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AIM (Assessment, Inspection,Maintenance) and Reliabilityof Offshore PlatformsR.G. Bea, PMB Systems Engineering, Inc., San Francisco, CaliforniaC.E. Smith, Minerals Management Service, Reston, Wrginia

ABSTRACT

Concerns for requalification ofexisting fixed offshore platforms haveserved to focus a need for developmentof a practical engineering approach tothe AIM (Assessment, Inspect ion,Maintenance) aspects of these struc-tures. The principal concerns forrequalification are focused on olderplatforms that are now in service, andthat are providing a resource criticalto Us. energy requirements.

This pape~ defines one approachto the AIM process for fixed offshoreplatforms. Probabilistic methods areapplied to several key parts of thisapproach. These include assessmentsof operating and environmental forces,the as-is and repaired capacities ofthe platform, and analyses of alter-native remedial mainte”a”ce programs.

INTRODUCTION

three primary interrelated elements inwhat will he termed the platform AIMtriangle [Fig. 1):

Fig.

Today, there are some 6,000 fixed 1.offshore platforms sited on thewxld,s Co”tine”tal shelves. Many ofthese structures have been in placefor over 30 years.

Renewed drilling activity tofurther develop kno”n reserves, andsupplemental recovery operationsindicate the need to re$.palify theseStructures for extended lives. Inaddit ion, there are extremely strongpressures to minimize costs, part ic”-larly in the light of depressed oilprices.

These developments have resultedin the vital concern with requalifica-tion of existing platforms. Fixed 2.platforms have had an enviable safetyrecord , and the objective is tomaintain this record as platformsenter their twilight years.

AIM REQUAL1F1CATIONS

The A1f4 engineering approach torzq. alif ication of platform. involves

1 Platform & 1 M Triangle

= - those engineeringappraisals intended toevaluate present and futureplatform serviceability,determine the desirablecharacteristics of presentand future serviceability,and examine alternativeplatform maintenance programswith the objective of identi-fying practical maintenanceprograms that will developacceptable platform service-ability characteristics whilepreserving essential safety,economic, and environmentalobjectives.

Inspect - those engineeringand operations programsdirected toward detection anddocumentation of significantdefects or damage in aplatform that can lead topotentially significantreductions in platformcapacities and serviceabilitycharacteristics.

57

,. m.in. a.n - cnose engineeringand operations programsdeveloped and implemented topreserve or enable a ?Iatformto develoo acc.eotablecapacities and ~erviceabilitycharacteristics.

The AIM triangle indicates acontinuing process of platform requali -ficatio”s intended to keep platformsin service by using preventative andremedial enaineerina/operationstechniques. “ The A1; ~rociss isintended to be one of progressive andcontinued reduction of risks totolerable and acceptable levels.

The AIM approach is positive.Inspection, definition of defects anddamage, and repairs or impcoveme”tsare given high priority in platformoperations, with an objective ofestablishing and maintaining theintegrity of a given structure at theleast possible cost. Practicalityimplicates an incremental in”estme”tin identifying a“d remedying platformdefects in the order of the hazardsthey might represent. This is aprioritized, learn-your-way-throughapproach. ..

The focus of the AIM approach(Fig. 2) is on identification of highhazard potential structures that maypossess significant defects or damage,and how to define cost ef feet ive,professionally acceptable, and prac-tical solutions for these structures.The benefits of AIM engineering andoperations activities must be justi–fied by the benefits that are achievedand the resources that can be investedto keep a vital resource flowing tothe market place.

Tbe basis of the AIM approach isthat the problem of a major platformwith potentially significant defectsis one that should be approachedwithout rigid conformance to “conven-tional pract ice, ” maintaining a highlevel of technical and operationalexcellence, and defining creative andpractical ways to lessen risks withinthe unavoidable constraints of cur-rently available knowledge, manpower,money and time. This is a structureand problem-specific approach. It isnot an engineering cede or rigidguideline approach.

Unfortunately, at this time,there are no established engineeringcodes or guidelines for platform AIM.In this vacuum, many engineers wouldadopt current platform design g.ide–lines and practices as a basis forevaluation of existing platforms.

0,,..,0.,9,.,,,,,s .,...,,,

.-i

11’”’ 1

Fig. 2 A I M Approach

This can be a dramatic “istake forseveral reasons. Codes are generalguides for practice. They camotrecognize many site, platform, a“doperation specific factors criticalto platform req”alifications. Codesare oriented to elements in a plat-form, and a general framework ofcommon engineering practice. Codesare intended to result i“ a struc-ture that is serviceable, safe, andeconomic.

Platform requalification have aseries of objectives that differ sub-stantially from those of codes a“dguidelines intended for design of astr. ct”ce. These object ives are thoseof realistically evaluating an existingplatform which is frequently defective,and attempting to answer the question,“will this structure, in its presentcondition, perform acceptably duringits remaining life?!, Alternatively,this question can be posed, “What canor should be done to allow thisplatform to perform acceptably duringits next AIM Cycle?mm These object ivessuggest a different set of engineeringphilosophies and approaches.

1N1T1ATION

The AIM approach (Fig . 2) isinitiated with the platform operatoridentifying a candidate platform.There are tw principal considerations:1) which platforms should be selected,and 2) how many platforms should beselected.

