Name: Jakob Tougaard
Address: Department of Bioscience, Aarhus University, DK-4000, Roskilde, Denmark
Underwater noise was recorded from the Wavestar wave energy converter; a full-scale hydraulic point absorber, placed on a jack-up rig on the Danish North Sea coast. Noise was recorded 25 m from the converter with an autonomous recording unit (10 Hz to 20 kHz bandwidth). Median sound pressure levels (Leq) in third-octave bands during operation of the converter were 106–109 dB re. 1 μPa in the range 125–250 Hz, 1–2 dB above ambient noise levels (statistically significant). Outside the range 125–250 Hz the noise from the converter was undetectable above the ambient noise. During start and stop of the converter a more powerful tone at 150 Hz (sound pressure level (Leq) 121–125 dB re 1 μPa) was easily detectable. This tone likely originated from the hydraulic pump which was used to lower the absorbers into the water and lift them out of the water at shutdown. Noise levels from the operating wave converter were so low that they would barely be audible to marine mammals and the likelihood of negative impact from the noise appears minimal. A likely explanation for the low noise emissions is the construction of the converter where all moving parts, except for the absorbers themselves, are placed above water on a jack-up rig. The results may thus not be directly transferable to other wave converter designs but do demonstrate that it is possible to harness wave energy without noise pollution to the marine environment.
This study was commissioned and funded by Wavestar A/S.
To characterize the underwater noise of the Wavestar wave energy converter
The noise recorded from the wave energy converter was barely detectable above ambient noise and in order to discuss possible negative effects of the converter noise on marine mammals it is relevant first to ask whether they are able to hear the noise above ambient at all. Third octave band levels in the bands 125–250 Hz are compared to ambient noise and audiograms for harbour seals and harbour porpoises, the two most common marine mammals in the North Sea. Harbour seals have good low frequency hearing and third-octave levels of the converter noise are well above the hearing threshold. Harbour seals are thus expected to be able to hear the converter noise, although the elevation in noise levels is so low (1–2 dB) that it is likely to be close to inaudible even at the close range where recordings were obtained. The exception is the noise generated by the hydraulic pump during lifting and lowering of the absorbers. This noise was 20–25 dB above ambient and should have been clearly audible to seals. In contrast to seals, harbour porpoises have poor low frequency hearing and it seems unlikely that the converter noise would have been audible to porpoises. Even the noise from the hydraulic pump is below the hearing threshold for porpoises and unlikely to have been audible.
Tougaard, J. (2015). Underwater Noise from a Wave Energy Converter Is Unlikely to Affect Marine Mammals. Plos One, 10(7), 1-7. https://tethys.pnnl.gov/publications/underwater-noise-wave-energy-converter-unlikely-affect-marine-mammals