.

60

1.00

12

.56

5.16

4.

17

1.02

1.

10

-10.

23

-5.5

5 -1

5.78

-1

0%

GS2

0.

68

1.25

12

.56

5.20

4.

17

1.02

1.

10

-12.

25

-11.

23

-23.

47

20%

GS3

0.

74

1.45

12

.56

5.20

4.

17

1.02

1.

10

-13.

33

-15.

43

-28.

76

17%

GS4

0.

78

1.58

12

.56

5.18

4.

17

1.02

1.

10

-13.

58

-17.

55

-31.

13

12%

GS5

0.

82

1.69

12

.56

5.20

4.

17

1.02

1.

10

-14.

87

-20.

59

-35.

46

13%

GS6

0.

86

1.80

12

.56

5.20

4.

17

1.02

1.

10

-15.

39

-22.

68

-38.

06

5%

D-0

1 0.

81

1.68

12

.45

4.97

3.

90

0.99

1.

00

-11.

94

-13.

53

-25.

47

8%

D-0

2 0.

81

1.68

12

.56

5.20

3.

93

0.99

1.

00

-15.

78

-18.

19

-33.

97

7%

D-0

3 0.

81

1.68

12

.56

5.20

3.

95

1.00

1.

00

-18.

97

-21.

15

-40.

12

6%

C-0

3 0.

60

1.00

12

.56

5.20

3.

95

0.99

1.

00

-13.

49

-5.2

8 -1

8.77

-7

%

C-0

2 0.

60

1.00

12

.56

5.20

3.

93

0.99

1.

00

-11.

67

-4.7

2 -1

6.38

-1

4%

C-0

1 0.

60

1.00

12

.56

5.16

3.

93

0.99

1.

00

-11.

09

-4.3

8 -1

5.47

-1

1%


3-30


Chapter 3

Figure 33.2. Observved spudcanforce comp

n breakout faponents (afte

ailure mecher Purwana

hanism and det al., 2009

diagram of 9)

3-31

breakout

1

Predicting Spuudcan Extractio

Figu

on Resistance in

Figure 3.3

ure 3.4. Sha

n Soft Clay

3. Variables

ape factor (a

s defined in

after Merifie

Table 3.2

eld et al., 20

003)

3-32

Chapter 3

Figure 3.5

Figur

5. Dissipatio

re 3.6. Pore

on of exces(a

pressure re

ss pore pressafter Purwan

esponses at

sure at spudna et al., 20

spudcan top

dcan base du005

p (after Purw

uring opera

wana et al.,

3-33

tion period

2005)

3


3-34

Figure 3.7. Predicted uplift force based on the reference method proposed by Purwana et al. (2009)

Chapter 3

3-35

Figure 3.8. Change in shear strength at top of the spudcan during the operation time


3-36

Figure 3.9. Gain in shear strength at base of the spudcan during the operation time

Chapter 3

3-37

Figure 3.10. Results of improved prediction method


3-38

Figure 3.11. Comparing performance of reference method with updated formulation

Chapter 3

3-39

Appendix A

A practical example for estimating the net extraction resistance is presented here. Below the breakout force for Test 2.0D2.0Y has been calculated.

Figure A3.1. Input data (Test 2.0D2.0Y)


3-40

Figure A3.2. Calculating parameters (Test 2.0D2.0Y)

Chapter 3

3-41

Figure A3.3. Net extraction load (Test 2.0D2.0Y)


3-42

4-1

CHAPTER 4

Centrifuge Experiments to Study Extraction of a

Deeply Embedded Spudcan Using Top Jetting

4.1 Abstract

Extracting the spudcan footings of mobile jack-up rigs from the seabed at the end of

their operations is challenging when the capacity of the rig to pull is low compared to

the extraction resistance of the spudcans. This is particularly the case when the spudcans

are deeply embedded in soft clay and subjected to long periods of operation that place

load on the foundations and allow for consolidation to occur in the soil. A technical

solution used by the offshore industry to overcome spudcan extraction resistance is to

use a water jetting system that ejects pressurised water through nozzles on the spudcan

face. The aim of using water jetting with nozzles located at the top of the spudcan is to

reduce extraction resistance through fracturing and softening of the upper soil.

However, the efficiencies of such systems are not known to offshore jack-up operators.

Top jetting efficiency is therefore addressed in this paper, which reports a series of

Centrifuge Experiments to Study Extraction of a Deeply Embedded Spudcan Using Top Jetting

4-2

physical experiments on jetted spudcan extraction conducted in a geotechnical beam

centrifuge. The efficiency of water jetting is reported for extraction from depths of up to

three diameters in normally consolidated clay, for different jetting flow rates. The

excess pore pressure and maximum breakout force measured reveal insights into the

extraction process with top jetting. The maximum extraction resistance is shown to be

unaffected by top jetting but relates to the suction developed at the spudcan base, which

can be reduced by jetting at the spudcan base (Bienen et al. 2009; Gaudin et al. 2011).

Top jetting can, however, reduce the extraction resistance post breakout as indicated by

the experimental results of this study.

4.2 Introduction

Self-elevated mobile jack-up units play an important role in offshore drilling in shallow

waters of up to approximately 120 m in depth (Figure 4.1). The inverted conical

footings of jack-ups, which are known as spudcans and can be in excess of 20 m in

diameter in a modern jack-up (Cassidy et al. 2009), can be penetrated in a wide range of

soil conditions. In softer soils, spudcans require large penetration to account for the

jack-up’s self-weight and the expected operational loads. Penetration of up to two or

three spudcan diameters may be necessary before reaching equilibrium during the

preloading process (Endley et al. 1981; Menzies and Roper 2008).

When a jack-up rig is removed from a site for redeployment, its legs and spudcans must

be extracted from the seabed. To overcome the soil resistance, the hull is floated, and its

buoyancy is used to create the pull-out force. In clayey soils and in situations of deep

spudcan penetration, the buoyancy of the hull generates only 20% to 50% of the

installation vertical force (Purwana et al. 2009; Bienen et al. 2009). In many

circumstances, this may not be sufficient to extract the spudcan. It is reported that

Chapter 4

4-3

spudcan extraction from penetration depths of one or two spudcan diameters can require

one or two weeks, and in some extreme cases, up to ten weeks have been required to

free the legs (InSafeJIP 2008). Therefore, the spudcan extraction process, especially

from soft clay, is a time-consuming process. Considering that the average day rate of a

jack-up of up to US$150 000 this carries significant financial cost.

To overcome this issue, most of the modern mobile drilling rigs are equipped with an

integrated water jetting system to assist in the extraction of the foundations. In sandy

and silty material, the water jetting system aims to fluidise the soil surrounding the

spudcan, resulting in a dramatic drop of the effective stresses and hence a significant

reduction of the extraction resistance (Lin 1987). In clay material, where significant

suction may be developed at the spudcan invert, the jetting aims to reduce the suction

and hence decrease the extraction resistance (Gaudin et al. 2011). A conceptual

framework for estimating the required flow rate for the purpose of successful undrained

spudcan extraction based on data from centrifuge experiments has been proposed

(Bienen et al. 2009; Gaudin et al. 2011).

The authors are unaware of any study on the effect of top water jetting on spudcan

extraction from clay soils. Therefore, a series of centrifuge tests were performed in the

beam geotechnical centrifuge at the University of Western Australia with the objective

to investigate the extraction of deeply embedded spudcans using top jetting. The

insights obtained from the experimental data provide the basis for the discussion on the

efficiency of top jetting, which concludes the paper.


4-4

4.3 Experimental setup

4.3.1 Facility and setup

The experiments were performed using the beam geotechnical centrifuge facility housed

at the University of Western Australia (Randolph et al. 1991).

Jetting was applied using a syringe pump housed within the centrifuge platform (House

2002). It consists of a piston moving inside a cylinder, which is connected by a hose to

an inlet in the strongbox. The cylinder has an inner diameter of 50 mm for a stroke of

220 mm. This provides an inner total volume of 432 cm3, available for the jetting. The

maximum drive rate of the motor shaft is 3 mm/s, corresponding to fluid rates of 5.9

cm3/s (in model dimensions).

The spudcan is penetrated into and extracted from the soil sample using an actuator that

is controlled by software written in-house using a Labview interface (De Catania et al.

2010).

4.3.2 Model and instrumentation

A 40 mm diameter model spudcan was fabricated to investigate the extraction of deeply

embedded spudcans with top jetting (Figure 4.2 and Figure 4.3). The spudcan was

manufactured from aluminium alloy 6061-T6 in two separately parts (top and bottom)

which were attached to each other by using four M2.95 mm screws (Figure 4.4). The

spudcan diameter was chosen sufficiently large (in comparison to the leg) to provide

insight into the mechanisms relevant to the prototype, but sufficiently small to perform

deep penetration tests to three diameters and to maximise the number of test sites in the

soil sample.

Chapter 4

4-5

The model spudcan was instrumented with two pore pressure transducers (one at the top

face and one at the base) that were installed at approximately half the distance between

the centre and edge of the spudcan (Figure 4.4). Note that in Figure 4.3, pore pressure

transducers are not shown for clarity.

Though provisions were made for bottom jetting and top jetting closer to the spudcan

centre, only the outer nozzles at the top of the spudcan (Figure 4.2) were used in the

tests reported here. The nozzles that were not required were blocked with screws (that

lay flush with the spudcan face). Each set of nozzles is inter-connected via a ring

channel with a diameter of 1.5 mm. The internal pipes are 2 mm in diameter and are

connected to the inlet located at the top of the leg. The nozzles feature an M1.2 mm

thread into which a M1.2 mm screw can be inserted to block the flow. Then, a 0.5 mm

diameter hole was drilled into each screw to allow water jets. Top jetting in action is

shown in Figure 4.5. It should be noted that providing holes smaller than 0.5 mm

diameter was not possible due to technical constraints. For dimensional analysis of the

nozzle diameter the reader is referred to Chapter 5.

No attempt was made to model the trusswork of a typical jack-up leg. Instead, a

cylindrical hollow leg, 9 mm in external diameter and 4 mm in internal diameter, was

fixed to the spudcan at one extremity and to the centrifuge actuator at the other

extremity through a 1 kN load cell.

4.3.3 Test procedure

Six tests were performed at an acceleration of 200 g in the beam geotechnical centrifuge

at the University of Western Australia. Tests were designed to study the effect of the top

jetting on spudcan extraction. In all tests, the spudcan installation depth was three times


4-6

the spudcan diameter, and spudcan extraction occurred after two years (in prototype

scale) operating load. Details of the test programme are provided in Table 4.1.

Spudcan penetration and extraction was undertaken at a penetration rate of 0.3 mm/s,

resulting in a normalised velocity V=vD/cv of 81, where v is the velocity of the spudcan

installation and extraction, D the spudcan diameter, and cv is the coefficient of

consolidation at a stress level consistent with the spudcan embedment. Finnie and

Randolph (1994) demonstrated that spudcan penetration occurred under undrained

condition provided that a normalised velocity greater than 30 is achieved. In the field,

successful spudcan extraction may require between 6 hours and 30 hours. Considering

spudcan diameters in the range 10 to 20 m and coefficient of consolidation in the range

0.1 to 100 m2/year, normalised extraction velocity in–situ is typically greater than 30.

The normalised extraction velocity of 81 is therefore considered to be in the undrained

condition and consistent with the field values.


stage, spudcan penetration was performed in-flight in the displacement-control mode.

The embedment depth was approximately three spudcan diameters. In the second stage,

the jack-up operation period was simulated (in a simplified fashion) by holding a

constant vertical load of approximately 85% of the maximum installation load for two

years in the load-control mode (prototype scale). This length of operation time was

chosen to induce a high degree of consolidation (with pore pressure measurements at

the spudcan invert indicating at least 90%), in order to create a situation that would

allow clear interpretation of the effectiveness of top jetting. Finally, in the third stage,

spudcan extraction was performed under displacement-control at a constant rate of 0.3

mm/s, and top water jetting was injected using the syringe pump. The testing

Chapter 4

4-7

programme (Table 4.1) was designed to study the effect of a variation of the jetting flow

rate, while the extraction load was recorded and gave an indication of the jetting

efficiency.

For all stages (penetration resistance in stage one, operating period in stage two, and

extraction resistance in stage three) the vertical force on the spudcan and pore pressure

at the top and the invert of the spudcan were monitored. The tests were separated by a

distance of three spudcan diameters to minimise disturbance between tests and

boundary effects.

One test (Nojet1) was performed without jetting as a base case in order to compare

results that included top jetting. Note that test Topjet3 was repeated as Topjet4, because

it was failed (therefore, only the results of Topjet4 are discussed). Jetting and extraction

were triggered simultaneously in all tests except Topject5. In this latter test, jetting was

started six seconds (model scale), corresponding to 66 hours (prototype scale), before

extraction to provide insights into the influence of sequence of events.

4.3.4 Soil preparation and characterisation

Commercial kaolin clay with characteristics provided in Table 4.2 was used to create a

soft normally consolidated soil sample in the beam centrifuge. The kaolin slurry was

prepared at a moisture level of twice the liquid limit, which was poured into a

rectangular strongbox with internal dimensions of 650x390x325 mm over a 15 mm

thick drainage sand layer. Thereafter, the sample was consolidated under self-weight in

the centrifuge at an acceleration of 200 g for a period of approximately five days.


4-8

The final height of the soil specimen was approximately 220 mm. Once the pore

pressures were dissipated, T-bar penetrometer tests were performed to confirm full

consolidation and assess the shear strength profile within the sample.

A T-bar penetrometer was used to derive the undrained shear strength profile of the

sample. The test was performed in flight (i.e. when the centrifuge was spinning) at a rate

of 1 mm/s to ensure that the undrained conditions were measured (Chung et al. 2006). A

bearing capacity factor of 10.5 (NT-bar) was used to derive the profile of the undrained

soil shear strength (Watson 1999; House et al. 2001; Lehane et al. 2009). Figure 4.6

shows the undrained soil shear strength profile in which the average shear strength

gradient is approximately 1.08 kPa/m leading to a shear strength ratio (su/'v) of about

0.15, common for normally consolidated kaolin in the centrifuge (Lehane et al. 2009).

The undrained shear strength profile also is presented versus the depth (H) normalized

by spudcan diameter (H/D) for the purpose of being consistent with comparing to the

other figures presented later in this paper.

4.4 Results and discussion

The load displacement response and excess pore pressure generated at the top and

bottom of the spudcan during installation, operating period, and extraction are

illustrated in Figure 4.7, Figure 4.9, and Figure 4.10 respectively (Figure 4.8 highlights

the extraction stage at the breakout points). Note that all results, unless stated otherwise,

are presented in prototype dimensions, i.e. lengths are scaled by N, load by N2 and

pressure by 1. Figure 4.7 and Figure 4.10 show all three stages, whereas Figure 4.9

shows the extraction phase only for clarity.

