d bb pipeline isolation paper

Double block and bleed isolation

by Angus Bowie STATS, Aberdeen, UK

Pipeline Pigging and Integrity Management Conference

Marriott Westchase Hotel, Houston, USA 8-9 February, 2012

Organized by Clarion Technical Conferences

and Tiratsoo Technical and supported by

The Professional Institute of Pipeline Engineers

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Pipelinepiggingandintegritymanagementconference,Houston,February2012

Proceedingsofthe2012PipelinePiggingandIntegrityManagementconference.Copyright2012byClarionTechnicalConferences,TiratsooTechnical(adivisionofGreatSouthernPress)andtheauthor(s).

Allrightsreserved.Thisdocumentmaynotbereproducedinanyformwithoutpermissionfromthecopyrightowners.

DoubleBlockandBleedDefinitionThebaselinedefinitionforincumbentvalvesistwovalves,testedandproventofulllinepressureinadirectionconsistentwiththerequiredisolationwiththevoidbetweenthemventedandmonitoredforpressurerise.Althoughthis is idealisticotherthanonbrandnewplant, itdoes formastartingpointforassessment.Theintentofthislevelofisolationisthatnosinglefailurecaninvalidatetheisolation. Failure (or inadvertentoperation)of theprimaryvalvewouldonlyallowpressure toenterthe bled zone between the valves. The second valvewould prevent any hydrocarbons (or otherpipelinepressure)fromaffectingtheworksite. Failure(orinadvertentoperation)ofthesecondaryvalvewouldonlyopentheambientvoidtotheworksiteofferingnodirectrisk.Theprimaryvalvewouldpreventanyhydrocarbons(orotherpipelinepressure)fromaffectingtheworksite.The requirement for the secondary valve to be tested to verify the isolation capacity is key toensuringthatanybypassoftheprimaryvalveisretained.The requirement for thevoidbetween thevalves tobevented toambient isnecessary toensurethatanypressurebuildupfromapassingprimaryvalvewillbedetectedandthatitcanbeventedtoasafearea,whistbeingretainedbythesecondaryvalve.Preventinganyhydrocarbonsorpressurefromaffectingtheworksite.Thisisasimpleconceptwhichiscomplicatedbytherealityofplantdesignandvalvewear.Theresultisthatmostofthetimetwoleaktightvalvesarenotavailable.Asaresultsomepracticalcompromiseisusuallyrequiredwhenincumbentpipelinevalvesareusedtoprovidedoubleblockandbleedisolation.Thefirstpartofthisisthedefinitionofacceptableleakrates on valves. Most operators have an acceptable leak rate criteria based on pipe diameter,pressure, and hazardous nature of the fluid being isolated.Other factors such as available ventcapacityandevenfirefightingcapacitycanalsobeusedwhenassessingwhatleakratecanbesafelyaccommodated.With thiscompromise, thesecondaryvalve (downstream) is testedusingpipelinepressuretovalidateitissealingwithinthedocumentedacceptableleakrate.Thevoidbetweenthevalves is ventedand locked in tomeasure theprimary valve (upstream) leak rate.Once the leakratesareacceptedaswithinallowable levels,thebleed isopenedtoflaretoensurenodifferentialpressurebuildsacrossthedownstreamvalve.Whereasinglevalve istobeusedtoofferequivalent isolation,thesamerulesshouldbeapplied.Where the valve design energises both the upstream and downstream sealswith the upstreampressure, the valve canbe considered fordoubleblock andbleed.However the singlemodesoffailurerequiretobeassessedtoensurethatsufficientsafeguardsareputinplacetomitigateagainstthesefailuremodes.Oneofthemainsinglefailurepointsisthatasingleactuatorfunctioncanremoveboththeprimaryand secondary seals at once, it is critical that inadvertent or accidental operation of the valve

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edpressureushingittowulicactuatioportedbyth theadditioltogenerate

