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DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION 2013

DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION

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Contents

1 Foreword 32 Key Findings 43 Introduction 64 Pipelines In the North Sea 7 4.1 Pipeline Types 7 4.2 PipelineConfiguration 9 4.3 PipelineInventoryandFunction 10 4.4 PipelineAncillary/AssociatedEquipment 115 DecommissioningRegulationsforPipelines 13 5.1 Overview 13 5.2 NotificationofDisusedPipelines 136 DecommissioningOptions 15 6.1 SelectionofDecommissioningOptions 15 6.2 Pipelines 15 6.3 AncillaryEquipment 187 Safety 19 7.1 Overview 19 7.2 ShortTermOperational HealthandSafetyChallenges 19 7.3 LongTermHealthandSafetyChallenges 208 Environmental Impact 21 8.1 Overview 21 8.2 EnvironmentalImpacts 219 MonitoringandLiability 2410 CostofPipelineDecommissioning 2511 PipelineDecommissioningtoDate 2612 Technology 27 12.1 Overview 27 12.2 PipelineCleaning 27 12.3 TrenchingandBurial 27 12.4 DeburialandDredging 30 12.5 SubseaCutting 30 12.6 Lifting 32 12.7 ReverseInstallationMethods 32 12.8 MattressRecovery 3413 RecyclingandReuse 36 13.1 Recycling 36 13.2 Reuse 3614 PublicConsultation 37 14.1 RequirementsforConsultation 37 14.2 StatutoryConsultees(UKCS) 37 14.3 ConsultationProcess 3715 References 38

Appendix A:TableofDecommissionedPipelines IntheNorthSeaRegion 39

Appendix B:CaseStudiesofPipelineDecommissioningProjects 44


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Abbreviations

CA ComparativeAssessmentEIA EnvironmentalImpactAssessmentDECC TheDepartmentForEnergyAndClimateChange,UKGovernmentHSE HealthAndSafetyExecutive,UKJNCC JointNatureConservationCommittee,UKMPA MarineProtectedAreaNORM NaturallyOccurringRadioactiveMaterialPLUTO PipelinesUnderTheOceanPWA PipelineWorksAuthorisationUKCS UnitedKingdomContinentalShelf

Definitions Pig PipelinemaintenancetoolusedforcleaningorinspectingtheinsideofapipelinePiggy-back A smalldiameterpipelinewhich isphysicallyattached toa largerdiameterpipelineusing

strapstofacilitateitsinstallationand/orlongtermprotectionPipelinespan Asectionofpipelinewhereseabedsedimentshavebeenerodedorscouredfromundera

pipeline,resultinginanunsupportedsectionofpipeS-lay Pipelineinstallationmethodforlargerdiameterpipelines


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1 Foreword

ThefirstmajoroffshorepipelineconstructionprojectintheUKwasthePipeLinesUndertheOcean(PLUTO1)projectwhichinstalled1,000milesofpipelinebetweenthesouthcoastofEnglandandFrancetoprovidefuelfortheinvasionofFranceduringWorldWarII[Ref.1].Inthemoderneraofoilandgasproduction,thefirstpipelineswereinstalledin1966totransportgasfromBP’sWestSolefieldtoareceivingterminalatEasingtonon the Lincolnshire coast. Since then, inexcessof 45,000kilometresofpipeline,umbilical and cablehasbeeninstalledacrosstheNorthSearegiontoenablethegatheringanddeliveryofhydrocarbonstoreceivingfacilitiesandend-usersacrossEurope.

Naturally, in amature province such as theNorth Sea,when fields reach the end of their economic life,sectionsofthetransportationinfrastructurebecomeredundantandmustbedecommissioned.TheprocessofdecommissioningredundantNorthSeaassetshasbeenongoingsincetheearly1990swiththedecommissioningoftheCrawfordfieldbyBHP.Sincethenpipelineinfrastructurehasbeendecommissionedatamodestratewhensystemsaredeemedtohavenofutureeconomiclife,andnoalternativeusecanbefound.

Thisreportaimstoprovideanoverviewofthedecommissioningperformedtodateofpipelinesandtheirassociated infrastructure. ItseekstocoverallareasoftheNorthSea, includingthe IrishSeaandWestofShetland,underthejurisdictionoftheUK,Norway,Denmark,theNetherlandsandGermany,althoughitisnotedthatdatafromsomeareasismorelimitedthanothers.

IntheUKandNorway,thedecommissioningofoilandgas-relatedpipelinesisconsideredonacase-by-casebasis,usingtheComparativeAssessment(CA)processtodeterminethebestoptionfordecommissioning.Thisenablestheparticulardiameter,lengthandconfigurationofindividualpipelinestobetakenintoaccountwhenconsideringdecommissioningoptionsagainst thecriteriaofsafety,environmental impact,costandtechnicalfeasibility.

ThedocumentdrawsonresearchperformedbyOil&GasUKovertheperiodfrom2010to2013,alongwithpubliclyavailableoilandgasindustrydata.ItprovidesapictureofthescaleofpipelineinfrastructureintheNorthSea,andtheindustry’sachievementsindecommissioningpartsofthatinfrastructure.Italsohighlightsthetechnicalcapabilitiesand limitationsthat impact thedecommissioningoptionsavailable toownersofpipelinesystems.

ThereportisdesignedasareferenceforindustryandothersinterestedinthedecommissioningofpipelineinfrastructureintheNorthSeaRegion.

11942and1943sawthedevelopmentofthefirstflexiblepipeline,the3inch‘Haiscable’;andthefirstrigidreeledpipe,the3inchsteel‘Hamelpipe’whichwereinstalledacrosstheEnglishChannel.TheserevolutionarypipelinedesignsprovidedtheallieswiththecapacitytotransportonemilliongallonsoffuelperdaytonorthernFrance.


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2 Keyfindings

Experience to-date

• SincetheWestSolegasexportpipelinewasinstalledin1966,anestimated2,500individualpipelines,umbilicalsandpowercableswithalengthinexcessof45,000kilometreshavebeeninstalledintheNorthSearegion,includingtheEastIrishSeaandWestofShetland.

• Thepipelineinventoryismadeupofrigid(steel)pipelinesandflexibleflowlines,andvariesindiameterfrom2inchestoamaximumof44inches.ThelongestpipelinecurrentlyoperatingintheNorthSearegionisFranpipe,withadiameterof42inchestransportinggas840kilometresfromDraupnerEintheNorwegianNorthSea,toDunkirkinFrance.

• Lessthan2percentoftheNorthSeapipelineinventoryhasbeendecommissionedsofar.Ofthepipelineswhichhavebeendecommissioned,80percentarelessthan16inchesindiameter.Halfofthelargerdiameterpipelines(16inchesorgreater)decommissionedtodatewereremoved:thesewereallunder1kilometreinlengthandinfieldpipelines.

• Somepipelines, in particular large diameter trunklines, represent important infrastructurewhichprovidesthemeansoftransportingcurrentoilandgasproductionbetweenfacilitiesandtoshore.Thisinfrastructurealsoprovidesopportunitiesforfuturedevelopmentofhydrocarbonsreserves,orstorageofcarbondioxideorgasinthebasin.ThisisakeyreasonwhythereiscurrentlyonlylimitedexperienceofdecommissioningsuchpipelinesintheNorthSea.

Processes:

• UndercurrentregulationsacrosstheNorthSea,pipelinedecommissioningiscarriedoutonacase-by-case basis, with the decommissioning option selected for each pipeline, umbilical and cableconfirmedbydetailedCA.

• HealthandsafetyisadominantfactorinanyCA,withthefocusbeingonminimisingrisksinthelongtermtootherusersofthesea,andintheshorttermtothosecarryingoutthedecommissioningoperations.

• An Environmental Impact Assessment (EIA) is prepared to support all pipeline decommissioningplans.Potentialenvironmentalimpactsarereasonablywellunderstoodfortheshorterlengthinfieldpipelines,andmitigationmeasureshavebeenestablishedtominimisetheeffectsduringandafterdecommissioning. At present, due to the limited experience of decommissioning larger diameterpipelinesintheNorthSea,itisdifficulttoquantifytheenvironmentalimpactofsuchdecommissioning.

• In themajority of decommissioning cases, it has been demonstrated that the best option is toleaveapipelineinplace,eitherontheseabed,orleftburiedbelowtheseafloor.Thisapproachiscomplementedbywhateverremedialactionisdeemednecessarytofurtherreduceanyriskstootherusersofthesea,forexamplethecuttingandremovalofexposedpipelineends.

• Anumberoftoolshavebeenproveninthecuttingofsteelandflexiblepipelines,includingthosewithmultiplecoatings,concrete,anti-corrosionlayersandinsulation.Cuttingcanbetimeconsuming,andcanleadtoextendedriskexposuretodiversifmultiplecutsareundertakensubsea.

• If removal is identifiedasthebestoption,reversereelinghasbeenusedasameansofremovingsmallerdiameterrigidsteelandflexibleflowlines.OntheUKContinentalShelf(UKCS),45kilometresofsmalldiameterpipelinesareknowntohavebeenremovedusingthethismethod.Althoughithasonlybeenusedforasmallnumberofpipelines,reelvesselshavebeenadaptedinthepasttoreversetheirnormalinstallationmodetoremovepipelines.


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• ThereisverylimitedexperiencegloballyofremovingpipelinesusingareversaloftheS-layinstallationmethod.Anumberoftechnicalchallengesexistintheapplicationofthismethodtolargediameterageingpipelines,andinparticularconcrete-coatedtrunklines.Theseissuesrelatetotheintegrityoftheconcreteweightcoatingandthesteelpipewallitselfaftermanyyearsofservice,bothofwhichwouldbesubjecttohighforcesduringrecovery.

• The reverse S-laymethod of pipeline recovery during a decommissioning programme has neverbeenusedintheNorthSeaandcannotbeconsideredproven,particularlyintheapplicationtolargediameterconcretecoatedpipelines.

• Overall,acase–by-caseapproachisconsideredappropriateforpipelinedecommissioning.AspartoftheCAprocessthewidevariationinpipelinetype,diameter,length,integrityandin-placeconditionareexamined.Whensafety,environmentalandcostconsiderationsarealsotakenintoaccount,thebestdecommissioningoptionforeachpipelinecanbeidentified.

Cost and reuse:

• EstimatingthecostsofdecommissioningthetotalpipelineinventoryintheNorthSearepresentsanon-goingchallengefortheindustry.Factorssuchaslimitedexperience,technicalunknowns,integrityuncertaintiesandthesignificantvariationinpipelineconfigurationsmakeitverydifficulttoforecastcostswithanyrealdegreeofaccuracy.

• Reuseopportunities for rigidsteelpipelinesrecoveredbythereversereelingprocessare limited.Subjectingapipetomultiplecyclesofplasticdeformationduringboththereelingandreversereelingprocesseswouldlikelycompromiseitsintegrity.

• Rigidsteelpipelinescanberecycledalongwithsomeofthecoatingsthatmaybeappliedtothem.Likewise,flexiblepipelines,umbilicalsandpowercablescanbeprocessedtoseparatetheirmetallicandplasticcomponentsandthenrecycled.

• Potential opportunities may exist for the reuse of flexible pipelines and umbilicals if their postrecoveryintegritycanbeconfirmed.


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3 Introduction

InOctober2010,Oil&GasUKinitiatedtheDecommissioningBaselineStudytocompiledata,experiencesandlessonslearnedonthedecommissioningofNorthSeaoilandgasinfrastructure.AmajoroutputfromthisworkwastheOil&GasUKreport‘TheDecommissioningofSteelPiledJacketsintheNorthSeaRegion’whichwaspublishedinOctober2012.

In addition to providing this visibility on recent work performed in the decommissioning of steel piledjackets,theDecommissioningBaselineStudyalsoprovidedsignificantinsightintothedecommissioningof oilandgaspipelines.

The networks of pipelines currently installed in the North Sea collectively provide the transportationinfrastructurethatallowsNorthSeaoilandgasproductiontobedeliveredtohostplatformsortoshore.Inmanycases,theexistenceofnearbypipelineinfrastructurehasleddirectlytotheeconomicexploitationofmarginalfields,whichwouldotherwisebeconsidereduneconomic.Suchopportunitiesremainakeyfactorinthetimingofanypipelinedecommissioning.

Asfieldshavereachedtheendoftheireconomiclife,specificpartsofthepipelinesystemnaturallybecomeredundant,andwithnopotentialfutureuse,theyareavailabletobedecommissioned.OilandgaspipelinedecommissioninghasbeentakingplaceintheNorthSeasincetheearly1990s,whentheCrawfordpipelineswere decommissioned. Since then, pipeline decommissioning has continued at a modest rate and onlywhen all potential re-use options for the infrastructure, including new field developments, have been carefullyconsidered.

ThisreporthasbeencompiledusingtheoutputfromtheDecommissioningBaselineStudyandadditionaldatafromtheindustrytoestablishareferenceonpipelinedecommissioningintheNorthSea.Itprovidesanoverviewofthepipelineinventoryandthedecommissioningperformedtodate.Italsoincludesasummaryof the applicable regulations, health and safety and environmental challenges, and an overview of thetechnologyavailable,itsapplicationsanditslimitations.

