Page 99 - Air and gas Drilling Field Guide 3rd Edition

90 CHAPTER 4 Downhole Equipment

The axial tension force (or load), F (lb, N), at the top of the drill string in a ver-
tical well is
h i

F ¼ L p w p þðL c w c Þ K b ; (4-1)
where L p is the length of the drill pipe (ft, m), w p is the weight per unit length of
drill pipe (lb/ft, N/m), L c is the length of the drill collars (ft, m), w c is the weight
per unit length of drill collars (lb/ft, N/m), and K b is the buoyancy factor.
The buoyancy factor is

g m
K b ¼ 1 ; (4-2)
g s
3
3
where g m is the specific weight of the drill fluid (lb/ft , N/m or lb/gal, kg/liter)
3
3
and g s is the specific weight of steel (lb/ft , N/m or lb/gal, kg/liter).
The maximum allowable design axial tension force, F a (lb, N), is
F a ¼ 0:9 F y ; (4-3)
where F y is the drill pipe tension force to produce material (steel) yield (incipient
failure of the drill pipe)(lb, N).
For most drill string designs, a factor of safety is used to ensure that there is a
margin of overpull (MOP) to allow for a stuck drill string. The drill string design
factor of safety, FS, is given by
F a
FS ¼ : (4-4)
F
The MOP is determined by the rotary drilling rig hoisting capacity. Thus, MOP is
MOP ¼ F c F; (4-5)

where F c is the hoisting capacity of the rotary drill rig (lb, N). However, the total
hoisting axial force cannot exceed F a .
Illustrative Example 4.1 A section of the example vertical well (see Figure
2-15) is to be rotary air drilled from 7,000 ft (2133.5 m) to 10,000 ft (3047.9 m)
7
with a 7 8 = -in (200 mm) tricone roller cutter drill bit. This is to be a direct circula-
tion drilling operation. Above the drill bit the BHA is made up of 500 ft (152.4 m)
13
1
of 6 4 = -in (158 mm)by 2 = -in (71.4 mm) drill collars and similar diameter survey
16
subs and nonmagnetic drill collars. The drill pipe available from the drilling con-
1
tractor is API 4 2 = -in (114 mm), 16.60 lb/ft (24.70 kg/m) nominal, IEU-S135,
NC46, new drill pipe. Determine the FS and the effective MOP associated with
using this drill string to drill this section of the well. The drilling rig has a hook load
capability of 300,000 lb (1334.4 kN).
USCS units solution: Reference 1 gives 83 lb/ft (123.48 kg/m) for the unit
13
1
weight of the 6 4 = -in (158 mm)by2 = -in (71.4 mm) drill collar, and an actual
16
1
unit weight of the API 4 2 = -in (114 mm) drill pipe is 19.00 lb/ft (28.27 kg/m)for
the drill pipe. Because the drilling fluid is air, then g m ¼ 0, and K b ¼ 1.0. The
maximum axial tension force in the top drill pipe element of the drill string is when