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There are two plug designs currently on themarket, onewith two lock and sealmodules, eachsimilartotheoriginaldesign,andtheotherbasedonthedualsealandsinglelockarray.Wewilllookat how the two designs compare to theDoubleBlock andBleed Isolation definition later in thisdocument.STATShas also taken the isolationplug selfenergised, fail safe, sealdesign todevelop ahot tapinstalled isolation toolwhich canbe installed through a singlehot tappenetration andoffer theadvantagesofboththemechanicalisolationplugandlinestoptooling.Theothermechanismwhichisusedfortemporaryisolationistheuseoffreezeplugs,wherealiquidoptimally water inside the pipe is frozen, normally by the application of liquid Nitrogen. Thesolidifyingprocesswouldgeneratean iceplugwhichboth sealsandgrips thepipe.This isolationtechnique isattractiveas itdoesnot require fullboreaccess to thepipebore for installationandrecovery,butdoeshave some challenges,particularly the loadingon thepipe fromboth the iceexpansion and the cryogenic effect on pipe. There is also the asphyxiation potential with theNitrogenevaporationandthepotentialtotallossofisolationifthecoolingsystemiscompromised.DoubleBlockandBleedIsolationToolingDouble Block and Bleed is a valve definition, so all temporary isolation techniques and devicesrequire tobe riskassessed tovalidate that theyofferanequivalent isolation level in the requiredapplication. Inorder toassess the comparisonof isolationprovided,weneed first to identify therequirements.Temporaryisolationtechniquesofferdifferentchallengestovalvessocantbeassessedinadirectlyequivalentmanner.Valves are designed to be installed in pipes for many years and a high number of operationsbetweenmaintenance.Asaresulttheyarenormallyprovidedwithacceptableleakrateshigherthanthoseofanewvalve.This,resultsinminimumbleedcapacitiesbeingspecifiedwhereanopenbleedis used. The wear, erosion and corrosion associated with old valves can cause many concerns,although valves in good condition should always at a minimum provide a secure mechanicalrestraint.Temporaryisolationdevicesontheotherhandarenotpermanentlyconnectedtothepipeworkandcan be inspected and tested immediately before use and tend to bemaintained after very fewoperations.The seals canbe replaced foreverydeployment soofferinga consistent levelof sealintegrity.Theonly limitationon this ispipebore conditionwhere the temporary isolationdevicesealsagainst.Allthehighpressureisolationdevicesidentifiedheredoapplysomelocalstressestothepipeatthesetlocationwhichneedtobeassessed.Theabilityoftheisolationintegritytobemonitorediscriticalinthetemporaryisolationassessment.Withvalves,whereanopenbleed isused, themonitoringof the isolation isminimal,asa riseofpressurewouldindicatealossofcontrolovertheisolationratherthananindicationofdeteriorationofeithertheprimarybarrierorventcapability.ThehighspecificationoftheBPrequirementabove,withthebleedlockedinwillofferthefullmonitorbutmaynotbeviablewithvalveleakrates.




The independenceofcontrolsystemstoensurenosingleoperationcould invalidatebothbarriers,which isacause forconcern inthesingledoubleblockandbleedvalve,mustbeaddressed intheassessmentoftemporaryisolationdevices.Accidentaloperation,whichcausesrestrictionsintheuseofESDVvalvesindoubleblockandbleed,iseasiertocontrolintemporaryisolationsystemssincetheytendtobeindependentofautomatedplantsystems,howeverneedstobecoveredinariskassessment.Whereassessingtheintegrityofanisolation,wethereforeneedtoassessthefollowing:

Twoindependentbarriers Abilitytotestbothbarriersinthecorrectdirectiontoverifytheisolationintegrity Any single failuremodes are understood and addressed. E.g. single bulkheads such as in

doubleblockandbleedvalves Bleedlimitations Monitoringcapability Sealintegrity Restraintintegrity Exposuretosingleseal Stabilityofisolationunderunstableloadingfrompressurefluctuations Effectonthepipingsystem

TheseareallassessedinitiallyinaFMECAforthetoolingthenaHAZIDonthedeploymentforSTATSsuppliedequipment.Thispaperwill lookattheSTATSTecnoPlugandBISEP indetailthencomparewithalternativetechnology.TecnoPlugThe current Tecno Plug is derived from the dual seal tethered isolation plug deployed byTecnomarineSystemssincethemid1980s.Thebasicconceptisfortwocompressionsealsmountedin series outboard of a taper lock array. There are several configurations from single to triplemodulewith themultimodule configurationsbeing connectedby structuralball joints.Thebasicloadingandoperationisthesameforallvarieties.

