Compliant liquid dampers-inerter for mitigating wind-, wave-, and earthquake-induced vibrations of monopile offshore wind turbines.

This work explores the possibility of installing compliant liquid dampers-inerter (CLDI) on monopile offshore wind turbines (OWTs) for controlling combined vibrations from wind, wave, and earthquakes. For numerical demonstration, the 5-MW NREL monopile OWT is taken up. Seven different wind speeds and a set of 20 earthquakes are considered. Hence, the means of structural response quantities are chosen. The peak and root mean square (RMS) values of displacement and acceleration responses of tower-top are monitored. A genetic algorithm-based multi-objective optimization is performed, and the optimum CLDI parameters are estimated for different ranges of inertance ratio and inerter topology. The optimization comprises four cases: firstly, minimizing the mean of peak responses and second, minimizing the mean of RMS responses in fore-aft (FA) as well as in side-side (SS) directions. Hence, each case has four objective functions, covering two loading scenarios, wind & wave, and earthquake. With the obtained optimum parameters, considering all these cases, the control effectiveness of CLDI is further investigated in time and frequency domains. The time domain results reveal that the CLDI controls the SS vibrations effectively, whereas, in FA direction, the efficiency is marginally less. Additionally, the frequency domain plots indicate the multi-mode control capability of CLDI. • Proposal of a CLDI-based vibration control mechanism for monopile WTs. • Development of a multi-objective optimization framework for WTs with CLDI. • Combined effects of wind, wave, and earthquake loadings on CLDI's performance. • Investigation of the control performance of CLDI in both FA and SS directions. • Illustration of vibration reduction mechanism of CLDI in time and frequency domains. [ABSTRACT FROM AUTHOR]