OTe 4429

Evaluation of Hutton TLP Response to Environmental Loadsby John Allen Mercier, Conoeo (U.K.) Ltd.; Steven J. Leverette, Gulf Research and DevelopmentCo., and Allen L. Bliault, va Offshore Ltd.

COPYRIGHT 1982 OFFSHORE TECHNOLOGY CONFERENCEThis paper was presented at the 14th Annu~l OTC in Houston, Texas, May 3-6, 1982. The material is subjectto correction by the author. Permission to copy is restricted to an abstract of not more than 300 words.

585

INTRODUCTION

ABSTRACT sway and yaw in response to the action of imposedenvironmental loads due to wind, waves and currents:surge, sway and yaw natural periods are long comparedto wave periods. The TLP's dynamic behaviour issimilar to that of an inverted pendulum where theTLP's excess buoyancy provides the restoring forceinstead of gravity. The amount of excess buoyancy isdesigned so that the tension leg loads never go slackunder worst-case design combinations of environmentaland on-board loads.

o the proportion of buoyancy provided by thecolumns, compared to that provided by thebracing and pontoons which connect thebottoms of the columns is rather larger thanfor a floating semi-submersible. This is aconsequence of the TLP's design objectivebeing to minimize tension leg loads .due towaves rather than wave-induced heave motion.

While the Hutton TLP is the first of a kind ofpermanently-installed compliant platform, methods ofenvironmental"response ~nalyses which have beenapplied in the design of other compliant platforms,such as semi-submersibles and articulated columns,provide a sound base for TLP design practices.Environmental loads and movements of semi­submersibles, articulated columns and TLPs are, inprinciple, amenable to rational analyses as illust­rated in numerous published papers (References 6 - 18are selected examples of available rational analysismethods). Nonetheless, this first-of-a-kind completeTLP design programme entailed considerable developmentof, for instance, approaches to combining extremeenvironmental conditions, identifying worst-case modesof responses, choosing specific analysis methods andaccounting for unexpected features of responses. Thispaper is an abbreviated account of key features ofthis part of the design programme.

The TLP structure is generally similar inconfiguration to a semi-suDlliersible drilling rig.Three characteristics of the Hutton TLP are, however,distinctive:

a com­scale modelApplic­

selected

Response predictions are obtained bybination of theoretical calculations withtests in wind tunnels and in wave basins.ations of theory and experiment to assessresponses are illustrated.

While the basic method of using analyses togetherwith experiments is usual for seakeeping evaluation ofships, semi-submersibles, etc., the nature of workdone as part of the Hutton design is especiallyextensive. Some results concerning evaluation of"secondary" responses of the Hutton TLP may beimportant consideration of other TLP designs.

Environmental data used in the design aresummarized and a brief account given of special workdone for application to TLP performance assessment.

The programme for evaluating environmentalloadings and responses of the Hutton Tension LegPlatform (TLP) is described.

The TLP, shown in Figure 1, is a compliantstructure which allows lateral movements of surge,

The TLP for the Hutton Field in the North Seawill be the first floating oil drilling andproduction platform which is designed to be mooredby vertical tension legs for the full duration of the·field' s productive life. General descriptions of theoverall Hutton TLP configuration and design have beenpresented elsewhere, including the preliminary design(Reference 1) and the final design (References 2 and3). More complete descriptions of two importantHutton TLP subsystems, the mooring system and thefloating vessel (hull and deck) are being given incompanion papers to this Offshore TechnologyConference (References 4 and 5). Environmental loadsand responses, which are the subject of this paper,have importantly influenced the design of those sub­systems.

THEORETICAL CONSIDERATIONS

586

•••••• (2)X/T = X/L cos c{

T = T +~T +~T +~T (3)preset wt wl set-down

~T =¥A z . . . . . . . . (4)set-down wp

Z = L (1 - coso( ) . . . . . . (5)

Wave-drift forces are non-linear and are relatedto wave diffraction and certain other phenomena (cf;Reference 18). Theoretical analyses suggest the wavedrift forces are proportional to the square of thewave height and experiments tend to support thisprediction. Drift forces depend importantly on wave

Current-induced force may be estimated bycalculations and/or measured by model tests, eitherin a towing tank or by using an under-water modelin a wind tunnel. Expected currents at the Huttonfield are moderate, such that current effects areappreciably less significant than wind and waveeffects.

Wind-induced forces may be estimated accordingto published procedures (cf; References 19 and 20)for combining the drag force estimates for componentsof the TLP above-water structure. An importantinfluence of wind direction is to vary the profilearea exposed to the wind. Such estimates forhorizontal force are typically found to correlatereasonably well with results of wind tunnel tests ofcompleted scale models. Results of wind tunnel testsfor the Hutton TLP have been used as the basis forwind load estimates and these include measurements ofother force components including lift, pitch momentand yaw moment. The effects of vertical windgradient should, of course, be accounted for incalculations and experiments. The programme of windtunnel tests for the Hutton TLP will be discussed morefully later.

x = Xwind + Xcurrent + Xwave drift • . • • • (1)

Total tension in the mooring legs is mainly dueto pretension, established when the TLP is installed,for reference conditions of water elevation andplatform weight. In service, platform weight variesdue to consumption, resupply and ballasting and waterlevel varies with the tide and storm surge: inaddition, under the action of platform offset the TLPis set-down deeper in the water as illustrated inFigure 2 and the platform's buoyancy is therebyslightly increased.

will be outlined to illustrate points relevant to theoverall design/analysis methods used.

