It set sail from San Francisco Bay in September amid high hopes and more than a little fanfare, its destination the Great Pacific Garbage Patch.

The goal was for the giant floating boom to make a dent in the vast archipelago of plastic that is choking the seas between California and Hawaii.

But last week, the nonprofit group that developed the boom said it would be towed back to port — in two pieces.

“We are, of course, quite bummed about this,” Dutch organization Ocean Cleanup wrote on a blog chronicling the device’s triumphs and travails.

By some estimates, the Great Pacific Garbage Patch consists of more than 80,000 tons of waste and debris, tossed together by the currents into a sort of island of lost toys, minus the island. As it grows, scientists say, so does the danger it poses to the health of the ocean.

The 2,000-foot-long boom, which arrived at the garbage patch after a voyage of about 1,400 miles, was designed to trap the trash so that it could be returned to shore. The Ocean Cleanup’s goals were ambitious: Collect 150,000 pounds of plastic in the first year, with more booms to follow. Within five years, the group hoped, half the debris would be collected.

But Monday, the organization said that in a routine inspection over the previous weekend, it found that an end section of the boom almost 60 feet long had detached. The boom will be taken back to shore as soon as weather allows, the group said.

Ocean Cleanup said it appeared that material fatigue and “local stress concentration” might have caused the fracture in the multimillion-dollar structure.

Skeptics had raised doubts about whether the boom, known as Wilson, would do much good and whether it could hold up to the forces of nature.

“It is very difficult to predict what will take place in these very dynamic ocean environments,” Nicholas Mallos, director of the Trash Free Seas program at the Ocean Conservancy, said Thursday.

Mallos praised Ocean Cleanup for helping turn world attention to a major environmental problem. But he said stopping plastics before they enter the ocean from beaches and local waterways was a more effective approach, along with reducing overall plastic use.

The inventor behind the boom, Boyan Slat, had himself warned of possible setbacks. “First of all,” he said before the boom was sent out, “it’s something that we haven’t really been able to test very well.”

And, in fact, in the early weeks of its deployment, the device, which has no crew and is powered by the sun, had trouble retaining the debris it collected. Its return to port, Ocean Cleanup said, offers a chance to address that.

“Although we would have liked to end the year on a more positive note,” the group said, “we believe these teething troubles are solvable, and the cleanup of the Great Pacific Garbage Patch will be operational in 2019.”