Proceedi

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ondesignedtoduction teeplugcanhow

byanarraythe conting

ialacrossthewhichenergternativede

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This offers the closest monitoring to the valve scenario. Both barriers are proven insitu to fullpipelinepressureandthevoidbetweenthesealsisbledtoambientandlockedinformonitoring.Asthe isolationplugsofferhigh integrity sealing, this annuluspressure isnot anticipated to requireventingduringtheisolation.However,iftheannuluspressuredidrise,thentheloadingonthesealwouldbeunchangedandthesystemhasalreadybeentestedtothesecondarysealloadingsodoesnotgooutsidethetestedcondition.Once the isolation certificate is issued, the control system is put in watchdog mode whichautomaticallymonitors theTecnoPlugsensorsand transmitschanges toalert theoperator.Theplugwillnotacceptanycommandwhichwillchangethestatuswhile inwatchdogwhich locksthesystemfrom inadvertentoperation.Thehydraulicunset isalso limitedandhighpressure isolationscantbeunsethydraulicallywithoutequalisingthepressurefirst.Singlefailureassessmentofthisplugistotheextentthattheoutboardmonitorisprotectedbyashuttlevalvetoprotectfromaleakingfitting.ComparingthistotheDoubleBlockandBleedCriteria:

Both primary and secondary seal are capable and tested to the pipeline pressure in thecorrectdirectionduringthedeployment

Annulus between the seals is vented and locked in minimising exposure from loss ofsecondarysealintegrity

Effectofthe loadingonthepipe iswellunderstoodandengineeredfortheapplication.AllplugsaretestedinequivalentpipetoHydrotestpressures

Both seals and the locks are fully activated by the differential pressure and by hydraulicactivationpressure.Twototallyindependentactuationsystems

Controlsystemlockedfrominadvertentoperation Automaticmonitorofoutboardpressure, inboardpressure,annuluspressure,hydraulicset

pressure,hydraulicunsetpressure.Thesesensorswouldgiveadvancenoticeofanychangeofstatusinthesystem.Thereistheabilitytointerveneinacontrolledmannerifrequired

LookingattheFailureModes:Thepressureheadisasteeldiscsimilarindesigntoablindflangewiththedesignto1.5timestheisolationplugdesignpressure.AsthestandardTecnoPlugworkingpressureisinexcessof3000psiandmostdeploymentsarelessthan2000psi,thenthisoffersadditionalcontingency.Everyisolationtoolistestedinsimilarpipetothecodehydrotestpressurespriortodeployment.Thiscomparestovalvegateswhichonlyrequirea1.1timesMAOPtestpressure.Thefrontcompressionsealistheprimarybarrier.Thissealisheldactivatedbyboththedifferentialpressure and hydraulic set pressure, both of which would require to be lost to invalidate theactivation.Failuremodesassessedarea leakdevelopingandacatastrophic lossofseal.Thesearehighlyunlikelyevents,howevertheconsequencesneedtobeassessed.Aleakpasttheprimarysealwould only restore the annulus pressure to the prevented plug statuswhich had already been




