The hydrodynamics of monopile foundations – Experimental measurements of near bed and free stream turbulence

Monopile foundations for offshore wind turbines affect the local flow conditions, with corresponding changes to scour and sediment transport processes. As the influence of wave climate on turbulence is still poorly understood, this is an important study area for modelling environmental impacts. Results of flume and basin experiments are presented, investigating the effect of a monopile foundation on turbulence in the presence of waves. A 10 cm diameter model monopile in 50 cm water depth was tested. Monochromatic, regular waves were used, with wave heights between 4 cm and 10 cm and a period of 1.13 s. A current velocity of 14 cm/s was used, and the direction of wave propagation was varied between 60° and 120° to the current. These model values were derived by scaling typical prototype values at 1:50. Turbulent Kinetic Energy measurements are presented for both current only and combined wave-current conditions, and for a range of incident wave angles and wave heights. The wave climate enhances dissipation of the turbulence generated by the monopile. It is shown that the modification of the turbulent wake is most significant when waves and currents are aligned, and less important when wave propagation is perpendicular to the current. Further analysis of the shear stress and turbulence datasets is ongoing. These results aid our understanding of scour processes at monopile foundations, and the transport of suspended sediments in their wake.