JPT2009_02_17OffshoreFocus

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After many years of development work, as well as many unsuccessful attempts,the first liquefied-natural-gas (LNG) floating production, storage, and offloading(FPSO) initiative was launched in 1976 by Linde, Technigaz, et al. under thename Consortium 76offshore LNG now is a reality. The first LNG receivingterminal installed on a gravity-based structure offshore Rovigo, Italy, will becommissioned soon. The first floating storage and regasification unit (FSRU)permanently moored in open sea offshore Livorno, Italy, is under construction,

and several LNG carriers equipped with regasification facilities are able to delivergas at several offshore ports in North America.

Offshore liquefaction also seems close to materialization, with several par-ticipants (including oil companies; engineering, procurement, and constructioncontractors; ship owners; and others) developing LNG FPSO solutions for stand-alone gas fields or for associated-gas developments.

Two main ways to approach these facilities seem to be pursued by the variousparticipants. The big way is followed mainly by major oil companies: It aims atdeveloping large-capacity liquefaction units (3106tonnes/a and larger) by useof processes generally derived from latest-generation onshore liquefaction cycles.A second trend, which could be called the small way, targets smaller capacities,by use of less-efficient processes often based on nitrogen cycles, but having better

suitability for use on board a floater.

While the latter probably has a wider range of potential application cases andwould ease topside layout and safety concerns for a lower overall capital expen-diture, the former will provide higher liquefaction efficiency and will targeteconomies of scale. In turn, it might be applicable to only a limited number ofrelatively large gasfield developments.

Today, both approaches have equal chances to open the way for offshorefloating liquefaction, but, certainly, both will need to rely on experiencedcompanies that can gather and leverage experience in offshore regasificationgained from the first FSRUs, combined with experience in large oil FPSOs andin onshore liquefaction.

All the industry needs now is a good gas field!

Offshore Facilities additional reading availableat OnePetro: www.onepetro.org

OTC 19429 A Coupled FE-SPH Approach for Simulation of StructuralResponse to Extreme Wave and Green Water Loading by J.C. Campbell,Cranfield University, et al.

OTC 19239 Innovative Pipe System for Offshore LNG Transfer by Ing. C.Frohne, Nexans Deutschland Industries, et al.

OTC 19315 Deepwater Moorings With High-Stiffness Polyester and PEN-Fiber Ropes by P. Davies, IFREMER Brest, et al.

Offshore Facilities

TECHNOLOGY FOCUS

74 JPT FEBRUARY 2009

JPT

Claude Valenchon, SPE, is Manager,Offshore Technology Development, forSaipem in Paris. In 1981, after 7 yearswith CG Doris, he joined BouyguesOffshore, which became Saipem S.A. in2002. Currently, Valenchon is in charge

of offshore technology developments,aiming at providing solutions, products,or concepts for tenders and design com-petitions, with current focus in the areaof deepwater-field developments, Arctic,and offshore LNG production. He serveson the JPT Editorial Committee andholds an engineering degree from thecole Nationale des Ponts et Chaussesin Paris.


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Petrobras has been investigating thesteel-catenary-riser (SCR) alternativesince the beginning of the 1990s. Sincethen, fatigue verification has been animportant issue, demanding good rep-resentation of the loading conditionsthat occur during the lifetime of theriser. The concern with fatigue hasmotivated Petrobras to research several

areas, such as metocean data acquisi-tion, hull design for motion optimiza-tion, special touch-down-point (TDP)joints, accurate models for vortex-induced-vibration (VIV) analysis, andthe corrosion-fatigue effect.

IntroductionInstallation of the P-18 SCR was a pioneerproject of a free-hanging SCR connectedto a semisubmersible, and it provedthe technical feasibility of the concept.Although this riser was installed as aprototype, it is still working in the gastransfer from platform P-18 to platformP-26. It has been monitored since 1999,and the results are being compared withthe design data and with simulationsperformed with in-house computer pro-grams and other commercial packagesthat include the complete design meth-odology. Other SCRs were studied, suchas the 12-in. oil-export riser for the P-19

semisubmersible in 770 m of water andthe 10-in. oil- and gas-export lines forthe P-36 semisubmersible at a waterdepth of 1360 m.

The free-hanging SCR configurationis considered as an available technologyfor semisubmersible applications, andthere is interest in the application ofSCRs connected to floating production,

storage, and offloading units (FPSOs)because of the trend to use these unitsfor exploration and production in deepwater. This has caused a need to studythis concept carefully, given the highoffsets and heave motions imposed bythe vessel at the top of the riser.

Fatigue verification is an importantissue that requires accurate evaluationof the loading conditions that occurduring the riser lifetime, and it alsorequires a precise knowledge of con-struction aspects that could decrease orchange riser-materials resistance.