Unlessnotedotherwisethegeneralreferenceto‘pipelines’throughoutthisdocumentreferstotrunklines,rigidflowlines,flexibleflowlines,umbilicalsandpowercables.

Oil&GasUKwouldliketoacknowledgethevaluablecontributionmadebythefollowinggroupsandorganisationsinthepreparationofthisdocument:TheDecommissioningBaselineStudyJIPSponsors2,PremierOilPlcandAtkinsLimitedinthepreparationofthisdocument.TheauthorswouldparticularlyliketothankthemembersofOilandGasUK’sDecommissioningSteeringGroupTaskGroup2fortheirsignificantcontribution.

2ApacheNorthSea,BPExplorationOperatingCompanyLimited,CNRInternational(UK)Limited,ConocoPhillipsUKLimited,DONGE&PAS,FairfieldEnergyLimited,MarathonOilDecommissioning ServicesLimited,MobilNorthSeaLLC,ShellUKLimited,StatoilAS,TalismanEnergy(UK)Limited, TotalE&PUKLimited,VentureNorthSeaGasLimited


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4 Pipelines in the North Sea

Oil andgasproduction involves the transportationofmanydifferentfluidsunderdifferent conditions, invaryingwaterdepthsandoceanographicenvironments.Thishas led toa rangeof typeofpipelinebeinginstalledacrosstheNorthSea.Thissectionprovidesadescriptionand inventoryofthedifferenttypesofpipelinecurrentlyinstalledandoperationalintheregion.

4.1 Pipeline TypesFigure1providesahighlevelcategorisationofthetypesofpipelinesinoperationintheNorthSeaRegion.

Figure 1 Pipeline Category Descriptions

Pipeline Description1

Typical Dimensions1

Applications PrimaryMaterialsofConstruction

AdditionalCoatings

Trunklines Upto44inchesdiameter,upto840kilometreslong

Majorexportinfrastructureforoilandgas

Carbonsteel Anti-corrosion2 coatingplusconcreteweightcoating3

Rigidflowlines Upto16inches,diameter,lessthan50kilometreslong

Infieldflowlinesandtie-inspools

Carbonsteelorhighspecificationalloy

Polymeranti-corrosioncoating

Flexibleflowline Upto16inchesdiameter,upto10kilometreslong

Infieldflowlinesandtie-inspools

Carcassofhighspecificationalloysandpolymerlayers;alloyend-fittings

Polymerexternalcoatings

Umbilical Between2and8inchesdiameter,upto50kilometreslong

Chemical,hydraulicandcommunicationdistribution

Thermoplasticpolymerorhighalloysteeltubes;wirearmouredprotection


PowerCables4 Between2and4-inchesdiameter;upto300kmlong

Powerdistributionbetweenandwithinfields

Coppercoreswithwirearmouredprotection


Notes1.Pipelinedescriptionsandtypicaldimensionsreflecttheiruseinthisdocument:othersourcesmaydifferin

theapplicationofthisterminology.2.Anti-corrosioncoatingsusedforthesepipelinesinclude:coal-tarenamel,bitumenandfusionbondedepoxy.3.Concreteweightcoatingsusuallyincludereinforcingwireorbars.4.Powerdistributioncablesareoftenincludedinanumbilicalstructure.

4.1.1 TrunklinesTrunklinesaremajorelementsofinfrastructuretransportinglargequantitiesofoilorgastoonshorereceivingfacilities.Trunklinesaccountfor18percentofthetotalnumberofpipelinesand63percentofthetotalpipeline length in theNorthSea inventory.Typically thesepipelinesareownedandoperatedbya singleoperator,orgroupofoperators,andtransportproductionfromanumberoffieldsonbehalfofthedifferentfieldowners.SuchpipelinesincludesomeofthelongestintheNorthSea,oftenhavingdiametersinexcessof


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30inches,withthelargestbeing44inchesindiameter.ThelongesttrunklinecurrentlyoperatingintheNorthSearegionisFranpipe,(seeSection2).

Largediametertrunklinesareinstalledutilisingthe‘S-lay’pipelaymethodfromaspecialistlay-vessel.Thisinvolvesweldingsectionsofpipetogetheronthedeckofthevessel,thenloweringthepipelinetotheseabedasacontinuousstringofpipe,asthevesselmovesforward.Thisprocesscancontinueformanykilometres,subjectonlytothesupplyofpipesectionsandsuitableweatherconditions.

4.1.2 RigidflowlinesFlowlinesaresmallerdiameter,shorterpipelinesusuallyassociatedwithasingleoilorgasfield.Socalled‘rigidflowlines’aremanufacturedfromcarbonsteelorahighperformancesteelalloy,withadditionalcoatingsprovidingcorrosionprotection,andinsomecasesinsulation.Thesepipelinesaccountforapproximatelyhalfofthetotalnumberofpipelinesand27percentofthetotallengthintheNorthSeapipelineinventory.Rigidflowlinesusuallytransportoilandgasbetweensubseainfrastructuretoahostplatformforprocessing.Theycanalsobeused to transport injectionwater to subseawells forpressuremaintenancepurposes. Thesepipelinesaretypicallylessthan16inchesindiameterandaremostofteninstalledbythereelingmethod.Thisinvolvesfabricatingtherequiredlengthofpipelineonshorebeforereelingthesteelpipearoundalargedrumonaspecialistreel-shipfortransportationtothefield.Theendofthepipelineisanchoredintherequiredlocationand thepipeunreeledas thevesselmovesalong theproposedpipeline route. The steelpipe isstraightenedasitisdeployed.

Flowlinesmaybeasshortas10metreslongwheninstalledbetweenasubseawellandmanifold(asocalledtie-inspool),butinmanycasestheyarealotlonger.Forexample,Total’sNUGGETSfieldN4wellislinkedtotheAlwynplatformviaflowlinestotalling67kilometresinlength(Ref2].

4.1.3 FlexibleFlowlinesFlexibleflowlineshavethesameapplicationasrigidflowlines,butaremanufactureddifferently.Insteadofhavingaconventionalhomogeneoussteelwalltocontainthefluid,thewallofaflexibleflowlineismadeupofcompositelayersofsteelwireandpolymersheathing,eachprovidingadifferentfunctioninthestructureofthepipewall.Collectivelytheselayersprovidetheflexibilityinthepipeline.

Unlikearigidflowline,whichisterminatedbyweldingonastandardendflangeatanappropriatelocation,flexible flowlines have specially made ‘end-fittings’ which are connected to each end of the pipeline atmanufactureandcannotbeeasilyremovedtoadjustthelengthon-site.Thesetypesofpipelineareinstalledusingavesselequippedwithalargecarousel,oftencapableofinstallinganumberofsimilarflowlinesinthesamecampaign.

Thepreferenceforusingeitherarigidorflexibleflowlineforagivenapplicationisdrivenbymanyfactorsincludingspecificdesignrequirements,installationconstraints,costorschedule.


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4.1.4 UmbilicalsandPowerCablesUmbilicals are commonplace in subsea developments, providing chemical injection, hydraulic andcommunicationsupporttowells.Theyaremadeupofabundleof‘cores’,eachofwhichmaybeuptooneinch in diameter, transporting chemicals or hydraulic fluid. The bundle also often contains fibre-optic orinstrumentationcableslinkingthesubseacontrolstothehostfacility.Theoutersheatharoundthebundleofcoresisprotectedbywirearmouring,givingtheappearanceofasinglecablewithanoutsidediameterofanythingupto8inches.Umbilicalsaretypicallyinstalledalongsideflowlinesystemsusingsimilarequipmenttothatusedtoinstallflexiblepipelines.Likeflowlines,umbilicalsareroutinelytrenchedbelowtheseabedlevel.

Power cables have a similar structure and installationmethod to umbilicals except they carry dedicatedpowertoasubseasystemorbetweenplatformfacilities.Oftenapowercablewillbeincludedinanumbilical.Wheninstalledseparatelytheyareprotectedinthesamewayasumbilicals,usinganexternalsheathmadeupofwirearmoursandinstalledinatrenchbelowseabedlevel.

4.2 PipelineConfigurationOfparticularsignificancewhenconsideringthedecommissioningofapipelineisitson-bottomstatus,post-installation.Thedesignprocessforanewpipelinedetermineswhetheritisinstalledrestingontheseabed,inanopentrenchcutintheseabed,orinstalledinatrenchandthenburiedusingseabedsoiltoalevelbelowthesurrounding levelof theseabed.Anyof theseconfigurations,orvariationsof them,maybespecifiedat installation.However,changescanoccurduringthe lifeofthepipelineduetotheactionofwavesandcurrentsonseabedsediments,orfromaccidentalinterferencebyotherusersofthesea,e.g.fishinggear,anchors,etc.

Thevariouspipelineon-bottomconfigurationscanbegeneralisedintothefourcategoriesshowninFigure2below.

Figure 2 Pipeline Configuration on the Seabed (Source Atkins)

Fullyexposedonunmodifiedseabed.(Somenaturalsettlement/embedmentmayhaveoccured.)

Fullyorpredominantlyexposedintrench:crownofpipebelowambientseabedlevel.

Crownofpipe–max25%diameter–exposedafternaturalbackfilloftrench:topofpipebelowambientseabedlevel.

Fullyburiedbyeithernaturalorploughedbackfill: topofpipebelowambientseabedlevel.

(A)

(B)

(C)

(D)

DECOMMISSIONINGOFPIPELINESINTHENORTHSEAREGION

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Theas-designedburialstatusofapipelineisdrivenbyanumberoffactors.Theremaybearequirementtoprotectapipelinefromnearbyoilandgasoperationalactivities,suchasaroundplatforms,ortoreduceriskstoandfromotherusersoftheseasuchasfishermanoranchorsfrommooredvessels.Insomecasesitmaybenecessarytolowerapipelineintotheseabedinareasofhighon-bottomcurrentstoensureitslong-termstability.Inothercasesburialandbackfillingofthetrenchmayassistininsulatingthepipelineforoperationalreasons,orbyprovidingresistancetoupheavalbucklingofthepipe.

Afterinstallationofapipeline,whetheritistrenchedornot,theseabedarounditmaymoveundertheactionofwavesandcurrents.Overtimethismayleadtoanunburiedpipelinebeingburied(socalledself-burial),oraburiedpipelinebecomingexposed,potentiallyleadingtospanning.(Thishappenswhereseabedsedimentshavebeenerodedorscouredfromunderasectionofpipeline,whichthenbecomesunsupported.)Incaseswhereapipelinebecomesexposed,thedegreeofexposuremayvaryalongitslengthfromfullyexposedontheseabed(Figure1,exampleA),tofullyburiedinabackfilledtrench(Figure1,exampleD).

Theon-bottomconfigurationofapipelineismonitoredoveritslifetimesothatwhenthedecommissioningplanisprepared,itsburialhistorycanbeusedtoassistindeterminingthepreferredmethodofdecommissioning.

4.3 PipelineInventoryandFunctionAspreviouslydetailed,around45,000kilometresofpipelineshavebeeninstalledintheNorthSeaRegionsince1966.Figure3illustratestheaggregatedlengthofpipelinesbydiameterinstalledandtheirrangeofservice.

Figure 3 Pipelines Installed in the North Sea by Diameter and Service (Source Xodus)

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

5000.0

Aggr

egat

e Pi

pelin

e Le

ngth

(km

)

OD (inches)

Aggregate Pipeline Lengths by Service

Unknown/Other

Methanol

Mixed Hydrate

Water

Condensate

Chemical

Oil

Gas


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Figure3showsthatasignificantproportion(63percent)ofthetotallengthofallpipelinesinstalledhaveadiameterinexcessof16inchesandmaybeconsideredas‘trunklines’asdefinedinTable1.Aswouldbeexpected,suchpipelinestransportmainlyoilandgas.Itcanalsobeseenthatthemanykilometresofsmallerdiameterpipelinescarryamuchwiderrangeofproductsfromoilandgastowaterandchemicalstoassistintheproductionofdifferenthydrocarbonstreams.

4.4 Pipeline Ancillary/Associated EquipmentIn addition to the pipelines themselves, there are twoother groups of associated equipment,which areusuallydealtwithinthesamedecommissioningplanasapipeline.Theseareconcretemattresses,includinggroutbags,andpipelinecrossings.

4.4.1 MattressesConcretemattresseshavebeenusedextensivelyintheNorthSeatoprovideprotectionand/orstabilitytosubseapipelinesandumbilicals,includingthejumperspoolsthatfacilitatethetie-instoplatforms,manifoldsandwellheads.Theyhavealsobeenusedasaneffectiveinterventiondevicefortherectificationofpipelinespans.FlexiblemattressesaretypicallymanufacturedbyjoiningdifferentshapesofconcreteblockstogetherwithpolypropyleneorKevlarrope.