Steady (time-average) horizontal forces due towind, current and wave-drift produce mean offsets ofplatform surge, sway and yaw. The restoring forcetending to resist these forces is the horizontalcomponent of inclined tension leg tension (see Figure2) :the Hutton TLP design does not embody

diagonal'bracing members providing trusssupport of the structure. This feature waschosen to enhance internal and externalinspectability and to avoid additionalcomplex structural junctions.

the TLP's overall depth is greater thanthat of semi-submersibles because the TLPuses a deep draft to attenuate wave forcesand, because it requires high deck elevatiorto avoid severe wave slams in case of hightide together with high waves. Anotherconsequence of the TLP deck elevation isthat the vertical centre-of-gravity, whichaffects certain dynamic responses, isrelatively high.

o

o

o to assure the TLP configuration hasreasonably good hydro-dynamic performance(viz., motions and tension leg loadings),subject to the additional objective ofhaving a relatively simple, constructableand inspectable hull configuration.

o to provide specific predictions of globaland distributed loads, motions and otherdynamic responses for use in design of TLPsystem components, including structuresand mooring systems.

The paper provides an outline of some rudimentsof the theories of environmental responses in thenext section. Data analyses and selection ofenvironmental parameters used as inputs to theresponse analyses are discussed in the subsequentsection where the matter of combinations of para­meters describing an "event" is considered. Thekinds of scale model tests and their role in thedesign programme are covered next. Some examples ofselected response predictions and techniques aregiven in a section which also touches on aspects offinal configuration development.

The method for designing the Hutton TLP systemfeatures and components is based on combiningtheoretical techniques, environmental input para­meters, experimental results, relevant acceptancecriteria and engineering judgement. Sometheoretical features of Bnvironmental responses,including steady forces and offsets, wave frequencyoscillations, and low and high frequency motions,

The principal particulars of the Hutton TLPdesign are listed in Table 1, which identifies themain dimensions and relevant mass and restoringforce propertie~. More complete information ondimensional and structural configurations may befound in References 4 and 5.

These and other features of the configurationhave affected, and been affected by, the designanalyses.

The main objectives of the Environmentalresponse evaluation work have been:

period and may be estimated by available advancedcalculation techniques or derived from scale modeltests in waves. For Hutton design work, model testsin realistic irregular waves were used as the basisof design predictions.

(~A )wp

lever for moment due to surgeadded mass

pitch added mass moment-of-,inertia

height of C. of G. of platforinmass above origin

surge added mass

platform mass

platform mass moment-of-inertiain pitch

heave added mass

Cxx

Czz

a,xx

a zz

.Q.

MCfe' = I

Mzz

where:

Design of configuration for wave forcecancellation

o

Engineering analysis and prediction can adaptmuch that is useful from linear techniques even incases where non-linearities are important. Non­linear effects on the several modes of response of aTLP are most readily and convincingly obtained fromscale model experiments although time-domainnumerical solution techniques may provide certainguidance with respect to some response modes (CfiReferences 13 - 16 especially). Useful and adequatetheoretical guidance can be gleaned from linearequations of motion concerning, several matters:

o Tension leg loads due to combined heaveforce and pitch moment

o Effect on predictions of changes in TLPvessel internal configuration (mass,vertical centre-of-gravity, etc.)

For simplicity, the casemotion in a plane of symmetryx, z, C") will be considered.equations of harmonic motion,system of Figure 3, are:

of excitation and(heave, surge, pitch:

The linearizedfor the coordinate

C.,..O' = snkTL + (DISPLACEMENT x GM

L)

bii's = hydrodynamic damping co-efficients

F Magnitude of wave-inducedz heave force

F Magnitude of wave-inducedx

surge force

Me' = Magnitude of wave-inducedpitch moment

Heave

2f W (M + a ) cos (~t + E )zz zz z

- wb sin (U)t + l: ) + C cos (wt + € )J zzz z zz z

(6)

2[- W (M + a ) cos (a,)t + E )

xx xx x

For the Hutton TLP the ,elastic restoringstiffness for heave and pitch, due to tension legstretch, greatly exceed the hydrostatic restoringeffects (which contribute less than two per cent ofthe total restoring coefficients for these "stiff"modes of response). The choice of origin of co­ordinates at the elevation of the tension leg cross­load bearings, near the bottom of the columns,simplifies th~ description of cross-coupling of surgeinto pitch (with this origin of coordinates thereshould be, strictly, a coupling of pitch into surgein equation 7, which is not consequential since pitchmotions are limited by the stiff mooring restraints).

- lOb sin (wt + c: ) + Cxx x xx

cos (wt + e i] xx

(7)

Platform natural periods may be estimated bythe following equation:

Pitch

2[- U) (MO'O" + aO'<:!) cos (wt + Co- )

-wbO"O'sin (u)t + l;",) + Co-crcos (wt +€aiJO"

+ [- w2(M J. + a .Q') cos (wt + €. i] x

xx xx x

Moo cos (wt + ~) • • . . • . • • •• (8)

For the Hutton TLP the natural periods are approx­imately 2 seconds heave, pitch and roll, 50 - 60seconds for surge and sway (depending on water leveland pretension) and 42 - 48 seconds for yaw.

