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This study aims to comprehensively investigate the dynamic characteristics of the tower of a scaled wind turbine model through wind tunnel tests. A model was scaled from the NREL 5 MW prototype wind turbine with a geometric scale ratio of 1/75, based on the similarity rules in thrust coefficient and dynamic characteristics. A series of wind tunnel tests were carried out on the scaled wind turbine model under different operating conditions and parked conditions with different yaw angles, and the modal parameters of the scaled model were identified by the stochastic subspace identification method and rotor stop tests. It was found that the vibration response of the tower in the fore–aft direction achieved its maximum value when the yaw angle was 90° with feathered blades, while the tower vibration response in the side–side direction was relatively severe with the yaw angle ranging from 10° to 50°. These observations are found to be well aligned with the aerodynamic characteristics of the airfoil. Moreover, the experimental results indicate that the scaled wind turbine model can reflect the vibration responses of its full-scale counterpart in the fore–aft direction. The natural frequencies and mode shapes of the scaled model can be accurately identified by different methods, but the identified damping ratios are relatively scattered. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWTs) are generally located in the harsh deep-sea environment and are highly susceptible to extreme loads. In order to ensure the normal operation of FOWTs, this article takes the semi-submersible FOWT as an example, proposes a new double-rope mooring system, and studies the dynamic performance of the FOWT with the double-rope mooring system and its effectiveness in reducing the dynamic response of the wind turbine. At the same time, the tuned mass damper (TMD) is installed in the nacelle of the wind turbine, and the TMD parameters are optimized considering the space limitation of the nacelle by limiting the TMD's stroke, which further reduces the dynamic response of the FOWT and improves its stability. Numerical simulation and analytical studies show that the new double-rope mooring system can reduce the dynamic response of the wind turbine to a greater extent than the traditional single-rope mooring system. Considering the stroke restriction, the control performance of TMD will be slightly weakened, but it is more in line with the actual engineering requirements. Compared with the original FOWT, the proposed new type of FOWT has better dynamic stability and has the prospect of extending to real engineering applications. [ABSTRACT FROM AUTHOR]

Accurate evaluation of aerodynamic damping and hydrodynamic damping is critical to response prediction and vibration control of spar-type floating offshore wind turbines. This paper presents a comprehensive analytical study of these two damping sources with respect to platform motions. Aerodynamic damping is evaluated by a newly derived aerodynamic damping matrix based on linearisation of aerodynamic resultant forces at tower top. Both radiation and viscous drag effects are considered for hydrodynamic damping. The former is analytically expressed in the form of a radiation damping matrix, while the latter is derived based on Morison's equation and strip theory. A simplified model is established based on the analytical damping expressions and successfully verified against FAST and Aqwa. Under various operational conditions, the damping ratios for different degrees of freedom of the platform are estimated using complex modal analysis. It is found that the modal damping ratios arising from different sources can be very different for different vibration modes of the platform, and the tower flexibility is proved to have negligible impact on the platform damping. For a typical operational state, surge, sway, pitch and yaw motions are highly damped, with the roll motion intermediately damped and the heave motion nearly undamped. [ABSTRACT FROM AUTHOR]