Wave-Induced FatigueOver the past few decades, Petrobrashas acquired Campos basin wave, cur-rent, and wind data, resulting in ametocean database containing morethan 7,000 records. Within these data,the occurrence of multimodal/multidi-rectional sea states was identified. Touse this database in riser design, thein-house software tools for structuralfatigue analysis were upgraded to con-sider bimodal/bidirectional sea states.

Because fatigue verification is animportant issue in steel-riser design, agood representation of loading condi-tions that occur during the riser lifetimeis needed and use of the entire databaseis recommended. However, the riserdesign schedule can be affected if arandom time-domain analysis is used.To minimize this, one solution adoptedwas to develop a statistical procedureto reduce the database to a reasonablenumber of representative loading cases

to be used in fatigue-damage verifi-cation. This method resulted in theadoption of approximately 150 fatigue-loading cases. The combined wave,current, and wind data are preserved inthe loading conditions that were cho-sen to represent all the usual metoceansituations in Campos basin.

Hull Design To Reduce Motion. Inthe Campos basin, the wave fatigueenvironment has been shown to bethe limiting factor for SCR feasibility.The alternative of optimizing platformmotions has been one of the ways toincrease the possibility of SCR applica-tions. Among dynamic motions, heavehas been identified as the most damag-ing. For the P-52 design, in 1800 m ofwater in the Roncador field, a limit for amaximum heave at an extreme point inthe hull has been established, and thesedata determined the choice betweenexistent hull models.

When starting a new hull design forthe P-55 unit, more-complete criteriawas used, with a set of operationalwaves chosen from the traditionallymost damaging ones. The P-52 motionswere taken as a reference. There wasinterest in knowing what level ofminimized motions could be obtainedconsidering a deep-draft-hull concept.A study was conducted with a largenumber of hull geometries, and themost adequate ones, in terms of con-

structability and other naval aspects,were chosen. The amplitudes of heavemotions for these hull models were ver-ified to be from 15 to 20% smaller thanthose obtained for the P-52 hull. On thebasis of this, a set of maximum motionsunder medium- and high-fatigue waveconditions has been established.

The monohull concept also has beendesigned to achieve minimized heavemotions. The same criterion has beenapplied as a guide to the models stud-

This article, written by Assistant Tech-nology Editor Karen Bybee, containshighlights of paper OTC 19249,

Influence of Fatigue Issues on theDesign of SCRs for Deepwater OffshoreBrazil, by A.L.F.L. Torres, M.M.Mourelle, S.F. Senra, E.C. Gonzalez,

andJ.M.T. da Gama Lima,PetrobrasS.A., originally prepared for the 2008Offshore Technology Conference,Houston, 58 May. The paper has notbeen peer reviewed.

Copyright 2008 Offshore TechnologyConference. Reproduced by permission.

Design of Steel Catenary Risers for Deepwater Offshore Brazil

OFFSHORE FACILITIES

The full-length paper is available for purchase at OnePetro: www.onepetro.org.

JPT FEBRUARY 2009 75


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ied for the monobore design, with andwithout storage capacity.

Coupled Models. The use of coupledanalysis tools in the design of SCRsbecomes even more important becausea large number of risers and mooringlines are connected to the platform andthe system is in deep water like the P-18,P-52, and P-55 systems. Petrobras hasdeveloped in-house software programsto analyze the coupling between thenonlinear hydrodynamic behavior of thehull and the structural and hydrodynam-ic behavior of the lines. Hybrid methodsthat combine the use of programs basedon coupled and uncoupled formulationshave been considered the possible road-map toward a fully coupled analysis anddesign methodology. The coupled analy-

sis carried out with the hybrid modelis attractive in contrast to the excessivecomputer cost of the fully coupled meth-od because a large number of analysesfor calculation of the fatigue behavior onrisers are necessary with it.

In recent studies of SCR design forthe P-55, the numerical model of thesystem was generated in three differentcoupled programs, and the results werecompared with the empirical data fromthe model tests in terms of platformmotions and line tensions. Calibrations

could be adjusted to obtain a morereliable numerical model of the entiresystem. Some investigations have beenconducted related to riser fatigue-dam-age response, comparing the use ofcoupled and uncoupled methods.

Another issue identified in the modeltest of the P-55 was vortex-inducedmotions (VIMs) in this deep-draft semi-submersible platform. The VIMs canresult in additional oscillations in riserand mooring-line tension, as well asadditional fatigue loading of the riserTDP. These effects are under investiga-tion through towing tests and compu-tational-fluid-dynamics (CFD) calcula-tions to verify if it is necessary to usesome mitigation device on the hull.

Frequency-Domain Approach.A non-

linear random time-domain analysis hasbeen adopted in fatigue-analysis verifica-tion because model nonlinearities aremodeled properly and the environmental-loadings random behavior is considered.The disadvantage is the high computertime required. Because fatigue-damagecalculation depends on stress variationsduring the lifetime of the structure, theset of loads used in the analysis shouldbe sufficiently complete to represent allpossible situations. Because Petrobrasuses its own measured environmental

database that contains a large numberof data points, the use of time-domainanalysis may affect the design sched-ule. Another solution, besides the sta-tistical treatment procedure to reducethe database, was the development of afrequency-domain methodology, basedon linearization techniques, that wasimplemented in in-house software to beused as an alternative tool for the initialphase of riser design.

