Thanks for the research Oliver!
I know that one of the Arctic U-Boats was U-28, which was a very successful boat - U-28 sank after sailing too close to the ammunition ship "Olive Branch" - gunfire from the deck gun blew up "Olive Branch", but sank the U-Boat too....
http://www.uboat.net/wwi/boats/index.html?boat=28
It was a controversial incident, because the "Olive Branch" sailors refused to let the Germans into the lifeboats, and all of U-28's men froze to death.
