AU2002311304B2

AU2002311304B2 - Reverse circulation hammer

Info

Publication number: AU2002311304B2
Application number: AU2002311304A
Authority: AU
Inventors: Ian Graeme Rear
Original assignee: Sandvik Mining and Construction Australia Production Supply Pty Ltd
Current assignee: Sandvik Mining and Construction Australia Production Supply Pty Ltd
Priority date: 2001-09-21
Filing date: 2002-11-29
Publication date: 2008-04-03
Anticipated expiration: 2022-11-29

Description

29-NOV-2002 16:27 WRAY AND ASSOCIATES NO. 717 P. P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Name of Applicant: Actual Inventor Address for service is: lan Graeme Rear Ian Graeme Rear WRAY ASSOCIATES Level 4, The Quadrant 1 William Street Perth, WA 6000 Attorney code: WR Invention Title: "Reverse Circulation Hammer" Details of Associated Provisional Application No: PR7898 filed on 21 September 2001 The following statement is a full description of this invention, including the best method of performing it known to me:- 29-NOV-2002 16:27 WRAY AND ASSOCIATES NO. 717 F. 6 -2- "Reverse Circulation Hammer" Field of the Invention This invention relates to a reverse circulation hammers.

Background Art Reverse circulation hammers are commonly used in downhole drilling especially in hard rock drilling. A difficultly with such hammers relates to the circumstance that the flow of exhaust air which carries cuttings from the bore hole can be retarded which results in the grinding of rock chips which are generated by the action of the drill bit. This is not only inefficient but can also be undesirable since in analysing the results of a drilling operation geologists generally tend to prefer large chip samples.

In addition when drilling in soft clay formations there is a tendency for the return passageways of the drill bit to become blocked when inactive and particularly as the hammer is being lowered into the bore hole. The fluid pressure which is generated within the bore hole on activation of the hammer can often be insufficient to clear the return passageways.

In addition in mud formations there can be a tendency for mud and clay to enter into the exhaust passageways within the drill bit support when the hammer is unpressurised which can occur when it becomes necessary to introduce additional lengths of drill rod into the drill string. Such material can block the exhaust passageways which are provided in the hammer enable the escape of exhaust fluid from the internal piston chamber. As a result, when the piston chamber is pressurised there is no exhaust from the upper end of the piston chamber which prevents the piston from commencing its cyclic reciprocation within the piston chamber.

In addition, as a result of normal wear and tear within the hammer there can be an increased degree of leakage within the piston chamber resulting in the 29. NOV. 2002 16:28 WRAY AND ASSOCIATES NO. 717 P. 7 -3development of a high pressure condition between the piston and the upper end of the chamber (particularly under "blow down" conditions) which can result in a difficulty in initiating the reciprocation of the piston when the hammer is to be reactivated.

In addition in the past in the use of reverse circulation down hole hammers it has been deemed necessary to seal the outer annulus between the borehole and the hammer in order to maximise the collection of cuttings and minimise the flow of fluid into the annulus above the drill bit and above the hammer. In this regard reference is made to Australian patent specifications 629250, 620785, 638571 and 656724. In practice it has been found that this practice presents difficulties which can result in the inefficient operation of the hammer and can cause jamming of the hammer and drill string in the borehole. One of the difficulties results from the requirement that for a hammer to operate effectively the pressure differential to each side of the piston needs to be maximised and therefore the pressure differential between the fluid pressure delivered to the piston chamber and the pressure at the exhaust of the hammer needs to be maximised. However with the fluid being exhausted from the hammer into the sealed space within the borehole surrounding the drill bit, all of that fluid must be exhausted though the exhaust ports in the drill bit. In this regard the cross sectional area of the hammer which delivers the pressurised fluid into the hammer is in many cases the same if only a little less than the total cross sectional area of the exhaust ports in the drill bit that conduct the fluid and cuttings to the return conduit and thence to the surface. Since the fluid being exhausted into the bore hole is of a reduced pressure the volume of that exhausted fluid will be greater resulting in a higher flow rate through the exhaust ports which in turn will generate a back pressure in the borehole which in turn reduces the pressure differential across the piston and reduces the efficiency of the a hammer. It is therefore desirable to maximise the exhaustion of fluid from the hammer whilst ensuring that there is sufficient fluid flow into the exhaust ports of the drill bit to carry the cuttings out of the borehole as soon as possible after they have been generated.

