API16A  annular blowout preventer/BOP installation could consist of both annular and ram type BOPs.assembled into a stack. Also, there can be a kill line valve and a choke line valve. The choke line valve is used to redirect the mud from the well bore to the choke manifold during a kick. The kill line valve is used to direct drilling fluid to the BOP during a kick. Annular BOPs are designed to form a seal in the annular space between the drill pipe and the wellbore and are usually mounted at the top of the BOP stack.