Abstract: A sea drilling jig comprises a tool carrier ship having a hull with a drill tower mounted on the hull over a hull opening which extends downwardly into the sea below the tower. A riser head is engageble in the hull opening and it may be coupled to the hull. The riser head includes a float portion with a ballast tank and it has a vertically extending riser tube opening therethrough for accommodating a riser tube and for holding it in position relative to the tower. The riser head is detachable from the carrier ship and may be ballasted to float at a level below the ship out of the turbulence area of foul weather whenever foul weather occurs.