Environmental activists rappelled off Portland's tallest bridge early Wednesday in an effort to stop a Shell Oil Arctic icebreaker from leaving the city.

Thirteen protesters dangled from the St. Johns Bridge while another 13 remained on the bridge as lookouts. Greenpeace USA executive director Annie Leonard said the activists have enough water and food to last for days, and can hoist themselves to allow other marine traffic to pass.

The Royal Dutch Shell PLC icebreaker Fennica arrived in Portland for repairs last week. The vessel was damaged earlier this month in the Aleutian Islands when it struck an underwater obstruction, tearing a gash in its hull.

The icebreaker is a vital part of Shell's exploration and spill-response plan off Alaska's northwest coast. It protects Shell's fleet from ice and carries equipment that can stop gushing oil.

Opponents of Arctic drilling worry that the area's remoteness and rough conditions will hamper cleanup efforts should a spill occur.

"These climbers hanging on the bridge really are at this point the last thing standing between Shell's plan to drill in the Arctic and the Arctic," Leonard said.

Shell spokesman Curtis Smith said in an email the Fennica will return to Alaska once final preparations are complete.

"As for the activities of the day, we respect the choice that anyone might make to protest based on Shell's Arctic aspirations; we just ask that they do so safely and within the boundaries of the law," Smith wrote.

Environmental groups had wanted the Obama administration to reject permits sought by Shell to drill in the Chukchi Sea because of the absence of the icebreaker.

The government, however, gave Shell approval to begin limited exploratory oil drilling, with conditions. Shell can only drill the top sections of wells because the company doesn't have critical emergency response equipment on site to cap a well in case of a leak. That equipment is aboard the Fennica.

Activists hope any delay will give the Obama administration time to reconsider granting the final permit. They also want to use up days in the short window for summer drilling.

"Shell's under enormous pressure to get this thing back up there," Leonard said.

Supporters of arctic drilling say it can be conducted safely with existing technologies and that future production will help sustain the country's energy needs and limit reliance on imports.

The U.S. Geological Survey estimates the Arctic offshore reserves in the Chukchi and Beaufort seas at 26 billion barrels of recoverable oil.

Sgt. Pete Simpson, a Portland Police Bureau spokesman, said officers were monitoring the protest and no arrests had been made. The bridge remained open to vehicles, but pedestrians were told to stay off.