In general, results indicated agreementbetween frequency- and time-domainapproaches in identification of critical-joint and critical-loading cases. In termsof fatigue-damage calculation, frequen-cy-domain analysis when compared totime-domain analysis furnished betterresults for the lazy-wave SCR configu-ration. For the free-hanging configura-

tion, larger differences were found andthe frequency-domain approach tendsto be more conservative. Research toevaluate the soil-structure interaction,aiming at representing the TDP varia-tion that is significant in the free-hang-ing configuration is ongoing.

VIV FatigueCampos Basin Currents. Campos basincurrent profiles for deep water are com-posed of two layers coming from differ-ent sources. These layers have different

Fig. 1Fatigue-damage contribution at the TDP.

Infield and Export RisersD-Class Weld at Outer Diameter

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Production A Production B Water Injection Gas Export Oil Export A Oil Export B

Damage,

%

VIV long term VIV short term Heave-induced VIV(1-year wave) *

Heave-induced VIV

(100-year wave) *

1st- and 2nd-order damage

* Relatively low damage values


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directions. The first layer covers thedepths from sea level to approximately300 to 400 m, going predominantlyto the south and southwest directions.Below this level, another layer becomesdominant going to the north and north-east direction.

From the first VIV calculations, as afunction of the 2D characteristic of the

software, the current velocities alongthe depth were projected to the riser in-plane and out-of-plane directions. Thiswas supposed to capture the charac-teristic of directionality of the Camposbasin current profiles.

Short-Term Response.The idea of con-sidering a short-term response camefrom the necessity of predicting theriser response when facing a 100-yearcurrent event during its operational life.No extreme stresses were expected, but

it was necessary to know the magnitudeof the induced fatigue damage. Theapproach used assumes that the dam-age from the worst-possible extremeevent will be resisted by the riser. Theextreme events, however, are not verywell characterized in terms of durationand the way the phenomenon evolves.In recent applications, the short-termdamage represented a significant per-centage of the total damage, as the casefor the P-55 design for an 1800-m water-depth application for the Roncador field,shown in Fig. 1.

Alternative CFD Model.As an alterna-tive to VIV traditional-model use, theinitiative was to incorporate a CFD pro-cedure into the in-house riser-analysispackage. The discrete-vortex method hasbeen implemented and is being testedand compared to the traditional-modelresults for some real applications. Themethod brings the possibility of usingthe current profiles with their directionalcharacteristics, and results obtained so farindicate some less conservative results.

Materials

The first SCR applications developedby Petrobras were related to importand export lines. When the use of SCRsstarted to be planned for productionlines, as in the P-52 project, the problemof how to face the highly corrosive envi-ronment turned out to be a major one.The presence of carbon dioxide (CO2)and/or hydrogen sulfide (H2S) in theproduced stream created doubt aboutthe applicability of the S-N curves used.

P-18 SCR

The P-18 SCR is the only operatingSCR currently in the Campos basin. Atthe time it was installed in 1998, theriser was the first SCR to be installedon a semisubmersible unit. A completemonitoring system was installed, andthe measurement campaign lasted forapproximately 2.5 years.

Many issues were investigated, andtoday the generated database still isbeing used in studies. The confirmationof the expected riser behavior, char-acterization of platform motions, andidentification of critical current profilesfor VIV response have been some of theresults obtained.

The measured data at strain gaugesat the riser top, associated with themeasured flex-joint-angle variations,are being used in the reassessment offlex-joint fatigue life. The data, besides

including real values, include VIV-induced axial vibrations that were notincluded in the design phase of the flexjoint, and today it is the main reasonfor revisional work regarding the topconnection system.

The riser was installed without anysuppressor device for VIV. The updatedmethod regarding VIV and wave fatigueis being applied to define what will beconsidered as the riser updated-designfatigue life. The TDP is the main focusbecause it is the region that suffersdamage from platform-induced wavemotions and also from VIV.

The reassessment plan includes thegeneration of an updated engineeringcritical assessment and inspection ofwelds by two methods, one performedexternally and another by umbilicalpig. The accuracy of the field inspec-tion may not correspond to ideal val-ues; neither may have the same levelof accuracy obtained during riser con-struction, but both will give importantreference values that will support theriser-integrity evaluation. A permanent

monitoring system has been designedfor the top section of the P-18 SCR that,once field tested, will be considered asa model for other SCRs to be installedin the future.

