Page 157 - Introduction to Petroleum Engineering

144 DRILLING

tip of the drill bit. The drill bit is used to grind, break, or shear the rock at the bottom
of the well. Optional attachments on the drill string may include stabilizers, jars, junk
baskets, mud motors, shock absorbers, and so forth.
Drill pipe and collars are rated by their size (outer diameter), weight per unit of
length, grade (the steel material and manufacturing process), and connections used to
screw the string together. The ratings are specified by international standards. Joints
of drill pipe and drill collar are compared in Figure 8.6. Connections from one pipe
to another are called box and pin connections.
The drill string is subjected to torsional stress caused by the torque associated
with the rotary drilling. Shear or torsional stress τ of a pipe subjected to a torque is
given by (Hibbeler, 2011)
Tr
τ = outer (8.5)
J

where T is the torque, r outer is the outer radius of the pipe, and J is the polar moment
of inertia. The polar moment of inertia is
π 4 r )
4
r
J = ( outer − inner (8.6)
2
for a pipe with inner and outer radii r and r . The angle of twist is
inner outer
TL
φ = (8.7)
GJ
where ϕ is the angle of twist, L is the length of the pipe (or pipe string), and G is the
shear modulus.

Drill pipe Drill collar

Box
connection

Pin
connection

FIGuRe 8.6 Drill pipe and drill collar.