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ESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture154.1: OffshoreStructures: GeneralIntroductionOBJECTIVE/SCOPE To identifu the basic vocabulary, to inhoduce the major.on".Oo for offshore platform stuctures, and to explain where the basic structural requirements for desigr are generated. PREREQIIISITES None. SUMMARY The lecturestartswith a_presentation ofthe importanceofoffshore hydro-carbonexploitation,the basicstepsin the development process(from seismicexplorationto platform removal)and the introductionof the major structuraiconcepri(jackerbase,GBS-based, fff, nuti". h" major codesareidentified. For the fixed platform concepts(lacket and GBS), the different execution phasesare briefly explained: design, fabrication and installation. Specialattentionis given to someprinciplesoftopside design. A basicintroductionto costaspects presented. is Finally terms ae intoduced through a glossary.

1. INTRODUCTIONOffshoreplatforms areconstructed producethe hydrocar-bons and gas.The contributionofoffshore oil productionin the year lggg to the to oil world energyconsumption was9Yoand is estimatedtobe 24%o 2000. in The investnent (CAPE requiredat presentto produceone barrel ofoil per day ($/B/D) and the productioncosts(OpE per barrel are depictedin the tablebelow. Condition Conventional Average Middle East Non-Opec Offshorc North Sea Deepwater CAPEX $18/D

OPEX$/B

4000 8000 500- 3000 3000 12000

5I 8

10000 25000 15000 35000

5-10 l 0 - l5

World oil production in 1988w_as million barrel/day. These figures clearly indicate the challenge for the offshore designer: a growing 63 contribution is required from offshore exploitation, a very capital intensive activity. Figure 1 showsthe distributionofthe oil and gas elds in the North Se4 a major conaibutionto the world offshorehydrocarbons. also It indicates the onshorefields in England, the Netherlands and Germany

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GEFMANY

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FigureI North Sea, oil and gas fields (source\Vorld Oil. Augusr 19BB) 2. OFFSHORE PLATFORMS2.1 Introduction of Basic TypesThe overwhelning majority of platformsarepiled-jacketwith deck suctures, built in steel(seeSlides 1 and 2). all

Slide 1 : Jacket basedplatform - Southem sector North Sea

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Slide 2 : Jacket basedplatform - Northem sector North Sea A secondmajor type is the gravity concretestructure (seeFigure 2), which is employed in the North Seain the Norwegian and British sectors.

Figure2 Gravity based substructureplatform primarily constructed for the British and Norwegianfields in the Northern North SeaA third type is the floating production rmit. 2.2 Environment The offshore environment can be characterizedby: . . r . . o water depth at location soil, at seabottom in-depth and wind speed, aiitemperahre waves, tide and storm surge, cr[rent ice (fixed, floes, icebergs) (if earthquakes necessary)

The topside saucture also must be kept clear of the wave crest. The clearance(airgap) usually is taken at approximately 1,50 m, but should be increasedifreservoi depletion will createsignificant subsidence.

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The envronment well as financial aspects as requirethat a high degreeofprefabricationmust be performedonshore.It is necessary designto to limit offshore work to a minimum. The overall cost of a man-how offshore is approximately five times that of an onshore man-hour. The cost of construction equipment requiredto handleloads,andthe cost for logisticsare also a maitudehigher offshore. Thesefactorscombinedwith the size and weight of the items,requirethat a designer must carefully considerall constructionactivitiesbetween shop fabricationand offshoreinstallation. 2.4 Codes Structuraldesigr hasto comply with specificoffshorestructuralcodes,The worldwide leadingstructuralcodeis the API-RP2A [1]. The recentlyissuedLloyds rules [2] and the DnV rules [3] are alsoimportant. Specificgovernment requirements have io be compliedwrth, e.g. in the rules of Deparnent Energy @oE), Norwegian PetroleumDirektorate of (NPD). For the detail desigr of the topsidestructwethe AISC-code[4] is frequent used,andthe AWS-code [5] is usedfor welding. In the UK the Piper alpha diaster has led to a completely new appr

