The decades-long effort to steer the famed ocean liner SS United States away from the scrap yard took a major step forward Thursday with the announcement that a luxury cruise ship line has signed an agreement with the nonprofit group that owns the vessel to renovate it and return it to sea if practicable.

The announcement Thursday at the Manhattan Cruise Terminal that Crystal Cruises has an option to renovate and eventualy buy the ship came as a surprise because the SS United States 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.

Crystal has agreed to pay the $60,000-a-month carrying charges to keep the ship docked in Philadelphia until the rest of the year while studying the feasibility of upgrading the ship. If the deal goes through, Crystal would reconfigure the ship to add balconies on its sides to resemble modern cruise ships. That’s not something that preservationists would be happy about, but they probably would agree that it beats the alternative — scrapping the ship.

“We think this is the best way to save the ship,” Susan Gibbs, executive director of the SS United States Conservancy, which owns the ship, and granddaughter of the ship’s designer, William Francis Gibbs, said to an audience at the terminal on the Hudson River that included maritime historians and industry officials.

If the renovation goes forward, the conservancy would then focus on building a museum ashore, looking for a New York site, Gibbs said.

Crystal president and chief executive Edie Rodriguez said Thursday the renovation costs could be in excess of $700 million, but that environmental cleanup issues are the only thing that could stop the project.

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 Its role superseded by passenger jets, it was taken out of service in 1969 and later stripped of its interior. The rusting hull, larger than the Titanic, has been moored in Philadelphia for nearly two decades.

“Crystal’s ambitious vision for the SS United States will ensure our nation’s flagship is once again a global ambassador for the highest standards of American innovation, quality and design,” Gibbs has said. “We are thrilled that the SS United States is now poised to make a triumphant return to sea and that the ship’s historical legacy will continue to intrigue and inspire a new generation.”

“The prospect of revitalizing the SS United States and re-establishing her as ‘America’s Flagship’ once again is a thrilling one,” Crystal’s Rodriguez said. “It will be a very challenging undertaking, but we are determined to apply the dedication and innovation that has always been the ship’s hallmark.”

Crystal, which is in the midst of the biggest expansion in its history, said the United States will have to be extensively rebuilt to meet more than 60 years of the maritime rules and regulations. It will be transformed into an 800-guest vessel with 400 luxury suites measuring about 350 square feet; it will have dining, entertainment, spa and other amenities. Such original features of the ship as the Promenade and Navajo Lounge will be retained. A new propulsion system will be installed.

Crystal said the ship will make some traditional trans-Atlantic voyages from New York City but also cruise to other U.S. ports and make international voyages.

To lead the feasibility study, Crystal has hired retired U.S. Coast Guard Rear Adm. Tim Sullivan to put together a team of experts.

The conservancy will continue to expand its museum collection and would set up shipboard displays and other programs aboard.

Rodriguez estimates that a makeover of the ship could cost between $700 million and $800 million, potentially somewhat cheaper than building a similar ship from the keel up.

The ship in his heyday carried 2,000 passengers.

There will be some environmental issues to resolve, including the presence of PCBs in some engineering areas, according to Crystal Cruises and conservancy officials. Part of the ship has already been cleaned up as part of the removal of its interior structure and furnishings, they said.