WO2005024173A1

WO2005024173A1 - Drilling arrangement

Info

Publication number: WO2005024173A1
Application number: PCT/AU2004/001209
Authority: WO
Inventors: Hendrik Corporaal
Original assignee: Camstar Nominees Pty Ltd
Priority date: 2003-09-08
Filing date: 2004-09-07
Publication date: 2005-03-17

Abstract

A drilling arrangement including a drill rod (10) having an inner drill pipe (11) coaxially mounted within an outer drill pipe (12) and defining a hollow annulus (25) therebetween. A roller drill bit (40) is fixed at one end of said drill rod (10) and the drill bit (40) includes a body (41) having axially spaced leading and trailing ends and an opening (55) through the leading end and in communication with the inside of the inner drill pipe (11). Roller cutters (42) are located at the leading end for engagement with ground being drilled, while the trailing end is connected to each of the inner pipe (11) and the outer pipe (12). A passage (50) extends through the body (41) in communication with the annulus (25) and facilitates discharge of flushing medium from the annulus (25) at the drill bit (40) for flushing drill cuttings through the opening (55) into the inner drill pipe (11) for transport away from the drill bit (40) through the inner drill pipe (11).

Description

DRILLING ARRANGEMENT

The present invention relates to a drilling arrangement, in particular for ground drilling for the purpose of water, mineral, oil or other resource recovery, and for underground investigation of installations such as slime dumps and settling pools.

Various forms of ground drilling arrangements exist. In most, it is necessary to include an arrangement for flushing away drill cuttings which are cut from the ground being drilled. In absence of a suitable flushing arrangement, the drill cuttings can build up at the drilling end of the drill clogging the drill bit and inhibiting further drilling. It is therefore known to introduce flushing medium at the drilling end of the drill to flush cuttings away from the drill bit and out of the hole being drilled, and the flushing medium can be gas, liquid, air or mist. This is known as "bit face cleaning".

In one form of drilling arrangement, a drill bit is fixed to a leading end of a cylindrical drill pipe, with the drill bit having a greater diameter than the drill pipe. In this arrangement, high pressure air or liquid can be injected through the interior of the drill pipe to flush past the drill bit and to carry drill cuttings in the opposite direction out of the drilled hole, between the outer surface of the drill pipe and the inner surface of the hole. While this arrangement is effective in removing drill cuttings, the cuttings removed do not form reliable samples for later analysis as to what type of materials or resources exist at certain depths of drilling. This is because the cuttings removed by flushing mix, and therefore become contaminated with ground particles or elements of the wall of the drilled hole rearwardly of the cutting face as they are flushed out of the hole.

To attend to this difficulty, dual tube drill strings have been developed in which an inner drill pipe of a smaller diameter is co-axially mounted within an outer pipe of a larger diameter, to create a hollow annulus between the two pipes. By this arrangement, the flushing medium can be injected to the drill bit through the annulus, to flush the drill cuttings away from the drill bit and rearwardly back through the inside of the inner drill pipe. By this arrangement, the drill cuttings are isolated from the wall of the drilled hole, so that sample cuttings can be retrieved which are not contaminated with other particles or elements.

The above dual tube arrangement has been effective for certain types of drilling, in particular those types known in the industry as down hole hammer drilling, and blade or drag bit drilling. However, that effectiveness has not been achieved to an acceptable level in drilling arrangements that employ roller drill bits, which is a drill bit having a body fitted with rollers on bearings and cutting teeth or tungsten carbide buttons applied to the rollers. Arrangements employed to date still do not provide satisfactory bit face cleaning, so affecting cutting performance and drill bit life. The present arrangements also provide poor sample recovery and the samples recovered can still be contaminated.

It is an object of the invention to provide a dual tube drilling arrangement that employs a roller drill bit, which provides an operating improvement over similar drilling arrangements of the prior art.

In a first aspect, the present invention provides a drilling arrangement including a drill rod having an inner drill pipe coaxially mounted within an outer drill pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of the drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of the inner drill pipe, roller cutters located at the leading end for engagement with ground being drilled, the trailing end being connected to each of the outer pipe and the inner pipe, a passage extending through the body in communication with the annulus and facilitating discharge of flushing medium from the annulus at the drill bit for flushing drill cuttings through the drill bit opening into the inner drill pipe for transport away from the drill bit through the inner drill pipe.

