Offshore wind turbine (OWT) structures are highly sensitive to complex ambient excitations, especially extreme winds. To mitigate the vibrations of OWT structures under windstorm or typhoon conditions, a new eddy current with tuned mass damper (EC-TMD) system that combines the advantages of the eddy current damper and the tuned mass damper is proposed to install at the top of them. In the present study, the electromagnetic theory is applied to estimate the damping feature of the eddy current within the EC-TMD system. Then, the effectiveness of the EC-TMD system for vibration mitigation is demonstrated by small-scale tests. Furthermore, the EC-TMD system is used to alleviate structural vibrations of the OWT supported by composite bucket foundations (CBF) under extreme winds at the Xiangshui Wind Farm of China. It is found that the damping of the EC-TMD system can be ideally treated as having linear viscous damping characteristics, which are influenced by the gaps between the permanent magnets and the conductive materials as well as the permanent magnet layouts. Meanwhile, the RMS values of displacements of the OWT structure can be mitigated by 16% to 28%, and the acceleration can also be reduced significantly. Therefore, the excellent vibration-reducing performance of the EC-TMD system is confirmed, which provides meaningful guidance for application in the practical engineering of OWTs.

Wind turbines (WTs) are exposed to a dynamic/cyclic load environment, and are subjected to 1P/3P loads under operational conditions. Recent studies introduced the Sommerfeld Effect to explain the dynamic response amplification induced by 1P/3P loads. This study establishes a theoretical model to analyze the resonance of WTs under 1P/3P loads. Sensitiveness analysis was conducted for parameters b, c, S, F, and T to explore their influence on the dynamic response. The resonance phenomenon induced by 1P/3P frequency passing the natural frequency is discussed. The results show that there is no Sommerfeld Effect in soft–stiff WTs. Only if the imbalance reaches a much higher value (19200 kg·m in this study) should the Sommerfeld Effect be considered for soft WTs; otherwise, it can be ignored. The 3P resonance appeared when the 3P frequency approached the natural frequency, but it was not the Sommerfeld Effect.

A composite bucket foundation (CBF) is a new type of supporting structure in offshore wind engineering. Its huge transition part is the key difference compared to other offshore foundations. Firstly, the vibration measurement system of a wind turbine with the CBF is introduced. A finite element method (FEM) was developed, and the rigid deformation performance of the transition part was characterized. Then, to clarify the influence of the transition part brings to wind turbines with CBFs, a three-DOF theoretical model was established by simplifying the transition part as a rigid body. Horizontal and rotational foundation stiffness were considered to present the constraint effect below the mudline. Sensitivity studies were conducted on the parameters (including mass, moment of inertia and mass center height) of the transition part. Further, the vibration properties of the CBF structures under different operation load conditions were compared through the theoretical model and the in situ data. The results show that the relative errors between the theoretical model and FEM model are 3.78% to 5.03%, satisfying the accuracy requirements. The parameters of the transition part have varying degrees of influence on the natural frequency, foundation stiffness and vibration response of the wind turbines with CBFs. Compared to wind and 1P loads, the 3P load has a greater influence if the 3P frequency is close to the natural frequency of the wind turbine.