EUROPIPE. The world trusts us.
Materials and design of high strength pipelines
Yong BaiStavanger University College, Stavanger, Norway
Gerhard KnaufMannesmann Forschungsinstitut, Duisburg, Germany
Hans-Georg HillenbrandEuropipe, Ratingen, Germany
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Materials and Design of High Strength Pipelines
Yong BaiStavanger University College, Stavanger, Norway
Gerhard KnaufMFI (Mannesmann Forschungsinstitut), Duisburg, Germany
Hans-Georg HillenbrandEuropipe, Ratingen, Germany
ABSTRACT
The demand for high-strength linepipe foroffshore applications has increased considerablybecause of the challenges that the offshorepipelines should be contracted in ever-deeperwaters and that for reasons of reducingoperational costs pipelines should be operated atincreased pressures. The development of newsteels and improved pipe manufacturingcapabilities enable high strength linepipe withappropriate toughness to be supplied.
In this paper, the following subjects related to theuse of high strength linepipe are discussed:
• Materials properties• Evaluation of the use of high strength steel
from design viewpoints;• Assessment of loading conditions for
installation and in-service conditions;• Development of additional design criteria
for the subjects not covered by codes, e.g.strength design of linepipes with yieldanisotropy.
The paper describes practical considerations onmaterial properties, design loads, coderequirements and concludes with thedevelopments in design criteria for strengthdesign of high strength pipes with yieldanisotropy.
KEYWORDS: Linepipe, Materials, Design, HighStrength,
Pipeline
INTRODUCTION
The demand for high-strength linepipe foroffshore applications has increased considerablybecause of the challenges that the offshore
pipelines should be constructed in ever-deeperwaters and that for reasons of reducingoperational costs pipelines should be operated atincreased pressures. The development of newsteels and improved pipe manufacturingcapabilities enable high strength linepipe withappropriate toughness to be supplied.
These developments cover linepipe for both sourand non-sour service. The materials underconsideration are grades X70 and X80 for non-sour service and grades X65 and X70 with a wallthickness of up to 40 mm for sour service.
Apart from structural strength, key considerationsare:
• Toughness of parent linepipe material and allwelded joints;
• Corrosion performance of lines that operate‘wet’;
• Weldability, including repairs and hyperbaricrequirements;
• Compatibility with external environment;• Availability of bends and fittings required to
complete a piping system;• Suitability for operational modifications
repairs and hot taps;• Cost.
In this paper, materials properties and practicaldesign considerations will be given. Theanisotropy with respect to tensile properties oflinepipe and longitudinal and hoop design loadswill be discussed for S- and J-laid pipelines.Existing codes have been evaluated and areas ofimprovement have been identified. Finally, ananalytical capacity equation is outlined to designpipe with yield anisotropy.
MATERIAL PROPERTIES OF HIGHSTRENGTH LINEPIPE
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The desire to increase the through-put byincreasing the operating pressure or byincreasing the usage factor has led to everincreasing demands for large-diameter steel pipe.These requirements refer in particular to strengthproperties and tolerances on dimensions. At thesame time, it is endeavored not to compromiseon operational safety and even to improve it,where possible.
Thanks to the intensive research anddevelopment work carried out and the qualityassurance measures consistently implemented inpipe production, it has been possible so far tomeet the requirements placed by the market.
However, the limits of physical and technicalfeasibility have almost been reached whenproducing high strength pipe that can meet theever-increasing requirements. As the strengthincreases, it becomes extremely difficult, if notimpossible to achieve the specified limits for theyield-to-tensile ratio or to fulfill increasedtoughness requirements.
In addition to the tensile properties in thetransverse direction, the tensile properties in thelongitudinal direction of the pipe play a crucial rolein the context of offshore pipelines. This fact hasbeen taken into account in the usual offshorepipeline codes in that the values specified foryield and tensile strength for the longitudinaldirection are the same as those specified for thetransverse direction. So far, the anisotropy withrespect to the strength properties of the pipeproduced by the UOE method has not been takeninto account adequately in the codes.
Figure 1 shows schematically the yield and tensilestrength frequency distribution curves for thetransverse and longitudinal directions for plateand pipe. Because of the elongation of themicrostructure in the rolling direction, the yieldand tensile strength values for the longitudinaldirection are lower than those for the transversedirection.
Figure 1. Comparison of strength distributions for plateand pipe specimens in transverse and longitudinaldirection.
As a result of the pipe forming and subsequentcold expanding, there is a marginal increase inthe tensile strength for both directions and in yieldstrength for the longitudinal direction. In contrast,the yield strength of the transverse, flattened stripspecimen is reduced. Comparison of these datawith those for the round bar specimen indicatesthat the reduction of the yield strength in the caseof the strip specimen can be attributed to theBauschinger effect resulting from the flatteningoperation of the specimen prior to the tensile test.
