Under combined wind and wave loads, offshore wind turbines (OWTs) may undergo excessive vibrations, leading to increased structural fatigue loading and reduced service life. In this paper, an analytical nonlinear model of an OWT is developed based on the geometrically exact beam theory. A statistical analysis of various wind and wave properties is performed in order to represent long-term metocean conditions. The fatigue damage of an IEA 15 MW reference wind turbine is evaluated under realistic environmental conditions using the rainflow counting method and Miner's Rule. The results show that the nonlinear model predicts larger stress fluctuations and shifts fatigue damage toward higher stress ranges, particularly under the wind turbine's rated operational conditions. Moreover, fatigue damage is maximized under orthogonal wind-wave misalignment. A precise linear relationship is identified between the misalignment angle and the fatigue damage at the critical mudline cross-section location. These findings highlight the necessity of incorporating realistic environmental conditions, directional misalignment and structural nonlinearity in the fatigue design of large-scale OWTs.

In this study, the dynamic responses of the IEA-15MW offshore wind turbine (OWT) subjected to the wind and wave forces are numerically investigated. An offshore wind turbine model based on a mixed-form geometrically exact beam formulation embedded in nonlinear Winkler foundation is developed, in which the blade-tower and monopile-soil coupling are taken into accounted. The dynamic analyses of the OWT under separate wind excitation shows that the rotational effect of the rotor on the response of the tower cannot be neglected. The dynamic analyses of the OWT under wave excitation alone shows that the displacement of the tower is not proportional to the magnitude of wave loads due to frequency detuning. Under combined wind and wave action, stochastic phase cancellation between wind turbulence and wave spectral energy may cause the structural responses to be lower than those under wind or wave excitation alone. The dynamic analyses considering soil-structure interactions (SSI) show that the frequency softening induced by SSI effect can increase the resonance sensitivity of OWTs on soft soil sites by bringing the fundamental frequency of the tower closer to the energy concentration of wave spectrum. Geometrical nonlinearities have a significant effect on the dynamic response of the blades and the interaction between shear deformation-induced warping effect and torsional deformation further amplify the torsional displacement. These findings highlight the necessity of using mixed-form geometrically exact beam theory to model the OWT system for aeroelastic coupling analysis in this paper.

Offshore wind turbines experience undesirable vibrations under harsh environmental loads, which can cause an increase in the probability of failure and decrease the structural reliability. In addition, there has been a growing interest in highly flexible structural components as the increase of wind turbine capacity, in which large deflection has a significant impact on the structural vibrations. The tuned mass damper (TMD) is one of the most widely utilised vibration attenuation methods in offshore wind turbines due to its easy implementation and cost issues. However, the TMD performance in offshore wind turbine vibration mitigation that considers large structural deflection and geometrical nonlinearity has not been extensively explored yet. The present study has taken large-scale offshore wind turbines as the research object and established a nonlinear dynamic model based on geometrically exact beam theory, in which the key components of offshore wind turbines are all considered and coupled, such as the blade, nacelle, and tower. To mitigate the vibrations, TMD is designed and introduced to the analytical model based on the tower vibration characteristics. The results indicate the effectiveness of employing a nonlinear analytical approach for the design of TMD in vibration mitigation of the offshore wind turbine, which will lay the foundation for nonlinear dynamic analysis techniques on advanced vibration controllers.

The cost of support structures in offshore wind turbines accounts for about a quarter of the total construction cost, and therefore the optimized design of support structures has significant economic benefits. Based on the guide-weight method which is a gradient-based optimization algorithm, a gradual optimization algorithm is proposed to optimize the monopile support structure that is widely used in offshore wind turbines. The optimization variables are defined based on the actual tower and pile sections, and the multi-constraint optimization problem is established by considering the constraints including displacement, natural frequency, strength, and geometric size. The interactions between the support structures and loads of wind, waves and currents are considered through multiple rounds of progressive optimization. The application of automatic differentiation technology makes it possible to efficiently evaluate the sensitivities of constraints under the condition that the constraints are expressed on basis of relative design specifications. Through the optimization of an established case, the proposed method is found to be stable, efficient and effective to reduce the cost of the monopile support structure.