General Comments

When planning for a platform with alarge number of SCRs, the interferencebetween the adjacent risers can becomean issue. Besides working on the dis-tance between supports and in thedifference between the azimuths of the

risers, frequently it is necessary to con-sider a difference in top angles betweenneighboring risers. As a function of this,the necessity of using top-angle valuesof 15 to 17 for some risers, in thevicinity of other risers with 20 of topangle, became the usual practice dur-ing design. For reduced-heave-motionunits, such as the deep-draft unit P-55,

the adoption of 17 or even 15 did notcause problems.

Conclusion

From the last results obtained withPetrobras design methodology andupdated data, it is impossible not toconsider the option of using VIV sup-pressors for an SCR. The questionis the relative length to be used, butwhen analyzing total length variationof strakes, their VIV efficiency, andtheir location along the SCR, the results

obtained through traditional modelingdo not present monotonic results, thusmaking it difficult to make decisionsabout the length of the strakes.

The successive evolution in the setof design currents applied for long-term and short-term response calcula-tions has caused the design to be morerobust and realistic, but up to now,always increasing the effect of VIV onthe overall riser design.

The incorporation of CFD proce-dures, which can keep the Camposbasin current profiles directionalitycharacteristic, is a promising way toobtain less conservative results. Up tonow, the design for production riserssubjected to corrosive fluids has ledto clad sections, corresponding to lessthan 20% of total riser length. Thistrend represents specific conditions ofthe Roncador field. More-severe situa-tions may occur that can force the useof longer clad sections. Applicationof clad pipes in the critical regionsrequires a better understanding of theweld behavior, geometrical imperfec-

tions, and nondestructive-test results.Monitoring new risers to be installed

is a key point that can support theevolution of design methodology forVIV and also with respect to wave-induced platform motions. The moni-toring system can be planned to becomposed of some equipment on thetop section that will operate duringthe entire riser lifetime, and anotherset of equipment that will be used fora limited duration for evaluation of thedesign methodology. JPT


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The use of icebreakers in support ofoffshore-ice operations, and specifi-cally their efficiency in support of ves-sel-shaped floating platforms in ice, isdiscussed. New-technology icebreak-ers equipped with azimuth thrustersachieve high levels of operability withvarious levels of ice management in awide range of effective ice thickness.

IntroductionThe oil industry has increased its inter-est in ice-covered waters. Operations,especially in water deeper than 100 m,use various vessels for drilling or pro-duction, with icebreakers support-ing their station keeping. The use oficebreakers enables stationary opera-tions in ice to continue with increasingdegrees of difficulty. The use of ves-sels in such offshore-ice operations issubstantially different from and moredemanding than traditional ice-transitoperations or port operations in ice-covered ports and terminals.

Offshore-ice projects, which oper-ate in moving pack ice, must ensurethat station-keeping operations con-tinue with a high degree of confidencethat the station-keeping limits of thefloating platform will not be exceeded.Rigorous risk control also is needed toensure that, if needed, all operationscan be stopped and the platform canbe removed safely from the location. Ahigh confidence required in the capa-

bility to stay on location before suchan operation is justifiable. The overallscope of ice management is more accu-rately risk management for offshore-ice operations. The following are anintegral part of the ice management.

Ice and environment intelligence Ice and environment forecasting Defining operability and safe ice

operational envelope for the stationaryvessel

Operational-risk evaluation andassessment

Alerting of operationsOnly after the above are completed

can the actual physical ice manage-ment (i.e., breaking and clearing ice)take place. Safe ice-management logicapplies to all floating platforms and toany platforms intended to be remov-able from their stationary operationallocations. This paper focuses on themost demanding version of the icemanagement, operations of ship-shapedplatforms. Vessel-shaped platforms areunique in that they offer a highly favor-able ice-interaction condition when ice

moves in the direction of the vessel.However, the challenge is that if icecomes from the side, the ice loads areso much higher that the benefit of thelow loads in one direction is totally lostand operation might not be feasible.

Azimuth-Icebreaker TechnologyThe most important development inphysical ice-management technologysince traditionally propelled icebreakersis azimuth thrusters. The wake of azi-muth thrusters can be more powerful interms of breaking ice than the hull of anicebreaker. Also, the azimuth-thruster

wake can clear the ice in a highlyefficient mannere.g., from ports, ter-minals, and the paths of escorted/sup-ported vessels or various offshore-iceplatforms (both bottom-founded andfloating). It is possible to place an ice-breaker accurately, even in moving ice,at a stationary location or to move it ina highly controlled fashion to locationswhere ice management is required atany time. Figs. 1 and 2, respectively,show the efficiency of azimuth thrusters

This article, written by Senior TechnologyEditor Dennis Denney, contains highlightsof paper OTC 19275, Ice Managementfor Ice Offshore Operations, by A.J.Keinonen,AKAC Inc., prepared for the2008 Offshore Technology Conference,Houston, 58 May. The paper has notbeen peer reviewed.

Copyright 2008 Offshore TechnologyConference. Reproduced by permission.

Management of Offshore-Ice Operations

OFFSHORE FACILITIES


Fig. 1Pacific Endeavorclearing ice with the wake of its propulsion inlow ice pressure.



