Investigation of viscous damping effect on the resonant heave response of a floating wind turbine

Abstract: Semi-submersibles are considered to be a promising substructure for the deployment of floating wind turbines (FWTs). Swells, characterized by long wave period, are particularly relevant to Australia. When subjected to swells, semi-submersible FWTs may experience resonant heave responses, resulting in large amplitude motions. At resonance, hydrodynamic damping, especially viscous damping induced by heave plates, becomes vital in determining the response amplitudes. This paper investigates nonlinear viscous damping effect on the linearly excited resonant heave motion of a floating wind turbine with thick heave plates. A scaled model test was carried out for a 10MW concept based on OC4-DeepCwind semi-submersible FWT at the scale of 1:40. Concurrently, a linear numerical simulation was conducted, serving as a basis for comparison. Heave motion transfer functions are obtained, showing that viscous damping is associated with a phase shift. We also identify that sea-state dependent viscous damping is correlated with a modified KC number that depends on the relative velocity amplitude between the platform and the water particles. Keywords: Floating wind turbine, heave plates, viscous damping, Keulegan Carpenter number