"Casing" is run into the well only one time after the well is drilled to "total depth" or "TD". The operation is called "setting pipe". Since the casing is picked up by the rig one length at a time when it is run in the ground it can be about any length as long as there is room for it to fit upright into the derrick. Tubing and drill pipe make a lot of "trips" in and out of the hole so they have to be able to be "stood back" in the derrick for bit changes and other mechanical in teh bottom hole assemblies.