r'

sn l(o

Particle trajectories

Circular orbits

wavelenE fi = 2 where6"=2-

1

H

Pviors I Ne\t I Conrents

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Lecture 154.3: Loads (II) - Other LoadsOBJECTIVE/SCOPE To presentand briefly describeall loads, exceptenvironmentalloads,and the load combinationsfor which a fxed offshore structuremust be designed. PREREQIIISITES for A basicknowledgeofstructuralanalysis stic anddynamicloadings. SUMMARY Thesecategoriesinclude The various categoriesofloads, except envonmental,for which a pile-sipportedsteel offshore platform must be designedare presented. permanent(ded) loads, operating(live) loads, loads generatedduring fabrication and installtion (due to lifu, loadout, transportion, launching and upending) and by accidentlloads. In addition, the different load combnationsfor all types ofloads, including environmental,as required (or suggested) applicableregulations (or codesofpractice)aregiven. hereinarethe following: The categories ofloads described 1. 2. 3. 4. loads Permanent(dead) (live) loads Operating andinstallation loads Fabrication Accidentalloads

loadsarenot included.They aedealtwith in LectureI 54.2 of The major categories environmental

1. PERMNENT (DEAD) LOADSPermanent loadsincludethe following: in a. Weightofthe structure ai, includingthe weightofgrout andballast,ifnecessary. or structureswhich are permanentlymountedon the platform. b. Weights ofequipment, attachments associated pressures. forcesincludebuoyancy andhydrosratic belowthe waterline.These forceson the variousmembers c. Hydrostatic mustbe designed the worst conditionwhenfloodedor non-flooded. for Sealed tubulamembers .. :

2. OPERATTNG(L[VE) LOADS

:

equipment material,as well asforcesgenerated or during on loadsarisefrom the operations the platfom and includethe weightofall non-permanent Operating operating loadsincludethe following: operation ofequipment.More specifically, a.Theweightofallnon-permanentequipment(e.g.drilling'production),facilities(e.g.livingquaters,fmiture,lifeSupportsystems,heliport'e liquids, etc. supplies, etc. moonng,helicopterlanding,crae operations, during operations, drilling, vessel e.g. b. Forcesgenerated The necessary data for computationofall operatingloadsare provided by the operatorand the equipmentmanufacturers.The data needto be critically evaluatedby the columnsarefor design ofthe portionsofthe is designer. exampleofdeiled live load specification given in Table 1 wherethe valuesin the first and second An stnrctwe directly affected by the loads and the reducedvalues in the last column ae for the sucture as a whole. In the absenceof suchdata,the following values are recommended B56235 [1]: in 3,2 and a. crew quarters passageways: KN/m2 b. workms areas: I(N/m-) c. storageareas:yH KN/mz where y is the specificweight ofstoredmaterials, to be takenlessthan6,87KN/m3, not H is the storage height(m). Forcesgenerateddung operationsare often dynamic or impulsive in natue and must be treatedas such.For example,according to the 856235 rules, two types of helicopter landing should be considered,heavy and emergencylanding. The impact load in the first caseis to be taken as 1,5 times the maximum take-offweight, while in the secondcasethis factor becomes2,5. In addition, a horizontal load applied at the points ofimpact and taken equal to halfthe maximum take-offweight must be considered.Loads from rotating machinery, drilling equipment,etc. may normally be treatedas harmonic forces. For vessel mooring, designforces are computed for the is Accordingto 856235, the minimum impactto be considered ofa vesselof2500 tonnesat 0,5 m/s. at speeds. largestship likely to approach operational^ -.-.., a

