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Offshore wind turbines threatened by scour

Offshore wind turbines appear to be more susceptible to scour than previously predicted, according to recent research in the latest issue of the ICE’s Maritime Engineering journal.

Research by a team of engineers from the Technical University of Denmark found that stone armour around the 4.2m diameter monopile foundations at the 160MW capacity Horns Rev offshore wind farm in the North Sea was found to have unexpectedly sunk by up to 1.5m.

Lead author Anders Nielsen says the sinking stones are a problem as they can, “reduce the stability of the monopole and change for instance the natural frequency of the dynamic response of an offshore wind turbine in an unfavourable manner”. While the armour can be repaired in the short term, finding a long term solution requires much greater analysis of how powerful sea currents behave around the bottom of offshore turbine towers, the authors said.

Nielsen added that the tests have identified that the sinking appears to be primarily caused by a “horseshoe vortex” effect relating to the flow of water surrounding the base of the monopile. Nielson added the results also reveal that, “a larger pile diameter relative to the size of the protection stones will cause a larger sinking and that two layers of stone will decrease the sinking relative to one layer of stones with the same size”.

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