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)TC 6724

‘he Design of Flexible Marine Risers in Deep and Shallow WaterI.Hoffman, HMC Offshore Corp.; N.M. Ismail and R. Nielsen, Wellstream Corp.; andL Chandwani, Zentech

opyright 1991, Offshore Technology Conference

nis paper was presented et the 23rd Annual OTC in Houston, Texas, Mey 6-9, 1991.

his peper was eelected for presentation by the OTC Program Committee following review of Information contained In an abstract submitted by the author(s). Contente of the paper,s presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(a). The material, as presented, doea not necessarily refleot7YPosition of tha Offshore Technology Conference or ha offfcera. Permlaslon to 00PY Is restricted to an ebs!raot of not more than 300 words. Illustrations may not be copied. The abstract?ould contain censplcuoua acknowledgment of where and by whom the paper Is presented.

iBSTRACT

This paper reviews the design methodology ofnarine riser systems and the analysis performed to sub-stantiatethe design corresponding to two field conditionsn deep and shallow water,

The analytical and numerical tools adopted are:apable of handling riser dynamic analysis for cases of;urvival, operation, interference, installation, and design,ife verification.

The results of computer analyses are presented inMS paper in graphical form for the riser system selectedIn the two cases. Output includes envelopes of riser co-~rdinates, axial force and time histories of forces and wavewrface profiles. The obtained results highlight the signifi-cance of design parameters for the combined flow ofwaves and currents on the seleetion of riser configuration.

NT’RODUCTION

The adoption of flexible pipe for marine risers asm integral part of offshore production systems is nolonger viewed as exploratory in nature. Recent installa-tions of key systems world wide have proved the conceptto be technically acceptable, economically attractive andDftenrepresenting a unique solution.

References and Illustrations at end of paper

Though flexible pipe as a marine product was in-troduced to the offshore market in the early seventies itwas not till 1978 that flexible risers were specified and in-stalled in the Enchova field offshore Brazil [1] as part of afloating production system.

Acceptance of flexible marine risers as compo-nents of floating production systems or for connection ofan export lines to a loading buoy as a viable long termengineering solution was up to recently dependent on theability of the prospective user or the manufacturer todemonstrate the adequacy of the design configuration cho-sen. Such evaluation requires certain dynamic analyticalmodels which could only be evaluated through model testor the luxury of full scale validation by actual experiencein the field alone with simulated life cycle tests. Theavailability of such proven tools today along withextensive variety of tests performed so far on samples ofthe flexible pipe itself and the actual field experience inEnchova [1] Balmoral [2] Jolliet [3] and Green CanyonBlock 29 [4] to mention a few, help remove the barriers ofacceptance but not necessarily lower the guard in so far asthe careful and often lengthy design procedure which suchan application calls for. Similar to other engineeringtechnological break throughs, developments oftendepends on the availability and ease of use of the ana-lytical tools capable of properly simulating the behavior offlexible marine riser in the ocean environment in which itis expected to operate survive, installed and removed if

253

necessary. Life expectancy of 15 - 20 years in notuncommon andlife cycle analysis is therefore required inaddition to the above mentioned design criteria,

The most convincing proof of acceptance of flexi-ble marine risers is best demonstrated if one reviews theoutstanding projects currently committed to such engi-neering solutions in Brazil, Canada, Australia, South EastChina, North Sea and the Gulf of Mexico. Some projectsrequire an extensive trade off study in order to comparethe flexible and rigid pipe options, taking into considera-tion the product, its installation, removal, and its ability tosurvive the environment. Technology and economy eachplay a major role in such a comparison and the designoften represent a rational compromise. The two cases se-lected for presentation and discussion in this paper are dif-ferent, since it involves the design of a riser system for twoextreme water depth condition one in over 2000 ft and theother below 100 ft. In either case a trade off study was notnecessary, since flexible riser was the only rational solutionavailable in such water depth and the associated environ-mental conditions on site. These cases were selected inorder to illustrate the differences in riser configurationsand the approach necessary to be taken in each case aswell as to emphasize similarities, such as the temporarynature of the installation and the expected removal withina short time; reuse of system components and the consid-eration as later discussed.

