Definition - What does BOP Stack mean?
A BOP Stack is an installation style of blowout preventers which is useful in controlling the wellhead pressure from an oil well. In the stacking of BOP's, generally two or more BOP's are connected on one stack. In an oilfield, a regular stack of blowout preventers consists of one to six ram-type blowout preventers and optionally, one or two annular-type blowout preventers. A typical stack configuration has the ram preventers on the bottom and the annular preventers at the top.
Petropedia explains BOP Stack
When drilling a well the mud system is a primary barrier and a BOP system is a secondary barrier for well pressure control. When the primary barrier fails, hydrocarbons may enter the wellbore and a kick will occur. To gain control on this situation the drilling crew will operate the BOP system to close the well and control the pressure. When there are multiple oil wells in close vicinity, BOP stacks are preferred.
BOP is of two types- Annular and RAM type which come in different sizes and pressure ratings. These are installed on top of the wellhead in stacks and allow access to the formation. They are designed to close the borehole completely when there is no string in the well. BOP's are major devices for safety of the rig and personnel, so they are inspected regularly and pressure tested every 21 days as per standards. On jackup rigs a BOP is mounted below the rotary while in floating drilling rigs it is installed on surface of the seabed.
The following are the functions of a BOP:
- Control the flow of well fluids.
- Allow pumping of fluids into well.
- Monitor the wellbore pressure and allow the removal of excess pressure.
- Shut in the well, i.e., seal the annular space between drill pipe and casing.
- Completely seal the wellbore and shear the string.