Pitch motion mitigation of spar-type floating substructure for offshore wind turbine using multilayer tuned liquid damper

The possibility of controlling the pitch motion of a spar-type floating substructure for offshore wind turbines by using a tuned liquid damper (TLD) was numerically investigated. First, the motion of a spar-type substructure scale model excited by surface waves was predicted, and the results were compared to the measured data. There was reasonably good agreement between the two results, which confirmed the validity of the present numerical methods. Then, the effects of a TLD on the motion of the floating substructure were quantitatively assessed by a comparison between the predicted results with and without a TLD. The pitch motion of the scale model could be reduced by using a TLD. Based on this result, multilayer TLDs were proposed to effectively reduce the pitch motion of the floating substructure at the fixed target frequency. This expectation was confirmed when the predicted results were compared with that of the single-layer TLD. Finally, a theoretical model was derived to compute the rate of reduction in the pitch motion of floating substructures depending on the number of TLD layers. This model allowed the minimum number of layers for TLDs to reduce the pitch motion of the substructure by the required amount to be determined.