DECISION-MAKIN GFOR OFFSHORE RESOURCE DEVELOP MENT
by
CMark:Fugle m.B.Sc.
A ThesisSubmitted to the SchoolofGrad uateStudies inPartialFulf1lmentoftheReq uirementsfortheDegreeof
Doctor ofPhilosophyinEngineering
Facultyof Engineeringand AppliedScience Mem orial UniversityofNewfoundland
1997
St.John'sNewfo undlan dCanada
TableorContents ABsTR.Acr•.
ACKNOW1...EDGEMENTS• TABLE OFCO NTENTS ••
usr
OFFIGURES UsTOFTABLES••NOMENCUTURE INTRODUCTION•••••
BA CKGROUND •
2.1 Overview 2.2 Petrclecmresources.. ...
2.3 Environme ntfacto rs
2.4 Systems.strateg ies.and crit eriafor evaluation. 2.5 Riskand reliability-based design
....ii ... . .. iv
...viii
... . .... 1 .j .5 ... .5
. 9
. 15
2.8 Decision andprobab ility theory...
PROBAB nJSTlC APPROACH•• •••
3.1 Overview... 3.2 Introd uction 3.3 Probabilityand itsevaluation 3.4 Exchangeability and mathe ma ticalinference. 2.6
2.7
Economics....
Considera tio n ofhumanlifeandthe environmen t.
...31 .....•J1
... . . . so
... 60
60 .. . 50 ..64 ..66
Integra tiontechni q uesfor determining probab ilitiesof failure 78 3.5
3.6 3.7
Partialexchangeability._ Extremalanalysisand designloads
.__.•.70 ...72
BASICMETHODOLOGY••.•• • ....• •.••.82
4.1 Overview . ... .••82
4.2 Evaluation of systemeconomics... 4.3 Reliability· based des ign_ NUMBER OF ENCOUNTERSWITHICEBERGS•
.85
. 113
.122
5.1 Introduction 122
5.2 Areal densityand relevantcharacteristics of ice bergs.. . . . 124
5.3 Enviro nme ntal characteristics 13 1
5.4 Icebergdriftvelocity ..137
5.5 Estimationof ice berg flux andapplications 141
5.6 Estimationof encounterratesand applications 145 5.7 Collisionvelocities. locations.and hydrodynamic effects. .. . 150
5.8 Numberofscourevents 160
QpER.All0NAL ASPEcrS ._•••163
6.1 Introduction ....163
6.2 Overview... ... _.163
6.3 DetectionofIcebergs.. ... .171
6.4 Iceber gTowing .. . . ... ... 184
6.5 Reliability
or
DisconnectProcedures. . . ....188vi
ApPUCATIONS 7.1 Overview ..
7.2 Estimatio nof global designloads 7.3 Downtime dueto iceberg incursion
...189 ....189 . ...189 ... _207
. 224
...230 7A EconomicanalysisofFPSOtype systemsfor smal lfields 213 CONCLUSIONS•.
REFERENCES..
ApPENDIX
A SUMMARYOF RESULTSFROMTHESTUDY"~1AxIMUMBowFORCE" 241 A.I Introdu ction
Probabili stic cal ibrationofpressure-arearel ation ship 251 A.2
A.3
Icevesse linteraction model.
...242 ...246
AA References..
vii
. 277
...7 .'0
.31 ..38
....43 44 Economic indicators
ListofFicura Locationsofdisco vered oil and gasfields General motionof icebergs throughregi o n.. .
Historical exchange rates•••.•.•••••• Changein fatality rateinUSworkplace.
Numbe rof injuriesandfatalitiesinUK offshoreind ustrybyyear.
Comparisonof number of injuries and fatalitiesin
selectedUKindustries. . 44
AJlernativecon ce ptualframew orkfor the value oflife. 46 Figure2.1
Figure2.2 Figure2.3 Figure2.4 Figure2.5 Figure2.6 Figure2.7
Figure 2.8 Figure2.9 Figun: 2.10 Figun: 3.1 Figure3.2 Figure3.3
Example decisiontree •• ••52
PDF'sforauributeXco rrespo nding to optionsAandB .S4 Effectof reducinguncertaintyon design load ..62
Effectofuncenaintyon optimization .64
Uncen.a.inty on estimateof themeanofa Poisso nprocessgivena eon-infonnativepriorand differentnumbersof samples. . 69 Figure3.4 Uncertaintyon estimateofthemean ofaPo issonprocess
Figure3.S Figure 3.6
given an informati veprior anddifferentnumbers of samples (Ptadlcares prio rdistribu tion )...
Exampleapplicationof partialexc hangeab ility.. ..
Parentload distrib ution
.10 .. ..71
.. .77 Figure3.7 Comparison ofusing averageannual Dumber of icebergs
viii
Figurc3.8 Figure4.1 Figure42 Figure4.3 Figure4.4 Figure4.5 Figure4.6 Figure4.7 Figu re4.8 Figu re4.9 Figure4.10
versusdistribution .
D1ustration ofimportance samplingtechnique..
Overall methodologyfor evaluati ngproductionsystems Confi gurati on ofsubsea system.
Effect of downtimeon economics
AverageCOStofUSoffshore wellsbydepthdrilled. ...
Cost ofcompleted we lls as a functionof depthdrilled.
Cost ofsingle anddouble hulled tankers.(NRC,199 1) Cargo dead wei ghtIvesseldeadweigh t...
Annualmanning cost DailyfuelcollSum ption . Examplecase - breakdownof costs
.. ...17
..81 .... 82 84 ... .89 ....93
.9'
.97 ...98.. ...98 98 ...100 Figure4.11 Overvie wofmethod ol ogy fordetermini ngdesigniceberg
impactloads
Figu re4.12 Variationsin average:ice crushing pressure versusnominal contact area
... .114
..118
Figure4.13 Ice bergimpact withGSS ..121
Figure5.1 Seasonaldistributionsofnumber ofice bergs and signi ficant wave height. . Figure 5.2 Iceberg waterline length disoi bution Figure 5.3 Averagenumberof icebergsinadegree sq uare
..l25 .127
based onlIPcountsexcludinggrowlers.
ix
. 129
Fi~5.4 Fi~5.5
lIPicebergcocee-averagesfor year/month . Proportion oficebergsdetectedbywate rlinelength (lIPAirborneSLAR detection) . Figure5.6 Seasonaldistributionsofnumberoficebergsand
..130
... . . .130
significantwave height...
Figure:5.7 Distributionof significantwave heightrepresentative of theicebergseason.. .
•.. . . • • • • . . . .132
. 132
Figure5.8 Comp arison of simulated andobserved distributio ns of windvelocity .
Figu re5.9 Compariso n ofsimulated andobserve d distributi on s of
current velocity .
Figure5.10 Wavedrift coefficientSfor sphericallyand cylindrica.l.ly
shapedice bergs .
Figure:5.11 Comparisonof simulated and observeddistributio nsof icebergdriftvelocities..
Figure5.12 Illustrationfordeterminingnumber ofice bergs crossing a linesegment
..135
. 137
. ....139
. ... . .142
....143 Figure5.13
Figure 5.14 Figure5.15 Figure: 6.1
llIustration for determiningnumber oficebergs entering
analert zone ... .... . 144
Possible imp actlocaric nsfor astruct ure:of arbitrarygeo metry 145 Illustrationoficebergcontact positionsarounda ship. .. . 147 Event tree foriceberg detection. management. avoidan ce. .164
Fi~6.2
Figure 6.3 Figure 6.4 Figure 6.5
Model forsearchparh byaircraft.•••••••• ••••••.••
Othermethods fordetectingicebergs . Illustrati onofdetecti oncapabilities fromdifferentsources Icebergalenzones .
.... .165 .165
166 .168
Figure 6.6 Effectof icebergsize on platform mountedSbandradar
performance 177
Figure 6.7 Effectof rainon platformmountedSbandradar performance 177 Figure6.8
Figure6.9
Effect offogonplatform mountedXbandradarperformance Effect oflookdirectionwithrespecttowindonS bandradar
pcrfonnance .
178
.•••118
Figure 6.10 Approximatedetection limitsin tenus oficebergwater linelength asafunctionofsignific ant wave height.
figure 6.11 Method forcalcu lati ng probability of detectionfrom
asupport vessel .