In order to demonstrate the design methodologyof such flexible risers, a short discussion of the design ap-proach and the analytical tools available for it must begiven first followed by the design verification stages whichone should undertake as part of the riser analysis prior todiscussing the effects on the pipe itself namely resultingstresses, and the product capabilities to withstand suchloads.

The results of the analysis for the two cases are theessence of this paper. Typical outputs illustrating thecritical aspects of the riser configuration selected and theirsensitivity to the assumptions made in each of the analyti-cal phases used are discussed.

FLEXIBLE RISER ANALYSIS AND DESIGN

Flexible pipes and risers are critical components ofoffshore field developments because they provide themeans of transferring fluids or power between subseaunits and a topside floating platform or buoys. These ris-ers accommodate floating platform motion and hydrody-namic loading by being flexible. In storm conditions theyundergo large dynamic deflections and must remain intension throughout their response. They are consequentlymanufactured to possess high structural axial stiffness andrelatively low structural bending stiffness. These struc-tural properties provide only a small resistance to lateraldisturbances caused by wave and current induced hydro-dynamic loadings. Their global dynamic behavior can

I

therefore be considered as more mechanical, or force de-pendent, than structural. In contrast, behavior near theend connectors of a system is governed by local structuralstiffness properties.

The design of a flexible riser system has to accountfor a combination of complex loading and motion phe-nomena. A major part of the design is therefore the systemanalysis where it is necessary to perform large deflectionanalysis of those tensile structures when subjected to dy-namic boundary conditions and non-linear hydrodynamicloading. Such ana~ysis must be performed by a softwarepackage which is fast enough to enable the engineer to ad-equately assess the effect of different parameters on thesystem and yet is rigorous in its structural modeling andsolution of the inherent equations of motion. [51[61

Characteristics of Flexible Riser Svstems

Flexible pipe is defined as a composite of layeredmaterials which form a pressure containing conduit. Thepipe structure allows large defection without a significantincrease in bending stresses. [7] The pipe is therefore de-signed so that it has a low bending stiffness and can ac-commodate high internal and external pressures. The pipeconstruction will either be of a bonded type (whereby lay-ers are bonded together using adhesive and are then vul-canized in an oven to form a homogeneous structure) ornon-bonded (whereby individual layers remain separatedallowing internal relative movements). Typical materialsused for construction include polymers, textile, steel andfabrics.

From an engineering analysis point of view, thetechnical characteristics of a flexible riser system arc

c tension dominated structure;● hydrodynamic loading due to waves and current;s dynamic boundary condition due to movement of

vessel;● pinned/clamped boundary conditions;“ system can be partially in air/partially submerged;c possible connection to a subsurface body;. possible change in weight along length of system;● possible surface contact at seabed.

On consideration of the above characteristics themain complexity in the analysis of flexible riser systems isdue to non-linearities arising from hydrodynamic loadingand dynamic boundary conditions.

Desi~n Methodolozv CriteriaI

Efficient design of flexible riser systems is only ‘made possible by using as computer-based solution tech-niques. The basic steps required in design of a flexibleriser system are set out in this section.

The design of flexible riser systems is usuallybased on allowable pipe curvatures and tensions, which

254

me prescribed by the pipe manufacturer, and clearances)etween the riser and other structures and boundaries dur-ng its dynamic response. The allowable curvatures andensions are based on full-scale test procedures and stressmalysis carried out by the manufacturer and these limitsinsure that the pipe is not over-stressed when respondingo dynamic loads and vessel motions. The system is gen-?rally designed such that the pipe is always in tensionhroughout its dynamic response cycle. Minimum clear-mces are also specified to avoid clashing problems be-ween riser and seabed or riser and vessel and between theiser or other adjacent risers, cables or mooring systems.