Classical flutter of a wind turbine blade is a concerned issue to hinder the wind utilization to a large extent. Recent predictions showed a decreasing flutter margin (the ratio of flutter speed to rated rotor speed) with the increase in wind turbine size. To address this issue, a new blade configuration called the double-pitched blade is proposed and analytically investigated for its potential to enhance the flutter suppressing capability of modern large-size wind turbine blades. This new blade comprise an inner part and a tip part, where the tip part can rotate (or pitch) independently with respect to the inner part through a tip actuator commanded by a feedback control law. The aerodynamic loads of blade tip due to the actively controlled rotation of the tip part provide a torque on the inner part, which provides damping for the torsional mode of the wind turbine blade. The effectiveness of this new double-pitched blade for suppressing flutter is verified through a simulation study conducted on a 907-DOF aero-servo-elastic wind turbine model. Parametric studies are performed on two main design parameters, i.e. the length of the tip part and the associated chordwise location of tip shaft with respected to the blade cross section, and flutter control performance are obtained by numerical optimization process. Simulation results show the optimal length of tip part is around 3.3 % of blade length, and the associated chordwise location of tip shaft is around 45 % of chord length, the flutter amplitude of the conventional blade can be mitigated to around 4 % using this double-pitched blade. • Anew double-pitched bladefor suppressing wind turbineflutter is proposed. • An aero-servo-elastic wind turbine model is developedto validate this design. • Parametric study on structural design parameters of this double-pitched blade is performed. • Flutter amplitude of blade can be significantly reduced. [ABSTRACT FROM AUTHOR]

Classical flutter of wind turbine blades is one of the most destructive instability phenomena of wind turbines especially for several-MW-scale turbines. In the present work, flutter performance of the DTU 10-MW offshore wind turbine is investigated using a 907-degree-of-freedom aero-hydro-servo-elastic wind turbine model. This model involves the couplings between tower, blades and drivetrain vibrations. Furthermore, the three-dimensional aerodynamic effects on wind turbine blade tip have also been considered through the blade element momentum theory with Bak's stall delay model and Shen's tip loss correction model. Numerical simulations have been carried out using data calibrated to the referential DTU 10-MW offshore wind turbine. Comparison of the aeroelastic responses between the onshore and offshore wind turbines is made. Effect of structural damping on the flutter speed of this 10-MW offshore wind turbine is investigated. Results show that the damping in the torsional mode has predominant impact on the flutter limits in comparison with that in the bending mode. Furthermore, for shallow water offshore wind turbines, hydrodynamic loads have small effects on its aeroelastic response. [ABSTRACT FROM AUTHOR]

The semi-submersible floating offshore wind turbine has been studied in detail due to its good stability. However, the occurrence of typhoons are very frequent in China's offshore area, putting forward a higher requirement for the stability of the floating wind turbine system. By changing the connection mode of the mooring line as well as the structural form of the platform based on the original OC4 model, two groups of models were examined by an in-house developed code named as the Analysis Tool of Floating Wind Turbine (AFWT). The influence of the arrangement of the mooring lines and the inclination angle of the upper columns on the motion response were clarified. It was found that the surge motion of the platform would be obviously decreased by decreasing the length of the upper segments of the mooring lines, while the heave motion of the platform would be significantly decreased as increasing the inclined angle of the columns. Therefore, a new model integrating the optimized multi-segmented mooring lines and the optimized inclined columns was proposed. The examinations showed that compared with the response motions of the original OC4 semi-submersible model, the proposed model could reduce both the surge and heave motions of the platform effectively. [ABSTRACT FROM AUTHOR]

• Deformation behaviors of large diameter steel tubes from a floating offshore wind tube (FOWT) against impact loads were investigated by experiments and numerical simulations. • Global motions of the tubes in the experiments were accounted for by a single-degree-of-freedom model for the rigid body motions of the FOWT. • Impact force-deformation curves, tube deformation patterns and energy dissipation were compared and discussed. • A nondimensional force-deformation curve was recommended for the design of large diameter steel tubes under impact loads. Large diameter steel tubes are widely used in bottom fixed and floating offshore wind turbines. Offshore wind turbines (OWTs) operating in the oceans are exposed to the risk of collisions when ships pass and dock at turbines. Thus, it is extremely important to investigate the impact mechanics of ship-OWT collisions and propose practical designs for methods to protect OWTs from collision loads. This paper presents a series of experimental and numerical studies on the deformation behaviors of large diameter steel tubes from a NREL 5 MW spar-type floating offshore wind turbine (FOWT) under lateral impact loads. These studies consider the effects of different impact velocities, attached masses, diameters and thicknesses of the tubes on their response to impact loading. In these experiments, a rigid indenter was mounted on a pendulum system and accelerated to strike the tubes; the scale was 1:30. The dimensions of the indenter head were much smaller than the tube diameter in order to concentrate the impact load. Global motions of the impacted tubes were modeled by springs introduced at the boundaries, and their stiffnesses were determined according to an equivalent single-degree-of-freedom (SDOF) model. Numerical simulations of the experiments were conducted using the nonlinear finite element (FE) software LS-DYNA. The experimental and numerical results were compared and discussed with respect to force-deformation curves, deformation modes and energy dissipation. Existing theoretical solutions for the lateral indentation resistance of tubes were also compared to the experimental and numerical simulation data. The results indicated needs for new solutions when the impact loads become concentrated. [ABSTRACT FROM AUTHOR]

