Dynamic Analysis of a 10 MW Floating Offshore Wind Turbine Considering the Tower and Platform Flexibility

Recently,semisubmersible floating offshore wind turbine technologies have received considerable attention.For the coupled simulation of semisubmersible floating offshore wind energy,the platform is usually considered a rigid model,which could affect the calculation accuracy of the dynamic responses.The dynamic responses of a TripleSpar floating offshore wind turbine equipped with a 10 MW offshore wind turbine are discussed herein.The simulation of a floating offshore wind turbine under regular waves,white noise waves,and combined wind-wave conditions is conducted.The effects of the tower and platform flexibility on the motion and force responses of the TripleSpar semisubmersible floating offshore wind turbine are investigated.The results show that the flexibility of the tower and platform can influence the dynamic responses of a TripleSpar semisubmersible floating offshore wind turbine.Considering the flexibility of the tower and platform,the tower and platform pitch motions markedly increased compared with the fully rigid model.Moreover,the force responses,particularly for tower base loads,are considerably influenced by the flexibility of the tower and platform.Thus,the flexibility of the tower and platform for the coupled simulation of floating offshore wind turbines must be appropriately examined.

Aero-Hydrodynamic Coupled Dynamic Characteristics of Semi-Submersible Floating Offshore Wind Turbines Under Inflow Turbulence

In this study,the frequency characteristics of the turbulent wind and the effects of wind-wave coupling on the low-and high-frequency responses of semi-submersible floating offshore wind turbines(FOWT)are investigated.Various wave load components,such as first-order wave loads,combined first-and second-order difference-frequency wave loads,combined first-and second-order sum-frequency wave loads,and first-and complete second-order wave loads are taken into consideration,while different turbulent environments are considered in aerodynamic loads.The com-parison is based on time histories and frequency spectra of platform motions and structural load responses and statistical values.The findings indicate that the second-order difference-frequency wave loads will significantly increase the natural frequency of low-frequency motion in the responses of the platform motion and structure load of the semi-submersible platform,which will cause structural fatigue damage.Under the action of turbulent wind,the influences of second-order wave loads on the platform motion and structural load response cannot be ignored,especially under extreme sea conditions.Therefore,in order to evaluate the dynamic responses of semi-submersible FOWT more accurately,the actual environment should be simulated more realistically.

Coupled Time-Domain Investigation on a Vertical Axis Wind Turbine Supported on a Floating Platform

The dynamic responses of a floating vertical axis wind turbine(VAWT)are assessed on the basis of an aero-hydro-mooring coupled model.The aerodynamic loads on the rotor are acquired with double-multiple stream tube method.First-and second-order wave loads are calculated on the basis of 3D potential theory.The mooring loads are simulated by catenary theory.The coupled model is established,and a numerical code is programmed to investigate the dynamic response of the semi-submersible VAWT.A model test is then conducted,and the numerical code is validated considering the hydrodynamic performance of the floating buoy.The responses of the floating VAWT are studied through the numerical simulation under the sea states of wind and regular/irregular waves.The effects of the second-order wave force on the motions are also investigated.Results show that the slow-drift responses in surge and pitch motions are significantly excited by the second-order wave forces.Furthermore,the effect of foundation motion on aerodynamic loads is examined.The normal and tangential forces of the blades demonstrate a slight increase due to the coupling effect between the buoy motion and the aerodynamic loads.

Effects of incident wind/wave directions on dynamic response of a SPAR-type floating offshore wind turbine system

As a promising renewable energy,offshore wind energy currently is gaining more attention,by which the economic and efficient operation of floating wind turbine systems is a potential research direction.This study is primarily devoted to the analysis of dynamic response of the NREL-5 MW reference wind turbine supported by an OC3-Hywind SPAR-type platform using a recompiled code which combines FAST with WAMIT.To verify the reliability of the recompiled code,the free decay motions of a floating wind turbine system in still water are examined with satisfactory results.After that,thirteen scenarios with different angles between wind and wave from 0°to 90°are investigated.The dynamic responses of the turbine system in various degrees of freedom(DOFs)for different incident wind/wave directions are presented in both time and frequency domains via the fast Fourier transform.

Study on Rigid-Flexible Coupling Effects of Floating Offshore Wind Turbines

In order to account for rigid-flexible coupling effects of floating offshore wind turbines, a nonlinear rigid-flexible coupled dynamic model is proposed in this paper. The proposed nonlinear coupled model takes the higher-order axial displacements into account, which are usually neglected in the conventional linear dynamic model. Subsequently,investigations on the dynamic differences between the proposed nonlinear dynamic model and the linear one are conducted. The results demonstrate that the stiffness of the turbine blades in the proposed nonlinear dynamic model increases with larger overall motions but that in the linear dynamic model declines with larger overall motions.Deformation of the blades in the nonlinear dynamic model is more reasonable than that in the linear model as well.Additionally, more distinct coupling effects are observed in the proposed nonlinear model than those in the linear model. Finally, it shows that the aerodynamic loads, the structural loads and global dynamic responses of floating offshore wind turbines using the nonlinear dynamic model are slightly smaller than those using the linear dynamic model. In summary, compared with the conventional linear dynamic model, the proposed nonlinear coupling dynamic model is a higher-order dynamic model in consideration of the rigid-flexible coupling effects of floating offshore wind turbines, and accord more perfectly with the engineering facts.

