Reinforcement learning-based structural control of floating wind turbines

The structural control of floating wind turbines using active tuned mass damper is investigated in this article. To our knowledge, this is for the first time that reinforcement learning-based control approach is employed to this type of application. Specifically, an adaptive dynamic programming (ADP) algorithm is used to derive the optimal control law based on the nonlinear structural dynamics, and the large-scale machine learning platform Tensorflow is employed for the design and implementation of the neural network (NN) structure. Three fully connected NNs, i.e., a plant network, a critic network, and an action network, are included in the proposed NN structure. Their training requires the gradient information flowing through the whole network, which is tackled by automatic differentiation, a popular technique for deriving the gradients of complex networks automatically. While to our knowledge, the network structures in the existing literature are rather simple and the training of the hidden layer is usually ignored. This allows their gradients to be derived analytically, which is infeasible with complex network structures. Thus, automatic differentiation greatly improves the employed ADP algorithm's ability in solving complex problems. The simulation results of structural control of floating wind turbines show that ADP controller performs very well in both normal and extreme conditions, with the standard deviation of the platform pitch displacement being reduced by around 40%. A clear advantage of ADP controllers over the H∞ controller is observed, especially in extreme conditions. Moreover, our design considers the tradeoff between the control performance and power consumption

Fluid inerter for optimal vibration control of floating offshore wind turbine towers

This paper proposes the use of a tuned mass damper fluid-inerter (TMDFI) for vibration control of spar-type floating offshore wind turbine towers. The use of an inerter in parallel with the spring and damper of a tuned mass damper (TMD) is a relatively new concept. The ideal inerter has a mass amplification effect on the classical TMD leading to greater vibration control capabilities. Previous work by the authors has shown that inerter based TMDs have great potential in vibration control of floating offshore wind turbines where enhanced vibration mitigation can be achieved using a relatively lighter device than classical TMDs. However, this previous work was based on the assumption of an ideal inerter that assumes the use of a mechanical flywheel type inerter. Mechanical inerters have some inherent disadvantages due to their complexity in design and high cost of maintenance. The use of a fluid inerter can alleviate these disadvantages as its design is rather simple and it comes with very low maintenance. Such devices have been proposed and investigated in the literature, however, their applicability in vibration control of floating wind turbines has not been investigated by researchers. The optimal design of a TMDFI is presented in this paper. It has been shown that optimization of a TMDFI is a six-dimensional non-linear optimization problem whose solution hyperplane contains multiple local minima. A systematic way has been developed in this paper, avoiding the use of metaheuristic search techniques, to optimize the damper while providing greater insight into the damper properties that offers a set of guidance to the designer. Numerical results demonstrate impressive vibration control capabilities of this new device under various stochastic wind-wave loads. It has been shown that the fluid-inerter performs as well as the ideal mechanical inerter. The considerable advantages of a TMDFI over the classical TMD demonstrated in this paper makes it an exciting candidate for vibration control

Complementary Airflow Control of Oscillating Water Columns for Floating Offshore Wind Turbine Stabilization

The implementation and integration of new methods and control techniques to floating offshore wind turbines (FOWTs) have the potential to significantly improve its structural response. This paper discusses the idea of integrating oscillating water columns (OWCs) into the barge platform of the FOWT to transform it into a multi-purpose platform for harnessing both wind and wave energies. Moreover, the OWCs will be operated in order to help stabilize the FOWT platform by means of an airflow control strategy used to reduce the platform pitch and tower top fore-aft displacement. This objective is achieved by a proposed complementary airflow control strategy to control the valves within the OWCs. The comparative study between a standard FOWT and the proposed OWC-based FOWT shows an improvement in the platform’s stability.This work was supported in part by the Basque Government, through project IT1207-19 and by the MCIU/MINECO through the projects RTI2018-094902-B-C21 and RTI2018-094902-B-C22 (MCIU/AEI/FEDER, UE)

Robust structural control of an underactuated floating wind turbine

This paper investigates the dynamic modeling and robust control of an underactuated floating wind turbine for vibration suppression. The offshore wind turbine is equipped with a tuned mass damper on the floating platform. The Lagrange's equation is employed to establish the limited degree‐of‐freedom dynamic model. A novel disturbance observer‐based hierarchical sliding mode control system is developed for mitigating loads of the underactuated floating wind turbine. In the proposed control scheme, two prescribed performance nonlinear disturbance observers are developed to estimate and counteract unknown disturbances, where the load induced by wave is considered as a mismatched disturbance while the load caused by wind is treated as a matched disturbance. The hierarchical sliding mode controller regulates the states of such an underactuated nonlinear system. In particular, the first‐order sliding mode differentiator is used to avoid the tedious analytic computation in the sliding mode control design. The stability of the whole closed‐loop system is rigorously analyzed, and some sufficient conditions are derived to guarantee the convergence of the states for the considered system. Numerical simulations deployed on both the design model and the National Renewable Energy Laboratory 5‐MW wind turbine model are provided, which demonstrate great effectiveness and strong robustness of the proposed control scheme

