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A drill ship at the heart of Shell's hunt for Arctic oil flunked a Coast Guard inspection last month when a piece of anti-pollution gear that already cost its owner millions in fines failed again.

The Coast Guard held the Noble Discoverer in Honolulu for a day until engineers could repair the device that separates oil from the water in the ship's bilges, said Lt. Scott Carr, a spokesman for the service. The April 23 inspection occurred less than five months after vessel owner Noble Drilling pleaded guilty to a variety of federal charges and paid $12.2 million in fines, partly for dumping oily water overboard when the same machine didn't work.

"They attempted to fix it. They couldn't get it fixed," Carr told VICE News. "They couldn't get it operating, and they were given a detention hold. Then they got the part, got it fixed, and got it out the door."

The 514-foot ship is now en route to Seattle, where Shell plans to muster another expedition into the remote Chukchi Sea off Alaska. It will be the multinational oil giant's first attempt to drill there since the ill-fated summer of 2012, when Noble Discoverer was plagued by engine trouble and the drill platform Kulluk ran aground in a storm at the season's end.

The environmental activist group Greenpeace, which is fighting those plans, called the latest breakdown aboard the drill ship a bad omen.

"Noble had problems with the oil water separator in 2012 which led them to plead guilty to environmental and maritime crimes — felonies," Greenpeace spokesman Travis Nichols told VICE News. "That doesn't bode well for the Arctic drilling program."

Noble Drilling did not respond to a request for comment. But Shell spokesman Curtis Smith told VICE News that the oil company still has full confidence in the vessel and its contractor.

"This system has since been upgraded and passed inspections prior," Smith wrote in an e-mailed response to questions. "This is a case of mechanical repairs, which from time to time are required on any equipment."

International law requires all large vessels to pull all but trace amounts of oil out of the water they dump overboard. In December, Noble admitted that its ship's oil separator was out of commission, as was an alarm that would have warned of the presence of oil.

Noble Discoverer's crew struggled to deal with a buildup of water below decks and rigged up a makeshift system to discharge water from the engine room straight overboard — then tried to hide that system from the Coast Guard, federal investigators concluded.

Carr said Coast Guard inspectors usually flag about 550 ships for marine pollution violations in a typical year. The service ordered 121 of the nearly 9,300 vessels they surveyed in 2013 to remain in port to fix safety or environmental deficiencies before sailing.

Shell is proceeding with Arctic exploration plans despite sharply reduced oil prices that have led other companies to put similar projects on ice. Smith told VICE News that Shell is looking to the long term: If it finds oil this year, it would take a decade or more for the first wells to start producing.

"In the meantime, it's widely accepted that global demand for energy will double by the year 2050 — so, we'll need energy in all forms, and Alaska's outer continental shelf resources could play a crucial role in helping meet that energy challenge," he wrote.

And the company is betting that the Arctic will be more hospitable this year. Carbon emissions from fossil fuels, which scientists blame for driving global temperatures upward, are warming the region roughly twice as fast as lower latitudes. The decline of sea ice cover has opened new opportunities to tap into the nearly 90 billion barrels of oil and trillions of cubic feet of natural gas that geologists estimate lies beneath.

The Interior Department approved Shell's exploration plans in late March. But the effort has raised fears about the safety of drilling in those unpredictable conditions, particularly after the 2010 Deepwater Horizon disaster in the Gulf of Mexico.

Greenpeace activists have tried to disrupt Shell's plans by boarding the drill rig Polar Pioneer and camping out on its main deck as it heads for Seattle aboard a heavy-lift ship. And Seattle Mayor Ed Murray has tried to deny Shell's flotilla a berth at the city's port, with his administration finding the facility needs to get a new permit before allowing Shell to oil rigs to dock there.

"To prevent the full force of climate change, it's time to turn the page on things like coal trains, oil trains and oil drilling rigs," Murray said in a statement issued earlier this week. He urged port officials "to pause and rethink this issue" before seeking new permits.

"This is an opportunity for the port and all of us to make a bold statement about how oil companies contribute to climate change, oil spills and other environmental disasters — and reject this short-term lease," he said.

Smith told VICE News that Shell "will continue to watch closely the actions of all interested parties in the coming days." The port terminal where its ships had been scheduled to tie up "remains our optimal choice," he wrote, "but we have other options."

But Nichols cheered the decision, which he said was driven by grassroots pressure, as a step toward tighter oversight of Shell and Noble.

"Doing due diligence on this company, because of their track record, is the only prudent way forward," he said. "And if you don't do your due diligence, you'll find later, based on their record, that they cut corners."