Effect of nonlinear hydrodynamics on global dynamic responses of a semi-submersible wind turbine

The natural frequencies of rigid body motion of a semi-submersible wind turbine are outside the linear wave-excitation range and can be excited by nonlinear hydrodynamics, which significantly increases the low-frequency dynamic responses. The difference-frequency wave force quadratic transfer functions (QTFs) and the frequency-dependent added mass and damping around the natural frequencies of rigid body motions of the OC4-DeepCwind semi-submersible wind turbine estimated from the potential flow theory in a time-domain coupled numerical model of wind turbine (OpenFAST) are modified based on a series of computational fluid dynamic (CFD) simulations in the current research. The dynamic responses of wind turbine under three groups of wave and wind conditions are investigated using the numerical models with different combinations of nonlinear hydrodynamics. The low-frequency components of the responses which include the surge motions, pitch motions, tower base bending moments and mooring line tensions increase by at least 50 % under irregular wave conditions when the difference-frequency wave force QTFs are modified. However, under the irregular wave and turbulent wind conditions, this increment will reduce, especially for the surge motions and upwind mooring line tensions. The effect of the difference-frequency wave force QTFs is negligible under turbulent wind conditions. The modified frequency-dependent damping leads to a decreasing in the low-frequency components of all responses under all wind and wave conditions, and this reduction will be up to 50 %. In addition, the modified frequency-dependent added mass has no influence on the resonance frequencies.