The anisotropy described and its development inthe course of pipe forming is typical of standardpipe and depends on the material grade, chemicalcomposition and pipe geometry. This dependencyis readily seen in the case of low carbon sourservice grades.
Figure 2 shows, by way of example, thedistribution curves determined on a production lotof grade X65 pipe intended for sour service. Ascan be seen, the distributions for the transversespecimens are shifted to the right relative tothose for the longitudinal specimens. It had beennecessary to optimize the rolling process to raisethe strength values for the transverse specimensso that the tensile requirements specified for thetransverse specimens are also met by thelongitudinal specimens.
Figure 2. Results on 610 mm OD x 14.3 mm W.T. X65production line pipe for sour service (PH3).
Such measures adopted to compensate for theanisotropy result in pipe with transverse strengthproperties corresponding to those of a nexthigher material grade. Of course, thesemeasures are cost intensive and may have anunfavorable effect on other material properties(toughness, Y/T, corrosion resistance).It is therefore prudent to check whether thepipeline design can tolerate the anisotropy in that
00,05
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it can accept reduced yield and tensile strengthvalues for the longitudinal direction.
DESIGN EVALUATIONOF HIGH STRENGTH STEEL
Review of the Usage of High Strength SteelLinepipes in Offshore Pipelines
For offshore pipelines, the current trend istowards linepipe in grade X70 with a wallthickness up to 40 mm. Fulfillment of therequirements for DWTT transition temperaturewill be progressively difficult as the wall thicknessincreases. For wall thickness in excess of 30mm, low transition temperatures can only beachieved by means of highly expensive rollingprocesses.
Until now, there has been only limited offshoreuse of X70 material. The main installationcontractors have completed three projects withX70 and have two planned until 1997. Again,these references are only indicative and notcomprehensive.
One example use of X70 linepipe in offshoreapplications is the Britannia pipeline for whichEuropipe supplied the linepipe. The Britannia fieldis a gas condense reservoir in the Central NorthSea approximately 200 km northeast ofAberdeen and 45 km north of Forties.
The Gas Export Pipeline, 682.4 mm OD ×15.9mm WT, is 186 km long. The pipeline designpressure is 179.3 barg and the design life of thepipeline is 30 years. The pipe grade is X70. Themechanical properties of the pipe used are givenin Figure 3. The pipeline was subject to reliability-based limit state design techniques in order tojustify a wall-thickness thinner than that permittedby BS8010. The Britannia pipelines werecompleted in 1997.
Figure 3. Production results on 682.4 mm OD x 15.9 mm W.T., API grade X70 linepipe.
Another large offshore project in grade X70 is thepipeline in the North Sea operated by Statoil,connecting Karstø, Norway, together withDornum, Germany. This pipeline has a length of600 km and is built of pipe 42” x 25 to 30 mm WT.
Europipe completed in the 1990's thedevelopment of grade X80 pipe 48" in OD and18.3 to 19.4 mm in wall thickness for onshorepipelines. It has been demonstrated that it isfeasible to manufacture commercially largediameter X80 pipe consistently for longtransmission pipelines, see Gräf and Hillenbrand(1995).
As regards offshore applications, a series ofpipes have been supplied for qualification testingwith respect to pipelaying. Use of X80 linepipe foroffshore field development is being qualified by ajoint industry project EXPIPE.
For low-alloy steel pipelines operating in sourservice, X65 is currently the established material.Special treatment in the steelmaking shop andfulfillment of special requirements for chemicalcomposition help prevent the formation ofnucleation sites for HIC. Production trials showbig potential for the development of higher gradesup to X80 for slightly sour conditions, see Gräfand Hillenbrand (2000).
Potential Benefits of Using High StrengthSteel
It is clear that the obvious advantage for usinghigher strength steels is cost saving. However,new approaches to design, manufacture andconstruction and the use of high-grade materialswill expose potential pipeline projects toincreased levels of technical and commercialrisks. This section of the paper identifies thebenefits and disadvantages associated with theuse of high strength steels.
Potential Cost ReductionIncreasing the grade of linepipe used forconstruction of a pipeline provides the opportunityto reduce overall material costs. The costreduction is based on the premise that increasingmaterial yield strength reduces the wall thicknessrequired for internal pressure containment andhence the overall quantity of steel required. The
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implications of using high-grade material areconsidered in relation to linepipe manufacturingand pipeline construction.A published study (Price (1993)), whichconsidered both direct and indirectconsequences of using a high strength steel,suggested a 7.5% overall project saving for a 42-inch offshore line laid with X80 instead of X65.Although the X80 pipe cost 10% more per ton, itwas 6% less per meter. Further savings wereidentified for transportation, weldingconsumables, welding equipment rental andoverall lay time.
