Before 1922, blowouts (gushers) were common. In these early wells, the hydrostatic pressure of the drilling mud in the annulus (open cylindrical space) between the drill string and the casing was the only thing standing between a successful drill and a blowout. Even today, one of the purposes of drilling mud is to control and balance formation pressures, so that highly pressurised formation fluid (oil, natural gas, methane) cannot flow into the wellbore during drilling and cause a blowout.
The problem with using only the hydrostatic pressure of drilling mud to prevent blowouts is that the same hydrostatic pressure that could prevent a possible blowout can also fracture the formation in unwanted ways. Such fractures can create thief zones, which in turn drain away drilling mud, leaving the well vulnerable to blowout. The result is a constant pressure balancing act, using the lowest density of mud possible.
In 1922, the invention of the blowout preventer created a new, much safer method of well control. For the first time ever, there was a physical way to seal off the wellbore in case of a sudden rise in formation pressure (kick).
Blowout preventers work by sealing off the annulus in case of sudden pressure change, preventing methane bubbles and other formation fluid from rising any further. The earliest ram-type blowout preventers had valves that had to be manually wrenched closed. Newer ram blowout preventers use either manual or automated hydraulic rams to close off the annulus, sometimes by actually shearing through the drill string. Annular blowout preventers automatically seal off the annulus with a ring of rubber, which tightens around the drill string as the formation pressures force it upward.
All blowout preventers are massive things the size of small houses. They have to be, to resist the kinds of geological pressures that can lead to blowouts. Most rigs have more than one set of blowout preventers. In land drilling, the blowout preventer is usually placed at the surface wellhead. When used with tie-back drilling risers in sea floor drilling, blowout preventers are located at the sea surface. On floating rigs with marine drilling risers, the blowout preventer is located on the sea floor.
Blowout preventers should be inspected and tested every month. Even with optimal maintenance; however, they do fail, especially under the extreme and somewhat experimental conditions of deep sea drilling. Failure to properly inspect and maintain blowout preventers increases their failure rate exponentially. If a blowout preventer fails, a pressure-related accident, possibly a blowout, becomes a near certainty.
The most notorious failure of a blowout preventer to date is the one that led to the 2010 Deepwater Horizon drilling rig disaster. Although several serious maintenance issues have been identified (hydraulic system leak, dead battery for the deadman switch), the specific causes of this failure are as yet unknown. It is; however, known that the Cameron TL blowout preventer may not have been the proper model for the conditions. For one thing, its shear ram would not have been capable of cutting through the threaded joints of a deep sea drill pipe: leaving as much as 10% of the drill pipe completely unprotected. Even the unit discovered on the sea floor doesn't seem to match the schematics given to British Petroleum (BP).
So crucial is a properly sized, properly maintained blowout preventer to safe drilling, that in 2003, it was designated a Mechanical Engineering Landmark by the American Society of Mechanical Engineers.