Concept Design and Coupled Dynamic Response Analysis on 6-MW Spar-Type Floating Offshore Wind Turbine

Tower,Spar platform and mooring system are designed in the project based on a given 6-MW wind turbine.Under wind-induced only,wave-induced only and combined wind and wave induced loads,dynamic response is analyzed for a 6-MW Spar-type floating offshore wind turbine(FOWT) under operating conditions and parked conditions respectively.Comparison with a platform-fixed system(land-based system) of a 6-MW wind turbine is carried out as well.Results demonstrate that the maximal out-of-plane deflection of the blade of a Spar-type system is 3.1% larger than that of a land-based system;the maximum response value of the nacelle acceleration is 215% larger for all the designed load cases being considered;the ultimate tower base fore-aft bending moment of the Spar-type system is92% larger than that of the land-based system in all of the Design Load Cases(DLCs) being considered;the fluctuations of the mooring tension is mainly wave-induced,and the safety factor of the mooring tension is adequate for the 6-MW FOWT.The results can provide relevant modifications to the initial design for the Spar-type system,the detailed design and model basin test of the 6-MW Spar-type system.

Platform motion minimization using model predictive control of a floating offshore wind turbine

Wind turbines are installed offshore with the assistance of a floating platform to help meet the world’s increasing energy needs.However,the incident wind and extra incident wave disturbances have an impact on the performance and operation of the floating offshore wind turbine(FOWT)in comparison to bottom-fixed wind turbines.In this paper,model predictive control(MPC)is utilized to overcome the limitation caused by platform motion.Due to the ease of control synthesis,the MPC is developed using a simplified model instead of high fidelity simulation model.The performance of the controller is verified in the presence of realistic wind and wave disturbances.The study demonstrates the effectiveness of MPC in reducing platform motions and rotor/generator speed regulation of FOWTs.

Dynamic Responses of A Semi-Type Offshore Floating Wind Turbine During Normal State and Emergency Shutdown

This paper addresses joint wind-wave induced dynamic responses of a semi-type offshore floating wind turbine(OFWT) under normal states and fault event conditions. The analysis in this paper is conducted in time domain, using an aero-hydro-servo-elastic simulation code-FAST. Owing to the unique viscous features of the reference system, the original viscous damping model implemented in FAST is replaced with a quadratic one to gain an accurate capture of viscous effects. Simulation cases involve free-decay motion in still water, steady motions in the presence of regular waves and wind as well as dynamic response in operational sea states with and without wind. Simulations also include the cases for transient responses induced by fast blade pitching after emergency shutdown. The features of platform motions, local structural loads and a typical mooring line tension force under a variety of excitations are obtained and investigated.

Motion Performance and Mooring System of a Floating Offshore Wind Turbine

发展近海风农场原来在浅水区域被执行与修理(装的海床) 结构。然而，有有限的浅水区域的国家要求创新漂浮的平台部署风汽轮机近海以便利用风精力在深海产生电。运动和系在的系统动力学的表演对设计一个合算、持久的漂浮的平台重要。这份报纸描述一个数字模型模仿漂浮的一种新半能沉入水中的类型的动态行为近海风汽轮机(FOWT ) 系统。风汽轮机与某个戳系数作为风块被建模，并且平台的水动力学和绳索系统动力学由 SESAM 软件是计算的。处于在装载的波浪和风下面的系统的动态反应上的环境条件的变化的效果被检验。结果显示半能沉入水中的概念有优秀性能， SESAM 能是为漂浮的风汽轮机设计和分析的一个有效工具。

Coupled Aerodynamic and Hydrodynamic Analysis of Floating Offshore Wind Turbine Using CFD Method

To simulate floating offshore wind turbine(FOWT)in coupled wind-wave domain via CFD method,the NREL 5MW wind turbine supported by the OC3-Hywind Spar platform is modeled in the STAR-CCM+ software.Based on the Reynolds-averaged Navier-Stokes(RANS)equations and re-normalisation group(RNG)k-εturbulence model,the rotor aerodynamic simulation for wind turbine is conducted.Numerical results agree well with the NREL data.Taking advantage with the volume of fluid(VOF)method and dynamic fluid body interaction(DFBI)technology,the dynamic responses of the floating system with mooring lines are simulated under the coupled wind-wave sea condition.The free-decay tests for rigid-body degrees of freedom(DOFs)in still water and hydrodynamic tests in a regular wave are performed to validate the numerical model by comparing its result with the results simulated by FAST.Finally,the simulations of the overall FOWT system in the coupled wind-wave flow field are carried out.The relationship between the power output and dynamic motion responses of the platform is investigated.The numerical results show that the dynamic response of wind turbine performance and platform motions all vary in the same frequency as the inlet wave.During platform motion,the power output of wind turbine is more sensitive than the thrust force.This study may provide some reference for further research in the coupled aero-hydro simulation of FOWT.

