Slender and flexible offshore wind turbines (OWTs) are vulnerable to external dynamic excitations, and passive tuned mass dampers (TMDs) have been widely used to control excessive vibrations of OWTs under harsh marine environments (e.g., strong winds and irregular sea waves). However, TMDs are only effective in the vicinity of the controlled frequency, i.e., in a narrow frequency band. Compared to passive TMDs, active control methods are normally considered to possess better control performances but at the cost of a large amount of external energy input. To this end, the present study proposes a novel energy-adaptive self-powered active mass damper (SPAMD) to mitigate the responses of OWT towers. The proposed control device can harvest energies from OWTs and then use them as the power to drive an active mass damper for structural vibration control. Specifically, a representative OWT is selected as a prototype structure and its tower is modeled as a multi-degree-of-freedom system by simplifying the rotor-nacelle assembly as a lumped mass and moment of inertia. The dynamic characteristics (mainly natural frequency and mode shape) of the tower obtained by the developed model are validated against a finite element model. Subsequently, the system configuration and working mechanism of SPAMD are introduced and SPAMD is incorporated into the developed model to simultaneously harvest energy and mitigate the fore-aft responses of the tower under wind and sea wave loads. The control effectiveness of SPAMD is further compared to the traditional TMD. Results show that SPAMD has a superior effect over TMD in controlling OWT responses.

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