Jackable oil rigs and corner columns for producing legs in an oil rig

A prefabricated column for an oil rig leg of a jackable oil rig comprising

• an outer pipe with a constant outer diameter along the length thereof;
• an inner pipe fixedly cast with said outer pipe to define an annular space with said outer pipe and having a constant outer diameter along the length thereof and an increasing wall thickness from a top section thereof to a bottom section thereof to support an increasing static vertical loading;
• and
• a hardenable material in said space between said pipes and securing said pipes together to form a prefabricated stratified structure.

A jackable oil rig comprising

• a deck;
• a base foot for placement on an ocean floor;
• at least one leg for supporting said deck on said base foot, said leg having a plurality of prefabricated corner columns and a plurality of transverse stays interconnecting said columns, each said prefabricated column having an outer pipe with a constant outer diameter along the length thereof, an inner pipe defining an annular space with said outer pipe and having a constant outer diameter along the length thereof and an increasing wall thickness from a top section thereof to a bottom section thereof to support an increasing static vertical loading, and a hardenable material in said space between said pipes;
• and
• a jack system on said deck for moving said deck relative to said leg.