..180
.183 Figure 7.1 ·log.o! (Hs•L)givenan iceberg ispresent. . . ..191 Figure 7.2 Icebergdrift velocit yVo (ml s) as a functionof HsandL. . ..19 1 figure 7.3 Probability a) of detectingan iceberg and
b)towing itgiven detection . .... ..192 Figure 7.4 Probabilityof successfullydetectingand lowingandiceberg. ...193 Figure7.5 a) .10glo'7tfor ImHsby5 mLintervai
b) increase in probability ofLandH,conditionalon impact 194 Figure7.6 -loglO'7,for l m Hsby 5 mLiotervais . . ...195
Figure7.7 a)Significant surge velocity(mls).and b)Averageimpactvelocity(mls).... . Figure7.8
Figure7.9
-log1o'7,forImls Vby5mLintervals -log llt (~'7I)forImlsVby5mL intervals•••••
...195 . .•196 ..196 Figure 7.10 a)Maximum impactpenetration (m) as afunctionof Van<!L.and
b}Maximum impaaforce (MN) asa function ofVandL. ...197 Figure 7.11 a) CombinationsofLandVcontributingto1001design load
b) Comb inationsof L and V contributing toI~design load. . ...198 Figure7.12 GBS-Sampling Distributions Usedand Contributi on to Interval
Around thc Design.Load .202
Figure7.1J Areascfccntn butio n rc IWandI~dcsignloads... • . . 204 Figure7.14 GBS•Sampling Distribu tions Used andContributio nlO
Interval Around
me
DesignLoad..•. . •..207Figure 7.1.5 Minim umtravel distances(kIn)givendrift velocity 209 Figure 7.16 Numbcrof icebergs(10--)peryearenteringzones of radii
R- o
210 Figure7017 Numberof icebergs (10")per yearentering8houralertzone. . ...211 Figure7.18 Remainingreservesand dailyproduction rate for50 millionbarrel field.
Figure:7.19 Cashflows and NPVsfor50 million barrel field . Figure 7.20 Remainingreservesand dailyproduction ratefor
100 million barrelfield .._..
Figure 7.21 Cash flowsand NPVsfor100 million barrelfield
.. . 219 .219
...220 .220
Figure722 Remaining reservesanddailyproductionratefo r 200million barrel field..
Figure 723 Cash flowsand NPVsfor 200 millionbarretfield Figurc:724 Sensitivity o fNPVs totheprice of oil . FigureA.I Description of thelce-Vessellnteraction Model
...221 221 ..•••.•223 ..260
FigureA.2 Definitionof Pen etration Geometry for a Wedge-Shaped Bo w 261 Figure A3 MeasuredPressure-AreaRelationships(Mastersoneral.•199 2) .262
Figure A4 Schematic ofa Higb PressureZOne.•. ..263
FigureAS CriticalZones of HighPressureA.tl'AIQ••.•and Des ign
Window (Jordaan etal.•1993). . . ..264
Figu reA.6 Example Tune Traces ofVerticalBowForce fromthe CanmarKigoriakOctober198 1 Trials . FigureA.7 ExampleTimeTracesofVerticalBowForcefrom the
MYArctic1984Trial s.
.265
...266
..269 FigureA.8 Time TnIUS ofForceDuringSimulatedRams ofthe
Canmar Kigcriak,p '"3a-o.-MPa;IceThiekness'"tom ..267 Figure A.9 TimeTracesof ForceDuringSimulatedRamsof theM.V.Arct ic
P '"3a.(l.~MPa;Ice Th.ickness'"10 m.... . . . .. . . ... . . . ..268 FigureA.IO TimeTnIUS offo«:eDuring S imularedRamsofthe N.LDesign
p'"3a-o.~MPa;IceThickness '"10 m . Figu re A.II TimeTracesofForceDuring Simula tedRams ofthe
Canmar Kigoriak.p'"6-o.~MPa;IceThickness=10m.. ... .•...270
Figure A.12 TimeTraces of Force During Simulated RamsoflheM.V.An:tic.
p=3a~uMPa;Ice Thicknesse10 m 271
FigureA.13 TimeTracesofForceDuring Simula ted Rams of the N.LDesign.
p::6a.o~MPa;Ice Thicknesse10m.. .
FigureA.14 Histogramand exceedaaceprobabilitiesofindividual(Parent)Rams
case
4:p::3a.o··MPa: 0c::1.5MPa; 0D" 0.2.CanmarKigoriakSpri ng1983
272
. .273 FigureA.IS Histogramand Exceedance Probabilities of Individual(Parent)Rams.
Case4:p= 3a.o.·MPa;0c= 1.5MFa; aD=0.2.
CanmarKigoriakOctober1983 . . ...274
Figure A.16 Histogramand ExceedanceProbabilitiesof Individual (Parent) Rams.
Case 4:p=3a4•MPa;0c=I.S MPa;00=0.2.M.V.Arctic1984.. .275 FigureA.17 HistogramandExceedanceProbabilitiesofIndividual(Parent) Rams
Case 4:p=3a.o·MPa;0c=I.S MPa;aD=0.2.
Manhattan Trials1969 .
xiv
. ...276
LlstorTables
Tab le 2.1 Sizes ofdiscovered fields 8
Tab le 22 Generic RoyaltyRegime 4{)
Table4. 1 Co mponen ts consideredincostmode l. 86
Ta ble4.2 Breakd ow nofcostsbyproportionforadouble-b ulledtanker. 101 Tab le 4.J Results from study on costs associatedwithicc~ngtbening
ofvessels....
... .
IIITable6.1 Marine RadarSpecifications 176
Table 6.2 APS-504MSRadarSpecifications. 176
Table 6.3 Appro :rimate Detection RangesofIcebergsand Limiting
DelectableIce berg SizesasaFunctionofSeaState 17.
Table 6.4 To wing Succes sRateby IcebergSizeClass. 187
Tab le7.1 Design impactloads(MN)associated with
specifie dprobabilities ofexceedan ce .199
Table 7.2 Parameters used forimportancesamplingdistributions ..200 Table7.3 Design loads(MN)for10 consecutivesimulatio ns using both
constantandrando mpressureareacoe fficients. ...201 Table7.4 Res ultsofanalyses of10'" designloads (in MN)
forthebowof an FPSO ..204
Tabl e 7.5 Parametersusedfor importance samplingdistri butions .205 Table 7.6 Designloads(MN)for10consecutivesimulatio nsusingboth
constant andrandom pressureareacoefficients.. ... . ..206
Table7.7 Expected numberorincucsionevems per year .•. .' .. '.' ...212 Table7.8 Downtimeduetoicebecgi.ncuniODS(days) .. . 212 Table 7.9 Economic Analyses orFPSO Typc Systems(l oD pages) ..216 Table 7.10 EconomicAnalyses or FPSO Type Syslems(2 0r3 pages). 217 Table 7.11 Economic Analyses orFPSO TypcSystems(3 or 3 pages) ...218 Table7.12 Economic Analyses of FPSO Typc Systems
.,.,':t
Table A.I VesselParameters... ...•. ... TableA.2 Items Recorded During RammingTrials
Table A.3 IceStrengthParamete rs .
. ... 257 258 ...258
TableA.4 EvaJuationor theFourBest Sets or Ice Strength Parame ters .259 TableA.5 Evaluation orIceStrengthParameters for Canmar Kigoriak.
M.V.Arcticand Manbanan .
xvi
... 259
Nomenclature Generalrema rks
Inthetext. symbolsareshowninitalics.Asa gene ralrule. sym bols forrandom quantitiesshownincapual lerrersandsymbolsforspecific or constantvaluesan: shown in small lerters.
Units
All moneyisinCanadi andoUarsunless indicatedotherwise. Millions of dollarsare denotedas MS orMSUSforUScurrency.
DWT dead weight tonaes
Abbre via tions
pdf probability density function cdf cumulativedisDibution function edf exceedance dislributionfunction
Acronyms
AES Atmospheric and Environmental Services(Can ada) API American Petroleum Institute
ASPPR Arctic Shipp ing PoUutio nPreventionRegu lations CAe CanadianArcticClass(icebreakers) COOlE Canadian Offshore DesignforIceEnvironments
x;vii
CSA fPSO
GBS lIP
lMO MARPOL NOAA NOlA NPV
NRC OPA90 OPEC
PIP RAO
StAR SORM
swors
Symbols
~
Canadian Stand3rds Association
Roari ng Producti on.Storage.aDd Offloading System Gravity Based Structure
lntem ationalIce Patrol Intern ational Maritime Organization
InternationalConventio n for lhe Prevention ofPollution from Ships NationalOceanicand AtmosphericAdmini stm.io n NewfoundlandOcean IndustriesAssociation NetPresentValue
Natio nal Research Council(US) OilPollutionAct (1990).US
Organizationof the Petroleum ExportingCountries PetroleumIncentivePackage
ResponseAmplitude:Operator Side-LookingAirbo rne Radar SecondOrderReliabilityMethod Single WellOilProducti on Syste m
C, cashflo winperiodt NPV netprese ntvalue
xviii
CRF capitalrecovery factor
DecisionTheo ry
a;.i=l. J1 j·thom possible alternatives
8".j=1.J1, j'thofn,possibleoutcomesassociatedwithalternative i
J.'y.j=l..n, theutilityassoc iated withfthofn,possible outcomes associatedwith alternativei
p ,,,
PI the distribution of outcomesP1is preferred tothe distribution of outcomesP, P,. PI the distributio nofoutcomesP,is equally preferred tothedistrib ution ofoutcomes Pl
p<dJ
expectedutility associatedwithdecisioni Pc utilityassociatedwithacertain equiValentevent~
P1{A) probab ility of event....
