The SSP: A New Class of Hull for the Oil Industry

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The SSP: A New Class of Hull for the Oil Industry

The SSP: A New Class of Hull for the Oil IndustryKåre Syvertsen, MSc / Sevan Marine AS, Clovis Lopes, Ph.D. / Sevan Marine do BrasilThe cost of producing petroleum through offshorefacilities increases sharply as the water depthsincreases. In recent years, to ensure profitability indeep-water and ultra-deepwater fields, the size ofprocessing plants and the number of risers haveincreased. The result is a number of large floatingunits capable of taking on big loads and offeringlarge deck areas and storage capacities.The increased size of floating units solves someproblems but compounds others. Besides raisingcosts, due to the increase in steel weight andequipment, the big platforms bring aboutcomplexity in a new scale, demandingdevelopment of new technologies. These changes,in turn, add to the risks. The larger size also limitsthe number of available shipyards capable ofdelivering such a hull on time and on budget.In a broad economic sense, with currenttechnology and in waters deeper than 1,000 m, oilfields must have reserves greater than 300 MMbbl,of a crude lighter than ~20° API, to warrantdevelopment.The first challenge, then, is how to exploitprofitably the smaller accumulations located inremote deep water. The second challenge is theheavy crude case. Even when heavy oil isdiscovered in sizeable quantities, flow assuranceis made difficult by the high crude viscosity andlow water temperatures.Sevan Marine is proposing a new type of floatingsupport able to offer greater deck areas and loadcapacities along with storage and low motioncharacteristics for lower cost. The SSP – SevanStabilized Platform – concept relies heavily ontwo features for cost reduction. The features areomni-directionality to weather forces and amodular design, thanks to the concept's intrinsicsymmetries.SSPThe mono-hull concept for offshore oil production,typified by the FPSO unit, is considered a lowercostoption for large, deep-water projects. Thisdesign is compared to other concepts, such as thesemisubmer-sibles, TLPs, and spars. FPSOs havebeen in use throughout the world and are a wellestablished technology. With the development ofturret and swivel systems, FPSOs have provedreliable in harsher environments. In more benignenvironments, the preference is for moreconventional spread-mooring FPSOs. Thesevessels are less expensive because they do notrequire turret or swivel. Among the FPSO'sadvantages are large storage capacity and highdeck load. The downside is that FPSOs aresensitive to wave direction even in moderate seas.The SSP concept is basically a mono-hull withcircular shape, depending on the same stabilityprinciples. Unlike the spar design, the SSP hull isThe Sevan Stabilized Platform concept is basically amono-hull with circular shape, depending on the samestability principles.wider than it is tall, because the operational draftis typically less than a third of the diameter. Thecircular shape dispenses the unit of any headingchanges, because the seas face the same vesselhydrodynamic resistance, whichever directionthey are coming. Consequently, the SSP does notneed turrets or swivels. This saves on fabricationand maintenance.The characteristic dimension for the SSP is thediameter. This makes it a highly modular design,with the diameter determining the size of theplatform. Sevan Marine has developed designs forthree SSP sizes based on the required oil-storagecapacities. From these, an intermediate size couldbe achieved. The SSP stability principles are thesame as for a ship-shaped vessel. The large waterplane area provides high stability, resulting in alarge deck load capacity. The available deck areais large and can be increased with a cantilevereddeck. This allows all activities to be conductedabove deck, with cargo, ballast, and liquidsupplies placed inside the hull for imp-rovedoperational safety. The internal tank layout places1


