Extreme Load Estimation for Wind Turbines: Issues and Opportunities for Improved Practice

Current design load estimation procedures for wind turbines often do not accurately treat the statistical nature of loads. Current practice for wind turbine load analysis is reviewed. The authors’ opinions on the shortcomings of these practices are discussed. Experience gained from current research on statistical load extrapolation methods is reviewed. Statistical modeling techniques are presented. Open questions on current techniques are summarized and critical issues that need to be resolved for an accurate statistical load extrapolation method are discussed. INTRODUCTION AND BACKGROUND Stochastic Environment Wind turbines must be designed to operate in a very stochastic environment for at least 20 years according to International Electrotechnical Commission (IEC) standards. In addition to the cyclic nature of loads induced by their own inertial effects, loads result from spatial and temporal changes in wind speed, direction, shear and vorticity. This has challenged designers for many years. Initially designers felt this level of detail in wind modeling was impossible and unnecessary. Very simple techniques were used in the late 70s and early 80s. These techniques worked when the designs were very simple and conservatively designed or the wind conditions were benign. As wind turbines became larger it was too expensive to use large safety margins. They were also being installed in very turbulent sites. 1 This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the U.S. Department of Energy under contract DE-AC04-94AL85000. Failures caused by inaccurate estimation of design loads mandated more accurate prediction techniques which did account for more detail in the inflow. Detailed structural dynamic models were developed and became the workhorses for the wind industry by the mid 1990s. Included in these computer codes were turbulence models, which simulated stochastic inflow fields, aerodynamic models, which predicted aerodynamic loads from the turbulent inflow, and control algorithms, which commanded pitch, yaw, and braking actions. The aerodynamic loads were applied to the structural dynamic model which was then run in a time marching fashion. Figure 1 shows a flow chart of the general analysis models. With this approach designers finally had tools which could simulate all the important operational features of the entire wind turbine, even the control system. One can argue about the accuracy of the various models that make this overall system of models, but even in their current state they provide a far more accurate and power tool than ever before. Armed with these new tools the designer must decide how to use them. He/she is now able to simulate almost any wind and operational condition. The designer is still faced with estimating the fatigue life and peak loads over 20 years. This implies running 20 years of computer simulations which, at the present time, only run near real time. Obviously this is not practical. How should a subset of simulations be used to extrapolate to a representative 20 year spectrum of loads?