29-NOV202 16:28 WRAY AND ASSOCIATES NO. 717 P. 8 -4- In addition in drilling deep wells and drilling under the water table it has been the custom to increase the fluid pressure delivered to the hammer to create sufficient pressure to overcome the ambient pressure at the exhaust of the hammer and to create sufficient pressure differential to activate to activate the hammer. On that high pressure fluid being exhausted a difficulty has existed in adequately exhausting the exhaust fluid from the bore hole through the ports provided in the drill bit and the return conduit to ensure an adequate pressure differential is maintained.

In addition it has been found that the absence of fluid flow through the annulus between the hammer and the borehole which is as a result of the sea[ which has been provided by a shroud or transmission sleeve presents difficulties. The absence of such fluid flow results in debris becoming deposited in the annulus above the seal (which is usually provided by a shroud or transmission sleeve) which creates a difficulty when it becomes necessary to extract the hammer from the borehole. In addition the accumulation of such debris above the seal increases the wear on the casing of the hammer.

In addition the pressurisation of the borehole by the presence of the seal (particularly under blowdown conditions) can result in the surrounding ground becoming pressurised, particularly in broken or porous ground conditions. Such pressurisation can result in the cuttings entrained in the fluid being carried into the ground which can reduce the cuttings being carned to the surface and affect the accuracy of the sampling of the cuttings being delivered from the borehole.

In practise it has been found that in some ground conditions on the hammer and seal moving past the pressurised ground that the ground will swell to reduce the diameter of the borehole above the seal which can result in jamming of the hammer or drill string in the borehole or difficulties when it becomes necessary to extract the hammer from the borehole in addition the delivery of all of the exhaust fluid into the base of the borehole (particularly under blowdown conditions) can result in considerable turbulence being generated in the base of the borehole. In loose, moist, wet or puggy 29-NOV-2002 16:28 WRAY AND ASSOCIATES NO. 717 P, 9 ground conditions such turbulence can result in the exhaust conduits and/or ports in the drill bit becoming blocked. This circumstance can arise with the introduction of a hammer into the borehole or placing the hammer into blowdown mode. When the hammer is to be reactivated and lowered to the base of the bore hole the conduits and/or exhaust ports in the drill bit become even more firmly blocked. Furthermore the turbulence can result in the ground surrounding the borehole being eroded and undesirable cavities being formed which can render the surrounding ground structure unstable.

Throughout this specification the term "reverse circulation hammer" shall be taken as relating to a hammer assembly comprising a casing supporting a top sub at one end and a drill bit support at the other end, a chamber defined within the casing between the ends thereof, an inner tube or conduit supported in the chamber from the top sub to be coaxial within the casing and adapted to communicate with an exhaust passage provided in a drill bit to be supported by the drill bit support, a piston slidably supported in the chamber over the inner tube for reciprocation between the ends of the casing, porting means adapted to deliver pressurised fluid from the top sub to the chamber to case said reciprocation of the piston within the chamber, exhaust passageways provided in the drill bit support adapted to exhaust fluid from the chamber to the drill bit, where the exhausted fluid is caused to be exhausted from the bore hole through conduits and/or ports provided in the drill bit and thence to the inner tube or conduit for ultimate delivery to the surface through the drill string.

Disclosure of the Invention Accordingly, the invention resides in a reverse circulation hammer comprising a casing supporting a top sub at one end and a drill bit support at the other end, a chamber defined within the casing between the ends thereof, an inner tube supported in the chamber from the top sub be coaxial within the casing and adapted to communicate with an exhaust passage provided in a drill bit to be supported by the drill bit support, a piston slidably supported in the chamber over the inner tube for reciprocation between the ends of the casing, porting means oO adapted to deliver pressurised fluid from the top sub to the chamber to case said 0 reciprocation of the piston within the chamber, exhaust passageways provided in the drill bit support adapted to exhaust fluid from the chamber to the drill bit, fluid ports provided in the inner tube to provide restricted fluid flow to the interior of the tube to N 5 generate a reduced pressure within the inner tube upstream of the fluid ports, the casing and drill bit support being dimensioned to provide an annular space between the hammer and the borehole to permit the flow of some fluid exhausted from the drill bit support through the annulus.

S0 According to a preferred feature of the invention the outer face of the drill bit support is N wear resistant.

According to a further preferred feature of the invention the fluid ports are located in the inner tube towards the outer end of the casing.

According to a further preferred feature of the invention the fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing.

0 According to a preferred feature of the invention, the fluid ports are spaced around the inner tube. According to one embodiment the fluid ports are spaced at equidistant intervals around the inner tube.

According to a preferred feature of the invention the fluid ports provide said communication substantially constantly throughout the reciprocation of the piston.