tested. The secondary seal is proven as a full barrier. The Tecno Plug control system allowscontrolledventingofthispressureifrequired.Acatastrophicfailureoftheprimarysealcouldallowmovementofthepressureheadtowardsthetaperlocks.TheTecnoPlugisthereforedesignedtoaccommodate thismovement to the extent of the full loss on the primary seal rubber volume.However,additionalto thisdefence,thesealwouldnotextrudeontothepipelinepressureasthepipelinepressure isactingagainst it.Extrusiontowardsthesecondarysealwouldbe limitedbythefreevolumearoundtheannulusringsoafullseallossisnotpossible.Theplugdesignhashoweverbeendesignedtoaccommodatetotallossofthisseal.Theannulusringisasolidsteelplateundercompressionwhichissupportedacrossthesurfacewiththecompressionsealsandisdesignedofsufficientthicknesstoretainrigidity.The rear compression seal is the secondary seal. This seal activation is maintained by both thedifferentialacrosstheplugandthehydraulicsetpressure,whetherthere ispressurebetweenthesealsornot.ThesealistestedaspartoftheTecnoPlugdeploymentandheldduringtheisolationwithminimalpressurelockedintheannulus.Thissealisassessedfortwofailurescenarios.Firstlyaleak developing. A leak in this sealwould be identified by the loss of the small pressure in theannulus.Thesmallvolumeandpressureminimisesthepotentialhazard.Thesecondhazardassessedis the catastrophic loss of the secondary seal. Like the primary seal the Tecno Plug stroke isdesigned to accommodate thismovement.However like the primary seal, the sealmovement islimited. For the seal to move forward into the annulus, the limited free annulus volumewouldpreventsealloss.Ifthesealmovedbackwards,themovementwouldbelimitedbythelocks.Hencethecatastrophicsealfailurealsoprotectedagainstbythenatureofthedesign.The lockbowl isasolidsteelringdesignedtotakethefulltestpressure inasimilarmannertotheannulusringandpressurehead.The taper lock segments havemore than 100% contingency arrayed around the lock bowl. Thisensuresthateccentricpipesurfacedegradation(e.g6oclockcorrosion)canbeaccommodated.Thelocksare fullyengagedbyboth thehydraulicsetpressureand thedifferentialpressureacross thepluginasimilarmannertotheseals.Theselocksaretestedtothefullpressureduringtheisolationplugdeploymentandthe loadingfromthedifferentialpressure isconstantwithpipelinepressure.Thealternativeofaxiallyseparating thecontingency locks,placingasetbetween theseals,wouldresultintheloadingonthelockgripchangingiftheannulusbetweenthesealschangesofferingthepotentialonanuntested conditionbeinggeneratedduringan isolation.STATShaveassessed thisriskasbeingmorehazardousthanasinglefullytestedandstablegrip.TecnoPlugshavebeenacceptedbymostmajoroperatorsoverthelast20years.

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Proceedingsofthe2012PipelinePiggingandIntegrityManagementconference.Copyright2012byClarionTechnicalConferences,TiratsooTechnical(adivisionofGreatSouthernPress)andtheauthor(s).Allrightsreserved.Thisdocumentmaynotbereproducedinanyformwithoutpermissionfromthecopyrightowners.

Isolationsystem Primarybarriercapability

Primarysealtest

Secondarybarriercapability

Secondarysealtest

Bleed/monitordescription

Controlsystemisolation

CommentsDualvalveandbleed

Blockvalvepotentialforminorleakratedependantonvalveservicehistory

Bleedbetweenvalvescanbeusedforafullservicetestattimeofisolation

Blockvalvepotentialforminorleakratedependantonvalveservicehistory

Pressurisingthevoidbetweenthevalvestopipelinepressurecanprovesecondaryvalve

Bleedvalveandmonitorbetweenthevalvesofferstheopportunitytobleedthevoidtoflareandlockinformonitoring.Alsooptiontoopenthebleedtoflaretopreventpressurebuildup.

Controlvalvestoisolated.Blockvalvestobelockedortagged

High levelofisolationbutrequiresincumbentvalvesinthesystemtobeavailableandoperatingwithinrequiredacceptancecriteria.

Singlesplitgatevalve

Sealonoutboardslab.Actuationonthetaperofthesplitactivatesthesealsonotreliantonpressureenergisation

Bleedonvalvecavitycanbeusedforafullservicetestattimeofisolation

Sealoninboardslab.Actuationonthetaperofthesplitactivatesthesealsonotreliantonpressureenergisation,howeverpressureactingonslaboffersadditionalenergisation

Pressurisingthevalvecavitytopipelinepressurecanprovesecondaryseal

Bleedvalveandmonitorofvalvecavityofferstheopportunitytobleedthevoidtoflareandlockinformonitoring.Alsooptiontoopenthebleedtoflaretopreventpressurebuildup.