The first consideration isbasically one of identifying thepriorities of the AIM process. Thesecond consideration is one of deter-mining the allocation of resources forthe AIM process.

There are a wide variety ofquantitative and qualitative rankingprocedures which can be used i“ theplatform selection process. Onepractical approach consists of t“oqualitative priority evaluationattributes: 1) consequence potential,and 2) defect potential. The conseq-uence potential is the likelihood,given an extreme loading event, thatthere could be extensive damage toproperty, lives, resources, and theenvironment. The defect ptentialis the likelihood of deficiencies i“design, comtruct ion, and/or operat ionof the platform. The essence of thedefects is as they might affect tbecapacity of the platform to resistextreme events (Fig. 3) .

&

Fig. 3 Intact and DamagedPlatform Capacities

Both of these potentials can beevaluated cm a relative ranking scale,e.g. , H = Hiqh, M = Mode Kate, L = Low.Knowledge of the structure, qualifiedjudgment, a“d most importantly, thehistory of performance of the struc–ture become the bases for the evalua–t ion. The two evaluations are com-bined (Fig. 4) to result in ninedifferent possible combinations ofConsequence md defect potentials.The first priorities for introducing aparticular platform to the AIM processare give” to those Structures whichpossess both high consequence anddefect Pote”ti.als.

“R”“’”Rr” la...... m,..,,,=‘H”” “’”””” Im

Fig. 4 Platform InspectionPriorities

Now, give” large number ofplatforms, the auestion becomes one ofbw m.my 6truct&es should be intro-duced to the AIM process? This isfundamental ly a question of how muchresource a particular operator feelsis appropriate to imest in AIMprograms, either for a specificplatform or a fleet of platformsassociated with some particulardevelopment.

At this time, there are nogeneral or easy answ?rs to thisquestion. Quantitative cost-bemfitanalyses could be made based on theoverall economics of a particularplatform or development to assure thata reasonable investment of resourceswill be made to maintain the plat-form’s abilities to perform acceptablyduring project operatiom [1-3] .

COND1TION SURVEY

In this AIM step, a data bank isinitiated or co”timed cm a particularstructure, including all availablepert inent information on the desiqn,construct io”, a“d operational historyof this str”ct”re. Of major import-ance are ident ifying a“d recordingexceptional events OK developments.The qreater tbe knowledge about aparticular structure, the” the morerealistic is the eval”atio”, and themore effecti”e the AIM program results.It is impossible to realisticallyevaluate a platforms performance orsafety without definite information o“the structure. The primary componentsthat should be incorporated into aplatform data bank are summarized inTable 1.

59

——— .—.

An AIM program can only beeffective if there is an adequatestore of information on the plat-form or fleet of platforms. Thisstore of information, or data bank,should contain information on thedesign, construction, operation,maintenance, and as-is condition ofthe platform. This data bank becomes“ital in directing the course ofinspection surveys -- determiningwhich elements to inspect, what tolook for, the methods to inspect, andthe timing or frequency of inspections[4-12] .

SCREENING

The next two AIM steps areconcerned with assessing or screeningthe candidate platform’s need fordefect mitigation. Examples ofplatform defects are given in Table 11If there appears to be no potentiallysignificant defects in the structure,the procedure is concerned with theengineering of tbe next inspection andmaintenance cycle. If there appear tobe significant defects, tbe next stepis to determine if mitigation of thesedefects is necessary.

Mitigation of defects refers to aprioritization of remedying thosedefects, and identification of prac-tical alternative remedial act ions.The evaluation necessarily depends onthe hazard potent ial of a givenplatform; given that tbe platformwould not perform adequately duringtbe next AIM cycle, and on the Fx3ten–t ial for such performance. If nomitigation appears to be warranted ,the procedure again branches to the

1.

2.

3.

4.

5.

6.

design of the next inspection andmaintenance cycle for tbe platform.

EVALU?+TION OF M1TIGATION ALTERNATIVES

If mitigation appears to bewarranted, the AIM process branches tothe detailed evaluation of tbe alter–natives for mitigation (Table III) .The alternatives include:

a.

b.

c.

d.

The structure itself -repairs to damaged, in-place,load-carrying members.

Loadings - removal of deckequipment, removal of marinefouling, removal of unused orunneeded elements (e.g. boatlandings, risers, etc. ).

Operations - improvement ofcorrosion protection, instal-lation of additional well andproduction safety equipment,installation of additionalpersonnel safety equipment,demanning in advance of storms.

Information - on-site ins Pee-tions and measurements to-improve detail of data onpresent condition of thestructure, development ofdetailed information on paStloading events.

1MPLEMENTATION AND DESIGN OF NEXT AIMCYCLE

Once the mitigation alternativehas been defined, the next step is toengineer that alternative and imple-ment it in tbe platform operations.

E%2!z : site data! s~+te~ia, .Wdslines, e~o-du=, d~awiwst etc.pertaining to the lnxtlal engineering phase of the structure.

Fabrication - Specifications, materials, equipment, quality assuranceprocedures and reports, ‘etc. pertaining to the onshore constructionphase of the structure.

Transportation - Specifications, equipment, quality assurance proceduresand reports, etc. , pertaining to the load–out and transport of thestructure to the offshore installation site.

