Bottomhole Assembly Required for a Desired Weight on Bit

The bottomhole assembly  (BHA) is a major part of the drilling system that influences how effectively weight is transferred to the drill bit, as well as the directional control, stability, and torque.  Selecting or configuring the BHA to achieve a desired weight on bit (WOB) is needed for efficient drilling, bit performance, and achieving the desired penetration rate.

Weight on Bit (WOB)

WOB is the force applied on the drill bit by the drill string, essential for efficient rock cutting.
Too much or too little WOB can damage the bit, reduce penetration rates, or increase the likelihood of drill string failure.

 

Length of Bottomhole Assembly Formula
\( L \;=\;  \dfrac{ W_b \cdot FS }{ DC \cdot BF  }\)     (Length of Bottomhole Assembly) \( W_b \;=\; \dfrac{ L \cdot DC \cdot BF }{ FS }\) \( FS \;=\; \dfrac{ L \cdot DC \cdot BF }{ W_b }\) \( DC \;=\;  \dfrac{ W_b \cdot FS }{ L \cdot BF  }\) \( BF \;=\;  \dfrac{ W_b \cdot FS }{ L \cdot DC  }\)
Symbol	English	Metric
\( L \)  = Length of Bottomhole Assembly	\(ft\)	\(m\)
\( W_b \) = Desired Weight to be used While Drilling	\(lbf\)	\(N\)
\( FS \) = Factor of Safety	\(dimensionless\)	\(dimensionless\)
\( DC \) = Drill Collar Weight	\(lbf\)	\(N\)
\( BF \) = Bouyancy Factor	\(dimensionless\)	\(dimensionless\)