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13th Congress of Intl. Maritime Assoc. of MediterraneanIMAM 2009, stanbul, Turkey, 12-15 Oct. 2009

Deepwater marine riser systems lifetime and fracture integrity prediction

and assurance

A.M. Suliman(PDI - Nipetco, Cairo, Egypt)

M.M. El-Gammal, Y.A. Abdel-Nasser & A.M. Rashwan(University of Alexandria, Alexandria, Egypt)

ABSTRACT: Few years ago, it has been noted that the the oil wells in the vicinity of the shore are getting dry

and dry every day. So, it is vital now to realize the impact of searching for new wells in the very deepwater

areas.Deepwater needs dictate that tighter tolerances on base materials and increased tolerances of accepted

flaws being deployed thus deviating from the recommended codes and the more leaning towards rationally as

well as the based optimized design approaches and the critical tolerable flaw studies without compromising

the safety and integrity of the structure should be required. Many codes have come to recognize this approach

and placed technical statements and reviewed guidelines.

The intent of the present paper is to highlight and cover technical and theoritical tools of predictions of

lifetime estimations based on brittle fracture inititiations and collapse countermeasures in accordance with

codes guidelines on flaw acceptance which in turn are based on engineering criticality studies. Practical case

studies have been included to technically demonstrate and prove the simplicity as well as the adaptaion of the

application of the proposed approaches and the accuracy of the predicted estimates.

1 INTRODUCTION

The world demand for energy is ever increasing and

the adequacy of energy supply is sharply challenged,

particularly the hydrocarbon based fuel resources.

The international reserve of mineral hydrocarbon

resources of recoverable amount of the mineral

resources available to disposition is heavily

degraded and recently deteriorated. This is theanticipated results that account for both technical

and economical feasibility of recovery of mineral

resources. Therefore cost of recovery and market

price are deterministic factors of the international oil

reserve. Therefore, any drastic increase in energy

price will tend to increase the cost of the recoverable

reserve as well as will require more adoption of

complicated technologies. The latter can provide

economical recovery options particularly where the

developed fields are far out offshore or for those

with marginal reservoir. Many oil fields that wereconsidered, not long ago, marginal and/or

uneconomical and were not included in the

international reserve are increasingly being

developed and used. Deepwater offshore oil fields

are a typical example of such fields due to its

challenging nature which is progressively being

overcome by emerging enabling technologies. This

is clearly as shown in Figure 1.

Typical challenges associated with deepwater

offshore explorations and developments are feasibly

seen in the increased pressures, decreased

temperatures, harder floorbeds, higher current

motions and more seabed magnitudes of waves. Inaddition to the common applications of the

deepwater marine risers are also challenged by

increased top tension, reduced buoyancy materials

efficiency, increased vibrations and increased

internal fluids pressures.

The need for more stronger, more viable

reduction exploitation times and the lesser

maintenance periods have all been devoted to study

structural integrity of offshore structures to

emphasize both the verification of functions as well

as the lifetime validity. The avialability of cost

effective computer technologies has led to

increasing application of risk and fitness for

performance and fitness for service based design

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approaches which resulted in many codes

recognizing and placing technical statements and

reviewed guidelines.

The British standard B.S.7910, the European

Committee for Standardisations (CEN) SINTAP

and Eurofitnet, APIs RP 579 are all dedicated to

address the fitness for purpose (FFS) based

assessment of sturctures. These codes and standards

provide eminant solutions for the limit state and

stress intensity factors for some standard member

and sections. Flawed or deformed members

requiring a decision based on their level of integrity

on whether to repair, replace, rerate or leave requires

a dedicated study which has a conservation level

inversely proportional to the avialable data and the

extent of the analysis.

Examples and applications from the media of the

available literature survey that cover non-standard or

deformed sections have been added tocomprehensively test the accuracy of the validity of

the theoretical approach. The described cases prove

that the utility and the validity of the theoretical

approaches are being confidently and remarkably

high.

2 STATE OF THE ART

Basic knowledge areas for the study that needed to

be thoroughly reviewed are: marine risers; fracture

mechanics; J-integral method; relevant codes;

engineering criticality analysis, Failure AssessmentDiagrams (FADs), and; finite element method.

