Study on the Structural Vibration Control of a 10 MW Offshore Wind Turbine with a Jacket Foundation Under Combined Wind, Wave, and Seismic Loads

To effectively extract the vast wind resource, offshore wind turbines are designed with large rotor and slender tower, which makes them vulnerable to external vibration sources such as wind and wave loads. Substantial research efforts have been devoted to mitigate the unwanted vibrations of offshore wind turbines to ensure their serviceability and safety in the normal working condition. However, most previous studies investigated the vibration control of wind turbines in one direction only, i.e., either the out-of-plane or in-plane direction. In reality, wind turbines inevitably vibrate in both directions when they are subjected to the external excitations. The studies on both the in-plane and out-of-plane vibration control of wind turbines are, however, scarce. In the present study, the NREL 5 MW wind turbine is taken as an example, a detailed three-dimensional (3D) Finite Element (FE) model of the wind turbine is developed in ABAQUS. To simultaneously control the in-plane and out-of-plane vibrations induced by the combined wind and wave loads, another carefully designed (i.e., tuned) spring and dashpot are added to the perpendicular direction of each Tuned Mass Damper (TMD) system that is used to control the vibrations of the tower and blades in one particular direction. With this simple modification, a bi-directional TMD system is formed and the vibrations in both the out-of-plane and in-plane directions are simultaneously suppressed. To examine the control effectiveness, the responses of the wind turbine without control, with separate TMD system and the proposed bi-directional TMD system are calculated and compared. Numerical results show that the bi-directional TMD system can simultaneously control the out-of-plane and in-plane vibrations of the wind turbine without changing too much of the conventional design of the control system. The bi-directional control system therefore could be a cost-effective solution to mitigate the bi-directional vibrations of offshore wind turbines.

Shaking table tests and numerical analysis of monopile-supported offshore wind turbines under combined wind, wave and seismic loads

Vibration control of a pentapod offshore wind turbine under combined seismic wind and wave loads using multiple tuned mass damper

Experimental study on seismic vibration control of an offshore wind turbine with TMD considering soil liquefaction effect

Offshore wind turbines (OWTs) are dynamically sensitive structures to low-frequency wind-wave loadings due to their low damping and high flexibility. This makes them vulnerable to unwanted vibrations in ocean environments. Therefore, there is a need to implement innovative vibration control devices to suppress undesired vibration and ensure their structural safety. A tuned mass damper (TMD) has been a promising solution to control excessive vibrations recently in OWTs due to higher efficiency and low installation cost. However, TMD performance in vibration mitigation of OWTs when placed at a nacelle is still challenging to investigate because the generator torque and pitch control may affect the dynamics of the overall system. In this paper, an optimal multiple TMD system is designed by placing TMDs at optimal locations along the tower, which are defined using the maximum amplitude of the displacements for the first three natural frequencies of the tower. In addition, extensive simulations are carried out with integrated OWT-TMD systems to find the optimal mass ratio values and quantity of the TMDs. The TMD system is also tuned for several scenarios, including operating, parked, and idling conditions under the combined wind-wave loadings. A numerical model of the 5 MW NREL OWT developed in OpenFAST is considered as the baseline in this study. The results show a root mean square (RMS) response reduction of 10.2% and 42% in fore-aft (FA) and side-side (SS) tower displacement with optimally designed multiple TMDs. Moreover, improved mitigation effects on tower base moment of RMS 43.6% and 20.8% are observed in orthogonal directions, respectively. The findings of this paper may have the potential for designing passive TMD systems for vibration reductions in OWT.

The steady state response of a torsionally coupled system with tuned mass dampers (TMDs) to external wind-induced harmonic excitation is presented. The torsionally coupled system is considered as one-way eccentric system. The eccentricity considered in the system is accidental eccentricity only. The performance of single tuned mass damper (TMD) optimally designed without considering the torsion is investigated for the torsionally coupled system and found that the effectiveness of a single TMD is significantly reduced due to torsion in the system. However, the design of TMD system without considering the torsion is only justified for torsionally stiff systems. Further, the optimum parameters of a single TMD considering the accidental eccentricity are obtained using numerical searching technique for different values of uncoupled torsional to lateral frequency ratio and aspect ratio of the system. The optimally designed single TMD system is found to be less effective for torsionally coupled system in comparison to uncoupled system. This is due to the fact that a torsionally coupled system has two natural frequencies of vibration, as a result, at least two TMDs are required which can control both lateral and torsional response of the system. The optimum damper parameters of different alternate arrangements such as (i) two identical TMDs placed at opposite corners, (ii) two independent TMDs and (iii) four TMDs are evaluated for minimum response of the system. The comparative performance of the above TMDs arrangements is also studied for both torsionally coupled and uncoupled systems. It is found that four TMDs arrangement is quite effective solution for vibration control of torsionally coupled system.

