GB2620417A

GB2620417A - Drilling apparatus

Info

Publication number: GB2620417A
Application number: GB2209970.9A
Authority: GB
Inventors: Barnes Moyes Peter
Original assignee: Rotojar Innovations Ltd
Current assignee: Rotojar Innovations Ltd
Priority date: 2022-07-07
Filing date: 2022-07-07
Publication date: 2024-01-10
Also published as: GB202209970D0; WO2024008585A1

Abstract

A percussive drilling apparatus 14 comprising a support 74 and a transmission shaft 78 axially moveable relative to the support. The transmission shaft is configured to be connected to a drill bit and be engaged with a percussive drive 12 such that the percussive drive can repeatedly apply axial impulsive forces. The apparatus includes a depth of cut control mechanism 84 for providing a predetermined limit on axial movement permitted to the transmission shaft relative to the support in response to the applied impulse forces. The depth of cut mechanism may be variable to allow a user to set a predetermined limit using an adjustable interface and mechanism, such as a telescoping arrangement. The depth of cut control mechanism may comprise a plurality of limit elements. The percussive drive may comprise a hammer 134.

Description

DRILLING APPARATUS

FIELD

The present disclosure relates to a drilling apparatus and method for use in percussive drilling.

BACKGROUND

The drilling of bore holes is required in numerous industries and applications, such as in the oil and gas exploration and production industry, geothermal industry, in carbon capture and storage applications, in water production applications and the like.

Numerous drilling techniques exist for such purpose, such as rotary drilling and percussive (or percussion) drilling.

In rotary drilling a drill bit is rotated against the rock being drilled, for example via a connected drill string and/or in-hole motor, with weight on bit (WOB) applied. The physical rock failure mechanism during rotary drilling can include shearing action, such as the case when using PDC bits, with sheared rock fragments or chips normally flushed from the bit/rock interface via circulated drilling fluid, typically known as drilling mud. The rock failure mechanism in some types of rotary drilling, such as using roller cone bits can include compressive failure, again with rock fragments flushed using a drilling fluid.

Percussive (or percussion) drilling typically involves applying repeated impulsive forces on a drill bit, for example via a reciprocating hammer. The physical rock failure mechanism during percussive drilling is understood to be related to compressive failure (crushing) and crack propagation, again with drilling fluid used to flush removed rock fragments away from the bit/rock interface. Percussive drilling may also be performed with rotation of the bit. For example, incremental rotation may be applied to index the drill bit to align cutting features with fresh rock surfaces during application of the impulsive forces. However, in some applications continuous rotation may be applied to provide a hybrid rotary-percussive drilling which penetrates the rock via multiple rock failure mechanisms. Such rotary-percussive drilling in some cases may assist to improve bit stability and drilling dynamics, and improve the rate of penetration (ROP), which is a key consideration in the economics (from time and cost perspectives) of any drilling operation.

Irrespective of the type of drilling performed the reliability of the drilling tools and equipment is a major factor in design and operational considerations, particularly as drilling is associated with extremely high loads and harsh conditions. The requirement to maximise the life of drilling equipment may need to be carefully balanced against the desire to maximise ROP.

Various drill bit designs exist, such as rolling cone bits, fixed cutter bits, hybrid cutter bits, flat headed drill bits, concave headed drill bits, convex headed drill bits, and the like. Specific drill bit designs, or species of design, may be associated with a particular type of drilling (rotary, percussive or rotary-percussive). That is, a drill bit designed for rotary drilling may not necessarily be suitable or reliably used in percussive drilling, and vice versa.

SUMMARY

An aspect of the present disclosure relates to a wellbore percussive drilling apparatus, comprising: a support; a transmission shaft axially moveable relative to the support in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with a percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a depth of cut control mechanism for providing a predetermined limit of permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.

The depth of cut control mechanism thus functions to control or limit, to a predetermined extent, a permitted maximum axial movement of the transmission shaft, and thus a connected drill bit when in use, in the first direction during each applied impulsive force. As will be described in more detail below, the predetermined limit is something which in intentionally provided by the functionality of the depth of cut control mechanism, for the specific purpose of providing a desired or intended limit to axial movement of the transmission shaft in the first direction.

The depth of cut control mechanism may provide a degree of control of the maximum achievable depth of cut or axial advancement of a connected drill bit associated with each applied impulsive force. The ability to limit, to a predetermined extent, the maximum permitted axial movement of the transmission shaft, and thus corresponding maximum permitted depth of cut during each impulsive force may provide a number of significant benefits and advantages, some of which are exemplified below. The depth of cut control mechanism may alternatively be defined as a depth of cut limit mechanism, or simply a limit mechanism.

In some applications weight on bit (NOB) may be applied with the repeated impulsive forces superimposed on top of this by operation of the percussive drive. The total depth of cut may therefore be a function of both the WOB and the impulsive force. In such an application or use the depth of cut control mechanism may limit, to a predetermined extent, the depth of cut associated with each applied impulsive load.

Control over the total depth of cut may therefore be achieved by a combination of operator control of the WOB and the function of the depth of cut control mechanism. By virtue of the depth of cut control mechanism, at least in some examples, providing a form of passive control (albeit intended control by virtue of the predetermined limit) over the depth of cut associated with the percussive drive, overall operator control requirements may be minimised, particularly in that an operator (e.g., a driller) may not necessarily need to deviate from conventional drilling control procedures and protocols, such as controlling the tension in a topside hoisting system and/or the like. However, some features and aspects of the present disclosure may advantageously permit additional control capabilities to be provided.

The ability to limit the axial movement of the transmission shaft may provide a degree of protection to associated equipment, such as a connected drill bit, bottom hole equipment and/or the like. For example, the inventor realises that resistance to penetration increases with some degree of proportionality to the depth of penetration during the course of a single applied impulsive force. As such, a greater extent of penetration during an applied impulsive force will result in increased peak loading on the drill bit. Limiting the depth of cut to a predetermined extent as described herein may therefore assist in preventing peak loading on the bit reaching excessive levels during rock penetration.

Limiting the maximum permitted depth of cut to a predetermined extent may mitigate damage to infrastructure (e.g., drill bit) without necessarily requiring the magnitude of the applied impulsive forces to be reduced. That is, one solution to reduce peak loading on the drill bit (or other infrastructure) could involve reducing the magnitude of the impulsive forces applied. This, however, may not always be possible and/or desirable. For example, an associated percussive drive may not permit adjustment of its output in terms of the magnitude of the impulsive force applied on the transmission shaft. Further, reducing the magnitude of the impulsive forces may reduce the ability of the drill bit to actually penetrate the rock, for example as the relevant mechanical rock failure thresholds cannot be exceeded, which may mean little benefit is gained from the percussive drive. Seeking to protect the drill bit by limiting its maximum permitted depth of cut to a predetermined extent may thus allow the magnitude of the impulsive forces to be optimised (e.g., set according to the mechanical properties of the rock, maximising rate of penetration (ROP) etc.), rather than restricted in view of concerns over damaging the drill bit, maximising or optimising the benefit of the percussive drive. In some cases, larger impulsive loading may be readily applied over the restricted depth of cut without peak loading exceeding damage thresholds.

The ability to provide a degree of protection to a connected drill bit may permit a wider range of drill bit designs to be used, perhaps avoiding the requirement to employ highly specialised drill bits which might typically be more expensive, and perhaps with more limited availability. For example, drill bits designed for rotary drilling applications may have a poorer resistance to impact based operational loads generated during percussive drilling. As an example, some rotary drill bits incorporate extremely hard or hardened cutting inserts, such as polycrystalline diamond inserts, which might be mounted on the drill bit and/or have mechanical properties suited to shearing of rock, but perhaps have a higher propensity to failure (e.g., fracturing, dislocation etc.) when subjected to impulsive loading axially against the rock surface. Roller or similar drill bits typically incorporate bearing or journal features which may be subject to failure under extremely high peak loading.

