Floating Wind Turbine System Research Articles

Fault-Tolerant Control of Floating Wind Turbine With Switched Adaptive Sliding Mode Controller

The fast-growing development of floating wind turbines demands control systems capable of both reducing output power fluctuations and fault-tolerant function. In this paper, we propose an adaptive switched sliding mode controller to enhance the performance of floating wind turbine systems in the presence of environmental uncertainties and actuator faults. A control-oriented switched linear model for floating wind turbines is introduced, considering the average dwell time. Based on the proposed controller, a full-order state observer and an adaptive law compensate for the debilitated control outputs resulting from detecting errors, disturbances, and faults. The control parameters are derived by solving stability theorems, which are proved by the Lyapunov stability theory, linear matrix inequality technique, and average dwell time technique. The proposed model and controller are validated on the NREL 5MW wind turbine and spar-buoy platform using the high-fidelity fatigue, aerodynamics, structures, and turbulence (FAST) code. The performance of the proposed controller is compared with an optimal gain-scheduling proportional-integral controller under different wind-wave combined conditions. The results show that the proposed controller improves the power quality and attenuates mechanical loads of floating wind turbine under healthy and faulty conditions. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —Floating wind turbines have drawn significant interest in renewable energy. When operating in the ocean far from shore, it is of great importance for floating wind turbines to have fault-tolerance capabilities for achieving stable power quality when faults occur in one or more components. A challenging problem is how to mitigate the fault effects on the wind turbine system. This paper proposes an adaptive fault-tolerant control strategy in the case of actuator fault occurrence in floating wind turbines.

Exploring Motion Stability of a Novel Semi-Submersible Platform for Offshore Wind Turbines

The stability of offshore floating wind turbine foundation platforms is a fundamental requirement for the efficiency and safety of wind power generation systems. This paper proposes a novel small-diameter float-type semi-submersible platform to improve system stability. To evaluate the superior motion stability of the proposed floating platform, a comprehensive frequency–domain response analysis and experimental study were conducted in comparison with the OC4-DeepCwind platform developed by the National Renewable Energy Laboratory (NREL). The respective comparison of the frequency–domain response analysis and the experimental results demonstrated that the proposed floating wind turbine platform shows better hydrodynamic characteristics and resonance avoidance capability. This not only reduces the Response Amplitude Operators (RAOs), but also enhances the system stability, namely, effectively avoiding the regions of concentrated wave loading and low-frequency ranges. Furthermore, the proposed small-diameter semi-submersible platform has the potential to reduce manufacturing costs, providing valuable insights for the manufacturing of offshore floating wind turbine systems.

A Review of Numerical and Physical Methods for Analyzing the Coupled Hydro–Aero–Structural Dynamics of Floating Wind Turbine Systems

Recently, more wind turbine systems have been installed in deep waters far from the coast. Several concepts of floating wind turbine systems (FWTS) have been developed, among which, the semi-submersible platform—due to its applicability in different water depths, good hydrodynamic performance, and facility in the installation process—constitutes the most explored technology compared to the others. However, a significant obstacle to the industrialization of this technology is the design of a cost-effective FWTS, which can be achieved by optimizing the geometry, size, and weight of the floating platform, together with the mooring system. This is only possible by selecting a method capable of accurately analyzing the FWTS-coupled hydro–aero–structural dynamics at each design stage. Accordingly, this paper provides a detailed overview of the most commonly coupled numerical and physical methods—including their basic assumptions, formulations, limitations, and costs used for analyzing the dynamics of FWTS, mainly those supported by a semi-submersible—to assist in the choice of the most suitable method at each design phase of the FWTS. Finally, this article discusses possible future research directions to address the challenges in modeling FWTS dynamics that persist to date.

