Underwater Noise

Belwind seen from RV Belgica

The construction and operation of offshore wind farms generates noise both above and under water that may be of environmental concern. At the European level, the new EU Marine Strategy Framework Directive (MSFD) has identified noise as one of the pressures that needs to be controlled to achieve the ‘good environmental status’ of European marine waters. The Belgian part of the North Sea hosts numerous human activities generating noise, including sand and gravel extraction, the installation of pipelines and cables, military exercises, intense shipping and offshore wind farms. We monitor the noise above and under water, during the construction and the operational phase of wind farms on the Belgian part of the North Sea. You can find more information in the 2013 Report on Impact of offshore windfarms.

Above and underwater noise

During the piling of a steel wind turbine foundation, we measured a maximum source power level of the above water noise of 145 dB(A). Piling activities could be detected in low background noise conditions at a distance of up to 10 km from the source and hence cannot be heard from the coast.

Gravity based foundation
Wind farm-generated noise: underwater noise

As the installation of gravity-based foundations (GBF) do not require piling, the construction of GBF wind turbines may be considered relatively silent as most of the noise is coming from an increase of shipping and dredging operations with noise levels of about 115 dB re 1 μPa, i.e. little higher than the ambient noise level under water. Piling events however are known to produce much higher peaks in noise levels. Even if these emissions are limited in time, they have to be considered of the same order of magnitude as those produced by airguns. Operational noise was measured both for the wind turbines at the Thorntonbank and the Blighbank. A 3 MW monopile wind turbine typically generated a sound pressure twice as high as that of a 6.15 MW jacket foundation turbine, in its turn emitting higher noise levels than a 5 MW GBF wind turbine.

Noise
Legislation

With regards to operational above water noise, environmental noise limits for onshore wind turbines are given in VLAREM. For a park of 100 wind turbines, this distance would need to be increased to at least 3-4 km. The residential areas nearest to the offshore wind farms are located at a distance of 30 km at present and 21 km when the whole Belgian wind farm zone will be developed. Residents along the Belgian coast will hence never experience noise pollution from the offshore wind farms.

For underwater generated noise, limits are not yet fully implemented in Belgian legislation. These underwater noise level limits are of course not directly related to human welfare, but rather to its disturbance of marine life, with a focus on marine mammals. Given the seasonally high density of harbour porpoises in Belgian waters (over 2 ind./km² on average), excessive noise may have an impact.

In the future we will use moored instruments to monitor underwater noise. These instruments record long samples of underwater noise covering one or more complete sequences of piling events. The instruments will also be used for operational noise recordings throughout a wide range of weather conditions.

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