Since the end of April, 2010, news stories have been filled with unfamiliar words and phrases about drilling for oil in deep water. We've heard about risers, drilling mud, semi-submersible drill ships, and blowout preventers. One phrase that was already familiar is “oil spill,” but 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 exploration companies hope will never be used, a machine with only one task: to stop oil and gas from gushing unchecked from a well. To understand the job of the preventer requires that you know what a blowout is. We’ll start there:
Oil reservoirs are under huge pressure, mostly because they are deeply buried. For every foot a well penetrates into the earth, the pressure increases by about 0.43 pounds per square inch (psi). At the bottom of a 10,000-foot well, the expected pressure is more than two tons per square inch. Compare that to the pressure in the tires on a car, which are usually inflated to about 35 psi – and a 10,000-foot well is just an average depth.
To combat that immense pressure, a well that is being drilled is filled with a dense liquid called drilling mud. The weight of the column of mud that fills the well is kept high enough to offset the pressure on any fluids discovered by the well, and keep them in the ground until they can be safely extracted.
Some zones deep underground are under higher pressure than their depth would predict, a condition petroleum geologists call “overpressured.” When a well penetrates one of these zones unexpectedly, the pressure underground forces the drilling mud back up the well, often 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 a component of the pipe that makes up a wellbore. They sit below the drilling rig on the ground surface or the seafloor. They are bolted to the top of the pipe, or casing, which forms the wall of the well at depth. Another length of pipe, the 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, which is considerably smaller. Drilling mud fills the space between the casing and the drillstring, or the annulus.
BOPs are designed to close the wellbore in case of a blowout, and to keep 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 supposed 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, the second type of blowout preventer is used. This design has hydraulic rams that drive hardened steel plates into the wellbore. The steel plates are designed to 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 newspaper stories have claimed:
Major manufacturers of BOPS include Hydril and Cameron (maker of the BOP that failed at the BP spill).
What is a blowout preventer? It’s a machine that exploration companies hope will never be used, a machine with only one task: to stop oil and gas from gushing unchecked from a well. To understand the job of the preventer requires that you know what a blowout is. We’ll start there:
Oil reservoirs are under huge pressure, mostly because they are deeply buried. For every foot a well penetrates into the earth, the pressure increases by about 0.43 pounds per square inch (psi). At the bottom of a 10,000-foot well, the expected pressure is more than two tons per square inch. Compare that to the pressure in the tires on a car, which are usually inflated to about 35 psi – and a 10,000-foot well is just an average depth.
To combat that immense pressure, a well that is being drilled is filled with a dense liquid called drilling mud. The weight of the column of mud that fills the well is kept high enough to offset the pressure on any fluids discovered by the well, and keep them in the ground until they can be safely extracted.
Some zones deep underground are under higher pressure than their depth would predict, a condition petroleum geologists call “overpressured.” When a well penetrates one of these zones unexpectedly, the pressure underground forces the drilling mud back up the well, often 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 a component of the pipe that makes up a wellbore. They sit below the drilling rig on the ground surface or the seafloor. They are bolted to the top of the pipe, or casing, which forms the wall of the well at depth. Another length of pipe, the 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, which is considerably smaller. Drilling mud fills the space between the casing and the drillstring, or the annulus.
Some facts about blowout preventers, regardless of what newspaper stories have claimed:
- They’re not necessarily the size of a small house: a blowout preventer’s size is a function of 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).
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