A new Dutch noise reduction system is able to keep monopile installation noise below the German legal limit. This is good news for sea mammals like whales and seals.
By Kent Krøyer
A fully working underwater noise mitigation system has now been demonstrated in the North Sea by the Dutch company IHC Offshore Systems, part of IHC Merwede, a known supplier of large hydraulic hammers for wind turbine monopile installation.
The patented system, called NMS-6900 (Noise Mitigation System), consists largely of a double-wall steel casing which completely embraces the submerged part of the monopile as it is hammered firmly into the sea floor.
Between the casing and the monopile there is a curtain of rising air bubbles that helps to disrupt the transmission of sound to the ocean. The bubble curtain is maintained by an air compressor.
The 90-foot long hearing protection tube is in place around the monopile which is then ready to be hammered into the seabed. The water depth at Riffgat is 20-27 meters. Photo: Riffgat Offshore Windpark.
The 360 ton NMS-6900 rests on a mudmat flange on the sea floor and is partly supported by the monopile. This gives a stable position during piling.
The system is able to adapt to differently sized monopiles through a variable guiding system which keeps the monopile concentrically aligned within the outer casing and is able to follow a tapered section. The variable guiding also isolates the mechanical contact between monopile and casing.
Meets the German limits
The system was used in the summer of 2012 during the installation of the 30 monopiles for the Borkum Riffgat wind farm on the coast of Germany. The water depth was approximately 25 meters.
“We have proved that the system works. The underwater noise level has been monitored by independent agencies, and the sound exposure level (SEL) was well below 160 dB at all frequencies,” says business and development manager Henk van Vessem of IHC Merwede.
This means that the noise levels fulfilled the German legal maximum underwater noise limit of 160 dB. The official report was not published at the time of the interview.
Henk van Vessem believes that the result is a breakthrough in the offshore wind farms installations market, which will be expanding in Northern Europe in the near future.
“This was our first commercial system. Now we are working to adjust the construction to make use of the experience we have gained to improve the system,” he says.
Adaptable to different foundations
IHC Offshore Systems is also working on noise mitigation solutions for pre-piling of jacket foundations. The Noise Mitigation System is integrated in a pre-piling template, the so called TINS. Using the system gives a technical advantage as it is also used for precision aligning and guiding of the jacket piles during the installation process.
“In some applications, such as in a jacket foundation, the noise reduction system can speed up the installation, since the system solves several adjustment problems, such as template adjustment, stick-up length adjustment, and inclination of all piles simultaneously,” says Henk van Vessem.