Design of an Innovative Point Array Offshore Floating Wind Turbine System and Analysis of Hydrodynamic Coupling Response Law

The new lattice floating wind turbine integrated system (also known as dot matrix floating wind turbine, and hereinafter referred to as DMF) is proposed as a new concept. More far reaching, it has obvious advantages over the traditional floating wind turbine scheme in terms of structural cost and motion stability, which provides a new idea for the development of offshore wind power energy. First, the structural parameters and mechanical model of DMF are analyzed to determine the feasibility and superiority of the overall scheme of the new lattice foundation. It is concluded that the stability of DMF in pitching motion is 70% higher than that of the traditional OC4 system. In order to further verify the feasibility of the DMF system and the accuracy of the theoretical model, based on the similarity theory, the research carried out the small‐scale prototype processing of DMF and the simulation experiment of wind wave flume. The test results are in good agreement with the simulation data. This study provides a reference and theoretical basis for the research and development of offshore multi wind turbine combined equipment and hydrodynamic stability optimization. It has certain theoretical guiding significance and economic development value.

Hydrodynamic and Static Stability Analysis of a Hybrid Offshore Wind-Wave Energy Generation

Marine structures like Floating Wind Turbine (FWT) is exposed to the oncoming waves and wind that can cause oscillatory motions within the system. These undesired oscillations can have negative impacts on the efficiency of the system, reduce its lifespan, hinder energy extraction, increase stress levels, and raise maintenance costs. To mitigate these negative impacts, the integration of Wave Energy Converters (WECs) into the FWT system has been proposed. This hybrid system may be capable of extracting coupled wind-wave energy and transferring electrical power to the shared grid. This paper presents an investigation of the use of Oscillating Water Columns (OWCs), a type of WECs, within a FWT system. The purpose of using an OWC to increase the hydrodynamic damping and reduce the resonant motions of the floating wind turbines under environmental loads, including both wind and wave loads. This is because the wave energy from OWC would be very small as compared to the wind energy. However, OWCs can provide a damping source for reducing the resonant motions of the floater, especially the pitch resonant motions. This would be very beneficial for the power performance of the floating wind turbine and the structural design of the floater. The purpose of this paper is to redesign the original FWT platform to accommodate the additional OWCs by considering the hydrostatic stability and hydrodynamics since the new elements, the OWCs, can significantly change the response of the platform. The redesign of the original FWT involves the integration of OWCs within two out of three columns of an existing semisubmersible platform for a 12 MW FWT. To do this, two moonpools, which are consistent with OWC air chambers, have been created within two columns of the FWT. The water ballast was designed for the columns with and without OWCs. After that the redesign is done hydrostatic stability and hydrodynamics analyses are evaluated. The hydrodynamics properties are discussed in terms of the hybrid platform response as compared to the original platform. The hybrid platform was modeled using GeniE and the hydrostatic stability and hydrodynamics of the system was evaluated by HydroD, tools developed and marketed by DNV. The results of this study demonstrate the potential benefits of integrating OWCs within a FWT system in terms of reducing the platform oscillatory motion.&#x0D;

Wind/Wave Testing of a 1:70-Scale Performance-Matched Model of the IEA Wind 15 MW Reference Wind Turbine with Real-Time ROSCO Control and Floating Feedback

Experimental results from the Floating Offshore-wind and Controls Advanced Laboratory (FOCAL) experimental program, which tested a performance-matched model of the IEA Wind 15 MW Reference Turbine on a 1:70 scale floating semisubmersible platform, are compared with OpenFAST simulations. Four experimental campaigns were performed, and data from the fourth campaign, which focused on wind and wave testing of the scaled floating wind turbine system, are considered. Simulations of wave-only, wind-only, and wind/wave environments are performed in OpenFAST, and results for key metrics are compared with the experiment. Performance of the real-time Reference OpenSource COntroller (ROSCO) in above-rated wind conditions, including the effects of the floating feedback loop, are investigated. Results show good agreement in mean values for key metrics, and hydrodynamic effects are matched well. Differences in the surge resonant behavior of the platform are identified and discussed. The effect of the controller and floating feedback loop is evident in both the experiment and OpenFAST, showing significant reduction in platform pitch response and tower base bending load near the platform pitch natural frequency.

