The wind industry has attracted attention and is growing rapidly due to the environmental concerns over conventional energy resources. The offshore wind industry, in particular, is becoming more attractive because of its advantages over onshore wind. Offshore wind turbines (OWT) are subject to undesirable vibrations caused by environmental loadings, seismic excitations, and rotor frequency excitations, and these excessive vibrations need to be minimized to increase serviceability and lifetime of the system. A number of structural control devices using vibration control mechanisms have been developed to mitigate the aforementioned excessive vibrations. Tuned mass dampers (TMDs) and tuned liquid dampers (TLDs) are two main vibration control devices widely used.
Various vibration control mechanisms originally from civil engineering field have been proposed to control the level of vibrations in the wind industry. Three main vibration control methods such as passive, semi-active, and active exist. The applications and descriptions of these methods utilized in buildings and wind turbine structures were reviewed by Symans and Constantinou [1
] and Chen and Georgakis [2
]. The passive control system improves damping and stiffness of the main structure without the need of employing external forces [3
]. The vibrations are not tracked via sensors in this method as curations of this system are constant. This method is widely used due to easy implementation and maintenance. Active vibration control system is a more sophisticated method in which not only mechanical properties are adjusted in the time domain but also external forces are employed. Thus, active control method requires the presence of active forces from external sources, resulting in high cost and complexity of the system. Semi-active vibration control system is the modified version of the passive control system with the capability of adjusting the properties of the system in the time domain with respect to certain properties of vibration forces such as frequency content and amplitudes. Vibration amplitudes and frequencies are tracked down using sensors and signal processing techniques in order to adjust the structural properties. Therefore, semi-active system optimizes vibration control capacity without employing external forces. In other words, semi-active system enjoys the best of both active and passive systems; therefore, it can be a more reliable and economically viable option for offshore wind turbines which are subjected to changes in their natural frequencies.
There is a considerable amount of literature on passive vibration control devices for wind turbines. One of the early studies in this field was done by Enevoldsen and Mørk [5
] in which effects of passive tuned mass dampers on a 500 kW wind turbine were studied and a cost-effective design was achieved owing to the implementation of structural control devices. Later on, Murtagh et al. [6
] investigated the use of tuned mass dampers (TMD) for mitigating along-wind vibrations of wind turbines. They concluded that the dynamic responses could be reduced providing that the device is tuned to the fundamental frequency. Colwell and Basu [7
] examined effects of tuned liquid column dampers (TLCDs) on offshore wind turbine systems to suppress the excessive vibrations and found that TLCD can minimize vibrations up to 55% of peak responses of OWTs compared to the uncontrolled system. Stewart and Lackner [8
] examined the impact of passive tuned mass dampers (PTMD) considering wind–wave misalignment on offshore wind turbine loads for monopile foundations. The results demonstrated that TMDs are effective in damage reduction of towers, especially in side–side directions. Stewart and Lackner [9
] in another study investigated the effectiveness of TMD systems for four different types of platforms including monopile, barge, spar buoy, and tension-leg and they observed tower fatigue damage reductions of up to 20% for various TMD configurations. In addition, passive multiple tuned mass dampers (MTMD) were proposed to improve the effectiveness of the vibration control system [10
]. Dinh and Basu [10
] investigated the use of passive MTMDs for structural control of nacelle and tower of spar floating wind turbines and concluded that MTMDs are more effective in displacement reduction. Whilst passive control methods can reduce the dynamic vibrations to some extent provided they are tuned properly, they can be easily off-tuned as soon as the dominating frequency of the system changes, resulting in ineffectiveness of the system and even increased vibrations compared to the uncontrolled system. Highly dynamic nature of offshore wind turbines subjected to a number of dynamic loadings and interacting with nonlinear soil conditions results in fluctuations in fundamental frequency. Abrupt pulsed nature excitations such as earthquake motions can lead to degradation of soil stiffness and, consequently, reduction in natural frequency. Furthermore, the cyclic vibration of surrounding soil can cause natural frequency reduction. In addition, natural frequency increase might occur when there is stiffening phenomena in certain soil types. Therefore, this raises many questions regarding the use of passive control devices in offshore wind turbines for the whole lifetime in which there are variations in dominating frequency and the application of more sophisticated vibration control devices needs to be examined.
