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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.