EP2358971A1

EP2358971A1 - Drill string

Info

Publication number: EP2358971A1
Application number: EP09828432A
Authority: EP
Inventors: Hendrik Corporaal
Original assignee: Camstar Nominees Pty Ltd
Current assignee: Camstar Nominees Pty Ltd
Priority date: 2008-11-25
Filing date: 2009-11-24
Publication date: 2011-08-24
Also published as: WO2010060134A1; AU2009321521A1

Abstract

A reverse circulation drill string including two or more multi-walled drill pipe sections coupled together in series. Each drill pipe section includes at least three concentrically arranged annular sections generally coaxially aligned with a drilling axis. The annular sections include an outer pipe, an intermediate tube enclosed within the outer pipe, the outer pipe and intermediate tube having a first axial conduit there between, and an inner tube arranged within the intermediate tube. The intermediate tube and the inner tube have a second axial conduit there between, the interior of the inner tube providing a third axial conduit therein. The second axial conduit is arranged for the passage of pressurised fluid to a drilling head of the drill string, the third axial conduit is configured as a cuttings passage through which cuttings from the drill head can travel, and the first axial conduit is arranged for communication with the environment external of the drill string.

Description

DRILL STRING

Field of the Invention

The present invention generally relates to a drill string and more particularly to a drill string which is suitable for reverse circulation drilling.

Background of the Invention

The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

Reverse circulation drill strings are used in the drilling industry to extract representative and substantially continuous samples of material in a drilling zone. Most reverse circulation drill strings known to the applicant include a drill rod formed from a series of interconnecting drill pipe sections. Such rods typically have a dual wall construction consisting of an inner tube enclosed within an outer pipe. This is the nomenclature used in the industry to describe these two parts of the drill string. Pressurised fluid such as compressed air, water, oil or mixtures thereof is pumped along the axial length of the drill rod to the drill head through an axial pressure conduit defined by the annular space between the outer pipe and inner tube. The fluid is used to flush cuttings from the drill face of the drill head axially upwardly through the interior of the inner tube to the surface where the cuttings can be collected for analysis. Air compressed up to pressures of more than I OOOpsi can be pumped through the axial pressure conduit to flush cuttings to the surface. Each of the inner tube and outer pipe must therefore have a construction similar to a high pressure vessel to withstand the large pressures pumped between these sections.

However, the outer pipe can undergo a certain unpredictable amount of abrasive wear during drilling when the outer surface of the outer pipe engages and rubs against the wall of a drill hole. Such wear can weaken the integrity of the outer pipe to the point of failure. If failure occurs when the pressure conduit contains high pressure air, the outer pipe can explode and discharge shrapnel dangerously. The severity of the explosion can be exacerbated through the use of an air and oil mixture used in some drilling operations to lubricate the drill head. As can be readily appreciated, such an occurrence is a significant safety concern in the drilling industry.

It would therefore be desirable to provide an alternative reverse circulation drill pipe assembly which reduces or substantially alleviates the risk of failure of a reverse circulation drill string.

Summary of the Invention

According to the present invention, there is provided a reverse circulation drill string including two or more multi-walled drill pipe sections coupled together in series. Each drill pipe section includes at least three concentrically arranged annular sections generally coaxially aligned with a drilling axis. The annular sections include an outer pipe, an intermediate tube enclosed within the outer pipe, the outer pipe and intermediate tube having a first axial conduit therebetween, and an inner tube arranged within the intermediate tube. The intermediate tube and the inner tube have a second axial conduit therebetween, the interior of the inner tube providing a third axial conduit therein. The second axial conduit is arranged for the passage of pressurised fluid to a drilling head of the drill string, the third axial conduit is configured as a cuttings passage through which cuttings from the drill head can travel, and the first axial conduit is arranged for communication with the environment external of the drill string.

Accordingly, the reverse circulation drill string includes at least three concentric annular sections which are axially aligned with a drilling axis, being the axis about which the drill string rotates. Unlike previous reverse circulation drill string configurations known to the applicant, a pressurised fluid, such as compressed air, flows through a passageway which is remote to the outer pipe of the drill string. This passageway, or pressure conduit, is an axial conduit located between the intermediate tube and the inner tube of each drill pipe section. The passageway is thus separated from the wall of a drill hole by the first axial conduit and by the outer pipe.

