Page 158 - Introduction to Petroleum Engineering

ROTARY DRILLING RIGS 145

example 8.2 Torsion in a Drill String
Torsion in a drill string is caused by a twisting moment (aka rotary torque T).
Calculate the angle of twist (in degrees) for a 9.144 m length of pipe subjected
⋅
to a rotary torque = 5400 Nm. The shear modulus of elasticity is 75 GPa, and
−
×
5
4
.
the polar moment of inertia of the drill string is 10 10 m .
Answer
⋅ (9 144m
TL 5400Nm . ) 360 °
Angle of twist φ = = = 0 0658 rad. × = 377 °
.
π
−5
GJ 75 ×10 Pa (10. ×10 m 4 ) 2π
9
Rotary bits are either roller‐cone (tricone) bits or drag bits. Materials used for
cutting surfaces of bits depend on what type of formation the drill bit will encounter.
Roller‐cone bits can have steel teeth or tungsten carbide buttons as shown in
Figure 8.7. Steel teeth are sufficient for softer formations, while tungsten carbide
buttons are used for harder formations. Diamond is used in two different forms for
drill bits: whole diamond and fused diamond grit, also known as polycrystalline dia-
mond compact (PDC). PDC bits have PDC discs bonded to tungsten carbide posts
mounted on the surface of a bit. PDC bits are good for drilling hard formations.
Diamond‐impregnated bits have whole diamonds bonded to the surface of a bit.
These bits can be used for the hardest formations.
In addition to cutting surfaces, bits must have nozzles for scouring cuttings from
the rock surface. The nozzles allow drilling fluids to pass through the drill string and
drill bit into the wellbore. Roller‐cone bits also have bearings. Bits operate in harsh
and abrasive conditions that produce significant wear to cutting surfaces, nozzles,
and bearings, eventually ending in bit failure as indicated by declining rate of

Pin
connection

Mud
nozzle or
jet
Leg

Cone
with
buttons

FIGuRe 8.7 Roller‐cone or tricone drill bit.