An assessment of axial loading on a five-turbine array

A structure that supports five turbines with a power coefficient of 0·40 (efficiency of 68%) has been studied using computational fluid dynamics to assess the power extracted and the flow field in a 3 m/s (6 knot) tidal flow. Peak axial sliding forces were assessed to determine anchorage requirements. While it is recognised that the turbines will most likely be positioned in relatively deep water in areas of steep tidal velocity gradients, this study considers the worst-case scenario for the axial sliding forces – that is, a uniform 3 m/s tidal velocity profile. The analysis shows that the fluid velocity increases around the structure, which could possibly be used advantageously in the placing of multiple turbine arrays. There is minimal interference between the wakes of the individual turbines, but there is interference between the wakes of some turbines and the bracing that forms part of the structure. The axial sliding force was found to be highest when the frame apex is head into the flow, and it is estimated that the coefficient of friction between the seabed and the array frame must be lower than 0·43 for sliding to occur with no additional ballast or anchorage