Installation - Specifications, equipment, materials, quality assuranceprocedures and reports, etc. , pertaining to the preparation for place-ment and replacement of the structure at the location.

Operations - Information pertaining to plaform loading and capacitycharacteristics and iwdifications that are developed during the drillingphase and during the production phase of operations of the structure.

Maintenance - Specifications, equipment, materials, procedures used topreserve or modify the capacity of or loadings on the platform.

Table 1 Platform Data Bank Components

60

—

1. Design

a. Storm wave and current forces underestimated

b. Earthquake forces underestimated

c. Tubular joint design results in low capacity and short fatiguelives

d. Conductor and riser wear on supports

e. Insufficient corrosion protection

f. Unanticipated scour

9. Gravity load underestimated

2. Construction

a.

b.

c.

d.

e.

f.

9.

Misalignments of legs, braces, and joints

Undercut welds

Insufficient penetration welds

Tank welds

Lamellar tearing

Insufficient pile penetration (lowering axial capacity, loweringlateral capacity)

Load-out, transportation, and launch damage to primary structuralelements

3. Operat ion

a. Corrosion protection not maintained (above and below water)

b. Boat bwnpers .md landings “ot maintaimsd (result img in damage toprimary structural elements)

c. Trash dumping (cables, pipe) resulting in damage to legs and braces

d. Field modifications to structure (cutting boles in members, addingrisers and riser supprts, adding deck sections and deck cantilevers)

e. Addition of “E1l conductors and producticm risers above design

f. Addition of deck eq”ipme”t and loadings abode design

9. Poorly engineered and implemented repairs to primary structuralelements

4. Accidental

a.

b.

c.

d.

e.

Boat and barge collisions, resulting i“ damage to primary str”ct”ralelements.

Dropped objects res.lti”g i“ damage to primary str”ct.ral elements.

Workover operations fires and explos ions resulting in damage toprimary structural elements.

Prcd”ctim equiprne”t fires and explosions resulting i“ damage toprimary structural elements.

Drilling fires and explosions resulting i“ damage to primary struc-tural elements.

Table 11 Examples of Platform Defects

61

R~lNG PLATFORM DEMANDS - MINIMIZE LOADS AND LOAD EFFECTS

“ Reduce deck loads - dead loads from equipment and facilities, live loadsfrom storage

“ Reduce wave and current forces - removal and prevent ion of marine fouling;removal of non-essential components and appurtenances

“ Reduce wane and current forces - re-evaluation of wave ?md current condi -tions based on site (bathymetric) , platform and operatiom (exposureperiod ) Specific conditions .md BAST*

INCREASING PLATPORN CAPACITIES - MAXIMIZE STRENGTH OF ELEMENTS

“ Increase strength of joints by qrouti”g, welding, profiling , replacement

“ Increase strength of primary and secondary members by doubler wraps,replacement, grout fill, secmdary bracing, soil strengthening(foundation members)

“ Add members - braces, piles, beams

“ Re-evaluate Serviceability a“d ultimate Limit States resistances a“dcapacities based on platform, site, and operations specific co”ditio”sand BAST

REDUCING OPERATIONS EXPOSURES

“ Reduce operations carried out onboard or adjacent to the platform

“ Reduce deck equipment

“ Reduce storage

“ Reduce wells and risers

“ Increase pollution control , clean-up equipment and measures

“ Increase well and prcductio” protection eq”ipme”t and measures

“ Reduce manning requirements

“ De-manning in advance of anticipated/forecast hazardous events

“ Reduce boat/barge transfer operatiom with equipment tied to platforms orin hazardous conditions

“ Reduce frequency of “en work-over operations

“ Additional effective life-saving equipment and injury treatment facilitiesand procedures

“ Additional training of operations persomel in conduct of safe operationsand maintenance of facilities

“ Reduce unengineered field alterations to the structure

INCREASING MAINTENANCE EFFECTIVENESS

“ Increase corrosion protection - above and belo” water

“ Increase scour protection

“ Increase frequency and extent of inspect ions and conditions surveys

“ Increase effectiveness of operations to maintenance engineering reportingsystems

●Best Available and Safest Technology

Table 111 Examples of Platform Hazard Mitigation Alternatives

62

-–

The results of this implementation areincorporated into the platform condi-tion survey/inspection data bank.

The final step concluding an AIMcycle for a platform is that ofdesigning and implementing the nextinspect ion and maintemmce cycle.The Ienath of tbe cvcle will deDendupon th~ projected ~iiforma”ce ”>b.arac-teristics of the platform, a“d theneed for and benefits of improvingknowledge and data on the platformcondition and performance .

RISK ANALYSIS

The risk analysis that will bediscussed is basic ! appropriate for apractical emgineerlnq state-of-pract ice to develop AIM programs. Tbeceader is referred to references [3-21]for background on more comprehend iverisk anal yses.

The approach (Fig. 5 ) has beencast i“ a demand versus capacityformat. “Demand” refers to futureloadings that may be imposed o“ thestructure. “Capacity” refers to

future resistances (or ability tocarry loadings) of the strwt”re. Thecapacities that will be of primaryconcern are those that connote primaryconsequences of the 10SS of service-ability of the platform. The proba-bilities of the demands exceeding theCapacities of the structure will betermed the probabilities of failure.