The surge mode of motion is effectively un­coupled from other responses according to theselinearized equations and, since wave periods aremuch shorter than the natural period, the surge

587

motion is dynamically attenuated and is directly out-of–phase with the surge force: that is, the surgeoffset is maximum positive when the surge force ismaximum negative. Wave-induced surge force is pre-dominantly due to horizontal pressure gradient andfluid acceleration excitation. For waves which arelonger than about twice the column spacing, the surgeforce lags wave crest passage by a phase of 90degrees: therefore maximum surge motion (in thedirection from which waves are propagating) leadswave crest passage by 90 degrees. Thus, for longwaves the TLP surge velocity is in the same directioxas horizontal wave velocities in way of wave crestsand troughs and relative velocities are diminished.

For the “stiff” modes of response (heave, pitch)the restoring force terms of the equation dominatethe acceleration terms, which are nearly negligible.Damping terms are relatively unimportant for wave-frequency responses in general. The wave inducedheave force excitation can be substantiallycontrolled by choice of platform geometriccharacteristics, as described by Horton, et al.(Reference 9). Heave force “cancellation” can, intheory, be effected for a particular wave period bybalancing the vertical forces on surface-piercingcolumns against oppositely-directed vertical forceson submerged buoyant components. Horton et al.(Reference 9) use simplified hydrodynamic analyses

appropriate to structures composed of slender,widely-spaced cylindrical members, to show that theimportant parameters for vertical force cancel-lation include platform draft, column spacing,displacement and proportion of total displacementembodied in the vertical surface-piercing columns.For configurations like the Hutton TLP, in whichcolumns and pontoons are relatively closely-spacedand not really “slender”, more complete hydro-dynamic analyses such as surface-distributed sourcetechniques (cf; References 11,12, 18) give betterestimates of vertical wave forces, including nearly-null net force predictions.

However, the designers’ main concern for a TLPis to limit tension leg loads per se, and thesedepend on both heave force and pitch moment;referring to the simplified case of Figure 3:

T1+T2= Fzcos(@t+~z) . . . . . .(10)

s(T -1

T2) =MdcOS (Gt+he) . . . . . . (11)

Other limitations on pursuing the line of forcecancellation as a design objective are that theocean wave environment contains waves having a broadrange of periods, so force cancellation at aparticular period is only a partial solution anyway,and considerations of cost, constructability andoperability (including tow-out stability)significantly influence configuration.

Inspection of equations (6) to (8), (neglectin9dynamic terms of (6) and (8) and damping terms)shows that heave and pitch motions, and hencetension leg loads, depend on:

o wave-induced excitations, which depend onlyon the external geometry, and

o pitch moment due to surge, partly associatedwith fluid added mass (depends on geometry)and partly with platform mass and verticalcentre-of-gravity. Both depend on surgeamplitude, x.

While scale model experiments are the mostreliable means of identifying hydrodynamic excitationand directly-related responses, effects of change inplatform mass and centre-of-gravity from a testedcondition can be readily accounted for bycalculations.

Low Frequency Motions---------------------

“Compliant” modes of response (surge, sway andyaw) include slow motions occurring at periods closeto the natural periods. These may be induced byvarious causes, including second–order wave driftforces and, possibly, wind gustiness (both of whichare broad-banded excitations)and possible unstablemotion behaviour, such as that described by Raineyand others (References 21, 17, 18).

Methods for theoretical estimating of second-order wave drift responses in surge or sway, whichhave little damping, give reasonable results whencompared with tests, but uncertainties concerningactual damping in the presence of wave-induced flowsand platform motions remain. Scale model testsprovide the most convenient and reliable basis forassessing such responses. Both theory and ex-periment indicate that short-period waves producegreater “wave-drifteffects (both mean and slowmotions) than long-period waves. Since wave-frequency surge is greater for long-period waves, thetotal surge offset is about the same for long- andshort-period seas.

Wind gusts are typically broad-banded and maycontain energy which could excite surge motions atthe natural period. These would also be controlled

by surge damping. Theoretical and experimentalresearch is required to clarify the importance ofthis matter.

Slow motions due to dynamic instabilities maypossibly be excited, in theory, by waves whoseperiods correspond to nTk/2, where n = 1, 2, 3, . . .For the Hutton TLP the shortest waves which mightexcite these instabilities have periods of 20 - 30seconds and such waves are rarely observed to contairmuch energy in the real oceans. In realistic random

wave tests, where slow motions may be excited by themechanism of slow drift, observations of unstablemotions might be masked even if they did occur. Someevidence of yaw motions due to Mathieu-type in-stabilities was noted in the model testing of a pre-liminary design for the Hutton TLP; such motionswere small (less than 2 degrees maximum) forrealistic extreme irregular waves, but largeoscillations would be excited by large amplitudeperiodic waves having periods close to Tyaw/2. (Such

588

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— —.——J—— .————- -——

-—:=—

results are of interest for research but do notrelate to design predictions.) No such instabilitieswere observed in the comprehensive tests carried outon the final Hutton TLP design.

