Floating Wind Turbine System Research Articles

Dynamic response in frequency and time domains of a floating foundation for offshore wind turbines

In this paper, the dynamic behaviors are studied for the semi-submersible floating foundation of a 600kW wind turbine in 60m deep water considering the coupled load effects of wind turbine–tower-floating foundation and mooring lines and ocean environment. The paper focuses on the key issues of floating foundation design, such as coupling dynamic analysis model and calculating methods. The finite element models are established and dynamic responses of floating wind turbine system under different combinations of turbulent wind, constant current and irregular wave are calculated in frequency and time domains by the use of the SESAM software. The motion performance and line tension are investigated. The results show that the semi-submersible floating foundation and mooring system can work in the ocean environment with significant wave height less than 4m.

Research on the Dynamic Response of Floating Foundation of a Tri-Floater Offshore Wind Turbine

In this paper, the overall finite element model is established, to analyze the small-sized floating foundation of a tri-floater and to make a local optimization on the stress concentration area. The transfer functions and the response spectrums of wave load and motion of floating wind turbine system are calculated by AQWA. Besides the concept of the floating foundation group is put forward in this paper. It is small in structure, easy to assemble, and it can be developed for any power of wind field.This concept has a certain reference value for the development of offshore wind industry in China.

Influence of Wind Turbine Aero-Elastic Load on Dynamic Response of Floating Platform

A numerical code, known as COUPLE, which has been developed to perform hydrodynamic analysis of floating body with a mooring system, is extended to collaborate with FAST to evaluate the interactions between wind turbine and its floating base. FAST is developed by National Renewable Energy Lab (NREL) for aeroelastic simulation of wind turbines. A dynamic response analysis of a spar type floating wind turbine system is carried out by the method. Two types of simulation of wind load are used in the analysis. One type is a constant steady force and the other is a six-component dynamic load from a turbulent wind model. Numerical results of related platform motions under random sea conditions are presented in time and frequency domain. Comparison of results is performed to explain the difference of two analyses. The conclusions derived in this study may provide reference for the design of offshore floating wind turbines.

A Coupled Method for Dynamic Analysis of Offshore Floating Wind Turbine System

A coupled dynamic analysis method is introduced for numerical simulation of floating wind turbine systems in this paper. A numerical code，which has been developed to perform couple hydrodynamic analysis of floating body together with its mooring system, is extended to collaborate with wind turbine simulator to evaluate the interactions between wind turbine and its floating base. To verify the coupled method, a dynamic response analysis of a spar type floating wind turbine system (NREL offshore-5MW baseline wind turbine) is carried out separately by the coupled Morison method and radiation-diffraction theory. Numerical results and comparison are presented. It turns out that this coupled method is competent enough to predict hydrodynamic performance of floating wind turbine system. The numerical results derived in this study may provide crucial information for the design of a floating wind turbine in the near future.

The wind-wave tunnel test of a tension-leg platform type floating offshore wind turbine

In this work, a tension-leg platform (TLP) type floating offshore wind turbine (FOWT) system was proposed, which was based on the National Renewable Energy Laboratory 5 MW offshore wind turbine model. Taking the coupled effect of dynamic response of the top wind turbine, support tower structure, and lower mooring system into consideration, the 1/60 scale model test for investigating the coupled wind-wave effect on performance of the floating wind turbine system was done in Harbin Institute of Technology's wind tunnel and wave flume joint laboratory. In addition, numerical simulations corresponding to the scale model tests have been performed by advanced numerical tools. The results of model tests and numerical simulations have a good agreement, so the availability of the numerical model has been verified. Furthermore, to improve the performance of the TLP system, one tentative strategy adding mooring lines to the TLP system was proposed, and the model test results of the two TLP systems were compared with each other. As a result, the motion responses of the floating platform and the force levels of tension legs were effectively reduced by the additional mooring chains. The new TLP FOWT system might play an active and instructive role in the development of future FOWT system.