Coupled aero-hydro-mooring dynamic analysis of floating offshore wind turbine under blade pitch motion

Accurate prediction of the dynamic responses of the floating offshore wind turbine (FOWT) under the blade pitch motion is quite challenging because of the strong nonlinear effects. In this study, a fully coupled and highly elaborated model was established based on the computational fluid dynamics, with the dynamic fluid body interaction method. The multi-stage movements consisting of the six degrees of freedom motions of the platform, the rotation of the rotor, and the blade pitch motion were defined by the superposition motion technologies. The blade pitch control module was created through the user-defined function to regulate the blade pitch motion. Then, several coupled dynamic simulations of the full-configuration DeepCwind floating wind turbine system were performed in power production, shutdown, and startup cases. The simulation results in the power production case indicate that the blade pitch motion decreases the generated aerodynamic loads and amplifies the response amplitude of the platform as negative damping is introduced in the FOWT system. The simulation results in the shutdown and startup cases indicate that the extreme motion responses are enlarged, and the mooring line tension oscillates dramatically when it is in high-tension states. In addition, the nonlinear interference effects in the unsteady flow fields, such as the shedding vortices broken by the blade pitch motion, are visualized and investigated in detail.

Normal Operating Performance Study of 15 MW Floating Wind Turbine System Using Semisubmersible Taida Floating Platform in Hsinchu Offshore Area

This study predicted the motion response and power performance of a floating wind turbine system equipped with a semisubmersible Taida platform, an IEA 15 MW wind turbine, and a 3 × 2 mooring design in the Hsinchu offshore area in the Taiwan Strait. The hydrodynamic properties were calculated using ANSYS-AQWA and STAR-CCM+. The motion equations were solved by OrcaFlex to obtain the motion response and generator power, as well as the dynamics of the mooring system and aerodynamics of the wind turbine. The waves were assumed to share the same direction as the wind. This study compared the mean values and standard deviations of the motion response, generator power, and mooring line tension between the potential- and viscous-flow approaches by considering the combination of seven wind directions and four current directions under two wave conditions in the Hsinchu offshore area. The numerical prediction shows that the viscous effect has a larger impact on the hydrodynamic properties in the heave, roll, and pitch motions. The angle between the leading mooring line of the system and dominant wind direction in the Taiwan Strait, which comes from the northeast, should be from 120° to 180° in order to deliver a relatively favorable performance of the system.

Roll motion mitigation of a barge-type floating wind turbine under random excitation using a tuned liquid column damper

ABSTRACT This paper aims to numerically evaluate the performance of tuned liquid column damper (TLCD) on mitigating roll motion of a barge-type floating wind turbine system subjected to random excitation. The governing equations for modeling the interaction between the roll motion of the floating system and liquid sloshing in the TLCD were derived using a Lagrangian approach. Random excitation based on wave spectrum was generated to simulate applied moment. Then, under the constraints of barge dimension and vertical liquid column movement, Den Hartog’s method was used to obtain optimum TLCD parameters. The results show that the optimally designed TLCD can efficiently mitigate the roll motion of the floating wind turbine.

Design of adaptive continuous barrier function finite time stabilizer for TLP systems in floating offshore wind turbines