Chapter 4

4-9

The results of installation resistance (stage one) and the two year operating period (stage

two) are tabulated in Table 4.3 and Table 4.4 respectively. In general, the maximum

penetration resistances can be consistently predicted with the numerical formulation for

analysing spudcan penetration by Hossain and Randolph (2009). The penetration

resistance of first test, Nojet1, is slightly lower than those of the subsequently

performed jetting tests. This is because of the increase in the undrained shear strength

within the sample due to additional consolidation (for more details, see Table 5.2 and

Figure 5.4 in Chapter 5).

During the two year operating period, excess pore pressure at the top and bottom of the

spudcan dissipated. It is noteworthy that a degree of consolidation ranging from 89.5%

to 94.7% was achieved at the spudcan base at the end of operation period for all tests,

whereas at the top of the spudcan, a degree of consolidation of approximately 40%

(average) was observed. This is best explained because the drainage path away from the

pore pressure at the top of the spudcan is dominated by clay soil that has been

significantly remoulded. The coefficient of consolidation of the remoulded soil would

be expected to be of a higher value.

The results of the extraction stage are summarised in Table 4.5. The normalised net

vertical loads, as reported in Table 4.3 and Table 4.5, were calculated based on the

undrained shear strength, su, at the depth of installation taken from Figure 4.6.

4.4.1 Influence of flow rate

Tests (Topjet1 to Topjet4) were performed with three different syringe pumping flow

ranging from 0.09 mm3/s to 0.94 mm3/s (model scale), corresponding to a flow of 1.04

to 10.86 US gpm (the prototype flow rate was calculated assuming a prototype nozzle


4-10

diameter of 38 mm). This allows direct comparisons with the non-jetting test and

isolates the influence of the top jetting (Figure 4.7 to Figure 4.10).

The maximum extraction resistance was similar in all tests, irrespective of jetting and its

flow rate. It was mobilised after approximately 0.2 spudcan diameters of upward

movement. The excess pore pressures (suction) at the spudcan base at this point, often

termed “breakout”, are also similar and account for the majority of the resistance. While

this compares well with experimental results of tests investigating bottom jetting

(Gaudin et al. 2011), it also indicates that the maximum extraction resistance is

governed by processes at the spudcan base with top jetting not having a significant

influence (at least at the flow rates tested here).

Top jetting can, however, alter the extraction resistance following breakout as shown in

Figure 4.7 and summarised in Table 4.5. A low flow rate of 0.03 mm/s (drive rate of the

motor shaft), (Topjet2) resulted in a slight decrease in extraction resistance compared to

non-jetted extraction (Figure 4.7). Interestingly, this stems from the negation of suction

at the spudcan base (approximately zero excess pore pressure, Figure 4.10) rather than a

significant difference in response above the spudcan (the excess pore pressure at the

spudcan top is similar to that recorded for the non-jetted extraction, Figure 4.9).

An intermediate flow rate of 0.1 mm/s (Topjet4) resulted in a momentary sharp drop in

extraction resistance (down to -4.2 MN following a peak of 13.2 MN), before

proceeding very similarly to Topjet2 (Figure 4.7) with the lower flow rate of 0.03 mm/s.

This behaviour may be explained by analysing the behaviour of the excess pore pressure

during extraction, as the same pattern is observed in Figure 4.10. The suction developed

its second peak at -43.2 kPa after the maximum suction (-161.9 kPa) dropped to a

Chapter 4

4-11

positive value of 78.28 kPa which produced an upward load resulting in decreasing pull

out resistance to -4.2 MN.

The highest flow rate investigated here, 0.3 mm/s (Topjet1) caused the extraction

resistance to reduce to approximately zero immediately post breakout (Figure 4.7).

Examination of Figure 4.10 gives a better insight into the mechanism developed. The

beginning of the extraction resulted in the development of suction at the base of the

spudcan, reaching a maximum value of -213.9 kPa which is very close to the suction

generated in test Nojet1. After the extraction resistance reached its peak (after 0.19

spudcan diameters of upward movement), the spudcan base experienced pore pressures

generated by top jetting. This resulted in a break of the suction and the generation of

excess pore pressure at the base of the spudcan of 125 kPa (Figure 4.10). This indicates

that post-peak, top jetting caused an increase in the pore pressure around the spudcan

edge resulting in negation of the suction pressure generated by the extraction.

Surprisingly, the pore pressure beyond that point remains positive. This means that the

top jetting applied an upward force at the invert of the spudcan, contributing to the

reduction of extraction resistance. This upward component from the top jetting explains

the pattern exhibited by the extraction resistance (Topjet1) a constant force of about -1.2

MN can still be observed after the breakout point.

This is because we believe that the water and soil are flowing from the outer top nozzles

around the spudcan to the bottom; effectively following the deep spudcan failure

mechanism. This is shown in diagram of Figure 4.11.

Because of this, and the location of the top pore pressure transducer, the excess pore

pressure measurement at the spudcan top did not show significant differences between

the tests (Figure 4.9). This could mean that the top jetting had little influence on the


4-12

pore pressure response above the spudcan. More probably though, it hints at the fact that

the single point measurement (and its location) was insufficient to capture the behaviour

fully.

For spudcan extraction from up to 1.5 diameters of embedment, the mechanism at the

beginning of the extraction was shown to be a combination of the weight of the soil

above the spudcan, the resistance along a shear plane generated above the spudcan and

negative excess pore pressure or suction that is developed at the spudcan base in

undrained extraction which contributed to about 70% of the total pull out load (Gaudin

et al. 2011). The failure mechanism after peak is then replaced by a localised flow

around mechanism, still associated with an uplift mechanism of the soil above the

spudcan (Gaudin et al. 2011). Assuming the same mechanisms still hold at 3 diameters

of embedment, the experimental results suggest that top jetting remoulded the soil (and

probably increased its moisture content) that displaced around the edge of the upward

moving spudcan, thus diminishing suction at the base. Note that no piping or soil

boiling were observed at the surface of the sample during the jetting tests, indicating

that for the flow rates investigated here, the additional water remained in the soil body.

At the same time, the soil column above the spudcan visually looked similar at the end

of the pullout and therefore is believed to have remained relatively unaffected by the

jetting.

Nozzles located closer to the spudcan edge may assist in the reversal of back-flow and

will probably benefit from low pressure jetting. We speculate that jetting using nozzles

closer to the spudcan centre may prove beneficial through remoulding of the soil

column, which would be achieved through high pressure jetting. However, in both

cases, this is unlikely to reduce the maximum extraction resistance (at breakout), as this

Chapter 4

4-13

appears to be governed by processes at the spudcan base. Both require experimental

evidence.

4.4.2 Influence of commencement of jetting

In test Topjet5, undrained extraction was performed six seconds after the initiation of

top jetting to examine the influence of commencing top jetting well before mobilisation

of pull-out force. The results of test Topjet5 compared with the results of test Topjet4 in

which jetting and extraction took place simultaneously.

As it can be seen in Figure 4.12 and Figure 4.13, the net extraction resistance and the

excess pore pressure for tests Topjet4 and Topjet5 are similar, albeit with differently

shaped curves around the breakout point. The initiation of top jetting well before

extraction commenced did not have a beneficial effect on the resistance.

4.5 Conclusions

Centrifuge tests were performed to assess the effectiveness of the use of top water

jetting in spudcan extraction and to quantify the potential reduction of extraction

resistance. Tests were performed in normally consolidated Kaolin clay at an

acceleration of 200 g in a geotechnical beam centrifuge.

The following conclusions can be drawn from the experimental results of this study:

1. The application of top jetting pressure did not reduce the maximum extraction

resistance. This is in contrast to the effectiveness of bottom jetting (Bienen et al.

2009; Gaudin et al. 2011). Practically, in an offshore scenario, concentrating on

increasing the volume of water released under the spudcan is more beneficial

than splitting the volume of water between bottom and top.


4-14

2. Top jetting was shown to reduce the extraction resistance post breakout, with

increased flow rates resulting in larger benefits. This was linked to positive

excess pore pressure generated at the base of the spudcan resulting in an upward

force being applied at the invert of the spudcan. Offshore, this may allow a faster

removal of the spudcan.

3. The application of top jetting at the nozzle location under investigation (17 mm

from centre or 3 mm from edge of the spudcan) did not change the measured

excess pore pressure above the spudcan, which could imply that a different

nozzle location may be more beneficial in terms of remoulding the soil column

carried up with the spudcan.

4. No additonal benefit was found from jetting for a period of time before

attempting to extract the spudcan.

It should be noted that these conclusions are limited to the experiments undertaken, but

are believed to provide relevant insight into the mechanisms taking place in situ. Further

studies are necessary, notably focusing on the size and location of the top jetting

nozzles, and modelling more accurately the extraction process in the field, which is

performed under load control rather than displacement control.

Chapter 4

4-15

References

Bienen, B., Gaudin, C., and Cassidy, M.J., 2009. The influence of pull-out load on the

efficiency of jetting during spudcan extraction, Applied Ocean Research, Vol.

31, No. 3, 202-211.

Cassidy, M.J., Quah, C.K., Foo, K.S., 2009. Experimental investigation of the

reinstallation of spudcan footing close to existing footprints, Journal of


Chung, S.F., Randolph, M.F., and Schneider, J.A., 2006. Effect of penetration rate on

penetrometer in clay, Journal of Geotechnical and Geoenvironmental

Engineering, Vol. 132, No. 9, 1188-1196.

De Catania, S., Breen, J., Gaudin, C., White, D.J., 2010. Development of a multiple axis

actuator control system, Proceedings of the 7th International Conference on


Einav, I., and Randolph, M.F., 2005. Combining upper bound and strain path methods

for evaluating penetration resistance, Int. J. of Num. Meth. in Eng., Vol. 63, No

14, 1991-2016.

Endley, S.N., Rapoport, V., Thompson, P. J., and Baglioni, V.P., 1981. Prediction of

jack-up rig footing penetration, Proc. 13th Offshore Technology Conference,

Houston, OTC 4144.

Finnie, I.M.S., and Randolph, M.F., 1994. Punch-through and liquefaction induced

failure of shallow foundations on calcareous sediments, Proc Inter Conf on


Gaudin, C., Bienen, B. and Cassidy, M.J., 2011. Investigation of the potential of bottom

water jetting to ease spudcan extraction in soft clay, Géotechnique, Vol. 61, No.

112, 1043-1054.

Hossain, M.S., and Randolph, M.F., 2009. Effect of strain rate and strain sofening on

the penetration resistance of spudcan foundations on clay, International Journal

of Geomechanics, Vol. 9, No. 3, 122-132.


4-16

House, A. R., 2002. Suction caisson foundations for buoyant offshore foundations,

Ph.D. Thesis, University of Western Australia, Australia.

InSafe JIP, 2008. Minutes of the 2nd progress meeting of the InSafe JIP, Singapore, 20

November 2008.

Lehane, B.M., O’Loughlin, C.D., Gaudin, C., and Randolph, M.F., 2009. Rate effects

on penetrometer resistance in kaolin, Géotechnique, Vol. 59, No. 1, 41-52.

Lin, S.S., 1987. A universal footing with jetting, Proc. Offshore Technology Conference

1987, Houston, USA, 299-306.

Menzies, D., and Roper, R., 2008. Comparison of Jackup rig spudcan penetration

methods in clay, Proc. 40th Offshore Technology Conference, Houston, USA,

OTC 19545.

Purwana, O.A., Leung, C.F., Chow, Y.K., and Foo, K.S., 2005. Influence of base

suction on extraction of jack-up spudcans, Géotechnique, Vol. 55, No. 10, 741-

753.

Purwana, O.A., Quah, M., Foo, K.S., Nowak, S., and Handidjaja, P., 2009. Leg

Extraction / Pullout Resistance - Theoretical and Practical Perspectives, In Proc.

12th Jack up Conf., London.

Randolph, M.F, Jewell, R.J, Stone, K.J.L, Brown, T.A., 1991, Establishing a new

centrifuge facility, Proc Inter Conf Centrifuge 91, Boulder, Colorado, 3-9.

Reardon, M.J. Review of the geotechnical aspects of jack-up unit operations. Ground

Engineering 1986; 19 (7):21 6.

Stewart, D.P., and Randolph, M.F., 1991. T-bar penetration testing in soft clay, Journal

of Geotechnical Engineering, Vol. 120, No. 12, 2230-2236.

Chapter 4

4-17

Table 4.1. Testing Programme

Test name

Penetration depth ratio

Operation time (year)

Operation load level

Jetting location

Flow (model) (mm3/s) Note

Nojet1 3.0D 2 85% NA NA

Topjet1 3.0D 2 85% Outer 0.94



Topjet5 3.0D 2 85% Outer 0.31 Jetting started earlyTopjet3 was failed and the results were not reported.







Consolidation coefficient, cv (at OCR=1 and v =165 kPa) 4.67 m2/year

Submerged unit weight, (at v =165 kPa) 7.5 kN/m3


4-18

Table 4.3. Summary of experimental results – at end of the installation stage

Test name Maximum

load (MN)

Maximum pressure

(kPa)

Normalised net vertical load,

Q/Asu (-)

u Bottom (kPa)

u Top

(kPa) Nojet1 15.2 299.2 11.4 309.5 94.8

Topjet1 16.6 327.3 12.6 316.9 72.4

Topjet2 15.9 315.1 12.1 311.2 64.9

Topjet4 17.6 347.7 13.2 324.7 75.1

Topjet5 17.8 351.4 13.5 326.6 73.7

Table 4.4. Summary of experimental results – at the end of the operation stage

Test name Maximum

load (MN)

Maximum pressure

(kPa)

Consolidation degree

(%)

u* Bottom (kPa)

u Top

(kPa) Nojet1 12.7 249.4 89.7 31.9 69.4

Topjet1 14.0 277.3 94.6 17.1 48.3

Topjet2 13.6 269.8 94.7 16.4 39.5

Topjet4 14.9 293.3 94.3 18.7 36.4

Topjet5 15.1 297.6 89.5 19.9 45.5

Table 4.5. Summary of experimental results – at the peak of the extraction stage

Test name Maximum

load (MN)

Maximum pressure

(kPa)

Normalised net vertical load,

Q/Asu (-)

u Bottom (kPa)

u Top

(kPa) Nojet1 14.2 281.6 10.8 -205.2 172.3

Topjet1 13.1 259.9 10.3 -213.9 141.4

Topjet2 14.3 284.6 11.4 -203.1 147.6

Topjet4 13.2 261.9 10.1 -161.9 180.5

Topjet5 13.4 265.6 10.4 -207.7 153.2

Chapter 4

4-19



4-20

Figure 4.2. Spudcan model

Figure 4.3. Spudcan model cross section (dimensions in mm or degree)

Outer ring channel

Pipe

Chapter 4

4-21

Figure 4.4. Spudcan before test

Figure 4.5. Top jetting in action

Kaolin clay

Leg

Outer nozzles Pore pressure transducer

M2.95 mm screw


4-22

Figure 4.6. Undrained shear strength profile

Chapter 4

4-23

Figure 4.7. Penetration and extraction loads


4-24

Figure 4.8. The extraction stage at the breakout points

Chapter 4

4-25

Figure 4.9. Excess pore pressure at top of the spudcan (for extraction only)


4-26

Figure 4.10. Excess pore pressure at the spudcan base

Chapter 4

4-27

Figure 4.11. Diagram of flow path (believed to be occurring)


4-28

Figure 4.12. Comparing extraction loads when jetting occurred before or simultaneous with extraction

Chapter 4

4-29

Figure 4.13. Comparing excess pore pressure at the spudcan invert when jetting occurred before or simultaneous with extraction


4-30

5-1

CHAPTER 5

The Effect of Water Jetting on Spudcan

Extraction from Deep Embedment in Soft Clay

5.1 Abstract

Self-elevating mobile jack-ups units are designed to move to a new field after operation

is completed, requiring extraction of the jack-up legs from the soil. However, the pull-

out force mobilised by hull buoyancy alone may not be sufficient especially when the

spudcan is deeply embedded in soft soil, where extraction in the field has been reported

to take weeks in some cases. A technical solution used by the offshore industry to

reduce spudcan extraction resistance is to employ a jetting system that ejects water

through nozzles on the spudcan. A conceptual framework for estimating the required

jetting flow rate applied at the spudcan base to enable successful extraction from soft

clay soil has previously been proposed for embedment depths of up to 1.5 diameters.