3. FABRICTION AND INSTA.LLATION LOADSDuing fabrication, erection lifu ofvarious strucual Theseloadsare temporary and ariseduring fabrication and insllation ofthe platform or its components. during platform loadout,ansportation to the site, launching and upending, as componentsgeneratelifting forces,while in the installation phaseforces are generated well as during lifts elatedto installation. According to the DNV rules [2], the retum period for computing designenvironmentalconditions for instllation as well as fabrication should normally be three times phasethis designretum periodup to the owner,while the 856235 rules[1] recommend a API-RP2A,on the otherhand[3], leaves the durationofthe corresponding wth tansportation ofthe structureto the offshore site. minimum recurenceinterual of 10 yearsfor the design environmentalloads associated

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3.1 Lifting Forcesbeinglifted, the numberand locationoflifting eyesusedfor the lift, the anglebetween Lifting forcesarefunctionsofthe weightofthe structuralcomponent eachsling (Figwe 1). All members conections and theverticalaxis andthe conditions underwhich the lifr is performed and ofa lifted component mustbe desigedfor the forces resultingfrom staticequilibriumofthe lifted weight andthe sling tensions. Moreover,API-RP2A recommends in orderto compensate any sidemovements, that for to structural members shouldbe designed the combinedactionofthe staticsling loadand a horizontalforce equalto for lifting eyesandthe connections the supporting 5% this load, appliedperpendicular the padeyeat the centreofthe pin hole.All thesedesignforcesareappliedas staticloadsifthe lifts areperformedin the to yard. If, however, lifting derrickor the structure be lifted is on a foating vessel, to thendynamicload factorsshouldbe appliedto the staticlifing forces. fabrcation te In particular, lifts madeoffshoreAPI-RP2A recommends minimumvaluesofdynamic load factors:2,0 ad 1,35.The first is for designingthe padeyes well for two as while the second for all othermembers asall members their endconections and framingthejoint wherethe padeye ttached, is is transmitting lifting forces.For loadout at shelteredlocations,the conespondngminimum load factors for the two groupsof structual componentsbecome,accordingto API-RP2A, 1,5 and 1,I 5, resoectivelv.

{a} Derrick and structuro on land

sJ

s

(b) Denick on land. Gtructure on floating barge

(c Derrick and strusture in the sea

Figure 1 Lifts under vrious conditions3.2 Loadout Forceswhen thejacket is loaded fiom the fabrication yard onto the barge.Ifthe loadout is carried out by direct lift, then, unlessthe lifting Theseae forces generated arrangement different from that to be usedfo installation, lifting forces neednot be computed,because is lifting in the open seacreatesa more severeloading condition which requires higherdynamicload factos.Ifloadout is doneby skiddingthe stucture onto the barge,a numberofstatic loadingconditions mustbe considered, with (as thejacketsupported its side.Suchloadingconditionsarisefrom the differentpositionsofthejacket during the loadoutphases, shownin Figure 2), from on marinetraftic or change supportsettlements. movement ofthe bargedueto tidal fluctuations, ofdraft andfrom possible Sincemovement ofthejacket is sloq all loading conditions can be taken as statc.Typical valuesoffriction coefcientsfor calculation ofskidding forces are the following: . o o o ............. 0-25 steelon steelwithout lubrication-. with lubrication...... ............ 0,15 steel steel on s t e eo n t e f l o n . . l . . . . . . . . . . . . - . . . . . . . . . . . .0 . - .0. . . . . -,1 . teflonon teflon ....... 0.08

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M 5 4 3 2 1

Tide curve lor May 5th phases H W O

'l979 'l i

o3 4 5 6 7 I I 10 1't1213141't61718

Phsse D W6ler pmped oul : 1.600lBi

Pro9t Fcket

Wdghl on the aEe

5,54m

37.O00rPhaee'l W6ler pumped out: 83Ol Balls6l trnsler: 17ot

o

6;14m

40.220t

65m

8.6r

Q59m

43.OOOt

95m

12.BOot

7,2O

47.7BOr

125m

't7.ooor

7,4m

50.500r

142m

17.OOOr

Figure 2 Various phses of jacket loadout by skidding

3,3 Transportation Forces(acket, deckfare transportedoffshore on bargesor self-floating. They dependupon the weight, geometryand Theseforcesare generatedwhen platform components during support conditions ofthe structure(by bargeor by buoyancy)and also on the environmentalconditions (waves, winds and currents) that are encountered ansporration. The types ofmotion that a floating structue may experienceare shown schematicallyin Figue 3.

