Nonlinear energy sink for vibration reduction in offshore wind turbines under rotating blade-wind-wave loads: Parameter selection and test verification

As offshore wind turbines (OWTs) evolve towards larger sizes, they become more susceptible to significant vibrations under the complex load conditions such as winds, waves, currents and blade rotation, accelerating structural fatigue damage and impacting both power generation efficiency and operational safety. This paper is aimed at investigating the vibration reduction capabilities of Nonlinear Energy Sink (NES) for OWTs under rotating blade-wind-wave load conditions. A 1:80 scale model of the 5 MW monopile OWT is fabricated in the laboratory. The corresponding parameters of NES and Tuned Mass Damper (TMD) are selected by the optimization for the first order natural frequency of the tower. A free-decay test and vibration tests on the scale model integrated with NES and TMD separately under different rotating blade-wind-wave loadings. The results have shown that NES exhibits superior robustness to variations in damper stiffness compared to TMD. NES can also reduce other vibrational frequency components induced by wave excitation and blade flapwise due to the nonlinear and broadband vibration frequency characteristics of NES. It is demonstrated that NES provides overall better vibration reduction capabilities than TMD in multi-modal vibration structures. This research offers valuable insights into the vibration reduction of OWTs under complex operational conditions.