While the inner and outer drill rod pipes can extend into connection with the drill bit body, in a preferred arrangement, the drill bit includes radially spaced outer and inner annular sleeves depending from the trailing end thereof and defining a space or spaces between them, preferably an annular space therebetween. In one arrangement, the annular space is co-extensive with the annulus between the inner and outer pipes. In the preferred arrangement, the outer annular sleeve is arranged for connection to the outer drill rod pipe, while the inner annular sleeve is arranged for connection to the inner drill rod pipe and the passage which facilitates discharge of flushing medium extends from the annular space.

The outer annular sleeve can be connected to the outer drill rod pipe in any suitable arrangement, such as in threaded connection, with either the sleeve fixed radially outside or inside the drill rod pipe. Alternatively, the sleeve and pipe can be in abutting engagement and be fixed in connection with a suitable external or internal sleeve. In the preferred arrangement, the sleeve is threadably connected to an inside surface of the drill rod pipe and a compression sleeve is disposed about the connection, radially outside thereof.

The inner annular sleeve can be connected to the inner drill rod pipe by the same connection arrangements discussed above, although in the preferred arrangement, the pipe includes a radially outwardly extending stepped portion to accommodate an end portion of the inner sleeve and a seal is disposed between the facing surfaces at the connection. Preferably the connection is a close sliding fit. The connection of the inner annular sleeve with the inner drill rod pipe preferably is such as to provide a generally uninterrupted internal passage for flushing of drill cuttings. That is, it is preferred that the internal surfaces of both the inner sleeve and pipe be generally co-extensive.

The passage which extends through the body can take any suitable route for the discharge of flushing medium as long as the flushing medium is directed in a manner to flush drill cuttings through the opening of the drill bit into the interior of the inner drill rod pipe. Preferably a plurality of passages is provided and in one form, the passages can extend to discharge ports or jets formed in the drill bit body which are spaced radially outwardly of the drill bit opening provided centrally of the drill bit. Preferably three discharge ports are provided with a passage extending to each, but more or less could be provided as long as satisfactory flushing requirements are met. The preference for three discharge ports and associated passages is because most roller drill bits employ three drilling rollers spaced equidistantly about the leading end of the bit, i.e., at 120° to each other (although there are two and four roller drill bit arrangements) and the discharge ports are conveniently positioned between the respective rollers.

By the connection of the inner drill rod pipe to the body of the drill bit, or to the inner annular sleeve of the drill bit, the annulus and the passage, and the annular space if provided, are isolated from the return path of flushing medium and drill cuttings through the inner pipe away from the drill bit. Advantageously, and in contrast to some prior art arrangements, the bearings on which the rollers of the drill bit are mounted, are separated from the return flow of flushing medium so that the drill cuttings in transport with the flushing medium do not come into contact with the bearings, which otherwise can increase the rate of wear of the bearings, or cause them to fail entirely. Moreover, by extending the passage through the body of the drill bit, the passage can be arranged to connect with lubrication passages for lubrication and cooling of the bearings by the flushing medium. This is appropriate for open bearings which require lubrication, although the invention is also applicable to sealed bearings and for such bearings, each passage extends to the discharge port without feeding flushing medium to the bearings.

In the drilling arrangement described above, the flushing medium flows through the hollow annulus, the annular sleeve (if provided) and through the passage of the drill bit body, for discharge at or towards the leading end of the drill bit body. In an alternative arrangement, a portion of the flushing medium is bled from one or more of the hollow annulus, the annular space, or the passage of the drill bit body, for flow in generally the reverse direction within the inner drill pipe. Suitable ports may be provided for this to occur, although in the preferred arrangement, the inner drill pipe is separated from the trailing end of the drill bit body, so that a passage extends between the inner drill pipe and the drill bit body, thereby facilitating flow of flushing medium into the inner drill pipe. That flow is constrained to flow in the reverse direction within the inner drill pipe. The reverse flow causes a vacuum or suction effect that assists the flow of drill cuttings away from the drill bit and through the inner drill pipe. Accordingly, in a second aspect, the present invention provides a drilling arrangement including a drill rod having an inner drill rod pipe coaxially mounted within an outer drill rod pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of the drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of the inner drill rod pipe, roller cutters located at the leading end for engagement with ground being drilled, the trailing end being connected to the outer drill rod pipe, a first passage extending through the body in communication with the annulus and facilitating discharge of flushing medium from the annulus at the drill bit for flushing drill cuttings through the drill bit opening into the inner drill rod pipe for transport away from the drill bit through the inner drill rod pipe, a second passage facilitating discharge of flushing medium into the inner drill rod pipe upstream of the roller cutters of the drill bit for flow within the inner drill rod pipe in a reverse direction to the flow of flushing medium within the hollow annulus, for creation of suction within the inner drill rod pipe away from the drill bit to assist transport of drill cuttings away from the drill bit.

