STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 767
(4-73)
where P, = tension load capacity of next (upper) section of drill pipe in lb
Ldp2 = length of section (2) in ft
Wdpz = unit weight of drill pipe (section 2) in lb/ft
Normally, not more than two sections are designed but, if absolutely necessary,
even three sections can be used. To calculate the tensile load capacity of drill
pipe, it is suggested to apply Equation 4-58 and use the recommended makeup
torque of the weakest tool joint for the rotary torque.
The magnitude of the safety factor is very important and usually ranges from
1.4 to 2.8 depending upon downhole conditions, drill pipe quality and acceptable
degree of risk. It is recommended that a value of safety factor be selected
to produce a margin of overpull of at least about 70,000 lb.
Additional checkup, especially in deep drilling, should be done to avoid drill
pipe crushing in the slip area. The maximum load that can be suspended in
the slips can be found from Equation 4-74:
D
(4-74)
where W,= = maximum allowable drill string load that can be suspended in the
slips in lb
Pt = load capacity of drill pipe based on minimum yield strength in lb
Ddp = outside diameter of drill pipe in in
Ls = length of slips (Ls = 12-16 in.)
K = lateral load factor of slip, K = (1 - f tan a)/(f + tan a)
f = friction coefficient between slips and bushing
a = slip taper (a = 9" 27'45")
SF = safety factor to account for dynamic loads when slips are set on
moving drill pipe (SF = 1.1)
Normally, if the drill pipe is sufficiently strong for tension, it will have
satisfactory strength in torsion, collapse and burst; however, if there is any doubt,
additional checkup calculations must be performed.
Example
Design a drill string for conditions as specified below:
Hole depth: 10,000 ft
Hole size: 93 in.
Mud weight: 12 lb/gal
Maximum weight on bit: 60,000 Ib
Neutral point design factor: 1.15
No crooked hole tendency
Safety factor for tension, SF = 1.4
Required margin of overpull: 100,000 lb