The transmission shaft may be configured to be directly connected to a drill bit, for example via a suitable connection, such as a threaded connection. The transmission shaft may be configured to be indirectly connected to a drill bit, for example via one or a number of intervening features or components. In one example the transmission shaft may be configured to be connected to (e.g., threadedly connected to) a drilling Bottom Hole Assembly (BHA) which comprises a drill bit. In such an example the drilling BHA may be moved by the transmission shaft in the first direction in response to an applied impulsive load.

The drilling apparatus may be useable with different forms of drill bit or drilling BHA, with the various forms having different masses. In this respect, different drill bits or drilling BHAs may have different masses and thus may be subject to different dynamic responses (e.g., acceleration) during an applied impulsive force. However, the function of the depth of cut control mechanism may be such that inherent protection is provided to the drill bit or drilling BHA irrespective of the properties, such as mass, inertia etc. of the drill bit or drilling BHA. Furthermore, any variable nature of the depth of cut control mechanism, as described in more detail below, may advantageously allow a user to optimise the apparatus for the specifically connected drill bit or drilling BHA.

Further, during use it is important for the applied impulsive forces to be sufficiently transmitted to the drill bit/rock interface. With this in mind the mass or inertia of the drill bit and/or BHA will attenuate some of the energy from the impulsive forces, such that in some circumstances it would be desirable to ensure large impulsive force are applied.

However, while large impulsive forces may be preferred for heavy BHAs, this may generate some issues where lower mass BHAs are utilised. In this regard, the function of the depth of cut control mechanism may permit larger impulsive forces to be more uniformly applied irrespective of the form of BHA utilised, on the understanding that the desired protections afforded by the depth of cut control mechanism are in place, such as allowing higher impulsive forces to be transmitted while mitigating against excessive peak loading, as described above.

As will be described in more detail below, the drilling apparatus may be used in rotary-percussive drilling, with the depth of cut control mechanism providing advantages when such drilling is performed. For example, conventional rotary drill bits (e.g., PDC bits) may be utilised, optimising the efficiency of the rotary drilling component, while providing a degree of protection to such rotary drill bits against possible damage due to the percussive drilling component.

Further, should the drill bit penetrate too far into the rock surface in response to an applied axial impulsive force the ability for the drill bit to be rotated to cut or shear the rock during rotation may be compromised, in that too large of a "bite" is taken. This could even result in stalling of rotation of the drill bit in that the drilling torque applied may not be sufficient to overcome the resistance generated by the degree of bit penetration. This could also contribute to stick-slip drilling phenomena in which cyclical torsional oscillations are established along the drill string, which could cause issues such as backing-off threaded connections, poor rate of penetration and the like. Limiting the maximum permitted axial movement of the transmission shaft, and thus depth of cut of the drill bit, may mitigate against such issues.

Additional benefits may be achieved during rotary-percussive drilling. For example, applying a percussive component while rotary drilling may ensure the drill bit remains sufficiently engaged with the rock face to facilitate efficient rotary drilling, without the risk of the percussive element causing the bit to penetrate too far and thus avoiding or mitigating one or more of the various issues as set out above. In this example the sufficiency of engagement of the drill bit with the rock face may be related to providing a desired level of engagement force (i.e., sufficient bite) and/or related to providing more continuous engagement. This might have benefits where it is difficult or not practical to apply WOB, or consistent WOB, for example in highly deviated bores (e.g., horizontal bores), in very deep bores where drag friction between bore hole and drill string is significant, and/or the like. If the bit is not engaged with the rock surface with sufficient force the bit may only superficially scrape along the rock surface during rotation. Furthermore, if engagement is discontinuous adverse dynamic responses may be established.

As noted, the transmission shaft is moveable axially in reverse first and second directions, and as such is reciprocal relative to the support. The transmission shaft may be defined as a reciprocating transmission shaft. The transmission shaft is configured to be moved in the first direction (direction of drilling) in response to applied axial impulsive forces. In order to facilitate repeated (i.e., reciprocal) operation the transmission shaft is capable of being returned in the second direction following each applied impulsive force. Such return may be achieved by virtue of operational conditions, for example by virtue of WOB. Where WOB is applied the return of the transmission shaft in the second axial direction may be automatic following each applied impulsive force.

In some examples the apparatus may comprise a return mechanism, configured to return the transmission shaft in the second direction following each applied impulsive force. The return mechanism may comprise a biasing arrangement configured to apply a biasing force on the transmission shaft. The biasing arrangement may comprise a spring or similar mechanism. The return mechanism may be operated intermittently with each successively applied impulsive force. Alternatively, the return mechanism may be configured to apply a substantially continuous return force on the transmission shaft, such that each impulsive force must overcome the continuous return force to allow the transmission shaft to be moved in the first direction, and as each impulsive force diminishes the return force will become dominant to return the transmission shaft in the reverse second direction.

As noted above, the depth of cut control mechanism functions to limit the maximum permitted axial movement of the transmission shaft to a predetermined extent (i.e., predetermined limit). In this respect the predetermined extent or limit may be something which is user defined or set in advance of use. That is, the depth of cut control mechanism is intentionally configured, set, formed etc. prior to deployment or use of the wellbore percussive drilling apparatus to provide the predetermined limit. A user may define or set the predetermined limit in accordance with one or a number of parameters. For example, a user may define the predetermined limit in accordance with factors such as the geology being drilled, drill bit design, BHA design or make-up, the magnitude of the impulsive forces, to percussive drive design, a desired ROP and/or the like.

The depth of cut control mechanism may be configured to provide the desired predetermined limit of maximum permitted axial movement of the transmission shaft during a manufacturing and/or assembly stage of the apparatus. Such configuration of the depth of cut control mechanism may be a function of the specific design of the apparatus, with the design made or created in accordance with the requirement to provide the predetermined limit. Alternatively, or additionally, and as will be defined in more detail below, such configuration of the depth of cut control mechanism may be something which can be achievable by a form of adjustment prior to use.

The limit mechanism may be settable to define the predetermined limit of maximum permitted movement of the transmission shaft in the first direction. That is, a user may set the depth of cut control mechanism in accordance with a desired movement limit to be achieved (i.e., predetermined limit).

The depth of cut control mechanism may be variable (i.e., a variable depth of cut control mechanism) to allow the maximum permitted movement of the transmission shaft in the first direction to be varied (i.e., selectively made larger or smaller). The variable nature of the depth of cut control mechanism may permit a user to set the predetermined limit. The depth of cut control mechanism may be infinitely variable.

Alternatively, the depth of cut control mechanism may be incrementally variable.

The depth of cut control mechanism may comprise a variable or adjustable interface between the support and the transmission shaft, which includes between components connected to the support and/or transmission shaft.

The depth of cut control mechanism may comprise an adjustment mechanism, wherein adjustment of the adjustment mechanism provides for variability of the depth of cut control mechanism. The adjustment mechanism may be reconfigurable to change the effect or setting of the depth of cut control mechanism.

The adjustment mechanism may comprise a telescoping arrangement. One of extending and retracting the telescoping arrangement may increase the maximum permitted movement of the transmission shaft in the first direction, and the other of extending and retracting the telescoping arrangement may decrease the maximum permitted movement of the transmission shaft. The adjustment mechanism may comprise one or more adjustable sleeves, for example one or more threaded sleeves. In one example the adjustment mechanism may comprise a threaded region on one of the support and transmission shaft or one or more components connected thereto, and a threaded sleeve configured to be engaged with the threaded region to allow adjustment (i.e., axial adjustment) of the threaded sleeve relative to the threaded region.

The adjustment mechanism may be mounted on or relative to one or both of the support and transmission shaft. The adjustment mechanism may be provided directly on one or both of the support and transmission shaft. The adjustment mechanism may be provided indirectly on one or both of the support and transmission shaft, for example on a separate component which is connected (for example axially rigidly connected) to the support and/or transmission shaft. In one particular example the adjustment mechanism may be mounted on the support. In some examples the adjustment mechanism may be provided at least in part by an associated percussive drive. For example, the associated percussive drive may comprise a telescoping arrangement.

