Modelling and analysis of the gearbox in a floating spar-type wind turbine

Abstract

This thesis seeks to reveal and investigate important drivetrain dynamics in relation to offshore wind turbines. Emphasis is placed on drivetrains of the spar-type floating wind turbines (FWTs). FWTs are proposed to be used for offshore wind power extraction at the deeper-water sites where fixed foundations are economically infeasible. The FWT is a complex machine that is subjected to tough offshore environmental conditions and access for maintenance, repair and overhaul is limited and expensive. It is therefore important to understand the FWT drivetrain because gearbox failures have consistently plagued the wind energy industry and have not been able to reach the 20 year design life. Moreover, gearbox failures often result in massive downtime. Some industry players have even labelled the gearbox as the ’missing link’. There is a need for new insight into the understanding of drivetrain dynamics from an offshore perspective in order to improve its design and better predict its life. The findings in this thesis contribute to the de-risking process of offshore wind.

The drivetrain and wind turbine from the National Renewable Energy Laboratory’s (NREL’s) Gearbox Reliability Collaborative (GRC) project are used as the case studies. There are three main topics in this thesis.

In the first topic, the modelling of the planet carrier is investigated in detail. The study is performed in two parts. First, the influence of subcomponents mated to the planet carrier in the gearbox assembly is investigated in detail. These components consist of the planet pins, bearings and the main shaft. In the second part of the study, the flexible body modelling of the planet carrier for use in multi-body simulations is examined through the use of condensed finite element and multi-body simulation models.

The second topic is the collaborative work with NREL which compared the multi-body models with the measurements taken from the GRC test campaigns. Data from the planetary stage is used to evaluate the accuracy and computation time of numerical models of the gearbox. A set of models that represent different levels of fidelity are established and compared to the measurement test results.

The last topic, which is also the main topic, deals with the modelling and analysis of the FWT drivetrain. Various spar platforms with different drafts in the spar buoys and mooring line designs for the GRC wind turbine are specially designed for this purpose.

A decoupled solution is used, i.e., the drivetrain is assumed to have no feedback on the rotor. Global aero-hydro-elastic-servo simulations are performed and the main shaft loads, generator speed and nacelle motions are calculated. These are used as inputs to a multi-body drivetrain model. Comparisons of the FWT and the land-based wind turbine (WT) are performed. Detailed sensitivity studies of the influences of the design parameters on the drivetrain loads and other wind turbine component loads are also performed.