Numerical Study on Cause and Characteristics of Low-Frequency Yaw Motion of a Slack-Moored FOWT

Slack-moored floating offshore wind turbines (FOWTs) may experience low-frequency motions. In fact, in the Fukushima demonstration FOWT project, low-frequency motions were observed not only in storms but also in relatively calm sea states. Low-frequency motions could cause damages to the slack-mooring system and the power cables that transmit electricity generated by the turbine. Furthermore, the low-frequency motion in yaw could cause damages to the connection part of the nacelle and the tower. Suppressing low-frequency yaw motion requires an accurate understanding of the cause and the characteristics of the motion. To do so, numerical simulation of a slack-moored 15 MW FOWT has been conducted in two or three wave components. The results indicate that the low-frequency yaw motion and resonance could occur at various frequencies.