Impact of offshore wind farm monopiles on hydrodynamics interacting with wind-driven waves

Abstract

This paper investigates the local and regional impact of offshore wind farm (OWF) foundations on hydrodynamics in interaction with wind-induced waves at the Meerwind-OWF site (German Bight, North Sea) on tidal and monthly time scales. For this purpose, a 3D high-resolution coupled circulation-wave model based on unstructured grids is employed, which enables an effective transition in resolution from ∼2 km in marine open boundaries to ∼2 m near the foundations. The OWF monopiles induce different local and regional changes of the monthly mean velocity at mid-depth: a decrease of ∼5 % near the piles and an increase of ∼1 % in a wider region surrounding the OWF. The latter can be attributed to the relevant regional reduction in water density of ∼0.02 %. Consequently, the monthly potential energy anomaly increases by ∼5 % outside the OWF, while it decreases by 40 % inside it. The monopiles reduce the monthly significant wave height (Hs) from ∼5 % within the OWF to <1 % over distances of ∼20 km. The prevailing westerly waves can affect the tidal asymmetry, particularly on the eastern side of the piles. This results in an asymmetry in the intensity of turbulent wakes on either side of the piles, in both monthly and tidal timescales. However, wave intensification can disrupt the periodic tidal pattern of the wake. An extreme event with Hs>4 m creates a peak wake during the slack water that is higher than those at times of maximum tidal currents during spring tides.