A hang-off assembly for supporting a riser string from an off-shore drilling rig. The hang-off assembly includes a housing with a passage through the housing and an open section allowing access to the entire length of the passage from outside the housing. The assembly also includes a gate member movable relative to the housing from an open position to a closed position, the gate member preventing access to the passage through the open section when in the closed position. A motor moves the gate member between the open and closed positions. An adapter attachable to the riser string includes a profile landable in the housing to support the riser string when connected to the adapter.