Tuning of passive TMD for floating offshore wind turbine using linear matrix inequalities

The study of a monopile offshore wind turbine with the soil–structure interaction effect is most challenging in structural design under multiple hazards, i.e., the combined wind, sea wave, and earthquake excitations. Different arrangements of passive tuned mass dampers (TMDs) were used to mitigate the service and seismic loads affecting an offshore wind turbine (OWT) including the pile–soil–structure interaction (PSSI) effect. Different schemes of passive TMDs, placed at the top of the OWT tower or also at the center of gravity (CG) of the OWT tower or at the connection between the OWT tower and monopile, were tested. Various arrangements of TMDs including the proposed herein top radial TMDs arrangements have been investigated to determine their validity in resisting vibrations resulting from service and earthquake loads. The lateral displacements, shear forces and bending moments in both horizontal directions and the axial forces all over the OWT tower and monopile heights were recorded to compare the performance of each mitigation scheme of TMDs. The comparison results showed that the TMDs placement should be at the top of the OWT tower and the top radial 6 TMDs arrangement was found to be the most effective mitigation scheme for all straining actions in the tower and the monopile of the OWT subjected to service and earthquake loads.

A vehicle-bridge tuned mass damper (TMD) coupled dynamic analysis and vibration-control model was established to optimize TMD damping effects on a steel-box girder bridge bearing vehicle loads. It was also used to investigate optimization efficiency of different algorithms in TMD design parameters. This model simulated bridges and vehicles with the use of a 7 degrees of freedom curved-beam element model and a 7 degrees of freedom vehicle model, respectively. The TMD system was simulated with the use of multiple rigid-body systems linked with springs and dampers. Road surface condition, as a vibration source, was simulated with the use of a frequency equivalent method based on a power spectrum. A variably-accelerated pattern search algorithm was proposed in line with the initial TMD parameters calculated by Den Hartog formula. Visual software was compiled by Fortran and used for an optimization study of vibration reduction. A three-span, curved, continuous steel-box girder bridge was used as the numerical example. Optimized effects and computational efficiency of vibration reduction under different methods were compared. The comparison included a single variable optimization based on Den Hartog formula, an ergodic search method, an integer programming method, a traditional genetic algorithm, a traditional pattern search algorithm, and a variably-accelerated pattern search algorithm. The results indicate that variably-accelerated pattern search algorithm is more efficient at improving TMD optimal parameter design. Final TMD parameter optimization values obtained by different methods are quite close to each other and tends verify the reliability of the optimization results.

Passive control of jacket–type offshore wind turbine vibrations by single and multiple tuned mass dampers

This study examines the optimal design of a tuned mass damper (TMD) in the frequency domain so that the dynamic response of cantilever beams can be decreased. Random vibration theory is applied to identify the mean square acceleration of the endpoint of a cantilever beam as the objective function to be reduced. In addition, to determine the optimal TMD coefficient of mass, stiffness, and damping, a differential evolution (DE) optimization algorithm is employed. The upper and lower limit values of these parameters are taken into account. A majority of the previous studies have concentrated on determining just the stiffness and damping parameters of TMD. Nonetheless, in this study there is also the optimization of TMD mass parameters to determine the mass quantity. In addition, there has been inefficient use of the stochastic DE optimization algorithm method for the optimization of TMD parameters in previous studies. Hence, to obtain optimal TMD parameters, this algorithm is precisely used on the objective function. Tests are carried out on the cantilever beam with the TMD system following this optimization method with harmonic base excitations that resonate the foremost modes of the beam and white noise excitation. The method proposed here is reasonably practical and successful regarding the optimal TMD design. When a TMD is designed appropriately, the response of the cantilever beam under dynamic interactions undergoes a considerable reduction.

The seismic performance of conventional tuned mass damper (TMD) has been often improved when more TMD mass ratio is utilized. One limitation in using higher TMD mass ratios for tall buildings is the challenges of designers from the practical point of view. So far, conventional TMD has been more uneconomical. The research on the seismic performance of friction tuned mass dampers (FTMD) is still going on. This paper aimed at evaluating the advantages of the optimal design of friction TMD over conventional TMD for tall structures. For this aim, an optimal design was developed based on a multi-objective cuckoo search optimization algorithm to find the optimal TMD and FTMD parameters, including mass, damping, frequency ratios, and the friction coefficient. Here, the seismic performances of a 40-storey tall building were evaluated and compared from structural responses and energy. Results showed that both dampers could significantly reduce the maximum floor displacement, drift, and acceleration. Furthermore, the FTMD system exhibited a better performance in reducing the roof displacement against the TMD system when the mass ratio was less than 0.03. These advantages are considered to be very important from a practical point of view.

