(Portland, OR)  --  More than a dozen Greenpeace activists are dangling from the St. Johns Bridge above the Willamette River, in an effort to prevent a Shell icebreaker from leaving Portland.  The icebreaker "Fennica" arrived in Portland last Saturday for repairs, and is set to leave for the Arctic on an oil-drilling mission.  The vessel was scheduled to depart yesterday, but after the activists rappelled over the bridge, Shell officials have refrained from saying when the Fennica will leave.  Greenpeace officials told KOIN-TV that 13 people have secured themselves in place below the bridge with enough supplies to last for days.