TECHNICAL FIELD

A bit for a down hole hammer is disclosed. Also disclosed is a bit drive and retention system for a down the hole (DTH) hammer and an associated shroud and porting system.

BACKGROUND ART

A down the hole (DTH) hammer drill (hereinafter referred to as either“hammer drill” or simply“drill”), including a reverse circulation hammer drill, is used for drilling holes in the ground. These drills are particularly well suited to drilling in hard ground. While they may also be used for drilling in soft or plastic grounds they often require greater operator skill and often a change in air flow and/or drill bit to improve efficiency.

The DTH hammer drill is screwed onto a downhole end of a drill string. The drill string is connected at an up-hole end to a rotation head. Drilling depth is advanced by adding lengths of drill pipe, one at time to the drill. The hammer drill is an assembly of numerous components including a hammer bit (hereinafter referred to as“hammer bit” or simply“bit”), a drive sub, a piston, a shroud, and a porting sleeve which controls air flow and the pressure field about the piston. The hammer bit is coupled to the DTH hammer via the drive sub. The hammer bit as a shank, a shoulder and a cutting face which is usually provided with a plurality of hardened buttons. The shank is formed with a plurality of splines and grooves extending axially from an up-hole end to the shoulder.

The drive sub is also formed with axially extending splines and grooves that are received in the grooves and splines respectively on the bit shank. This allows the bit to slide in an axial direction relative to the drive sub while also allowing the drive sub to impart torque from the drill string to the bit. A downhole end of the drive sub is arranged to bear on the shoulder of the bit when the bit is at the top of its stroke. However, when the hammer drill is in a blowdown mode the end of the drive sub is separated from the shoulder of the bit by a distance equal to the length of the bit stroke. A retaining ring up hole of the drive sub is used to prevent the hammer bit from falling out of the drive sub.

A shroud is located over a portion of the bit and the drive sub. The shroud assists in directing airflow about an outside of the bit. A downhole end of the shroud is also used as a backup to prevent the bit from falling out of the drive sub if the shank fractures or the retaining ring fails. To achieve this function, the downhole end of the shroud is formed with internal screw thread. This thread screws over a thread formed about the shoulder on the bit. When assembling the hammer bit, the shroud is screwed onto and then off the thread on the shoulder so that it lies downhole of the thread of the shoulder.

As the threads can only pass each other by way of rotation in a direction opposite to that of the drill string, if there is a failure in the shank or the retaining ring, the thread on the shroud can retain the bit. This is because the thread on the shoulder of the bit cannot pass the thread on the retaining ring by way of axial motion. Also, the threads on the shoulder and the shroud have an opposite sense to that of the rotation of the hammer drill. Therefore, the rotation of hammer drill cannot result in the thread of a broken bit screwing through and past the shroud and therefore falling into a hole being drilled.

In normal use of the hammer drill the thread on the shroud is spaced from the thread on the shoulder by distance greater than the stroke of the bit. Therefore except for in the case of a failure of the bit or the retaining ring the thread on the shroud is not impacted by the thread on the shoulder of the bit.

For the thread on the shroud to pass over the drive sub it is formed on a portion of the cylindrical wall of the shroud having a reduced thickness. The reduced thickness forms an area of weakness in shroud. It is not unusual for the reduced thickness portion of the shroud to be distorted and in particular knocked out of round through normal use of the hammer drill. When this occurs, in order to change the bit the shroud cannot be unscrewed but rather has to be cut off. This significantly increases the time to change the bit with the consequence of decreasing efficiency and increasing the cost of drilling.

SUMMARY THE DISCLOSURE

In one aspect there is disclosed a shroud for a down the hole hammer drill comprising:

a cylindrical portion having an uphole end and an opposite downhole end and an

intermediate portion with an inner circumferential surface of constant inner diameter extending from the downhole end to the uphole end;

a stop structure at the uphole end, the stop structure extending radially inward from the inner circumferential surface; and

a thread structure at the downhole end, the thread structure extending radially inward of the inner circumferential surface. In one embodiment the thread structure comprises a screw thread that extends continuously for at least one revolution of the inner circumferential surface.

In one embodiment the thread structure comprises a plurality of circumferentially spaced thread segments.

In one embodiment the body has a wall of uniform thickness for a length extending from a downhole end of the stop structure and an uphole end of the thread structure.

In a second aspect there is disclosed a shroud for a down the hole hammer having a hammer bit with a shank, a shoulder extending radially outward of the shank and a bit face, and they drive sub located over the shank and configured to bear on the shoulder, the shroud comprising:

a cylindrical portion having an uphole end and an opposite downhole end and an

intermediate portion with an inner circumferential surface of constant inner diameter extending from the downhole end to the uphole end;

a stop structure at the uphole end, the stop structure extending radially inward from the inner circumferential surface, the stop structure configured to prevent the shroud from falling from the drive sub; and

a thread structure at the downhole end, the thread structure extending radially inward of the inner circumferential surface and arranged to lie downhole of the shoulder of the bit when the stop structure bears on the drive sub.

In a third aspect there is disclosed a bit drive and retention system for a downhole hammer having a drill string and a hammer bit having a shank, a shoulder and a bit face, the system comprising:

a drive sub arranged to transfer torque from the drill string to the hammer bit and a shroud arranged to fit over a portion of the drive sub and the hammer bit;

the drive sub comprising an uphole portion and a downhole portion, the downhole portion arranged to bear on the bit shoulder and having an outer diameter less than that of the bit shoulder;

the shroud having: a cylindrical portion having an uphole end and an opposite downhole end and an intermediate portion with an inner circumferential surface of constant inner diameter extending from the downhole end to the uphole end; a stop structure at the uphole end, the stop structure extending radially inward from the inner circumferential surface and configured to prevent the shroud from falling from the drive sub; and a thread structure at the downhole end, the thread structure extending radially inward of the inner circumferential surface and arranged to lie downhole of the shoulder of the bit when the stop structure bears on the drive sub.