However, the validity of the framework for deeper embedments is unclear. This paper

reports a series of geotechnical centrifuge experiments on jetted spudcan extraction

The Effect of Water Jetting on Spudcan Extraction from Deep Embedment in Soft Clay

5-2

from depths of up to three diameters in normally consolidated clay. The measurements

of the generated suction and the maximum breakout force confirm that the existing

conceptual framework remains valid for this deep embedment; an important result as

these are the problematic depths in the field.

5.2 Introduction

Self-elevating mobile jack-ups units (Figure 5.1) are employed for offshore exploration

and development purposes in shallow waters, up to approximately 150 m depth. These

rigs have become the most common type of offshore drilling units and currently

perform approximately half of the world’s drilling activity. At present, the fleet consists

of more than 400 jack-ups around the world (plus more than 100 under construction)

with an utilisation of about 85% (rigzone, 2014).

Jack-up rigs comprise of a triangular shaped buoyant hull having interaction with three

or four movable legs through a rack and pinion system, capable of raising the hull over

the surface of the sea. Each independent leg has its conical footing known as spudcan.

The biggest spudcan built to date has a diameter of 23 m, though sizes up to 20 m in

diameter are more common.

Jack-ups are designed to move to a new site after the operation is completed. It is

therefore required that the spudcan footings, which in soft soil can be embedded up to

three diameters (Endley et al., 1981; Menzies and Roper, 2008), are extracted by

jacking down the hull into the water, and then lowering it beyond neutral draft. The

maximum allowable overdraft provides the maximum pull out force available to

overcome the soil resistance. However, this tensile load may not be sufficient to extract

the foundation especially in softer soils and corresponding deep embedment.

Chapter 5

5-3

It is acknowledged that spudcan extraction can require one or two weeks, though up to

ten weeks have been reported (InSafe JIP, 2008). The spudcan extraction process,

especially from soft clay, may therefore be a time-consuming process. Considering that

the average day rate of a jack up is between US$60 000 to US$160 000 (depending on

the water depth rating; rigzone, 2014) this has significant financial implications. To

overcome this issue, most of the modern mobile drilling rigs are equipped with a water

jetting system integrated into the spudcan to assist in the leg extraction. The water is

supplied from pumps located on the hull, through hoses down the jack-up legs and is

jetted through nozzles located on the spudcan top and/or bottom faces.

Water jetting at the spudcan top face aims to reduce extraction resistance through

fracturing and softening the upper soil. The effectiveness of top jetting in reducing the

extraction resistance from deep embedment was investigated by Kohan et al. (2013). A

40 mm in diameter model spudcan, featuring sixteen 0.5 mm jetting nozzles arranged on

a concentric circle and located 3 mm from edge of the spudcan, was extracted from an

embedment of three diameters under undrained condition. Three diameters was chosen

as it is exceptional for large spudcans to be buried further (see Menzies and Roper,

2008). Tests were performed in normally consolidated kaolin clay and in displacement

control at an acceleration of 200 g. It was concluded that the application of top jetting

pressure did not reduce the maximum extraction resistance, but reduced the post

breakout resistance with increasing flow rates. This is linked to positive excess pore

pressure generated at the base of the spudcan resulting from the water and soil that is

flowing from the outer top nozzles around the spudcan to the bottom effectively

following the deep spudcan failure mechanism.


5-4

This is in contrast to the effectiveness of bottom jetting which has been demonstrated

through centrifuge tests to reduce the extraction resistance by diminishing the negative

excess pore pressure, or suction, generated at the spudcan invert. The tests conducted at

an acceleration of 200g modelled a 17.11 m (prototype) diameter spudcan equipped

with three sets of twelve jetting nozzles at the spudcan base (Bienen et al., 2009; Gaudin

et al., 2011a). The embedment depth was up to 1.46 spudcan diameters corresponding to

25 m in prototype scale. The term ‘filling ratio’, f, was introduced to relate the reduction

in maximum extraction resistance, or breakout force, to the water volume required in

the jetting operation. The filling ratio is defined as the ratio of the volume of water

jetted into a theoretical void left by the extracting spudcan, and is calculated as the total

jetting flow rate divided by the product of the spudcan extraction rate and spudcan

invert contact area. The reduction in extraction resistance was demonstrated to depend

on the filling ratio rather than the jetting pressure, and at a filling ratio of 0.7 the jetting

flow rate was found to negate the generation of negative excess pore pressure at the

spudcan invert. Bienen et al. (2009) and Gaudin et al. (2011a) proposed a conceptual

framework to estimate the required bottom jetting flow rate based on the centrifuge

experimental data. Note that the conclusions are valid only if the behaviour of the

surrounding soil is considered undrained during extraction process.

The previous findings on the effectiveness of bottom jetting, and the proposed

framework, are limited to spudcan extraction from a maximum embedment depth of 1.5

diameters. Kohan et al. (2014) showed that the undrained unjetted extraction governing

failure mechanism remain unchanged for embedment from 1.5 to 3 diameters by

performing eight centrifuge tests at an acceleration of 200 g designed to investigate the

effects of the embedment depth and the duration of operation time on spudcan

extraction. In the first stage of undrained extraction, the mechanism is a combination of

Chapter 5

5-5

reverse end bearing at the spudcan invert and uplift of the soil column above the

spudcan (Gaudin et al. 2011a, Figure 5.2). At peak extraction resistance the failure

mechanism transitions a localised flow around mechanism, still associated with an uplift

mechanism of the soil above the spudcan.

The purpose of the present study is to verify the validity and reliability of the bottom

jetting framework for spudcan extraction from embedment up to 3 diameters. This was

undertaken by performing centrifuge tests modelling spudcan extraction with bottom

jetting in the beam geotechnical centrifuge at the University of Western Australia.


5.3.1 Facility and setup

The experiments were performed using the beam geotechnical centrifuge facility housed

at the University of Western Australia (Randolph et al., 1991; Gaudin et al., 2011b).

The centrifuge has a maximum payload of 200 kg at the maximum acceleration of 200 g

providing a 40 g-tonne capacity with a radius of 1.8 m. The sample was prepared in a

strongbox with internal dimensions of 650 mm by 390 mm. The final sample height was

220 mm when tested at an acceleration of 200 g. This represents a prototype test bed of

130 m in length, 78 m in width, and 44 m in depth. The tests were separated by a

distance of three spudcan diameters to minimise disturbance between tests and

boundary effects. A two-dimensional actuator mounted on top of the strongbox was

employed to penetrate and extract the spudcan in the underlying soil. The actuator

controlled by an in-house interface based on Labview (De Catania et al., 2010) was

connected to a 1 kN load cell, which measured vertical loads on the spudcan during

penetration and extraction.


5-6

Jetting was applied using a syringe pump housed within the centrifuge platform (House,

2002). It consists of a piston moving inside a cylinder, which is connected by a hose to

an inlet in the strongbox. The cylinder has an inner diameter of 50 mm for a stroke of

220 mm. This provides an inner total volume of 432 cm3 available for jetting. The

maximum drive rate of the motor shaft is 3 mm/s, corresponding to fluid rates of 5.9

cm3/s (in model dimensions).


The same 40 mm diameter model spudcan (8 m in prototype) used by Kohan et al.

(2013) to investigate the extraction of deeply embedded spudcans with application of

top jetting was used to examine the influence of bottom water jetting application on

spudcan extraction from deep embedment in soft clay. One additional test with

combined top and bottom jetting is also included. The spudcan diameter was chosen

sufficiently large (in comparison to the leg) to provide insight into the mechanisms

relevant to the prototype, but sufficiently small to perform deep penetration tests to

three diameters and to maximise the number of test sites in the soil sample. The spudcan

was manufactured from aluminium alloy 6061-T6 in two separate parts (top and

bottom) which were connected by four M2.95 mm screws (Figure 5.3a).

Each set of nozzles is inter-connected via a ring channel with a diameter of 1.5 mm

(Figure 5.3b). The internal pipes are 2 mm in diameter and are connected to the inlet

located at the top of the leg. The nozzles feature a M1.2 mm thread into which a screw

can be inserted to block the flow. A 0.5 mm diameter hole was drilled into each screw

of the nozzles in use to allow water jetting. This was the minimum opening diameter

that was technically feasible, and scaling considerations are outlined below.

Chapter 5

5-7

The model spudcan was instrumented with two pore pressure transducers (one at the top

face and one at the base) that were installed at approximately half the distance between

the centre and edge of the spudcan (Figure 5.3c). In this study, for the test reported for

application of both top and bottom jetting, only the outer nozzles at the top of the

spudcan were used though provision was made for top jetting closer to the spudcan leg.

The nozzles that were not required were blocked with screws (that lay flush with the

spudcan face).

No attempt was made to model the trusswork of a typical jack-up leg. Instead, a

cylindrical hollow leg, 9 mm in external diameter and 4 mm in internal diameter, was

fixed to the spudcan at one extremity and to the centrifuge actuator at the other

extremity through the load cell.

5.3.3 Centrifuge scaling factor

In centrifuge modelling, linear dimensions of the model are scaled down by a factor of

N relative to the prototype where N is the ratio of the centrifugal acceleration to the

gravity of earth. Scale factors for other parameters such as load, pressure, and time, can

be found in Garnier et al. (2007) who made an inventory of the scaling laws and

similitude questions related to centrifuge modelling. Gaudin et al. (2011a) established

the scaling factor on the flow rate to be N2/3, where is the reduction factor between

the correctly scaled and the actual model nozzle diameters. For instance, prototype

jetting nozzles of 38 mm diameter would require 0.19 mm diameter nozzles in a 1/200th

reduced scale model. If the actual model nozzle diameter is 0.5 mm, is equal to 2.63

and the scaling factor on the flow rate is 2002/2.633. Hence, though the model nozzle

diameter is not scaled directly, the dimensionless group that characterises the response,

Qjv/gdn3 (Gaudin et al., 2011a), is satisfied. In this expression Qj is the flow rate, v is the


5-8

spudcan extraction velocity, g is the gravity acceleration and dn is the diameter of the

jetting nozzle.


Commercial kaolin clay with characteristics provided in Table 5.1 was used to create

two soft normally consolidated soil samples in the beam centrifuge. The kaolin slurry

was prepared at a moisture content of twice the liquid limit, which was poured into the

rectangular strongbox over a 15 mm thick drainage sand layer. Thereafter, the sample

was consolidated under self-weight in the centrifuge at an acceleration of 200 g for a

period of approximately five days. Over the consolidation time, sample settlement was

measured. The final height of the soil specimen was approximately 220 mm in both

samples.

Once the pore pressures were dissipated, T-bar penetrometer tests were performed to

confirm full consolidation and to derive the undrained shear strength profile of the

samples. The tests were performed in flight at a rate of 1 mm/s to ensure undrained

conditions (Finnie and Randolph, 1994; Chung et al., 2006). A bearing capacity factor

(NT-bar) of 10.5 (Lehane et al., 2009) was used to assess the profile of the undrained soil

shear strength. The profiles can be represented by an average shear strength gradients of

0.98 kPa/m to 1.04 kPa/m in sample 1 and 1.08 kPa/m to 1.19 kPa/m in sample 2

(Figure 5.4).

5.3.5 Test programme and test procedure

The experimental program included a total of nine tests performed at an acceleration of

200 g in the beam geotechnical centrifuge at the University of Western Australia (Table

5.2). Two tests (Nojet1 and Nojet2) were performed without jetting as reference cases.

Six tests investigated the effect of bottom jetting on the spudcan extraction resistance.

Chapter 5

5-9

One additional test (Topbotjet1) was designed to understand whether the application of

combined top and bottom water jetting is beneficial.


stage, the spudcan was penetrated in-flight under constant velocity up to an embedment

depth of three spudcan diameters. In the second stage, the jack-up operation period was

simulated by holding a constant vertical load of approximately 85% of the maximum

installation load for two years in prototype scale (26.3 minutes in model scale). This

length of operation time was chosen to induce at least 85% of consolidation, hence

maximising the extraction resistance (Koahn et al., 2014) and allowing clear

interpretation of the effectiveness of bottom jetting. The nozzle openings in the jetting

system allow the possibility of additional drainage (though the volume would be

limited). Comparisons between the dissipation curves with similar tests performed

without nozzles show a slight increase in the dissipation rate, though this cannot be

attributed definitively to the nozzles. Finally, in the third stage, the spudcan was

extracted at constant velocity while water jetting was applied using the syringe pump.

Spudcan penetration and extraction were undertaken at a rate of 0.3 mm/s, resulting in a

normalised velocity V=vD/cv of 81 at the embedment depth of three spudcan diameters,

where v is the velocity of the spudcan installation and extraction, D the spudcan

diameter, and cv is the coefficient of consolidation (taken at a stress level consistent

with the spudcan embedment). This ensured that fully undrained conditions were

satisfied according to the criterion established by Finnie and Randolph (1994) and

further validated for spudcan in Cassidy (2012). In the field, successful spudcan

extraction may require between 6 hours and 30 hours. Considering spudcan diameters in


5-10

the range 10 to 20 m and coefficient of consolidation in the range 0.1 to 100 m2/year,

normalised extraction velocity in–situ is typically greater than 30.

The testing programme (Table 5.2) was designed to study the effect of a variation of the

jetting flow rate (and thereby the filling ratio f) on the recorded extraction load, which

provides an indication of jetting efficiency. For all stages the vertical force on the

spudcan and pore pressure at the top and the invert of the spudcan were monitored.

