The Interaction of Tidal Currents with Offshore Wind Turbine Monopiles : An Experimental Study of Flow, Turbulence, Scour and the Reduction of Scour Around the Monopile.

Mc'Govern, David John (2010) The Interaction of Tidal Currents with Offshore Wind Turbine Monopiles : An Experimental Study of Flow, Turbulence, Scour and the Reduction of Scour Around the Monopile. PhD thesis, Lancaster University.

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Abstract

The installation of offshore wind turbines involves the driving of a cylindrical steel monopile into the sea bed. The presence of the monopile in the water column causes changes in the local flow and turbulence field resulting in increased flow velocity, turbulence and bed shear stress. This causes a scour hole to develop around the base of the monopile, adversely affecting its stability. An extensive laboratory campaign was run to study how tidal currents may change the flow, turbulence, bed shear stress amplification, scour time development and scour depth around a scaled cylinder representing the monopile. The experiments yielded detailed results showing a highly three-dimensional flow field whose characteristics vary significantly with changes in velocity and water depth. The scour hole resulting from reversing time/depth-varying currents was symmetrical in shape and its time development was slower than in unidirectional currents. The results implied this scour depth to be lower than unidirectional current scour depth. The possibility of reducing scour by attaching collars and helical strakes to the cylinder was investigated. The flow and turbulence field around was found to be markedly different to that around the regular cylinders, exhibiting large horizontal-axis rotation in the downstream wake. For the collared cylinder, these differences translated to a lower scour depth during the early stages of the reversing time/depth-varying current in comparison to that of a steady unidirectional current. However, the scour depth at the end of the test was the same for both the smooth and collared cylinders. From the presentation and discussion of the model results, it is suggested that prototype tidal variation in flow depth and velocity does not simplify to unidirectional currents and that the continued use of unidirectional current based prediction methods may overestimate scour in tidal conditions and lead to unnecessary scour protection and resource expenditure.