We mention substructures (a.k.a. “subs”) a lot on the blog, often in relation to moving or walking systems. Located directly over the well, the substructure raises, lowers and supports the rig floor, derrick and other rig floor components like the drawworks.
A common substructure is the slingshot sub. They begin in a folded position and are then unfolded as hydraulic cylinders or winches raise the rig into place. On a slingshot sub, all rig floor components are installed and rigged up with the floor lowered.