In recent years, the concept of the prefabricated assembled steel-concrete hybrid tower (SCHT) has been shown to be a very economical option for multi-megawatt wind turbines. However, with the common application of SCHT for wind turbines, concrete cracking problems caused by resonance effects have been observed in SCHTs after the operation of wind turbines. First, field measurements of a SCHT for a 6.0 MW wind turbine located in a mountainous area of Yunnan Province, China, revealed a significant deviation between the measured frequency and the designed frequency. Subsequently, a nonlinear dynamic model based on the mixed form of geometrically exact beam theory was developed, in which the blades, nacelle, and tower were explicitly taken into account. A simplified spring-dashpot-mass model was used to account for the soil-structure interaction (SSI) effect. The accuracy and efficiency of the proposed numerical model were validated by comparing it with the modal identification results based on the measured data. Finally, the effects of the modulus of elasticity of concrete, the top mass of the tower and the shear modulus of the foundation soil on the natural frequencies of the SCHT were investigated.

SummaryThe prestressed anchor bolt system is a reasonable connection mode between the upper steel tower and the bottom concrete foundation for multi‐megawatt wind turbines. This prestressed anchor bolt connection with forged flanges has a similar form to the prestressed high‐strength bolt connection with forged flanges between steel tubes for the tower. However, their mechanical performances have a great difference because of the influence of the stiffness of the anchorage zone in the concrete foundation. Based on the Petersen's method, the engineering calculation method of the prestressed anchor bolt system for wind turbine foundation is derived. The tensile force of the anchor bolt, the anchorage stiffness and clamping force of the base are all deduced according to the theories of mechanics of materials. The numerical models of the segment foundation with the unfavorable anchor bolt and the overall foundation with all anchor bolts are developed for researching the influence of the spatial effect of adjacent segments on the restraint stiffness of the concrete foundation. The numerical analyses of four engineering cases designed by engineering calculation method are carried out for verifying the effectiveness of engineering calculation method. The analysis results show the spatial effect of adjacent segments can be neglected and the Petersen's method can be directly applied for the design of the prestressed anchor bolt system for wind turbine foundation in engineering practice. The engineering calculation method meets the accuracy requirements in engineering practice and can be used to design the prestressed anchor bolt system of wind turbine foundation.

With the development of wind energy, the unit capacity of wind turbine is becoming larger and the height of the support tower is getting higher. Limited by the transport conditions, the diameter of the tubular steel tower cannot further increase with the increase of the tower height, which results in the lower natural frequencies of the tower. The higher and more slender wind turbine towers are more sensitive to the external excitation and many tower collapses have occurred in recent years because of the interplay with the longer rotor blades. A 2 MW wind turbine system with a 120 m full steel tubular tower is taken as an example in this study to research the nonlinear coupling vibration mechanism under the normal operation case and the emergency shut-down case. An integrated model of large-scale wind turbine system including the tower, blades, hub, and nacelle is established by the open-source software FAST to study its nonlinear interaction vibration mechanism. Under the normal operation load case, the acceleration response of the steel wind turbine tower is greatly affected by the rotational frequency of the rotor. The maximum accelerations in the fore-aft and side-side directions both appear around the middle-upper height of the tower. Under the emergency shut-down load case, the maximum fore-aft acceleration response appears at the top of the tower, but the maximum side-side response still appears around the middle-upper height of the tower. The first-order vibration mode plays a dominant role in the top fore-aft acceleration and the second-order vibration mode has great contribution to the side-side acceleration under the emergency shut-down load case. The numerical results of this study can provide insight into the vibration control mechanism of the full steel tower.