According to an alternative preferred feature of the reverse circulation hammer, fluid ports provide communication when a piston is in the vicinity of the piston's end positions within a chamber. According to a preferred feature of the invention, the drill bit support is adapted to enable the drill bit to be moveable outwardly from the casing to a "blowdown" position at which the piston is retained in engagement with the drill bit support, wherein when the hammer is at that condition the fluid ports provide said communication.

00 According to a preferred feature of the invention a second set of fluid ports are provided

O

in the inner tube in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the S second fluid ports.

According to a further preferred feature of the invention the second set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the S other end of the casing.

S0 According to a preferred feature of the invention, the second set of fluid ports are spaced N around the inner tube. According to one embodiment the second set of fluid ports are spaced at equidistant intervals around the inner tube.

According to a preferred feature of the invention, the second set of fluid ports are controlled by the check value of the top-sub such that the second set of ports are opened on the check valve opening to deliver fluid into the chamber.

According to a preferred feature of the invention a third set of fluid ports are provided in the inner tube in the region of the top-sub to provide restricted fluid flow to the interior 0 of the tube to generate a reduced pressure within the inner tube upstream of the third set of fluid ports. According to a preferred feature of the invention the third set of fluid ports are located downstream from the second set of fluid ports.

According to a further preferred feature of the invention the third set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing.

According to a preferred feature of the invention, the third set of fluid ports are spaced around the inner tube. According to one embodiment the third set of fluid ports are spaced at equidistant intervals around the inner tube.

According to a preferred feature of the invention the hammer is provided with a crossover sub located proximate to the top-sub, said cross-over sub having at 29-NOV-2002 16:29 WRAY ANO ASSOCIATES NO. 7 17 F. 12 -8least one delivery port providing communication between the interior of the inner tube and the annulus, said at least one delivery port being inclined with respect to the central axis of the hammer such that the predominate direction of flow is in a direction away from the hammer. According to a preferred feature of the invention the cross-over sub is located adjacent the end of the top-sub which is remote from the chamber. According to preferred feature of the invention the cross-over sub is incorporated with the top-sub.

According to a further aspect the invention resides in an inner tube of a reverse circulation hammer, said inner tube having fluid ports provided in the wall of the inner tube to in use provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the fluid ports.

According to a preferred feature of the invention the fluid ports are located towards the one end of the inner tube to be proximate the drill bit.

According to a further preferred feature of the invention the fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the inner tube.

According to a preferred feature of the invention, the fluid ports are spaced around the inner tube. According to one embodiment the fluid ports are spaced at equidistant intervals around the tube.

According to a preferred feature of the invention a second set of fluid ports are provided in the inner tube intermediate of the ends and at apposition which in use is to be in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the second fluid ports.

According to a further preferred feature of the invention the second set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing.

00 According to a preferred feature of the invention, the second set of fluid ports are

O

0 spaced around the inner tube. According to one embodiment the second set of fluid

(N

ports are spaced at equidistant intervals around the tube.

N 5 According to a preferred feature of the invention, the second set of fluid ports are located such that in use they will be controlled by the check valve of the top-sub such 0 that the second set of ports will open on the check valve opening.

According to a preferred feature of the invention a third set of fluid ports are C 0 provided in the inner tube towards the other end to in use to be located in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the third set of fluid ports.

According to a preferred feature of the invention, the third set of fluid ports are spaced around the inner tube. According to one embodiment the third set of fluid 0 ports are spaced at equidistant intervals around the tube.

According to a further aspect the invention resides in a reverse circulation hammer adapted to be connected to a single walled drill string, said hammer including a cross-over sub located proximate to a top-sub, said providing communication between the drill string and a chamber of the hammer to enable the delivery of pressurised fluid from the drill string to the top-sub, said cross-over sub having at least one delivery port providing communication between the interior of an inner tube and the exterior of the crossover sub, said at least one delivery port being inclined with respect to the central axis of the hammer such that the predominate direction of flow is in a direction away from the hammer. According to a preferred feature of the invention the cross-over sub is located adjacent the end of the top-sub which is remote from the chamber. According to 29.NOV.202 16:30 WRAY AND ASSOCIATES NO. 717 P. 14 preferred feature of the invention the cross-over sub is incorporated with the topsub.

The invention will be more fully understood in the light of the following description of several specific embodiments.