)esim Parameters and Procedures

The main problem in design of flexible riser sys-ems is that the number of design parameters is large. Themvironmental conditions, vessel motions and riser proper-tiesare usually well-defined. The main design parametersme therefore the choice of configuration, the length of‘iser, the system geometry and the sizing of buoyancynodules, subsurface buoy or arch. The choice of riser con-figuration is usually based on economic criteria, position of#ells etc. and in most cases can be considered as known.

The length of riser, sizing of buoyancy compo-nents and system geometry need to be determined by theilesigner.

The first stage in the design of a flexible riser system.s to determine an acceptable system layout. The first;tage is based on static analysis and it is normal to carrymt a parametric study to assess the effect of changing the~esign parameters (i.e., system geometry and length) onthe static curvature and tension, Based on the results of:his parametric study, the design selects a suitable range of~ystemgeometries and lengths that satisfy the design cri te-ria. The parametric study will also assess the static effects>f vessel offset (displacement of the top end) and currentloading in different directions.

The second stage in the design procedure is to per-form dynamic analysis of the system to assess the globalciynamic response. A system layout and length is chosenFromstage one and a series of dynamic load cases are thenconsidered. These load cases combine different wave andcurrent conditions, vessel positions and riser contents inmder to prove an overall assessment of the riser suitabilityyin operational and survival conditions. The correspondinganalyses are then carried out and dynamic curvatures, ten-sions and clearances are checked against the design limits.The design may need to be modified at any stage of thisprocedure — it is therefore essential that the solution tech-nique is fast.

The third stage in the design procedure is to pcr-forrn detailed static and dynamic analyses of local areas todesign particular components. This state is presented in aseparate publication [8].

Riser Confhzuration Selection

Industry practice calls for several types of riserconfigurations which are typically used in conjunctionwith Floating Production/Loading Systems. The standardfive configurations generally used are: Free-hangingCatenary, Lazy-S, Steep-S, Lazy Wave and Steep Wave.Figure 1 illustrates the two types of configurations whichare discussed in this paper.

Although dynamic response of a riser system toextreme and operational environmental conditions playsthe key role in selection of a particular configuration, otherimportant factors that should also be considered duringthk phase are

●

●

s

●

Interference with other riser systems andmooring linesActivity of other vessels in the vicinityEase of laying and retrieval and future re-quirements of maintenanceInspection and workover operations.

The dynamic response of a particular risers stemzis directly related to the environmental loadings ue to

the combined wave-current field flow coupled with the in-teraction arising from the structural non-linear behavior ofthe riser itself. The spatial and temporal distribution of theintegral properties of wave such as mass, momentum,pressure and ener ,

Yset the ground for the selection of a

particular riser con iguration for a particular depth of wa-ter.

Consider a progressive wave moving in the posi-tive horizontal axis (figure 2). For simplicity consideringsmall amplitude wave theory, the wave kinetic energyconcentration averaged over one wave period at anyelevation above the bottom is given by:

~zopzkzKE(s).-=

402(cosh kh)2 Cosh2ks..............................(1)

In order to represent the above relation in dimen-sionless form, the ratio of the kinetic energy at any eleva-tion S to that at the mean free surface is given by

KE(s) cosh 2ksKE(h)- cosh 2kh........................................(2)

Equation (2) is plotted in figure 3 for the case ofh/L= 0.05 (conventional shallow water limit). The figureshows that for the case of shallow water waves the energyconcentration is nearly uniform with depth, while for caseof deep water waves the energy is concentrated near thesurface.

It should be highlighted that the effects due towave-currents interaction should be always considered indesign and analysis as it might cause a significant changein the magnitude and distribution of fluid forces. This

255

change of fluid #orces could be dramatic, as seen by Ismail[9], in coastal waters not only due to the strong currentshear but also due to the characteristics of shoaling waters.The parameters which also usually sensitive to wave-cur-rent interaction effects, include hydrodynamic force coeffi-cients, current velocity profiles and relative direction ofwaves and currents.

