Page 242 - Air and Gas Drilling Manual

5-84 Air and Gas Drilling Manual


[
2
503
.27
P
=  ( ,695
1
)
27
,
27
+ ,431
3
ai


2 (503 . ) 2 ] e 2 (. 0 026 ) ( , 1 200 ) − 3 431 (503 . ) 2      . 05
P ai = 10 960 lb/ft abs
,
P ai
p ai =
144
.
p ai = 76 1 psia
This is the pressure at the bottom of the inside of the innertube in the dual wall pipe
just above the drill bit.
With the large center opening in the drill bit, the pressure change in the air flow
from the bottom of the annulus to the bottom of the inside of the drill string (inside
the drill bit) is assumed to be negligible. The inside of the bottom of the drill
string at the drill bit is assumed to have the same pressure as that at the bottom of
the annulus. Thus,
p bh = 76 1 psia
.
P = p 144
bh bh
2
P bh = 10 960 lb/ft abs
,
The outside diameter of the innertube of the dual wall pipe is 2.750 inches and
the inside diameter of the outer tube of the dual wall pipe is 4.000 inches (see Table
3-6). These pipe surfaces form the annulus of the dual wall pipe. Thus,
d oi = 2 750 inches
.
d oi
D =
oi
12
.
D oi = 0 229 ft
and
.
d io = 4 000 inches
d io
D io =
12