On the recently completed Britannia gas pipeline,cost studies during detailed engineering showedthat by increasing the linepipe material gradefrom X65 to X70, an approximate cost reductionof US$ 3.5 million could be achieved. The projectCAPEX is approximately US$ 225 million.
Although not directly related to the use of highstrength material, other potential cost savingsidentified in the same study include:
• Tighter than normal (API 5L) definition ofdimensions. Consideration should be givento reducing linepipe tolerances on ovalityand wall thickness from API 5Lrequirements. The actual tolerancesrequired will be determined by evaluatingpotential cost reductions anticipated duringpipeline construction and mechanicaldesign, compared to the expected increasein linepipe manufacturing.
• Use of fracture mechanics acceptancecriteria for determination of maximumallowable defect sizes in pipeline girthwelds. Traditionally, the acceptance criteriafor weld defects are based on workmanshipstandards. More recently, alternativecriteria such as Engineering CriticalityAssessment (ECA) have been used todetermine the acceptability of defects, seeKnauf and Hopkins (1996).
• Non-standard pipeline diameters should beconsidered. Optimization of the pipe IDbased on modeling of the pipelines indetailed design may demonstrate that thelinepipe cost can be reduced by procuringpipe of the exact ID required as opposed toselecting the larger standard size, forexamples on the Britannia gas pipeline.Conversely, it may be of benefit to modifythe design flowrates to enable selection of amore economical size of pipe.
A quick and reliable inspection of girth welds isrequired in the context of pipelaying, especially ofhigh strength pipe. There have been considerableadvancements in recent years in this field.Starting from conventional radiography, the NDTequipment used for pipeline inspection has beenimproved. Radiography systems are availablewhich produce a real-time image of the weldbeing inspected. Such systems can also be usedfor the quality control of production welds in pipemanufacture, followed by automated evaluation ofthe data. As an alternative to radiography, high-speed ultrasonic inspection is available. Theradiographic images and also the ultrasonicindications are stored electronically and offerinstant retrieval. The time to inspect each weld isreduced compared to traditional methods, andthereby significantly reducing construction costs.
Wall Thickness and ConstructionGiven two similar design conditions, increasingthe grade of linepipe in simplistic terms willcorrespondingly decrease the wall thickness andtherefore provide cost benefits. In addition to this,a thinner wall thickness will also have variousimpacts on construction activities. A thinner wallthickness will require less field welding andtherefore, in theory, has the potential to reduceconstruction/lay time.
Increasing the material grade and strength oflinepipe is beneficial to laying pipe in deeperwaters. Furthermore, certain projects can only beimplemented with pipe having reduced weight andoptimized strength and toughness.
The maximum water depth by conventional S-laymethod is being stretched to the extent thatALLSEAS have installed a 12-inch pipeline usingX70 steel in 1600 m water depth in the Gulf ofMexico. However, it is questionable that the samelay method can be used for a larger pipe diameterin the same water depth. It is widelyacknowledged that the J-lay method is the mostsuitable for laying pipe in waters beyond 1000 m.A thinner wall thickness has a direct impact onthis installation method since the requirements forlay barge tensioners is related to the water depthand weight of pipe.
Pigging RequirementsThe thicker walled sections of the pipeline indeeper waters may restrict the full capabilities ofintelligent pigging. There is a limitation on the wall
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thickness depending on the type of pigging toolused, which can be overcome by the use of high-strength steels
Potential Disadvantages of UsingHigh Strength Steels
Limited SuppliersThe worldwide availability of proven suppliers formaterial grades above X70 is relatively limited.
Welding RestrictionsWith regards to the weldability of X80 steel, thereis a medium risk of schedule extension and costincrease since it has only been used on a smallnumber of onshore projects and there is noexperience offshore. Welding to the requiredquality may be slowed by more processrestrictions and more complex controls. Due tothe limited worldwide experience of welding X80linepipe, certain key welding issues will have to beaddressed in further studies, particularly that ofwelding consumables.
Limited Offshore Installation CapabilitiesThe number of offshore pipelay contractors withproven experience of welding X70 steel linepipeis limited. Additionally, the experience of layingdeepwater pipelines by the J-lay method is limitedto relatively small diameter pipelines.
Fatigue and Fracture of High Strength SteelWhile an onshore pipeline is essential subjectedto fatigue essentially due to internal pressurefluctuations, an offshore pipeline experiencesfatigue stresses in the form of longitudinal tensionand bending. Fatigue life under offshoreconditions is used as the basis for many of thelimits placed on pipeline strength design. Theselimits have often been established based onempirical data from tests on low strength steels,with a safety margin applied. In general, the abilityof steels to resist fatigue failure increases withincreasing strength. But, the notch severityincreases with increasing strength, a finding thatis of significance to the configuration of the girthweld in the context of the fatigue life of offshorepipelines.