Brief Description of the Drawings The description is made with reference to the accompanying drawings of which: Figure I is a sectional side elevation of a downhole hammer according to the first embodiment showing the piston in the "blowdown" position; Figure 2 is a part sectional side elevation of the hammer according to the first embodiment illustrating the drill bit end of the hammer with the drill bit in the closed position; and Figure 3 is a part sectional side elevation of the hammer according to the first embodiment illustrating the drill bit end of the hammer with the drill bit in the blowdown position; Detailed Description of Specific Embodiments The first embodiment as shown in the accompanying drawings comprises a downhole reverse circulation hammer which is used to be utilised in the formation of bore holes in the ground. The hammer comprises a casing 11 which supports a top sub 13 at one end and a drill bit support 15 at the other end. The drill bit support retains a drill bit 17 by means of a retaining ring 19 provided in the drill bit support 15 which is engagable with an annular splined collar 21 provided at the inner end of the drill bit such that the drill bit is capable of limited slidable movement within the drill bit support The interior of the casing 11 provides a piston chamber 23 between the top sub 13 and the drill bit support 15. In addition, the top sub 13 supports an inner tube 29.NCY.2002 16:30 WRAY AND ASSOCIATES NO. 717 P. -11 which is coaxial with the casing 11 and extends from the top sub into the drill bit support such that its outermost end is slidably receivable in an exhaust passageway 27 provided in the drill bit 17 which is formed with exhaust ports.29 in its outer axial face which extend between the outer axial face of the drill bit 17 and the exhaust passageway 27. The engagement of the inner tube 25 within the drill bit 17 ensures that there is constant communication provided between the interior of the inner tube 25 and the exhaust ports 29 when the drill bit is in its closed position as shown at Figure 2 and in the "blow down" position as shown at Figures1 and 3.

The piston chamber 23 accommodates a piston 30 which is slidably received within the piston chamber 23 and over the inner tube 25 such that the piston is capable of reciprocation between an impact position at which it is in engagement with the inner end 39 of the drill bit 17 and a raised position at which is in closely spaced relation with the top sub. The internal bore of the piston is greater than 16 the external diameter of the inner tube for most of the length of the inner tube to define an annular passageway therebetween. The portion of the inner tube adjacent the top sub has a diameter corresponding to the internal diameter of the piston in order that the piston sealing engages the inner tube when in its raised position. The piston is formed with set of annular ribs 33 at spaced intervals along its length.

In addition the interior of the casing is associated with an inner sleeve 31 which extends from the top sub 13 to form the internal face of the casing in the region proximate the top sub and to define a plenum between the external face of the sleeve and the opposed internal face of the casing which is in communication with the top sub. The casing is provided with two sets of inlet ports 35 and 37 which open into the piston chamber to provide communication between the plenum and the piston chamber 23 at spaced axial positions along the length of the sleeve. The ribs 33 of the piston are slidably and sealingly engaged with the internal face of the casing and sleeve to define a first sub-space A between the drill bit support and the adjacent end of the piston and a second space B between the top-sub and the adjacent end of the piston.

29-NOV-2002 16:30 WRAY AND ASSOCIATES NO, ?17 P 16 -12- With reciprocation of the piston within the piston chamber the ribs cooperate with the inlet ports 35 and 37 to sequentially permit the admission of fluid into the first space A when the piston is in the impact position to cause the piston to move to its raised position and then to the second space B when the piston is at its raised position as shown to cause the piston to be driven to the impact position as shown. As a result, reciprocation of the piston within the piston chamber is effected through the controlled delivery of pressurised fluid into the piston chamber through the ports 35 and 37 which delivery is controlled by virtue of the ribs 33 provided on the piston 30. The fluid in each space is exhausted from each space as that space is contracting where the exhausted fluid flows from the piston chamber through fluid exhaust passageways defined between the drill bit and the drill bit support. In the case of the second space B the exhausted fluid initially passes from that space through the annular passageway defined between the inner tube and the bore of the piston. The fluid which is exhausted from the hammer is delivered into the bore hole through the splined passageway provided between the drill bit support 15 and the drill bit 17 to enter the borehole and return to the surface through both the annulus that exists between the walls of the borehole and the casing and through the exhaust ports 29 provided in the outer face of the drill bit, the exhaust passage 27 and the inner tube In addition the inner tube of the hammer according to the embodiment is provided with a set of first fluid ports 40 which are provided around the inner tube at a position located proximate its free end. The first fluid ports 40 provide communication between the annular space defined between the inner tube and the bore of the piston, In order to provide the first fluid ports 40 without adversely affecting the strength of the inner tube, the outer wall of the inner tube is provided with an enlarged portion through which each fluid port passes.