p,.{r) probabilitythat the discreterandom quantityNtakesvaluer(probability massfunc tion )
/;"x j probabilitythatthe randomquantityXlakes value.l"(proba bility density function)
F;"x) probabilitythatthe randomquantityXisless thanor equal to thevaluex (cumulative distributi onfunction)
I-F.J,x) probabilitythaIlhenmdomquanlityXisgrcalerthanthevaluex{ex~
distribu tionfunction)
probability density function for random quantityX conditionalonspecific value1for randomquantityA
fXlJxlfJ) probabilityofx givenparametric distribution/withparameter8 /tlf8)for8 priordistribution forparame ter8
!l'I(lJ;xl9} likelihood of x givendistributionwithparameter8
/Mx't81x} posteriorprobabilitydistributi onforparametric8given observationx
Reliability-basedde!fign
l...o designicebergimpact load
Pp peakpressureassociat edwith nominal contact area a duringanice-structure interaction
averageiceaushingpressure associated withnominalcontact area nominalcoo taetarea
VN componentof impactvelocitynonnaltofaceofstructure Fmax maxi mum force duringimpact
Fmaxomaximum forceduringimpactgivenDOroeauoeof iceberg EO eccentricityofimpac:l.
radius ofgyration of iceberg
icebergwaterl inelength (m)
A~ projected abovewaterarea(m~)of theiceberg perpendicular tothewind directio n
A. projected below water area (mJ)oftheiceberg perpendicular toitsdirecucn ofmovement relativeto the curren t(whenconside red uniform) 0, characteristic dimensionof theicebe rg(m)for wavedriftforces p averagearealde nsityof icebergs(icebergsper mJ)
Hs signifi cantwave height
T, peakperiod
L, wavelengthassociatedwithpeak period S.JJ) spectralwave ene rgyat frequency
f
H: averageheightofbac kgroundsweu(m)
ve wind speedaveragedover the time interval,ata heigh t0(;:metersabove sea le vel(where t mustbeoneof the times(orwhichaand
P
areprovided )\'"" wind speedave ragedoverIhour at10mabovesea level gustfactor forI,referen ced10V..,.
P
heightexponentfor rUA windveloc ity(mfs )
H regular wave height(m)
numberof encount ersoficebergswid!ch arac ters
i
in environmental conditionsdefinedby1
'IF annualexpected fluxoficebergsacross a linesegment
IcebergMan agemem
rangeof icebergfromproductionsite
PM pro bab ili ty ofsuccess fullydete ctin g the iceberg andavoidi ngcollis ion Pw/..r) probabilityof avoiding collisiongiven detection at ranger Pt/..r) prob ab ility offirstdetectingtheice berg atranger P,(r) probabilityofsucce ss fully towingthe iceberg
Pir) probabilityof success fully disconnecti ngthe:producti onvesse land moving offsite.
HVdrodynam icsand wave·in ducedIDoriQns
fJ.u) probabilitydensity functionforinstantane o us ice bergvelocityu
cr
vari ance:of theiceberg heavecrsurge veloc ityinopen wa ter ffio zeroth momentof theiceber gopen water veloc ityspectru m Vs isthe signific an tveloc itycompo ne ntofinterest.f,J.u..0) probabilitydensityfunctiooforinstantaneOusforwardiceberg veloc ityu (Ra yleig h distri butio n)givenGaussian motion andzeronetdrift f~(u..a.k) probabilitydensit yfunc tionforinstant aneou s forward iceberg ve locityu
(Speci al Rayleighdistribution)given Gaussianmotion and netdriftofk:mfs
xxiii
USIl meanof SpecialRayleighdistribution
s(C1J normalizin g co nstant when updating Gauss ian distributio n with forward velocitytoget Rayleighdistribution
~
'1..(wJnumbc:rofscoerevenrsoverasubseastructure ofwithw, '1Js) numberofscoureve nts overasubsea pipelinesegment of lengthI
Implementa tion
& bin sizefOf"icebergwaterlinelength
M bin size for Hs
xxiv
IS TR ODUCTI ON
Thisthesisdescribes amethodologyforoptimizingthechoice and designof oil productionsystems inoffshore regionswhere there is a significant iceberghazard.The work focusesin particular on the future of oil fields on the Grand Banksoff Canada' s ea...t coast.
The problem of designingfor possibleiceberg impacts has manyof the features associated with offshore development such assignificant uncertainty regarding environmen tal parameter s, and implementatio nof complexsystems requiringa rangeofexperti se. A number ofsignificant oil and gas fieldshave been discovered on the Grand Banksand further discoveriesare expected.At present.thelargeHibernia and TerraNovafieldsare being for developed .
When considering the development ofaparticularoffshoreresource.oneneeds to make a number of decisionsincluding whetheror not the projectis viable. which production systems should be used, whatthe best operatingstrategies are.whatenvironmentalloads can be expected. and whetherfurther data acquisitionor research and development are required.
Itisnecessaryin eachcase to identify the problem and the associated criteriafor success. to determine alternativecoursesof actionand theirpossibleoutcomes, toassessthe probability ofoccurrence of each outcome, and finally choose the bestalternatives.Theamountof effort whichshould be expended in evaluatingeach case dependson the potentialincreasein benefits.
Choosinga productionsystem isusuallyan iterativeprocess.Inthe prelim inary stages,theadvantagesand disadvantagesof different types of systems areidentified and requirements for further environmental data. researchand development.andspecificstudies
are iden tified. Approximate analysessuffiCienttounderstandthemainaspectsofthe problemand tonarrow downthenumber of alternativ esisrequired.Inthelaterdesign stages.detailedoptimization.mode ltesting.andanalysismaybeperformedfor severalof themorepromising options.
Themaincriteriawhenevaluatingandcomparing productionsystems aresafely and econo mics.First.itmust be shownthat the systems proposedpose acceptab ly smallrisks 10 personneland the environmentInaccessing risksofsuuctura.Ifailure.wave.iceberg impact.andship collisionloads need tobeconsidered. Sourcesof other risk include fire, explosions. blowouts.andcapsize.The totalriskdueto allsourcesis oftendifficuh 10 estimatebecauseof factors suchasmisconception in design.poorcommunication.
computatiooaJ error.poorfabrication.poormaintenance,andhuman errorduring operatio n.
Thesetypes of risks are generally reduced throughpropertraining andchec kingprocedures;
they canalsobe reducedthrough simpler designandthrough designswith redundantload paths.
Inthecase of structuralloads, design criteriaspecifiedincodesareimposed toensure that me structure issafe.These criteriamay be derivedbased on experience or calibrated using probabi listicmethods.The presenceof icebergson theGrand Banks has resulted in special designrequirements.Fixed systems. suchas theHiberni a gravitybasedproduction platform.must be abletowithstand impactsbylarge icebergs.Forsuch large icebergs.
wave-ind ucedvelocitiesare smalland driftvelocitiesrangeuptoaboutI mls inextreme cases.Designfor suchlargeicebergsresults in moremassive and therefore expensive platforms than those usedintheNorthSea.whichare made slenderat the waterline to
minimizewave loads . Iffloating syste ms ace tobe used.they must be designedtoavoid impacts by larger icebergs.Thisis done bytowingthe icebergs or by movingthe vessel off site. Someicestre ngthe ning will be required as smal l icebergsace difficuh to detect in moderate and highsea states.Wave-inducedmotions ace important insuchcasesandimpact velocities could be as higha.sfou r tofive m/sfor very smallicebergs. Shuttle tanke rs should beabletodetect larger icebergsand manoeuvretoavoid them.but may requirestre ngtheni ng forsmalle r icebergs which ace difficult to detect. Impactspeeds may rangeup to 10mls depend ingon thevessel speed. In thecase of subsea equipment such as wellheads.
manifolds.and flowlines. scouringof the seabed bylargeicebe rgs is a concern . Possibl e solutions includeburial of equipmentdeep enoughto avoid damage. placement of equipment insubseaglory holesdeepenoughto avoid contact.oracceptanceof occasional damagewith repairs and replacement where the systems can be madefai l safetoavoidenvironmen tal pollut ion.
To evalua te the economicviabilit y of a developmentit isnecessaryto estimate the magnitudesand timingofcash flows. A common mea.sure for evaluatingand compari ng systemsisthe net presentvaluewhichindicates the presentvaluewhe nallfuturecashflows arediscountedto the presentat a given thres hold rate of return.Generallyone want s to reduce initialcapital costsand reduce the timerequiredbefore revenues ace achieved.
Revenuesare determinedby the price ofoiland the rate of production.Capital and operating costs are dictatedbythecapaci ty required and the particulardesign .There isusuall ya tradeoff betweeninitialcapitalcosts andlater costs for repair and maintenance.Capitalcosts associated with icebergsincluding ice strengtheningof structures and vesselsand burial of
subsea equipment.Increased operatingCOstSresultfrom ice managementandrepairof damaged equipment.Theexpected amount ofproductiondowntime associated with a given syste m isalsovery imponant.Downtime willresultwhen moving offlocation toavoid ice bergsand whenrepairing equipment.1be downtimewill be affectedbytheamountof timewaiting foran appropriat e weather windowto reconnectorenact repai rs.