Oldermattressesinstalledinthe1970sweremadefrombitumenoraggregatepouredintomattressbags.Theuseofbitumenmattressesstoppedintheearly1980s. TheexactnumberofconcretemattressesintheNorthSeaisnotreadilyavailable.Oil&GasUKestimatessuggestthatbetween35,000and40,000mattresseshavebeendeployedonandaroundoilandgassubseainfrastructuresinceoperationsbeganintheNorthSea.

4.4.2 CrossingsTheneedtocrossotherpipelinesalongadesignatedpipelinerouteisinevitableinawelldevelopedbasinsuchastheNorthSea.Manycrossingshavebeenconstructedovertheproductivelifeofthebasinwithincreasingnumbersbeingrequiredastheregionmatures.Pipelinecrossingsaresignificantintermsofdecommissioningbecauseofthedirectimpacttheycanhaveondecommissioningoperationsandtheoptionselected.Ifapipelinewhichisbeingdecommissionedcrossesoriscrossedbyanotheroperatingpipeline,thesectionofthepipelineatthecrossingwillbeleftinsituuntilsuchtimeastheoperatingpipelineisalsodecommissioned.Thismeansthatadecommissionedpipeline iscutsomedistanceawayfromanycrossingsalong its length, typicallynocloserthan50metresfromtheoperatingpipeline.Ineachcasetheresponsibilityforthedecommissioningofthesectionsleftinplaceneedstobeestablishedbetweentheparties.Unlessotherwiseagreed,thefinancialresponsibilitywill remainwith theownerof the respectivepipelines. There aremanyways that a pipelinecrossingmaybeconstructed.TheexamplesshowninFigure4illustratehowaconfigurationchangesdependingonburialstatusandwhattypicalpipelineconfigurationsmustbeconstructed.


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Figure 4 Examples of Pipeline Crossing Configurations (Source Atkins)

Crossings are usually constructed using concretemattresses, grout-filled bags or bespoke cast concretestructures.Theremustbeaminimumclearancebetweentwopipelinesatacrossingof300millimetres,andaconcretemattressusuallyprovidesthisclearanceandprotectionbetweenthetwopipes.Theothermaterialsusedtoconstructthecrossingdependontherequiredheightofthecrossingpipeline.Smallercrossingscanbebuiltfromasmallnumberofmattressesand/orgroutfilledbags.Largercrossingsarerequiredforlargerdiameterpipelinesandareoftenmadeusingpurpose-builtcastconcretesections.

Thisshortensinstallationtimeswhencomparedtotheplacementoflargenumbersofmattressesorgroutbags.Inmanycasescrossingsareburiedinrockdumptoprovideprotectiontothecrossing,andtoreduceanypotentialsnagginghazards.

Exposedpipelinecrossesburied3rdpartypipeline–removalreadilyachieved:insituburialnotfeasible

Exposedpipelinecrossesexposed3rdpartypipeline–removalsubjectto3rdpartyapproval,andsignificantmaterialtoberemoved:insituburialnotfeasible

3rdpartypipelinecrossesburiedpipeline–notanissuesincepipelinecan remainburied

3rdpartypipelinecrossesexposedpipeline–sectionofpipeline‘trapped’ifcrossingpipeline is live


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5 DecommissioningRegulationsforPipelines

5.1 OverviewAlthoughanumberofinternationaltreatiesgovernthedisposalofwasteatsea,includingthemanagementofdecommissionedoffshorestructures,therearenointernationalregulationsorguidelines,relatingspecificallyto the decommissioning of pipelines. At present, pipeline decommissioning is covered within nationallegislation.

IntheUK,thePetroleumAct1998[Ref3]outlinestherequirementsforownersofinstallationsandpipelinestoobtainapprovalfortheirdecommissioningprogrammefromtheSecretaryofState.Thedecommissioningprogrammeshouldcontaindetailsofcostandproposalsforremovalanddisposal.ItmustbesupportedbyanEIAandissubmittedtotheDepartmentforEnergyandClimateChange(DECC).

Pipelinesshouldbethesubjectofaseparatedecommissioningprogrammeunlesstheyarelocatedwithinthesamefieldasotherequipmentorinstallationstobedecommissionedatthesametime.

Inadditiontotheapprovalofthedecommissioningprogrammeforapipeline,thefollowingmayalsoberequired:• ConfirmationthattherequirementsoftheCoastProtectionAct1949Section34PartIIhavebeensatisfied• FulfilmentofnotificationrequirementsfortheHealthandSafetyExecutive(HSE)underregulation22

ofthePipelineSafetyRegulations1996[Ref7]• Anyenvironmentalconsentsorpermitsrequiredduringdecommissioningactivity• Disposalofmaterialsonshoremustcomplywithrelevanthealthandsafety,pollutionpreventionand

wasterequirements/permits

IfpartortheentirepipelineistoberemovedorthedecommissioningprogrammewouldresultinachangetoanypartoftheTableAinformationintheoriginalPipelineWorksAuthorisation(PWA)thenaPWAVariationwouldalsoberequired.

Iftheapproveddecommissioningprogrammeforapipelinecontainsproposalsfortheplacementofassociatedmaterialsontheseabedsuchasrockdump,thenalicencemustbeobtainedundertheMarineandCoastalAccessAct2009[Ref4]inEnglandandWalesortheMarine(Scotland)Act2010[Ref5].

InNorway,pipelinesandcablesarenotspecificallyreferredtoinChapter5Decommissioning,ofthePetroleumAct1996.Theyare,however,coveredbyaseparateWhitePaper47(1999–2000),‘DisposalofPipelinesandCablesontheNorwegianContinentalShelf’.

5.2 NotificationofDisusedPipelinesIntheUK,theownerofapipelinemustnotifytheDECCwhenapipelinereachestheendofitsoperationallife.Undercertaincircumstances,thismaybebeforeotherfacilitiesinthesamefield.InsuchcasestheDECCmayconsiderthedeferralofdecommissioningforthepipelineuntiltheendofthewholefieldlife.

SomepipelinesmayrepresentimportantUKCSinfrastructureandprovidethemeansforfuturedevelopmentofhydrocarbonsreserves,orstorageofcarbondioxideorgasinthebasin.Toallowforthefuturereuse,thedecommissioningofsuchpipelinesmayalsobedeferred.


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Thedeferralofpipelinedecommissioningtotheendoffieldlifeorforpossiblereuseiscarriedoutunderthe‘InterimPipelineRegime’(IPR).TheDECCwillsendthepipelineowneraDisusedPipelineNotificationformrequestingdetailsonthestatusofthepipeline.TheDECCwillconsultwithothergovernmentdepartmentsandthen issuea letteroutliningtheconditionsunderwhich it ispreparedtodeferdecommissioningtoaspecifieddate.Ifreuseofthepipelineisconsideredviable,thensuitableandsufficientmaintenanceofthepipelineisrequiredoftheowner.


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6 DecommissioningOptions

6.1 SelectionofDecommissioningOptionsAsnotedpreviously,pipelinedecommissioningintheUKisregulatedbytheDECCandguidanceisprovidedintheDECCGuidanceNotes[Ref6].Inaddition,thePipelineSafetyRegulations1996[Ref7]providerequirementsforthesafedecommissioningofpipelines.Thereareanumberofoptionsforthedecommissioningofoffshorepipelines,andtheseareevaluatedbycomparativeassessmentinaccordancewiththeDECCGuidance.

Optionsforthedecommissioningofpipelines,mattressesandpipelinecrossingsaredescribedbelow.Thesedescriptionspresentthetechnicaloptionsfordecommissioning:theCAprocesswouldalsotakeaccountofsafety,environmentalandsocietal impactandcost inorder todeterminetheoptimumdecommissioningoptionforaspecificpipelineandassociatedinfrastructure.

6.2 PipelinesWhendevelopingtheoptionsfordecommissioningapipeline,theprimaryoptionscanbegroupedintosub-optionsofeitherleaveinsituorremoval.Typicallytheyaresummarisedas:• Leaveinsitu–minimalintervention• Leaveinsitu–minorintervention• Leaveinsitu–majorintervention• Removalbyreversereeling• RemovalbyreverseS-lay• Removalbycutandlift

6.2.1 LeaveInSituIn theUK, theDECCprovidesguidanceonpipelines, includinganypiggy-backpipelineorumbilicalwhichcannoteasilybeseparated,whichmaybecandidatesforinsitudecommissioning.ThecaseshighlightedbytheDECCare:• Thosewhichareadequatelyburiedortrenchedandwhicharenotsubjecttodevelopmentofspans

andareexpectedtoremainso• Thosewhichwerenotburiedortrenchedatinstallationbutwhichareexpectedtoself-buryovera

sufficientlengthwithinareasonabletimeandremainsoburied• Thosewhereburialortrenchingoftheexposedsectionsisundertakentoasufficientdepthandis

expectedtobepermanent• Thosewhicharenottrenchedorburied,butwhichneverthelessarecandidatesforleavinginplaceif

theCAshowsthattobethepreferredoption(e.g.trunklines)• Thosewhereexceptionalandunforeseencircumstancesduetostructuraldamageordeterioration,

orothercause,meanstheycannotberecoveredsafelyandefficiently

Thevarioussub-optionsofinterventionpriortoinsitudecommissioningofpipelinesaredescribedbelow.Inallcases,pipelinesarecleanedtoanappropriatelevelaspartofthedecommissioningoperations.

6.2.1.1 MinimalInterventionForapipelinethatwastrenchedandburiedatinstallationandcanbeshowntohaveremainedburiedalongitslengthoveritslifetime,theoptiontodecommissionthepipelineinsitumayrequireminimalintervention.After cleaning, apipeline isusually leftfilledwith seawaterwith theends leftopen to the sea.Potentialsnagginghazardsatthepipelineendswouldberemovedtocompletethedecommissioningplan.Thiswouldrepresent a ‘minimal intervention’ decommissioning option and may include cases where a pipeline isexpectedtoself-buryovertime.


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6.2.1.2MinorInterventionInadditiontothetasksdescribedaboveforastableburiedpipeline,theremaybeaneedforselectedremovalorremedialburialofshortsectionsofpipelinealongitslength,whichcouldpresentapotentialhazardtootherusersofthesea.Thiscouldincludesectionsofpipelineswhichlieontheseabedbetweenthetrenchandtheformerlocationofasubseastructure.Likewise,sectionsofpipelinethathavebecomescouredandformedspansmayalsoberemovedaspartofadecommissioningplan.

Underthesecircumstances,sectionswouldtypicallyberemovedbysubseacuttingandliftedtothesurfacebyasuitablyequippedsupportvessel.Otheroptionsavailableareremedialtrenchingofexposedsections,orusingrock-dumptoremovethesnagginghazard.

ForatrunklinewhichisinstalledontheseabedandwhereCAhasshownthatinsitudecommissioningisthebestoption,similarminorinterventionmayberequired.Thiscouldinvolverectificationworksonsectionspronetoscourandthedevelopmentofspans,andmanagementofthepipelineends.

6.2.1.3MajorInterventionApipeline initially installedontheseabed,orwhichwasoriginallytrenchedmayhavesignificantsectionsthathave required interventionover its lifetime. In thesecircumstances, thepreferredoptionmaybe todecommissionthepipelineinsituandcarryoutmajorinterventionworks,ratherthancompleteremoval.Aftercleaningandremovalofthetie-insateachend,thepipeline,orsignificantsectionsofit,maybetrenchedbelowthesurroundingseabedlevel.Alternatively,significantsectionsmayberemovedbyutilisingthecutandliftorreverseinstallationmethods.

Whereapipelineistrenched,thedepthoftrenchingisdeterminedbytheneedtoremoveanyhazardstootherusersofthesea,takingaccountseabedandsoilconditionsandotherdeterminingfactors.AtypicaltargetdepthsuggestedbytheDECC[Ref6]is0.6metrestothetopofthepipe.

6.2.2 RemovalFor small diameterpipelines, flexibleflowlines andumbilicalswhich are installedon the seabedandnottrenched,theDECCguidance[Ref6]isthattheseshallnormallyberemoved.Formoresignificantremovalsthefollowingsectiondescribestheoptions.

6.2.2.1 ReverseReelingForpipelineswith adiameterof 16 inchesor less,which arenot concrete coated, apossiblemethodofremoval isbyareversalofthereelinginstallationprocess.Reeling istheinstallationmethoddescribedinSection4.1.2andhasbeenusedextensivelyacrosstheNorthSeaforbothrigidandflexibleflowlines.

Theinstallationofrigidpipelinesbythereelingmethodreliesontheplasticdeformationofthepipewallduring installation to ensure the reeled pipelinewill subsequently lie straight on the seabed.When theprocessisreversedfortheremovalofapipeline,thepipeisreeledontothespecialistreelvesselandisonceagainplasticallydeformedsothatitsitsontherecoveryreel.Thelengthofpipelinethatcanberecoveredislimitedbythesizeandcapacityofthereel.Oncethepipelineisonthereelitistakentoashore-basedfacilityandremovedbyreversingtheprocessonceagain.

Duetothenatureofthereelingandunreelingprocess,itisunlikelythatarigidpipelinerecoveredusingthismethodcouldbereused.Themultiplecyclesofplasticdeformationof thepipelinewallcouldpotentiallycompromiseitslongtermintegrity.Thesteelfromrecoveredrigidpipelinesisrecycled.


