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Approach to the fatigue analysis of vertical-axis wind-turbine blades

Description: A cursory analysis of the stress history of wind turbine blades indicates that a single stress level at each wind speed does not adequately describe the blade stress history. A statistical description is required. Blade stress data collected from the DOE/ALCOA Low Cost experimental turbines indicate that the Rayleigh probability density function adequately describes the distribution of vibratory stresses at each wind speed. The Rayleigh probability density function allows the distribution of vibratory stresses to be described by the RMS of the stress vs. time signal. With the RMS stress level described for all wind speeds, the complete stress history of the turbine blades is known. Miner's linear cumulative damage rule is used as a basis for summing the fatigue damage over all operating conditions. An analytical expression is derived to predict blade fatigue life.
Date: September 1, 1981
Creator: Veers, P.S.
Partner: UNT Libraries Government Documents Department

All wind farm uncertainty is not the same: The economics of common versus independent causes

Description: There is uncertainty in the performance of wind energy installations due to unknowns in the local wind environment, machine response to the environment, and the durability of materials. Some of the unknowns are inherently independent from machine to machine while other uncertainties are common to the entire fleet equally. The FAROW computer software for fatigue and reliability of wind turbines is used to calculate the probability of component failure due to a combination of all sources of uncertainty. Although the total probability of component failure due to all effects is sometimes interpreted as the percentage of components likely to fail, this perception is often far from correct. Different amounts of common versus independent uncertainty are reflected in economic risk due to either high probabilities that a small percentage of the fleet will experience problems or low probabilities that the entire fleet will have problems. The average, or expected cost is the same as would be calculated by combining all sources of uncertainty, but the risk to the fleet may be quite different in nature. Present values of replacement costs are compared for two examples reflecting different stages in the design and development process. Results emphasize that an engineering effort to test and evaluate the design assumptions is necessary to advance a design from the high uncertainty of the conceptual stages to the lower uncertainty of a well engineered and tested machine.
Date: December 31, 1994
Creator: Veers, P.S.
Partner: UNT Libraries Government Documents Department

Fatigue reliability of wind turbine fleets: The effect of uncertainty of projected costs

Description: The cost of repairing or replacing failed components depends on the number and timing of failures. Although the total probability of individual component failure is sometimes interpreted as the percentage of components likely to fail, this perception is often far from correct. Different amounts of common versus independent uncertainty can cause different numbers of components to be at risk of failure. The FAROW tool for fatigue and reliability analysis of wind turbines makes it possible for the first time to conduct a detailed economic analysis of the effects of uncertainty on fleet costs. By dividing the uncertainty into common and independent parts, the percentage of components expected to fail in each year of operation is estimated. Costs are assigned to the failures and the yearly costs and present values are computed. If replacement cost is simply a constant multiple of the number of failures, the average, or expected cost is the same as would be calculated by multiplying by the probability of individual component failure. However, more complicated cost models require a break down of how many components are likely to fail. This break down enables the calculation of costs associated with various probability of occurrence levels, illustrating the variability in projected costs. Estimating how the numbers of components expected to fail evolves over time is also useful in calculating the present value of projected costs and in understanding the nature of the financial risk.
Date: December 31, 1995
Creator: Veers, P.S.
Partner: UNT Libraries Government Documents Department

Fatigue case study and reliability analyses for wind turbines

Description: Modern wind turbines are fatigue critical machines used to produce electrical power. To insure long term, reliable operation, their structure must be optimized if they are to be economically viable. The fatigue and reliability projects in Sandia`s Wind Energy Program are developing the analysis tools required to accomplish these design requirements. The first section of the paper formulates the fatigue analysis of a wind turbine using a cumulative damage technique. The second section uses reliability analysis for quantifying the uncertainties and the inherent randomness associated with turbine performance and the prediction of service lifetimes. Both research areas are highlighted with typical results.
Date: December 31, 1994
Creator: Sutherland, H.J. & Veers, P.S.
Partner: UNT Libraries Government Documents Department

