Offshore Wind Power Projects in the Great Lakes - Ministry of ...

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Offshore wind power projects in the Great Lakes: Background and science considerations for fish and fish habitat Although both methods are effective at reducing sound levels from pile driving, providing at least as great a sound reduction as air bubble curtains (Caltrans 2009), the larger air space created by cofferdams gives these devices greater sound attenuation value than the smaller isolation casings. Cofferdams have been used extensively during piling works for the San Francisco Bay bridge with good success in reducing potentially injurious sound levels, although site depths were generally less than 15 m. Depending on the additional effort required to install and dewater these temporary structures, particularly for projects with large numbers of individual monopiles or in greater depths, this option might not be well suited for use during offshore wind turbine installation. Here again field-testing under specific site conditions would be required to assess the feasibility and effectiveness of such devices in the Great Lakes. pile driving hammer monopile de-watered cofferdam Figure 2. Cofferdam concept. Cofferdams constructed of sheet piling can be temporarily installed around larger piles or project areas to isolate pile driving sound waves from the water column, reducing the noise injury potential to fish. De-watered cofferdams achieve greater sound pressure level reductions owing to the difference in sound wave propagation between air and water. Reducing noise levels at the source As discussed above, mitigation options to reduce the effects of pile driving noise on fish include warning or deterrence techniques to keep fish away from the zone of impact, or devices to reduce the propagation of sound pulses into the water column to minimize the zone of impact. They also Aquatic Research and Development Section 24
Offshore wind power projects in the Great Lakes: Background and science considerations for fish and fish habitat include less noisy methods of pile installation or devices that reduce sound levels directly at the pile driving source. Vibropiling – A technique of vibropiling has recently emerged as an effective means of reducing the noise levels associated with pile driving in the aquatic environment. In this case vibratory hammers rather than impact hammers are used to deliver light, rapid beats to the pile to make it vibrate. These vibrations cause the surrounding sediment at the base to liquefy, allowing the pile to sink in. Vibratory hammers produce sound energy that is generally 10 to 20 dB lower than impact hammers, although the accumulated sound exposure level from vibropiling could be higher because this technique requires considerably more time to install the pile (Caltrans 2009). The reduced sound pressure levels generated by vibropiling might also reduce impacts on nearby fish. For example, caged brown trout held in the field as close as 25 m to the source were found to elicit no behavioural reactions to vibropiling activity (Nedwell et al. 2003), indicating that this method could minimize effects to certain fish species. Unfortunately, vibropiling is only suited for small or medium-sized piles, specific substrate types, and relatively shallow depths. In addition, impact pile driving is still required initially in order to “proof” the piles and test their load bearing capacity. As a result, while it should still be considered as an option in appropriate field conditions, vibropiling might need further development before it is a viable option for noise mitigation during offshore pile driving. Where vibropiling is not feasible, pre-drilling or water-jetting in the piles could help to attenuate some of the noise generated during impact pile driving by loosening the substrate and allowing the pile to be driven further with each blow (PND Engineering 2005). In this way the number of strikes required to install a pile could potentially be reduced, thereby reducing the temporal extent of hydroacoustic impacts. However, these options have yet to be employed extensively in offshore construction projects, and could act to further increase turbidity at the pile driving site, and thereby impact fish in other ways. Press-in piling – Another technique that could replace conventional impact pile driving is the use of a press-in piling machine. These are self-contained devices that use static forces i.e. the leverage of a previously installed pile, to install the monopiles. Because no impulsive type noises are created using this technique, underwater sound levels are expected to be significantly less Aquatic Research and Development Section 25
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