Service reliability of offshore wind turbines

A probabilistic formulation is proposed to assess the performance of the support structure of offshore wind turbines based on their probability and expected time of exceeding specified drift thresholds. To this end, novel probabilistic models are developed to predict the mean and standard deviation of the drift ratio response of wind turbine support structures operating under day-to-day loads as a function of the wind turbine geometry and material properties, and loading conditions. The proposed models are assessed using a database of virtual experiments generated using detailed three-dimensional (3D) nonlinear finite element (FE) models of a set of representative wind turbine configurations. The developed models are then used in a random vibration formulation to estimate the probability and expected time of exceeding specified drift thresholds. As an example, the probability and expected time of exceeding specified drift thresholds are estimated for a typical offshore wind turbine at different wind speeds. A comparison is made between the results obtained based on the proposed models, those obtained using simulators commonly used in practice and detailed 3D nonlinear FE analyses.     

Wind turbines and seismic hazard: a state‐of‐the‐art review

Wind energy is a rapidly growing field of renewable energy, and as such, intensive scientific and societal interest has been already attracted. Research on wind turbine structures has been mostly focused on the structural analysis, design and/or assessment of wind turbines mainly against normal (environmental) exposures while, so far, only marginal attention has been spent on considering extreme natural hazards that threat the reliability of the lifetime‐oriented wind turbine's performance. Especially, recent installations of numerous wind turbines in earthquake prone areas worldwide (e.g., China, USA, India, Southern Europe and East Asia) highlight the necessity for thorough consideration of the seismic implications on these energy harnessing systems. Along these lines, this state‐of‐the‐art paper presents a comparative survey of the published research relevant to the seismic analysis, design and assessment of wind turbines. Based on numerical simulation, either deterministic or probabilistic approaches are reviewed, because they have been adopted to investigate the sensitivity of wind turbines' structural capacity and reliability in earthquake‐induced loading. The relevance of seismic hazard for wind turbines is further enlightened by available experimental studies, being also comprehensively reported through this paper. The main contribution of the study presented herein is to identify the key factors for wind turbines' seismic performance, while important milestones for ongoing and future advancement are emphasized. Copyright © 2016 John Wiley & Sons, Ltd.     

不同海况及伺服系统作用下10 MW单桩式风力机地震易损性研究

为探究不同海况及伺服系统下单桩式近海风力机的地震易损性,以DTU 10 MW风力机为研究对象,建立风浪相关的地震-湍流风-波浪多物理场模型,研究其在变速变桨伺服系统下的动力特性,基于增量动力分析方法评估其地震易损性。结果表明：变速变桨伺服系统可有效缓解风力机高风速下无地震作用时的塔顶振动;当风轮在大推力下,较小的波浪载荷一定程度上可降低风力机塔顶振动及塔底弯矩;随地震动强度增加,风力机各临界损伤状态失效概率逐渐增加;风力机地震易损性主要由地震动强度决定,波浪载荷与湍流风载荷对风力机地震易损性影响较小。     

Seismic evaluation of offshore wind turbine by geotechnical centrifuge test

As offshore wind turbines are now planned to be installed at seismic activity areas around Asia in large numbers, understanding of the seismic behavior of offshore wind turbine has become essential to evade structural hazards due to earthquake. Although the seismic behavior of the structure is largely affected by soil‐foundation‐structure interaction (SFSI), there is only a few experimental data about this subject as conventional offshore wind turbines are mostly located in the area where earthquakes are scarce. Geotechnical centrifuge experiment can provide reliable experimental data for this subject as it can reproduce field stress condition of the soil and simulate earthquake motion in a scaled model test. In this research, three case studies using centrifuge model test were performed to evaluate the seismic behavior of offshore wind turbine during the earthquake and permanent deformation after the earthquake. The results were compared with conventional seismic evaluation methods. Monopile, Monopod, and Tripod foundations were chosen for the experiment. Peak acceleration and rotational displacement of the wind turbine for three cases were evaluated under various intensities of seismic loading applied by centrifuge‐mounted shaking table. Results were compared with conventional evaluation method for design acceleration and conventional rotational displacement criteria suggested in DNV‐OS‐J101.     