1rlthis thesis,a methodologyfor optimizingdesignispresented, First. available probabilisticmethods applicableto situations wherethere is limiteddataand uncertainty regardingprocesses are reviewed .Inparticular.theBayesian frameworkis co nsidered.The poSSibilityofextendi ngthe rangeofBayesianapplicationstoempl oymo re complex likelihoodfunctionsisex plored.. Emphasisis givento problems relevan ttooffshore development. Second.criteria and modelsforcomparing the economicsof differentsystems arepresented.Theseinclu de models to estimate thecapital and operati ngcosts of the prod uction systems.andto estimate los t revenuesdue todownti me.Third.methodsfor determi ning the numberofincidences involvingicebergsare developed.Fo r determinin g designimpactloads.reliabili ty-based designmethods are imple mente d.Fac to rs that are co nsideredinclude theiceberg population. the environment,iceberg detection. iceberg
mana gement,avoidancestra tegies.andthe ice-structureinteractionprocesses.To illustrate themethodology. prelimi nary analysesarecond uctedfora numberof examplefield scenariosand systems.
BACK GR OUfloT)
2.1 Overview
Inthischapter.anoverviewisgiven oftheproblemof designingproductionsystems foroffshoreregionswhereicebergsare present,thetypes of productionsys te ms being proposed. availab lemethods for analysing iceberg loads. risks. and downtime.and areas where imp rovementsarerequired.inSection2.2.the importanceof tbcpetroleum resources off Newfoundlandisdiscussed.anda briefoverviewisgiven of discoveredand potential fields.InSectio n2.3.anoverviewis give noftheenviro nmentalconditions00theGrand Banks.withemphasis on !hoseparametersneededinanalyses.Anoverview isgiven of the availabledata.includingmethodso( collection.parametersrecorded.andlimh auens.In Sec tion2.4.the requireme ntsfor developin g afieldandthetypesofproduc tion systemsthat havebeenproposedwillbediscussed.InSection 25.the importanccoficebergimpaa risks inthe overall design isdiscussedandareview is givenof published methodsfordetermining design icebergimpact loads.risk.anddownti me.InSection2.6 , thegeneral factors determiningthe economic viabilityora fieldarediscussed. InSection2.7.therequirements forgocx1decisio nmakingatediscussedand aspects offormaldecisiontheory.includingthe:
useof deci siontrees.pro babilitytheory.andutili ty theoryatebrieflyoutlined.
2.2 Petroleumresources
Total energy demand in Canadaisexpected10risefrom 9.600Peta Joules in1990 to13.800PetaJoulesin2010.with oil andgascomprisingapproximately60% ofthis (CroasdaIeandMcDougall.1994).Conventioaal reserves arebeingdepletedinCanadaand
if theycannotbereplaced domestically . theymust be replacedthrough imports.Possible sourcesfor inc reasing the domesticsupply of oilincl udeimprovedrecoverymethods.the develop men t of lal"sands andheavyoildeposits,and the development of frontier oilandgas.
Totalestimatedrecoverable reservesinthe Canadian frontierregio nsare about3.4 billion barre lsofoil and44 trillion cubicfeetofgas.Ofthis, approxi m ately1.6billion barrel s of oil.4trillion cubic feet of gas,and240million barrels ofnaturalgas liquids,havebeen found ODtheGrand Banks . Explorarion costshave been relativelylow.averaginglessthanS2US per barrel,andlicenceshave beengrantedforfurtherexploration workintheimmedi ate vicin ityof existing discoveries . Itwas not indicated bow thesecosts reflectedthe governmentPetro leum Incenti ve Package (PIP) grants avail a bleatthattime. Future potentialdiscoveries ontheGrand Banksareestimated at3billio nbarrels of oil and5trillion cubicfeetof gas.
Forthe objectivesof thisstudy,itisnecessary tode termi nethe likely field charac teri sticsof futuredevelopme ntsso that examplefieldscenarios canbeset upfor analyses.Themainparameters requiredaretheamount of reserves,the reservoir depths(for drillingCOSts).theextentandcontinuityof thc reservoirs(affectingthenumberand typesof wells ),thelikely flow rates,andanyrequirementsforwater and gasinjection.special treatme nts forhydrat es.wax.COl'andH;rS,or abnormal tempe ratures or press ures.
Adesc ripti on ofoiland gas fields alreadydisco vered ontheGrandSanies and off Labradormay be foundin Chipman(1992).The locationsof the differentfindson the Grand Banks and off LabradorareshowninFigure2.1andthe magnitudes ofthefindsareshown inTableI.Allofthesignifican toildiscoveriestodate areon theGrand Banks.Ofthese
the majorityofoilisfoundinfour fields (Hibernia, Tetra Nova,Hebro n. and 'Whiterose ).
Themnaining discovered fieldsallhaveprovenreserves of less tbao 2S millionbarrels.All of tbeoilfields arefound in the Jeanne d'Arcbasin, except South Tem pes t, Fivegas fields with greaterthan 20billion cubicmetreshave been found..These areWhite rose and Hibernia on theGrand BanksandNorth Bjami,Bjami and Gudridoff Labrador. Inaddition, significantnaIUra1gasliquidsassociated with gas arefound.inHibernia,Whitcrose,andBen NevisOn theGrandBanks..and NorthBj ami and Bjami off Labrador.TheHebron field comprisesfourn:servo irs.Tbese are Ben Nevis (129 million barrels),Hibernia(46 million barrels ),FortuneBay(14 million barre ls ),andJeann e d'Arc (6 millionbarrels ).TheBen Nevisoil is relativelyheavy soanificialliftwillberequired.
Figure2.1 Locations ofdiscove red oilandgasfields(Chipman, 1992)
Table1 Sizes of discovered fields(Chipman.1992) Fields on the Oil (millions) Gas (billions ) NGL's (mi llio ns) GrandBanks
m' bbl m' ",.Ct. m' bbl
Hibernia 106.0 666 28.7 1017 17.7 III
Terra Nova 64.6 406 7.6 269 2.2 14
Hebron 31.0 195
wbnercse 28.4 178 42.7 1509 9.2 58
WestBcoNevis 4.0 2S
MM» 3.6 23
BenNe vis 3.0 19 65 229 4.7 30
North BenNevis 2.9 18 3.3 115 0.7 4
Springdale 22 \4 6.7 236
Nautilus 2.1 13
SouthTempest U 8
Fortune 0.9 6
South Mara 0.6 4 4.\ 144 1.2 8
North Dana 13.3 <70 1.8 II
Trave 0.8 30 02 \
Subtotals 250.6 157 5 113.7 4'1I 9 37.7 237
Fields off Oil (millions) Gas (billions ) NGL's (mill io ns ) Labrador
m '
bbl m' cu.ft. m' bblNorth Bjami 63.3 2235 13.1 82
Gudrid 26.0 920 1.0 6
Hopedale 24.3 859 5.0 31
Snorri 3.0 105 0.4 2
Subtotals 119.6 4224 19.9 123
Most of the oilinthe Whiterose field is foundintheBenNevis reservoir (158 million barrels)whichconsists of anum berofdifferent pools.One particular pool has122 million barrelsinasmallareawith goodproductionpotential.
Oearly.the developmentofthereservesoff Newfoundland and Labradorwill play asignificantrole inensuringCanada'senergy selfsufficiencyinthenear future.Whilethe tou.!reservesareverysignificant.theenviromnentinwhich theyarelecared is very severe andformostofthe smaller fields,the cost of developmen tisprohibitiveat prese nt,To allocate prese ntresearchand developmen teffortsefficiently to reducethese costs,it is important toidentifythemostimportan t factorsandtodetenni.ne those systems and strategies whichhavethe bestpromise ofleading to reduced costsand risks.
2.3 Environmentfadors
Oneof themaindeterrents to thedevelopmentofsmaller fieldson the Grand Banks isthecombinationofrelativelyhighsea stalesinthe regionandthe seas onal occurrence of icebergs.A number of other environmen talfactors ontheGrand Bankswhichrequire spec ialatten tioninclude cold air and watertemperatures andicinginthewinter.and the prese nceof seaiceand extremefogconditionsin thespring.
IcebergsoriginatefromglaciersinGreenland andarctic Canada. Themain transpo n mechanism bringing icebergssouth isthe Latndor current.Thecurrentflows northalong thewestcoastof Greenland, then south along the east coasts ofBaffinIslandand Labrador.
WherethecurrentreachesNewfoundland,itswings tothe east andthensplits aroundthe Grand Banks(Figure2.2).One branch followsthe eastCoastof Newfoundlandwhile the
~\
'. , := .1 .l
....
~.~ ~,- - ~ J
"'~>.,.J
Figure2.2 General motionofice bergs through region other moves east along the nonbem edge of the banksto theAemisb pass andsouth.Thetwo branchesmeetthewarmDOI'lberly flowingGulfStream currentnear thesouthernedgeof the Grand Banks.It takesapproximately two seasonsforaniceberg to reach the Granj:! Banks.
the exact time depends on when and where the icebergiscalved. the variati onsin the stre ngth of theLabradorCU1ttI11,the winterseaiceconditions, and thelocal windswhich maytraptheiceberginsbot!:or moveitotf the main current.Atthe GrandBanks,persistent windpatternscancausetheicebergstobeblown wellto east or westofthe banks fora signifi cant portionof aseason.Whenthewinds are cnshcre.lar genumbers of icebergs can bebeldalongtheshoresofNewfo undl and.