Bienen et al. (2009) recommended to initiate jetting as early as safe and practical.

Therefore, in order to prevent any delays between water jetting at the nozzles and

mobilisation of pull-out force by the spudcan, jetting commenced slightly earlier than

extraction (Table 5.2). However, jetting and extraction were triggered simultaneously in

Botjet1 and Botjet6 to examine the influence of the sequence of events.

5.4 Experimental results

Results of spudcan extraction without jetting are briefly presented before the effects of

jetting are discussed. All results are presented in prototype dimensions, unless otherwise

stated.

5.4.1 Reference case, extraction without jetting

The bearing resistance of the reference cases without jetting increases with depth during

installation due to the increasing undrained shear strength of the soil (Figure 5.5).

Similar to observations by Gaudin et al. (2011a) and Kohan et al. (2014), the excess

pore pressure at the spudcan invert is higher than the penetration bearing resistance,

indicating contribution of the soil back-flow to an increase of the penetration load, due

to the self-weight of the soil. Spudcan installation results in heavy remoulding of the

soil such that shearing takes place under very low effective stresses.

Chapter 5

5-11

During the operational period the load remained constant while the excess pore pressure

dissipated (Figure 5.5 and Figure 5.6). Degrees of consolidation of approximately 86%

(Nojet1) and 89% (Nojet2), respectively, were achieved. On the other hand, at the top of

the spudcan, degrees of consolidation of about 34.5% and 43.0% were inferred for tests

Nojet1 and Nojet2 respectively, which is explained by the lower coefficient of

consolidation of the highly remoulded soil and consolidation taking place under much

lower stresses (soil self-weight only) compared to the high stresses applied under the

spudcan base.

The maximum extraction resistance of -276.0 kPa and peak negative excess pore

pressure of -241.2 kPa (Nojet2) were mobilised simultaneously after about 0.1D of

spudcan displacement (Figure 5.5 and Figure 5.6). The bearing resistance responses and

excess pore pressures at the top and bottom of the spudcan at the end of the installation

and operation, and at the breakout point are summarised in Table 5.3 and Table 5.4.

5.4.2 Undrained jetted extractions – Bottom jetting

Tests in which spudcan extraction was conducted with the aid of bottom jetting are

tabulated in Table 5.2. The aforementioned test procedure for installation, operation,

and extraction is similar in all tests, which allows direct comparison of the jetted tests

with the non-jetted tests in each sample. In the other words, jetted tests performed in

sample 1 are compared with non-jetted Test Nojet2 and jetted tests in sample 2 with

Nojet1 (Table 5.2). Results of the penetration resistance and excess pore pressure

responses during installation and operating period are summarised in Table 5.3 and

Table 5.4.

Typical extraction resistances and the suction developed at the spudcan invert for three

jetted tests with different filling ratios ranging from 0.2 to 0.7 and one non-jetted test for


5-12

comparison are shown in Figure 5.5 and Figure 5.6, respectively. In order to prevent any

delays in effectiveness of the water jetting, this was initiated slightly before

commencement of extraction (Table 5.2). Additional tests were performed to investigate

the effect the timing of commencement of jetting has on the extraction response, which

is discussed in a later section.

As previously concluded by Gaudin et al. (2011a), the application of bottom jetting

limits the development of negative excess pore pressure or suction at the spudcan base,

which accounts for the majority of the extraction resistance. The response is described

in terms of the extraction resistance ratio, defined as the ratio of the maximum jetted

extraction resistance qj to the maximum non-jetted extraction resistance qnj, and in terms

of the suction ratio, which is the ratio of the maximum suction pressure developed at the

spudcan invert with jetting, psj, to the that of without jetting, psnj. The reduction in both

ratios is a function of the filling ratio, i.e. the higher the filling ratio, the lower

extraction resistance and suction ratios (Bienen et al., 2009; Gaudin et al., 2011a), and

this is confirmed here (Table 5.5).

Figure 5.5 shows that the value of the maximum extraction resistance decreased from -

276.0 kPa in the reference case to -105.0 kPa at a filling ratio of 0.7 in test Botjet2

(corresponding to an extraction resistance ratio of 0.38). This filling ratio resulted in

excess pore pressure (with respect to hydrostatic pressure) being developed at the

spudcan invert, which peaked at 55.9 kPa (or a suction ratio of -0.23) at the breakout

point (Figure 5.6). Filling ratios of 0.5 (Botjet4) and 0.2 (Botjet5) result in intermediate

behaviour with reduced (though still negative) excess pore pressures corresponding to

the volume of water jetted and hence reduced breakout resistance of -156.8 kPa and -

207.2 kPa, respectively (Table 5.4 and Table 5.5; Figure 5.5 and Figure 5.6).

Chapter 5

5-13

The effect of water jetting and the resulting changes to the negative excess pore pressure

response, compared to the reference case of spudcan extraction without jetting, is

evident also in the mobilisation distance of breakout resistance. Figure 5.5 shows that

the mobilisation distance of the maximum extraction resistance for jetted tests increases

with the filling ratio. In the test with the highest investigated filling ratio of 0.7

(Botjet2), breakout resistance was measured after approximately 0.2 spudcan diameter

of upward movement, which is about twice that of the non-jetted test. This softer

response was also observed by Kohan et al. (2014) when spudcan extraction occurred

after shorter operation times that do not allow excess pore pressure generated during

installation to dissipate completely (Figure 5.7). This potentially indicates that the

reverse end bearing mechanism is not fully mobilised before it transitions to a localised

flow mechanism at the breakout point, and jetting has a similar effect to lower degrees

of consolidation prior to extraction.

Different mechanism was also observed after post peak. The pressure resistance curve

during extraction after post peak in Figure 5.5 shows a smooth reduction post peak for

Botjet2 in comparison with Nojet2 which a sharper reduction is observed. This

behaviour is similar to behaviour of the "immediate extraction" test in Figure 5.7,

indicating a flow around mechanism for extraction with bottom jetting application.

Figure 5.8 illustrates the extraction resistance ratio plotted against the filling ratio,

which provides insights into the water jetting performance. As depicted, the extraction

resistance ratios are in good agreement with the line of successful undrained jetting

extraction proposed by Gaudin et al. (2011a). The failure mechanism governing

spudcan extraction without jetting remains unchanged despite significantly deeper

spudcan embedment of up to 3 times the spudcan diameter (Kohan et al., 2014). The


5-14

experimental data shown here provide evidence that this finding also holds for spudcan

extraction with water jetting applied at the invert. The bearing capacity factors during

jetting extraction, Nc, plotted in Figure 5.9 against the filling ratio, underlines this

conclusion. These were back-calculated as the bearing resistance, qj, divided by the

original undrained shear strength of the soil, su, at the relevant depth. The bearing

factors reduced from a value of about 11.3 for undrained extraction without jetting to an

average value of 4.8 at a filling ratio of 0.7 (Table 5.4). Bearing capacity factors back-

calculated in a similar fashion from the data of Gaudin et al. (2011a) are in good

agreement with the bearing capacity factors calculated in the present study (Figure 5.9).

This further strengthens the argument of unchanged mechanisms at embedment of up to

3 diameters in the present study compared to maximum embedment depth of 1.5

diameters in Bienen et al. (2009) and Gaudin et al. (2011a).

The negative excess pore pressure response at first glance does not appear to support

these conclusions. Although the negative excess pore pressure (suction) ratio reduces

with increasing filling ratio (Figure 5.10), the gradient differs from that established by

Gaudin et al. (2011a). However, this is easily explained as a result of differences in soil

shear strength, spudcan geometry and location of the localised pore pressure

measurement. Negative excess pore pressure generation is a function of soil shear

strength at the installation depth which is higher in tests reported by Gaudin et al.

(2011a). Further, the base of the spudcan tested previously represents a steeper cone

with an enclosed angle of 140°, while the spudcan model used in the present study

featured a flatter base with an enclosed angle of 160°. The excess pore pressure

generated during spudcan penetration varies over its diameter, with the maximum at the

centre and reducing towards the edge (Purwana et al., 2005), and steeper cones are

expected to generate higher excess pore pressure. The geometry of the spudcan invert

Chapter 5

5-15

and the location of the pore pressure sensor will therefore result in differences of the

value that is measured at a particular point. The effects are evident already in the excess

pore pressures generated during spudcan penetration (Figure 5.11), and consequently

result in the apparent differences between the two data sets at breakout shown in Figure

5.10. The measurements of extraction resistance, however, demonstrate that the

effectiveness of bottom jetting in reducing the extraction resistance from shallower

embedment also holds true for deeper spudcan penetration. Therefore, the level of

jetting rate required can be estimated as set out in Bienen et al. (2009). This is important

as the majority of spudcan extractions requiring jetting in the field commence from deep

embedment.

Additional tests performed to investigate the influence of commencement of jetting on

the extraction response did not show any systematic changes as Table 5.2, Table 5.4,

and Table 5.5 summarise. Three tests with filling ratios of 0.7 were carried out where

jetting was initiated 0.0, 2.8 and 9.0 s prior to extraction, respectively. Similarly, two

tests with delays of 0.0 and 3.3 s in commencement of extraction, respectively, were

performed with filling ratios of 0.5. The results are graphically illustrated in Figure 5.8,

Figure 5.9, and Figure 5.10.

5.4.3 Application of both top and bottom jetting

One additional test was performed to provide an indication of the effect of combined

jetting through nozzles at the spudcan top and invert (Topbotjet1), with a syringe

pumping flow rate of 0.1 mm3/s (model scale). The results are compared with those of

tests with similar flow rates, applied at the spudcan base only (Botjet6) or exclusively at

the top (Topjet4- see Kohan et al., 2013). All relevant test data are included in Table 5.2


5-16

to 5.5. Note that the calculation of the filling ratio is not meaningful unless water is

jetted through the bottom nozzles only.

While Figure 5.12 shows that the peak extraction resistance is largest in the case of top

jetting only (Topjet4), the response in the tests with jetting at the base only (Botjet6)

and combined top and bottom jetting (Topbotjet1) are similar. As the water supply was

shared between the top and bottom outlets, it is possible that the jetting flow directs

itself primarily towards the spudcan base (though this cannot be confirmed). However,

as top jetting by itself has been shown to be ineffective to relieve the maximum

extraction resistance (see also Kohan et al. (2013), though it can reduce subsequent

resistance), it is desirable for jetting to be directed at the spudcan base initially. It is

noted that the majority of jack-ups in the field have the same water supply the top and

bottom jetting systems.

Thereafter, the response is similar, though the extraction resistance in the case of only

top jetting is initially still a little larger. This difference is due to the excess pore

pressure response at the spudcan invert (Figure 5.13). It is noteworthy that in all three

tests, even that with no bottom jetting, positive excess pore pressures were generated at

the spudcan invert (Figure 5.13). This results from the relative movement of spudcan

and soil as initially described in Kohan et al. (2013), such that top jetting eventually

becomes effective at the spudcan base. However, the excess pore pressure resulting

from top jetting only are slightly lower than those in the tests with combined or bottom

only jetting.

The top pore pressure measurements showed no significant difference between the tests.

However, this may simply be due to the location of this point measurement relative to

the nozzles and the soil and water flow and is therefore inconclusive.

Chapter 5

5-17

While the results of top jetting and combined top and bottom jetting suggest that a

reduction of the breakout resistance through jetting can only be achieved by directing

the water flow at the spudcan invert (for a full discussion on the series of top jetting

tests see Kohan et al., 2013), it should be noted that in the centrifuge tests top jetting

could not be performed at the high pressures expected to be applied when using top

jetting in the field.

5.5 Conclusions

Centrifuge experiments have been performed to investigate the application of bottom

jetting to ease the extraction of deeply embedment spudcans in normally consolidated

clay soil. Tests were performed on a 1:200 scale spudcan model with an embedment

ratio of 3 times the spudcan diameter, and different flow rates.

Results demonstrated that the existing conceptual framework for estimating the required

flow rate for successful undrained spudcan extraction from shallow embedment in soft

clay (Gaudin et al. 2011a) is also valid for deep embedment (up to 3 spudcan

diameters), since the governing mechanisms were shown here to remain unchanged.

This is significant as it allows application of the framework to the entire range of

spudcan embedments typically encountered in the field, with the procedure to estimate

the required jetting flow rate proposed by Bienen et al. (2009) remaining valid for deep

embedment also.

In addition, jetting through nozzles located at the spudcan top was demonstrated to

eventually result in similar extraction behaviour as the relative movement between

spudcan and soil carries the change in pore pressure response created by the water flow

to be effective at the spudcan base. Note, however, that only jetting at the spudcan

invert enables the peak extraction resistance to be reduced.


5-18

References



31, No. 3, 202-211.

Cassidy, M.J. (2012). Experimental observations of the penetration of spudcan footings

in silt. Géotechnique, Vol. 62, No. 8, 727-732.









Houston, OTC 4144.




Gaudin, C., Bienen, B. and Cassidy, M.J. (2011a). Investigation of the potential of


61, No. 112, 1043-1054.

Gaudin, C., Cassidy, M.J., Bienen, B., and Hossain, M.S. (2011b). Recent contributions


behaviour. Ocean Engineering, Vol. 38, No. 7, 900-914.





Chapter 5

5-19

House, A. R. (2002). Suction caisson foundations for buoyant offshore foundations.

PhD thesis, University of Western Australia.


November 2008.




(OMAE), Nantes.

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014). Spudcan extraction from

deep embedment in soft clay. Applied Ocean Research, In Press, (doi):

10.1016/j.apor.2014.08.001.

Lehane, B.M., O’Loughlin, C.D., Gaudin, C., and Randolph, M.F. (2009). Rate effects

on penetrometer resistance in kaolin. Géotechnique, Vol. 59, No. 1, 41-52.



OTC 19545.



753.







Reardon, M.J. (1986). Review of the geotechnical aspects of jack-up unit operations.

Ground Engineering, 19(7):21 6.

Rigzone.com, accessed 7 January 2014.


5-20

Stewart, D.P. (1992). Lateral loading of pile bridge abutments due to embankment

construction. PhD thesis, University of Western Australia.

Chapter 5

5-21







Consolidation coefficient, cv (at OCR=1 and v =180 kPa) 4.74 m2/year

Submerged unit weight, (at v =180 kPa) 7.5 kN/m3

The

Effe

ct o

f Wat

er Je

tting

on

Spud

can

Extra

ctio

n fr

om D

eep

Embe

dmen

t in

Soft

Cla

y

5-22

Tabl

e 5.

2. T

estin

g Pr

ogra

mm

e

Test

N

umbe

r Te

st

Nam

e D

ay

Sam

ple

Shea

r st

reng

th

(k

Pa/m

)

Syrin

ge

rate

(mm

/s)

Filli

ng

Rat

io

(-)

Extra

ctio

n af

ter

effe

ctiv

enes

s of

jetti

ng**

(s

) 1

Bot

jet1

1

1 0.

