A submersible mobile caisson having ice crush resistance for adapting semi-submersible mobile offshore drilling units and mobile offshore oil well production rigs, which are ice crush sensitive for use in ice bearing offshore ocean waters, is provided. The caisson comprises a floatable generally four sided structure composed of front and rear outer walls, and opposing side outer walls, with the rear outer wall having a removable entrance door. Inner walls are positioned in a spaced apart, opposing relationship with each of the outer walls, and so that the inner and outer walls define a perimeter structure and the inner walls define a cavity which is located within the perimeter structure. The perimeter structure has a plurality of ballast compartments which permit the caisson to float or be sunk so as to rest on the ocean floor. A drilling/production moon pool system is also provided in the perimeter structure.