A review of these methods is beyond the scope of

this work. However, where simple clarifications are

a must to communicate an idea are briefly given.

2.1 Acting loads on risers

Examples of metalic riser configuration and floaters

are illustrated in Figure 1.

Risers are normally subjected to the following

loads:

External and internal pressure; Axial tension/ compression; Vibrations; Temperature; Bending moment (Akram, 2008).

These loads are typically resulting from:

Applied top tensions. Self weight. Hydrodynamic forces caused by waves

and currents.

Vessel motion. Riser resonance (Akram, 2008).

(a)The new developed examples of deeper risers,

(b)Some of the recent examples of deeper risers citedto tension rigs

Figure 1. Example of metallic riser configuration and floaters

2.2 Modes of failure of loaded cracked risers

Straining rate, fluctuating stresses, stress

concentrations, metallurgical flaws, high and low

temperatures, corrosion and other special effects cancause an engineering component to fail by fracture

which may lead to a catastrophic disaster.

The load bearing capacity of a body is reduced by

the presence of a crack. As shown in Figure 2(a), the

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n

crack

stress is increased locally at the crack tip as a result

of a notch effect. There are two ways in which this

can lead to permanent failure under Plastic collapse

and Fracture.

(a)Plastic collapseFor every ductile material, increasing the load on

the body leads first to yield at the crack tip and the

stress then rises locally to the flow stress (usually

assumed to be the average of the yield stress and the

tensile strength). If the external load is increased

further, the plastic zone spreads across the

remaining ligament Figure 2(b). Failure by plastic

collapse follows when the average stress in the

ligament reaches the material flow stress.

(b)FractureWhere the material is less ductile, the intense

damage is limited to a highly stressed region ahead

of the crack tip and failure of the ligament is by

propagation of the crack. In a purely elastic material,this is an entirely brittle process with no plastic

flow. However, in metal, the high stresses in the

crack tip region usually cause yielding to form a

local plastic zone Figure 2(c). Nevertheless, when

this zone is small, the mode of propagation is still

essentially brittle. In less brittle materials, the plastic

zone is larger and the facture process becomes one

of ductile tearing. This mechanism may persist even

where the plastic zone extends across the whole

ligament but, in this case, the failure mode may be

by plastic collapse as described above.

Figure 2. Modes of Failure of Cracked Structures

Figure 3. J-integral contour path

2.3 Review of J-integral method

The J-integral provides a means for describing the

severity of conditions at a crack tip in a non-linear

elastic material, Figure 3.

The J contour integral approach is based on thefinding that for a two-dimensional crack situation,

the sum of the strain energy density and the work

terms along a path completely enclosing the crack

tip are independent of the path taken.

The energy line integral J is defined for either the

elastic or elastic-plastic behavior as follows:

dsx

uTdywJ

p

= . (1)

Where p = any contour path surrounding the crack

tip; w = strain energy density =

=

0

.dw ;

T = traction vector defined according to the outward

normal n along path p; u = displacement vector; and

s = arc length.

2.4 FAD construction using J-integral method

A failure assessment diagram (FAD) gives a two

parameter approach to assessing a defect. It accountsfor the possibility of fracture and plastic collapse

separately. These possibilities are plotted on the

axes of the FAD as Kr (fracture resistance factor)

against Lr (collapse resistance factor). The concept

of the FAD requires the use of fracture parameters

that cater for large scale plasticity. The j-integral is

widely used for this purpose. Once the diagram is

generated, an assessment point is considered which

may lie in the "acceptable" or "unacceptable" region

of the diagram, Figure 4.

Several codes of practice provide guidelines for useof the finite element method in creating a failure

assessment diagram e.g. R6, API 579, BS7910. The

approach is similar in all of these codes:

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a. undertake a linear elastic FEA and determine theJ-Integral values: Jelas

b. carry out a plastic FEA and determine the J-Integral values as a function of load, P:

Jtotal=Jelas+Jplas

c. determine the reference load of plastic collapse(Pref)

d. draw the failure assessment diagram using points(Kr,Lr) where:

Kr=(Jelas/Jtotal)

Lr= P / Prefe. draw the vertical cut-off on the Lr axis; this

depends on the material properties; for some

materials:

Lr(max) = flow stress / yield stress

f. Then assess a point on the FAD and verify thetheoretical analysis with the obtained results.