In a preferred arrangement, the inner drill rod pipe terminates close to, but separated from the trailing end of the drill bit body, so that a passage in the form of a gap extends between the inner drill rod pipe and the drill bit body. In this arrangement, the hollow annulus therefore is in communication with the inside of the inner drill pipe through the gap. The gap preferably is annular, so that there is complete separation between the inner drill rod pipe and the drill bit body.

In the preferred arrangement, the drill bit includes radially spaced outer and inner annular sleeves depending from the trailing end thereof and defining an annular space therebetween. In this respect, the drill bit takes substantially the same form discussed above in relation to the first aspect of the invention. Thus, the outer annular sleeve can be connected to the outer drill rod pipe in any suitable arrangement, such as in threaded connection, with either the sleeve fixed radially outside or inside the outer drill pipe. Likewise, other arrangements as discussed above can be adopted in relation to the outer annular sleeve in its connection to the outer drill pipe.

In this embodiment of the invention, the inner annular sleeve can be separated from the inner drill pipe to provide the required passage for flow of flushing medium into the inner drill pipe. In one preferred arrangement, the end of the inner drill pipe adjacent the drill bit body, extends into the annular space defined between the inner and outer annular sleeves. For this, the end of the inner drill pipe can be stepped, in one arrangement radially outwardly, so that the end of the inner drill pipe extends substantially centrally of the annular space.

In the above arrangement, flushing medium flows through the hollow annulus towards the drill bit body, and while a portion of the fluid medium flows through the first passage of the drill bit body, a second portion of the flushing medium is diverted around the end of the inner drill pipe into a further passage extending between that end of the inner drill pipe and the inner annular sleeve. That third passage extends in communication with the interior of the inner drill pipe for flow of flushing medium into the inner drill pipe.

In a third aspect, the present invention provides a drilling arrangement including a drill rod having an inner drill pipe coaxially mounted within an outer drill pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of said drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of said inner drill pipe, roller cutters located at said leading end for engagement with ground being drilled, said trailing end being connected to said outer pipe, a passage extending through said body in communication with said annulus and facilitating discharge of flushing medium from said annulus at said drill bit for flushing drill cuttings through said drill bit opening into said inner drill pipe for transport away from said drill bit through said inner drill pipe.

The attached drawings show an example embodiment of the invention of the foregoing kind. The particularity of those drawings and the associated description does not supersede the generality of the preceding broad description of the invention.

Figure 1 is a 120° cross-sectional view of a drill rod according to one embodiment of the present invention.

Figure 2 is a cross-sectional view of the drill rod of Figure 1 , rotated through 60°.

Figure 3 is a further cross-sectional view of the drill rod Figure 1 , taken through one of the roller cutters only.

Figure 4 is a cross-sectional view of an alternative embodiment of the invention.

Figures 1 , 2 and 3 all show a drill rod 10 of generally elongate form, and including an inner drill pipe 11 and an outer drill pipe 12. The major difference between Figures 1 , 2 and 3, is the section taken through the drill bit of the drilling arrangement. In Figure 1 , the section is taken through a 120° cross- section while in Figures 2 and 3, the section is taken through only one of the roller cutters. In Figure 2, the second roller cutter is not shown, while in Figure 3, simply to illustrate the form of roller cutter envisaged to be used in a drilling arrangement of the present invention, the roller cutter taken out of the section of Figure 3, is shown.

