FAQ

In very rare cases, if turbine failure occurs, the fall of an object into water can create an impact wave which can propagate. If this phenomenon occurs, is the wave dangerous?

Impact waves are generated by a non-natural falling object. The impact of an object on water generates a splash and concentric radiation waves which create an impact wave. Concentric waves are restricted to the surface and dissipate rapidly. The splash is a mixture of air and water which dissipates part of the impact energy. It does not contribute to the generation of an impact wave and is not dangerous. The smoother the surface, the greater the contact surface area between the object and the water will be, promoting energy transfer. The impact will therefore be highest in calm seas and lowest in rough seas.

The physics at play when objects hit the water are very complex as they involve the air compressibility and water viscosity. They also feature the fluid/structure interaction governing the dissipation of energy through the deformation of impacting and impacted media related to their difference in density and shock resistance. Thus, only a very complex chain of calculations involving highly non-linear physics is able to accurately resolve this problem, which is not the aim here. The philosophy behind our method is therefore to put forward a simplified approach while ensuring that the hypotheses at each stage remain conservative, in order to conclude with a maximum theoretical value which will never be exceeded.

We note that the probability of a wind turbine suffering a major failure to the extent of causing a falling object is very low. To determine whether the wave generated in this case is dangerous, the experts first calculated the height of the wave generated by a falling object detached from a turbine. In order to remain conservative in subsequent calculations, the worst-case scenario was taken, i.e. a flat surface hit by an object. It should be noted that this sea state does not exist in the natural environment.

In the event of rupture, the tower would hit the water progressively from the base to the nacelle. The impact would therefore be minimal. Furthermore, the curved shape of the tower means that the surface area instantaneously in contact with the water is reduced. Given that the impact force of an object on water is proportional to the instantaneous contact surface area and its speed squared, the fall of a tower into the water will not generate a dangerous wave.

As it is almost impossible for three blades to break at the same time, the scenario of the rotor falling was not studied. The scenario considered here consists in a 86 m-long blade from a 10 MW turbine falling off at maximum rotation speed (10 rpm) from a horizontal position, assuming that the blade stops rotating and falls flat onto the sea surface. This study shows that the wave generated by such an impact theoretically would not exceed 3.4 m, and 600 m from the impact point would be less than 1 m high. This wave is not considered as dangerous given that offshore wind farms are located over 10 km from the coast and the turbines are spaced 1 km apart.

In short: we note that the probability of a wind turbine suffering a major failure to the extent of causing a falling object is very low. If this phenomenon occurred in a calm sea, the wave generated by the falling object would be less than 3.4 m high and would dissipate very quickly. This height would be far less if the blade fell into a rough sea. Fixed-foundation wind farms therefore cannot generate impact waves that are dangerous for humans.

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.

A tsunami is a long wave which propagates with a depth far lower than its wavelength (non-dispersive conditions). The movement associated with this wave concerns the entire water column. Tsunamis are most frequently of geological origin (submarine earthquake, volcanic eruption, coastal or submarine landslide), meteorological origin, or triggered by asteroid impact. A tsunami causes the rapid, exceptional inundation of coastal areas by the sea, due to a temporary and abnormal rise in sea level.

In the case of a fixed-foundation offshore wind farm, only submarine and coastal landslides can trigger tsunamis. Wind farms cannot cause volcanic eruptions, earthquakes or asteroid impact. Coastal landslides or collapses result from the combined effects of geological (coastal sediment deficit, bedrock alteration), oceanographic (swell, tides, current) and meteorological (precipitation, frost) phenomena. Fixed-foundation wind farms can have a very minor influence on swell, currents and sediment transport which could destabilise coastal cliffs.

