The dynamic response of a Spar-type floating wind turbine under freak waves with different properties

Abstract

In the present work, the dynamic responses of a Spar-type floating offshore wind turbine (SFWT) under freak waves with different properties are simulated and analyzed. The properties of freak wave are represented by changing different decision parameters a1, a2, a3 and a4, corresponding to the ratios of the wave elevations around the freak wave crest (FWC). The improved phase modulation method is adopted to generate the original freak wave (OFW). On the basis, the freak wave elevation is further modulated to increase the decision parameters with different degrees, and the modulated freak waves (MFWs) are obtained. The SFWT motions under the OFW and MFWs are simulated in the time domain. We firstly investigated the influence of freak wave on the SFWT transient dynamic performances, including the surge, heave, pitch motions, the turbine output power and the mooring tensions. Then, the SFWT dynamic response under different MFWs corresponding to the specific factors were analyzed. The results show that the freak wave fluctuation represented by the factor a1 significantly affects the surge, heave, pitch transient motion, output power and extreme mooring tension of the SFWT. The output power of the SFWT decreases less after the FWC with the wave fluctuation increasing. At the same time, the wave fluctuation significantly affects the mooring tension, which greatly increases both near the focus time and the extreme mooring tension time. The height of the previous wave (PW) related to a2 affects the heave motion at the trough after PW impact, which in turn affects the transient heave. a3 represents the height of the next wave (NW) and has little effect on the dynamic performance. The impact height of freak waves represented by a4 has a particularly significant effect on heave. It not only affects the transient motion at the focus time, but also has a significant effect on heave after the freak wave subsides.