Fatigue Life Fragilities and Performance-Based Design of Wind Turbine Tower Base Connections

Abstract

Wind turbines are typically designed for a target service life of 20 years because of fatigue-related failures of the moving components such as the rotors and blades. Designing the tower to have the same lifetime as other components is clearly desirable for optimization of the wind turbine system. This study focuses on the fatigue life of the tower base connection subjected to wind loading. A simplified finite-element model for wind turbines subjected to nonlinear wind loading in time domain was developed specifically for application in this study. The relative motion between the wind speed and moving blades in the flapwise direction only creates force nonlinearity for the applied wind load and hence, necessitates application of a simple fluid/structure interaction model. Then, a model for fatigue assessment (Mode I), including crack propagation, was developed for the tower base connection. The inclusion of crack propagation is expected to extend the service life of the tower compared with conventional fatigue life analysis using the characteristic S-N approach. By varying the tower thickness, diameter, and considering predefined levels of crack propagation, fragility curves based on a fatigue life limit state are developed for application of performance-based design. The desired fatigue life of a wind turbine tower for different wind sites can be obtained based on the fragilities. Finally, performance-based design for a typical 5 MW wind turbine is used as an illustrative example in this study.