Upheaval Buckling Failures of Insulated Buried Pipelines

OTC 6488

Upheaval Buckling Failures of Insulated Buried Pipelines:A Case StoryN-J.R. Nielsen and B. Lyngberg, Maersk Olie & Gas AS, and P.T. Pedersen,Technical U. of Denmark

Copyright 1990. Offshore Technology Conference

This paper was presented at the 22nd Annual OTC in Houston. Texas, May 7-10,1990.

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(3). Contents of the paper,as ç nted, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, doesnot necessarily retiediany ;ion of the Offshore Technology Conference or its officers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. Theabstract should contain conspicuous acknowledgment of where and by whom the paper is presented.

TECHNISCHE INIVERUTEETLaboratorium voor

ScheepshydromecbanicaArchief

Makelweg 2,2628 CD DelftTel.: 015-7868'73 -Fax: 015.781836

terfield pipelines in the Danish Sector ofthe North Sea.

It is the objective of this paper to pre-sent the case story of this pipeline up-heaval giving a chronological descriptionof all actions taken, in order to re-estab-lish the pipeline integrity.

Based on the upheaval buckling experiencewithin Mmrsk Olie og Gas AS, a recommenda-tion for the design and installation ofburied, hot pipelines is presented.

INTRODUCTION

L_zing the annual pipeline inspectionsurvey in July 1986 along the buried RolfA/Gorm E two phase pipeline in the DanishSector of the North Sea, a pipeline sectionwas discovered to have protruded the sea-bottom, and was standing in an arch. The17 km long pipeline is an O.D. 8.625" x WT14.3 mm carbon steel line (API 5L grade X52). The pipe is insulated with a 2" thickpolyurethane foam (PUF) of min. density 96kg/m3 encased in a high density polyethylene(PE) jacket. The exposed PUF at the endsof the PE jacket is sealed with water tightend cap sleeves. A 2" thick concrete weightcoating is applied on top of the PE jacketresulting in an overall pipe diameter of0.45 m. A 3" gas lift line is "piggy bac-ked" to the 8" pipeline. The pipeline

References and illustrations at end ofpaoer.

The pipeline was laid in the summer of 1985in 40 m water depth utilizing conventionallay barge techniques. Trenching of the linewas carried out using water jetting equip-ment.

The pipeline was brought into serviceJanuary 1986 for transport of unstabilizedhydrocarbons with a temperature of up to180°F (82°C) from the Rolf satellite fieldto the central process facility at Gorm.

Upheaval buckling analysis of the 8" linewas carried out as part of the designdocumentation resulting in a loweringrequirement of 1.15 m to top of the con-crete coated pipe. The certified upheavalbuckling calculations were performed accor-ding to the state-of-art at that time, i.e.using the "classical" upheaval bucklinganalysis, where the design criteria againstupheaval snap buckling is based upon postbuckling equilibrium curves assuming a pipeof uniform weight on a rigid foundation.

Following the detection of the exposure anupheaval buckling research programme wasimmediately initiated at the TechnicalUniversity of Denmark. The results from thestudy showed that the "classical" designapproach did not always yield conservativeresults. Thus, a new and improved theoreti-cal model was developed and made availablelate 1986, and formed the basis for therepair work. The new design approach hasbeen published in ref. [2], [3] and [4].

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ABSTRACT

The first recognized upheaval buckling of build-up is shown in Figure 1, and a detai-a subsea, buried pipeline took place in led description of the pipeline is given1986 in one of Mmrsk Olie og Gas AS' in- by Pallesen et.al. (1985) in ref. [1].

Sr--

DETECTION OF EXPOSED PIPE AND ACTIONS TAKEN

AA-found Survey (let Upheaval). f0F.n

During the 1986 annual inspection surveyalong the buried 8" Rolf A /Gorm E pipe-line, an exposed pipe section was detectedfrom side scan sonar records at a distanceof 0.3 km (KP 0.3) from the Rolf platform.A subsequent diver inspection of the ex-posure revealed a very localized pipelineupheaval,where the apex of the buckle mea-sured to the bottom of the pipe protruded1.1 m above seabed level leaving the pipe-line free spanning over a 10 in section,see Fig. 2. The apex of the buckle coin-cided with a field joint, and it becameevident that the steel pipe at this loca-tion was subjected to a relatively severebending curvature. An estimation of theinduced strain showed that a strain in theorder of 3.5-8.0% was likely to have oc-cured, which meant that the pipe crosssection had undergone plastic deformation.

