Design and Dynamic Analysis of a 10-MV Medium-Speed Drivetrain in Offshore Wind Turbines
MetadataShow full item record
- Institutt for marin teknikk 
This thesis deals with design and dynamic analysis of 10-MW medium-speed drivetrains for large-scale offshore wind turbines. Many issues related to two main topics, drivetrain design and dynamic analysis, have been investigated. A detailed drivetrain design methodology is described, including design principle, layouts, basis, criteria, loads and optimization process. Based on this methodology, a four-point drivetrain consisting of a conventional three-stage medium-speed gearbox is designed for the Technical University of Denmark (DTU) 10-MW wind turbine. Further, a bedplate is designed for the four-point drivetrain. Moreover, a novel gearbox consisting of a fixed planetary stage and a differential compound epicyclic stage is designed based on the same design methodology as for the conventional gearbox. Highfidelity drivetrain dynamic models are established using a multi-body system (MBS) method and they differ in the layout with respect to conventional versus compact gearboxes as well as rigid versus flexible bedplates. The rationality of the drivetrain model with conventional gearbox is checked via the first order natural frequency comparison between theoretical and simulation results. Meanwhile, the rationality of the dynamic model of the compact gearbox is checked via the comparison of power distribution ratios on two transfer paths that are calculated using theoretical and numerical simulation methods. The risk of resonance risk is checked both for the drivetrain models with conventional and compact gearboxes. Several case studies related to the drivetrain dynamic analysis are performed. They cover the topics of drivetrain modelling fidelity, drivetrain dynamic analysis methods, drivetrain layouts, drivetrain supports as well as drivetrain external excitation and drivetrain dynamic load-sharing, response statistics as well as fatigue damage. The effect of bedplate flexiblity on drivetrain dynamic behaviour is studied. Natural frequencies of the coupled rotor-drivetrain-bedplate-tower model for rigid and flexible bedplates are compared. Moreover, load effects and fatigue damage of drivetrain gears and bearings for the rigid and flexible bedplates are compared. The results indicates that the bedplate flexibility would increase the load effects on bearings inside the gearbox, especially for bearings at low- and high-speed stages, while it would reduce the load effects on the main bearings. In addition, sensitivity analysis of the bedplate modelling fidelity is conducted in order to provide a reasonable recommendation for drivetrain numerical modelling. Dynamic responses of the 10-MW drivetrain calculated by a fully coupled and a de-couple analysis methods are compared. A fully coupled dynamic model of a 10-MW floating wind turbine with a detailed drivetrain is established using the software SIMPACK and it is verified by code-to-code comparisons with respect to natural frequencies and dynamic responses in the time domain. In addition, short-term fatigue damage of the drivetrain gears and bearings obtained by the fully coupled and the de-coupled models is compared in order to unveil the question whether the traditional decoupled method yields sufficiently accurate results for drivetrain dynamic responses. The results indicate that the de-coupled method would yield to conservative results in the drivetrain fatigue damage if the natural frequencies of the drivetrain are sufficiently separated from that of the global system. The dynamic behaviour of the wind turbine gearbox in conventional and compact layouts is compared. Dynamic load-sharing behaviour and shortterm fatigue damage of the two gearboxes are comprehensively evaluated and compared under pure torque load cases, tangential pin position error conditions and non-torque load cases, respectively. The results demonstrate that the compact gearbox has better dynamic performance under different torque load cases and is more robust to withstand the effects of manufacturing errors and rotor non-torque loads compared to the conventional gearbox. The dynamic behaviour of the 10-MW wind turbine drivetrain supported on a bottom-fixed monopile and a spar-type floating offshore wind turbines is compared. The effect of global loads and nacelle motions on drivetrain dynamic responses is studied based on a de-coupled method. The results demonstrate that different global loads between the bottom-fixed and the spar floating turbines are the main cause for the different drivetrain responses. Nacelle motions of the spar floating turbine have a large effect on axial force of planet carrier bearing in the first stage of the gearbox, while they have minor effects on other components in the drivetrain. The fundamental reasons for the different load effects between the two turbines and the influence for the influence of nacelle motions of the spar turbine are identified. The effect of variability and uncertainty in environmental conditions on short-term fatigue damage of floating wind turbine drivetrains is investigated. Variability and uncertainties of several environmental variables are discussed, including mean wind speed, turbulence intensity, wind shear exponent, significant wave height and wave peak period, wind-wave misalignment as well as number of wind and wave samples, and their effects and relative importance on drivetrain fatigue damage are evaluated. It is found that variability and uncertainties of the stochastic wind field are the main sources that determine the variation of drivetrain fatigue damage, while the variability and uncertainties of the irregular waves generally make small effects, at least for the concept investigated. The original contributions of this thesis includes the development of drivetrain design for large-scale offshore wind turbines, providing a drivetrain dynamic model for benchmark studies, carrying out a series of drivetrain dynamic characteristic analysis for various topics, providing a deeper insight into drivetrain dynamics in relation to offshore wind turbines and hence facilitate the development of multi-megawatt offshore wind turbine drivetrains.