Assessing the seismic wavefield of a wind turbine using polarization analysis

Ambient seismic noise can often be seen as problematic but with the right analysis can act as a tool to image the Earth. Wind turbines are known to generate low frequency vibrations; however, the wave types that are generated are currently unknown. Characterizing these vibrations will allow wind turbines to be used as a seismic source and be of value to geotechnical applications and seismic interferometry. This paper uses polarization analysis of the seismic wavefield around a small wind turbine to identify the type of wave being generated by the turbine and to clarify the source. The seismic data recorded 190 m from the wind turbine are processed using a window length of 0.1 s and bandpass filtered on a selection of frequency ranges. Polarization analysis is performed for two different wind speed ranges, in order to show the variation of wave characteristics between operational and non‐operational modes of the wind turbine. Polarized surface waves are identified as the predominant wave type at blade rotation harmonics, making this work particularly relevant to multichannel analysis of surface waves and seismic interferometry. Copyright © 2017 John Wiley & Sons, Ltd.     

Impact of fault ride‐through requirements on fixed‐speed wind turbine structural loads

The emphasis in this article is on the impact of fault ride‐through requirements on wind turbines structural loads. Nowadays, this aspect is a matter of high priority as wind turbines are required more and more to act as active components in the grid, i.e. to support the grid even during grid faults. This article proposes a computer approach for the quantification of the wind turbines structural loads caused by the fault ride‐through grid requirements. This approach, exemplified for the case of a 2MW active stall wind turbine, relies on the combination of knowledge from complimentary simulation tools, which have expertise in different specialized wind turbines design areas. Two complimentary simulation tools are considered i.e. the detailed power system simulation tool PowerFactory from DIgSILENT and the advanced aeroelastic computer code HAWC2, in order to assess of the dynamic response of wind turbines to grid faults. These two tools are coupled sequently in an offline approach, in order to achieve a thorough insight both into the structural as well as the electrical wind turbine response during grid faults. The impact of grid requirements on wind turbines structural loads is quantified by performing a rainflow and a statistical analysis for fatigue and ultimate structural loads, respectively. Two cases are compared i.e. one where the turbine is immediately disconnected from the grid when a grid fault occurs and one where the turbine is equipped with a fault ride‐through controller and therefore it is able to remain connected to the grid during the grid fault. Copyright copy; 2010 John Wiley & Sons, Ltd.     

Uncertainty in wind climate parameters and their influence on wind turbine fatigue loads

According to the wind turbine standard IEC 61400-1, structural integrity of wind turbines is determined either by direct reference to wind data or by load calculation. In both cases, deterministic values are applied and uncertainties neglected for the wind climate parameters and the structural resistance.The uncertainty related to the wind climate parameters depends highly on the presence, duration and quality of on-site wind measurements, and the perturbations introduced by flow modelling. For the wind speed distribution, the uncertainty is considered in assessment of the annual energy production. For other wind climate parameters which potentially have a large influence on the wind turbine loads, the uncertainty is often not well investigated.This paper presents a probabilistic framework for assessment of the structural reliability level of wind turbines in fatigue loading. Uncertainty of the site specific wind climate parameters at each turbine position is estimated based on the local wind measurements, speed-up factors and the distance between the wind turbine and the measuring position. The framework is demonstrated for a wind turbine project in flat terrain. The results show that the uncertainty in the site specific wind climate parameters normally accounts for 10–30% of the total uncertainty in the structural reliability analyses.     

Effects of seismic and aerodynamic load interaction on structural dynamic response of multi-megawatt utility scale horizontal axis wind turbines

Horizontal axis wind turbines can experience significant time varying aerodynamic loads that has the potential to cause adverse effects on structural, mechanical, and power production. The progress in the wind industry has caused the construction of wind farms in areas prone to high seismic activity. With the advances in computational tools, a more realistic representation of the behavior of wind turbines should be performed. One of the simulation platforms was developed using the 5 MW NREL utility scale reference turbine model. The performed simulations will be used to evaluate the effects of aerodynamic and seismic load coupling on the power generation and structural dynamics behavior of this structure. Different turbine operational scenarios such as (i) normal operational condition with no earthquake, (ii) idling condition with the presence of seismic loads, (iii) normal operational condition with earthquake, and (iv) earthquake-induced emergency shutdown will be simulated with various loading conditions to show the differences in generated power and dynamic response. The results of this paper provide formulations for calculating generated power and design deriving parameters by considering different intensity measures. Moreover, the effects of aerodynamic damping and pitch control system are presented to shows reduction in the resulting design demand loads.     