98

0.13

4 0.

7 0.

0

2 B

otje

t2

1 1

0.98

0.

134

0.7

2.8

3 B

otje

t3

1 1

0.98

0.

134

0.7

9.0

4 B

otje

t4

2 1

1.04

0.

096

0.5

3.3

5 B

otje

t5

2 1

1.04

0.

038

0.2

6.1

6 B

otje

t6

1 2

1.08

0.

096

0.5

0.0

8 N

ojet

1 1

2 1.

08

NA

N

A

NA

7 N

ojet

2 2

1 1.

04

NA

N

A

NA

9 To

pbot

jet1

5

2 1.

19

0.10

0 N

A

0.0

To

pjet

4*

1 2

1.08

0.

100

NA

0.

0 *:

See

Koh

an e

t al.

(201

3)

**: A

s th

e je

tting

took

eff

ect,

an in

crea

se in

por

e pr

essu

re w

as r

ecor

ded

by th

e po

re p

ress

ure

trans

duce

r at

the

spud

can

inve

rt. T

his

colu

mn

deta

ils th

e nu

mbe

r of

se

cond

s bet

wee

n th

e in

crea

se in

por

e pr

essu

re a

nd th

e co

mm

ence

men

t of s

pudc

an e

xtra

ctio

n.

Cha

pter

5

5-23

Tabl

e 5.

3. S

umm

ary

of e

xper

imen

tal r

esul

ts (i

nsta

llatio

n an

d op

erat

ion)

Test

nam

e Pe

netra

tion

dept

h ra

tio

Pene

tratio

n re

sist

ance

N

orm

alis

ed

net v

ertic

al

load

Exce

ss p

ore

pres

sure

at

the

spud

can

inve

rt Ex

cess

por

e pr

essu

re a

t th

e sp

udca

n to

p

en

d of

the

inst

alla

tion

end

of th

e op

erat

ing

perio

d

end

of th

e in

stal

latio

n en

d of

the

oper

atin

g pe

riod

H/D

Q

q

Q/(A

.s u)

u

u

u

u

(-)

(MN

) (k

Pa)

(-)

(kPa

) (k

Pa)

(kPa

) (k

Pa)

Bot

jet1

3.

02

15.8

31

1.7

13.1

32

2.4

23.9

81

.8

36.2

Bot

jet2

3.

00

14.4

28

4.8

12.1

28

9.9

46.6

76

.3

47.5

Bot

jet3

3.

00

14.4

28

4.6

12.1

32

3.7

213.

8 80

.2

88.3

Bot

jet4

2.

96

15.2

30

2.0

12.3

33

7.4

156.

5 86

.8

59.0

Bot

jet5

2.

99

16.2

32

0.4

12.8

34

6.6

135.

3 84

.7

52.6

Bot

jet6

3.

02

15.6

30

8.8

11.8

33

8.3

39.4

97

.3

81.2

Noj

et1

3.02

15

.2

299.

2 11

.4

309.

5 31

.9

105.

8 69

.4

Noj

et2

3.00

15

.8

311.

3 12

.4

337.

3 35

.9

90.2

50

.3

Topb

otje

t1

2.95

17

.1

339.

7 13

.3

355.

1 44

.6

86.2

72

.2

Topj

et4

3.0

17.6

34

7.7

13.2

32

4.7

18.7

75

.1

36.4

The

Effe

ct o

f Wat

er Je

tting

on

Spud

can

Extra

ctio

n fr

om D

eep

Embe

dmen

t in

Soft

Cla

y

5-24

Tabl

e 5.

4. S

umm

ary

of e

xper

imen

tal r

esul

ts (e

xtra

ctio

n)

Test

nam

e Ex

tract

ion

resi

stan

ce

Nor

mal

ised

ne

t ver

tical

lo

ad

Bre

ak-o

ut

dept

h Ex

cess

por

e pr

essu

re a

t the

sp

udca

n in

vert

Exce

ss p

ore

pres

sure

at t

he

spud

can

top

Q

q

Q/(A

.s u)

u

u

(MN

) (k

Pa)

(-)

(m)

(kPa

) (k

Pa)

Bot

jet1

-5

.5

-109

.3

-4.9

22

.7

61.9

16

8.2

Bot

jet2

-5

.3

-105

.0

-4.7

22

.7

55.9

15

8.8

Bot

jet3

-4

.5

-89.

8 -4

.1

22.6

73

.2

176.

0

Bot

jet4

-7

.9

-156

.8

-6.5

23

.4

-5.7

16

1.5

Bot

jet5

-1

0.5

-207

.2

-8.4

23

.6

-111

.7

152.

1

Bot

jet6

-7

.1

-141

.5

-5.7

23

.1

30.2

18

2.8

Noj

et1

-14.

2 -2

81.6

-1

0.8

24.1

-2

05.2

17

2.3

Noj

et2

-13.

9 -2

76.0

-1

1.3

23.5

-2

41.2

15

9.9

Topb

otje

t1

-8.4

-1

67.3

-6

.6

23.4

5.

1 16

9.2

Topj

et4

-13.

2 -2

61.9

-1

0.1

24.0

-1

61.9

18

0.5

Cha

pter

5

5-25

Tabl

e 5.

5. S

umm

ary

of je

tting

ext

ract

ion

perf

orm

ance

Test

Nam

e Fi

lling

ratio

Ex

tract

ion

resi

stan

ce

ratio

Su

ctio

n ra

tio

f q j

/ q n

j p s

j / p

snj

(-

) (-

) (-

)

Bot

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40

-0.2

6

Bot

jet2

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-0.2

3

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jet3

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7 0.

33

-0.3

0

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57

0.02

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jet5

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75

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5 -0

.15

Noj

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00

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00

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-0.0

2


5-26


Chapter 5

5-27

Figure 5.2. Failure mechanisms during undrained extraction (after Gaudin et al., 2011a)


5-28

Figure 5.3. Spudcan model (dimensions in mm or degree)

Outer ring channel

Pipe

b

a

c

Kaolin clay

Leg

Outer nozzlesPore pressure

transducer

M2.95 mm screw

Chapter 5

5-29

Figure 5.4. Undrained shear strength profile


5-30

Figure 5.5. Penetration and extraction resistances for jetted tests

Chapter 5

5-31

Figure 5.6. Excess pore pressure development at the spudcan invert for jetted tests


5-32

Figure 5.7. Penetration and extraction resistances for different operating periods at an embedment ratio of 1.5, without jetting (after Kohan et al., 2014)

Chapter 5

5-33

Figure 5.8. Jetting extraction performance in terms of extraction resistance reduction


5-34

Figure 5.9. Net bearing factors

Chapter 5

5-35

Figure 5.10. Jetting extraction performance in terms of suction at spudcan invert


5-36

Figure 5.11. Excess pore pressure at the end of the installation, operation and at the breakout for reference cases

Chapter 5

5-37

Figure 5.12. Penetration and extraction resistances for combination of top and bottom jetted test


5-38

Figure 5.13. Excess pore pressure development at the spudcan invert for combination of top and bottom jetted test

6-1

CHAPTER 6

Experimental Investigation of the Effect of Cyclic

Loading on Spudcan Extraction

6.1 Abstract

Self-elevating mobile jack-up units have been employed in offshore exploration and

development in shallow waters at depths of up to approximately 150 m. Jack-ups are

designed to move to a new site after operations are completed. The spudcan footings,

which can be embedded up to three diameters deep in soft soil, must therefore be

extracted by jacking down the hull into the water and then floating it beyond the neutral

draft. This provides the maximum pull out force to overcome the soil resistance to the

jack-ups, but this force may not be sufficient. Problematic cases of this offshore are

reported to take up to ten weeks to extract, a costly exercise for the industry. A method

sometimes used offshore is to cycle the spudcans vertically in an attempt to free them.

This can be achieved by pushing and pulling the leg by leaving the hull afloat in the

water and allowing the impact of small amplitude waves on the hull to generate cyclic

Experimental Investigation of the Effect of Cyclic Loading on Spudcan Extraction

6-2

loads on the spudcan. This paper reports a series of centrifuge tests investigating the

ability to extract a spudcan under regular and irregular cyclic loading. Spudcan

extraction tests were performed from a depth of three spudcan diameters in normally

consolidated clay in a geotechnical beam centrifuge. The results demonstrate that

successful extraction is dependent on the combination of mean pull out load and the

amplitude of the cycling. It is also shown that insufficient tensile static loads and

prolonged small cyclic loads result in the dissipation of the negative excess pore

pressure at the spudcan invert caused by the buoyancy of the hull in excess of neutral

draft. It results in consolidation of soil and changes in the shear strength of the soil, and

consequently either extraction of the spudcan after a long period of time or unsuccessful

leg extraction.

6.2 Introduction

A self-elevating jack-up unit is a type of mobile platform that is mostly used for drilling

in shallow waters at depths up to approximately 150 m. Jack-ups are involved in drilling

activities worldwide, such as in the Gulf of Mexico and West Africa, where the seabed

sediments comprise soft, normally consolidated or lightly overconsolidated clay. A

typical unit consists of a triangular buoyant hull that is fitted with three or four long

support legs that are capable of raising and lowering the hull over or into the sea. Each

independent retractable leg has a conical footing that is known as a spudcan. The largest

spudcan built to date is 23 m in diameter, though sizes up to 20 m in diameter are more

common.

Jack-ups are towed to a new site after completion of drilling operations. It is therefore

essential to extract the preloaded spudcan footings by jacking down the hull into the

water and then lowering it beyond neutral draft. Although the allowable overdraft is

Chapter 6

6-3

approximately 0.3 m an overdraft of 1.6 m has also been reported (Purwana et al.,

2009). This usually provides a maximum pull out load of between approximately 30%

and 50% of the maximum installation load on each leg. The maximum pull out

resistance in clay can be predicted by the method developed by Purwana et al. (2009)

and later improved by Kohan et al. (2014a), which is based on the results of centrifuge

tests on Malaysian and UWA kaolin clay. Nevertheless, the maximum pull out load

supplied by the hull buoyancy of the jack-up may not be sufficient to extract the

foundation, especially in softer soils in which the spudcans require large penetrations

before attaining a sufficient bearing capacity to withstand the jack-up’s self-weight and

the expected operational loads. A penetration of up to two or three spudcan diameters

may be necessary to reach equilibrium during the preloading process (Endley et al.,

1981; Menzies and Roper, 2008).

Spudcan extractions can be problematic and require one to two weeks, or even ten

weeks in the most extreme cases (InSafe JIP, 2008), making extraction a time-

consuming process with significant financial impacts. The extraction mechanism is a

combination of the uplift of the soil at the top of the spudcan and reverse end bearing at

the spudcan invert due to suction (Gaudin et al., 2011a; Kohan et al., 2014b); the latter

contributes significantly to the peak undrained extraction resistance (Purwana et al.,

2005; Kohan et al., 2014b). The components of the mechanism are influenced by the

magnitude of the operational load and the length of time that the jack-up operates at a

site. This is associated with soil strength recovery, arising from the dissipation of the

excess pore pressures that are generated by the penetration process and the resulting

increase in effective stresses in the soil around the spudcan.


6-4

A number of methods can be used when difficulties are met to facilitate spudcan

extraction. This includes jetting water at the spudcan top and invert to notably relieve

the suction generated by extraction (Bienen et al., 2009; Gaudin et al., 2011a; Kohan et

al., 2013, 2014c) and applying cyclic loading to the legs, also in an attempt to reduce the

suction through generation of excess pore pressures (InSafe JIP, 2011). The latter can be

achieved by mechanically cycling the legs up and down or by allowing small amplitude

waves to hit the hull lowered in the water. The assumption is that the cycles assist in

spudcan retrieval by reducing the breakout force. This is the focus of the centrifuge tests

reported in this paper.

The behaviour of soils under undrained cyclic loading is different from that under

monotonic loading and is dependent on the mode (e.g. "One-way" or "Two-way"),

amplitude and mean stress and frequency of the cyclic loading (Andersen, 2004). Cyclic

loading generates excess pore pressures that decrease the effective stresses and increase

the permanent shear strains in the soils with continued cycles, which could ultimately

lead to a loss of shear strength or stiffness of the seabed sediments (Andersen, 2009).

This behaviour is applicable to all clays; however, high plasticity clays tend to show

greater strain rate dependency, and subsequently greater cyclic shear strength.

To the best of the authors' knowledge no experimental tests have been conducted on

cyclic spudcan extraction, though cyclic loading has been applied on a centrifuge

spducan model to replicate in-situ storm loading conditions by Dean et al. (1998). The

centrifuge tests of Dean et al. (1998) were performed on overconsolidated Speswhite

kaolin clay on spudcan of a three leg jack-up models after installation at scale 1/112th

and 1/225th (i.e at centrifuge accelerations of 112g and 225g). Three 57.8 mm diameter

model spudcans with conical and flat bases were fitted to the ends of the three leg jack-

Chapter 6

6-5

up models and were used to transfer the cyclic horizontal loading to the clayey soil.

Results show a steady increase in settlement for all the three spudcans.

This paper presents a series of centrifuge tests performed to assess whether

environmentally induced cyclic loading can be actively sought to facilitate spudcan

extraction in clay soil. The mean uplift cyclic load and the amplitude of the cycles were

the two main parameters investigated. Moreover, to evaluate the difference between

regular and irregular cyclic loads (i.e. with cycles of constant and variable amplitude,

respectively), a hydrodynamic analysis of a submerged jack-up hull based on the three

dimensional diffraction theory in the time domain was used to define the cyclic loading

on the spudcan due to wave action. The resulting irregular load cycles were used as

input in the centrifuge tests. The insights obtained from the experimental data provide

the basis for the discussion of the effect of cyclic loading on spudcan extraction.


6.3.1 Facility

The experiments were performed using the beam geotechnical centrifuge facility at the

University of Western Australia (Randolph et al., 1991; Gaudin et al., 2011b). Scaling

relationships can be found in Garnier et al. (2007), who developed an inventory of

scaling laws and similitude questions related to centrifuge modelling. The spudcan

penetrates into and is extracted from the soil sample using an actuator that is controlled

by software written in-house using a Labview interface (De Catania et al., 2010). This

allows the environmental load combinations that occur during jack-up installation,

operation, and extraction (such as complex combined buoyancy and both regular and

complex irregular cyclic loads) to be simulated in the centrifuge geotechnical

laboratory.


6-6


A model spudcan with a diameter D of 30 mm was fabricated, representing a 6 m

diameter spudcan at 200 g (Figure 6.1a). The spudcan was manufactured from

aluminium alloy 6061-T6 and was connected to a two-dimensional actuator. The model

spudcan was instrumented with two pore pressure transducers (one at the top face and

one at the base) that were installed at approximately half the distance between the centre

and the edge of the spudcan. The cross-section of the pore pressure transducers at the

top and base of the spudcan is shown in Figure 6.1b.

