Analysis of the Hull Structure of Sevan Arctic Mobile Offshore Drilling Unit subjected to accidental

133 Pages·2015·13.1 MB·English

by  Patrick Bratlie

Analysis of the Hull Structure of Sevan
Arctic Mobile Offshore Drilling Unit
subjected to accidental Ice Loads
Patrick Bratlie
Marine Technology
Submission date: June 2015
Supervisor: Bernt Johan Leira, IMT
Norwegian University of Science and Technology
Department of Marine Technology

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MasterThesis,Spring2015
for
Stud. Techn. PatrickBratlie
AnalysisoftheHullStructureofSevanArcticMobileOffshoreDrillingUnitsubjectedto
accidentalIceLoads
AnalyseavskrogkonstruksjonenpåSevanArcticMODUutsattforuforutsetteislaster
AcontinuousdevelopmentofstructuraldesigncriteriaforfloatingstationaryunitsinArctic
areasistakingplace. Withinsuchaframeworkitisalsoimportantthatadequatealgorithmsfor
analysisofnon-linearstructuralbehaviorareavailableinordertoquantifyreservestrength
propertiesinthecasethatdamagetostructuralcomponentsmightoccur. Theobjectiveofthe
presentprojectworkistoaddresstheseissuesinsomedetail.
Thecandidateshalladdressthefollowingtopics:
1. A summary is to be made of ice properties, ice mechanics and ice load formulations for
floatinghullstructures.
2. RelevantloadingconditionsfortheSevanArcticMODUunitaretobediscussedwithfocus
on local strength requirements (e.g. related to plate thickness and bending stiffeners).
Corresponding methods for computation of linear and non-linear load-effects and the
associatedstructuralresistanceasimplementedinthecomputerprogramAbaqusareto
behighlighted.
3. LocalresponsecalculationsareperformedforaparticularpartoftheSevanArcticMODU
byapplicationofAbaqus. Theexamplepartselectedbasedondiscussionwiththesuper-
visor.
4. The degree of global platform motion due to ice impact is to be assessed. Alternative
structuralcalculationmodelsshouldbeconsiderediffoundrelevant.
5. Parameterstudiesarecarriedouttotheextentthattimeallows. Acorrespondingmatrix
of parameter values for the different cases to be analyzed should be established prior to
theanalysisitself,andthematrixshouldalsobediscussedwiththesupervisorbeforethe
calculationsareperformed.
Theworkscopemayprovetobelargerthaninitiallyanticipated. Subjecttoapprovalfromthe
supervisor,topicsmaybedeletedfromthelistaboveorreducedinextent.
Inthethesisthecandidateshallpresenthispersonalcontributiontotheresolutionof
problemswithinthescopeofthethesiswork.
Theoriesandconclusionsshouldbebasedonmathematicalderivationsand/orlogicreasoning
identifyingthevariousstepsinthededuction.
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Thecandidateshouldutilisetheexistingpossibilitiesforobtainingrelevantliterature.
Thethesisshouldbeorganisedinarationalmannertogiveaclearexpositionofresults,
assessments,andconclusions. Thetextshouldbebriefandtothepoint,withaclearlanguage.
Telegraphiclanguageshouldbeavoided.
Thethesisshallcontainthefollowingelements: Atextdefiningthescope,preface,listof
contents,summary,mainbodyofthesis,conclusionswithrecommendationsforfurtherwork,
listofsymbolsandacronyms,referencesand(optional)appendices. Allfigures,tablesand
equationsshallbenumbered.
Thesupervisormayrequirethatthecandidate,inanearlystageofthework,presentsawritten
planforthecompletionofthework.
Theoriginalcontributionofthecandidateandmaterialtakenfromothersourcesshallbe
clearlydefined. Workfromothersourcesshallbeproperlyreferencedusinganacknowledged
referencingsystem.
Thethesisshallbesubmittedin3copies:
• Signedbythecandidate
• Thetextdefiningthescopeincluded
• Inboundvolume(s)
• Drawingsand/orcomputerprintswhichcannotbeboundshouldbeorganizedinasepa-
ratefolder.
Supervisor: ProfessorBerntJ.Leira
Deadline: June24th2015
Summary
Duetoanincreasingenergydemandintheworld,theinterestoftheArcticregionforoffshoreoil
and gas exploration is increasing. The demand ofstructures capable ofoperatingin the harsh
Arctic environment is therefore expected to increase. Such structures need to overcome chal-
lengeslikedealingwithextremelylowtemperatureandiceloads. TheSevanArcticMobileOff-
shoreDrillingUnit(SevanArcticMODU)isanArcticdrillingunitconceptdevelopedbySevan
Marine. Themainscopeofthethesishasbeentoinvestigatehowthisstructurearecapableof
dealingwithlargeandunexpectediceloading.
Ice is a complicated material. When compressed, zones of high pressure forces may develop,
accompaniedbyeffectslikespalling,splittingandrecrystallization. Becauseofthecomplicated