To illustrate the suitability of a particular configu-ration for a particular water depth, two riser desi~ caseswere studied in deep and shallow waters. The results ofthese studies are in the following section. The computeranalyses were carried out at Wellstream’s engineeringoffices using computer program FLEXRISER-4 developedby Zentech, London.

~nalvsis Tools and Validation

Three distinct stages have been previously identi-fied as essential for the design procedures of flexible risersystems. The designer must be provided with the neces-sary tools to complete all three stages efficiently. Due tothe complexity of the system, these tools have to be com-

~uter-based solutions of the inherent equations of motion.ince each design stage is essentially based on the same

configuration, the most efficient package to use will havecommon data input and result output modules for all 3 de-sign stages. This approach reduces time and error on datainput, since much of the system data is not changed be-tween stages. Several software packages are commerciallyavailable which basically meet the design approach out-lined above. While the obvious approach to analysismight be to select one such program to simplify and speedthe design through familiarization with that particularprogram, it has been Iearned that as a result of some dif-ferences in methodology and approach to the analyticalsolution, more than one program is often required to vali-date and investigate specific results which are particularlysensitive to assumptions and input data used by the par-ticular program selected. While general agreement existsbetween three of the programs which were evaluated,some differences may show up particularly in sensitive ar-eas.

Two techniques are generally available and accept-able for solutions of the governing equations systems fordesign stages 1 to 3. These techniques are the finite differ-ence and finite element schemes. Both methods areequally applicable to solution of flexible riser problems —however the finite difference scheme has distinct advan-tages in terms of both speed of solution and computermemory requirements, particularly when performing ran-dom seastate analysis. One of the programs used namely“Flexriser 4“ by Zentech [10] is based on finite differences,direct space and time discretization of governing equa-tions, the other two programs, Flexcom- 3D by M.C.S. andSeaflex [10] by Marintek are based on finite elementmethod and the virtual work principle. Though the abovetwo approaches to solution of the governing equation sys-tem may distinguish the one program from the other two,

numerous other aspects of the mechanical behavior offlexible risers must be considered and treated numericallyleaving extensive room for substantial differences betweenthe two solutions both using the same method of analysisnamely finite element. Such consideration, for example,due to non-linearities, ma result from large rotations and

Jdisplacements, large di ferences between the bendingverses axial and torsional stiffness, contact with other rigidsurface which may lead to interference, contact with theseabed and last but not least, modeling of the hydrody-namic loading.

A general software evaluation project flexible risersystem analysis was reported in [101 and included three ofthe programs mentioned about or close versions of it.Static and dynamic analysis of two typical riser configura-tions using four different software packages all designedto rovide cost effective engineering solutions was given

rin 10]. All programs were three dimensional, capable ofhanding axial, bending and torsional stiffness, regular andirregular waves, seabed friction and use compatible inputand output data presentation.

The main evaluation of the four software packagesevolved a lazy S’ configuration in 77 M water depth, withthe current and wave headings assumed to be perpendicu-lar to the plane of the catenanes in mean position. There-sults of all programs were within plus or minus 10% ofeach other. It should also be noted that analysis obtainedby a “cable elements” 3-D program was used as a basis forcomparison and yielded a reasonable “mean” for all fourprograms. An additional analysis comparing results fromjust two of the proWams for J tube riser clamped on a lay-over arch was also given in [10]. It was concluded in [101that for most standard riser configurations the programsevaluated as well as other dynamic programs using moresimplified approach seem to yield similar results for ten-sions, angular deflection, bending radius and other rele-vant parameters. This conclusion a eed with an earlier

Fone in[11] both advocating the use o simplified programsin general and more elaborate approach for riser configu-rations where bending and torsional behavior could becritical. A combination of an efficient general analysisprogram with a capability to investigate specific detailwhen circumstances call for is not generally availablewithin one commercial software package. However, theuse of two or even three such packages as part of the de-sign process in conjunction with a sensitivity analysis tocritical inputs could most likely yield satisfactory en@wer-ing solution in most cases,

A flexible riser analysis package should ideally bevalidated using the following four sources

(a) Comparing with generalized theoretical solutionsand checking result consistency using differentmodeling techniques;

(b) Comparing with results predicted by compatiblesoftware packages;

I

I

I

I

256

(c)

(d)

Comparing with results predicted by experimentusing scaled model tests:

Comparing with results obtained from full-scaleoffshore tests.

