Sources of fatigue damage to wind turbine blades

Abstract

At the University of Utah, we have developed an integrated approach to analyzing fatigue damage sources to wind turbine systems. Using a suite of wind turbine specific computer codes, we can isolate parameters of interest and evaluate the relative importance each plays in the fatigue damage sustained over the life of a wind turbine. Models of free-yaw, yaw driven, rigid hub and teetering hub turbines have been investigated to determine load scenarios which lead to the most fatigue damage for each type of machine. Some results have been presented previously (Pierce and Laino, 1996; Laino and Hansen, 1997; Laino, 1997). These will be reviewed and additional results discussed leading to overall conclusions about this project. Conclusions include: • Atmospheric stability is a major factor in wind turbulence activity and hence fatigue life. • Controlling turbine response to turbulence can reduce fatigue damage accumulation. Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. and the American Society of Mechanical Engineers. All rights reserved. 304 • Discrete events (e.g., emergency shutdown, extreme gusts, etc.) can be a primary source of fatigue damage to teetering hub machines, but have little effect on rigid rotors. • Control system design can be a factor in fatigue damage accumulation, even when loads are not a control parameter. • Yaw behavior affects fatigue loads for both free yaw and yaw driven machines. Conclusions are drawn upon results with the focus on which factors in a wind turbine design and environment generate the most damaging fatigue loads. With this knowledge, designers can concentrate their efforts on the variables which best serve to alleviate these loads. The importance of the results to wind turbine load standard criteria is also discussed. Finally, recommendations for future work are presented. Introduction Task specific computer codes for wind turbine analysis have made it possible to conduct Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. fully computer based fatigue analyses of various machine and inflow conditions. Using this approach, analyses of three wind turbine models were made under various operating conditions to determine the relative importance of various parameters on fatigue life. The results reveal areas of particular concern for each turbine design type.