Advantageously, a drill string according to the present invention can facilitate pressure relief of the pressurised fluid from the second axial conduit to the external environment in the event that the intermediate tube fails. If the intermediate tube does fail, the first axial conduit between the outer pipe and intermediate tube is configured to provide a pressure relief passage through which pressurised fluid leaking from the second axial conduit can vent to an environment external to the drill string. In most cases, the external environment would be the ambient atmosphere, although in undersea drilling the external environment may be a body of water. Consequently, the drill string arrangement according to the present invention ensures that the first axial conduit between the outer pipe and the intermediate tube can never be substantially pressurized. This is an important safety measure which can reduce or even substantially alleviate damage caused by failure of the outer pipe when under pressure.

Moreover, whilst the outer pipe undergoes abrasive wear during drilling operations, the intermediate tube is substantially untouched, thereby providing greater certainty of the integrity of the pressure conduit during drilling operations.

Still further if the intermediate tube fails, any shrapnel which is discharged will be substantially or fully confined within the outer pipe.

The outer pipe can include any suitable venting means that allows the first axial conduit to communicate with an environment external to the drill string. In one embodiment, the first axial conduit is configured to be open to the external atmosphere. This can be accomplished using any number of arrangements which provide an opening or other form of door, passageway, link, or the like in a section of the outer pipe. In one form, at least a portion of the outer pipe of at least one drill pipe section includes one or more vent holes which allow fluid communication between the first axial conduit and the atmosphere around the outer pipe. The vent holes can comprise two or more bore holes which can be axially spaced about the circumference of the outer pipe of the at least one drill pipe section. It is preferable that the at least one drill pipe section in which the vent holes are located is positioned substantially above the surface of the drilling zone during operation of the drill string to allow the pressurised fluid to vent into the atmosphere. Preferably, the vent holes are located in a saver sub type drill pipe section. The vent holes can therefore be located in one or more of a pin-to-pin, box-to-box, pin-to-box and box-to-pin saver sub section which is connected directly below an air swivel section of the drill string.

One or more of the vent holes can include a pressure relief valve which allows the first conduit to communicate with the external atmosphere when pressure in the first conduit exceeds a set pressure. The set pressure can be any pressure equal to or greater than atmospheric pressure. However, it is preferred that the set pressure is greater than 100 psi, more preferably greater than 200psi.

The outer wall of the pressure conduit of a drill string according to the present invention is not subject to a comparable degree of unpredictable wear as an equivalent two pipe drill string known to the applicant because this wall is enclosed within the outer pipe of each drill pipe section in the drill string according to the present invention. As can be appreciated, such a configuration provides a greater degree of predictability in the integrity of the walls of the pressure conduit. Consequently, the pressure conduit can undergo pressure testing, and be given a substantially predictable pressure rating. Accordingly, in some embodiments, the second axial conduit is tested and is officially certified to hold a particular pressure. Preferably, the certified pressure is equal to or greater than 1000 psi. More preferably, the certified pressure is equal to or greater than 1500 psi.

Each of the first and second axial conduits can have any number of configurations (axial shape and radial cross-section) along their length. However it is preferred that the first axial conduit substantially comprises the annular cavity defined between the inner wall of the outer pipe and outer wall of the intermediate tube and the second axial conduit substantially comprises the annular cavity defined between the inner wall of the intermediate tube and outer wall of the inner tube. The first and second axial conduits therefore have a substantially annular radial cross-section which generally runs the axial length of the pipe section.

The drill string of the present invention can include three, four, five or more concentrically arranged annular sections. However, one preferred form of the drill string according to the present invention includes three concentrically arranged annular sections, being an inner tube which functions as a cuttings passage through which drill cuttings travel from the operation end or drilling head of the drill string to the surface; an intermediate tube concentrically arranged around the inner tube to form an axial annular conduit therebetween through which pressurised air can be pumped to the operational end of the drill string; and an outer pipe concentrically arranged around the intermediate tube which functions as an outer casing for the drill section and is subject to wear. When all three annular sections are assembled into a single unit, the annular sections form a drill pipe section of the drill string. The drill string is constructed by coupling together adjacent upper and lower ends of each individual drill pipe section. In this respect, each of the outer pipes, intermediate tube and inner tube include coupling sections to interconnect with complementary coupling sections of axially adjacent drill pipe sections.

The coupling sections of the outer pipe, intermediate tube and inner tube can include any suitable coupling that interlinks the axial ends of adjacent coupling sections. It is preferable however that each coupling section releasably engages the coupling section of each adjacent outer pipe, intermediate tube and inner tube respectively to allow the drill string to be assembled and disassembled when in use.