Pipe materials, as a rule, should possessadequate toughness to prevent the initiation andthe propagation of long-running fractures in thepipeline, see Re et al (1995), ISO 3138 and DNVOS F101 (2000). For pipes with increasingstrength also an increasing toughness level isrequired. This relationship is also valid for girth
welds, i.e. with increasing strength of linepipe,weld metals of increased strength and sufficienttoughness are required to ensure overmatchingbehavior of girth welds.LONGITUDINAL LOADS DURINGINSTALLATION AND IN-SERVICE
General
The objectives of this chapter are to describe thelongitudinal loads acting on pipelines as below:
• S-lay installation loads;• J-lay installation loads;• Reeling installation loads;• In-service loads for cold pipelines;• In-service loads for HP/HT (high pressure
high temperature) pipelines.
For large diameter pipelines, the preferredinstallation method is S-lay when water depth isshallower than e.g. 500 m. For deepwaterinstallation, J-lay is considered more suitablebecause it demands less laying tension. The layrate for S-lay is 2-3 times faster than J-lay.Typically a lay rate for S-lay is 3 km per day and aday rate is approximately 3 millions NOK (0.4millions USD) for both S-lay and J-lay.
Unless specifically defined, “strain” is meant tobe total strain.
Installation Loads for S-laid Pipelines
For deep water pipe lay with S-lay method, thepipe’s departure angle on the stinger has to beincreased to minimize tension requirements andprovide seabed maneuverability. The requirementof stinger departure angle is linked to theminimum curvature radius, or laying straincriterion. Less conservative laying straincriterion, is being developed by Deepipe JIP,Expipe JIP and design projects by operators andengineering companies.
Recent research projects document thatallowable static strain may be 0.28%. Due to areduction of strain hardening for high strengthsteel, the allowable laying strain for high strengthpipe is lower than that for X65 pipes. Furtherresearch is therefore required to study thefeasibility of relaxing strain criterion for highstrength pipe in deep water.
High strength steels are beneficial both withrespect to wall-thickness sizing and less tension
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requirements resulting from smaller weight duringinstallation. The limiting failure modes withrespect to pipeline installation are considered tobe
• Girth weld fracture from weld defects;• Concrete tensile failure due to overstressing;• Low cycle fatigue due to installation and
in-service load cycling;• Buckling as a result of external-over pressure in
sagbend;• Collapse due to bending moment and internal
pressure during operation.
The girth welds are the potential “week” section ifone of the following situations is presented duringinstallation:
• Strain concentration due to heavy concretecoating or use of thick buckle arrestors;
• Fracture due to less strict requirements of welddefect inspection and repair;
• Fatigue due to long holding period in a roughwave condition during installation.
The assessment of acceptable defect depth andlength represents a key element evaluating thestrain level that can be accepted on the stingerand during in-service condition. The acceptabledefect sizes depend heavily on the materialstrength and fracture toughness. These materialproperties are associated with significantstatistical scatter and systematic variationsaround the girth weld. Two kinds of laboratorytests can be useful to justify an increase ofallowable tensile strain:
• Ductile fracture tests for improved fractureresistance assessment;
• Low-cycle fatigue tests.
Installation Loads for J-laid Pipelines
The J-lay method involves installing pipeline in avertical mode from a dynamically positionedvessel and therefore allows installation in waterdepth beyond the limits of the S-lay and Reelingmethods.
The feasibility of vertical J-pipelay, in particularfrom smaller vessels, was examined by DeepStarJIP (Ekvall et al, 1994) for deepwater installation.The pipe diameter for such deepwater pipelinesis typically 10” to 20” while that in the case of thecurrent Blue stream project in the Black Sea is24”. The major technical difficulties are e.g.
strength against collapse under combined loads,vessel positioning, stinger integrity, and pipehandling. For vertical J-lay, the vessel can beoriented arbitrarily with respect to the pipelineroute to minimize the wind and wave forces actingon the vessel, allowing J-lay installation tocontinue under a wider range of weatherconditions. For offshore pipeline installation,regardless of the pipelay method, a stinger isnormally used to control the deflection of thesuspended pipe span and to keep the bendingstrains within an acceptable limit. A shorter andless curved stinger is required for J-laying(compared to S-laying) pipelines in deep waters,since the pipe span lifts off at a less steep angle.
The maximum bending stress along thesuspended span occurs in the sagbend or aroundthe stinger.
