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.

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.

Correlation Analysis of Wind Turbine Temperature Rise and Exergy Efficiency Based on Field-Path Coupling

To solve the problems of large losses and low productivity of permanent magnet synchronous generators used in wind power systems,the field-circuit coupling method is used to accurately solve the electromagnetic field and temperature field of the generator.The loss distribution of the motor is accurately obtained by considering the influence of external circuit characteristics on its internal physical field.By mapping the losses to the corresponding part of the three-dimensional finite element model of the motor,the temperature field is solved,and the global temperature distribution of the generator,considering the influence of end windings,is obtained.By changing the air gap length,permanent magnet thickness,and winding conductivity,the relationship between the loss,temperature rise,and exergy efficiency can be obtained.By optimizing the air gap length,permanent magnet thickness,and winding conductivity,the best configuration and material properties can improve the efficiency of the motor by up to 4%.

Coupled Dynamic Response Analysis of A Multi-Column Tension-Leg-Type Floating Wind Turbine

This paper presents a coupled dynamic response analysis of a multi-column tension-leg-type floating wind turbine（Wind Star TLP system） under normal operation and parked conditions. Wind-only load cases, wave-only load cases and combined wind and wave load cases were analyzed separately for the Wind Star TLP system to identify the dominant excitation loads. Comparisons between an NREL offshore 5-MW baseline wind turbine installed on land and the Wind Star TLP system were performed. Statistics of selected response variables in specified design load cases（DLCs） were obtained and analyzed. It is found that the proposed Wind Star TLP system has small dynamic responses to environmental loads and it thus has almost the same mean generator power output under operating conditions as the land-based system. The tension mooring system has a sufficient safety factor, and the minimum tendon tension is always positive in all selected DLCs. The ratio of ultimate load of the tower base fore-aft bending moment for the Wind Star TLP system versus the land-based system can be as high as 1.9 in all of the DLCs considered. These results will help elucidate the dynamic characteristics of the proposed Wind Star TLP system, identify the difference in load effect between it and land-based systems, and thus make relevant modifications to the initial design for the Wind Star TLP system.

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） ofa 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 is 92% 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.

Evaluating effectiveness of multiple tuned mass dampers for vibration control of jacket offshore wind turbines under onshore and seafloor earthquakes

The dynamic characteristics and structural responses of operation and grid loss offshore wind turbines(OWTs)under onshore and seafloor earthquakes are analyzed based on the established coupled seismic analysis model.In addition to the remarkable influence of the rotor system on the responses of the operation OWT under earthquakes,interactions among the natural modes of the grid loss OWT in the fore-aft and side-to-side directions are revealed.By comparing with the onshore earthquakes,the more significant differences of structural response are observed under the selected seafloor earthquakes,due to the longer duration and more abundant energy distribution around the natural frequencies of OWT.Concurrently,a multiple tuned mass damper(MTMD)is designed and applied to the operation and grid loss OWTs.Then,the comparisons of the mitigation effects under onshore and seafloor ground motions are carried out,and the necessity of applying seafloor ground motions to OWTs are proved.Moreover,in comparison to the operation OWT,more effective reductions are observed for the grid loss OWT under onshore and seafloor earthquakes using the designed MTMD.Therefore,the combined shutdown procedures and MTMD vibration control strategy is suggested for OWTs under earthquakes.

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.

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.

A computational platform for considering the effects of aerodynamic and seismic load combination for utility scale horizontal axis wind turbines

The wide deployment of wind turbines in locations with high seismic hazard has led engineers to take into account a more comprehensive seismic design of such structures. Turbine specific guidelines usually use simplified methods and consider many assumptions to combine seismic demand with the other operational loads effecting the design of these structures. As the turbines increase in size and capacity, the interaction between seismic loads and aerodynamic loads becomes even more important. In response to the need for a computational tool that can perform coupled simulations of wind and seismic loads, a seismic module is developed for the FAST code and described in this research. This platform allows engineers working in this industry to directly consider interaction between seismic and other environmental loads for turbines. This paper details the practical application and theory of this platform and provides examples for the use of different capabilities. The platform is then used to show the suitable earthquake and operational load combination with the implicit consideration of aerodynamic damping by estimating appropriate load factors.

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.

Ship collision impact on the structural load of an offshore wind turbine

When a maintenance and operations ship is berthing,there is a chance the ship may collide into the wind turbine.When these ships collide into wind turbine structures,this can result in significant changes to the foundation and structure of the wind turbine.In this paper,the structural load of a 4 MW offshore wind turbine was analyzed during a collision with an operations and maintenance ship.The variations in the wind speeds on hub height,waves,and the sea currents were measured.The dynamic simulation of the wind turbine was carried out using the test data as the input parameters.As a result,the load condition of the turbine without a collision was obtained.Finally,the measured turbine load was compared with the simulation results.This study shows that the collision of the operation and the maintenance ship increases the bending moments at the tower’s bottom and the blade’s roots.

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.

An Integrated Structural Strength Analysis Method for Spar Type Floating Wind Turbine

An integrated structural strength analysis method for a Spar type floating wind turbine is proposed in this paper,and technical issues related to turbine structure modeling and stress combination are also addressed.The NREL-5MW ＂Hywind＂ Spar type wind turbine is adopted as study object.Time-domain dynamic coupled simulations are performed by a fully-coupled aero-hydro-servo-elastic tool,FAST,on the purpose of obtaining the dynamic characteristics of the floating wind turbine,and determining parameters for design load cases of finite element calculation.Then design load cases are identified,and finite element analyses are performed for these design load cases.The structural stresses due to wave-induced loads and wind-induced loads are calculated,and then combined to assess the structural strength of the floating wind turbine.The feasibility of the proposed structural strength analysis method for floating wind turbines is then validated.

