Page 100 - Air and gas Drilling Field Guide 3rd Edition

4.6 Drill String Design 91

the depth is 10,000 ft (3047.9 m) and when the drill bit is lifted off the bottom of
the well (after the target depth has been reached). The maximum axial tension
force is determined from Equation (4-1). This is
h i
ð
F ¼ ð 9500Þ 19:00Þ þ 500Þ 83:00Þ 1:0Þ
ð
ð
ð
F ¼ 222000 lb:
This tension force to yield is 595,004 lb (2646.6 kN)[1]. Equation (4-3) can be
used to determine the maximum allowable axial tension force for the drill pipe.
Thus, Equation (4-3) is
F a ¼ 0:9 595004Þ
ð
F a ¼ 535504 lb:

The factor of safety is determined from Equation (4-4). Equation (4-4) gives
535504
FS ¼
222000
FS ¼ 2:41:

The selected rotary drilling rig has a maximum hoisting capacity of 300,000 lb,
thus using Equation (4-5) the effective MOP is

MOP ¼ 300000 222000
MOP ¼ 78000 lb:
The calculations just given show that an additional 78,000 lb (above the weight of
the drill string) can be pulled by the hoist system or with the assurance that the
drill string will not fail structurally.
SI units solution: The unit mass of the 158-mm by 71.4-mm drill collar is 123.48
kg/m, and the actual unit mass of the API 114 mm drill pipe is 28.27 kg/m. Because
the drilling fluid is air, then g m ¼ 0and K b ¼ 1.0. The maximum axial tension force
in the top drill pipe element of the drill string iswhen the depth is 3047.9 m and when
the drill bit is lifted off the bottom of the well (after the target depth has been
reached). The maximum axial tension force is determined from Equation (4-1). This is
h i
ð
ð
ð
ð
F ¼ ð 2895:5Þ 28:27Þ 9:81Þ þ 152:4Þ 123:48Þ 9:81Þ 1:0Þ
ð
ð
F ¼ 987613 N:
This tension force to yield is 2646.6 kN [1]. Equation (4-3) can be used to
determine the maximum allowable axial tension force for the drill pipe. Thus,
Equation (4-3) is
F a ¼ 0:9 2646600Þ
ð
F a ¼ 2381940 N: