A riser control device, particularly designed to be used in connection with spool or horizontal production trees (12) for wells in sub-sea oil and gas installations. Within a housing (1,2) of the device is provided, in opposed direction, radially movable pair of rams (6) for isolating (sealing off) the well and simultaniously, in opposed direction a radially movable pair of shear blades (7) for cutting off an intervention string or the like. The rams (6) and blades (7) are driven by means of a vertically disposed actuator (8,9,23) integrated in the housing (1, 2). The actuator may be in the form of a hydralically driven annular piston (23)/annular chamber (29,30) device, which via piston rods (14) and translation beams (8) transforms the movement of the piston (23) to open or close the rams (6) and shear blades (7).