To determine the overall buckling confi-guration, diver probing of the burial depthon both sides of the exposed pipe wascarried out. The results of the probingshowed that the buckle wave length wasconfined to 24 m corresponding to two pipejoints.

As the burial depth to bottom of the "un-disturbed" pipeline sections to either sideof the upheaval was approx. 1.5 m, theoverall buckling amplitude became of theorder of 2.6 m. Fig. 3 shows the buckledpipe section after having been retrievedfrom the seabottom and transported toshore. It is noticed that the resulting up-heaval configuration has similarities witha plastic "hinge" failure.

Immediate Safety Measures

At the time, in July 1986, when the pipeli-ne upheaval was discovered, the line wasoperating at a pressure of 1,000 psig tran-sporting approx. 10,000 barrels daily ofunstabilized oil with a flowing tubinghead temperature (FTHT) of 176°F (80°C).

During the service period (7 months) priorto detection of the upheaval buckle the pi-peline had been subjected to 15 major shut-downs varying between 1 and 17 hours (aver-age duration was 5 hours). Assuming thatthe upheaval had occured at an early stagein the service period, the susceptibilityto "low cycle high strain" fatigue was eva-luated to be critical, in particular in thewelding area, where relatively large stressconcentrations exist.

Consequently, in order to minimize the riskof a rupture in the plastically deformedpipe, the following safety precautionswere taken:

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Maintain steady production (constanttemperature and pressure) .

Monitor the upheaved pipeline configura-tion.

Dedicate a guard vessel to patrol at theupheaval location for protection againstthird part damage (e.g. fishing activi-ty).

Establish procedures defining remedialactions in case of observed abnorma-lities.

The 3" piggy back line had not yet beentaken into service as a gas lift line andtherefore did not present a potentialsafety hazard.

The relevance of precaution No. 1 (elimina-tion of low cycle fatigue) was verifiedupon shut-in of the line prior to therepair work as contraction of the line uponcooling down lowered the apex of the uphea-val section from 1.1 m to 0.55 in aboveseabed level, see Fig. 2,

ASSESSMENT OF DAMAGE

In order to find the technically and econo-mically most feasible repair strategy, adetailed assessment of the damage causedby the upheaval was undertaken. In particu-lar the investigations were concentratedon

Strain/ovalizationLow cycle fatigueAmount of foam damageDetermination of time ot upheaval

The ratio between the outer diameter andthe wall thickness of the 8" steel pipe is15.3. Such "compact" pipes normally havesufficient capacity to form a plasticbending mechanism without significantflattening. Thus, it was concluded that thecross section was not subjected to severeova lization.

The "low cycle high strain" fatigue lifeof the pipe at the apex of the buckle wasestimated to 20-25 cycles of 3-3.5% strain.Based on the production history (number ofshutdowns), it was calculated that: theremaining fatigue life at the time ofdetection was 10-15 strain cycles of 3-3.5%. The strain of 3-3.5% was estimatedto correspond to one complete shut-downassuming an initial induced strain of 8%.However, as no "low cycle high strain"fatigue data was available for the materi-al in question, these calculations weresubjected to some uncertainty.

2 UPHEAVAL BUCKLING FAILURES OF INSULATED BURIED PIPELINES - A CASE STORY OTC 6488

3.

ue to the longitudinal expansion of the8" steel pipe at the upheaval location,relative movements occurred between thePUF/PE-sleeve interface causing the foamto slide approx. 50 mm out of the sleeveand damaging the end cap sealings. Thisbond breakage could have taken place fora considerably distance on both sides ofthe upheaval. Also, there were indicationsthat a shrinkage of the foam had takenplace. After having retrieved the damagedsection, the severity of the foam shrinkagebecame evident. The foam had collapsed tohalf the original thickness, see Figure 4.

