Potential Impacts of Offshore Wind Farms on North Sea Stratification

Abstract

Advances in offshore wind farm (OWF) technology have recently led to their construction in coastal waters that are deep enough to be seasonally stratified. As tidal currents move past the OWF foundation structures they generate a turbulent wake that will contribute to a mixing of the stratified water column. In this study we show that the mixing generated in this way may have a significant impact on the large-scale stratification of the German Bight region of the North Sea. This region is chosen as the focus of this study since the planning of OWFs is particularly widespread. Using a combination of idealised modelling and in situ measurements, we provide order-of-magnitude estimates of two important time scales that are key to understanding the impacts of OWFs: (i) a mixing time scale, describing how long a complete mixing of the stratification takes, and (ii) an advective time scale, quantifying for how long a water parcel is expected to undergo enhanced wind farm mixing. The results are especially sensitive to both the drag coefficient and type of foundation structure, as well as the evolution of the pycnocline under enhanced mixing conditions—both of which are not well known. With these limitations in mind, the results show that OWFs could impact the large-scale stratification, but only when they occupy extensive shelf regions. They are expected to have very little impact on large-scale stratification at the current capacity in the North Sea, but the impact could be significant in future large-scale development scenarios.

Turbulence and Mixing in a Shallow Shelf Sea From Underwater Gliders

“…The installation of wind turbines in a region strongly influenced by tidal currents generate a turbulent wake. Assuming that the North Sea would be entirely covered by equally spaced (700–800 m) wind turbines, Carpenter et al () parameterized and tabulated turbulent production values for the German Bight sector of the North Sea based on standard bulk drag models. Depending on expected variations in the drag coefficient and on the foundation structure geometries, the average power production by turbine foundations that could be fed in to turbulence was estimated to be × × (Carpenter et al, ).…”

“…Additional to the naturally occurring mixing processes, the increased interest in renewable energies and the development of the technology to build wind turbines offshore in greater water depths have led to the planning and construction of offshore wind farms (OWFs) at coastal regions (Carpenter et al, ; Ho et al, ). Turbine foundations generate additional turbulence in the water column that can contribute to mix a stratified regime.…”

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

“…As the latest development, the massive construction of offshore wind farms -underway or planned -is likely to have a significant impact on marine mammals (Koschinky et al, 2003) and seabirds (Garthe and Hüppop, 2004;M. Busch et al, 2013), but possibly also mixing (Lass et al, 2008;Ludewig, 2015;Carpenter et al, 2016) and nutrient transport.…”

“…Due to their operational flexibility and a long endurance of the order of months, gliders sample the oceans at low cost in a way no other platforms currently do (Testor et al, 2010). Measurements showing the observed buildup of stratification φ over the summer months unaffected by offshore construction (dots; Carpenter et al, 2016) and the estimated rate of stratification removal by the turbine foundation structures in offshore wind farms (straight lines). The stratification is computed as φ (t) = H 0 ρ mix − ρ (z, t) gzdz, with water depth H , density ρ, gravitational acceleration g, vertical coordinate z, and time t. Measurements are from a thermistor mooring at Marnet station NSB3 in 2009 (black dots); glider data are collected in the vicinity (54 • 40.8 N, 6 • 43.9 E) in 2014 (green dots) and from larger-scale transects passing through NSB3 in 2012 (blue points).…”

“…The stratification is computed as φ (t) = H 0 ρ mix − ρ (z, t) gzdz, with water depth H , density ρ, gravitational acceleration g, vertical coordinate z, and time t. Measurements are from a thermistor mooring at Marnet station NSB3 in 2009 (black dots); glider data are collected in the vicinity (54 • 40.8 N, 6 • 43.9 E) in 2014 (green dots) and from larger-scale transects passing through NSB3 in 2012 (blue points). The rate of stratification removal for thermocline thicknesses b = 6, 9 m is based on a simple 1-D analytical model (Carpenter et al, 2016).…”