The BP operated Greater Plutonio field development offshore Angola comprises a spread-moored FPSO in 1,300 m water depth, serving as a hub processing the fluids produced from or injected into the subsea wells. The selected riser system is a riser tower tensioned by a steel buoyancy tank at its top end and distributed foam buoyancy along a central structural tubular. The riser bundle is asymmetric in cross-section and this paper presents the work performed to determine the specific hydrodynamic characteristics of the design. Both basin tests and CFD analysis results are presented with discussion on some specific hydrodynamic issues: vortex-induced vibration (VIV) of the global riser tower system, VIV of individual risers, and the dynamic stability of the global system (i.e. galloping). Finally, guidelines for the assessment of the hydrodynamic behaviour of such system geometries are proposed. The results of this paper demonstrate that the Greater Plutonio riser bundle represents an effective solution in term of hydrodynamic behaviour and is not sensitive to VIV fatigue or galloping.
The Greater Plutonio Riser Tower is installed in Angola in a Water depth of 1310 meters (4300 feet), and gathers 11 risers (two production loops, three water injection, one gas injection and three gas lift lines) and one gas lift umbilical. The total mass of this unique riser system is more than 4000 tons. This paper aims at describing the main challenges associated to the Riser Tower design. After a description of the riser tower main elements (flexible jumpers, buoyancy tank, guiding frames welded to the core pipe, gas lift manifolds...), numerical analyses are presented (in place extreme and fatigue analyses). Assessment of the friction developed by the risers against the guiding frames and associated effects on the global behaviour of the riser tower are detailed. Various sources of conservatism, lessons learned and ways of improvements are also shared for future projects.
The process used to select a riser concept for a deepwater Floating Storage and Offloading system for the Gulf of Mexico is presented. Numerous riser concepts were screened with three taken forward through the more rigorous concept selection process. The three riser configurations were the Steel Lazy Wave Riser, the Single Line Hybrid Riser and the Tension Leg Riser. A system approach was adopted where the turret location, the mooring system and the risers were designed together. The process involved rigorous evaluation to verify both technical and installation feasibility, along with engineering definition sufficient to establish cost estimates. Analysis results for the three risers are presented to demonstrate important design issues that must be investigated to make an informed riser selection. Introduction The recent increase in discoveries in deep and ultra-deep water, coupled with fast-paced development schedules for Floating Production Systems, has led to a rapid evolution in the design of risers in terms of their complexity as well as variety. In addition, ship-shaped systems, including both Floating Production, Storage and Offloading systems (FPSOs) and Floating Storage and Offloading systems (FSOs), are often the preferred option for various technical and commercial reasons. Equipped with turrets, they can be highly functional solutions even in harsh environments. However, the motions of ship-shaped systems are typically more severe than the motions of other types of floating facilities due to their heave, roll and pitch motions being in the same frequency range as the wave energy. This further complicates the design of the risers. Consequently, riser systems that may be feasible on a Tension Leg Platform, Spar or semi-submersible-based facility may not always work on an F(P)SO. Particularly in harsher environments, this requires the F(P)SO riser system to have either a more compliant or a de-coupled configuration compared to simple catenary or toptensioned vertical risers. A study was performed to assess the technical feasibility and commercial viability of several riser options for a 2- million barrel new-build FSO studied for possible deployment in approximately 1,370 meters water depth in the Gulf of Mexico. Steel Lazy Wave Riser (SLWR), Single Line Hybrid Riser (SLHR) and Tension Leg Riser (TLR) concepts were investigated in detail. To establish technical feasibility, an integrated system approach was adopted where the FSO hull form, turret location, mooring system and riser system were investigated jointly to capture the important interactions between key components. To assess technical feasibility of the risers, extreme hurricane conditions were investigated to check allowable stresses and compression in the riser and to determine top termination requirements in terms of tension and rotation. Fatigue analyses were also performed. Both wave frequency fatigue and slow drift fatigue were examined along with fatigue due to Vortex Induced Vibration (VIV). The technical assessment confirmed that the three riser concepts are feasible for the conditions specified in the study. Note, however, that the SLWR was close to the fatigue limit while the SLHR and TLR, which de-couple their steel riser segments from the motions of the vessel, exhibited good fatigue performance.
The paper presents a general overview of the Greater Plutonio Hybrid Riser Tower. It covers the various phases that led to the realization of this unique system: conception, procurement, fabrication, offshore installation and start up. This Riser Tower is now installed and operating in Angola in a water depth of 1310m (4300 feet). It gathers 11 risers and one umbilical. It is the largest in diameter and weight installed in the world so far. After concept selection and a detailed design phase, all elements were procured from numerous suppliers and assembled on the Lobito yard in Angola. The tower was then towed in a submerged configuration and upended on the Greater Plutonio site. It was then anchored in position, connected to the flowlines and the FPSO before being tested and used for production. Some lessons learned for future projects by the Oil Company and the Contractor with particular emphasis on the possibility of extending the use of this concept in ultra deepwater are presented. Introduction The Hybrid Riser Tower (HRT) is one of the riser system concepts suitable for deep and ultra deep water field developments. This concept has been used previously on the Garden Banks, Girassol and Rosa Lirio fields. The Greater Plutonio HRT is believed to be the largest installed in the world today. It allows conveying all production and injection fluids of the five BP operated fields on the Greater Plutonio site which are: Galio, Cromio, Paladio, Cobalto and Plutonio. In comparison, the oil production passing through this tower is about the same as the one passing through the three towers of the Girassol field. The Greater Plutonio HRT was part of a turnkey contract executed by Acergy to provide Umbilicals, Risers and Flowlines to BP and its partners to allow production, injection, mooring, offloading and control functions between the Subsea Production Systems and a spread moored FPSO. The contract was awarded in February 2004 and the first oil production occurred in October 2007. The present paper provides an overview of the main phases leading to the delivery of the Riser Tower package within the project and some associated Lessons Learned. Description of the tower The riser configuration selected for the Greater Plutonio Field Development consists of one single Riser Tower, with a buoyancy tank. The overall configuration is shown hereunder. A riser tower is a bundle of several risers, anchored to the seabed and tensioned by means of buoyancy. It is connected to the FPSO by means of flexible jumpers and to the seabed flowlines termination assemblies by means of spools. These spools are designed to accommodate tower inclination (mainly due to the FPSO excursions), as well as expansion of both risers and flowlines.
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