An investigation of variable power collective pitch control for load mitigation of floating offshore wind turbines

ABSTRACTThis paper investigates the loads on offshore floating wind turbines and a new control method that can be used to reduce these loads. In this variable power collective pitch control method, the rated generator speed, which is the set point that the collective pitch control attempts to drive the actual generator speed towards, is no longer a constant value but instead a variable that depends on the platform pitch velocity. At a basic physical level, this controller achieves the following: as the rotor of a floating turbine pitches upwind, the controller adjusts so as to extract more energy from the wind by increasing the rated generator speed and thus damps the motion; as the rotor pitches downwind, less energy is extracted because the controller reduces the rated generator speed and again damps the motion. This method is applied to the NREL 5 MW wind turbine model, in above‐rated conditions where the platform motion is most problematic. The results indicate significant load reductions on key structural components, at the expense of minor increases in power and speed variability. The loads on the blades and tower are investigated more generally, and simple dynamic models are used to gain insight into the behavior of floating wind turbine systems. It is clear that for this particular design, aerodynamic methods for reducing platform motion and tower loads are likely inadequate to allow for a viable design, and so new designs or possibly new control degrees of freedom are needed. Copyright © 2012 John Wiley & Sons, Ltd.

Study on the Dynamic Characteristic for Spar Type Floating Foundation of Offshore Wind Turbine

In this paper, the dynamic behaviors are studied for Spar type floating foundation of a 3kW in the 10m deep water considering the coupled wind turbine-tower-floating foundation and mooring lines and ocean environment load effects. The paper focus on the key issues of design of floating foundation, such as coupling dynamic analysis model and calculating method. The finite element models are established and dynamic responses of floating wind turbine system under different combinations of turbulent wind, constant current and irregular wave are calculated in frequency and time domain with SESAM software. The motion performance and lines’ tension are investigated, and some valuable conclusions are drawn. The results show that the Spar type floating foundation and mooring system can work in the ocean environment which significant wave height less than 2m, the designed large water-entrapment plate can minimized the motion of floating foundation obviously.

The Effect of Additional Mooring Chains on the Motion Performance of a Floating Wind Turbine with a Tension Leg Platform

In this study, two types of floating offshore wind turbine (FOWT) systems were proposed: a traditional tension leg platform (TLP) type and a new TLP type with additional mooring chains. They were both based on the National Renewable Energy Laboratory 5 MW offshore wind turbine model. Taking the coupled effect of dynamic response of the top wind turbine, tower support structure and lower mooring system into consideration, not only were the 1/60 scale model tests for the two floating wind turbine systems done in HIT’s wind-wave tunnel according to the typical design conditions in IEC61400-3 code, but also the numerical simulations corresponding to the scaled model tests were performed by advanced numerical tools. As a result, the numerical results displayed good agreement with the test data. Moreover, the additional mooring chains could play an active role in reducing the surge displacement, surge acceleration and typical tension leg force responses of the FOWT system, which is very beneficial for ensuring the good operational performance and the safety of the FOWT system.

An investigation of variable power collective pitch control for load mitigation of floating offshore wind turbines

ABSTRACTThis paper investigates the loads on offshore floating wind turbines and a new control method that can be used to reduce these loads. In this variable power collective pitch control method, the rated generator speed, which is the set point that the collective pitch control attempts to drive the actual generator speed towards, is no longer a constant value but instead is a variable that depends on the platform pitch velocity. At a basic physical level, this controller achieves the following: as the rotor of a floating turbine pitches upwind, the controller adjusts so as to extract more energy from the wind by increasing the rated generator speed and thus damps the motion; as the rotor pitches downwind, less energy is extracted because the controller reduces the rated generator speed and again damps the motion. This method is applied to the NREL 5 MW wind turbine model, in above rated conditions where the platform motion is most problematic. The results indicate significant load reductions on key structural components, at the expense of minor increases in power and speed variability. The loads on the blades and tower are investigated more generally, and simple dynamic models are used to gain insight into the behavior of floating wind turbine systems. It is clear that for this particular design, aerodynamic methods for reducing platform motion and tower loads are likely inadequate to allow for a viable design, so new designs or possibly new control degrees of freedom are needed. Copyright © 2012 John Wiley & Sons, Ltd.

