SYSTEM

Technical Field

A drill string and an associated reverse circulation drilling system drill are disclosed for use in drilling a hole in the ground, such as for example, but not limited to: exploration or production holes, water bores and geothermal wells.

Background Art

Many types of ground drilling systems are available for drilling holes for particular purposes and in specific ground conditions. In reverse circulation (RC) drilling, a down the hole (DTH) hammer or other cutting/fracturing mechanism is coupled to a downhole end of a drill string having mutually isolated first and second fluid flow paths. A compressed or pressurised fluid, typically a gas such as air, is delivered through one of the fluid flow paths to drive the hammer or other cutting mechanism. This fluid together with entrained drill cuttings is returned to the surface through the second fluid flow path. The cuttings can be separated from the fluid and subsequently assayed.

The drill string is made of a plurality of end to end connected dual wall drill pipes each having an inner tube and an outer tube. An annular area formed between the inner and outer tubes creates the first fluid path. Compressed air is delivered to the drilling tool at the downhole end of the string through the first fluid path. Once the drilling tool has been powered or the fluid jetted against the bottom of the hole to mobilise drill cuttings, the exhaust fluid and cuttings are directed back to the surface through the second fluid path constituted by the ensemble of the inner tubes.

Issues that challenge the ability of these RC drilling systems to work well at increasing depths relate to increasing back pressures in the two fluid paths of the drill pipes and corresponding drill string.

In order for the total fluid mass flowing through the first fluid path down the drill string and through the hammer or (other drilling/cutting tool) to subsequently return back up the drill string through the second fluid path, the pressure at the bottom of the string must be suitably higher than the outlet pressure at surface and the sum of all pressure losses along the return path, In addition, ideally the up hole velocity (i.e. through the second fluid path) should at a minimum be between 4200 and 6000 feet per minute to entrain cuttings and convey them to the surface if the drilling fluid is air.

There are several problems with the return system when the fluid contains gases as follows:

1 The flow through the second fluid path (i.e. up the string from the downhole end to the uphole end) is met by friction resulting in a pressure loss, a corresponding increase in volume and, in turn, an increasing internal velocity in the inner tube.

2 An increase in fluid velocity results in increased frictional/pressure losses over a given length of the return path.

3 An increase in velocity results in increased wear on the bore of the inner

tubes and other components of the return system.

4 The back pressures associated with the flow of the returning fluid can

increase to a point where the velocity of the returning fluid is not great enough to lift the cuttings, causing the system to block.

Additionally, in and through the first (annual) fluid path where a compressible fluid is being delivered to the hammer or other drilling tool an inverse problem arises so that when the pressure is very high at the inlet to the drilling system the velocity is low and as the compressed air flows down the annulus it is meet with frictional losses which must be overcome to ensure flow occurs.

When total frictional losses are minimised then the greatest efficiency occurs in a given system.

One of the factors limiting the depth of a well formed by RC drilling is that the pressure at which fluid is introduced into the first path needs to overcome the pressure at the bottom of the drill string (i.e. back pressure), as well as producing sufficient fluid exit velocity to carry the drill cuttings back to the surface.

The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the drill string and associated reverse circulation drilling system. Summary of the Disclosure in a first aspect there is disclosed drill string suitable for a reverse circulation drilling system, the drill string comprising:

a first fluid path; and

a second fluid path isolated from the first fluid path;

the drill string having an up hole end and a downhole end, the first and second fluid paths extending between the up hole and downhole ends wherein: a fluid is able to flow through the first fluid path from the up hole end to the downhole end and return to the up hole end through the second flow path; the second fluid path being configured to provide a substantially constant velocity of fluid flowing from the downhole end to the up hole end.

In a second aspect there is disclosed drill string for a reverse circulation drilling system, the drill string comprising:

a first fluid path; and

a second fluid path isolated from the first fluid path;

the drill string having an up hole end and a downhole end, the first and second fluid paths extending between the up hole and downhole ends wherein: a fluid is able to flow through the first fluid path from the up hole end to the downhole end and return to the up hole end through the second flow path; the second fluid path has a cross-sectional area which is smaller at the downhole end than at the up hole end.

In one embodiment the cross-sectional area of the second fluid path increases at a plurality of spaced apart locations from the downhole end to the up-hole end and wherein at each successive location in an up hole direction the cross-sectional area of the second fluid path increases.

