PORTLAND, Ore. - Greenpeace protesters dangling from a bridge in Portland, Oregon, on Thursday forced back an icebreaker that Royal Dutch Shell needs in Alaska to start drilling in the Arctic, as a U.S. judge ordered the group to pay daily fines starting at $2,500 per hour. The 13 Greenpeace protesters, who rappelled down from the bridge over the Willamette River on Wednesday, are hoping to shorten Shell’s Arctic drilling season by stopping the Fennica icebreaker, which is carrying emergency equipment that would cap any blown-out well. Shell is not allowed to start drilling without it and the drilling season ends in October, when sea ice forms. Greenpeace says Arctic drilling could be damaging to populations of whales, polar bears and walrus in the event of an oil spill. Greenpeace is the latest group to stage demonstrations over the past three months in a bid to block Shell vessels from heading north from Pacific Northwest ports. The Fennica stalled once it neared the St. John’s bridge on the Willamette, which leads to the Pacific Ocean. Protesters dangling from ropes with large red and yellow banners, at times chanting “Shell No”, prevented the ship from passing under the span, forcing the vessel back to port. “When that ship turned, that was history,” kayaker Michael Foster told Reuters. A Coast Guard official told reporters the Fennica now wanted to depart later on Thursday afternoon. Later on Thursday, a firefighter descended from the bridge and slashed part of a rope linking protesters, local broadcast footage showed. A few minutes later, two activists lowered themselves down from the bridge toward a boat and were arrested, Multnomah County Sheriff’s Office spokesman Steve Alexander said. Dozens of kayakers were pushed to one side of the river by waterborne U.S. Coast Guard backed by sheriff’s deputies on loud speakers. The bridge and the river were closed to traffic. Onlookers sympathetic to the protests gathered in a nearby park and occasionally cheered in support. Two activists were issued police citations, Greenpeace said. Shell hopes to return to the Arctic for the first time since 2012, when it experienced a series of mishaps including the grounding of an oil rig. Company spokesman Curtis Smith said Shell respects the rights of individuals to protest but that “the staging of protesters in Portland was not safe nor was it lawful.” As the protests stretched into the afternoon, a U.S. judge in Alaska held Greenpeace in contempt and ordered it to pay fines of $2,500 per hour if the protest continues, with fines increasing daily to a rolling $10,000-per-hour after Aug. 2. Greenpeace USA Executive Director Annie Leonard said in a statement its protesters would stay in place for the time being: “While we respect the courts, we also respect the increasingly urgent science that tells us Arctic oil needs to stay underground.” (Additional reporting by Timothy Gardner in Washington and Eric M. Johnson in Seattle; Editing by Eric M. Johnson and Sandra Maler)