An efficient method for restraining the large vibration displacements and loads of offshore floating wind turbines under harsh marine environment is proposed by putting tuned mass dampers in the cabin. A dynamics model for a barge-type offshore floating wind turbine with a fore–aft tuned mass damper is established based on Lagrange’s equations; the nonlinear least squares Levenberg–Marquardt algorithm is employed to identify the parameters of the wind turbine; different parameter optimization methods are adopted to optimize tuned mass damper parameters by considering the standard deviation of the tower top longitudinal displacement as the objective function. Aiming at five typical combined wind and wave load cases under normal running state of the wind turbine, the dynamic responses of the wind turbine with/without tuned mass damper are simulated and the suppression effect of the tuned mass damper is investigated over the wide range of load cases. The results show that when the wind turbine vibrates in the state of damped free vibration, the standard deviation of the tower top longitudinal displacement is decreased approximately 60% in 100 s by the optimized tuned mass damper with the optimum tuned mass damper mass ratio 1.8%. The standard deviation suppression rates of the longitudinal displacements and loads in the tower and blades increase with the tuned mass damper mass ratio when the wind turbine vibrates under the combined wind and wave load cases. When the mass ratio changes from 0.5% to 2%, the maximum suppression rates vary from 20% to 50% correspondingly, which effectively reduce vibration responses of the offshore floating wind turbine. The results of this article preliminarily verify the feasibilities of using a tuned mass damper for restraining vibration of the barge-type offshore floating wind turbine.

Structural control methods are a promising way to improve the dynamic response of offshore wind turbines. In this study, the effectiveness of passive tuned mass damper (TMD) and hybrid mass damper (HMD) control is examined for suppressing the vibration in a monopile offshore wind turbine subjected to a combination of wind, wave, and seismic loads. A high-fidelity wind turbine model is established based on the multi-body dynamics simulation code SIMPACK. A reduced-order model of the wind turbine is, then, extracted from input-output time-domain response data, which is used to design an HMD controller using the H∞ loop shaping approach. The controller is, then, applied in the high-fidelity multi-body model of the wind turbine, and an additional control force is applied using feedback from the displacement acceleration at the tower top. The performance of the passive TMD and HMD control systems is examined and compared in terms of the suppression of tower-top displacements under normal operating and parked conditions. The results revealed that the HMD control system can better reduce the tower-top displacement as compared to the passive TMD system before and during earthquakes, albeit at the expense of high input control power and large TMD displacements. However, the two control systems have a negative impact on the dynamic response of tower after the earthquake. Moreover, the investigation of controller gains indicated that the vibration suppression effect of HMD improves with the increase in control power, leading to larger TMD strokes.

Offshore wind turbines (OWTs) are becoming more attractive than their onshore counterparts due to advantages such as high wind speed, less visual impact and less noise constraints in the marine area. However, the combined wind, wave and seismic loading and other environmental effects render the offshore wind turbines suffering from excessive vibration which adversely influences the system performance and structural integrity. In this regard, the present dissertation aims to develop effective structural control techniques to mitigate three-dimensional structural vibrations of offshore wind turbines. Analytical models of a fixed-bottom and floating offshore wind turbine are established using Euler-Lagrangian equation wherein the interaction between the blades and the tower is modeled. The aerodynamic loading, hydrodynamic loading, hydrostatic effect and mooring cables are also incorporated in the model. The turbulent wind field profile has been generated using Kaimal spectrum and Matlab codes have been developed to map the full wind field profile onto each span rotation of the rotating blades. The aerodynamic loading is calculated using the blade element momentum method where the Prandtl’s tip loss factor and the Glauert correction are considered. JONSWAP spectrum is used to generate wave time histories, and the hydrodynamic loading is estimated using Morison’s equation. The hydrostatic effect on the floating wind turbine is modeled based on the Archimedes principle. A dynamic linking library, MoorDyn, is used to model the mooring cables. To reduce the bi-directional vibrations of the fixed-bottom monopile OWT, a three dimensional pendulum tuned mass damper (3d-PTMD) and a three dimensional pounding pendulum tuned mass damper (3d-PPTMD) are proposed. Fatigue damage of the tower is calculated for the controlled and uncontrolled OWT using Miner’s rule and rain-flow cycle counting method. It is found that the proposed 3d-PTMD can reduce the fatigue damage under real metocean conditions. Also, research results indicate that the 3d-PPTMD can improve the performance of the 3d-PTMD facing off-tuning issues and reduce the stroke. Next, two novel controllers are proposed to reduce a spar type floating offshore wind turbine structural responses in roll, pitch and heave directions. Dual linear pounding TMDs (2PTMDs) and a three-dimensional nonlinear tuned mass damper(3d-NTMD) are proposed. The 2PTMDs mitigation effect is evaluated and compared with traditional TMDs and it is observed that the 2PTMDs can provide effective reduction in pitch and roll directions with a 50% smaller stroke compared to the traditional TMDs. To reduce the heave, pitch and roll responses, a 3d-NTMD is proposed and