As shown, the inner drill pipe 11 is disposed within the outer drill pipe 12, and the respective pipes are generally coaxial. The inner pipe 11 is in sealed connection with a further inner drill pipe 13 by way of a stepped end 14 of the pipe 11 , which steps radially outwardly to accommodate the upper end of the pipe 13. A suitable seal 15 is disposed between the upper end of the pipe 13 and the stepped end 14. The respective inner drill pipes 11 and 13 are generally of similar construction, although as shown in Figure 1 , the pipe 11, has a greater lengthwise extent than the pipe 13. A second outer drill pipe 16 is shown also, and each of the outer drill pipes 12 and 16 include a cooperating threaded engagement 17 and axially facing abutting surfaces 18. The pipes 12 and 16 thread together in the direction in which the drill rod 10 rotates during drilling, to ensure that the respective drill pipes do not disengage during drilling operations. As is well known in the art, the length of the drill rod 10 can be increased by adding further pipes 11 and 12 to the existing of those pipes.

To maintain the inner pipes 11 and 13 properly spaced from the outer pipes 12 and 16, retaining lugs 19 are provided. The retaining lugs 19 nest against a suitable step 20 and are maintained in engagement against the step 20 by circlips 21.

The arrangement of the inner and outer pipes is such as to form a hollow annulus 25 between the pipes, which extends the length of the drill rod 10.

The inner drill pipe 13 includes a stepped end 26 which is radially stepped to engage about an inner annular sleeve 27. Disposed between the stepped end 26 and the sleeve 27 is a seal 28 which can be of any suitable kind, and likely to be of the same kind as the seal 15.

Disposed radially outwardly from the sleeve 27, is an outer annular sleeve 29 which is in threaded engagement with an end section 30 of the outer drill pipe 16. A compression sleeve 31 extends about the outer surface of the end section 30, and the compression sleeve 31 defines a radially inwardly extending abutment 32 for abutting engagement with the axially facing end 33 of the end section 30.

Threadably connected to the outer drill pipe 16, is a roller drill bit 40, which has a body 41 from which each of the annular sleeves 27 and 29 extend. Those sleeves extend from a trailing end of the body 41 , while the roller assembly 42 is mounted on a leading end of the body 41. The roller assembly 42 can take a known configuration, and typically will comprise three separate cutting rollers, disposed equally about the axis A at about 120° spacing, and each drill bit face 43 includes a plurality of tungsten carbide buttons on an outer face thereof, and a suitable bearing race on an inner face thereof, to accommodate thrust bearings 45 and roller bearings 46.

In Figure 1 , the drill bit 40 is shown to have an open bearing arrangement on the left hand side of the axis A, and a sealed bearing arrangement on the right hand side of that axis. Figure 1 is shown like this, to illustrate that either open or sealed bearing arrangements can be employed in the present invention, but in practice, each bearing arrangement of a drill bit would be either an open bearing or a sealed bearing type.

The open bearing type on the left hand side of the axis A, includes a lubrication channel 50 which extends from an annular space 51 between the inner and outer annular sleeves 27 and 29. The passage 50 extends to a further lubrication channel 52 that extend to bearings of the roller assemblies 42, to lubricate and cool those bearings for rotation of the drill bit faces 43. As shown in the right hand side of Figure 1 , an equivalent passage 50 does not extend to the sealed bearing arrangement because that bearing arrangement does not require external lubrication.

Figure 2 is a cross-sectional view of the drill rod 10 of Figure 1 , but rotated through 60°, while Figure 2 also shows the drill rod 10 disposed within a drill hole or bore 53 which has been drilled.

While Figure 2 shows the same drill string 10 of Figure 1 , it shows the drill string 10 with further inner and outer drill pipes 11 and 12 fixed to those drill pipes shown in Figure 1. It will be readily be appreciated, that by continually adding inner and outer drill pipes 11 and 12 to the existing drill pipes 11 and 12, the length of the drill rod 10 can be extended.

By rotating the drill rod 10 of Figure 1 through 60°, only a single roller assembly 42 is shown although in Figure 3, a second roller assembly 42 is shown, but not in cross-section. In Figure 2 the right hand side of the axis A shows a void 54 and this view also shows by arrows, the passage of flushing medium through the hollow annulus 25, the annular space 51 , the passage 50 and into the void 54. Likewise, Figure 3 shows this passage of flushing medium also, as shown, the flushing medium exits the passage 50 closely adjacent the leading end of the drill bit 40 and drill cuttings are flushed into the central opening 55 of the drill bit 40 and then into the interior of the string of inner drill pipes 11 and 13.

