Design of a Non-Ship-Shaped FPSO for Sakhalin-V Deepwater
Authors: Srinivasan Nagan , Singh Sanjay , Chakrabarti Subrata , Sundaravadivelu R. , Kanotra Rahul 
Published: October 2008

The purpose of the paper is to propose a vessel design that could work in clear water open-sea and in ice-covered arctic environments with least ice-management requirement. With that aim, a state of the art non-ship-shaped circular FPSO design concept is described here that provides the capability for round-the-year drilling and production operations within Arctic Frontier Regions. The vessel is purposely sized with storage capacity to make it massive and to provide an inherent inertia and momentum to break ice effortless and to provide an economic solution to the industry.

The proposed arctic-class floating vessel has several advantages over fixed arctic-structures and over the existing floating designs proposed for arctic applications. The proposed vessel is independent of water depth in its operation, functions well both in ice-infested and clear water, is compliant to ice/ridges loads, and has ice-breaking capability by vessel induced swing. The vessel thus has both active and passive icebreaking mechanism built into the design concept.

The key vessel design challenges for the arctic applications are considered in developing the concept including ice-structure interaction, stability, storage, constant draft, motion, moon pool resonance, moorings and station keeping, flexible riser systems, disconnectable turret system, offloading operation, fabrication, transportation, and installation. The vessel hull with most of its topside load is designed such that it could be fabricated elsewhere and dry-transported on transportation vessel. During its operation, the oil/water ballast storage may be adjusted to maintain the same draft.

The capability to withstand and break the ice and ice ridge forces are built into the design more efficiently. This arctic class vessel is designed with a dis/re-connectable turret to support flexible risers and vessel moorings. Thus, dual mooring system is utilized. In the case of the very harsh environment beyond control, say with the threat of ice-bergs, the vessel could be disconnected & moved away, if needed, effortless with the help of the turret buoy system. The re-attachment of the turret is feasible. Overall, the proposed floating vessel is considered suitable for water depths ranging from shallow to deep water, suitable for harsh environment with/without ice. This paper presents the design of a non-ship-shaped FPSO adapted to deep arctic fields. For this study, the condition of the Russian Barents Sea and Sakhalin-V are selected. The vessel is designed to resist the first and second year sea ice and the large and deep ridges. The predominant and most innovative features of the vessel design are presented. Wave tank model test with 1:45 scale model was carried out to ensure the hydrodynamic properties of the vessel and its sea-keeping performance in waves for severe summer condition in the arctic sea. Innovative damping system is designed to control the near resonant response of the heave and the roll behavior of the vessel. Conical shaped moon-pool used is first of the kind in the technology to increase the heave natural period without spoiling the stability of the vessel.