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Reviewed by: Jennifer Amaral
Last Update: November 13th, 2020
At the 17th Ronald C. Baird Sea Grant Science Symposium Session #3: Sediment and Sound, Jennifer Amaral (Ph.D. student, URI Ocean Engineering and Lead Scientist & Engineer, Marine Acoustics, Inc.) explained the existing methods that are used to reduce the sounds of pile driving during the construction of offshore wind farms.
One common method is by using a bubble curtain (or a double bubble curtain) in which rings of bubbles are released at the seafloor that then rise to the surface resulting in a curtain that helps to reduce sound levels. This method is most effective in shallower waters and calm conditions whereas in areas of deeper water and faster currents, the bubbles are swept away before they reach the surface resulting in less of an effect.
According to the Discovery of Sound in the Sea (DOSITS) website, developed by the University of Rhode Island’s (URI) Graduate School of Oceanography (GSO) in partnership with Inspire Environmental of Newport, RI, bubble curtains can be deployed to reduce or mitigate impacts from high noise levels produced by pile-driving1. This is effective in reducing effects felt by species such as toothed whales; however, other marine species like seals and sea lions hear better at lower frequencies2. For this reason, other technologies and research are still necessary to minimize sound impacts.
Amaral suggests that other considerations may be plausible as the development of offshore wind farms increases. These include foundations and installation methods that could generate less sound such as floating foundations which are tethered to the seafloor with anchors. She also recommends the potential use of a combination of vibratory and impact pile driving in which the pile is vibrated into the sediment rather than being forced in. Another consideration is the use of quieter hammer technology which would reduce the sound levels generated directly at the source.
The last consideration that Amaral mentions is scheduling within the seasons. She explains that sound propagates differently in each season due to the temperature and resulting sound speeds in the water column; so, these seasonal differences could be used to help reduce sound levels during construction at further ranges. As an example, Amaral suggests that offshore wind construction is scheduled at times of year that have higher acoustic transmission loss so that the sound will not propagate as far out in range at such high levels.
The DOSITS website elaborates upon other mitigation strategies such as the avoidance of marine mammal habitats, detection of animals and modification of the sound-producing activities, modification or removal of the sound source, ramp-up of the sound signal intensity and sound screening. Read more about this here.