Copyright 2006, Offshore Technology Conference

This paper was prepared for presentation at the 2006 Offshore Technology Conference held inHouston, Texas, U.S.A., 14 May 2006.

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AbstractDeep water field developments have introduced a new set ofgeneral conditions which can significantly affect the design ofsubsea pipelines compared to more conventional water depthsexperienced in the past. Deep water pipelines do not require aconcrete weight coating due to the low wave and currentloads, as a result these pipelines have a lower submergedweight. This combined with the lower resistance provided bythe soft soil found on the sea bed in deep water have resultedin pipelines being more susceptable to movements both in thelateral and axial directions as a result of pressure andtemperature driven loads. Consequenthly the instability ofpipelines on the sea bed induced by pressure and temperatureeffects have taken a much larger importance. This paper willcompare a range of solutions which can be used to mitigatethese issues of lateral buckling and pipe walking, which cancumulated their effect and induce fatigue failures of thepipeline. The paper will be based upon the practicalexperienced gain in implementing these mitigation measuresin deep water projedcts in West Africa. Based upon anunderstanding of the instability mechanisms and the practicalaspects of installation, each mitigation method will bediscussed and the impact on pipeline design, installation, andproject cost and planning will be highlighted.

IntroductionThe expansion of submarine pipelines under the influence ofpressure and temperature is not a new subject area for thedesign of pipelines, risers and tie-in spools. However,previously, the issues were mainly concerned with thedisplacements of the pipe ends and the resulting stresses andstrains in the pipe wall. In addition for trenched and buriedpipelines the issues of up-heaval buckling have requiredspecific attention in recent years. It is important to note that

the pipe end displacements under these conditions can beconsidered to be more or less reversible. That is the expansionof the pipeline experienced during production start up is moreor less equal to the retraction of the pipeline during a shutdown situation. This paper deals with a relatively newphenomena influencing deep water flowlines layed directly onthe seabed, where the displacements induced by pressure andtemperature variations can result in progressive andcumulative movements of the pipeline. The factors influencingthe progressive movements include :-

Constant and asymmetrical tension applied to thepipeline from a Steel Catenary Riser (SCR)

Relatively lightweight pipelines due to the absence ofa concrete weight coating and use of insulationcoatings which can have specific gravity less than 1.0.

Short in field flowlines, consequently the whole line isaffected by the pressure and temperaturedisplacements

In general deep water soils are very soft with a highwater content and consequently provide relativelylittle resistance to pipeline movements

The conditions described above for deep water flowlines canresult in two different types of displacements. These are :-Axial Displacement commonly known as Pipe Walkingwhere the progressive displacement of the pipeline over manyproduction start up cycles can cumulate resulting in globalaxial movement of the pipeline.Lateral Displacement where the pressure and temperatureinduced displacements can cause the pipeline to move laterallyand can result in lateral buckling of the pipeline.

Impact of Pipeline MovementsThe pipeline displacements induced by pressure andtemperature will result in stresses in the pipeline itself and theconnection spools and subsea structures. The continuousproduction cycles of start up and shut down result in cyclicloading and progressive movements of the flowlines. This willresult in cumulative fatigue damage to the flowlines, which ifnot considered in the overall design of the flowline system canresult in failure and rupture of the flowline. Under theseconditions excessive bending and subsequent failure of thepipeline are extremely unlikely, the fatigue of the line beingthe most likely failure mechanism. It is important that adistinction is made between the design conditions for apipeline and the operating conditions. The design pressure and

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Mitigation Methods for Deepwater Pipeline Instability Induced by Pressure andTemperature VariationsD. Perinet and I. Frazer, Acergy

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temperature by their nature should be extreme not to beexceeded values, which are used for the ultimate design of thepipeline. However the design temperature and pressure shouldnot be used when assessing the pipe walking and lateralbuckling of the pipeline, as this may result in overlyconservative results. The operational temperature and pressuremust be used when assessing the pipe walking and lateralbuckling.

It is not just the cyclic pressure and temperature loadingcaused by production shut downs, which need to beconsidered, but also the pressure fluctuations resulting fromthe transient nature of the fluid flow inside the pipeline.Immediately after a production shut down the flow is notstable and pressure fluctuations can add to the cumulativefatigue damage in the pipeline.