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Pitch

Sway

Surge

Yaw

Figure 3 Types of moton of a floating object.I order to minimize the ascitid.risksad securesafetransport from the fabrication yard to the platform site, it is important to plan the operatipncarefully by- ,\' . considering, according Io..\PI-RP2A[3], ihe followrng: -

. 1 . Previousexperie4cealo4g the tow route wiidows" 2. Exposure time and reliability ofpredicted"weatherAccessibilityof safehavens weathersystem Seonal designwind, wave and current conditions, taking into accountcharacteristicsofthe tow suchas size, structure, 5 . Appropriateretum periodfor determining sensitivitvand cost.J.

Transportationforces are generated the motion ofthe to\H,i.e. the structureand supportingbarge.They are determinedfrom the design winds, wavesand curents.If by mustbe based the results on ofmodel basintestsor directly.Accordingto API-RP2A [3], towing analyses is the the stncture self-floating, loadscanbe calculated appropriateanalytical methodsad must consideru/ind and wave directibns parallel, perpendicularand at 45o to the tow axis. Inertial loadsmay be computedfrom a ofthe seastateandexperience makesuch rigid body analysis ofthe tow by combiningroll andpitch with heavemotions,when tle sizeofthe tow, magnitude as values: the assumptions reasonable. openseaconditions, following may be considered typical design For roll: 20' Single- amplitude Single- amplitudepitch: 10' Periodofroll or pitch: 10 second 0,2 Heaveacceleration: g When tanspofting a largejacket by barge,stability againstcapsizingis a primary design considerationbecauseofthe high centreofgrav ofthejacket. Moreover, the relative stiffiress ofjacket and bargemay needto be taken into accounttogether with the wave Slammingforces that could result during a heavyroll motion ofthe tow (Figure 4) when structural aaalyses caried out for designingthe tie-down bracesand tlejacket membersaffected by the induced loads. Specialcomputerprograms ae for systemand the resulting stresses any specified envionmentalcondition. are available to computethe transportationloads in the structure-barge

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Tie downs

F : Componerrt of gravity plus inertia G r - Cenlr of gravity of lacket z : Centre of gravity of the tow M = Metacentre of the towAreas of potential impact

Figur 4 Schematic vew of launch barge and jacket undergongmoton3.4Launchingand UpendingForcesupendinginto its proper vertical position to rest on the during the launch ofajacket from the bargeinto the seaand during the subsequent Theseforcesare generated can view oftheseoperations be seenin Figure5. seabed. schematic A

Figure 5 Launchingand upending sequences of a platform jacketThere are five stagesin a launch-upendingoperation: a. Jcketslidesalongthe skid beams b. Jacketrotateson the rocker arms c. Jacketrotates md slidessimultaneously completely comesto its floatingequilibriumposition ard d. Jacketdeches e. Jacketis upendedby a combinationofcontolled flooding and simultaneouslifting by a derrick barge. The loads, static as well as dynamic, induced during eachofthese stagesand the force required to set thejacket into motion can be evaluatedby appropriateanalyses, which also considerthe action ofwind, waves and currentsexpectedduring the operation. To start the launcb,the bargemust be ballastedto an appropriatedraft and rim angle and subsequentlythe jacket must be pulled towards the stem by a winch. Sliding ofthe jacket startsas soonas the downward force (gravity componentald winch pull) exceedsthe friction force. As the jacket slides, its qeight is supportedon the two a and reches minimum, equal to the length ofthe rocker beams,when rotation starts.lt rs legs that arepart ofthe lamch russes. The supportlength keepsdecreasing ganerally at this instant that the most severelaunching forces developas reactionsto the weight ofthejacket. During sages (d) and (e), variable hydrostatic forces arise which haveto be consideredat all membersaffected.Buoyancy calculationsare required for every stageoffe operationto ensurefly contolled, stablemotion. required for launching and upending and also to porray the whole operationgraphically. Computerprogms are availableto perform the stressanalyses