The paper presents a wavelet-based linearization method for evaluating aerodynamic damping of a wind turbine during operation. The method is used to estimate the aerodynamic damping solely from actual measurements of the dynamic response of the operating wind turbine due to ambient excitation from air turbulence and control forces. Based on the response measurements the generalised displacement, velocity and acceleration vectors related to a given aeroelastic model and an available aeroelastic code are estimated by a state observer. Then, the external generalised load vector, depending on the generalised velocity vector, is obtained from the aeroelastic code. Next, the external generalised load vector is linearized into two parts: a quasi-static load vector independent on the generalised velocity vector and a first order term linearly proportional to the velocity vector indicating the aerodynamic damping matrix. Filtering technique is applied to evaluate the quasi-static load vector from the actual measurements of the structural stiffness force, made up as a product of the time-dependent stiffness matrix and the estimated generalised displacement vector. Finally, the time-dependent aerodynamic damping matrix has been evaluated by wavelet analysis at each time step. Unlike other inverse-based approaches, this wavelet-based method can avoid calculating the inverse of the velocity vector covariance matrix, which is singular. The proposed method has been illustrated by a reduced 13-DOF aeroelastic model, which is used to mimic the in situ response measured on the wind turbine. [ABSTRACT FROM AUTHOR]

Blade edgewise vibration is a growing concern for modern megawatt wind turbines, as it may significantly shorten the blade lifetime. In this paper, optimal calibration (tuning) of a blade-mounted tuned liquid column damper (TLCD) using closed-form formulas is investigated. A 2-DOF model for the blade-TLCD system is first established, acting as the basis for analytical derivation of the optimal tuning formulas. Two different frequency-tuning criteria are employed, one based on the equal dynamic amplification at two neutral frequencies and the other one based on the equal modal damping ratio of the two free vibration modes. Two optimal frequency-tuning (frequency ratio) formulas having similar expressions are derived based on these two criteria, and both formulas are dependent on the rotational speed, the mounting position, the blade edgewise eigenfrequency and the TLCD prescribed parameters. Next, an explicit expression for the optimal equivalent TLCD damping ratio is obtained by equalizing the dynamic amplification at three excitation frequencies, and it also exhibits dependence on the rotational speed and the edgewise eigenfrequency. A closed-form formula for the optimal TLCD head loss coefficient is then proposed, providing a straightforward method to determine the optimal value without resorting to iteration. Comparisons of the two optimal frequency-tuning formulas are conducted in terms of the dynamic amplification, the root loci of the 2-DOF system, and the parametric sensitivity, using data from the NREL 5MW reference wind turbine. The results indicate that these two formulas lead to very similar optimal design of the blade-mounted TLCD as well as the damper performance, although the 1st formula only guarantees the equal dynamic amplification while the 2nd one only guarantees the equal modal damping ratio. The set of closed-form formulas developed in this study facilitates a convenient and robust design of blade-mounted-TLCD for suppressing edgewise vibrations, and forms the basis for developing semi-active control algorithms. [ABSTRACT FROM AUTHOR]

