GIGLIO, Italy -- Engineers embarked on a costly, painstaking and potentially perilous operation on Monday to raise the wreck of the Costa Concordia cruise liner, saying they had successfully begun coaxing its battered hull away from two granite reefs where it ran aground 20 months ago, killing 32 people.
By late afternoon, however, almost eight hours after the recovery began, engineers said the operation might take longer than initially planned. Parts of the vessel had emerged, discolored and rusting, from the waters where it has languished on its side.
Salvage experts have said the dimensions of the rusting 951-foot vessel make the operation unparalleled in the annals of marine salvage, running up enormous bills as more than 500 divers, technicians, engineers and biologists prepared the ship for what is known as "parbuckling" to bring it upright and minimize environmental risks to Giglio Island, a marine sanctuary.
"It's an extraordinary operation that has never been done before," Franco Porcellacchia, project manager for Costa Cruises, the ship's operator, said at a news conference in Rome last week.
Engineers had predicted that the first phase of the recovery, involving a rotation of 20 degrees, would be complete in about six hours. But as that forecast slipped, it seemed likely that the recovery would take longer.
"The ship has now rotated about 10 degrees," said Sergio Girotto, project manager at the salvage company Micoperi. "The hull is completely detached from the rocks" on which it settled after smashing into them with 4,229 passengers and crew members aboard.
Asked why the operation was delayed, Mr. Porcellacchia told reporters: "I wouldn't call it a delay. We didn't make any appointments. We supposed that this incredible operation could take place in 12 hours. If it takes 15, it's O.K."
Using huge jacks, cables, pulleys and specialized equipment, the salvage attempt had been scheduled to begin at first light, but a sudden storm prevented workers from moving a barge and rubber booms close to the ship.
Three hours after work finally started, engineers said the first phase of the operation -- to ease the vessel away from its rocky perch -- was going according to plan.
"These hours were the most uncertain, as we could not establish how much the hull was wedged," Mr. Girotto said. "Now we have to guide it into the desired position."
Pressed down for months by water and the ship's own weight, the hull of the Concordia displayed "great deformations," Mr. Girotto said.
The next phase of the salvage, engineers say, is to settle the wreck on an artificial seabed made of bags of cement next to underwater steel platforms. To achieve that, the ship would need to have rotated about 65 degrees, engineers said.
Italian authorities said there had been no visible seepage of pollutants from the vessel so far.
Franco Gabrielli, the head of Italy's Civil Protection Agency, said the location of the bodies of two passengers who are still missing, an Italian woman and an Indian man, was unclear.
Television crews and residents clustered on the island to peer at the wreck.
The initial movement of the vessel on Monday seemed almost imperceptible, such were the huge forces of nature, gravity and human endeavor pulling against one another. But slowly, the appearance above the waterline of a narrow band of rust in part of the ship that had been submerged seemed to indicate that the vessel was shifting.
The size and the location of the 114,500-ton ship are the most challenging aspects of the salvage project, experts said.
After running aground, the vessel settled at an extreme angle about 50 yards from the shore. Preparations for the salvage operation took 14 months, and the cost has risen to $799 million from $300 million and could rise further, according to Costa Cruises.
The Concordia has been stabilized through anchors and cement bags, and steel platforms have been built underwater on the port side. On Monday, salvage crews used pulleys, strand jacks and steel cables, placed on nine caissons attached to the left side of the ship, to slowly dislodge it from the two rocks where it has been laying.
After about 20 degrees of rotation, the caissons will start taking in water. The downward force of the water will decrease the rotation speed and help complete the parbuckling, a word that originally referred to the sling used to lift a barrel with a double rope passed around it.
The operation will be monitored by engineers and remotely operated vehicle pilots from a control room on a barge close to the bow of the Concordia. If images or sonar show dangerous twisting, the technicians can adjust the process.
A command center on shore will closely follow the salvage operation. If the ship does not rotate, or does not rotate properly, another crew of engineers can intervene.
"There are a lot of unknown facts, so we made a lot of assumptions," said Nick Sloane, an engineer and senior salvage master with Titan Salvage of Fort Lauderdale, Fla. "Some are conservative assumptions, and some are optimistic."
The operation carried many risks, Mr. Sloane said, including unpredictable weather. Its second enemy is time: the longer the vessel stays where it is, the higher the risk is that it cannot be removed in one piece.
Salvage masters and the Italian authorities have prepared for possible complications. Most of the fuel was siphoned off within months of the shipwreck. But the floating city that once transported and entertained more than 4,000 people still contains chemicals, lubricants and diesel fuel from the engine rooms that could leak into the pristine waters for which Giglio, a popular tourist spot, is known.
During the rotation process, the region's environmental agency is taking samples and monitoring the water quality. Salvage officials and the Italian authorities expressed confidence that the operation would succeed and said the chance that the ship could break apart was "remote."
"The whole project inevitably had many questions marks, such an operation on such a big ship is unprecedented," said Emilio Campana, the director of the research office for naval and maritime engineering at Italy's National Research Council. "They need to extract the ship from the rocks and rotate it almost at the same time. They've never tried anything like this on an intact vessel."
There are many uncertainties about the structural damage that the ship has sustained and how the attempt to right it will be affected.
"Ships are designed to float upright, not to lie down under their own weight. It's an unnatural position," Mr. Campana said. "The structure is broken and somewhat deformed. No one knows how it will react to the rotation movements."
North of Giglio, in a Grosseto courtroom, the ship's captain, Francesco Schettino, is scheduled to defend himself this fall from charges of multiple manslaughter, causing a shipwreck and abandoning the vessel before everyone was safe. He denies wrongdoing and has said his maneuver to take the vessel close to the shore after the impact saved many lives. He is the only defendant in the criminal trial; five others, a company official and four crew members, have pleaded guilty to reduced charges.
Gaia Pianigiani reported from Giglio, and Alan Cowell from London.
This article originally appeared in The New York Times.