Instead of modelling a typical truss jack-up leg, a cylindrical hollow leg with an

external diameter of 6.5 mm and an internal diameter of 4 mm was fixed to the spudcan

at one end and to the centrifuge actuator at the other end through a 350 N load cell,

which measured the vertical loads on the spudcan during penetration and extraction.


UWA kaolin clay with the characteristics shown in Table 6.1 was used to create two

normally consolidated soil samples in the beam centrifuge. The samples were prepared

in a strongbox with internal plane dimensions of 650 mm by 390 mm. The kaolin slurry

was prepared at a moisture content of twice the liquid limit and was poured into a

rectangular strongbox over a 15-mm-thick drainage sand layer. The sample was then

consolidated under self-weight in the centrifuge at an acceleration of 200 g for

approximately five days. The settlement of the sample was measured during

consolidation. The final height of the soil specimens was approximately 200 mm. These

dimensions represent a prototype test bed that is 130 m long, 78 m wide, and 40 m deep.

T-bar penetrometer tests were used to characterise the undrained shear strength profile

of the samples, both before and after testing. The tests were performed in flight at a rate

Chapter 6

6-7

of 1 mm/s which was sufficiently fast to ensure undrained behaviour in the kaolin

(Finnie and Randolph, 1994; Chung et al., 2006). A bearing capacity factor of 10.5 (NT-

bar) was used to derive the undrained soil shear strength profiles for the two samples that

were tested (Lehane et al., 2009). The samples exhibit a linear increase in strength with

depth, with an average shear strength gradient of 1.06 kPa/m, characteristic of normally

consolidated kaolin clay in the beam centrifuge.

6.4 Regular and irregular cyclic loading

Spudcan extraction was investigated under regular and irregular cyclic loading. The

average peak amplitudes of the regular cyclic loading Vcyc varied from 0.04 to 0.36 of

the maximum extraction resistance Vex under monotonic loading (how these forces are

estimated is explained in the testing procedures section of this Chapter) (Table 6.2).

The shape of the irregular wave was developed through hydrodynamic analysis of a

submerged jack-up hull assuming a water depth of 100 m. Details of this calculation are

provided in Appendix A. The shape of the irregular waves is shown in Figure A6.5.

This shape was scaled in the centrifuge tests as follows:

i) the average of the highest one-third of the peaks was calculated,

ii) the average of the lowest one-third of the peaks was calculated,

iii) the peak to peak amplitude (difference between the averages of the highest

one-third and lowest one-third of the peaks) was calculated,

iv) the peak amplitude was scaled down to a percentage of the maximum

extraction resistance measured in the monotonic case, and

v) the hull buoyancy load was added to the scaled spudcan responses.


6-8

Figure 6.2 compares regular cyclic loading with irregular cyclic loading. The peak

amplitude of regular cyclic loading and the highest one-third of irregular cyclic loading

are 50 N (model scale) which corresponds to approximately 0.3 of the maximum

extraction load. These patterns of cyclic loadings are similar to what were used in

Test02 to Test04 (regular cyclic tests), and Test06 and Test07 (irregular cyclic tests). It

is evident from Figure 6.2 that the period of irregular cyclic loads is longer, reflecting

the period of a significant wave height Hs of 1.5 m (Appendix A).

6.5 Experimental programme and procedure

The experimental program included sixteen tests that were performed at an acceleration

of 200 g. Details of the test programme are provided in Table 6.2 and schematically

shown in Figure 6.3. The tests were separated by a distance of three spudcan diameters

to minimise the disturbance between the tests and boundary effects.

6.5.1 Monotonic tests

One test in each sample (Test01 and Test08) was performed without cyclic loading as

reference monotonic cases , in which the spudcan was extracted at a rate of 0.3 mm/s

using displacement control which resulted in a normalised velocity (vD/cv) of greater

than 30. In this normalisation, v is the spudcan extraction velocity, D is the diameter of

the spudcan, and cv is the coefficient of consolidation of the soil. In the field, successful

spudcan extraction may require between 6 and 30 hours. For spudcan diameters of 10 to

20 m and coefficients of consolidation from 0.1 to 100 m2/year, the in–situ normalised

extraction velocities are therefore consistent with the experiments at values typically

greater than 30.

Chapter 6

6-9

6.5.2 Cyclic tests

The remainder of the tests were designed to cover a range of cyclic amplitudes about

different mean tensile pulls. Twelve tests investigated the effect of regular cyclic

loading on the spudcan extraction resistance, and two tests (Test06 and Test07)

examined the behaviour of the spudcan during extraction under irregular cyclic loading.

The cyclic loading tests were performed at a frequency of 0.5 Hz, which corresponded

to a period of 2 s. The frequency was limited to 0.5 Hz to ensure a good control of the

load applied and the application of accurate cyclic sequences.

6.5.3 Testing procedure

The same test procedure was used for all tests and consisted of four stages.

In the first stage, the spudcan penetrated in-flight to an embedment depth of

three spudcan diameters at a penetration rate of 0.3 mm/s, which resulted in a

normalised velocity V=vD/cv of greater than 30 at the embedment depth, where

v is the velocity of the spudcan installation, D is the spudcan diameter, and cv is

the coefficient of consolidation (taken at a stress level that is consistent with the

spudcan embedment, see Table 6.1). This ensured that fully undrained

conditions were satisfied according to the criterion established by Finnie and

Randolph (1994) and further validated for spudcans by Cassidy (2012).

In the second stage, the jack-up operational period was simulated by maintaining

a constant vertical load of approximately 85% of the maximum installation load

for two years (prototype scale, 26.3 minutes model scale).

In the third stage, the spudcan was extracted to the mean vertical tensile force

Vmean corresponding to the hull buoyancy, which values are presented in Table

6.2 as a proportion of the maximum spudcan extraction resistance Vex. To ensure


6-10

consistency between tests, the maximum extraction loads in the cyclic loading

Vex had to be estimated by scaling the monotonic extraction resistance (Vmon as

measured) by the ratio of maximum penetration measured between the

monotonic and cyclic test. That is Vex = Vmon (Vp-cyc / Vp-mon). This is shown in

Figure 6.3.

In the last stage, regular or irregular cyclic loads were applied to the spudcan.

The magnitude of these cyclic loadings are reported as Vcyc/Vex in Table 6.2.

The testing programme (Table 6.2) was designed to investigate the effect of variations

in Vmean and the amplitude of cyclic loading Vcyc. The effect of irregular cyclic wave

loads was also tested. Of the twelve regular wave tests, nine remained in tension

throughout the cyclic pull, while three went into compression for at least part of the test.

The former are called "One-way" cycles in Table 6.2 and the latter "Two-way".

The vertical forces in stage one and the applied loads on the spudcan in stages 2, 3, and

4 were monitored. In addition, in all cases, the pore pressures at the top and the invert of

the spudcan were measured.

6.6 Experimental results

The results of the spudcan installation and operation stages are briefly explained first.

The results of the regular and irregular cyclic loadings are then discussed. Success or

failure in extraction is dominated by combination of the mean pull out load Vmean and

the amplitude of the cycling Vcyc.

Two preliminary graphs are provided to exhibit the relation between the required

number of cycles to failure and combinations of ratios of the static and cyclic loading to

the breakout load. Unless stated otherwise, all of the results are presented in the

Chapter 6

6-11

prototype dimensions; i.e., the lengths are scaled by N, the loads are scaled by N2, and

the pressures are scaled by 1.

6.6.1 Installation and operation stages

The typical load displacement response and excess pore pressures generated at the

bottom of the spudcan during installation, the operating period and extraction are

illustrated in Figure 6.4 and Figure 6.5, respectively. During installation, the excess pore

pressures at both the top and the invert of the spudcan increase linearly with depth. The

values of the excess pore pressures at the top and bottom of the spudcan at the end of

the installation reflect the changes in total stress and the reduction in effective stress due

to (at least partial) remoulding, which results in a change in the net penetration

resistance. The excess pore pressure at the spudcan invert exceeds the penetration

resistance as was observed by Gaudin et al. (2011a) and Kohan et al. (2013, 2014b,

2014c)). This indicates that soil back-flow contributes to an increase of the penetration

load due to the self-weight of the soil. Therefore, shearing during penetration is

generated under very low effective stresses (i.e., under no friction) and hence under

undrained conditions.

During the two year operating period, the load remained constant, while the excess pore

pressure dissipated. Average degrees of consolidation of 57% and 84% were achieved at

the top and bottom of the spudcan, respectively. The lower degree of consolidation at

the top of the spudcan is explained by the lower coefficient of consolidation of the

highly remoulded soil and by consolidation taking place under much lower stresses

(only the soil self-weight) compared to the high stresses under the spudcan base.


6-12

6.6.2 Vertical pull Vmean

At the end of the operation period, the compressive holding load was replaced by a

tensile vertical load to simulate the hull buoyancy load (stage 3). This resulted in

negative excess pore pressures being developed at the spudcan invert; their magnitudes

are dependent on the level of the hull buoyancy load; i.e., higher tensile loads result in

higher suction forces. In Test05, in which Vmean/Vex is 0.11, the induced excess pore

pressure is approximately 47% of that developed in the monotonic test (Test01), which

demonstrates that a floating hull beyond the transit draft generates a significant suction

force irrespective of its depth. The results of this stage are summarised in Table 6.3.

6.6.3 Regular cyclic loading

The behaviour of the soil during regular cyclic loading was governed by the hull

buoyancy load Vmean and the amplitude of the cyclic loads Vcyc. Examination of two

typical cyclic tests in which the spudcan failed to be extracted (Test02) through cyclic

loading and succeeded in being extracted (Test03) from deep embedment soil provides

insight into the effects of the cyclic loads on spudcan extraction. These tests were

performed with a constant tensile hull buoyancy load and different amplitudes in one-

way mode (Table 6.2).

In Test02, failure did not occur while the spudcan was under cyclic loading. A total of

5000 cycles were applied to the spudcan with a peak tensile load of 0.45 (Vmean/Vex +

Vcyc/Vex; 0.34 + 0.11) of the maximum predicted extraction load. The negative excess

pore pressure of -112 kPa that was generated at the spudcan invert due to the simulated

hull buoyancy load gradually dissipated during application of the cyclic loads and

reached an average of 15 kPa with an amplitude of 10 kPa (Figure 6.5 and Figure 6.6).

After applying 5000 cycles, the spudcan was extracted at 0.3 mm/s (displacement

Chapter 6

6-13

control; undrained conditions) and exhibited a maximum extraction resistance

approximately 1.5 times higher than that of the monotonic test (Figure 6.4).

Failure did not occur because the small amplitude of the cyclic loading did not generate

cyclic degradation. The accumulated upward displacement is expected to increase

during the one-way cyclic loading; however, Figure 6.4 indicates that the vertical

displacement of the spudcan ceased during continued cyclic loading. In the first 500

cycles, an upward displacement of 1.4 mm (model scale; 5% of the spudcan diameter)

was measured, at an extraction rate of 0.0014 mm/s (model scale), i.e. a normalised

velocity of 0.3. No further upward movement was observed with additional cyclic loads.

The normalised extraction velocity achieved indicates that the soil experienced drained

conditions, which resulted in soil consolidation. Figure 6.6 illustrates the cyclical excess

pore pressure responses at the top and base of the spudcan to the first 500 cycles of

regular cyclic loading and shows that the cyclically induced excess pore pressure at the

top of the spudcan dissipated, which increased the effective stresses and caused the

material above the spudcan to have a higher contribution to the extraction resistance.

Extraction of the spudcan in displacement control mode resulted in a mobilising suction

pressure of -141 kPa at the spudcan invert after 5000 cycles, which was approximately

78% of that of the monotonic test. This indicates that the reverse end bearing

mechanism at the spudcan invert contributed in the extraction mechanism, which in

combination with the consolidated soil on top of the spudcan led to a higher extraction

resistance.

In contrast to Test02, failure occurred in Test03 after 2294 cycles were applied. This

occurred a normalised upward displacement of 1.48 D (Figure 6.7 and Table 6.3). In

this test, 350 cycles with a peak tensile load of 0.62 (0.33 (Vmean/Vex) + 0.29 (Vcyc/Vex))


6-14

of the maximum extraction load were sufficient to diminish the suction pressure of -108

kPa that developed at the spudcan base (Figure 6.8). On the other hand, a comparison of

the excess pore pressure that developed at the top of the spudcan in Test03 with the

monotonic test (Figure 6.9) demonstrates that the excess pore pressure accumulated

during cycles at the top dissipated, resulting in an increase in effective stress under

partially drained conditions that was similar to Test02. The measurement of higher

loads (Figure 6.7) and observations of greater amounts of soil on top of the spudcan

once the spudcan was fully extracted demonstrates that a wider wedge of soil was

involved during the application of the cycling loads during the partially drained

extraction. Therefore, only the top soil contributed to spudcan extraction (as suction at

the base was measured to be negligible), which occurred at an embedment depth of

approximately 1.5 times the spudcan diameter, where the peak tensile load could

overcome the resistance of the top soil because of the reduction in plug weight as the

spudcan approaches the surface. However, these assumptions would need to be

validated through a particle image velocimetry (PIV) analysis of applying cyclic loading

to a deeply embedded spudcan in soft clay.

Increasing the amplitude of the cyclic loads resulted in reaching failure with fewer

cycles and less normalised upward displacement (as a case in point, compare Test03

with Test04 in Table 6.3). In addition, for the tests in which the peak tensile load is

greater than 0.70 (Vmean/Vex + Vcyc/Vex) of the extraction resistance, failure occurs after

a few cycles. In these cases, the suction pressure that developed at the spudcan invert at

the end of stage 3 did not diminish completely, and the effective stress of the top soil

did not change significantly. This indicates that both the top soil and the reverse end

bearing mechanism are mobilised before the cyclic loading overcomes the extraction

resistance.

Chapter 6

6-15

6.6.4 Irregular cyclic loading

Two tests, Test06 and Test07, were performed to provide an indication of the effect of

irregular cyclic loading on spudcan extraction with Vmean/Vex of 0.32 and 0.33,

respectively. The averages of the difference between the highest one-third and the

lowest one-third of the peaks to the maximum extraction resistance of the monotonic

test are 0.35 for Test06 and 0.48 for Test07. The results are compared to those of the

tests with a similar Vmean/Vex but with regular waves.

Figure 6.10 shows the differences in the number of cycles to failure for Test04 and

Test06 with similar peak amplitudes (one-third for Test06) (Table 6.3). Failure occurred

in Test06 after 3162 irregular cycles and an uplift displacement of 1.3 D. The load

displacement responses in Test04 and Test06 initially do not appear to correspond to

what is expected. Although the peak maximum vertical loads are similar to the

maximum extraction resistance, failure takes place after applying another 3075 cycles.

