Quick assessment of semi-submersible floating offshore wind turbines under misaligned wind, wind-wave and swell-wave loading

Abstract

Computer-intensive coupled time-domain simulations are used by both academia and industry to capture the nonlinear behavior of floating offshore wind turbines. To assist in pre-design and optimization, computationally efficient methods such as frequency-domain methods are required. However, state-of-the-art frequency-domain methods consider simplified cases which do not represent operational design cases of industry projects. This paper compares the effectiveness of a frequency-domain method based on Response Amplitudes of Floating Turbines (developed by the NREL) to time-domain simulations carried out in BHawC-OrcaFlex by Siemens Gamesa Renewable Energy. The load cases are part of IEC Design Load Case 1.2 (fatigue in normal power production) and are selected from an industry project. Reasonable agreement is found for the pitch/roll rigid body modes around rated wind speed. However, the mean response and dynamic response is overestimated by the frequency-domain method due to underestimation of the mooring stiffness and the underestimation of hydrodynamic damping. For the estimation of the fore-aft and side-to-side bending moment, main differences are identified due to the absence of the tower bending, excitation due to rotor rotation and second-order wave forcing in the frequency-domain method. The results show the need for extending the frequency-domain method with tower bending and rotor rotation as degrees of freedom.