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.     

浮式风机气动—水动—气弹性耦合响应数值模拟

随着海上风电产业的快速发展,大型浮式风机逐渐从概念设计走向工程应用,但仍面临较大的挑战。一方面,在风、浪等环境载荷的作用下,浮式风机的气动载荷和水动力响应之间存在明显的相互干扰作用;另一方面,风力机大型化使得叶片细、长、薄的特点愈发突出,叶片柔性变形十分显著,这会影响到浮式风机的耦合性能。基于两相流CFD求解器naoe-FOAM-SJTU,结合弹性致动线模型和等效梁理论,建立了浮式风机气动—水动—气弹性耦合响应计算模型,并对规则波和剪切风作用下Spar型浮式风机的气动—水动—气弹性耦合响应进行了数值模拟分析。结果表明,风力机气动载荷使得叶片挥舞变形十分显著,而叶片的扭转变形会明显降低风力机的气动载荷。此外,风力机气动载荷会增大浮式平台的纵荡位移和纵摇角,同时,浮式平台运动响应会导致风力机气动载荷产生大幅度周期性变化。进一步地,叶片结构变形响应会使得浮式风机尾流场的速度损失和湍动能有所降低。     

Probabilistic analysis of offshore wind turbines under extreme resonant response: Application of environmental contour method

Offshore wind turbines can exhibit dynamic resonant behavior due to sea states with wave excitation frequencies coinciding with the structural eigenfrequencies. In addition to significant contributions to fatigue actions, dynamic load amplification can govern extreme wind turbine responses. However, current design requirements lack specifications for assessment of resonant loads, particularly during parked or idling conditions where aerodynamic damping contributions are significantly reduced. This study demonstrates a probabilistic approach for assessment of offshore wind turbines under extreme resonant responses during parked situations. Based on in-situ metocean observations on the North Sea, the environmental contour method is used to establish relevant design conditions. A case study on a feasible large monopile design showed that resonant loads can govern the design loads. The presented framework can be applied to assess the reliability of wave-sensitive offshore wind turbine structures for a given site-specific metocean conditions and support structure design.     

Dynamic Response of Offshore Wind Turbine on 3×3 Barge Array Floating Platform under Extreme Sea Conditions

Offshore wind farm construction is nowadays state of the art in the wind power generation technology.However,deep water areas with huge amount of wind energy require innovative floating platforms to arrange and install wind turbines in order to harness wind energy and generate electricity.The conventional floating offshore wind turbine system is typically in the state of force imbalance due to the unique sway characteristics caused by the unfixed foundation and the high center of gravity of the platform.Therefore,a floating wind farm for 3×3 barge array platforms with shared mooring system is presented here to increase stability for floating platform.The NREL 5 MW wind turbine and ITI Energy barge reference model is taken as a basis for this work.Furthermore,the unsteady aerodynamic load solution model of the floating wind turbine is established considering the tip loss,hub loss and dynamic stall correction based on the blade element momentum(BEM)theory.The second development of AQWA is realized by FORTRAN programming language,and aerodynamic-hydrodynamic-Mooring coupled dynamics model is established to realize the algorithm solution of the model.Finally,the 6 degrees of freedom(DOF)dynamic response of single barge platform and barge array under extreme sea condition considering the coupling effect of wind and wave were observed and investigated in detail.The research results validate the feasibility of establishing barge array floating wind farm,and provide theoretical basis for further research on new floating wind farm.     

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。     

基于非定常面元法的海上浮式风机气动性能研究

对于海上浮式风机而言,由于受到剪切风、塔影效应、浮式基础运动等因素的共同影响,其气动载荷会更加复杂,因此如何准确快速地对海上风力机的气动性能进行预估显得尤为重要。基于速度势的非定常面元法理论,研究海上浮式风机气动载荷特性,编制了相关的计算程序。以NREL 5 MW风机为例,建立了叶片和尾流的三维数值模型,计算得到了不同风速下风机的输出功率以及叶片表面的压力分布,对比数据结果分析了该方法的可靠性。针对非定常流动,模拟了剪切风和塔影效应的作用,并重点分析了浮式基础运动对风机气动载荷的影响。研究表明,浮式基础的纵荡和纵摇会增加输出功率的波动幅值,艏摇运动会导致单个叶片上的气动载荷产生较大的波动,为浮式风机叶片控制提供了参考。     

超大型和大型半潜浮式海上风力机动力响应对比

以超大型风力机(DTU 10 MW)为研究对象,对现有的大型(NREL 5 MW)无撑杆半潜浮式风力机支撑平台进行放大设计,用于支撑超大型风力机,基于气动-水动-伺服-弹性全耦合计算模型,根据设定的典型工况,使用FAST软件对超大型和大型无撑杆的半潜浮式风力机系统进行时域耦合分析,并依据计算结果对超大型和大型浮式风力机系统的运动响应和结构动力反应等特性进行对比分析。研究发现:半潜浮式风力机大型化后,气动荷载效应对风力机系统的激励作用更为突出,使得浮式平台运动由风荷载激励的低频共振反应比例增大,波频运动比例减小,这也导致由浮式平台低频运动激励的锚链张力反应增大。此外,高倍的飞轮转动频率对大型半潜浮式风力机叶片、塔架结构的激励作用较超大型半潜浮式风力机更为显著。     

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.     

