Dynamic Analyses and Vibration Mitigation of Offshore Monopile Wind Turbines

To more effectively extract the wind resources in marine areas, offshore monopile wind turbines are built with larger rotor and slender tower, which are susceptible to the external vibration sources. This study develops sophisticated numerical models to more accurately capture the dynamic responses of these structures, and proposes practical passive control strategies to mitigate their excessive vibrations under wind, wave and/or seismic loads to enhance the structural performance in the normal operational conditions

Fragility reduction of offshore wind turbines using tuned liquid column dampers

High flexibility of offshore wind turbines (OWTs) makes them vulnerable to excessive vibrations. This paper studies vibration control of offshore wind turbines induced by multi-hazard excitations. A model consisting of entire offshore wind turbine foundation and tower controlled by tuned liquid column dampers(TLCD) considering nonlinear soil pile interaction is established. The model is subjected to wave, wind, and seismic loading. The effect of severity of earthquake on the performance of the structural control device is investigated. A fragility analysis based on acceleration capacity thresholds is performed to estimate reliability improvement using the structural control devices. The fitted fragility functions based on multiple stripes analysis are constructed and compared with the empirical cumulative distribution curves. The results suggest that the use of an optimal TLCD with a mass ratio of 2.5% reduces the fragility of the system by as much as 6% and 12% for operational and parked conditions, respectively

Load reduction of a monopile wind turbine tower using optimal tuned mass dampers

We investigate to apply tuned mass dampers (TMDs) (one in the fore–aft direction, one in the side– side direction) to suppress the vibration of a monopile wind turbine tower. Using the spectral element method, we derive a finite-dimensional state-space model d from an infinite-dimensional model d of a monopile wind turbine tower stabilised by a TMD located in the nacelle. and d can be used to represent the dynamics of the tower and TMD in either the fore–aft direction or the side– side direction. The wind turbine tower subsystem of is modelled as a non-uniform SCOLE (NASA Spacecraft Control Laboratory Experiment) system consisting of an Euler–Bernoulli beam equation describing the dynamics of the flexible tower and the Newton–Euler rigid body equations describing the dynamics of the heavy rotor-nacelle assembly (RNA) by neglecting any coupling with blade motions. d can be used for fast and accurate simulation for the dynamics of the wind turbine tower as well as for optimal TMD designs. We show that d agrees very well with the FAST (fatigue, aerodynamics, structures and turbulence) simulation of the NREL 5-MW wind turbine model. We optimise the parameters of the TMD by minimising the frequency-limited H2-norm of the transfer function matrix of d which has input of force and torque acting on the RNA, and output of tower-top displacement. The performances of the optimal TMDs in the fore–aft and side–side directions are tested through FAST simulations, which achieve substantial fatigue load reductions. This research also demonstrates how to optimally tune TMDs to reduce vibrations of flexible structures described by partial differential equations

STRUCTURAL CONTROL OF OFFSHORE WIND TURBINES USING PASSIVE AND SEMI-ACTIVE CONTROL

Offshore wind energy has the potential to generate substantial electricity production compared to onshore locations, due to the high-quality wind resource. Offshore wind turbines must endure severe offshore environmental conditions and be cost effective, in order to be sustainable. As a result, load mitigation becomes crucial in successfully enabling deployment of offshore wind turbines. A direct approach to reduce loads in offshore wind turbines is the application of structural control techniques. So far, the application of structural control techniques to offshore wind turbines has shown to be effective in reducing fatigue and extreme loads of turbine structures. However, the majority of previous research regarding the application of structural control to offshore wind turbine noted the needs for the high-fidelity analysis for structural control using a computer aided engineering (CAE) tool, such as FASTv8. In this dissertation, a structural control module coupled with FASTv8 is developed to meet the needs for high-fidelity analysis of structural control techniques for various OWTs. In addition, the developed control module is updated to analyze various structural control devices operating both passively and semi-actively. The dynamics of an omni-directional pendulum-type tuned mass damper and orthogonal tuned liquid column dampers (TLCDs) are mathematically modeled and incorporated into the structural control module. With the developed control module, several structural control devices are optimized through a variety of techniques (parametric study, exhaustive search and multi-objective optimization). Solving optimization problems not only provides the parameters for each control device that can be applicable to other multi-megawatts offshore wind turbines, but also provides insight into the effects of design variables on the control performance. Site-specific meteorological and oceanographic data that consists of a combination of wind and wave data are processed and compiled in order to establish key design load cases. With the optimal designs of structural control devices, non-linear fully-coupled time marching simulations are conducted by running a series of design load cases in order to investigate the impacts of passive and semi-active structural control on improving fatigue and extreme behaviors of fixed-bottom and floating offshore wind turbines. The simulation results demonstrate the effectiveness of various structural control techniques on reducing fatigue and extreme loadings

