Fatigue Life Of Wind Turbines Research Articles

Time-domain fatigue damage assessment for wind turbine tower bolts under yaw optimization control at offshore wind farm

Yaw optimization control is recognized as the most effective active wake control strategy for enhancing the overall power generation of wind farms. However, the potential adverse effects of yaw optimization control on the fatigue life of wind turbines remain unclear. This study examined the effect of yaw optimization control on the fatigue life of offshore wind turbines using tower bolts. Initially, the yaw misalignment of the wind turbines was optimized using the open-source FLORIS package to maximize wind farm power generation. Subsequently, the time-varying load of the wind turbines was obtained through the OpenFAST software, using the optimal yaw misalignment, wind speed, and turbulence intensity at the hub height as inputs. The fatigue life of the wind turbines was then calculated by integrating the Schmidt–Neuper engineering stress algorithm, rain flow counting method, and Miner-Palmgren fatigue damage accumulation theory. Finally, a case study of the Horns Rev I large-scale offshore wind farm, comprising 80 wind turbines, revealed that yaw optimization control could significantly enhance the annual power generation of the wind farm by approximately 1.818%. However, optimization comes at the cost of reducing the fatigue life of wind turbine tower bolts by approximately 3–9 years.

Optimal Semi-Active Control for a Hybrid Wind-Wave Energy System on Motion Reduction

Integration of wave energy converters and floating wind turbine can potentially reduce the levelized cost of energy. The coupling effects between the wave energy converters and the foundation, however, could amplify the foundation motion so that both the efficiency and the fatigue life of wind turbine would be degraded. In this study, wave energy converters are designed as a controlled suspension system for a novel hybrid wind-wave energy system, and an optimal semi-active control method having predictive ability is proposed for reducing the foundation motion. In the controller design, the physical constraints and the passivity of the wave energy converters are also taken into consideration. A full numerical model taking account of aero-hydro-servo-mooring coupled dynamics is proposed for a comprehensive analysis of the controller under various environmental conditions. Numerical results show that the foundation motion can be reduced by 16% even in very rough sea states with the help of the controller.

Wind farm layout optimization to minimize the wake induced turbulence effect on wind turbines

High turbulence intensity induced by the wake effect in wind farms is of great significance to the fatigue life of wind turbines. However, previous wind farm layout optimization (WFLO) only considers the shading effect between wind turbines. To fill this critical gap, we propose a novel multi-objective WFLO (MO-WFLO) framework considering both the power generation and the distribution of streamwise turbulence intensity in the wind farm. In this framework, a newly developed turbulence intensity model of turbine wake is used to calculate the turbulence intensity distribution, and a two-dimensional Gaussian wake model is adopted to calculate the wake losses of the wind farm, in which the wake expansion rate of turbine wake is determined accounting for the turbulence intensity of the turbine’s inflow. The nondomination sorting genetic algorithm with elite strategy (NSGA-II) is resorted to maximize the total power generation and to minimize the comprehensive streamwise turbulence intensity of the turbines’ inflow. The results show that under the same annual energy production, our proposed framework can effectively reduce the maximum turbulence intensity level of the turbines with high fatigue loads, and shorten the cumulative operation time of the wind turbines in high turbulence conditions in the wind farm. By applying the proposed framework to a realistic wind farm, a new layout can be obtained with increased total power by 0.8% and a reduced maximum comprehensive streamwise turbulence intensity of the turbines by 8.1% compared with the original layout.

Quantifying the fatigue life of wind turbines in cyclone-prone regions

Wind turbines are designed to harvest kinetic energy of the wind, which are more susceptible to the impact of tropical cyclones compared with other high-rise structures. Most of the existing research of cyclone (hurricane/typhoon) impacts on wind turbine merely concentrated on its short-term failure (strength or stability failure). However in this paper, it is evidently demonstrated that cyclones can also have a significant impact on the long-term failure (fatigue failure) of a wind turbine. In this study, a novel framework is developed in the fatigue life evaluation of a wind turbine, in which two external factors, i.e., the progressive change of cyclone direction and intensity observed at a specific site, combined with one internal factor, i.e., a parked wind turbine with feathered or unfeathered blades are considered. Subsequently, the effect of cyclone-normal-wind direction misalignment and cyclone average recurrence intervals are included to have a synthetic assessment on the damage potential of cyclones. The proposed fatigue analysis framework can be also extended to other structures, e.g., the hybrid wind-tidal energy conversion system in cyclone-prone regions.

