The drilling riser of the Deepwater Horizon was a large diameter pipe that connected the drilling rig to the blowout preventer (BOP) anchored to the top of the well casing at the seabed. The weight of the drilling mud that filled the Macondo wellbore and the riser allowed the crew to control pore pressures and prevent kicks during drilling and completion activities. In addition, steel casing and cement was in place to protect the sides of the wellbore and prevent hydrocarbons from entering the well. Cement had also been placed at the bottom of the well to seal the hydrocarbon bearing zone. The crew of the Deepwater Horizon relied on these physical barriers to drill safely. They also relied on their ability to run and interpret tests to ensure that these physical barriers had been properly established in the well before taking additional critical steps as part of temporary abandonment operations.
Acceptance of the negative test as successful indicated the Deepwater Horizon crew believed the well had been sealed.
As previously mentioned, the well was not sealed and the configuration of the well was not what they had planned. The kill line was plugged by the spacer injected to displace mud from the drillpipe and upper wellbore. This unrecognized factor complicated interpretation of the negative pressure test.
Displacement of Mud from the Riser
The crew proceeded to open the BOP and displace the remaining drilling mud from the Macondo well in preparation for setting a surface cement plug. With the drilling mud removed, the open BOP was the only physical barrier against a kick. The ability of the blowout preventer to act as this barrier was contingent upon human detection of the kick and timely activation of the BOP.
Diverter System Route
The diverter system is one of the pieces of equipment on a drilling rig designed to limit oil and gas from inundating the rig floor during excessive flow from the riser by routing the well fluids to a safer location. Using a control panel, the Deepwater Horizon crew could preset the route to one of two locations, either the mud gas separator located on the rig, or overboard. The standard preset route was to the mud gas separator, and this was the route preset on April 20, 2010. In this configuration, if the crew wanted to change the route before or during an emergency, they needed to complete a multi-step process to divert overboard.
Eruption of Fluids and Subsequent Explosion
During the process of displacing the riser, a mixture of seawater, drilling mud, and hydrocarbons erupted onto the drilling rig, which the crew immediately tried to divert to the mud gas separator. Within a minute after diverting, mud overwhelmed the mud gas separator and erupted out of it and multiple other locations. The blowout preventers (BOP) were now critical. The crew activated both the annular preventer, which did not seal well, and several pipe rams in the BOP stack. From the time well fluids released onto the deck until the first explosion, the crew had 9 minutes to understand what was happening, determine the best well control responses, and implement them. At ~9:49 p.m., the first explosions occurred on the rig, and data transmission from the rig to shore ceased.
Image Credits: U.S. Chemical Safety and Hazard Investigation Board