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Characterisation of impacts on the environment of an idealised offshore wind farm foundation, under waves and the combination of waves and currents

Abstract : Offshore wind technology is currently the most widespread and advanced source of marine renewable energy. Offshore wind farms populate waters through the North Sea and the English Channel. The UK and French governments devised deadlines to achieve percentages of electricity from renewable sources by 2020, these deadlines and the direct translation of land based wind farm technology to the offshore environment resulted in the rapid expansion of the offshore wind energy. New wind farms have been designed with a larger number of masts and are moving from shallow offshore banks to deeper waters and in order to produce more power the diameters of monopoles masts are becoming larger to support larger turbines. The three-partner EU INTERREG funded project OFELIA (http://www.interreg-ofelia.eu/) aims to establish a cross-channel (between the UK and France) research collaboration to improve understanding of the environmental impacts of offshore wind farm foundations. The objective of the present study is to characterise changes in the hydrodynamics and sea bed in the vicinity of an offshore wind farm mast and in the wake area under wave and wave-current conditions corresponding to events in the French wind farm site of Courseulles-sur-mer (offshore of Lower Normandy, in the English Channel). Experiments were carried out in two laboratory facilities: a wave flume of 35 m long, 0.9 m wide and 1.2 m in depth with regular and irregular waves (García-Hermosa et al., 2014); and a wave and current flume of 17 m long, 0.5 m wide and 0.4 m depth with regular waves, currents from 180° to the waves and a mobile bed (Gunnoo et al., 2014). Flow velocity measurements were taken with an Acoustic Dopple Velocimeter (ADV) at various points around the cylinder and Particle Image Velocitmetry (PIV) techniques were applied to larger areas upstream and downstream of the cylinder. During the assessment of waves and currents' effects on the bed evolution were assessed using a laser and camera system photographing the bed (Marin & Ezersky, 2007, and Jarno-Druaux et al., 2004). Velocity fields, and flow structures around the cylinder at low KC numbers (KC~1) were characterised and parameters such as vorticity, turbulent kinetic energy and bed shear stresses derived where possible. During the experiments vortex structures with a horizontal axis were observed in the vicinity of the cylinder and the bed even at low KC. The Keulegan-Carpenter number (KC) is defined as: KC = UmT- D, where Um is the bottom orbital velocity, T the peak period and D the pile diameter. As part of the project, the findings from the experiments fed into a regional numerical modelling (Rivier et al., 2014) to improve parametrisation of the representation of the within-cell processes (local to the mast). References García-Hermosa, M. I., Brossard, J., Cohen, Z., Perret, G. (2014). Experimental characterisation of wave induced flow fields due to an offshore wind farm mast. First International Conference on Renewable Energies Offshore (RENEW) Lisbon, Portugal. November 2014. Gunnoo, H., Abcha, N., Mouazé, D., Ezersky, A., García-Hermosa, M. I. (2014). Laboratory simulation of resonance amplification of the hydrodynamic fields in the vicinity of wind farm masts. Proceedings of the First International Conference on Renewable Energies Offshore (RENEW) Lisbon, Portugal. November 2014. Jarno-Druaux, A., Brossard, J., Marin, F. (2004). Dynamical evolution of ripples in a wave channel, European Journal of Mechanics B/Fluids 23: 695-708. Marin, F. and Ezersky, A. B. (2007). Formation dynamics of sand bedforms under solitons and bound states of solitons in a wave flume used in resonant mode. European Journal of Mechanics - B/Fluids, Elsevier, 2008, 27 (3), pp.251-267. Rivier, A., Bennis, A.-C., Pinon, G., Gross, M., Magar, V. (2014). Regional numerical modelling of offshore monopile wind turbine impacts on hydrodynamics and sediment transport. Proceeding of the 1st International Conference on Renewable Energies Offshore (RENEW) Lisbon, Portugal. November 2014.
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Submitted on : Friday, March 29, 2019 - 6:13:28 PM
Last modification on : Monday, April 27, 2020 - 4:26:30 PM

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  • HAL Id : hal-02084821, version 1

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Isabel García-Hermosa, Nizar Abcha, Jerôme Brossard, Anne-Claire Bennis, Alexander Ezersky, et al.. Characterisation of impacts on the environment of an idealised offshore wind farm foundation, under waves and the combination of waves and currents. EGU General Assembly, Apr 2015, Vienne, Austria. ⟨hal-02084821⟩

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