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. 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. Variations Rotary assembly Rotary assemblies are commonly used where formations are predictable and the rig economics are an issue. In such an assembly the weight of the drill collars gives the BHA the tendency to sag or flex to the low side of the hole, collar stiffness length and stabiliser diameter and placement are engineered as a means of controlling the flex of the BHA. This will bring about the desired hold, build or drop tendency. The ability to vary the directional tendency of the assembly comes from varying the weight on the bit. A fixed assembly has only one directional tendency. The weight on bit allows you to tune that tendency. The bottom-hole assembly (BHA) can be: slick with no stabilisers a pendulum bottom-hole assembly for reducing the deviation of a well packed with stabilisers for a straight hole / a short lock assembly a fulcrum for increasing the deviation of the well / a long lock assembly Bottom-hole assemblies are also described as specialized, such as articulated, steerable, and oriented or non-oriented. Fulcrum / Long Lock This assembly is used to build angle. The assembly usually incorporates a near bit stabiliser. Behind this would be a selection of drill collars and Heavy Weight Drill Pipe (HWDP). The length of the section AFTER the near