The arrangement of Figures 1 to 3 advantageously maintains the drill cuttings away from the interior wall of the hole 53, so that those cuttings are transported to the trailing end of the drill rod 10 and can be sampled for analysis, without concern regarding potential contamination through contact with the hole wall 53. Additionally, the drill cuttings are maintained isolated from the lubrication passages of the open bearings shown on the left hand side of axis A in each of Figures 1 and 2. In some prior art arrangements, the lubrication passages have been open to the rearward flow of drill cuttings in flushing medium, and the drill cuttings have entered the bearings and caused their increased wear and/or failure. A further advantage of the illustrated arrangement, is that the flushing medium is discharged closely adjacent to the forward end 56 of the hole 53 so that the flushing action is maximised for removal of drill cuttings at that end. In some prior art arrangements, the flushing medium is discharged away from the forward end 56, which reduces the pressure of the flushing medium to clear drill cuttings.

Referring to Figure 4, this shows an alternative embodiment of a drilling arrangement according to the invention. The drill rod 100 includes an inner drill pipe 101 and an outer drill pipe 102. The inner and outer drill pipes 101 and 102 are coaxial, and are held in that arrangement by support lugs 103. In relation to the upper support lug 103, a circlip 104 maintains the lug in position against an internal step 105 formed in the outer drill pipe 102. The coaxial alignment of the inner drill pipe 101 with the outer drill pipe 102, provides a hollow annulus 106 between the respective pipes.

The outer pipe 102 includes a thread 107 at one end, for threaded engagement with additional drill pipe members, such as those shown in Figures 1 and 2, and further includes another thread 108 at the opposite end thereof, for threaded engagement with a roller drill bit 109. The roller drill bit has the form of the roller drill bit 40 shown in Figure 3. Thus, further discussion in relation to the drill bit

109 is not necessary. The roller drill bit 109 includes an outer annular sleeve

110 which is engaged in threaded connection with the thread 108 of the outer drill pipe 102, and an inner annular sleeve 111 which is generally coaxial with the outer annular sleeve 110. Between the outer and inner annular sleeves 110, 111 , is an annular space 112. Disposed generally centrally of that space, is the annular end or distal region 113 of the inner drill pipe 101.

The position of the annular end region 13 within the annular space 112 is such as to partition, generally centrally, the annular space 112. As shown, the annular space 112 is in communication with a passage 114 which extends through the body 115 of the roller drill bit 109. As shown, the annular space 112 is also in communication with a lubrication channel 116 for lubricating the various bearings 117 of the roller drill bit 109.

Flushing medium which is introduced into the hollow annulus 106, enters the annular space 112 and in the arrangement illustrated, the flushing medium can take one of two separate paths. The first path is equivalent to that shown in Figure 2, in which the flushing medium continues to flow towards the roller assemblies 118 or in other words, towards the face of the ground being drilled, to entrain drill cuttings, for lifting away from the drill face and for transport to within the inner drill pipe 101.

The second path that the flushing medium can take, is to effectively reverse its flow direction about the annular end region 113 to enter the passage between the radially inner surface of the end region 113 and the radially outer surface of the inner annular sleeve 111. Flushing medium that takes that path also enters the interior of the inner drill pipe 101 but the action of reversing of flow, is such as to create a suction at the point of entry of that flushing medium into the inner drill pipe 101. By creating that suction, drill cuttings which are lifted away from the ground being drilled, are assisted in their passage through the inner drill pipe 101 , away from the roller drill bit 109. Thus, the embodiment illustrated in Figure 4 provides enhanced removal of drill cuttings from the ground being drilled.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.

Claims

CLAIMS:

1. A drilling arrangement including a drill rod having an inner drill pipe coaxially mounted within an outer drill pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of said drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of said inner drill pipe, roller cutters located at said leading end for engagement with ground being drilled, said trailing end being connected to each of said outer pipe and said inner pipe, a passage extending through said body in communication with said annulus and facilitating discharge of flushing medium from said annulus at said drill bit for flushing drill cuttings through said drill bit opening into said inner drill pipe for transport away from said drill bit through said inner drill pipe.

