Equivalent Fatigue Loads Research Articles

Vertical Axis Wind Turbine Design Load Cases Investigation and Comparison with Horizontal Axis Wind Turbine

The paper studies the applicability of the IEC 61400-1 ed.3, 2005 International Standard of wind turbine minimum design requirements in the case of an onshore Darrieus VAWT and compares the results of basic Design Load Cases (DLCs) with those of a 3-bladed HAWT. The study is based on aeroelastic computations using the HAWC2 aero-servo-elastic code A 2-bladed 5 MW VAWT rotor is used based on a modified version of the DeepWind rotor For the HAWT simulations the NREL 3-bladed 5 MW reference wind turbine model is utilized Various DLCs are examined including normal power production, emergency shut down and parked situations, from cut-in to cut-out and extreme wind conditions. The ultimate and 1 Hz equivalent fatigue loads of the blade root and turbine base bottom are extracted and compared in order to give an insight of the load levels between the two concepts. According to the analysis the IEC 61400-1 ed.3 can be used to a large extent with proper interpretation of the DLCs and choice of parameters such as the hub-height. In addition, the design drivers for the VAWT appear to differ from the ones of the HAWT. Normal operation results in the highest tower bottom and blade root loads for the VAWT, where parked under storm situation (DLC 6.2) and extreme operating gust (DLC 2.3) are more severe for the HAWT. Turbine base bottom and blade root edgewise fatigue loads are much higher for the VAWT compared to the HAWT. The interpretation and simulation of DLC 6.2 for the VAWT lead to blade instabilities, while extreme wind shear and extreme wind direction change are not critical in terms of loading of the VAWT structure. Finally, the extreme operating gust wind condition simulations revealed that the emerging loads depend on the combination of the rotor orientation and the time stamp that the frontal passage of gust goes through the rotor plane.

Developing a passive load reduction blade for the DTU 10 MW reference turbine

This paper presents the development of a passive load reduction blade for the DTU 10 MW reference wind turbine, using the aero-hydro-servo-elastic analysis tool 3DFloat. Passive load reduction is achieved by introducing sweep to the path of the blade elastic axis, so that out-of-plane bending deflections result in load alleviating torsional deformations of the blade. Swept blades are designed to yield similar annual energy production as a rotor with a reference straight blade. This is achieved by modifying the aerodynamic twist distribution for swept blades based on non-linear blade deflection under steady state loads. The passive load reduction capability of a blade design is evaluated by running a selection of fatigue- and extreme load cases with the analysis tool 3DFloat and determining equivalent fatigue loads, fatigue damage and extreme loads at the blade root and tower base. The influence of sweep on the flutter speed of a blade design is also investigated. A large number of blade designs are evaluated by varying the parameters defining the sweep path of a blade's elastic axis. Results show that a moderate amount of sweep can effectively reduce equivalent fatigue damage and extreme loads, without significantly reducing the flutter speed, or compromising annual energy production.

Adaptation of Controller Concepts for Support Structure Load Mitigation of Offshore Wind Turbines

Different controller concepts can be employed for the support structure load reduction of offshore wind turbines, however, they entail unfavorable collateral effects like additional actuator wear or load fluctuations in other turbine components. Hence, there is a need to identify possibilities of employing such controller concepts only under particular loading and operational conditionse.g. sea states with high waves or large wind-wave misalignment when their load reduction potential is high in relation to their unfavorable side effects. This paper introduces a multi-objective optimization method to perform a trade-off analysis between the reduction of damage equivalent fatigue loads at the monopile support structure and the collateral effects of the different controller concepts. The optimization is performed considering the baseline controller, plus tower fore-aft controller to reduce the tower fore-aft bending moment and the active generator torque controller to reduce the side-to-side load. These two concepts increase the pitch activity and drive train torque variability as collateral effect, respectively. With the optimization methodology presented, it is possible to identify the most efficient operation time to activate different control concepts under each load case, utilizing their advantageous load reduction while limiting their penalty in collateral effects.

