The Care and Feeding of Blowout Preventers
Beginning in April, 2010, the news was filled with the unfamiliar terminology of deep-water drilling. We learned about risers, drilling mud, semi-submersible drill ships, and blowout preventers. One already familiar phrase was “oil spill,” but did how the mess that is the giant ecological disaster in the Gulf of Mexico happen? There was a blowout, and the blowout preventer didn’t work.
What is a blowout preventer? It’s a machine that drillers pray will never be used, a machine with one task: to stop oil and gas from gushing unchecked from a well–a blowout.
Some zones deep underground are under higher pressure than their depth would predict, a condition oil geologists call “overpressured.” When a well unexpectedly penetrates one of these zones, underground pressure forces drilling fluid out of the borehole back to the surface, potentially emptying the well in just seconds: a blowout. Blowouts can be so powerful that they also force the drillstring–thousands of feet of steel pipe–out of the well with the mud. Needless to say, a blowout is not just dangerous; it can be disastrous.
Blowout preventers (BOPs) are part of the pipe that makes up a wellbore. They sit beneath a drilling rig on the ground surface or seafloor. They are bolted to the top of the casing, which forms the outer wall of the borehole. Another length of pipe, a riser, is bolted to the top of the BOP and extends to the drilling rig above it, a distance of a few to several thousand feet. The casing contains the drillstring, a long string of pipe of smaller diameter. Drilling mud fills the space between casing and drillstring, called the annulus.
BOPs are supposed to seal the wellbore in case of a blowout, keeping the fluids deep underground where they belong. There are two kinds of BOPs, which are usually stacked together: the first is a thick rubber donut that is designed to clamp down on the drill string and seal off the annulus. The annular BOP sits on top of the blowout stack.
If the annular BOP fails to seal the well, a second type of blowout preventer is activated immediately. This design has hydraulic rams that drive hardened steel plates into the wellbore. The steel plates act like giant shears, cutting through the drillstring and creating a seal inside the BOP itself. When a blowout is detected on the rig floor–the mud begins to boil out of the casing or the gas detectors sound an alarm–rig personnel are trained to hit one of the many panic buttons all around the drill rig. That is supposed to activate first the annular blowout preventer and, if that fails, the hydraulic rams. At the BP Macondo well, the blowout preventer is presumed to have failed.
Some facts about blowout preventers, regardless of what reporters who know nothing of the technology say:
• They’re not necessarily the size of a small house: a blowout preventer’s size is depends on the depth of the well and the diameter of the pipe. Some stacks are only four or five feet tall.
• Not all blowout preventers sit “on the sea bottom”: wells drilled on land can also hit overpressured zones that mandate use of BOPs.
• Not all drilling wells have blowout preventers. In fact, most don’t. Overpressured zones that can cause major blowouts occur only in a limited and fairly predictable set of areas and subsurface environments.
Major manufacturers of BOPS include Hydril and Cameron (maker of the BOP that failed at the BP spill)
Images of BOPs can be found in this slide show.