Therate of deterioration oficebergs determines howfarsou th theytravel.Icebergs deterioratemainlythroughmelting , erosion, calving ,andspli tting.Therate of melting
10
increases proportionatelytothetemperature gradientinthe wateratthesurfaceorthe iceber g. Thisin tumisafunctionortheambient watertemperature.waveaction. andthe motion of theiceberg relativetotheSUlItIWldi.ngament.Themost signifi can terosionoccurs aroundthewaterline orthe icebergduetowaveactiooandcanresultinundercuttin g and eventuall y calvingoricepiecesfrom theresulting overhangs.lbe~oceof sea iccreduces the influence onthe erosi onoricebergsbydamping wavesandreducingwater temperature.
Thenumber oricebergsreachingthe GrandBanksissignifi can tlyhigherinyearswhen sea ice offLabrador extends out overthe mainpanorthe Labradorcurrent(Marko.1993).Most icebergsarriveon theGrand Banksinthe spring andearlysumme r;thou gh they have occurred insmallnum bers at othertimesofthe year.Icebergsgenerall y do not travelrat south of the Grand Banks. asthe relativelywarm Gulfstre am causesquickerosion and mel ting.
Thecollectio nor data onthe populationoricebergsisbothdiffic ult andexpensi ve.
Theannualvariationinthe number oficebergsis such thatasignificantnumberor years are requiredtogel agood estimateorthc:distributionofm.aldensity oriceberg:sata given
sue .
Theprob lemis made more difftCUltbecausesmallertce:bergs canbedifficu ltto dc:tc:ctexcc:pt invery moderate environmental conditions.Todescri bethe:ice bergsha peandsize:parameters andtheirccrrelaricnsaceunuc:ly,threedimc:nsional contoursare required.Above wate rprofiles can be determinedfromstereoscop ic photograp hs,howeve rthe:measurements orunderwatershapes using sop histicated sonarsys temssuspe nded from ashi p arequ ite expensive.More often. icebergs are classifiedaccordingtosiz e classes (growler.hergybit,
Ll
small. medium.large . verylarge) orby estimates oftheir simplest visible above-water"
dimensions(walerlineleugth. waIerline width.and beigbt).
Relativelygood bistorical data is available on the number and location soficebergs 00theGrandBanks from thelntem ati ooallcePatrol(lIP) ,theoil indusuy, and research instit utes.TheUP wassetupin 19 12.afterthesinking oftheTitani c.with the mandateto notify marinersofthepresen ceof icebergin thenormal shipping lanes.To dothis.theUP composes maps ofthepositionsof sited icebergsbasedon shipreports anddedicated overflights.ThelIP no w uses an icebergtrajectorymodeltopredict wheretosearchfor sighted icebe rgsonsubseq uenttrips; to reduce the number of doublecountsdueto resightings .andtobeable toaccountfor anyicebergsinsubsequentmapsthat can notbe found duetoeitherbadweatheror insufficienttime. TIle lIP has also perfonnednumerous scientificstudies overthe years(0improvetheirprediction capabilities.Tbe reportsproduced by thelIP providea uniqueandvaluablesourceofinfonn ationon thenumbers and movementsoficeber gs overthepast 90 years.
Thereare a numberoflimitatio ns in applyingthe lIP data forrisk analyseswhich shouldbenoeed,Flightpathswere chosen to followthesouthern extent oftheiceberg il'JCUl'Sionandtorelocateicebergswhich had been IRvious ly sighted.thereforethecoverage at anygivenlocation may bebiased.Itisoften diffICulttodetermine ifareason themaps whichshow no icebergsresult because thereisno coverage.the conditionsare poorfor detection. orthereare no icebergs.Furthermorethedata collection proced ures have changed over timeastechnologyandde mandsforinformati on changed. Forexampl e.dedicated overflights were introducedafter
ee
second worldwar.the use ofthe iceberg trajectory12
forecasting modelwasintroduced in 1979,and theuse of SLAR(Side-LookingAirborne Radar)wasintroduced in1982. Oear documenlationoftheproceduresused each yearis00 lODgeravailable.makingitdifficult10estimatetheeffectS oflbesechanges. Foreumple, the weather conditionsalongtheroutes and!bereasonsforchoosingparticu larroutesarenOI alwaysprovided.Neitherarethe methods used for determinin gwhether or not agive n icebergwas arecountwhenusingtheforecastmodel.While"theUPmap sprovidegood informationon thep:lSitio nsofthe icebergs,onlyasimple sizeclassifica tio nwas used;this specifiesonlythe classes iceberg andgrowler.ItisIa10wnthai:thenumberofsmallicebergs issomewhatunderestimaledand that the nwnbc:rofgrowlersissignificantlyunderestimated.
Data on thenumbersand types oficebergsatdifferent drilling sitesonthe:Grand Banks havebeen collected byoil companies over thelas t15years.Availabledata includes iceberg trajecto rypositions determined by radar,records ofice berg positio nsfrom overfl igh ts,and measurements ofthephysic al dimensionsof icebergs from suppo rtships.
Areasonably largedatabase of waterline len gth,height ,width.andshape class observed fromsupportvessels has been collected, As well.a smaller highquality data-base of detailed measurements of above and below waterprofilesisavailable.Whenconside ri ngestimati ng the averagearul densitiesof icebergsfrom thisdata.itshould be noted that therigsareonly located at a given site longenoughtodrillthe well.Also, the oil companieshavethe same problemswithdetermi ning resighti ngsofice bergs anddet ectin gsmall icebergsas theUP.
Thewindis themajor drivingforce duringstormcondi tions.It acts directly on the icebergs and indirectly throughthegen er ationofsurface wavesandCUl1"e Dt.Thewind also plays a majocrole in limiting theradardetection capabilitythrough the generationofsmall
13
capillarywaves whichcausebackscatter(sea clutter);lhis canmaskoutthesignal returned from.the iceberg. Distributionsgiving thewindvelocity as functionsofdirectionandmonth may befoundintheAES Windand Wave Atlas (MacL,an,n PlansearcbLzd.,199 1).OIlthe GrandBanks.windsare mostcommon fromthe southwestandarestroegestinJan uuy.
Tbesea state isoftenmodelled as a(Xunbinationoflocalwindgeneratedwa vesand lo w frequencyswe ll.'Theinteesiry ofthelocally generatedwaves isa function ofthe strength,duration.andfelChofthewind.OntheGrandBanIes,thewind isusually associated with cyclo nic weatherpatterns.Ata given loc ati on,the windusuallyslowly turns in direction.This affects the generationof waves(andsurfacecurrent)by limiting the effecti ve fetch .Theresulting seaStales can be quite com plex. containingwave energyatanumber of frequenciesanddirections.Distributionsfor parameterssucbasthe signi fican t waveheight Hscanbefoundin theAESWindandWaveClimat eAtlas for theEastCoast (Macl...aren Plansean: h. (991).Tbis datais based on wave-riderandNOAAwave -buoy data for the nocthemGrandBanks duringtheperiod1970 10 1989.Wherea seaspectrumis required. the Jons wap spectrum recommended byleBlo nd et.al.(1982) can beused,
Theocean currentsan:the resultant ofa num ber of forcingfunctions which include thestresses associatedwithlarge-scalewindpatterns (resulting in geosuophic currents).tidal forces , differences inwater densitiesatdiffere nt locati ons.and local winds.Tbese forces can resultincom plexcurrentpatterns. Anapproximateestimateof the locally gene ratedcurrent canbefoundusing Ekman'smodel(see Pond and Pickard .1983.pg.109).
Othe r param eters whichinfluence detection include lighting,visibility,fog ,and prec ipitation.Theligh tingandvisibili ty determinethevisualdet ectioncapabilities;the
14
visibilityceilingaffects wben flyingispermitted;and fogand~piutionaffectradar detection.
2..4 Syste ms,strategies,an dcri teria foren l ua tio n
Thenumber.types.spaci ng. and timing of wellsand theprod uctionrates achieved will be determinedlargelybased on reservoirengineeringrequirements.Theamount of oil thou:canbe producedisroughlypro portioaaltothesizeof the reservoir.butalso dependson the conti nu ity ofthe reservoir.meshapesand orie ntations ofthe indivi d ual pay zones,the permeab ili ty ofthe rock.the characteristics ofthefluid,. andtheamountof natural drive available.Tbe producedfluids may come from differentreservoirssitua ted atdiffere nt vertic al and horizontaloffsetsandthesereservoirsmaybebroken into numerous individual poolsthrough faulting andotherprocesses.Tbenumber. sizes. shapes andrelativelocations oftheseindi vidualpoolsaffectsthenumber andtypeof developmentwellsrequire d.where theyare loc ated .and how deeptheymust be drilled.Horizo ntal drilling techniqu esareused
[0controlthepamwttichthedrill pipetakes throughthe reservo irandthusinc rease contact with thepay zone.Extendedreach drilling techniq ues are usedto reachpools atlarge horizontal displacementsfromthedrill site.Drilling reachestypicall y reach 9 km(Le, 2 km down and 7 kIn horizontal).thoughreaches of about4kInare morecommon and there may bedifficu lty indrillin glongersections (Lever.1995).Thedrillreach matcanbeachie ved islargelydetermined by thesizeofthedrill rig.Therig sare ratedforparticularwe lldepths and havelimi ta tions intermsof pump capacityand bookcapacity (the abilityto bring the
15
drill pipe out ofthe well),'Ibereisnota significantdiffereeceinthe distances achievable fromfloatingandfixedprod uctionsystems.