Simulating high-frequency wind for long durations

Description: A wind simulator that can provide high-frequency synthetic wind data indefinitely has been developed to numerically test various aspects of wind turbine control systems. The wind simulator builds synthetic wind data in two basic steps. First, using an auto-regressive method, the simulator creates low-frequency wind speeds at a small number of time steps for each one-hour interval. This low-frequency part of the wind has a Weibull distribution and an exponential auto correlation. Second, it creates high-frequency data and superimposes it on top of the low-frequency wind speed data. The high-frequency part has zero mean, Gaussian distribution and frequency content selected from a library of turbulence pad`s. The low- and high-frequency parts are summed to produce the final result. This process repeats itself for as many one-hour intervals as the user desires and lends itself to the eventual addition of diurnal and seasonal effects.
Date: November 1, 1993
Creator: McFarlane, A.; Veers, P. & Schluter, L.
Partner: UNT Libraries Government Documents Department

Application of measured loads to wind turbine fatigue and reliability analysis

Description: Cyclic loadings produce progressive damage that can ultimately result in wind turbine structural failure. There are many issues that must be dealt with in turning load measurements into estimates of component fatigue life. This paper deals with how the measured loads can be analyzed and processed to meet the needs of both fatigue life calculations and reliability estimates. It is recommended that moments of the distribution of rainflow-range load amplitudes be calculated and used to characterize the fatigue loading. These moments reflect successively more detailed physical characteristics of the loading (mean, spread, tail behavior). Moments can be calculated from data samples and functional forms can be fitted to wind conditions, such as wind speed and turbulence intensity, with standard recession techniques. Distributions of load amplitudes that accurately reflect the damaging potential of the loadings can be estimated from the moments at any, wind condition of interest. Fatigue life can then be calculated from the estimated load distributions, and the overall, long-term, or design spectrum can be generated for any particular wind-speed distribution. Characterizing the uncertainty in the distribution of cyclic loads is facilitated by using a small set of descriptive statistics for which uncertainties can be estimated. The effects of loading parameter uncertainty can then be transferred to the fatigue life estimate and compared with other uncertainties, such as material durability.
Date: January 1, 1997
Creator: Veers, P.S. & Winterstein, S.R.
Partner: UNT Libraries Government Documents Department

Aeroelastic behavior of twist-coupled HAWT blades

Description: As the technology for horizontal axis wind turbines (HAWT) development matures, more novel techniques are required for the capture of additional amounts of energy, alleviation of loads and control of the rotor. One such technique employs the use of an adaptive blade that could sense the wind velocity or rotational speed in some fashion and accordingly modify its aerodynamic configuration to meet a desired objective. This could be achieved in either an active or passive manner, although the passive approach is much more attractive due to its simplicity and economy. As an example, a blade design might employ coupling between bending and/or extension, and twisting so that, as it bends and extends due to the action of the aerodynamic and inertial loads, it also twists modifying the aerodynamic performance in some way. These performance modifications also have associated aeroelastic effects, including effects on aeroelastic instability. To address the scope and magnitude of these effects a tool has been developed for investigating classical flutter and divergence of HAWT blades. As a starting point, an adaptive version of the uniform Combined Experiment Blade will be investigated. Flutter and divergence airspeeds will be reported as a function of the strength of the coupling and also be compared to those of generic blade counterparts.
Date: December 31, 1998
Creator: Lobitz, D.W. & Veers, P.S.
Partner: UNT Libraries Government Documents Department

Performance of twist-coupled blades on variable speed rotors

Description: The load mitigation and energy capture characteristics of twist-coupled HAWT blades that are mounted on a variable speed rotor are investigated in this paper. These blades are designed to twist toward feather as they bend with pretwist set to achieve a desirable twist distribution at rated power. For this investigation, the ADAMS-WT software has been modified to include blade models with bending-twist coupling. Using twist-coupled and uncoupled models, the ADAMS software is exercised for steady wind environments to generate C{sub p} curves at a number of operating speeds to compare the efficiencies of the two models. The ADAMS software is also used to generate the response of a twist-coupled variable speed rotor to a spectrum of stochastic wind time series. This spectrum contains time series with two mean wind speeds at two turbulence levels. Power control is achieved by imposing a reactive torque on the low speed shaft proportional to the RPM squared with the coefficient specified so that the rotor operates at peak efficiency in the linear aerodynamic range, and by limiting the maximum RPM to take advantage of the stall controlled nature of the rotor. Fatigue calculations are done for the generated load histories using a range of material exponents that represent materials from welded steel to aluminum to composites, and results are compared with the damage computed for the rotor without twist-coupling. Results indicate that significant reductions in damage are achieved across the spectrum of applied wind loading without any degradation in power production.
Date: December 7, 1999
Creator: Lobitz, D.W.; Veers, P.S. & Laino, D.J.
Partner: UNT Libraries Government Documents Department

