Published: Sep 04, 2010 1:32 PM EDT
Updated: Sep 05, 2010 4:37 AM EDT

ON THE GULF OF MEXICO (AP) — Icelike crystals had formed Saturday on the 300-ton blowout preventer that failed to stop oil from spewing into the Gulf of Mexico, forcing BP crews to wait before they could safely hoist the device to the surface.

The hydrates — which have caused the oil giant problems before when the company was trying to contain the oil spilling into the Gulf — need to melt because they are combustible.

"We don't want to lift it and risk an uncontrolled release of gas because that's inherently dangerous," said Darin Hilton, the captain of the Helix Q4000 vessel, which is raising the device with a giant crane.

Crews must take care not to damage the device, which is considered a key piece of evidence in the spill investigation.

The device would be lifted the final 500 feet to the surface once it was assured the hydrates had dissipated. It was not an unexpected delay, Hilton said. Before the stop, it had been painstakingly raised at a rate of about 450 feet to 500 feet per hour.

One man on board is using a joystick to guide the crane holding the blowout preventer upward. It will ultimately be raised through a large hatch in the underbelly of the Q4000, where it will then be placed on what is essentially a huge, metal holding device called a shipping skid.

The device likely wouldn't be hoisted onto the vessel until about 7 p.m. EDT, said BP PLC spokesman Neil Chapman.

There are 137 people aboard the ship, including FBI agents who are waiting to take possession of the device after its mile-long journey. It will eventually be taken to a NASA facility in Michoud, La., to be analyzed.

Hydrates form when gases such as methane mix with water under high pressure and cold temperatures. The crystals caused BP PLC problems in May, when the company tried to place a 100-ton, four-story dome over the leak to contain it.

The 50-foot device was detached from the wellhead Friday afternoon. Another blowout preventer had successfully been placed on the blown-out well, the government said later.

The April 20 explosion aboard the Deepwater Horizon killed 11 workers and led to 206 million gallons of oil spewing from BP's undersea well.

Investigators know the explosion was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before igniting.

But they don't know exactly how or why the gas escaped. And they don't know why the blowout preventer didn't seal the well pipe at the sea bottom after the eruption, as it was supposed to. While the device didn't close — or may have closed partially — hearings have produced no clear picture of why it didn't plug the well.

Lawyers will be watching closely, as hundreds of lawsuits have been filed over the oil spill. Future liabilities faced by a number of corporations could be riding on what the analysis of the blowout preventer shows.

The raising of the blowout preventer followed Thursday's removal of a temporary cap that stopped oil from gushing into the Gulf in mid-July. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.

The government wanted to replace the failed blowout preventer first to deal with any pressure that is caused when a relief well BP has been drilling intersects the blown-out well.

Once that intersection occurs sometime after Labor Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.