More and more prefabricated steel-concrete hybrid wind turbine towers have been built because of their better lateral stiffness than those of the full steel towers, in which epoxy resin joints are commonly adopted at the horizontal joint between two ring units for improving the erection speed. In fact, epoxy resin joints are designed in the same way as dry joints due to the very thin thickness of epoxy resin layer, in which epoxy resin only acts as a leveling blanket and sealer for jointing and compensates for the unevenness of the contact surface between two ring units. The current design method for the resistance to torsional moment at the horizontal joint is not reasonable because of the unreasonable assumption of Saint-Venant's torsional theory. The integral expressions of the ultimate torsional moment at the horizontal joint with and without shear force are derived, respectively. The solution of the integral expressions for the ultimate torsional moment is realized by Python programming. The refined finite element analyses of two cases are compared with the existing small-scale tests with segmental aluminum tubes, which verifies the calculation accuracy of the proposed integral method. In the modified integral model of the ultimate torsional moment, a correction term of the resistance to torsional moment and a more suitable distribution of shear stress under the action of horizontal shear force are proposed to obtain a more accurate ultimate torsional moment. Finally, 36 sets of cases with typical dimensions and axial forces in practical engineering are analyzed by the proposed integral model in the absence of horizontal shear force. One six-parameter model for calculating the ultimate torsional moment is fitted by the least square method. A discount factor is proposed to consider the influence of the horizontal shear force on the ultimate torsional moment.

为研究大型风电机组的振动机理,基于贝兹理论选取用于低风速区2MW风电机组的叶片,基于PID控制系统标定相关参数.采用FAST软件建立塔架、叶片、轮毂和机舱一体化模型,对该2MW风电机组正常运行工况和紧急停机工况下的动力响应进行分析.分析结果表明,该风电机组一阶频率较低,塔架前后方向为主要振动方向;正常运行工况下,风电机组塔架前后方向加速度响应受3P、6P影响较大(1P为风轮额定转动频率),二阶振型影响明显;紧急停机工况会加剧塔架的振动,塔架前后方向加速度响应主要受塔架一阶振型影响,塔架二阶振型对塔架侧向加速度响应影响明显;该结果为后续振动控制研究奠定基础.

The finite element (FE) analyses based on linear elastic fracture mechanics (LEFM) for the crack propagation and failure of the grout layer in the wind turbine foundation with pre-stressed anchor bolts are presented. Firstly, one half-symmetric planar numerical model is developed. According to the linear elastic analytical results, one initial crack is set to be the zone of the maximum tensile streess and its direction is perpendicular to the direction of the maximum principle tensile stress. The crack will propagate in the direction of the zero mode-II stress intensity factor during the failure of the grout layer. The numerical simulations with different prarmeters are performed. The histories of mode-I stress intensity factor are analyzed and design formulas predicting the failure and ultimate load capacity of the grout layer are proposed based on parametric studies. The increasing width or the decreasing depth of the grout layer can significantly mitigate the failure. One case study of the grout layer with the typical size and the corresponding fatigue analyses are carried out. The numerical results show that the proposed equation can agree well with the cracking status and failure of the grout layer in practical engineering and help predict the fatigue life of the grout layer under the certain intial crack length.

预应力锚栓式风力发电塔基础的灌浆层经常出现开裂掉角现象,断面整齐光滑.采用FRANC-2D建立灌浆层部位的线弹性断裂力学模型,分析裂缝发展路径及断裂强度因子的变化情况.模拟结果显示:裂缝由灌浆层和钢板的交界处产生,开始沿着水平方向向外缘发展,随后沿着45°向下开展直至开裂至灌浆层边缘;I型断裂强度因子在裂缝开展过程中先减小后增大再减小,且在裂缝开展至0.2倍总长附近达到局部最小值,此点可用于评估灌浆层的断裂承载力.不同灌浆层厚度和外伸宽度的参数化分析结果显示:厚度增大后,裂缝较易开展;宽度增大后,裂缝较难开展,且宽度影响更大.基于参数化分析及量纲分析,提出灌浆层断裂承载力的计算公式和可供工程参考的设计建议,并结合工程算例的线弹性断裂力学和疲劳断裂分析,验证了公式的有效性和适用性.