A major design concern is that pipe strength inthe sagbend is very sensitive to collapse during avertical J-lay installation, since the pipe in thisregion is subjected to very high stresses due tocombined bending and external pressure.
At the touchdown point, the stress due to changeof the configuration as well as contact force fromthe seabed, can be very high. This may inducesome cross-sectional ovalisation that may furtherreduce pipe collapse strength. At the touchdownpoint, the bending collapse is a displacement-controlled situation. The laying strain-limit may bedetermined using external pressure – curvatureinteraction equations. Typical strain is 0.2%during J-installation of pipelines.
Fatigue loads should also be included to designfor an abnormal weather situation, where cyclicloads may be repeated if the pipe is on-hold for along period, due to e.g. repair needs. Thecalculation of fatigue loads may be conductedusing dynamic installation analysis.
The methods of strength design for S-layinginstallation, as discussed earlier, are generallyapplicable to J-laying situations.
Installation Loads for Reeled Flowlines
Strain Level During ReelingThe different stages of the reeling process are:
• Reeling on;• Reeling off;• Pipe passing the entry guide and ;
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• Bending at pipe straightener.
Strain as Pipe is Lowered to the SeabedAs in traditional S-lay, the reeled pipe may bebent as it passes over a stinger leaving thevessel, and bent in the opposite direction as itmeets the seabed. These bending scenarios areoften referred to as overbend and sagbend,respectively.
Strain During Pressure TestingPrior to putting a pipeline into operation the pipewill be hydrotested to a test pressure higher thannormal operating pressure. The procedure andthe test pressure depend on the pipeline codeused for design. The longitudinal strain at a hydrotest is about 0.2% and the equivalent stress isclose to SMYS (Specified Minimum Yield Stress)at critical locations (e.g. spans) in a hydro test.
In-service Loads for Cold Pipelines
For cold pipelines, the in-service loads are:
• Fatigue due to extensive repair period or roughwave conditions during installations;
• Functional loads (e.g. pressure, temperature,weight and support reaction);
• Environmental loads (e.g. wave and currentloads);
• Accidental loads (e.g. impacts, dropped objects,explosion, fire and anchoring);
• Trawling loads (fishing gear loads during impact,pull-over and hooking process).
The internal pressure may be reversiblyestimated from wall-thickness and material gradeusing (pressure containment) hoop stresscriterion. Typically internal pressure is 200 bargfor gas export lines and 350 barg for infieldflowlines, although the exact value is to be givencase by case. The intention to list the abovevalues is to show that high pressure in offshorepipelines is far higher than that experienced inonshore pipelines.
The weight is to be estimated considering thevolume of steel, pipe contents density, coatingthickness and density. The contact force betweena cold pipeline and the seabed may be simplycalculated based on force equilibrium.
The wave and current loads on pipeline areestimated using Morisson’s equations given bydesign codes. The wave and current velocitiesare calculated based on water depth and the gapdistance between the pipe and the seabed.Again, equations are available from the codes tocalculate wave and current velocities. Statisticalvalues are used to estimate leads:
• For ultimate strength analysis, the extremevalues corresponding to n years return periodare used;
• For fatigue strength analysis, characteristicvalues are used;
• For design against accidental loads, normaloperating loads are to be used;
• For design against fishing gear pull-over loads,loads corresponding to the specific spanheights are to be used.
The characteristic fatigue loads for cold pipelinesare:
• Cyclic loads during installation phase, e.g.induced by wave loads or reeling loads;
• Cyclic loads due to free-spans, e.g. due tovortex-induced vibrations or caused by cyclicwave force in a shallow water.
For cold pipelines, the temperature induced axialdisplacement is negligible. It is perhaps correct toassume that the typical normal operating loadsare internal pressure. However, global buckling(e.g. upheaval buckling & lateral buckling) shouldnot be excluded from design if operating pressureis high and soil friction is low.
Unless fishing gear loads are large, longitudinalloads are not a demanding requirement. Normallyvery little seabed intervention is required for thesafety of a pipeline in operating conditions.Unfortunately for design of pipelines in the NorthSea, pull-over loads are governing designparameter where fishing activities are frequentand water depth is less than 350 m. The pull-overloads consist of vertical (downward) componentand horizontal component. Both are functions ofspan height, and trawling velocity. The time-history of the pull-over loads are available fromdesign guidelines made by the pipeline industry.
As daily practice in design offices, finite elementin-place analysis is conducted to estimate thestructural response due to fishing gear pull-overloads, and comparisons with limit-state designcriteria are carried out to ensure the structuralresponse is acceptable.
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In-service Loads for HP/HT Pipelines
GeneralHP/HT (High Pressure High Temperature)pipelines are defined as
• Design internal-over pressure istypically tD
t−
28.0 SMYS
• Operating temperature is 130ºC and above.