In one embodiment the cross-sectional area of the second fluid path progressively increases in an up-hole direction.

In one embodiment the first fluid flow path has a cross sectional area that is larger at the down hole end than the up hole end. ln one embodiment the cross-sectional area of the first fluid path increases at a plurality of spaced apart locations from the up-hole end to the downhole end.

In one embodiment wherein the cross-sectional area of the first fluid flow path progressively increases in the downhole direction.

In one embodiment the drill string comprises a plurality of end to end coupled drill pipes each drill pipe having an up-hole end and a downhole end, and wherein the first and second fluid paths extend through each of the drill pipes.

In one embodiment each drill pipe comprises an outer tube and an inner tube with an annular space formed between the outer tube and the inner tube, and wherein the annular space forms a portion of the first fluid path and an inside of the inner tube forms a portion of the second fluid path.

In one embodiment the inner tube for at least one of the drill pipes has a larger cross-sectional area at the up-hole end than at the downhole end.

In one embodiment the drill pipes have the same outer diameter.

In one embodiment an outer diameter of at least one of the drill pipes is larger at an up-hole end in comparison to a drill pipe at a downhole end.

In one embodiment a plurality of drill pipes comprise a first set of drill pipes in which the inner tube has a larger cross-sectional area at the up hole end than the downhole end and wherein the cross-sectional area of the inner tube at the up hole end is different for each of the drill pipes in the first set.

In one embodiment the drill string comprises a second set of drill pipes wherein the inner tube of each drill pipe in the second set has a constant cross-sectional area from the up-hole end to the downhole end.

In one embodiment the second set of to the drill pipes comprises a plurality of sub sets of drill pipes wherein for any one subset the cross-sectional area of the inner drill pipe is substantially the same, and wherein the cross-sectional area of the inner drill pipe in different subsets is different. ln a second aspect there is disclosed a reverse circulation drilling system comprising: a drill string in accordance with the first aspect; and a down hole tool attached to a downhole end of the drill string, the downhole tool having an outer casing and a bit.

In one embodiment the reverse circulation drilling system comprises at least one shroud between the string and the tool to enable a flow of fluid to a region between the outer casing and the at least one shroud.

In a third aspect there is disclosed down the hole hammer comprising: an outer casing, a hammer bit wherein the outer casing or a sub above the hammer further includes one or more ports enabling a bleed of fluid from within the hammer to an exterior of the hammer from a location above the hammer bit.

In one embodiment the down hole hammer comprises one or more ring is disposed: about the outer casing or between the outer casing and the sub, wherein the one or more rings extend radially from the outer casing.

In one embodiment the rings are arranged to form a labyrinth seal between the downhole hammer and a wall of a borehole.

In one embodiment the bleed holes include one or more bleed holes between: the one or more rings and the sub, or between at least two of the rings.

In a fourth aspect there is disclosed a drill pipe for a reverse circulation drilling system comprising:

an outer tube and an inner tube with an annular space formed between the outer tube and the inner tube, and wherein the annular space forms a first fluid path and an inside of the inner tube forms a portion of a second fluid path; the drill pipe having axially opposite first and second ends and wherein a cross-sectional area of the second fluid path at the first end is greater than that at the second end.

In a fifth aspect there is disclosed a reverse circulation hammer drilling system comprising:

a dual wall drill string;

an RC hammer coupled to an end of the drill string the RC hammer having an air driven piston and a hammer bit;

at least one air vent formed in one or both of the drill string and the RC hammer enabling a portion of air being delivered in a down hole direction for driving the RC hammer to bleed into a hole being formed by the drilling system prior to the air driving the piston; and

one or more seals between the at least one air vent and hammer bit providing resistance to air flow from the one or more vents in a direction downhole of the seals.

In one embodiment the dual wall drill string has an inner tube of constant inner and outer diameter along an entire length of the string, and an outer tube of constant inner and outer diameter along the entire length of the string.

Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of the system and method 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 is a schematic representation of an embodiment of the disclosed drill string and corresponding reverse circulation drilling system;

Figure 2 is a schematic representation of a second embodiment of the disclosed drill string and corresponding reverse circulation drilling system;

Figure 3 is a schematic representation of a third embodiment of the disclosed drill string and corresponding reverse circulation drilling system;

Figure 4 is a schematic representation of a fourth embodiment of the disclosed drill string and corresponding reverse circulation drilling system,

Figure 5 is a schematic representation of a fifth embodiment of the disclosed drill string and corresponding reverse circulation drilling system; and

Figure 6 is a schematic representation of a further embodiment of the disclosed reverse circulation drilling system.

Detailed Description of Specific Embodiment

Figure 1 is a schematic and conceptual representation of an embodiment of the disclosed drill string 10. The drill string 10 is disposed in a well 100. The drill string 10 has a downhole end 102 and an opposite up hole end 104. A hammer 24 is coupled to the downhole end 102 of the drill string for boring the well 100 in a known manner. The hammer 24 includes an outer case 25, a bit 26, a fluid driven piston 27 for cyclically impacting the bit 26, and a shroud 28 which assists in fluid flow out of the hammer 24. The form of the hammer 24 is not critical to the construction and functionality of the disclosed drill string and drill pipe.

The drill string 10 has a first fluid path 16 and a second fluid path 18 isolated from the first fluid path16, both extending between the up-hole end 104 and downhole end 102. A fluid flows through the first fluid path 16 from the up-hole end 104 to the downhole end 102 and returns to the up-hole end through the second flow path 18. The second fluid path 18 is configured to provide a substantially constant velocity of fluid flowing from the downhole end 102 to the up-hole end 104.

To provide the substantially constant fluid velocity, the second fluid path 18 is arranged so that it’s cross-sectional area at the downhole end 102 is smaller than at the up-hole end 104. That is, there is an increase in the cross-sectional area (and corresponding diameter) of the second fluid path from the downhole end 102. As explained in greater detail below, this change in cross-sectional area may be a continuous change but more practically the change will be stepped over a number of stages from the downhole end to the up-hole end.

For illustrative purposes only, the drill string 10 is illustrated as comprising five drill pipes 12A, 12B, 12C, 12D and 12E (hereinafter referred to in general as“drill pipes 12”). (In reality an embodiment of the disclosed drill string 10, when the

corresponding well has reached its target depth, is likely to comprise tens, hundreds or thousands of drill pipes connected in an end to end fashion). Each pipe 12 has an inner tube 20 and an outer tube 22. An annular space is formed between the inner and outer tubes of each drill pipe. The annular space forms a portion of the first fluid path 16; and, an inside of the inner tube 20 forms a portion of the second fluid path 18.

Collectively the ensemble of the annular space between the outer tubes 22 and the inner tubes 20 of the pipes 12 that make up the drill string 10 form the first fluid path 16 while the ensemble of the inner tubes 20 form the second fluid path 18.

Figure 1 is illustrative of the broad concept on which embodiments of do the disclosed drill string are based. In an ideal, though somewhat unpractical form of the drill string 10, the bore of the inner tube 20 of successive drill pipes 12 is formed with progressively increasing cross-sectional area (i.e. progressively increasing inner diameter) in the direction of fluid flow through the second fluid path 18, which in this instance is from the downhole 102 end toward the up hole end 104. In one

embodiment this can be achieved by tapering the bore of the inner tubes 20, while maintaining a constant outer diameter of the inner tubes 20. Alternately this can be achieved by forming the inner tube 20 wall to have a constant thickness with a progressively increasing inner and outer diameter in the direction of fluid flow through the second fluid path 18. In the ideal system the latter is preferable as this also provides the drill string 10 with a progressively increasing annular cross-section in the direction of fluid flow through the first flow path 16 which in this instance is from the up hole end 100 for the downhole end 102.

The embodiment of the drill string 10 shown in Figure 1 is somewhat impractical as it requires the large number of drill pipes 12 to be made with inner tubes 20 of different diameters and correctly sequencing the drill pipes 12 when screwing them into the drill string 10 as the depth of the well 100 advances.

Figure 2 illustrates a more practical, although again still largely conceptual, embodiment of the disclosed drill string numbered here as 10A. In this embodiment the same reference numbers are used to denote the same features as in the first embodiment shown in Figure 1.