Novel parameter settings for gain-scheduled feedback control of rotational speed in a floating offshore wind turbine–generator system

Novel parameter settings for gain-scheduled feedback control of rotational speed using collective blade pitch manipulation in a spar-buoy floating offshore wind turbine–generator system were developed in order to reduce both power-output fluctuation and platform motion. The development was conducted through numerical simulation using the aeroelastic simulation model (FAST), measured high wind speed data, and simulated irregular sea waves. In this gain-scheduled feedback control of rotational speed, a proportional-plus-integral action was employed, and the proportional gain was varied to the blade pitch angle. First, the sensitivity analysis of the control parameter settings, that is, the proportional gain, integral time, and the generator torque manipulation strategy, to the system performances was carried out. Then, a new guideline of parameter settings for the gain-scheduled feedback control was theoretically revealed by investigating the damped oscillation characteristics of the feedback control loop. Novel parameter settings yielded a natural frequency of the feedback control loop higher than that of the platform pitching motion and a damping coefficient larger than 1.0. Under these parameter settings, the platform pitching motion was similar to that under settings previously developed for this floating offshore system, while the power-output fluctuation was drastically reduced. High power–generation performance and a significant reduction in the damage equivalent fatigue loads at the main parts of the system were also obtained.

Wind Climate Parameters for Wind Turbine Fatigue Load Assessment

Site-specific assessment of wind turbine design requires verification that the individual wind turbine components can survive the site-specific wind climate. The wind turbine design standard, IEC 61400-1 (third edition), describes how this should be done using a simplified, equivalent wind climate established from the on-site distribution functions of the horizontal mean wind speeds, the 90% quantile of turbulence along with average values of vertical wind shear and air density and the maximum flow inclination. This paper investigates the accuracy of fatigue loads estimated using this equivalent wind climate required by the current design standard by comparing damage equivalent fatigue loads estimated based on wind climate parameters for each 10 min time-series with fatigue loads estimated based on the equivalent wind climate parameters. Wind measurements from Boulder, CO, in the United States and Høvsøre in Denmark have been used to estimate the natural variation in the wind conditions between 10 min time periods. The structural wind turbine loads have been simulated using the aero-elastic model FAST. The results show that using a 90% quantile for the turbulence leads to an accurate assessment of the blade root flapwise bending moment and a conservative assessment of the tower bottom for-aft bending moment and low speed shaft torque. Currently, IEC 61400-1 (third edition) neglects the variation in wind shear by using the average value. This may lead to a nonconservative assessment of blade root flapwise fatigue loads, which are sensitive to wind shear. The results in this paper indicate that using a 75% quantile for the wind shear at each wind speed bin leads to an appropriate, but conservative, assessment of the fatigue loads. However, care should be taken when using this approach for components where low or negative wind shears can lead to large fatigue loads. This is the case for some drivetrain components where a lower quantile may be required.

Fatigue assessment of steel half-pipes bolted connections using local approaches

This paper proposes a multiaxial fatigue assessment of a steel half-pipe bolted connection using a local energy-based approach. This bolted connection has been proposed for onshore wind turbine towers. A global linear-elastic model with beam elements for the onshore wind turbine tower was developed taking into account the stiffness of the joint. Damage equivalent fatigue loads were applied in global beam model. To assess the stiffness of the joint a linear-elastic model considering the preload on the bolts was made. A local elastoplastic model was built to obtain the maximum principal stresses and strains. Using this analysis was possible to determine the SWT damage parameter and the number of cycles to failure for the steel half-pipes bolted connection under investigation.

Dynamical measurement system for wind turbine fatigue load

A measurement system specially used in wind turbine fatigue load assessment is developed based on Labview platform and Control Area Network (CAN). By applying CAN bus communication technology, the system can perform data automatic acquisition, data stable transmission and data real-time monitoring. By adopting the technology of virtual instrument modular design, the system is designed to analyze the wind turbine mechanical load levels against wind and power, equivalent loads and lifetime fatigue loads, etc. Considering the effects of small load strengthening, low amplitude load damaging and multilevel load interaction, a novel fatigue lifetime prediction model is proposed to obtain more accurate and reliable prediction of blade fatigue life. With the developed measurement system, the in-field load measurements are performed and the results showed the system has satisfactory accuracy and good adaption, convenient operation, high integration, low cost and great practicality to load measurement of large wind turbine. And based on the proposed model the fatigue life of WT blade can be estimated more trustworthily and reliably.