Ideally, all the above methods should be used tovalidate a package. In practice, (a) (b), and (c) can be im-plemented with relative ease whereas (d) requires complexmonitoring equipment therefore a large capital invest-ment. A recent attempt to illustrate such a validation waspresented in [12]. Results of full scale measurement werereasonably correlated with the numerical model yieldingthe same order of magnitude values at frequencies corre-sponding to the numerical model capabilities. Vortexshedding effects were not accounted for in the numericalmodel used in [12].

It is apparent from the above, that to achieve a aflexible riser system design, one must use a suitable com-puter software package to perform static and dynamicanalysis. Such a package should form the basis of 3 dis-tinct design stages — determination of system layout, dy-namic global response analysis and detailed localizedanalysis and details design.

A study of the effects of variation of a certain pa-rameter on the system dynamic response can be usefuland sometimes essential to assessing trends and investi-~te what seems to be ‘trouble spots’. This type of anal-

!

sis allows the designer to make intelligent estimates ofynamic magnifications for other similar configurationsnd load cases.

1Furthermore, the sensitivity of the system re-

ponse to small variations in environmental loads and ves-el displacements can also be analyzed and studied. Thisacilitates determination of the system dynamic responseo small changes in forcing function magnitude and defi-“tion and thus identifies the possible existence of criticalynamic magnification trends.

~NGINEERING ANALYSIS AND DESIGN VERIFI-2AT10N

IWith any one or more of the software packages

vailable as a design tool, the successful completion andedification of a flexible riser depends to a large extent one methodology developed by the designer in using suchftware.

Static and dynamic analysis of the riser system arearried out to determine the layout and demonstrate theuII feasibility of the riser systems under all operating.~odes. The same type of analysis is also applied to theiser and the remaining static pipe life work for configura-ons likely to be encountered during riser installation andlipe layout phase.

The static and dynamic response analysis of theflexible pipe presented in this paper simulates the dynamicbehavior of single and multiple flexible riser systems sub-jected to hydrodynamic loading and vessel motion. Thesoftware utilized a 3dimensional non-linear analysis ap-proach in the time domain. The software was used for fea-sibility and conceptual studies, as well as for global anddetail design for installation and operating conditions. Itwas also ~sed for an interference-analysis to determineclearances between risers, mooring lines, pipe protectionagainst abrasion as a result of interaction with the seabed,design of pull-in tubes clearances, and similar potentialobstructions.

The software is applicable for both the preliminarystatic analysis to determine system layout and general con-figuration as well as for the full dynamic response analy-sis.

The scope of work generally consists of fourphases: ,

Phase I: Riser Configuration Design and Analysisfor Survival and Marginal OperationConditions

Phase 11: Interference StudyPhase III: Installation AnalysisPhase ~ Fatigue Analysis Input Data Preparation

The above applies to all flexible pipe componentsused as part of the riser system, which require design veri-fication.

The above engineering design approach was ap-plied to two specific riser configurations representing twoextreme water depth conditions, i.e., 600M and 40M: Thefirst a Free-hanging catenary riser and the second a SteepWave riser with buoyancy. These two cases were selectedsince it represents a design challenge on the one hand anda case where flexible riser is the only acceptable engineer-ing solution.

The results of the analysis, including the inputdata, output in tabular and graphical form along with in-terpretation of the results is presented in the followingsection.

PRESENTATION AND INTERPRETATION OF RESUL~

In order to illustrate the potential and limitationsof adopting specific riser configurations in a particularwater depth, the results of riser dynamics analyses for tworiser configurations in deep water and shallow water arepresented. The computer analyses were conducted atWellstream’s engineering office and made use of theFLEXRISE-4 personal computer program developed byZentech, U.S.