2.5 Different levels of FAD

FADs are generally classified in current codes into

three different types according to the material

properties available for FFS assessment and the

conservatism of the diagram; higher-level FADs

require more complex data but are less conservative.

Level 1 Figure 4(a), a preliminary FAD based on the

crack tip opening displacement (CTOD) design-

curve method, is the basis of the elasticplastic

fracture assessment procedure in BS 7910. Level 2

Figure 4(b) is an alternative FAD based on the lower

bound of many curves obtained from experimentaldata on general austenitic steel. Both level 1 and

level 2 contain universal Failure Assessment Line

(FAL) which is the criterion line of FAD

independent of material properties, as shown in

Figure 4. However, level 3 Figure 4(c) is a material-

specific FAD based on the reference stress model.

Where the structure did not prove safe after being

assessed using a level 3 FAD, a designated material

and geometry specific FAD generated using fracture

mechanics analysis can be used to assess the

structure.A general flow chart of the procedure of a level

3 FAD diagram is shown in Figure 5.

2.6 Flaw Assessment

The following is the recommended sequence of

operations for carrying out an assessment for a

known flaw:

a. Identify the flaw type, i.e. planar, non-planer orshape;

b. Establish the essential data, relevant to theparticular structure;

Figure 4, Schematics of various FADs: (a) level 1, (b) level 2and (c) level 3 FAD.

c. Determine the size of the flaw;d. Assess possible material damage mechanisms

and damage rates;

e. Based on the damage rate, assess whether theflaw would grow to this final size within the

remaining life of the structure or the in-serviceinspection interval, by sub-critical crack growth;

f. Assess the consequences of failure;g. Carry out sensitivity analysis;h. If the flaw would not grow to the limiting size,

including appropriate factors of safety, it is

acceptable. Ideally, the safety factors should take

account of both the confidence in the assessment

and the consequences of failure.

i. Determine the limiting size corresponding to thefinal modes of failure.

Appendixe I gives a brief summary of theprocedure for unstable crack growth assessment.

3 DISCUSSION AND CASE APPLICATION

STUDY

Most of the existing recommended practices and

standards that address fitness for purpose provide

limit state solutions for standard sections and

standard joints. The limit state solutions are

expressed in terms of empirical formulas that

establish FALs of FADs. Needles to say here, thatthese tend to be conservative solutions.

To realize the potential of these methods it is

important to seek an alternative solution, which is

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Figure 5. Level 3 FAD flow chart.

advanced assessment using fracture mechanics and

finite element methods.

The case which was studied here was for a riser joint(pipe) suffering from both a dent and a crack.

Consideration for using materials with different

tensile strength was also studied.

3.1 Difficulties in FAD construction

As summarized in Table 1, different levels of FAD

have different requirements with regards to available

material data. Lower levels require less data and

tend to be more conservative. Nevertheless, the

more rationale and optimized options, require more

detailed material data as well as material and

geometry specific FAD being constructed using a

carefully carried out finite element model analysis.

3.2 Validation of plain pipe FAD

Burdekin et al., has published work for actual testing

carried out on cracked plain pipes. This data was

used as reference value to extensively validate the

finite element model. The finite element model

showed good correlation with the published results.

3.3 difficulty in modeling dented pipe

The dent location involves 3D. deformation of pipe

geometry. In addition to the normal inherent

difficulties involved with 3D. modeling,

Figure 6. Dented craked tube model

constructing a finite element model sustaining a 3D.

deformation has the extra difficulty of meshing the

dented portion of the geometry.

3.4 Validation of dented pipe model

Abdel-Nasser et al., had previously extensivelyinvestigated modeled dented pipes using finite

element method and comparing them to physical

experiments results.

3.5 FAD for a dented pipe with circumferential

crack

Both the validated plain-pipe model and the dented

pipe model were combined to produce the model for

the dented pipe with a circumferential crack, Figure

6 and 7.

Results of the validation analysis are plotted in

Figure 8.

Figure 7. Meshed finite elemet model

3.6 Usage of HTS

Given two similar design conditions, increasing the

grade of steel of line-pipe in simplistic terms will

correspondingly decrease the wall thickness and

therefore provide cost benefits. In addition to this, a

thinner wall thickness will also have various impacts

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Part Thickness

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Lr

Kr

Table 1. Selection of Analysis Levels from Tensile Data.