The prediction of dynamic fatigue life of multi-axial loaded system

The purpose of this paper is to compare with estimation of equivalent fatigue load in time domain and frequency domain and estimate the fatigue life of structure with multi-axial vibration loading. The fatigue analysis with two methods is implemented with various signals like random, sinusoidal signals. Also, an equivalent fatigue life estimated by rainflow cycle counting in time domain is compared with results estimated with probability density function of each signal in frequency domain. In case of frequency domain, equivalent fatigue life can be estimated through Dirlik’s method with probability density function. The work proposed in this paper compared the fatigue damage accumulated under uni-axial loading to that induced by multi-axial loading. The comparison was performed for a simple cantilever beam exposed to vibrations of several directions. For verification of estimation performance of fatigue life, results are compared to those of FEM analysis (ANSYS).

0510 浮体式洋上風力発電システムの翼ピッチと発電機トルクの併用操作による出力および浮体動揺制御

To reduce both the power output fluctuation and the platform motion, a feedback control algorithm based on blade pitch and generator torque manipulations for a spar-type floating offshore wind turbine-generator system was developed through numerical analysis using the aeroelastic simulation model (FAST), observed high wind speed data, and irregular sea waves. The blade pitch is collectively manipulated to maintain the generator power at a rated value, and the generator torque is manipulated to dampen the nacelle fore-aft motion. The concept of this control approach is to increase the generator torque in response to the platform motion to the leeward side and to provide a positive damping effect immediately to the platform motion. The numerical simulation showed the effectiveness of the generator torque manipulation and the filter of the nacelle fore-aft speed for the reduction in the platform motion. This feedback control approach had an advantage in not only the power output fluctuation and platform motion but also damage equivalent fatigue load characteristics over previous control approaches.

Dependence of offshore wind turbine fatigue loads on atmospheric stratification

The stratification of the atmospheric boundary layer (ABL) is classified in terms of the M-O length and subsequently used to determine the relationship between ABL stability and the fatigue loads of a wind turbine located inside an offshore wind farm. Recorded equivalent fatigue loads, representing blade-bending and tower bottom bending, are combined with the operational statistics from the instrumented wind turbine as well as with meteorological statistics defining the inflow conditions. Only a part of all possible inflow conditions are covered through the approximately 8200 hours of combined measurements. The fatigue polar has been determined for an (almost) complete 360° inflow sector for both load sensors, representing mean wind speeds below and above rated wind speed, respectively, with the inflow conditions classified into three different stratification regimes: unstable, neutral and stable conditions. In general, impact of ABL stratification is clearly seen on wake affected inflow cases for both blade and tower fatigue loads. However, the character of this dependence varies significantly with the type of inflow conditions – e.g. single wake inflow or multiple wake inflow.

Combined wind turbine fatigue and ultimate load reduction by individual blade control

If each blade of the wind turbine has individual pitch actuator, there is possibility of employing the pitch system to mitigate structural loads through advanced control methods. Previously, considerable reduction of blade lifetime equivalent fatigue loads has been achieved by Individual Blade Control (IBC) and in addition, it has also been shown the potential in blade ultimate loads reduction. However, both fatigue and ultimate loads impact on the design and life of wind turbine blades. In this paper, the design and application of IBC that concurrently reduce both blade fatigue and ultimate loads is investigated. The contributions of blade load spectral components, which are 1P, 2P and edgewise mode from blade in-plane and/or out-of-plane bending moments, are firstly explored. Four different control options for reducing various combinations of these load components are compared. In response to the different spectral peaks of both fatigue and ultimate loads, the controller has been designed so that it can act on different frequency components which vary with wind speed. The performance of the IBC controller on fatigue and ultimate load reduction is assessed by simulating a 5MW exemplar wind turbine. Simulation results show that with a proper selection of controlling inputs at different wind speed, the use of a single combined IBC can achieve satisfactory reduction on both fatigue and ultimate loads.

