* Key pressure test misinterpreted by rig workers-BP
* Cementing failed at some point-BP
WASHINGTON, May 26 (Reuters) - Workers on the ill-fated Deepwater Horizon rig misinterpreted a key test indicating a critical problem with the operation in the hours before the explosion that led to the massive Gulf oil spill, BPofficials said on Wednesday.
A negative pressure test performed on the day of the explosion showed pressure on the drill pipe was at 1,400 lbs per square inch, said BP officials helping to complete the company's internal investigation of the accident.
During this same period, the rig's kill line data showed basically no pressure. The workers seem to have interpreted these results to mean the pressure test was successful and that no hydrocarbons could enter the well, said officials. Some of these investigators declined to be identified because of sensitive nature of the accident.
"This is, we believe, where a mistake was made," an official told reporters at a press briefing. "Looking at the information suggests the negative tests in fact failed."
Some details about why key decisions were made may never be known because some of those involved did not survive the accident, the officials said.
"The integrity testing that was done was not understood by the people on the rig. They believed they had achieved isolation from the reservoir, when indeed it hadn't been achieved," said Mark Bly, BP group head of safety and operations.
BP briefed reporters on the details of their investigation so far after releasing this initial report to lawmakers earlier this week. [ID:nN26010464]
The officials said pressure should have measured the same on both the drill pipe and kill pipe.
The officials refused to identify who was responsible for the decision to move forward with finishing the well. The rig was owned an operated by Transocean, but was leased by BP.
Eventually workers recognized there was a problem on rig and shut down the pump, but it was not enough to prevent the explosion.
Both BP and Transocean have lobbied blame at each other since the accident. The rig's chief mechanic told federal investigators on Wednesday that there was a "skirmish" between BP and Transocean staff about whether to proceed with a procedure that may have been a fundamental mistake that led to the explosion. [ID:nN26206690]
In addition to the misread test, the investigators said the cementing on the well must have failed at some point to completely plug the well. Some cementing on the rig was completed by Halliburton. Halliburton has defended its work, saying that all of its cementing procedures were dictated by BP. (Reporting by Ayesha Rascoe; Editing by Lisa Shumaker)
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