The depth of cut control mechanism may comprise a plurality of limit elements configured to be selectively mounted on the apparatus. A user may select one or more of the plurality of limit elements to be incorporated within the apparatus in accordance with a desired predetermined movement limit to be set. As such, different selections and/or combinations of limit elements may provide for variability of the depth of cut control mechanism. In one example at least two limit elements may be different (e.g., of a different length) and a user may select one of the at least two limit elements in accordance with a predetermined limit to be set. In this respect at least two of the limit elements may be interchangeable.

In an example a user may select multiple limit elements to be mounted together, for example axially stacked, within the apparatus in accordance with a desired predetermined limit to be set.

One or more of the plurality of limit elements may comprise a sleeve, shim, spacer, collar and/or the like. The plurality of limit elements may collectively define a kit of parts, for example a movement limit kit of parts.

The depth of cut control mechanism may limit a maximum permitted stroke length of the transmission shaft in the first direction. As such, the depth of cut control mechanism may define, in a predetermined manner, the maximum permitted stroke length. The depth of cut control mechanism may be variable (for example as described above) to allow variation of the maximum permitted stroke length of the transmission shaft in the first direction. In this example, when in use, the stroke length set may correspond to the maximum achievable depth of cut. In some examples the variable depth of cut control mechanism may permit adjustment of the maximum permitted movement of the transmission shaft in the range of, for example, 0.01mm to 10mm, 0.02mm to 5mm, 0.03mm to 1mm or 0.05mm to 0.1mm.

The apparatus may define a first interface between the support and the transmission shaft. The first interface may be provided directly between the support and the transmission shaft. However, in some examples the first interface may be provided indirectly between the support and the transmission shaft, for example via interconnected components or features. The first interface may comprise a first pair of cooperating features (e.g., surface). One of the first pair of cooperating features may be provided on or associated with the support and the other of the first pair of cooperating features may be provided on or associated with the transmission shaft. The first pair of cooperating features may be of any suitable form to facilitate axial engagement therebetween. For example, the first pair of cooperating features may comprise one or more of a load shoulder, annular lip, a no-go profile, and/or the like.

The first pair of cooperating features may be the same or different.

The first interface may be configured to be registered (i.e., the first pair of cooperating features brought together into engagement) when the transmission shaft is axially moved relative to the support in the first direction in response to an applied impulsive force. In this example the first interface may function to define an end limit of movement of the transmission shaft in the first direction, at the predetermined movement limit. That is, when the first interface is registered further movement of the transmission shaft in the first direction relative to the support is prevented.

The first interface may be configured to be deregistered (i.e., the first pair of cooperating features moved apart out of engagement) when the transmission shaft is axially moved relative to the support in the second direction.

The first interface, when registered, may function to divert an applied impulsive force from the transmission shaft (and connected drill bit) and into the support, and any connected equipment or infrastructure (e.g., a drill string). This may assist in providing benefits in terms of protecting the drill bit from excessive peak loading. In this example the first interface may alternatively be defined as a force diverter. As such, and in accordance with the benefits described above, the ability to divert an applied impulsive force at the predetermined limit may be such that larger impulsive forces may be uniformly applied.

In some examples, the depth of cut control mechanism may be variable by virtue of adjusting at least a portion of the first interface, such as adjusting the position of one or both of the first pair of cooperating features.

Movement of the transmission shaft in the first direction may result in the transmission shaft being extended relative to the support. Conversely, movement of the transmission shaft in the second direction may result in the transmission shaft being retracted relative to the support. The depth of cut control mechanism may be variable by adjusting the extent of permitted extension and/or retraction of the transmission shaft.

The transmission shaft may be axially moveable in the reverse first and second directions over a reciprocation length. The reciprocation length may be a maximum permitted length of relative axial movement between the transmission shaft and the support. The reciprocation length may be defined between fully retracted and fully extended positions of the transmission shaft relative to the support.

The depth of cut control mechanism may define (i.e., limit or delimit) the reciprocation length. The depth of cut control mechanism may be variable to permit variation of the reciprocation length, for example in accordance with one or more of the examples above.

The apparatus may define a second interface between the support and the transmission shaft. The reciprocation length may be defined between the first and second interfaces. The depth of cut control mechanism may be variable by virtue of adjusting one or both of the first and second interfaces.

The second interface may be provided directly between the support and the transmission shaft. However, in some examples the second interface may be provided indirectly between the support and the transmission shaft, for example via interconnected components or features. The second interface may comprise a second pair of cooperating features (e.g., surface). One of the second pair of cooperating features may be provided on or associated with the support and the other of the second pair of cooperating features may be provided on or associated with the transmission shaft. The second pair of cooperating features may be of any suitable form to facilitate axial engagement therebetween. For example, the second pair of cooperating features may comprise one or more of a load shoulder, annular lip, a no-go profile, and/or the like. The second pair of cooperating features may be the same or different.

The second interface may be configured to be registered (i.e., the second pair of cooperating features brought together into engagement) when the transmission shaft is axially moved relative to the support in the second direction. As such, when the second interface is registered further movement of the transmission shaft in the second direction relative to the support is prevented. The second interface may be configured to be deregistered (i.e., the second pair of cooperating features moved apart out of engagement) when the transmission shaft is axially moved relative to the support in the first direction.

The depth of cut control mechanism may be variable by virtue of adjusting one or both of the first and second interfaces. Such adjustment may effectively vary the reciprocation length.

In some examples, the depth of cut control mechanism may be variable by virtue of adjusting at least a portion of the second interface, such as adjusting the position of one or both of the second pair of cooperating features.

The support may define a housing, wherein the transmission shaft is at least partially located within the housing. Alternatively, the support may define a mandrel, wherein the transmission shaft is at least partially located externally of the mandrel.

In some examples the support may be axially longer or shorter than the transmission shaft. In one example the support may be shorter than the transmission shaft, wherein the transmission shaft protrudes from opposing axial ends of the support. In this example a first axial end of the support may define a portion of the first interface (e.g., one of the first pair of cooperating features). A second axial end of the support may define a portion of the second interface (e.g., one of the second pair of cooperating features).

The support may be configured to be secured to a drill string or assembly. In one example the support may be configured to be secured to an associated percussive drive. In such an example the percussive drive may form part of a drill string or assembly. The support may be configured to be secured to a housing of an associated percussive drive.

The support may be rotatably coupled to a drill string or assembly such that a drive torque from the drill string assembly may be transmitted to the support. In this example the drill string or assembly may define a rotary drive.

The support may comprise a connector to permit connection with a drill string or assembly. The connector may be configured to permit torque transmission between the drill string or assembly and the support. The connector may be of any suitable form. For example, the connector may comprise a threaded connector. The connector may facilitate a rigid axial connection between the support and drill string or assembly.

In some examples the support may form part of a drill string or assembly. For example, the support may form part of an associate percussive drive.

In one example a connected drill bit or drilling BHA may define a portion of the second interface (e.g., one of the second pair of cooperating features).

The apparatus may comprise a bit connector configured to facilitate connection with a drill bit or a drilling BHA. The bit connector may comprise a threaded connector. The bit connector may be adjustable to permit variability of the depth of cut control mechanism. Such adjustability may, for example, alter or modify the second interface.

In one example the bit connector may be telescopic to facilitate adjustment. The bit connector may be part of an assembly which incorporates one or a number of spacer components, such as shims, collars etc. to facilitate adjustment. The size and/or number of spacer components used or incorporated in to the bit connector will determine the extent by which the bit connector may be made up.

The transmission shaft may be rotatably coupled to a drill string or assembly such that a drive torque from the drill string or assembly may be transmitted to the transmission shaft, with torque thus applied to a connected drill bit. The drill string or assembly may thus define a rotary drive. In this example the transmission shaft may be configured to transmit both the applied axial impulsive forces and drive torque to a connected drill bit.

