Measurement and interpretation of underwater noise during construction and operation of offshore windfarms in UK waters.

Open Access Report 2007

Subacoustech

Subacoustech Ltd has been contracted by The Crown Estate on behalf of the Collaborative Offshore Wind Research Into the Environment (COWRIE) to measure the underwater noise generated by offshore windfarms during construction and operation. The purpose of the programme of measurements is to provide information which will aid the estimation and minimisation of the impact of noise during the lifecycle (construction, operation and decommissioning) of windfarms. Previous reports have established that pile driving during windfarm construction creates high levels of underwater noise. Injury of marine species could be caused by the pile driving noise at distances of the order of 100 metres, and behavioural effects at ranges of the order of 10 kilometres or more. To further document the noise levels created during pile driving, further measurements are presented herein, taken during pile driving on five windfarms, at North Hoyle, Scroby Sands, Kentish Flats, Barrow and Burbo Bank. The measurements indicate that the Source Levels of these five pile driving operations varied between 243 and 257 dB re 1 Pa @ 1 metre, having an average value of 250 dB re 1 Pa @ 1 metre. The Transmission Losses were characterised by values of geometric loss factor N of 17 to 21, and absorption factor of 0.0003 to 0.0047 dB/m. The measurements are analysed in the dBht metric, which weights noise according to the hearing sensitivity of marine species of animals, and thus indicates the likely loudness of the noise. The concept of noise dose, used to estimate the cumulative effect of noise on humans, may also be extended to marine mammals by this means. On this basis, a method which is relatively simple to calculate and apply is proposed for estimating areas around a pile driving operation within which the two key auditory effects of noise will occur. This method may be summarised as “Provided animals are free to flee the noise, those within the area bounded by the 90 dBht level contour will strongly avoid the noise. Animals within the area bounded by the 130 dBht level contour may suffer injury or permanent damage to hearing”. Since the injury ranges indicated by the measurements do not exceed a few hundred metres, they indicate that observation by marine mammal observers and soft start procedures might be effective in reducing these effects of the noise. Measurements are also presented of noise during the operation of the North Hoyle, Scroby Sands, Kentish Flats and Barrow offshore windfarms. In general, the level of noise created by operational windfarms was found to be very low and no evidence was found of noise levels that might have the capacity to cause marine animals to avoid the area. The environment of a windfarm was found to be on average about 2 dB noisier for fish, and no noisier for marine mammals, than the surrounding area. This is no more than variations which might be encountered by these animals during their normal course of activity.

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