In 2012, an 18-year-old Dutch entrepreneur told a TEDx audience he was designing a device to clean-up the Great Pacific Garbage Patch.

Boyan Slat's idea sounded like a novel concept that would quickly succumb to the insurmountable logistical issues of cleaning up 1.6 million square kilometres of plastic in deep, remote, open ocean.

However, six years on Mr Slat's organisation The Ocean Cleanup is about to test a 120-metre section, ahead of deploying the first 600-metre system to the Northern Pacific Ocean later this year.

Boyan Slat, now 23 years old, has brought the launch of his project forward to 2018. ( Supplied: The Ocean Cleanup )

With millions in crowdfunding cash behind them, they aim to send up to 60 devices to the region between Hawaii and the north-west coast of the United States by 2020.

At a launch in 2017, Mr Slat told a large crowd — including many of his financial backers — the devices will be able to "clean-up 50 per cent of the patch in just five years".

Critics say the apparatus will act as an aggregating device that will attract and trap marine life, and that environmental impact studies have been insufficient to eliminate risk.

Many also believe powerful swells in the North Pacific will smash apart the structures, adding even more garbage to the world's biggest floating dump.

But The Ocean Cleanup team say they are confident they can iron out any kinks, and pull off "the largest clean-up in history".

'To catch plastic, act like plastic'

The pilot being launched this year — called a "floater" — consists of a 600-metre-long, floating hard-walled tube made from high-density polyurethane (HDPE).

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A screen a few metres deep will run the length of the floater, "able to catch anything from one-centimetre plastic particles up to large, discarded fishing nets".

Suspended from the floater a few hundred metres below, will be a free-floating sea anchor.

The idea is that the sea anchor creates enough drag that the apparatus moves slower than the floating plastic debris on the surface, which will be pushed into the net by prevailing winds and currents.

At the same time, the fact that the apparatus is free floating means in theory, it should be pushed to the same areas as the rubbish.

Spokesperson Erika Traskvik said The Ocean Cleanup was on track to launch soon.

"The assembly is going on really well. The first segment, which is 120 metres [long] has been put together and will be towed out for a tow-test in the [San Francisco] Bay area next week," she said.

The tow-test will be used to establish the speed at which they can make the 1000-kilometre-plus journey out to the garbage patch.

Once in place, the system will be allowed to float at sea for about six weeks. A ship will then empty the nets of plastic and take it back to the mainland for recycling.

Plastics collected during an expedition by The Ocean Cleanup in 2015. ( Supplied: The Ocean Cleanup )

And if all goes to plan, the launch of the second, larger system will not be far behind.

"The following systems we plan on making a bit larger to increase the capture efficiency," Ms Traskvik said.

"The second system might be deployed early next year, but then after that it could go fast. So we think that we could get up to 60 systems by 2020."

To finance such a large fleet, The Ocean Cleanup say they are in talks with corporate sponsors; the idea being to offer companies the chance to sponsor and monitor their own system.

"They then get their logo on it and they can follow it through an app and through our website," Ms Traskvik said.

"We plan on having a map of where all the systems are, how much plastic they've collected and what the wave and wind conditions are."

They say they are waiting on the proof of concept before they, or their potential corporate sponsors, commit to anything.

Failure may create 'world's biggest piece of marine debris'

If objects are allowed to drift long enough in the ocean, micro-ecosystems start to form around them.

Within a few weeks to months, molluscs and algae colonise, fish use the structure to shelter from larger predators, and seabirds and marine mammals move in to hunt and rest.

Fishers have learnt to exploit this by deploying artificial structures called fish aggregating devices or FADs.

If left long enough, FADs can create micro-ecosystems. ( Supplied: Will Rose/Greenpeace )

They can be as simple as a few lengths of bamboo strung together, or large platforms fitted with biomass sensors that beam information to mother ships, letting them know when to move in with nets.

Physical oceanographer Kim Martini says The Ocean Cleanup apparatus is essentially a giant FAD.

Worse still she says, it will be congregating fish and marine life in an area of the ocean with the biggest plastic problem on earth.