In one embodiment the thread structure comprises a screw thread that extends continuously for at least one revolution of the inner circumferential surface.

In one embodiment the downhole portion of the drive sub is formed with the continuous screw thread to engage with the thread structure and wherein the thread structure can be screwed onto and subsequently off the downhole portion to a location downhole of the drive sub and the shoulder of the bit.

In one embodiment the thread structure comprises a plurality of circumferentially spaced thread segments.

In one embodiment the downhole portion of the drive sub is provided with a plurality of grooves configured to enable the thread segments to slide in an axial direction to a location downhole of the drive sub and the shoulder of the bit.

In one embodiment the body has a wall of uniform thickness for a length extending from a downhole end of the stop structure and an up-hole end of the thread structure.

In one embodiment the downhole portion of the drive sub is provided with a plurality of through holes.

In a fourth aspect there is disclosed porting sleeve for a hammer drill assembly the porting sleeve comprising a tubular body having an up-hole end and a down hole end and a set of ports having an axial length, wherein at least one of the ports has have a non-symmetrical open area about a transverse line midway between opposite axial ends of the port.

In one embodiment the at least one of the ports has a non-symmetrical configuration on opposite sides of the transverse line.

In one embodiment the at least one of the ports has a first pair of parallel edges and second pair of parallel edges axially spaced from the first pair of parallel edges and where a distance about a common radius between the edges in the first pair is different to that between the second pair. In one embodiment the least one port comprises a plurality of holes wherein each of the holes has a different open area.

In a fifth aspect there is disclosed a drill bit for a down the hole hammer comprising:

a shank having axially extending and circumferentially alternating grooves and splines; lugs extending radially outward from one end of the shank; and

a bit head formed at an opposite end of the shank;

wherein the lugs have an axial length of between about 24 mm and 41 mm.

In a sixth aspect there is disclosed drill bit for a down the hole hammer comprising:

a shank having axially extending and circumferentially alternating grooves and splines; lugs extending radially outward from one end of the shank; and

a bit head formed at an opposite end of the shank;

wherein the bit has a maximum bit drop of less than 46mm.

In an embodiment the bit has a weight of between 10% and 20% less than the weight of an industry standard same diameter bit.

In one embodiment the bit is a weight of between about 9.5 kg and 10.5 kg for a drill bit diameter of between 133 mm and 143 mm.

In a seventh aspect there is disclosed a porting sleeve for a hammer drill assembly the porting sleeve having an up-hole end and a down hole end and a set of porting holes near the downhole end, the porting holes having an overall axial length OAL and an area A wherein at least one of the porting holes has an open area for a portion from an up-hole end of the porting hole to a distant of 0.25OAL which no more than 0.2A.

In one embodiment the at least one of the porting holes has an open area for a portion from an up-hole end of the porting hole to a distant of 0.25 of the OAL which is no more than 0.15A.

In one embodiment the least one of the porting holes has an open area for a portion from an up-hole end of the porting hole to a distant of 0.17of the OAL which is no more than 0.12A and preferably no more than 0.1 A. In one embodiment the least one of the porting holes has an open area for a portion from an up-hole end of the porting hole to a distant of 0.08 of the OAL which is no more than 0.05A, and preferably no more than 0.4A.

In one embodiment the least one of the porting holes has an open area for a portion from an up-hole end of the porting hole to a distant of 0.5 of the OAL which is no more than 0.45A, and preferably no more than 0.42A.

In an eighth aspect there is disclosed a porting sleeve for a hammer drill assembly the porting sleeve having an up-hole end and a down hole end and a set of porting holes near the downhole end, the porting holes having first and second pairs of parallel edges wherein the first pair of parallel edges are up-hole of the second pair of parallel edge, the second pair of parallel edges are spaced by a distance of W ₂ mm and the first pair of parallel edges are spaced by a distance Wi mm where of between 0.2W ₂ £ Wi £ 0.8W ₂ .

In one embodiment 0.4W ₂ £ Wi £ 0.6W ₂ .

In one embodiment WI=0.5W ₂ .

In one embodiment the at least one porting hole has a curved edge at an up-hole end of the first pair of parallel edges with a radius of 0.5 Wi mm.

In one embodiment the at least one porting hole has a curved edge at a downhole end of the second pair of parallel edges with a radius of 0.5 W ₂ mm.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the shroud and bit retention system as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 a is a section view through a prior art hammer assembly having a hammer bit, a drive sub and a shroud;

Figure 1 b is a schematic representation of a portion of a prior art hammer assembly when in a blowdown mode; Figure 1 c is a schematic representation of a prior art hammer bit;

Figure 2a depicts a prior art hammer assembly where a piston of the hammer is at the top of its stroke and with the hammer bit tagging at of a hole being drilled;

Figure 2b shows the piston just before striking the top of the bit;

Figure 2c shows the hammer assembly in the feathering mode;

Figure 2d shows the hammer assembly in the blowdown mode;

Figure 3 is a section view of a hammer assembly incorporating embodiment of the disclosed shroud and associated bit drive and retention system;

Figure 4 is an exploded isometric view of the hammer assembly shown in Figure 3;

Figure 5 is a schematic representation of an embodiment of the disclosed drive sub and a hammer bit;

Figure 6 is a schematic representation of a prior art drive sub and prior art full shoulder hammer bit;

Figure 7 is a schematic representation of another type of prior art drive sub and a prior art cut shoulder hammer bit;

Figure 8a is a schematic representation of a downhole portion of a conventional prior art porting sleeve;