Singlevalveactuationmustbemechanicallylockedorisolatedfromtheactuatortopreventinadvertentoperation

Highlevelofisolationbutrequiresincumbentvalveinthesystemtobeavailableandoperatingwithinrequiredacceptancecriteria.

Floatingslabvalvewithpistonsealontheoutboard

Pistoneffectsealontheoutboardsideoftheslabprovidespressureenergisedsealwhilevalvecavityisvented.Springactivationofthesealonlywherethevalvecavityisatpipelinepressure


Sealoninboardslab.Pressureactingonslaboffersmainsealenergisation.Sealremainsenergisedindependentofthevalvecavitypressure







Ballvalvewithdualpistoneffectoninboardseal

Pistoneffectsealontheoutboardsideoftheballprovidespressureenergisedsealwhilevalvecavityisvented.Springactivationofthesealonlywherethevalvecavityisatpipelinepressure


Doublepistonusedontheinboardsealtoenergisethesealwiththevalvecavitypressure.Pistonwillalsoworkinreversewithinboardpressureactivatingtheseal.





Standardtrunionballvalve

Pistoneffectsealontheoutboardsideoftheballprovidespressureenergisedsealwhilevalvecavityisvented.Springactivationofthesealonlywherethevalvecavityisatpipelinepressure


Inboardseatisonlyenergisedbyinboardpressuresocanbeusedforverificationifprimaryseal,butnotsecondarybarrier

Ventvalvecavitywithpipelinepressureonbothsidesofthevalve.



Providessingleprovenblockisolation

STATSslabgatevalvewithdualinboardseals

Sealoninboardslab(outerof2concentricseals).Pressureactingonslaboffersmainsealenergisation.Sealremainsenergisedindependentofthevalvecavitypressure

Bleedbetweenthesealscanbeusedforafullservicetestattimeofisolation

Sealoninboardslab(innerof2concentricseals).Pressureactingonslaboffersmainsealenergisation.Sealremainsenergisedindependentofthevalvecavitypressure

Pressurisingtheannulusvoidbetweenthesealstoorabovethepipelinepressureoffersfullsealverification

Smallvolumebetweenthesealsoffersaverysensitivemonitor.Thisisatemporaryvalveusedforhottappingsosealsarechangedregularlyofferingleaktightsealing.Annuluscanbepressurisedeitherexternallyorwithpipelinepressure.Abilitytobleedpassingsealislimitedduetosmallvolume.

Differentialpressureacrossthevalvegeneratesfrictionsufficienttopreventvalveoperation.Howeverisolationofactuationsystemismandatory.

Notsuitableforpermanentinstallationduetosensitivitytosealdamagewhichcouldpreventisolationbeingachieved.



TecnoPlugconfiguration

CompressionsealmaintainedbothbyselfenergisationfromfullDPacrosstheplugandhydraulics


CompressionsealmaintainedbothbyselfenergisationfromfullDPacrosstheplugandhydraulics

Pipelinepressuretrappedbysettingsequenceallowsfullservicetestattimeofisolation

Smallvolumeheldjustaboveambientandcontinuouslymonitored

Controlsystemactuationsystemiselectronicallylockedbythewatchdogfunction.UnsetsystemisloadlimitedtoensureDifferentialpressurerequirestobelowtoallowunsetting.

Singlearrayoflocksisfullytestedtotheisolatedpressureandloadingisnoteffectedbythepressurevariationbetweentheseals.Zeroenergyzoneoutboardofsecondarysealinnormaloperation

Alternativeremoteisolationplugconfiguration

CompressionsealwithindependentlockringmaintainedbothbyselfenergisationfromhalfDPacrosstheplugandhydraulics

BleedbetweenthesealslimitedtomaintainDPacrossthesecondarymodulecanbeusedforapartialservicetestattimeofisolation.Fullsealtestperformedpriortomobilisation

CompressionsealmaintainedbothbyselfenergisationfromhalfDPacrosstheplugandhydraulics

Sequentialsettingoftheplugmodulescouldallowfullservicetestattimeofisolation

Significantvolumeheldathalfthepipelinepressure.Pressurelockedinandmonitoredforfluctuation

NotknowntoSTATS

Fluctuationsinpipelinepressureorbetweensealannuluspressurescanchangetheplugstatusduringisolation(posttest)e.g.loweringofpipelinepressurecouldbackpressureprimaryseal.Alossofintegrityinsecondarysealwouldbothcontaminatetheworksitewiththetrappedannuluscontentsandapplyanuntestedloadtotheprimarymodule.



