Numerical modelling of morphological impacts of offshore wind farms

• Main Author: Christie, Elizabeth Katherine
• Published: University of Liverpool 2014
• Subjects: 620
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.634460

Demand for renewable energy resources to reduce greenhouse gases for EU targets, has led to a recent rapid development of Offshore Wind Farms (OWF). As OWFs become larger and multiple sites are developed, it becomes increasingly important to determine the wind farms impact on the coastal environment for design and planning. It is well established that the wind turbine monopiles at OWFs modify the flow in the localised area of the structure, to create a complex 3D flow structure, which ultimately results in scour hole formation. This present research aims to determine the impact of an OWF at both the localised and coastal scale through large scale modelling with the structures represented as islands in the mesh. This method is thought to be an improvement on the typical method of representing the structures as a resistance term in the grid, as it is able to capture some of the complex flow at the structure. The TELEMAC modelling package is used, with the hydrodynamics determined by the TELEMAC-3D module, the waves by the TOMAWAC module, and sediment transport by the SISYPHE module. Validation of the model at the structure showed good agreement with empirical data in the near field of the structure. Tidal flows are well predicted across the water depth, whilst scour formation is well predicted in front of the pile, there are areas of accretion in the wake which are unexpected. The large scale impact of the wind farm on coastal processes was assessed and compared over two wind farm sites, representing different coastal environments. The Burbo Bank wind farm is situated in a coastal bay, whereas Scroby Sands OWF is an open coast site. In both cases the wind farm was seen to block the flow and influence the large scale coastal sediment pathways. At Burbo Bank the wind farm enabled stirring of sediment into suspension, and influenced the sediment transport over the south east corner of Liverpool Bay. The Scroby Sands wind farm was found to reduce the sediment flux magnitude in the vicinity of the array. The long term morphological impact is also determined for the Burbo Bank OWF over a year period, with a morphological acceleration factor. Two methods are compared for generation of a set of representative waves, based on frequency of occurrence and wave energy. Both methods indicate that over a year period the wind farm has a large influence on sediment transport pathways, and increases sediment flux across the Great Burbo Flats. Maximum scour depth predictions at the structures showed good agreement with empirical formula. The pattern of scour for the representative waves based on frequency of occurrence, fits well with measured scour at the wind farm array.