The first fluid ports 40 are spaced at equi-distant intervals around the inner tube and are inclined inwardly with respect to the central axis of the inner tube in the direction of the top sub. As a result the first fluid ports provide a restricted flow of exhaust fluid from the first and second space into the interior of the inner tube during reciprocation of the piston 30 within the piston chamber 23. As a result there is a constant delivery (although restricted) of pressurised fluid into the 29-NOV-2002 16:31 WRAY AND ASSOCIATES NO. 7 17 P. 17 -13interior of the inner tube 25 which is in a direction corresponding to the flow of exhaust fluid and cuttings within the inner tube In addition the inner tube of the hammer according to the embodiment is provided with a set of second fluid ports 43 which are provided around the inner tube 25 at a position located within the top-sub at the location of the check valve which is provided in the top-sub. The check valve 45 is illustrated in Figure 1 in both its open and its dosed position. The second fluid ports 43 provide communication between the annular space defined between the inner tube and the pressurised region of the top-sub whereby the communication is controlled by the check valve 45 such that communication will occur only when the check valve is open. The second fluid ports 43 are spaced at equi-distant intervals around the inner tube and are inclined inwardly with respect to the central axis of the inner tube in the direction of the top sub. As a result the second fluid ports provide a restricted flow of exhaust fluid from the top-sub into the interior of the inner tube 25 only when pressurised fluid is being delivered to the chamber of the hammer during reciprocation of the piston 30 within the piston chamber 23 and when the hammer is in blowdown mode). As a result there is a constant delivery (although restricted) under those conditions, of pressurised fluid into the interior of the inner tube 25 which is in a direction corresponding to the flow of exhaust fluid and cuttings within the inner tube In addition the inner tube of the hammer according to the embodiment is provided with a set of third fluid ports 47 which are provided around the inner tube 25 at a position within the top-sub to the side of the check valve 45 which is remote from the chamber. The third fluid ports 47 provide communication between the annular space defined between the inner tube 25 and the inlet portion of the top-sub whereby the communication is constant irrespective of the operative condition of the hammer. The third fluid ports 47 are spaced at equidistant intervals around the inner tube and are inclined inwardly with respect to the central axis of the inner tube in the direction of the top sub. As a result the third fluid ports 47 provide a restricted flow of exhaust fluid from the inlet of the top-sub into the interior of the inner tube 25 at all times when pressurised fluid is 29. NOV. 2002 16:31 WRAY AND ASSOCIATES NO. 717 P. 16 -14being delivered into the top-sub. As a result there is a constant delivery (although restricted) of pressurised fluid into the interior of the inner tube which is in a direction corresponding to intended the flow of exhaust fluid and cuttings within the inner tube 25 which creates a negative pressure upstream of the third fluid ports 47.

When it is desired that the hammer be deactivated the drill string is raised from the bore hole which causes the drill bit 17 to slide outwardly within the drill bit support such that the annular collar 21 becomes engaged with the retaining ring 19, and the hammer adopts the "blow down" position as shown at Figures 1 and 3. At this position the piston 30 is retained in its lower most position within the casing and the second space B between the piston and the top sub is pressurised to a greater extent than the first space A between the piston and the drill bit support in that pressurised fluid is delivered directly into the space B to be exhausted through the annular space provided between the inner tube 25 and the internal bore of the pfston 30. When in this mode pressurised fluid is caused to be introduced in to the inner tube 25 through the first fluid ports 40, the second fluid ports 43 and the third fluid ports 47 to maintain a flow of fluid along the inner tube 25 away from the drill bit 17 and to promote the flow of exhaust fluid from the borehole.

It is believed that the promotion of fluid flow into the exhaust passageways in the drill bit will reduce the turbulence which would otherwise be created by the delivery of pressurised fluid into the borehole. In addition because of the promotion of fluid flow through the inner tube which is promoted by the presence of the fluid ports in the inner tube when the hammer is inactive and in blowdown mode this serves to promote the cleaning of the bottom of the borehole and as a result on the drilling operation recommencing only fresh rock is drilled and the quality of the resultant sample will be improved.

In addition the drill bit support 15 is dimensioned such that it has an external diameter which is slightly greater than the diameter of the casing and which is less than the cutting diameter of the drill bit 17. As a result an annulus is created 29-NOV-2002 16:31 WRAY AND ASSOCIATES NO. 717 K. 19 between the hammer and the side wall of the bore hole which will permit the flow of some of the exhaust fluid from the hammer through that annulus. Because of the promotion of the flow which is generated in the inner tube by the first, second and third fluid ports the flow of the exhaust fluid with the entrained cuttings will be predominantly to the exhaust ports within the drill bit and the flow of exhaust fluid through the annulus will be substantially free of cuttings.

In addition the outer radial face of the drill bit support at least is formed to be resistant to wear.