By evaluating the biological parametersof the marine growth on the exposed pipesection, it was concluded that the upheavalwas most likely to have taken place inJanuary 1986, i.e. within the same monththe pipeline had been brought into service.

DETAILED PIPELINE SURVEY

ollowing the detection of the pipelineupheaval at KP 0.3, it was decided to carryout a comprehensive sub-bottom profile(SBP) survey along the entire length of thepipeline in order to determine the in-situcondition of the pipeline burial and pipe-line profile.

The SBP survey started in September 1986with pipeline crossings at approx. 10 in

intervals utilizing a remotely operatedvehicle (ROV). The 10 in crossing intervalwas selected as a compromise between surveycosts and required number of SBP measure-ments in order to resolve pipeline undula-tions with 20-30 m wavelength.

During the execution of the SBP survey asecond pipeline exposure was found at adistance of 2.2 km (KP 2.2) from the Rolfplatform.

This exposure did not exist at the time of-he annual pipeline survey in July 1986,_.e. the pipe had upheaved between lateJuly and early September 1986. The sub-sequent diver inspection revealed a 5 in

pipeline exposure with a maximum height of0.2 m above seabed level measured to thebottom of the pipe. As for the first up-heaval at KP 0.3 the imperfection wavelength was confined to 24 m. The overallheight of the buckle was of the order of2 m.

No further exposures were found along thepipeline. However, at 26 locations thepipeline was subjected to severe verticalundulations, i.e. imperfection amplitudesof 0.5-1.0 in with a corresponding wavelength of 50-70 m. The minimum depth ofcover at these locations varied between 0.4m and 1.4 in as compared to the generallyobtained lowering depth of 1.6-1.9 in totop of pipe.

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Thus, upon completion of the detailed SBPsurvey the state of the pipeline could besummarized as follows:

Upheaval buckle at KP 0.3

Upheaval buckle at KP 2.2

Excessive vertical undulations of unex-posed pipeline sections at 26 locations.

CAUSES OF OBSERVED UNSTABLE PIPELINE BE-HAVIOUR

Installation Induced Imperfections

In order to explain the causes of theobserved unstable pipeline behaviour thehistory of the pipeline was closely exami-ned.

It was known and documented that the pipe-line had been subjected to trawl gear im-pact at about 10 locations along the pipe-line route prior to trenching of the line.The upheaved section at KP 0.3 had experi-enced such trawl gear impact causing da-mage to the 8" line and laterally displa-cing 6 joints (72 m) of the 3" piggy backline up to 2 in from the 8" pipe, leavinga permanent bend in the 3" line. Trawl gear"pull-over" calculations indicated thatalso the 8" steel pipe had been stressedbeyond the elastic limit. At a further 5locations the 3" line had been damagedwith displacements up to a maximum of 10m from the main line.

During lowering of the pipeline all sec-tions, which had been hit by trawl boardswere temporarily left untrenched to provideaccess for detailed inspection and sub-sequent repair work, if required. Upontrenching of the repaired sections, theresulting pipeline configuration in thetransition zones becomes as shown on Fig.5.

An analysis of the bending stresses inducedin this condition is shown in Fig. 6, asa function of the untrenched length fordifferent depths of burial. In the presentcase, it was found that the linear cal-culation model predicted stresses whichexceeded the yield stress by typically 23%.Therefore, some plastic deformation of thepipeline was expected to have been introdu-ced during the subsequent trenching opera-tion. Furthermore, the starting and stop-ping of the trenching at these locationswould almost certainly have introduced somelevel of foundation imperfection over andabove that to be found in sections wheretrenching had been performed in one conti-nuous pass.

A change in the operating parameters of thetrenching equipment and variable soilconditions along the pipeline route couldfurther result in an irregular pipelineprofile.

OTC 6488 N.J.RISHOJ NIELSEN, B.LYNGBERG, P.TERNDRUP PEDERSEN 3

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for this idealized model, where the foun-dation is assumed to be rigid. The bifur-cation load is infinitely high. As thelimiting permissible temperature rise istaken the minimum temperature on the U-shaped postbuckling equilibrium curves inthe temperature-buckling wave length plane,such as Fig. 8, or temperature-bucklingamplitude plane. For perfect pipelineswithout initial imperfections such a pro-cedure will be conservative.