. .. .. .. . ,.,”,s.,,.O,.”T,ALAF,L,C.,,ON0, ,..., s,,,.,,.,“.,...,

Fig. 5 Risk Analysis Approach

Uncertainties and probabilisticare important ways of describing thestr. ct”re. They are based on tbeknowledge provided by the conditionsurvey(s) , tbe future demands (e.g.kmwl.edge of enviromnental forces) ,and the future capacities (ea.. .kno”ledge of the load-carryin~ capacityof the platform) . The basics of the

approach are deterministic; assess-ments of uncertainties are added tomake the picture more complete.Experience a“d performance dataon either the demands or the capaci–ties play a vital role in assuringreasonable characterizations of theseitems.

Risk (Pf) will be defined asthe probability (P) that the platform-slateral capacity (Rc) iS equal to orless than the maximwm lateral loading(Sm) imposed cm the platform duringtbe exposure period (L) :

PfL = P (Rc $ Sm) (1)

The platforms capacity will betaken as tbe ultimate Limit state(ULS ) resistance or the maximumlateral force that m“ be imposed o“tbe platform before collapse (unableto suppart vertical loadings) .

Note that the platform’s capacitywill be dependent UFCIn tbe as-iscondition of the platform’s membersand upon any changes that might takeplace in kbis condition. Such changesmight take place as the result ofstrengthening or rehabilitationmeasures, or as the result of fatigue,corrosion, or operations damige.Further, note that the platform 1slateral capacity will be conditionaluFOn its, vertical loadings (as-is,altered in future) .

The platform’s demands will beexpressed as tbe maximum lateralloadings or forces that could bedeveloped by storms (combination ofwind, wave, and current forces] orother similar events that could occurduring the platforms exposure period.

The platform’s exposure periodrisk (pfL) “ill be related to itsannual risk (Pfa) as follows [22] :

PfL = 1 - (1 -Pfa)L (2)

or approximately,

PfL = (Pfa) . L (3)

The environmental lateral loadingswill be taken as the dominant sourceof variability .md uncertainty. TheUncertainties a“d variabilitiesassociated with the platforms lateral

63

capacity can be evaluated by deter-mining the changes in risk thatdevelop as a result of changes in theevaluated capacity.

The annual platform risk (annualprobability that demand will exceedcapacity) is determined as a functionof the return period (RPC) of thestorm that develops lateral loadsequal to the platforms capacity orULS resistance:

Pfa ‘ l/RPc (4)

DEMANDS

Characterization of the demandsthat can be impsed on a platformstarts with evaluation of the likeli-hood of experiencing various intensi-ties of events. These events could beenvironmental [e.g. developed byhurricanes or earthquakes) , or theycould be operational (e.g. due todrilling and production activities) .

For example , measurerne”ts andanalyses of hurricanes affecting thenorthwest Gulf of Mexico [23,241 coulddevelop information on_the AverageReturn Periods (ARP) , T, associatedwith different possible maximum waveheights occ”rri”g at a give” platformlocation (Fig. 6). The ARP, s expressthe average time between occurrencesof wave heights that equal or exceed agiven maximum wave heiqht (Hm) .

,.l/,0 m, moo ‘,,00,

,,,”.Nw,,.. i “mm

Fig. 6 Wave Heights Vs.Return Period

A similar illustration could bedeveloped for .my source of demands.For example, for earthquakes, meaning-ful measures of the intensity of

ground motions (acceleration, velocity,displacement) would replace the waveheights [25] .

The key element is to chooseparameters that adequately describethe force effects developed by thesource of the demand.

Generally, there are two primarysources of variability with regard toenvironmental demands: intensity a“dproximity. For example, the heavyline i“ Fig. 6 represents a typicalsite in a gecqraphical region. Thewave heights are primarily a functionof variable storm intern ities. Thescatter band indicated around thehea”y line indicates the uncertaintiescontributed by proximity, or stormtracks in the case of hurricanes.Both sources of uncertainty should beconsidered to determine the resultantuncertainty of the occurrence ofmaximum wave heights at a givenplatform location.

It is important to recognizesite-specific effects, and see thatthese are properly reflected in theevaluation of uncertainties. Forexample, shoaling effects at a givenlocation could indicate wave heiqhtsthat are substantially different froma typical or “average” site condition[261 . Also note that the site canexert important limiting or truncatingeffects on what could otherwise be acontinuous or unlimited distributionof potential wave heights (Fig. 6) .Water depth and breaking wave processesplace a physical limit on the maxiumnave heights that can be developed inshallow water locations. SimilartYPe S Of site-specific and source-specific factors can place importantlimitations on the maximum magnitudesof many types of environmental demands.

The next step of the platfOLIIIdemand characterization concerns theprediction of loadings or forces,given the measure of the demandintensities. In the case of hurri-canes, given the maximum wave heigbtstthis step determines the loadings onthe platform of concern (Fig. 7 ).This involves computing hydrodynamicforces for the range of wave heightsof interest. Other forces of potentialconcern are those generated by thewater currents and winds that occur atthe time of the maximum waves. Notethe conditionality of the combinationof winds, waves, and currents. It isnot the maximum wind, wave and current.It is the combination that produces~maximm forces or force effects onthe platform. Tbe conditionality notonlv aDolies to the magnitudes of theoth~r ~~urces of loadi~g, but also ontheir directions.

