Abstract
During the construction of the OWEZ wind farm facility 8 - 18 km off the Dutch coast of Egmond aan Zee the underwater emitted sound signatures of 6 out of 36 monopile driving cases were measured and analysed. The broadband Source Levels estimates measured over the highest amplitude of three pile driving cases matched remarkably well and were between 242 and 249 dB re 1 μPa (rms) with the energy mainly peaking between 150 and 500 Hz. The attenuation of the sound exposures of these three pile driving cases show also consistency and was in the range of 21 to 23 log (Distance). The analysis of the sound spectra expressed a low frequency cut-off, indicating the Source Level results could be an underestimate. The present results are a close match with other projects of similar physical dimensions. The measured Source Levels are in the category of very heavy impulsive sound exposures comparable with large seismic airgun arrays and about 40 times higher than the firing of a single mid-sized airgun. Sound pressure levels were plotted against the trends of the progressive developed hammering energy and penetration depth of the piles. In terms of applied hammering energy, the six selected cases were representative for the hammering of the 36 monopiles with the highest energy case as one of the six measured hammering cases. The energy relation was only recognised in a single distance range of only one hammering case, in the very first part of the hammering cycle, but on average the progressive energy trend in the hammering cycle was not expressed in sound pressure levels and levels measured at a fixed distance at the start and end of the hammering cycle were similar in most cases. A diffracted component of the seabed-borne seismic shockwave arrived just in front of the seaborne received signal of the hammering blow. This seismic component was found proportional with the applied energy and/or penetration depth (+12 and +8 dB) at both low and nominal energy conditions. According the available guidelines on safe gradients for cetaceans the average sound exposure of 247 dB re 1μPa and the minimum/maximum regression (21/23 Log Distance) would cause a temporary threshold shift to the hearing sense of harbour porpoise within a radius of 3300-7200 m from the source, and a permanent shift when present within a radius of 600-1100 m. Behaviour studies on harbour porpoise during the construction of a similar type of construction in the North Sea (Horns Reef) showed that an effect was found at 11 km from the construction site (Tougaard et al. 2005). In advance of the hammering of the monopiles measures were taken by the contractor/commissioner to deter marine mammals from the exposed area.