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in clearing ice in low ice pressure andin clearing ice in the presence of morethan9/10(very-close-pack) ice.

Design IntegrationA highly competent ice-managementsystem can keep a platform on locationby use of a highly aggressive and activeapproach for time periods correspondingto exploration drilling in ice. However,in this case, vulnerabilities to critical iceinteractions most likely will be accept-able or even optimal to the platform.Designing for long-term low-probabilityevents typically does not make sense forexploration platforms. Accepting even adrastically increased risk of downtimemight be the best option. In the caseof production, the criteria change andit is essential that the whole systembe designed to remove significant vul-nerabilities risking frequent downtime.The biggest risk may be that real-lifeoperations might introduce unaccept-able ice interaction, which could lead tofrequent unplanned downtime.

Station Keeping in IceThere are several types of ship-shapedplatform experiences in ice.

A moored drillship with limited sta-tion-keeping capability and no possibil-ity to vane results in high ice loads whenpushing ice to the side as ice comes fromthe bow and when pushing ice down,depending on the bow angles. However,with ice coming from the side, the iceaccumulates and pushes the keel part ofthe ice accumulation down. Severe icecan get into the mooring lines when iceis either pushed down or significantlyaccumulated at the platform. This typeof platform relies on ice management to

break most of the severe ice even thoughit may be able to handle some lighterice by itself. Returning to location in ice

requires significant ice-management andanchor-handling support. A dynamically positioned construc-

tion vessel can have low station-keepingcapability and very high inherent ice-breaking resistance because of the verti-cal stem of the vessel and near-vertical-sided wedge-shaped bow. It has a verygood ice-clearing performance to thesides, and keeps ice from going underthe vessel. It has no protection againstice getting into its moonpools, but issurprisingly efficient at keeping ice awayfrom them, despite its shallow draft. Ithas excellent ice-vaning capability. Inice-pressure situations, ice is pusheddown and into the moonpools. This ves-sel relies totally on ice management tobreak the ice into small enough piecesto enable station keeping. In ice pres-sure, the vessel is able to stay on locationonly if the pressure and resulting icecompacting can be cleared to ensure thatno ice enters the moonpools. Returningto location is easy because it is self-pro-pelled. However, getting out of the severeice that has stopped operations requires

significant icebreaker-escort support, ifit is not possible to let the vessel simplydrift with the general ice drift.

A manually positioned icebreakerwith relatively low station-keeping capa-bility has a dynamic-positioning systemthat has proved to be unworkable insevere ice. It has relatively low inherentice loads from the bow and has limitedvaning capability because of the longparallel sides of the vessel when usingonly its main thrusters. The medium-icebreaking bow partially pushes ice

down and to the sides. A custom protec-tion skirt was built into the moonpool ofthe Vidar Vikingto prevent ice from get-ting into it. The Vidar Vikinghad to relyheavily on ice management and a largeoperational radius available of 50 m, tobe able to vane into ice drift. Also, it hadto rely on ice management to keep icefloes small enough to enable staying on

location without exceeding allowableradius of offset. Returning to location inice was relatively easy.

A floating storage and offloading(FSO) vessel connected to a single-anchor-leg-mooring (SALM) buoyhas highly limited ice tolerance andmedium ice-load capability with van-ing capability but no ice-drift-reversalcapability and has inherently very highice loads on the basis of the shape ofthe FSO. There was limited protec-tion against ice interfering with the oil

hose exiting the SALM. Deep access ofthe oil hose to the FSO provided goodprotection from ice. The system reliestotally on ice management in the pres-ence of any ice to ensure small ice-floesize, giving low ice loads, and ensureprevention of ice interfering with theoil hose from the SALM. Returning tolocation and all aspects of operation inthe presence of ice require continuousand significant ice-management sup-port, including ice clearing.

A four-point-moored icebreakerwith highly limited vaning capabilityhas floating stern lines, low station-keeping capability, and inherentlymedium-to-high ice loads. There isno protection against ice getting intothe moonpool, and ice gets into iteasily. The operation had to acceptice-drift reversals and relied heavily onice management. Returning to locationrequired competent ice-managementsupport including ice clearing.

The main lesson from these experi-ences was that it is a high priority todesign a system specifically for each

geographic region and its custom appli-cation. This process must take intoaccount the following key factors.

Ice load in the unidirectional icedrift

Ability to respond to rapid changesin the direction of ice drift

Ability to respond fast enough toice-drift loops, quick turns, and driftreversals

Confidence in full ice clearing, notallowing ice to get into the risers, moor-ings, and other structures

Fig. 2Pacific Endeavorclearing ice with the wake of its propulsion in9/10original ice cover.


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Returning to location in the pres-ence of ice

Training operators

Ice Clearing and DismantlingThe main innovations of the use of azi-muth thrusters on icebreakers in termsof ice-management work are as follows.

Ice can be broken by the wake of

thrusters, which can be even more effi-cient than breaking ice with the hull ofthe vessel.