In some examples the transmission shaft may form part of a drill string or assembly.

In one example the transmission shaft may be rotatably coupled to a drill string or assembly via the support. That is, any drive torque delivered to the support may be transmitted to the transmission shaft.

A rotary connection may be provided between the transmission shaft and the drill string or assembly. In one example, a rotary connection may be provided between the transmission shaft and the support. The rotary connection may be axially compliant to accommodate relative axial movement between the support and transmission shaft. The rotary connection may comprise one or more of a splined connection, key and key way connection and/or the like.

The percussive drive may be provided separately from the drilling apparatus. That is, the drilling apparatus may be provided separately from the percussive drive and connected thereto. In this respect, the drilling apparatus may be capable of being used with various types and forms of percussive drive.

However, in some examples the percussive drive may form part of the drilling apparatus. Thus, as aspect of the present disclosure may relate to a wellbore percussive drilling apparatus, comprising: a percussive drive; a support; a transmission shaft axially moveable relative to the support in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with the percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a depth of cut control mechanism for providing a predetermined limit of permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.

The percussive drive, as noted above, is configured to repeatedly apply axial impulsive forces on the transmission shaft in the first direction. Each impulsive force may be considered to act momentarily, or non-continuous. It should be understood that the term "percussive" (which is often interchangeable with percussion) is not intended to limit the impulsive forces as being generated only by impact action between two structures. Instead, any means by which impulsive forces can be generated is intended, examples of which will be provided below.

In one example the percussive drive may be impact-based. For example, the percussive drive may comprise a hammer. The transmission shaft may be configured to be repeatedly struck by the hammer in the first direction. In this respect the transmission shaft may comprise an anvil surface to be struck by the hammer.

The hammer may be reciprocated within the percussive drive in the first and second directions. In this example the hammer may engage (i.e., strike) the transmission shaft when the hammer is moved in the first direction, and then disengaged (i.e., lifted) from the transmission shaft when the hammer is moved in the second direction. As such, in use, the hammer may accelerate towards the transmission shaft over a distance until striking the transmission shaft, at which point the kinetic energy of the hammer is transferred to the transmission shaft.

The percussive drive may comprise a fixed hammer system. For example, the hammer may be configured to travel over a set or fixed distance prior to striking the transmission shaft. Further, the hammer may be of a fixed mass. However, in other examples the percussive drive may comprise an adjustable hammer system. For example, adjustment in the permitted hammer travel distance may be made to provide for a variation in the kinetic energy which can be generated. That is, the distance over which the hammer may be subject to acceleration may be increased or decreased, thus allowing the velocity of the hammer at impact with transmission shaft to be varied. The hammer travel length may be adjustable by a telescoping hammer arrangement.

The mass of the hammer may be changed, for example by interchanging the hammer, by adding or removing hammer mass and/or the like.

The drilling apparatus may comprise an impact control mechanism. The impact control mechanism may function to providing a degree of control, for example damping, of the effect of a hammer impact on the transmission shaft. The impact control mechanism may be configured to alter the profile of energy transference from a hammer to the transmission shaft. For example, the impact control mechanism may function to extend the time period over which the energy from the hammer is applied, thus extending the period of the impulsive force. Extending the time period of energy application may result in the application of a lower peak impulsive force. Extending the time period of energy application may assist to maintain the drill bit engaged with the formation for a longer time period during application of each impulsive force. Damping the effect of the hammer may assist to provide additional protection to connected infrastructure, such as a drill bit. The impact control mechanism may comprise a damping structure, such as a resilient member, a spring, a fluid damper and/or the like. In one example the damping structure may be interposed between the hammer and the transmission shaft. For example, the damping structure may be mounted on one or both of a hammer face of the hammer and an anvil surface of the transmission shaft. In some examples the damping structure may be located at an interface (e.g the first interface) between the transmission shaft and the support.

The percussive drive may comprise a reciprocating drive shaft. Such a reciprocating drive shaft may define or support a hammer.

In one example the transmission shaft may be connected to the reciprocating drive shaft of the percussive drive, such that the transmission shaft is directly driven to reciprocate. The transmission shaft may be releasably connected to the reciprocating drive shaft. Alternatively, the transmission shaft may be integrally formed with the reciprocating drive shaft.

The percussive drive, for example the reciprocating drive shaft of the percussive drive, may function to limit the maximum permitted movement of the transmission shaft in the first direction. In one example the percussive drive, for example the reciprocating drive shaft, may be variable (e.g., telescopic) to provide variation of the depth of cut control mechanism.

The percussive drive may comprise any suitable drive arrangement. The drive arrangement may be configured to provide a reciprocal drive output. The percussive drive may be mechanically operated, for example via a motor. The percussive drive may be fluid operated, for example hydraulically operated. That is, the percussive drive may be operated by pressure and/or flow within a fluid system.

The percussive drive may comprise a housing and a mandrel, wherein the mandrel and the housing are configurable to be rotated relative to each other. In one example one or both of the support and the transmission shaft may be rotatably connected to one of the housing and the mandrel. It will be understood that while the terms "housing" and "mandrel" have been used herein for convenience, these components may alternatively be referred to as a first structure or first body portion and a second structure or second body portion of the percussive drive, the first structure or body portion and the second structure or body portion being rotatable relative to each other. In this respect the term "housing" is not intended to be limiting in terms of a component which houses another component (e.g., the second structure/body or mandrel), and similarly the term "mandrel" is not intended to be limiting in terms of a component about which another component (e.g., the first structure/body or housing) is mounted.

The percussive drive may comprise a reciprocating piston mounted within a piston housing to define a piston chamber, wherein the piston is moveable in the first and second axial directions. The percussive drive may comprise a rotary valve assembly for facilitating the reverse movement of the reciprocating piston. The rotary valve assembly may comprise a valve inlet for communicating with a pressure region and a valve exhaust for communicating with an exhaust region, the rotary valve assembly being operated by relative rotation between the mandrel and the housing to be cyclically reconfigured between: a pressure configuration in which the piston chamber is in pressure communication with the valve inlet and isolated from the valve exhaust to permit the piston to move in the first axial direction in accordance with the piston chamber being pressurised via the valve inlet; and an exhaust configuration in which the piston chamber is isolated from the valve inlet and in pressure communication with the valve exhaust to permit the piston chamber to be depressurised and the piston to move in the second axial direction.

In this example movement of the piston in the first axial direction generates or facilitates the generation of an impulsive force.

In use, the apparatus may be used in combination with the pressure and exhaust regions such that a pressure differential is applied across the rotary valve assembly.

Specifically, the pressure within the pressure region may be elevated above the pressure in the exhaust region. In particular, the pressure within the pressure region may be sufficient (for example sufficiently high) to pressurise the piston chamber to permit the piston to move in the first axial direction, and the pressure within the exhaust region may be sufficient (for example sufficiently low) to permit the piston chamber to be depressurised and the piston to move in the second axial direction.

The pressure region may include one of a region internally and externally of the apparatus. In one example the pressure region may include a region internally of the drilling apparatus (e.g., a flow path through the apparatus). In this example pressure within the pressure region may be controlled by modifying the pressure within an associated drill string. Such pressure modification may be achieved via fluid control equipment; such as pump equipment. Alternatively, or additionally, such pressure modification may be achieved via one or more flow restrictors (variable, fixed or otherwise) within the drill string (e.g., within the drilling apparatus). In some examples exit ports (e.g., nozzles) within the drill bit for permitting outflow of drilling fluid during drilling may generate a back pressure within the pressure region.

The exhaust region may include one of a region internally and externally of the drilling apparatus. In one example the exhaust region may include a region externally of the drilling apparatus, for example within an annulus region surrounding the drilling apparatus/drill string.

