Investigation and Prediction of Yaw Bearing Fatigue in Floating Offshore Wind Turbines

Abstract

Onshore and offshore wind turbines are increasingly important renewable energy generation devices. With increasing energy demand and limited land and nearshore coastline resource, installation of wind turbines is moving to deeper waters. Floating offshore wind turbines (FOWTs) are sensitive to the complex environment loads they are subjected to, which are more extreme than for onshore fixed and offshore fixed wind turbines. This thesis assesses a floating offshore wind turbine’s yaw bearing asset integrity during its operational lifetime. The yaw bearing is a connection component which is located at the top of the wind turbine tower. It is a critical component that connects the wind turbine nacelle to the tower. Although the yaw bearing fails less frequently when compared to other components in a wind turbine, its location on the tower top increases the difficulties of maintenance and replacement, resulting in a higher cost of maintenance and replacement than the other components in the nacelle. The fatigue life and crack propagation of yaw bearings are complex as they are exposed to varying working conditions. Current industrial standards and guidelines for wind turbine yaw bearings are limited and focus on yaw bearings which are used in traditional fixed wind turbines. Some parameters used to assess them are from accumulated experience which is limited for FOWTs. This thesis develops a systematic assessment method for assessing the integrity of yaw bearings in wind turbines. For FOWTs, the influence of complex dynamic loads is considered. Fatigue life assessments of yaw bearings and crack propagation results in yaw bearings are compared in 5 MW wind turbines with six different support foundations: onshore, monopile, ITI barge, Tension Leg Platform (TLP), spar and semi-submersible. A time-domain method is applied to predict the wind and hydrodynamic loads on the yaw bearings using three wind velocities. A Gumbel distribution, rain flow counting algorithm, linear cumulative damage law and S–N curve theory are used to generate the life-time damage equivalent loads. The fatigue life of the yaw bearing is then calculated using a finite element method (FEM) and a code-based approach. The crack propagation analysis of critical locations points in yaw bearings is conducted using a submodel technique with extended finite element method (XFEM). Finally, the corresponding fatigue lives and fatigue crack propagation in yaw bearing are analyzed and compared. Fatigue life, crack propagation and stress intensity factor (SIF) results show that yaw bearing’s life is significantly affected by both the environment conditions and support foundations. Thus, it is necessary to assess the integrity of yaw bearings in wind turbines especially the ones installed in novel FOWTs. Compared with traditional analytical methods, which are designed only for yaw bearings in fixed onshore and offshore wind turbines, the results show that the fatigue lives of yaw bearings calculated using code-based methods are non-conservative. The proposed method is more practical to assess yaw bearing service life under actual operational conditions and can be used as part of asset integrity management of FOWTs.