Development of novel track nonlinear energy sinks for seismic performance improvement of offshore wind turbine towers

High-rise and flexible offshore wind turbines (OWTs) are frequently constructed in high-seismicity regions, and passive linear control strategies are commonly adopted to mitigate the seismic responses of OWTs. However, the effectiveness of linear control devices is often sensitive to the frequency contents of seismic motions as well as structural frequencies. Track nonlinear energy sinks (NESs) have recently been proven effective in broadband vibration suppression but also possess amplitude-dependent control performance. Two novel NESs with newly-designed track profiles combining quadratic and quartic polynomials are proposed in the present study to improve the effectiveness of track NESs. One (hybrid track NES) combines nonlinear and linear positive stiffness, whereas the other (bistable track NES) combines nonlinear positive and linear negative stiffness. A statistical linearization technique is adopted to optimize the profiles and damping coefficients of the track NESs. A 3D finite element model of a typical OWT is established, and the restoring force composition of the track NESs is analyzed to simplify their numerical modeling. The effectiveness and robustness of the hybrid and bistable track NESs in OWT vibration control are systematically investigated under a set of synthetic and real ground motions considering different structural frequencies and further compared to the tuned mass damper (TMD) and the traditional track NES with a fourth-order polynomial profile. Numerical results show that hybrid and bistable track NESs outperform their counterpart of the traditional track NES. NESs exhibit a robust behavior against frequency changes despite their slightly less control effectiveness than that of the TMD. Moreover, large response reductions are achieved using the bistable track NES under low and moderate input energy levels.