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.

Effects of Second-Order Difference-Frequency Wave Forces on Floating Wind Turbine Under Survival Condition

In this paper, the effects of second-order difference-frequency wave forces on the global motion of an offshore wind turbine system with a large displacement under the survival condition are studied. In this case, the hydrodynamic force is the main force because the blades are feathered to reduce the lifting force. The first-order hydrodynamic forces are calculated by WADAM, while the second-order wave forces are calculated by a customized MATLAB module. Then the hydrodynamic coefficients are transferred to the wind turbine analytical code FAST. Through the comparisons of dynamic responses between the first-and second-order numerical models, it is found that the second-order wave forces significantly influence the motion of floating wind turbine under the survival condition. Moreover, neglecting the second-order force significantly underestimates the tension forces in the mooring lines.

浮式风力机若干特征动力学问题综述

总结浮式风力机类型及其对应的特征动力学问题,针对浮式风力机气动荷载、水动荷载的计算方法以及结构动力学、控制动力学典型问题进行论述。讨论了气动—水动—结构—伺服耦合分析的难点,重点分析了二阶波浪力、畸形波等非线性波浪荷载、流荷载及涡激运动对浮式风力机特征动力响应的影响。阐述了浮式风力机动力学研究的试验方法、数值仿真方法、样机测试方法,并对模型试验技术的相似理论、气动模型的实现和难点以及数值仿真的频域方法、时域方法和分析工具进行了归纳对比。研究表明:浮式风力机多场、多体耦合动力分析机理及相关技术仍不成熟,气动荷载、高阶非线性波浪荷载耦合模型的建立是动力学问题研究的重点,数值仿真及模型试验是浮式风力机动力响应研究的主要方法,样机测试技术的积累将促进设计标准的完善及浮式风电的产业化发展。

附加阻尼控制下风电机组机网耦合载荷建模及分析

针对附加阻尼控制下风电机组机械载荷问题,开展风电机组机网耦合载荷建模及载荷特性分析研究。采用FAST与Matlab/Simulink软件联合方法,建立综合考虑机组机械特性和电气特性的并网风电机组机网耦合载荷模型。采用所建模型开展附加阻尼控制对机组机械载荷影响的仿真分析,并复现附加阻尼控制下风电机组机械系统与电力系统的机网共振现象。结果表明,所建模型能够表征附加阻尼控制下风电机组机械系统与电力系统的耦合互作用,系统低频振荡会导致机组关键结构出现明显振动载荷,且系统振荡频率与机组机械结构固有振动频率接近时两者共振会急剧加大机组振动载荷。

风浪联合作用下分布式调谐质量阻尼器对海上半潜漂浮式风机的减振控制

海上半潜漂浮式风机在复杂深海环境下产生有害振动会威胁风机的安全性和耐久性,针对该问题并结合美国NREL的5 MW样机的漂浮平台几何结构构造,提出利用分布式调谐质量阻尼器(Tuned Mass Dampers,TMDs),即分别在漂浮平台的3根浮筒中布置TMD,形成等边三角形布置,对随机风浪联合作用下海上半潜漂浮式风机的平台纵摇振动进行控制。为了更好地描述分布式TMDs对海上半潜漂浮式风机的减振效果,基于拉格朗日方程和模态叠加法,对海上半潜漂浮式风机-TMDs耦合系统提出并建立了9自由度多体动力学模型。基于H_(∞)算法,即以平台纵摇频响函数的峰值为优化目标,对分布式TMDs的参数进行优化设计,优化设计中考虑了3个TMDs之间的耦合关系。对风机-TMDs耦合系统开展了风浪联合作用下的数值模拟,分析了分布式TMDs对平台纵摇响应的减振效果。结果表明:最优设计下的分布式TMDs对海上半潜漂浮式风机平台纵摇振动具有良好的减振性能;在三种不同工况的随机风浪荷载作用下,分布式TMDs对平台纵摇固有频率附近的功率谱密度曲线峰值减振率和标准差减振率能分别达到39%和52%以上。

海上浮式风机多体系统耦合动力模型研究

海上浮式风机是近年来随着海上风电的快速发展,为了捕获深海更丰富、更持久的风能而提出的一种风力发电装置,已成为当今风能开发的主要方向。作为一种多体系统,由于海上浮式风机结构特殊,加上环境复杂,对其进行准确的计算和分析尤为重要。本文对海上浮式风机的耦合动力模型进行了研究,建立了复杂工况下Spar型海上浮式风机改进的14-DOF耦合动力模型,包括气动力模块、水动力模块和结构分析模块等,用于扩展其适用范围和准确计算风机的动力响应,并通过数值仿真对所建模型进行了分析和验证。主要的改进有:不对平台和塔架的转动角度作小量近似,扩展其适用范围;考虑角速度和欧拉角速度的换算关系,不作等化处理。此外,所建模型考虑风机叶片扭转角对叶片变形的影响,得到了较为准确的叶片面内外响应。同时采用线性势流理论对水动力进行计算,较之Morison方程适用性更广。仿真分析表明,本文所建模型可以更准确地计算海上浮式风机系统的动力响应,且具有更广的适用范围。