Design Waves and low-frequency responses of a semi-sub floating wind turbine in spread seas

• Experiments resolving subharmonic response of floating wind turbine in spread seas. • Design Wave reproduction for surge and pitch responses. • First- and second-order pitch response well reproduced by Design Waves. • Second-order response mapped. This paper presents an experimental analysis of the rigid-body responses of a design variant of Stiesdal Offshore’s TetraSub substructure in severe uni-directional and spread waves. Harmonic separation of extreme responses shows that linear (odd) response crests are slightly reduced in spread seas, while changes in time history shape prevented similar quantification for the corresponding second-order subharmonic (even) responses. Uni-directional Design Waves targeting extreme surge and pitch responses were identified and tested in the tank. The odd and even pitch response were well reproduced, and the odd harmonic surge response matched predictions within a 95 % confidence interval. Even harmonic surge responses deviated due to 1) lack of drift offset, 2) temporal asymmetry, and 3) damping effects. The relative balance between odd and even response varied with conditioning target, for averaged and Design Wave responses most notably for surge and to a smaller degree for pitch. We link this difference to damping. Spreading was found to further reduce this difference for pitch. Further analysis of the odd and even response signals showed that spreading reduced the standard deviation of the total and odd response. This reduction was stronger for the even response. Surprisingly, the reduction levels for odd and even were similar for both surge and pitch, when based on the averaged largest events. We link this to damping.