GB2183694A - Improvements in or relating to rotary drill bits - Google Patents

Abstract

A rotary drill bit for drilling deep holes in subsurface formations comprises a bit body 10 having a shank 14 for connection to a drill string and a plurality of elements 22, 16 mounted on the bit body for cutting, abrading or bearing on the formation being drilled. The bit body includes a fixed structure 11 and a movable structure 17, each carrying elements for acting on the formation, the movable structure being capable of reversible movement relatively to the fixed structure between two limiting positions, the relative movement providing at least two configurations in which there are different distributions, between said elements 22, 16 of the loads applied to the bit during its engagement with the formation. Control means, such as hydraulic means 29, 30 or spring means, are provided to control the movement of the movable structure 17, and hence the load distribution between the elements 22, 16 automatically in response to the torque and/or axial loads applied to the bit. <IMAGE>

Description

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SPECIFICATION

Improvements in or relating to rotary drill bits

5 The invention relates to rotary drill bitsforuse in drilling or coring deep holes in subsurface formations.

Rotary drill bits of the kind to which the invention relates comprise a bit body having a shankforcon-nection to a drill string and a plurality of elements 10 mounted on the bit body for acting on the formation being drilled. The elements may comprise cutting elements, abrading elements or wear pads. The invention is particularly, but not exclusively, applicableto drill bits ofthe kind where the cutting elementsare 15 preform elements, each element being in the form of a tablet, usually circular, having a hard cutting face formed of polycrystalline diamond or other super-hard material. Typically, abrasion elements may be studs of hard material, such as tungsten carbide, in 20 which diamond particles, for example neutral diamond particles, are embedded.

Usually the bit body will incorporate a passagefor supplying drilling fluid to the surface ofthe bitfor cleaning and/orcooling ofthe cutting elements. 25 In such bits, the cutting elements, particularly if they are ofthe polycrystalline diamond preform type, are liable to fail or wear excessively when subjected to substantial overheating. Such overheating may arise as a result of excessive weight-on-bit while 30 drilling, i.e. excessive downward force on the cutters, orasa result of excessivedrag onthecutters due, for example, to the nature ofthe formation being cut. It is difficultto prevent such overheating by operator control ofthe weight-on-bit ortorque 35 since it is not possible to determine with any certainty whether overheating is in fact occurring and, in any case, the conditions causing overheating may be encountered only intermittently and for short periods, for example as a result of temporary changes in 40 the nature ofthe formation through which the drill is passing.

Cutting elements are also liable to fail due to sudden overload, for example due to impact as a result ofthe bit being dropped into the hole. 45 The present invention sets outto provide a form of drill bit in which the configuration ofthe bit is variable to enable it to cope with such overloading, whether it be only momentary or of continuing duration. The change in configuration may be effected 50 automatically in response to the overloading, or may be under operator control.

According to the invention there is provided a drill bit comprising a bit body having a shank for connection to a drill string and, mounted on the body bit, a 55 plurality of elements for cutting, abrading of bearing ontheformation being drilled, the bit body including at least two relatively movable structures, each carrying elements for acting ontheformation, which structures are capable of reversible movement re-60 latively to one another between two limiting positions, said relative movement providing at least two configurations in which there are different distributions, between said elements, ofthe loads applied to the bit during its engagement with theform-65 ation, means being provided to controls said limited relative movement between said two structures and hence to control the distribution between said elements ofthe loads applied to the bit.

Said control means may comprise means respon-70 sive to loads applied to the bit body during drilling in such manner as to change the configuration ofthe bit body automatically in accordance with variation in said loads. Alternatively or additionally, said control means may be operator controlled from the sur-75 face while the bit is in the hole being drilled. Such operator control may, for example, be effected by a signal, such as an hydraulic signal from the surface to the bit, or may be effected by operation ofthe bit in such mannerthatthe loads therein effect the requi-80 red change in configuration.

Preferably the arrangement is such that the change in configuration ofthe bit body is reversed upon reversal ofthe variation in the loads applied to the bit body. Such reversal may be subject to a hysteresis 85 effect.

The load responsive means may be responsive to variation in the weight-on-bit, or to the torque applied to the bit, or to both.

