Wind Turbine Platform Research Articles

Spatial coherence of ocean waves in multiline anchor systems for floating offshore wind turbines

A shared multiline anchor system may reduce material and installation costs for floating offshore wind farms, but the concept also expands the physical space over which the analysis of the system must be considered. The multiline anchor force is a combination of forces that originate from different floating offshore wind turbine (FOWT) platforms, and this interconnectivity mandates investigation of spatial coherence of waves over the area of the connected turbines. The goal of this work is to determine if anchor force characteristics are sensitive to spatial coherence of the wave field, or if an assumption of independent wave fields at different FOWT locations provides sufficiently accurate anchor load characterizations. Spatially coherent irregular wave fields are generated and applied to FOWT systems in 3-line and 6-line anchor systems, revealing negligible sensitivity of multiline anchor force to wave spatial coherence. These results are continued with a parametric study showing how wave characteristics affect correlation lengths of spatially coherent waves. A review of installed offshore wind turbine spacings is presented as a context for these results. In each section, regular waves are first used to examine the dynamics in a simple context, then irregular waves are used to more realistically simulate ocean conditions.

Validation of a 750 kW semi-submersible floating offshore wind turbine numerical model with model test data, part I: Model-I

This paper describes a model test and numerical simulation of a 750-kW-semi-submersible platform wind turbine under several wind and wave conditions for validation of the numerical simulation model. The semi-submersible platform was designed to support the 750-kW-wind turbine class and operate at a water depth of 50 m. The model tests were performed to estimate the performance characteristics of the wind turbine system in the wide tank of the University of Ulsan. Motions and loads of the wind turbine system under the wind and wave conditions were measured and analyzed. The NREL-FAST code was used to simulate the wind turbine system, and the results were compared with those of the test model. The results demonstrate that the numerical simulation captures noticeably the fully coupled floating wind turbine dynamic responses. Also, the model shows a good stability and small responses during waves, wind, and operation of the 750-kW-floating offshore wind turbine.

Proposal of a Novel Semi-Submersible Floating Wind Turbine Platform Composed of Inclined Columns and Multi-Segmented Mooring Lines

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.

Ocean wave active compensation analysis of inverse kinematics for hybrid boarding system based on fuzzy algorithm

In this paper, a method of solving inverse kinematics based on fuzzy algorithm for hybrid system is presented. This approach could compensate the wave disturbances when the hybrid mechanism is conducting a certain task, such as, transporting the repairmen into wind turbine platform. By deriving the Jacobian matrix of the hybrid mechanism, the relationship between the angular and linear velocities of variables in joint space and gangway's end-effector in task-space can be obtained. Thereafter, the numerical integral method is used to obtain the inverse kinematic solution. Structurally, the hybrid mechanism consists of a 6-DOFs Stewart parallel platform and a 3-DOFs gangway. Due to the kinematical redundant of the hybrid boarding system, its inverse kinematic solutions are not unique. Thus, a fuzzy strategy is put forward to plan the variable values reasonably. Moreover, it can satisfy the requirement of variable constraints. When conducting ocean wave compensation, specifically, the Stewart parallel platform is used to compensate the wave disturbance in roll direction. Moreover, it is efficient to compensate other disturbances by adjusting the joint angles of gangway. The effectiveness and the robustness of the proposed method is demonstrated using numerical simulation when the wave disturbance is added into the hybrid boarding system.

High Voltage Ride through Control of PMSG-Based Wind Turbine Generation System Using Supercapacitor

Regarding PMSG-based wind turbine generation system, this paper proposes a supercapacitor energy storage unit (SCESU) which is connected in parallel with the DC-link of the back-to-back converter to enhance its high voltage ride through performance. The analysis of the operation and control for the grid-side converter and SCESU are conducted. Based on real time digital simulators (RTDS), a model and a Hardware-in-the-Loop (HiL) platform of PMSG-based wind turbine with SCESU is developed, and the simulation results show that the SCESU absorbs the imbalanced energy and the grid-side converter absorbs inductive reactive power during the period of voltage swell and verify the correctness and feasibility of the high voltage ride through control strategy.

