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Summary: The aim of this study is to reduce the deformation of large horizontal axis wind turbine blades using shape memory alloy (SMA)‐based centrifugal stiffening. A discrete model considering dominant modes of the tower, drive train and blades is developed in this study to demonstrate the performance of the proposed stiffening strategy. Here, super‐elastic behaviour of SMA is characterized by Graesser‐Cozzarelli model. Aerodynamic loads acting on the blades are evaluated using blade element momentum theory. The response is simulated using aerodynamic damping, which is estimated in each mode of vibration. Numerical results presented in this paper clearly show the significance of the proposed SMA‐based stiffening to reduce blade vibration. Sensitivity analysis is also carried out to demonstrate the performance envelop of the proposed stiffening strategy over the operational range of the benchmark 5‐MW wind turbine. The study clearly highlights the performance enhancement in terms of deformation in two orthogonal directions and design in terms of longitudinal stress that ultimately improve the serviceability of the blade. [ABSTRACT FROM AUTHOR]

Vibration control is an essential component of safe and sustained operation of modern large wind turbine towers. As the blade-tower assembly offers time-dependent system matrices that periodically evolves with the rotational speed of the turbine, control theories (e.g. linear quadratic Gaussian) can not be directly adopted in this case. With this in view, the present study aims to develop a semi-active strategy using multiple magneto-rheological tuned liquid column dampers (MR-TLCDs) to mitigate excessive vibration of the horizontal axis wind turbine towers. The design of the controller is achieved by the multi-blade coordinate transformation that converts the system matrices into a non-rotating framework followed by time averaging to enable the application of linear control law. Parameters of the non-linear MR-TLCDs are optimally tuned to dissipate maximum energy. Aerodynamic loads are evaluated using modified blade element momentum theory. The performance of the proposed control strategy is assessed against different wind speeds. Clipping laws are employed to keep the semi-active control force in the feasible range. Two different clipping laws are compared in this paper. Finally, sensitivity analysis is carried out to demonstrate the performance envelope and robustness of the proposed controller. • Semi-active control of wind turbine tower using multiple MR-TLCDs. • Response characterization using blade-tower interaction. • Aerodynamic load estimation using BEM theory and rotationally sampled spectra. • Optimal control force estimation in LQG/LTR framework using MBC transformation. • Sensitivity analysis using actual large scale wind turbine parameters is carried out. [ABSTRACT FROM AUTHOR]

Spar type floating wind turbines exposed to marine environment experience significant vibration, which is more prominent under wind-wave misalignment. This unavoidable vibration, particularly in the sway direction, affects overall performance and can induce significant damage to sensitive electro-mechanical components leading to downtime for maintenance. With this in view, the present study aims to propose a modified spar-torus combination by introducing spring and dashpot in between, so that it can work as an isolator. First, a comprehensive mathematical model for this multi-body system is developed using Kane's approach with proper aero-elastic and hydrodynamic simulation. For this purpose, 3D wind fields are generated in TurbSim and the waves are simulated from Pierson-Moskovitch spectrum considering wind-wave correlation and misalignment. Aerodynamic loads are computed using modified Blade Element Momentum theory while and hydrodynamic loads are generated using Morison's equation. Using these inputs, the response of the modified spar-torus combination is solved, which demonstrates the efficiency and advantage of the proposed vibration isolation. Different sea states and wind conditions are simulated replicating the actual scenario to investigate the performance envelope of the proposed controller for spar-type floating wind turbines. • Spar-Torus combination is modified so that it can act as an isolator for wave induced load. • Multi-body dynamic formulation is developed in Kane's approach considering aeroelastic and hydro-dynamicsimulation. • Significant vibration control is achieved in sway direction under wave-wind misalignment. [ABSTRACT FROM AUTHOR]

This paper proposes a wavelet multi-resolution based individual pitch control strategy for spar-type floating offshore wind turbines (FOWTs) and investigates its performance under joint wind-wave-current loads considering the effects of wave-current interactions. A multi-resolution analysis (MRA) based wavelet controller that modifies an optimal control problem cast in linear quadratic regulator (LQR) form constrained to a band of frequency has been used in this paper. The weighting matrices of the LQ regulator are varied in different frequency bands depending on the emphasis to be placed on the response energy and control effort to minimize the cost functional of that frequency band. This formulation results in frequency band dependent controller gains that lead to a time-varying controller. Daubechies wavelet is used in the MRA based filter that ensures perfect decomposition of the time signal over a finite interval and fast numerical implementation for control application. The multi-resolution wavelet-LQR individual blade pitch controller is used to control blade out-of-plane vibrations with additional emphasis on 1P frequency of the wind turbine. The emphasis on 1P frequency along with the blade's out-of-plane natural frequency is shown to reduce aerodynamic loads corresponding to 1st rotational frequency of the wind turbine which in turn reduces vibrations in other modes of the wind turbine. The proposed controller is simulated using a 5-MW baseline offshore wind turbine with realistic operational conditions including wave-current interactions. The controller has been proved to be effective in every analyzed met-ocean condition. [ABSTRACT FROM AUTHOR]