It has been found that in using conventional down hole reverse circulation hammers which do not incorporate first, or second, or third fluid ports 40, 43 and 47 of the form described above several difficulties can arise. One such difficulty relates to the regrinding of cuttings generated by the action of the drill bit in the bore hole as a result of an inadequate flow of cuttings from the space between the cutting face of the drill bit and the bore hole. This is not only inefficient but also disadvantageous since in analysing the results of a drilling program geologists generally prefer larger rock chips to be recovered from the drilling action. In addition, in clay formations there is a tendency that the exhaust ports 29 in the drill bit can become blocked on introduction of the drill bit into the ground and can be difficult to unblock. A further difficulty that arises is that in mud formations there is a tendency for mud and clay to enter the exhaust passageways provided between the drill bit and the drill bit support when pressurised fluid is not being delivered to the drill bit (which can occur during rod changes) which will cause the exhaust passageways to become blocked and prevent the reactivation of the piston. In addition once a hammer has been subjected to some wear, there can be a tendency for a leakage of pressurised fluid into the second space B provided between the piston and the top sub which will cause pressurisation of the second space when the hammer is in the "blow down" mode which can create difficulties when its necessary to reactivate the hammer.

29. NOV. 2002 16:31 WRAY AND ASSOCIATES NO. 717 P. -16- Furthermore for the reasons referred to earlier it has been found the sealing the space in the bore hole in the region of the drill bit by the use of a shroud or transmission sleeve can result in inefficiencies in the operation of the hammer and result in the hammer and or drill string becoming jammed. In the case of the first embodiment described above there is no seal between the hammer and the borehole which permits the flow of exhaust fluid through the space between the borehole and the hammer. The effect of such is that the pressure differential to each side of the piston within the hammer is increased and that space is cleared of debris, cuttings and water. Furthermore because of the promotion of fluid flow through the inner tube by the presence of the first, second and third fluid ports the exhaust fluid flowing into the space between the hammer and the drill bit is substantially dclean.

It has been found in testing of a hammer according to the embodiment that the presence of the fluid ports in the inner tube promotes a substantially constant fluid flow into the inner tube in the direction away from the drill bit. This tends to produce a low pressure zone in the exhaust passage 27 and exhaust ports 29 of the drill bit. This negative pressure serves to enhance the flow of exhaust fluid and cuttings from the bore hole which reduces the degree of regrinding of the cuttings generated by the action of the drill bit in the bore hole and result in larger chips being delivered to the surface, In addition the negative pressure that is created serves to reduce the likelihood of blockage of the exhaust ports and exhaust passageway which may otherwise occur in clay or mud formations. The effect of the fluid ports on the operation of the hammer is further enhanced by the escape of some clean exhaust fluid through the annular space between the hammer and the borehole which improves the efficiency of the hammer and reduces the likelihood of jamming. Furthermore, it has been found that penetration rates of the drill bit increase noticeably under hard rock conditions and appreciably in soft ground conditions. A further advantage which has been found is that in mud formations where there is a tendency for mud or clay to enter the exhaust passageways defined in the absence of pressurised fluid being delivered to the hammer, that when the hammer is reactivated by the introduction of pressurised fluid into the hammer the controlled bleed-off of air from the upper 29-NOV-2002 16:32 WRAY AND ASSOCIATES NO. 717 P. 21 -17piston chamber through the fluid ports serves to assist in overcoming the back pressure so developed and the pressure developed in the lower piston chamber between the piston and the drill bit support, to cause activation of the piston and its cycling within the chamber and in so doing impacting on the drill bit and promoting the dislodgement of clay and mud in the exhaust passageways.

Furthermore, in the case of worn hammers it is expected that the bleeding off of pressurised air from the second space B will serve to depressurise the second space B between the piston and the top sub when the piston is in its impact position to minimise the difficulty of reactivating the piston. In addition it has been found that there is reduced wear on the drill bit at the bit face and the face exhaust ports when compared to arrangements where the borehole is sealed.

In addition the hammer of the first embodiment incorporates a cross-over sub 49 which is fixed to the top-sub 13 and which is in turn connected to a single walled drill rod 51 which delivers pressurised fluid from the surface to the hammer. The cross-over sub provides a connection between the drill rod 51 and the top-sub 13 in order that the pressurised fluid from the drill rod can be delivered to the hammer. In addition the cross-over sub 49 includes a delivery port 53 which opens at its inner end into the inner most end of the inner tube 25 to receive the fluid flow from the inner tube, The other end of the delivery port 53 opens to the outer face of the cross-over sub to deliver the fluid flow from the inner tube into the space between the drill string and the borehole. The axis of the delivery duct is oblique to the wall of the bore hole and is directed away from the hammer to minimise erosion of the borehole.