The substantive difference between the drill string 10 and drill string 10A is that the inner tube 20 of each of the constituent drill pipes 12 is formed with a substantive length 21 having a constant inner and outer diameter and a relatively short length 23 at one end formed with a progressive or a stepped change in inner diameter. The next downhole successive drill pipe in the string has an inner tube 20 with a substantive length 21 having constant inner and outer diameter matching that of the length 23 of the previous drill pipe; and a relatively short end length 23 at one end formed with a progressive or stepped change in inner diameter. And so on.

In this embodiment it will be recognised that the cross-sectional area or inner diameter of the second fluid path 18 is again smaller at the downhole end 102 than the up hole end 104. Flowever instead of this change being continuous as in the drill string 10 shown in Figure 1 change occurs in a stepwise manner at spaced apart locations along the length of the drill string 10A. Simultaneously in this embodiment the annular area of the first fluid path 16 changes in a complimentary manner, i.e. the annular area is smaller at the up hole end 104 than the downhole end 102.

Figure 3 illustrates a portion of a further embodiment of the disclosed drill string designated as 10B. Here the drill string 10B is composed of a plurality of sets of drill pipes 12. There is a first set of drill pipes 12A1 -12An. In the first set each drill pipe has an outer tube 22Ay (where y is an integer 1 -n) of constant inner and outer diameter; and an inner tube 20Ay of constant inner and outer diameter. The drill string 10B also has a second set of drill pipes 12B1 - 12By (where y is an integer 1 - n). In the second set each drill pipe has an outer tube 22By (where y is an integer 1 - n) of constant inner and outer diameter, and which is the same as that for the outer pipes 22Ay; and an inner tube 20By of constant inner and outer diameter. However, the inner tube 20By has a smaller inner (and outer) diameter than for the inner tube 20Ay, being the inner tube of the drill pipes for the adjacent up hole set of drill pipes.

The drill string 10B also includes one or more transition drill pipes 12Xn, thought only one transition drill pipe 12X1 is shown in Figure 3. The respective transition pipe 12Xn is located between each of the different sets of drill pipes 12 which are formed with inner tubes 20 of constant inner diameter. Each transition pipe 12Xn has an outer tube of the same inner and outer diameter as all the other drill pipes 12 in each of the set 12An, 12Bn, but has an inner tube 20Xn which reduces in inner diameter in the downhole direction so as to match at its opposite ends of the inner diameter of the immediately adjacent drill pipes of the two sets in-between which it is disposed. Further sets of drill pipes 12Cn, 12Dn etc. may be added with appropriate transitions drill pipes therebetween. In this embodiment there may be different numbers of drill pipes in each set.

If the drill pipe 12A1 is the upmost drill pipe in the drill string 10B and a drill pipe 12Bn is the lowermost drill pipe in the drill string 10B then, as with all previous embodiments, the second fluid path 18 has a smaller cross-sectional area or inner diameter at the downhole end than at the up hole end. Additionally, the cross- sectional area of the annular fluid path 16 is larger at the downhole end 102 than the up-hole end 104.

The length of the drill pipe 10B can of course be extended by using additional sets of drill pipes each of which has an inner tube 20 of the same inner diameter, but where this is different to that of the inner tubes in an adjacent set of drill pipes, and using corresponding transition pipes 12 X to transition between the different inner diameter of different adjacent sets. In this described embodiment the transition pipes 12X are shown as being shorter in length than the other drill pipes 12. In such circumstances the transition pipes 12X may be more properly called or considered to be transition subs. However, in an alternate embodiment in order to facilitate easier coupling and decoupling of the drill pipes the transition pipes 12X may more conveniently be of the same length as all the other drill pipes. This will allow the adding in and breaking out of the transition pipe to be done in the same way as all the other drill pipes 12.

As a fictitious, non-limiting example only and for illustrative purposes the drill string 10B could be made up of:

• 20 drill pipes 12A1 -12A19 each having an outer pipe 22A with an inner

diameter of 172mm and an inner pipe 20A with an inner diameter of 155mm

• 10 drill pipes 12B1 -12B9 each having an outer pipe 22B with an inner

diameter of 172mm and an inner pipe 20B with an inner diameter of 145mm

• 5 drill pipes 12C1 -12C4 each having an outer pipe 22B with an inner diameter of 172mm and an inner pipe 20B with an inner diameter of 135mm