The percussive drive may comprise a biasing arrangement configured to bias the reciprocating piston in the second axial direction. In such an example, when the rotary valve assembly is in its pressure configuration pressure applied within the piston chamber will act to bias or move the reciprocating piston in the first axial direction against the bias of the biasing arrangement, and when the rotary valve assembly is in its exhaust configuration the biasing arrangement will act to bias or move the reciprocating piston in the second axial direction.

The biasing arrangement may provide a sufficient biasing force to exceed the pressure force applied by the exhaust region at the valve exhaust.

In one example the biasing arrangement may define or provide a threshold (non-zero threshold) biasing force acting in the second axial direction. When the rotary valve assembly is in its pressure configuration pressure within the piston chamber will need to exceed the threshold biasing force to enable movement of the reciprocating piston in the first axial direction. Accordingly, the biasing arrangement may define a pre-activation pressure range, in which pressure within this pre-activation pressure range may be applied within the piston chamber without causing operation of the reciprocating piston, and thus the transmission shaft. This may provide significant benefits in that pressure operations, within the pre-activation pressure range, may be performed within the percussion drilling apparatus and/or any infrastructure associated therewith, without causing operation of the drilling apparatus (i.e., without causing percussive drilling). Such pressure operations may be associated with tool manipulation operations, such as tool operation, activation and/or the like. Such pressure operations may be associated with telemetry operations, such as transmitting signals (e.g., data signals, communication signals etc.) using pressure based telemetry. Such telemetry may be improved in that interference or noise associated with operation of the percussive drilling apparatus is not present or is at least minimised.

The biasing arrangement may comprise a spring, or similar structure.

The biasing arrangement may be interchangeable and/or adjustable to permit a used to set a predefined threshold biasing force, and thus a predefined pre-activation pressure range.

The drilling apparatus may define a flow path therethrough, for example an axial flow path. The flow path may permit a fluid, such as a drilling fluid to be communicated through the drilling apparatus to the drill bit. The transmission shaft may comprise or form part of the flow path. The transmission shaft may comprise an axial throughbore.

An aspect of the present disclosure relates to a wellbore percussive drilling apparatus, comprising: a transmission shaft axially moveable in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with a percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a depth of cut control mechanism (or a limit mechanism) for limiting the maximum permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.

An aspect of the present disclosure relates to a wellbore percussive drilling apparatus, comprising: a support; a transmission shaft axially moveable relative to the support in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with a percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a settable depth of cut control mechanism for setting a predetermined limit to the maximum permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.

The user settable depth of cut control mechanism may be variable, in that a user can set different predetermined limits within the same drilling apparatus.

An aspect of the present disclosure relates to a method for performing percussive drilling of a bore hole, comprising: mounting a drill bit to a transmission shaft of a drilling apparatus, the transmission shaft being axially moveable in reverse first and second directions, the first direction corresponding to a direction of drilling; setting a depth of cut control mechanism to define a predetermined limit to the permitted axial movement of the transmission shaft in the first direction; engaging the drill bit with a rock to be drilled; and operating a percussive drive to repeatedly apply axial impulsive forces on the transmission shaft in the first direction.

The drilling apparatus may be provided in accordance with any other aspect. Thus, features defined in relation to any other aspect, and their described function and modes of operation apply here.

The percussive drive may be provided in accordance with any other example disclosed herein.

The method may comprise rotating the drill bit while the percussive drive is operating to provide rotary-percussive drilling. Rotating the drill bit may be achieved by applying a rotary drive to the transmission shaft.

As noted, the method involves setting a depth of cut control mechanism to define a predetermined limit to the permitted axial movement of the transmission shaft in the first direction. This arrangement may effectively limit the maximum depth of cut achievable for each applied impulsive force. Thus, the method may alternatively be defined as a method for controlling the depth of cut during percussive drilling.

An aspect of the present disclosure relates to a percussive drilling assembly, comprising: a percussive drilling assembly according to any other aspect; and a percussive drive engaged with the percussive drilling assembly.

An aspect of the present disclosure relates to a reciprocating drive apparatus, comprising: a housing and a mandrel configurable to be rotated relative to each other; a reciprocating piston mounted within a piston housing to define a piston chamber, wherein the piston is moveable in reverse first and second axial directions; a rotary valve assembly comprising a valve inlet for communicating with a pressure region and a valve exhaust for communicating with an exhaust region, the rotary valve assembly being operated by relative rotation between the mandrel and the housing to be cyclically reconfigured between: a pressure configuration in which the piston chamber is in pressure communication with the valve inlet and isolated from the valve exhaust to permit the piston to move in the first axial direction in accordance with the piston chamber being pressurised via the valve inlet; and an exhaust configuration in which the piston chamber is isolated from the valve inlet and in pressure communication with the valve exhaust to permit the piston chamber to be depressurised and the piston to move in the second axial direction; and a biasing arrangement configured to bias the reciprocating piston in the second axial direction, the biasing arrangement defining a pre-activation pressure range, in which pressure within this pre-activation pressure range may be applied within the piston chamber without causing movement of the reciprocating piston in the first direction, wherein movement of the piston in at least one of the first and second axial directions generates an applied force within the apparatus and/or an applied force output from the apparatus.

It should be understood that the features defined in relation to one aspect may be applied in combination with any other aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a percussive drilling assembly which includes a percussive drilling apparatus, with the assembly illustrated in a first phase of operation: Figure 2 is a cross-sectional view of the percussive drilling assembly of Figure 1, illustrated in a subsequent, second phase of operation; Figures 3 and 4 provide an enlarged view of the drilling apparatus of the drilling assembly of Figure 1, with a variable depth of cut control mechanism set in different configurations; Figures 5 and 6 illustrate the drilling apparatus of the drilling assembly of Figure 1, with alternative forms of depth of cut control mechanism; Figure 7 is a cross sectional view of the drilling apparatus of Figure 1, incorporating an optional impact damping arrangement; Figure 8 illustrates the drilling apparatus of Figure 7 in a hammer strike configuration; Figure 9 is a cross sectional view of the drilling apparatus of Figure 1 with an alternative form of impact damping arrangement; Figure 10 is a cross sectional view of an alternative percussive drilling apparatus with an adjustable hammer, wherein the hammer is illustrated in a first configuration; Figure 11 illustrates the percussive drilling apparatus of Figure 10 with the adjustable hammer illustrated in a second configuration; Figure 12 is a cross sectional view of a further alternative percussive drilling apparatus illustrated in a first configuration; Figure 13 illustrates the percussive drilling apparatus of Figure 10 in a second configuration; Figure 14 is a cross sectional view of a still further alternative percussive drilling apparatus with an adjustable percussive drive, wherein the adjustable percussive drive is set in a first configuration; and Figure 15 illustrates the percussive drilling apparatus of Figure 14 with the adjustable percussive drive in a second configuration.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of a percussive drilling assembly 10 for use in drilling a bore hole, for example into and through geological formations. For the purposes of the present disclosure reference will be made to drilling through rock, although it should be recognised that the assembly 10 may be used to drill through any formation or material.

The assembly 10 includes a percussive drive 12, a percussive drilling apparatus 14 and a drill bit 16 connected end-to-end in series, with the drill bit 16 defining the distal end 18 of the assembly 10. The drill bit 16 in the present example forms part of a drilling BHA 17, although in other examples the drill bit 16 may be directly connected to the drilling apparatus 14. A flow path 19 extends axially through the drilling apparatus 10 to facilitate the flow of a drilling fluid towards the drill bit 16 (or alternatively from the drill bit 16 during reverse circulation). It should be understood that although the present description identifies the drilling apparatus 14 as a separate integer which is connected to the percussive drive 12, the percussive drive 12 and drilling apparatus 14 may, in some examples, be provided as a single integrated unit. Further, a generic drill bit 16 has been illustrated. However, any drill bit design may be used.