High Frequency Motions.---------------------

“Rigid” mode responses (heaver pitch and roll)may be excited at periods very much shorter than waveexcitation periods. Such responses might be causedby impulsive fluid loading (transient response) or,possibly, by superharmonic,excitation such as thatinvestigated by Yoshida, et al. (Reference 17). Someexperimental evidence of trans?ent responses, termed“ringing”, will be described in a later section ofthis report. Superharmonic excitation responses werenot perceived. Mechanical excitation of these modesof response by rotating equipment or drillingoperations (jarring, etc.) might also occur but thesepossibilities will not be discussed here.

ENVIRONMENTAL CRITERIA

Design criteria for environmental conditionswere re–examined at the start of the final design.Data for wind, waves, currents and tides have beenassembled for the Hutton site (Reference 22). Thisincludes wind data collected at a number of stationsin the vicinity of the Hutton Field over long periodsof time as well as the considerable body ofinstrumentally recorded wave data from the NorthernNorth Sea acquired for the UK Offshore OperatorsAssociation.

The specific requirements of the Hutton TLPresponse prediction methods, including analyses andscale model testing, influenced the final choice ofcombinations of environmental effects to produce themost severe (design case) loads. Recommendedpractice for designing for environmental loadsrecognises that natural simultaneous occurrence ofindividual extreme environmental conditions should beaccounted for. The environmental condition reportsexamine correlations between several environmentalphenomena (wind and waves, wind and current, wavesand current, tides and waves, etc.) and provides someinformation on joint probabilities of extremeconditions. While available evidence on wind, wavesand currents indicates that the maximum values areunlikely to occur simultaneously, it is commonpractice to assume they do. For oceanic platformslike TLPs, whose environmental responses aresignificantly dependent on a number of parameters,including some which have secondary effect forconventional or existing structures, there is a needfor better appreciation of probable combinations ofloads. The urgency of Hutton project designdevelopment did not allow time to research probablecombinations. Two areas of criteria specificationwere, however, analyzed and interpreted in somedepth; namely, combined wave height and wave period,and combined tide and meteorological surge waterlevel variation.

Wave data extrapolation does not fully identifyextreme waves: either a deterministic (periodic)

“design”J

wave can be assumed or a realistic irregularwave train applied. Both premises have bee used inthe Hutton design programme, for differentapplications. Further, the combinations ofcharacteristic heights and periods must be chosenbased on analysis.of the data together with suitablejudgement concerning “acceptable” probabilitylies ofexceedance. Usual wave data extrapolation ~rovidesmore definitive information on wave height dhan onwave period.

!

The joint probability of heig t combinewith period must be used for TLP design whe e, forsome kinds of dynamic responses, wave period may beequally or more important than wave height. I Designwave heights and period combinations were e ‘timated

7based on advanced oceanographic analysis me~hods(References 23 and 24) which were somewhat

The enve%~~rthan earlier North Sea estimates.regular design wave heights and periods is s!howninFigure 4.

Design extreme water level range affec~s bothmaximum and minimum tension leg tension and~ hence,payload capacity. Predictions of water lev 1variations due to combined astronomical tidd and

1

meteorological surge based on a joint proba ilityanalysis method devised by Pugh and Vassie Reference25) ‘have been used. Results of one month’s tidaldata obtained at the Hutton site in the cou se of abottom topographical.survey have been used 3improved tidal range estimates. The extrem

{:;level range forone hundred year return perxod IS

~

obtained from a sea level frequency distrib tion asschematically indicated in Figure 5, based nReference 25. Use of extreme water levels ,ogetherwith extreme storm-induced winds and waves

ireduces

extra conservatism for design and analysis ssimultaneous occurrence of all events must dave verylow probability of occurrence.

dTable 2 lists selected environmental c iteriawhich will be recognized to be similar to t ose usedfor other Northern North Sea design projects!.

ISCALE MODEL TESTS IInstalled-Condition Tests-------------------------

!The basic method of using analyses tog ther withexperiments is usual for seakeeping evaluation of

1

ships, semi-submersibles and other floating andoceanic systems. For the Hutton TLP design programmethe use of scale model testing has been esp ciallyextensive.

IThe size and proportions of the Hutton TLP hull

are such that the wave-induced hydrodynamic forcesare largely associated with wave pressure g adientand fluid acceleration. It was therefore e pettedthat both experimental and theoretical meth ds shouldprove suitable for predicting performance.

1Analyses

and wave tank tests for the preliminary des”gn wereencouraging in this respect but subsequent destingrevealed unwanted surprises. t!”Wave tank tes lng, for

!

the installed condition, played a central r le infinal design development: it was found to p ovide themost reliable and complete information on complexphenomena.

589 I.—

–-

——— ____ _

— ————.

-—m= - _

— = -— - .— — -

~_ ~—. .. — — —>-——= -= = =—

— — ._ __ _ -=-—

.

— — ——

— .—

—

—

_=*= —==== >–.——

— .