This essay discusses the finite-time stabilization of Tension Leg Platforms (TLP), one of the most considerable offshore floating wind turbine systems. The most critical issue in installing such turbines is controlling and balancing them on the water. In order to stabilize the TLP systems in floating offshore wind turbines, an adaptive terminal sliding mode controller is designed. Because the discontinuous signal function is one of the most important disadvantages of common sliding mode controllers, which causes an undesirable chattering phenomenon in the control law, the suggested approach introduces a new Lyapunov candidate function based on the adaptive continuous barrier function technique, so that the planned control law on TLP systems is obtained continuously and smoothly. The mentioned improved control method uses Lyapunov's stability theory and satisfies the convergence of states around the switching surface in a finite time. In this way, the proposed method removes the chattering phenomenon and operates with higher accuracy than the conventional controllers. In addition, the unknown bound of external disturbance is estimated using the adaptive control scheme. Finally, based on the simulation results, it can be seen that all the state variables of the TLP system are asymptotically stable in the presence of external disturbance and converge to zero, which indicates the efficiency of the proposed technique for stabilizing the tension leg platforms in offshore floating wind turbines.

Coupled dynamic response analysis of multi-column floating offshore wind turbine with low center of gravity

To realize the application of the floating offshore wind turbine (FOWT) from deep to relatively shallow waters, a new concept of multi-column floating wind turbine platform with low center of gravity (CG) is designed and validated. The multi-column low CG platform is designed to support a 6MW wind turbine class and operated at a water depth of 50m in the South China Sea. The frequency domain software WADAM and time domain software NREL-FAST are used to simulate coupled dynamic responses of the floating wind turbine system with second-order wave loads considering. The dynamic behaviors of multi-column low CG FOWT system under normal operation and parked conditions are presented. The influence of second-order wave force on the motion responses of the multi-column platform, fore-aft force and moment of the tower base and mooring force are researched respectively. The results demonstrate that the coupled dynamic responses at rated operating condition and extreme condition meet the normal operating requirements and extreme survival requirements of FOWT system in the shallow water (50m) of South China Sea. In addition, it is found that, the wave frequency response gradually replaces the second-order low frequency response as the main influencing factor of the coupled dynamic response of the FOWT system with the increasing severity of the sea states. However, in general, the magnitude of second-order low frequency response increases with the increasing severity of the design load case. Thus, in the subsequent design of the shallow water FOWT system, the second-order effects should be paid enough attention.

On the hydrodynamic responses of a multi-column TLP floating offshore wind turbine model

The offshore floating wind turbine system is a key piece of equipment for harvesting wind energy from the ocean field. However, the structural safety analysis and design of such equipment largely require a suitable platform to accurately obtain the dynamic characteristics. In the present study, an attempt was made to investigate the dynamic responses of the proposed 5-MW WindStar multi-column tension-leg-platform (TLP) system under different ocean environment conditions. Typical motion responses of the WindStar system, including the surge and pitch, and a series of measurements obtained from wave basin tests, such as nacelle acceleration and tendon tension, were compared with those of simulated results from the FAST code under four hydrodynamic load conditions. Additionally, the second-order wave load was incorporated with the first-order wave load, so as to allow for accurate evaluation of the high-frequency response of the floating offshore wind turbine (FOWT). Further, the effect of high-order wave loads on the high-frequency responses of the TLP FOWT was elucidated. The second-order frequency wave load was found to be a significant factor in determining the motion responses of the WindStar TLP system, and the results from FAST were found to agree well with those of the experiment. The coupled method of experiments and numerical analysis can be effectively used to understand the high-frequency dynamic responses of TLP FOWT systems, such as the WindStar TLP.

An Open-Source Frequency-Domain Model for Floating Wind Turbine Design Optimization

A new frequency-domain dynamics model has been developed that uses open-source components to efficiently represent a complete floating wind turbine system. The model, called RAFT (Response Amplitudes of Floating Turbines), incorporates quasi-static mooring reactions, strip-theory and potential-flow hydrodynamics, blade-element-momentum aerodynamics, and linear turbine control. The formulation is compatible with a wide variety of support structure configurations and no manual or time-domain preprocessing steps are required, making RAFT very practical in design and optimization workflows. The model is applied to three reference floating wind turbine designs and its predictions are compared with results from time-domain OpenFAST simulations. There is good agreement in mean offsets as well the statistics and spectra of the dynamic response, verifying RAFT’s general suitability for floating wind analysis. Follow-on work will include verification of potential-flow and turbine-control features and application to optimization problems.