Proposal of an evaluation method of equivalent fatigue load for small wind turbine blade

This study is carried out for the purpose of collecting data that is based on the evaluation of the equivalent fatigue load for a small wind turbine. The equivalent fatigue load is indicated as the method of designing load calculation in IEC 61400-2 ed.2 DLC1.1. In this study, it is performed fatigue load test for the commercial model of the small wind turbine, and it is evaluated the applicability of S-N diagram as material specification and the material factor “m” for S-N diagram model. It is cleared that S-N diagram as material specification is able to apply to the evaluation for blade fatigue load. Moreover, it is indicated experimentally the material factor “m” of 10 to a single material of G-FRP that is composed as general small wind turbine's blades. Furthermore, it is suggested the estimated method of the equivalent fatigue load by using the fatigue load function of Simplified Load Model (SLM) and it is made a trial calculation by this method.

J0540203 浮体式洋上風力発電システムの動特性解析 : 自然風況下での出力変動と浮体動揺の相関分析([J054-02]再生可能エネルギー(2))

The correlation in a spar-type offshore floating wind turbine-generator system under a field wind condition between the power output fluctuations and the floating platform motion is analyzed through a numerical simulation, in which various control parameters for the rotor speed by the pitch angle manipulation and the two types of load torque manipulations are employed. The results show the trade-off relationship in high wind speeds and regular wave between them and the significant influence of the floating platform motion on the damage equivalent fatigue loads in the main parts of the system. Based on these results, a control guideline, in which the floating platform motion should be suppressed for the reduction in the damage equivalent fatigue loads in priority to the reduction in the power output fluctuations, is derived.

Aerodynamic Performance Analysis of a Large Scale Wind Turbine Blade under Variable Condition

Based on BLADED software, the aerodynamic performance of a large scale wind turbine blade was analyzed under variable condition. The results show that the rated power of the blade under variable condition is increased 10%, when the rated wind speed is changed from 10.5m/s to 11.0 m/s. The blade’s wind power coefficient is above 0.46, and its tip speed ratio is between 7.8 and 11.4. When its tip speed ratio is 9.5, the blade’s maximum wind power coefficient is 0.486. It is indicated that the blade has good aerodynamic performance and wide scope of wind speed adaptive capacity. The blade root’s equivalent fatigue load is 2.11 MN•m, and its extreme flapwise load is 4.61 MN•m. The loads under variable condition are both less than that of the designed condition, so the blade’s application under variable condition is safe.

다축 하중 구조물의 동적 피로수명 예측

The purpose of this paper is to compare with estimation of equivalent fatigue load in time domain and frequency domain and estimate the fatigue life of structure with multi-axial vibration loading. The fatigue analysis with two methods is implemented with various signals like random, sinusoidal signals. Also an equivalent fatigue life estimated by rainflow cycle counting in time domain is compared with results estimated with probability density function of each signal in frequency domain. In case of frequency domain, equivalent fatigue life can estimate through Dirlik’s method with probability density function. And the work proposed in this paper compared the fatigue damage accumulated under uni-axial loading to that induced by multi-axial loading. The comparison is preformed for a simple cantilever beam, which is exposed to vibrations of several directions. For verification of estimation performance of fatigue life, results are compared to those of FEM analysis (ANSYS).

Multi-parameter Fatigue Equivalence Loadings for Specification Applications

This paper deals with a method to define Equivalent Fatigue Loads (EFL) from in-service load measurement in the case of a structure subjected to multiple variable fatigue loadings. EFL can be used in order to define tests on full scale structures for an experimental validation approach. It can be also used for the evaluation of the severity of the in-service loads, useful step in a reliability approach, in order to optimize the geometry with respect to the fatigue damage. The aim of the paper is to define the mathematical and mechanical concept of the Equivalence Fatigue Method in the case of multi-parameters loading for specification application. This method uses fatigue damage evaluation from statistical in-service measurement data. The specificity of the method presented here is to use a fully multi-parameters equivalence method. Damages is evaluated using life prediction method based on multiaxial criteria. The method is used for rotating structures, applied to a specific family of structure: train wheels. Train wheels are critical safety railway components mainly subjected to random multiaxial fatigue induced by multi-parameters loading. In this paper an approach to evaluate damage from this measurement in order to find the EFL with a damage equivalence optimization is developed.