海上风机吸力式桶形基础承载特性研究综述

风机基础作为海上风机整体结构的重要组成部分,承受着上部风机所受到的风浪流荷载,并且对风机的安全性及可靠性至关重要。吸力式桶形基础由于其安装简单和可重复利用等优点,在海洋平台基础中得到了广泛应用,并逐步应用于海上风机基础中。但由于海上风机与海洋平台在海洋环境中的荷载工况有一定的差别,仍需要通过对其承载特性研究现状进行全面认识,以实现吸力式桶形基础在海上风机基础中的可靠应用。文中通过总结和评价现有研究对桶形基础在不同土体条件以及荷载条件下进行试验及数值模拟分析得到的研究结果,综述了静荷载和循环荷载作用下砂土和黏土中的吸力式桶形基础的承载特性研究现状,以及海上风机吸力式桶形基础的相关研究。文章展望了目前应用于海上风机基础的桶形基础仍缺乏的研究,为海上风机吸力式桶形基础的可靠应用及后续研究提供重要参考。     

偏航工况下海上浮式风力机基础运动响应及锚泊载荷影响分析

海上浮式风力机受风、浪、流等外部载荷影响,运营期间经常处于偏航工况,给风力机基础运动响应和锚泊载荷带来重要影响.基于经典叶素动量理论及势流理论,建立海上浮式风力机水—气动力耦合分析模型,对在非定常风、不规则波浪联合作用下,风力机偏航时基础运动响应及锚泊载荷等进行分析.研究发现,额定风速工况下,风力机偏航对平台纵荡和纵摇运动影响较大,偏航30°时纵荡和纵摇平均值比偏航0°时分别下降20.68％和37.36％,垂荡运动响应受风力机偏航影响较小;锚泊载荷变化趋势与平台运动及锚链布置有关,平台纵荡对锚泊载荷影响较大,偏航30°时锚链#1有效张力平均值比偏航0°时下降12.98％.     

支撑结构频率对海上风机动力响应的影响机理研究

海上支撑结构的优化设计是海上风机技术发展的必然趋势,降低支撑结构的载荷是保证风机安全运行的有效途径。海上支撑结构受到风浪复杂环境荷载作用,风、浪载荷决定着塔底承受较大的剪力和倾覆力矩,同时风浪的随机性和周期性会影响塔架的疲劳载荷。基于海上风机支撑结构频率对载荷影响的研究,分析海上支撑结构频率对叶片根部挥舞和摆振响应的影响,探究频率对风机响应的影响机理。研究表明：波浪频率诱导是基础疲劳载荷响应的主要原因;开展单桩基础设计,当整机频率确定时,基础变径段可设置于浪溅区下部区域范围;叶片摆振响应受1P频率影响较大,在风机设计时可增加叶片摆振方向的阻尼;当整机频率邻近3P频率时,塔筒刚度变化对基础载荷响应的影响大于基础刚度变化;海上支撑结构设计时可优先考虑塔筒刚度。     

Dynamic Analysis of A 5-MW Tripod Offshore Wind Turbine by Considering Fluid–Structure Interaction

Fixed offshore wind turbines usually have large underwater supporting structures.The fluid influences the dynamic characteristics of the structure system.The dynamic model of a 5-MW tripod offshore wind turbine considering the pile–soil system and fluid structure interaction(FSI) is established,and the structural modes in air and in water are obtained by use of ANSYS.By comparing low-order natural frequencies and mode shapes,the influence of sea water on the free vibration characteristics of offshore wind turbine is analyzed.On basis of the above work,seismic responses under excitation by El-Centro waves are calculated by the time-history analysis method.The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water.The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater.     

Dynamic Analysis of A 5-MW Tripod Offshore Wind Turbine by Considering Fluid–Structure Interaction

Fixed offshore wind turbines usually have large underwater supporting structures. The fluid influences the dynamic characteristics of the structure system. The dynamic model of a 5-MW tripod offshore wind turbine considering the pile–soil system and fluid structure interaction (FSI) is established, and the structural modes in air and in water are obtained by use of ANSYS. By comparing low-order natural frequencies and mode shapes, the influence of sea water on the free vibration characteristics of offshore wind turbine is analyzed. On basis of the above work, seismic responses under excitation by El-Centro waves are calculated by the time-history analysis method. The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water. The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater.     

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 (WindStar 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 WindStar TLP system to identify the dominant excitation loads. Comparisons between an NREL offshore 5-MW baseline wind turbine installed on land and the WindStar 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 WindStar 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 WindStar 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 WindStar 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 WindStar TLP system.     

