A Bottom hole assembly is a component of a drilling rig. It is the lower part of the drill string, extending from the bit to the drill pipe. The assembly can consist of drill collars, subs such as stabilizers, reamers, shocks, hole-openers, and the bit sub and bit.
The bottom-hole assembly (BHA) can be:
Bottom-hole assemblies are also described as, a) specialized, such as articulated, b) steerable, and c) oriented or non-oriented.