An improved frequency domain model for floating offshore wind turbines

Abstract A cost- and time-efficient design evaluation tool can assist the FOWT industry in evaluating many different concepts. An open source frequency domain model for FOWT applications shows promising results for the integral, coupled response, but clear differences to higher fidelity time-domain studies are observed. In this work, the hydrodynamic modelling is improved by using a potential flow diffraction code, and the implementation of the floating feedback mechanism has been improved. Attempts have been made to improve the mooring system modelling by implementing a lumped mass approach, and to include the excitation of a turbulent wind spectrum. A good match in the system’s motion responses is obtained in the wave frequent range when compared to time-domain results. At lower frequencies, the excitation from the turbulent wind spectrum remains to be improved on. At higher frequencies, the mooring line modelling is to be improved for reliable results. For both effects, a clear outline is set up to improve in future work. The current model is capable of accurately modelling fully coupled wave-frequent motions in combined wind-wave environments in an efficient manner. When the suggested improvements are incorporated in future work, it is expected that a good match in the low- and high-frequency ranges can be obtained as well, allowing to study the full motion- and mooring tension response. Finally the tool will give developers and designers the opportunity to evaluate a large amount of designs in a cost and time efficient manner.