采用FAST软件对同一风场、2 MW风电机组的纯钢塔架和钢-混凝土组合塔架进行正常运行工况和急停工况的动力响应分析.结果表明:两种工况下,纯钢塔架塔顶位移响应均大于钢混塔架;在正常运行工况下,两类塔架塔顶处加速度响应受3倍、6倍风轮转速对应的频率影响较大,且纯钢塔架受二阶振型影响明显;急停工况下,塔架振动加剧,纯钢塔架相比于钢混塔架更加敏感,两种塔架前后振动方向加速度响应主要受塔架一阶振型影响,而侧向振动方向加速度响应受塔架二阶振型影响明显.

The steel–concrete hybrid wind turbine tower is characterized by the concrete tubular segment at the lower part and the traditional steel tubular segment at the upper part. Because of the great change of mass and stiffness along the height of the tower at the connection of steel segment and concrete segment, its dynamic responses under seismic ground motions are significantly different from those of the traditional steel tubular wind turbine tower. Two detailed finite element models of a full steel tubular tower and a steel–concrete hybrid tower for 2.0 MW wind turbine built in the same wind farm are, respectively, developed by using the finite element software ABAQUS. The response spectrum method is applied to analyze the seismic action effects of these two towers under three different ground types. Three groups of ground motions corresponding to three ground types are used to analyze the dynamic response of the steel–concrete hybrid tower by the nonlinear time history method. The numerical results show that the seismic action effect by the response spectrum method is lower than those by the nonlinear time history method. And then it can be concluded that the response spectrum method is not suitable for calculating the seismic action effects of the steel–concrete hybrid tower directly and the time history analyses should be a necessary supplement for its seismic design. The first three modes have obvious contributions on the dynamic response of the steel–concrete hybrid tower.

In order to solve the common cracking and slurry emitting problems in the embedded-ring foundation，the epoxy resin grouting material is proposed to repair the cracks and fill the cavities near the T-flange. The local finite element models at the weakest section of the undamaged foundation and the reinforced foundation by epoxy resin grouting material are developed by the finite element software ABAQUS. The energy dissipation capacity of the models under the reciprocating loading are studied . Numerical results show that the energy dissipation capacity is significantly improved after reinforcement. Then，two overall finite element models are established for the reinforced foundations grouted by epoxy-resin and cement-based materials，respectively. The stress and strain distribution of the foundations are investigated under monotone loading of the standard value and design value of ultimate load. Results show that the distribution of compressive stress on the interface between the T-flange and the foundation concrete for the foundation reinforced by epoxy resin material is more uniform. The stress concentration phenomenon is obviously alleviated and the plastic zone of concrete above the T-flange is obviously reduced.

基于一种适用于风力发电塔架振动控制的盆式调谐/颗粒阻尼器,设计多组对照试验,研究阻尼器减振效果受结构自振频率、阻尼器质量比以及容器内颗粒数量3个设计参数的影响规律.采用摇摆测试台等效模拟风力发电塔架在急停工况下的一阶振动,使用激光位移传感器测得试验平台的位移响应,采用峰值法分析得到试验平台的阻尼比.试验结果表明,不同设计参数下的阻尼器均可有效提高结构的阻尼比,归纳结构自振频率、阻尼器质量比、容器内颗粒数量对阻尼器减振效果的影响规律,发现该阻尼器具有对结构自振频率不敏感的优点,可用于风力发电塔架的振动控制,可为该类型阻尼器的优化设计与推广应用提供参考.

