The double bubble keeping sea life out of trouble

Protecting sea life

Hammering the foundations of wind turbines into the sea bed causes noise, and surrounding sea life can be severely disturbed, even harmed, if no precautions are taken. Underwater species, such as the harbour porpoise, are dependent on sounds to navigate, and the installation noise chases them away over very long distances.

How does it work?

The concept of bubble curtains is fairly simple: a large hose is placed on the sea bed, forming a large circle surrounding the monopile that is to be installed. Many compressors pump air into this hose, generating bubbles through small holes, until a solid bubbly curtain has been created.

In reality, however, numerous variables may affect a bubble curtain and its performance, each with a significant risk of lowering the curtain’s effectiveness. Hence, by recreating such bubble curtains in their basin, MARIN seeks, together with the project partners, to understand these variables and the impacts they might have on the bubbles’ efficiency.

Such variables include bubble distribution, bubble size, and the fraction between air and water. Other parameters like the current, the mass flow of air going into the hose, the diameter of the circular bubble curtain, and the porousness of the hose, are also included in different stages of the research.

Additionally, research regarding the sound propagation of noise from piling through water and soil will also be targeted, as well as the creation of useful bubble measurement technology and the establishment of improved understandings of current noise levels and frequency ranges.

Essentially, the research wants to establish “what do you have to tune about your bubble curtain to ensure that the noise mitigation is enough to install offshore wind farms, without causing harm to the sea life,” as Linda Kemp summarises.

What’s unique?

Unsurprisingly, the extensiveness of the project is unique and also the essential collaboration between various expertises of industry and research institutes. Though the literature on similar experiments to mitigate underwater noises exists, this often targets near-field systems, rather than far-field systems. This is a significant distinction.

With near-field noise mitigation the noise is mitigated directly at its source. With far-field noise mitigation, there is a certain distance between sound source and applied mitigation measures.

Since a certain distance is kept between the monopile and the mitigation measure - in this case, the bubble curtain. The ‘near’ systems generally focus on the lower frequencies, and are less efficient in reducing the sound radiated from the into the sediment and then propagated into the water. Yet research on far-field noise mitigation barely exists.

For Linda Kemp specifically, the whole sound topic is quite new: “It’s very cool that we as MARIN have knowledge about water and waves and are doing tests, and are incorporating such a new topic, namely noise in combination with bubbles, in our basins. It was quite challenging, but that’s also the nice part, because you’re developing something new.”

Add to this a measurement system that was built from scratch, and a ground-breaking project is born - one that is, Linda Kemp hopes, “future-proof”.

Cooperation is key

Apart from its professional excitement, the project is also well-received on a personal level. As is in line with GROW’s beliefs, namely that cooperation is necessary to provide big solutions to the problems at hand, Bubbles JIP (Joint Industry Project) is the result of a close collaboration with industry partners and research institutes. For example, whereas TNO provides its expertise in acoustics, TU Delft specialises in process system modelling and contributes to the acoustical research as well. And do not forget MARIN’s hydrodynamic expertise.

Moreover, industry partners provide knowledge about current practices and environmental conditions. Together, this results in interesting conversations about what the industry wants to do, what they want to know, and what can be done to facilitate that.

Linda Kemp stresses how fun and inspiring the project becomes as a result. “Developing something new, together, to protect sea life and to improve ocean sustainability is incredibly motivating, and all those involved now visit the basin to watch the magic of the bubbles.”

What happens next?

As the Bubbles JIP project page outlines, a best practice report will be drawn up to offer information and advice to industry partners. Not only will this equip the offshore wind industry with the knowledge and abilities to protect marine life, it also reduces the legal, financial, and technological risks that are often posed due to incorrect or excessive usage of bubble curtains.

A proper execution of this project will, as Linda Kemp highlights, “help to install a further amount of wind turbines, with a negligible impact on the environment” and support the industry in making the energy transition happen. This, clearly, is another revolutionary step towards a better world.