—. —

—

Installed condition tests were all conducted in measure forces and moments due to wind for both thethe same test facility, the number 3 tank of the U.K. installed condition and in heeled conditionsNational Maritime Institute, and the same scale ratio related to the free-floating TLP during tow-out.(1:64) was used. The”facility can simulate largelong-crested waves of either irregular (varying Tests used a wind-speed profile corresponding toheight and period) or regular shape. Both kinds of open-sea surface and measured turbulence intensitywave systems were used for deriving various kinds of agreed reasonably with expected open-sea turbulenceresponses. Real”ocean waves are irregular and up to a height corresponding to about 40m elevation.certain kinds of responses can only be reliably The TLP model’s main columns were coated with grit inderived from tests in these kinds of waves. The use order to stimulate boundary layer turbulence andof regular “design” waves is common practice in fixed represent high Reynolds number flow conditions.platform design. This practice permits some sim-plifications of complex calculations and inter- Tests were conducted to examine the effects ofpretations, and it embodies a degree of conservatism variations in certain design features, including deckin that observed non-linearities in large irregular elevation above sea level (freeboard), size ofwaves are consistently less than for regular waves. accommodation modules and size of helideck.

The models were instrumented to measure motions Other Testsof surge, sway and yaw, tension leg loadings (at the

-----------

bottom of the tension legs), relative wave elevation Scale model testing has been used for evaluatingwith respect to the deck as well as wave elevation at a wide range of responses in addition to motions anda number of fixed locations in the tank. A single loadings for the installed condition. These include:riser model was fitted at the TLP model wellbay andtensioned by a spring device. This model riser o Dynamic responses of groups of risersapproximately represented the effect on the model TLPwhich the array of similarly tensioned individual

o Resistance and seakeeping during tow–out

risers ~ill produce on the full-size TLP. Special o Motions, mooring and tension-leg installationinstrumentation was applied for certain special (preliminary tests)investigations. Effects of wind on platformbehaviour were approximately represented by applying

o Motions of TLP deck on load-out barge prior

a steady horizontal force to the centre of the modelto mating with TLP hull

by using a string with a weight over a pulley. This o Simulation of deck–to-hull mating, in lowproduced a mean offset in the direction of wave sea statespropagation and a corresponding mean platform set-down.

0 Evaluation of airflow around the structure,including generator exhaust plume dispersal

The re-analysis of North Sea wave data, leading(wind tunnel)

to predictions of extreme design waves of greatheight and steepness, required testing to strive to

It is not possible to provide descriptions and

produce such waves in the tank. Experience withresults of these mbdel tests as it would require more

generation of waves in a basin shows that certainspace than is available in the present paper.

regular waves do not propagate without changing shape.Because of small imperfections or disturbances, waves SOME RESPONSE PREDICTIONS AND TECHNIQUESthat are high and steep tend to distort andeventually break down after traveling some distance Wave-Induced Tension Loadsfrom the wave generator. The phenomena, whose

--------------------------

occurrence was predicted by Benjamin and Feir Measurements of tension leg loads in very broad-(Reference 25), is characteristic of steep regular banded irregular waves have been analyzed in awaves in general and not of the wave tank. Regular special way to permit comparison with theoreticalsteep, high waves can not propagate in the ocean predictions of total platform wave–induced heavewithout disintegrating into irregular waves: hence, force and pitch moment. Calculations were made withthe “design wave” is seen to be a useful fictitious the computer program NMIWAVE (Reference 12) whichevent. For testing of the Hutton TLP design in“design waves”

accounts for wave diffraction and column-pontoonthe model was situated close to the interaction effects. Experimental transfer functions

wave generator so that the effects of the steepest were derived by cross-spectral analysis. The waves

possible regular waves could be studied. Since these used for testing were of moderate height so thewaves proved to be not perfectly repetitive, data responses may be expected to be linearly related toanalysis procedures were devised to extract “average- the waves. Figure 6 compares the amplitude of thecycle” response measures. experimental transfer functions (solid lines) with

the theoretical heave force and pitch moment. The

Results of analyses of tank test measurementsdegree of agreement is encouraging except that

will be described in the following sections of thistheoretical pitch moment is underestimated for long

paper.wave periods. For long wave periods, however, thetension leg loads are mainly caused by heave forces.

Wind Tunnel Force MeasurementsThe phase relations between pitch moment and heave

------------------------------ force and the wave excitation are not shown, but the

A 1:200 scale model of the above-water portionagreement between theory and experiment is not so

of the Hutton TLP was tested in a wind tunnel togood.

590

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—— ____ _— — —

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— = -— - .— — -

~_ ~—. .. — — —>-——= -= = =—

—-— — ._ __ _ -=

.

— ———

— .—

—

—

—

_=*= —==== >–.——

.

—. —

—

~===— _~ _= __--=-.= .——__= –- .? ~–__—–=_

—= -.= ._— —

.- _e_: -== a.

— —.——J———— .——- -——

~—____-—:=—

Note that heave force cancellation is nearlyachieved for wave periods around 15 seconds. The“static” heave force per unit of water level variation

appropriate to very long period waves (tide, etc.) is1260 tonnes for this tested model (not a final design)so it can be seen that considerable success can beachieved in hydrodynamic force cancellation. Around‘&is period, of course, pitch moment controls tensionLeg loads and the short period extreme design waveproves to have the greatest effect on minimum tensionleg loads and required pretension.