During the design phase also the effect ofimperfections was discussed. This discus-sion was related to Fig. 8, which schema-tically shows how the response will differfrom the one predicted for the perfectgeometry case. It was concluded that sincethe adopted "trough" criterion is not verysensitive to imperfections the proposeddesign should be conservative also forpipelines, which invariably posses initialimperfections.

The result of these numerical calculationswas that for a design temperature of 200°F(93°C) and an internal pressure of 3,000psig (20.7 MN/m2) the required minimum soilcover was 0.90 in to top of the 8" pipe.The design trench depth was then taken as1.60 in giving approx. 1.15 in soil cover totop of the 8" line.

The trenching analysis performed for thepipeline was based on a method suggestedby Mousselli [5]. The theoretical back-ground for this analysis is a beam model,where the pipe is assumed elastically sup-ported by a concentrated soil spring at theedge of the trench, see Fig. 9. Assuminga relatively rigid foundation and an allo-wable maximum bending stress equal to 305MN/m2 (0.85% SMYS), it was found that themaximum trenching depth was 1.6m, and thatan increase in trench depth of 20 cm wouldincrease the stress level by 4%.

A New Design Procedure

In order to get a consistent theoreticalbasis for the repair of the pipeline, itwas obvious that a new mathematical modelneeded to be developed.

During this analysis work it was shownthat a buried and heated pipeline sectionwith an initial imperfection can liftitself upwards upon experiencing the opera-ting temperature, by slightly lifting theoverburden without necessarily being ableto break out of the soil, i.e. withoutupheaval snap buckling. During a Latershut-down the line will cool down and tryto return to its original position. Now,during the period with uplift, sand par-ticles will have tended to fill the "cavi-ty" below the pipeline created by theuplift. Thereby, a complete recovery to theoriginal position is prevented. Hence, atsections of the pipeline with initialimperfections above a certain limit the

Time Dependant Upheaval behaviour

Whereas the first upheaval at KP 0.3 wasconsidered a classic upheaval buckling case(upheaval snap buckling) the second uphea-val at KP 2.2 could not be so easily ex-plained owing to its apparent time depen-dency, i.e. the upheaval took place betweenthe annual inspection survey in July 1986and the SBP survey in September 1986. Also,from the time of detection of the firstupheaval until the decision of maintainingsteady production was taken the line wassubjected to 4 additional shut-down situa-tions of which two were major.

By comparing the detailed SBP (1986) surveywith the as-built (1985) survey documenta-tion, it became evident that certain pipe-line sections with initial vertical "imper-fections" had moved their way upwardsthrough the soil. An example of such obser-ved imperfection growth is presented inFig. 7, showing that an initial "imper-fection" height, which in 1985 was 0.5 m,had grown to 1.0 in in 1986, reducing thelocal soil cover from 1.2 m to 0.7 m. Rel-ating the 0.5 in increase in "imperfection"height to the totally 17 major shut-downs,the average growth rate becomes 30 mm pertemperature cycle.

Following the above observations, it becameclear that the classical upheaval bucklinganalysis applied during the design phasewas insufficient. It did not model thedetrimental effects of plastic deformationof the pipeline in combination with lackof straightness and it could not explainthat the geometric imperfection amplitudeswere growing with time.

DEVELOPMENT OF A THEORETICAL MODEL

Analyses performed prior to installation

Prior to installation, upheaval bucklinganalyses had been carried out in order todetermine the necessary depth of burial andan independent analysis appraisal had beenperformed by a classification society. Inboth cases the analysis procedure was iden-tical to the "classical" calculation per-formed for analysis of vertical stabilityof railroad tracks.

Following this approach the problem wasformulated as a heavy, elastic beam on arigid flat foundation where the weight ofthe beam includes the weight of the 8" and3" pipes and the weight of the cover soil.The solution procedure places the pipe intoa buckled configuration and seeks to findthe temperature and internal pressure whichcan maintain equilibrium with a prescritedlength L of the pipe in the buckled posi-tion. This analysis produces postbucklingequilibrium curves which can be reached,if the pipe is sufficiently disturbed. Anormal bifurcation behaviour does not exist

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I I


'-perfection amplitudes will grow with the.mberof temperature changes such that the

local soil cover will decrease and at acertain stage upheaval snap buckling cantake place.

