SAN FRANCISCO (CBS SF) — Work crews have begun the delicate process of lifting the wreckage of a large sunken barge off the bottom of the San Francisco Bay near BART’s Transbay Tube, officials said Tuesday.

The 112-foot freight barge Vengeance sunk in stormy weather last winter and settled gently on the sea floor above the Transbay Tube. The Coast Guard said at the time that the wreckage posed “no threat to the Bay Area Rapid Transit District Transbay Tube or salvage operations.”

“The barge has settled… above the subterranean BART tube, which is sheltered by a 25-to-30-foot protective layer of earth,” U.S. Coast Guard 11th District officials said in a statement.

Regular sonar scans and tube inspections were conducted to ensure the BART tube was never threatened. Eventually, crews righted the barge under water and moved it off the Tube.

Contractor Global Diving & Salvage said the process will take 30 to 60 hours.

“It’s kind of a slow process. It’s a careful process,” Petty Officer Sarah Wilson said. “It’s a complex and dynamic situation.”

The barge, which was being used by BART to perform maintenance work, sunk in gale force winds at about 12:22 a.m. April 7th in 50 feet of water between Treasure Island and the Transbay Tube off the Port of Oakland.

It was carrying 4,000 gallons of diesel fuel and 300 gallons of hydraulic fluid aboard when it sunk. Initially, the barge was leaking, but divers managed to quickly plug the leak.

About 2,700 gallons of fuel was removed from the barge on April 14.

Divers have cut the barge into sections in preparation for its removal.