A Blowout Preventer, or BOP, is a large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve, using remotely operated hydraulic actuators, the drilling crew can regain control of the reservoir. Procedures are then initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation.
BOPs come in a variety of styles, sizes and pressure ratings. Some can effectively close over an open wellbore, some are designed to seal around tubular components in the well (drill pipe, casing or tubing), and others are fitted with hardened steel shearing surfaces that can actually cut through drill pipe. Since BOPs are critically important to the safety of the site they are inspected, tested, and refurbished at regular intervals determined by a combination of risk assessment, local practice, well type and legal requirements.
BOP tests can vary from daily function testing to monthly or less frequent testing as deemed necessary based on company procedures and policies.