Vibration suppression for monopile and spar-buoy offshore wind turbines using the structure-immittance approach

Offshore wind turbines have the potential to capture the high-quality wind resource. However, the significant wind and wave excitations may result in excessive vibrations and decreased reliability. To reduce vibrations, passive structural control devices, such as the tuned mass damper (TMD), have been used. To further enhance the vibration suppression capability, inerter-based absorbers (IBAs) have been studied using the structure-based approach, that is, proposing specific stiffness-damping-inertance elements layouts for investigation. Such an approach has a critical limitation of being only able to cover specific IBA layouts, leaving numerous beneficial configurations not identified. This paper adopts the newly introduced structure-immittance approach, which is able to cover all network layout possibilities with a predetermined number of elements. Linear monopile and spar-buoy turbine models are first established for optimisation. Results show that the performance improvements can be up to 6.5% and 7.3% with four and six elements, respectively, compared with the TMD. Moreover, a complete set of beneficial IBA layouts with explicit element types and numbers have been obtained, which is essential for next-step real-life applications. In order to verify the effectiveness of the identified absorbers with OpenFAST, an approach has been established to integrate any IBA transfer functions. It has been shown that the performance benefits preserve under both the fatigue limit state (FLS) and the ultimate limit state (ULS). Furthermore, results show that the mass component of the optimum IBAs can be reduced by up to 25.1% (7,486 kg) to achieve the same performance as the TMD

Evaluating passive structural control of tidal turbines

The research focus of this project is to design a methodology to reduce fatigue and peak structural loading experienced by support structures used for tidal stream converters. The methodology is based on the dynamic analysis of a towermonopile support structure for offshore wind turbines. A tuned mass damper (TMD) is implemented in the nacelle in fore-aft direction by correcting the discrete equation of motion of a fixed tidal turbine. Parameters such as added mass and viscous damping were thus incorporated in the mass and damping matrix to study the effects of using a TMD on a tidal energy converter. Both frequency and time domain analysis are presented to compare the TMD effect in different conditions. Moreover a sensitivity analysis in soil effect and different tower-monopile shape is presented. The result shows the influence of the TMD for a fixed tidal turbine when the structure suffers an instant impact and under unsteady continuous wave-current coupled forces

Data-driven structural control of monopile wind turbine towers based on machine learning

This paper studies the data-driven structural control of monopile wind turbine towers based on machine learning approach, by using an active tuned mass damper (TMD) located in the nacelle. The adaptive dynamic programming (ADP) approach is employed to obtain the optimal controller which is derived on the modern large-scale machine learning platform Tensorflow. The proposed network structure includes three simple three-layer neural networks (NNs), i.e. a plant network, a critic network, and an action network. The plant network is used to capture the fully nonlinear dynamics of the structural system while the action network is used to approximate the optimal controller. Their training requires the gradient information flowing through the whole network. The automatic differentiation is used in this paper for all the gradient derivations, which greatly improves the employed ADP algorithm’s ability in solving complex practical problems. The simulation results of structural control of monopile turbine towers show that on average the active TMD achieves 15% performance improvement on tower fatigue load reduction over a passive TMD, with small active power consumption (less than 0.24% of the turbine’s nominal power production). Besides, the controller design considers the trade-off between control performance and power consumption