• A first transition pipe 12Xab connected between sets 12A and 12B with an inner diameter of 172mm and an inner pipe 20Xab with an inner diameter at an end adjacent pipe 12A19 of 155mm and an inner diameter at an end adjacent pipe 12B1 of 145mm

• A second transition pipe 12Xbc connected between sets 12B and 12C with an inner diameter of 172mm and an inner pipe 20Xbc with an inner diameter at an end adjacent pipe 12B9 of 145mm and an inner diameter at an end adjacent pipe 12C1 of 135mm

To assist in reducing the ingress of water into the drill string 10 which may flow through the second fluid path 18 a plurality of ports (not shown) can be provided in the outer case 25 to facilitate the bleeding of compressed air from the first flow path 16 into the well. This can create a high-pressure region which excludes the ingress of water.

Figure 4 illustrates a portion of a drill string 10C comprising three sets of drill pipes 12A, 12B and 12C (hereinafter referred to in general as“sets of drill pipes 12”). Each set of drill pipes 12 may comprise any number of substantially identical drill pipes. There is however a progressive change in the cross-sectional area of at least the second fluid path 18 in the same manner as described above in relation to the previous embodiments. That is, the cross-sectional area of the second fluid path 18 is at a minimum the upstream end with reference to the direction of flow in the fluid path 18 (which corresponds to the downhole end 102), and at a maximum at the downstream end of the fluid path 18 (which corresponds to the up hole end 104 of the drill string 10C). A number of transition subs or pipes 12X1 , 12X2 can be used to transition between the inner diameter is of the inner tubes 20 of the respective adjacent sets of drill pipes 12.

Additionally, in this embodiment of the drill string 10C has a stepwise change in the outer diameter of the outer tubes 22 of the drill pipes 12 in each of the sets. The transition subs or pipes 12X1 , and 12X2 are configured to accommodate for this change in the configuration of the outer tubes between adjacent sets of drill pipes 12.

By suitably arranging the diameters of the inner and outer tubes 20, 22 of the drill pipes in each of the sets, the drill string 10C is also formed with a change in the cross-sectional area of the second fluid path 18 in the same manner as described above in relation to the previous embodiments. That is, the cross-sectional area of the second fluid path 18 is at a minimum the upstream end with reference to the direction of flow in the fluid path 18 (which corresponds to the down hole end 102), and at a maximum at the downstream end of the fluid path 18 (which corresponds to the up-hole end 104 of the drill string 10C).

The drill string 10C is able to drill to greater depths as it has stronger drill pipes at the up-hole end (and therefore can support a greater load), and also has a larger internal diameter at the up hole end to enable air velocity to be maintained as close as possible to an optimum level.

In addition to the described variation in the cross-sectional area of the first and second fluid paths, embodiments of the drill string 10 or an associated RC drilling system which comprises a combination of the drill string 10 with the downhole hammer 24 may be enhanced by the provision of a high-pressure air vent enabling the bleed of high-pressure air into the well 100 at a location near or adjacent the hammer 24. The bleeding of a portion of the high-pressure air from the outer annular (i.e. first) flow path 16 into the well 100 will assist in minimising the ingress of water in through the hammer bit and up the second flow path 18. For example, consider an embodiment of the drill string 10 being operated to provide an air pressure of about 350 psig in the immediate vicinity of the hammer bit 26. One or more vents in the drill string 10 (and/or near an up-hole end of the hammer 24) may be provided to add say a further 300psig into the well up hole of the bit 24. This then provides a total of 650psig to exclude standing water in the well 100 from entering the second flow path 18 and mixing with the sample entrained in the up flowing.

An embodiment of this is depicted in Figure 5. This Figure shows an RC air hammer drilling system 500 having a drill string 10A similar to that depicted in Figure 2 and a hammer 24 but with the addition of air distributor rings 502, a sub 504 provided with one or more air vents 506 and a hammer 524 provided with a plurality of obstruction rings 508 which form substantial but not necessarily perfect seals along its length. While a plurality of rings 508 is shown, in other embodiments there may be only a single ring 508. The number and spacing of the rings 508 is to be determined on the basis of the well depth and level or head of standing water W within the well at a particular depth or range of depths. The air distribution rings 502 may provide a clearance of between about 1 -4 mm with the surface of the well 100. A 1 mm clearance is likely to be most common but it can be larger in certain circumstances such as when drilling clays or broken ground.