2. A drilling arrangement according to claim 1 wherein said trailing end of said drill bit includes radially spaced outer and inner annular sleeves and defining a space or spaces therebetween, and each of said outer and inner annular sleeves being connected to a respective one of said outer and inner drill pipes, and wherein said space or spaces is in communication with each of said hollow annulus and said drill bit body passage.

3. A drill arrangement according to claim 2, wherein said space or spaces is an annular space.

4. A drilling arrangement according to claim 3, wherein said annular space is coextensive with said hollow annulus.

5. A drilling arrangement according to any one of claims 2 to 4, wherein said outer annular sleeve is connected to said outer drill pipe by a threaded connection.

6. A drilling arrangement according to any one of claims 2 to 4, wherein said outer annular sleeve and said outer drill pipe are in abutting engagement and are fixed in connection by a threaded sleeve.

7. A drilling arrangement according to any one of claims 2 to 6, wherein a compression sleeve is disposed about the connection between the outer annular sleeve and the outer drill rod pipe radially outwardly thereof.

8. A drilling arrangement according to any one of claims 2 to 6, wherein the inner annular sleeve is in close facing relationship with a distal end region of said inner drill pipe and a seal is disposed between and in connection with the facing surfaces of said sleeve and said pipe.

9. A drilling arrangement according to claim 8, wherein radial inner surfaces of said inner annular sleeve and said inner drill pipe are generally coextensive, and said distal end region of said inner drill pipe is stepped radially outwardly to receive said inner annular sleeve in facing relationship.

10. A drilling arrangement according to claim 8 or claim 9, wherein said inner annular sleeve and said inner drill pipe are in sliding connection.

11. A drilling arrangement according to any one of claims 1 to 10, wherein said passage is in communication with lubrication channels that extend in communication with bearings of said roller cutters, wherein flushing medium flows through said passage and said lubrication channels for lubricating said bearings.

12. A drilling arrangement according to claim 11 , wherein said lubrication channels are isolated from contact with said drill cuttings.

13. A drilling arrangement according to any one of claims 1 to 12, wherein said drill bit body includes more than one said passage each of which extend to discharge ports for discharge of flushing medium radially outwardly of said drill bit opening.

14. A drilling arrangement according to claim 13 wherein said drill bit includes three roller cutters disposed at 120° to each other and three discharge ports, each of which is in communication with a said passage, and said discharge ports being disposed at 120° to each other and between adjacent roller cutters.

15. A drilling arrangement including a drill rod having an inner drill pipe coaxially mounted within an outer drill pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of the drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of the inner drill pipe, roller cutters located at the leading end for engagement with ground being drilled, the trailing end being connected to the outer drill pipe, a first passage extending through the body in communication with the annulus and facilitating discharge of flushing medium from the annulus at the drill bit for flushing drill cuttings through the drill bit opening into the inner drill pipe for transport away from the drill bit through the inner drill pipe, a second passage facilitating discharge of flushing medium into the inner drill pipe upstream of the roller cutters of the drill bit for flow within the inner drill pipe in a reverse direction to the flow of flushing medium within the hollow annulus, for creation of suction within the inner drill pipe away from the drill bit to assist transport of drill cuttings away from the drill bit.

16. A drilling arrangement according to claim 15, wherein said inner drill pipe terminates close to, but separated from said trailing end of said drill bit body so that said second passage takes the form of a gap which extends between said inner drill pipe and said drill bit body.

17. A drilling arrangement according to claim 16, wherein said gap is annular, providing complete separation between said inner drill pipe and said drill bit body.

18. A drilling arrangement according to any one of claims 15 to 17, wherein said trailing end of said drill bit includes radially spaced outer and inner annular sleeves and defining a space or spaces therebetween and wherein said outer annular sleeve is connected to said outer drill pipe.

19. A drilling arrangement according to claim 18, wherein said outer and inner annular sleeves are spaced apart coaxially.

20. A drilling arrangement according to claim 18 or claim 19, wherein said space or spaces is an annular space.

21. A drilling arrangement according to claim 20, wherein a distal end region of said inner drill pipe extends into said annular space, to divide said annular space into radially inner and outer sub-spaces.

22. A drilling arrangement according to claim 21 , wherein said distal end region of said inner drill pipe extends into said annular space substantially centrally of the radial width of said annular space.

