Experimental and numerical investigation of nonlinear diffraction wave loads on a semi-submersible wind turbine

In a severe sea state, nonlinear wave loads can excite resonant responses of floating wind turbines either at high (structural) or low (rigid body motions) natural frequencies. In the present work, a computational fluid dynamics (CFD) model and an engineering model based on potential-flow theory with Morison-type drag are developed to investigate nonlinear wave loads on a stationary, rigid semi-submersible wind turbine under regular and irregular waves. The numerical results are validated against experimental measurements. A trimmed floater is modelled to examine the change in nonlinear wave loads due to the mean pitch angle which occurs during operation of a floating wind turbine. Furthermore, wave loads on each column are investigated numerically. Compared to the experimental measurements, the CFD model gives better estimations than the engineering model for the first, second and third order wave diffraction loads. The engineering model based on the first- and second-order potential-flow theory has large discrepancies in the phase of high order wave diffraction loads and underpredicts the amplitude of low-frequency wave loads. In the CFD simulations for the studied wave period (12.1 s), the second and third harmonic surge forces on the starboard columns are significantly larger than those on the upstream column, while first harmonic results are consistent with potential flow. The trim angle (5°) results in an increasing surge force and pitch moment but a decreasing heave force.