This upheaval creep mechanism resembles thewell known phenomenon seen in farm fieldsin the spring time where the frost drivesburied stones up to the surface.

The theory behind this new design procedureagainst upheaval creep is semi-analyticaland easy to apply. It was implemented ona PC, and the theoretical background hassince been published in [3). It was decidedthat in order to avoid that heating andcooling of the pipeline shall cause agrowing foundation imperfection, a designcriterion should be that, at the maximumpipeline pressure and temperature, themaximum uplift displacement should berestricted to typically 20 mm. At KP 0.3the foundation imperfection amplitude wastimated to be 0.2-0.4 in and the wave

-_,ngth to be 24.5 m (two pipe joints). Dueto the large bending moments induced in thepipeline during trenching, a plastic defor-mation of the same form was assumed. Se-veral shapes of the imperfection wereanalyzed. As an example Fig. 10 showsrelations between the minimum trench depthH and the allowable temperature differenti-al AT for three different imperfectionamplitudes, all prop-shaped. From Fig. 10,it is clear that for a maximum temperaturerise equal to 80°C and the estimated imper-fection amplitudes upheaval creep had tobe expected to take place for a trenchdepth of 1.5 m.

This semi-analytical linearized analysisprocedure [3] is rapid and easy to apply.This is also illustrated in ref. [2], wherethe procedure was subsequently used tostudy the design and installation of va-rious pipelines and where requirements were9-itab1ished for acceptable out-of-straight-

3s of these pipelines. Of course, thelinearized analysis procedure can only beexpected to be sufficiently accurate foranalyses connected with design againstgradual upheaval caused by temperaturefluctuations, where the upheaval displace-ments are small.

To overcome this shortcoming, a non-lineartheoretical model was developed, whichcould be used to study the final upheavalevent where the pipeline comes out of thesea bottom, see Fig. 11. This non-linearanalysis procedure was also implementedon a PC. A detailed description of themathematical background for the method hasbeen presented in [4].

The iterative procedure uses, as startconfiguration, the linearized solutionproduced by the upheaval creep analysis andthe mathematical model includes non-line-arities caused by:

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Geometric non-linearities due to largedeflections of the pipeline.

Non-linear stress-strain behaviour ofthe pipeline material.

A soil uplift resistance, which varieswith the uplift displacement.

Variable or deformation-dependent axialfriction forces.

Numerical results are presented in Fig. 11showing temperature difference AT versusupheaval clearance A between the founda-tion and the pipeline at the apex, for thecase of a trench depth equal to 1.5 in in"cemented" sand. The foundation imperfec-tion amplitude is 0.20 in and, as before,the pipeline is assumed to be stress freein this shape. Included in this figure isthe result of the classical heavy elasticbeam calculation. It is noted that theequilibrium curve produced by the classicalprocedure differs considerably from theresult predicted by the more correct cal-culation procedure.

Besides the above-mentioned limitation onthe allowable pre-buckling displacements,in order to avoid upheaval creep, it isalso necessary to impose restrictions onhow close the post-buckling equilibriumcurve should be to the points of operationon the pre-buckling path. The latter cri-terion is necessary in order to avoid snap-through buckling due to small disturbances.

Verification of Mathematical Model

To illustrate the correlation betweenpredictions based upon the theoreticalmodel and the observed upheaval creepbehaviour, the "imperfect' pipe sectionshown in Fig. 7 will be considered. Thebasis is the assumption that the observedas-built out-of-straightness correspondsto the shape of a "propped" pipe section.The free span equilibrium wave length asso-ciated with a prop height of 0.5 in can becalculated to 46 in (airfilled pipeline).Such an imperfection configuration has beendrawn in Fig. 7. It is seen that the shapeof a "prop imperfection" shows good agree-ment with the as-built survey measurements.