One preferred coupling section includes a threaded connection so that respective coupling sections have interengaging male and female threaded surfaces which couple together to releasably secure the coupling sections together. It is preferred that adjacent axial ends of the outer pipe are connected by a coupling section which includes a threaded connection. One preferred form of the threaded connection used to releasably secure adjacent ends of a pair of the outer pipes together is a box and pin threaded coupling. The box and pin threaded coupling can include an internally threaded coupling sleeve (the box) which receives a cooperating externally threaded distal pipe section (the pin). The threaded surfaces of the coupling are preferably located on axially tapered sections. Preferably, the threaded sleeve is included on a lower axial end of an outer pipe section relative to the drilling direction and the externally threaded pin is included on an upper axial end of the outer pipe section relative to the drilling direction.

The coupling section of the outer pipe section can be integrally formed with the outer pipe section. However, for convenience of manufacture it is preferred that if the coupling section of the outer pipe is threaded it is formed separately to a body portion, typically an elongate generally tubular section, of the outer pipe. The separate threaded coupling section is coupled to the body portion prior to assembly of the drill pipe section. Preferably, the threaded coupling section is welded to the body portion of the outer pipe.

Whilst the coupling sections of the intermediate tube and inner tube could also include a threaded connection, it is preferred that these sections are coupled together using a different connection configuration. Preferably, axially adjacent sections which form the intermediate tube and/or the inner tube are coupled together using a sliding connection. One preferred form of sliding connection includes a sleeve section configured to tightly receive an adjacent distal end of an intermediate tube or inner tube respectively in abutting engagement.

Preferably, the sliding connection includes one or more circumferential o-rings which provide a pressure seal between the engaging ends of the sliding connection. More preferably, the one or more o-rings are housed on an inner surface of the sleeve section. The pressure seal between the engaging ends of the intermediate tube or inner tube can preferably withstand a pressure greater than 1000 psi, and more preferably greater than 1500 psi.

The sliding connections between respective adjacent ends of pairs of intermediate tubes and adjacent ends of pairs of inner tubes can be of generally the same or similar configurations. However, it is preferable to configure each axial end of a drill pipe section with the sliding connection between intermediate tubes axially offset from the corresponding sliding connection between inner tubes. Such a configuration offsets any point of weakness that may be provided by a seal of each sliding connection relative to the drilling axis. It is also preferable that the sleeve section and distal end of the intermediate tubes that form a sliding connection has the opposite axial orientation to the axial orientation of the sleeve section and distal end of the inner tubes that form a further sliding connection. In one preferred embodiment, the sliding connection of a pair of intermediate tubes includes a sleeve on a lower end of one intermediate tube which receives an upper distal end of a lower axially adjacent intermediate tube and the sliding connection of a pair of inner tubes includes a sleeve on an upper end of one inner tube which receives a lower distal end of an upper axially adjacent inner tube.

Each pipe section of the reverse circulation drill string can be preassembled to form a single assembly prior to assembly in the drill string. In some embodiments, each of the outer pipe, intermediate tube and inner tube are interconnected, typically welded, at certain locations along the length of the pipe section. However, in a preferred embodiment, the outer pipe is assembled on the drill string as a separate section, with the intermediate and inner tubes being connected together at certain locations along the length of each tube to form a separate assembly to the outer pipe. In this form, the intermediate and inner tube assembly can be assembled on the drill string and then the outer pipe can be slid or otherwise inserted over and around this assembly.

The assembly of the intermediate and inner tubes can be formed by concentrically interconnecting each respective tube about the drilling axis using one or more radial connectors. Such radial connectors can be welded to each of the intermediate and inner tubes, thereby radially interconnecting the intermediate and inner tubes into a single assembly. Other radial connectors can be welded to the intermediate tube or the inner tube to function as radial spacers that radially space the intermediate tube from the inner tube along the axial length of the pipe section. Similarly, radial spacers can be provided between the intermediate tube and outer pipe within the first conduit to radially space the outer pipe from the intermediate tube in a concentric arrangement. Such spacers can be affixed to the outer surface of the intermediate tube or inner surface of the outer pipe.

The configuration of the pipe sections according to the present invention can require certain elements of the drill string assembly to be modified to operate. In this respect, the new configuration requires a modified air swivel device to fed pressurised fluid, such as pressurised air or an air mixture (air and oil or the like), into the second axial conduit. The pressurised fluid is preferably fed into the second axial conduit using an air swivel device having an air inlet having a circumferential inlet connection with the second conduit, with the first axial conduit terminating in a section of the air swivel device.

Brief Description of the Drawings

The present invention will now be described with reference to the figures of the accompanying drawings, some of which illustrate particular preferred embodiments of the present invention, wherein:

Figure 1 is a cross sectional schematic view of a prior art reverse circulation drilling assembly having a two pipe construction.