Probabilistic risk assessment on wind turbine towers subjected to cyclone‐induced wind loads

A probabilistic risk assessment to estimate the economic impact on wind turbine towers subjected to cyclone‐induced wind loads is applied. An event‐based probabilistic framework is used to assess the risk parameters for selected hazard events. The results are integrated considering all uncertainties related to each part of the process in a probabilistic formal way. The hazard is defined by a set of nonoverlapping stochastic events that exhaust the considered sample space, including the spatial distribution, the annual frequency, and the randomness of the hazard intensity. The vulnerability is defined in terms of the first and second statistical moments: the expected damage and its corresponding variance (or standard deviation) for a certain type of wind turbine towers. The vulnerability component of the model is an important contribution of the study; it uses a realistic modeling of the wind field, and instead of one (as in other studies), three failure modes are considered to characterize the vulnerability. The risk is expressed in economic terms, namely, the single‐event expected loss, the average annual loss, the pure premium, and the loss exceedance curve. The approach is applied to a probabilistic tropical cyclone wind risk assessment on 3001 wind turbine towers from 65 wind farms in Mexico. It is concluded that the selected metrics are of especial importance for risk retention (financing), particularly if schemes or risk transfer instruments are to be defined, and therefore, they will be a very valuable contribution to further studies in defining strategies for financial protection of national infrastructure against wind disasters induced by cyclones.     

Passive structural control of offshore wind turbines

The application of control techniques to offshore wind turbines has the potential to significantly improve the structural response of these systems. A new simulation tool is developed that can be utilized to model passive, semi‐active and active structural control systems in wind turbines. Two independent, single degree of freedom (DOF) tuned mass‐ damper (TMD) devices are incorporated into a modified version of the aero‐elastic code FAST (Fatigue, Aerodynamics, Structures and Turbulence). The TMDs are located in the nacelle of the turbine model, with one TMD translating in the fore‐aft direction, and the other in the side‐side direction. The equations of motion of the TMDs are incorporated into the source code of FAST, yielding a more realistic system for modeling structural control in wind turbines than has previously been modeled. The stiffness, damping and commanded force of each TMD are controllable through the FAST‐Simulink interface, and so idealizations of semi‐active and active control approaches can be implemented. A parametric study is performed to determine the optimal parameters of a passive single DOF, fore‐aft, TMD system in both a barge‐type and monopile support structure. The wind turbine models equipped with TMDs are then simulated and the performance of these new systems is evaluated. The results indicate that passive control approaches can be used to improve the structural response of offshore wind turbines. The results also demonstrate the potential for active control approaches. Copyright © 2010 John Wiley & Sons, Ltd.     

A new method for spatial allocation of turbines in a wind farm based on lightning protection efficiency

Existing studies of the spatial allocation of wind farms are typically based on turbine power generation efficiency and rarely consider the damage caused by lightning strikes. However, lightning damage seriously affects the economic performance of wind farms because of the high cost of repairing or replacing damaged blades. This paper proposes a method for the spatial optimization of multiple turbines based on lightning protection dependability. Firstly, the lightning protection efficiency of turbine blade protection systems is analyzed by combining the physical mechanisms of lightning leader progression with a conventional electro‐geometric model to develop an electro‐geometric model of turbine blades (EGMTB). Then, the optimized spatial allocation of multiple turbines in a wind farm is investigated using the EGMTB. The results are illustrated from an example wind farm with 1.5 MW turbines, which shows that the optimal spacing between two turbines perpendicular to the prevailing wind direction L_⊥ is 4R‐6R, where R is the length of a turbine blade. This spacing is shown to effectively shield turbine blades from lightning damage over a wide range of lightning currents (>26‐60 kA). Note that, the suggested L_⊥ will be smaller considering the influence of lightning polarity as it takes more difficulty developing upward leader (UL) in the condition of positive lightning striking. Experiments verify the effectiveness and correctness of this method.     