Active vibration control is not only suitable but also urgently needed in order to mitigate excessive vibration for dynamic systems such as wind turbine towers. However, there is a challenge of directly observing displacement and developing an efficient reduced-order control strategy. With this in view, a reduced-order controller based on displacement observation is developed for the vibration mitigation of flexible high towers, where the problems of low signal-to-noise ratio (SNR) and intricate acceleration-based control algorithms are eliminated completely. The displacement can be measured by laser or modern videometrics technology. A reduced-order model with an active mass damper (AMD) at the top of the tower is created using modal truncation approach based on modal participation factor (MPF), which successfully preserves the dominant modes of the original structure, and reduces the influence of high-frequency uncertainties. Considering structure uncertainties caused by environmental factors and aging process, an unscented Kalman filter (UKF) is introduced to identify the modal stiffness of the reduced-order model. The present study, using a 5 MW onshore wind turbine as an example, investigates active vibration control under wind and seismic loads and compares it with traditional tuned mass damper (TMD). From a dynamic perspective, the simplified model of wind turbine tower takes the form of a multi-degree-of-freedom series model. Consequently, a six-story steel frame platform has been constructed to experimentally validate the efficacy of the reduced-order controller. According to numerical simulation and laboratory testing, the control effect of the reduced-order controller with a few displacement states as feedback can be utilized as an active control strategy for high-rise flexible towers. • A displacement feedback controller is designed for active control of practical engineering of HAWT. • The work simplifies the complicated wind turbine active reduction problems into a reduced-order active control issue. • The latest achievement of displacement observation technology with videometrics was introduced. • This study provides a complete platform involving model development - algorithm simulation - experimental verification. [ABSTRACT FROM AUTHOR]

The fatigue life prediction for offshore wind turbine support structures is computationally demanding, requiring the consideration of a large number of combinations of environmental conditions and load cases. In this study, a computationally efficient methodology combining aerodynamic decoupling and modal reduction techniques is developed for fatigue life prediction. Aerodynamic decoupling is implemented to separate the support structure and rotor-nacelle assembly. The rotor dynamics were modelled using an aerodynamic damping matrix that accurately captures the aerodynamic damping coupling between the fore-aft and side-side motions. Soil-structure interaction is modelled using p-y curves, and wave loading calculated based on linear irregular waves and Morison's equation at a European (North Sea) site. A modal reduction technique is applied to significantly reduce the required number of degrees of freedom, allowing the efficient and accurate calculation of hotspot stresses and fatigue damage accumulation. The modal model was verified against a fully coupled model for a case-study, monopile supported offshore wind turbine in terms of response prediction and fatigue life evaluation. The modal model accurately predicts fatigue life (within 2%) for a range of parameters at a fraction of computational cost (0.5%) compared to the fully coupled model. [ABSTRACT FROM AUTHOR]

• An aero-servo-elastic wind turbine model is presented. • The generalized load vector is calculated based on actual measurements and aeroelastic model. • Linearizing the generalized load into quasi-static load and aerodynamic damping load. • A DWT-based online identification method is proposed to estimate the time-varying aerodynamic damping matrix A new online identification method for estimating the time-varying aerodynamic damping of a wind turbine is proposed. Through a combination of measured aeroelastic response data and an aeroelastic wind turbine model, the time-varying aerodynamic damping matrix can be identified with the aid of the real-time Discrete Wavelet Transform (DWT) technique. The proposed method is validated by means of a 13-DOF aero-servo-elastic wind turbine model, which is used to mimic the in situ aeroelastic response of the NREL 5 MW wind turbine. A measurement noise sensitivity survey is conducted. Results from current DWT-based estimation method are compared with those from previously proposed Continues Wavelet Transform (CWT)-based estimation method and found to be superior in terms of accuracy and efficiency. [ABSTRACT FROM AUTHOR]