Accelerated Controller Tuning for Wind Turbines Under Multiple Hazards

During their lifecycle, wind turbines can be subjected to multiple hazard loads, such as high-intensity wind, earthquake, wave, and mechanical unbalance. Excessive vibrations, due to these loads, can have detrimental effects on energy production, structural lifecycle, and the initial cost of wind turbines. Vibration control by various means, such as passive, active, and semi-active control systems provide crucial solutions to these issues. We developed a novel control theory that enables semi-active controller tuning under the complex structural behavior and inherent system nonlinearity. The proposed theory enables the evaluation of semi-active controllers’ performance of multi-degrees-of-freedom systems, without the need for time-consuming simulations. A wide range of controllers can be tested in a fraction of a second, and their parameters can be tuned to achieve system-level performance for different optimization objectives

Fuzzy Airflow-Based Active Structural Control of Integrated Oscillating Water Columns for the Enhancement of Floating Offshore Wind Turbine Stabilization

This paper presents the modeling and stabilization of a floating offshore wind turbine (FOWT) using oscillating water columns (OWCs) as active structural control. The novel concept of this work is to design a new FOWT platform using the ITI Energy barge with incorporated OWCs at opposite sides of the tower, in order to alleviate the unwanted system oscillations. The OWCs provide the necessary opposing forces to the bending moment of the wind upon the tower and the waves upon the floating barge platform. However, the forces have to be synchronized with the tilting of the system which will be ensured by the proposed fuzzy airflow control strategy. Using the platform pitch angle, the fuzzy airflow control opens the valve of one side and closes the valve of the other side accordingly. Results of simulation in comparison with the standard FOWT and a PID-based airflow control show the efficiency of the fuzzy airflow control and its superiority to decrease the platform pitching and the top tower fore-aft displacement.The authors would like to thank the Basque Government for funding their research work through project IT1555-22 and the Ministry of Science and Innovation (MCIN) for funding their research work through projects PID2021-123543OB-C21 and PID2021-123543OB-C22 by MCIN/AEI/10.13039/501100011033/FEDER, UE, and the University of the Basque Country (UPV/EHU) through the María Zambrano grant MAZAM22/15 funded by UPV-EHU/MIU/Next Generation, EU

Structural Control Strategies for Load Reduction of Floating Wind Turbines

Doktorgradsavhandling ved Fakultet for Teknologi og realfag, Universitetet i Agder, 2015Offshore wind energy has attracted great worldwide attention in recent years, while strong potentials have been found in deep sea areas in many places, such as the coastal lines of the United States, north Europe, and east Asia. According to extensive experiences in offshore industry, floating foundation for wind turbines is considered as an economical and applicable solution. So far, plenty of numerical investigations have been conducted by world-wide research institutions, and different kinds of prototype programs have also been launched, including OC3-Hywind, MIT/NREL TLP, ITI Barge, and Principle Power WindFloat, etc. One big challenge for floating windmills different from fixed bottom installations is the extra platform motion, which will heavily increase the load on turbine structure due to the high inertial and gravitational forces or even cause the failure of turbine control strategy. Special mechanical design or advanced control technique is required to improve wind turbine reliability, and effective load reduction methods are needed for the design of floating wind turbines. Among different approaches for load mitigation, structural control has offered a direct solution to dynamically compensate the vibrations of turbine structures and reduce their loads. This dissertation is mainly about the numerical investigations of different structural control ideas for load reduction of floating wind turbines. The state-of-the-art wind turbine simulator FAST-SC (customized for structural control analysis) is used in the simulation analysis, and different scenarios, including the below rated, rated, and parked situations, are considered respectively. Papers A and B are dealing with the parameter optimization problem of a spar-type floating wind turbine equipped with tuned mass dampers (TMDs). The passive structural control devices can either be installed inside the platform (Paper A) or along the nacelle (Paper B). Different performance indices and parameter optimization methods are adopted for TMD parameter determination, including frequency analysis, exhaustive search, and intelligent algorithms. Particularly, a mathematical model for wind turbine surge-heavepitch motion is established based on the D’Alembert’s principle of inertial forces. Paper C investigates the idea of installing tuned liquid column dampers (TLCDs) in floating wind turbines for load reduction, and the code FAST-SCTLCD is implemented based on FAST-SC for fully coupled high-fidelity wind turbine simulation with semi-active structural control channel. Optimal parameters are computed by using genetic algorithm based on the established model, while how to tune the head loss coefficient remains to be investigated. Paper D proposes a gain scheduling H2/H∞ active structural control deign for a hybrid mass damper (HMD) installed at the tower top of a floating wind turbine. The wind turbine dynamic model is improved in this work based on polynomial curve fitting approach, and different steady-state points are derived. The state feedback controller is designed by solving linear matrix inequalities (LMIs). However, full-state feedback controller is technically impossible to implement due to lack of sensors, while the observer-based control design could be a possible solution. Then, Paper E discusses this idea, and an observer-based guaranteed cost structural controller is developed