Figure 2 is a part cross sectional view of a pipe section of a reverse circulation drill string incorporating the present invention.

Figure 3 is a second cross sectional view of a portion of two pipe sections of the reverse circulation drill string of Figure 2, highlighting in greater detail the features of the connection sections of the drill string.

Figure 4 is a cross sectional view of the connection sections of the outer pipe of the reverse circulation drill string of Figures 2 and 3. Figure 5 is a cross sectional view of an air swivel and an attached saver sub section of a reverse circulation drill string according to the present invention.

Detailed Description

It should be appreciated that in the following description the designation of upper and lower orientations are used relative to the drilling direction of the drilling string assembly 10, as shown in the figures. In this respect, a drilling string typically has a downwardly directed drilling direction into a drilling medium 11 such as the ground, rock or the like as shown by arrow D in Figure 1.

Referring to Figure 1 , there is shown one existing form of a two pipe reverse circulation drill string duel pipe assembly 10 known to the applicant. The illustrated assembly 10 shows the general arrangement of components of a reverse circulation drill string assembly.

The two pipe reverse circulation drill string assembly 10 illustrated in Figure 1 , consists of two basic assemblies, an upper assembly 12 and a lower drilling assembly 14.

The upper assembly 12 includes those components which remain generally above the ground during drilling operation of the drill assembly 10. The upper assembly 12 includes a deflector channel 16 which connects the drill string assembly 10 to a cyclone (not illustrated), a rotary drive device 18 which drives the axial rotation of a drill string 17 including the drilling assembly 14, and an air swivel 20 which pumps pressurised air into a pressure conduit 22 within the drill string 17.

The deflector channel 16 is an elbow connector which deflects upwardly flowing cuttings 69 from the cutting channel 24 by about 45 degrees to re-direct the flow to a cyclone (not illustrated) for separation. Given the abrasive nature of most cuttings 69, the deflector channel 16 includes a wear plate 26 on an upper surface of the deflector channel 16 against which cuttings 69 collide. The wear plate 26 can be replaced once worn to an unusable condition. The rotary drive device 18 is connected axially below the deflector channel 16 relative to the drilling direction through connector 27, which in this case is a sample swivel section. The rotary drive device 18 comprises an axially mounted stationary housing 19 which is drivingly engaged with a driven spindle section 34. The spindle section 34 is therefore driven about the drilling axis X-X of the drill string assembly 10. The housing 19 includes two or more drive motors 28, typically hydraulic drive motors, which are drivingly connected to peripheral drive pinions (not illustrated) which inturn engage a belt or chain 30 which drive a central bull gear 32. Gearing can also be provided between the drive motors 28 and drive pinions. The central bull gear 32 is drivingly engaged to a spindle section 34 axially connected to the drill string 17.

The air swivel 20 is directly connected to the lower axial end of the drive spindle 34. The air swivel 20 provides an annular inlet 35 into the pressurised fluid conduit 22 through which pressurised fluid, such as air, can be pumped. The air swivel 20 includes two operatively engaged sections, being a first stationary outer housing 36, and a rotary swivel spindle 38 which is threadably connected to the lower axial end of the drive spindle 34 of the drive device 18 and rotated by this spindle 34 about the drilling axis X-X. The outer housing 36 provides a stationary fixture onto which a pressurised fluid source (not illustrated) can be connected. In this respect, the outer housing 36 includes a fluid inlet 40 which provides a sealed conduit between the pressurised fluid source and annular inlet 35. The outer housing 36 also functions as a mount in which the spindle 34 can freely rotate. The cuttings conduit 24 passes through the centre of each of the air swivel 20 and rotary drive device 18.

The lower drilling assembly 14 includes the drilling components of the drill string assembly 10 which are generally near or down the drill hole 13 during drilling operations. The lower drilling assembly 14 includes a number of drill pipe sections including a saver sub section 42 and multi-section drill rod 44, and a drilling tool 46, in this case a pneumatically driven percussion hammer, attached to the operative end of the drill string assembly 10. Each section of the lower drilling assembly 14 is rotatable about the drilling axis X-X. The saver sub section 42 comprises two connector sections 48 and 50 axially connected to the lower end of the swivel spindle 38 of the air swivel 20. The saver sub sections 48 and 50 are short drill pipe sections which are used to protect the connection thread of the air swivel 20. The first saver sub connector section is a box-to-box sub 48, which as the name suggests is a connector section having two internally threaded box connector sections 49 at each axial end. The box-to-box sub 48 is threadably engaged to the lower end of the swivel spindle 38. The second saver sub connector section is a pin-to-pin sub 50, which as the name suggests is a connector section having two externally threaded pin connector sections 51 at each axial end. The pin-to-pin sub 50 connects to an upper box connector 53 of the drill rod 44. Each saver sub 48 and 50 has essentially the same dual pipe construction as the drill rod 44.