In recent years, the criticality of fore-aft vibrations induced by winds on wind turbine towers has increased; these vibrations are generally evaluated using equivalent fatigue load (EFL). This is the first study to adopt wind speed histories from large eddy simulations as input for dynamic analysis of wind turbines, and to evaluate EFL against real wind distribution. Tuned mass damper (TMD) and rotational inertial double tuned mass damper (RIDTMD) were employed to control these vibrations. Parametric analysis of the damper parameters was conducted. An innovative global optimization tool was developed based on a radial basis function neural network and genetic algorithm. Moreover, for the first time, GPU acceleration technologies were adopted to enable the optimizations of dampers through massive cases. Numerical results show that damper optimizations under real wind distributions are essential, and that optimized dampers reduce 44% EFL. The performance of RIDTMD is better than TMD but has a narrower system control bandwidth. The optimized dampers are significantly affected by wind speed; however, they are least affected by wind direction. The developed GPU-based codes can run 2001 times faster than the CPU-based ones, and the optimization tool can further reduce 85% computational time, which is open to other researchers. • TMD and RIDTMD are employed to control fore-aft vibration of wind turbine tower. • Novel GPU-accelerated codes are developed for dynamic response analysis and are 2000 times faster than CPU-based ones. • Newly developed RBFNN-GA-based tool is utilized to globally optimize dampers and reduces 85% computational time. • For the first time TMD and RIDTMD are optimized against real wind distribution and obtain about 44% EFL reductions. • For TMD, μ = 0.21 and v = 1.05; for RIDTMD, μ = 0.21, ξ = 0.42, and v 2 = 1.6 are recommended for the wind turbine. [ABSTRACT FROM AUTHOR]

Excessive fore-aft vibrations of wind turbine tower are the major reason for tower collapsing, which can be evaluated using the equivalent fatigue load (EFL). Wake effects are generated by the former wind turbines on the latter ones, and can greatly increase EFL and reduce lifetime of the latter ones, but they were seldom considered. Consequently, this study calculated EFL countering the wake effects under real wind distributions. The effects of wind turbine spacing on EFL indicate that the wake effects between wind turbines with spacing of approximately 2 times of rotor radius should be considered. Subsequently, tuned mass damper (TMD) was placed in the nacelle to passively control the fore-aft vibrations. An optimization tool for TMDs was developed based on the radial basis function neural network and genetic algorithm, which was compared with the theoretical equations and parametric analysis. GPU acceleration technology was utilized. Numerical results show that, under real wind distributions, the TMDs obtain at least 40.1% EFL reduction, which is 8.9% higher than the theoretically optimized one. Importantly, the GPU-based codes can run 2001 times faster than the CPU-based ones, and the optimization tool can reduce 44.9% computational time further. The codes are made available for other researchers. • PyGAOWT is validated against commonly used GH-bladed and obtains well agreements. • GPU-accelerated PyGAOWT can run 2001 times faster than CPU version and is opened. • RBFNN-GA-based optimization tool performs better and reduces 45% time compared with parametric analysis. • For wind turbines with spacing of about 2 times of rotor radius, TMDs should be applied. • TMD is optimized under real wind distribution instead of white noise with wake effects countered. [ABSTRACT FROM AUTHOR]

A steel-ultra high performance fibre reinforced composites (steel-UHPFRC) hybrid tubular tower structure (≥100m) is proposed and its feasibility to support an offshore wind turbine (OWT) with a 10 MW rotor is investigated in this paper. The hybrid tower combines a reinforced UHPFRC tube at the bottom and a steel tube at the top. Two numerical models are established to calculate the ultimate and fatigue loads for the hybrid tower and a standard steel tube tower as a reference. A simplified method is proposed to estimate the ultimate strength of the reinforced UHPFRC tube section, while a method based on the S–N curves is used to check its fatigue strength. It is found that the hybrid tower can satisfy design requirements and has a much longer fatigue life compared to the reference steel tower. Furthermore, the total material cost of the hybrid tower is found to be only 58% of that of the reference steel tower, while the masses of these two towers are kept close. • A steel – ultra high performance fiber reinforced concrete (steel-UHPFRC) hybrid tower structure is proposed for tall offshore wind turbines (OWTs). • The feasibility of the proposed hybrid tower with height of 149m above mudline is investigated comprehensively and compared with typical steel tower structure. • The efficient use of materials and much longer fatigue life of the proposed hybrid tower is highlighted. • The total material cost of the hybrid tower is less than 60% of that from the reference steel tower, while the masses of both towers are comparable. [ABSTRACT FROM AUTHOR]

