Dynamic Analysis and Design of Gearboxes in Offshore Wind Turbines in a Structural Reliability Perspective

Abstract

The drivetrain is the heart of the wind turbine. In wind turbines, the wind's kinetic energy is converted to electrical power through the drivetrain which often consists of a gearbox and a generator. The drivetrain is exposed to a variety of load conditions through its 20-year service life. Among the drivetrain components, the gearbox failure has been of concern, primarily due to its long repair or replacement downtime. Thus it is always important to improve the gearbox reliability in order to reduce the downtime cost and make the wind-generated power more competitive.

This thesis addresses modelling of the wind turbine gearbox dynamic behaviour, load and load effect analysis and design based on first principles. Forces and moments acting on the drivetrain shaft and motions were obtained by global analysis based on models of the whole wind turbine system.

These forces/moments and motions were then applied on the refined gearbox models in multi-body simulation tools. A 750-kW land-based gear- box was initially used in the analyses, but as it differs from state of the art designs of large offshore multi-megawatts gearboxes, a 5-MW reference gearbox was designed and used in the studies. The 5-MW reference offshore gearbox was developed following the most commonly used state of the art design practices and first principles.

The wind turbine gearbox dynamic behaviour and response analysis un- der gear geometrical errors and imperfections, shaft deections or misalign- ments were also studied. The methods which could reduce their impacts were investigated. Such methods including oating sun gear in planetary stage were studied in details.

Moreover, the methods to predict the long-term extreme loads and re- sponses in the gearbox were investigated. For the gear load analysis and apart from the multi-body simulation model, a simpli ed analytical method

was discussed. Long-term fatigue analysis and reliability assessment were also carried out. A modified ISO procedure was established for the long-term fatigue damage calculation of gears in wind turbines. The uncertainties in loads and load effects were taken into the account in the reliability analysis. The application of structural reliability methods for mechanical components was demonstrated by fatigue reliability analysis of the gears. Furthermore, a model-based gearbox fault detection was developed. A prognostic model-based gearbox fault detection was introduced and was ex- amined on the 750-kW gearbox. In addition, the fatigue reliability method developed earlier was employed for creating a reliability-based inspection map which can be used to identify the faulty components by inspecting those with the higher probability of fatigue failure. Such maps can reduce the downtime during inspection and maintenance. In addition, the 5-MW reference gearbox was used for the comparative study of load effect and fatigue damage in the land-based and oating wind turbines. Four oating wind turbines were investigated: spar, TLP and two semi-submersibles. The oating structures with highest thrust load on the main shaft were found to be the one with the highest drivetrain damages, mainly on the main bearings. The original contributions of this thesis include developing a systematic design approach based on rst principles for wind turbine gearboxes, developing a 5-MW reference gearbox for offshore research and development, developing a model-based fault detection method and evaluating feasibility of using land-based designed gearboxes on oating turbines. They provide the background for further studies of wind turbine gearboxes and for different fixed or floating wind turbine concepts. This thesis presents methods for gearbox load effect and fatigue analysis mainly applied on offshore wind turbines, but they are in general generic and applicable to land-based turbines too.