Designers of a floating system to collect plastic marine debris from the Pacific Ocean are pointing to two reasons why the system failed: The debris moved faster than creators expected, and a weld failed in the plastic pipe system.
In an update on the Ocean Cleanup — which combines a floating high density polyethylene pipe with a polyester skirt to corral and collect floating plastic — project founder and CEO Boyan Slat said floating plastic moved faster than the U-shaped system.
"Not only was the system then moving too slow, but the plastic was also moving faster than we had predicted," Slat wrote in his latest website update for the Netherlands-based project.
As a result, the ocean sweeper dubbed System 001 only collected 2.2 tons of plastic for recycling before its structural failure.
"At times, large amounts of plastic would gather, only for some -- or all -- of it to float out again," Slat said in his update. "Plastic was also occasionally observed to accumulate on the convex side of the cleanup system. The plastic count within the system did not remain high enough for an efficient extraction process."
Then, on Dec. 29, 2018, a 26-foot end section (18 meters) of the 2,000-foot long HDPE floater broke off. A root-cause analysis shows the structural failure was caused by a stress crack at a point where the collection skirt is welded to the floater pipe. The point is called a dovetail connection.
"Because the dovetails were welded in 1-meter segments, due to the fabrication possibilities of our supplier, this created a discontinuity, or gaps, between each weld," Slat said.
Plastic beads were used to fill the gaps, but they did not form a high-quality bond with the pipe, and the cleaning system fractured at one of the weld points. An underwater photo of the break showed concentric rings increasing up through the HDPE pipe, Slat noted.
The broken pieces of System 001 were towed to Hilo Bay, Hawaii for study. Experts from HDPE pipe and component suppliers, university professors, research institutes and consultants looked at the system parts and data collected with the Ocean Cleanup staff.
The group's engineering team is using the latest results to update the system design and prepare for relaunch while other experts look at ways to speed it up so that it retains captured plastic. A sail rig is under consideration.
The cleanup system will be redesigned and repaired in California and then relaunched from Hawaii to the garbage patch's chaotic environment of waves, winds, currents and drift forces, possibly in a few months.
If the Ocean Cleanup can solve all issues, the system is expected to harvest about a ton of debris per week.
The group is working with experts at Agru Kunststofftechnik GmbH and its Georgetown, S.C.-based Agru America Inc.