An assembly of a drill string element adapted to be held by the drill pipe rams of a blowout preventer (BOP) to enable the drill string to be disconnected at the BOP and a detachable driving element is described. The drill string element comprises a housing portion defining a longitudinal bore along a longitudinal axis X-X of the housing portion. At least one recess is arranged in the longitudinal bore, said at least one said recess (8) being arranged to engage at least one lug of a detachable driving element arranged to be disposed in the longitudinal bore to rotationally lock the housing portion and detachable driving element together.