In one particulararrangementthere may be moun-90 ted a number of elements on each of said two relatively movable structures, the elements on one structure being so located thatthey do not act significantly on theformation in one configuration ofthe bit body, but act significantly on the formation inan-95 other configuration ofthe bitbody.

One structure on the bit body may be fixed in relation to the shank, the other structure being movable relatively to the fixed structure in a direction having at least an axial component, and/or a rotational com-100 ponent, whereby the weight-on-bit load and/or torque tends to move the movable structu re relatively to the fixed structure.

Preferably means are provided to oppose relative movement between said relatively movable struc-105 tures ofthe bit body from said one configuration to said other configuration, whereby said movement takes places only when the load acting on the bit reaches a value sufficient to overcome the opposing force provided by said means. The means for oppos-110 ing relative movement between said relatively movable structures preferably applies a force tending to restore said structures to said one configuration upon reduction ofthe load applied to the bit. Said means may comprise, for example, hydraulic means 115 orspring means.

In one particular embodiment ofthe invention, the fixed structure isformed with a cylinder in which is slideable a piston member movable with said other structure, said cylinder being in communication with 120 a passage in the bit body for supplying drilling fluid to the surface ofthe bit, whereby the hydraulic pressure of the drilling fluid urges the movable structure towards one limit of its movement, and opposes movement thereof relatively to the fixed structure. 125 In an alternative embodiment according to the invention spring means couple the two structures of the bit body together and are such that rotational deformation ofthe spring means, resulting from applied torque, is accompanied by axial deformation 130 thereof, whereby a change in the torque applied to

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said other structure causes relative rotational movement between the structures and rotational deformation ofthe spring means, and the accompanying axial deformation ofthe spring means effects relat-5 ive axial movement between the structures.

In any ofthe above arrangements said other structure ofthe bit body, which is movable relatively to the shank, may have an outerface on which are mounted a plurality of main cutting elements which, 10 under normal drilling loads and in said one configuration ofthe bit body, perform at least a major part of the cutting and abrading of the formation, and said one structure ofthe bit body, which is fixed in rela-tiontotheshank, may havean outerface on which 15 are mounted a plurality of secondary elements which, under normal drilling loads and in said one configuration ofthe bit body, are so located in relation to said main cutting elements thatthey do not act significantly ontheformation, relative movement 20 between said structures, under abnormal increased drilling loads, causing the parts to move to said other configuration in which said secondary elements are so located in relation to said main cutting elements thatthey act on theformation to a significant extent. 25 The secondary elements may include cutting elements similarto the main cutting elements, abrading elements, wearpadsora roller cone assembly.

The following is a detailed description, by way of example, of embodiments of the invention, refer-30 ence being made to the accompanying drawings, in which:

Figure 1 is a diagrammatic vertical section through a drilI bit in accordanee with the invention;

Figure 2 is a diagrammatic end view ofthe bit 35 shown in Figure 1;

Figures3 and 4, Figures5and 6, and Figures 7and Sare similar views to Figures 1 and 2 showing dia-grammatically alternative embodiments;

Figures Sand 10 are a side view and end view re-40 spectively of a spring device for coupling thetwo parts of a drill bit according to the invention;

Figure 11 is a diagrammatic vertical section through an alternative form of drill bit according to the invention;

45 Figure 72isanendviewofthedrillbitshownin Figure 11;

Figure 73 is a section through a further form of drill bit according to the invention;

Figure 14 is an end view ofthe bit shown Figure 13; 50 and

Figure 75is a half-section through a drill bit assembly in which the means for controlling the variable configuration ofthe bit is incorporated in a separate su b-assem bly.

55 Referring to Figures 1 and2,themain bitbody 10 comprises an outerfixed part 11 having at its upper end a reduced diameter portion 12 which is secured within the lower end of a sub-assembly 13, the upper end of which is formed with a threaded shank 14for 60 connectiontothedrillstring.

At its lower end, the fixed part 11 is formed with two end face portions 15 on which are mounted abrasion elements 16. The abrasion elements 16 may be of any suitable form, for example they may comprise 65 tungsten carbide studs in which are embedded particles of natural diamond. The end face portions, and the abrasion elements thereon, constitute a secondary cutting structure.