4. ACCIDENTAL LOADSAccording to the DIW rules [2], accidenal loads are loads, ill-defined with respectto intensity and frequency,which may occur as a result ofccident or exceptional in as category the NPD regulations but not in API-RP2A [3], B56235 [] or the DOE-OG rules [5]. circumstances. Accidentalloadsarealsospecified a separate [4], dropped objects,and mintendedflooding ofbouyancytanks.Specialmeasures fire Examples ofaccidenl loadsareloadsdueto collision with vessels, or explosion, protection ofwellheads or other critical equipmentfrom a droppedobject can be provided by are normally taken to reducethe risk from accidentalloads. For example, specially designed,impact resistantcovers.According to the NPD regulations[4], an accidentalload can be disregardedif its annualprobability of occurrece is less

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Earthquakes treated an environmental are as loadin offshore and than loa. This numberis meantas an orderofmagnitudeestimate is extremelydifcult to compute. structue design.

5. LOAD COMBINATIONSdesign employed. is uponthe designmethodused,i.e. whetherlimit stateor allowablestress fixed offshorestructuesdepend The load combinations usedfor designing procedures are: recommended usewith allowableshess for The load combinations ofthe platform. to environmental loadsplus maximumlive loads,appropriate normal operations a-Deadloadsplus operating b. Dead loads plus operatingenvironmentalloads plus minimum live loads, appropriateto normal operationsofthe plaform. c- Dead loadsplus exeme(design) environmentalloads plus maximum live loads, appropriatefor combining with extremeconditionsr d. Dead loads plus exheme(design) environmentalloads plus minimum live loads, appropriatefor combining with extremeconditions. Moreover, environmentalloads, with the exceptionofearthquake loads,should be combined in a mannerconsistentwith theirjoint probability ofoccunence during the with waves,wind, etc. as environmental loa4 i.e.,not 1obe combined Earthquake loads,ifapplicable,areto be imposed a separate loadingconditionconsidered. would requirecessation ofplaform limiting conditions that, ifexceeded, ofseverebut not necessarily environmental conditionsaredefinedas representative Operating operauons. the limit statedesignmethod,which the NPD rulesalsorequire[4]- 856235 design recommend semi-probabilistic but The DNV rules[2] permit allowablestress not method[1]. API-RP2Ais very specificin recommending to applylimit ste permitsboth methods the designequations givesarefor the allowablestress it but must be checked: methods. Accordingto the DNV andthe NPD rulesfor limit statedesigrfour limit states l. Ultimatelimit state mustbe used: Fr this limit stethe following two loadingcombinations Ordinary: P + 1,3L + 1,0D + 0,7E, and 1,3 : E x t r e m e 1 , 0 P+ 1 , 0 L + 1 , 0 D + 1 , 3E (live), Deformation (e.g.,temperature, (dead), differentialsettlement) Environmental and loadsrespectively. Operating whereP, L, D andE standfor Permanent For well controlled deadand live loadsduring fabrication and installation, the load factor 1,3 may be reducedto 1,2. Furthermore,for structuresthat are gas,the 1,3 load factor for environmentalloads - except earthquakes may be unmannedduring storm conditions and which ae not usedfor storageofoil and I,15. reducedto 2. Fatiguelimit state All loadfactorsaeto be takenas 1,0 limit state 3. Progressive Collapse All load factorsaeto be takenas 1.0. 4. Sewiceabilitylimit state All load factorsareto be takenas i.0. and are in characteristic valuesofthe loadsusedin the abovecombinations limit states summarized Table2, takenfrom the NPD rules. The so-called