Ice can be cleared in a highly effec-tive manner by use of the wake of azi-muth thrusters.

The icebreaker can remain stationaryin moving ice while managing ice or canmove in any direction while doing so.

The wake of the thrusters can dis-mantle large first-year ridges by blow-ing away their keels, causing collapsefrom lack of buoyancy.

The ability of the thrusters to breakice was tested on several occasions,and a model was developed. The azi-muth-thruster icebreaker can preparea wide channel in unbroken level iceand a significantly wider channel in

an area where ice has been prebrokenby a primary-ice-management vessel.Because azimuth thrusters can clearice efficiently with the wake of theirpropulsion, they offer powerful oppor-tunities for managing and clearing icefrom any location. For offshore-iceapplications, this ability is a majorimprovement in terms of managing

the exact ice that needs to be managed.Orienting two azimuth thrusters to actopposite to one another will keep thevessel stationary even in moving ice.Orienting them slightly forward or aft,will move the vessel slowly longitudi-nally; and by use of well-known tech-niques of operating a twin-azimuth-thruster-equipped vessel, it also can bemoved sideways.

Integration of IceManagement Into Design

A powerful ice-management systemcan keep a ship-shaped platform, or aconical one, on location under surpris-ingly high ice forces, even with theuse of traditional icebreakers. Addingthe major improvements available to

the ice-management systems, it couldbe claimed that almost any platformcould be kept on location in movingice, provided a sufficiently powerfulice-management system is used. Eventhough this is not advisable, it showsthat there should not be a specific needto develop very high station-keepingcapability nor should the platform need

to be designed to handle a major quan-tity of ice independently. It may bemore advantageous to have the plat-form not break ice very efficiently;possibly, clearing the ice to the sides ismore important in certain situations.

There clearly is no single optimalsolution to a general ship-shaped plat-form operating in ice or to an ice-management system to support such aplatform. However, the choices avail-able for a designer are from the wholeoperational philosophy and approach to

handling risk and include the support-ing icebreaker and azimuth technology.So far, each project has been a pioneer-ing one, with much work remaininguntil some level of routine developmentand solutions is established. JPT


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Lessons learned from use of a floating-production unit (FPU), made of pre-stressed high-performance concrete, inoperation for Total E&P Congo are pre-sented. The unit has been in use for 12years on the NKossa oil field in 170-mwater depth. The focus is on structural-modeling techniques, aging process-es, and development of an inspection

program. The paper is not intendedto make a recommendation betweensteel and concrete, which would entailmany other considerations.

IntroductionThe FPU NKP is 220 m long, 46 mwide, and 16 m high, with a displace-ment of 107 000 tonnes. It contains27 000 m3 of concrete, 2350 tonnesof prestressed steel, and 5000 tonnesof passive steel. During its 12 years ofoperation, the FPU has undergone onetechnical stop for process maintenance,as scheduled in the design; otherwise,it has been in uninterrupted service.The unit, shown in Fig. 1, was builtin southern France in 199495 andinstalled 1 year later in the NKossa oilfield in 170-m water depth, 60 km offthe Congo coast.

The production facilities and livingquarters for 160 people are fitted on the10 000-m2 deck, which for construc-tion purposes was subdivided into sixmodules: accommodation and centralcontrol, utilities, electric-power genera-tion, gas compression for re-injection,crude oil, and gas. Design production is16 000 tonne/d of oil and 1300 tonne/dof liquefied petroleum gas. The unit isheld in place, 70 m away from the NKF2platform, with a spread-moored configu-

ration by means of 12 mooring lines.

Asset-Integrity ManagementA special method was developed toanalyze and monitor the condition ofthe units. The aim of the floating-unitsintegrity-management process is toensure management and continuousfollow-up of floating units from safety,environmental, operational, mainte-nance, and quality-management view-points. It includes recommendations on

inspection, maintenance, and repairs.This process calls for the following.

Structural and anchoring modelingand analysis

Qualitative [risk-based inspection(RBI)]

Yearly reviews of the inspection,repair, and maintenance (IRM) plan

Data management and storage(including reports)

Assistance for emergency response Framework for analysis

The program is divided into fourcomplementary, interacting modules,as shown on Fig. 2.

Structural nonlinear finite-element-analysis (FEA) model and dynamic-mooring model

IRM plan and schedule incorporat-ing class requirements (e.g., renewal ofcertificates and repairs), and incorpo-rating the risk-based inspection

Database (e.g., plans, results ofmodels, inspection reports, and class

This article, written by Senior TechnologyEditor Dennis Denney, contains highlightsof paper IPTC 12546, Lessons LearnedFrom 12 Years of Operations of a HugeFloating-Production Unit Made of

Prestressed High-Performance Concrete,by Bertrand Lanquetin, HeidiDendani, and Pascal Collet, Total,and Jose Esteve, Bureau Veritas, pre-pared for the 2008 InternationalPetroleum Technology Conference, KualaLumpur, 35 December. The paper hasnot been peer reviewed.