Wind Turbine Controller to Mitigate Structural Loads on a Floating Wind Turbine Platform

This paper summarizes the control design work that was performed to optimize the controller of a wind turbine on the WindFloat structure. The WindFloat is a semisubmersible floating platform designed to be a support structure for a multimegawatt power-generating wind turbine. A controller developed for a bottom-fixed wind turbine configuration was modified for use when the turbine is mounted on the WindFloat platform. This results in an efficient platform heel resonance mitigation scheme. In addition, several control modules, designed with a coupled linear model, were added to the fixed-bottom baseline controller. The approach was tested in a fully coupled nonlinear aero-hydro-elastic simulation tool in which wind and wave disturbances were modeled. This testing yielded significant improvements in platform global performance and tower-base bending loading.

Numerical Analysis of a Floating Offshore Wind Turbine by Coupled Aero-Hydrodynamic Simulation

The exploration for renewable and clean energies has become crucial due to environmental issues such as global warming and the energy crisis. In recent years, floating offshore wind turbines (FOWTs) have attracted a considerable amount of attention as a means to exploit steady and strong wind sources available in deep-sea areas. In this study, the coupled aero-hydrodynamic characteristics of a spar-type 5-MW wind turbine are analyzed. An unsteady actuator line model (UALM) coupled with a two-phase computational fluid dynamics solver naoe-FOAM-SJTU is applied to solve three-dimensional Reynolds-averaged Navier–Stokes equations. Simulations with different complexities are performed. First, the wind turbine is parked. Second, the impact of the wind turbine is simplified into equivalent forces and moments. Third, fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing the UALM. From the simulation, aerodynamic forces, including the unsteady aerodynamic power and thrust, can be obtained, and hydrodynamic responses such as the six-degrees-of-freedom motions of the floating platform and the mooring tensions are also available. The coupled responses of the FOWT for cases of different complexities are analyzed based on the simulation results. Findings indicate that the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform are obvious. The aerodynamic loads have a significant effect on the dynamic responses of the floating platform, and the aerodynamic performance of the wind turbine has highly unsteady characteristics due to the motions of the floating platform. A spar-type FOWT consisting of NREL-5-MW baseline wind turbine and OC3-Hywind platform system is investigated. The aerodynamic forces can be obtained by the UALM. The 6DoF motions and mooring tensions are predicted by the naoe-FOAM-SJTU. To research the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform, simulations with different complexities are performed. Fully coupled aero-hydrodynamic characteristics of FOWTs, including aerodynamic loads, wake vortex, motion responses, and mooring tensions, are compared and analyzed.

Loads and Response of a Tension Leg Platform Wind Turbine with Non-Rotating Blades: An Experimental Study

This paper describes model testing of a Tension Leg Platform Wind Turbine (TLPWT) with non-rotating blades to better understand its motion and tendon responses when subjected to combined wind and unidirectional regular wave conditions. The TLPWT structure is closely based on the National Renewable Energy Laboratory (NREL) 5 MW concept. Multiple free decay tests were performed to evaluate the natural periods of the model in the key degrees of freedom, whilst Response Amplitude Operators (RAOs) were derived to show the motion and tendon characteristics. The natural periods in surge and pitch motions evaluated from the decay tests had a relatively close agreement to the theoretical values. Overall, the tested TLPWT model exhibited typical motion responses to that of a generalised TLP with significant surge offsets along with stiff heave and pitch motions. The maximum magnitudes for the RAOs of surge motion and all tendons occurred at the longest wave period of 1.23 s (~13.0 s at full-scale) tested in this study. From the attained results, there was evidence that static wind loading on the turbine structure had some impact on the motions and tendon response, particularly in the heave direction, with an average increase of 13.1% in motion amplitude for the tested wind conditions. The wind had a negligible effect on the surge motion and slightly decreased the tendon tensions in all tendons. The results also showed the set-down magnitudes amounting to approximately 2–5% of the offset. Furthermore, the waves are the dominant factor contributing to the set-down of the TLPWT, with a minimal contribution from the static wind loading. The results of this study could be used for calibrating numerical tools such as CFD codes.