In order to estimate the probable influence of wind farms on sediment transport, experts have calculated scour, i.e. erosion at the base of the turbine. The scour depth can reach up to 1.7 times the diameter of the monopile and the scour distance 10 times the diameter. In the case of a gravity foundation with a 36 m diameter base, there is no major influence on the sediment dynamics beyond 360 m. For other types of foundation (jacket, monopile) which have a smaller diameter, this distance is shorter. As wind farms are located around ten kilometres from the coast, they cannot increase the risk of tsunamis caused by coastal landslides. Submarine landslides occur when the continental slope is steep. Fixed-foundation wind farms are located on a part of the continental shelf with a very low slope. A fixed-foundation offshore wind farm therefore cannot generate submarine landslides.

In short: fixed-foundation wind farms cannot trigger tsunamis.

Several types of offshore renewable energy (ORE) farm components are built, stored and maintained in ports (e.g. gravity foundations, floats) before being transported to their area of operation. Certain components are then brought back to port for maintenance. As ports are international shipping hubs, they are colonised by non-indigenous species. These organisms could colonise turbine components, which constitute bare substrates. During the transportation and installation of these components in the environment, species attached to their surface could be dispersed in the environment.

The experts recommand a quick transport of ORE components on site to reduce exposure time to nonindigenous species more commonly present in areas close to ports and on the shoreline. The experts also advise to implement a protocol for monitoring turbine foundations or floats in order to observe and report the development of non-indigenous species.

Ballast water may be taken onboard by ships forstability. It can contain thousands of aquatic or marine microbes, plants and animals, which are then carried across the globe. These organisms may be released when the ballast water is discharged. The International Convention for the Control and Management of Ships’ Ballast Water and Sediments (BWM Convention) was adopted in 2004. It entered into force in 2017 to introduce global regulations to control the transfer of potentially invasive species. As a temporary solution, pending the installation of an on-board ballast water treatment system, ships should exchange ballast water mid-ocean.

If regulations are complied with, untreated ballast water should not be discharged near wind farms. The convention requires ships to discharge ballast water at a considerable distance from the coast (see text box and diagram below) to prevent the spread of non-indigenous species.

Certain marine organisms that are sessile (i.e. attached to a substrate) at adult stage produce pelagic larvae (living in the water column). These larvae move passively under the influence of currents before finding a hard substrate to attach to and develop. For these species, offshore wind farms can provide new hard substrate habitats where sessile organisms develop and in turn reproduce. The larvae produced disperse from this new point and can thus reach new rocky areas: this is the so-called stepping stones. In order to illustrate the stepping stones that wind turbines could have at sea, we chose to take a simple example: a rocky reef at a certain distance from other reefs. A wind farm is built between the reef and the rocky areas.

Before farm installation, the organisms on the reefs in ecosystem A produce larvae. The maximum larval dispersal distance means that they cannot reach the rocky reefs in ecosystem B. With the wind farm in place, the organisms on the natural reef in ecosystem A produce larvae. The wind turbine foundations and moorings, and floats in the case of floating wind turbines, act as an artificial reef. Larvae attach themselves to and colonise the wind turbine foundations. Once attached, if conditions are conducive, the organisms will develop and reproduce, in turn releasing pelagic larvae into the water column. Carried by favourable currents, the larvae will reach the rocky reefs of ecosystem B, where the species was initially absent. This is what is referred to as the stepping stones. In this way, the larvae of organisms in ecosystem A, which cannot directly colonise ecosystem B, may possibly do so in the presence of wind turbines.

French offshore wind farms are located close to the French coastline; there is therefore a low risk of

creating a stepping stones between the English and French coasts for example. The stepping stones at the scale of the French coasts would have little impact as many other structures and activities transport species or already act as a relay on this scale (shipping traffic, aquaculture, buoys, etc.). The risk of non-indigenous species spreading through the stepping stones produced by French wind farms appears limited.

In short: the risk of non-indigenous species spreading through the installation of ORE farms in France appears limited but not negligible. The experts recommend developing a protocol for monitoring turbine foundations in order to observe and report the development of non-indigenous species.