Drilling Industry Glossary

Blowout Preventer Stack

The blowout preventer assembly is mounted on the top of the casing head during drilling operations. Its purpose is to prevent the contents of the annulus from being aggressively emptied into the atmosphere when the bit drills into an unexpectedly high pressure zone. A “blowout” is prevented by closing off the annulus by sealing against the drillpipe.
There are several different elements in the blowout preventer stack. At the top of the stack is the annular preventer. This device maintains a seal against the drillpipe even as the drillstring is passed upward from the borehole.
There are several different elements below the annular in the blowout preventer stack. Four different elements in the blowout preventer stack are:

1. Pipe rams: These units have a semi-circular sealing element that fits around the drill pipe.
2. Blind Rams: These units will seal when there is no drill pipe in the borehole.
3. Shear Rams: These will cut the drillpipe and then seal the borehole.
4. Drilling Spools: These are spacers between pipe rams, which provide access to the wellbore to either add or withdraw fluid.

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