Problems and issues arising in the design and operation of mono-hull FPSOs have been apparent for many years. This has resulted in shortcomings of current design philosophy being highlighted. Environmental downtime of 10 to 14 days per year is not uncommon in both new-built and converted FPSOs. The financial loss resulting from this downtime during the life of a field, is a significant factor in the overall field economics. The importance of selecting the principal dimensions of the hull to optimise the natural periods of the FPSO is discussed in this paper. Converted FPSOs are therefore at a disadvantage as the natural periods are already set by the principal dimensions and mass properties, which may not suit the environmental conditions at site, potentially resulting in high downtime. However, it is also noted that preliminary design of new built-FPSOs has also tended to neglect this aspect of design and opportunities to optimise motion performance have often been lost by following standard ship building practices in selecting initial principal hull dimensions. This paper explores a new rational approach to the design of the FPSOs, relative to the field environmental conditions, particularly for deep water locations. The main objective is to achieve minimum downtime to avoid financial penalties caused by the associated loss of production. Preliminary hull sizing software for optimising the hull principal dimensions, relative to the mooring and riser requirements and the field environmental conditions for a given storage capacity and topside load is presented. Benefits for the riser and mooring systems and for hull material and fabrication costs are also outlined.
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