The U.S. Coast Guard announced Thursday that hydrographic surveyors discovered a previously uncharted shoal in the area where a key ice-handling ship for Shell's Arctic drilling program was gouged last Friday.
But the Coast Guard is not yet saying exactly what gashed the Fennica's hull.
The ship was headed to the Chukchi Sea drilling grounds to assist in the company's controversial efforts to seek oil there, but was forced to return to Dutch Harbor after the crew realized the ship had been damaged and found a leak in the ballast tank.
Coast Guard investigators are not saying the shoal is the cause of the gouging, said Petty Officer Shawn Eggert, a USCG spokesman.
The incident is under investigation, he said.
Shell said the breach in the 380-foot-long Finnish vessel -- tasked with deploying the capping stack designed to stop an oil spill -- is about 39 inches long and about 2 inches wide.
Eggert said a notice has been sent to mariners about the shoal. The area was hydrographically surveyed Wednesday by a small boat from the National Oceanic and Atmospheric Administration survey vessel Fairweather, coincidentally docked in Dutch Harbor on a break from surveying in Western Alaska.
Eggert said the Coast Guard has not confirmed the size of the shoal. The information gathered in the hydrographic survey is still being analyzed by NOAA, he said.
The Coast Guard called the Alaska Marine Pilots group based in Dutch Harbor late Wednesday to provide some information about the shoal, said Rick Entenmann, president of the group's Western Alaska region.
He said the sea depth that was supposed to be about 45 feet in that area is 21 feet shallower than previously thought, he said.
"It's a frightening realization," he said.
Ships commonly traverse the area east of Hog Island near Dutch Harbor without incident, he said.
He just watched one traveling "dead center" over the spot where the damage is believed to have occurred. But it was a fishing vessel probably drawing about 12 feet of water, so it was able to clear the hurdle with no problem, he said.
The Fennica's draft is listed at 27 feet.
Entenmann said the shoal may have been overlooked in past surveys in part because surveying technology and techniques weren't as good as they are today.
"It's amazing we haven't had this done before but sometimes it takes an incident for them to say, 'let's check other areas,' " he said.
What exactly caused the tear is unknown, said Entenmann.
"It could have been a sharp rock, an anchor with barnacles on it, who knows," he said.
Shell has filed a repair plan that the Coast Guard must approve, Eggert said. A review of that plan is underway.
Dan Magone, of ship repair company Resolve-Magone Marine Services in Dutch Harbor, where the Fennica now awaits repairs, said the gash is an easy fix.
It's a common problem for ships in the area, he said.
"It's a minor deal. It's only major because of the oversight and bureaucracy involved. It would have already been gone from here if not for that," he said.