Numerical Modeling and Analysis of a Hybrid-Spar Floating Wind Turbine

In this paper, numerical modeling and analysis of a hybrid-spar floating wind turbine is presented. The hybrid-spar consists of steel at the upper part and precast prestressed concrete at the lower part. Such a configuration is referred to as a hybrid-spar in this paper. The hybrid spar was successfully installed offshore of Kabashima island, Goto city, Nagasaki prefecture, Japan, on Oct. 18, 2013 (see Utsunomiya et al., 2015, “Design and Installation of a Hybrid-Spar Floating Wind Turbine Platform,” ASME Paper No. OMAE2015-41544 for details). In this paper, some details on numerical modeling of the hybrid-spar for design load analysis are presented. Then, the validation of the numerical analysis model is presented for a full-scale hybrid-spar model with 2-MW wind turbine. The comparison has been made for the natural periods and the response during rated power production test. Basically, both comparisons have shown good agreement between the measured values and the simulation, showing reliability of the developed code and the numerical model.

Time-domain analysis of a flexible floating platform of offshore wind turbine using potential flow theory

Time-domain analysis of a flexible floating platform of offshore wind turbine using potential flow theory

Response and Motion Performance of Offshore 5MW National Renewable Energy Laboratory Wind Turbine Platform Based on Nicobar Costal Ocean State (Short Communication)

Wind turbines on floating support platforms are designed to be installed in a deep offshore environment several miles off the coast and in water depths greater than 60m. Effects from sea ice, varying mean sea level, and marine growth constitute additional loads that must be considered in a real design process. Design Modeler is the ANSYS tool used to create geometry for hydrodynamic systems. Surface bodies are only supported by ANSYS AQWA thus entire solid body which is created using the design modeler is transformed in to surface body. Four different mini Tension leg platforms were prepared all the dimensions and standards are followed from the guidelines of national renewable energy laboratory United States. This paper focused on the motion performance of tension leg platform supported wind turbine prototypes in Nicobar coast of India. The wind, wave and ocean current data were obtained and inputted in to the AQWA modules and the result is validated with respect to time domain.

Out-of-plane bending stiffness and hotspot stress based on advanced numerical analysis techniques

ABSTRACTThis paper aims to presents an integrated procedure to reach the out-of-plane bending (OPB) moment-induced hotspot stress distribution for the mooring chains of an 8MW class floating offshore wind turbine platform (FOWT). Three-link models of the mooring chains were used to obtain the OPB stiffness, mid-span stress distribution, hotspot stress, and stress concentration factor based on the prescribed rotation approach (PRA), direct tension approach (DTA), and empirical formula. Multi-link analyses were also conducted to build a tension angle-interlink angle relationship between two successive links at a given tension level. A combination of three-link and multi-link analyses produced the relation of the hotspot stress-tension angle. After a stress interpolation procedure for the hotspot stress history, the hotspot stress range distributions were presented using rainflow counting technique. The tension-induced stress range distribution well followed the Rayleigh distribution, while the OPB moment elevated the hotspot stress which will seriously affect the fatigue accumulation.

Dynamic modeling and vibration suppression for an offshore wind turbine with a tuned mass damper in floating platform

The worldwide demand for renewable energy is increasing rapidly. Wind energy appears as a good solution to copy with the energy shortage situation. In recent years, offshore wind energy has become an attractive option due to the increasing development of the multitudinous offshore wind turbines. Because of the unstable vibration for the barge-type offshore wind turbine in various maritime conditions, an ameliorative method incorporating a tuned mass damper (TMD) in offshore wind turbine platform is proposed to demonstrate the improvement of the structural dynamic performance in this investigation. The Lagrange's equations are applied to establish a limited degree-of-freedom (DOF) mathematical model for the barge-type offshore wind turbine. The objective function is defined as the suppression rate of the standard deviation for the tower top deflection due to the fact that the tower top deflection is essential to the tower bottom fatigue loads, then frequency tuning method and genetic algorithm (GA) are employed respectively to obtain the globally optimum TMD design parameters using this objective function. Numerical simulations based on FAST have been carried out in typical load cases in order to evaluate the effect of the passive control system. The need to prevent the platform mass increasing obviously has become apparent due to the installation of a heavy TMD in the barge-type platform. In this case, partial ballast is substituted for the equal mass of the tuned mass damper, and then the vibration mitigation is simulated in five typical load cases. The results show that the passive control can improve the dynamic responses of the barge-type wind turbine by placing a TMD in the floating platform. Through replacing partial ballast with a uniform mass of the tuned mass damper, a significant reduction of the dynamic response is also observed in simulation results for the barge-type floating structure.