Copyright 2008 International PetroleumTechnology Conference. Reproduced bypermission.

Lessons Learned From 12 Years of Operationsof a Prestressed High-Performance-Concrete

Floating Production Unit

OFFSHORE FACILITIES


Fig. 1NKPFPU.



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status), with information shared on anetwork system

Emergency-response serviceIn alignment with the integrity-man-

agement program, the unit was placedwithin a classification scope. Surveysand maintenance actions required bythe Classification Society are intro-duced and accounted for within thesystem. Falling within the classificationscope of work are the mooring, hulland marine systems, accommodationquarters, and the helideck structuresand topside connection to deck. Insome cases, risers and subsea equip-ment also are included.

Prestressed-Concrete FPU

The unit was constructed with 26 lat-eral compartments (B and T compart-

ments) and 13 central compartments(C compartments). The lateral com-partments can be used as ballast tanks,but only the four tanks in the cornersare used on site to maintain trim andpitch. The central compartments arevoid spaces. Running through the cen-tral void spaces as a spine is the tech-nical gallery that connects the aft andfore pump rooms and provides accessto the internal compartments.

The nose on the fore end is a con-crete cantilever that supports the flaretower, as far away as possible from theaccommodation quarters.

The shell is not a fully watertightcontinuous skin. It is pierced in severallocations for water intake for process-plant cooling, ballast, fire-extinctionmeans, freshwater production, and

other uses. The most important comesin through Space T9 with a pipe(metallic outside and concrete inside)that penetrates the side shell and runsthrough two compartments beforereaching the piscine, an enclosedbasin within Compartment C10. Threepumps (two in service, one spare) thenmove the water from the piscine to the

process plant for cooling purposes.Fig. 3 shows that the hulls self-

supporting structure is made of lon-gitudinal and transversal walls (calledbulkheads) that also provide the inter-nal subdivision. They are made of rein-forced concrete through which longitu-dinal, vertical, and transversal tendons(prestressed cables) extend, providingcompression in the shell-plane direc-tions. Depending on location, wallthickness varies from 40 to 80 cm.

Concrete Prestressing. Concrete-structure prestressing consists of apply-ing a compressive load to enhancestrength. The main aim is to maintainthe concrete in compression under theforecasted external loads. Here, ten-dons made of several ultrahigh-tensile-steel strands going through metallicconduits set within the concrete sectionwere used. The path of these ductswas defined carefully at the designstage to solve practical constraints (e.g.,access holes and equipment founda-tions) while maintaining the requiredcompressive load. To protect the steeltendons and provide a solid-concretesection, the conduits are filled withinjection grout.

To ensure appropriate load distribu-tion and shear capacity, passive rein-forcement steel is embedded in theconcrete. These bars are the same asthose typically used in concrete beamsin any building, as shown in Fig. 4, aphoto taken during construction of theNKPbarge.

FEA ModelA numerical model was built to assessactual and future conditions. Thedynamic loads (motions and sea pres-sures) were assessed with 3D diffraction-radiation-analysis software taking intoaccount the latest metocean data fromthe site. An interface was developed totransfer the hydrodynamic loads directlyto the structural model. The model wasbuilt with the following constraints.

Reinforced concrete was consid-ered to be a homogeneous material

Fig. 2Floating-unit integrity-management modules.

FEA,anchoringmodels and

management

tool

RBI analysis

and IRM cycle

Datamanagement

and reporting

Emergencyresponse

= interaction

Periodical(re)assessment

Fig. 3Compartment plan and starboard profile.


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with yield-capacity values according totests performed at construction time.

Prestressed cables were modeledexplicitly. The modeled cable tensionaccounted for the variation of the pre-stressing load along each cable causedby friction and losses caused by anchor-age penetration.

Concrete-creep and -shrinkageproperties were considered bias coef-ficients applied directly on the tendonloads.

Topside loads were introduced asconcentrated masses placed at eachtopside-module center of gravity andconnected to its supporting stools atthe deck through rigid connections.

Operational ExperienceInspection and Maintenance Plan.On the basis of drawings review, surveyreports, and FEA results, an inspectioncampaign was set up. It includes theclassification requirements and addi-tional tasks to maximize the unit effi-ciency. The main objectives are

Identify any defect and its deterio-ration process.

Chemical attack Corrosion Crack Coating deterioration Accident Define the severity of damage Provide recommendations for

repair Provide an image of the condition

of the unit to be compared in futurecampaigns

Means of Survey. Depending on loca-tion and inspection time, differentmeans of survey are to be used.

Global-visual inspection (GVI), oroverall survey. Intended to report onthe overall condition of the hull struc-ture and determine the extent of addi-tional close-up surveys.

Close-visual inspection (CVI), orclose-up survey. Details of structuralcomponents are inspected at close visu-al range (i.e., normally within reach ofhand).