257

Free-hanting Catenary

A free-hanging catenary configuration wasadopted for riser systems in two design cases which havesimilar overall design parameters but very different waterdepths at the respective project sites. The first case is in600 M water depth and the second on in 90 M water depth.Table 1 illustrates the main design parameters used as in-put for the computer analyses. For the first case, Figure 4shows a snapshot of the riser configuration and the distri-bution of axial force along the length of the riser depictinga small amount of compression near the seabed. Alsoshown are the time variation of the axial force, near thesea bottom. In contrast to this, for the 90M water depthcase, figure 5 shows an appreciable amount of compressiveforce in the riser near the seabed. To maintain the stabilityof a flexible riser system, it is imperative that the riser piperemains in tension throughout its operational life.Because of the development of this excessive amount ofcompressive force in the pipe in shallow waters it can besafely concluded that free-hanging catenary configurationis unsuitable for shallow water depths. Studies have alsoshown that sometimes even in deep water locations, be-cause of heavy seas and the resulting heave motion of thevessel, appreciable amount of compression in the riserpipe near the seabed in free-hanging catenary configura-tions might also occur. To mitigate the effects of this lossof tension in the riser pipe which could adversely affect theservice life of the pipe, one of the following measures maybe adopted:

● Increase departure angle of the riser at the endconnector at the floater level.

. Increase the weight of the riser pipe section nearthe seabed by wrapping with some heavymaterial.

● Modify the vessel to reduce the heave motion.

● Attach a subsurface buoy.

Each of the above remedies could be viewed as apossible solution depending on the technical andeconomical constraints of the project in general and theriser system in particular.

$teeu Wave

Dynamic riser analyses were also conducted for adesign of a single oil export system in a 40 m water depthsite. An extensive analysis was carried out prior to arriv-ing at the proposed steep riser configuration (either 6-inchor 8-inch flexible pipe diameter) which resulted in manyunacceptable configurations primarily due to the riser in-ability to withstand the environmental conditions im-posed, extreme wave heights of 15 m for the 1 year-stormand 20 m for the 100 year-storm. The analyses showedthat any Lazy or Free-hanging system are not feasible atall. Moreover, the need to eliminate the Steep S configura-

tion, as an alternative, became appa~ent when the sensitiv-ity of the riser dynamic response to the intensity of addi-tional buoyancy distribution was determined. Thus, theSteep Wave was left as the only feasible configuration forthe riser of this oil export system. For the Steep Waveriser, the distribution of buoyancy along the riser was de-termined (figure 6) to prevent any buckling instability and.to reduce any excessive angle at the riser base. A snapshotof the riser configuration under the dynamic effects ofwaves and currents is shown in figure 7 for both cases offar and near field. The corresponding axial tension forcealong the Steep Wave riser is shown on figure 8.

It is apparent that the Steep wave configuration,though a natural choice for extreme conditions as in thecase above, does not necessarily offer an overall solution toall problems of flexible riser design.

CONCLUSIONS

For the design cases of flexible marine risers indeep and shallow water considered in this paper, it wasevident that the most apparent riser configuration does notnecessarily provide the appropriate solution. The deep-water case emphasizes a solution based on a simple riserconfiguration to facilitate modularity and hence ease ofinstallation and removal. In the shallow-water case, thedesign is a more complex riser configuration due to thesevere environment loads requiring particular designconfimration loads. Both cases represent new frontiers foruse of flexible pipe as marine riser~.

The sensitivity of the riser dynamic response, inparticular configuration, to environmental data andvessel /floater motion data warrants a careful review ofdesign basis prior to the dynamic analysis and design ofmarine risers.