Figure 8b is a schematic representation of an embodiment of a downhole portion of the disclosed porting sleeve;

Figure 9a is a laid out or development view providing a comparison of ports in the porting sleeve is shown in Figure 8a and 8b;

Figure 9b is a representation of an alternate configuration of ports in a second embodiment of the disclosed porting sleeve; Figure 9c is a representation of an alternate configuration of ports in a third embodiment of the disclosed porting sleeve;

Figure 9d is a representation of an alternate configuration of ports in a fourth embodiment of the disclosed porting sleeve;

Figure 10a is a partial section view of the prior art hammer assembly shown in Figure 1 b;

Figure 10b is a schematic representation of a hammer assembly incorporating the disclosed bit, bit retention system, porting sleeve and associated feathering system;

Figure 1 1 is an isometric view of a second embodiment of the disclosed shroud and associated bit drive and retention system; and

Figure 12 is a section view of a hammer assembly incorporating the second embodiment of the shroud and associated bit drive and retention system.

Recap of prior art confiqurations and qeneral operation of a prior art hammer drill

Figures 1 a-1 c show a portion of prior art hammer assembly 10 for a DTFI reverse circulation hammer drill. The hammer assembly 10 comprise a hammer bit 12, a drive sub 14 and a shroud 16. A bit retaining ring 18 is also shown at an up-hole end of the drive sub 14. The drill bit 12 has an elongated shank 20, a shoulder 22 and cutting face 24. The shoulder 22 is formed with a thread 26 about its outer circumferential surface. The cutting face 24 is provided with a plurality of hardened buttons 28. An axial channel 30 is formed through the bit 12 and splits into a plurality of branches 32 at this downhole end. The channel 30 and branches 32 allow a driving/working fluid such as compressed air, after exiting the hammer, to return up through the bit 12 carrying drill cutting and thus also acting to clear cuttings from the bit face 24.

The shank 20 is formed with axially extending splines 33 and grooves 35. The splines and grooves extend from the up-hole end of the drill bit toward the bit face 24. The drive sub 14 is also provided with a plurality of axially extending grooves and splines on its inner circumferential surface that mate with the splines 33 and grooves 35. This allows axial motion of the drill bit 12 relative to the drive sub 14 as well as the transfer of torque from the rotation head and drill string to the bit 12. The splines and grooves are of different width forming gaps between the engaged bit and drive sub to form axial passages for the flow of air which is used to drive a piston that impacts on the bit 12. This air flows between the shank 20 and the drive sub 14 and out through the shroud 16. The air subsequently returns up the drill bit 12 through the branches 32 and channel 30 carrying drill cuttings to the surface.

The bit 12 in this form of prior art assembly is known as a“cut shoulder bit” and is

characterised by the grooves 35 extending axially beyond the shoulder 22 to provide a continuation of the air flow path formed by the grooves in the shank 20. Therefore, with the cut shoulder bit when the shoulder 22 is abutting or located adjacent the downhole end of the drive sub 14 air is still able to flow past the drive sub 14 and out of the shroud 16.

An up-hole end of the bit 12 is formed with a plurality of circumferential spaced apart lugs 34. The lugs 34 together with the retaining ring 18 form the primary retaining system for the hammer assembly 10. The stroke length of the drill bit 12 is set by the distance between the bottom of the lugs 34 and the retaining ring 18. When the bit 12 is that the top of its stroke, the drive sub 14 is in contact with the shoulder 22. When the drill is in the blowdown mode (i.e. when the drill string is lifted from the toe of the hole being drilled) the drill bit can slide axially in a down hole direction by a maximum distance equal to the bit stroke length.

The drive sub 14 has an external thread 15 on an up-hole portion 36 to provide a screw connection to an outer barrel 41 of the hammer assembly that in turn is screw coupled to a downhole end of the drill string. A downhole portion 38 of the drive sub 14 is formed with an increased outer diameter. A shoulder 40 is formed at the transition between the up-hole portion 36 on the downhole portion 38. The reduced outer diameter of the up-hole portion 36 is accommodated in a lower portion of the outer barrel.

With reference to Figs 1 a, and 1 b the shroud 16 is of a generally cylindrical configuration and has a stop structure in the form of an internal shoulder 42 at an up-hole end and a screw thread 44 at a downhole end. A circumferential wall 46 of the shroud 16 has a constant outer diameter, an upper part 48 of a first inner diameter and a lower part 50 of a second larger inner diameter. Therefore, the part 50 has a smaller wall thickness. This is necessary to enable the shroud 16 to fit over and locate about the drive sub 14 and bit 12.

When attaching the bit 12 to the hammer drill assembly the drive sub 14 is first slid over the shank 12 so that it rests on the shoulder 22. The shroud 16 is then placed over the drive sub 14. When initially doing this the thread 44 contacts the beginning of the thread 26 on the shoulder 22. The shroud 16 is now rotated so that the threads 44 and 26 engage each other and shroud 16 can screw over the shoulder 22. Eventually the threads 44 fully pass the threads 26 and the shroud 16 is lowered until the shoulders 40 and 42 contact each other as shown in Figure 1 a.

It is because the threads 44 are formed the internal wall of the shroud 16, and the shroud needs to pass over the downhole portion 38, that the part 50 of the shroud 16 has an increased inner diameter and thus a reduced wall thickness. If wall of the shroud at part 50 had the same inner diameter is the portion 46 for its entire length, then the thread 44 could not pass over the portion 38. On the other hand if the wall thickness of the shroud 16 were reduced to the same as a part 50 then the hoop strength of the shroud would be reduced for its entire axial length.