Several kinds of data analyses have beenspecially tailored for use by the Hutton designproject. To derive information on non-linear

Yaw---,

Yaw motion is composed of steady and slowly–varying effects of winds, currents and waves and asmall wave frequency component. Asymmetry of theplatform geometry, including risers located off-centreis not of sufficient importance for the wave frequencyeffects to produce significant responses. In fact,the measuring devices used for the wave tests had tohave increased sensitivity to perceive the motions inyaw, which rarely exceeded about 2 degrees and whichwere mostly slowly-varying. Statistical inter-pretations of these measurements were also used fordesign predictions.

responses to regular extreme design waves test data I Relative Wave Elevationhas been conditionally averaged.

------------___________

Figure 7 shows analyzed test records for waveelevation, surge motion and anchor vertical load foran 18 second, 31m high (nominal) regular wave.Averages for a large number of cycles are determinedand plotted along with spots for + 2 standarddeviations from the mean. This t~chnique provides

improved definition of non-linear variations ofresponses for extreme wave conditions where test wavesare not perfectly repetitive. Surge motion is seen irFigure 7 to be very nearly sinusoidal (with a small

mean offset, due to”wave “drift” force), while thewave profile has a sharp crest (negative ordinate) an~the tension leg force record reveals some repetitivehigh-frequency perturbations as well as some othernon-sinusoidalness. Test results like these have beerused to determine empirical factors for adjustingtheoretical analyses to predict extreme tension legloads using a “design wave” approach.

The extent of non-linearity of tension leg loadsin extreme waves may be seen in Table 3, ‘which givesthe ratio of experimental tension leg load range(peak-to-peak) divided by wave height for the mostheavily loaded corner tension leg. Results are for32m draft and 45 degree heading, for three testconditions: 14m and 18m significant irregular wavesand 30m “regular” design waves.

Surge Motion--- --------

In the analysis and prediction of extreme surgemotion due to wave forces it was found that theinfluence of steady and slowly-varying wave driftmotions could most conveniently be treated by notattempting to distinguish these motions from the wavefrequency motions and applying statistical methods toextrapolate results. The maximum measured surgemotion for a set of realistic irregular wave testsis determined and normalized with respect to thestandard deviation of the surge motion. It is foundthat, to a good approximation, results of tests invarying headings, varying draft (and therefore,pretension) and varying weather intensity (1 year and100 year return period waves) yield about the samevalue for this normalized maximum. The maximum down-wave excursion exceeds the maximum up–wave excursionby about 50 per cent. The contribution to maximumsurge, of about 25m, which is due to waves is about15m for the Hutton TLP.

Some tests revealed the occurrence of waveprofile modifications due to the presence and motionsof the TLP. These modifications included wave run-uparound the columns, particularly the down-weathercolumns, and some local up-well~ng under the middle ofthe deck. These effects were most important for highwaves with shorter periods. An experimental approachto quantifying the extent and intensity of local waveelevations was used to provide a basis forconservative theoretical estimates of possible dynamicloadings on the underside of the deck which areaccounted for in structural design.

Tension Leg Ringing-----------------—-

High frequency tension variations were measuredin some tests, especially in high, steep waves, whichare resonant vertical vibrations of the TLP platform.The response is impulsive and decays because ofsystem damping which corresponds to about 5 per centof critical. To quantify the probable intensity ofthis mode of response in various severe sea states,special tests were carried out in irregular waves.Theoretical considerations of possible mechanismswhich might be responsible for these impulsiveresponses indicated the full-size response would notbe more intense than the model test results. Whileseveral plausible causes were identified, no firmconclusions could be drawn about the mechanismresponsible for this ringing, which may be similar toa kind of anomalous wave response reported by Lonergan(Reference 26). Expected ringing intensities areaccounted for in designjanalysis of maximum tensionleg loads and fatigue damage estimates.

CONCLUSIONS

The path to final design of a new system, such asthe Hutton TLP, wilL teach designers and analysts newlessons. The work done to complete the Hutton designrelied heavily on experimentation. Further designprogrammed may benefit from lessons of this programmeand improved analysis techniques will surelycontribute to effective design, but model testingshould be planned for as part of the overall designactivity. More extensive testing during preliminarydesign would be advantageous.

591

I.—

I

ACKNOWLEDGEMENTS 9. Horton, E.E., L.B. McCammon, J.P. Murtha andJ.R. Paulling, “Optimization of Stable

The authors are grateful to the managements of Platform Characteristics”, Offshoreall participating companies in the Hutton Field Technology Conference, paper OTC 1553 (1972).Development for granting permission to present thispaper. 10. Yashima, N., “The Experimental and Theoretical

Study of a Tension Leg Platform in Deep Water”,Participants in the development are: Offshore Technology Conference, paper OTC 269o

(1976).0 Conoco (U.K.) Limited (Operator)

o British National Oil Corporation 11. Faltinsen, O., and F. Michelsen, “Motions ofLarge Structures in Waves at Zero Froude

o Gulf Oil Corporation Number”, Proc. Intl. Symp. on Dynamics of

o Amoco (UK) Exploration CompanyMarine Vehicles and Structures in Waves, Inst.Mech. Engrs., London, April 1974.