Aeroelastic tailoring in wind-turbine blade applications

Description: This paper reviews issues related to the use of aeroelastic tailoring as a cost-effective, passive means to shape the power curve and reduce loads. Wind turbine blades bend and twist during operation, effectively altering the angle of attack, which in turn affects loads and energy production. There are blades now in use that have significant aeroelastic couplings, either on purpose or because of flexible and light-weight designs. Since aeroelastic effects are almost unavoidable in flexible blade designs, it may be desirable to tailor these effects to the authors advantage. Efforts have been directed at adding flexible devices to a blade, or blade tip, to passively regulate power (or speed) in high winds. It is also possible to build a small amount of desirable twisting into the load response of a blade with proper asymmetric fiber lay up in the blade skin. (Such coupling is akin to distributed {delta}{sub 3} without mechanical hinges.) The tailored twisting can create an aeroelastic effect that has payoff in either better power production or in vibration alleviation, or both. Several research efforts have addressed different parts of this issue. Research and development in the use of aeroelastic tailoring on helicopter rotors is reviewed. Potential energy gains as a function of twist coupling are reviewed. The effects of such coupling on rotor stability have been studied and are presented here. The ability to design in twist coupling with either stretching or bending loads is examined also.
Date: April 1, 1998
Creator: Veers, P.; Lobitz, D. & Bir, G.
Partner: UNT Libraries Government Documents Department

Probabilistic analysis of mechanical systems

Description: This paper proposes a framework for the comprehensive analysis of complex problems in probabilistic structural mechanics. Tools that can be used to accurately estimate the probabilistic behavior of mechanical systems are discussed, and some of the techniques proposed in the paper are developed and used in the solution of a problem in nonlinear structural dynamics.
Date: September 1, 1993
Creator: Priddy, T. G.; Paez, T. L. & Veers, P. S.
Partner: UNT Libraries Government Documents Department

Analysis/test correlation using VAWT-SDS on a step-relaxation test for the rotating Sandia 34 m test bed

Description: The combined analysis/test effort described in this paper compares predictions with measured data from a step-relaxation test in the absence of significant wind-driven aerodynamic loading. The process described here is intended to illustrate a method for validation of time domain codes for structural analysis of wind turbine structures. Preliminary analyses were performed to investigate the transient dynamic response that the rotating Sandia 34 m Vertical Axis Wind Turbine (VAWT) would undergo when one of the two blades was excited by step-relaxation. The calculations served two purposes. The first was for pretest planning to evaluate the relative importance of the various forces that would be acting on the structure during the test and to determine if the applied force in the step-relaxation would be sufficient to produce an excitation that was distinguishable from that produced by the aerodynamic loads. The second was to provide predictions that could subsequently be compared to the data from the test. The test was carried out specifically to help in the validation of the time-domain structural dynamics code, VAWT-SDS, which predicts the dynamic response of VAWTs subject to transient events. Post-test comparisons with the data were performed and showed a qualitative agreement between pretest predictions and measured response. However, they also showed that there was significantly more damping in the measurements than included in the predictions. Efforts to resolve this difference, including post-test analyses, were undertaken and are reported herein. The overall effort described in this paper represents a major step in the process of arriving at a validated structural dynamics code.
Date: November 1, 1993
Creator: Argueello, J. G.; Dohrmann, C. R.; Carne, T. G. & Veers, P. S.
Partner: UNT Libraries Government Documents Department

User`s manual for FAROW: Fatigue and reliability of wind turbine components: Version 1.1

