Oil spill latest: Why the macondo well exploded

By Ross Densley

“Drill pipe pressure had risen rapidly to 5,800 psi. It is thought that the annular preventer may have been closed at this time, but since the drill pipe valve was not closed, the pressure would have reached the pumps where the relief valve pressure could have been exceeded and tripped gas would have flooded the pump room.”

Tony Heyward, BP's beleaguered CEO, was accused of negligence by the US Senate last week regarding the ineptitude of BP's practices toward the Macondo well that ruptured gas and caused the Deepwater Horizon rig to catch fire and sink.

The finger-pointing was due in part because of a letter sent to BP prior to the explosion, which outlined the inefficiencies with the well, something BP supposedly ignored prior to the catastrophe.

Many commentators have expressed concern over deepwater drilling, and cited the issues with drilling at depths up to four miles under the surface, however, it appears that the explosion on the Deepwater Horizon rig would have materialised in shallow-water regardless.

The well status before the explosion

According to the oil drum, the drilling depth at the Macondo well had reached a total depth of 18,360 feet, with the previous casing shoe at 17,168 feet. The annulus or drill hole was eight and a half inches in diameter, with the Rotary Kelly Bushing (RKB) to Mud line 5,067 feet. The open hole had been logged over a four-day period.

A casing was run the entire length of the drill pipe, at seven inches by nine 7/8 inches from the total depth up to the wellhead.

The casing had been cemented using +/-100 bbls of slurry. There were no losses and the plug was bumped. No back flow was observed after displacement. Top of cement is estimated at 16,200 feet.

The casing was tested to 2,650 pounds per square inch, with the blind shear rams closed.

Sequence of events leading to the explosion

After 16.5 hours of bumping the cement plug, the well integrity was trialed with a negative test. Then 14.3 pounds per gallon (ppg) of oil based mud was extracted from the well at a depth of 8,367 feet.

This test was carried out part-way through the displacement of the well to seawater including a complex spacer pill, with the well shut in and the kill line open and full of seawater. Kill line pressure was zero but there was 1,400 psi on the drill pipe.

Despite some probable issues with the inflow/draw down test, as cited in BP's report, the overall reaction to the test was satisfactory.

As a result, the annular or drill pipe, would have been opened up to seawater, which meant that oil-based mud was transferred to a supply boat at around 25 to 31 barrels a minute.

In the 45 minutes proceeding the explosion, the pressure in the annulus had dropped and increased, presumably due to escaping gas, which eventually led to the explosion. Records show 4 telephone calls between the rig floor and the Toolpusher (drilling manager responsible for all operations) during this time. At 21.45 the pressure had built up to 5,800 psi, and contact with the rig was lost, presumably to the explosion.

"Drill pipe pressure had risen rapidly to 5,800 psi. It is thought that the annular preventer may have been closed at this time. But since the drill pipe valve (Kelly cock/stab in valve) was not closed, the pressure would have reached the pumps where the relief valve pressure could have been exceeded and tripped gas would have flooded the pump room (this is speculation but quite likely)," says the Oil Drum.

From there, the emergency disconnect system (EDS), responsible for closing off valves and rams and blind shear rams on the blowout preventer was reportedly pressed, but failed to trigger and did not work.

Despite the EDS not functioning, the automatic mode failure system (AMF), on the blowout preventer should have functioned to close off the blowout preventer rams, but this also failed to fire.

Concluding the errors

There are a number of corresponding inadequacies from BP during the 45 minutes that led to the gas leak, some of which were mechanical flaws, but most were failure in policy, planning and general rig practice.

Gas leaked because the hangar was running without a lock ring, which could have provided sufficient pressure to affect seal integrity. The hangar was also a single barrier, which meant the cement was not tested. As a result, gas from the annulus likely got past the hangar seal and kick-started the blowout.

Throughout the process, especially with the repeated drop and rise in pressure, basic rig practices should have dictated the well being shut down. However pumping operations continued; returns were dumped and the return flow meter was bypassed, so the rig was effectively blind until things started to get quite serious.

Once the explosion occurred, the Deepwater Horizon structure was compromised and collapsed into the sea, where the riser pipe, bringing oil up to the rig snapped and left oil, now under pressure and rising to the surface, to gush into the Gulf of Mexico at around 17,000 barrels a day.

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