Yaw motion of floating wind turbine platforms induced by pitch actuator fault in storm conditions

The paper discusses the factors affecting the large amplitude yaw responses of the DeepCwind wind turbine platform in storms with a faulty blade pitch actuator. The behaviour in operational and storm conditions without faults is provided as a base of comparison. They are followed by the responses to 50-year extreme values defined by the IEC standards, North Sea statistics, and Portugal's coastal storm data. All environmental conditions are head waves upwind sets, presumably causing minimal yaw excitation despite differing in wave excitation frequencies and maximum wind speeds. These scenarios are expanded further to cases where one of the pitch actuators stops functioning and fails to feather the blade. The result reveals large angle yaw motions of the platform. The reasoning behind this motion is explained and a possible method of mitigation through a revision of the HSS brake capacity is discussed. In this process, a frequently applied form of the hydrostatic matrix is reviewed as it includes overlooked coupling terms.

Comparison of pilot-scale floating offshore wind farms with shared moorings

As floating wind turbine technology matures and wind farms are considered at larger scales in deeper waters, moorings may take a larger share of the costs. In response, floating wind farms could be designed such that the moorings are shared between turbines, using interconnecting mooring lines between adjacent floating offshore wind turbine platforms and also possibly connecting multiple mooring lines to the same anchor. This may reduce the total length of mooring line, the wind farm footprint, and the number of anchors; worthwhile advantages if not overcome by increases in mooring strength requirements.A design algorithm is presented for preliminary sizing of shared mooring systems for pilot-scale floating offshore wind farms. Quasi-static modelling is used to analyze the performance of shared-mooring systems produced by the algorithm. The model accounts for steady wind thrust forces and nonlinear mooring line reactions. A four-turbine floating wind farm is designed and analyzed with three alternative shared-mooring configurations and four water depths. Results are presented in terms of platform displacements, mooring tensions, and cost, providing an initial comparison of shared mooring options and their potential for cost savings over conventional individually-moored configurations.

Evaluating a new concept to integrate compressed air energy storage in spar-type floating offshore wind turbine structures

This paper presents a new concept for integrating compressed air energy storage (CAES) into spar-type floating wind turbine platforms. A preliminary investigation of the implications of integrating the proposed concept on the design and dynamic characteristics of a 5 MW floating offshore wind turbine (FOWT) system is presented. A simplified numerical model for sizing the spar to cater for a given compressed air pressure and energy storage capacity is presented. This is then used in a parametric analysis to establish the relationship between the storage capacity, the spar geometry and the additional mass of the floating configuration to support high air pressures. Numerical simulations for the dynamic response are then performed using the marine engineering software tool ANSYS Aqwa© to study the effect of the added steel mass of the spar resulting from CAES integration on the dynamic response characteristics under irregular wave conditions. The results are compared to those derived for a conventional FOWT-spar configuration without an energy storage system. It is shown that it is technically feasible to integrate short-term energy storage capacities on the order of Megawatt-hours. Although integration of CAES in the floating spar results in a significant increase in the weight of the floating structure, it is shown that this also results in a reduction in the motion experienced by the FOWT for the met ocean conditions considered in this study.