Two non-linear upheaval buckling analysesof this "prop imperfection" are presentedin Fig. 12 showing pre- and post-bucklingequilibrium curves in a temperature versusuplift displacement coordinate system. Thedifference in the response of the twocurves is caused by the effect of plasticdeformation of the pipe, i.e. the uppercurve corresponds to a pipe without aninitial geometric pipe imperfection whereasthe lower curve includes an initial geo-metric pipe imperfection equal to thefoundation imperfection. That is, in thelatter case the pipeline is assumed to bestress free in the propped configuration.

.


During the 7 months service period, thetemperature difference at the "imperfect"location did not exceed 55°C. Compared tothe calculated peak (instability) tempera-ture of 65-70°C no upheaval snap bucklingshould be expected, which is in agreementwith observations. However, it is noticedthat the crown uplift at a temperaturedifference of 55°C for the pipe in the pre-buckling stage varies between 10-40 mmdependent upon the degree of plasticityassumed for the pipe section. With a depthof burial at the apex of the imperfectionof 1.2 m, the uplift displacement corre-sponding to peak uplift resistance in thesoil would typically be in the order of 20mm. Consequently, for uplift movements inexcess of 20 mm local shear failure couldbe expected in the soil resulting in a non-reversible flow of sand around the pipe.Upon cooling of the line the redistributedsand particles and the non-linear pipe/soilinteraction prevents the line from retur-ning to the original position resulting inan upward ratcheting effect.

The conclusion is that even if it is notpossible, yet, to predict the speed of theupheaval creep, then correlation studiessuch as the one presented here show thatthe new mathematical model is a usefultool to predict when upheaval creep is apotential problem. It also gives a goodindication of the order of magnitude of thedisplacement growth for each load cycle.

REPAIR

On the basis of the damage assessment per-formed on the upheaved sections KP 0.3 andKP 2.2, the detailed out-of-straightnesssurvey and the developed theoretical modelthe required remedial repair work wasdefined. The repair work included thereplacement of line pipe at the two exposedsections and rock dumping of areas whichwere susceptible to upheaval bucklingfailure.

Repair at KP 0.3 and KP 2.2

It was decided to replace 6 joints of 8"and 3" line pipe at each of the two exposedsections by deburying the affected linepipes, cutting out the damaged pipe jointsand replacing them by a spool piece, 6

joints (i.e. 72 m) in length using hyper-baric welding. Following the hyperbaricwelding of the new spool pieces rock dum-ping was carried out to backfill the ex-cavated trenches. The rock dump was re-quired to ensure the necessary overburdenon the pipeline and thus prevent upheavalof the sections upon start-up of the pipe-line. The repair work was carried out inOctober/November 1986 and each hyperbaricrepair took about 3 weeks.

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The decision to replace line pipes at theexposed areas was taken because:

The upheaved sections had been subjectedto significant, but partly unknownaccumulated plastic strains.

The apparent breakage of the bond bet-ween the polyurethane foam and the poly-ethylene sleeve at KP 0.3 reduced thelongitudinal frictional resistance oneither side of the upheaval increasingthe pipeline's susceptibility to largeupheaval movements. The upheaval at KP2.2 would most likely have been subjec-ted to "similar" bond breakage.

An additional important aspect of therepair at KP 0.3 was the pipeline expansionat the Rolf riser. Because the observedbond breakage between the insulation foamand the sleeve could extend for a consi-derably distance around the region at KP0.3 the max. allowable expansion at theriser could be exceeded. As the riser wasdesigned to take up the pipeline expansionby bending of the riser pipe an excessivepipeline expansion could overstress theriser. Thus, in order to ensure a "high"longitudinal frictional resistance for therepair spool regardless of the conditionof the bond between the foam and sleeve theexpansion forces in the steel pipe weretransferred directly to the concrete coa-ting via "anchors" welded to the steel pipeat the spool ends. Further, all fieldjoints in the spool were filled with con-crete instead of the original foam, inorder to assure that the compressive loadswere transmitted through the length of thespool piece.

