What is the main job of the big valve called a BOP, besides just closing the well?

Besides the immediate action of closing the well, the main job of the big valve called a Blowout Preventer, or BOP, is to ensure the safe and controlled management of wellbore pressure to prevent an uncontrolled influx of formation fluids, known as a kick, from escalating into a blowout. A kick occurs when the pressure exerted by the drilling fluid in the wellbore is less than the pressure of the fluids in the rock formation, allowing these formation fluids to flow into the well. Once the BOP closes the well, its primary ongoing function is to contain this wellbore pressure safely, acting as a critical barrier. It does this by sealing off the annular space, which is the area between the drill pipe and the casing, or by sealing off the open wellbore if no pipe is present. This containment allows operators to regain hydrostatic control over the well, which means establishing a column of fluid heavy enough to counteract the formation pressure. To regain control, the BOP facilitates the circulation of kill fluid. This involves pumping heavier drilling fluid, often called mud, down the drill pipe and out of the well through specific lines connected to the BOP, such as the choke line. This process removes the influx of formation fluid and increases the hydrostatic pressure inside the wellbore, eventually overcoming the formation pressure and bringing the well under control without allowing uncontrolled flow to the surface. Furthermore, the BOP system allows for the controlled movement of drill pipe into or out of the well while pressure is being contained, a process known as stripping or snubbing. This is critical for performing well control operations or for preparing to kill the well under pressure. In the most severe emergency, the BOP also has the capability to shear, or cut, the drill pipe and then seal the wellbore completely, providing a final safeguard against a blowout.