The HP/HT pipelines are typically infield flowlineswhere oil and gas are transported withoutexpensive cooling process.
Seawater is a good cooling system. At a distanceof a couple of kilometers from a platform or atemplate, the temperature of the pipe containmentbecomes lower than 10 ºC.
As for cold pipelines, the in-service loads forHP/HT pipelines are:
• Functional loads (e.g. pressure, temperature,weight and support reaction);
• Environmental loads (e.g. wave and currentloads);
• Accidental loads (e.g. impacts, droppedobjects, explosion, fire and anchoring);
• Trawling loads (fishing gear loads duringimpact, pull-over and hooking process).
Differences between Cold Linesand HP/HT linesIn the following, the difference between HP/HTpipelines and cold pipelines are described:
• The major difference is temperature-induced strain and thermal buckling. Asknown by pipeline industry for many years, aHP/HT pipeline may experience upheavalbuckling if the pipeline is rock-covered.Lateral buckling (snaking) may occur if theline is free on the seabed.
• Design of HP/HT pipeline against fishinggear loads becomes a crucial issue sincelarge stress and moment may be observedunder pull-over loading and the pipelineindustry does not allow strain-based designfor pull-over loads yet. The moment criteria
for load-controlled situations from designcodes are rather conservative.
• Seabed intervention cost for protection ofin-service pipeline is governed by pull-overloads.
Strain level in operating flowlinesThe main source of cyclic loading duringoperation is repeated heating-up and cooling-down due to shut-downs/start-ups. For a pipelaying on the seabed with no rock cover, thethermal expansion may cause the pipe to deformlaterally or feed pipe into free-spans, resulting inbending strain. Similarly, for a fully constrainedburied pipeline there will be radial and hoop strainvariations resulting from the start-up/shut-downcycles.
Summary of Loads and Load Combinations
Actuallongitudinalloads
Hooploads
CodeRequirements
Remarks
Reeling Maximum2% longitu-dinal strain
Nohooploads
Fracture& localbucklingchecks
For smalldiameterflowlines
S-Lay Maximum0.3 %strain
Externalpressureforsag-bend
Fracture,Rotation,Collapsein sag-bend
Forshallowwater,largediameter
J-lay Maximum0.3 %strain
Externalpressurefor sagbend
Fracture,Collapsein sagbend
For deepwater,large &small pipe
Coldopera-tion
pull-overinduced0.3% strainor stressof 0.9SMYS
Hoopstress of0.8SMYS
Limitstatebaseddesigncriteria
Crucialfor sea-bed inter-ventiondesign
HP/HTopera-tion
pull-overinduced0.3% strainor stressof 0.9SMYS
Hoopstress of0.8SMYS
Limitstatebaseddesigncriteria
Crucialfor sea-bed inter-ventiondesign
Table 1. Summary of Installation and OperationLoads.
DESIGN EXPERIENCEON LOADS AND STRENGTH
Limit-state Design of Offshore Pipelines
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Limit state based strength criteria may bedeveloped for pipelines covering the potentialfailure modes:
• Out of roundness for serviceability;• Bursting due to internal pressure, longitudinal
force and bending;• Buckling/collapse due to pressure,
longitudinal force and bending;• Fracture of welds due to bending/tension;• Low-cycle fatigue due to shut-downs;• Ratcheting due to reeling and shut-downs;• Accumulated plastic strain.
The limit-states are to be defined for the followingload situations:
• Installation condition;• Empty condition;• Water filled condition;• Pressure test condition;• Operational conditions;• Shut-down conditions.
The strength criteria are to be defined for thefollowing design situations:
• Static and dynamic installation criteria;• In-place behavior;• Trawl pull-over response;• Static free-spans;• Dynamic free-spans.
It should be documented that adequate structuralsafety is maintained against the potential failuremodes for the given design situations when thestrength criteria developed are satisfied.
Pipe dimensions, operating conditions andmaterial dictate the allowable moments, stressesand strains.
The experience from design of North Seapipelines is summarized in the following sections.
Experience from Design of Large DiameterExport Pipelines
The following is a summary of design experienceon loads and strength:
• When water depth is less than 350 m, thewall-thickness design is normally governedby internal pressure containmentrequirement, e.g. hoop stress criterion. In
order to achieve cost saving, it isnecessary to use high strength steel pipe.
• When water depth is greater than 350 m, astudy is required to investigate the nonlinearrelation between the costs and steel gradefor different water depths. Higher yieldstrength also helps increase the pipebuckling/collapse capacity for external-overpressure situations, however, thisrelationship is no longer linear.
