Abstract: A snubbing unit for running and retrieving a string of tubing into a well under pressure selectively operates in a stand alone mode and a rig assist mode. The unit has a lower frame for mounting during the stand alone mode on a snubber blowout preventer. Stationary weight supporting slips and snubbing slips are carried by the lower frame during the stand alone mode. The unit has four hydraulic cylinders with weight supporting slips and snubbing slips carried by a traveling base plate on the piston rods. A stationary base plate is mounted between the lower frame and the traveling base plate. A spacer tube is connected to and between the lower frame and the stationary base plate in the stand alone mode. In the rig assist mode, the lower frame along with the stationary weight supporting slips and the spacer tube are removed to enable a rig blowout preventer to connect to the stationary base plate.