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 operations at the FINO 3 offshore wind research platform, where the water depth was approximately 25 m (Matuschek & Betke 2009). To compensate for bubble drift due to currents, the bubble ring was installed at an extended radius of 70 m around the pile. While this measure resulted in sound pressure level reductions of 7-12 dB, the degree of attenuation varied with direction indicating that the integrity of the bubble curtain was compromised due to current in some places (Matuschek & Betke 2009). Furthermore, for a bubble ring of this size, with a circumference exceeding hundreds of meters, much higher-capacity air compressors, energy supplies and related equipment would be needed (Nehls et al. 2007). Multistage air bubble curtains – The complication of greater depth and current speeds was encountered during pile installation by the California Department of Transportation, where local conditions rendered existing unconfined air curtain systems ineffective. As a result, engineers developed several solutions to overcome these challenges, including the use of either confined or multistage air bubble curtains. Multistage air bubble curtains involve a vertically stacked array of perforated rings placed around the pile at different depths. Although the bubbles are still subject to horizontal movement from currents, the idea here is that the ring above would generate additional bubbles, in this way ensuring that a uniform curtain of air bubbles is maintained around the pile. While more effective than the original air bubble curtain design in deeper waters, this type of apparatus would still be limited to currents less than 1 knot (Christopherson & Wilson 2002). Confined air bubble curtains – In areas with currents or tidal forces exceeding 1 knot, the use of confined or enclosed air bubble curtains would likely be required. For smaller piles, a large, hollow cylinder placed around the pile with the air bubble ring placed within it is an effective means of preventing currents from sweeping the bubbles away, while still ensuring sound attenuation. However, for larger piles, specially manufactured confined air curtain systems might be required. For example, during the pile installation demonstration project for the San Francisco-Oakland Bay Bridge, a thick fabric mantle was constructed to confine the air bubble curtain, resulting in increased sound attenuation over the unconfined system (Illingworth & Rodkin Inc. 2001). Another existing model is the Gunderboom® Sound Attenuation System, comprised of two layers of fabric between which air is bubbled. While this proprietary system has proven highly effective at attenuating pile driving noise in the field in up to 3-knot currents, Aquatic Research and Development Section 22
Offshore wind power projects in the Great Lakes: Background and science considerations for fish and fish habitat the high cost of this alternative might be prohibitive for some developers (PND Engineering 2005). Although confined air bubble curtain systems have been successfully applied in harbour works where local wave and current conditions are relatively benign, it remains to be seen whether these options would be viable for offshore pile driving where conditions are generally harsher. The use of air bubble curtains to attenuate pile driving sound during offshore wind power project installation in the marine environment has thus far been limited. However, the success of the air bubble curtain system during pile driving at the FINO 3 offshore wind research platform in the North Sea indicates that this strategy should be further explored, field-tested and considered for use in other offshore wind development projects—including those proposed within the Great Lakes. Owing to the performance limitations of current bubble curtain technologies, project sites that are at depths of less than 25 m would be best suited to this approach. As this will likely be the depth range of most Great Lakes projects, air curtain systems have the potential to be used during pile driving operations for turbine installation within the Great Lakes. Recognizing that improvements continue to be made to enclosed air bubble curtain technologies, it has recently been acknowledged that they could be soon be applicable for pile driving in deeper water (Bailey et al. 2010). Isolation casings and cofferdams – Other devices that exploit the density differential between air and water to decouple sound wave transmission from a hammered pile to the surrounding water column are isolation casings and cofferdams. Isolation casings are often hollow cylindrical piles slightly larger in diameter than the pile being driven, which are temporarily installed and dewatered to create an air space around the monopile. Isolation piles were used during construction of the Benicia Bridge in California, and successfully reduced sound pressure levels by 20-25 dB (Illingworth & Rodkin Inc. 2001). Alternatively, to isolate larger piles or project areas from the water column, cofferdams can be temporarily constructed around the work site (Fig. 2). These are commonly fabricated from sheet piling, and like isolation casings are most effective when the water inside is pumped out leaving a large air space between the exposed pile and the water column. In both cases, the sound waves emanating from the hammered pile would have to pass through the air space before reaching the surrounding water, which interferes with sound propagation. Aquatic Research and Development Section 23
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