Mooring system design considerations for FPSOs

This paper identifies the critical issues related to mooring systems designed for ship-shaped vessels, such as the Floating Production, Storage and Offloading units (FPSOs), in general and under the effects of Revolving Tropical Storms (RTS) in particular, FPSOs have been employed worldwide as an economic solution for the development of offshore oil and gas. Most current applications are in relatively benign environmental areas, such as Southeast Asia, West Africa and Offshore Brazil near the Equator, Some applications are in temperate latitudes, e.g. the North Sea, in which the design events are winter storms. Winter storms have a limiting wind speed of around 100-mph and are more predictable. In contrast, RTSs, i.e. Hurricanes in the Gulf of Mexico (GOM) and Typhoons in the South China Sea (SCS), can evolve rapidly from Category 1 with sustained wind speeds of just over 74 mph to Category 5 with wind speeds over 155 mph. The much more unpredictable nature of tropical cyclones and the consequences of mooring failure causing possible property loss and environmental damage have prompted the concern of regulatory bodies for utilizing FPSOs in the GOM. This paper addresses design options for FPSO mooring systems. Firstly, the design environmental conditions of winds, currents and waves encountered in GOM and SCS are discussed. Secondly, mooring design issues and options are outlined. Finally, advantages and possible drawbacks of proposed options are highlighted. It is hoped that suggestions in the paper will assist the designer in enhancing the reliability of FPSO mooring systems.