6. CONCLUDING SUMMARY. . o o o . In addition to environniental lods,an offshore structue must be designedfor deadand live loads,fabrication and inslallation loads as well as accidentalloads are rulesof practice,listedin the references, usuallyfollowed for specifring suchloads. Widely accepted usedfor the correspondingphases. The type and magnitudeof fabrication, transportationand installation loads dependupon the methodsand sequences Dynamic and impact effects are normally taken into accountby meansof appropriatedynamic load factors. Accidental loads aenot well defined with respectto intensity and probability of occunence.They will typically require special protective mesues. Load combinationsand load factors dependupon the designmethodto be used.API-RP2A is basedon allowable stess design and recommendsagainstlimit state limit statedesig. desig,while DNV andNPD recommend desiga, BSI favoursallowablestress

7. REFERENCESInstitution,London, 1982. British Standards Fl 856235, "CodeofPracticefor Fixed OffshoreStructwes", Det and of [2] "Rulesfor the Design,Construction Inspection OffshoreStructures", NorskeVeritas@NV), Oslo, 1977(with conections1982). Institute,Washington, D.C., 18th Fixed OffshorePlaforms", AmericanPetroleum DesigningandConstructing Practice Plalming, for [3] API-RP2A, "Recommended ed.,1989. Norwegian Petroleum Directorate (MD), 1985 [4] "Regulation for Structural Desig of Load-bearingStructues Intendedfor.Exploitation of Petroleum Resources", ofEnergy, London 1985. [5] DOE-OG, "OffshoreInstallation:Guidaceon Designand Constuction",U.K. Department

8. ADDITIONAL READING1. 2. 3. 4. OCS, "Requirementsfor Verif,ing the StructuralIntegity of OCS Platforms"., United StatesGeologic Survey,National Cente, Reston,Virginia, 1980. Gulf PublishingCo.;Houston,1981. Hsu, H.T., "Applied OffshoreStructal Engineering", Gulf PublishingCo.,Houston,1981. Grafl W.G., "Introductionto OffshoreStructures", New York, 1986. Gerwick,B.C. Jr., "Construction ofOffshore Stuctues", John!r'iley,

Table 1 Minimum designlive load specification Loads to be taken into account For portionsofthe structure For the structureas a whole

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(k/m2) Zone considered Flooring and joists 5 (1) Other components 5(1) (3)

zone(aroundwells and Process machines) large-scale Drilling zone Catwalks and walking platforms (except emergencyexits) Stairways (except emergenc] exits) Module roofing Emergency exits STORAGE Storagefloors - heavy Storagefloors - light Delivery zone Non-attributed area

2.5 2.5I

5(1)

5( t ) 2.5 3

)

0

)

l.)

I 0

5

l8 9 l0o

t2o

8 (2) 4 (2)

10J

as partlikely to be remtived, with a minimum valueof5 kN. Point loadsareassumed being (1) Accumulated with a point loadequalto the weight ofthe heaviest appliedto a 0,3m x 0,3m surface. (2) Applied on the entirety ofthe flooring surface(including trafc). runs. These valuesaethe input for the computer for (3) This columngivesthe loadsto be takeninto account the structe'soverallcalculation. Table 2 CharacteristicLoads accordingto NPD [4]

LOADTYPE

LIMIT STATFS FOR TEMPORARY PHASES Progressive Collapse 3ewiceability Fatigue Ultimate Abnornal effects Damage

LIMIT STATES FOR NORMAL OPERATIONS Prosressive Collapse Serviceability Fatigue Ultimate Abnormal effects Damage coridition

DEAD LIVEDEFORMATION ENVIRONMENTAL Dependent on operational requirenmts Expected load history

