While some data were being transmitted to shore for safekeeping right up until the blast, officials from Transocean, the rig owner, told Congress that the last seven hours of its information are missing and that all written logs were lost in the explosion. Earlier tests that suggested explosive gas was leaking were preserved.
The gap poses a mystery for investigators: What decisions were made - and what warnings might have been ignored?
Special Section: Gulf Coast Oil Disaster
"There is some delay in the replication of our data, so our operational data, our sequence of events ends at 3 o'clock in the afternoon on the 20th," Steven Newman, president and CEO of Transocean Ltd, told a Senate panel. The rig blew up at 10 p.m., killing 11 workers and unleashing the gusher.
Meanwhile, undersea robots were trying to thread a small tube into the jagged pipe that is pouring oil into the Gulf of Mexico in to cut down on the spill from a blown-out well that has pumped out more than 4 million gallons of crude.
The company planned to move the 6-inch tube into the leaking 21-inch pipe - known as the riser - on Thursday night. The smaller tube will be surrounded by a stopper to keep oil from leaking into the sea, which could be in place by Friday, BP said. The plan is for the tube to siphon the oil to a tanker at the surface.
Since the April 20 drilling rig explosion set off the catastrophic spill, BP PLC has tried several ideas to plug the leak that is spewing at least 210,000 gallons of oil into the Gulf a day. The size of the undulating spill was about 3,650 square miles, or the size of Delaware and Rhode Island combined, said Hans Graber, director of the University of Miami's Center for Southeastern Tropical Advanced Remote Sensing.
Houston attorney Tony Buzbee, who represents several rig workers involved in the accident, questioned whether the fourth and final test claimed by BP and Transocean,what he called "the phantom test," was even performed.
"I can just tell you that the Halliburton hands were scratching their heads," said Buzbee, whose clients include one of the Halliburton crew members responsible for cementing the well to prepare for moving the drilling rig to another site.
Details of a likely blowout scenario emerged this week for the first time from congressional and administrative hearings. They suggest there were both at key points the day of the explosion.
Buzbee said that when Halliburton showed BP PLC and Transocean officials the results of the pressure tests that suggested gas was leaking, the rig workers were put on "standby." BP is the rig operator and leaseholder.
Buzbee said one of his clients told him the "Transocean and BP company people got their heads together," and 40 minutes later gave the green light.
The attorney said the Halliburton crew members were not shown any new test results.
"They said they did their own tests, and they came out OK," he said. "But with the phantom test that Transocean and BP allegedly did, there was no real record or real-time recordation of that test."
None of the three companies would comment Thursday on whether any data or test results were purposely not sent to shore, or on exactly who made the final decision to continue the operations that day.
Five thousand feet under the sea, if the tube being inserted doesn't work, BP could try a , which would be placed over the well and also would siphon the oil to the surface.
In another interesting experiment, BP might wind up shooting junk of all shapes and sizes to plug the nooks and crannies into the blowout preventer - a giant piece of machinery that's allowing some of the oil to escape. In the aptly named "junk shot," engineers would shoot pieces of tires, golf balls, knotted rope and other items into it in hopes the right size stuff makes its way to the appropriate holes. Once the leak is clogged, heavy mud will be poured in. It would then be sealed off with cement.
BP also has sprayed chemicals on the oil to break it up into smaller droplets with about 4 million gallons of oily water recovered.
The global oil giant said Thursday its costs for fighting the massive spill in the Gulf had risen by $100 million in three days to .
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The size of the spill, as measured from satellites, seems to have grown about 50 percent from May 10 to late Thursday, said Graber from the University of Miami.
"There's a hell of a lot coming out," Graber said of the oil.
The 210,000 gallons estimate that is leaking daily comes from the National Oceanic and Atmospheric Administration and has frequently been cited by BP and the Coast Guard. Some scientists have said based on an analysis of BP's video of the leak that the flow rate is much higher, while others have concluded the video is too grainy to draw any such conclusions.
As for the missing data, rigs like Deepwater Horizon keep a daily drilling report. It is the version of that report given to Congress that cuts off at 3 p.m.
The log confirms that three pressure tests, conducted from the morning to the early afternoon of April 20, indicated unseen underground leakage into the well. But there is no mention of a fourth test that BP and Transocean say was conducted and that they say indicated it was safe to proceed.
In the hours leading up to the explosion, workers finished pumping cement into the exploratory well to bolster and seal it against leaks until a later production phase. After the tests that indicated leakage, workers debated the next step and eventually decided to resume work, for reasons that remain unclear.
At the same time, heavy drilling fluid - or mud - was being pumped out of a pipe rising to the surface from the wellhead, further whittling the well's defenses. It was replaced with lighter seawater in preparation for dropping a final blob of cement into the well as a temporary plug for the pipe.
When underground gas surged up uncontrollably through the well, desperate rig workers tried to cap it with a set of supersized emergency cutoff valves known as a blowout preventer. However, the device was leaking hydraulic fluid and missing at least one battery, and one of its valves had been swapped with a useless testing part.
Rep. Bruce Braley, D-Iowa, said the lack of offsite data storage is something he intends to look into further.
"I'm sure we'll be taking action to follow up with those requirements," he said. "Because it's critical information that would give rise to understanding of what happened and why more wasn't done to shut off the flow of oil and prevent this from happening."