The role of the nonlinear wave kinematics on the global responses of an OWT in parked and operating conditions

High accuracy in the prediction of design loads for offshore wind turbines is a crucial prerequisite to achieve safe and economic designs. In this paper it is shown that numerical tools used to reproduce the wave-induced loads on offshore wind turbines are often based on overly simplistic mathematical models, which lead to important inaccuracies in the assessment of the system response. The study investigates the effects of nonlinear waves on the structural response of a bottom-supported offshore wind turbine. The main concern addresses the different behavior of the system when the turbine is in parked condition or when the turbine is in power production. In the parked configuration, the nonlinear wave kinematics used in the Morison equation causes dangerous effects in terms of internal stresses and resonant vibrations of the dynamical system. They are entirely missed when a linear wave kinematics is used. When the turbine is in power production the global damping increases dramatically due to the aeroelastic interaction of the rotor. Significant growth of the system response still occurs due to the nonlinear wave forcing; however, the resonant vibrations are completely damped out. A simple quantification of the equivalent linear damping ratio, modeling the first fore-aft mode of the tower as a 1-DOF linear damped mass–spring system, permits a rough estimation of the contribution to the global damping of the aeroelastic effects. During power production the system exhibits ten-time larger damping capabilities with respect to the parked condition.