Changes in surge and pitch decay periods of floating wind turbines for varying wind speed

Recently published experimental results show variations in surge and pitch decay periods of floating wind turbines (FWTs) subjected to different incident wind velocities. This paper explores the external loads acting on a FWT, with special attention to nonlinearities which affect its low-frequency global motions. The period variations in surge and pitch are found to have different sources. In surge, the mooring system nonlinearities dominate, while for pitch the relative phase between the nacelle velocity and the thrust induced in the rotor leads to an apparent inertia/damping effect. Simplified 2-DOF models using linearized stiffness coefficients for surge and modified inertia and damping matrices for pitch are developed. Comparisons with state-of-the-art aero-hydro-servo-elastic time-domain simulations show excellent agreement for three distinct catenary-moored FWT designs.