I wanted to call your attention to this excellent story from the Houston Chronicle describing some of the potential causes of the Deepwater Horizon spill. The short version appears to be that they were switching the drill chamber over from mud to water, which exposed what may be a potentially faulty concrete job, which brought gas to the surface. When that happened, and the blowout preventer was activated, the BOP failed, potentially because of leaky hydraulics.
As the Chron story explains, BP should not have been replacing the mud with water unless they were very sure of the cement job done the day before.
Experts say well-capping poses special hazards. One arose that day as crews were replacing the mud with seawater in pipes going from the ocean floor to the rig.
Deep gases exert astounding upward pressure on a well. “Drilling mud,” a heavy fluid used to lubricate the drill and bring up bits and pieces of rock, is used as the main line of defense against the upward pressure, or a disastrous eruption of gas.
The mud was being displaced so the riser could be detached from the rig and the wellhead, and the well could be capped with a final cement plug. But seawater is much lighter than mud. The pressure the riser was applying to the well would have lessened by as much as 38 percent, experts said.
That could prove significant.
Investigators likely will be considering whether the drill hole and the casing pipe were secured properly with cement a day earlier.
“The big question is how confident were they in the casing cementing job,” said Elmer “Bud” Danenberger, who recently retired as chief of offshore regulatory programs for the Minerals Management Service. “They shouldn’t have begun this (riser) operation until they were confident in that.”
Now, as the MMS recently found, problems with the cementing process have been one (but not necessarily the only) cause in a plurality of blowouts in recent years. Though most of those cementing-related blowouts occurred in far shallower waters than this well.
Cementing problems increased significantly during the current period as these problems were associated with 18 of the 39 blowouts, compared with 18 of the 70 blowouts with identified contributing factors during the previous study. During the current period, all but one of the blowouts associated with cementing problems occurred in wells with water depths less than 400 ft.