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To more effectively extract the vast wind energy in marine areas, offshore wind turbines have been constructed with slender tower and large rotor. External vibration sources such as aerodynamic, sea wave and seismic loadings can threaten the safety of these energy infrastructures. It is important to evaluate the reliability of offshore wind turbines subjected to external vibration sources. Previous research works on the wind turbine fragility analyses only considered the fragility of the tower by assuming the wind turbine was in the parked condition with the blade mass lumped at the top of the tower. The study of the fragility of the blade which is one of the most important components of a wind turbine has not been reported. In the present study, a detailed three-dimensional (3D) finite element (FE) model of the NREL 5 MW wind turbine is developed in ABAQUS, and the tower and blades are explicitly modelled to realistically estimate the aerodynamic loads and structural behaviours of the wind turbine. The uncertainties of the structural mass, stiffness and damping are taken into account to develop the probabilistic wind-induced demand models for the tower and blades. The dynamic behaviours of the wind turbine subjected to the simultaneous aerodynamic and sea wave loadings are investigated in a probabilistic frame and the fragility curves for both the tower and blades under the parked and operating conditions are derived and discussed.

Offshore wind turbines (OWTs) are more and more widely used to produce electrical energy nowadays. Besides the constant wind and wave loads, earthquake excitation can be another important vibration source to the OWTs since many OWTs have been/will be constructed in the seismic prone areas. Extensive research works have been carried out to understand the dynamic behaviours of OWTs when they are subjected to multi-hazards (e.g. the simultaneous wind, wave and/or earthquake loadings). However, when seismic excitations are considered, the onshore earthquake records are normally used as inputs in the analyses due to the lack of offshore data and the difficulty in synthesizing the offshore seismic motions. This practice may lead to inaccurate structural response estimations since it is well known that the seawater can significantly suppress the seafloor vertical motions near the P wave resonant frequencies of the seawater layer, which in turn results in the different characteristics of onshore and offshore earthquake recordings. Moreover, the earthquake motions along the pile of OWTs are different from those at the ground surface, i.e. the earthquake motions vary with the soil depth. Recently, a method to stochastically simulate the earthquake ground motions on the offshore site was proposed, in which the influence of seawater layer was considered and the earthquake motions at any soil depth could be obtained. This paper carries out numerical simulations on the dynamic behaviours of OWTs subjected to the combined wind, wave and earthquake loadings, and the depth-varying offshore seismic motions are used as inputs in the analyses. The seismic responses of OWTs obtained from the onshore and offshore earthquake motions are calculated and compared. The influence of depth-varying ground motions on the dynamic responses of OWTs is discussed.

This study proposes replacing the spring and dashpot elements in the conventional tuned mass damper (TMD) by shape memory alloy (SMA) wires to form an SMA-based TMD to control the seismic responses of wind turbine tower. An equivalent linearization method is adopted to optimize the initial stiffness of SMA-based TMD by minimizing the root mean square (RMS) displacement of the tower. To have an insight into the control performance of SMA-based TMDs, a three-dimensional (3D) finite element model of a typical wind turbine tower is developed in ABAQUS, and tower responses without and with SMA-based TMDs when subjected to simulated and recorded ground motions are systematically analysed. For comparison, seismic responses of the tower controlled by the linear TMDs are also simulated. Numerical results show that the SMA-based TMDs can substantially mitigate seismic responses of the tower with almost the same reduction ratios as the linear TMDs, while the strokes of the SMA-based TMDs are much smaller than those of the linear TMDs. This merit makes the proposed method more practical compared to the conventional TMD-based method since the space in the nacelle and tower is normally very limited.

Offshore wind turbines can be built larger and lighter than they used to be due to the application of new materials. These large and flexible structures are vulnerable to external vibration sources such as wind, sea wave and earthquake excitations. It is necessary to mitigate the dynamic responses of offshore wind turbines to ensure the safety of these structures. Extensive research works have been carried out to mitigate the vibrations of the tower and/or blades of offshore wind turbines. Almost all the previous studies on the offshore wind turbine tower vibration control propose installing the control device at the top of the tower, i.e. in the nacelle. This method is effective to suppress the fundamental vibration mode of the tower, in which the maximum displacement occurs at the top of the tower. This practice is reasonable when wind and/or sea wave loadings are of interest since the energies of these vibration sources are concentrated in the low frequency range, and normally only the fundamental vibration mode of the tower is excited. On the other hand, offshore wind turbines may locate in the seismic prone areas, earthquake loading can be another vibration source during their lifetimes. When offshore wind turbines are subjected to earthquake excitation, higher vibration modes might be also excited. These higher vibration modes can further contribute to the structural responses and in certain circumstances they may even dominate the structural responses. In this case, installing the control device only in the nacelle will not be effective and more control devices should be installed at certain locations along the tower. In other words, one single control device will not be effective to control the tower vibrations if both the fundamental and higher vibration modes are of interest. This paper proposes using multiple tuned mass dampers (MTMDs) to control vibrations from the fundamental and higher modes of offshore wind turbine tower under multiple hazards, i.e. under the combined wind, sea wave and earthquake excitations. The effectiveness of the proposed method is numerically investigated. It should be noted that only the vibration of the tower is of interest in the present study. The vibration control of the blades is out of the scope of this paper, which will be further investigated.

