STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 747
(4-63)
where Pc = minimum collapse pressure resistance as specified by API in psi
PCc = corrected collapse pressure resistance for effect of tension in psi
Ym = minimal yield strength of pipe in psi
Substituting Equation 4-63 and Equation 455 into Equation 4-54 (note: or = 0,
z = 0, oe = Y,) and solving PCc yields
(4-64)
or
P,, =
(4-65)
Equation 4-65 indicates that increased tensile load results in decreased collapse
pressure resistance. The decrement of collapse pressure resistance during normal
DST operations is relatively small; nevertheless, under certain conditions, it may
be quite considerable.
Example
Determine if the drill pipe is strong enough to satisfy the safety factor on
collapse of 1.1 for the DST conditions as below:
Drill pipe: 4+-in., 16.6-lb/ft nominal weight, G-105 steel grade, class 2
Drilling fluid with a density of 12 lb/gal and drill pipe empty inside
Packer set at the depth of 8,500 ft
Tension load of 45,000 lb, applied to the drill pipe
From Table 4-84, the collapse pressure resistance in uniaxial state of stress,
Pc - 6,010 psi. Reduced wall thickness for class 2 drill pipe = (0.65)(0.337) =
0.219 in. Reduced Ddp for class 2 drill pipe = 3.826 + (2)(0.219) = 4.264 in.
Reduced cross-sectional area of class 2 drill pipe equals:
IC
IC
-(4.264)'--(3.826)'
4 4
= 2.783in.'
The axial tensile stress at packer level is
o z = L = 45 Oo0 16,170psi
2.783