Noise barier with air bubbles for piling

Marine Performance Systems (MPS) develops a system with air bubbles that absorb and reduce noise. On 9 December 2019, the company won with its concept the Offshore Wind Innovation Challenge, organised by Innovatielink and Offshore Wind Innovators.

Driving wind turbine monopiles in the seafbed results in a large amount of noise underwater and in the seabed. The hammering by the pile driver can produce 170 to 180 dB of sound; that is comparable with the sound produced by a firing cannon. Should you swim nearby in the sea during the pile driving, you would turn instantly deaf.

Moreover, in the future monopile diameters will increase, and therefore there is a growing demand for technological solutions that reduce the underwater noise. After all, authorities around the North Sea and elsewhere enforce strict limits in order to minimise damage to nature, fish and sea mammals. For this purpose, Marine Performance Systems (MPS) develops a system with air bubbles that absorb and reduce noise.

When the company won with its concept the Offshore Wind Innovation Challenge, Van Oord and other large offshore contractors responded immediately and already in January, a month later, meetings were planned to discuss cooperation and carry out experiments in Wageningen at the Marin (Maritime Research Institute Netherlands).

There were three other finalists in the Challenge. Two of them were focussed on the design of new oyster cages that must enable the growth of oysters on the seabed near offshore wind farms in the North Sea. The fourth contestant was a company that uses sound to measure friction, tension and wear in jointed bolts and thus increases the efficiency of the maintenance management of wind turbines.

“We know a lot about bubbles”, says Pieter Kapteijn, Chief Technology Officer at MPS, in his presentation in the Oude Bibliotheek in Delft. He mentioned an additional reason for his evident selfconfidence; he found himself back on familiar territory because once in this building he was cramming to complete his study as civil engineer.

His company, with facilities in Rotterdam and Copenhagen, started in 2014 with the development of Air Lubrication, an air bubble drag reduction system for vessels. By emitting air bubbles along a vessel’s hull, the drag of a vessel can be reduced with 30 to 40 percent. In that case it is of paramount importance that the air bubbles are emitted in the boundary layer between
hull and water, that the bubble size is kept constant and that the bubbles can have their effect along the whole length of the vessel.

Kapteijn’s colleagues and co-founders at MPS, Frode Lundsteen Hansen and Fulko Roos, made contact with Van Oord, and that company encouraged them to work out the idea of a noise barrier with air bubbles. “Sound forces an air bubble to vibrate and that resonance supplies energy or heat to the bubble”, explains Kapteijn. “At the same time, that bubble spreads the sound pulses around to other bubbles, that will in turn absorb the vibration. It reflects in all directions. Thus, it is about absorbing – from vibration to heat – and scattering. That results in noise reduction.”

During the piling of the tubular monopile in the seabed the hammering of the pile driver causes sound waves that propagate from up to down and back up through the pipe. A part of the noise is radiated to the surrounding water, and another part causes a low frequency vibration in the seabed, that will also be reflected upwards to the water surface.

“For the noisiest part of the sound spectrum, between 30 and 3000 hertz, we introduce air bubbles with a specific size, 7 to 0,3 millimetre diameter. It is essential that we continuously tune the diameter of the bubbles to the changing frequencies, because when the monopile is driven deeper into the seabed, the sound frequencies change”, explains Kapteijn. “Noise mitigation is all about control.”

“We are now working on an active system in which continuously performed measurements and feedback result in the oscillators producing air bubbles with the optimal diameter. For instance, when the strongest noise is around 400 Hz, the oscillator will create bubbles with a diameter of 1 to 2 millimetres.”

“We know how it works in theory, we are all in agreement about that. Now we have reached the engineering phase in which we continue the development with other partners, such as Van Oord, towards a practical, working system. We are confident that this approach will offer a solution and mitigate the underwater sound explosions”, according to Kapteijn.

Text: Benno Boeters

Photo: Pieter Kapteijn (L) and Frode Lundsteen Hansen (right), resp. CTO and CEO of Marine Performance Systems

This article appeared in the 2020 January edition of Wind Energy Magazine

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