LEVEL DATA NEEDEDWHEN TO

USE

DEFAULT LEVEL

Yield or proof strength When no othertensile dataavailable

STANDARD LEVELS

1. BasicYield or Proof

Strength: UltimateTensile Strength

For quickestresult. Mismatch inproperties less than

10%

2. Mismatch

Yield or ProofStrength: UltimateTensile Strength.

Mismatch limit loads

Allows formismatch in yieldstrengths of weldand base material.

Use when

mismatch isgreater than 10%of yield or proof

strength (optional).

3. SS (Stress-strain defined)

Full Stress-StrainCurves.

More accurateand lessconservative than

levels 1 and 2.

Weld mismatchoption included.

ADVANCED LEVELS

4. Constraint

Allowance

Estimates of fracture

toughness for cracktip constraint

conditions relevant tothose of cracked

structure.

Allows for lossof constraint inthin sections or

predominantlytensile loadings

5. J-Integral

Analysis

Needs numericalcracked body

analysis

6. SpecialCase : Leak

before BreakAnalysis

As per level 1 butwith additional

information on crackgrowth mechanism

and estimationmethods for

determining cracklength at

breakthrough.

Pressurisedcomponents when

a conventionalapproach does not

indicate sufficientsafety margin.

on construction activities. A thinner wall thicknesswill require less field welding and therefore, in

theory, has the potential to reduce construction/ lay

time. At present there is

Figure 8. Results of analysis to validate F.E. model.

Table 2. Results of F.E. model analysis in tablature format .

StandardFAD

UM 10

UM 10

Dent

UM 10Dent

X65

UM 10

X65

1 1 1 1 1

0.9944 1.00083 9.96E-019.96E-01

1.00081

0.9758 1.00247 9.95E-019.95E-01 1.0024

0.9297 1.00491 9.95E-019.95E-01

1.00478

0.8106 1.00814 9.95E-019.95E-01

1.00792

0.5723 1.01213 1.00359 1.00348 1.0118

0.31989.60E-01 1.00502 1.00488

1.00727

insufficient data to make a direct like-for-like

comparison between, say, X70 and X65 for a given

pipe diameter. By increasing the material grade, it is

possible to lay pipeline in deeper waters. A thinner

wall thickness has a direct impact on this installationmethod since the requirements for lay barge

tensioners are related to the water depth and weight

of pipe.

4 RESULTS

Figure 8 displays the results of the carried out

analysis on tubular joints subjected to both a dent

and a crack and comparing them to results for pipes

sustaining a crack only. The analysis was repeatedfor all cases utilizing grade X65 high tensile steel.

The same results are presented in tabulated format in

Table 2.

The analysis is designed around the fracture ratio

Kr which in turn is based on stress intensity factor

which is a geometry dependent parameter that

increases with defect size and the applied stress.

Since all the cases were studied to investigate the

effect of variation of the material and including a

dent in the geometry of the tubular member and

identical load ratio Lr stations were used, the produced FADs met the expectations of being

closely correlated to each other. However from

Table 2, it could be noted that there is a slight

decrease in fracture capacity for all the cases

containing a dent compared to cases that did not.

Also where the X65 material was used, it is noted

that there is also a slight decrease in fracture

performance probably due to increased yield point

and reduced elongation to fracture.

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Figure 9. Results of dented & cracked joint F.E. modelanalysis.

5 CONCLUSION

From this work it can be safely concluded that using

standard FAD to assess damage to tubular member

containing a dent and a crack is safe and rather

conservative.This technique is most valuable when building

safety cases based on the proofing of employed non

destructive testing method utilized by establishing a

FAD for the concerned structure and assesses the

smallest detectable crack or the largest crack to

escape detection and finding out its potential impact

on the structure.

Defects revealed during in-service routine

inspections that does not endanger the integrity of

the structure, need no longer be repaired as with this

technique, the behavior of the crack and its stabilitycan easily be modeled.