As will be described in detail below, the percussive drive 12 functions to apply repeated impulsive forces on the percussive drilling apparatus 14, which are then transmitted to the drill bit 16 to cause the drill bit 16 to be axially reciprocated relative to the percussive drive 12 in reverse first and second directions, identified by arrows 20, 22, with the first direction 20 corresponding to the direction of drilling. The drilling apparatus 14 provides a function of limiting, to a predetermined extent, the maximum permitted axial movement of the drill bit 16 in the first direction, which can provide a number of advantages as described herein.

The percussive drive 12 includes an upper mandrel 24 which includes a connector 26, in this case a female box connector, which facilitates connection to a rotary drill string (not shown), such that the mandrel 24 may be rotated by the drill string. The drill string may be rotated at surface, for example via rotary table, top drive etc., and/or may be rotated by a down hole motor, such as a mud motor.

The percussive drive 12 further includes a housing 28 which is rotatably connected to the mandrel 24 via a connector 30 (e.g., a threaded connector). An impulsive force assembly 32 is mounted within the housing 28 and is arranged to reciprocate in the first and second directions 20, 22. In the present example the percussive drive 12 is configured to apply impulsive forces to the percussive drilling apparatus 14 via repeated hammer impacts, and as such the impulsive force assembly 32 may be referred to as a hammer assembly. The hammer assembly 32 includes a hammer 34 having a striking face 36 at one end, and a piston 38 at an opposite end, wherein the piston 38 is dynamically sealed within an annular space 40 defined between the housing 28 and a cylindrical extension 42 of the mandrel 24. The hammer assembly 32 further comprises a spring 44 which acts axially between the housing 28 and the piston 38 to bias the hammer 34 in the second direction 22.

The piston 38 divides the annular space 40 into first and second piston chambers 46, 48 (the first piston chamber 46 is more clearly shown in Figure 2), wherein the second piston chamber 48 is also sealed by a sealing arrangement 50 defined between the housing 28 and hammer 34. The first piston chamber 46 is in communication with a control bore 52 formed within the mandrel 24, wherein the control bore 52 extends to a valve assembly 54. As will be described in more detail below the valve assembly 54 functions to alternately present the control bore 52 in communication with the internal flow path 19 or an external region 56, which in use may be an annulus between the assembly 10 and the bore being drilled. The second piston chamber 48 is in communication with the external region 56 via a port 58.

The percussive drive 12 further comprises a reaction sleeve 60 rotatably mounted about the mandrel 24. Although not specifically illustrated, a bearing assembly may be present between the mandrel 24 and reaction sleeve 60. In the present example the reaction sleeve 60 includes a series of circumferentially distributed and axially extending blades 62 which function to engage the wall of a bore being drilled to generate rotational drag between the reaction sleeve 60 and bore wall. Any alternative structure which provides this drag function may be used. Thus, during operation relative rotation between the mandrel 24 and the reaction sleeve 60 may be established, which as noted below in more detail functions to operate the valve assembly 54. The relative rotation established may include the mandrel 24 rotating while the reaction sleeve 60 is rotatably fixed (by engagement with the bore wall) or the mandrel 24 rotating faster than the reaction sleeve 60 (by virtue of some rotation of the reaction sleeve 60), or successive periods of each.

The valve assembly 54 includes a valve inlet 64 formed in the mandrel 24 and which communicates with the internal flow path 19, and a valve outlet 66 formed in the reaction sleeve 60 and which communicates with the external region 56. The valve assembly 54 further includes a valve selector (or valve body) 68 which is in the form of a sleeve and is rotatably fixed to the reaction sleeve 60, thus permitting relative rotation between the mandrel 24 and the valve selector 68 to be established during use. The valve selector 68 includes a pressure port 70 and an exhaust port 72 which are circumferentially separated by 180 degrees in the present example.

When in the configuration illustrated in Figure 1 the mandrel 24 and valve selector 68 are positioned relative to each other such that the valve inlet 64 is closed and the exhaust port 72 of the valve selector 68 is aligned or registered with the control port 52, thus presenting the first piston chamber 46 in pressure communication with the external region 56 via the valve outlet 66. When in this configuration both the first and second piston chambers 46, 48 are in pressure communication with the external region 56 and are thus pressure balanced, allowing the spring 44 to move the hammer 34 in the second direction 22.

The percussive drilling apparatus 14 comprises a support housing 74 which is rotatably connected to the housing 28 of the percussive drive 12 via a connector 76 (e.g., a threaded connector). In alternative examples the support housing may be provided as part of the housing 28, such as a component of the housing 28 or an integral part of the housing 28. A transmission shaft 78 is mounted within the support housing 74 and rotatably connected thereto via a connection arrangement 80, wherein the connection arrangement 80 facilitates the rotary connection while permitting the transmission shaft 78 to reciprocate relative to the support housing 74 in the first and second directions 20, 22. In an alternative example, the transmission shaft 78 may be directly rotatably secured to the housing 28 of the percussive drive 12.

One end of the transmission shaft 78 includes a head 81 defining an anvil surface 82 which is arranged to be repeatedly engaged or struck by the striking surface 36 of the hammer 34. The transmission shaft 78 is connected at an opposite end to the drilling BHA 17 via a connector sub 85. While a connector sub 85 is illustrated, any means of connecting the transmission shaft to the drilling BHA 17 (or directly to the drill bit 16) may be utilised. Further, the connector sub 85 may be integrally formed with the transmission shaft 78.

The drilling apparatus 14 provides a function of limiting the maximum permitted axial movement of the transmission shaft 78, and thus connected drill bit 16, in the first direction in response to each impulsive force applied by the hammer 34. For this purpose, the drilling apparatus 14 includes a limit mechanism, specifically a variable limit mechanism in the present example, which allows a user to set a predetermined movement limit in advance. The limit mechanism may be provided by multiple elements of the drilling apparatus 14, as will become apparent from the following description.

The drilling apparatus 14 defines a first interface 84 which includes a first pair of cooperating features in the form of load shoulders 86, 88 provided, respectively, on the transmission shaft 78 (specifically on the head 81) and the support housing 74. In the configuration illustrated in Figure 1 the first interface 84 is deregistered in that the first pair of load shoulders 86, 88 are axially separated and disengaged. However, as will be described in more detail below, the first interface 84 is configured to be registered by the first pair of load shoulders 86, 88 being brought together into engagement when the transmission shaft 78 is moved in the first direction in response to an applied impulsive force. As such, the first interface 84 defines an end limit of movement of the transmission shaft 78 in the first direction.

The first interface 84, when registered, may function to divert an applied impulsive force from the transmission shaft 78 (and connected drill bit 16) and into the support housing 74, and thus percussive drive 12 and connected drill string. This may assist in providing benefits in terms of protecting the drill bit from excessive peak loading.

The apparatus 14 defines a second interface 90 between the support housing 74 and the transmission shaft 78, and more specifically the connector sub 85 connected to the transmission shaft 78. The second interface includes a second pair of cooperating features in the form of load shoulders 92, 94 provided, respectively, on the support housing 74 and the connector sub 85. In the configuration illustrated in Figure 1 the second interface 90 is registered in that the second pair of load shoulders 92, 94 are engaged. Such engagement occurs when the transmission shaft 78 is axially moved in the second direction. As such, the second interface 90 defines an end limit of movement of the transmission shaft 78 in the second direction 22.

The transmission shaft 78 may therefore be axially moveable in the reverse first and second directions 20, 22 over a reciprocation length defined between the limits of the first and second interfaces 84, 90. As will be described in more detail below, in the present example the reciprocation length may be varied by adjusting the second interface 90, thus effectively providing a means to adjust the maximum permitted movement of the transmission shaft 78 (and thus connected drill bit 16) in the first direction. In this example the drilling apparatus 14 includes an adjustment sleeve 95 mounted on the support housing 74 for use in adjusting the second interface 90. In the example illustrated in Figure 1 the adjustment sleeve 95 is positioned such that the reciprocation length is at a maximum.