Description: FAROW is a Computer program that assists in the probabilistic analysis of the Fatigue and Reliability of wind turbines. The fatigue lifetime of wind turbine components is calculated using functional forms for important input quantities. Parameters of these functions are defined in an input file as either constants or random variables. The user can select from a library of random variable distribution functions. FAROW uses structural reliability techniques to calculate the mean time to failure, probability of failure before a target lifetime, relative importance of each of the random inputs, and the sensitivity of the reliability to all input parameters. Monte Carlo simulation is also available. This user`s manual is intended to provide sufficient information to knowledgeably run the program and meaningfully interpret the results. The first chapter provides an overview of the approach and the results. Chapter 2 describes the formulation and assumptions used in the fatigue life calculations. Each of the input parameters is described in detail in Chapter 3 along with hints and warnings on usage. An explanation of the outputs is provided in Chapter 4. Two example problems are described and solved in Chapter 5, one for the case where extensive data are available and the other with limited data where the uncertainty is higher. A typical input file and the output files for the example problems are included in the appendices.
Date: November 1, 1994
Creator: Veers, P. S.; Winterstein, S. R.; Lange, C. H. & Wilson, T. A.
Partner: UNT Libraries Government Documents Department

FAROW: A tool for fatigue and reliability of wind turbines

Description: FAROW is a computer program that evaluates the fatigue and reliability of wind turbine components using structural reliability methods. A deterministic fatigue life formulation is based on functional forms of three basic parts of wind turbine fatigue calculation: (1) the loading environment, (2) the gross level of structural response given the load environment, and (3) the local failure criterion given both load environment and gross stress response. The calculated lifetime is compared with a user specific target lifetime to assess probabilities of premature failure. The parameters of the functional forms can be defined as either constants or random variables. The reliability analysis uses the deterministic lifetime calculation as the limit state function of a FORM/SORM (first and second order reliability methods) calculation based on techniques developed by Rackwitz. Besides probability of premature failure, FAROW calculates the mean lifetime, the relative importance of each of the random variables, and the sensitivity of the results to all of the input parameters, both constant inputs and the parameters that define the random variable inputs. The ability to check the probability of failure with Monte Carlo simulation is included as an option.
Date: July 1, 1993
Creator: Veers, P. S.; Lange, C. H. & Winterstein, S. R.
Partner: UNT Libraries Government Documents Department

Elastic dynamics of a complete wind turbine structure: Theoretical development

Description: This paper develops a pseodo-linear formulation of the equations of motion for analyzing elastic bodies which undergo large rotations relative to one another with an emphasis on its application to HAWTs. This procedure greatly simplifies the computational aspects of the solution algorithm over the nonlinear alternatives and should yield a significant improvement in computer speed. Additional speed can be achieved by ordering the nodes such that a minimum bandwidth can be realized (leading to approximately 64 multiplications per degree of freedom per solution step). The formulation utilizes a set of nested moving coordinate systems, each of which is loosely tied to one of the elastic bodies such that the displacements in the body relative to its coordinate system remain small. The formulation also includes a scheme for handling the nonlinear geometric stiffness that occurs in the blades as a result of the centrifugal loads in a pseudo-linear fashion.
Date: January 1, 1992
Creator: Lobitz, D.W.; Arguello, J.G. & Veers, P.S.
Partner: UNT Libraries Government Documents Department

Fatigue reliability of wind turbine components

Description: Fatigue life estimates for wind turbine components can be extremely variable due to both inherently random and uncertain parameters. A structural reliability analysis is used to qualify the probability that the fatigue life will fall short of a selected target. Reliability analysis also produces measures of the relative importance of the various sources of uncertainty and the sensitivity of the reliability to each input parameter. The process of obtaining reliability estimates is briefly outlined. An example fatigue reliability calculation for a blade joint is formulated; reliability estimates, importance factors, and sensitivities are produced. Guidance in selecting distribution functions for the random variables used to model the random and uncertain parameters is also provided. 5 refs., 9 figs., 1 tab.
Date: January 1, 1990
Creator: Veers, P.S.
Partner: UNT Libraries Government Documents Department