Normally for an in-service dent some yielding

must occur, thus changing the material properties in

that region. In this study a homogenous material was

assumed throughout the model. Future work is

suggested to be carried out on accommodating

plastic deformation outcome in the material

definition in that region. This should not affect the

accuracy of this study significantly as behavior of

materials possessing higher toughness in general

was studied using the X65 model.

REFERENCES

Abdel-Nasser, Yehia A. and Rashwan, Ahmad M. 2006. NewSimplified Equation for Predicting Ultimate Strength ofDamaged Tubular Members,Alexandra University Journal.

Annex B of Volume III of the FITNET FFS.

API 1993. Recommended Practice for Design, Selection,Operation and Maintenance of Marine Drilling RiserSystems.API RP 16Q.

API 1998. Specification for Casing and Tubing. Spec. 5CT:

Sixth Edition.

Bai, Yong 2001. Pipelines and Risers. Oxford: Elsevier

Science Ltd.

British Stainless Steel Association website;WWW.bssa.org.uk.

BSI 1999. Guide on Methods for Assessing the Acceptabilityof Flaws in Metallic Structures. British StandardInstitution. BS 7910.

Burdeking, F.M. 2001. Experimental validation of the ultimatestrength of brace members with circumferential cracks.U.K. HSE Executive Offshore Technology Report, 081.

Burdekin, F.M. 2001. The Static Strength of Cracked Joints inTubular Members. U.K. HSE Executive OffshoreTechnology Report, 080.

DNV 2001. Dynamic Risers. Offshore Standard. DNV-OS-F201.

El Gammal, M. M. 1975. A new method for estimating fatiguelife for ship structures. Shipbuilding, 22: 254.

El-Gammal, M. M. 2003. Fatigue Life Prediction in thePresence of Inherited Defects and Corrosion with MarineApplications. Journal of Marine Design and Operations,Proceedings of the Institute of Marine Engineering,

Science and Technology. B: 33.

Euro FitNet 1999. SINTAP Procedure. Final Version.

General Electric Energy website; WWW.gepower.com.

Suliman, Akram 2009. Fracture Lifetime Prediction of MarineRiser Systems. PhD. Thesis.

The Welding Institute website; WWW.TWI.Org.

Walters, D., Thomas, D. and Hatton S. 2000. Design and

Optimization of Top Tension Risers for Ultra Deep Water.Proceedings of Conference (1) Floating Production

Systems.

APPENDIX I: UNSTABLE CRACK AS FAILURE

CRITERIA

The condition when the assessed point crosses the

FAL can cause the crack growth initiation.

Engineering Criticality Analysis (ECA) also

considers the unstable crack growth as failurecriteria. In evaluating the margin on unstable crack

growth, J-Resistance curve is required. The J-

Resistance curve is converted into fracture

toughness Kmat vs. crack extension a data. Once the

assessed point is outside the safe region, small

increment to crack size a, is given. This modifies

the assessed point. Because of increase in crack size,

K increases while the limit load decreases. The main

change is in the value of Kmat, which increases

appreciably. Hence the, assessed point has a lowerKr value and a marginally increased Lr value. This

process is repeated to check if the assessed point

enters the safe region. In such a case, the crack

arrest takes place. If the assessed point fails to enter

the safe region, the unstable crack growth occurs.

The unstable crack growth load is the load at which

the locus of the Lr-Kr just touches the FAL. The

procedure is breifly summarized in Figure i.1.

Consider Figure i.2. The procedure for

estimating load to cause unstable crack growth is as

follows:1. The original assessed point is A.2. The load is increased, the crack growth

initiation occurs at point B.

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3. On further increasing the load, crack growthinitiation occurs. Consider point L1. As the

stable crack growth takes place, the assessed

point is updated based on increased crack

size and increased Kmat. The locus followed

is L1-L1. The assessed point re-enters the

safe region and the crack is arrested. The

amount of crack extension done is decided

by the availability of J-Resistance data.

4. Now consider the load L3. Here, afterexhausting the J-Resistance, still the assessed

point is in the unsafe region. The unstable

crack growth takes place at this load.

Figure i.1. Methodology for crack growth analysis.

Figure i.2. Unstable crack growth assessment.

5. The objective now is to find a load L2 suchthat the locus L2-L2 is tangent to the FAL.

This is the limiting load for unstable crack

growth.

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