Tuned mass dampers (TMDs) are widely adopted to control the adverse vibrations of engineering structures. To further improve the effectiveness of TMD, inerter was introduced into TMD recently to form inerter-based TMD systems. Similar to TMD, inerter-based TMDs should be carefully designed (optimized) in order to get their best control performances. In the previous studies on using inerter-based devices for seismic induced vibration control, the external excitation was normally simplified as a white noise and the inherent structural damping was ignored. However, it is well known that seismic excitation cannot be simply assumed as a white noise and damping always exists in the structure. The parameters obtained by the previous optimization procedures thus do not necessarily result in the best performance of the device. In the present study, the equations of motion of a single-degree-of-freedom (SDOF) structure equipped with three types of inerter-based TMDs subjected to seismic excitation are firstly developed. Instead of using a white noise as input, the filtered Kanai-Tajimi spectrum, which is characterized by the site damping and frequency, is adopted to model seismic ground motion. Then the effects of site damping, site frequency and structural damping on the inerter-based TMDs are comprehensively investigated and formulas are proposed to estimate the optimal parameters. Lastly, the responses of a structure without control and controlled by an inerter-based TMD under simulated and recorded earthquake ground motions are analysed by using MATLAB/Simulink. Numerical results show that the optimal parameters of inerter-based TMDs are significantly dependent on the site frequency and structural damping, while the site damping has little influence. Moreover, the accuracy of the proposed formulas is validated, and the control effectiveness of the inerter-based TMD is confirmed.

Wind energy as one of the renewable energies is serving as an indispensable role in generating new electric power. The worldwide installation of wind farms has considerably increased recently. To extract more wind resources, multi-megawatt wind turbines are usually designed and constructed with large rotors and slender tower. These flexible structures are susceptible to external dynamic excitations such as wind, wave and seismic loads. The excessive vibrations can compromise the wind energy conversion, lead to the structural fatigue damage and even result in the catastrophic failure of wind turbines in harsh environmental conditions. Various control devices have been proposed and used to mitigate the unwanted vibrations of wind turbines to enhance their safety and serviceability. This paper aims to provide a state-of-the-art review of the current vibration control techniques and their applications to wind turbines. Firstly, the widely used control strategies in engineering structures are briefly introduced. Their applications to suppress the adverse vibrations of the structural components of wind turbines, mainly the tower and blades, are then reviewed and discussed in detail. It can be concluded that the vibration mitigation of wind turbines is very challenging due to the fact that the dynamic behaviours of wind turbines are very complicated, which are associated with the aerodynamics, rotation of the blades, interaction between the tower and rotating blades, and soil-structure interaction, etc. Moreover, it is a challenge to straightforwardly use many of the conventional control devices because of the limited spaces in the tower and blades.

This paper presents a multi-scale stochastic dynamic analysis method for offshore structures. The uncertainties in the structural material parameters, such as mass density and Young's modulus, are considered. They are assumed to be lognormal distributions and represented by using the Karhunen–Loeve (KL) and Polynomial Chaos (PC) expansions. Since the variance of the output responses is unknown, the output vibration response is represented by using PC expansion. The multi-scale stochastic analysis is conducted with PC expansions of different orders representing responses at different DOFs defined as three categories, namely, important, less important and the least important ones. Iterated Order Reduced (IOR) model reduction technique is employed to remove the PC coefficients of slave DOFs. Two numerical examples are taken to verify the accuracy and efficiency of the proposed method for the multi-scale stochastic dynamic response analysis of offshore risers. The response statistics such as mean value and variance can be obtained from the proposed method. The results are compared with those from Monte Carlo Simulation (MCS) and Stochastic Finite Element Method (SFEM). Results demonstrate that the computational demand for uncertainty evaluation is greatly reduced, and the accuracy of the results is maintained.