Numerical modelling and validation of a semisubmersible floating offshore wind turbine under wind and wave misalignment

Coupled aero-hydro-servo-elastic simulation tools play an important role in the design of offshore floating wind turbines. For rational design of the system, accuracy of the numerical tool is important in predicting the system responses. While the load cases where the wind and wave are aligned are sometimes the largest contributor to the design, evaluation of the load cases where the wind and wave are a misaligned condition are also required in the design codes. In this study, first a series of water tank tests is performed for a 1/50 scale semisubmersible floater and results for irregular wave tests with aligned and misaligned wind were analysed. Then, an in-house numerical tool, NK-UTWind is used to model the full scale system. Results showed that measured motions in surge, heave, and pitch are similar for the aligned and misaligned cases, and these were well reproduced by the calculation. Measured sway and roll motion in the misaligned case were characterized by the components in wave frequencies, which calculation expressed well for the roll motion while underestimated several peaks for the sway motion. The characteristics of frequency distribution of measured tower-base shear force in the x direction and moment around the y axis forces were similar for both aligned and misaligned cases, which agreed approximately well with the calculation. The peak frequencies of the measured tower-base shear force in the y direction were similar for aligned and misaligned cases, and the measured tower-base moment around the x axis force included additional components in wave frequencies for the misaligned case. Calculations reproduced the natural frequency component for the tower-base moment around the x axis in both cases, while the accuracy was low for shear force in y direction.