The German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety [EU2] has identified as potential methods of reducing underwater noise radiated from offshore wind turbines the following mitigation techniques, prototypes of which have been demonstrated: bubble screens, pile sleeves, hydrodynamic sound dampers, BEKA jacket, ring of fire hoses, cofferdam, gravity foundation, and suction bucket. A DOE report prepared for Congress [US5] adds several mitigation techniques to this list that are specific to pile driving operations during the construction phase. Considerable attention has been devoted to bubble screens because of cost and relative ease of implementation [US1, US2, US3, US4, EU1]. A modeling study of pile driving noise in shallow water (15 m to 30 m depth) [US6], based on analytical and numerical techniques similar to those described in Appendix C2, led to conclusions that bubble screens and compliant surface treatments reduce noise levels by approximately 10 dB, compared with massive dewatered cofferdams that reduce noise levels by approximately 20 dB. These aforementioned applications of bubble screens to noise abatement have relied either on the principle of acoustic impedance mismatch between gas and water, or the mass-spring resonance produced by the compliance of a bubble layer and the mass loading of the surrounding water. Very recently it has been demonstrated in lake experiments that exploiting bubble resonances and the associated losses can reduce noise levels by more than 40 dB at frequencies of several hundred Hz radiated from sound sources near the water surface, and not in contact with the bottom [US7].
US.1
"Acoustic wave propagation in air-bubble curtains in water-Parts I (History and theory) and II (Field experiment)"
S. N. Domenico
Geophysics 47, 345-375 (1982).
US.2
"Development of an air bubble curtain to reduce underwater noise of percussive piling"
B. Wursig. C. R. Greene, Jr., and T. A. Jefferson
Mar. Env. Res. 49, 79-93 (2000).
US.3
"Mitigating seismic noise with an acoustic blanket the promise and the challenge"
W. S. Ross, P. J. Lee, S. E. Heiney, J. V. Young, E. N. Drake, R. Tengham, and A. Stenzel
Leading Edge March, 303-313 (2005).
US.4
"Underwater Acoustic Measurements from Washington State Ferries 2006 Mukilteo FerryTerminal Test Pile Project"
A. MacGillivray, E. Ziegler, and J. Laughlin
Technical report prepared by JASCO Research, Ltd (Victoria, British Columbia, Canada) for
Washington State Ferries and Washington State Department of Transportation
March 6, 2007 (Version 2)
US.5
"Report to Congress on the Potential Environmental Effects of Marine and Hydrokinetic Energy Technologies"
Department of Energy, Wind and Hydropower Technologies Program
Dec. 2009
US.6
"Mitigation of Underwater Pile Driving Noise During Offshore Construction: Final Report"
A. Stokes, K. Cockrell, J. Wilson, D. Davis, D. Warwick
Applied Physical Sciences Corp., Groton, CT 06340
January 27, 2010
US.7
"Mitigation of Low-Frequency Underwater Sound Using Large Encapsulated Bubbles and Freely-Rising Bubble Clouds"
K. M. Lee, K. T. Hinojosa, M. S. Wochner, T. F. Argo IV, and P. S. Wilson
Proceedings of Meetings on Acoustics, Vol. 12, 070006
2012
EU.1
"The use of an air bubble curtain to reduce the received sound levels for harbor porpoises (Phocoena phocoena)"
Klaus Luckea, Paul A. Lepper, Marie-Anne Blanchet, Ursula Siebert
J. Acoust. Soc. Am. 3406, vol. 130
November 2011
EU.2
"Noise Mitigation Measures and Low-Noise Foundation Concepts-State of the Art"
Tobias Verfuß, Projektträger Jülich
BfN-Symposium Towards an Environmentally Sound Offshore Wind Energy Deployment
Stralsund, Germany
January 25, 2012