Remedial Rock Dumping

At a total of 31 locations (incl. the tworepairs at KP 0.3 and KP 2.2) along thepipeline route the safety margin againstupheaval buckling was found inadequate.Therefore to re-establish the integrity ofthe line in accordance with the originaldesign condition an increase of the over-burden at these locations was necessary.The most feasible solution was found to berock dumping of the affected areas. Therock dump requirements could be related tothe following criteria:

To prevent upheaval of the newly re-paired sections.

To prevent upheaval buckling of sectionswhere the foam/sleeve bond was broken.

To avoid excessive riser expansion atthe Rolf platform.

To prevent a growing "imperfection"amplitude of buried pipeline sectionswith significant vertical undulations.


pon the completion of the remedial rockdump a total of 4.5 km pipeline had beencovered with a rock berm of height varyingfrom 0.8-1.6 m. The berm plateau width wastaken as 4.0 m to account for lateralsurvey inaccuracy.

INSPECTION OF RETRIEVED PIPELINE SECTION

The section retrieved from KP 0.3 was takeninto the testing laboratory to test theremaining fatigue life of the most strainedsection, which had caused great concernjust after the detection of the firstupheaval. The pipe was first subjected to50 cycles of 1% strain after which thestrain was increased to 3.5% and the fai-lure occurred after 6 full cycles. Thesetests confirmed the estimated remainingfatigue life made at the time of the detec-tion of the first upheaval.

Following the observed shrinkage of thensulation foam at the first upheaval_ocation, KP 0.3, laboratory tests werecarried out using foam samples from theretrieved pipe sections as well as fromspare pipe joints taken from the "emergen-cy" storage. The results of the investiga-tions showed that the foam tended to shrinkto approximately half its original thick-ness when exposed to a combination ofwater, temperature and hydrostatic pres-sure. None of these effects in isolationwere found to cause the shrinkage.

Thus, water tight end cap sleeves are ofparamount importance in order to preventa detonation of the polyurethane foam.

RECOMMENDATION FOR DESIGN AND INSTALLATIONOF BURIED HOT PIPELINES

Following the Rolf upheaval buckling in-cident and the Mmrsk Olie og Gas AS in-house experience from installation of "hot"nipelines, a summary of essential aspects1.thin upheaval buckling design of buried

pipelines is presented:

It shall be documented that buriedpipelines will not be subjected to aprogressive upheaval creep failure(limitation of crown uplift movement).

It shall be documented that the pipelinehas sufficient safety against a snapthrough buckling failure (width oftemperature-upliftdisplacementcurve).

Upheaval buckling analyses shall bebased upon a consistent mathematicaltheory, which is capable of modellingthe uplift behaviour of "imperfect"pipelines taking into account the non-linear pipe/soil interaction and non-linearities of the pipe material.

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The imperfection configurations used atthe design stage should reflect typicalimperfection configurations, which couldbe generated during the installation(e.g. a prop type imperfection).

A scenario of critical pipe out-of-stra-ightness configurations for given burialdepth shall be established (requirementto trenching contractor).

Upon installation of the pipeline theachieved out-of-straightness and depth-of-burial shall be measured with suffi-cient accuracy to verify that the in-stalled pipeline fulfills the designrequirements. This will enable remedialactions, e.g. rock dumping, to be em-ployed in case of a violation of thedesign requirements.

The as-built documentation shall formthe basis for the subsequent annualinspection surveys such that "critical"pipeline sections can be monitored forpossible upheaval creep (preventivemeasure).

ACKNOWLEDGEMENT

The authors wish to thank the managementof Marsk Olie og Gas AS for their permis-sion to publish this paper.

REFERENCES

(1] Pallesen, T.R., Braestrup M.W., Jor-gensen 0. and Andersen J.B.:"Insulated Pipeline Design for theDanish North Sea", 6th InternationalConference on the Internal and Ex-ternal Protection of Pipes, Nice,France 5-7 November 1985, pp. 189-202.

Nielsen, N.J.R.; Pedersen, P. Tern-drup; Grundy, A.R. & Lyngberg B.: "NewDesign Criteria for Upheaval Creep ofBuried Subsea Pipelines", Proc. of theSeventh Inter. Conf. on OffshoreMechanics and Artic Eng., OMAE, Hous-ton, vol. v, pp. 243-250, 1988.

