Alaska ? A tugboat put in service to help prevent another oil spill disaster in Prince William Sound ran aground on the same reef as the Exxon Valdez 20 years ago in what remains the nation’s worst oil spill.

The Coast Guard said Thursday that the 136-foot tug with six crew aboard had just completed an ice survey and was heading back to port in Valdez when it grounded on Bligh Reef. The tug reported the grounding in a radio call at 6:15 p.m. Wednesday.

Two of the tug’s fuel tanks were damaged. The tanks contain an estimated 33,500 gallons of diesel fuel, about a quarter of their total capacity.

The Coast Guard said Thursday that there was a fuel sheen about 3 miles long and 30 yards wide that had drifted away from the vessel. There was no sheen visible around the tug.