The drilling assembly 10 is illustrated in Figure 1 with the hammer 34 retracted by virtue of the valve assembly 54 presenting the first piston chamber 46 in communication with the external region 56 and the action of the spring 44, as described above. As such, the hammer 34 is disengaged from the transmission shaft 78, allowing the transmission shaft 78 to be fully stroked in the second direction (limited by the second interface 90 as described above) by virtue of weight set down on the drill bit 16 (WOB) when engaged with the rock being drilled.

Rotation of a connected drill string will cause corresponding rotation of the drill bit 16, by virtue of the various rotary connections 30, 76, 80, 84 within the assembly 12, thus providing rotary drilling with cutting elements of the drill bit 16 shearing the rock being drilled. Such rotation will also cause the mandrel 24 to rotate relative to the reaction sleeve 60 (which is engaged with the drilled bore wall), allowing the valve assembly 54 to be reconfigured until the mandrel 24 and valve selector 68 are positioned relative to each other as illustrated in Figure 2. When in this configuration the exhaust port 72 of the valve selector 68 becomes misaligned from the control port 52 and the pressure port 70 becomes aligned with both the control port 52 and the valve inlet 64. As such, the first piston chamber 46 is presented in communication with the internal flow path 19 and isolated from the external region 56.

During operation a pressure differential is applied between the internal flow path 19 and the external region 56, with the pressure of the internal flow path 19 higher than that in the external region 56. In this respect pressure may be applied within the flow path 19 by control of fluid flow therethrough. In one example the drill bit 16 comprises outflow ports 96 which function as flow nozzles or restrictors, thus generating an elevated back pressure within the flow path 19 and establishing the desired pressure differential. In an alternative example a flow choke, such as a variable valve, may be provided within the flow path, downstream of the valve assembly 54.

When the appropriate pressure differential is applied, and the valve assembly 54 reaches the configuration shown in Figure 2 the first piston chamber 46 will be rapidly pressurised, such that the pressure differential is applied across the piston 38, resulting in the hammer assembly 34 being rapidly accelerated and driven in the first direction 20, with the hammer 34 striking the head 81 of the transmission shaft 78 and thus applying an impulsive force thereto. This impulsive force rapidly drives the transmission shaft in the first direction 20, effectively causing the drill bit 16 to apply an impulsive compressive force against the rock being drilled. This compressive force against the rock is intended to induce compressive failure and crack propagation allowing the bit to penetrate into the rock. Thus, the bore is advanced by a combination of rotary and percussive drilling. However, in other examples percussive drilling may be used alone, with the drill bit 16 isolated from the rotation required to operate the valve assembly, such as by removing the rotary connection 80 between the transmission shaft 78 and the support housing 74 (optionally utilising a bearing instead), and perhaps incorporating a rotary drag device within the drilling BHA 17, similar to the reaction sleeve 60.

It should be noted that in order for the piston 38 to be moved in the first direction the pressure applied within the first piston chamber 46 must apply a force on the piston which can overcome the bias of the spring 44. As such, the spring 44 applies a threshold operational force, in that operation will only be initiated when the force threshold is exceeded. This threshold operational force may translate to an operational pressure threshold, such that the spring 44 may define a pre-activation pressure range, in which pressure within this pre-activation pressure range may be applied within the first piston chamber 46 without causing percussive drilling. This may provide significant benefits in that pressure operations, within the pre-activation pressure range, may be performed within the percussion drilling assembly 10 and/or any infrastructure associated therewith, without causing operation of the drilling apparatus 14. Such pressure operations may be associated with tool manipulation operations, such as tool operation, activation and/or the like. Such pressure operations may be associated with telemetry operations, such as transmitting signals (e.g., data signals, communication signals etc.) using pressure based telemetry. Such telemetry may be improved in that interference or noise associated with operation of the percussive drilling apparatus 14 is not present or is at least minimised.

As illustrated in Figure 2, the transmission shaft 78 is capable of moving in the first direction 20 until the first interface 84 registers or engages, thus limiting the maximum permitted movement of the transmission shaft 78 and connected drill bit 16 in the first direction. As such, the maximum depth of bit penetration into the rock is limited.

Furthermore, any residual impulsive force will be diverted into the support housing 74, and thus connected percussive drive 12 and drill string. This may provide a degree of protection to the drilling BHA 17, particularly to the drill bit 16.

For example, resistance to penetration will typically increase with some degree of proportionality to the depth of penetration during the course of a single applied impulsive force. As such, a greater extent of penetration during an applied impulsive force will result in increased peak loading on the drill bit 16. Limiting the depth of cut as described herein may therefore assist in preventing peak loading on the bit 16 reaching excessive levels during rock penetration.

Further, providing a degree of protection to the drill bit 16 in the manner described may avoid any requirement to limit the magnitude of the impulsive forces in view of concerns over damaging the drill bit 16, maximising or optimising the benefit of the percussive drive. In some cases, larger impulsive loading may be readily applied over the restricted depth of cut without peak loading exceeding damage thresholds.

The ability to provide a degree of protection in the manner described may also permit a wider range of drill bit designs to be used, perhaps avoiding the requirement to employ highly specialised drill bits which might typically be more expensive, and perhaps with more limited availability.

Further, during rotary-percussive drilling should the drill bit 16 penetrate too far into the rock surface in response to an applied axial impulsive force the ability for the drill bit 16 to be rotated to cut or shear the rock during rotation may be compromised, in that too large of a "bite" is taken. Limiting the maximum permitted axial movement of the transmission shaft 78, and thus depth of cut of the drill bit 16, may mitigate against this issue.

Additional benefits may be achieved during rotary-percussive drilling. For example, applying a percussive component while rotary drilling may ensure the drill bit 16 remains sufficiently engaged with the rock face to facilitate efficient rotary drilling, without the risk of the percussive element causing the bit 16 to penetrate too far and thus avoiding or mitigating one or more of the various issues as set out above.

Further rotation of the drill string causes the assembly 10 to again return to the configuration of Figure 1, in which the first piston chamber 46 is presented in communication with the external region 56, allowing the fluid in the first piston chamber 38 to be exhausted and the hammer assembly 32 to return to a retracted position. WOB then causes the transmission shaft 78 to move in the second direction 22 until the second interface 90 registers or engages. As such, continuous rotation of the drill string will cause repeated percussive operation of the assembly 10.

The form of the exemplary valve assembly 54 is such that an impulsive force (hammer strike) is applied once for each rotation. In this respect, an operator may readily vary the percussive frequency by simply modifying the speed of rotation. This degree of control, in combination with the ability of the operator to set a predetermined maximum bit penetration may advantageously allow the operator to more readily control the rate of penetration (ROP).

In alternative examples the valve assembly 54 may include multiple pressure and exhaust ports 70, 72 to allow multiple hammer strikes per revolution. For example, four pressure ports 70 may be provided with four exhaust ports 72, with the ports intermittently provided at a 45-degree spacing, thus providing for 4 hammer strikes per revolution.

With the degree of control provided in terms of the frequency of percussive drilling and setting the maximum depth of cut an operator may have the capacity to more accurately achieve a desired ROP. For example, should an operator require a ROP of 12 m/h using a percussive drilling assembly which generates four impulsive forces per revolution, and requires to limit the bit penetration to 0.5mm (for example accounting for the drill bit type and protection required, BHA design, the geology etc.) a nominal drill string rotation of 100RPM may be set. Some account may need to be made of any rotary drilling component, should that be utilised, in the usual manner.

It should be recognised that any number of pressure and exhaust ports 70, 72 may be provided. As another example, a 16 valve array may be provided, which includes 16 pressure ports 70 and 16 exhaust ports 72 which are evenly circumferentially distributed. This arrangement will provide 16 cycles per revolution. If a rotational speed of 90 rpm is provided, then a frequency of 24 Hz will be generated.