mechanical behaviour, the forces exerted from ice onto a structure are often simulated with a
pressure-arearelationship. However, usingsucharelationshipinacollisionanalysisprovesto
beinsufficientsinceitdoesnottakeintoaccountlocaleffectsinagoodway.
Abaqus has been used to model a part of the Sevan Arctic MODU. The model considered is a
part of the platforms outer wall. The structural models have been used in non-linear finite el-
ementsanalysestoinvestigaterelevantloadingconditions. Themodelshavebeenusedassess
two topics concerning a collision between a small iceberg and a floating structure. The first
topicwastoseehowthedrillingunitreactstohighconcentratedpressureloadswhichmaybe
presentinacollision. Itwasfoundthatthestructureisabletoresisthighforceswithoutexperi-
encecriticaldamage. Itwasalsofoundthattheimpactlocationislessimportantinaniceberg
collision than when the structure is subjected to level ice and smaller ice features. This is be-
causethestructureisdesignedwithaslopinghullformintheicedraft,reducingtheforcesfrom
incominglevelicebyforcingtheicetofailinbending.
The second topic was to simulate a collision between the platform and an iceberg. The forces
from the iceberg acting on the structure were calculated by considering the energy in the col-
lision. In these calculations it was assumed that all of the kinetic energy in the collision was
dissipatedascrushingenergyintheiceberg. Theresultsfromtheseenergyconsiderationswere
implementedinAbaqusintermsofapressure-areadependentload.
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Sammendrag
På grunn av en økende energietterspørsel er interessen for Arktis som et fremtidig område for
offshoreolje-oggassutvinningøkende. Detvildermedblietbehovforstrukturersomkanop-
erere i de tøffe arktiske områdene. Slike strukturer må takle utfordringene som ekstremt lave
temperaturerogstoreislaster. SevanArcticMobileOffshoreDrillingUnit(SevanArcticMODU)
er et konsept på en arktisk borerigg som er utviklet av Sevan Marine. Hovedoppgaven i dette
arbeidetharværtåundersøkehvordandennestruktureneristandtilåhåndterestoreoguvent-
edeislaster.
Is er et komplisert materiale. Når det komprimeres kan det dannes områder med veldig store
trykk. Pågrunnavdenkompliserteoppførseltilissimuleresoftekreftenesomutøvesfraispå
enstrukturvedhjelpavtrykk-arealrelasjoner.Brukavslikeforenkledemetodertarderimotikke
hensyntillokaleeffekterpåengodmåteogvisersegåværeutilstrekkeligienanalyseavenkol-
lisjonmellomisfjellogenflytendestruktur.
Abaqus har blitt brukt til å modellere en del av skroget til Sevan Arctic MODU. Modellen er
inkludererenutvalgtdelavplattformsyttervegg.Strukturmodelleneharblittbruktiulineæreel-
ementanalyserforåundersøkerelevantelastkondisjoner.Modelleneharhovedsakligblittbrukt
for å analysere en kollisjon mellom et lite isfjell og en konstruksjon. To hovedtemaer har blitt
settnærmerepå. Detførstetemaetharsomhensiktåvurderehvordanboreenhetenreagererpå
store,konsentrertetrykkbelastningersomkanforekommeienkollisjon.
Det andre temaet har som hensikt å simulere og modellere en kollisjon mellom plattformen
ogetisfjell. Kreftenefrasomvirkerpåstrukturenbleberegnetvedåsepåenergienisammen-
støtet. I disse beregningene ble det forutsatt at all den kinetiske energien i sammenstøtet ble
tattoppavisen.ResultatenefradisseberegningenbleimplementertiAbaqusiformavettrykk-
areaavhengiglasttilfelle.
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Preface
ThisreportisaMasterThesisinthefieldofmarinestructuralengineering. Thethesiswaswrit-
tenattheDepartmentofMarineTechnology,NorwegianUniversityofTechnologyandScience
(NTNU),inTrondheim. Theworkwasconductedduringthespringsemester2015.
AlargepartofthetimespentonthethesisworkwasusedtomakethemodelsusedinFEanal-
yses. Frombefore,IhadlittleknowledgeinusingAbaqus,bothintermsofmakingmodelsand
running analyses. Because this part turned out to be more time consuming than anticipated,
the initial scope of work had to be changed during the semester. Nevertheless, I am pleased
withhowtheendresultturnedout.
Iwouldliketothankmysupervisor,ProfessorBerntJohanLeira,forguidanceandsupportdur-
ing the work process. I would also like to thank Hans Olav Sele and Einar Bernt Glomnes in
Sevan Marine for providing information about the Sevan Arctic MODU and ice engineering in
general. Lastly, I would like to thank fellow student Erlend Hopsdal Skjetne for the numerous
discussionswehavehadregardingmodelingandrunninganalysesinAbaqus.
PatrickBratlie
Trondheim,June22nd2015
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