NOMENCLATURE

a =

g“

k =

L=

s =

T=

P =

a=

wave amplitude

acceleration of gravity

27’Cwave number P L wave

length

Wave Length

elevation above sea bottom

Wave Period

fluid density2n

wave frequency ~

258

?EFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Machado, Z.L., and Dumay, J.M., “DynamicProduction Riser on Enchova Field OffshoreBrazil”. Offshore Brazil Conference, LatinAmerica Oil Show, Rio de Janeiro, June 1980.

Mahoney, T.R. and Bouvard, M.J. “FlexibleProduction Riser System for Floating ProductApplication in the North Sea”. ProceedingsOffshore Technology Conference, Paper OTC 8163May 1987.

Tilling Hast, W. S. “The Deepwater PipelineSystem on the Jolliet Project”. Proceedings -Offshore Technical - Conference” paper OTC6403, May 1990.

Espinasse, P.F., Schnitticer, D.A. and Sturdevant,L.A. “Design and application of Dynamic FlexiblePipe Array for Placid B-C 29 FPS” ProceedingsOffshore Technology Conference Paper OTC 6164,May 1989.

Brown, P.A., Chandwani, R. and Larsen, L,“Flexible Riser Dynamics Modelled in 13-Dimensions”, Proc. F. Conf, on Mobil OffshoreStructures, The City University, London 1987.

Soltanahmadi, A. and Barnes, M.R., “DynamicResponse and Modeling of Flexible Risers”, Proc.Int. Conf. on Non-Conventional Structures, Inst. ofCivil Eng., London, 1987.

American Petroleum Institute, “RP176 -Recommended Practice for Flexible Pipe”,Houston, 1987.

Brown, P.A., Soltanahmadi, A. and Chandwani,R., ‘Troblems Encountered in Detailed Design ofFlexible Riser Systems”, Int. Seminar on FlexibleRisers, Univeristy College, London, 1989.

Ismail, N.M., ‘WaveCurrent Models for Design ofMarine Structures”, Journal of the Waterway, Port,Coastal and Ocean Division, ASCE. Vol 110, No. 4,NOV.1984.

Kodaissi, E., Lemerchand, E., Narzul, P., “State ofthe Art on Dynamic Programs for Flexible RiserSystems ASME Ninth International Conference onOffshore Mechanics and Attic Engineering,Houston, 1990.

Kokkinowrachos, Giese, K. , and Markoulikis,

12. Lopes, A. P., de silva Neto S.F. Estefgn S. F. andDa Silveria M.P.R. “Dynamic Behavior of aFlexible Line Design Installation” ProceedingOffshore Technolo~ Conference Paper No. OTC6437, May 1990.

“Comparative Evolution of Numerical Methodsfor Non-Linear Riser Problems”, Workshop onFloating Systems and Offshore Operations,Wageningen, Nov. 1987.

259

Table 1 Data for Catenary and Steep Wave Rfsers

FfexibleRiser Data:

fnternd Diameter, mOutside Dtameter, mAxial Stiffness,NBen,% Stiffness, tim2Wt m am,Kg/m

Environmental Data

Water depth, mWave hzight, mWave Period, sCurr2nt Sp2ed, m/s2c

Top, Bottom

Ve5sel/Fi0at2r Excitation

Surge, mHeave, m

Catenary

Case A

0.10160,1631SA6S72.00E344

62511.B11.2

1.1,50

1.s2.0

CaseB

0.10160.16315.46S72.00s344

9011,811.2

1.1,.5

LB2.0

%xp Wave

4015.013.0

2.3,1.07

6.S07.25

A FLOAllNGPLAIFoRIJ

CALMBOVY — !&wyg&~

— —C--8-=

bba-.ti J /VORTE4WODINC

AIRING44 UN[S

,//,/ - m5ER

SEEP-WAX ,/

R12ZR /’/’

+’/// ///

//

/’ ///

SEA BOllCM // r,l’rumv mcm

RISIRS&I~

/// ///19W7

///

f19.1-fNUdY cues of two Ilexlble clam cm ffguratlms.

t---Wove Length L

1

Wave Crest _

- ,,... w“,”. , “,,.,

‘-----------”“’?T----%?FZZ:H

I I i / Actualpressure head // / I

!“’’()’mi4-l%I

// /.