Pedersen, P. Terndrup & Jensen, J.Juncher: "Upheaval Creep of BuriedHeated Pipeline with Initial Imper-fections", J. of Marine Structures,Vol. 1 pp. 11-22, 1988.

Pedersen, P. Terndrup & Michelsen,J.: "Large Deflection Upheaval Buck-ling of Marine Pipelines", Proc.Behaviour of Offshore Structures(BOSS), Trondheim, Norway, Vol. 3, pp.965-980 June 1988.

Mousselli, A.M.: "Pipe Stresses atthe Seabed during Installation andTrenching Operations", Proc. OffshoreTechnology Conference, Paper no. OTC2965, 1977.

f3]

(a)

(b)

2" PUPEpoxy Coating

Fig. 1 Pipeline Build-up for the 8./3. Rolf to GormPiggy Back Line.

2 meter 3

Epoxy Coating

3" GAS Lift Line

2" Concrete Coating

PE Sleeve

8" OIL! GAS Line

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Geometry of the Exposed Pipe Section at KP 0.3.As-found Configuration.Configuration after Complete Cool Down.

FloId Joint 025rorn InfIll

455 530

309 400

Fig. 3 Retrieved Pipe Section Fig. 4 r Severe Foam Shrinkage infrom KP 0.3. Line Pipe from KP 0.3.

1

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Fig.(a)

kb)

.5

.4

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Line of symmetry

Moment distribution

Fig. 5 : Equilibrium Form and Moment Distributionfor a Partly Untrenched Pipeline.

Max. Moment normalized by ELI Lc

""\\

H/Lc ..I0NU= .08

.06

Non-dimensional Bending Momentas a Function of Half theUntrenched Length.

589

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Fig. 6:

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-

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350

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250

200

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0

Or 10 20 30 40 50 meterv---Symbols

: As-built Survey Measurements lune 1985

FOOLF

.3

0: SBP Survey Measurements September *88

0.5

A P0 ,k40D = 0.219'mt 0.014 m

IniViol Impert Length 5010Pipe Total Length .200Rigid Flat Foundation

46 m

25 50 75, 100 125 150 175

Uplifted Length. tit

Fig, a : Example of "Classical UpheavalResponse Calculations.

590

Sea bed

Curve fittedAs-built "Prop imperfection

through 1986 SBP Survey

1m

Fig_ 1 Observed Upheaval Creep, of an Initially 'Imperfect" Pipe Section-1

Fig. 9 Pipe Configuration at theof the Trench.,

11

2:rn

LU q.

'Edge

!I

100-

0

Allowable upliftA.0.02 m

Sea Floor

_ No

A.2cm6f

-0-'.../ ....

../ .../. k"t<C(3'''.""'

./..-. .......°- .....-...--- --- --- 40,--- .---. _----"bk''--- .------ ---

_--- -- ------- --...--- -- _----0-- ....------ .,----- --------

I 1

2L0 = 24.5m

No e-°

50-

Heavy, linear beam on flat foundation.-'

Non-I ineor method. elasto-plastic pipe (X52)

Upheaved PipelineSea Floor

Uplift A (m)

Fig. 11 : Post-buckling Equilibrium Curvesfor the 8" Rolf Pipeline.

591

115 2.0

04 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2D

Minimum trench depth H cm)

Fig. 10 : Critical Temperature to preventUpheaval Creep of an "Imperfect",Buried Pipe Section.

0 10

Lift-off oint

X

T.(C)100

50

200

AT°(C°)

150-

6I-0.2m

I

60

50-

40-

30-

20-

10-

0o

Temperature at "IMPERFECT LOCATION"

II

Sea bed

11.7m

i0 5m

46m

0:1 0.2 0;3 0:4 05 0.6 0.7 08UPLIFT DISPLACEMENT (m)

Fig. 12 : Influence of Geometric PipeImperfection on UpheavalBehaviour.

592

80

AT Pipe with NO GEOMETRIC IMPERFECTION

70- "STRESS FREE" PIPE

(°C)

Upheaval Buckling Failures of Insulated Buried Pipelines

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