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Thismethodisalsousedintherecoveryofflexibleflowlines.Thestructureofthewallofaflexibleflowlinemeansitdoesn’texperiencethesamedeformationcyclesastherigidpipelineduringthereelingandunreelingprocess.Multiplereelingandunreelingcyclesshouldnot,therefore,compromisethelongtermintegrityofaflexibleflowline.Intheory,suchpipelineshavethepotentialforreuseifasuitableapplicationisfound.Itis,however,theresponsibilityoftheend-usertodemonstratetheintegrityofarecoveredflowline(seeSection13.2).

6.2.2.2 ReverseS-layLargerdiameterandconcretecoatedtrunklinesaretypicallyinstalledusingtheS-laymethodasdescribedinSection4.1.1.AlthoughithasneverbeenusedbeforeintheNorthSea,apotentialremovalmethodisthereversalof theS-lay installationprocess.Thismethod isoftenconsidered in theCA fordecommissioningpipelinesinexcessof16inchesdiameterand/orconcretecoated.

ThismethodwouldinvolverecoveringapipelineendtothedeckofaspecialistS-layvessel.Thevesselwouldthenmovealongtherouteofthepipeline,stoppingatsuitablepointswhereacutwouldbemadetoremoveasectionofpipefromtherecoveredpipelinestringonthedeckofthevessel.Thesesectionswouldthenbetransferredtoasuitabletransportationbargeforonshorerecycling.

Althoughtherehavebeensomeexamplesoftheapplicationofthismethodintheshallowwater(lessthan24metreswaterdepth)oftheGulfofMexico,anumberofsignificanttechnicallimitationscurrentlyexistwhichprecludeitslargescaleapplication,i.e.:• Hightensionforceswouldneedtobeappliedtothepipelineduringrecoveryfromthevesseltensioner

systemtotheoutersurfaceoftheconcreteweightcoattobringthepipeontothedeckandholditinplaceforcutting.Theintegrityofagedconcreteweightcoatingcannotbeassuredandwouldneedtobecarefullyassessedtoconfirmthatthenecessarytensioncouldbegenerated,withouttheconcretecoatingdisintegratingandthecontrolofthepipelinebeingcompromised.

• This tension would also be applied into the steel wall of the pipeline and after many years ofoperation,theintegrityofthepipewallalongitslengthunderthehighrecoveryloadswouldneedtobeconfirmed.

• Thereisthepotentialforverylargequantitiesofmaterialstoberecoveredduringthedecommissioningofalargediametertrunkline.Thereisnoestablishedsupplychain/disposalrouteforthequantitiesofconcrete,steelandanti-corrosioncoatingswhichwouldbetakenonshoreduringamajorpipelineremovalcampaign.

6.2.2.3 CutandLiftAnotherpossiblemethodusedfortheremovalofpipelinesectionsistheso-called‘cutandlift’method.Thiscanbeusedforanydiameterorlengthofpipeline.Thisistheprocesswherebyapipelineiscutintosectionssubseabydiver-operatedcuttingtoolsorusingremotelyoperatedcuttingequipment,andthesectionsarethenrecoveredtoasurfacevesselusinganon-boardcrane.

Thisoptionhasbeenwidelyusedforremovingshortersectionsofpipe,eitherfortheremovalofashortpipeline in its entirety, or when discrete sections are being removed under a decommissioning plan. Itis usually thepreferred removal option for short sections of pipe,when it is impractical or prohibitivelyexpensivetomobilisemajorremovalequipment.

Mostsignificantly,thecutandliftmethoddoescreategreaterriskstothepersonnelcarryingouttheoffshoreoperations,especiallydivers.Ithasthereforebeenpreferabletolimitthatriskexposurebyavoidingextensiveoffshorecutandliftprogrammes.


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6.3 AncillaryEquipment

6.3.1 MattressesThe DECC guidance on mattresses and grout bags is that they should be removed from the seabed atdecommissioning. The guidancedoes, however, recognise that in some circumstances itmight be betterfor badly degradedmattresses tobedecommissioned in situ. In such circumstances, a CA is required todemonstrate that the best decommissioning option has been chosen. It is common practice to removemattressesandgroutbagsduringthedecommissioningofapipelineandassociatedtie-ins,withoutaseparateCAbeingperformed.

Thefeasibilityofremovaldependsmainlyontheageofthemattress,anditsburialstatus.Bitumenmattressescan be difficult to recover as they can break up when lifted. Similarly, older block-type mattresses candisintegrateduringrecoveryduetothedegradationofthepolypropyleneropeholdingtheblockstogether.Insuchcircumstances, therisktopersonnelperformingthedecommissioning increasesandtheoperatormayrequestpermissiontodecommissionsuchmattressesinsitu.

Somemattressesarefittedwithfrondstopromotesedimentdepositionafterdeployment.These,andothermattresses,canbecomeburiedovertime,andundersuchcircumstancestheoperatorcouldrequestthattheyaredecommissionedinsitu.

TechnologyoptionsfortheremovalofmattressesaredescribedinSection12.8.

6.3.2 Crossings,GroutBagsandConcreteFormworkIf a pipeline being decommissioned crosses other operational pipelines, it is usual practice to leave theconstructedcrossing inplaceuntilallpipelinesaredecommissioned.Thisavoidsunnecessaryrisks tothe‘live’infrastructure.Thisrepresentsadeferralofthedecommissioningworks.

Asforallpipelineinfrastructure,operatorsarerequiredtoconsideralloptionsfordecommissioningacrossing.Anyproposaltoleaveallorpartofacrossinginsitumustbesupportedwithevidencedemonstratingthereasonswhythisispreferred.Suchreasonsmayincludesufficientburial,impracticalitiesorsafetyconcernswithremoval,oranyotherexceptionalcircumstance.Manycrossingsarerockdumpedforprotection,whichmaybeavalidreasonwhythecrossingshouldbeleftinsitu.

Formworkusedtoconstructlargercrossingsisinstalledusingdedicatedliftingpad-eyesorslings,builtintotheconcretestructure.Thefeasibilityofremovingsuchitemsbyreversingthisprocessrequiresconfirmationoftheintegrityoftheliftpoints.Undersuchcircumstancesitmaybedemonstratedthatdecommissioninginsituispreferred.


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7 Safety

7.1 OverviewIntheUKundertheSafetyCaseRegulations[Ref.8]andpriortoanydecommissioningworkbeginning,theSafetyCaseforaninstallationmustbeupdatedandsubmittedtotheHSE.TheSafetyCaseforaninstallationwill include thoseelementsof apipeline that are safety critical and in closeproximity toan installation,for example isolation valves. The Safety Case must demonstrate that the proposed decommissioningarrangementsreducetherisktopeopletothelowestlevelthatisreasonablypracticable.

ThenotificationrequirementsunderthePipelineSafetyRegulations1996[Ref7]mustalsobefulfilled.Theseregulations ensure that a pipeline is designed, constructed andoperated safely, andprovide ameansofensuringpipelineintegrity,therebyreducingriskstopersonnelandtheenvironment.Undertheregulations,pipelinesshouldbedecommissionedinsuchamannerthattheydonotbecomeadangertopeople.Offshore,theextentoftheobligationtoremoveapipelinewilldependonthediameterofthepipeline,itslocationontheseabed,itsstabilityandthelocalsubseaconditions[Ref7].

Safetyisparamountandintegraltoallphasesofdecommissioningprojects,andsoformsakeypartoftheCAofthepipelinedecommissioningoptions.IntheCA,safetyistypicallyconsideredontwodifferenttimescales:• Thehealthandsafetychallengesthatmayposearisktopersonnelduringdecommissioningoperations

intheshortterm• Thehealthandsafetychallengesthatmayposearisktootherusersoftheseainthelongterm

7.2 ShortTermOperationalHealthandSafetyChallengesThemainhealthandsafetychallengesthatmayposearisktopersonnelduringdecommissioningoperations,arecommontoallpipelinedecommissioningoptions.However,thoseoptionswhichrequiretheleastintervention,andthereforetheuseoffewervesselsandoffshoreworkers,mayrepresentalowerrisktopersonnel.

Themainhealthandsafetychallengesareasfollows:• Lifting – the potential for large numbers of vessel-based lifts and the uncertainties surrounding

structural integrity of an aged pipe section, concrete mattresses, or the lift points of concreteformwork

• Diving–significantdiverinterventionmayberequiredtosupportextensivesubseacuttingandliftingoperations

• Hazardoussubstances–residualmaterialswithinpipelinessuchasmethanol,chemicalsfromumbilicalcores,waxdeposits,hydrocarbonsorNaturallyOccurringRadioactiveMaterial(NORM)scaling

• Integrity–hiddenflawsand structuraldegradation in the steelpipewallor concrete coatingsofagedpipelines,orauxiliaryequipmentsuchasgroutbagsormattresseswhichwerenotdesignedforremovalaftermanyyearsinservice

• Highlevelsofactivity–therearemanyworkersatallstagesofadecommissioningproject,onshoreandoffshore,potentiallyworkinginadynamic,constantlychangingenvironment

• Poorweather–thisextendsthedurationofoffshoretasksbyprohibitingwork,andincreasesthenumberofman-hoursrequiredoffshore

• Marinegrowth–managementofwasteandodour

Anumberoftechniquesareemployedtoreduceand/ormitigatetheriskstopersonnel.Thosemethodswhichhaveprovensuccessfulincluderegularupdatingofworkplansandemergencyproceduresthroughouttheproject;permit-to-worksystems;safetyinitiativessuchasgoodquality‘toolboxtalks’,sharingofexperienceandlessonslearned;andtechnologyimprovementsandtraining.


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7.3 LongTermHealthandSafetyChallengesTheinsitudecommissioningoptionsforpipelinespotentiallyposealongtermhealthandsafetychallengeintheformofsnaggingriskstootherusersofthesea.Atparticularriskarefishermenwhousedemersalbottomtrawlinggearoffshore,orfixedgearfisheriesnearshore.Asnaggingriskonadecommissionedpipelinemaybecausedby:• Pipelinespansduetoseabedscourunderapipeline• Exposedpipelineends• Longtermridgesintheseabedfromtrenchingoperations• Exposedpipelinecrownsduetodeburialofpipelines• Unevendegradationofexposedpipelinesovertime• Anchorscarsormounds• Steepsidedrockdumpprofiles

Anumberofinitiativesareemployedtoreducethepotentialsnaggingrisktofishermen.Immediatelyafterdecommissioningoperationsarecomplete,debrisontheseabedisremoved,andtypically,trawlsweepsbyfishingvesselswithchaintrawlsarecarriedoutalongthedecommissionedpipelinecorridor.Thishelpstoidentifyanypotentialsnagginghazards,whichcanthenbemanaged.

Theendsofdecommissionedpipelines,orcutsectionsofpipelines,poseapotentialrisktofishermen.Thiscanbereducedwithremedialmeasuressuchastheplacementofrockdumporgroutbagsattheendstoroundthemoff,andcreateanover-trawlableprofile.Earlyconsultationwiththefishingindustryassistsinestablishingthemostappropriateremedialmeasurestoreduceorremovethehazard.

Ownersofpipelinesdecommissionedinsituwillcarryoutregularsurveystomonitorandinspecttheconditionofthepipeline.ThedetailsofdisusedpipelinesarereportedtotheHydrographicOfficeandrecordedintheFishSAFEdatabase[Ref9].ThisdatabasecontainsinformationonalloilandgasinfrastructureontheUKCS.Itisprovidedtofishermentwiceayearasanoverlaytotheiron-boardnavigationalplotters.TheprovisionofdatathroughtheFishSAFEprojectisfundedbytheoilandgasindustry.FurtherdetailsoninteractionwiththefishingindustryduringoperationsanddecommissioningcanbefoundintheOil&GasUKFisheriesLiaisonOfficerGuidelines[Ref10].


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8 Environmental Impact

8.1 OverviewIntheUK,thedecommissioningprogrammeforapipelinemustbesupportedbyaCAoftheoptionsand anEIA.

TheCAhelps select thebest decommissioningoptionby comparing eachonbasis of complexity, safety,economicsandimpacttotheenvironment.

TheEIAidentifiesthelikelyenvironmentalandsocietalimpactsofdecommissioningactivities,andproposesmitigationmeasurestoavoid,orreducetoacceptablelevels,anysignificanteffects.TheEIAalsoassessescumulativeimpactsaswellasthosethathavethepotentialtoaffectMarineProtectedAreas(MPAs).

TheregulationsoftheNorwegianPetroleumActof1996alsorequirethatanEIAiscarriedoutaspartofthepreparationfordecommissioninginfrastructureassetsincludingpipelines.

Thepotential environmental impacts andareas formitigation that are considered aspart of theEIA arehighlightedbelow.Thepotentialsignificanceoftheenvironmentalimpactisrelatedtothelengthofpipelinetobedecommissioned.