大型海上风力机地震冲击角动力学响应分析

为研究海上风力机在不同地震冲击角下的动力学响应,基于p-y曲线法构建土-构耦合模型,基于DTU 10 MW 单桩式近海风力机建立有限元模型,研究地震冲击角变化对大型海上风力机地震动力学响应的影响。结果表明：0°和90°地震冲击角下风力机结构受载荷响应最剧烈;当地震冲击角为锐角时,塔顶前后向和侧向位移幅值均下降,总应变能集聚现象显著缓解;地震冲击角为15°和30°时风力机等效应力均值相对其他角度有明显下降。因此,主动调整风力机叶轮朝向以调整地震冲击角可能成为风力机受地震冲击后降低损害的有效控制方式。     

Large eddy simulation of dynamically controlled wind turbines in an offshore environment

Accurate modelling of transient wind turbine wakes is an important component in the siting of turbines within wind farms because of wake structures that affect downwind turbine performance and loading. Many current industry tools for modelling these effects are limited to empirically derived predictions. A technique is described for coupling transient wind modelling with an aero‐elastic simulation to dynamically model both turbine operation and wake structures. The important feature of this approach is a turbine model in a flow simulation, which actively responds to transient wind events through the inclusion of controller actions such as blade pitching and regulation of generator torque. The coupled nature of the aero‐elastic/flow simulation also allows recording of load and control data, which permits the analysis of turbine interaction in multiple turbine systems. An aero‐elastic turbine simulation code and a large eddy simulation (LES) solver using an actuator disc model were adapted for this work. Coupling of the codes was implemented with the use of a software framework to transfer data between simulations in a synchronous manner. A computationally efficient simulation was developed with the ability to model turbines exhibiting standard baseline control operating in an offshore environment. Single and multiple wind turbine instances were modelled in a transient flow domain to investigate wake structures and wake interaction effects. Blade loading data were analysed to quantify the increased fluctuating loads on downwind turbines. The results demonstrate the successful implementation of the coupled simulation and quantify the effect of the dynamic‐turbine model. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of spatial and temporal resolution of the turbulent inflow on wind turbine performance estimation

The effects of spatial and temporal resolution of wind inflows generated using large eddy simulations (LES) on the scales of turbulence present in the wind inflow, and the resulting changes in wind turbine performance were investigated for neutral atmospheric boundary layer conditions. Wind inflows with four different spatial resolutions and five different temporal resolutions were used to produce different turbine responses. An aero‐elastic code assessed the dynamic response of two wind turbines to the different inflows. Auto‐spectral density functions (ASDF) of turbine responses, such as blade deflection and bending moment, that are representative of the turbine response were used to assess the effect of the inflow. The results indicated that, as additional turbulence scales were resolved, the wind turbines showed a similar increased response that was evident in both the ASDF and variance of the different wind turbine performance parameters. As a result, the amount to which turbulence is resolved in the inflow, particularly using tools such as LES, will be important to consider when using these inflows for wind turbine design and performance prediction. Copyright © 2015 John Wiley & Sons, Ltd.     

极限环境载荷诱导风力机结构振动影响分析

风力机桩基、塔架及连接部件构成的支撑结构属顶部承担较大质量的力学结构,地震对其造成的影响远大于常规建筑.针对上述问题,基于NREL开发计算平台,联合TurbSim、AeroDyn、FAST及Seismic,对变风载荷、变地震载荷(波形、强度)下的风力机动力学响应进行研究.发现:地震横波对风力机结构响应造成剧烈影响,纵波...     

