A clamp for supporting a subsea wellhead and attached casing string preparatory to installing same in a well, and slip bowl assembly for running a drill pipe cementing string inside the casing string. The clamp comprises two hinged together clamp halves with a connector to permit the clamp to be locked about a subsea wellhead. Half-circle cutouts in each clamp half form a clamp bore when the clamp is closed. The clamp bore has a grooved profile to mate with a grooved profile on the exterior of the subsea wellhead. When the subsea wellhead is clamped in place, with the clamp resting on the rig floor, a slip bowl assembly having a grooved base plate is placed atop the clamp.