Current and wave effects around windfarm monopile foundations

Abstract

Laboratory measurements were undertaken to investigate wave and current velocities in the vicinity of a wind turbine monopile foundation, in order to inform environmental impact assessments and to quantify flow variability in the region of the power take off cable. Flow measurements were made up to 15.5 pile diameters (D) downstream of the pile. Measurements were also taken around the perimeter of the pile (~0.75 D from the pile centre) at the approximate representative height of the power cable. In current-only tests, the mean flow was reduced immediately downstream of the pile, but returned to within 5% of background levels by 8.3 D downstream of the pile centre in representative conditions. A new parameterisation of the velocity recovery is given. The turbulent eddy shedding frequency was well predicted by the Strouhal number. Turbulence peaked at 1.5 D from the pile centre, and the subsequent decay was parameterised. Velocity magnitudes at the side of the pile were up to 1.35 times greater than background flow rates, in line with potential flow theory. Velocities in the wake region were much less than predicted by potential flow theory, corresponding with increased turbulence. Tests with waves indicated that oscillatory velocities reduced immediately down-wave of the pile, but returned quickly to background levels (by 1.65 to 3.5 D of the pile centre). The general near-pile distribution of the orbital velocity maximum was well represented by potential flow theory. Orbital velocities were reduced immediately up-wave and down-wave of the pile. At the side of the pile in wind sea conditions, the velocity increased up to 1.66 times the background level. This increased to 1.85 times in swell conditions. For orthogonal currents and waves, a velocity parameter was calculated as the mean current plus wave orbital velocity, resolved. With the mean current direction as a reference, the maximum flow was observed at the side of the pile. At 0.75 D from the pile centre, the flow was enhanced by up to 1.2 times the no–pile case. Spectral peaks in the velocity were evident at both wave frequency and at the Strouhal frequency, immediately down current from the pile.