Why noise mitigation?

The majority of offshore wind turbines in the North Sea is supported by monopile foundations. These large steel cylindrical piles are installed by driving them into the soil with around 3000 hammer strokes. Every stroke is accompanied by a shockwave of underwater noise, which forms an increasing threat to sea animals. Mammals like dolphins, porpoises and seals use echo-location to find prey and a partner. Increased levels of underwater noise have shown to disturb the behavior of marine mammals. Wind installation contractors therefore use noise mitigation systems to limit the noise levels and comply with regulations.

Image courtesy of A. Tsouvalas and A. Metrikine.

Initial idea

Current noise mitigation systems do not adequately reduce low-frequency noise. They have their best performance at high frequencies although current monopile sizes are producing mainly low-frequency noise. Therefore it is SeaState5's ambition to develop a low-frequency noise mitigation system. The main parts is our newly developed hydro-acoustic absorber mounted in a carrier system. The absorbers are semi-active elements that form the acoustical workhorse for reducing the acoustic energy. The carrier system holds the absorbers in place and consists of multiple vertical lines between seabed and buoys.

Absorber technology

Our technology is a hybrid design between a mechanical and electrical system. Combining the best of both worlds, we can tune the absorber to the exact (low) frequency of the shockwave and to the desired noise reduction level. The semi-active element is depth-independent which makes it suitable for deeper waters.

We combine several absorbers together to fully absorb the main shockwave that comes from the monopile. Because the size of the absorber is much smaller than the wavelengths contained in the shock wave, multiple elements work like one single barrier (as shown in the simulation gif).

Current status

After a successful indoor test-campaign, which showed the potential of our absorber design, we scaled up to open-water tests in the area of Leiden. We are now working on the first (single element) prototype for offshore use. More to follow...

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