The drill rod 44 is formed from a series of interconnected drill pipe sections 52, which are coupled together at each adjacent axial end. The illustrated drill rod 44 has a two pipe construction, having an outer pipe 54 enclosing a smaller diameter inner tube 56. The inner tube 56 is formed from a series of slidably connecting pipe lengths 57 which are sealed together using one or more o-rings 59. The outer pipe 54 is formed of a series of annular pipe segments 60 having threaded ends, which allow each segment to be fitted together. The annular cavity 62 between the outer pipe 54 and inner tube 56 provides the pressurised fluid conduit 22 between the air swivel 20 and the hammer 46, and the interior of the inner tube 56 provides the cuttings conduit 24 through which cuttings flow from the cutting face 65 of the drill string 10 to the deflection channel 16.

The hammer 46 forms the operational end of the drill string 10 which operatively engages the drilling face to drill, hammer, cut or otherwise extract material to extend the drill hole 13. In the illustrated case, the hammer 46 comprises a substantially cylindrical body having a generally planar work surface 65. The hammer head 46 is provided with one or more sample extraction passages 67 which are in fluid communication with the cuttings conduit 24 within the inner tube 56. The pressurised fluid conduit 22 connects to the cuttings conduit 24 at an intersection zone 68 within the hammer 46. The pressurised air is used to drive the pneumatic action of the hammer 46 and also flush the cuttings 69 from the workface 65 into the cuttings conduit 24 to the surface.

In operation, the drill string including the drilling assembly 14 is rotated by the rotary drive means 18. Pressurised air is pumped into the pressurised fluid conduit 22 from the air swivel, and flows axially downwardly through the drill string to the hammer 46 as indicated by the arrows A. The pressurised air actuates the hammer 46 action of the drill string 10 and also flows through to the intersection zone 68 within the hammer 46 where the air moves upwardly through the cuttings conduit 24 as indicated by the arrows B. The differential pressure between the intersection zone 68 and the cutting face 65 draws any cuttings 69 from the cutting face, into the cuttings conduit 24. The cuttings 69 are thereby flushed from the cuttings face 65, upwardly through the cuttings conduit 24 to the surface to the deflector channel 16.

The present invention differs from the assembly 10 illustrated in Figure 1 principally by the configuration of the air swivel 20, the saver sub 42 and the drill rod 44, which will now be described in more detail. It should be appreciated that a drilling assembly according to the present invention operates in the same general manner as described for the assembly 10 of Figure 1.

Referring now to Figure 2, there is illustrated a cross-sectional view of one drill pipe section 70 in accordance to one preferred embodiment of the present invention. For the sake of clarity, the entire axial length of the middle body section 7OA of the drill pipe section 70 has not been illustrated as indicated by the two break lines W drawn through the centre of the drill pipe section 70. It should be appreciated that this middle body section 7OA comprises an elongate section having a similar configuration as the drill pipe section 70 either side of the break lines W.

As best illustrated in Figures 2 and 3, the drill pipe section 70 includes three generally coaxial substantially tubular sections, being: an inner tube 72, the interior of which functions as a cuttings passage 77 through which drill cuttings 69 (Figure 1 ) travel from the cutting face 65 (Figure 1 ) at the operational end of the drill string to the surface;

an intermediate tube 74 which has a larger diameter than the inner tube 72 and is concentrically arranged around the inner tube 72; and

an outer pipe 76 which has a larger diameter than the intermediate tube 74 and is concentrically arranged around the intermediate tube 74. The outer pipe 76 functions as an outer casing for the drill pipe section 70 and as it contacts the walls of the drill hole 13 is subject to abrasive wear.

As can be clearly seen in Figure 2, the inner wall of the intermediate tube 74 and outer wall of the inner tube 72 frame an axial annular pressure conduit 78 through which pressurised air can be pumped to the operational end of the drill string. Similarly, the inner wall of the outer pipe 76 and outer wall of the intermediate tube 74 frame another annular passage, in this case a safety conduit 79 which runs the axial length of each drill pipe section 70. The safety conduit 79 runs parallel to the pressure conduit 78 and is provided as a pressure relief passage to allow venting of pressurised fluid from the pressure conduit 78 if ever a section of the intermediate tube 74 fails.

