The invention provides a managed pressure drilling system. The system comprises a rotating sealing device and a drill string assembly comprising a plurality of drill pipe members. Each drill pipe member has a first tool joint having a first tool joint outer diameter; a second tool joint having a second tool joint outer diameter; and a tubular body between the first and second tool joints having a tubular body outer diameter. In at least one section of the drill string the first tool joint outer diameter, the second tool joint outer diameter and the tubular body outer diameter are substantially the same. The rotating sealing device is configured to form a fluid seal against at least a part of the at least one section of drill string.