CEMENT CASING:

The well being drilled by the Deepwater Horizon rig, Mississippi Canyon Block 252, was in the process of being sealed on the day of the disaster. Exploratory rigs like the Deepwater Horizon only do the initial drilling before production rigs come in to actually extract the oil.

The first step in the sealing process is to lock the casing pipe in place by pouring cement between it and the borehole.

Halliburton, which was contracted for the task, decided to use a cement blend containing nitrogen. The mixture can provide a stronger bond than other cement compounds, but its properties make it trickier to use.

Halliburton official Tim Probert told a Senate committee Tuesday that the cement around the casing had been tested "in accordance with accepted industry practice."

"The results of the positive test were reviewed by the well owner and the decision was made to proceed with the well program."

Cementing is a difficult process, according to F.E. Beck, an associate professor of petroleum at Texas A&M University.

"Cement is perhaps the most difficult barrier to install and control... because cement is installed as a liquid but acts as a barrier as a solid," he told lawmakers.

Halliburton has defended its work on cementing the casing, but faces accusations it mishandled the job.

"Was the well properly cemented? Were there problems with the well casing? Were all appropriate tests run on the cement and casings? These are some of the critical questions," said Steven Newman, chief executive officer at Transocean, which leased the rig to British Petroleum.

CEMENT PLUG:

Once cementing around the casing is complete, ordinary practice would be to "plug" the well with cement. Until that final step, a heavy drilling fluid called "mud" sits over it, preventing flammable gases and materials from shooting up.

A cement plug involves workers pouring liquid cement into the casing pipe which sinks through the "mud" and hardens. The fluid can then be flushed out with sea water.

Halliburton testified Tuesday that the cement was never poured in on this occasion because the process was in fact being carried out in reverse.

"We understand that the drilling contractor then proceeded to displace the riser with seawater prior to the planned placement of the final cement plug," Halliburton executive Tim Probert said.

"Prior to the point in the well construction plan that the Halliburton personnel would have set the final cement plug, the catastrophic incident occurred. As a result, the final cement plug was never set."

It was unclear whether BP had requested the process be done in reverse order. Such a maneuver is unusual but not unprecedented, experts said.

Steven Newman, chief executive of rig owner Transocean, said in his testimony that the blowout had to be caused by "a sudden, catastrophic failure of the cement, the casing or both."

HUMAN ERROR:

By the time the decision was made to flush the pipe, pressure was likely building up in the heavy drilling fluid -- a leak in the cement casing would have made the situation even worse.

A rise in pressure is something rig workers are trained to watch for, as a key warning sign that something is wrong. Increased pressure often precedes a well blow-out.

In this case, the rig workers do not seem to have detected the increasing pressure, which could have been eased by shutting valves. Oblivious, they began pumping sea water into the drilling pipes to clear them. Once the heavier drilling fluid was displaced, the pressure was able to escape, pushing up water and mud first, and then natural gas that was quickly ignited.

Investigators will examine why the crew did not notice the warning sign, and whether eagerness to finish the project quickly played a role. Some reports have suggested the workers though they might receive a bonus for finishing the well early.

BLOW-OUT PREVENTER:

The rig was equipped with a key device that should have been able to seal the well, the so-called blow-out preventer. But the massive instrument, a 450-ton mechanism sitting on top of the wellhead, some 5,000 feet below the sea's surface, failed.

The crew made the first unsuccessful attempts to activate the device before the rig sank. Since then, efforts to activate the preventer using remote-controlled submarines have also failed.

Newman told lawmakers the preventer could not have caused the accident but that once activated it could have prevented some of the environmental damage resulting from the massive oil slick that has leaked from the rig.

BP America Chairman and President Lamar McKay told lawmakers the preventer's failure was still a mystery.

"The systems are intended to be... fail-safe; sadly, and for reasons we do not yet understand, in this case they were not," he said.

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