23. A drilling arrangement according to claim 22, wherein radial inner surfaces of said inner annular sleeve and said inner drill pipe are generally coextensive, and said distal end region of said inner drill pipe is stepped radially outwardly to extend into said annular space.

24. A drilling arrangement according to any one of claims 15 to 23, wherein said connection between said outer annular sleeve and said outer drill pipe, is a threaded connection.

25. A drilling arrangement including a drill rod having an inner drill pipe coaxially mounted within an outer drill pipe and defining a hollow annulus therebetween, a roller drill bit fixed at one end of said drill rod, said drill bit including a body having axially spaced leading and trailing ends, an opening through the leading end in communication with the inside of said inner drill pipe, roller cutters located at said leading end for engagement with ground being drilled, said trailing end being connected to said outer pipe, a passage extending through said body in communication with said annulus and facilitating discharge of flushing medium from said annulus at said drill bit for flushing drill cuttings through said drill bit opening into said inner drill pipe for transport away from said drill bit through said inner drill pipe.

26. A drilling arrangement according to claim 23 wherein said drill bit includes radially spaced outer and inner annular sleeves depending from said trailing end and defining a space or spaces therebetween, and said outer annular sleeve being connected to said outer drill pipe, and wherein said space or spaces is in communication with each of said hollow annulus and said drill bit body passage.

27. A drilling arrangement according to claim 26, wherein said space or spaces is an annular space.

28. A drilling arrangement according to claim 26 or claim 27, wherein said inner annular sleeve is connected to said inner drill pipe.

29. A drilling arrangement according to claim 28, wherein flow of flushing medium through said annulus, said annular space and said body passage is isolated from flow of flushing medium and drill cuttings through said inner drill pipe.

30. A drilling arrangement according to any one of claims 27 to 29, wherein said annular space is coextensive with said hollow annulus.

31. A drilling arrangement according to any one of claims 25 to 30, wherein said outer annular sleeve is connected to said outer drill pipe by a threaded connection.

32. A drilling arrangement according to any one of claims 25 to 30, wherein said outer annular sleeve and said outer drill pipe are in abutting engagement and are fixed in connection by a threaded sleeve.

33. A drilling arrangement according to any one of claims 25 to 32, a compression sleeve is disposed about the connection between the outer annular sleeve and the outer drill pipe radially outwardly thereof.

34. A drilling arrangement according to any one of claims 25 to 32, wherein the inner annular sleeve is in close facing relationship with distal end region of said inner drill pipe and seal is disposed between and in connection with the facing surfaces of said sleeve and said pipe.

35. A drilling arrangement according to claim 34, wherein radial inner surfaces of said inner annular sleeve and said inner drill pipe are generally coextensive, and said distal end region of said inner drill pipe is stepped radially outwardly to receive said inner annular sleeve in facing relationship.

36. A drilling arrangement according to claim 34 or claim 35, wherein said inner annular sleeve and said inner drill pipe are in sliding fit connection.

37. A drilling arrangement according to any one of claims 25 to 36, wherein said drill bit body includes more than one said passage each of which extend to discharge ports for discharge of flushing medium radially outwardly of said drill bit opening.

38. A drilling arrangement according to claim 37, wherein said drill bit includes three roller cutters disposed at 120° to each other and three discharge ports, each of which is in communication with a said passage, and said discharge ports being disposed at 120° to each other and between adjacent roller cutters.

39. A drilling arrangement according to any one of claims 25 to 27, or any one of claims 30 to 33 or claims 37 or 38 when dependent on any one of claims

25 to 27, including a second passage facilitating discharge of flushing medium into the inner drill rod pipe upstream of the roller cutters of the drill bit for flow within the inner drill rod pipe in a reverse direction to the flow of flushing medium within the hollow annulus, for creation of suction within the inner drill rod pipe away from the drill bit to assist transport of drill cuttings away from the drill bit.

40. A drilling arrangement according to claim 39, wherein said inner drill pipe terminates close to, but separated from said trailing end of said drill bit body so that said second passage takes the form of a gap which extends between said inner drill pipe and said drill bit body.

41. A drilling arrangement according to claim 40, wherein said gap is annular, providing complete separation between said inner drill pipe and said drill bit body.