J056035 小形風車ブレード試験による等価疲労荷重評価(〔J056-03〕再生可能エネルギー(風力発電)(3))

This study is carried out for the purpose of collecting data that is based on the evaluation of the equivalent fatigue load for a small wind turbine. The equivalent fatigue load is indicated as the method of designing load calculation in IEC 61400-2 ed.2 DLC1.1. In this study, it is performed fatigue load test for the commercial model of the small wind turbine, and it is evaluated the applicability of S-N curve as material specification and estimated the material factor "m" for S-N curve model. It is cleared that S-N curve as material specification is able to apply to the evaluation for blade fatigue load. Furthermore, it is able to estimate the material factor "m" of 10 to a single material of G-FRP that is composed as general small wind turbine's blades. Furthermore, it is suggested the estimated method of the equivalent fatigue load by using SLM fatigue load function and it is made a trial calculation by this method.

A Fatigue Analysis Method Research for Main Frame of the Large-Scale Wind Turbine

This paper presents a method to calculate the main frame damage of fatigue directly by the equivalent fatigue load of the wind turbine. This method can take advantage of the equivalent fatigue load to check the damage of the main frame fast. This paper also analyses the weight of each components of the fatigue load, which is convenient for the optimization design of the structure for main frame. Comparing this method with traditional fatigue calculation method, we can find this method is reasonable and veracious. Using this method can greatly improve the efficiency of the damage estimate for the main frame. It can be analogized to other mechanical components, and this method is very valuable in practical engineering applications.

Indicial lift response function: an empirical relation for finite‐thickness airfoils, and effects on aeroelastic simulations

ABSTRACTThe aeroelastic response of wind turbines is often simulated in the time domain by using indicial response techniques. Unsteady aerodynamics in attached flow are usually based on Jones's approximation of the flat plate indicial response, although the response for finite‐thickness airfoils differs from the flat plate one.The indicial lift response of finite‐thickness airfoils is simulated with a panel code, and an empirical relation is outlined connecting the airfoil indicial response to its geometric characteristics. The effects of different indicial approximations are evaluated on a 2D profile undergoing harmonic pitching motion in the attached flow region; the resulting lift forces are compared with computational fluid dynamics (CFD) simulations. The relevance for aeroelastic simulations of a wind turbine is also evaluated, and the effects are quantified in terms of variations of equivalent fatigue loads, ultimate loads, and stability limits.The agreement with CFD computations of a 2D profile in harmonic motion is improved by the indicial function accounting for the finite‐thickness of the airfoil. Concerning the full wind turbine aeroelastic behavior, the differences between simulations on the basis of Jones's and finite‐thickness indicial response functions are rather small; Jones's flat‐plate approximation results in only slightly larger fatigue and ultimate loads, and lower stability limits. Copyright © 2012 John Wiley & Sons, Ltd.

후류에 의한 풍력터빈의 피로하중 특성

The wake generated by a wind turbine has an effect on performance of a downstream wind turbine as well as mechanical loads. This paper investigated characteristics of fatigue load at the blade root due to the wake effects and quantitatively analyzed its effects at operating condition of a 5MW tripod offshore wind turbine using Bladed 4.1 software. The wake effects was studied the way the wake`s center position move from the rotor center to the blade tip to the far-away position where the wake doesn`t affect the wind turbine. When wake`s center was located on the blade tip or the rotor center, damage equivalent fatigue load was higher than other positions. It was up to 10~14% compared to those of non-wake case. Results of this study would be helpful to design wind turbines and wind farms to have lifetimes more than 20 years of the wind turbine.