Design and Optimization of Mooring Systems for Floating Wind Turbines

Hywind Demo is the world s first full scale floating offshore wind turbine, developed and operated by Statoil ASA. Throughout this thesis, a time domain model of Hywind Demo is used to design an optimized mooring system which can reduce the overall cost of floating wind turbines. This thesis describes different mooring system design ideas, consisting of commonly used configurations such as catenary and taut mooring, and a less common configuration which uses buoyancy elements to increase the length of a fibre rope and increase flexibility without increasing the footprint. Rules and regulations that govern the design of the mooring system for a floating offshore wind turbine are described to their full extent with respect to ULS, ALS and FLS. In accordance with the rules and regulations, two environmental conditions were established. These correspond to rated wind velocity for the wind turbine and 50-year storm. For these two conditions the significant wave height and peak period; current velocity; and wind velocity were determined. A time domain model of Hywind Demo with catenary mooring is established in SIMORIFLEX through SIMA, this model is compared against a correlating frequency domain model in MIMOSA. The results show that the SIMO-RIFLEX model and MIMOSA model are in correspondence, and that the MIMOSA model yield more conservative results. With the aim of maintaining the station keeping properties of the catenary system, several fibre mooring designs were investigated, including plain taut mooring; fibre mooring with clump weights; and fibre mooring with buoyancy elements. It was found that by combining the elasticity of fibre ropes of 1000 m with the geometric flexibility added by a buoy with 30 t net buoyancy, the same restoring properties as the catenary system was achieved. Environmental response analyses on the fibre mooring system was conducted, verifying it s applicability and compliance with the governing rules and regulations. The proposed fibre mooring system results in a cost reduction of 70% and a weight reduction of 60% for the mooring system, while maintaining similar floater motion responses, mooring utilization factors and footprint as the original, catenary mooring system. A brief study on the use of fibre mooring in 130 m water depth was performed with regard to Statoil ASA s proposed relocation of Hywind Demo. The study yielded positive results, and the utilization of the fibre mooring system in shallow water is seen to be feasible, though further investigation should be performed