8.2 Environmental Impacts

8.2.1 GaseousEmissions/EnergyUsageGaseousemissionsmaycausealocalreductioninairqualityandcontributetowiderclimatechangeprocesses.EmissionsofprimarilyCO2,butalsosmallerquantitiesofCO,NOx,SOxandVOC,aregeneratedduringthecombustionoffuelbyvesselsusedforcutting,liftingandtransportationofrecoveredpipelines.Emissionswouldalsobegeneratedthroughtheproductionofnewrawmaterialssuchassteel, toreplaceanequalquantityofmaterialinpipelineswhicharedecommissionedinsitu.

Emissions can be calculated from industry standard data, and project-specific estimates of likely fuelconsumptionbyvesselsandthereplacementoflostmaterialwillbeincludedintheEIA.

8.2.2 Discharges to SeaDischargeofsewageandfoodwaste,ballastwaterandtreatedbilgewatermayoccurduringvesseloperations.Thesewouldcauselocalisedandtransientdeteriorationinwaterquality,butposenoreallong-termhazardstobirds,fish,benthosorplankton.

Any chemicals that are used to clean and flush pipework during decommissioning are strictly controlledthroughtheOffshoreChemicalRegulations2002[Ref11].Pipeworkisflushedthroughtheexistingprocessingroutetotheonshoreterminalortransportedbyshuttletankerashore.

All pipelines are cleanedbeforedecommissioning, however, there is a possibility that a small amountofresidualdepositswillremainontheinsideofthepipe.Asapipelinewhichhasbeendecommissionedinsitudegrades,thereisapossibilitythatsuchdepositsontheinsideofthepipelinewillbreakdownandbereleasedintothewatercolumn.Anysuchreleasewouldbeverygradualandanyimpactwouldbehighlylocalised.

AlldischargestoseaduringdecommissioningoperationsarepermittedactivitiesthatareregulatedbyDECC.


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8.2.3 Underwater NoiseUnderwater noise is generated from vessel operations, particularly from the use of dynamic positioningsystems,aswellasfromcuttingandseabedexcavationworks.Thishasthepotentialtocausedisturbancetoanymarinemammalsincloseproximitytothedecommissioningoperations.

The potential disturbance to marine mammals will be assessed in the EIA process. Decommissioningoperations followthe JointNatureConservationCouncil (JNCC)guidanceonmitigatingthe impactofanynoise.MarineMammal Observers are used on board vessels andmitigationmeasures may include notcommencingoperationsuntiltheareaisshowntobeclearofmammals,andperhapsasoft-starttonoisyoperations.

8.2.4 PhysicalDisturbancetotheSeabedDecommissioningoperationsmayresultinlimiteddisturbancetotheseabedaroundpipelines.Forexample,sedimentcouldbedisturbedtoenableaccessforcuttingandlifting,forpipelineburialthroughjetting,orthrough the placement of remedialmaterials such as rock dump. Each of these operationswould resultin various degrees of physical disruption to the seabed, localised sediment re-suspension and potentialsmotheringofbenthicanimals.

Theextentofphysicaldisturbanceislikelytobesimilarorlessthanthatcausedduringinstallation,andwouldoccurinnarrowcorridorsalongtherouteofthepipeline.ThepotentialimpactwouldbeassessedthroughtheEIAprocess.Recoveryratesforbenthiccommunitiesarelikelytobeveryrapid.

Anyadditionalmaterialsplaced,suchasrockdump,willhaveaverysmallfootprintontheseabedandmayprovideadditionalhardsubstratumwhichcanbecolonisedbymobileandencrustingorganismcommunities.ThelongtermeffectoftheintroductionofsmallareasofsubstratumintopartsoftheNorthSeawithnaturallysandyormuddyseabedsisnotfullyunderstoodatpresent,andiscarefullyconsideredintheEIAandbytheregulators.

8.2.5 WasteandNORMInsomecasestheselecteddecommissioningoptionforapipelineistobringthepipelineonshorefordisposal.It is likelythatthemajorityofapipelinewouldberecycled.Theremaybesomematerials thatwouldbeconsignedfordisposal(suchassomeplastichoses).Thismayincludetheresidualcontentsofpipelineswhichmayhavebuiltupduringtheiroperationallife;forexample,waxydeposits,oilysludgesorNORMscale.Allresidualcontentswouldberecoveredanddeposedof inaccordancewithcurrentpermitrequirementsatlicensedsites.

Disposalofmaterialstolandfillwillreducethefuturecapacityforsuchdisposal,whichmayresultinalandfillresourceissueinfutureasmoreinfrastructureisdecommissioned.

The transportation toanonshore facilityand subsequentdisposalofpipelinesmaycausedisturbance tolocalcommunitiesthroughnoise,odourfrommarinegrowth,dustandincreasedtraffic.ThepotentialimpactdependsonthelocationofthesiteandvolumestobeprocessedandwillallbeassessedwithintheEIA.

8.2.6 MetalsSacrificialanodesareusedasprotectionstructuresonpipelinestoreducecorrosionandmaintainintegrityduringitsoperationallife.Theseanodesaremadefromzincoraluminium-zinc-indiumandmaycontaintraceamountsofmercury,copper,cadmiumorlead.


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Astheanodesdepleteovermanyyears,there isapossibilitythattraceamountsofmetalscouldmigratethroughthesedimentandinsomecircumstancescouldbeaccumulatedbysomemarinespecies.Theimpactofsuchmetalsdependsontheratesatwhichtheydissolve,migratethroughthesediment,anddissolveinthewatercolumn,andthedegreetowhichtheyarebioavailable.Italsodependsonphysicalfactorssuchaswaterdepth,temperature,oxygenlevelsandflowoverthesurfaceofthepipeline.Thisis,however,likelytobeanimpactoflowsignificance.

Thelevelsoflead,cadmiumandmercuryfoundinsedimentintheNorthSeahasbeenfallingsince1990,andinputsfrompipelineanodesareconsideredtobeinsignificantincomparisonwithothersourcessuchasriverineandcoastalindustries[Ref12].


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9 MonitoringandLiability

ResidualliabilityfordecommissionedoffshoreinfrastructureisdeterminedintheUKbythePetroleumAct1998[Ref3],wherebytheliabilityforanystructuresleftinsiturestswiththefacilityownerinperpetuity.NorwegianlegislationislessdefinitiveandfutureliabilityisagreedbetweenthefacilityownerandtheState,andmaybeassumedbytheStatebasedonanagreedfinancialcompensation.

Thus,ifapipelineisdecommissionedinsituintheUK,thereremainsaliabilityonitsownertomonitoritsconditionandtoensureitremainssafeforotherusersofthesea.GuidancefromtheDECC[Ref6]providesforapost-decommissioningsurveyalongthepipelinecorridor,typicallyextendingto100metreseithersideofthepipelinealignment.

Asecondsurveyistypicallyperformedayearlaterfromwhichthestabilityoftheremnantinfrastructure isconfirmed.Anenvironmentalsurveyisalsoperformedpost-decommissioningduringwhichsamplesarecollectedforanalysis.Thefuturemonitoringplanforthesite isagreedwiththeregulatorthrougharisk-basedapproach.InNorway,pipelinesthatarelaidontheseabedaresurveyedafterdecommissioningusuallywithanROV.Environmental surveys (chemical,physical andbiota) are carriedoutat the site twiceafteroperationofthepipelineshasceased.


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10 CostofPipelineDecommissioning

Asnoted,themethodsusedfordecommissioningpipelinesacrosstheNorthSeaarebasedonacase-by-caseevaluationofoptionsusingtheCAapproach.Withatotalinventoryinexcessof2,500individualpipelinesandatotallengthof45,000kilometres,determininganoverallcostofdecommissioningthisinventorywithevenamodestdegreeofaccuracyrepresentsasignificantchallenge.Anumberofotherfactorsemphasisethecomplexityofthetask:• With limitedactualpipelinedecommissioningexperience, there isminimalcostdataavailable for

benchmarking• Some of the principle methods being considered for decommissioning large diameter pipelines

are unproven and hence the actual cost of applying these methods is yet to be determined (i.e.reverseS-lay)

• An execution model, which seeks to realise economies of scale by combining decommissioningcampaignsacrossgroupsoffieldsandoperators,hasyettobedetermined

Oil&GasUKseekstoprovideindicativecostsbasedonoperatorprovideddatainitsannualDecommissioningInsightSurvey[Ref13],andthiswork iscontinuing.Furthercollaborativeeffort isunderwaytodevelopacredibleanddetailedcostmodelforthetotalcostofpipelinedecommissioning.


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11 Pipeline Decommissioning to Date

ThepipelinesdecommissionedintheNorthSeatodateanddescribedassuchinpublicallyavailablesourcesarepresentedinAppendixAandsummarisedinFigure5.

Figure 5 Pipelines and Umbilicals Decommissioned in the North Sea To Date (2013)

Pipeline Description

Diameter Range

Numberof Pipelines

TotalLength(km)Estimated1

Trunklines 16to32inches 17 62Flowlines(RigidandFlexible)

upto14inches 123 692

Umbilicals and PowerCables

upto8inches 20 79

Totals - 1601 833

Note1:Thisdataisfrompublicallyavailableresources,anddiameterandlengthdataisnotavailableforallpipelines/umbilicalsdecommissionedsofar.

Case studies for the Shelley, NorthWest Hutton and Tristan NW pipeline decommissioning projects areincludedinAppendixB.TheseprojectsillustratetherangeofprojectsperformedtodateintheNorthSeaRegion.

Figure5showsthatpipelinedecommissioningintheNorthSeaRegionisstillataveryearlystage.Lessthan2percentofthetotallengthoftheexistingNorthSeapipelineinventoryhassofarbeendecommissioned.

Ofthetotalnumberofpipelineswhichhavebeendecommissioned80percentarelessthan16inchesindiameter.Halfofthelargerdiameterpipelinesalreadydecommissionedwereremoved.Thesewereallunderonekilometreinlengthandinfieldpipelines.Theremainingpipelineshavebeenleftinsitu.Thelongestlargediametertrunklinetobedecommissionedsofaristhe35kilometrePiperAtoClaymore30incheexportline,whichwasdecommissionedinsitu.


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12 Technology

12.1 OverviewWhen evaluating a preferred option for decommissioning a pipeline and its associated equipment, theavailability and track record of technology provides the context for the other key CA criteria of safety,environmentalimpactandcost.Keytechnologyareasinpipelinedecommissioningare:• Pipelinecleaning• Trenching,burialandde-burial• Subseacutting• Lifting• Reverseinstallationmethods• Mattressremoval

Thecurrentstatusoftechnologyintheseareasisnotedinthefollowingsections.ExamplesofthepipelinedecommissioningprojectscompletedtodateareprovidedinAppendixB.

12.2 Pipeline CleaningPriortodecommissioning,apipelinewillbedepressurizedandanyhydrocarbonsremoved.ItisthenpurgedofitscontentsandcleanedinaccordancewiththePipelinesSafetyRegulations[Ref7].Thismayinvolvetheuseofpigs,whicharepipelinemaintenancetoolsusedforcleaningorinspectingtheinsideofapipeline.

Whetherapipelineisremovedordecommissionedin-situ,itisthoroughlycleanedtoensurethatpollutantsare not released to the environment in unacceptable quantities. For a pipeline decommissioned in-situ,thepipelineiscleanedtominimisepotentialcontaminationofthemarineenvironmentbydischargeofanyresidualhydrocarbonsfromthepipelineasitdegradesovertime.

Cleaningandpurgingiscarriedoutfollowingcessationofproduction,pipelinesystemdepressurisationandremovalofbulkhydrocarbons.Thecleaningprogrammeisdevelopedbasedonthespecificneedsofeachsystem,butatypicalprogrammemayinclude:• Chemicalcleaningtodetachhydrocarbonresiduefromthepipewall(usingbulksurfactantsorgelpigs)• Bi-directionalmagneticcleaningtoremoveferrousdebris• Bi-directionalbrushcleaningtoremoveotherloosedebris• Bi-directionaldisccleaningpigtoscrapetheremainingsoftermaterialfromthepipewall

Bi-directionalmagneticcleaningandbrushcleaningmayrequiremultiplepassesuntilthelineisattherequiredcleanlinessi.e.thewaterqualityemergingwiththepigsiswithinallowablecontaminantlevels.Dependingontheconditionofthepipeline,andthecleaningscheduleadoptedduringoperation,thecleaningprogrammeatdecommissioningmayincludefoampigsorspecialistmechanicalcleaningpigs.

12.3 Trenching and BurialThetechnologyfortrenchingandburialofpipelinesiswellestablished.Anumberofcontractorsofferarangeoftrenchingtoolscapableoftrenchingandburyingpipelinesofvariousdiametersinallsoiltypes.

Technologyexistsforpost-laytrenchingandburialofpipelines,andforremedialburial,thistechnologyisreadilyapplicabletodecommissioning.Thereislimitedexperienceofpipelinesbeingburiedspecificallyfordecommissioninginsitu.Itwas,however,thepreferredoptionfordecommissioningthe20inchoilexportpipelineinBP’sNorthWestHuttondecommissioningprogramme.SimilarlytheFrigg-Oseberg‘Frostpipe’oilpipelinewasapprovedforselectiveburialofexposedsectionsalongitsroute.


