The Care and Feeding of Blowout Preventers

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:

• 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).

The Anatomy of a Blowout

To comprehend a catastrophic oil well blowout, we first need basic understanding of how petroleum collects in underground reservoirs and how exploration for those reservoirs works. For starters; oil, natural gas, and water collect in underground layers when their path to shallower layers is blocked by an impenetrable zone. Instead of collecting in “lakes” or “rivers” of oil, however, hydrocarbons accumulate in tiny pores within huge volumes of rock.

Being buried under miles of solid rock means that hydrocarbon reservoirs are under enormous pressure. The pressure increases, on average, by a factor of 0.433 psi/foot or 9.792 kPa/meter. This regular pressure gradient means that pressure at the bottom of a ten-thousand-foot well is more than 4300 pounds per square inch; compared to a pressure of about 30-35 psi for a car tire. Since liquids cannot be compressed, deeply-buried reservoir fluids seek any possible pressure relief.

Drilling a hole ten or fifteen inches in diameter from the surface to a deep reservoir provides just such relief. To keep oil and water from spurting out of a wellbore, drillers fill the hole with fluid of their own. Called “mud” or “drilling mud,” this fluid is carefully designed to carry out several different functions, one of which is to match the pressure in the underground layers and prevent the crude oil from rushing to the surface. Maintaining balance is relatively simple in areas of normal pressure, where pressure at depth can be predicted from the standard pressure gradient (above).

There are, however, subsurface layers in which the pressure is much higher than that predicted by the pressure gradient. Unexpected penetration of such an overpressured zone can result in a blowout, as can improper drilling practices or poor well design.

When a blowout occurs liquids in the reservoir stream into the wellbore, forcing tons of drilling mud and thousands of feet of steel pipe from the mouth of the well at the surface. The rising column of oil, water, and natural gas are under such vast pressure that they can reach supersonic speeds; more than 1100 feet per second. Crude oil and natural gas are both flammable, and are often ignited by the heat of friction in the moving column or by sparks as metal and chunks of rock smash against one another. In the early days of exploration, drill rigs often “burned to the ground” after a blowout; though such gushers were looked on favorably before scientists understood the environmental havoc wreaked by such a disaster.

A blowout is both an environmental and an economic disaster, for not only are large quantities of a valuable resource wasted, the infrastructure at the surface is destroyed. In the April, 2010, blowout in the Gulf of Mexico, the semi-submersible drillship Deepwater Horizon burned and sank at a cost of $600 million and eleven lives. Five thousand barrels of oil per day, valued at some $400,000, poured out of the breached drill pipe. Because of such costs, exploration companies take expensive measures to prevent blowouts.

The first line of defense against blowouts is the drilling mud, described above. Before drilling into potential overpressured zones, mud engineers “mud up” to increase the density of the fluid in the well. The second line of defense is casing, heavy-weight large-diameter pipe that is cemented in place to line the hole and isolate zones of different pressure. The final line of defense is a massive mechanical device called a blowout preventer or BOP.

Blowout preventers come in several designs depending on the manufacturer (leading makers include FMC, Hydrill, and Cameron). A BOP is placed at the ground surface or, for offshore work, at the seafloor; between the drill rig and the well head. BOPs are designed to trigger automatically upon detection of rapid uphole flow, or trigger remotely on command. Blowout preventers come in two types: the first is basically a giant rubber doughnut that can be activated to seal off the annulus – the space between the drill pipe and the casing. The second type consists of massive hydraulic rams that force hardened, edged surfaces inward to cut the drill string and seal the well with a thick metal wedge.

The worst-case scenario of a blowout is one in which reservoir fluids breach the cement holding the casing in place and reach the surface around the outside of the pipe – in this instance, even BOPs are of no use. There has been speculation that this is what happened at British Petroleum’s Macondo well off the Mississippi Delta (April, 2010).

If the BOPs fail and a blowout occurs, options for recovery of the well are few. One option is to collapse the wellhead with shaped charges (compare to John Wayne’s portrayal of Red Adair in the movie “Hellfighters”). A more likely scenario is to drill a relief well that intersects the blown well – a technological challenge, to be certain, but doable. The relief well is used to dump high density “kill fluids” – super-weight drilling mud – into the wellbore of the flowing well and, eventually, bring it under control. Drilling a relief well takes weeks or months, while the blowout continues to spew crude oil, and can cost tens of millions of dollars.

In spite of all the technology and wellfield expertise, blowouts still occur. The April 2010 is one of the largest ever, a list that includes the Pemex IXTOC I blowout, which poured 10,000 barrels of oil per day into the Gulf of Mexico in 1979-80; and the 1969 blowout of a Unocal well in the Santa Barbara Channel off southern California. The environmental damage caused by the Unocal blowout is responsible for California’s strict regulation of offshore drilling.


Glossary: http://www.glossary.oilfield.slb.com/search.cfm
more information: http://www.chron.com/disp/story.mpl/business/deepwaterhorizon/6973912.html