Nevertheless, when renormalizing these data to the installed power, the message is different with Seliciclib CDK inhibitor a little lower average energy per MW used for the jacket foundation (665MJ/MW) than that for the monopile foundation (721MJ/MW). While jacket piling used less piling energy per MW, the average duration of piling per installed MW remained 26% higher with 55 minutes for a jacket and only 41 minutes for a monopile. However, an even better normalization would be obtained when standardising to the MW produced instead of the MW installed. Such standardisation would, however, be premature at this moment, since the wind farms are either operational for a short period of time (Belwind) or not yet operational at all (C-Power, phases 2 and 3). For both monopiling and jacket installed in the BPNS, cumulative SEL of 196dB re 1��Pa2s @750m was found.

Comparison with the available data for the Q7 wind farm [10] located in Dutch waters and featuring 4m diameter monopiles was possible after a renormalization at 750m. Some 13dB higher cumulative SEL was computed (209dB re 1��Pa2s). Unfortunately, other comparisons based on that variable are difficult to make since primary data are missing. Adapted from [11], zero to peak levels ranging between 185 and 199dB re 1��Pa for a pile diameter ranging between 3,3 and 4,7m were observed in various wind farms located in German and UK waters. These results are of the same order of magnitude and coherent with what was observed in the BPNS wind farms.Some of the levels observed here for both the monopile or jacket type foundations installation exceed the 185dB re 1��Pa permitted by the Belgian MSFD descriptor 11.

This indicates that future offshore wind farms will need to take mitigating measures during construction. Different methods exist [11, 14]. One of these is the air bubble curtain method [15] that could reduce the levels (both Lz?p and SEL) by about 14dB. These values were obtained inside a port and such technique remains to be validated at sea, with, for example, strong tidal current. A current of 1m/s, which is not uncommon for the BPNS, may indeed induce a drift of the bubble curtain of about 70m for a bottom depth of 20m [11]. New difficulties may arise when the sleeve may be in contact with the pile due to the tidal current. For bubble curtains, size of the bubble has an impact on sound insulation [14, 15].

A second method often preferred by the industry for sound isolation is the use of pile sleeves made from various material including foam or air [11, 14]. This last method can achieve a sound reduction of 20 to 25dB for low frequencies where the maximum noise is produced (Figure 1). These methods, if they were used in conjunction Dacomitinib with piling works, would have reduced the produced noise to levels below the Belgian MSFD requirements.