The decades-long effort to steer the famed ocean liner SS United States away from the scrap yard took a major step forward Thursday when a luxury cruise ship line announced plans to renovate it and return it to sea.

The announcement by Crystal Cruises and the SS United States Conservancy that the 64-year-old ship could return to trans-Atlantic service from New York and other voyages from the city and additional American ports came as a surprise.

The conservancy has been focused in the past few years on trying to return the 990-foot ship, now mothballed in Philadelphia, to New York for a new life as a stationary attraction with a hotel, museum and retail, office and conference space. But finding a developer willing to do that and a permanent site for the ship has proved frustrating.

“We would be remiss if we would pass up an opportunity to restore such an important symbol of luxury travel and a bygone era of Americana,” said Crystal chief executive Edie Rodriguez.

The company will pay the $60,000-a-month carrying charges to keep the ship docked in Philadelphia while studying the feasibility of upgrading the ship. If the determination, expected by the end of the year, is favorable, Crystal will negotiate the purchase of the ship from its owner, the conservancy.

Crystal would reconfigure the exterior of the ship to add balconies on its sides in keeping with most modern cruise ships. It would give the SS United States a slightly more boxy look amidships. Changing the classic rakish lines of the ship might offend some preservationists, but the conservancy thinks it’s better than having the ship broken up and melted down.

“We think this is the best way to save the ship,” said Susan Gibbs, conservancy executive director and granddaughter of the ship’s designer, the late William Francis Gibbs, a part-time Locust Valley resident.

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Rodriguez said rebuilding the vessel with its entirely gutted interior would cost in excess of $700 million, potentially cheaper than building a similar ship. That would include an environmental cleanup of PCBs and other pollutants and installing a new propulsion system.

“Failure is not an option,” Rodriguez said. She said the only thing that would stop the project is not higher than anticipated costs but if the U.S. Environmental Protection Agency found pollution on the ship could not be remediated.

If Crystal buys the ship, it would be the third oceangoing vessel for the Los Angeles-based company. The ship carried 2,000 passengers when it was launched in 1952. It would be transformed into an 800-guest vessel with 400 luxury suites measuring about 350 square feet. Original features including the Promenade Deck and Navajo Lounge will be retained.

If the renovation goes forward, Gibbs said, the conservancy would focus on building a museum ashore, ideally in New York City where it originally sailed from Pier 86, just south of where Thursday’s announcement was made. The conservancy would also provide artifacts and historical exhibits for the ship.

“My grandfather would be thrilled to see his ship reemerge as a technologically innovative, beautiful, luxurious and modern vessel,” Gibbs said. “The essence of her will remain front and center.”

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The United States’ maiden voyage in 1952 set the still-current record for the fastest trans-Atlantic passenger ship crossing. Its role superseded by passenger jets, it was taken out of service in 1969. The rusting hull, larger than the Titanic, has been moored in Philadelphia for nearly two decades.