64

‘“’”””~-—””

’0000

/

0-,,,.,,.,,0..,,mo300,,.‘#Am..6,,!+

,.000/

,;]

w,” .“..,.,, “o SPREADING .’:..’ ,

,.0.0

..

,,000 .Y,,..’ ,$

..

2,000-.,’,

w,,”CURRENT, , w,,”s,,,,.,..

1,s.0

1 ~~.,.

Fig. 7 Influence of Currentand wave spreading onHydrodynamic Forces

The problem of force predictionis complex, and thus it may be neces–sary to characterize the forces in aparametric manner, investigating thepotential influences of variouselements immlved i“ the prediction offorces. For example, one might waitto investigate the effects of incl.dingand excluding the hydrodynamic forcesassociated with directional wavespreading (understanding that thegreatest amounts of directionalspreading are associated with themaximum wave heights mar the stormcenter) .

In this regard, results fromrecent wave force measurerne”t programscan pro”ide important sources ofinformation to calibrate or verifyConventional hydrodynamic force models(Fig. 8). Results from a full-scaleinstrumented platform in a water depthof 175 feet subjected to maximum waveheights up to 40 feet, and associatedsurface cwre”ts in the range of 1.9to 3.8 feet per second, indicate thata co””entional hydrodynamic forcemodel tends to substantially over-predict the true forces [27,28] . Theo“erprediction exceeds 100 percentwhen plausible combinations of currentsand marine fo”linq effects are included:the mea” ratio of measured to Computedforce ramges between 0.44 a“d 0.8. Thecoefficient of “ariatio” (measure ofscatter) o“ the predicted versusmeasured forces falls in the range of 20to i“ excess of 45 percent.

,.,,,,,8,, ,,,,9,,0

,5.0

,0,2

,0,0

,..600,0.04003..0

,0.0

,.,.

5,0

2..,..0.5

0.20.!..2 0.30.4 .6 m !0 20 3..

MEASURED,PREDICTED FORCE

(PREDICTED FOmE = AH Gummw vfAvEFORCE,%= o.e.%=7.5. No CURRENTI

Fiq. 8 Correlations Between‘Measured and Predicted

Wave Forces

Since the hydrodynamic force computa–tions involve major modeli”q assump–tio”s concerning computations of wanekinematics, wave propagation throughthe structure, forces and forcecoefficients shieldimg, roughness,etc. , the questions regarding the trueor best estimate forces must becarefully weighed i“ contrast toconvent ional (and intentionallyconservative) formulations usedin design practice [29] .

Note that in this step, it isimportant to “se force predicticmmethods that result in unbiasedpredictions of the forces. Bias isde fired as the ratio of the true orexpected value of the parameter to itsnominal or computed “alue. In thisstage of the demand characterization,we want the ratio of true to predictedforce to be near unity.

Also note that one could chooseto focus the analyses o“ global loadeffects [such ?.s total lateral baseshear or total overturning moment atthe seafloor) or local member loadeffects (such as the maximum foxces i“a brace, joint, or pile). ln theremainder of the discussion thatfollows, the focus will be on globaldemands exerted o“ the platform. Thisis “Ot meant to exclude the possibilityof cimd. cting similar analyses ofelements or structural s“bsysternswithin the platform, for these may beof “ital interest as well.

65

The final step in the demandcharacterization is the one of com-put ing the maximum forces or forceeffects on the platform, based on theresults of the first two steps [30] .The result (Fig. 9) is the characteri-zation of the likelihood (expressedas the ARP) of various passible mag-nitudes of maximum demands (expressedas the total lateral force on theplatform) .

, r

1,0 !00 ,,000 ,0,00.3

..,”.,FSR,CO T -,,..,

Fig. 9 Platform Demands

CAPACITIES

Platform capacities can bedescribed in Serviceability LimitState and Ultimate Limit Statebehaviors where:

a. Serviceability Limit State(SLS ) - demands or loading-deformation conditions underwhich the functicm of thestructure may be impaired;damage results, but col-lapse is not imminent, noris the platform renderedunserviceable.

b. Ultimate Limit State (ULS) -demands or loading-deformationconditions under which thestructure is no longerserviceable, or is unable tofulfill its intended functions.

Capacities generally have beencharacterized in a resistance–deformation format for the ent irestructure system (Fig. 10) . General lYthe capacities are described by loador force resistances, although dis-placements or deformations may even bemore descriptive. For example, if the

structure is loaded so that a signifi-cant permanent tilt is developed i“the structure, its load resistance maybe relatively unaffected, b“t theStructure is rendered “nser”iceable.

‘z~

Fig. 10 Capacities of original,Defective and Strengthened

Platform

Capacity of the platform must becharacterized in terms that areimportant to the serviceabilityDroDerties of the svstemi. In this~ap;r, capacity of ihe system will bedefined in terms of the static,lateral load resistance at which thesystem is mable to supprt itsgravity or vertical loadings (Rc,Fig. 10). This state will be termedcollapse or failure.

Due to the transient nature ofmost environmental loadings, thestructure may or may not be in a truestate of failure at this point.Limitat ions in practical analyticalmethods to define platform capacitymake the definition only an index orimperfect reflector of the behavior ofa structure at ULS.