Figure 1 b also shows in part an air driven piston 43 and a lower end of porting sleeve 45 of the hammer assembly. The piston 43 reciprocates within the outer barrel 41 striking the up- hole end of the bit 12. This transfers energy to the bit 12 which is used to fracture the strata at the toe of the hole being drilled. The piston 43 is caused to reciprocate by the flow of air and an arrangement of ports and porting surfaces on the inside surface of the outer barrel 41 and in and on the porting sleeve 45. The porting sleeve 45 has a tubular body and includes a plurality of ports 47 and are formed with long parallel sides that run in the axial direction and shorter radiused ends that extend in the circumferential direction of the sleeve 45.

FIGs 2a-2d illustrate in a general sense the operation of the hammer assembly with particular reference to motion of the piston 43. Flere the hammer assembly is also shown with its inner tube 49 that extends from an up-hole end of the hammer assembly through the centre of the outer barrel 41 and into the channel 30 of the bit 12. The piston 43 reciprocates along the tube 49. An exhaust path 51 (shown as dashed lines in Fig 2b) is formed between the outside of the inner tube 49 and the inner circumferential surface of the piston 43. The exhaust path 51 is opened and closed depending on the position of the piston 43 along the inner tube 49. The inner tube 49 includes a shoulder 49a at an up-hole location that forms a seal with the inside surface of the piston 43 when the piston 43 is at, or near, the top of its stroke.

It will also be seen that the inner circumferential surface of the outer barrel 41 is formed with a circumferential recess 53 at location next to the ports 47 of the porting sleeve 45. This recess 53 and the internal adjacent region of the outer barrel 41 is often termed as the“top chamber” of the hammer drill. The hammer drill 10 also has a“bottom chamber” 55 downhole of the top chamber. An intermediate chamber 57 exists between the top chamber and the bottom chamber.

Figure 2a shows the piston 43 at the top of its stroke and with the hammer bit 12 tagging at of a hole being drilled. In this configuration the top chamber 53 is open with air pressure at a maximum. The intermediate chamber 57 is open and in fluid communication with the top chamber 53, while the bottom chamber 55 is sealed from the top chamber and intermediate chamber by virtue of a surface 43a of the piston being radially opposite and adjacent and inner an intermediate inner wall 41 a of the outer barrel 41 . But the bottom chamber 55 is open at its lower end and therefore subject to the pressure within the hole. The exhaust path 51 is closed. Under these conditions air pressure differential between the top and the bottom chambers which is acting on the piston 43 causes the piston 43 to move in a down hole direction toward the bit 12.

Figure 2b shows the piston 43 just before striking the top of the bit 12. Because of the interaction of different surfaces on the piston 43, the inside surface of the outer barrel 41 and the porting sleeve 45, the top chamber 53 is now closed and the exhaust path 51 is open. Therefore, pressure in the top chamber 53 begins to decrease as air pressure bleeds through the path 51 . The bottom of the bottom chamber 55 is shut at its down hole end but open at its up-hole end to the intermediate chamber 57. The intermediate chamber 57 in turn is open to the driving air pressure through the ports 47 of the porting sleeve 45. Therefore, the air pressure equalises about the piston 43 just before it strikes the bit 12.

Figure 2c shows the hammer assembly in the feathering mode. The feathering mode is a transition state between the normal drilling mode and the blowdown mode described in relation to Figure 2D below. In the feathering mode the bit 16 slides in a downward direction and thus reaches from (i.e. is now spaced from) the end of the shroud 16. Flowever, the bit 16 has not slid down to the maximum extent required to enter the blowdown mode. The feathering mode may be arrived at in at least two ways. The first is when drilling in softer plastic grounds in which the bit when impacted by the piston penetrates into the softer ground and reaches from the shroud 16. A second way of entering the feathering mode is by a drill operator deliberately applying pullback force on to the drill string either to assist the drilling process when in soft or plastic grounds, or on the way to the blowdown mode for deliberately applying maximum airflow to the hole and back up the bit for clearing of the hole. During the feathering mode the piston is able to strike the bit 16. If the bit 16 is displaced sufficiently so that the piston 43 is able to slide down below the top of the ports of 47 in the porting sleeve 45 airflow will commence through the ports 47 through the exhaust path 57 and commence the process of the hammer assembly and intentionally, or prematurely entering the blowdown mode.

Figure 2d shows the hammer assembly in the blowdown mode where the hammer assembly is lifted from the toe of the hole enabling the bit 12 to slide in a down hole direction. This allows the piston 43 to also slide in a down hole direction beyond the up hole end of the ports 47. The full air pressure at the top chamber 53 is now able to flow through the exhaust path 51 , this is shown by the dots in this Figure. This air pressure is passed through between the drive sub 14 and bit 12 out of the shroud 16 into the hole and then back up the inside of the bit 12 to assist in clearing the hole.

When drilling in soft or plastic ground e.g. in clay an operator of the hammer drill will attempt to“feather the drill” in which they seek to slightly lift the hammer assembly to take some of the weight off the bit to prevent or minimise the risk of clogging the internal branches and channel of the bit 12. The feathering range however is particularly small and requires great skill because very soon after the ports 47 open, the entire width of the ports available to vent the full air pressure to the hole through the exhaust path rather than being available to reciprocate the piston 43.

Detailed description of specific embodiments

Figures 3 - 5 illustrate an embodiment of the disclosed shroud, drive sub and an associated bit retention system. This is also illustrated with respect to a modified drill bit 1 12 that is smaller and lighter than the prior art bit, the significance of which will become apparent later in this description. For ease of cross-reference with the prior art shown in Figures 1 a-2d, features of the disclosed shroud and bit retention system that are equivalent to those of the prior art are referenced with the prior art reference numbers incremented by 100.

The shroud 1 16 is formed with a circumferential wall of uniform thickness from a downhole end of the stop structure constituted by the shoulder 142 to the commencement of a thread structure 144 at the downhole end of the shroud 1 16.