The use of the cross-over sub as described above enables a reverse circulation hammer to be used with conventional single walled drill rod in situation where conventional hammers have been used in the past without the need to utilise specialised fluid delivery system which have become established practice in the use of reverse circulation hammers in the past. As a result a reverse circulation hammer according to the first embodiment can be used in blast hole and construction drilling applications for mining and micropile drilling in soft ground conditions where the use of conventional hammers prior art reverse circulation 29-NOV-2002 16:32 WRAY AND ASSOCIATES NO. 717 P. 22 18 hammers have not been able to be used because of the potential damage to the surrounding ground formation that can result from their use. In addition since the annulus between the hammer and the borehole is not required to carry cuttings past the hammer, the diameter of the hammer and therefore the piston diameter can be increased to increase the energy output and the effectiveness of the hammer.

According to a second embodiment of the invention the hammer and top-sub as described in relation to the first embodiment but without the cross-over sub is connected to a dual walled drill rod whereby the inner tube 25 is connected to a central return passageway provided within the drill rod and the pressurised fluid is delivered to the top sub of the hammer through the outer annulus provided in the drill rod. The pressurised fluid is delivered from the drill rod into the top sub and then to the plenum defined between the casing 11 and the sleeve 31.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

It should be appreciated that the scope of the present invention need not be limited to the particular scope of the embodiment described above.