A novel optimised nacelle to alleviate wind turbine unsteady loads

A comparative study of the effectiveness of an innovative optimised nacelle on fatigue loads alleviation and performance improvement in both upwind and downwind configurations has been conducted. The idea is to optimise wind turbine nacelles using flaps to push the flow towards outer spans, which leads to improved turbine performance, as well as mitigates unsteady loads, therefore, extending the fatigue life of wind turbine's components. Theoretical explanations indicate that in the case of upwind configuration, increasing downstream velocity leads to a reduction in turbine power. However, lower wind speeds seen by inner spans lead to mitigate thrust load generated by inner spans. In the case of downwind configuration, increasing upstream velocity leads to improve turbine power based on the turbine Euler equation, and pushing flow towards outer spans with higher lift/drag ratios leads to mitigate unsteady loads generated by inner spans. Experimental results show that using flaps to optimise the nacelle shape not only mitigates unsteady loads and therefore extends the fatigue lifetime of blades but also improves turbine power and yaw stability. Furthermore, using flaps in downwind configuration can compensate for the adverse effect of wind turbine yaw misalignment on the wind turbine performance and unsteady loads.

Individual Blade Pitch Control for Extended Fatigue Lifetime of Multi-Megawatt Wind Turbines

A novel experimental study on the use of individual pitch control to extend the fatigue life of wind turbines is conducted. The experiment is accomplished using a sub-scale model of a multi-megawatt turbine in both upwind and downwind configurations in a water towing tank. It is shown that individual pitch control based on a sinusoidal pitching scheme that is locked to the phase of the rotor rotation can mitigate unsteady loads caused by the velocity variation that is induced by the effect of rotor tilt and therefore extends the fatigue lifetime of blades. A positive pitching amplitude in the case of upwind configuration and a negative pitching amplitude in the case of downwind configuration eliminate the cumulative damage fractions due to the velocity variation.

Influence of non-Gaussian characteristics of wind load on fatigue damage of wind turbine

Based on translation models, both Gaussian and non-Gaussian wind fields are generated using spectral representation method for investigating the influence of non-Gaussian characteristics and directivity effect of wind load on fatigue damage of wind turbine. Using the blade aerodynamic model and multi-body dynamics, dynamic responses are calculated. Using linear damage accumulation theory and linear crack propagation theory, crack initiation life and crack propagation life are discussed with consideration of the joint probability density distribution of the wind direction and mean wind speed in detail. The result shows that non-Gaussian characteristics of wind load have less influence on fatigue life of wind turbine in the area with smaller annual mean wind speeds. Whereas, the influence becomes significant with the increase of the annual mean wind speed. When the annual mean wind speeds are 7 m/s and 9 m/s at hub height of 90 m, the crack initiation lives under softening non-Gaussian wind decrease by 10% compared with Gaussian wind fields or at higher hub height. The study indicates that the consideration of the influence of softening non-Gaussian characteristics of wind inflows can significantly decrease the fatigue life, and, if neglected, it can result in non-conservative fatigue life estimates for the areas with higher annual mean wind speeds.

Matching experimental and numerical data of dynamic power train loads by modeling of defects

AbstractDefects on wind turbines such as power train misalignments or blade pitch angle deviations are dealt with. These defects cause additional dynamic excitations and thus can reduce the fatigue life of wind turbine components.In order to improve the reliability of dynamic load computations and related fatigue dimensioning of wind turbines, a highly discretized simulation model that incorporates potential system defects is set up. A sensitivity analysis of the impact of system defects on power train dynamics is performed. Experimental measurements of gearbox orbital paths and of the corresponding torque arm loads could be reproduced with good correlation when the simulation model was complemented by power train misalignments and by blade pitch angle deviations. Comparisons of experimental and numerical data are presented in time and frequency domains. Feasible consequences about the impact of alignment defects on the resulting fatigue damage are presented. Copyright © 2014 John Wiley & Sons, Ltd.

