Protecting porpoises from harmful noises during the driving of foundation piles into the seabed remains a critical factor in the construction of offshore wind farms in the North and Baltic seas. In the past, the noise emission limit of 160 dB (decibels) at 750 m from the source was often exceeded by 10 dB, i.e. by a factor of 10. As a comparison, a jet plane emits around 140 dB at 30 m. To at least ensure that the legal limit is respected, eight builders and operators of offshore wind farms carried out a study last year, sponsored by the German Federal Environment Ministry, to evaluate various noise reduction systems.
Five noise protection systems were compared: a pipe with an internal bubble curtain from the company IHC, the fire hose method from Menck, a "Little Bubble Curtain" (small, staged bubble curtain) from Weyres, noise reduction shells with two bubble curtains from Weyres and the Hydro Sound Damper from the TU Braunschweig.
The pile-driving operations were evaluated by the Institute for Technical and Applied Physics of Oldenburg. In the prescribed radius of 750 m from the sound source, and in the 100 to 300 Hertz range, the range with the most energy, the sound damping effect was found to be between 0 and 10 dB. In the range up to around 5000 Hertz, to which sea mammals are particularly sensitive, the highest reduction effect was 25 dB.
The authors of the study call for further research to improve the understanding of the factors influencing sound during pile-driving operations for offshore wind turbine foundations.
Volker Buddensiek
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