Analysis of Key Disciplinary Parameters in Floating Offshore Wind Turbines with An AI-Based SADA Method

Floating offshore wind turbines(FOWTs)are a promising offshore renewable energy harvesting facility but requesting multiple-disciplinary analysis for their dynamic performance predictions.However,engineering-fidelity level tools and the empirical parameters pose challenges due to the strong nonlinear coupling effects of FOWTs.A novel method,named SADA,was proposed by Chen and Hu(2021)for optimizing the design and dynamic performance prediction of FOWTs in combination with AI technology.In the SADA method,the concept of Key Disciplinary Parameters(KDPs)is also proposed,and it is of crucial importance in the SADA method.The purpose of this paper is to make an in-depth investigation of the characters of KDPs and the internal correlations between different KDPs in the dynamic performance prediction of FOWTs.Firstly,a brief description of SADA is given,and the basin experimental data are used to conduct the training process of SADA.Secondly,categories and boundary conditions of KDPs are introduced.Three types of KDPs are given,and different boundary conditions are used to analyze KDPs.The results show that the wind and current in Environmental KDPs are strongly correlated with the percentage difference of dynamic response rather than that by wave parameters.In general,the optimization results of SADA consider the specific basin environment and the coupling results between different KDPs help the designers further understand the factors that have a more significant impact on the FOWTs system in a specific domain.

Dynamic Response of 6MW Spar Type Floating Offshore Wind Turbine by Experiment and Numerical Analyses

The floating offshore wind turbine(FOWT) is widely used for harvesting marine wind energy. Its dynamic responses under offshore wind and wave environment provide essential reference for the design and installation. In this study,the dynamic responses of a 6 MW Spar type FOWT designed for the water depth of 100 m are investigated by means of the wave tank experiment and numerical analysis. A scaled model is manufactured for the experiment at a ratio of65.3, while the numerical model is constructed on the open-source platform FAST(Fatigue, Aerodynamics,Structures, and Turbulence). Still water tests, wind-induced only tests, wave-induced only tests and combined windwave-current tests are all conducted experimentally and numerically. The accuracy of the experimental set-up as well as the loading generation has been verified. Surge, pitch and heave motions are selected to analyze and the numerical results agree well with the experimental values. Even though results obtained by using the FOWT calculation model established in FAST software show some deviations from the test results, the trends are always consistent. Both experimental and numerical studies demonstrate that they are reliable for the designed 6 MW Spar type FOWT.

Motion Characteristics of Novel Floating Foundation for Offshore Wind Turbine

A novel floating foundation to support the NREL offshore 5 MW wind turbine was designed conceptually by combining the characteristics of barge and Spar. The main focus was structural design and hydrodynamic modelling. Based on this novel floating foundation, the hydrodynamic performance was investigated in the frequency domain and time domain by using the wave analysis software Hydro D and Deep C from Det Norske Veritas. The frequency domain analysis was conducted to investigate the effects of the incident wave angle and water depth. The time-domain analysis was carried out to evaluate the response of the floating foundation under a selected operational condition. The hydrodynamic performances of this floating foundation with respect to time series and response spectra were also investigated in this study.

A Novel Dynamics Analysis Method for Spar-Type Floating Offshore Wind Turbine

The dynamic behavior of floating offshore wind turbine (FOWT) is crucial for its design and optimization. A novel dynamics analysis method for the spar-type FOWT system is proposed in this paper based on the theorem of moment of momentum and the Newton’s second law. The full nonlinearity of the equations of motion (EOMs) and the full nonlinear coupling between external loads and the motions are preserved in this method. Compared with the conventional methods, this method is more transparent and it can be applied directly to the large-amplitude rotation cases. An in-house code is developed to implement this method. The capability of in-house code is verified by comparing its simulation results with those predicted by FAST. Based on the in-house code, the dynamic responses of a spar-type FOWT system are investigated under various conditions.

