Correlation between Acceleration and Drivetrain Load Effects for Monopile Offshore Wind Turbines

This paper investigates the correlation between the tower-top axial acceleration and the load effects in drivetrain components in a monopile (bottom-fixed) offshore wind turbine. In designing offshore wind turbines, it is a common practice to set a limit for axial acceleration. The main objective of this work is to evaluate the rationality of this assumption as a design criterion for critical components in the drivetrain such as gears and bearings, and to provide guidance for designing the drivetrains in monopile wind turbines. In this study, a 5-MW offshore wind turbine on a monopile structure is modelled and the load effects in the drivetrain are calculated through a de-coupled analysis approach. For each chosen wind speed, the most probable significant wave height and period was chosen from a site in the North Sea with water depth of 29 m which is similar to Dogger Bank wind farm. The results reveal that the life of components inside the gearbox are not correlated with the maximum axial acceleration for the monopile structure. The load effect or life of the first main bearing in the 4-point support is directly correlated with the wind speed while the second main bearing, which carries the axial loads is uncorrelated. The tower-top bending moment, which is an important parameter in designing the main shaft, is found to be highly correlated with the wind speed, not necessarily with the axial acceleration.