A fully coupled frequency domain model for floating offshore wind turbines

This paper presents a new frequency domain modeling approach for floating offshore wind turbines with coupled wind turbine, floating platform, and mooring system sub-models. The sub-models are generated using the validated numerical tools FAST and WAMIT to obtain the frequency domain aerodynamic and hydrodynamic characteristics, respectively, for any given design candidate. The turbulent wind and irregular wave loads are incorporated in the frequency domain model using wind and wave power spectral density functions, the JONSWAP and Kaimal spectra, respectively. To validate the proposed 6 DOF frequency domain framework across standard operational environmental conditions, predicted system responses of a 5 MW NREL offshore wind turbine with three classes of reference platforms including the OC3-Hywind, the MIT/NREL TLP, and the OC4-DeepCwind semisubmersible were compared to the outputs of 6 DOF and 22 DOF FAST time domain simulations. The comparison over an aggregate of eleven environmental conditions focused on differences in predicted platform rigid body motions and structural considerations including platform surge, roll, and pitch, and rotor thrust, total blade root and tower base bending moments/fatigue loads, fairlead and anchor tensions/fatigue loads. In terms of platform motions, the worst match of frequency and time domain model predictions was seen for the OC4-DeepCwind semisubmersible with errors of 13.2% in peak displacement values. The frequency domain model predictions of rotor thrust, blade root and tower base bending moments demonstrated the largest error in the case of the OC3-Hywind spar buoy with the peak loads differing by up to 12.8%. Errors in the predictions of maximum fairlead and anchor tensions were less than 11.5% with maximum error occurring for the MIT/NREL TLP. In terms of fatigue load comparison, the blade root and tower base fatigue load predictions showed less than 9.8% errors for all the reference platforms. Comparison of the fairlead and anchor fatigue loads showed that errors were less than 13.8% with the largest error seen for the OC3-Hywind spar buoy platform. Overall, the frequency domain model provides reliable means for assessing platforms dynamics at the conceptual stage of the design process.