Mooring system optimization for floating wind turbines using frequency domain analysis

Abstract This paper describes a new tool for the optimization of catenary mooring systems for floating wind turbines with a semi-submersible support structure. Each source of environmental loading -wind, current and waves -is spread over a range of compass directions, based upon probability distributions determined by meteorological and oceanographic measurements. Frequency domain analysis is applied to determine the linear response of the platform. Mean forces due to wind and current are determined to find the mean position of the platform and thereby the linearized mooring stiffness matrix. Subsequently, the stochastic loads from wind sea, ocean swell and turbulent wind are applied in the form of spectral loading matrices. The resulting mooring system design is site-specific, accounting for the directionality of environmental loads. A large number of different scenarios can be evaluated quickly and approximately. Hence the tool is useful for preliminary design of floating wind turbines.As an example application, a symmetric semi-submersible design with three columns is analyzed. Results for two sites in the North Sea, with different water depths and distinct directionality of environmental loads, are compared and indicate that spectral wind loads should be considered when designing mooring systems for floating wind turbines.