Looking in more detail, the shroud 1 16 has a body 200 with an up-hole end 202, an opposite a downhole end 204, and an intermediate portion 206. The intermediate portion 206 has an inner circumferential surface 208 of constant inner diameter extending from the up-hole end 202 to the up-hole end 204. The shoulder 142 at the up-hole end 202 extends radially inward from the inner circumferential surface 208. Similarly, the thread structure 144 at the downhole end 204 extends radially inward of the inner circumferential surface 208. Hence the intermediate portion 206 has a uniform wall thickness.

However, the thread structure 144 of the shroud 1 16 is different to the thread 44 on the prior art shroud 16. The thread structure 144 in this embodiment is in the form of a plurality of circumferential spaced thread segments 210, as shown most clearly in Figure 4. The shroud 1 16 is used in conjunction with the modified drive sub 1 14. The drive sub 1 14 is modified by the provision of longitudinal slots 212 that extend for the full length of the down hole portion 138 of the drive sub 1 14. The thread structure 144 can pass over the downhole portion 138 of the drive sub 1 14 by travelling through and along the slots 212 on the downhole portion 138.

The slots 212 have a circumferential width marginally greater than that of the thread segments 210. Therefore, when placing the shroud 1 16 over the drive sub 1 14 the thread segments 210 are aligned with respective slots 212 allowing the shroud 1 16 to slide down until the thread segments 210 reach the shoulder 122 on the bit 1 12. The shroud 1 16 can now be rotated so that the thread segments 210 engage and screw over the threads 126 on the bit 1 12. Once the threads have disengaged the shroud 1 16 can be slid down until the shoulder 142 abuts the shoulder 140. Now the thread segments 210 of the thread structure 144 lie downhole of the shoulder 122 and the thread 126.

The thread segments 210 are located outside of the normal stroke range of the bit 1 12 and therefore would not normally be impacted by the bit 1 12. However, if the shank 120 of the bit 1 12 were to fracture the thread segments 144 can act to retain the broken bit in the same way as the threads 44 described above in relation to the prior art.

The bit 1 12 shown in Figures 3-5 is formed with a cut or segmented shoulder 122. However, this is not material to the working of the shroud 1 16. The shroud 1 16 and associated drive sub 1 14 can be used with a bit 1 12 having either a continuous or a segmented shoulder 122. However, when the bit 1 12 has a cut shoulder 122 the thread segments 144 on the shroud 1 16 have a circumferential width greater than the circumferential spacing between the shoulder segments so that the thread segments 144 cannot slide axially between the shoulder segments. The shoulder 122 is segmented in this embodiment by way of extending the axial grooves 135 of the bit 1 12 through and past the shoulder 122.

The combination of the shroud 1 16 and the drive sub 1 14 constitutes a bit drive and retention system 220. As previously mentioned the drive sub 1 14 differs from the prior art drive sub 14 by way of the provision of axially extending slots 212 in the downhole portion 138.

Additionally, in this embodiment (but not required for all embodiments) the drive sub 1 14 is also formed with a plurality of through holes 137. In this embodiment one hole 137 is formed in each of the slots 212. The purpose of the through holes 137 is to allow flow of air from the grooves 135 to flow between the shroud 1 16 and the bit 1 12 and subsequently out from the downhole end 204 of the shroud 1 16.

The through holes 137 are not needed in the presently illustrated embodiment of Figures 3-5 where the bit 122 has the segmented shoulder 122 to provide airflow as described above. This is because such airflow already exists with the bit 1 12 due to the extension of the grooves 214 through the shoulder 122.

However predominantly, prior art bits have continuous shoulders rather than segmented shoulders. The seating of a conventional drive sub on a continuous shoulder would substantially block this airflow. Having the through holes 137 provides the bit drive and retention system 220 with flexibility enabling it to be used with the continuous shoulder standard prior art drill bits 12, as well as segmented or cut shoulder bits 122.

Figure 5 is a side view of the drive sub 1 14 formed with the longitudinal slots 212 and the through holes 137 abutting against the shoulder 122 of the modified drill bit 1 12. The shroud 1 16 is able to engage the sub 1 14 and connect to the other components of a standard down the hole reverse circulation hammer assembly.

For comparison Figure 6 shows a prior art cut shoulder bit 12 with a prior art drive sub 14 which also includes through holes 37 however does not have the full length of the slot 212 of the drive sub 1 14, but rather a partial slot 212A. The slots 212A are closed end by ridge 213 at the up-hole end of portion 38. The present shroud 1 16 will not work with the prior art drive sub 14 because the thread segments 210 on sub 1 14 have no way of passing over the portion 38 of the drive sub 14. However, the standard drill bit 12 can be used in hammer drill assemblies incorporating embodiments of the present strengthened and improved shroud 1 16 by also utilising the drive sub 1 14 instead of the prior art drive sub 14.

Figure 7 is also provided for comparison purposes showing a prior art drive sub 14 without any holes 37 or slots 212 in portion 38. This drive sub 14 is used with a cut shoulder bit. Because of the existence of the cut shoulder the drive sub 14 does not require the holes 37 for air to flow between the sub 14 and the shank 20 past the shoulder 22, because the air is able to flow in groove 35 which continues beyond the shoulder 22.

The prior art bits 12 of Figures 6 and 7 and the modified bit 1 12 of Figures 3-5 have the same length shank 120 that extends from the up-hole end of the respective bits to their respective shoulders 22, 122. Flowever, these bits have the following differences: a) the overall length of the bit 1 12 is less than the overall length of the bits 12, this

difference in length being in the respective heads of the bits downhole of their shoulders; b) the outer diameter of the bit 1 12 from its threads 126 to the commencement of its

crown 127 is less than the diameter in the same region of the bits 12; c) the length of the lugs 134 on the bit 1 12 are longer than the lugs 34 on the bits 12 and that therefore the stroke length (i.e. the bit drop) of the bit 1 12 is less than that of the bit 12; d) the bits 1 12 are lighter due to the reduced axial length of the bit head/crown 127.