Theamount of nauuaJdrivein a reservoir basasignificant effect on thefraaionof oil whichcanbeeconomically produced. Soun:esof natural drive includegravity .natura) gascaps. natural gasin solution.and waterdrivethrough aquifers.Theviscos ityof the oil andtheamountofgasinsolutio ndeterminehow easily thefluidsflo w.Also. asthereservoir isdeveloped. therelativeproportionsof oil. gas.andwater can change,affectingthe flow properties.When::narunl driveis insufficient,.recovery canbeenhancedusin g gasor water injection. Thisrequiresadditio nalwe lls.tubing.and flowlines.aswellasadditio na.l equipmen ton thep1.aIfonnfor com press ing gas and cleaning water.Where waterinjection isused. largeamounts ofproduced water mayneed tobe separatedOUIandcleaned.
Thenature of Ibe producedfluids also can affectthedesign.Largeamountsof gas can causemuluflcw problemsifthe oil and gassepara te out.Sandandcorros ion dueto sulphurcancause high rates ofdeteriorationto the welland subseaequi pme nt. requiring freq uentwork overs.When::wax and hydratesare present, these can plugequipm en tifnot handledcorrectly.Problemssuchas corrosion. wax.andhydratescanbe reduced through chemicalinjecti on . thoughthese require additionalflo wlincs and controls.Special subsea equipment maybe required(0handl e high fluidpressuresandtemperatures.Inaddition.
pressuresandtemperatures mayneed10bemaintained toreduce wax and hydrate build up.
The basic processing require mentson theplatform include removalofsan d and water.separationof oiland gas.andpreparati on ofoil andgas for storageor transportation.
Differentseparat ors may be requiredif tbe products fromthe different wellshave separa te
16
linesor flowsfromindiv idual wellsneed tobe periodi caUy tested.Theoptimal type of sepanuor in eacb casewilldepend onthe volumeof flow and on the composition. pressure.
andtemperatureof the produced fluids.Before theproduced oil can bestormOl'"transported.
itmay needtobe stabilizedsothat itdoesnot separateout intocomponents.Excesswater inthe producedgasesneed to beremovedbefore thegasiseitbertransported,usedasa fuel.
or reinjected. Where the gasisremjected,compressors will berequired.Equipmentisalso requiredtodean produced water before itis disposed of orreinjected.Ifwaterinjectionis increased asnaturaldrive is depleted.signific antamountsofwatermayneedto be handled latein the life of thefield de velopm ent.
Either a gravitybasedplatform ,ashiporsemi-s ubm ersiblefloating system.ora subseatic-incan be used to develop a field ontheGrandBanks.The mainadvantage ofusing a fixedplatformisthatmany.if not allofthe wellscan bedriUedand completedatthe su rface oftheplarform.Where floatingplatforms are used.allof the wells are completed subsea. resulting insigni fican tly higher capitalecmpieuc cand workove rcosts.Anothe r advantag e ofusingafixed platformis that down time duetowavesandicebergsis signi fw:antly reduced.lbemain disadv an tageof usinggravitybased platf orms are significan tly higher capital costs.Whereas in theNorthSea gravitybasedsrrucr uresare slenderto reduce the amount of concre te require dand toreducewavelo ads. struc tureson theGrandBanks mustbemuchlarger to be abletowiths tand iceberg impacts loads.An additionaldisadv antageistheinability to movethestructureduringorafter thefiel d develop ment.
17
Aoatingproductionsyscemsconsist ofthe subseadevelopment,ariserbasemanifold andriserwithflo wIioesandcontrollinesbetweentheseabed and thevesse l.andthevessel itself whichhousesthe proccs.sing equi pmen t andpossiblystorage.Bothship orsemi- submersiblesysrems havebeenused,The semi-sub mersiblehas somewhatbettermotion characteristics butlessdeckweightandstoragecapacity. Whereshipproductionsystemsare considered.turret-mooredsystemswilllikelytobe usedtoreduceen viron men talloa ds and allow quickdisconnc:cLThemaindesignparametersfor afloa tingproductionsyste mare the wate rdepth,theproductionrate.theDumberof flowlines and controllinesrequired.the environme n tal conditions ,and thestorage requirements.1bewaterdepth determinestheCOSt of the mooringand riser systems.The production rate and Uterequirements for waterand gasinjection determi nethecapacity of equipmentrequired and thusthe deck space and weightrequirements.Thenumberof flo w-linesandumbilicalsrequiredaffect the designof the risersystemrequired.Th.i.swillalso significan tlyaffect thedes ignand cost ofturre t- mooringsystems.Tbeenvironmentdetermineshowstro ng thevessel andmoo ring system mustbeand influences down-time.The requiredstoragecapacityisdetermined basedon[he productionrate andtheexpecteddown -ti me oftheshuaJe tankersyste m.Som estora ge capacity may alsobe desirable for wetproduction (before the water isseparated out) incase the processingequipmen tbreaks down.Adedicatedsto rage tankermaybe usedtoreduce downti me when shuttletankersareno tavailab le or cannot moor.
Ifa well isnOIcompletedat the surface. it mustbecompletedat theseabed and a subseaflowlinesuse d totransportthe prod ucedfluidsto theproductionsite.The subsea syste m willalsoincludecontrollinesto adjust the press ureatthe well head. and injection
18
lines tobringchemicals.wate r.andgastothe wellboreand reservoir.1bc: wellbeaditself consists of a productioo tree with whichflowstoand from thewe llborecan be controlled and access canbe: gained for wort. overs.Wells maybedrilled separately orin close proximity from a template.Wheretemplatesare not used, subseawells are generallyspaced at leas t2Smapartto protect the wellheadsfromfalli ng drilling andworkoverequipment. Origi nally.many wellswoul dbedrilledfrom asinglesubsea template ,thishasbeenlargely replaced by the use ofwe~whicharecomp leted individuall y.Individualwe ll com pletio ns arc:less complex than integra ted templates.and aremore flexi bleasdrillin g can lake place before instal1ing the manifold, oreven designing it,Because:oftberisk of ice be rg scour on the GrandBanks, well beads willbeencasedinacoocrcteglory bolejustbelowthe seabed.
Flowlinesmay either beburied ineeec bes orleft on thesurface,dependi ng onthe expected numberof incidencesandwhethe r ornotthey can bemadefailsafe. Atpresen t,produced fluids fromsu bse a well sarctrans ported as a multiph asefluid to the hos tplatfonn.The maxim umdistan ce over whichmulti-phasefluid can be transportedisabout ISkm,the abilityto achievethis distance depends onthe particularcircumstances.such as the extent of natural driveavailable.[fthe distancesover whichproducedfluidscan be transported subseacan be increased, then the number of productinn sitesrequired can be reduced, and additionalmarginalwells maybecome profitable.A number of areasof research are being followedtoinc rease subsea transport distance.these include betterpum pin gsystemsfor multiphasefluids;better methods to suppressseparationof multiphasefluidsandpro blems relatedtowax.hydrates .and corrosion ;and the developmentofequ ipme ntforsubse a separa tio nsothat liquidandgas canbetransportedinsepara telines.Whenconsidering
19
development of smaller oilfields,itis ofnotethaJ:ifsubseatransportdistancescouldbe increased to2SIan.many of the smallerfieldswould be within ther.lngeofboth Hibernia andTe rn. Nova.Hiberniacould potentially receive tie-ins10 years after production starts, thismaybelongerifthereserve baseis increased.
Theability to avoidice berg s successfullywhenchoosingafloaling production syste m. and the amountof downtime that willbeincurred,ace influe ncedbythe iceberg detection.manage ment,and avoidance system.The operatorswill most likely useice berg detecti onand management systemssimilar to that usedfor drilling cperaec ns . 1bc procedures taken when an icebergisdetectedwilldepend on therange from the platformat which theicebergis initially detected, its approach speed, theweathercoDditio ns. andthe operationsunderway on the platform.lbe actions likely will be specifiedinterms ofalert zonessimilar to thosethaihave beenusedfor drillingoperations.For drillingoperations.
three alen zonesaredefinedbased onthe required time10 cease operationsand disco nnect the vesse lmooringsystem. andthe estimatedtime in whic htheice bergcould reachthe platform.Ifan icebergis detected inthe outer lone3. itismonitored.Ifit appeared10be approac hingthe platforman attempt to deflect it awayby lOwing will be made.Oncethe icebergreacheszone 2,exif itisdetectedinzone 2,theoperators attempt todeflect the iceberg by rowin g, shutdownoperations .anddisconnectthe mooring system.Oncethe iceberg reaches theinnerlone Iorif itisdetected in zone1. the operatorsmove off site as quicklyas possible.