EXPECTEDVALUESPECIFIED VALUE

VALUE EXTREME E)(PECTEDVa]ue dependent on measures taken Dependent on operational requiremmts Expected Annual Anual load probabiJity probability history Amual exceedance probability 702

to-z

104

ACCIDENTAL

NOTAPPUCABLE

Dependent on operational requiremm

NOTAPPUCABLE

Annual NOT exceedance APPLICABLE probability LO4

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Lecture 15^.4 - AnalysisIOBJECTI\rE/SCOPETo presentthe main analysisproceduresfor offshore structures. PREREQTIISITES Lecture 15,A..1: Offshore Structures:General Introduction Lecture 154.2: Loads I: Introduction and Environmental Loads Lecture 154.3: Loads II: Other Loads RELATED LECTURES Lecture 154.5: Analysis II SUMMARY

Acceptance criteriafor the ale Analyticalmodelsusedin ofhore engineering briefly described. are verificationof offshorestructures presented. for Simplerulesfor preliminarymembersizingaregiven andprocedrnes staticin-placeand dynamic analysis described. are

1. A}{ALYTICAL MODELThe analysisof an offshore structure is an extensivetask, embracing consideration of the different stages,i.e. execution, installation, and in-service stages,dwing its life. Many disciplines, e.g. structural, geotechnical,naval architecture,metallurgy are involved. This lecture and Lecture 154.5 are purposely limited to presenting an overview of available analysis proceduresand providing benchmarksfor the readerto appreciatethe valid of his assumptionsand jackets, which are more unusual structurescomparedto decks and results. They primarily address more closely resemble onshorepetro-chemical plants. modules, and which

2. ANALYTICAL MODELThe analytical models used in offshore engineeringare in some respectssimilar to those adoptedfor other types of steel structures.Only the salient featuresof offshore models are presentedhere. The samemodel is used tlroughout the analysisprocesswith only minor adjustmentsbeing made to suit the specific conditions, e.g. at supportsin particular, relating to each analysis.

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Stick models (beam elementsassembledin frames) are used extensively for tubular structures fiackets, bridges, flare booms) and lattice trusses(modules, decks). 2.1.1Jotnt Each member is normally rigidly fixed at its endsto other elementsin the model. If more accuracyis required, particularly for the assessment natural vibration modes, local of joint stiffriess matrix. flexibility of the connectionsmay be represented a by 2.1.2 Members In addition to its geometrical and material properties, eachmember is charactetisedby hydrodynamic coefficientS,e.g.relating to drag, inertia, and marine growth, to allow wave forces to be automatically generated.

2.2 PlateModelsIntegrated decks and hulls of floating platforms involving large bulkheads are describedby plate elements.The characteristicsassumedfor the plate elementsdependon the principal state of stress which they are subjectedto. Membrane stresses taken when the element is subjectedmerely to are axial load and shear.Plate stresses adoptedwhen bending and lateral presswe are to be taken into are account.

3. ACCEPTAI\CE CRITERIA3.1 Code ChecksThe verification of an element consistsof comparing its characteristicresistance(s) a design force to or stress.It includes: o a strength check, where the characteristicresistanceis related to the yield strength of the element, o a stability check for elementsin compressionwhere the characteristicresistancerelates to the buckling limit of the element. An element (member or plate) is checked at typical sections(at least both ends and midspan) against resistanceand buckling. This verification also includes the effect of water pressurefor deepwater structures. Tubular joints are checkedagainstpunching under various load patterns. Thesechecksmay indicate the need for local reinforcement of the chord using overthicknessor internal ring-stiffeners. Elements should also be verified against fatigue, corrosion, temperatwe or durability wherever relevant.

3.2 Allowable StressMethodThis method is presently specified by American codes (API, AISC).

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The loads remain unfactored and a unique coefficient is applied to the characteristicresistanceto obtain an allowable stressas follows: Condition

Axial

Strong axis bending

'Weak axis bending 0,75

Normal Extreme

0,60 0,80

0,66 0,88

1,00

"Normal" and "Extreme" respectively representthe most severeconditions: o under which the plant is to operatewithout shut-down. . the platform is to endureover its lifetime.