Numerical Simulation of Icing on Nrel 5-MW Reference Offshore Wind Turbine Blades Under Different Icing Conditions

Offshore wind energy resources are operational in cold regions, while offshore wind turbines will face the threat of icing. Therefore, it is necessary to study icing of offshore wind turbines under different icing conditions. In this study, icing sensitivity of offshore wind turbine blades are performed using a combination of FLUENT and FENSAP-ICE software, and the effects of liquid water content (LWC), medium volume diameter (MVD), wind speed and air temperature on blade icing shape are analyzed by two types of ice, namely rime ice and glaze ice. The results show that the increase of LWC and MVD will increase the amount of ice that forms on the blade surface for either glaze ice or rime ice, and an increase of MVD will expand the adhesion surface between ice and blade. Before reaching the rated wind speed of 11.4 m/s, it does not directly affect the icing shape. However, after reaching the rated wind speed, the attack angle of the incoming flow decreases obviously, and the amount of ice increases markedly. When the ambient air temperature meets the icing conditions of glaze ice (i.e., −5°C to 0°C), the lower the temperature, the more glaze ice freezes, whereas air temperature has no impact on the icing of rime ice. Compared with onshore wind turbines, offshore wind turbines might face extreme meteorological conditions, and the wind speed has no impact on the amount of ice that forms on the blade surface for most wind speeds

     

海上风力发电单立柱支撑结构拟静力分析

海上风电支撑结构不同于一般海洋结构物,它受到复杂的风机气动荷载、机械控制荷载和海洋环境荷载的多重作用。文章针对海上某单立柱风电支撑结构,通过分析其结构固有频率的约束限制以及外环境荷载的动力特性,综合考虑外环境荷载尤其是风机荷载的动力放大影响,给出海上单立柱风电支撑结构的拟静力分析思路。并进行极端及操作工况下支撑结构在风、浪、流环境荷载组合作用的应力计算和强度分析。提出该种结构在使用现有海洋结构物设计规范和风机设计规范时的注意事项。该分析比较结果及结论可作为海上类似风电支撑结构的设计参考。     

畸形波作用下半潜浮式风机系泊失效停机响应分析

半潜浮式风机逐渐在深海风电开发中受到关注,建立风机、平台与系泊结构耦合数值计算模型,通过FAST与AQWA链接进行风机塔基荷载及平台运动响应相互耦合传递,基于随机波与极限波组合模型生成畸形波时程序列,进行半潜浮式风机系泊失效全过程时域模拟计算分析,得出系泊锚链张力、风机、塔筒和平台运动时程响应,探究系泊失效、风机停机和叶片变桨速率对浮式风机平台系泊结构动力响应的影响。结果表明：畸形波作用下浮式平台和系泊结构动力响应显著,系泊失效导致塔基剪力增加,平台纵荡和纵摇运动响应显著增大;风机停机会引起系泊锚链张力显著减小,转子推力、塔基剪力和叶尖挥舞位移响应逐渐衰减,平台纵荡、纵摇和横摇运动响应显著减小;随着叶片变桨速率增加,风机转子推力和塔基剪力波动幅值增大。     

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 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.     

考虑流固耦合效应的单桩式海上风电机组结构响应模拟

针对海上风电整机系统,建立了一体化流固耦合分析方法。在模拟过程中,采用浸入边界法解决风机叶片旋转引起的静动干涉问题,利用改进的守恒式 level set（简称 ICLS）方法捕捉海浪自由面,并使用交错迭代法求解流固耦合方程。通过构建 “风机—塔架—基础”一体化流固耦合数值模拟方法,能够在一次仿真计算中实现海上风电整机系统的全过程数值模拟,准确求解多荷载耦合作用下“风机—塔架—基础”的整体结构动力响应。以某单桩式海上风电工程为例,验证了本方法能够实现对海上风电整体系统的一体化分析。     

Incorporating irregular nonlinear waves in coupled simulation and reliability studies of offshore wind turbines

Design of an offshore wind turbine requires estimation of loads on its rotor, tower and supporting structure. These loads are obtained by time-domain simulations of the coupled aero-servo-hydro-elastic model of the wind turbine. Accuracy of predicted loads depends on assumptions made in the simulation models employed, both for the turbine and for the input wind and wave conditions. Currently, waves are simulated using a linear irregular wave theory that is not appropriate for nonlinear waves, which are even more pronounced in shallow water depths where wind farms are typically sited. The present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on the support structure (monopile) of an offshore wind turbine. We present the theory for the irregular nonlinear model and incorporate it in the commonly used wind turbine simulation software, FAST, which had been developed by National Renewable Energy Laboratory (NREL), but which had the modeling capability only for irregular linear waves. We use an efficient algorithm for computation of nonlinear wave elevation and kinematics, so that a large number of time-domain simulations, which are required for prediction of long-term loads using statistical extrapolation, can easily be performed. To illustrate the influence of the alternative wave models, we compute loads at the base of the monopile of the NREL 5MW baseline wind turbine model using linear and nonlinear irregular wave models. We show that for a given environmental condition (i.e., the mean wind speed and the significant wave height), extreme loads are larger when computed using the nonlinear wave model. We finally compute long-term loads, which are required for a design load case according to the International Electrotechnical Commission guidelines, using the inverse first-order reliability method. We discuss a convergence criteria that may be used to predict accurate 20-year loads and discuss wind versus wave dominance in the load prediction. We show that 20-year long-term loads can be significantly higher when the nonlinear wave model is used.