A bottom hole assembly (BHA) is a component of a drilling rig. It is the lowest part of the drill string, extending from the bit to the drill pipe. The assembly can consist of drill collars, subs such as stabilisers, reamers, shocks, hole-openers, and the bit sub and bit.[1]
The BHA design is based upon the requirements of having enough weight transfer to the bit (WOB) to be able to drill and achieve a sufficient Rate of Penetration (ROP), giving the Driller or Directional Driller directional control to drill as per the planned trajectory and to also include whatever Logging While Drilling (LWD) / Measurement While Drilling (MWD) tools for formation evaluation. As such BHA design can vary greatly from simple vertical wells with little or no LWD requirements to complex directional wells which must run multi-combo LWD suites.
Prior to running a BHA most oilfield service providers have software to model the BHA behaviour such as the maximum WOB achievable, the directional tendencies & capabilities and even the natural harmonics of the assembly as to avoid vibration brought about by exciting natural frequencies.