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Therearethreemaintypesoftoolincommonuseonsubseapipelines:• Jettingmachine• Cuttingmachine• Plough

Theapplicabilityofeachtrenchingmethodtoaburialoperationwilldependonanumberoffactors,mostgenerally the sizeof thepipeline and the type and strengthof soil. Figure 6 gives a general viewof theapplicabilityofthetypesoftoolsavailable.Therearealsohybridtoolsavailablethatcombine jettingandtrenchingfunctionstocoverawiderrangeofsoilconditions.

Figure 6 Trenching Method Suitability (Source Atkins)

Jettingsystemsvaryfromcomplexexcavatorstosimpletrenchingsleds.Jettingtoolscanworkinsand,siltandmediumclay.Jettingexcavatorscanalsobeusedindeburialoperationstoremovenon-cohesivematerials,includingrockdump.

Jettingproduceswideshallowtrenches in loosesand,andthereforemaynotprovidesufficientburial fordecommissioning.Indensersandsandweakercohesivesoilsthetrenchshapeiswelldefined.

J E T T E R S

P L O U G H SC U T T E R S

Denisty Very loose

Very Soft Soft Firm Sti� Hard RockStrength

ASSETSELECTION

COHESIONLESS SOIL - SAND

COHESIVE SOIL - CLAY

Loose Medium Dense Dense Very Dense Cemented


page29

Cuttingtrenchersareessentiallythesameas jettrenchingvehiclesbutusemechanicalmeansofcreatinganopentrenchsuchaschaincutters,wheels,disks,etc.Thesoil iscutunderthepipeandthematerial isentrainedusingadredgepumpsystemandejectedtothesideofthetrench. Atrenchingploughoperatesbybeingpositionedastridethepipelinewiththecuttingshareopen.Thepipelineispickedupbyforeandaftgrabscreatingaspaninthepipeline.Rollersareclosedaroundthepipetosupporttheloadduringburialprocessandtheshareisclosedbeneaththepipe.Therigidpipelineis loweredintoa‘V’cuttrench,formedbymechanicaldeformationoftheseabedbythepipelineploughastheploughis pulledforward.

Theexcavatedtrenchmaterialisdepositedinbermsonbothsidesoftheformedtrenchandcanberemovedoncompletionofthetrenchingpassbyabackfillingprocess.Sometrenchingploughsexistthatcanbeusedtobackfilltrenchesonasecondpass.Otherwise,aseparatebackfillploughwillbeusedincombinationwiththetrenchingplough.

Figure 7 Pipeline Trenching Plough

This image is reproduced with kind permission from Deep Ocean.Further use of this image, of any kind in any format, must first have written consent from Deep Ocean.


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12.4 DeburialandDredgingThereareanumberofcontractorsofferingMassFlowExcavators(MFEs)capableofdredginganddeburialoperationsduringpipelinedecommissioning,aswellaslocalpipelineburial.MFEscanbeemployedinmostsoilconditions,andarecapableofexcavatingrockdump.

MFEsworkusingrotatingpropellerstocreateahighspeed,lowpressureaeratedcolumnofwatertofluidisetheseabedmaterialforeitherburialordeburialoperations.AMFEduringdeploymentisshowninFigure8.

Figure 8 Mass Flow Excavator

This image is reproduced with kind permission from Reef Subsea.

Further use of this image, of any kind in any format, must first have written consent from Reef Subsea.

12.5SubseaCuttingThere are several differentmethods and types of equipment for cutting pipelines subsea as part of thedecommissioningprogramme.Themaintypesofcuttingequipmentfallintothefollowingcategories:• AbrasiveWaterJet• DiamondWireCutting• ReciprocatingCutting• HydraulicShears

Theseareall termed‘coldcuttingtools’andcanbeoperatedbydivers,mountedtoanROV,oroperateddirectly fromonboardavessel. Socalled ‘hot’ cutting toolsalsoexist,but thesehave limitedapplicationduringdecommissioningandarenotdiscussedfurtherhere.


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In all pipeline-cuttingapplications, the cuttingdevicemust gain access aroundall orpartof thepipelinecircumference,andthismayrequiredredgingunderneaththepipelinetopositionthetool. Inadditiontopotentialaccessconstraints,theselectionoftheappropriatetoolforcuttingpipelinesectionswilldependonthesizeofthepipelineandthecoatingsappliedtoit.Inallcases,toolselectionwillbebasedonfindingthesafestapproach,whichminimizestherisksposedtopersonnelbytheoperation.

Abrasivewaterjetisacommonlyusedsubseacuttingmethodandconsistsofahigh-pressurejetofwaterandsand/gritmix,whichisdirectedontotheitemtobecut.Typically,theabrasivewaterjetwilloperateat10,000-15,000psi.Thismethodisveryversatile,andduetotherelativelysmallcuttingheadtheprocesscanbeusedwhereaccessisrestricted.Insometools,thewaterjetiscapableofcuttingthroughbothsidesofthepipesimultaneously,andhenceneedonlyrotatehalfwayaroundthepipetocompleteacut.Othersrequireaccessaroundthecompletecircumference.

Diamond wire cutting tools are commonly used on pipeline decommissioning and intervention projectsaround theworld. A diamondwire cuttingmachine consists of a continuous loopof diamond-encrustedwiremountedonapulleysystemwhich isdriveneitherhydraulicallybyahydraulicpowerunitonboardthedeploymentvessel,orsubseabyaworkclassROV.Theycanalsobedrivenelectricallywhenworkingatgreaterwaterdepths.Aswithabrasivewaterjetting,dredgingofpipelinesontheseabedmayberequiredtopositionthetoolatthelocationtobecut.

Dependingonthenumberandtypeofcutsbeingmade,adiamondwiremayrequireregularreplacement.Thisisanexpensiveandtime-consumingoperation,whichisperformedonthedeckofthesupportvessel.Forlargerdiameter,concretecoatedpipelines,thediamondwiremayneedreplacingaftereverycut,whichwouldlimititsapplicabilityinamajorcutandliftoperationonalongdistancetrunkline.

Therearetwomaintypesofreciprocatingtool:thebandsawandtheguillotine.Bothtoolsuseaserratedsteelblade:theguillotinecutterusesareciprocatingmechanismto‘slice’downintothepipewithaback-and-forthmotion,whilethebandsawhasacontinuousflexiblebladedrivenroundanumberofpulleys.Bothtoolsareclampedtothepipeinordertoperformthecut.

Theguillotinecuttercancutamaximumdiameterof32inchesandcanbefullyROVcontrolled.Likewise,thebandsawiseitherdiverorROVcontrolled,buttoolingupto48inchescutdiameterisavailable.Forbothtools,thespeedofcuttingandthelifespanofthebladesaredependentonthematerialsbeingcut.Largediameterconcretecoatedpipewillbeparticularlyslowtocutandwillleadtothehighestbladeconsumption.Thismakesthesecuttersalessattractiveoptionforlong-distancetrunklines.

Hydraulicshearsaretraditionallyusedonshoreoronthedeckofanoffshoreplatform.Thesearemountedontheboomofanexcavatoranddismantlefacilitiesusingthe‘piece-small’method.Recentlyhowever,theyhavebeenusedforsubseaapplication,andalsoinmajorprojectssuchasthedecommissioningoftheNorthWestHuttonplatform.

For subseaapplication theshear is suspended fromthevessel craneandplaced inposition foreachcut.Although the tool has had limited use subsea, its simple operation could make it suitable for makingmultiple cuts along longpipelineswithout recovery to thedeck for replacementof consumables, etc.Atpresentthetoolisonlydeployablewithdiverassistance,butitisunderstoodthatanROVoperabledeviceis underdevelopment.


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Onedisadvantageof thehydraulicshear is that itdoesnotproducea ‘clean’cut,whichmayrepresentahazardtopersonnelduringhandlingandmaymakethehandlingofcutpipesectionsthemselvesdifficult.Itisnotedthatthereisnopublishedevidenceofshearsbeingusedforpipelinedecommissioningtodate.

12.6 LiftingThereisaneedduringthe‘cutandlift’processofdecommissioningtoliftthecutpipelinesectionsfromtheseabedtoatransportationvessel.Thisisperformedusingroutineliftingtechniques,butwillusuallyrequirediversupport.Asnotedabove,somecuttingtechniquesproduceacoarsecut,whichcaninfluencehowtheliftingequipmentisattachedtothepipesection.Thiscanslowdowntheliftingoperationsandmayhaveasignificantimpactonthedurationofliftingoperationsforlonglengthsofpipeline.

12.7 ReverseInstallationMethods

12.7.1 Reverse ReelingReversereelingistheprocessbywhichrigidorflexiblepipelinescanberecoveredfromtheseabedbyreelingthemfromtheseabedusingaspecialistreelvessel.TheprocessisdescribedinSection6.2.2.1.

For rigidpipe, therearea limitednumberof specialist reelvesselsavailable fromthe leading installationcontractors.Thesevesselsareusuallyengagedininstallationactivities,butcanbeandhavebeenadaptedtorecoverpipelinesaspartofadecommissioningproject.VesselssuchasTechnip’sApacheII(Figure9)andSubsea7’sSevenNavica(Figure10)arecapableofperformingthiswork.

Figure 9 Apache II Reeling Vessel

This image is reproduced with kind permission from Technip. Further use of this image, of any kind in any format, must first have written consent from Technip.


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Figure 10 Seven Navica Reeling Vessel

This image is reproduced with kind permission from Subsea 7. Further use of this image, of any kind in any format, must first have written consent from Subsea 7.

Flexibleflowlinesandumbilicalsalsorequirespecialistequipmenttocarryouttherecoveryoperationbutmorevesselsareavailabletoperformthiswork.

Althoughtheuseofthesevesselsforbothrigidandflexiblepipelinerecoveryhasnotbeencommon,bothmethodshavebeenusedindecommissioningprojectsandcanbeconsideredproven.

12.7.2 Reverse S-layTheprocessbywhichpipelinescouldberemovedatdecommissioningbythereverseS-layprocessisdescribedinSection6.2.2.2.Asnoted,thisisnotanoperationthathasbeencarriedoutintheNorthSea,althoughitisunderstoodthatthereissomeexperienceofremovingshortlengthsofsmalldiameterpipelinesusingthismethodinshallowwaterintheGulfofMexico.

ExamplesofpipelayvesselsutilisingtheS-laymethodforinstallationofpipelinesareSaipem’sCastoroSeiandAllseas’Audacia(Figure11).


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Figure 11 Dynamically Positioned Pipelay Vessel Audacia

This image is reproduced with kind permission from Allseas.

Further use of this image, of any kind in any format, must first have written consent from Allseas.

ThesevesselsareexamplesofhighspecificationinstallationvesselsthathaveoperatedregularlyintheNorthSeainstallinglargediameterpipelines.Neitherhasbeenusedtoremovelonglengthsofpipelineaspartofadecommissioningproject.

Ingeneral,reverseS-layhasnotbeenusedforthedecommissioningoflargediameterpipelines,howevera number of issues have been identified regarding the feasibility of the process. These are noted in Section6.2.2.2.

FurtherstudyisnecessarybeforethereverseS-layprocesscanbeconsideredfeasiblefordecommissioninglongdistancelargediameterpipelines.

12.8 MattressRecoveryTherecoveryofmattressesisadiverandvessel-intensiveoperation,withthetimetakentoperformtheworkverymuchdependentontheageandconditionofthemattressesbeingrecovered.

Asyet,noestablishedtechniqueortechnologyhasbeenuniversallyadoptedformattressrecovery.Itislikelythatthemajorityofnewermattresses(iethoseinstalledwithinthelast10years)willhaveblockslinkedwithpolypropyleneorKevlarropes.The lifting loopscouldbeexpectedtobe ingoodcondition,althoughthiswouldhavetobeconfirmed,andthesemattressescouldberecoveredtosurfaceusinghandlingframesand/orspeedloaders.Speedloadersarealiftingarrangementdeployedontheseabedontowhichanumberofmattressescanbeliftedusingliftingframesbeforethespeedloaderisrecoveredtothesurface.


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Foroldermattresses,orformattressesthathavebrokenup,aconventionalgrabtoolcanbedeployedtorecoverthemattresspiecesdirectlytotheservicevessel,ortoabasketsubseaandthenrecovered.

Areviewofmattressrecoveryprojectsto-dateshowsthatitispossibleinsomecasesfornewermattressestoberecoveredinlessthananhour.Foroldermattresses,whichmayhavebeensubseafor20+years,therecoverytimecanbeupwardsof12hourspermattress.

Forspecificdecommissioningprogrammes,samplemattressescanberecoveredduringthepre-commissioningsurveystoreviewtheirconditionandtoconfirmtherequiredrecoverymethod,orthefeasibilityofreusingthemattresseswithinthefield.


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13 Recycling and Reuse

13.1 RecyclingWhenlengthsorsectionsofsteelrigidpipelinesarerecoveredaspartofadecommissioningprogramme,thesteelitselfisrecycledusingaprovensupplychain.Allsteelpipelineshaveananti-corrosioncoatingandoftenhaveinsulationcoatingsapplied.Wherepossiblethesecoatingsareremovedandrecycled,otherwisetheyaresenttolandfill.