There is a variety of engineeringanalytical methods to define theplatform capacities. Convent ionallinear elastic analytical methods,such as those used in most designpractice, have deficiencies in theirabilities to characterize platform ULScapacities. This is due to nonlinear,behavior of platform elements at highloading levels.

Nonlinear, inelastic analyticalmethods, such as those used in designpractice for severe earthquake proneregions, are the best presentlyavailable approach for characterizing

-—-

behavior of the platform at the ULS.However, this approach suffers fromcomplexity and from the generalunfamiliarity of most engineers withthe technology of nonlinear, inelasticanalyses. Even in their simplestform, “o”Ii”ear static pushoveranalyses represent a significantengineering effort although the ana–lytical tools are available [31-35] .

To perform time-domain, nonlinear,inelastic structural analyses thatproperly track the ULS bebavior of thestructure requires a“ eve” moresignificant effort. Such analyseshave been performed for relatively fewstructures [36-38] .

In between these two extremes ofanalytical eipproacbes lie eq”ivalei-,tlinear methods that attempt to mimictbe essential elements of nonlinearbehavior, yet retain the basic compu-tational tools of linear elasticanalyses [16,39–41] . Such approachesrequire extensive studies of nonlinearbehavior i“ typical platform systemsor sub-systems. The results are wedto guide the equivalent linear approxi-mations to the true or best estimatenonlinear behavior. The guidancebecomes deoe”de”t on the m.rticularplatform s~stems or s“b-s~stemsstudied, and thus lacks generality.However, given sufficient development,this approach represents a practicalalternative to either conventional1inear elastic methods or nonlinearinelastic methods .

Generally, major AIM concernswith evaluating platform capacity willaddress the platform i“ its original(designed) , as-is defective (damaged) ,and possibly strengthened (rehabili-tated) conditions (Fig’. 10) . Thesecapacities become the basis forjudging the integrity of the structureand evaluating alternatives for itsrehabilitation.

Repair Effects

The effectiveness of alternativerepair or rehabilitation measures ontbe platform capacity should becarefully considered from severalstandpoints. Experience has shownthat repairs to tubular joints a“dmembers and foundation elements aremajor e“gineerimg challenges [4,44-47] .These repairs are limited by thepracticalities of “hat can reasonablybe accomplished offshore (and of tem““derwater) . They call for innovativeand ef feet ive eng ineering strategiesfor repairs that balance strength,stiffness, a“d ductility of thesingle component and the str”ct. resystem.

Experience has also shown thatthere is a potential liability thatoccurs from a poorly designed orpoorly executed repair. 1“ more thano“e case, attempts to repair a com-ponent have done more damage thanleaving it alone. In some cases,damage has bee” done to other parts ofthe platform in the course of attemp-ting the repairs. The potentialeffects of plausible downside outcomesof platform repairs should be investi-gated and considered before a repairscheme is selected.

Careful engineering analyses ofalternative repair schemes can defineeffective a“d practical solutions todifficult repair problems. Evaluationsof the results, of repairs shouldinclude an evaluation of the expected,

up-side and down-side effects of thereDairs on the Dlatform caDacitv.

1). “-

——7 1

I,.5 !.0 !.5 2.0 *.5

Mm.., .,S,LACEMENT,, .,,. -,,,,

Fig. 11 Effect of Repairson Platform Capacity

Pro”isi.a” of a rational basis forselecting a remedial alternative(mitigation measure) in the case ofa damaged or defective platform iscritically important. when conditionsurveys indicate that some platformelement is damaged, the first tendencyis to conclude that it must berepaired. In some cases, this is avalid conclusion. Ho”ever, in manyinstances repair is .“”ecessary forthe following! reasons:

a. Platforms are generallydesigned to be highlyredu”da”t. This redundancyis a“ investment i“ producimga highly damage tolerantStr”ct”re. E“en though it isdamaged, the str”ct”reps

67

capacity may not be seriouslyaffected. Alternative loadpaths may provide thenecessary backup.

b. Platforms are generallydesigned and constructed withmany explicit and implicitsources of conservat isrns.Site- and platform-specificconditions may be such thateven if there has been somereduction in tbe capacity ofthe structure as the resultof damage or defects, thestructure is still acceptableand highly serviceable.

c. There may be more effectivemitigation measures to assuresafe operation of the struc-ture. Load (or demand )controls and operationcontrols can frequently bemore effective and lesscostly in providing a struc–ture that has acceptableserviceability characteristics.

d. There are many elements i“ aplatform that are not im~rt-ant to in-place capacity orperformance. Generally theseelements are associated withconstruct ion (fabricatio”,transportation, i“stallatio”)of the structure. Damage tothese ‘“secondary” elementsdoes not necessarily implythat there has been a“important decrease i“ thein-place capacity of theplatform. Note that thesesecondary elements or systemscan provide important back-upsources of strength to theprimary in-place load-carryingmembers.

Fatigue Considerations

One im~rtant AIM concern ispotential fatigue damage to platformelements. Fatigue damage, or tbereduction in capacity and stiffnessas the result of repeated loadings,is present to some extent in all ofthe platform superstructure a“dfoundation elements. Current fatiguedesign approaches for those elementsare intended to minimize the potentialfor fatigue damage during the life ofthe structure [48-50] .

