A ''passive pickup'' device to clean plastics from the ocean is to face one of its toughest challenges yet: force 8 or 9 gales and seas to match.
The idea, first floated by Dutchman Boyan Slat, is that the current could deliver waste plastics trapped by the ocean gyres to a collection mechanism much more efficiently than going out to fish for it.
However Joost Dubois of the Ocean Cleanup project told MJ that the next step is to test a ‘near full-scale’ 100m segment of the collection barrier “for real-life survivability”. It will have an 8 to 12 month deployment in the North Sea with its gale force wind and wave conditions, “which is exactly the point of putting it out there” he explained.
The boom, a 1.5m diameter floating tube with a 1.3m skirt, will be anchored about 12nm off the Hague at Scheveningen this summer: he pointed out that “a 'regular' North Sea storm with its very short and steep chops, models for an event that would occur only once in 50 years on the Pacific with its longer and slower waves”. It may come sooner than expected: although normally speaking extreme weather is only expected in autumn, July last year turned out to be a particularly bad month.
However, he remains confident: “It’s been designed to meet offshore industry requirements... and there’s plenty of redundancy in the anchors.”
Despite this, it’s still a tough job to bring the various requirements together. Mark Paalvast, one of the project’s hydrodynamic analysts explained that the final boom will be over 100km long, but this makes for an “engineering paradox”.
“We need a system to perfectly follow the waves,” he said, but “over such a long span, you get huge loads, and these huge loads make the system’s behaviour very ‘stiff’.”
Therefore recently a 1:18 scale model of the boom was taken to Marin’s testing tanks to put it through combinations of waves and currents “to find the limits of the system” he explained.
These results are allowing different structural elements to be refined to more closely match the wave profiles, and are being fed back into the necessary numerical flow modelling: this is important because there are countercycles inside the big whorls, “and if you miss-orient the array then it won’t work” explained Mr Dubois. However, it’s relying on not just one, but three mathematical models, varying in scale.
While the macroscale and mesoscale models look at it from a wider perspective, the microscale model is barrier specific said his colleague, Bruno Sainte-Rose. In simpler terms, where the macro- and mesoscale models answer questions on location and shape of the array, the microscale model is designed to find out how the barrier interacts with the vertical distribution of plastic – necessary for analysing how the material can be most efficiently captured.
At the same time, the Ocean Cleanup project has itself been gathering real-world interest: the plastic pollution problem in the waters around Tsushima island, located between Japan and South Korea, is climbing by approximately one cubic metre per person per year and the local government is looking for innovative solutions to stop it reaching the island’s shores.
Expected to operate for at least two years, the system will span 2,000m, becoming the longest floating structure ever deployed in the ocean (beating the current record of 1,000m held by the Tokyo Mega-Float).
However, time is of the essence: “There are two messages” emerging from the largest-ever sampling of the Great Pacific Garbage Patch last year, said Mr Dubois.
The expedition saw 30 ships with sampling nets crossing the rubbish-trapping gyre: although he was careful to say these are “preliminary findings - not yet confirmed” the results underline the need to take action, and quickly.
“Firstly, initial results suggest there is more plastic out there than is currently expected from previous research,” he said. “Secondly it seems most of the plastic – taken by volume - is present in larger pieces....” This he realises is in contrast with some other, earlier research and is keen to say the analysis “is not yet complete” but will be presented in a peer-reviewed scientific article by the end of the year.He added: “The findings from our Mega Expedition could be perceived as controversial by the oceanic research community, as they are not in line with today's general understanding of the problem.”
However dramatic the results, the aim was not to be contentious: “Primarily we did it to make sure we are not preparing a ‘mission impossible’. It’s not good news for the oceans but a useful confirmation for us.”
By Stevie Knight
