An Arctic-bound Royal Dutch Shell PLC icebreaker left Portland, Oregon, on Thursday after authorities cleared environmental protesters, who had rappelled off a bridge to block its path. The vessel is heading for an oil drilling operation in the Arctic Ocean, which, according to Greenpeace, could threaten polar bears and walruses in the region.

As many as 13 Greenpeace environmentalists were dangling from the St. Johns Bridge to block the course of Shell’s MSV Fennica icebreaker. On Thursday, the vessel managed to make its way down the Willamette River toward the Pacific Ocean after authorities forced the protesters from the river and the bridge, the Associated Press (AP) reported.

Authorities arrested “a number of people,” Sgt. Pete Simpson, a Portland police spokesman, told the AP, adding that charges against the detained demonstrators are yet to be determined. “This is, obviously, a very unique situation.”

The Fennica, which arrived in Portland for repairs last week, tried to leave earlier on Thursday, but failed as activists refused to let it pass the bridge. Greenpeace USA had also been ordered by a federal judge in Alaska to pay a fine of $2,500 for every hour that protesters suspended themselves from the bridge to block the ship, which had received the final approval by the Obama administration to drill two new exploration wells about 70 miles off Alaska’s coast.

What an amazing group of heroes! We majorly delayed Fennica and brought worldwide attn to @Shell 's reckless drilling pic.twitter.com/MuVZgGIpYz — 350 PDX (@350_PDX) July 31, 2015

The hourly fine against Greenpeace could increase to $5,000 an hour on Friday, $7,500 an hour on Saturday, and $10,000 an hour on Sunday if the organization does not lift the blockade, the AP reported, citing U.S. District Judge Sharon Gleason.

“Shell isn’t just threatening polar bears and walruses with its drilling plans. By tapping into a new source of oil—only accessible because of melting ice—it’s threatening the entire world with worsening climate change,” Greenpeace said on its website.

Environmentalists want to delay the ship so that winter weather in the Arctic can prevent it from drilling until 2016. Activists believe that the U.S. government would also change its stance on the issue by that time, the AP reported.