Structural Control for an Offshore Wind Turbine with a Tuned Mass Damper in Floating Platform

In recent years, offshore wind energy has become an attractive option due to the increased demand for the renewable energy. A method incorporating a tuned mass damper (TMD) in offshore wind turbine platform is proposed to demonstrate the improvement on structural dynamic performance in this investigation. The Lagrange's equations are applied to establish a limited degree-of-freedom (DOF) mathematical model for the barge-type offshore wind turbine. Genetic algorithm (GA) is then employed to find the globally optimum TMD design parameters. Numerical simulations based on FAST have been carried out to evaluate the effect of the passive control system. A changeable mass for the floating wind turbine will be brought for installing a heavy tuned mass damper in the platform. In this case, partial ballast is substituted for the equal mass of the tuned mass damper, and the vibration mitigation is simulated in five typical load cases. Results show that the passive control approaches can improve the dynamic responses of the barge-type wind turbine by placing a tuned mass damper in floating platform. Through replacing partial ballast with the equal mass of the tuned mass damper, a significant reduction of dynamic responses is also observed in simulation results for the barge-type floating structure.

Vibration control using double-response damper and site measurements on wind turbine

Based on the work principles of tuned mass damper and particle damper, an innovative double-response damper combined tuned mass damper with particle damper is proposed to suppress the wind-induced vibrations of wind turbine towers in this study. The proposed damper was designed, fabricated, and installed on the top platform of an onshore wind turbine with a rated power output of 1.5 MW. The site measurements were carried out to investigate its control effectiveness. The dynamic responses under both emergency shutdown case and normal operation case were obtained by acceleration transducers installed on the wall near the top platform of the wind turbine tower. Because of the distinguishing dynamic response characteristics under these 2 cases, different data processing methods are used to analyze the measurement data. For emergency shutdown case, the exponential decay method in time domain is applied to obtain the first order modal damping ratio of the wind turbine tower in different measurement phases. For normal operation case, the scatter graph method is used to analyze the control effectivenesses of the damper in different measurement phases. The measurement results show that the damper could not only significantly increase the first order modal damping ratio of the wind turbine tower under the emergency shutdown case but also effectively suppress its wind-induced vibration under the operation case.

Non-conventional vortex generators calculated with CFD

Wind turbine blades size has increased during last years due to wind turbine platform growth especially for offshore applications. The interest of wind turbine manufacturers in blade add-ons have raised to mitigate flow separation and blade loading. One type of add-ons are the vortex generators (VGs).VGs are passive flow control devices that are usually triangular or rectangular vanes inclined to the flow and are dimensioned with regard to the local boundary layer thickness. VGs are known for their capability of delaying separation and increasing lift force on the blade. Their main drawback is the drag penalty added to the blades. In this work, novel shaped VGs are computed with OpenFoam using a fully resolved approach to test if they are efficient with regard to the drag penalty. The use of non-conventional VGs reduce the separated flow zone in the leading-edge of the vane resulting in a lower residual drag and in an increase on the overall efficiency. This fact would reduce the loads associated with the vane.Three VG shapes are simulated. Two of them have a cross section based on thin airfoils and the other one is a conventional rectangular VG used as reference. The VGs are mounted on a flat plate and they are calculated for the same flow characteristics and size than the ones used in the experiments performed under the scope of the AVATAR project [1]. Relevant quantities, such as the peak vorticity, the vortex vertical and lateral paths, wall shear stress, velocity contours and drag coefficient, will be compared in order to evaluate the VGs performance.

An Advanced Control Technique for Floating Offshore Wind Turbines Based on More Compact Barge Platforms

Hydrodynamic Floating Offshore Wind Turbine (FOWT) platform specifications are typically dominated by seaworthiness and maximum operating platform-pitch angle-related requirements. However, such specifications directly impact the challenge posed by an FOWT in terms of control design. The conventional FOWT systems are typically based on large, heavy floating platforms, which are less likely to suffer from the negative damping effect caused by the excessive coupling between blade-pitch control and platform-pitch motion. An advanced control technique is presented here to increase system stability for barge type platforms. Such a technique mitigates platform-pitch motions and improves the generator speed regulation, while maintaining blade-pitch activity and reducing blade and tower loads. The NREL’s 5MW + ITI Energy barge reference model is taken as a basis for this work. Furthermore, the capabilities of the proposed controller for performing with a more compact and less hydrodynamically stable barge platform is analysed, with encouraging results.