Modes of response of an offshore wind turbine with directional wind and waves

A coupled dynamic analysis of a floating wind turbine system has been performed to investigate effect of wave direction relative to wind on the system. Hydrodynamic loads are calculated by linear frequency domain approach and aerodynamic effect is taken into account by increasing hydrodynamic damping and restoring matrices with aerodynamic damping and gyroscopic stiffness. A modal analysis of the system was made to explain the calculated motions. It brings out the natural frequencies, natural modes and modal damping coefficients of the system. Excitation of natural modes, by waves explains the motion observed in the response amplitude operators, and the effect of wave direction relative to wind. This modal analysis helps to better understand the behaviour of floating wind turbine systems.

Simulation of large-amplitude motion of floating wind turbines using conservation of momentum

A new method is presented to directly derive the nonlinear equations of motion (EOMs) of a floating wind turbine system using the theorem of conservation of angular momentum and Newton's second law. The methodology considers the system as two rigid bodies: the tower and the rotor-nacelle assembly (RNA). The large-amplitude rotation of the tower is described by the 1-2-3 sequence Euler angles, which offer accurate nonlinear coupling between motions in 6 degrees of freedom (DOFs). Two additional DOFs of the RNA relative to the tower, nacelle yaw and rotor spin, are prescribed by mechanical control and are also included in the EOMs of the entire system. Results from the EOMs are transformed among different coordinate systems at every time-step for use in the computation of hydrodynamics, aerodynamics and restoring forces, which preserves the nonlinearity between external excitation and structural dynamics. The new method is verified by critical comparison of simulation results with those of the popular wind turbine dynamics software FAST. The concept of highly compliant floating wind turbines is introduced. The large-amplitude motions and gyroscopic moments of one of these smaller, lighter structures is simulated in an example.

Individual blade pitch control of floating offshore wind turbines

AbstractFloating wind turbines offer a feasible solution for going further offshore into deeper waters. However, using a floating platform introduces additional motions that must be taken into account in the design stage. Therefore, the control system becomes an important component in controlling these motions. Several controllers have been developed specifically for floating wind turbines. Some controllers were designed to avoid structural resonance, while others were used to regulate rotor speed and platform pitching. The development of a periodic state space controller that utilizes individual blade pitching to improve power output and reduce platform motions in above rated wind speed region is presented. Individual blade pitching creates asymmetric aerodynamic loads in addition to the symmetric loads created by collective blade pitching to increase the platform restoring moments. Simulation results using a high‐fidelity non‐linear turbine model show that the individual blade pitch controller reduces power fluctuations, platform rolling rate and platform pitching rate by 44%, 39% and 43%, respectively, relative to a baseline controller (gain scheduled proportional–integral blade pitch controller) developed specifically for floating wind turbine systems. Turbine fatigue loads were also reduced; tower side–side fatigue loads were reduced by 39%. Copyright © 2009 John Wiley & Sons, Ltd.

Technical and economic aspects of a floating offshore wind farm

Abstract Development of the conceptual design for FLOAT – an offshore floating wind turbine – is described in this paper. This design represents a marriage of the wind power and offshore oil and gas technology. The objective of the FLOAT project is to develop a floating wind turbine system enabling the economic generation of electricity from wind power in offshore locations, typically between 100 and 300 m water depth. The technical and economic aspect of installing an offshore wind farm based on the FLOAT concept is addressed. Issues with regard to legal, environmental, fabrication and installation operations are considered.