A movable central part 17 ofthe bit is axially 70 slideable within a bore 18 in the part 11, inter-

engaging splines 19 on the part 17 and in the bore 18 being provided forthe transmission of torque between thetwo parts. The lower end ofthe movable part 17 isformed with a head portion 20 on which are 75 provided blades 21 which carry preform cutting elements 22 in known manner. The head portion 20 andthecutting elements thereon constitute the primary cutting structure ofthe bit. Nozzles 23 mounted in the end surface ofthe head portion 20 com-80 municate through passages 24 with a central passage 25 in the movable part 17 ofthe bit, which passage communicates in turn with a central passage 26 in the sub-assembly 13. In use ofthe bit, drilling fluid under pressure is supplied through the pas-85 sage 26, passage 25, passages 24 and nozzles 23for cleaning and cooling the cutting elements.

A piston assembly 27, including a heavy duty seal 28 and scraper ring 29, is mounted on the upper end ofthe movable bit body part 17 and is slideable 90 within a cylinder 30 integrally formed with the subassembly 13. The lower end ofthe cylinder 30 is in communication, through low pressure link passages 31, with the annular space between the subassembly 13 and the walls ofthe bore, (normally re-95 ferredtoastheannulus).

As previously discussed, the cutting elements 22 mounted on the end face 20 may be susceptible to overheating, and consequent damage orfailure, as a resu It of excessive weig ht-on-bit and/or excessive 100 torque and the configuration ofthe bit is such as automatically to compensate for such excessive loads. The configuration also protects the cutters against momentary overloads due to impact, for example as a result ofthe bit being dropped in the hole. 105 In normal use ofthe bit shown in Figures 1 and 2, the hydraulic pressure ofthe drilling fluid in the passage 26, which is lowerthan the hydraulic pressure in the annulus and at theface ofthe bit, urges the piston assembly 27 downwardly in the cylinder 30 so 110 thatthe movable bit part 17 is in its lowermost position in relation to the fixed bit part 11. In this position the main cutting action atthe bottom ofthe hole being drilled is effected by the primary cutting structure comprising the cutters 22 on the central mov-115 able part 17 ofthe bit. The part 17 may be so positioned normally in relation to thefixed part 11 that when the cutters 22 are in operation under normal weight-on-bit loads the abrasion elements 16 on the face portions 15 are either out of engagement with 120 theformation or perform only a subsidiary cutting effect on the formation.

However, should there be a momentary or continuing overload on the cutters 22, resulting in increased weight-on-bit, the overload will causethe 125 central part 17 to retract upwardly relatively to the outer part 11 againstthe axial restraint provided by the hydraulic pressure ofthe drilling fluid. This retraction ofthe central part 17 will re-distribute the loads on the end face ofthe bit so thatthe abrasion 130 elements on the secondary cutting structure carry a

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higher proportion ofthe load, thus relieving the overload on the more vulnerable cutters 22.

The particularform of the cutting elements and abrasion elements, and their distribution and mount-5 ing overthe surface ofthe bit body do not form a part ofthe present invention and will nottherefore be described in detail. As previously mentioned, however, the invention is particularly applicable to drill bits where the cutting elements are in the form of pre-10 forms, for example comprising a front hard facing layer of polycrystalline diamond orothersuperhard material bonded to a backing layer of less hard material, such as tungsten carbide. Alternatively, the preforms may comprise a unitary layer of thermally 15 stable polycrystalline diamond material. The preforms may be directly mounted on the bit body or may be bonded to studs, for example of tungsten carbide, which are mounted in sockets in the bit body. The bit body itself may be machined from steel 20 or may be formed of tungsten carbide matrix infiltrated with a binder alloy, or may be a combination of such materials. Again, the precise method of construction ofthe bit body does notform part ofthe present invention.

25 In the alternative arrangement shown dia-grammatically in Figures 3 and 4, the drill bit comprises a bit body 110 formed at one end with a threaded shank 111 for connection to the drill string. The operative end face 112 of the bit body is formed with 30 a plurality of cutting elements (not shown).

The bit has a gauge section including kickers 113 which contactthe walls ofthe boreholetostabilise the bit in the bore hole. A central passage 114 in the bit body and shank delivers drilling fluid through 35 nozzles in the end face 112 in known manner to clean and/or cool the cutting elements. A socket for one such nozzle is indicated diagrammatically at 117.

