A comparative study of fully coupled and de-coupled methods on dynamic behaviour of floating wind turbine drivetrains

Traditionally, drivetrain responses are obtained by a de-coupled analysis, which first involves a global analysis with a simplified representation of the drivetrain, followed by a detailed analysis of the drivetrain with the input of global response on the drivetrain interface. As the wind turbine size increases, it is questionable whether this de-coupled analysis method yields sufficiently accurate results. To address this question, a comparative study of the drivetrain dynamic behaviour obtained by a fully coupled method and a de-coupled one, is conducted and reported in this paper. A 10-MW fully coupled aero-hydro-servo-elastic floating wind turbine dynamic model is developed, including a high-fidelity drivetrain. The developed fully coupled model is assessed to be reasonable via the comparison of drivetrain first-order natural frequency and code-to-code comparisons in terms of global responses between two simulation tools Simpack and Fast. Resonance analysis of the 10-MW drivetrain in the fully coupled model is performed, with focus on rotor-drivetrain-bedplate-tower coupled modes in the low frequency range. Time domain simulations of the drivetrain in the fully coupled and the de-coupled models are carried out in different environmental conditions. One-hour fatigue damage of drivetrain gears and bearings in the fully coupled and de-coupled models are compared. Effect of nacelle motion on drivetrain fatigue damage in the de-coupled analysis is discussed. The results are presented to demonstrate whether the de-coupled method could be confidently used for drivetrain dynamic analysis. This study provides a basis for drivetrain design and dynamic analysis in floating wind turbines