Conductor Casing Pipe Specs, Installation | Oil & Gas
The conductor pipe runs from the seafloor/surface to relatively shallow depths providing a conduit to allow circulation of drilling fluid as well as case off unconsolidated formations. It is the first string that is used to support well control equipment, typically a drilling diverter. A common size for the conductor casing pipe is 24", 26" or 30" and it is set from 40m to 5OO m below the seafloor/surface.
https://www.drillingmanual.com/conductor-casing-pipe-installation-oil-gas-specifications-connectors/