The Noise Mitigation System is a new system to reduce noise during offshore pile driving. Just like the muffler in an exhaust of a car, the Noise Mitigation System targets specific low frequencies that produce most noise. This method is what distinguishes the Noise Mitigation System from other systems.
Targeting the specific frequencies is achieved by special acoustic resonators. These resonators that look like inversed coffee cups are built in horizontal frames, which are folded out around the pile which has to be driven into the seabed. By bringing the Noise Mitigation System in the water, the cups get filled with air. The air pockets are dimensioned to absorb the sound frequencies that produce the most noise during offshore pile driving. As a result, the most harmful sound for mammals is dampened.
Another added value of the Noise Mitigation System is the fact that waves and currents have virtually no influence on the system. This is due to the open yet robust design. The blinds simply go up and down.
The Noise Mitigation System has been tested extensively in 2018. The results showed that the Noise Mitigation System targets the low frequencies. When used in combination with the Big Bubble Curtain, a system that reduces the higher peak frequencies, they meet the requirements set by the Dutch government.
The Big Bubble Curtain can best be described as a long hose with holes, in which air is pumped during pile driving. The air escapes through the holes and causes a curtain of bubbles. The Big Bubble Curtain not only mutes the high frequencies, but the hose also reduces the sound that emerges from the seabed further away from the piling location. The Noise Mitigation System is placed directly around the monopile, the Big Bubble Curtain is placed on the seabed at 100 meter around the place where pile driving takes place.
First commercial use
Because of the positive test results Van Oord started commercial use of the Noise Mitigation System earlier this year. During the installation of wind farms Borssele III and IV, the Noise Mitigation System was used with a Double Big Bubble Curtain. This was necessary due to the season in which the standards for noise emission were higher.
Van Oord used an improved version of the Noise Mitigation System. The company reduced the spacing between the resonator levels. Also, they expanded the Noise Mitigation System with ‘bubble diffusers’, to get as much air as possible into the resonators. Thus, the air in the water column of the Noise Mitigation System was maximised. These adjustments in the Noise Mitigation System enhanced the muting effect on the lower frequencies.
Result of the first commercial use
Altogether, the combination of the improved Noise Mitigation System and the addition of a Double Big Bubble Curtain resulted in a sound attenuation between 12 and 25 dB. Specifically, during pile driving at Borssele III and IV the combination produced a minimum of 157 and a maximum of 172 dB. Taking the season of pile driving into account, in which the standards are higher, the system amply met the Dutch standard for underwater noise.
During commissioning the Noise Mitigation System, it turned out that the predictability of the resonators appeared better than expected and that the muting of frequencies, could be predicted accurately. This indicates the system can be adapted to other circumstances, such as deeper water where other resonators may be necessary. This makes the Noise Mitigation System a suitable solution for future projects.
Van Oord intends to continue using the system and to improve it even further. The biggest challenge during operation at the Borssele sites was in the handling of the frame. The system had to have sufficient ballast to reach the seabed, but it should also not be too heavy that it could not be attached to a ship.
Noise Mitigation System; the future standard noise reduction method?
For Van Oord, the aim is for the Noise Mitigation System to become the standard method for noise reduction during offshore pile driving. Thanks to the first use at Borssele III and IV, knowledge has been accumulated to optimise the system. For example, improving the robustness of the system will contribute further silence to a broader frequency spectrum. To easily adapt the system to desired muting levels in specific frequency ranges, Van Oord is considering making the system more flexible. Besides this, the company is reflecting on the idea of placing more resonators in the water column.