• As long as strain-based design can beapplied for operating conditions, thelongitudinal loads are far below the capacity.Therefore, the required longitudinal yieldstrength is not so high – leading to apotential use of pipes whose hoop yieldstrength is far higher than longitudinal yieldstrength.
• When a pipe is under a load-controlledsituation, the buckling/collapse capacity ofthe pipe may be assessed using momentcriteria.
Experience from Design of Infield Flowlines
The following is a summary of design experienceon loads and strength:
• Flowlines are typically installed using reelingmethods. A detailed welding qualificationprogram is required to ensure that nofracture or local buckling occurs during thereeling process and there is no threat to thefatigue strength after line installations.
• For small diameter flowlines in the NorthSea, the governing design loads are thetrawling loads. In this instance,buckling/collapse criteria (moment criteria)are governing design parameter.
MATERIAL PROPERTY REQUIREMENTS
General
The purpose of this chapter is to describe thematerial requirements, and compare therequirements for longitudinal direction andcircumferential direction. Typically, the materialproperties requirement in hoop direction arerelated to pressure containment hoop stresscriterion and buckling/collapse under externalpressure, while longitudinal properties are directlyspecified for buckling/collapse under bending andtension, and weldability.
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It is beneficial from the viewpoint ofmanufacturing to allow hoop yield strength higherthan longitudinal yield strength. In the following,requirements will be described regarding CTOD,yield stress, ratio of SMYS and SMTS, fatigueproperties and wall-thickness tolerances.
Material Property Requirement in HoopDirection
Necessary CTOD value requirements for HAZand weld metal are to be established that arerelevant for the specific design conditions withregard to type and extent of longitudinal welddefects likely to exist. Typically, the requiredCTOD value is established through ECA(Engineering Criticality Assessment) using BritishStandard PD 6493.
The extent of longitudinal weld defects that likelyto exist, is defined in the operators’ weldingqualification specifications. Typical values are:depth 3 mm and width minimum of 25 mm andpipe wall-thickness.
The required CTOD value, as calculated basedon codes, is rather stringent, due to largescatters in the CTOD values from tests. Practicalexperience from field use of the line pipes have,demonstrated that there has been very littlestructural failure due to lack of CTOD value inhoop direction for line pipes. It is thereforesuggested to closely evaluate the following:
• CTOD testing methods, scatters and statisticalevaluation of scatters;
• Possibility to reduce the number of CTOD tests;• Safety factors used in ECA determination of
CTOD requirements;• ECA design equations and analysis methods.
Similar observations may be made on the CTODrequirements for the longitudinal direction.
It is likely that fracture occurs in the weldment.Then the CTOD requirements made to pipe basematerial are not relevant. However, the CTODvalue for HAZ (Heat Affect Zone) may berelevant for fracture in HAZ. Weldability of thepipe is a more important parameter than CTODvalue.
Material Property Requirement inLongitudinal Direction
The CTOD value for line pipes in longitudinaldirection is influential for fracture limit-state whenECA such as PD6493 is applied to calculate thelimiting loading condition to avoid fracture.
The CTOD value needed to avoid fracturedepends on the extent of girth weld defects likelyto exist and the applied load. For a defect depthof 3 mm, a wall thickness of 25.4 mm and loadingup to 0.5% total strain a defect length of 177 mm(7 x wall thickness) was shown to be safe whenCTOD is minimum 0.10 mm, see Knauf andHopkins (1996).
The discussions on unstable fracture and CTODfor hoop direction are also valid for longitudinaldirection.
The fact is that the yield stress in longitudinaldirection does not significantly affect pipestrength as long as strain-based design isapplicable to the design situation. The reasoningfor this statement is that strain acting on pipelinesin operating condition is typically as low as 0.2%unless the pipeline is under a high pull-over load.
With exception of some special materialproblems, the Y/T (SMYS/SMTS) ratiorequirements can be replaced by introducingstrain-hardening parameters such as σR and nused in a Ramberg-Osgood equation. In Bai et al(1994), a set of equations are given to relateSMYS and SMTS with strain-hardeningparameters σR and n.
The material strain-hardening effect may beaccounted for in fracture mechanics assessmentand local buckling/collapse checks through use ofthe stress-strain curves. In fact, a set of designequations was given by Bai et al (1997) and Baiet al (1999) for local buckling/collapse. In thepapers by Bai et al. (1997, 1999), the effect ofmaterial strain hardening parameter onbuckling/collapse have been discussed in detail.
The level-2 and level-3 failure assessmentdiagrams in PD6493 do also account for strain-hardening effects.