A primary concern is with elementsthat have been damaged , or poorlyfabricated, or perhaps Underdesigned

for fatigue. These problems areusually identified by inspections thatshow the presence of cracks in thejoints and members of the str”ct”re,or perhaps by indications that thefound.atio” elements are “sof teninga’(e.g. scour pits around the piles) .

For tubular joints and members, aconventional S-N (stress-number ofcycles to failure) – Miners Rule(damage accumulation rule for combiningdifferent stress-cycle history effects)can do little to assist in charac-terizing the remaining capacity of adamaged tubular member or joint[51-53] . Conventional foundationanalyses are also rarely appropriate[54] .

Alternative fatigue analysisapproaches have been and are beingdeveloped, such as linear elastic a“dnonlineac fracture mechanics approachesfor cracked tubular joints a“d members[52,551, and load cycle-by-cycle analy-ses for piles [56] . The approachesare complex. They are still underintensive development. It is notlikely at this time (1987) that theycan or should be incorporated into ageneral AIM approach at their presentlevel of developrne”t. Ho”ever, theycan provide useful i“formatio” in somespecial cases.

A viable approach for reccq”izi”gthe effects of fatigue damage is thatof determining how the capacity of thejoint or brace might be reduced byfatigue cracking a“d other damage[52,57-61]. Analyses and experimentalevidence can be used to assist suchevaluations.

Inspections seem to provide theonly reliable method of detectingfatigue damage [47,52,53] , identifyingthe defects as cracks associated withthis damage, and determi”i”g if thecracks are growing and leading toadditional decreases i“ the element *scapacity.

Projected decreases in the plat-form system capacity as a function oftime (Fig. 12) can be used to guidedefinition of inspection intervals andconsideration of alternative rehabili-tation measures (and the timing ofrepairs) . Uncertainties in fatigueeffects can be recqnized by injectingplausible changes in element capacitiesand stiff nesses as a function of time,and determining the resultant influ-ences of these changes on platformcapacity.

66

—

‘2r————

Fig. 12 Platform CapacityDecreases As A Function ofTime Due to Fatigue Damage

RISK QuANTIFICATIONS

The approach used in this paperto quantify risk consists of threebasic steps:

1. Based on a return period (RP)evaluation for the hazard ofurimarv concern [e.q. hurri -;anes ~enerating ‘maiimum waveheights, H“) and a bestestimate evaluation of thedemand (e.g. total lateralforce, Sm) associated withthe range of the hazard,determine the return periods[likelihoods) associatedwith the potent ial demands(Fig. 9).

2. Based on the structure in itsas-is condition, in thevarious conditions representedby PraCtiCal AIM a“d rehabili -tatmn measures, and in the“ario”s time periods of con-cecn (reflecting potentialfatigue effects) , determinethe best estimate and rangeof ULS resistances of theplatform (Rc) (Figs. 10-12).

3. Determine the annual (A) andexposure period (L) risks(PfA and PfL, respectively)fr“nl

PfA =

PfA =

PfL =

P (Rc $ Sm) (5)

(RPC)-l (6)

L. (Pf~) (7)

where RPC is the ReturnPeriod (years) of thedemands that exceed plat-form capacities.

The range of risks for the rangeof as-is conditions of the structure(Fig. 13) , for various alternativestrengthening or AIM measures(Fig. 14) , and for various periods oftime (Fig. 15) can be used to quanti–tatively evaluate alternative AIMprograms.

Fig. 13 As-As ConditionPlatform Risk

l\

,6[yH4\\

24. s m

“,s,,s,,,...s----,0,.,,s

Fig. 14 Effect ofStrengthening

on Platform Risk

69

Fig. 15 Effect of Time(Fatigue Damage) on

Platform Risk

EVALUATION OF AIM ALTERNATIVES

Evaluation of AIM program alter-natives to determine the “best”program is basically a problem ofdetermining tbe acceptable or tolerablelevel of risk associated with tbeplatform operations, and the definitionOf a Practical and affordable AIMprogram that will result in that levelof risk. Tbe level of acceptable riskis equivalent to an acceptable ULSresistance of tbe platform.

There are a wide variety of basesfor determining what constitutes atolerable level of risk. One ishistorical, i.e. , the level of riskthat has been developed by tbe industryand accepted by the public. Thedifficulty in equating actuarial(historical) and computed risks istwofold: (a) the data from whichactuarial risks are derived are verylimited (few failures) ! and (b) theinformation and analytical methcdsused to calculate risks result inaPPKOXimatiOns of the true risk[notional risk) . Because quantifiedzisks involve many approximations,they are only an index of the truerisk.

Another problem is that the pastrisks may not be a valid basis todefine acceptable future risks.Changing bases of engineering, con-struction and operations by theindustry, and changing values of thepublic can make past risk basesinvalid.

A second approach to defining anacceptable risk level is requiring

that the str”ct”re be returned to itsoriginal or as-designed condition. Inthe case of platforms that weredesigned with ““conservative criteria,the validity of such a“ approach isquestionable. Similarly, if theplatform were conservatively designedoriginally, then defects or damageneed not necessarily imply that theplatform risks are below an acceptablelevel.