However, this is explained by Figure 6.2, which shows the pattern of irregular cyclic

loads. Irregular cyclic loading features longer period between peak loads. This results in

consolidation occurring between peak loads, and associated increase in the shear

strength of the soil, so a higher number of cycles is required to fail the soil.

No significant difference was observed in the response of the excess pore pressures at

the spudcan top and invert to irregular cyclic loading (Figure 6.11 and Figure 6.12).

Likewise, while the permanent excess pore pressures were dissipated, irregular cyclic

loading generated cyclical excess pore pressures with peaks that are functions of the

peaks of the cyclic loading; i.e., higher amplitudes result in higher excess pore

pressures.


6-16

6.7 Preliminary contours of failure under cyclic loading

It is intuitive to expect a reduction in the number of cycles to failure with increases in

the average Vmean/Vex and the amplitude Vcyc/Vex of cyclic loading. However, the

extraction resistance for the lower values of Vmean/Vex and for low Vcyc/Vex is greater

than that of the monotonic case. That is, the cyclic loading caused strengthening of the

soil, making it difficult to extract. There is said to be infinite cycles for extraction in this

case. Figure 6.13 shows a preliminary contour plot of the mean tensile load ratio

Vmean/Vex that would need to be held for varying cyclic amplitude ratios Vcyc/Vex to

reach failure. This was constructed using all of the experimental test results, but with the

shape based on the ratios with larger number of points (see 0.45 for instance). However,

some of the data does not fit the exact trends shown in the contours; therefore, these

indicative contours should just be considered preliminary guidance. This mean tensile

load could be linked to the available hull buoyancy during extraction. Therefore, for

each hull buoyancy level, the number of cycles to reach failure can be determined for

any amplitude of the regular cyclic loads.

The same test results are replotted as contours lines of the number of cycles to failure in

various combinations of ratios of the static Vmean and regular cyclic loads Vcyc to the

maximum extraction load Vex, as shown in Figure 6.14. This chart is valid for values of

Vmean/Vex and Vcyc/Vex up to 0.50. Each curve is based on the experimental test results

and shows the required number of cycles for the peak tensile load (Vmean/Vex + Vcyc/Vex)

to overcome the spudcan resistance. Failure does not occur below the curve identified as

Nf=∞ with the peak tensile load of approximately 0.40 of the maximum extraction load.

The area above the line labelled as Nf=10 shows that fewer than 10 cycles are required

to achieve failure if the combination of the static and cyclic loads is greater than 0.75 of

Chapter 6

6-17

the extraction resistance. A hull buoyancy load level and regular cyclic amplitude level

of 0.50 results in Nf=1.

These are very preliminary results and the contour plots representing the best

estimations of contours considering the data available. Nevertheless, they are provided

as they do provide some evidence to what level of cycling would need to be performed,

and for how long, before extraction of the spudcan could be achieved. Further study is

recommended to develop a complete database for spudcans subjected to cyclic loading

during extraction.

6.8 Conclusions

Centrifuge experiments were performed to investigate the behaviour of spudcans that

are deeply embedded in clay soil and subjected to cyclic loads. Tests were performed on

a 1:200 scale spudcan model with an embedment ratio of 3 times the spudcan diameter

and different levels of mean vertical pull and amplitude of cyclic loads.

The results demonstrated that during cyclic loading, partially drained conditions occur

and result in an increase in the effective stresses at the top of the spudcan due to

consolidation. At the spudcan invert, the induced suction pressure due to the hull

buoyancy load decreases in the early stage of cyclic loading. If the peak tensile load

(Vmean/Vex + Vcyc/Vex) is sufficiently high, the effective stress and shear strength then

decrease due to remoulding of the soil, which overcomes the extraction resistance and

results in upward movement. Otherwise, the extraction fails due to increases in the shear

strength of the soil, and higher contributions of the suction pressure develop beneath the

spudcan, which in combination with the consolidated soil on top of the spudcan lead to

greater extraction resistance.


6-18

Two graphs were proposed to estimate the number of cycles required for failure as a

function of the static Vmean and the regular cyclic loads Vcyc of the monotonic extraction

resistance Vex. Peak tensile loads (Vmean/Vex + Vcyc/Vex) less than 0.40 of the maximum

extraction load indicate that failure will not take place. However, few cycles are

sufficient to achieve failure if the combination of static and cyclic loads is greater than

0.75 of the breakout load of the monotonic case. These graphs are preliminary in nature

and require further validation, but provide some guidance to the level of cycles needed

to extract a spudcan.

This study also provides an indication of the effect of irregular cyclic loading on

spudcan extraction. The results of irregular cyclic loading in comparison to those of

regular cyclic loading with similar hull buoyancy load levels demonstrates that more

cycles are required to overcome the extraction resistance. This is because of longer

periods between peaks which results in more consolidation, an increase in the soil’s

shear strength, and consequently more cycles required for successful extraction.

These conclusions are limited to the experiments performed in this study but are

believed to provide relevant insight into the behaviour that operate in-situ. Additional

studies are necessary, particularly performing PIV and numerical analyses, focusing on

shorter wave periods, and verifying the proposed charts by conducting more tests.

Additionally, field data of cyclic loading behaviour in offshore conditions would be

most useful in compering to these centrifuge results.

Chapter 6

6-19

References

Andersen, K.H. (2004). Cyclic clay data for foundation design of structures subjected to

wave loading. Proc. Int. Conference on Cyclic Behaviour of Soils and

Liquefaction Phenomena, Bochum, Germany, 371-387.

Andersen, K.H. (2009). Bearing capacity under cyclic loading - offshore along the coast

and on land. The 21st Bjerrum Lecture presented in Oslo, 23 November 2007.

Canadian Geotechnical Journal, Vol. 46, No. 5, 513-535.



31, No. 3, 202-211.

Cassidy, M.J. (2012). Experimental observations of the penetration of spudcan footings

in silt. Géotechnique, Vol. 62, No. 8, 727-732.







Dean, E.T.R., James, R.G., Schofield, A.N., and Tsukamoto, Y. (1998). Drum

centrifuge study of three-leg jackup models on clay. Géotechnique, Vol. 48, No.

6, 761-785.

DNV-RP-C205 (2010). Environmental conditions and environmental loads, October

2010.



Houston, OTC 4144.


6-20




Gaudin, C., Bienen, B. and Cassidy, M.J. (2011a). Investigation of the potential of


61, No. 112, 1043-1054.

Gaudin, C., Cassidy, M.J., Bienen, B., and Hossain, M.S. (2011b). Recent contributions


behaviour. Ocean Engineering, Vol. 38, No. 7, 900-914.





InSafe JIP (2008). Minutes of the 2nd progress meeting of the InSafe JIP. Singapore,

20th November 2008.

InSafe JIP (2011). Improved guidelines for the prediction of geotechnical performance

of spudcan foundations during installation and removal of jack-up units. Joint

Industry-funded Project. Authors: Osborne, J.J., Teh, K.L., Houlsby, G.T.,

Cassidy, M.J., Bienen, B., Leung, C.F. 28th March 2011.




(OMAE), Nantes.

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014a). Predicting spudcan

extraction resistance in soft clay. Geotechnical Engineering Journal of the

SEAGS & AGSSEA, Vol. 45, No. 4, 52-61.

Kohan, O., Gaudin, C., Cassidy, M.J., and Bienen, B. (2014b). Spudcan extraction from

deep embedment in soft clay. Applied Ocean Research, Vol. 48, 126-136.

Chapter 6

6-21

Kohan, O., Bienen, B., Gaudin, C., and Cassidy, M.J. (2014c). The effect of water

jetting on spudcan extraction from deep embedment in soft clay. Ocean

Engineering, Submitted revised version in November 2014.

Lehane, B.M., O’Loughlin, C.D., Gaudin, C., and Randolph, M.F. (2009). Rate effects

on penetrometer resistance in kaolin. Géotechnique, Vol. 59, No. 1, 41-52.



OTC 19545.



753.




Purwana, O.A., Krisdani, H., Zheng, X.Y., Quah, M., and Foo, K.S. (2010). An

assessment of jack up spudcan extraction. Proc. Int. Symp. on Frontiers in

Offshore Geotechnics, Perth, Australia, 679–684.





6-22

Table 6.1. Characteristics of kaolin clay

Sample 1 Sample 2






Consolidation coefficient (cv) (OCR=1 and at an embedment depth of 3D)

4.24 m2/year

4.14 m2/year

Submerged unit weight ( (at an embedment depth of 3D) 7.2 kN/m3 6.8 kN/m3

Cha

pter

6

6-23

Tabl

e 6.

2. T

estin

g Pr

ogra

mm

e

Test

N

ame

Sam

ple

Pene

tratio

n de

pth

ratio

O

pera

tion

time

Ope

ratio

n lo

ad

leve

l H

ull

buoy

ancy

lo

ad le

vel

Cyc

lic

ampl

itude

lo

ad le

vel

Cyc

lic

load

type

M

ode

of

cycl

ic

load

ing

- -

H/D

-

Vop

-mon

/Vp-

mon

; V

op-c

yc/V

p-cy

c V

mea

n/Vex

V

cyc/V

ex

- -

Test

01

1 3D

2

year

s 0.

85

NA

N

A

NA

N

A

Test

02

1 3D

2

year

s 0.

85

0.34

0.

11

Reg

ular

O

ne-w

ay

Test

03

1 3D

2

year

s 0.

85

0.33

0.

29

Reg

ular

O

ne-w

ay

Test

04

1 3D

2

year

s 0.

85

0.34

0.

34

Reg

ular

O

ne-w

ay

Test

05

1 3D

2

year

s 0.

85

0.11

0.

29

Reg

ular

Tw

o-w

ay

Test

06

1 3D

2

year

s 0.

85

0.32

0.

351

Irre

gula

r Tw

o-w

ay

Test

07

1 3D

2

year

s 0.

85

0.33

0.

481

Irre

gula

r Tw

o-w

ay

Test

08

2 3D

2

year

s 0.

85

NA

N

A

NA

N

A

Test

09

2 3D

2

year

s 0.

85

0.45

0.

04

Reg

ular

O

ne-w

ay

Test

10

2 3D

2

year

s 0.

85

0.45

0.

09

Reg

ular

O

ne-w

ay

Test

11

2 3D

2

year

s 0.

85

0.44

0.

18

Reg

ular

O

ne-w

ay

Test

12

2 3D

2

year

s 0.

85

0.43

0.

26

Reg

ular

O

ne-w

ay

Test

13

2 3D

2

year

s 0.

85

0.16

0.

36

Reg

ular

Tw

o-w

ay

Expe

rimen

tal I

nves

tigat

ion

of th

e Ef

fect

of C

yclic

Loa

ding

on

Spud

can

Extra

ctio

n

6-24

Test

N

ame

Sam

ple

Pene

tratio

n de

pth

ratio

O

pera

tion

time

Ope

ratio

n lo

ad

leve

l H

ull

buoy

ancy

lo

ad le

vel

Cyc

lic

ampl

itude

lo

ad le

vel

Cyc

lic

load

type

M

ode

of

cycl

ic

load

ing

- -

H/D

-

Vop

-mon

/Vp-

mon

; V

op-c

yc/V

p-cy

c V

mea

n/Vex

V

cyc/V

ex

- -

Test

14

2 3D

2

year

s 0.

85

0.17

0.

26

Reg

ular

Tw

o-w

ay

Test

15

2 3D

2

year

s 0.

85

0.19

0.

19

Reg

ular

O

ne-w

ay

Test

16

2 3D

2

year

s 0.

85

0.72

0.

05

Reg

ular

O

ne-w

ay

1 0.3

5 or

0.4

8 of

the

aver

age

diff

eren

ce b

etw

een

the

high

est o

ne-th

ird a

nd th

e lo

wes

t one

-third

of

the

peak

s to

the

max

imum

ext

ract

ion

resi

stan

ce o

f th

e m

onot

onic

test

.

Chapter 6

6-25

Table 6.3. Summary of experimental results for extraction

Test Name

Peak tensile load Normalised upward displacement at failure

Number of cycles to failure

Vmean/Vex + Vcyc/Vex f/D Nf

(-) (-) (-)

Test01 NA NA NA

Test02 0.45 0.05 ∞

Test03 0.62 1.48 2294

Test04 0.68 0.48 87

Test05 0.4 0.03 ∞

Test06 0.67 1.30 3162

Test07 0.81 0.11 30

Test08 NA NA NA

Test09 0.49 1.51 2952

Test10 0.54 1.20 2365

Test11 0.62 0.79 273

Test12 0.69 0.19 29

Test13 0.52 0.14 18

Test14 0.43 0.98 1182

Test15 0.38 0.05 ∞

Test16 0.77 0.12 8

Experimental

Figure 6.1

Investigation o

. Model spu

f the Effect of C

udcan and l

Cyclic Loading

ocation of tmm

g on Spudcan Ex

the pore prem)

xtraction

essure transdducers (dim

6-26

mensions in

Chapter 6

Figuree 6.2. Regullar versus irrregular

6-277

Experimental

Investigation of the Effect of C

Figure 6.3.

Cyclic Loading

Schematic

g on Spudcan Ex

of testing p

xtraction

programme

6-28

Chapter 6

Fi

igure 6.4. Penetration aand extractiion resistancces for Testt01 and Test

6-29

t02

9

Experimental

Figure 6.

Investigation o

5. Developm

f the Effect of C

ment of exc

Cyclic Loading

cess pore prTes

g on Spudcan Ex

ressure at thst02

xtraction

he spudcan invert for Te

6-30

est01 and

Chapter 6

Figu

ure 6.6. Exccess pore prressure responses to thee first 500 ccycles for T

6-31

est02

1

Experimental

Fig

Investigation o

gure 6.7. Pe

f the Effect of C

enetration a

Cyclic Loading

nd extractio

g on Spudcan Ex

on resistanc

xtraction

ces for Test001 and Test0

6-32

t03

Chapter 6

6-33

Figure 6.8. Development of excess pore pressure at the spudcan invert for Test01 and Test03

Experimental

Figure 6

Investigation o

6.9. Develop

f the Effect of C

pment of ex

Cyclic Loading

xcess pore pTes

g on Spudcan Ex

pressure at tst03

xtraction

the spudcann top for Tes

6-34

st01 and

Chapter 6

Figure

6.10. Penettration and extraction rresistances ffor Test01, Test04, and

6-35

d Test06

5

Experimental

Figure 6

Investigation o

6.11. Develo

f the Effect of C

opment of e

Cyclic Loading

excess pore Test04, an

g on Spudcan Ex

pressure at nd Test06

xtraction

the spudca

n invert for

6-36

r Test01,

Chapter 6

Figure 6.112. Development of exccess pore prand T

ressure at thTest06

he spudcan ttop for Test

6-37

t01, Test04,

7

,


6-38

Figure 6.13. Preliminary contours of hull buoyancy load level as a function of cyclic amplitude ratio to reach to failure

Chapter 6

6-39

Figure 6.14. Proposed contours for the number of cycles to failure (contours represent preliminary estimation)


6-40

Appendix A

A hydrodynamic analysis was performed to simulate wave loads on a jack-up hull that

was floating in shallow water 100 m deep using the MOSES software version 7.06. The

geometry of the jack-up hull used for the modelling is shown in Figure A6.1. A

significant wave height Hs of 1.5 m, which corresponds to the peak wave period Tp of

6.7 s (Tp = sqrt (30 x Hs)) (DNV, 2010), was assumed to pass through a hull that was 10

m deep with a draft of 7 m. The wave incidence angle was varied from 0° to 180° from

the stern at an interval of 22.5°. These assumptions, which are reported in Table A6.1

are similar to the conditions under which the removal of jack-up legs takes place in the

field.

