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Minimising the impacts of offshore wind farms


Example of noise reduction: use of a bubble curtain. Photo: Hero Lang (Trianel)
Example of noise reduction: use of a bubble curtain. Photo: Hero Lang (Trianel)

Noise mitigation

The harbour porpoise is a highly vulnerable protected species that is endangered by pile driving noise during construction of wind turbines (see: Construction and Operation-Related Impacts and Underwater Noise). Noise mitigation is needed to protect the harbour porpoise from death, injury and severe disturbance. Technical measures must therefore be taken during noise-intensive construction work and pile driving. Since 2008, BSH approvals have stipulated that the sound event level (SEL) may not exceed 160 dB at a distance of 750 m from a pile driving site. On 1 December 2013, the Federal Environment Ministry additionally introduced a strategy for the protection of harbour porpoise in the North Sea from the impacts of pile driving noise Schallschutzkonzept (only in German) with additional measures to limit noise-induced disturbance.


New noise mitigation techniques

BfN has worked for many years to promote the development of noise prevention or reduction techniques. New noise mitigation methods developed in recent years have been deployed in the construction of offshore wind turbines in German marine areas and significantly cut noise from pile driving:

  • Bubble curtain: A system of perforated hoses or pipes in a circle on the seabed around the pile driving site; air rising from the holes forms a curtain of ascending bubbles in the water that reflects or muffles the propagated sound.

    Bubble curtain noise mitigation. Diagram: Trianel
    Bubble curtain noise mitigation. Diagram: Trianel
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  • HSD (Hydro Sound Damper): Air-filled balloons or fixed elements of foam material attached to a net surrounding the pile driving site from sea floor to surface, with the quantity and the size of the balloons or elements selected according to the sound frequency to be dampened; works by the same basic principle as a bubble curtain.
  • Cofferdam: Structure such as a steel cylinder placed round a foundation pile during pile driving. The space between is pumped dry and the resulting air gap attenuates the sound propagated during pile driving.

New installation methods and foundation designs

New foundation types such as gravity foundations and suction buckets remove the need for pile driving. There are also alternative, low-noise methods of inserting conventional piles. Examples:

  • Vibratory pile driving: Foundation piles are inserted by a quieter, vibratory pile driver rather than an impact ram.
  • Suction buckets: An inverted steel ‘bucket’ is set down on the sea floor and sunk into the seabed by suction created by pumping the water out of it. The foundation gains its stability from a combination of ground pressure and hydrostatic pressure.

A comprehensive study on the state of the art in noise mitigation measures and alternative low-noise foundation designs is available as a PDF document:

Zwischen Naturschutz und Energiewende – Herausforderung Schallschutz beim Bau von Offshore-Windparks, 2014. (7,9 MB, only in German)

Minimising impacts on migrating birds and bats

The most important and most effective method of reducing the impacts of offshore wind farms on migrating birds and bats is to select sites outside of areas of special importance to bird and bat migration. Suitable measures include:

  • Keeping clear bird/bat migration corridors in future plans for offshore wind farms.
  • Bird-friendly marking or lighting of wind turbines and converter platforms.
  • Temporary shutdown during mass migration events to reduce collision risk (especially in bad weather and visibility conditions).
  • Rotating the rotor plane out of the direction of migration.

Implementation of the last two measures requires good prediction models for migration and surveys of migration intensity in the immediate surroundings of wind farms.


Migrating common scoter (Melanitta nigra) in the North Sea. Photo: Matthias Putze
Migrating common scoter (Melanitta nigra) in the North Sea. Photo: Matthias Putze

Minimising impacts on seabirds

The most important and most effective method of reducing the impacts of offshore wind farms on seabirds and is to select sites outside of areas that are of special importance to seabird species as resting, feeding or wintering locations. One wind farm has been built in the middle of the Eastern German Bight special protection area (SPA), while approval has been refused for two wind farm projects applied for in a registered SPA in the Baltic Sea. Suitable measures include:

  • Keeping clear areas where sensitive species concentrate and designated special protection areas in future plans for wind farms, with the particular aim of avoiding habitat loss and minimising collision risk.
  • Keeping clear corridors between seabird habitats to allow barrier-free, safe movement between habitats.

Minimising impacts on sediment and benthic fauna

To reduce the impacts on benthic organisms and communities near wind farms, preference should be given to foundation designs with the smallest possible impermeable footprint. Effort should be made during construction to avoid or minimise the relocation of sediment and the generation of turbidity plumes. Sensitive or protected habitats should also be avoided and thus protected from harm by the selection of suitable locations for individual wind turbines.

In cable laying as in the construction of wind turbines, the techniques used should be selected so as to keep sediment relocation and turbidity plumes to a minimum. Cables should be routed to avoid sensitive or protected habitats.

Negative effects on seabed organisms due to sediment warming around power cables must be avoided by burying the cables deep enough in the seabed. The temperature of the top 20 cm of the seabed must not increase by more than 2 Kelvin.

The greater distance from the sediment surface gained by burying submarine power cables also minimises the electromagnetic fields above the sea floor generated by the operation of such cables (see: Submarine Cables). While three-phase systems generate alternating magnetic fields, direct current cables create static magnetic fields. The three conductors in a three-phase system and the outward and return conductors of a direct current system must therefore be contained in a common bipolar or tripolar cable or laid close enough together for their magnetic fields to cancel each other out. Electric fields can be avoided by suitable shielding.

 

Map North Sea

Offshore Wind Farms, Grid Connections and Natura 2000 Sites in the German Exclusiv Economic Zone (EEZ) of the North Sea.

Offshore Wind Farms, Grid Connections and Natura 2000 Sites in the German Exclusiv Economic Zone (EEZ) of the North Sea. (As of 03.2015)

Map Baltic Sea

Offshore Wind Farms, Grid Connections and Natura 2000 Sites in the German Exclusiv Economic Zone (EEZ) of the Baltic Sea.

Offshore Wind Farms, Grid Connections and Natura 2000 Sites in the German Exclusiv Economic Zone (EEZ) of the Baltic Sea. (As of 03.2015)

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