The challenges of design are most keenly felt when there is no room for error or afterthought. Modern prototyping techniques have helped relieve these pressures, but is not always feasible, and the engineer has to get it all right in one shot.
A one megawatt wind turbine designed by Bennetts fits neatly into this category. With a 45m turbine tower and 52m diameter rotor, the size of this structure effectively eliminated the option of physical prototyping.
Generating power for around 625 Northern Ireland homes, the design brief was to create a turbine which would last for at least 20 years. The stresses and forces experienced by this structure meant that the team at Bennetts had to perform stringent finite element analysis tests (FEA).
The critical component on any wind turbine is the hub casting, whose sole purpose is to hold the blades in place. However, as the turbine pitch is regulated, space must also be made for actuation equipment, associated control and maintenance access.
Using ANSYS’ Multiphysics package, Bennetts examined both the hub static stresses and fatigue life under simulated loading.
The hub and shaft interface was recognised as a critical area. Loads applied were taken from the three blades and applied to the three bearings where the blades meet the hub. Each of the blades has three forces and three moments, giving a total of 18 inputs which are applied as distributed loads onto the bearing. Hub design was optimised by using ANSYS software to analyse the effect of static load on these inputs.
The turbine’s vigorous action coupled with shifting stresses and changing weather conditions, makes it highly susceptible to damage. On structures where the loads are monodirectional it is possible to draw up a simple time against load graph, but, with 18 inputs to consider this is not possible.
The solution was to take the FE model of the hub and apply to it 18 load cases, each one representing a unit of load for the input. The model was run 18 times with a unit load of 1,000N on each blade.
Using this technique produced results for any combination of load by super-imposing the unit load results on top of each other. In-house software was used to combine the 18 sets of results with time history inputs to examine the casting in 100 selected and 226 randomly chosen points over its surface. Refining these results established the single stress values for each of the selected points, and ultimately reduce the 18 traces to one single trace.
The turbine is now generating at 1MW.
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