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Underwater Noise from a Wave Energy Converter Is Unlikely to Affect Marine Mammals.

Tougaard J - PLoS ONE (2015)

Bottom Line: 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).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.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioscience, Aarhus University, DK-4000, Roskilde, Denmark.

ABSTRACT
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.

No MeSH data available.


Frequency spectra of noise recorded close to the wave energy converter with the converter stopped and in different stages of operation.Noise levels are expressed as median (L50) and upper and lower 5% percentile (L5 and L95) sound pressure levels in third-octave bands. a) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded with the wave energy converter running (red dashed line; percentiles: red fine dashed lines). b) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded during start and stop of the converter (red dashed line; percentiles: red fine dashed lines). Self-noise of the recorder was below 80 dB re. 1 μPa in all third-octave bands. c) Results of pair-wise Mann-Whitney tests (p-value) for all third-octave bands comparing converter running to ambient noise. Broken line indicates 5% significance level, adjusted for multiple comparisons (Dunn-Sidak correction). d) Same as c), but comparing start and stop of converter to ambient noise.
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pone.0132391.g002: Frequency spectra of noise recorded close to the wave energy converter with the converter stopped and in different stages of operation.Noise levels are expressed as median (L50) and upper and lower 5% percentile (L5 and L95) sound pressure levels in third-octave bands. a) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded with the wave energy converter running (red dashed line; percentiles: red fine dashed lines). b) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded during start and stop of the converter (red dashed line; percentiles: red fine dashed lines). Self-noise of the recorder was below 80 dB re. 1 μPa in all third-octave bands. c) Results of pair-wise Mann-Whitney tests (p-value) for all third-octave bands comparing converter running to ambient noise. Broken line indicates 5% significance level, adjusted for multiple comparisons (Dunn-Sidak correction). d) Same as c), but comparing start and stop of converter to ambient noise.

Mentions: Median broad band (10 Hz–20 kHz) sound pressure level (Leq) was 123 dB re. 1 μPa, irrespective of status of the wave energy converter (stopped, running or starting/stopping). The recorded sequence was subdivided into 10 s periods (186, 84 and 72 periods with the converter running, stopped and starting/stopping, respectively) and one-third octave levels were computed for each segment by means of the Matlab-function filtbank (Christophe Couvreur, Faculte Polytechnique de Mons, Belgium). Median (L50) and upper and lower 5% percentiles (L5 and L95) of third-octave sound pressure levels are shown in Fig 2A and 2B for the three states of the converter (running, stopped and start/stop procedure). Levels were compared pairwise within each 1/3 octave band by Mann-Whitney’s U-test to test for systematic differences among the three states (Fig 2C and 2D). A significance level α of 5% was used, but adjusted for multiple comparisons by the method of Dunn-Sidak, by which an adjusted value for α is obtained: α′ = 1 − 1(1 − α)1/n, where n is the number of pairwise comparisons (third-octave bands). An example of the noise from the running converter is found in the supplementary material as S1 Audio.


Underwater Noise from a Wave Energy Converter Is Unlikely to Affect Marine Mammals.

Tougaard J - PLoS ONE (2015)

Frequency spectra of noise recorded close to the wave energy converter with the converter stopped and in different stages of operation.Noise levels are expressed as median (L50) and upper and lower 5% percentile (L5 and L95) sound pressure levels in third-octave bands. a) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded with the wave energy converter running (red dashed line; percentiles: red fine dashed lines). b) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded during start and stop of the converter (red dashed line; percentiles: red fine dashed lines). Self-noise of the recorder was below 80 dB re. 1 μPa in all third-octave bands. c) Results of pair-wise Mann-Whitney tests (p-value) for all third-octave bands comparing converter running to ambient noise. Broken line indicates 5% significance level, adjusted for multiple comparisons (Dunn-Sidak correction). d) Same as c), but comparing start and stop of converter to ambient noise.
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pone.0132391.g002: Frequency spectra of noise recorded close to the wave energy converter with the converter stopped and in different stages of operation.Noise levels are expressed as median (L50) and upper and lower 5% percentile (L5 and L95) sound pressure levels in third-octave bands. a) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded with the wave energy converter running (red dashed line; percentiles: red fine dashed lines). b) Comparison between ambient noise spectrum (converter stopped, solid blue line; percentiles: dot-dashed blue lines) and noise spectrum recorded during start and stop of the converter (red dashed line; percentiles: red fine dashed lines). Self-noise of the recorder was below 80 dB re. 1 μPa in all third-octave bands. c) Results of pair-wise Mann-Whitney tests (p-value) for all third-octave bands comparing converter running to ambient noise. Broken line indicates 5% significance level, adjusted for multiple comparisons (Dunn-Sidak correction). d) Same as c), but comparing start and stop of converter to ambient noise.
Mentions: Median broad band (10 Hz–20 kHz) sound pressure level (Leq) was 123 dB re. 1 μPa, irrespective of status of the wave energy converter (stopped, running or starting/stopping). The recorded sequence was subdivided into 10 s periods (186, 84 and 72 periods with the converter running, stopped and starting/stopping, respectively) and one-third octave levels were computed for each segment by means of the Matlab-function filtbank (Christophe Couvreur, Faculte Polytechnique de Mons, Belgium). Median (L50) and upper and lower 5% percentiles (L5 and L95) of third-octave sound pressure levels are shown in Fig 2A and 2B for the three states of the converter (running, stopped and start/stop procedure). Levels were compared pairwise within each 1/3 octave band by Mann-Whitney’s U-test to test for systematic differences among the three states (Fig 2C and 2D). A significance level α of 5% was used, but adjusted for multiple comparisons by the method of Dunn-Sidak, by which an adjusted value for α is obtained: α′ = 1 − 1(1 − α)1/n, where n is the number of pairwise comparisons (third-octave bands). An example of the noise from the running converter is found in the supplementary material as S1 Audio.

Bottom Line: 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).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.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioscience, Aarhus University, DK-4000, Roskilde, Denmark.

ABSTRACT
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.

No MeSH data available.