Hysteretic nonlinear SMA-TMD for wind-wave-induced vibration mitigation of monopile offshore wind turbines considering stiffness degradation

During service, local damages in towering structures can induce structural stiffness degradation, which may lead to changes in structural dynamic characteristics, such as natural frequency. Thus, the control effect of the linear TMD with a narrow frequency band may decrease in the lifetime service. This paper proposes a novel hysteretic nonlinear shape memory alloy TMD (HNSMA-TMD) using the superelastic effect of the SMA and prestress method to improve the TMD's control effect when considering structural stiffness degradation. Firstly, the configuration and working principle of the proposed HNSMA-TMD is introduced. SMA's excellent superelastic performance can provide the hysteresis force for the HNSMA-TMD and broaden the control frequency bandwidth of HNSMA-TMDs. The mechanical model and finite element model (FEM) of the HNSMA-TMD are developed. Finally, a design method of the HNSMA-TMD is proposed, and the vibration responses of the offshore wind turbines without TMD, with TMD and with HNSMA-TMD under wind-wave loads are compared, considering structural stiffness degradation. Results show that the root mean square response reduction ratio of the wind turbine with HNSMA-TMD remains nearly 25% when considering 20% stiffness degradation. In comparison, the ratio of the wind turbine with TMD reduces from 36.3% to 6.4%.