Page 104 - Air and Gas Drilling Manual

3-32 Air and Gas Drilling Manual
from the annulus between the inside of the borehole and the air hammer housing to
the inside of the innertube of the dual wall pipe.

Figure 3-26: Skirted tri-cone roller cutter drill bit for dual pipe reverse circulation
drilling operations (courtesy of Lang Exploratory Drilling).

Table 3-6 gives the dimensions and mechanical properties of the dual wall pipe
designed with O-ring sealed innertubes and threaded outer tubes (i.e., concealed
innertube pipe of Drill Systems from Foremost Industries Incorporated). The
working torque values are the maximum recommended torque to be applied to the
dual wall drill string during rotary drilling operations. The tension to yield values
are the limiting axial tension loads that will initiate the material yield in the top
pipe element in the dual pipe drill string. This yield can be in the outer tube of the
dual pipe body or in the shouldered threaded connection, whichever is the weakest.

Table 3-6: Concealed innertube pipe (of Drill Systems) dual wall pipe dimensions and
mechanical properties (courtesy of Foremost Industries Incorporated).
Dual Wall Pipe OD 3.75 in 4.00 in 4.50 in
Outer Tube ID 3.25 in 3.50 in 4.00 in
Innertube OD 2.125 in 2.25 in 2.750 in
Innertube ID 1.875 in 2.00 in 2.375 in
Pipe Wt/ft 12.6 lb/ft 14.6 lb/ft 21.3 lb/ft
Working Torque 4,800 ft-lb 7,000 ft-lb 8,900 ft-lb
Tension to Yield 200,600 lb 244,200 lb 210,100 lb
Lengths Available 10 ft 10 ft 20 ft