STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 763
Unit weight of Hevi-Wate(') drill pipe = (41)(0.847) = 34.72 Ib/ft. Part of weight
on bit that may be created using drill collars = (93.18)(330)(cos 40) = 23,555 lb.
Required length of Hevi-Wate'') drill pipe is
(40000- 23555)(1.5) = ,llft
(34.72)(cos40)
Assuming an average length of one joint of Hevi-Wate(c) drill pipe to be 30 ft,
24 joints are required.
Fatigue Damage of Drill Pipe
It should be understood that the majority of drill pipe and tool joint failures
occur as a result of a fatigue damage. The problem of fatigue failure is not
adequately researched; however, it is basically agreed that tension and bending
(reversing tension and compression of the same drill pipe fiber), magnified by
vibrations, contribute the most to such type of failure. Cycling stress results in
a crack that spreads across the cross-section and causes ultimate failure. A
fracture begins at a point on or near the surface that is weaker than any other
due to a number of reasons (e.g., surface imperfections or stress raisers). Fatigue
cracks can also initiate at points below the surface of the drill string if the
proper conditions exist. It should also be remembered that drill pipe fatigue is
cumulative in nature, so the changes that affect failure are usually long delayed
and require a certain amount of time to be detected.
The drill collars and particularly their connections are also exposed to cyclic
stresses. Subsequently, these are susceptible to fatigue damage, but the changes
that may influence failure are more quickly discovered.
Based on work done by A. Lubinski, J. E. Hansford and R. W. Nicholson (API
RP 7G, Section 6), gives the formula for the maximum permissible hole curvature
in order to avoid fatigue damage to drill pipe.
432,000 G, tanh( KL)
c=-- = ED,, KL
K = (6)"
(4-66)
(4-6'7)
For Grade E drill pipe
(T, = 19,500 - (0.149)~~ = 1.34 (4
For Grade S-135 drill pipe
33,500)2
(4-68)
Ob = 20,000 1 -
(-
14;kOO)
T
0, = -
A
(4-69)
(4-70)