Design of a digital displacement pump piston for multi-megawatt offshore wind turbine transmissions

Concerns about anthropogenic climate change have significantly increased the demand for sustainable energy production. Wind power is recognised as one of the leading contributors for meeting this requirement. To meet energy demand, and to allow subsidy free wind energy to compete economically with fossil fuels, the rated power of wind turbines has been increasing. There are, however, many engineering challenges associated with achieving increased turbine size. Geared wind turbines have scaling issues related to gearbox stiffness and load sharing amongst gear teeth, which has hindered their size increasing much above 8MW. Direct drive turbines also have scaling issues related to high top-head mass and generator stiffness, which is necessary for holding tight flux gaps. A hydrostatic transmission employing Digital Displacement technology (DDT) is an alternative to the traditional gearbox. It uses modular pumping groups which theoretically allow scaling of rated power by simply increasing the number of pumping modules. There are benefits associated with increasing the displacement of individual modules such as reduced component number and machine architecture complexity. However, issues of robustness have been discovered with increasing displacement due to thermal effects. The objective of this thesis is to establish if the size of existing pump pistons can be increased to reduce component number without adverse effects on robustness. A multibody elasto-hydrodynamic simulation tool is used to predict fluid film thicknesses in piston to roller interfaces. This is combined with a lumped parameter thermal analysis in an iterative process to account for oil heating effects. To validate these simulation results, a test rig is designed and built. A novel fluid film measurement technique, which relies on ultrasound reflections, is developed and used to evaluate the piston under a range of operating conditions. The validated simulation model shows that the scope for scaling of the piston size to increase the pump module displacement is limited. A novel nacelle architecture for Digital Displacement (DD) machines is proposed which prevents over-constraining the main-shaft bearings and removes the need for compliant mounts between the pump and the nacelle casting. This novel design delivers similar benefits to the scaling of pistons, with increased robustness and efficiency