From the above discussion it will be apparent that the use of the disclosed drive sub 1 14 enables a reverse circulation hammer drill assembly to use standard cut shoulder and full shoulder drill bits, as well as the shorter lighter drill bit 1 12. When the disclosed drive sub 1 14 is used with a conventional drill bit 12 the corresponding conventional shroud 16 should also be used. The significance of this is that hammer drill operators using embodiments of the drive sub can source bits from multiple suppliers of standard bits as well as the disclosed light weigh bit 1 12, by use of the drive sub 1 14. Moreover, and possibly more significantly the disclosed shorted lighter bit 1 12 with cut shoulders can be used with a standard hammer drive sub and the shorter shroud 1 16 to accommodate the shorter bit drop.

The shorter lighter bit 1 12 for the same standard piston 43 naturally changes the piston to bit mass ratio providing greater energy transfer to the bit and thus the ground.

Testing and trials using the shorter lighter drill bit 1 12 showed drilling speed increases (i.e. the time to dill per meter reduces) considerably in hard rock in comparison to the standard drill bits 12. This is believed to be due to the greater energy transfer from the striking piston 43 into the hole. The energy required to move the bit 1 12 is less than for a prior art bit (due to its lower mass) and therefore due to the law of conservation of energy, more of the energy from the piston 43 is transferred into the strata at the toe of the hole thereby improving the degree of fracturing of the strata at impact with the bit 1 12.

The Applicant through research and testing has proposed the disclosed bit 1 12 with reduced weight in comparison to the standard bits 12 by having a smaller reduced weight head while the shank length remaining the same. This is achieved by forming the bit with a reduced stroke. The reduced stroke is facilitated by the formation of the longer length lugs 134 on the bit 1 12 in comparison to the prior art lugs.

To provide context to the significance of the disclosed drill bit 1 12, a standard hammer bit 12 has a 46 mm bit drop. This is the full possible axial displacement of the drill bit between when the hammer is tagging the toe of a borehole to when the hammer is in the blow down mode with the lugs 34 abut the retaining ring 18. However, the modified disclosed bit 1 12 can provide a reduction in the bit drop of between 0-18 mm by extending the length of the drive lugs 134 by the same length in comparison to the standard lugs 34. As result of this reduced bit drop the axial length of the shroud 1 16 can also be reduced by the same length. This also provides a more robust shroud 1 16 in comparison to the standard shroud 16. In a more specific example the length of the lugs 134 are increase the standard bit-ring lug on the upper end of the bit spline, from 23.5mm to approx 41 0mm (i.e. lug length increase for the lugs 134 of by 17.5mm) which provides an equal reduction in reduces the standard bit drop from 46mm to approximately 28.5mm for the bit 1 12. Of course, in other embodiments the increase in lug length and corresponding decrease in bit drop may be arranged for anywhere within the above described ranges, for example the length of the lug 134 may be between: 46 mm and 45 mm inclusive; or between 46 mm and 44 mm inclusive; or 46 mm and 43 mm inclusive; or 46 mm and 42 mm inclusive; or 46 mm and 40 mm inclusive; or 46 mm and 38 mm inclusive; or 46 mm and 34 mm inclusive; or 46 mm and 30 mm inclusive. In these examples of the bit drop will decrease by a commensurate amount.

In terms of weight reduction embodiment of the bit 1 12 may have weight reduction in comparison to a like for like industry standard bit 12 of about 1 kg-2kg. In terms of percentage reduction in comparison to a like for like industry standard bit this may be in the order of 10%-20%. In one specific example where the lugs 134 are increased by a length of 15.5 mm (e.g. so the lugs have a length of about 39 mm), the corresponding bit 1 12 would have a weight reduction of about 15%, or about 1.7 kg in relation to an industry standard 5 ¼ inch 52 hammer (which may have a weight of 1 1 .2 kg), or about 1 .8 kg in relation to an industry standard 5 5/8inch 52 hammer bit (which may have a weight of about 12.3 kg). In these examples the embodiment of bit 1 12 for a standard 52 hammer, the bit 1 12 would have a weight of about 9.5 kg for a 5 ¼ inch (133 mm) diameter bit, and a weight of about 10.5 kg for a 5 5/8 inch (143mm) diameter bit.

While the reduced weight bit 1 12 outperforms standard bits in hard rock, testing also indicates that the lightweight bit may have problems drilling in softer plastic materials. To address this, as disclosed herein, it is proposed to increase the feathering range of the hammer drill. To this end a feathering system is disclosed through modification of the standard porting sleeve 45 so as to reduce pressure differential between the top and bottom of the piston 43 while the weight on the bit is partially relieved by increasing the pullback force on the drill string. However, for harder grounds embodiments of the shorter lighter bit 1 12 can be used with the standard hammer with a standard porting arrangement. Indeed, the shorter lighter bit 112 can also be used in softer grounds with a standard porting arrangement but may have the same performance issues as the standard bit.

An embodiment of the feathering system 300 is incorporated in the porting sleeve 145 shown Figure 8a. The porting sleeve 145 has a tubular body with a plurality of ports 147 that extend in the axial direction. For comparative purposes a corresponding portion of a standard porting sleeve 45 is shown in Figure 8b. Figure 9 also shows a development of the corresponding ports of the porting sleeve 145 and 45. Figure 10a shows a prior art hammer assembly 10 with standard porting sleeve 45, bit 12, drive sub 14 and shroud 16 and 10b provide a comparison between a hammer assembly. Figure 10b shows a hammer assembly 100 with the disclosed feathering system 300 with porting sleeve 145, bit 1 12, drive sub 1 14 and shroud 1 16. The hammer assemblies in Figures 10a and 10b are in the same operating position.

