Wave spectral analysis for design of a spar floating wind turbine in Mediterranean Sea

Floating offshore wind turbines (FOWTs) represent one promising concept for renewable energy production. However, the quantitative description of their dynamic behavior is extremely challenging, since it depends on the coupled effects of turbine aerodynamics, control system, hull hydrodynamics and mooring dynamics. In this context, hydrodynamic damping is a major source of uncertainty, particularly for relatively slender structures, such as spars. The aim of the present paper is to provide quantitative estimations of hydrodynamic damping ratios, natural frequencies and response spectra of the six rigid body motions of a spar FOWT under a wide range of wave conditions, including wind-generated sea states, swells and mixed seas. To this aim, the results of an open-sea experiment on a 1:30 model of OC3-Hywind spar in parked rotor conditions, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy), are analyzed and discussed. The above mentioned dynamic properties of the model are estimated through the spectral analysis of the rigid body motion time series collected during the experiment, classified depending on the input wave conditions. As a result, quantitative estimations of the dynamic properties of the model are provided, which can be used as input values for numerical models of the full-scale structure. Hydrodynamic damping ratios are estimated, including their variability depending on sea conditions, due to nonlinearity. Also, motion response spectra obtained for wind generated waves can be easily scaled-up to achieve a reasonable representation of the full-scale structure response under a range of relevant design conditions. Finally, additional information about nonlinear effects, coupling between degrees of freedom and close-to-resonance behavior of the model are drawn and commented.