This reports reviews the possibilities to mitigate the noise arising from pile driving for the construction of offshore windfarms. It analyses mitigation measures which have been applied in related projects and assesses their applicability to offshore pile driving. Based on this, suggestions for new mitigation measures are made. The report identifies two methods which are promising to be both applicable and effective in reducing underwater noise arising from offshore pile driving. Both methods are considered to be generally compatible to the working processes at sea. Although further engineering work would be needed, they could be brought into practice within a few months.
Based on measurements of offshore pile driving, noise emissions of large piles are estimated to reach peak levels of 201-205 dB re 1μPa and sound exposure levels SEL of 175-178 dB re 1μPa at a distance of 500 m. Noise produced from offshore pile driving may be harmful and disturbing to marine wildlife. From literature data it is inferred, that physical impairment may occur above levels (SEL) of 180 dB re 1μPa. As a first proxy for disturbance of marine mammals, the report refers to a level of 140 dB re 1μPa.
Mitigation measures have so far mainly focussed on bubble curtains, which are made up from air bubbles released at the seafloor around a source of noise. Bubble curtains may efficiently reduce underwater noise but it is considered to be impossible to install bubble curtains in the offshore environment at great water depths and tidal currents. The main reason for this is the slow ascent rate of the bubbles resulting in large installation accounting for currents and water depths.
Attempts to mitigate noise from pile driving by prolonging the duration of the blows of the piling procedure through modification of the pile driver were rejected at this stage. As a prolongation of the blows may result in a loss of piling energy this may impair the success of the piling. However, further research on this method is recommended.
Two new methods are described in detail which are considered to be effective and practicable to construct a permanent noise barrier around the piles made up from foam or air: First, an inflatable piling sleeve which can be permanently mounted below the piling gate at the construction platform. The sleeve is meant to be released after insertion of the pile into the piling gate and inflated to a 50 mm layer of air during the piling operation. The sleeve is expected to reach an attenuation of 20 dB broadband. Second, a telescopic double-wall steel tube with an interspace filled with foam. The tube is constructed in several segments to reduce the height when released on the seafloor underneath the piling gate. The pile is inserted into the tube which is lifted to full length during the piling operation. A 100 mm foam layer is calculated to reach an attenuation of 15 dB broadband. Both methods are considered to be compatible to the piling process and costs are roughly estimated to reach about 20,000 € per pile in the inflatable sleeve and about 25,000 € per pile in the telescopic tube. The construction of the telescopic tube are lower than in the inflatable sleeve but overall costs are expected to be higher as handling at sea demands some extra time of the construction process. In this respect, there appears to be an advantage of the inflatable sleeve which would result in very little interference in the piling process. The attenuation from these methods is considered to be high enough to achieve a substantial reduction of the impacts on marine wildlife. Calculated radii of physical damage may be reduced by more than 90 % and radii of disturbance by two-third.
Suggestions for further investigations and towards the development of a programme for reducing underwater noise from pile driving are presented.
It is concluded, that noise mitigation measures offer good opportunities to reduce the impacts of underwater noise arising from the construction of offshore windfarms. For the offshore industry, noise mitigation may prove to be beneficial as their application may allow construction works in areas and times when restrictions are needed to protect sensitive species.