What specific type of BOP ram is used to seal the wellbore when no drill string is present?

The specific type of Blowout Preventer (BOP) ram used to seal the wellbore when no drill string is present is called a blind ram, also sometimes referred to as a blank ram. A blind ram is designed to completely seal off the wellbore, providing a pressure-tight barrier against the flow of fluids. Unlike pipe rams, which are designed to close around a specific size of drill pipe, blind rams have a flat or slightly concave face that can seal against itself, effectively closing off the entire bore of the well. Blind rams are typically located at the bottom of the BOP stack, providing a last line of defense in case other BOP elements fail. They are hydraulically actuated, meaning they are closed by applying hydraulic pressure to the ram cylinders. The sealing elements on the blind ram are designed to withstand high pressures and to maintain a seal even if the wellbore is not perfectly circular. In some cases, blind rams may also have the capability to shear the drill string before sealing the wellbore. These are known as blind shear rams, which provide an added level of protection in situations where the drill string cannot be removed from the wellbore before closure. Therefore, the blind ram is the designated BOP ram for sealing the wellbore in the absence of a drill string.