As best illustrated in Figures 3 and 4, each adjacent axial end of the outer pipe 76 is interconnected using a box and pin threaded coupling 87. This form of threaded connection 87 includes a first outer pipe 76 having a female box or sleeve 80 having an inwardly threaded surface 81 which is arranged to threadably receive a distal pipe end 82 of an adjacent outer pipe 76' having an outwardly threaded surface 83. Each of the threaded surfaces 81 and 83 have an axial taper, the distal pipe end 82 tapering from a smaller outer diameter at its distal end 84 to a greater outer diameter at the proximate end 85 of the thread 83, with the sleeve 80 having a cooperating taper 81 on its inner surface. The sleeve 76 has an internal diameter which closely corresponds to the outer diameter of the distal pipe end 82. In the outer pipe section 76 illustrated in Figure 2, the upper axial end of each outer pipe 76 includes the box connection section 80 and the lower axial end of the outer pipe 76 includes the pin connector section 82. It should be understood that in other embodiments, the orientation of these connector ends 80 and 82 relative to the drilling direction D can be reversed.

The outer pipe 76 of each drill pipe section 70 includes at least two inwardly stepped sections 91 spaced along the axial length of the section 70. These stepped sections 91 are spanner flat sections around which a spanner or similar tool can be engaged to tighten the engaged threaded ends of the outer pipe 76 together. The spanner flats 91 are formed by a number of circumferentially spaced axially aligned planar panels forming a polygonal axial cross-sectional section in the outer pipe 76.

As best illustrated in Figure 3, each adjacent axial end of the inner tube 72 and intermediate tube 74 are interconnected using a sliding connection 88 and 89 respectively. Each sliding connection 88, 89 comprises an annular sleeve 90, 9OA on a first tube 72 or 74' which steps radially outwardly to accommodate an adjacent distal end 92, 92A of an adjacent tube 72' or 74. A series of o-rings 93 are accommodated within the inner surface of the sleeve 92, 92A. The o-rings 93 provide a pressure seal between the inner surface of the sleeve 90, 9OA and the outer surface of the distal end 92, 92A of an adjacent tube. In this respect, each sliding connection 88, 89 is manufactured with high tolerances between the coupling sleeve 90, 9OA and distal end 92, 92A of each of the inner tube 72 and intermediate tubes 74 are such to ensure that the o-ring seal sets 93 form a high pressure seal between the inner surface of the sleeve 90, 9OA and the outer surface of the distal end 92, 92A. Preferably, the pressure seal can withstand pressures of greater than 1200 psi, and more preferably greater than 1500 psi.

As best illustrated in Figure 3, the sliding connections 88 and 89 between adjacent ends of the intermediate tube 74 and 74' and adjacent ends of the inner tube 72 and 72' have very similar configurations. However, as can be appreciated, the inner tube 72 and intermediate tube 74 define the walls of the pressure conduit 78 which is designed to contain pressurised fluid. The sliding connections 88 and 89 are therefore arranged with an axial offset between each connection 88 and 89. This axially offsets the points of weaknesses resulting from each sliding connection 88 and 89 in the walls of the pressure conduit 78. Furthermore, it can also be observed that the sliding connection 89 of the intermediate tube 74 includes a sleeve 90 on a lower axial end of an upper intermediate tube 74' which receives an upper distal end 92 of the lower adjacent intermediate tube 74 and the sliding connection 88 of the inner tube 72 includes a sleeve 90 on a upper end of an lower inner tube 72 which receives a lower distal end of the upper adjacent inner tube 72'. It should be understood that in other embodiments these orientations can be reversed.

As shown in Figure 4, each of the threaded coupling sections 87 of the outer pipe 76 is formed separately to a body portion 76A (Fig. 3) of the outer pipe 76, the threaded coupling section 87 being welded to the body portion 76A prior to assembly of the drill string using a circumferential weld 99. Similarly, each of the sliding connections 88 and 89 of the intermediate tube 74 and the inner tube 72 is formed separately to a body portion 74A and 72A of the intermediate tube 74 and the inner tube 72 respectively, the sliding connections 88 and 89 being welded to the body portion 74A and 72A prior to assembly of the drill string using a circumferential weld 101 and 102.

