A group of environmental activists have been removed from Shell-contracted icebreaker bound for Alaska.

On Thursday morning, six Greenpeace protesters boarded the Finnish icebreaker Nordica using inflatables, and then locked themselves to the vessel after tying up banners that criticize Arctic drilling. At the time of the boarding, the Nordica was underway in the Baltic Sea. The protesters lasted 10 hours aboard the ship before being arrested by Swedish authorities. According to Greenpeace, the arrested protesters were from Sweden, Finland, Austria, Israel, and Denmark.

This is the second time this week that activists have boarded the Nordica. On Tuesday, 20 protesters locked themselves to the vessel while it was in Helsinki, preparing to depart.

Right now, the U.S. Coast Guard is preparing for similar protests in Alaska. In June, they will be increasing their security forces in the Aleutians while Shell stages its fleet in Unalaska before heading to the Beaufort and Chukchi Seas to commence exploratory drilling.