Nondestructive testing (NDT). Aclose inspection made by electrical,electrochemical, or other methods todetect hidden damage.

Sample taking. In some cases, theNDT for concrete provides only pro-vides information only of the sur-face (less than 20mm), and if doubtsexist concerning the actual level ofchlorides penetration or carbonationdepth, samples may need to be taken.Adequate filling of the space left behindis necessary.

In-water survey. Survey is car-ried out underwater by divers and/orremotely operated vehicle. It usually isfor cleaning marine growth.

Inspection Program. The inspectionprogram was defined by dividing theasset into different zones.

The submerged zone is everythingbelow the water surface at the servicedraft.

The splash zone is the area submit-ted to intermittent wetting by waves.

The atmospheric zone comprisesstructure and equipment on and abovethe upper deck.

The internal zone includes all struc-ture, spaces, and reservoirs beneath theupper deck.

Each zone is divided into subar-eas, each of which envelopes structureand/or equipment with similar inspec-

tion scopes. The different surveys weredefined to detect typical concrete-deg-radation processes.

Effect of seawater on cements (i.e.,sulfate and chloride)

Lime leaching/carbonation Alkali/aggregate reaction Reduction in cement content and

strength Increase of permeability (permit-

ting chloride ingress) FatigueAs expected with the use of high-

quality concrete, only a few defectswere found.

Concrete Damage From SteelCorrosion. Even without mechani-cal degradation, there are two majorsituations in which corrosion of rein-forcing steel can occur: carbonationand chloride ingress. With either, theremoval of the protective passive filmleads to the galvanic corrosion. Whenthis occurs, the produced rust requiresmore space than the original steel,straining the surrounding concrete.Because concrete is relatively weak intension, cracks develop, exposing thesteel to even more chlorides, oxygen,and moisture, and the corrosion pro-cess accelerates.

Corrosion-protection systems wereset in place during construction.Cathodic protection (CP) is a tech-nique to control the corrosion of (rein-forcing) steel by making the steel thecathode of an electrochemical cell. CPis the reduction or elimination of corro-sion by making the metal a cathode by

connecting it to a sacrificial or galvanicanode, or by use of an impressed directcurrent. Cathodic areas in an electro-chemical cell do not corrode. If all theanode sites are forced to function ascurrent-receiving cathodes, then theentire metallic structure would be acathode and corrosion would be elimi-nated. Electrical continuity of all pas-sive steels and other structural metallicparts is necessary.

The 183 anodes (on internal shell inthe four water-ballast compartments)

Fig. 4NKPunder construction.


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JPT FEBRUARY 2009

provide a steel-reference electrodepotential of approximately 850 mV toprotect the reinforcing steel. Exposedsteel structures are painted. It is neces-sary to clean exposed steel and repaintit to avoid loss of steel and preventconcrete damage.

FEA Model vs. Real Life. The FEA-

model assessment has verified thatthe design keeps the overall struc-ture in compression. However, thestructural analysis highlighted somelocalized areas where lack of compres-sion could be found. These localizedareas coincide with the findings of aninspection showing superficial defects,mostly the result of the practical dif-ficulty of concrete reinforcement inthese areas. Examples are degradationof deck edges and flare-tower sup-port ends, the latter having prestressed

cables ending on them. These items areeasy to repair and do not need a high-tech qualification to restore them to as-built condition. Flare-tower-cantileversurface cracks were explained by the

FEA model showing that the designwas optimized for the site conditions.Particular surveys have been definedfor this member as a result of thenumerical calculations.

Conclusions

Thus far, CP is actively protecting rein-forcement steel. The high-quality con-

crete is providing adequate protectionagainst carbonation and chloride pen-etration and assuring satisfactory aging.

After 10 years, concrete-hull main-tenance and repairs consisted basicallyof restoring concrete-surface cover lostfrom abrasion and impacts. The designenhanced compartment inspectability.Spaces are open without intermedi-ate members blocking the view GVIis easily conducted with several fixedillumination sources, although addi-tional means of access for CVI of the

upper parts are necessary. The mainbackground danger will always besteel corrosion.

A better comprehension of thestructure-aging process could have

been obtained if samples had beenprepared during construction andleft on board (at ambient conditions)for later strength- and mechanical-properties tests. Similarly prestressedsamples for fatigue-capacity reas-sessment should have been preparedbefore construction, rather than bas-ing the base-design values on litera-

ture. Construction-quality control isrequired to ensure durability.

The definition of the floating-units-integrity-management program forNKP allowed changing from a pas-sive- and corrective-action frame to aproactive scheme. Main nonacciden-tal-degradation processes and most-exposed locations have been identified,enabling establishing an inspectionprogram particular to NKP. Creatingthe FEA model representing as closelyas possible the as-built and site condi-

tions, helped in understanding thesurvey outcome. In case of an accident,the FEA model could be used to evalu-ate the condition of the unit and helpmake the right decisions. JPT

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