The bit body is formed in two parts indicated generally at 115 and 116 in Figure 3. The part 115 is fixed in 40 relation to the shank 111, for example is welded thereto, and comprises a base portion 117 from which three pillars 118 extend downwardly. The pillars 118, as best seen in Figure 4, are equally spaced around the central vertical axis ofthe drill bit and 45 areofcircularcrosssection.

The second part 116 ofthe bit body is the main part on which most ofthe cutting elements are mounted and on which the kickers 113 are provided. The main body part 116 is provided with three cylindrical bores 50 119 which extend parallel to the central axis and receive the pillars 115 in sliding engagementtherein.

Thetwo parts 115 and 116 ofthe bit body are coupled together by a connector 120 the lower end of which is in screw-threaded engagement with a cen-55 tral threaded blind hole 121 in the bit body part 116. The upper end ofthe connector 120 passes slideably through a central aperture in the base portion 117 of the body part 15 and is formed at its upper end with a circularflange 122 which is received inacounterbore 60 123 in the base portion 117.

Surrounding the connector 120 is a resilient coupler comprising concentric spaced tubular elements 124and 125, between which is bonded an annular layerof elastomeric material 126. 65 Theinnerandoutertubularelements 124and 125

are staggered axial ly. The outer element is seated in a recess in the base part 117 and the inner element is seated in a similar recess in the body part 116, the arrangement being such that the body part 116 may 70 be displaced vertically with respecttothe body part

115 against the resilience ofthe elastomer 126.

The lower ends of the pillars 118 are formed with elements (not shown) for acting ontheformation being drilled. The elements on the lower ends ofthe 75 pillars may be abrasion elements or wear pads. For example, the abrasion elements may comprise studs of hard material, such as tungsten carbide, in which superhard particles such as natural diamond are embedded.

80 The elements on the lower ends ofthe pillars 118 might, alternatively, be cutting elements similarto the cutting elements mounted on the main bit body 116. In this case, however, it is probable thatthe pillars 118 would be located at a greater radial dis-85 tancefrom the axis of rotation ofthe bit so as to provide a back-upforthe cutting elements nearerthe gauge region, which are particularly susceptible to overheating and wear.

During normal drilling under design weight-on-bit 90 loads, the cutting oftheformation iscarried out entirely by the cutting elements and/or abrading elements mounted on the end face 112 ofthe main bit body part 116. The length ofthe pillars 118 is such thatthe elements which they carry at their lower 95 ends project less far than the operative elements on the main body part 116, so that they do not act on the formation. However, as the weight-on-bit increases the main body part 116 moves upwardly relatively to thefixed body part 115, against the action ofthe elas-100 tomer 126. When the weight-on-bit reaches a predetermined level the retraction ofthe main body part

116 is such thatthe elements on the lower ends ofthe pillars 118come into active engagement with theformation and serve to back up the action of the cutting

105 elements on the end surface 112.

If the elements on the pillars 118 are cutting elements they have the effect of increasing the number of cutting elements over which the weight-on-bit is distributed, thus reducing the downward load on 110 each individual cutting element. Alternatively, the elements on the lower ends ofthe pillars 118 may comprise simple abrasion elements or wear pads. Such elements are less efficient at cutting than the cutting elements but are much less susceptible to damage 115 by overheating. In this case the engagement ofthe abrasion elements or wear pads with theformation serves as a temporary back-upforthe cutting elements. Such arrangement means less risk ofthe main cutting elements being damaged dueto over-120 heating, but at the cost of a significant reduction in rate of penetration. Such arrangement is particularly suitable for use in formations where there may be layers or inclusions of harderformation to which the cutting elements are subjected only temporarily. The 125 back-up wear pads or abrading elements are thus brought into action for oniy brief periods to prevent damage to the main cutting elements.

In the alternative arrangement shown in Figures 5 and 6 the resistance to vertical displacement ofthe 130 main body part 216 relatively to the fixed body part

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215 is provided by hydraulic means ratherthan by an elastomer, inthiscasethe passage214fordrilling fluid communicates with a chamber227 betweenthe two parts ofthe bit body, and the passage 228 lead-5 ingtoeach nozzle leadsfrom this chamber.