The succes s of these operatio ns ultimately dependson theeffectivenessof the detectio nsystem, the to win gsystems, andthe mooringrelease systems.Aircraftprovide
20
good advancedetectioncapabilities forthegeneral region, butarcrestricted ...ben enviro nmen talconditio ns arepoor.SUPPOI1vessels canoperateinmost env ironme ntal conditions, however thedetectionrange for theirradarsystemsis generall ylessthan those on produ ctio n vessels ora.ircnft.Supportvessels extendthe overall detec tio nrange by cond ucti ng searcb patterns beyondtheradar delection rangeof the production vessel,and they can concentrate their efforts onareas fromwhich icebergsarcexpected[0approach.
Thoughtheredundancyprovided bythedifferent systems sbould resultinimp rovedoverall detectio n,allthe systems arclimited w henit comestodetectin g smallerice be rgs insto rm conditions.
Tomodelthe detectionof icebergs using radar,it isnecessary10consider the charac teris ticsoftheparticular radar system. the proportio nofreceived elecuomagnetic radiatio nthe iceber g returns .the strengthofcompetingsignalssuchas seaclutter.and the proportio nofsignallostdueto absorption byfogandrain.Detectiondepends on whetherthe returnedsourcesignal can be distinguishedfrom the competingsignals and noise generated withinthe radar system.Linlework.has been doneto determinetherisk. of an iceberg reachingtheplatfonn.Partofthereason maybethattherearelargeuncertain tiesre garding the magnitude of sea clutterinhighseastatesand the effectsof sea sprayand overwashon thereturnsfromthe iceberg.One oftheobjectives ofthis researcbis todo sensitivi tyanalysis todetenninebow much variationsintheradarsyste mcapab ilitiesaffecttheoverallrisk.
Thesucces s of rowingoperations dependsonthe environmental conditions,the size andshape ofthe iceberg,and howsoon it isdetected. Generally,tow ing capabilitydecreases withthe severity of the sea state. Towing isnotefficientwben signifJCat\twaveheights
21
exceed 4 m.Theability of the operators to disconnecr: the mooring systemiscriticalin avoidingapp roac hicebergswhichcacaoebetowed. Arough esumereof the~liability would range from 0 iflessthan 21 minutesareavailable to 0.98ifme re than 12haws are available(Berry,1992 )
2.5 Risk and relia bUlty- based.design
[t isimportan ltodistinguishbetweenthe Iotai.riskto personnelandthe environment on the one hand, and the target levels of safety used instruetural design.Risk topersonnel isoften definedinterms ofme annual probabilitiesofinjury andfatality foran individua l.
Published levels of risk, basedonStatistics. canbe found for different activities and occupations.Inthe case of an offshoresystem. the Iotai.risk may include causessuch as fire, shipcollision.capsize.and waveloads inaddition toiceloads.Otherfactondetermini ng total riskinclude humanerrorsinconceptu alizatio n,calc ulatio n.and fabrication, and improper installati o nand maintenance.Thetotallevelofsafetyis generally vel)'diffic ult topredict. thoughitmay bepossibleto qualitativelydifferentiatebetween syste ms.
Suuctura1design requirements,on the otherband, are often basedon"target"safetylevels speci fied in codes. Inlimi lSlate design. a number of safety levelsmaybe spec ified correspo nding to the consequences of failure.For example.itmay bespec ified that the probabilityof a majorstructural(ultimate)failure possiblyresulti ngin1055of lifemust be less than10"per year andthe probabilityofminor structural (servicea bility)failurerequiring repairislessthan lo-J peryear.Total failure ratesareappro xim ately anorderofmagni tude higher thanfail urerates resultingfromextremeloads.
22
Probabilistic methods are appliedtodes ign problemsbecause of theneedtogive explicit coosideration to the uncertainties involved.Madsen et.11.(19 86).giveabrief description of thede velo pmen tof sttuCtllla1 reliability.andpoint outsomeofthestre ngths and limitations of usingproba bilistic methods for design. 1bemainpurposeof probabilistic designmethodsistoprovide a rationalfnmewori::forthosepartsofthedesign processthatcan be controlled.
Structures are generallydesignedto meetspecifiedstandards as set out in national codes.Tbesecodes are developed to insureadequatele vels of safety[0personnel and tothe environment,and arc developedinconsensus byindustry,government, andother interested parties.The code forthe design ofoffs horestruc turesin Canada (CSA-S47 1.1992 ) is speci fied bythe Canadian StandardsAssociation (eSA);this isanon-profitinde pendent organi za tion. The designcriteriaare specifiedas a number ofloadcombinati o ns and corres ponding loadfactorswhich thestructuremust be able towithstand.Therationaleused forob tai ningthesecriteriaisdocu me ntedinJo rdaanand Ma.es (1991);the val ueswere determi ned usinga calibrationproceduresuch thattheminimalsafety levetto personneland the environmentissimilar to thataccepted in other indu stries.Inthecase ofice be rglo ads.
whic harerareeventson the Grand Banks,itisrecommendedthatthedesignloads be chosen based00a probabilityofexceedancebetweento·)and10"'".
Todeterminedesignloads ,analytical models are developedto predict theloa d corresponding to any setof input parameters. The modelsmustdetermi neappropriate probabilitydistribu tion s. both fortheseinput parame tersand for the numbe rofcollisio n
23
events. Thena probabilistic methodcan be used todctennioc
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resulting disttibution of loads andfromdlis.medesignvalue.Agood reviewoftheuseof the diff"crcnt probabilisticmcthcxts.suchasMonteCarlo sim ulation.. important sampling. and first andsecond order reliabili tymethods,mac can be usedrodctenniocmere:su1ting load distributionsordesign valuesisgive ninMe lchcrs (1987).Thechoice of a designload.givenan estimated distribution of loads and the estimatednumberof collisions.is anextreme valueproblem.Jordaan and Maes(1984) considerdifferentsolutio ntechniques for rareandfrequeol loadingevents.
Themeth ods ofsubjectiveprobability havebeen applied10the designprocessby Jordaanand Maes (1984).Theyassume thatthe load distributioncanbedescribedinterms ofadistributio nF:n/o (.d 1.Jwhich isa functio nof aSCiof parametersdefined by the array1.. Ir1.isdescribedby the distributionF
J,A>.
men by applyi ng dcFineni'stheorem.thcjoint probab ilitydistribution,Frforany sequenceofloads.Xl "•.•X.whic hare exc han geable.i.e.theoniecof the loads hasnoeffectontheir probability of occurrence.canbe expressed
(I) and the distribution forthemaximumof thenloads canbe expressed as
(2)
To findthe distrib utiononthemaximum of11future loadsif mloadshave already been obse rved.given aninitialpriordistributio n.Bayes ' theorem canbeused.
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Macs (1985) considers the applicationof subjective probabilityandexchangeab ility todes.ignproblemsinmoredetail.Hebriefly touches ontheroles offormal dccisioo.analysis anddesigncodesasapplied to designprob lems.andgivesan excetle ne review of the diffen: nttypesof extremal problemsandthemethods which have beenused to solve them. Maes thenshows howtheideas ofsubjectiveprobabilityand exchangeabilitycan be used to developimproved extremal modelstohandle problemssuc bassho rtdatarecords. random numbersofeven ts, and uncertaintiesindata and load scenarios.One particularlyuseful result isthe extensionofequation(2)tocaseswheretheloadvariesas afunction of the stateof nature0:whichisitselfassumedtohe exchangeable.Appli cations consideredincludeloads duetoearthquakes. waves,andice fearures.
Todeterminedistri buti o nsforicebergcollisionloads,modelsare required10 determinethe numberandtypesof ice bergsencounte red indifferentenvironment al conditions.the efficie ncyof the managementsystem, theinfl uen ce of hydro d ynamic effec ts onthecollisio n loc ations andveloc ities.andthecollisionloads.
One oftheearliest studies todetermin eice bergrisks tooffshore platformson the GrandBanks was carriedoutbyBlenkarnandKnapp(1969 ).Theyestimate dthe numberof icebe rgs passingthrough a 1/2degreerectangle based 00 Inremazicn alIcePaU'OIsighti ogs from1948!O1956.Their modelfortheannualimpactprobability for anice berg with a platfonn wasbasedon theassum ptionthattheicebergs traveledinastraightlinethroughthe rectangle.ReddyeraI.(1980)and Redd yandCbecma(198 7)showhowtouseMonteCarlo simulati onto determineconfidence limitsonthe impac t probabilitiesestimatedusing Blenkarn and Knapps method and show howto usc:EmpiricalBayesiantechniquestoreduce
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theuncertaintyas moredata00thesizes and directions ofmo tioo of theice bergs becomes available .
