Dual Wall Pipe

Dual wall drill pipe elements are used exclusively in reverse circulation drilling operations. This type of drill pipe is used for drilling shallow (~ 3,000 ft or less) water wells, environmental monitoring wells, geotechnical boreholes, and mining boreholes. These rotary drilling operations can be; a) rotation of the dual wall pipe drill string with a tri-cone or drag type bit, b) rotation of the dual wall pipe drill string with a standard downhole air hammer (with standard air hammer bit), or c) rotation of the dual wall pipe drill string with a reverse circulation downhole air hammer (with reverse circulation air hammer bit). These rotary drilling operations are carried out with hydraulic top drives (for single rotary drilling rigs) and with power swivels (for double and triple rotary drilling rigs).

Figure 3-25 shows the schematic of a typical dual wall drill string set up for five optional bottomhole assemblies. This schematic utilizes the concealed innertube pipe type of dual wall pipe. This type of dual wall pipe has an innertube that is O-ring slip fitted into a similar innertube in the next element of drill pipe when elements are made up to each other. The outer tube of the dual wall pipe are made up with non-API shouldered connections.

At the far right of Figure 3-25 is a tri-cone bit bottom assembly for conventional (reverse circulation) rotary drilling operations. This tri-cone bit threads to a shouldered innertube of the latching bit assembly. The outer tube fits over the top of the drill bit and allows the compressed air to flow from the annulus of the dual wall pipe to the roller cutters of the bit. The bit has a large inner hole that allows the compressed air with entrained cuttings to flow to the surface through the innertube of the dual wall pipe.

Next (moving from right to left) are two alternate conventional rotary drilling bottomhole assemblies. One is an open face bit (drag bit) for coring operations and the other is another tri-cone drill bit configuration. This latter tri-cone bit assembly makes use of a skirted rock bit sub which is often integral to the bit itself. Figure 326 shows a skirted tri-cone bit for a dual wall drilling operation.

The next two bottomhole assemblies are for rotary drilling operations using downhole air hammers. The first of these assemblies from the right is a reverse circulation hammer with a large opening in the air hammer bit for the returns (see Chapter 11). The last assembly is a standard direct circulation downhole air hammer with an interchange sub that allows the return air with entrained rock cuttings to flow

Borehole Hammer Bits

Figure 3-25: Dual wall pipe drill string schematic showing three drill bit string options and two downhole air hammer string options. Nomenclature of above; 1) adapter sub, 2) air swivel, 3) O-ring, 4) air swivel inlet, 5) O-rings, 6) shaft adapter, 7) O-rings, 8) saver sub adapter, 9) O-rings, 10) center tube, 11) saver sub, 12) O-rings, 13) dual wall drill pipe, 14) interchange connector tube, 15) O-rings, 16) interchange sub, 17) standard down-the-hole (DTH) air hammer, 18) connector tube for reverse circulation downhole air hammer, 19) O-rings, 20) reverse circulation downhole air hammer, 21) O-rings, 22) bit connector sleeve, 23) rock bit sub, 24) wear sleeve, 25) tri-cone bit, 26) open face drag bit for coring and, 27) latching bit sub (courtesy of Foremost Industries Incorporated).

Figure 3-25: Dual wall pipe drill string schematic showing three drill bit string options and two downhole air hammer string options. Nomenclature of above; 1) adapter sub, 2) air swivel, 3) O-ring, 4) air swivel inlet, 5) O-rings, 6) shaft adapter, 7) O-rings, 8) saver sub adapter, 9) O-rings, 10) center tube, 11) saver sub, 12) O-rings, 13) dual wall drill pipe, 14) interchange connector tube, 15) O-rings, 16) interchange sub, 17) standard down-the-hole (DTH) air hammer, 18) connector tube for reverse circulation downhole air hammer, 19) O-rings, 20) reverse circulation downhole air hammer, 21) O-rings, 22) bit connector sleeve, 23) rock bit sub, 24) wear sleeve, 25) tri-cone bit, 26) open face drag bit for coring and, 27) latching bit sub (courtesy of Foremost Industries Incorporated).

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.

Drill Bit Such Triple Head Drills
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

Table 3-6: Continued.

Dual Wall Pipe OD

5.50 in

7.00 in

9.625 in

Outer Tube ID

4.50 in

6.00 in

8.625 in

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 tube of the dual wall pipe (see Chapter 8). Therefore, the velocity of the air flow in the annulus is slow. The flow in the annulus at each connection is somewhat obstructed and therefore not suited for direct circulation. The inside tube, however, has no obstructions at the connections and allows free flow of the compressed air with entrained rock cuttings.

Drill Pipe Connection Table
Figure 3-27a: Pin threaded shouldered connection for dual wall pipe (courtesy of Holte Manufacturing Company).
Api Regular Connection

Figure 3-27b: Box threaded shouldered connection for dual wall pipe (courtesy of Holte Manufacturing Company).

In general dual wall pipe elements are structurally very rigid and stiff. Thus, this type of pipe can be used in compression. This eliminates the need for heavy drill collars to place WOB in a reverse circulation, dual wall pipe drilling operation.

Figure 3-28: Box and pin of the quick release hex head connections for dual wall pipe (courtesy of Holte Manufacturing Company).
Table 3-7: Fully threaded connection and quick release hex head connection dual wall pipe dimensions and mechanical properties (courtesy of Holte Manufacturing Company).

Dual Wall Pipe OD

4.50 in

6. 625 in

8.625 in

10.75 in

Outer Tube ID

4.00 in

5.921 in

7.825 in

9.850 in

Innertube OD

2.875 in

4.50 in

5.00 in

7.000 in

Innertube ID

2.259 in

4.00 in

4.408 in

6.276 in

Pipe Wt/ft

23.0 lb/ft

39.0 lb/ft

56.0 lb/ft

90.0 lb/ft

Working Torque

8,000 ft-lb

26,000 ft-lb

42,500 ft-lb

85,000 ft-lb

Tension to Yield

205,000 lb

380,000 lb

410,000 lb

430,000 lb

Lengths Available

10 ft

20 ft

20 ft

20 ft

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