Both operation and construction of offshore wind turbines induce underwater noise. While it is not yet clear if operating noise affects the behavior of marine animals, construction noise is considered crucial. Common foundation techniques require to drive steel tubes up to 30 m into the seabed. In general, hydraulic pile hammers are used for this purpose. During the erection of a 3.5 m monopile, peak sound pressure levels of more than 180 dB re 1 μPa have been measured at 1 km distance from the pile driver. These levels are potentially harmful to marine mammals like harbor porpoises and induce flee reactions in a large area. Due to larger piles requiring higher blow energies, even higher levels are expected in future projects. Hence, noise reduction is mandatory. Within a joint research project, a concept of practicable noise reducing methods is derived from measured results and numerical simulations on construction noise of offshore wind turbines. Theoretical background and technical realizations are discussed in this paper. Furthermore, results of numerical simulations and of scaled and near full scale experiments are shown.
There are two main approaches:
- Adjusting the parameters of the pile stroke,
- Use of sound barriers.
One of the key parameters of method 1 is stroke duration. Prolonging the impulse not only reduces the sound level, but also shifts the maximum of the acoustic spectrum to lower frequencies, which are less harmful to marine mammals. Vibration pile driving, where applicable, also considerably reduces the sound level with respect to impulse pile driving, in particular the peak level. Underwater noise measurements of vibrohammer are compared to impulse hammer. Method 2 includes various techniques like the well-known bubble curtain, but also noise barriers based on sound impedance mismatch between the barrier material and water. Both method 1 and 2 are mutually independent; when used in combination, their efficiency simply adds up in terms of dB numbers and a very high degree of noise reduction is achieved.
The research project is supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU).