Fatigue life analysis of threaded connections in offshore wind turbines

With the rapid expansion of the installed offshore wind capacity around the world, it is essential to improve the structural integrity of these energy structures for reduced electricity cost and prolonged operational lifespans. An important part of the offshore wind turbine structures is the connection between the monopile foundation and the transition piece. Currently the dominant technology for connecting the monopile to transition piece is using L flanges held together with large-scale bolts. Threaded connections have emerged as a prevalent technology for linking sections of wind turbines, boasting commendable performance despite some inherent drawbacks. This study conducts a comprehensive review of the recommended fatigue design curves for threaded connections in international standards and compares them with the existing fatigue data on medium to large scale bolt sizes. Additionally, the fatigue behaviour of M72 threaded connections has been further analysed by performing new tests with two different values of mean stress. The obtained data from this study have been discussed in terms of the level of conservatism in the recommended fatigue design curves available in international standards for threaded connections. Moreover, the experimental analysis has been combined with numerical and analytical investigations to provide further insight into the life prediction of the threaded connections under fatigue loading conditions.