If a pipeline is considered to be perfectly straight then theprimary movements will be in the axial direction. Howeverthis is very rarely the case and curvature of the pipeline canresult in lateral movements, which increase with increasingcurvature of the line. High levels of curvature can amplify theeffects of pressure variations resulting in relatively largelateral movements due to pressure variations.

Consequently it is clear that the production shut downfrequency of a flowline system can directly influence thefatigue damage incurred in the flowline. As an order ofmagnitude one shut down per week during the lifetime of aflowline can significantly influence the fatigue behavior of theline. For a 20 year field life this results in approximately 1000production shut down cycles. This will induce 1000 thermalcycles in the line which on there own are unlikely to result infatigue failure of the pipeline. However these must becombined with the pressure variations which can be orders ofmagnitude higher resulting in a potential fatigue problemunless mitigation measures are provided for in the pipelinesystem design.

A detailed description of the factors influencing both theaxial and lateral movements of pipelines subjected to pressureand temperature loads can be found in Reference /1/. Inaddition these issues are the subject of a Joint Industry ProjectSAFEBUCK /2/ where the uncertainties in both the soil andfatigue models are being studied together with theestablishment of design guidelines for dealing with theseconditions. This paper will subsequently concentrate on themethods, which can be employed to mitigate against both pipewalking and lateral buckling. Before discussing the mitigationmethods it is first necessary to describe the boundaryconditions existing on the pipelines, as these will influence theselection of the appropriate mitigation methods.

Boundary ConditionsThe boundary conditions have a major impact on the responseof a pipeline when subjected to pressure and temperatureloads. The boundary conditions to be considered not onlyinclude the conditions at the extremities of the pipeline such asrisers and subsea structures, but also the conditions along thelength of the line. This is particularly important for waterinjection lines, particularly in West Africa where several teestructures can be located along the length of the pipeline.These structures can act as anchors along the line and canprevent axial movements of the pipeline or be fitted with

sliding arrangements to provide some compliance with theaxial loads.

The risers have a significant influence on the stability ofthe pipelines. In the case of a Hybrid Riser Tower, Figure 1,the risers are rigidly linked to the flowlines through tie-inspools. Consequently these risers can be considered as fixedstructures at the end of the pipeline.

Figure 1 : Hybrid Riser Towers

However in the case of a Steel Catenary Riser, Figure 2,the riser connects directly to the pipeline at the touch downpoint of the catenary which will result in asymmetrical loadingof the pipeline.

Figure 2 : Field Development with Steel Catenary Risers

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At the touch down point region of the SCR, the suspendedpart of the riser will transfer a tensile force into the horizontalpipeline, which is in contact with the seabed. In the case oftemperature loading, if the pipeline is not anchored, thepipeline will experience an expansion at both ends of thepipeline, that is towards the SCR and towards the oppositeend. As a result the pipeline will expand and move on eitherside of a fixed point on the pipeline. When the pipelineexperiences a production shut down the pipeline will cooldown and on both sides of the fixed point. However thepipeline is still under the influence of the tensile load from theriser and consequently these fixed points are at differentlocations depending on whether the pipe is expanding orcontracting. In the shut down situation when the pipeline iscooling down and retracting the fixed point will be closer tothe SCR than during the start up situation. As a result thepipeline will experience a net displacement towards the SCRat every production Shut Down Cycle, this is further illustratedin Figure 3 below. A similar asymmetrical loading andcumulative displacement may result when pipelines are layeddown a relatively steep sloping seabed. In this case theasymmetrical loads are resulting from the gravity componentof the pipeline weight acting down the direction of the slopingseabed.

Figure 3 : Illustration of Pipe Walking

Mitigation MeasuresWhen dealing with lateral buckling and pipe walking it isconsidered safer to provide mitigation measures to preventthese conditions than attempt to design the pipeline system toaccommodate the loads and displacements.

From a general point of view there are two alternativeapproaches which can be taken with respect to mitigationmeasures. These are restraining the pipeline to prevent or limitthe resulting displacements and a compliant approach wherethe location for the displacements are selected to preventexcessive stress variations in the pipe. The anchoring approachis best suited to mitigating pipe walking, whereas thecompliant approach is best suited to the control of lateralbuckling.