• Time-invariant state space model of wind turbine allows modal analysis in operation. • Model combining aerodynamic decoupling and multi-blade coordinate transformation. • Complex-valued modal model shows modal coupling and influence of mean wind speed. • Tower and blade dynamic responses match fully coupled model in time and frequency domain. • Modal reduction significantly reduces computation cost, allowing fast fatigue analysis. An efficient modelling methodology for steady-state operating wind turbines is proposed, combining aerodynamic decoupling, multi-blade coordinate transformation, and modal reduction. This leads to a complex-valued, reduced-order modal model for prediction of dynamic responses of the tower-rotor-blade wind turbine system, considering rotor rotation, blade flexibility, and vibration coupling between rotor and tower. A fully coupled finite element model was first developed, with aerodynamic forces linearised and expressed as a viscous aerodynamic damping matrix. A time-invariant state space model is formed using multi-blade coordinate transformation, allowing standard modal analysis. The complex-valued eigenvalues and mode shapes were obtained, and it is shown that the operating wind turbine modes exhibit a combination of tower and blade vibrations. Various degrees of modal reduction are applied to the state space model to obtain a modal model with fewer degrees of freedom, whose performance was evaluated in the time and frequency domain for operating wind turbines in normal condition. The displacement and stress responses are in close agreement with those of the fully coupled model with the first 21 modes included. The model already performs well with 8 modes considered to capture relevant fundamental frequency peaks. This allows significantly reduced computational effort and can be particularly beneficial for fatigue prediction, reliability analysis, and structural identification. [ABSTRACT FROM AUTHOR]

• The collision between a spar-type floating offshore wind turbine (FOWT) and ship is numerically investigated by the nonlinear finite element (FEM) method. • Model verification of finite element model is performed for both FOWT and ship. • The collision forces, displacements and accelerations of FOWT with different impact velocities are obtained. • The incurred structural damage of FOWT and ship with different impact velocities is evaluated. • The energy conversion ratio between FOWT and ship subjected to collision is obtained. This paper investigated the dynamic responses and the incurred structural damage of a 5 MW spar-type floating offshore wind turbine (FOWT) and a ship when they collide. Ansys/LS-DYNA was used to establish the finite element models (FEMs) of the FOWT and the ship and to subsequently conduct the collision analysis. The global dynamics of the FOWT are captured well by the FEM comparing with FAST code and a simplified 8-degree of freedom (8-DOF) model established in MATLAB. The validity of the ship FEM was guaranteed by the fact that the model outputs agree well with empirical formulas. A series of simulations were then carried out using the FEMs to analyze the impact forces, displacements, accelerations, damages and energy dissipation during the ship collision for various impact velocities. The results show that the peak collision force, peak spar platform displacement and peak tower top displacement increase almost linearly with the initial velocity of the ship. The maximum tower top acceleration during the collision consistently exceeds 0.2 g, severely affecting the normal operation of the FOWT. The collision causes dents on the spar platform rather than the collapse of the whole wind turbine. After the collision, the initial kinetic energy of the ship is mainly transformed into the plastic deformation energy of the FOWT and the energy dissipated in the water. In addition, assuming a rigid ship bow results in accurate FOWT dynamic responses but fails to provide a good evaluation of the spar platform deformation. [ABSTRACT FROM AUTHOR]