Derrick:

A large load-bearing structure, usually of bolted construction.

In drilling, the standard derrick has four legs standing at the corners of the substructure and reaching to the crown block.

The substructure is an assembly of heavy beams used to elevate the derrick and provide space to install blowout preventers, casingheads, and so forth.

Because the standard derrick must be assembled piece by piece, it has largely been replaced by the mast, which can be lowered and raised without disassembly.