Drill String Theory

A ‘drill string’ is the name for the dynamic assembly used in the drilling process – typically consisting of the drill pipe, transition pipe, and bottom hole assembly.  Each component is essential in the drilling process.

Drill pipe is sections of thick steel tubing, threaded at both ends, that delivers drilling fluid, weight, and torque to the drill bit.  It is hollow such that drilling fluid can be pumped (via mud motors) through the string to the drill bit, where it provides lubrication, cooling, and flushing functions.  The drill pipe also offers a conduit in which downhole measuring and logging equipment can be used.  The sheer weight of the drill pipe contributes to the downforce applied to the drill bit, adding to its efficiency.  Rotational torque is generated by the drilling rig and is transmitted via drill pipe to the drill bit.  It’s this combination of torque, weight, and fluid that drive the well into the ground.

Nestled in between the drill pipe and bottom hole assembly, is the transition pipe.  This typically consists of heavyweight drill pipe (a thicker version of drill pipe already discussed).  The primary purpose of the transition pipe is to provide a flexible connection between the drill pipe and BHA.  This is necessary as, in general, the highest point of fatigue in the drill string is located at this point, and most failures occur here.  Another secondary purpose for the transition pipe, is to add extra weight to the BHA and thus drill bit – furthering the efficiency of the drilling process.

The final part of the drill string is the bottom hole assembly.  It usually consists of a drill bit, drill collars, and drilling stabilizers.  The drill bit is used to break up the rock formation and is at the very end of the drill string.  There are several different kinds of drill bits used in the industry, but most common are tricone and PDC.  Next are the drill collars – they are generally thicker sections of heavyweight drill pipe and are used primarily to add weight to the drill bit (in directional drilling, heavyweight drill pipe is often used in lieu of drill collars to allow greater flexibility and thus sharper direction changes).  The last component is the drilling stabilizer.  Used to keep the drill pipe centered in the hole, the drilling stabilizer’s job is to ensure the drill bit is being driven into the ground without sidetracking or deviation, to minimize vibration, and ensure the quality of the hole being drilled.  The bottom hole assembly is the ‘leading edge’ of the drill