Drill-string

Drill-string is the major component of a drilling system. Usually, it is made up of drill-pipes, drill-collars, stabilizers and a drill-bit. It transports the torque generated by the rotary table or top drive to the drill-bit to realize the drilling operation. And it also serves as a conduit for drilling fluid [3]. During drilling operation, the drillstring is subjected to various loads, such as hook load at the top, rotary system driven torque, weight on bit (WOB), torque on bit (TOB), drilling fluid buoyant force and its self-weight. The hoisting system can support the drill-string weight. The drill-pipe section is normally the longest part of the drill-string and it is normally in tension. Drill-collars are extra heavy steel pipes providing necessary WOB to assist the bit to cut the rock formation, which are placed right above the drill-bit and below the drill-pipe section. The significant strength of drill-collars helps stabilize the downhole assembly, facilitate straight drilling and protect itself from excessive fatigue or wear.

Drill-bit

Another important component of a drilling system is the drill-bit. Its main task is to fracture and penetrate into the rock formation at down-hole. Two main kinds of drillbits are commonly used: roller-cone bits (RC) and polycrystalline diamond compact bits (PDC). The property of the formation (soft or hard) determines which type of

Figure 1.1: Basic Elements of a Drill Rig System

the drill-bit to be selected. Figure 1.2 and Figure 1.3 show a tri-cone bit and a PDC bit, respectively