BISEP CompressionsealmaintainedbothbyselfenergisationfromfullDPacrosstheBISEPandhydraulics


CompressionsealmaintainedbothbyselfenergisationfromfullDPacrosstheBISEPandhydraulics

Pressurecanbeappliedtotheannulusbetweenthesealshigherthanthepipelinepressuretoperformfulltestonthesecondaryseal

Smallvolumeheldjustaboveambientandcontinuouslymonitored.Alsooptiontoopenthebleedtoflaretopreventpressurebuildup.

ControlsystemactuationsystemHydraulicallylocked.UnsetsystemisloadlimitedtoensureDifferentialpressurerequirestobelowtoallowunsetting.

Dualclevisarmsofferfullredundancyintheaxialrestraint.Zeroenergyzoneoutboardofsecondarysealinnormaloperation.

LineStoptoolwithgasbag

PressurecompetentLipsealleaktight~90%.Energisedbypipelinepressure

Bleedbetweenthelipsealandthegasbag

Gasbag notpressurecompetent.

Bleedbetweenthelipsealandthegasbag

Separatehottapprovidesbleedformonitoring.Bleedcanbeopenedtosafelocationtopreventpressurebuildupbetweenlinestopandbag

Isolatedpressurelockstoolinplace.Retractionsystemisnormallynotcapableofrecoverywithoutpressureequalisation.Gasbaginflationlockedin

Singleprovenisolation

Weldtesttool Mechanicallyorhydraulicallyactuatedcompressionsealnotenergisedbypipelinepressure

Pressuretestbetweensealsprovessealintegrity.Toolaxialrestraintishoweverlimitedtocontactfrictionandsolimitedcapability

Mechanicallyorhydraulicallyactuatedcompressionsealnotenergisedbypipelinepressure

Pressuretestbetweensealsprovessealintegrity.Toolaxialrestraintishoweverlimitedtocontactfrictionandsolimitedcapability

Bleedinthisnormallyreferstoanopenbleedthroughthetoolwhichisusedtopreventpressurebuildupoutboardofthetool

Mechanical/hydrauliclockforactivation.Openventforbleed

Singleprovenisolationforambientoperation.

Dualgasbag Inflatedgasbag Pressuretestbetweengasbagsprovesseal

Inflatedgasbag Pressuretestbetweengasbagsprovesseal

Pressuremaintainedbetweenthegasbagswithinertfluidensuredhydrocarbonvapoursdonotpass.

Lockinofgasbagpressure

Requiredambientisolationwithseparateopenvent.



Singlegasbag Inflatedgasbag None None None none Lockinofgasbagpressure

Offerssingleunprovenambientisolation.

Freezeplug Liquidplugfrozeninsidethepipe.Requirescontinuouscoolingtomaintainisolation

Pressuretestacrosstheplug

None None Candepressurisetheinboardvoidforservicepressuretest.

Noisolationofcontrolsystemfeasible.Failureofactivecontrolsystemwillresultinfailureofisolationpossiblecatastrophicfailure.

Normallyusedinconjunctionwithasecondarysysteme.g.singlevalvetoprovideamorerobustisolation.

Mudpack Pipeendfilledwithmudtopreventhydrocarbonsreachingworksite

None None None none Mudpackrequirestobemanuallyremoverordisplacedwithpipelinepressure.Nocontrolsystemtolock

Singleuntestedambientseal.

Gelplug Piggablegelslugprovidesambientbarrier

None Piggablegelslugprovidesambientbarrier

None None Gelplugsdesignedtomeltinproductionthereforenocontrolsystemlockingrequired.

Notsuitableforwarmclimates.

PPIM CoverDouble Block and Bleed Paper - PPIM 2012 web version

d bb pipeline isolation paper

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