Underwater noise during the construction of wind farms in the North Sea
During the construction phase of a wind farm, underwater noise can have a negative impact on marine mammals. To reduce this impact, noise limitations, amongst others, are enforced for the construction of offshore wind farms. Pondera, in collaboration with HWE and TNO and instructed by the Dutch Ministry of Economic Affairs and Climate Policy, have investigated whether the initially proposed noise restriction for future wind farms IJmuiden Ver and Nederwiek I is feasible.
To minimise the impact on marine mammals in the North Sea (such as porpoises and seals), a regulatory noise restriction has been set for noise generation during the piling of the turbine foundations which may not be exceeded. In the ‘Kader Ecologie en Cumulatie’, calculations show that a noise limitation of 160 dB re 1 μPa2s SELs (at 750 metres from installation) will protect marine mammal populations. The executed study examined the feasibility of this noise restriction based on available knowledge regarding effectiveness of current and developing mitigation systems.
There are various noise mitigation systems that achieve noise reduction during installation. For example, IHC Noise Mitigation Screen (IHC-NMS), Hydro Sound Damper (HSD) and bubble screens. To meet the noise limitation of 160 dB re 1 μPa2s SELss, a noise reduction of about 27 dB will be required. Current mitigation systems have not yet proven such a reduction in practice.
Meeting the noise restriction seems to become a challenge, even when combining different mitigation systems. Alternative piling methods are currently being explored, such as vibrating, which produce less underwater noise. However, these systems are not yet sufficiently tested or certified.
The full report can be read here.
Figure: A bubble screen around the piling site is one of the methods by which underwater noise can be attenuated. Source: Stichting De Noordzee