The port 47 in the standard porting sleeve 45 is formed with parallel sides 93 that run in an axial direction of the sleeve 45 and are spaced apart by a transverse distance W ₂ . Opposite ends of the port 47 are curved with a radius of W ₂ /2. In comparison in the feathering system 300 the corresponding ports 147 has a modified configuration somewhat similar to a keyhole with a reduced width portion 149 at an up-hole end. More specifically the port 147 has a first pair of parallel edges 95 and second pair of parallel edges 97. In this embodiment the spacing between the parallel edges 97 is a same as that of the parallel edges 93. There is a curved edge 99 at an up-hole end of the parallel edges 95 and a curved edge 101 at the downhole end of the parallel edges 97. The overall axial length OAL of the ports 47 and 147 is the same. Also, the maximum width of the ports 47 and 147 is the same, namely W ₂ . The reduced width portion 149 of the port 147 extends for between 25%-40% of the OAL and has a width of W ₁ mm where: O.2W ₂ £ W ₁ £ O.8W ₂ . Opposite sides of the reduced width portion 149 are parallel to the axis of the sleeve 145. The up-hole end of the reduced width portion 149 is curved with a radius of W ₂ /4.

As result of the reduced width portion 149 at the up-hole end of the piston 43 wipes across the port 147 for a same distance of displacement as for the standard port 47 a substantially smaller area of the port 147 is opened in comparison to the port 47. Figure 9 shows three lines T, U and V extending transversely across the ports 47 and 147. These lines can be considered to be the up-hole end of a piston 43 sliding inside of the corresponding sleeve in a down hole direction. When the up-hole end of the piston 43 has passed the top or starting point of the respective ports 47, 147 by a distance so that it now lies at the location T, which in this example is 3 mm, the opened area of the port 47 is Tmm ² , whereas the opened area of the port 147 is about 0.65Tmm ² . This represents about a 35% reduction in the open port area 147.

As the piston progresses to the location U, which may be 6 mm from the top of the respective ports, the open area of the port 47 is Umm ² , whereas the open area of port 147 is about .60Umm ² . When the piston traverses a further 3mm to position V the open area of port 47 of Vmm ² , whereas the open area of port 147 is about 0.57Vmm ² .

The reduced flow area of the port 147 at the initial stages of being swiped by the up-hole end of the piston 43 can also be viewed in terms of the total area A of the port hole 147. The porting hole 147 with the overall axial length OAL has an area A. The area of the porting hole 147 from its up hole edge to a distant of 0.25OAL (which coincides with the line V) is no more than 0.2A, and preferably no more than 0.15A.

The portion of the porting hole 147 from its up-hole end to a distant of 0.17 of the OAL (coinciding with the line U) is no more than 0.12A and preferably no more than 0.1 A.

The portion of the porting hole 147 from its up-hole end to a distant of 0.083 of the OAL (coinciding with the line T) is no more than 0.05A and preferably no more than .04A. Looking to the halfway distance of the OAL (i.e. 0.5 OAL), represented by the line R, the open area of the port 147 which is no more than 0.45A, and preferably no more than 0.42A. In a more general sense at least one of the ports 147 have a non-symmetrical open area about a transverse line midway between opposite axial ends of the port 147. (This line is shown as line R in Figure 9.) Thus, for a port 147 having a total area Amm ² then the port 147 will have a smaller open area (i.e. less than 0.5Amm ² ) one side of the transverse line than the other. Consequently, as a piston 43 travels for a first half of the axial length of the port 147 in a downward direction from an up-hole end of the port 147 to the transverse midway line (i.e. from edge 99 to R) it opens a smaller port area in comparison to the piston’s travel for a second half of axial length of the port 147 to the downhole end (i.e. from R to edge 101 ). Also, in this embodiment at least one of the ports 147 has a non- symmetrical configuration on opposite sides of the transverse centre line located midway of the axial length of the port 147.

The ports 147 may also be described by and differentiated from the prior art ports in terms of the spacing between respective pairs of parallel edges 95 and 97. For example at least one (in this embodiment all) of the ports has a first pair of parallel edges (e.g. 97) and second pair of parallel edges (e.g. 95) axially spaced from the first pair of parallel edges 97, where a distance about a common radius between the edges in the first pair 97 (equivalent to the width W ₂ referred to above) is different to that between the second pair 95 (equivalent to the width W ₂ referred to above). As a consequence of this configuration as the piston 43 travels from the up-hole end at edge 99 of the port 147 to the axially opposite end at edge 101 the initial rate of opening of the port 147 is slower for a first portion of the piston travel than a subsequent portion of the piston travel.

From this it can be understood that the rate of air flow and pressure bleed from the top chamber through the exhaust path 51 occurs more slowly with the feathering system 300. Consequently, the piston 43 needs to travel a greater distance with the feathering system 300 to transition between 100% percussion to 100% blowdown as the hammer assembly is lifted slightly off the top of the hole. This provides an increased feathering range where the piston 43 is able to reciprocate and impact the bit before full blowdown is achieved at which time the bit has slid out of impact range of the piston 43 and the piston 43 is held down by reason of the difference in exposed area to a uniform pressure field. This is a phenomenon well-known to those skilled in the art and therefore not described in great detail herein but in essence arises due to a difference in surface areas looking in the up hole direction and down direction of the piston 43. The surface area looking in a down hole direction is several times greater than that looking in the up-hole direction. As force is the product of pressure and area, with a uniform pressure field acting on the piston 43 greater force acts in the down hole direction to hold the piston 43 down when the hammer drill is in the blowdown mode. Returning back to the shroud and associated bit retention system, Figures 1 1 and 12 illustrate an alternate embodiment. In this embodiment the shroud is denoted by the reference number 1 16a and the bit retention system by reference number 220a. All features of the shroud 1 16a and the bit drive and retention system 220a which are in substances same as those of the shroud 1 16 in the system 220 of the first embodiment are denoted with the same reference numbers while those which materially differ are denoted with the same reference numbers but the addition of the suffix“a”.