FLOTANT concept: floater design, integrated modelling &amp; global performance

Abstract This paper presents the final design of the FLOTANT concept developed within the framework of a Cooperation Research Project funded by the European Union’s Horizon 2020 research and innovation programme. The FLOTANT concept is a hybrid concrete-plastic barge-type floating offshore substructure holding a 12MW wind turbine; buoyancy is achieved using plastic material (foam) fitted within the floater substructure. The paper describes the FLOTANT project innovations and presents the principal dimensions of the floating wind turbine system. The floating system has been developed to fulfil strict requirements at two different locations: Gran Canarias (GC) and West of Barra (WoB) sites. Therefore, two different mooring solutions including innovative elastomers to reduce peak loads are presented. The floating system’s intact and damaged static stability is checked along with the hydrostatic and hydrodynamic properties, showing the low-cost barge-type system’s feasibility. To ensure the desired behaviour of the floating system, a relevant subset of design load cases (DLCs) based on international standard requirements have been simulated checking the global performance of floater and moorings. Fully coupled hydro-servo-aero-elastic simulations have been performed to consider the wind and wave loading, the aerodynamic response of the rotor, the hydrodynamic response and structural dynamics of the turbine-floater-mooring assembly, and the control system actions. A summary of the results of the last design loop within the FLOTANT project is presented.

Data-driven control of the coupled loads for floating wind turbine systems based on deformable trailing edge flaps

Load controller design for floating wind turbine systems (FWTS) poses unique challenges owing to the couplings of platform motions, blade loads and rotor power. Any controller based on a single objective is likely to worsen other performance metrics. This paper presented a data-driven control system to comprehensively handle that muti-objective coupling problem. The control system was realized by introducing a model free adaptive control (MFAC) based on deformable trailing edge flap (DTEF) and a feedback compensation in the blade pitch control loop. The identification error and actuator saturation were considered while designing the control law. The proposed control system was numerically tested in the modified FAST code equipped with DTEF interfaces. Numerical results showed considerable load reductions on blades and floating platforms at the cost of the added DTEF actuation and a minor increase in blade pitch rate. Moreover, the generator power fluctuations and the fatigue loading on drive train and tower were also alleviated. In addition, the synchronous aero-hydro-elastic coupling relationships in the wind-blade-wave-platform system were impaired by introducing the servo-dynamics of DTEF and blade pitch feedback compensation. Hence the total energy flowing into FWTS was dissipated.

A Comparison of the Turbine Tower Damping Effects of a Series of Back Twisted Active Pitch-to-Stall Blades for a Spar and a Semi-Submersible FOWT

Abstract Floating offshore wind turbines are subjected to higher tower fatigue loads than their fixed-to-seabed counterparts, which could lead to reductions in turbine life. The worst increases are generally seen in the tower axial fatigue, associated with the tower fore-aft bending moment. For a spar type platform, this has been shown to increase by up to 2.5 times and for a semi-submersible platform, by up to 1.8 times. Reducing these loads would be beneficial, as the alternative of strengthening the towers leads to increase in cost. Here, two offshore floating wind turbine systems, of the spar type, are analyzed and selected responses and tower fatigue are compared: one incorporates a variable-speed, variable-pitch-to-stall blade control system and a back twisted blade, and the other a conventional pitch-to-feather control. The results are then compared with those obtained in an earlier study, where the same turbine configurations were coupled to a semi-submersible platform. A weighted wind frequency analysis at three mean turbulent wind speeds of 8, 13, and 18 m/s highlights that the impact of the back twist angle magnitude and initiation point on tower axial fatigue life extension was the same for both platform types. Compared with their respective feather base models, an increase in the tower axial fatigue life of 18.8% was seen with a spar platform and 10.2% with a semi-submersible platform, when a back twist angle to the tip of −6 deg was imposed along with the variable-speed, variable-pitch-to-stall control.