the ballast tanks at the SSP periphery, giving her adouble-hull configuration.The SSP symmetrical shape allows access to alltanks, ballast, and cargo from a centralcompartment. Even with the possible adoption ofa moon-pool, access can be made through ringmain lines for water and oil. No piping is neededinside any of the tanks, which greatly simplifiesthe engineering design, construction, andoperation. It is estimated that only around 30% ofpiping normally used in a conventional FPSO willbe needed in the SSP. In addition, the amount ofcabling will be reduced due to the compact design.The SSP double hull is designed to maximizesafety, both operational and environmental. Thecargo tanks are protected by the ballast tanks toprevent leaks to the sea in the event of a collision.Further, the double hull has an importantstructural role, adding stiffness in combinationwith the upper deck and the central shaft, to whichit is linked via vertical bulkheads and horizontalstiffener elements.The overall result is a lighter steel weight thatreduces construction costs. The cylindrical shapeprovides for much lower flexing moments beingimposed on the structure, which reduces saggingand hogging, and makes for low fatigue stresslevels. At the base of the SSP lies a bilge box thatprovides damping to the vertical and angularmotions of the vessel. The bilge box is responsiblefor enlarging the vessel added mass and increasingthe damping.Motion BehaviourThe SSP hydrodynamic behavior has beenextensively tested in the Marintek ocean basin, inTrondheim, Norway, where the SSP's excellentbehavior for both angular and vertical motions hasbeing verified. The tests simulated sea conditionsin the North Sea and in Brazil, corresponding toextreme weather loadings and multidirectionalseas. Both scenarios were investigated up to their100-year storm conditions.The SSP's response in pitch/roll motions wasexceedingly small across the board. Angularmotions are especially relevant for the operationalefficiency of a process plant, which would need tobe shut-down in cases of excessive roll angles.Large roll motions are responsible for fatigueinducedcracks in structures like long flare boomsand large turret bearings. Roll motions increasevertical riser motions, when risers are positionedaway from the geometrical center of the unit. Sucha riser arrangement is common onsemisubmersibles and spread-moored FPSOs.In turret-moored FPSOs, high angular motions caninduce high stress levels at the riser hangingpoints. The SSP heave response surpasses that ofsemisubmersibles a good range of the seaspectrum. Heave response is a significantconsideration for steel catenary riser fatigue.SCRs are primarily sensitive to high frequencyheave motions. These motions are found around11.5-sec period. Model tests demonstrated that,with a suitable bilge box design, the SSP motionsare well within the motion requirements to preventSCR fatigue. The low motion behavior shown bythe SSP not only increases the plant's efficiencyand riser life, but also increases the overalloperational safety levels.The Sevan Stabilized Platform is a new type offloating support, able to offer greater deck areas andload capacities, along with storage and low motioncharacteristics, for lower cost, the company says..OffloadingOffloading operations to shuttle tankers will bedone by using two stations, positioned 180° fromeach other, and away from the accommodations.These are designed as reel type stations, with asmall hawser winch at their side. Tests wereconducted at Marintek with a conventional tankersize model, without DP. The results indicated thatthe tanker yaw motions were in general moderateor small, since the SSP surge and sway motionsare much smaller than those for a ship-shapedvessel. This reduces the interaction between thetwo vessels during offloading operations,increasing the operational safety.The high degree of hull symmetry in the SSPconcept favors block modularity. The hull is madeup of eight types of construction modules. Withthis design, construction will benefit from alearning curve effect, where the efficiency inblock construction will increase as the workprogresses. This fact, coupled with the absence ofpiping and cabling inside the tanks, simplifies the2


detailed design work, and lowers overallconstruction costs.The SSP concept has a wide range of applications,from large units in deepwater, down to theproduction of small fields in shallow areas. TheSSP concept offers a flexible solution that may betailored for a large number of applications. TheSSP may be used as a full-fledged ultra-deepwaterFPSO. With a large number of risers, it may beused as a shallow water storage unit, for example,in combination with a fixed production platformor as a DP unit, designed for test productionand/or early production. When operating as a DPunit, the SSP can use one or more drill-pipe risers,since it does not need to turn. A derrick for riserhandling and well intervention, process plant, andstorage facilities would be available for this typeof operation. Due to the ease of construction, theSSP can be fabricated in a wide number ofshipyards.With the industry facing the ever-mounting costsfor deepwater developments, the size ofeconomically producible fields continues to grow.A less expensive design is needed to access thescores of smaller accumulations that represent idleinvestment and lost opportunities. The SSP isbased on proven design principles andconventional steel fabrication. This conceptincreases efficiency across the board, fromconstruction to operations, including safety andmaintenance.3. “Effects of Fully Coupled and Quasi-StaticSemi-Submersible Vessel Motions on SteelCatenary Riser’s Wave Loading Fatigue” by W.Ye, J. Shanks, J. Fang. Proceedings of 2003Offshore Technology Conference. OTC 15105.4. “Technical Challenges for Offshore Heavy OilField Developments” by C.D. Wehunt, N.E.Burke, S.G. Noonan. Proceedings of 2003Offshore Technology Conference. OTC 15281.5. “Can Heavy Oil and Deepwater be Mixed?” byJ. Haney. Proceedings of 2003 OffshoreTechnology Conference. OTC 15282.AuthorsKåre SyvertsenMSc in ship construction from NTNU inTrondheim in 1973; Professor in marinetechnology at NTNU, Trondheim from 1976 to1990; currently vice-president of Technology forSevan Marine AS.Clovis LopesPhD. in Petroleum Engineering by LSU(Louisiana, USA) 1997; MSc in PetroleumEngineering by Unicamp (Campinas, Brazil)1990; currently New Business Developmentmanager for Sevan Marine do Brasil.References1. “Brazil’s Evolving Deepwater Risk RewardProfile” by S.P. Thurston, and T.R. Bard.Proceedings of 2003 Offshore TechnologyConference. OTC 15052.2. “Creating a Sustainable E&P Business inBrazil” by J. Haney and M. Koot. Proceedings of2003 Offshore Technology Conference. OTC15054.3

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