Pipe Walking PreventionIn order to eliminate the effects of pipe walking it is

essential that the pipeline be anchored in some way. Achievingthis at the first end of the pipeline is relatively straightforward,as the anchoring system can be included within the initiationsystem for the pipelay operations. However anchoring thepipeline at the second end, where the pipelay is terminatedrequires a more complex solution. At the first end of thepipeline it is possible to use a suction anchor which can beconnected to the pipeline to anchor the pipeline end andprevent any axial displacement of the line. The suction anchorcan be positioned accurately on the seabed prior to the start ofpipelay and either used to initiate the pipelay or be connectedto the pipeline after the laying operation. The connection tothe pipeline can be made with a chain linking the suctionanchor to the flowline as shown in Figure 4.

Figure 4 : Axial Movement Mitigation with Anchor and Chain

It is necessary with this system to pretension the chainprior to lay down of the pipeline head on the seabed, which isan added operation and consequently adding to the cost of theoverall pipeline system.

An alternative to the chain connection system is a systemof direct mechanical connection of the pipeline end to thesuction anchor. This is shown in Figure 5 and Figure 6 wherethe Tee Bar arrangement on the end of the pipeline mates withthe receptacle on top of the suction anchor, providing a staband hinge over mechanism for both initiating the pipelay andanchoring the flowline end preventing pipe walking.

Figure 5 : Pipeline Anchor with Stab & Hinge Over System

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Figure 6 : Offshore Installation of Stab & Hinge Over System

The stab and hinge over system shown Figure 5 and 6above provide a rigid connection to the top of the suctionanchor. This has the effect that the pipeline end is axiallyrestrained in both directions which must be taken into accountin the overall pipeline system design. In addition with thissystem the pipeline loads are transferred to the top of thesuction anchor. This is a less efficient load transfer thanapplying the load part way down the anchor, as shown inFigure 4. As a result the suction anchors may need to be largerthan if the loads were applied at a more optimum location.

For anchoring the second end of a pipeline a preinstalledset of anchors can be used connected to the pipeline via a setof chains and a clamp on the pipeline, as shown in Figure 7.

Figure 7 : Anchor and Chain Restraint for Pipeline

If it is necessary to provide an axial restraint system for themid-section of a pipeline, a preinstalled suction pile can beused fitted with a set of guides on the top. The pipeline islayed through the guides and a set of chains used to providethe connection to the suction anchor to provide the axialrestraint to the pipeline, as shown in Figure 8.

Figure 8 : Mid Line Axial Restraint System

Lateral Buckling PreventionAs discussed above a compliant approach is the most

effective mitigation technique for lateral displacements.Basically this involves allowing the pipeline to move laterallyin a controlled manner to prevent the build up of highcurvature areas which can result in high fatigue loads. SnakeLay is one such method, which has been proposed as amitigation method for lateral buckling. This is based uponlaying the pipeline with some predetermined curves in the lineas shown in Figure 9.

Figure 9 : Snake Lay

The aim of snake lay is to provide an over length of thepipeline within the deliberate curves layed in the line, whichwill absorb the expansion of the pipeline. However it in theopinion of the authors the snake lay method of mitigation isnot suitable as a mitigation technique for deep water pipelines.The main reason for this is, in deep water the seabed soil tendsto be soft clay as a result it is very difficult to predict the levelof lateral resistance which can be achieved form the soil due tothe uncertainties related to pipeline emmbedment. As a resultthe expansion behavior of the pipeline in the overlengthregions can be very uncertain, consequently it is not a reliablemitigation technique for such deep water conditions. Inaddition it can be difficult to guarantee the accurate formationof the desired curved lay route in deep water conditions due tothe level of control available from the laybarge and the levelof resistance from the soft seabed.

One simple potential mitigation method for lateraldisplacements is to increase the lateral resistance of thepipeline on the seabed by progressive free flooding of thepipeline, thereby increasing the embeddment of the pipe intothe seabed. This requires the injection of chemicals into thewater such as corrosion inhibitor and biocide, which is anadded complication. The efficiency of this method is based onthe fact that the pipe-soil resistance at the Touch Down Pointis larger than in the case of post laying flooding. However ithas to be pointed out that the soil behavior may be differentfrom the consolidation and draining point of view in the caseof the short duration loading during laying and the final resultmay be less than anticipated.