Study on Gyroscopic Effect of Floating Offshore Wind Turbines

Compared with bottom-fixed wind turbines,the supporting platform of a floating offshore wind turbine has a larger range of motion,so the gyroscopic effects of the system will be more obvious.In this paper,the mathematical analytic expression of the gyroscopic moment of a floating offshore wind turbine is derived firstly.Then,FAST software is utilized to perform a numerical analysis on the model of a spar-type horizontal axis floating offshore wind turbine,OC3-Hywind,so as to verify the correctness of the theoretical analytical formula and take an investigation on the characteristics of gyroscopic effect.It is found that the gyroscopic moment of the horizontal axis floating offshore wind turbine is essentially caused by the vector change of the rotating rotor,which may be due to the pitch or yaw motion of the floating platform or the yawing motion of the nacelle.When the rotor is rotating,the pitch motion of the platform mainly excites the gyroscopic moment in the rotor’s yaw direction,and the yaw motion of the platform largely excites the rotor’s gyroscopic moment in pitch direction,accordingly.The results show that the gyroscopic moment of the FOWT is roughly linearly related to the rotor’s inertia,the rotor speed,and the angular velocity of the platform motion.

Current Status and Future Trends for Mooring Systems of Floating Offshore Wind Turbines

With the increasing demand of energy and the limitation of bottom-fixed wind turbines in moderate and deep waters,floating offshore wind turbines are doomed to be the right technical choice and they are bound to enter a new era of rapid development.The mooring system is a vital system of a floating wind turbine for station-keeping under harsh environmental con­ditions.In terms of existing floating wind turbine projects,this paper is devoted to discussing the current status of mooring systems and mooring equipment.This paper also presents the mooring analysis methods and points out the technical difficulties and challenges in mooring design,in­stallation,operation and maintenance stages.Finally,the developing trends of the mooring system are summarized,aiming to provide a reference for future mooring research.

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.

Experiment Study of Dynamics Response for Wind Turbine System of Floating Foundation

The floating foundation is designed to support a 1.5 MW wind turbine in 30 m water depth. With consideration of the viscous damping of foundation and heave plates, the amplitude-frequency response characteristics of the foundation are studied. By taking into account the elastic effect of blades and tower, the classic quasi-steady blade-element/momentum(BEM) theory is used to calculate the aerodynamic elastic loads. A coupled dynamic model of the turbine-foundationmooring lines is established to calculate the motion response of floating foundation under Kaimal wind spectrum and regular wave by using the FAST codes. The model experiment is carried out to test damping characteristics and natural motion behaviors of the wind turbine system. The dynamics response is tested by considering only waves and the joint action of wind and waves. It is shown that the wind turbine system can avoid resonances under the action of wind and waves. In addition, the heave motion of the floating foundation is induced by waves and the surge motion is induced by wind. The action of wind and waves is of significance for pitch.

Preliminary Design of a Submerged Support Structure for Floating Wind Turbines

Cost-effective floating wind turbines with efficient installations are highly desired in deep waters(>50 m).This paper presents a submerged floating offshore wind turbines(SFOWT)concept for intermediate water depths(50-200 m).The performance of SFOWTs can be improved through a judicious choice of configuration,pretension,and mooring line layout.Four SFOWTs with different configurations and a similar mass,named Cyl-4,Cub-4,Cyl-3,and Hex-3,were designed and analyzed.The responses of the four SFOWTs were predicted under operational condition and extreme condition.The results show that the four SFOWTs exhibited good performance under both conditions.The effect of platform configurations on power output was negligible under the operational condition.Under the extreme condition,among the four SFOWTs,the mean bending moments at the tower base were very close,while the maximum values differed by up to 21.5%,due to the configurations.The effect of wind-wave misalignment under the extreme condition was further analyzed.In general,the motion performances of the four-pontoon SFOWTs,Cyl-4 and Cub-4,were superior to those of the three-pontoon SFOWTs,Cyl-3 and Hex-3.Optimization studies of the mooring system were carried out on Cub-4 with different mooring line pretensions and four mooring layouts.The optimized Cub-4 could reduce the maximum motion responses in the surge,heave,and yaw by 97.7%,91.5%,and 98.7%,respectively.

Numerical Analysis on Motion of Multi-column Tension-Leg-Type Floating Wind Turbine Basement

The offshore wind energy presents a good solution for the green energy demand.The floating offshore wind turbine(FOWT)is one of the most potential choices of the basement construction for offshore wind turbines in deep water.Hydrodynamic performance of multi-column tension-leg-type floating wind turbine is investigated numerically,particularly at its motion responses.Based on the Navier-Stokes equations and the volume of fluid method,a numerical wave tank(NWT)is established to simulate the floating structure system.The analytical relaxation method is adopted to generate regular waves.Dynamic mesh method is used to calculate the motion of the floating body.Hydrostatic decay of motion and hydrodynamic forces in the regular wave are provided.The computation results agree with the experimental data available.Numerical results show that the wave force on the lower pontoon of the system is the greatest while that on the center column is the smallest.Detailed information about the changes of the wave forces on different elements of the floating system is discussed.