Technology Improvement Opportunities for Low Wind Speed Turbines and Implications for Cost of Energy Reduction: July 9, 2005 - July 8, 2006

Description: This report analyzes the status of wind energy technology in 2002 and describes the potential for technology advancements to reduce the cost and increase the performance of wind turbines.
Date: February 1, 2008
Creator: Cohen, J.; Schweizer, T.; Laxson, A.; Butterfield, S.; Schreck, S.; Fingersh, L. et al.
Partner: UNT Libraries Government Documents Department

Damping measurements using operational data

Description: We have measured modal damping using strain-gauge data from an operating wind turbine. Previously, such measurements were difficult and expensive. Auto-correlation and cross-correlation functions of the strain-gauge data have been shown to consist of decaying sinusoids which correspond to the modal frequencies and damping ratios of the wind turbine. We have verified the method by extracting damping values from an analytically generated data set. Actual operating response data from the DOE/Sandia 34-meter Test Bed has been used to calculate modal damping ratios as a function of rotor rotation rate. This capability will allow more accurate fatigue life prediction and control. 16 refs., 3 figs., 2 tabs.
Date: January 1, 1991
Creator: James, G.H.; Carne, T.G. & Veers, P.S.
Partner: UNT Libraries Government Documents Department

Estimation of uncertain material parameters using modal test data

Description: Analytical models of wind turbine blades have many uncertainties, particularly with composite construction where material properties and cross-sectional dimension may not be known or precisely controllable. In this paper the authors demonstrate how modal testing can be used to estimate important material parameters and to update and improve a finite-element (FE) model of a prototype wind turbine blade. An example of prototype blade is used here to demonstrate how model parameters can be identified. The starting point is an FE model of the blade, using best estimates for the material constants. Frequencies of the lowest fourteen modes are used as the basis for comparisons between model predictions and test data. Natural frequencies and mode shapes calculated with the FE model are used in an optimal test design code to select instrumentation (accelerometer) and excitation locations that capture all the desired mode shapes. The FE model is also used to calculate sensitivities of the modal frequencies to each of the uncertain material parameters. These parameters are estimated, or updated, using a weighted least-squares technique to minimize the difference between test frequencies and predicted results. Updated material properties are determined for axial, transverse, and shear moduli in two separate regions of the blade cross section: in the central box, and in the leading and trailing panels. Static FE analyses are then conducted with the updated material parameters to determine changes in effective beam stiffness and buckling loads.
Date: November 1, 1997
Creator: Veers, P.S.; Laird, D.L.; Carne, T.G. & Sagartz, M.J.
Partner: UNT Libraries Government Documents Department

Sensitivity Analysis of Offshore Wind Cost of Energy (Poster)

Description: No matter the source, offshore wind energy plant cost estimates are significantly higher than for land-based projects. For instance, a National Renewable Energy Laboratory (NREL) review on the 2010 cost of wind energy found baseline cost estimates for onshore wind energy systems to be 71 dollars per megawatt-hour ($/MWh), versus 225 $/MWh for offshore systems. There are many ways that innovation can be used to reduce the high costs of offshore wind energy. However, the use of such innovation impacts the cost of energy because of the highly coupled nature of the system. For example, the deployment of multimegawatt turbines can reduce the number of turbines, thereby reducing the operation and maintenance (O&M) costs associated with vessel acquisition and use. On the other hand, larger turbines may require more specialized vessels and infrastructure to perform the same operations, which could result in higher costs. To better understand the full impact of a design decision on offshore wind energy system performance and cost, a system analysis approach is needed. In 2011-2012, NREL began development of a wind energy systems engineering software tool to support offshore wind energy system analysis. The tool combines engineering and cost models to represent an entire offshore wind energy plant and to perform system cost sensitivity analysis and optimization. Initial results were collected by applying the tool to conduct a sensitivity analysis on a baseline offshore wind energy system using 5-MW and 6-MW NREL reference turbines. Results included information on rotor diameter, hub height, power rating, and maximum allowable tip speeds.
Date: October 1, 2012
Creator: Dykes, K.; Ning, A.; Graf, P.; Scott, G.; Damiami, R.; Hand, M. et al.
Partner: UNT Libraries Government Documents Department