0 Gas Council (Exploration) Limited

o Mobil North Sea Inc.12. IIogben,N. and R.G. Standing, “Research Related

Primarily to Gravity Type Structures ando Amerada Petroleum Corporation of the UK Tethered Buoyant Platforms”, National Maritime

o Texas Eastern North Sea Inc.Institute, Ship TM 443, February 1976 (paperpresented to NPL Seminar on Fluid Loading ofOffshore Structures, September 1975.

Design Contractor is:13. Paullingr J.R., “Time-Domain Simulation of

Brown & Root (U.K.) Limited, in association Semi-submersible Platform Motion withwith VO Offshore Limited. Application to the Tension–Leg Platform”, Spring

Meeting Society of Naval Architects and Marine

REFERENCESEngineers, (1977).

14.1.

Natvig, B.J., and Pendered, J.W., “Non-linearMercier, J.A., R,G. Goldsmith and L.B. Curtis, Motion Response of Floating Structures to Wave“The Hutton TLP: A Preliminary Design”, Excitation, Offshore Technology Conference,European Offshore Petroleum Conference and paper OTC 2796 (1977).Exhibition, paper EUR 264, London (October1980). 15. Albrechtr H.G., D. Koenig, and K. Kokkinowrachos

2.“Non-Linear Dynamic Analysis of Tension Leg

Mercier, J.A., and R.W. Marshall, “Design of a Platforms for Medium and Greater Depths”, Off-Tension Leg Platform”, Proceedings of a shore Technology Conference, paper OTC 3044Symposium on Offshore Engineering, Royal (1978).Institution of Naval Architects, London(November 1981). 16. Denise, J–P.F., and N.J. Heaf, “A Comparison

3.Between Linear and Non–Linear Response of a

Mercier, J.A., and T.O. Marr, “Design of the Proposed Tension Leg Production Platform”,Hutton TLP”, Offshore South East Asia Offshore Technology Conference, paper OTC 3555Conference and Exhibition, paper PC-82-090, (1979).Singapore (February 1982).

17. Yoshida, K., T.Yoneya, N. Oka and M. Ozaki,4. Tetlow, J.H., H. Bradshaw and M.J. Leece, “Motions and Leg Tensions of Tension Leg

“Hutton TLP Mooring System”, Offshore Platforms”, Offshore Technology Conference,Technology Conference, paper OTC 4428 (1982). paper OTC 4073 (1981).

5. Ellis, N., J.H. Tetlow, F. Anderson and A.L. 18. Gie, T.S., and W.C. de Boom, “The Wave-InducedWoodhead, “Hutton TLP Vessel Structural Motions of a Tension Leg Platform in Deep Water”Configuration and Design Features”, Offshore Offshore Technology Conference, paper OTC 4074Technology Conference, paper OTC 4427 (1982). (1981).

6. McClurer A.C., “Development of the Project 19. Det Worske Veritas, “Rules for the DesignMohole Drilling Platform”, Transactions, Society Construction and Inspection of Offshoreof Naval Architects and Marine Engineers (1965). Structures, Appendix B, Loads,” (1977).

7. Hooft, J.P., “A Mathematical Hethod of 20. American Bureau of Shipping, “Rules for BuildingDetermining Hydrodynamically’Induced Forces on and Classification of Offshore Mobile Drillinga ,%misubmersible”, Transactions, Society of Units”.Naval Architects and Marine Engineers (1971).

21.8.

Rainey, R.C.T., “The Dynamics of TetheredPaullingr J.R., Y..S.Hong, E.H. Chen and S.G. Platforms,” Transactions, Royal Institution ofStiansen, “Analysis of Semisubmersible Naval Architects (1978).Catamaran-Type Platforms”, Offshore TechnologyConference, paper OTC 1553 (1972).

592

–-

—— ____ _— — —

———.-—m= - _

— = -— - .— — -

~_ ~—. .. — — —>-——= -= = =—

—-— — ._ __ _ -=

.

— ———

— .—

—

—

—

_=*= —==== >–.——

.

—. —

—

~===— _~ _= __--=-.= .——__= –- .% ~–__—–=_

—= -.= ._— —

.- _e_: -=aa.— —.——J—

——— .——- -——~—____-—:=

—

22. Marex, “Environmental Conditions in the Hutton 25. Benjamin, T.B. and J.E. Feir, “TheField - Design Prediction Report”, Marine Disintegration of Wave Trains in Deep Water”,Exploration Ltd., proprietary report no. 429, Journal of Fluid Mechanisms, Vol. 27 part 3(February 1980). pp 417 - 430 (1967).

23. Longuet-Higgins, M.S., “On the Joint 26. Lonerganr J. , “Dynamic Behaviour of Models ofDistribution of the Periods and Amplitudes of Tethered Buoyant Platforms”, European OffshoreSea Waves”, Journal of Geophysical Research, Petroleqm Conference and Exhibition, paperVol. 80, No. 18, (1975). EUR 265, London (October 1980).