In the first step of the analysis, Response Amplitude Operators (RAOs) were generated

for all six degrees of freedom (surge, sway, heave, roll, pitch and yaw) while the jack-

up hull was floating freely at the centre of gravity. RAOs describe how the response of

the jack-up varies with frequency and are normally normalised by the wave height.

Figure A6.2 to Figure A6.4 show the RAOs for heave, roll, and pitch, respectively.

Figure A6.2 shows that the RAO approaches unity at low frequencies, which means that

the jack-up hull simply moves up and down with the waves. At high frequencies, the

response approaches zero because the effects of many very short waves cancel out over

the length of the vessel. An RAO value of greater than unity indicates that the jack-up's

response is greater than the wave amplitude. This occurs near the vessel’s natural

period, and the peak is due to resonance.

The JONSWAP (JOint North Sea WAve Project) spectrum was used to represent the

characteristics of real wave energy spectra. The JONSWAP spectrum is characterised

Chapter 6

6-41

by a peak enhancement parameter which controls the sharpness of the spectral peak.

In the North Sea, the values of range from 1 to 7 and have a mean value of 3.3.

In the second stage, a global analysis of the full three-dimensional jack-up model was

performed to obtain the spudcan responses to wave through a 30 minute simulation. In

the model, the connections between the legs and the hull were assumed to be fixed, and

a pinned connection was assumed to simplify the footing under retrieval conditions.

Therefore, the bending moments on the footing were transferred by vertical tension and

compression forces on opposite legs.

The results show that the maximum vertical load occurs at an incidence angle of 112.5

degrees from the stern, which is similar to the conclusions of Purwana et al. (2010). The

responses of the spudcans on each leg to the wave are presented in Figure A6.5. The

spudcan that is connected to Leg 1 (Figure A6.1) has the greatest response to the wave,

and the maximum tensile and compressive vertical loads are approximately 4.4 MN and

-3.6 MN, respectively. The highest one-third of the peaks and the lowest one-third of

the peaks are 0.72 MN and -0.77 MN, respectively.


6-42

Table A6.1. Hydrodynamic database

Parameters Assumptions

Total length of hull 80 m

Total width of hull 80 m

Cantilever part of hull 15 m

Depth of hull 10 m

Total height of legs 150 m

Water depth 100 m

Draft 7 m

Environmental heading From 0° to 180°; increment 22.5°

Significant wave height, Hs 0.8 m and 1.5 m

Spectral peak period, Tp 4.9 s and 6.7 s

Duration of the simulation 30 min

Chapter 6

Figu

ure A6.1. Geeometry of tthe jack-up hull used inn the hydroddynamic an

6-43

nalysis

3

Experimental

Investigation o

Figure A

f the Effect of C

A6.2. RAOs

Cyclic Loading

for heave f

g on Spudcan Ex

for freely flo

xtraction

oating jack-up barge

6-44

Chapter 6

Figuree A6.3. RAOOs for roll oof freely floaating jack-uup barge

6-455

Experimental

Investigation o

Figure A

f the Effect of C

A6.4. RAOs

Cyclic Loading

s for pitch fo

g on Spudcan Ex

or freely flo

xtraction

oating jack-uup barge

6-46

Chapter 6

Figure A6.55. Vertical sspudcan reaaction at legg1

6-477


6-48

7-1

CHAPTER 7

Concluding Remarks

7.1 Introduction

This dissertation focuses on undrained spudcan extraction from deep embedments in

normally consolidated clay. The behaviour of the spudcan during extraction was

investigated experimentally to address the 3 themes and 5 associated aims that were

outlined in Chapter 1. The results of this research and recommendations for future work

are presented in this chapter.

7.2 Main findings

7.2.1 Specifying the breakout failure mechanism of deeply embedded spudcans

To investigate the failure mechanism of undrained spudcan extraction from deep

embedments, a series of physical model experiments was conducted at an acceleration

of 200 g in the geotechnical beam centrifuge of the University of Western Australia.

The tests were performed in normally consolidated clay with a linearly increasing

Concluding Remarks

7-2

undrained shear strength gradient. The instrumented spudcan, which was embedded at

depths of 1.5 to 3 spudcan diameters, was extracted after varying operational periods.

The mechanism that governs the undrained extraction of a spudcan when it is subjected

to short or long periods of operation (i.e., the spudcan experiences vertical operating

loads) was identified as a reverse end bearing mechanism that is associated with plug

uplift at the point of maximum extraction resistance. For immediate extraction, the

mechanism is a full flow round mechanism with a ratio of the extraction to the

penetration resistance. This mechanism is valid for initial embedment ratios up to 3

times the spudcan diameter.

The contribution of the peak negative excess pore pressure or suction at the spudcan

invert is a function of the operational period. Longer operational periods result in higher

suction forces and consequently higher extraction resistances. This is in contrast to the

soil plug uplift, which is constant for all operational periods.

The magnitude of peak suction that develops at the spudcan invert also increases

linearly with the embedment depth and the initial effective stress during undrained

extraction.

7.2.2 Improving a predictive method for spudcan extraction based on data of

embedment depth and vertical load history

The performance of the existing analytical method that has been proposed to predict the

maximum spudcan extraction resistance for embedment depths of up to 1.5 spudcan

diameters was examined. The method was shown to require improvement.

Therefore, the spudcan extraction resistance was estimated in accordance with the

extraction mechanism that was identified for deeply embedded spudcans in soft clay and

Chapter 7

7-3

based on the centrifuge test data of spudcan extraction from UWA and Malaysian kaolin

clay. The resistance is essential for jack-up operators to anticipate potential extraction

issues and develop mitigation measures to facilitate spudcan extraction.

A set of recommendations based on insights obtained from the database for spudcan

embedments of up to 3 diameters was proposed to update and improve the existing

predictive method. The recommendations are related to the factors that characterise the

change in soil shear strength at the base and the top of the spudcan and two new factors

that consider the effects of the operational load and strength ratio on spudcan extraction

in clay.

The improved method demonstrated a higher degree of accuracy than the existing

method; the mean differences decreased from 57% to 8% for both types of clay.

7.2.3 Understanding the effectiveness of top jetting in reducing the extraction

resistance from deep embedments

In the case of deeply embedded spudcans in soft clay and for long periods of operation,

the capacity of the rig may not be sufficient to overcome the spudcan extraction

resistance. To mitigate this issue, jack-up operators employ a water jetting system that

ejects pressurised water through nozzles on the spudcan faces to reduce the spudcan

extraction resistance by fracturing and softening the upper soil, which is one of the two

components of the extraction mechanism.

To assess the efficiency of top jetting in spudcan extraction, centrifuge experiments

were performed in which the spudcan was extracted from embedment depths of 3

diameters in normally consolidated clay after a two year operating period with different

jetting flow rates. The tests were performed in normally consolidated kaolin clay at an

Concluding Remarks

7-4

acceleration of 200 g in the geotechnical beam centrifuge using a 40-mm-diameter

model spudcan with 16 outer nozzles at the top of the spudcan.

The analysis of the centrifuge results demonstrated that the application of top jetting

pressure did not reduce the maximum extraction resistance; however, it reduced the

post-breakout extraction resistance at increased flow rates. This is because positive

excess pore pressure is generated at the base of the spudcan by the flow of water and

soil from the outer top nozzles around the spudcan to the bottom, which results in an

upward force on the invert of the spudcan and consequently easier extraction.

7.2.4 Determining the performance of bottom jetting to ease the extraction of

deeply embedded spudcans

Following the investigation of spudcan extraction using top jetting, a similar study was

performed to examine the performance of bottom jetting in diminishing the suction

forces at the spudcan base and consequently reducing the extraction resistance from

deep embedments.

The experimental results were analysed to verify the validity of the existing conceptual

framework for estimating the optimal bottom jetting flow rate (i.e., no suction is

generated) for spudcan embedments of up to 3 diameters. The results showed that the

conceptual framework is valid for deep embedment depths. This conclusion is also

supported by the aforementioned extraction mechanism for embedments of 1.5 and 3

spudcan diameters, which remains unchanged.

In addition to these findings, the results demonstrated that only jetting at the spudcan

invert reduces the peak extraction resistance. This was investigated by testing the

application of combined top and bottom water jetting. The results showed that the

Chapter 7

7-5

extraction behaviour when using combined jetting is similar to that when only bottom

jetting is used but is different from when only top jetting is used.

7.2.5 Providing insight into the behaviour of the spudcan during extraction

under cyclic loading

Another method that is used by jack-up operators to retrieve legs and spudcans is to

apply cyclic loads on the spudcans by pushing and pulling the leg by leaving the hull

afloat in the water and allowing the impact of small amplitude waves on the hull to

reduce the maximum extraction load.

The same apparatus that was used to investigate the failure mechanism of deeply

embedded spudcans was employed to study the behaviour of the spudcan during

extraction from depths of 3 spudcan diameters in normally consolidated clay under both

regular and irregular cyclic loading for different levels of the buoyancy load (mean

vertical pull) and amplitudes of cyclic loads.

The Moses program was used to perform a hydrodynamic analysis of a submerged jack-

up hull based on 3 dimensional diffraction theory in the time domain. The responses of

the spudcan to wave action are subsequently used in the centrifuge to simulate cyclic

loading using an actuator that is controlled by in-house software that was written using a

Labview interface.

The results demonstrated that the behaviour of the soil during regular cyclic loading was

dominated by the level of the hull buoyancy load and the amplitude of the cyclic loads.

If the peak tensile load (the combination of the hull buoyancy load and the amplitude of

the cyclic loads) is sufficiently high in comparison to the maximum extraction load, the

effective stress and shear strength decrease due to remoulding of the soil, which

Concluding Remarks

7-6

overcomes the extraction resistance and upward movement. Otherwise, the extraction

fails due to dissipation of the induced excess pore pressures during cyclic loading,

which results in soil consolidation and increases the effective stresses and shear strength

of the soil. Therefore, the suction pressure that develops beneath the spudcan increases,

which, in combination with the consolidated soil on top of the spudcan, leads to greater

extraction resistance.

The results of irregular cyclic loading demonstrated that more cycles are required to

overcome the extraction resistance as the period of time between the peaks increases

compared to regular cyclic loading with similar hull buoyancy loads. Consolidation

occurs between the peak loads, which increases the shear strength of the soil, so more

cycles are required for successful extraction.

Two indicative graphs were proposed to estimate the number of cycles that is required

for failure as a function of the buoyancy load and the regular cyclic loads with respect to

the maximum extraction resistance. For the test conditions of this thesis, peak tensile

loads of less than 0.40 times the maximum extraction load indicate that failure will not

occur; however, few cycles are required to achieve failure if the combination of static

and cyclic loads is greater than 0.75 times the breakout load.

7.3 Recommendations for Future Work

The following areas of research would further develop the findings of this thesis.

Extraction of deeply embedded spudcans

1. The spudcan extraction tests in this study were performed in kaolin clay.

Although the results were compared with tests conducted in Malaysian kaolin

clay, it is recommended that future investigations of spudcan extraction be

Chapter 7

7-7

performed with offshore marine clay or other clayey soils with different

characteristics.

2. Additional studies are necessary to more accurately model the extraction process

in the field, which is performed under load control rather than displacement

control.

3. Spudcan installation depths up to a maximum of 3 spudcan diameters are

common offshore, although a spudcan embedment ratio of 5.6 has been reported,

which is an exceptionally high. Therefore, further studies should be performed

to determine whether the extraction mechanism is different for spudcan

embedment ratios greater than 3.

Extraction of deeply embedded spudcans using water jetting

4. A comprehensive understanding of the mechanisms that occur during the

extraction of deeply embedded spudcans using water jetting would be provided

by employing transparent soils and 3 dimensional visualisation techniques. It is,

however, noted that current Particle Image Velocimetry (PIV) techniques are

two-dimensional and cannot be applied because the jetting system is not

symmetrical. Development of a 3 dimensional system, possibly with transparent

soil, remains a challenge.

5. Spudcan extraction with top jetting could be improved if a different set of

nozzles (located closer to the centre of the spudcan) becomes active during

extraction because the jet would remould the soil column that is carried up with

the spudcan. Also, having two different systems for top (high pressure) and

bottom (high volume) jetting, not a common supply would be beneficial.

6. The use of water jetting was proven in this thesis, but still anecdotal evidence

that it does not work in the field remains, possibly this is due to the water jets

Concluding Remarks

7-8

becoming blocked. Further investigation of the use of water jetting in the field is

required and the results should be interpreted in the framework provided here in

Chapter 5.

Extraction of deeply embedded spudcans under cyclic loading

7. Particle Image Velocimetry (PIV) analysis of physical tests of spudcan

extraction under cyclic loading would provide a better understanding of the

mechanisms that take place during extraction.

8. Interactions between different legs and spudcans of a jack-up during cyclic

loading can be considered using a 3 leg jack-up model. In addition, the

behaviour of spudcans under a combination of moment, vertical and horizontal

cyclic loading should be investigated.

9. To obtain a practical chart in addition to the two preliminary graphs that were

introduced to estimate the number of cycles to failure as a function of the

buoyancy load and the regular cyclic loads, additional experimental tests that

consider different wave periods need to be performed.

Other comments

10. This thesis is limited to the experiments that were performed; however, relevant

insights into undrained spudcan extraction mechanisms are provided. Numerical

modelling of spudcan extraction that focuses on water jetting and cyclic loading

should be used to expand the current knowledge of the extraction mechanisms.

11. State of the art devices and techniques are required to study several relevant

items that are beneficial to spudcan extraction. For example, special tools and

methods are required to measure the increase in soil shear strength at the top and

bottom of the spudcan after the operational period, to fabricate smaller jetting

Chapter 7

7-9

nozzles, or to excavate the soil above the spudcan to ease the extraction of

deeply embedded spudcans.

12. Although this study provides some guidance on the extraction of spudcans from

deep embedment depths, close cooperation with jack-up operators is necessary

to verify the outcomes of this research with the conditions that operators face in

the field.

Concluding Remarks

7-10

improving spudcan extraction from … · vi the centrifuge modelling technique was used to...

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