"It really increases their exposure to plastic and also increases the risk of entanglement," she said.

"If it fails it actually becomes the world's biggest piece of marine debris."

Even if the device doesn't end up harming marine life, Ms Martini says it is based on a flawed premise and will do little to clean-up the patch.

A study Mr Slat co-authored, published in Biogeosciences in 2015, found "plastic concentrations drop exponentially with water depth".

The researchers based their findings in part, on a series of trawls in the North Atlantic sub-tropical gyre, at 0.5-metre intervals from the surface down to five metres depth.

The Ocean Cleanup team have designed their net accordingly.

"Since most of [the plastic] is located between the surface and a depth of five meters, the array will be designed to clean-up only the plastic which is located in this upper layer of the water column," their website states.

But according to Ms Martini, plastic mixing happens throughout the water column, and sampling the top five metres is insufficient to draw conclusions about plastic distribution in open ocean.

"They're saying that most of the plastic is in the upper one metre of the ocean," she said.

"Most of the plastic isn't at the surface. A lot of it's been found down deep. There's another 2,995 metres that they didn't actually study."

If a ship runs foul of a net, who pays?

A number of questions about the project remain largely unanswered.

Who's responsible if a ship becomes entangled in one of the devices at sea? Who pays for the salvage operation? And what happens if a device laden with plastic washes up on a foreign beach?

The Ocean Cleanup is wading into a legal grey area, according to University of Queensland professor of maritime and commercial law, Nick Gaskell.

Part of the pipe that will make up the 'floater section' on The Ocean Cleanup device is moved into place. ( Supplied: The Ocean Cleanup )

"Exactly what is this thing that's going to be out on the high seas? It's probably not a vessel or a ship, but is it an installation? Or is it marine scientific equipment?" Professor Gaskell said.

"Different rules apply under the Law of the Sea Convention depending on what it is."

For the project to be a success, liability issues will need to be addressed, he said.

"What happens if it washes up on shore with all this plastic debris? It's actually causing pollution," he said.

"Who would have the cost of removing it? You'd be looking at the company itself that's operating it."

A 'ghost net' of tangled fishing nets and rubbish, pulled up during a 2015 expedition by The Ocean Cleanup. ( Supplied: The Ocean Cleanup )

In the case of a ship becoming disabled by one of the devices, Professor Gaskell said there may be several parties that a liability lawyer could seek to attach blame to.

"Those issues would come back to the company which is operating this, and/or any vessel which deployed it — if there was negligence in the way it was deployed," he said.

"You'd hope that these sort of issues would have been raised at an international level … to provide mechanisms by which notification can be made as to where these things can be deployed."

Sea life impact 'small or negligible'

Throughout the long development process, Mr Slat and The Ocean Cleanup team have been transparent and have encouraged constructive feedback.

The Ocean Cleanup say marine mammal experts will be on hand to help prevent tragedies like this. ( Getty Images: Michael Pitts/Nature Picture Library )

They insist they are still in the development phase, and will be constantly updating and improving the design after the first device is deployed.

A ship will be permanently stationed alongside the first deployment, monitoring how it works and looking for flaws in the design.

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"There's also going to be marine mammal experts checking for what could be around there," Ms Traskvik said.

In response to fears about the environmental impact of the devices, they say they have commissioned another environmental impact assessment — yet to be published — which found "the [potential] impact to sea life is small or negligible".

The floaters will be fitted with lights, automatic identification systems (AIS) and radar to reduce the risk of collision, and they say they are currently working with "several organisations" in the US to establish protocols for launching from San Francisco.

"When we're towing out to the patch itself, it's in international waters, so we have to have a flag state as part of the system," Ms Traskvik said.

They plan, at this point, to fly under the flag of the Netherlands.

As the saying goes, the road to hell is paved with good intentions, and The Ocean Cleanup has that in spades.

But the project has garnered huge public interest and support around the world.

Many seem captivated by the potential to undo some of the impact we have had on the oceans — and thousands have put money toward making that dream a reality.