Forrecoveredflexibleflowlines,umbilicalsandpowercables,themetallicendfittingscanberemovedandrecycled,orinsomeinstancesreused.ThisisdescribedintheShelleyCloseOutReport[Ref.14].Themetallicelementsofthecarcassofflexibleflowlines,andthewiresusedinarmouringlayersinumbilicalsandpowercables,canalsoberecoveredusingspecialistequipmentandthenrecycled.Suchprocessesseparateouttheplasticmaterialsfromthedifferentlayers,whichcanthenberecycledifpossible.

Adecommissioningprojecttypicallyachievesrecyclingorreuseratesinexcessof95percentoftherecoveredmaterials,andinsomecasesupto98percent.Similarratescanbeachievedforpipelinedecommissioningprojects,dependingonthevolumeandtypeofnon-recyclablecoatingsrecoveredwiththepipelines.

13.2 ReuseIntegrityisakeyissuewhenconsideringthereuseofpipelinesorpipelinematerials.Forrigidsteelpipelines,recoveredinasinglelengthbythereversereelingprocess,thepipewallwillhavebeensubjecttosignificantreverse cycle plastic deformation during its original deployment and then recovery process. This cansignificantlyaffectthelongtermintegrityofthepipestructureandwouldruleitoutforreuse.Hence,nosteelpipelinerecoveredinthiswayhasbeenreused.

Itwouldbepossibletodemonstratethatadecommissionedpipelineleftinsitucouldbereusedforalternativeserviceandoperatingconditionsandthisisregularlyconsidered.

Flexible flowlines, umbilicals and power cables are readily recovered by reverse reeling as part of adecommissioningprogramme.Suchmaterialscantheoreticallybereused,butprovingthattheintegrityofthecomplexmulti-layeredstructureofsuchcomponentshasnotbeencompromisedduringthehandlingandoperationalprocessisdifficult,andoftenrecyclingistheonlyrealisticoption.


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14 PublicConsultation

14.1 RequirementsforConsultationIntheUKthereisastatutoryrequirementforoperatorstoconsultwithstakeholderswhomaybeaffectedby decommissioning proposals under section 29(3) of the PetroleumAct 1998 [Ref 3]. This includes thedecommissioningofoilandgaspipelines.

InNorwaythecessationofproductionplanrequiresaseparateimpactassessmentprogrammetobeprepared.Thisensures thepublicareproperly informedandprovidesvariousstakeholderswith theopportunity toexpressopinionsandinputsintothescopeandexecutionoftheproject.

14.2 StatutoryConsultees(UKCS)AnnexHoftheDECCGuidanceNotes[Ref6]specifiesthoseorganisationsthatshouldbecontactedaspartof statutory stakeholder consultation. These are Global Marine Systems, Northern Ireland Fishermen’sFederation,ScottishFishermen’sFederationandTheNationalFederationofFishermen’sOrganisationsUK.AnnexEandSection6.14oftheGuidanceNotesalsoidentifyGovernmentdepartmentswitharelevantroleandtowhomcopiesofadraftdecommissioningprogrammemustbesent.

14.3 ConsultationProcessOn theUKCS statutory consultation starts at the point atwhich a draft decommissioning programme issubmittedtotheDECC.Aperiodof30daysfortheconsultationisstatedintheDECCguidelines.

Decommissioning proposals are announced by placing a public notice in appropriate national and localnewspapersandjournals,andbyplacingdetailsontheInternet.Thisnoticeindicateswherecopiesofthedraftdecommissioningprogrammecanbeviewedandtowhomrepresentationsshouldbesubmitted.

TypicallytheprogrammeisavailabletodownloadfromtheInternetwithhardcopiesavailableforinspectionattheoperator’soffices.Theresultsofconsultationsarereportedinthedecommissioningprogrammewhenitissubmittedforfinalapproval.

FurtherguidancecanbefoundintheGuidelinesonStakeholderEngagementforDecommissioningActivitiesontheOilandGasUKwebsiteat:http://www.oilandgasuk.co.uk/knowledgecentre/decom_guidelines.cfmInNorwaytheplanforcessationofproductionmustbepresentedbytheoperatortwotofiveyearsaheadofthecessationofproduction.Theplanforcessationissubjecttopublichearing.


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15 References

1 OperationPluto,A.C.Hartley,ProceedingsoftheInstitutionofMechanicalEngineers1946154:433,DOI:10.1243/PIME_PROC_1946_154_054_02

http://pme.sagepub.com/content/154/1/433.citation

2 TotalNUGGETSFieldFactsheet http://www.uk.total.com/pdf/activities/NuggetsFactSheet.pdf

3 UKPetroleumAct1998 http://www.legislation.gov.uk/ukpga/1998/17/contents

4 MarineandCoastalAccessAct2009 http://www.legislation.gov.uk/ukpga/2009/23/contents

5 Marine(Scotland)Act2010 http://www.scotland.gov.uk/Topics/marine/seamanagement/marineact

6 DECCGuidanceNotesDecommissioningProgrammes https://www.og.decc.gov.uk/regulation/guidance/decomm_guide_v6.pdf

7 PipelineSafetyRegulations1996 http://www.hseni.gov.uk/l82_a_guide_to_the_pipelines_safety_regulations_1996.pdf

8 OffshoreSafetyCase http://www.hse.gov.uk/offshore/safetycases.htm

9 FishSAFEproject http://www.fishsafe.eu/en/home.aspx

10 FisheriesLiaisonOfficerGuidelines http://www.oilandgasuk.co.uk/knowledgecentre/Fisheries.cfm

11 OffshoreChemicalRegulations2002 http://www.legislation.gov.uk/uksi/2002/1355/contents/made

12 OSPARCEMPAssessmentReport2011 http://www.ospar.org/documents/dbase/publications/p00563_cemp_2011_assessment_report.pdf

13 Oil&GasUKDecommissioningInsightReport http://www.oilandgasuk.co.uk/knowledgecentre/market_information.cfm

14 ShelleyCloseoutReport https://www.gov.uk/oil-and-gas-decommissioning-of-offshore-installations-and-pipelines


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AppendixA:TableofDecommissionedPipelinesintheNorthSeaRegion