Floating wind turbine energy and fatigue loads estimation according to climate period scaled wind and waves

Offshore wind power is one of the fastest-growing renewable energy sources, as it is expected to play a major role in the transition towards sustainability and net zero emissions. Despite its potential, the interaction of the turbines with the oceanic waves, especially in case of floating turbines, is one of the main drawbacks associated to it. In fact, mechanical oscillations caused by the waves could potentially alter the operation and lifetime of the turbines. Hence, while the characterization of the wind is sufficient for the long-term design of onshore wind turbines, the procedure is more complex in case of offshore turbines, since the height, period and direction of the waves will affect the lifetime of the turbine. In this paper, a methodology for the evaluation of the energy generation and fatigue mechanical loads of a Floating Offshore Wind Turbine (FOWT) considering a 30-year period is proposed. To that end, meteorological data from 1991 to 2020 are characterized using a cluster analysis and reduced into a computationally affordable number of simulation cases. Results show negligible energy loss of a FOWT due to interaction with the oceanic waves. However, a substantial increment of the mechanical fatigue in the side-side and fore-aft bending moments of the tower are detected. Such analyses might be applied for the predictability of the lifetime of an offshore wind turbine, as well as the selection of potential optimal wind farm locations, based on climatic patterns and the evolution of meteorological data.The authors acknowledge grant PID2020-116153RB-I00 funded by MCIN/AEI/10.13039/501100011033 and, as appropriate, by “ERDF A way of making Europe”, by the “European Union” or by the “European Union NextGenerationEU/PRTR”. Additionally, financial support by the University of the Basque Country under the contract (UPV/EHU project GIU20/008) has been received

Active Blade Pitch and Hull-Based Structural Control of Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWTs) have the potential to bring renewable energy to waters too deep for traditional offshore wind turbines while still being able to harness strong coastal winds in areas near population centers. However, these floating wind turbines come at a higher capital cost relative to fixed foundations and are more susceptible to vibrations induced by waves. Advances in control technologies offer the potential to reduce fatigue loads due to these vibrations, extending the life of the platform and thereby spreading the capital costs of the turbine over a longer period of time. One such advance is in blade pitch control, a standard component of most modern wind turbines. Existing solutions for adapting the blade pitch controller for use on a floating platform either detune the controller with the result of slowed response, make use of complicated tuning methods, or incorporate a nacelle velocity feedback gain. With the goal of developing a simple control tuning method for the general FOWT researcher that is easily extensible to a wide array of turbine and hull configurations, this last idea is built upon by proposing a simple tuning strategy for the feedback gain. This strategy uses a two degree-of-freedom (DoF) turbine model that considers tower-top fore-aft and rotor angular displacements. For evaluation, the nacelle velocity term is added to an existing gain scheduled proportional-integral controller as a proportional gain. The modified controller is then compared to baseline land-based and detuned controllers on semisubmersible, spar, and TLP systems for several load cases. Results show that the new tuning method balances power production and fatigue load management effectively, demonstrating that it is adaptable to many different types of hulls. This makes it useful for prototype design. Advances in hull-based structural control are also considered through the evaluation and development of a gain schedule for a novel type of adjustable tuned mass damper known as a ducted fluid absorber. This type of tuned mass damper uses compressed air to adjust its natural frequency, and so the amount of power consumed by the compressors is evaluated relative to the output of the wind turbine. Performance of a hull designed for ducted fluid absorbers is evaluated for several incoming wave directions to ensure consistent performance, and the potential for extracting electricity from the ducted fluid absorbers is considered. Finding the dampers to be feasible for use, a method of scheduling the settings of these dampers to minimize the standard deviation of a platform rigid-body mode of choice is developed. The addition of the dampers is found to produce significant reductions in the magnitude of several vibration modes, though the advantages of actively controlling the damper setting are small relative to those of simply having the dampers

Performance Analysis on the Use of Oscillating Water Column in Barge-Based Floating Offshore Wind Turbines

Undesired motions in Floating Offshore Wind Turbines (FOWT) lead to reduction of system efficiency, the system’s lifespan, wind and wave energy mitigation and increment of stress on the system and maintenance costs. In this article, a new barge platform structure for a FOWT has been proposed with the objective of reducing these undesired platform motions. The newly proposed barge structure aims to reduce the tower displacements and platform’s oscillations, particularly in rotational movements. This is achieved by installing Oscillating Water Columns (OWC) within the barge to oppose the oscillatory motion of the waves. Response Amplitude Operator (RAO) is used to predict the motions of the system exposed to different wave frequencies. From the RAOs analysis, the system’s performance has been evaluated for representative regular wave periods. Simulations using numerical tools show the positive impact of the added OWCs on the system’s stability. The results prove that the proposed platform presents better performance by decreasing the oscillations for the given range of wave frequencies, compared to the traditional barge platform.This work was supported in part by the Basque Government, through project IT1207-19 and by the MCIU/MINECO through the projects RTI2018-094902-B-C21 and RTI2018-094902-B-C22 (MCIU/AEI/FEDER, UE)