Claims

29.NOY.2002 16:32 29. NV. 2C2 1632 RAY ANDJ ASSOCIATES N.1 .2 NO. 717 P. 23 -19- Claims The claims defining the invention are as follows: 11. A reverse circulation hammer comprising a casing supporting a top sub at one end and a drill bit support at the other end, a chamber defined within the casing between the ends thereof, an inner tube supported in the chamber from the top sub to be coaxial within the casing and adapted to communicate with an exhaust passage provided in a drill bit to be supported by the drill bit support, a piston sridably supported in the chamber over the inner tube for reciprocation between the ends of the casing, porting means adapted to deliver pressurised fluid from the top sub to the chamber to case said reciprocation of the piston Within the chamber, exhaust passageways provided in the drill bit support adapted to exhaust fluid from the chamber to the drill bit, fluid ports provided in the inner tube to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the fluid ports, the casing and drill bit support being dimensioned to provide an annular space between the hammer and the borehole to permit the flow of some fluid exhausted from the drill bit support through the annulus. 2. A reverse circulation hammer as claimed at claim 1 wherein the outer face of the drill bit support is wear resistant 3. A reverse circulation hammer as claimed at claim 1 or 2 wherein the fluid ports are located in the inner tube towards the other end of the casing. 4. A reverse circulation hammer as claimed at any one of the preceding claims wherein the fluid ports are Inclined inwardly with respect to the central axis of the inner tube in the direction of the ofthr end of the casing. A reverse circulation hammer as claimed at any one of the preceding claims wherein the fluid ports are spaced around the inner tube. S 6. A reverse circulation hammer as claimed at any one of the preceding claims O 0 wherein the fluid ports are spaced at equidistant intervals around the inner tube. 7. A reverse circulation hammer as claimed at any one of the preceding claims wherein the fluid ports provide said communication substantially constantly throughout the reciprocation of the piston. 8. A reverse circulation hammer as claimed at any one of the preceding claims wherein the fluid ports provide communication when a piston is in the vicinity of the .0 piston's end positions within a chamber. 9. A reverse circulation hammer as claimed at any one of the preceding claims wherein the drill bit support is adapted to enable the drill bit to be moveable outwardly from the casing to a "blowdown" position at which the piston is retained in engagement with the drill bit support, wherein when the hammer is at that condition the fluid ports provide said communication. A reverse circulation hammer as claimed at any one of the preceding claims wherein a second set of fluid ports are provided in the inner tube in the region of the !0 top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the second fluid ports. 11. A reverse circulation hammer as claimed at claim 10 wherein the second set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing. 12. A reverse circulation hammer as claimed at claim 10 or 11 wherein the second set of fluid ports are spaced around the inner tube. 13. A reverse circulation hammer as claimed at claim 10 or 11 or 12 wherein the second set of fluid ports are spaced at equidistant intervals around the inner tube. oO 14. A reverse circulation hammer as claimed at claim 10 or 11 or 12 or 13 wherein, 0 the second set of fluid ports are controlled by the check valve of the top-sub such that (N the second set of ports are opened on the check valve opening to deliver fluid into S the chamber. Ni A reverse circulation hammer as claimed at any one of the preceding claims 0 wherein a third set of fluid ports are provided in the inner tube in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the third set of fluid ports. S0 16. A reverse circulation hammer as claimed at claim 15 wherein the third set of fluid ports are located downstream from the second set of fluid ports. 17. A reverse circulation hammer as claimed at claim 15 or 16 wherein the third set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing. 18. A reverse circulation hammer as claimed at claim 15 or 16 or 17 wherein, the third set of fluid ports are spaced around the inner tube. 0 19. A reverse circulation hammer as claimed at claim 15 or 16 or 17 or 18 wherein the third set of fluid ports are spaced at equidistant intervals around the inner tube. A reverse circulation hammer as claimed at any one of the proceeding claims wherein the hammer is provided or in use to be associated with a cross-over sub located proximate to the top-sub, said cross-over sub having at least one delivery port providing communication between the interior of the inner tube and the annulus, said at least one delivery port being inclined with respect to the central axis of the hammer such that the predominate direction of flow is in a direction away from the hammer. 29.NOV-2002 16:33 WRAY AND ASSOCIATES NO. 717 P. 26 -22- 21.A reverse circulation hammer as claimed claim 20 wherein the cross-over sub is located adjacent the end of the top-sub which is remote from the chamber. 22.A reverse circulation hammer as claimed claim 20 or 21 wherein the cross-over sub is incorporated with the top-sub. 23.A reverse circulation hammer as herein described with reference to the accompanying drawings. 24.An inner tube of a reverse circulation hammer, said inner tube having fluid ports provided in the wall of the inner tube to in use provide restrncted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the fluid ports, inner tube as claimed at claim 24 wherein the fluid ports are located towards the one end of the inner tube to be proximate the drill bit 26.An inner tube as claimed at claim 24 or 25 wherein the fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the inner tube. 27.An inner tube as claimed at claim 24 or 25 or 26 wherein the fluid ports are spaced around the inner tube. 28.An inner tube as claimed at claim 24 or 25 or 26 or 27 wherein the fluid ports are spaced at equidistant intervals around the tube. 29.An inner tube as claimed at any one of claims 24 to 28 wherein a second set of fluid ports are provided in the inner tube intermediate of the ends and at apposition which in use is to be in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the second fluid ports. 29-NOV-2002 16:33 WRAY AND ASSOCIATES N]O. 717 P. 27 -23- inner tube as claimed at claim 29 wherein the second set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing. 31 .An inner tube as claimed at claim 29 or 30 wherein the second set of fluid ports are spaced around the inner tube.
32.An inner tube as claimed at claim 29 or 30 or 31 wherein the second set of fluid ports are spaced at equidistant intervals around the tube.
33.An inner tube as claimed at any one of claims 29 to 32 wherein the second set of fluid ports are located such that in use they will be controlled by the check valve of the top-sub such that the second set of ports will open on the check valve opening. 34,An inner tube as claimed at any one of claims 24 to 33 wherein a third set of fluid ports are provided in the inner tube towards the other end to in use to be located in the region of the top-sub to provide restricted fluid flow to the interior of the tube to generate a reduced pressure within the inner tube upstream of the third set of fluid ports. inner tube as claimed at claim 34 wherein the third set of fluid ports are inclined inwardly with respect to the central axis of the inner tube in the direction of the other end of the casing. 36,An inner tube as claimed at claim 34 or 35 wherein the third set of fluid ports are spaced around the inner tube.
37.An inner tube as claimed at claim 34 or 35 or 36 wherein the third set of fluid ports are spaced at equidistant intervals around the tube.
38.An inner tube as herein described with reference to the accompanying drawings. oo 39. A reverse circulation hammer adapted to be connected to a single walled drill 0 0 string, said hammer including a cross-over sub located proximate to a top-sub, said providing communication between the drill string and a chamber of the hammer to S enable the delivery of pressurised fluid from the drill string to the top-sub, said e¢3 C 5 cross-over sub having at least one delivery port providing communication between the interior of an inner tube and the exterior of the crossover sub, said at least one 0 delivery port being inclined with respect to the central axis of the hammer such that e¢3 the predominate direction of flow is in a direction away from the hammer. C, 0 40. A reverse circulation hammer as claimed at claim 38 wherein the cross-over sub is located adjacent the end of the top-sub which is remote from the chamber.
41. A reverse circulation hammer as claimed at claim 38 the cross-over sub is incorporated with the top-sub.
42. A reverse circulation hammer adapted to be connected to a single walled drill string substantially as herein described.