Country Operator Pipeline Type

Location Length Diameter Fluidtype InstallationDate

DecommissioningOption

From To km inches

NL GDFSuez L10-K L10-B/L10-A(s) 5.8 11 1984

NL GDFSuez K12-A/L10-A K12-E 3.9 2 1986

NL GDFSuez K12-E K12-C 6.3 11 1986

NL GDFSuez L10-S1 L10-AP 11.5 7 1988

NL GDFSuez K12-E L10-S1 4.6 4 1988

NL GDFSuez L14-S1 L11a-A 6.0 7 1990

NL GDFSuez K12-S1 K12-BP 4.9 7 1991

NL GDFSuez K11-B K12-C 16.1 14 1995

NL NAM K11-FA-1 K8-FA-1 6.0 7 1978

NL NGT L11-A NGT-pipe 11.8 11 1990

NL Taqa P15-B P15-C 3.4 10 1985

NL Taqa P15-B P15-C 3.4 6 1985

NL Taqa P15-C P15-B 3.4 6 1985

NL Taqa P15-B P15-C 3.4 4 1985

NL Total K5-EN/C K5-D 2.7 12 1997

NL Total L4-PN L4-A 11.4 10 1999

NL Total K4-BE K4-A 8.0 10 2000

NL Unocal Q!-Helder-B Q1-Helder-AW 1.8 9 1986

NL Unocal Q!-Helder-B Q1-Helder-AW 1.8 9 1987

NL Unocal Q1-Haven-A Q1-Heder-AW 5.8 9 1989

NL Wintershall K13-B K13-AP 9.2 10 1977

NL Wintershall K13-D K13-C 3.5 10 1978

NL Wintershall P12-C P12-SW 6.9 8 1990

NL Wintershall P14-A P15-D 12.6 10 1993

NL Wintershall P2-NE P6-A 38.2 10 1996

NO ExxonMobil Odin FriggTCP2 26.0 20 Gas

NO GasscoAS FriggTCP2 VesterledT _ 32 Gas

NO Total FriggTCP2 OsebergA 82.0 16 Oil

NO Total Trunkline DP2 TCP2 0.7 26 Gas Removal

NO Total Trunkline DP2 TCP2 0.7 26 Gas Removal

NO Total Flowlines DP2 TCP2 0.7 4 Condensate Removal

NO Total Flowlines DP2 TCP2 0.7 8 Chemical Removal

NO Total Power DP2 TCP2 0.7 3 Power Removal

UK _ Ardmore toSal2 1.4 _

UK _ Ardmore toSal2 1.4 _

UK Apache FortiesE FortiesA 4.0 6 Oil 1986

UK Apache FortiesD FortiesC 3.6 20 Oil 1975

UK BHP Flowlines Douglas Lennox 12 Gas Leaveinsitu-minorintervention

UK BHP Flowlines Esmond Forbes 10 Gas Leaveinsitu-minorintervention

UK BHP Flowlines Esmond Gordon 12 Gas Leaveinsitu-minorintervention


page 40




From To km inches

UK BHP Flowlines Esmond Forbes 2 Gas Leaveinsitu-minorintervention

UK BHP Flowlines Esmond Gordon 12 Gas Leaveinsitu-minorintervention

UK BP FlexibleFlowlines

Don Don 4 Oil Removal


Don Don 4 Water Removal

UK BP Risers Schiehallon&Loyal

Schiehallon&Loyal

0.8 10 Oil Removal


Schiehallon&Loyal

0.8 8 Oil Removal

UK BP Umbilical Schiehallon&Loyal

Schiehallon&Loyal

0.7 8 Removal

UK BP Umbilical Schiehallon&Loyal

Schiehallon&Loyal

0.7 8 Removal


Schiehallon&Loyal

0.7 10 Oil Removal


Schiehallon&Loyal

0.7 10 Oil Removal


Schiehallon&Loyal

0.7 10 Oil Removal


Schiehallon&Loyal

0.7 8 Oil Removal


Schiehallon&Loyal

1.4 8 Gas Removal


Schiehallon&Loyal

2.9 12 Water Removal


Schiehallon&Loyal

0.7 8 Oil Removal


Schiehallon&Loyal

0.8 10 Oil Removal

UK BP Flowlines NinanTee NorthWestHutton

10 Gas Leaveinsitu-minimalintervention

UK BP Trunkline NorthWestHutton

CormorantA 20 Oil Leaveinsitu-majorintervention

UK BP Trunkline Hutton(TLP) NorthWestHutton

<0.5 12 Oil Leaveinsitu-minimalintervention


Schiehallon&Loyal

0.7 10 Removal


Schiehallon&Loyal

0.7 10 Oil Removal

UK BP RigidFlowlines

Don Thistle 17.3 8 Production Leaveinsitu-minimalintervention

UK BP RigidFlowlines

Don Thistle 13.1 8 Water Leaveinsitu-minimalintervention

UK BP Umbilical Don Thistle 17.7 3 Chemical Leaveinsitu-minimalintervention


Don Don 4 Water Removal





UK BP Flexible Flowlines

Schiehallon&Loyal

Schiehallon&Loyal

1.9 12 Removal


page 41




From To km inches


Schiehallon&Loyal

Schiehallon&Loyal

4.5 _ Removal


Schiehallon&Loyal

Schiehallon&Loyal

2.0 10 Oil Removal


Schiehallon&Loyal

Schiehallon&Loyal

0.1 10 Oil Removal

UK BridgeEnergy

Flowlines TristianNW TristianNW 15.5 6 Production Leaveinsitu-minorintervention

UK BridgeEnergy

Umbilical TristianNW TristianNW 15.3 4 Leaveinsitu-minorintervention

UK CNRI Flowlines Staffa Ninan 9.6 9 Oil Removed

UK ConocoPhillips Hutton(TLP) NWHutton 8.0 12 Oil Leaveinsitu

UK ConocoPhillips Maureenplatform

ALC 24 Oil Leaveinsitu


Moira 10.0 6 Oil Removal-reversereel


Moira 10.0 2 Gas Removal-reversereel


Moira 10.0 4 Removal-reversereel

UK Hamilton Moira Maureen* 10.1 7 Oil

UK Hess FlexibleFlowline

Fife Fife 2.0 6 Production Leaveinsitu-minimalintervention


Fife Fife 1.7 7 Water


Fife Fife <0.1 4 Water Removal-reversereel

UK Hess Flowlines Fife Fife _ Gas Removal-reversereel

UK Hess Flowlines Fife Fife _ Chemical Removal-reversereel


Fergus Fergus 7.4 7 Leaveinsitu-majorintervention

UK Hess Flowlines Fergus FPSO 5.6 7 Oil Leaveinsitu

UK Hess Flowlines Fergus Fergus _ Chemical

UK Hess RigidFlowline

Flora Flora 8.0 8 Production Leaveinsitu-minimalintervention


Flora Flora 7.0 3 Gas Leaveinsitu-minimalintervention


Flora Flora 8.4 8 Water Leaveinsitu-minimalintervention

UK Hess Flowlines Flora Flora 9.5 6 Chemical


Fife Fife 2.0 6 Production Removal-reversereel


Flora Angus 18.8 8 Oil 2002 Leaveinsitu


Angus Flora 10.4 3 Gas 2002 Leaveinsitu

UK Hess Flowlines Ivanhoe/RobRoy

Claymore 40.0 14 Oil 1990 Leaveinsitu-minorintervention

UK Hess Flowlines Ivanhoe/RobRoy

Tartan 23.0 8 Gas 1990 Leaveinsitu-minorintervention

UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Production

UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Oil 2000 Removal

UK Hess Flowlines Ivanhoe RobRoy 1.6 5 2000 Removal


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From To km inches

UK Hess Flowlines Ivanhoe RobRoy 1.6 4 Gas 2000 Removal

UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Water 2000 Removal

UK Hess Flowlines RobRoy Ivanhoe/RobRoy

1.6 4 Chemical 2000 Removal

UK Hess Flowlines RobRoy RobRoy <0.1 4 Power Removal-reversereel

UK Hess Flowlines RobRoy RobRoy <0.1 2 Gas Removal-reversereel

UK Hess Flowlines RobRoy RobRoy <0.1 4 Water Removal-reversereel

UK Hess Flowlines RobRoy RobRoy <0.1 _ Chemical Removal-reversereel

UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Oil 2000

UK Hess Flowlines Ivanhoe RobRoy 1.6 5 2000

UK Hess Flowlines Ivanhoe RobRoy 1.6 4 Gas 2000

UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Water 2000

UK Hess Flowlines RobRoy Ivanhoe 1.5 _ Chemical 2000 Removalofdynamicsections

UK Hess Flowlines Ivanhoe Ivanhoe <0.1 4 Power Removal-reversereel

UK Hess Flowlines Ivanhoe Ivanhoe <0.1 2 Gas Removal-reversereel

UK Hess Flowlines Ivanhoe Ivanhoe <0.1 4 Water Removal-reversereel

UK Hess Flowlines Ivanhoe Ivanhoe <0.1 _ Chemical Removal-reversereel

UK Hess Umbilical Flora Angus 10.5 1 Chemical Leaveinsitu-minimalintervention

UK Maersk _ 9 Oil

UK Maersk _ 2 Fibre

UK Nexen SCOTT SCOTT 1.7 _

UK Oryx Flowlines Ninian HuttonTLP 8.6 8 Gas 2000

UK Perenco Flowlines Welland Thames 17.5 16 Production Leaveinsitu-minimalintervention

UK Perenco Flowlines Thames Welland 17.2 3 Chemical Leaveinsitu-minimalintervention

UK Perenco Flowlines Welland Welland 8.0 8 Gas Leaveinsitu-minimalintervention



UK Perenco Umbilical Welland Welland 8.0 4 Leaveinsitu-minimalintervention

UK Perenco Umbilical Welland Welland 5.8 4 Chemical Leaveinsitu-minimalintervention

UK Perenco Umbilical Welland Welland 4.2 4 Chemical Leaveinsitu-minimalintervention

UK Premier Umbilical Shelley FPSO 2.4 3 Power Removal-reversereel

UK Premier Flowlines Shelley FPSO 2.0 8 Production Leaveinsitu-minimalintervention

UK Shell BrentSouthUTA

TOBS-2 _ _ Condensate

UK Shell BrentSouthUTA

TOBS-1 _ _ Condensate

UK Shell BrentSouthWI2

TO OP4 _ _ Water

UK Shell Flowlines LemanAP LemanCD 8 Chemical


page 43

DatainthistablewasobtainedfrompubliclyavailablesourcesviaDECCandDEAL.Insomeinstancethefulldata-setrequiredforindividualpipelineswasnotavailable.




From To km inches

UK Shell LemanBT LemanBP 4

UK Shell Flowlines IndeM IndeJ 3.4 12 Gas 1985 Leaveinsitu-minorintervention

UK Shell Flowlines IndeJD IndeM 3.7 3 Condensate 1985 Leaveinsitu-minorintervention

UK Shell Flowlines IndeK IndeN 2.4 10 Gas 1987 Leaveinsitu-minorintervention

UK Shell Flowlines IndeK IndeN 2.4 3 Condensate 1987 Leaveinsitu-minorintervention

UK Shell Trunkline IndeJ IndeAT 3.9 20 Production 1971 Leaveinsitu-minorintervention

UK Shell Trunkline IndeK IndeAT 9.1 24 Production 1972 Leaveinsitu-minorintervention

UK Shell Flowlines IndeL IndeJ 3.2 16 Gas 1977 Leaveinsitu-minorintervention

UK Shell Flowlines Statfjord BrentSouth 10 Oil Notinuse

UK Silverstone Flowlines TristianNW DavyNUI 15.5 6 Gas Leavein-situ-minimalintervention

UK Talisman Trunkline PiperA Piper/Claymore 35.2 30 Oil

UK Talisman Trunkline Tartan MCP01 18 Gas

UK Talisman Claymore 2.0 14 1999

UK Talisman ClaymoreSpur 7.3 30 2000 Leavein-situ-minimalintervention

UK Total Power TP1 FP 0.5 3 Power Removal

UK Total Power TP1 FP 0.5 4 Power Removal

UK Total Power CDP1 TP1/QP 0.5 3 Power Removal

UK Total TP1 FP 0.5 3 Removal


UK Total Risers MCP01TalismanPipeline 0.5 18 Removal

UK Total Trunkline MCP01FriggPipeline 0.5 32

UK Total Trunkline MCP01VesterledPipeline 0.5 32

UK Total AlwynNorth 4.7 6 Water 2000

UK Total Umbilical CDP1 TP1/QP 0.5 8 Chemical Removal

UK Total Trunkline CDP1 TP1/QP 0.5 26 Gas Removal

UK Total Trunkline CDP1 TP1/QP 0.5 26 Gas Removal

UK Total Flowlines CDP2 TP1/QP 0.5 4 Condensate Removal

UK Total Trunkline TP1 FP 0.5 24 Gas Removal



UK Venture Umbilical Kittiwake Kittiwake _ 3 Chemical Removal-Cutandlift

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NorthWestHutton

Operator BP

Water depth 143metres

Pipelines 10inch12.6km OilImport

20inch,12.8km OilExport

Field Description

The NorthWest Hutton field is located in Block 211/27a of the UKCS. The facilities at NorthWestHuttoncomprisedasteelplatform,wellsandpipelinesthatwereinstalledtoproducehydrocarbonsandassociatedproductsfromtheNorthWestHuttonreservoir,discoveredin1975.

The North West Hutton facilities were installed and commissioned between 1981 and 1983. TheplatformisoperatedbyAmoco(UK)ExplorationCompany,onbehalfofAmocoUKPetroleumLimited,asubsidiaryofBPplc.BPown25.8percentofthefield,andtheotherownersareCIECOExplorationandProduction(UK)Limitedwith25.8percent,EnterpriseOilU.K.Limitedwith28.4%andEssoExplorationandProductionUKLimitedwith20percent.

TheNorthWestHuttonfieldbeganproductionin1983,andbythetimeofcessationofproductioninJanuary2003thefieldhadproducedsome126millionbarrelsofoil.

Pipeline Decommissioning Execution

DecommissioningoftheNorthWestHuttonpipelinestookplacein2011and2012.

The 10 inch, concrete coated, gas import pipeline carried gas from the Ninian Tee to the NorthWestHuttonplatform.Thepipelinewastrenchedat installationandremainedburiedat thetimeofdecommissioning,andtheCAofoptionsconcludedthatthepipelineshouldbedecommissionedinsitu.Thepipelineendsandancillaryequipment,suchasmattresses,wereremovedandrecoveredtoshoreforrecyclingordisposal.

At installation, the20 inch,concretecoatedoilexportpipelinewas laidontheseabedbetweentheNorthWestHuttonandCormorantAlphaplatforms.Toremoveanypotentiallong-termriskstootherusersofthesea,thedecommissioningoptionselectedusingCAwastotrenchandburythepipelinebelowtheseabed.At threecrossingsalong the lengthof thepipeline, sectionsof thepipelinewereremovedandrecoveredtoshore,withthepipelineendsremaininginthetrench.Allmattressesusedtoprotectthepipelinewererecoveredtoshoreforrecyclingordisposal.

AppendixB:CaseStudiesofPipelineDecommissioningProjects


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Shelley

Operator PremierOil

Water depth 92-96metres

Pipelines 8inch2.0km,Production 3.5inch,2.4km,

ControlUmbilical

Field Description

TheShelleyfieldislocatedinBlock22/02band22/03aoftheUKCS,approximately192kilometresfromthenortheastcoastofScotlandand32kilometresfromtheUK/Norwaymedianline.

TheShelleyfacilitiesconsistedoftwoproductionwellswithXmastreesandfishing-friendlyprotectivestructures,andasubseaproductionmanifold insideaprotectionstructure.TheseweretiedbacktotheSevanVoyageurFPSObya2.02kilometrelongtrenchedandrockdumpedrigid8inchproductionpipelineanda2.42kilometretrenchedelectro/hydrauliccontrolumbilical.Bothofthesewerelocatedina10metrewidecorridorbetweentheproductionmanifoldandtheFPSO.

Thefieldbeganproducing inAugust2009,but reservoirperformancedidnotmeetexpectations socessationofproductionoccurredinJuly2010.TheShelleyfacilitiesweredecommissionedduring2010and2011,anddetailscanbefoundinthepubliclyavailabledecommissioningclose-outreport.


Fromacomparativeassessmentofdecommissioningoptions,exposedsectionsof the trenchedandrockdumped8inchproductionpipelinewereremoved,withthepipelinecutasclosetotherockdumpbermaspossible. The remaining trenchedand rockdumped sectionsweredecommissioned in situ.Priortoremovaloperations,theproductionpipelinewasflushed,achievingoilinwaterconcentrationoflessthan30partspermillion.


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Theexposedsectionsoftherigidproductionpipelinewerecutusingahydraulicallypoweredsubseasaw,withtheresultingshortsectionsprotrudingfromtherockdumpprotectedfromsnaggingusinggrout bags. The cut sections of pipewere returned to shore for recycling. The 2.42 kilometre longelectro-hydraulicumbilicalwasremovedinsectionsandreturnedtoshorefordisposal.Theumbilicalwascutusingahydraulicguillotinecutteronthedeckoftherecoveryvessel.Theshorttie-insectionsofflexiblepipeateachendofthepipelinewererecoveredtoshoreforrefurbishmentandpotentialreuse.Allmattresseswererecoveredtoshoreandreusedasfoundationmaterialsbyalocalfarmer.


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Tristan NW

Operator BridgeEnergy (FormerlySilverstone EnergyLimited)

Water depth 25-35metres

Pipelines 6inch15.5km, Production

4inch,15.3km, ControlUmbilical

Field Description

The NorthWest Hutton field is located in Block 211/27a of the UKCS. The facilities at NorthWestHuttoncomprisedasteelplatform,wellsandpipelinesthatwereinstalledtoproducehydrocarbonsandassociatedproductsfromtheNorthWestHuttonreservoir,discoveredin1975.

The North West Hutton facilities were installed and commissioned between 1981 and 1983. TheplatformisoperatedbyAmoco(UK)ExplorationCompany,onbehalfofAmocoUKPetroleumLimited,asubsidiaryofBPplc.BPown25.8percentofthefield,andtheotherownersareCIECOExplorationandProduction(UK)Limitedwith25.8percent,EnterpriseOilU.K.Limitedwith28.4%andEssoExplorationandProductionUKLimitedwith20percent.

TheNorthWestHuttonfieldbeganproductionin1983,andbythetimeofcessationofproductioninJanuary2003thefieldhadproducedsome126millionbarrelsofoil.


AdetailedCAwasperformedto identifythebestoptionfordecommissioningthe14.8kilometre lengthof trenchedandburiedproductionpipelinewithpiggy-backedumbilical.Optionswere comprehensivelyassessedandcomparedonthebasisoftheirsafetyrisk,environmentalimpacts,CO2emissions,technicalfeasibility and cost. This assessment indicated that thebestoptionwouldbe to remove the sectionsofpipelineandumbilicallyingontheseabedateitherendoftheline,andleavethecombinedpipelineandumbilicalinitspresenttrench,buriedbynaturalsedimentandareasofspotrockdump.

Theproductionwell,umbilicalandproductionpipelinewereflushedandcleaned,withallproductionfluidsdisposedofviatheDavyNormallyUnmannedInstallation(NUI),or,fortheumbilical,tosea.

TheinfrastructureattheTristanNWFieldwasthencompletelyremovedtoshore,withtheexceptionofthe14.8kilometretrenchedsectionofthepipelineandpiggy-backedumbilical,itsexistingcoverofspotrockdump,andtherockdumppreviouslyprotectingthe250metrecoilofumbilicalattheTristanNWwellhead.

Thisequipmentwasdecommissionedinsituinaccordancewiththeapproveddecommissioningplan.Allmaterialsrecoveredtoshorewererecycled,andnothingwassenttolandfill.


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