A third approach is based onselection of an AIM alternative thatattempts to optimize tbe use ofresources, results in tbe highestpossible utility, or develops thegreatest present valued benefitsassociated with operations of thestructure [1-3] . This approach is onethat attempts to define the AIMprcqram that results in a minimumtotal expected cost, E(T)o, associatedwith AIM operations. The totalexpected cost, .E(T) is taken as tbesum of expected initial AIM costsE(1) , and expected future loss ofservice costs, E(L) :

E(T) = E(I) + E(L) (8)

The expected initial costs (Fig.16) are all of those investments thatare associated with implementing aparticular AIM alternative. Thesecosts could be those associated withstrengthening, inspection, and opera–tions changes intended to maintain orincrease the Dlatform caoacitv at somegiven level. -

,,. AtTfnNATlv.

“’1 “’l “’-l

4.8“,sR, S18,ANCE-.. ,0,.,,$

Fig. 16 Platform InitialA I M Alternatives Costs

In addition, initial costs couldbe associated with changing operationsexposures (Fig. 16). For example,reducing onboard oil storage, requiringplatform evacuations in advance ofstorms, and incrementing down-hole

70

-

safety shut-in equipment can substan-tially reduce the costs associatedwith a loss of serviceability of thestxucture.

Tbe expected costs associatedwith loss of serviceability (Fig. 17)can he computed as the product of thetotal costs given a 10ss of service-ability, Cf, the annual likelihood ofthe loss of serviceability, Pfa, andtbe period of time being considered, L:

E(L) = (Cf) . (Pfa) . L (9)

,,..,,,.”,.,”,

“’l “21 ●37

.,, ,

Fia. 17 Platform ExuectedLos= of Serviceabil it~-Costs

The loss of serviceability costsshould include the expected value ofall of those costs associated with theplatform reaching its ULS at thepoint in time of concern. Such anestimate could be based o“ a replace-ment cost or on a sal”age and abandon-ment cost (including the “alue of lostproduct ion or reserves) .

Since short periods of time areof usual concern, it may “ot benecessary to consider present-val”imgpotential future costs associated withloss of serviceability.

Each AIM program can be as.50ci–ated with maintaining tbe platform atsome ULS resistance for some period oftime. The objecti”e is to find theAIM progran that de”elops a minitn”mtotal expected cost [Fig. 1S).

It should be noted that the totalexpected cost associated with the AIMprograms over the life of the facilitymust be such that the opera tiom canb? mai”tai”ed at a“ ecomxnic lew?l.

Fig. 18 Total Expected A 1 MAlternatives Costs

SUMMARY

The objectiw of this paper hasbeen to outline a“ integrated , general ,and “on-prescriptive engineeringapproach to the requalification ofexist ing platforms. Practicality, inthe context of present engineeringcapabilities, was a key aspect indevelopment of this approach. Keep ingplatforms in service and establishingtheir integrity at the least possiblecost were key precepts.

Inspection, definition of defectsand damage, and repairs and improve-ments must be given high priority inplatform operations if structures areto retain high degcees of service-ability. Poorly maintained structurescost . It costs, scarce resources tomaintain str”ct”res . Howeverr AIMimestments ca” ret”r” significantdividends by increasing platformcapabilities, Io”ering the incidenceof serious down-time events, a“dlowezing future repair costs. Thebenef its of AIM engineering andoperations should be justified by thebe”ef its that are achieved , and theresources that an operator ca” investto keep a vital resource flowing tothe market place.

The AIM approach is one ofprogressive reduction of risks totolerable levels . The AIM approachproceeds in a step-wise manner throughplatform identification, a structurecondition survey , screening of pote”–tial defects .md the wed for mitiga-tion of these defects to determine the

71

nature of and justification foralternative AIM programs. Once aparticular AIM program has beenchosen, it is engineered, implemented,and its results recorded as a basisfor cent inuing the next AIM cycle.

Realistic analytical engineeringmodels are a particularly criticalelement in any platform requalifica-t ion. Realistic models are needed forcharacterizing the platforms futuredemands (loadings) and capacities(loading resistances) . It is herethat tbe best available currenttechnology needs to be implemented.It is also here that site- andplatform-specific factors may beinjected into the analytical models.Of particular importance are the‘experience factors. ‘v The experiencefactors pertain to knowledge of how aparticular olatform, or similarilatforms, have performed in the past,especially in high loading (demand)situations. This up-dating informationcan serve as proof-loading or proof-capacity ( resistance) data to assurereality of the analytical modelsresults. It is important that conven-tional design-oriented analyticalprocedures and methods be re-examined,site/ platform-specific conditionsrecognized, and sources of implicitconservatism removed from the charac-terizations of future demands andcapacities.

Uncertainties and the attendantrisks are an important aspect of AIMprocesses and programs. A basicapproach has been suggested to charac-terize platform demands, capacities,and performance. Broad scope AIMprcgrams to manage uncertainties,risks and potential consequences areevaluated in tbe context of theircosts and their benefits. Theseprcgrams are implemented in a repeti-tive, continuing process of improvingunderstanding and practices to lowerrisks associated with existingplatforms.

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73

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74 -.

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Metric Co”versio” Table—.

1 m = 3.28 ft1 mm = 0.04 in., ~2 = ,0., fjft2, ~3 s q~.~, f~2

1 kg m3 = O.O62 lb/ft31 kg = 2.20 lb.

75