Another relatively simple mitigation method is to bury thepipeline beneath the seabed. This can be achieved with asubsea trenching and burial system such as the one shown inFigure 10. This will obviously depend on the characteristics ofthe seabed soils, however in some circumstances can provide arelatively cost effect and reliable solution to the mitigation oflateral buckling.

Walking

ClampChain

Pile

Walking

ClampChain

Pile

Guidelateral

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Figure 10 : Pipeline Burial Machine

Lateral buckling can be mitigated by allowing the pipelineto expand by providing an axial sliding system on the subseastructures. With this system the thermal expansion of the pipeend does not generate buckling compression at the extremity.Such systems can be used in conjunction with a chain-anchoring device attached to the sliding part of the subseastructure. This is a method, which has been employed on anumber of projects in West Africa. In this situation a slidingdevice is built into the subsea structure, as shown in Figure 11.The sliding device is locked during the pipelay operations andthen released by an ROV once the pipeline is installed on theseabed.

Figure 11 : Mid Line Tee with Sliding System

Mitigation against lateral buckling can be achieved byproviding controlled lateral movements of the pipeline toavoid the situation of the expansion from various parts of thepipeline feeding into one particular area producing a verylarge lateral displacement with high curvature. This mitigationmethod involves providing areas along the length of thepipeline which have a significantly lower lateral resistancethan the majority of the line. Thereby forcing the pipeline tomove laterally in these areas to relieve the axial load in thepipe which can cause the high buckling loads. At each of these

locations a sleeper is placed on the seabed and the pipelinelayed over it. This has the effect of both providing a surfacewhich has less lateral resistance than the seabed, and alsoraising a length of pipeline above the seabed also reducing thelateral resistance of the pipe. A typical layout for this type ofmitigation system is shown in Figure 12.

Figure 12 : Sleeper System

The selection of the particular mitigation system which ismost suited to a particular situation will be very muchdependant on the conditions and constraints affecting aspecific project. Each one of the mitigation methods presentedin this paper are suitable for a number of physical situationsthe selection will be very much dependant on project specificconstraints. The philosophy of this mitigation method is thespreading of the extra length induced by thermal expansionover a long section of the pipeline in order to avoidconcentration of curvature.

The design of the pipeline has to be undertaken byeliminating any potential source of curvature concentration.Short sections with a lower stiffness in particularly when thepipeline includes a number of different components needparticular attention. Special care should also be given to theoverall layout of the flowline system on the seabed toeliminate areas where the pipe has a short radius of curvature.This can result in the situation of the lateral loading due topressure and temperature exceeding the lateral resistanceprovided by the soil.

At locations where the pipe is planned to move laterally,the pipe design is no longer governed by static loads.Consequently the fatigue behavior of the pipeline whensubjected to cyclic loads requires special attention in theseareas. This results in the need for extra attention to the weldingof the pipeline and in particularly the improvement of the weldprofile for the adjacent pipes in these areas.

The mitigation methods presented above can require asignificant amount of subsea intervention and consequentlycan have a relatively large cost associated with the mitigationsystems. It is not possible within the confines of this paper togo into the details costs of each mitigation system as this willbe very much dependant on the specifics of the situation underconsideration. However as a general guide for a large subsea

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field development the cost of the mitigation measures can beas much as 10% of the overall cost of the flowline system.However the cost of not including any mitigation measurescan be orders of magnitude higher than this.

ConclusionsThis paper has presented an overview of the drivers behindpipe walking and lateral buckling, highlighting the need formitigation systems. The mitigation systems, which are suitablefor control of both pipe walking and lateral buckling havebeen presented and discussed.

AcknowledgementsThe authors wish to acknowledge Acergy for supporting thispaper and the contribution from the various Project Teamswho have implemented many of the mitigation solutionspresented in this paper.

References/1/ Perinet, D., and Frazer, I. : Movements of Deep Water

Flowlines as a Result of Pressure and Temperature Loads, andthe Influence of Steel Catenary Risers, Deep OffshoreTechnology Conference, Vitoria, Brazil, 2005.

/2/ Bruton, D., Carr, M., Crawford, M., and Poiate, E. : The SafeDesign of Hot On-Bottom Pipelines with Lateral BucklingUsing the Guideline Developed by the SAFEBUCK JointIndustry Project, Deep Offshore Technology Conference,Vitoria, Brazil, 2005.

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