Page 105 - Air and Gas Drilling Manual

Table 3-6: Continued.
7.00 in
5.50 in
Dual Wall Pipe OD
9.625 in
6.00 in
8.625 in
Outer Tube ID 4.50 in Chapter 3: Downhole Equipment 3-33
Innertube OD 3.75 in 5.00 in 6.625 in
Innertube ID 3.25 in 4.50 in 6.063 in
Pipe Wt/ft 27.4 lb/ft 40.2 lb/ft 64.3 lb/ft
Working Torque 15,500 ft-lb 30,500 ft-lb 97,700 ft-lb
Tension to Yield 206,900 lb 466,000 lb 466,000 lb
Lengths Available 20 ft 20 ft 20 ft
Another type of dual wall pipe is fabricated with a threaded shouldered
connection (box and pin) across both the outer and innertubes at each end of the pipe
element. This dual wall pipe design gives a contiguous single pipe element (no
separate innertube). This threaded shouldered connection is a modified 3 1/2 inch
API Regular connection (1/2 inch longer and 1/2 inch larger diameter). Each box
and pin is fabricated with elliptical “windows” located approximately halfway
between the shoulders (see Figures 3-27a and 3-27b).
These windows allow flow communication from the annulus space of one dual
wall pipe element to the annulus space of another dual wall pipe element. The
central hole in the connection allows unobstructed return flow of air and entrained
rock cuttings through the inside of the inside tube of the pipe elements (see Figures
3-27a and 3-27b). This fully threaded dual wall pipe is available in 4.50 inch and
6.625 inch outside diameters. Table 3-6 gives the dimensions and mechanical
properties for the these fully threaded dual wall pipe sizes.
An alternate to the above fully threaded shouldered connection is the quick
release hex head connection dual wall pipe. This design also gives a contiguous
single pipe element (no separate innertube). Figure 3-28 shows these hex head
connections. The “box” and “pin” hex heads of ends of two separate pipe elements
are fitted together and hardened steel rods inserted in side slots to secure the
connections. The hex heads transmit the torque of the rotary drilling action through
the drill string. The hardened steel pins transmit the axial tension and compression
through the drill string. The four round holes in the faces of the box and pin
connections allow for flow communication from annulus space of one dual wall pipe
element to the annulus space of another dual wall pipe element. The central hole in
the connection allows unobstructed return flow of air and entrained rock cuttings
through the inside of the inside tube of the pipe element (see Figure 3-28). In
general, the quick release hex head connection is used for the larger diameter dual
wall pipe, e.g., 8.625 inch and 10.75 inch outside diameters (or larger).
Table 3-7 gives the dimensions and mechanical properties for these quick release
hex head dual wall pipe sizes. 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
solid rods that are used to make up the connections, which ever is the weakest.
Dual wall pipe are uniquely designed for reverse circulation operations with
compressed air (or other gases). The large annulus space in this type of drill string
acts as a pressure vessel. The air volumetric flow rates in a reverse circulation
operation are low since the rock cuttings are carried to the surface through the inside