Vibration reduction of floating offshore wind turbine with nonlinear vibration absorber: Concept, numerical analysis and experimental tests

Floating offshore wind turbines (FOWTs), as emerging energy systems, are subjected to environmental loads from winds, waves, and currents throughout their operational lifespan. Numerous strategies have been proposed to mitigate FOWT vibrations, aiming to enhance structural safety and stability. However, conventional linear vibration absorbers are often limited in their effectiveness, as they are typically designed for specific frequencies. In this paper, a nonlinear vibration absorber (NVA) is introduced, which achieves nonlinear stiffness through the orthogonal arrangement of springs. A numerical model of the FOWT equipped with the NVA is developed, and a semi-analytic technique is employed to reveal the targeted energy transfer (TET) and resonance capture cascading (RCC) phenomena occurring during the vibration reduction process, with the goal of achieving efficient mitigation performance and multi-modal vibration absorption. Additionally, an integrated FOWT-NVA wave basin test system is established, and extensive experiments are conducted under various offshore environments and absorber operating conditions. The results demonstrate that the NVA exhibits excellent vibration control performance and frequency robustness. Compared to conventional linear absorbers (such as tuned mass dampers, TMDs), the NVA significantly improves vibration reduction in both tower top responses and tower-base loads. This study validates the effectiveness of the NVA and provides a foundation for future experimental techniques and advanced controllers for FOWTs.