Comparisons of Material PropertyRequirements
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Which material properties are dominant in localbuckling/collapse? The answer is dependent onloads as the following:
• For internal pressure containment, hoop SMTS;• For external-pressure induced buckling, hoop SMYS;• For bending collapse, longitudinal SMYS;• For combined internal pressure and bending, hoop SMTS;
Longitudinal SMYS & SMTS;• For combined external pressure and bending, hoop SMYS;
Longitudinal SMYS & SMTS.Pipe strength under combined internal pressureand bending is an important design case, iffishing activities are frequent.
It is difficult to compare the requirements of thematerial property in hoop and longitudinaldirections. Rather the following is a discussionon cost-effectiveness of raising material’sperformance in hoop and longitudinal directions.
Raising hoop SMYS will directly result in aproportional reduction of the required wall-thickness of the line pipe for water depthshallower than 350 mm. However, if the designcodes, on buckling/collapse for external-overpressure case, are further upgraded, this waterdepth may be extended from 350 m to 450 m. It isthe authors' opinion that the existing designequations for external-over pressure situationsare rather conservative. To achieve yield andtensile strength values that conform to therequirements, as specified for the transversedirection, a corresponding increase in thestrength in the longitudinal direction is needed.This in turn leads to increased production costsand may lead to difficulties in meeting therequirements for yield-to-tensile ratio, toughnessand sour service suitability, etc..
As a conclusive remark on materials propertyrequirements, it is believed that:
• The minimum CTOD values in both hoop andlongitudinal directions typically should be0.1mm; the applicability of lower CTODvalues can be validated by ECA methods.
• It is economically beneficial and technicallyjustifiable that for pipe grades X60 to X80yield and tensile strength in longitudinaldirection can be lower by up to 10% thanthose in the transverse direction for waterdepths shallower than 450 m.
• For fracture and local/buckling failure modes,the Y/T value requirement can be removedif the strength analysis explicitly account for
the difference of strain-hardening whoseparameters (σR and n) are a function ofSMYS and SMTS as the equations given inBai et al(1997).
As a further study, it is proposed to compare theY/T ratio requirements from alternative codes(e.g. 0.93 from API for onshore pipelines, 0.85from EPRG and 0.87 from DNV’96 guideline). Itis perhaps possible to find some other rationalcriteria that can replace the Y/T ratio requirementin strength design. In order to develop alternativecriteria, it is necessary to understand thereasoning of using Y/T ratio as a designparameter.
STRENGTH DESIGN OF LINE PIPESWITH YIELD ANISOTROPY
Anisotropy has been taken into account for thefirst time in the recently established DNVoffshore standard F101 in that the minimumtensile strength required in the longitudinaldirection has been reduced by 5%, compared tothat in the transverse direction. It should beendeavoured to pursue other codes to adopt thisapproach and to apply this approach also to yieldstrength. Reduction of the strength levels in theorder of 10% for the longitudinal direction istechnically justified.
An analytical solution may be derived for thecalculation of the moment capacity of a pipe witha corrosion defect subjected to internal pressure,axial force and bending moment. The maximumcapacity is defined in the solution as the momentat which the entire cross section yields. Thecorrosion defect is conservatively assumed to besymmetrical to the bending plan.
Criteria for buckling/collapse calculations ofcorroded pipes with yield anisotropy were derivedby Bai et al (1999).
The moment criteria were re-visited and extendedfor design of high strength steel pipes with yieldanisotropy.
CONCLUSIONS
The paper provides technical information fromlinepipe manufacturing and design viewpoints topromote use of high strength linepipes. Thefollowing is conclusive remarks:
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1. Material properties are given for high strengthlinepipe.
2. Practical considerations on use of highstrength steel have been given, focusing oncost impact, welding, material and corrosionaspects.
3. Pipeline design loads have been summarizedfor S-laid large diameter export lines andsmall diameter infield flowlines.
4. The requirements of material properties havebeen discussed to justify use of yieldanisotropy line pipe.
5. Strength design equations have beendeveloped for high strength linepipes thathave yield anisotropy.
6. Regulatory bodies, specifications and designcodes should pay more attention to thetechnical feasibility of pipe properties. Closeco-operation among designers, pipelayingcontractors, pipeline operators and pipemanufacturers should be intensified.
REFERENCES
1. API 5L (1995): “Specification for Line Pipe”, 41stEdition.
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9. Graf, M. K. and Hillenbrand, H. G. (1995): “Productionof Large Diameter Line Pipe - State of The Art andFuture Development Trends” Europipe GmbH.
10. Gräf, M.K. and Hillenbrand, H.G. (2000):“Development of larger-diameter linepipe for offshoreapplications”, 3rd International Pipeline TechnologyConference, 22-24 May 2000, Brugge, Belgium.
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13. ISO 3138-2 (1996): “Petroleum and natural gasindustries - Steel pipe for pipelines - Technicaldelivery conditions - Part 2: Pipes of requirement classB”.
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