The shroud 1 16a as a body 200a with and up hole portion 202, downhole portion 204 and an intermediate portion 206. The intermediate portion 206 has an inner circumferential surface 208 of constant inner diameter extending from the up-hole end 202 to the up-hole end 204. The shoulder 142 at the up-hole end 202 extends radially inward from the inner

circumferential surface 208. Shroud 1 16a also has a thread structure 144a at the downhole end 204 that extends radially inward of the inner circumferential surface 208. Hence the intermediate portion 206 has a uniform wall thickness.

However, the thread structure 144a of the shroud 1 16a is different to the thread structure 144 on the shroud 1 16. Specifically, the thread structure 144a is now a continuous thread in that extends for at least one continuous revolution. While the thread structure 144a is the same as the thread structure on the prior art shroud 16 significantly the shroud 1 16a is still formed with a circumferential wall of uniform inner diameter and thickness from the downhole end of the shoulder 142 to the commencement of the thread 144a.

The shroud 1 16a is able to pass over the drive sub 114a of the retention system 220a, notwithstanding its uniform wall thickness, by forming the downhole portion 138a with a complimentary screw thread 224.

During assembly the drive sub 1 14a is placed on the shank of the bit 1 12 and lowered until its downhole end sits on the shoulder 122. The shroud 1 16a is then placed over the drive sub 1 14a until the thread structure 144a abuts the commencement of the thread 224 on the downhole portion 138a. The shroud 1 16a is now screwed onto thread 224. Eventually the downhole end of the thread 144a disengages from the thread 224.

Next the thread structure 144a will abut the thread 126 on the shoulder 122. Continued rotation of the shroud 1 16a enables the thread structure 144a to engage and subsequently screw over the thread 126. Initial engagement of the thread structure 144a with the thread 126 can be assisted by slightly lifting the drive sub 1 14a from the shoulder 122. However, this is not absolute necessity.

Once the thread structure 144a passes the thread 126 the shroud 1 16a can slide further down in the axial direction until the shoulder 142 abuts the shoulder 140 as shown in Figure 12.

The thread structure 144a now lies downhole of the thread 126 on the shoulder 122 and therefore again acts as to retain the bit 1 12 from falling from the drill in the event that the shank 120 breaks or the up hole retention ring fails. It should be noted that Figure 12 depicts the hammer assembly with the bit 1 12 at the bottom of its stroke. In this configuration the shoulder 122 is spaced from the downhole portion 138a and the thread structure 144a is still maintained downhole of the thread 126.

Whilst specific embodiments of the drive sub and associated bit drive retention system have been described numerous modifications and variations are possible. For example, the shroud 1 14a can also be formed with one or more axial slots on the downhole portion 138a and provided with through holes 222 similar to those shown in the drive sub 1 14. This would enable the shroud 1 14a and associated bit drive retention system 220a to be used with drill bits having either a continuous or a segmented shoulder 122. Also, the specific shape and configuration of the port 147 can of course be varied to achieve the same effect as that described in the above embodiment namely to provide an initial relatively slow rate of air flow and pressure bleed from the top chamber through the exhaust path 51 to enhance the feathering range over the prior art. For example, an up hole end of the port 147 can be tapered to a point. Multiple variations of this is shown as dashed lines 95a, 95b and 95c in Figure 9. Irrespective of the specific shape and configuration of the port 147, in order to achieve the reduced rate of opening of the port 147 for the initial travel of the piston 43 an open area of port 147 on opposite sides of a transverse line located midway along the axial length of the port 147, (i.e. midway along the direction of travel of the piston across the (port 147).

In another possible variation each port may include two or more holes of different open that are ion alignment with each other, so that a port is considered to comprise a plurality of holes that are aligned with the travel, extending from an upper most edge of an upper most hole to the lower most edge of the lower most hole. Examples of this are shown in Figs 9b and 9c. Figure 9b shows a port 147b which comprises an elliptical hole H1 and a rectangular hole H2. The holes H1 and H2 are axially off set from each other, i.e. are generally arranged along the direction of travel of a piston 43 within an associate downhole hammer. The port 147b has an upper most edge 99b and a lower most edge 101 b. As the piston 43 travels on its downward stroke from the edge 99b to the edge 101 b, for a first portion of the length of travel of the piston 43, with a progressively uncovers the hole H1 is a relatively slow bleed of air through the port 147b compared to where the piston uncovers the hole H2 which exposes a greater area per unit length of travel of the piston than the hole H1.

Figure 9c shows a variation where the port 147c comprises three circular holes H1 , H2, H3 of progressively increased diameter. Figure 9d shows yet a further variation is where the port 147d comprises a plurality of holes H1 , H2, H3 each of increased area and are both axially and circumferentially offset from each other. The ports 147, 147b, 147c, 147d in each embodiment produce the same effect of controlling the rate of air bleed, and in particular modifying by slowing down the of air bleed in comparison to standard porting sleeves. They should be readily apparent in each of these embodiments the configuration, and more particularly the open area, of the ports is non-symmetrical about a transverse line R midway between the upper most edges 99b, 99c, 99d and the lower most edges 101 b, 101 c, 101 d of the ports.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word“comprise” and variations such as“comprises” or“comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features of the container as disclosed herein.