24. Spillane, M.W., “Conoco/Hutton Wave Study”,Gulf Research Development Co. proprietaryreport, (June 1980).

TABLE 1 TABLE 2

PRINCIPAL PARTICULARS OF THE HUTTON TLP EXTREME DESIGN ENVIRONMENTAL CRITERIA

Dimensions------_-—_ Wind——_—

1 minute mean @ 10m elev.Deck

44 m/see---- Wind gradient variation with

Length 78m elevation according to l/8thBzeadth 74m power lawDepth 12mWeather Deck 69m Waves(elev. above keel)

——---

Regularr “Design” Waves---—--—-— ______________

Hull (pontoons)---- Height 30.3mHeight 10.8m Period 14.6 - 18.5 secWidth 8.om

Irregular Waves_______________

Columns------- Significant Height 16.6rn4 corners 17.7m diem. Average zero-crossing period 13.9 sec2 centre 14.5m diam.

Current—------

Operating Conditions 5 minute mean at lom depth——__________________ 85 cm/sec

Draft at MWL 33.2mDisplacement at MWL 63,300 tonnes

Water Level—----—--—_—

Range between HDWL and LDWL 2.9mRestoring Force Coefficients (for hydro–--———____--—________________

dynamics)

c22 (heave) about 100,000 tonnes/m

c (surge) about 130 tonnes/mxx

cam (pitch) about 7,800,000 tonne-m/m

593

TABLE 3

TENSION LEG LOADS DUE TO WAVES

TABLE GIVES’ RATIO OF EXPERIMENTAL TENSION LEG LOAD RANGE

(PEAK-TO-PEAK) D1vlDED BY WAVE HEIGHT FOR MOST HEAVILY

LOADED CORNER TENSION LEG. 32M DRAFT, 45 DEGREE HEADING

(RESULTS NORMALIZED BY DIVIDING BY THE RATIO FROM CASE

(a) FOR THE LONG PERIOD WAVES).

Wave Period-----------

Test Condition 14.5 sec., down- 18.5 sec., up-

weather tension weather tension

leg most heavily leg most heavily

loaded loaded

(a) 14m sig. wave height

(RAO from cross–

spectral analysis

(b) 18m sig. wave height

(RAO from cross-

spectral analysis)

(c) “Regular” wave approx

30m height (ratio of

peak-to-peak of wave

forms )

0.92 1.0

1.02 1.o6

1.13 1.17

–-

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-—:=—

.@=-’

i

,’

Figure 1. THE HUTTON TLP— . __.. _____

ii

‘$..

II.“

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— —.——J———— .——- -——

~—____-—:=—

T.L.I? OFFSETS

d,”, . ,

Fig. 2. SCHEMATIC REPRESENTATION OF

FORCES AND MOVEMENTS OF TLP

–-

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— =-—- .—— -

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.

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—

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.

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— —.——J———— .——- -——

~—____-—:=—

4?

1111

T/T1

s

—

—

——

——

—

INERTIAFORCE

-Mx ~

s1

SPRING RATE nkTLFOR HALF OF 7ALL TENSIONLEGS

Fig. 3. COORDINATE SYSTEM FOR SIMPLIFIED

ANALYSES (HEAVE, SURGE, PITCH )

34

I

I

I

, 1 I 1 , , 1

8 10 12 14 16 18 20 22 24 26

PERIOD SECONDS

Fig. 4. HEIGHT/PERIOD BOUNDARY FOR

EXTREME DESIGN (REGULAR ) WAVES

–-

——. . ___— — —

———.—-—m= -_

— =-—- .—— -

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.

— — ——

— .—

—

—

—

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.

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——— .——- -——~—____-—:=

—

HDWLA

HAT :z“.

gAwuza&

MWL WATER DEPTH

14902M

LAT WATER DEPTH-.

LDWL v

..

.“..

....

.“.

.“

..”..

.“..”

. . ..“

WATER LEVELS WATER LEVELFREQUENCYDISTRIBUTION

Fig.5. EXTREME WATER LEVELS, INDICATING

(SCHEMATICALLY) THE FREQUENCY DISTRIBUTION

OF WATER LEVELS

————_—_————._._._._—_—__

–-

——. . ___— — —

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— =-—- .—— -

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.

— ———

— .—

—

—

—

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.

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——— .——- -——~—____-—:=

—

10001

‘

THEORETICAL PREDICTl~SBY NMI kvAVEHEAVE FORCE oPITCH FORCE h

[

250

0! 0 o~ THEORY !5 “$0, m ,

15 20 25°

WAVE PERIOD, SEC

Fig. 6. COMPARISON OF THEORY (NMI WAVE) AND

EXPERIMENT FOR WAVE-INDUCED HEAVE

FORCE AND PITCH MOMENT

–-

——. . ___— — —

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— = -— - .— — -

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.

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~—____-—:=—

16 I

-8

-12

-16

-20

16

12

8

4

‘o

-4

-8

-12

-16

0

1000

2000

3000

4000

5000

6000

0

“., .”

7000

8000

+

. .

PHASE ANGLE (DEC)40 80 120 160 200 240 280 320 360

,++

“..

“.

Fig.7. WAVE TEST DATA (AVERAGED OVER

ABOUT 80 CYCLES) .WAVE HEIGHT,30M.

WAVE PERIOD,18 SEC.

–-

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.

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—