,!I Bottom

///)/ ///// l//// /l/////l/////////l///////////l//////l///////l

fig. Z-Ratio of wave klnatlcnnnrgyat slh t.aklnetlcenergyat MI$.1.0.

I 260

1.0

0.8

0.6

0.4

0,2

0

DEEPWATER

<f=o.5

Percent Kinetic Energy

FIKI.3-Vwilcal pmssum d!strlbution wlthln ● proorasslve wave.

400

:. 200

a

— 200

-10

-5

0

5

10

1s0

-300 VerticalX-Y SNAPSHOT

Xl Coord [ml

-200

-1oo

0 ,

100

200 \

?004Q0 \

tio\

600 \70060~900

-200 0 200 400 600 800 iooo 1200tkTriZOntal (Y) Coord [m]

-20

0

20

40

60

60

100

MO

Km

So

o

1 I I I 1 1 c

X-Y SNAPSHOT

%

r-150

\!

7 I

-100 -50 0 50 100 150Horizontal (Y) CL!MW[m]

200~p50

Tension Force Vs Distance

/ ~ “ i

t

B@9imnmw E1# Tunaion Force VS Distan-m -54i . m

-PM M _m u L

———————

6* -10 “m Fmfm M Wave Profile Vs Time

&!m I I I

WV* %’0film M Wave Profile Vs Time

I I I

/,

n

P ,n / \

\ _// \ / \ /

Tm#im F.mm [kNJ Tension Force Vs Time

... A nl.w. \ I \ / \ /

al \ / \ / / /\

\ /

o /v /

v /v v

-5 f ) A A n /) AI

o / \/

/\ /\ i \ /\

5a m . a m

w TunsimI Forcn km ‘iT&% &I’ Force Vs Time,

w

40

m

o

-al

-s4 10 I I I m m iTime [SK] Fig, 5 TypIca! Results of Free Hw@ng Riser Mkdysls in 90 H Vater Depth

FQ 4 Typical Results of Free Hnngng Riser Amdysis h 625 H Water Oepth

vERTIcAL

co0RD

(m)

—

%

S1 S2

S3

S4

Buoyancy per Unit Length - B2 1,37 kn/m I Length, Buoy End to Base - S4 O m

Buowncv Der Unit Lenath -63 2.29 kn/m Riser Oiometec 6 In.

Fig. 6–Design configuration of steep wave riser.

-sr––~––~––-,––~––,

–!––-–L--A

15 r——

,.L––l––J_I___L__i_i.-40 -.33 -m -10 0 10

HCW7.CNTAL(Y)&XRO (.)

(i) FAR FIELD

-5vEDRT5

c 10AL 15

l––-l–-–,–-–~-––-r---,

+ —— -1

r ——;L ——-1

1- ——4L —— -1

t ——-!

L —— J

1- —— 4.L–_l–_J___L__l__J-40 -M -20 -10 0 10

HCRIZC#TAL(YjCCCRO(.)

(ii]NEAR FIELD

Fig. 7-X-Y snapshot of steep wave riser configuration.

Curvature Vs Distance,W Cu=vatve Wn)

w A I

I.m - /

+m

-a4m1

/

-iIm u

Curvature Vs DkrhnceCwvttuw ah)

— —~

\

\

1

1

-4.m I I I I I I I IIwzl

‘Dkttn)” s “ n

1!

a

4

3!

m

n

1

-11

TensionForce Vs DistanceTenslcm Fcecr O@

m“---Iulon

O&w” * u “

FROMNODEMO.iTONOOENO.86

(i)FAR FIELD

44

2!

a

u

1

-u

TmgmFmeT&ion Force Vs Dktance

I /~ / “e

~. I // “

I1-

tua ‘mSt(n)* w “ n

FROM NODE NO.lTONOOENO.66

(II) NEAR FIELD

Fig,a Tensionand Curvmture Distributionalong a Steep Wave Riser,

262