Modelling of Spudcan and Suction Caisson PerformanceM.J. Cassidy, M.F. Randolph (University of Western Australia) and B.W. Byrne (Oxford University)

Project: An intensive study of shallow foundation behaviour

Purpose: Make a comparative assessment of the response of similarly sized spudcans and caisson.

Motivation: Investigation of shallow foundation alternatives for the long-term deployment of mobile offshore drilling units

For more details visit us at www.cofs.uwa.edu.au

Investigating Spudcan and Caisson Foundation Response in the UWA Drum Centrifuge

• Drum Centrifuge installed 1998

• 1.2m diameter; soil sample channel 300mm high (vertically) and 200mm radial depth

• max acceleration 485g

SOIL

The drum centrifuge at UWA has been used to comparatively assess the performance of spudcan and caisson foundations in both normally consolidated kaolin clay and normally consolidated calcareous silt (from the Gorgon area). The tests were designed to investigate footing behaviour under combined loading, and to provide data that can be used to validate existing as well as develop new plasticity models. The novelty of the testing was to compare the response of skirted and non-skirted foundations and therefore assess whether improvements in performance could be obtained by the addition of a skirt. In order to apply appropriate loading combinations to the model scale footings in a centrifuge, a hinged loading arm was developed. This allowed higher ratios of moment to horizontal load to be applied to the footings than the more conventional fixed arm approach.

Spudcan Caisson L/D=0.5

"poppet" valve

8 mm thread 8 mm thread

Loading legs used in the experiments Model spudcan and caisson footings

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 1 2 3 4 5 6

Vertical Penetration, w (m)

Ver

tica

l Loa

d, V

(M

N)

Spudcan Caisson

Example vertical installation data

Example combined loading data of a caisson

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-0.2 0 0.2 0.4 0.6Hor. Disp. (m)

Hor

. Loa

d (M

N)

-3.5

0.0

3.5

7.1

10.6

14.1

17.7

21.2

24.8

28.3

Hor

. Str

ess

(kP

a)

swipe 1swipe 2swipe 3swipe 4swipe 5swipe 6

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-0.2 0.8 1.8 2.8Vert. Load (MN)

Hor

. Loa

d (

MN

)

-3.5

0.0

3.5

7.1

10.6

14.1

17.7

21.2

24.8

28.3

-7.1 10.6 28.3 46.0 63.7 81.3 99.0Vert. stress (kPa)

Hor

. str

ess

(kPa

)

swipe 1swipe 2swipe 3swipe 4swipe 5swipe 6

All normalised results against theoretical yield surface

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-1.5 -0.5 0.5 1.5V/V u

M/D

Asu

Caisson

Spudcan

Theory

-1

0

1

2

3

4

5

-1.5 -0.5 0.5 1.5V/V u

H/A

su

Analysis of jack-up at the Sunrise site

Based on the experimental results (and existing theory) a force-resultant model for describing the behaviour of caissons in clays has been developed. Though plasticity models depicting spudcan behaviour exist they have been limited to the traditional inverted conical spudcan footings shape and the extension to the caisson configuration here represents a significant advance. As the model is developed within strain-hardening plasticity theory it allows it to be implemented with structural analysis codes. (1) The model has been verified by retrospective simulation of the centrifuge experiments.(2) Its practical application shown by a series of example analyses of the proposed production jack-up at

the Sunrise site.(3) The benefits of the caisson configuration compared with typical spudcan shapes have been shown.

Simplified model of proposed Sunrise jack-up and footing options

Mea

n w

ater

dep

th 1

43.1

m

120m

39.1m

Windward Leeward

Two legs

Single leg

84.375m

33.9m

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-100 -50 0 50 100time (s)

Ho

r. H

ull

Dis

p.

(m)

D30 L15

D30 L10

Spudcan V0=204.39MN

-8-6-4-202468

-100 -50 0 50 100

Wav

e el

ev.

(m) Windward Legs

Leeward Leg

Deck displacement response for 100-year conditions for different footing options

11.625 3.375

4.35

2.7

D = 30m

L =

15m

L=10

m

D = 30m

Project Conclusions and Benefits1. The caisson in the centrifuge tests displayed increased moment stiffness, increased horizontal capacity and

larger combined load capacity under tension, but surprisingly no significant improvement in moment capacity.

2. A simple strain-hardening plasticity model for the analysis of caisson was developed and implemented in UWA’s jack-up analysis package.

3. For the Woodside’s proposed Sunrise site, (a) optimum caisson geometries (b) penetration and relative movement of the foundations, including permanent deformation in extreme events (c) comparative behaviour to equivalent sized spudcan footings, were studied and the advantages of caisson foundations quantified.

(after Stewart, Boyle & Randolph, Centrifuge 98)