An apparatus for drilling a subterranean wellbore using a drill string. The apparatus includes a riser, a sealing device arranged in the riser so as to form a first riser chamber and a second riser chamber, and a riser booster pump. The riser at least partly contains the drill string and defines a substantially annular space around the drill string. The second chamber is in a fluid communication with the riser booster pump so that a kill fluid stored in the second riser chamber may be maintained at a first pressure which is greater than a second pressure of a drilling fluid in the first riser chamber.