The illustrated drill pipe section 70 is constructed from two separate assemblies which are slid together to form the three pipe structure shown in Figure 2. The first outer assembly is provided by the outer pipe 76, which comprises a series of sections which can be threadably connected together. The intermediate tubes 74 and inner tubes 72 are connected together at certain locations along the length of each tube to form a second assembly. The second assembly is formed by radially interconnecting each of the intermediate tubes 74 and inner tubes 72 about the drilling axis X'-X' using one or more radial connectors 94. Each radial connector 94 comprises a body or block which is welded to the inner surface of the intermediate tube 74 and the outer surface of the inner tube 72. Other radial spacers 96 are also provided in the drill pipe section 70 which radially space the intermediate tube 74 from the inner tube 72, but do not interconnect the two tubes 72, 74. These radial spacers 96 are spacer bodies or blocks which are welded to the inner surface of the intermediate tube 74 or the outer surface of the inner tube 72.

Radial spacers 98 are also provided between the outer pipe 76 and intermediate tube 74 to maintain the general radial spacing of the outer pipe 76 and intermediate tube 74 along the axial length of each drill pipe section 70. These radial spacers 98 are spacer bodies or blocks which are welded to the inner surface of the outer pipe 76 or the outer surface of the intermediate tube 74. The spacer 98 is only welded to one surface so as to accommodate any differential expansion between of the outer pipe 76 and intermediate tube 74 during drilling operation. As can be appreciated, such differential expansion can cause damage and even failure of a drill pipe section 70.

Referring now to Figure 5, there is shown an air swivel 100 and saver sub section 103 which can be attached to the upper end of a first drill pipe section 70 as shown in Figures 2 and 3. The illustrated air swivel 100 has the same general configuration as the air swivel 20 shown in Figure 1. Air swivel 100 is directly connected to the lower axial end of the drive spindle 34 which drives rotation of the spindle about the drilling axis X'-X' for this reverse circulation drilling assembly. The air swivel 100 provides an annular inlet 104 into the pressurised fluid conduit 78 through which pressurised fluid, such as air, can be pumped. The air swivel 100 includes two operatively engaged sections, being a first stationary outer housing 105 which includes a fluid inlet 106 to which a pressurised fluid source (not illustrated) can be connected. The outer housing 105 also functions as a mount having a set of upper and lower circumferential rollers 1 10 and 1 12 in which the spindle 108 can freely rotate. The configuration of the spindle section 108 differs slightly to the spindle section 38 shown in Figure 1 , with the spindle section 108 providing a fluid passage 114 to the pressure conduit 78 and having a blind passage 1 16 which acts as the upper terminal end of the safety conduit 79. Again, the cuttings conduit 77 passes through the centre of the air swivel 100. The illustrated saver sub 103 has a similar triple walled construction as drill pipe section 70, having an outer pipe 76A, intermediate tube 74A and inner tube 72A. Accordingly, the illustrated saver sub 103 effectively has substantially the same configuration as the drill pipe sections 70 but have a shorter axial length. Nevertheless, the saver sub 103 differs from the drill pipe sections 70 through the inclusion of a series of circumferentially spaced bore holes 116 drilled through the outer pipe 76A, which connect the safety conduit 79 with the external environment around the outer pipe 76. These bore holes 1 16 function as pressure relief holes in the event the intermediate tube 74 fails anywhere along its axial length in any drill pipe section 70. Accordingly if a portion of the intermediate tube 74 cracks, is punctured, or otherwise fails, the safety conduit 79 between the outer pipe 76 and intermediate tube 72 is configured to provide a pressure relief means through which pressurised fluid leaking from the pressure conduit 78 can vent to an environment external to the drill string. Typically, the external environment would be the ambient atmosphere.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Throughout the description and claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

Claims

1. A reverse circulation drill string including two or more multi-walled drill pipe sections coupled together in series, each drill pipe section including at least three concentrically arranged annular sections generally coaxially aligned with a drilling axis, the annular sections including: an outer pipe; an intermediate tube enclosed within the outer pipe, the outer pipe and intermediate tube having a first axial conduit therebetween; and an inner tube arranged within the intermediate tube, the intermediate tube and the inner tube having a second axial conduit therebetween, the interior of the inner tube providing a third axial conduit therein, wherein the second axial conduit is arranged for the passage of pressurised fluid to a drilling head of the drill string, the third axial conduit is configured as a cuttings passage through which cuttings from the drill head can travel, and the first axial conduit is arranged for communication with the environment external of the drill string.

2. A reverse circulation drill string according to claim 1 , wherein at least a portion of the outer pipe of at least one drill pipe section includes one or more vent holes providing fluid communication between the first axial conduit and the external environment around the outer pipe.

