Sea state changes within a fixed-foundation wind farm could be due to the diffraction phenomenon and the modulational instability caused by the turbine network.
A rogue wave is a short wave with a severe combination of height and steepness. Rogue waves are exceptionally large in relation to surrounding waves. Waves are generated by wind; a rogue wave can be caused by: – the effect of a strong current running counter to the wave direction, – the meeting of two wave systems, – focusing is the concentration of wave energy at a given place and time. A rogue wave can appear in a given sea state that has been underestimated: all phenomena triggering it may not have been taken into account in short- and long-term forecast models.
The greater the diameter of the foundations, the more diffracted the waves will be. Thus, in the case of jacket foundations (steel lattice), waves are not modified as the dimensions of the obstacle are far lower than the wavelength. In the case of monopile and gravity foundations, the waves that can be diffracted have a lower amplitude and shorter period. These waves propagate radially. 500 m from the turbine, only 5% of the height of the induced wave remains. In the worst-case scenario, i.e. gravity foundations with a diameter of approximately 30 m, the sea state at a distance of 2 km from the farm is identical to what it would be without the farm. There is therefore no increase in the risk of dangerous rogue waves being generated due to diffraction inherent to the geometry of the wind farm.
The modulational instability mechanism can also lead to the formation of rogue waves. This mechanism corresponds to non-linear resonance of the different waves present in a wave train. If there is low disturbance, the balance between a wave and neighbouring waves could be upset. One of the waves in the group will thus absorb the energy of its neighbours by resonance and thus become far larger. This growth could cause the wave to triple its initial height. However, this very large wave will have a limited lifetime, as it will ultimately return the energy to its neighbours. In practice, this mechanism applies to waves with very low spectral spread, occurring at great depth in relation to their wavelength.
The experts examined the disturbances that could be induced by the spacing between turbines. Can they trigger this instability mechanism? The sea state climatology in wind farms off the French coastline indicates that such conditions cannot occur, given the respective characteristic dimensions of the wind farms and sea states.
In short: the diffraction phenomenon within a fixed foundation wind farm network does not generate dangerous waves for humans. Similarly, the sea state climatology in wind farms off the French coastline indicates that dangerous waves cannot be created by the modulational instability mechanism.