Fatigue life assessment of 5 MW onshore wind turbines

Abstract

Renewable energy is a significant part of energy sector due to its economical and natural reasons. The modern wind turbines can be designed such that they are resistant to the heavy loads and and excessive forces because of the technological improvements. On the other hand, these type of systems should be durable against other design parameters such as fatigue. In this study, a 5 MW onshore wind turbine was analyzed against fatigue failure at the bolted joints. Initially, soil uncertainty is investigated in terms of soil stiffness that is represented by subgrade reaction of modulus. Change in soil stiffness due to soil uncertainty leads to change in overall period of the structural system. This situation may result in resonance and; therefore, the fatigue life of the structural parts changes significantly. A 5 MW onshore wind turbine model is created in the FAST (Fatigue, Aerodynamics, Structures and Turbulence) program which is a tool that can perform dynamic analyses for wind turbines under stochastic wind loading. Effect of resonance is investigated in terms of base shear and over-turning moment obtained from FAST analysis. Two different cases; namely, 'in-resonance' and 'out-of-resonance' cases have been considered to compare the fatigue life of the structural parts. It was found that the fatigue life of the 'in-resonance' case is decreased approximately to one third of the fatigue life of the 'out-of-resonance' case. This observation reveals the importance of accurate modelling of the overall i.e. soil-foundation-structure system to obtain the actual structural period.