3. A reverse circulation drill string according to claim 2, wherein the vent holes comprise two or more bore holes axially spaced about the circumference of the outer pipe of the at least one drill pipe section.

4. A reverse circulation drill string according to claim 3, wherein the at least one drill pipe section is located substantially outside a hole being drilled by the drill string.

5. A reverse circulation drill string according to claim 4, wherein the at least one drill pipe section is a saver sub type drill pipe section connected directly below an air swivel section of the drill string.

6. A reverse circulation drill string according to any one of the preceding claims, wherein the first axial conduit is open to the ambient atmosphere.

7. A reverse circulation drill string according to any one of claims 2 to 5, wherein one or more of the vent holes include a pressure relief valve which allows the first conduit to communicate with the external environment when pressure in the first axial conduit exceeds a set pressure.

8. A reverse circulation drill string according to any one of the preceding claims, wherein the second axial conduit is officially certified for holding a particular pressure.

9. A reverse circulation drill string according to claim 8, wherein the certified pressure is equal to or greater than 1000 psi.

10. A reverse circulation drill string according to any one of the preceding claims, wherein adjacent axial ends of the outer pipe are connected by a threaded connection.

11. A reverse circulation drill string according to claim 10, wherein the threaded connection includes a box and pin threaded coupling in which an upper end of the outer pipe includes an inwardly threaded coupling sleeve configured to couple with a lower end of an adjacent outer pipe section having an outwardly threaded pin end.

12. A reverse circulation drill string according to claim 1 1 , wherein the coupling section is formed separately to a body portion of the outer pipe, the coupling section being secured to the body portion prior to assembly of the drill string.

13. A reverse circulation drill string according to claim 12, wherein the coupling section is welded to the body portion of the outer pipe.

14. A reverse circulation drill string according to any one of the preceding claims, wherein adjacent sections which form the intermediate tube and/or the inner tube are coupled together through a sliding connection.

15. A reverse circulation drill string according to claim 14, wherein the sliding connection includes a sleeve section configured to tightly receive an adjacent distal end of an intermediate tube or inner tube respectively in abutting engagement.

16. A reverse circulation drill string according to claim 15, wherein the sliding connection includes one or more circumferential o-rings which provide a pressure seal between the engaging ends of the respective intermediate tube or inner tube.

17. A reverse circulation drill string according to claim 16, wherein the sleeve section houses the one or more o-rings on an inner surface thereof.

18. A reverse circulation drill string according to claim 16 or 17, wherein the pressure seal between the engaging ends of the intermediate tube or inner tube can withstand a pressure greater than 1000 psi.

19. A reverse circulation drill string according to any one of claims 16 to 18, wherein the sliding connection of the intermediate tube on each axial end of a drill pipe section is axially offset from the corresponding sliding connection of the inner tube on the same axial end of that drill pipe section.

20. A reverse circulation drill string according to claim 19, wherein the sleeve and distal end of the intermediate tube that form a sliding connection has the opposite axial orientation to the axial orientation of the sleeve section and distal end of the inner tube that form a further sliding connection.

21. A reverse circulation drill string according to claim 20, wherein the sliding connection of a pair of each intermediate tubes includes a sleeve on a lower end of one intermediate tube which receives an upper distal end of a lower axially adjacent intermediate tube and the sliding connection of a pair of inner tubes includes a sleeve on an upper end of one inner tube which receives a lower distal end of an upper axially adjacent inner tube.

22. A reverse circulation drill string according to any one of the preceding claims, wherein the intermediate and inner tubes form a separate assembly to the outer pipe.

23. A reverse circulation drill string according to claim 22, wherein the intermediate and inner tubes are radially interconnected about the drilling axis by one or more radial connectors.

24. A reverse circulation drill string according to claim 23, wherein the radial connectors are welded between and to each of the intermediate and inner tubes.

25. A reverse circulation drill string according to claim 24, wherein radial spacers are provided between the intermediate tube and outer pipe within the first conduit to radially space the outer pipe from the inner tube in a concentric arrangement.

26. A reverse circulation drill string according to claim 25, wherein the radial spacers are affixed to the outer surface of the intermediate tube or the inner surface of the outer pipe.

27. A reverse circulation drill string according to any one of the preceding claims, wherein pressurised fluid is fed into the second axial conduit using an air swivel device having an air inlet having a circumferential inlet connection with the second conduit, the first axial conduit terminating in the air swivel device.

28. A reverse circulation drill string according to any one of the preceding claims, in which the first axial conduit can facilitate pressure relief of the pressurised fluid from the second axial conduit to the external environment in the event that the intermediate tube fails.