Hydraulic pressure ofthe drilling fluid within the chamber 227 urges the body part 216 downwardly with respect to the fixed body part and so provides a predetermined resistance to upward displacement 10 ofthe movable body part as well as returning the movable part to its original position when the weight-on-bit returns to its normal value.

A pin 229 projects radially inwardly from the body part 216 into a recess 230 in the fixed body part 215 15 so as to limit the relative displacement between the parts.

Figures 7 and 8 shown an arrangement in which the configuration ofthe bit changes in response to variation in applied torque as well as to variation in 20 weight-on-bit.

As best seen in Figure 7 the arrangement is generally similarto the arrangement of Figure 3 except thatthe elastomer ofthe Figure 3 arrangement is replaced by a helical compression spring 331 encircl-25 ing the connector320. The bit reacts to variation in weight-on-bit in similar fash ion, therefore, to the arrangements of Figures 3 and 5.

However, in this arrangement the bores 319 in the main body part 316 which receive the pillars 318 are 30 elongate in section as shown in Figure 8. This permits rotational displacement of the main body part 316 with respect to the fixed body part 315. The body parts 316 and 315 are mechanically coupled by any suitable means, such as inter-engaging hel.ical 35 splines, whereby displacement between the parts has both rotational and axial components.

Alternatively, the body parts may be connected by a coupler of the kind shown in Figures 9 and 10. The coupler is generally in the form of a cylindrical steel 40 sleeve 332 the central portion ofthe sleeve being formed around its periphery with a plurality of equally spaced inclined slots 333. The configuration of the coupler 332 is such that when opposite ends thereof are twisted relatively to one another about its 45 central axisthe axial length ofthe element is reduced.

Consequently, when the main body part 316 ofthe bit is subjected to sufficienttorque to overcome the torsional resistance provided by the coupler332,the 50 body part 316 is rotationally displaced with respect tothefixed body part 315, such displacement being permitted by the elongate cross-section ofthe bores 319. The resultant rotational deformation ofthe coupler 332, which connects the body part 315 and 55 316 together, causes a reduction inthe axial length of the element and this retracts the main body part316 axially with respectto the fixed part315. The elements on the lower ends ofthe pillars are thus brought into effective action ontheformation aspre-60 viously described.

In the arrangements shown in Figures3to 10the pillars are shown as integral with the base portion. It will be appreciated thatthey might be in the form of separate elements secured to the base portion and, 65 indeed, theshapeand location ofthefixed body part may be of any other suitable configuration. Two such possible alternative configurations are shown in Figures 11 to 14.

In the alternative arrangement shown in Figures 11 70 and 12 the main bit body part 416 is provided with preform cutting elements 434 mounted on studs 435 received in the body part. Abrasion elements are mounted on the kickers, one of which elements is indicated at 436, and may comprise tungsten carbide 75 studs in which natural diamonds are embedded.

In this arrangementthe fixed part 415 ofthe bit body is integrally formed with downwardly extending arms437 on each of which is mounted a roller cone 438 which may be similarto the kind commonly 80 used in rock bits. The roller cone 438 may be of any conventional construction such as is wel l known in theartand will not be described in detail.

In the arrangement shown, hydraulic means similarto that shown in Figure 5 are provided to resist 85 upward displacement ofthe body part 416 with respectto thefixed body part 415, but itwill beapp-reciated that any other suitable form of resistance means, such as spring means, may be provided instead.

90 During normal operation ofthe drill bit, cutting of theformation is effected by the preforms 434, and the roller cones 438 are held out of effective engagement with theformation. However, upon retraction ofthe movable body part 416, due to increased 95 weight-on-bit and/ortorque due to increased drag forces, the roller cones 438 will be brought into operation and, as is well known, the roller cone type of construction is particularly effective in dealing with hard formations.

100 While it is known to provide combination bits in which roller cone assemblies are combined with preform cutters, the present invention providesforthe different types of cutting action to be brought into use depending on the nature ofthe formation being 105 cut.

In the alternative embodiment shown in Figures 13 and 14the downwardly extending arms 537 have at their lower ends surfaces 538 in which are embedded arrays of natural diamonds. Again, as is well known, 110 natural diamonds are more suited to certain types of harderformation than preform cutting elements, and the variable configuration bit according to the present invention ensures that such cutting elements are automatically brought into action if har-115 der formations are encountered which cannot be effectively dealt with by the preform cutters.