Active vibration control has also been studied for the application of wind turbines by a number of researchers [11
]. Most studies of active control systems for wind turbines were focused on the vibration control of the blades. For example, Stanio and Basu [14
] proposed an active vibration control system based on active tendons for wind turbines. The results of their numerical simulations showed that the proposed control approach is robust in improving the blade responses under vibrations due to the change of rotational speed of the blades. In addition, Fitzgerald and Basu [17
] proposed a cable connected to an active tuned mass damper for the reduction of in-plane blade vibration and they found that the proposed control system mitigates the vibrations of large and flexible blades more effectively. In addition, Kim et al. [18
] introduced a robust modal control of lightly damped structures using an active dynamic vibration absorber. They used a single active electrical dynamic absorber and tested its effectiveness in control of multiple modes both experimentally and analytically.
The semi-active control mechanism is more suitable for the systems with high time-variant parameters such as offshore wind turbines. Semi-active vibration control devices for the application of buildings have been actively studied by a number of researchers [1
] in the last few decades. However, their application in wind energy is a new field. One of the earliest studies on semi-active control mechanism for wind turbines was done by Kirkegaard et al. [24
], in which they presented an experimental and numerical investigation of semi-active vibration control of offshore wind turbines equipped with a magnetorheological (MR) fluid damper. The authors claimed that using MR dampers for offshore wind turbines results in considerable reduction of the lateral displacement compared to the uncontrolled system. Later on, Karimi et al. [25
] proposed a controllable valve in tuned liquid column dampers for the application of offshore wind turbines. In addition, the use of semi-active tuned mass dampers in control of flapwise vibrations of wind turbines was examined by Arrigan et al. [26
]. The authors proposed a frequency-tracking algorithm for retuning the vibration control device and they observed significant vibration reductions owing to the semi-active mechanism. Furthermore, Weber [27
] studied application of an adaptive tuned mass damper concept based on semi-active controller using MR dampers. Their results showed that the real-time controlled MR semi-active tuned mass damper is a robust device for reducing structural vibrations. Semi-active control mechanism for tuned liquid column dampers (TLCDs) was also studied by Sonmez et al. [28
]. The authors used a control algorithm based on short-time Fourier transformation (STFT) and investigated the effectiveness of the proposed device under random excitations. More recently, Sun [29
] explored semi-active tuned mass dampers for the NREL (National Renewable Energy Laboratory) 5 MW baseline wind turbine excited by environmental loadings in conjunction with seismic motions considering post-earthquake damage to soil and tower stiffnesses. The author demonstrated the superiority of semi-active vibration control over the passive one in multihazard conditions. Although Sun’s [29
] work is well founded, it is limited to only one earthquake record (1994 Northridge Newhall 90) and further study for a suite of earthquake records with different frequency contents and intensities is required. Another limitation of the aforementioned work is that soil–pile interaction was modeled using a simplified method (closed-form solution) in which the stiffness of embedded pile is considered with a constant rotation and lateral stiffness value in seabed level. More advanced soil–pile interaction model based on time-variant nonlinear stiffness considering soil damage phenomena can enhance the previous works. In addition, the effect of semi-active tuned mass dampers on other structural responses such as base shear and base moment should be investigated.
To fill this gap, this study investigates semi-active tuned mass dampers for offshore wind turbines under multihazard conditions considering time-variant nonlinear soil–pile interaction properties and time-variant damage. A detailed model of the modern NREL 5 MW wind turbine equipped with semi-active tuned mass dampers (STMD) is developed. Stochastically wave and wind loadings in conjunction with seismic loadings are applied to the system and dynamic responses such as displacement, base shear, and base moments are investigated. Compared to the previous models, the developed model has the capacity to consider soil–pile interactions more realistically. Furthermore, a suite of seismic records is used with the aim to consider a wider range of seismic characteristics in the simulations. This paper is organized into five sections. In Section 2
, the numerical model of the system including tuned mass dampers, the wind turbine, soil–pile interaction, and the control algorithm is presented. Section 3
defines the loading sources including wind, wave, and earthquake. The numerical results and discussions are presented in Section 4
and conclusions are made in Section 5