Vibration response of a 2.3 MW wind turbine to yaw motion and shut down events

Structural health monitoring (SHM) is a process of implementing a damage detection strategy for a mechanical system. Wind turbine machinery stands to benefit from SHM significantly as the ability to detect early stages of damage before significant malfunction or structural failure occurs would reduce costs of wind power projects by reducing maintenance costs. Vibration analysis of dynamic structural response is an approach to SHM that has been successfully applied to mechanical and civil systems and shows some promise for wind turbine application. Traditionally, a setback to turbine vibration‐based SHM techniques has been the unavailability of turbine vibration response data. This study begins to address this issue by presenting vibration response for a commercial 2.3 MW turbine to a limited number of operating conditions. A database of acquired vibration response signals detailing turbine response to yaw motion, start‐up, operation and shutdown has been assembled. A Daubechies sixth‐order wavelet was used to perform an eight‐level discrete wavelet decomposition such that general trends and patterns within the signals could be identified. With further development, the presented analysis of vibration response may be integrated into routines to reduce downtime and failure frequency of utility scale wind turbines. Copyright © 2011 John Wiley & Sons, Ltd.     

An experimental investigation on the effect of individual turbine control on wind farm dynamics

Individual wind turbines in a wind farm typically operate to maximize their performance with no consideration of the impact of wake effects on downstream turbines. There is potential to increase power and reduce structural loads within a wind farm by properly coordinating the turbines. To effectively design and analyze coordinated wind turbine controllers requires control‐oriented turbine wake models of sufficient accuracy. This paper focuses on constructing such a model from experiments. The experiments were conducted to better understand the wake interaction and impact on voltage production in a three‐turbine array. The upstream turbine operating condition was modulated in time, and the dynamic impact on the downstream turbine was recorded through the voltage output time signal. The flow dynamics observed in the experiments were used to improve a static wake model often used in the literature for wind farm control. These experiments were performed in the atmospheric boundary layer wind tunnel at the Saint Anthony Falls Laboratory at the University of Minnesota using particle image velocimetry for flow field analysis and turbine voltage modulation to capture the physical evolution in addition to the dynamics of turbine wake interactions. Copyright © 2015 John Wiley & Sons, Ltd.     

A new seismic‐based monitoring approach for wind turbines

Wind turbine performance and condition monitoring play vital roles in detecting and diagnosing suboptimal performance and guiding operations and maintenance. Here, a new seismic‐based approach to monitoring the health of individual wind turbine components is presented. Transfer functions are developed linking key condition monitoring properties (drivetrain and tower acceleration) to unique, robust, and repeatable seismic signatures. Predictive models for extreme (greater than 99th percentile) drivetrain and tower acceleration based on independent seismic data exhibit higher skill than reference models based on hub‐height wind speed. The seismic models detect extreme drivetrain and tower acceleration with proportions correct of 96% and 93%, hit rates of 91% and 82%, and low false alarm rates of 4% and 6%, respectively. Although new wind turbines incorporate many diagnostic sensors, seismic‐based condition/performance monitoring may be particularly useful in extending the productive lifetime of previous generation wind turbines.     

Dynamic behavior of wind turbines influenced by aerodynamic damping and earthquake intensity

In the present paper the effects of aerodynamic damping and earthquake loads on the dynamic response of flexible‐based wind turbines are studied. A numerical analysis framework (NAF) is developed and applied. NAF is based on a user‐compiled module that is developed for the purposes of the present paper and is fully coupled with an open source tool. The accuracy of the developed NAF is validated through comparisons with predictions that are calculated with the use of different numerical analysis methods and tools. The results indicate that the presence of the aerodynamic loads due to the reduction of the maximum displacement of the tower attributed to the dissipation of earthquake excitation energy in fore‐aft direction. Emergency shutdown triggered by strong earthquakes results to a rapid change of aerodynamic damping, resulting to short‐term instability of the wind turbine. After shutdown of the wind turbine, enhanced dynamic response is observed. For the case where the wind turbine is parked, the maxima displacement and acceleration of tower‐top increase linearly with the peak ground acceleration. With the use of the least‐square method a dimensionless slope of tower‐top displacements is presented representing the seismic response coefficient of tower that can be used to estimate the tower‐top acceleration demand. Moreover, on the basis of the seismic response coefficient, an improved model for the evaluation of load design demand is proposed. This model can provide accurate predictions.