Itwill be appreciated that, in the above described examples, the structure which controls the change in configuration ofthe bit body may still be operable 120 even when the cutting elements themselves have become so worn as to renderthe bit unusablefor further drill ing. There is therefore shown in Figure 15 an embodiment in which the meansfor controlling the configuration ofthe bit body forms a separate 125 sub-assembly to which the bit body is connected. This sub-assembly is therefore re-usable and only the bit body itself needs to be replaced when worn.

Referring to Figure 15, the main bit body 640 comprises an outerfixed part 641 having at its upper end 130 a threaded shank 642 and at its lower end two end

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face portions in which natural diamonds or abrasion elements are mounted in similarfashionto the arrangement of Figures 1 and 2. Nozzles are provided for the supply of drilling fluid to these face portions of 5 the bit and communicate with a central passage 647 in the bit body.

A movable central part 648 ofthe bit is axially slideable within a stepped central bore in the outer part 640 and is formed with a head portion 649 on 10 which are provided blades which carry preform cutting elements651 in known manner. Such preform cutting elements may be of any ofthe types previously described. Nozzles mounted intheendsurface ofthe head portion 649 communicate with the 15 central passage 647 in the bit body.

The upper end ofthe inner part 648, remote from the head portion 649, is connected to a force-transmitting sleeve 654 which is slideable in the central bore in the fixed outer part 640.

20 The control sub-assembly 643 comprises a fixed outertube 655 the upper end of which isformedwith a threaded shank 656 for connection to the drill string and the lower end of which isformed with any internally threaded socket 657 within which the threaded 25 shank642 ofthe main bit body may be engaged. Mounted within the tube 655 is a tubular spring structure 658 which is both longitudinally and rotati-onally resilient. The upper end 659 ofthe spring structure is secured to the outertube 655 and the op-30 posite end carries a coupler 660 which is connected totheend ofthe force-transmitting sleeve 654 when the main bit body is coupled to the control subassembly.

In normal use ofthe coupled bit and control sub-35 assembly, the main cutting action at the bottom of the hole being drilled is effected by the cutters 651 on the central movable part 648 ofthe bit. The part 648 may be so positioned normally in relation to the outer part 640 that when the cutters 651 are in oper-40 ation under normal weight-on-bit loads the natural diamonds mounted on the face portions 43 are either out of effective engagement with the formation or perform only a subsidiary cutting effect on the formation. However, should there be momentary orcon-45 tinuing overload on the cutters 651, resulting in increased torque and/or weight-on-bit, the overload will cause the central part 648 to retract relatively to the outer part 640 against the torsional and longitudinal resilient restraint provided by the structures 50 658 in the control sub-assembly. This retraction of thecentral part 648 will redistribute the loadsonthe end face ofthe bit so thatthe natural diamonds or abrasive elements on the outer part 640 carry a higher proportion ofthe load, thus relieving the 55 overload on the more vulnerable preform cutters 651.

All of the arrangements described above may relieve both continuing overloads, duefor exampleto the bit meeting a harderformation or being subjec-60 ted to excessive weight-on-bit, or momentary overloads due to impact, for example caused by dropping ofthe bit in the hole. In the arrangements described above such control ofthe bit is effected automatically, i.e. variation in the load on the bit causes the 65 change in configuration which enables the bitto cope with such increase in load. However, there may also be incorporated in the bit means whereby the configuration ofthe bit may be adjusted by command from the surface. For example, the relatively 70 movable parts ofthe bit may be arranged to be moved to different relative positions in response to codes of hydraulic pulses in the drilling fluid or by coded sequences of movement ofthe bit. For example, two relatively movable parts ofthe bit may be 75 arranged to be toggled between two different configurations upon a predetermined type of movement, or sequence of movements, ofthe bit while it is down the hole.

Claims (16)

80 CLAIMS

1. A drill bit comprising a bit body having a shank for connection to a drill string and, mounted on the bit body, a plurality of elements forcutting, abrading

85 or bearing ontheformation being drilled, the bit body including atleasttwo relatively movable structures, each carrying elements for acting ontheformation, which structures are capable of reversible movement relatively to one another between two 90 limiting positions, said relative movement providing at least two configurations in which there are different distributions, between said elements, ofthe loadsappliedtothe bit during its engagement with theformation, means being provided to control said 95 limited relative movement between said two structures and hence to control the distribution between said elements ofthe load applied to the bit.