1026 風レンズ風車ブレードの実働ひずみ計測と疲労強度・寿命推定(S04-2 実験力学における計測・解析法の新展開(2)新しい試験法と産業応用,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Recently, drains of fossil fuel and the increase of the amount of CO_2 discharge are fatal issues for earth environment. Wind turbine with brimmed diffuser has the power generation efficiency of 4 times as high as the conventional wind turbine. The accidents of wind turbine with brimmed diffuser were caused for several times. However, there are no researchers on the fatigue strength and the fatigue life of the new wind turbine. Then, the purpose of the study is to estimate the fatigue strength and the fatigue life of the wind turbine with brimmed diffuser. The service strain of blades with the variation of wind velocity and wind direction was measured by wireless strain measurement system. The results show that the higher the variation ratio of the wind velocity and the wind direction are, the larger strain variation of the wind turbine blades. In addition, fatigue life based on the modified Miner rule was estimated by the strain data. The fatigue life of the wind turbine blades was five years estimated on the base of the strain data at the wind tunnel equipment.

Influence of different wind profiles due to varying atmospheric stability on the fatigue life of wind turbines

Offshore wind energy is being developed on a very large scale in the European seas. The objective of developing wind energy offshore is to capture greater wind speeds than are encountered onshore and as a result more energy. With this also come more challenges in the design of wind turbines due to the hostile offshore environment. Currently the standards for offshore wind turbines prescribe a site specific design for the support structures and the design for the rotor nacelle assembly according to onshore standards. Wind turbines are designed to withstand fatigue and ultimate loads. For the fatigue loading several input conditions have been prescribed, amongst which wind profile is one of them. Wind profile is represented by power law or logarithmic law as given in the standards. A neutral stability of the atmosphere is considered while obtaining the wind profile using the logarithmic law. In this paper the atmospheric stability is varied in order to estimate different wind profiles and simulations are run in Bladed to check its influence on the fatigue damage at the blade root. The variations in the atmospheric stability has been taken into account by using some typical values of Obukhov length. From steady state simulations it has been found that atmospheric stability is important for fatigue damage. The analysis showed that variation in the distribution of atmospheric stability causes large variations in the fatigue damage for different sites. Thus, it is worthwhile to carry out a full scale study using the turbulent winds and real data for wind turbine and environmental conditions.

Prediction of Stochastic Blade Responses Using Measured Wind-Speed Data as Input to the Flap Code

Accurately predicting wind turbine blade loads and response is important in predicting the fatigue life of wind turbines. The necessity of including turbulent wind effects in structural dynamics models has long been recognized. At SERI, the structural dynamics model, or FLAP (Force and Loads Analysis Program), is being modified to include turbulent wind fluctuations in predicting rotor blade forces and moments. The objective of this paper is to show FLAP code predictions compared to measured blade loads using actual anemometer array data and a curve-fitting routine to form series expansion coefficients as the turbulence input to FLAP. The predictions are performed for a three-bladed upwind field test turbine. An array of nine anemometers was located 0.8 rotor diameters (D) upwind of the turbine, and data from each anemometer are used in a least-squares curve-fitting routine to obtain a series expansion of the turbulence field over the rotor disk. Three 10-min data cases are used to compare FLAP predictions to measured results. Each case represents a different mean wind speed and turbulence intensity. The time series of coefficients in the expansion of the turbulent velocity field are input to the FLAP code. Time series of predicted flap-bending moments at two blade radial stations are obtained, and power spectra of the predictions are then compared to power spectra of the measured blade bending moments. Conclusions are then drawn about the FLAP code’s ability to predict the blade loads for these three data cases. Recommendations for future work are also made.