Offshore field development requires significant investments: drilling of the wells, flow lines, platform and associated topsides, installation, and commissioning. It is well understood that the choices made for the field development during the initial project stages and through the project life are largely driven by economic considerations. In order to successfully identify the most interesting concepts during the early stages of a project, it is necessary to understand all the implications of each concept studied. One of the concepts often considered for development of offshore fields is the turret-moored ship shape FPU (FPSO/FPU, FLNG). Generally the FPU is permanently connected to risers and anchoring lines via a turret, allowing the FPU to freely weathervane and reduce environmental forces applied to the FPU. In geographical regions subjected to particularly harsh conditions, or submitted to hurricanes / cyclones / typhoons, permanent turret mooring systems can now be designed to moor any size of vessel unit up to 500 m long such as LNG barges, and the use of turret moorings is no longer considered a feasibility issue. Nevertheless the disconnectable turret mooring system may still provide an attractive alternative when harsh sea states result in problems for hull girder strength or in excessive freeboard requirements to deal with green water. For such situations, disconnecting the floater before the weather conditions become too severe could solve the overall feasibility of the FPU system concept, particularly when the system must be designed for severe 10,000-year return period conditions. This paper presents such large capacity disconnectable mooring systems, and shows how they can solve feasibility issues and provide a global cost optimization of the complete FPU concept compared to a permanently moored solution.
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