2. A drill bit according to Claim 1, wherein said control means comprises means responsive to loads

100 applied to the bit body during drilling in such manner as to change the configuration ofthe bit body automatically in accordance with variation in said loads.

3. A drill bit according to Claim 2, wherein the arrangement is such thatthe change in configuration

105 ofthe bit body is reversed upon reversal of the variation in the loads applied to the bit body.

4. A drill bit according to Claim 2 or Claim 3, wherein the load responsive means are responsive to variation in the weight-on-bit.

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5. Adrill bitaccordingtoanyofClaims2to4, wherein the load responsive means are responsive to variation in the torque applied to the bit.

6. Adrill bit according to any of Claims 1 to 5, comprising two relatively movable structures, each

115 having a number of said elements mounted thereon, the elements on one structure being so located that they do not act significantly ontheformation in one configuration ofthe bit body, but act significantly on theformation in another configuration ofthe bit

120 body.

7. Adrill bit according to Claim 6, wherein one of said structures on the bit body is fixed in relation to the shank, the other structure being movable relatively to thefixed structure in a direction having at

125 leastoneaxialcomponent,wherebytheweight-on-bit load tends to move the movable structure relatively to the fixed structure.

8. Adrill bit according to Claim 6, wherein one of said structures ofthe bit body is fixed in relation to

130 theshank,theotherstructure being movable re

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latively to thefixed structure in a direction having at least a rotational component, wherebythe torque load tends to move the movable structure relatively to the fixed structure.

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9. AdrillbitaccordingtoanyofClaims6to8, wherein means are provided to oppose relative movement between said relatively movable structures ofthe bit body from said one configuration to said other configuration, whereby said movement 10 takes place only when the load acting on the bit reaches a value sufficientto overcome the opposing force provided by said means.

10. Adrill bit according to Claim 9, wherein said meansforopposing relative movement between

15 said relatively movable structures applies a force tending to restore said structures to said one configuration upon reduction ofthe load applied to the bit.

11. Adrill bitaccording to Claim 10,wherein

20 hydraulicmeansareprovidedforopposingrelative movement between said relatively movable structures.

12. Adrill bitaccording to Claim 11, wherein said fixed structure isformed with a cylinder in which is

25 slideable a piston member movable with said other structure, said cylinder being in communication with a passage in the bit body for supplying drilling fluid to the surface ofthe bit, wherebythe hydraulic pressure ofthe drilling fluid urges the movable structure 30 towards one limit of its movement, and opposes movement thereof relatively to the fixed structure.

13. Adrill bit according to Claim 10, wherein spring means are provided for opposing movement between said relatively movable structures.

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14. Adrill bit according to Claim 13, whereinsaid spring means couple the two structures ofthe bit body together and are such that rotational deformation of the spring means, resulting from applied torque, is accompanied by axial deformation 40 thereof, whereby a change in the torque applied to said otherstructure causes relative rotational movement between the structures and rotational deformation ofthe spring means, and the accompanying axial deformation ofthe spring means effects relat-45 ive axial movement between the structures.

15. Adrill bit according to any of Claims7to 14, wherein said movable structure has an outerface on which are mounted a plurality of main cutting elements which, under normal drilling loads and in

50 said one configuration ofthe bit body, perform at least a major part ofthe cutting and abrading ofthe formation, and said fixed structure has an outerface on which are mounted a plurality of secondary elements which, under normal drilling loads and in 55 said one configuration ofthe bit body, are so located in relation to said main cutting elements that they do not actsignificantly on theformation, relative movement between said structures, under abnormal increased drilling loads, causing the parts to move to 60 said other configuration in which said secondary elements are so located in relation to said main cutting elements thatthey acton theformation to a significant extent.

16. Arotarydrill bit substantially as hereinbefore 65 described with reference to Figures 1 and 2, Figures 3

and 4, Figures 5 and 6, Figures 7 and 8, Figures 9 and 10, Figures11 and 12, Figures 13and 14orFigure15 ofthe accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 4/87, D8991685.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.