ThereareseveralweakDe:sseswiththe basicBlenkarnandKnappmodel.First.actual traject ori es ofice bergsthroughadegreesquare canbemucblongerthanstraight line approximation.Second,fluxdepends00boththenumberandvelocitiesoficebergspassing through aregionand canbedifficulttomeasure.Inthemodeldescribed,iceberg trajectories are sim ulated usioga Marlc.o v techni queinwhicbthestatisticalvariations in iceberg velocities anddirecucesarecaptured.lbeoumberof icebergsinthe modelis determined by calib rati ngthe modelagain stthe numberof iceberg sobservedpassingne ar drillsites.
During the extensive oilexplorati oninthe Arcticduring the early19 80's.geo me tric solutions were developed bytheoil industry fordeterminingtheprobabilitiesofimpactsby ice floes into fixedplatforms;thesehave been publishedin a numbe rof sources suchas Jonlaan (1983). Dunwoody (1983).and Sanderson(1988).Thesemethods canbe applied to theproblemof impacts with icebergs.Because theyaresim pler and lessproneto measurement errorsthan methods requiring estimates of iceberg Ilux, they arc used here.
Geomcuical solutions for determining the expectednumbersof encocmers withice features giventheir sizes and velocitieshave beenpresented by Maes andJordaan(19 84 )and Sanderson(1988).To determinethe numbe rof encountersindiffe rent conditions.it is nec e ssaryto obtainappropriate data onicebergs andenvironmentand10acCOUnIforthe ir seasonal variationsasdisc ussed earlier.Whenconsidering impacts with floatingprodu ction syste ms andshuttletankers.whichrely00detecting and avoidingicebergs.itis im po rtant toaccount fortheeffect oftheenvironmenton the probabilitiesof encounter as wellas on
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detection.Insevereconditions,icebergs may travelseveral times faster thanin ordinary condi tio ns. thusincreas in g theprobab ility of impactwith systemsatfixedloc ation.This factor hasnot been adequatelydealt withinpublish e d studies.
Modelsare availableto predict theradarprobabilities of detectionfor different sized ice bergs and environmentalconditio ns (Ryan and Jo hnso n,199 2).Furthe r verificationof thedetection probabilitiesin high sea states are required.Theresul ts arein terms cr 'racer"
whichrela tetotheproba bilityof a signal fromthe icebergbeing observeddurin g asingle radar scan.rather thanlbeprobabilitiesof detectionas required withinprobabilisticanalyses.
A numberof analysesof iceberg towing records are available(Hetzel andMiller. 1985 and Bishop, 1989) whichgivean indicationof the conditionsandnum ber of icebergs forwhich towing is possible.When applying these resultstorisk analyses. attentionsho uld be given tothedefinitionused fortowingsucce ss. Fcrreguteral.(1987)outlinea probabilistic method for dererminlngcollisionprobabilities for ships hitting icefeaturessuchas multi-year icefloes.The probabilityofcollis ionineac h case is detenninedfromthe probability of det ect ingthe icefeature as a function of range and the probability thatthevesselcan manoeuvrequicklyenoughtoavoidit.
Aspects of the hydrodynamic interactionproblem have been addressed by McTaggan (1989),Isaacson(19 88), and Wishahy (1988). Thesesources describethebas icprinciples involvedand provide analytic solutions foridealized situations.Lever etai.(1988) present a method fo rdeterminin gthedistributionof surge veloc ities oficebergsin random seas.
Wishahy.inCammacneral.(1993), hasexte nded thisanal ysis to consider themotions of aniceberg in the vicinity of a vessel.
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Almostallanalytic models used todale todetermineglobaliceberg loads model the icecrushingstrengtheitbcrasacoostantoras a functionof
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nominalcontactarea.lbe tarter~Iationsh.ipsare typicallydetermined by bestfits todata based on interactions involvingthecNShingofboth glacial.andsea ice i.r:ldiffere nt load scen arios(see for examp le San derson,19 88andJotdaan and Zou, 1993).Bas ic researchisbein gdon e on thefailure mechani csofglacial ice (seeforexampleJo rdaanetal.,199 3),but accurate predicti ons of global loadsfromflrsrprinciplesarenot yetpossible.Theoverall collision dynamicsinicebergstnICtW'einteracti ons have been considered ina numberof smdies(DutbinbandMarsden. 1986.Nevel 1986,andBruneau, in Cammaert et al.,199 3).A benerunderstanding of ice failure mechanicsis required10be able to model the effects of friction during eccentric collis ionsandthe variations in loadsbecause of differe ncesinthe sha peoftheicebergsatthepointof contact.
The number of published papers dealingwith comprehe nsivepro bab ilisticstudieson des ign iceberg collis ionloadsisfairly sm all.Twoexamples . whic htake significan tly differen tapproaches.arebrie fly outlined he re.
Undberg and Anderson(1987)conducted a preliminary study to determinethererum periodsassociatedwith various levelsofdamage dueto ice berg collis ionsfor a numberof differentsteel semi-submcrsibledesigns.Ilwas proposed thai: different level sof risk:shou ld be allowedfordifferenrdegreesofdamagetothestructure.Forexample: small deformations should be allowedwithminimu mreturnperiodsof 1 to50yearsdependingonthemembe r affected; collisions resulting in leakage or bracing failuresho uldbe allowedwith minimum returnperiods of1000years;andcollisio nsresultinginheavydamage of morethan1..5 m
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indentati on shouldbeallowedwilh minimum.returnperiodsof 10,000years.Collis.ion rerum periodsweregivenfor4 sizesoficebergs.Thesenmged from 5)UrSfor aSOOtooneiceberg to 2S yearsfora15 ,000 tonne iceberg.1bccollisio n velociti esweredetermined fromthe driftvelocities oftheice bergs(assumedatI mfs)plusthe wave- ind ucedrelative veloc iti es of tile twobodies.assumingseastates of 6. 7.and10 msignifi can t wave height.The problem ofdetennining whether the icebergcouldcollidemore than oncewas modelledbyassuming thatatmo st1collisionoccurs witheach columnand two collisions with each pontoon deck can occur.Thenum be r in eachcasewas determinedbased ontheinitialeccentrici ty aftbc collisioo. whichwaschosen randomly.1bccoUisioo loadsweredetermin edusing a 3 hinge analys isforthe platesandstiffeners and afiniteclementanalysis for thestringe rs and heavier mem bers.Themaximumcollisionareaand force wen: determined basedonthe initial kinetic ene rgyofthe iceberg and acons tanticecrushing press ure.Icestren gths ran ging fro m4to 10 MPawe reassumedand allof theinitial collisionenergywas assumed tobeabsorbe d in the crus hi ngof the ice.Thestudy providedcurves sho wi n g theforce versuspenetra tio n for impactsoncolumnbulkh eads and betwee n bulkhe ads.Theanalys isshowed that forthe de sign conditionsspec ified.itshouldbepossible to cccstrec t an appropri ate vessel.The icebergsare small;thiswill affect the conclusions significantly.
Oneof the most c:oaqmhensiveprobabilistic analysisfor determining design iceberg collision lo adsforafixed structure is thesecondorder reliabili ty method(SO RM) implementedby Isaacsonand McTaggart (19 89 ) andMcT agg art (1989).Though the speci fic examplespresentedwerenot meant10be used for design purposes (for example.an arbitrary collisionfrequency of 20 eventsper year was used)the methodology issoundand the cases
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runinstructive.Inthe probabilisticmodel.truncal:ed cylindricallysha ped ice ber gs colliding with acylindricalstructurewere considered.Theicebergs were assumed toapproachthe plalformwith valuesfortheice berginitialmass.aspectratio.driftvelocity.eccentnciryof approach.significant waveheight,referenceiceaushin gpress ure.and [heice friction coefficien trandomlycbosenfromgiven distnbutiocs.A maj orportion of McTa ggart'sthesis deaJswith hydrodynamic interaction effects. Formeprobab ilisticanalysishe useda simplified modeltoreduce ron times,Thecollis io nveloc ity was determinedas thesumof the final driftvelocity. when Iineardiffraction effects wereaccountedfor. and theopen water wave-ind ucedvel oc ity ofthe iceberg .The wave-induced.velocity was determinedasthe value of the calculated response amplitudeoperator(R.A.a.)forthe iceberg at thepeakwave periodof therandomsea.timesthe significantwaveheight.The force due tothecrushin g ofice ateach instantwas determinedasthe productofthe contac tarea ofthecrushed.zone normal tothe platf orm timesthecrushingstrength of the ice determinedas a functionof contactareaA tange ntial frictionalforceproportiooaJtothenonnalcrushingforcewas applied. in the modelwhenever the tangentialvelocityof theice bergrelati veto the structure was greaterthan zero.Theinp utparameterswere modelled using uniformandlognormal distributionsbased on means andstandarddeviations frommeasured.data.Theice berg size and veloci tydistri butions were updatedto account for the pro bab ilityofcollidingwiththe platfo rm. Asecond orderreliabili tymethod(SaRM)wasusedtointegra tetheprobabilities to get collisionloads and kinetic energies. The design force associatedwith a probabilityof exceedance of 10% overthe life of the structurewas determinedtobe0.43 ON.Themost probable values of tbe inputparametersassociated with the designloadwere as follows:an
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