3.3 Limit StateMethodThis method is enforced by Europeanand Norwegian Authorities and has now been adopted by API ' as it offers a more uniform reliability. Partial factors are applied to the loads and to the characteristicresistanceof the element, reflecting the amount of confidenceplaced in the design value of eachparameterand the degreeof risk acceptedunder a limit state,i.e: o Ultimate Limit State(ULS): correspondsto an ultimate event considering the structural resistancewith appropriate reserve. o Fatigue Limit State(FLS):

relates the possibilityof failure undercyclic loading. to (PLS): Limit State Progressive Collapsereflects the ability of the structure to resist collapse under accidental or abnormal conditions. o ServiceLimit State(SLS): correspondsto criteria for normal use or durability (often specified by the plant operator). 3.3.L Load factors Norwegian Authorities (2, 4) specify the following setsof load factors: Load Categories

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ULS (normal)

ULS (extreme)

PLS (accidental)

where the respectiveload categoriesare: P are permanentloads (structural weight, dry equipments,ballast, hydrostatic pressure). L are live loads (storage,persorxlel,liquids). . ,;

D are deformations (out-of-level supports, subsidence). E are environmental loads (wave, current, wind, earthquake). A are accidental loads (dropped object, ship impact, blast, fire). 3.3.2Material factors The materialpafal factors foi steel is normally taken'equalto 1,15 for ULS and tr,00for PL,Sand SLS design. 3.3.3 Classification of Design Conditions Guidancefor classiffing typical conditions into typical limit statesis given in the following table:Condition Loadings PIL E DA

Design Criterion

Construction

P

ULS.SLS

Load-Out

P

reducedwind

disp support

ULS

Transport

P

transpol wind and wave

IJLS

Tow-out (accidental)

P

floo-ded compart

PLS

Launch

P

Lifting

P

In-Place(normal)

P+L

wind. wave & snow

actual

G'"*-

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In-Place(extrem

P+L

wind & 100 year wave

actual

ULS SLS

In-Place(exceptional) ]

P+L

wind & 10000year wave

actual

PLS

P+L

l u - quaKe

ULS

Rare Earthquake

P+L

IExplosion P+L

roaquate

PLS

Iblasl PLS

Fire

o.oppe ouect I

F* Il ' . ' ]reducedwave & wind

fire

PLS

drill collar

PLS

boat impact

PLS

PI-Sr

4. PRELIMINARY MEMBER SIZINGThe analysis of a structure is an iterative processwhich requires progressiveadjusfinent of the, member sizeswith respectto the forces they transmit, until a safe and economical design is achieved. It is therefore of the utmost importanceto start the main analysis from a model which is close to the .: final optimized one. The simple rules given below provide an easy way of selectingrealistic sizesfor the main elements of offshore structuresin moderatewater depth (up to 80m) where dynamic effects are negligible,

4.1 Jacket Pile Sizeso calculatethe vertical resultant (deadweight, live loads, buoyancy), the overall shearand the overturning moment (environmental forces) at the mudline. . assumingthat the jacket behavesas a rigid body, derive the maximum axial and shearforce at the top of the pile. o selecta pile diameter in accordancewith the expectedleg diameter and the capacity of pile driving equipment. . derive the penetration from the shaft friction and tip bearing diagrams. o sSuming equivalent soil subgrademodulus and full fixity at the base of the jacket, an calculatethe maximum moment in the pile and derive its wall thickness.

4.2 Deck Leg Sizeso adaptthe diameter of the leg to that of the pile. r determinethe effective length from the degreeof fixity of the leg into the deck (depending upon the height of the cellar deck). o calculatethe moment causedby wind loads on topsides and derive the appropriatethickness.

4.3 Jacket Bracings

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. select the diameter in order to obtain a span/diameterratio between 30 and 40. calculate the axial force in the brace from the overall shearand the local bending causedby the wave assuming partial or total end restraint. . derive the thicknesssuch that the diameter/thickness ratio lies between 20 and70 and, eliminate any hydrostatic buckle tendency by imposin gDlt