The bleeding of air pressure from the flow path 16 prior to operating the hammer 24 (i.e. driving the piston in the hammer) and flowing back up the second path 18 also assists in reducing the average up hole velocity through the second path 18. There is a requirement for a minimum discharge pressure at the up-hole end of the path 18 in order for the sample to discharge into an associated sampling system. In some present RC drilling systems this pressure is in the order of 10 psig. This requires a minimum (and of course higher) inlet pressure at the up-hole end of the first path 18 to take account of the pressure drop through the drill string 10 and the hammer 524 and to provide sufficient pressure differential at the bottom of the well to enable operation of the hammer 24. Depending on the geometry of the drill string providing the inlet pressure to achieve a discharge pressure of 10 psig can result in up-hole velocities in excess of 600 mph. This results in significant wear of the drill string 10. Due to the variation in the area of the first fluid path, there is less pressure drop than in a conventional system and therefore while the drilling system can be operated with a lower inlet pressure, it could be used with a conventional inlet pressure to enable dumping of part of the air pressure into the well prior to operating the hammer 24 without adversely affecting the operation of the hammer.

The provision of the one or more rings 508 below the vents 506 restricts downhole flow of the bleed air. Accordingly, there is now air being delivered into the well 100 at the face of the hammer bit 26, as well as air being delivered into the well at locations commensurate with the vents 506 which are up hole of the bit 26. The combined pressure of these two air discharges acts to exclude standing water in the vicinity of the hammer, and in particular from the toe of the well to a location near the highest of the rings 508. Of course, the air dumped via the vents 506 is unable to flow up the second path 18 and accordingly there is a reduction in discharge air velocity.

The provision of the air vents 506 and rings 508 are described in the context of the drill string 10 having first and second flow paths of variable cross section. However, the benefits of the use of the air vents 506 and rings 508 can be used with conventional dual wall drill pipes having an inner tube and outer tube of constant diameter (and therefore a first fluid path of constant cross-sectional area and a second fluid path of constant cross-sectional area, which is usually different to that of the first fluid path). Having a plurality of rings 508 forms a labyrinth seal.

In view of this, and as exemplified by Figure 6, there is also disclosed a RC hammer drilling system 600 which in general terms comprises the combination of a dual wall drill string 610, an RC hammer 624 coupled to an end of the drill string 610, at least one air vent 606, and one or more seals/rings 608 between the air vent and RC hammer 624.

In the RC hammer drilling system 600 and the dual wall drill string 610 has an inner tube 620 of constant inner and outer diameter along the entire length of the string 610, and an outer tube 622 of constant inner and outer diameter along the entire length of the string 610.

One or more vents 606 are provided either in the drill string 610, or a corresponding vent sub 604 located between the downhole end of the string 610 and at an up-hole end of the hammer 624, or in an outer casing of the hammer 264 itself.

One or more seals 608 are provided in the RC hammer drilling system 600 between the air vents 606 and a downhole end of the RC hammer 624. The downhole end of the RC hammer 624 is constituted by a cutting face of the corresponding hammer bit 626. In this particular embodiment the seals 608 are formed on an outer casing of the RC hammer 624.

Optionally one or more distribution rings 602 are provided in the drilling system 600 up hole of the air vents 606.

The RC hammer drilling system 600 can be operated in the same manner as a conventional RC hammer but of course with a portion of the air flowing through the annular flow path (i.e. the first fluid path) being dumped into the hole via the air vents 606 to provide the same benefits in operation in terms of exclusion of water and reduced discharge air velocity as described above in relation to a system utilising the disclosed drill pipe 10 having fluid flow paths of varying cross-sectional area. Whilst a number of specific embodiments have been described, it should be appreciated that the drill string and associated drilling system may be embodied in many other forms. For example, the preferred embodiments of the drill string described in association with a down the hole hammer. However, the drill string may be used with different types of cutting or borehole forming mechanisms and systems such as tri-cone drilling heads. Also, the hammers or hammer systems provided with the seals 508, 608 may be further modified to bleed fluid pressure from ports provided in the outer case between at least two of the rings 508, 608.

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” or variations such as“comprises” or“comprising” is 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 in various embodiments of the drill string and drilling system disclosed herein.