Water Depth Variation Influence on the Mooring Line Design for FOWT within Shallow Water Region

The objective of this paper was to present the modeling and optimization of mooring lines for floating offshore wind turbines (FOWT) located in various water depths from 50 m to 100 m in Taiwan western offshore areas. A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCwind platform with the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine. The mooring lines proposed consist of a catenary mooring with studless chains. Three nominal sizes of the mooring chain links are taken into account with diameters of 95 mm, 115 mm and 135 mm. According to this configuration, a total of five mooring designs for different water depths (i.e., 50 m, 60 m, 70 m, 80 m, 100 m) are analyzed according to the rules and regulations of the two certification institutions, Det Norske Veritas (DNV) and American Petroleum Institute (API). Considering ultimate limit state (ULS), fatigue limit state (FLS) and maximum operating sea state (MOSS) based on a typhoon with a 50-year return period and current with a 10-year return period, 25-year design life, as well as 1-year return period, respectively, long-term predictions of breaking strength, fatigue and stability are performed. The software OrcaFlex version 10.3 d is used to simulate and design the mooring lines. The obtained results show that the shallow mooring design of 50 m water depth case presents the heaviest chains among the other water depths, increasing their mooring costs. On the other hand, the 100 m water design has much longer mooring lines, making this parameter the cost driving one. Thus, the minimum mooring cost range is from 60 m to 80 m water depth.

Individual/collective blade pitch control of floating wind turbine based on adaptive second order sliding mode

A new control strategy based on adaptive second order sliding mode approach is applied to a floating wind turbine system in the above rated region. This adaptive controller is well adapted to a highly nonlinear system as floating wind turbine, and can be easily implemented with very reduced knowledge of modeling. The proposed controller partially based on multi-blade coordinates transformation combines collective and individual collective blade pitch control, for power regulation, platform pitch motion reduction and reduction of blades fatigue load. The proposed controller is implemented on FAST simulator and shows high level of performances.

Boundary vibration control of a floating wind turbine system with mooring lines

In this paper, we investigate dynamic modeling, active boundary control design, and stability analysis for a coupled floating wind turbine (FWT) system, which is connected with two flexible mooring lines. It is a coupled beam-strings structure, and we design two boundary controllers to restrain the vibrations of this flexible system caused by external disturbances based on the coupled partial differential equations and ordinary differential equations (PDEs–ODEs) model. Meanwhile, significant performance of designed boundary controllers and system’s stability are theoretically analyzed, and a set of simulation results are provided to show efficacy of the proposed approach.

Coupled Dynamic Analysis of a Tension Leg Platform Floating Offshore Wind Turbine

The dynamic response of a tension leg platform (TLP) floating offshore wind turbine (FOWT) was analyzed with considering the aero-hydro characteristic of the whole floating wind turbine system including the wind turbine, TLP platform, and tethers. The “aero-hydro” coupled dynamic analysis was conducted in ansys-aqwa with a dynamic link library (DLL) calculating the aerodynamics loading at every steptime based on the blade element momentum theory. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and regular wave show that the wind loads influence mainly the low-frequency response of the TLP FOWT. The wind loads have a large impact on the offsets of the TLP away from the initial position while the wave loads influence mainly the fluctuation amplitude of the TLP FOWT. The average TLP pitch response under the wind load is significantly larger due to the large wind-induced heeling moment on the wind turbine. In addition, the tension of tethers at the upwind end is greater than that at the downwind end. The wind loads could reduce effectively the average tension of the tethers, and the tension of tethers is significantly affected by the pitch motion. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and irregular wave show that the surge and pitch of TLP result in an obvious increase of thrust of the turbine and the amplitude of torque fluctuation, more attention should be paid to the pitch and surge motion of TLP FOWT.