The embedded-ring wind turbine foundations were widely applied in the early development stage of wind power industries because of its properties such as easy installation and adjustment. However, different damages occurred on some embedded-ring wind turbine foundations in recent years. Based on the common damage phenomena of embedded-ring wind turbine foundations, the structural defects and damage mechanisms of embedded-ring wind turbine foundations are analyzed in a gradual way. Cheese head studs are proposed to be welded on the lateral wall of the steel ring to strengthen the connection between the steel ring and the foundation concrete. The foundation pier is elevated 1 m to increase the embedded depth of the steel ring. The circumferential confining pressure is applied on the lateral side of the foundation pier to lead it into a state of pressure. One simplified method is proposed to calculate the contribution of welding studs in this strengthening method. Taking an embedded-ring wind turbine foundation as an example, the numerical analyses for the original foundation and the reinforced one are carried out to compare the stress and strain distribution changes. Based on the numerical results corresponding to the peak and valley value loads, the fatigue life of the concrete and studs are evaluated according to the relevant design codes. The numerical results show that this strengthening method can coordinate the deformation of the embedded steel ring and the foundation concrete by circumferential prestressing and welding studs. The maximum principal stresses of the foundation pier and the fatigue stress range of the concrete around the bottom of the steel ring have been greatly reduced after strengthening. The gaps between the embedded steel ring and the foundation pier are also obviously decreased because of these strengthening measures. The stress concentration phenomena of the concrete around the T-shaped flange have been remarkably improved. The fatigue life can meet the requirements of relevant design codes after strengthening. Therefore, it can be concluded that the safety performance and service life of the embedded-ring foundation can be guaranteed.

吸热塔作为塔式光热电站的核心构筑物,虽其外形为典型的高耸结构,但因其顶部放置质量巨大的熔盐储罐,又使其受力特征不同于传统高耸结构.钢塔架是传统高耸结构中常见的结构形式,具有刚度大、加工安装方便、材料用量省等优点,可用作塔式光热电站的支承结构.文章以某100 MW熔盐塔式光热电站的200 m高吸热塔项目为工程背景,利用ABAQUS进行了连续体的拓扑优化,得到塔架杆件布置方式;在此基础上,基于遗传算法自编Matlab优化设计程序对钢结构吸热塔进行了截面优化和形状优化,在满足设计要求的前提下,降低了总造价.

根据风电机组基础环基础常见的破坏现象,深入研究基础环基础的受力机理和损伤破坏的原因.针对基础环T型法兰的构造缺陷,提出在露出柱墩顶面的基础环内外侧焊接栓钉并施加环向预应力加强基础环与混凝土柱墩之间的连接,给出适用于风电机组基础加固的栓钉计算算法.以某风电机组基础环基础为例,通过数值分析研究加固前后基础受力状态的变化,根据相关规范对其疲劳寿命进行评估.分析结果表明,该方法可在不损伤原基础结构的前提下,通过环向预应力和焊接在基础环上的栓钉,使钢制基础环和基础混凝土变形协调,改善T型法兰和基础环侧壁受力状态,缓解基础环埋深不足导致的基础环与基础顶面脱开问题及T型法兰附近应力集中问题,提高了基础的疲劳寿命.

吸热塔是塔式光热电站的核心构筑物,是典型的高耸结构,但因其顶部放置质量巨大的熔盐储罐,使其在地震作用下的受力性能不同于其他传统的高耸结构.为了研究格构式钢结构吸热塔的抗震性能,以某100 MW熔盐塔式光热电站的200 m高吸热塔项目为工程背景,采用SAP2000软件建立了三维有限元模型并进行了模态分析、反应谱分析、多遇地震下的弹性时程分析和罕遇地震作用下弹塑性时程分析.结果 表明,结构在多遇地震作用下所有构件处于弹性状态;罕遇地震作用下的位移角满足设计要求;塔架变坡处的层间位移角发生突变,为结构的薄弱部位;考虑几何非线性时,塔顶的大质量吸热器对顶层间位移角的影响较大,在抗震设计中要予以重视.