In some circumstances while an operator may be interested in achieving a particular ROP, and thus control drilling RPM accordingly, other operational factors may be accounted for. For example, in some cases a particular frequency or frequency range of percussive drilling may be understood to provide particular benefits. Such a desired or target percussive frequency may be readily achievable by controlling the drilling RPM. For example, in some cases an operator may opt to provide a percussive frequency in the range of 1 to 100 Hz, for example 5 to 80 HZ, 10 to 70 Hz, 20 to 50Hz, 20 to 40 Hz. In this respect multiple factors may be considered to achieve this, including the number of pressure and exhaust ports 70, 72 and the drilling RPM.

Of course, in the examples provided above considering a target ROP, an operator will be desirable to take into account the component of ROP which may be achieved via other concurrent drilling mechanisms, such as rotary drilling.

As noted above, the maximum permitted movement of the transmission shaft 78 in the first direction 20 is set in advance to provide a predetermined movement limit. This prior setting of a predetermined limit may be achieved during manufacture or assembly. However, in the present example setting is achieved via the adjustment sleeve 95, as will now be described in more detail with reference to Figures 3 and 4, wherein Figure 3 is an enlarged view in the region of the drilling apparatus 14 with the predetermined movement limit set at its maximum value, and Figure 4 is a corresponding view with the predetermined movement limit set at its minimum valve.

Referring first to Figure 3 the adjustment sleeve 95 is threadedly mounted on the support housing 74 and forms part of the second interface 90. Specifically, an end face of the adjustment sleeve 95 defines load shoulder 92, which is illustrated in engagement with cooperating load shoulder 94 of the connector sub 85. In this example the adjustable sleeve is fully threaded onto the support housing 74, against a stop 96, with screws 97 used to lock the adjustable sleeve 95 in this position. With this configuration or setting a maximum separation gap al is provided between the first pair of load shoulders 86, 88 of the first interface 84. As such, the transmission shaft 78 may only be permitted to move in the first direction 20 a maximum distance of al upon application of an impulsive force by the hammer 34.

If the predetermined movement limit is to be reduced the adjustment sleeve 95 may be manipulated to extend from the support housing 74, as illustrated in Figure 4, effectively moving the position of the load shoulder 94 of the second interface 92, and thus providing a smaller separation gap 02 between the first pair of load shoulders 86, 88 of the first interface 84. As such, the transmission shaft 78 may only be permitted to move in the first direction 20 a maximum distance of az upon application of an impulsive force by the hammer 34.

While the adjustment sleeve 95 is illustrated in Figures 3 and 4 at the extremes of its adjustment capabilities, it will be recognised that any intermediate setting may be achieved. In some examples the variable limit mechanism may permit adjustment in the maximum permitted movement of the transmission shaft in the range of, for example, 0.01mm to 10mm, 0.02mm to 5mm, 0.03mm to 1mm or 0.05mm to 0.1mm.

Alternative forms of adjustable limit mechanism (or depth of cut control mechanism) may be provided. For example, as illustrated in Figure 5, the adjustable sleeve 95 may be substituted by a spacer sleeve 98 of a longer length, to thus provide a desired maximum separation gap of a3 within the first interface 84. A range of spacer sleeves 98 with differing lengths may be available for selection by an operator, to allow the required predetermined maximum movement limit of the transmission shaft 78 to be set.

As a further example, as illustrated in Figure 6, a spacer sleeve 99 of a desired length may be used in combination with one or more shims 100 to provide a desired maximum separation gap of cia within the first interface 84. The number and/or thickness of the shims may be selected accordingly.

In the examples provided above the adjustment of the predetermined movement limit is achieved within the second interface 90. However, other options exist. For example, similar adjustment capabilities may additionally or alternatively be provided within the first interface 84. Further, a limit or depth of cut control mechanism may be provided remotely from the first and/or second interfaces 84, 90, such as associated with the percussive drive, and example of which will be presented later below.

In the example presented above, the hammer 34 strikes the transmission shaft 78 with an impulsive blow. In some examples an impact control mechanism may be provided to permit a degree of control, for example damping, of the effect of the hammer impact on the transmission shaft 78. An example is provided in Figure 7, which is an enlarged view in the region of the percussive drilling apparatus 14 incorporating an example of an impact control mechanism. The impact control mechanism includes a spring arrangement 101 (Belleville spring stack in this example) within the first interface 84, specifically between the first pair of load shoulders 86, 88. The drilling apparatus 14 is shown in Figure 7 prior to the transmission shaft 78 being struck by the hammer 34, with the spring 101 fully extended, and in Figure 8 during hammer impact and the spring 101 fully compressed. The spring 101 may therefore function to dampen some of the energy of the hammer impact, to limit the severity of the impulsive force being transmitted to the connected drill bit 16 (not shown in Figures 7 and 8). The impact control mechanism may function to extend the time period over which the energy from the hammer 34 is applied, thus extending the period of the impulsive force. Extending the time period of energy application may result in the application of a lower peak impulsive force. Extending the time period of energy application may assist to maintain the drill bit 16 engaged with the formation for a longer time period during application of each impulsive force As an alternative example, as illustrated in Figure 9, a spring arrangement 102 may be provided between the hammer 34 and the anvil surface 82 of the transmission shaft. In the specific example illustrated the spring arrangement is secured to the transmission shaft 78, but this could alternatively be secured to the striking face 36 of the hammer. In a further example spring arrangements may be secured to both the hammer 34 and the transmission shaft 78. A combination of the examples of Figures 7 and 9 may also be possible, for example to include a spring arrangement 101 within the first interface 84 and a separate spring arrangement 102 between the hammer 34 and transmission shaft 78.

Furthermore, although a spring is presented in the examples above, any structure which provides a similar function may be used, such as a fluid damper, compressible material (e.g., elastomer) and/or the like.

In the example percussive drilling assembly 10 described above the hammer 34 is fixed within the hammer assembly 32. However, in alternative examples the hammer may be adjustable to vary the permitted stroke length of the hammer and achievable impulsive force, as will be described with reference to Figures 10 and 11, which illustrate, in cross-section, an alternative percussive drilling assembly 110 in different hammer configurations. The percussive drilling assembly 110 is largely similar to assembly 10 first shown in Figure 1 and as such like features share like reference numerals, incremented by 100. In view of the similarities between assemblies 10, 110 only those features necessary to identify the differences will be described.

Referring initially to Figure 10 the assembly 110 includes a percussive drive 112, percussive drilling apparatus 114 and drill bit 116 provided as part of a drilling BHA 117. As before, the percussive drive 112 includes a hammer assembly 132 which is caused to reciprocate by operation of a valve assembly 154, and the drilling apparatus 114 includes a transmission shaft 178 to be struck by the hammer assembly 132. The hammer assembly 132 in this example includes an adjustable hammer 134 which is mounted via an adjustable connector 104. The adjustable connector 104 is telescopic in order to provide adjustment of the effective length of the hammer assembly 132.

The telescoping adjustable connector 104 may be provided in any suitable form, for example a threaded connector. The hammer assembly 132 is shown in Figure 10 with the hammer 134 fully retracted such that the hammer assembly 132 defines a minimum length. When in this configuration a separation gap 105 between the hammer 134 and transmission shaft 178 is maximised, such that the hammer 134 can be accelerated to achieve maximum kinetic energy prior to impact. This arrangement may thus maximise the magnitude of the associated impulsive force applied on the transmission shaft 178.

However, and as illustrated in Figure 11, the hammer 134 can be extended by adjustment of the connector to increase the effective length of the hammer assembly 132 and thus reduce the separation gap 105 between the hammer 134 and transmission shaft 178. As such, the hammer 134 will be accelerated over a shorter distance and will thus accumulate less kinetic energy (with respect to the configuration of Figure 10), resulting in impulsive forces of a lower magnitude.

In a further variation, the hammer 134 could be interchanged for a hammer of a different mass and/or modified to add or remove mass to provide a further method by which the maximum achievable kinetic energy of the hammer 134 can be modified (thus modifying the magnitude of the impulsive forces applied).

It should be recognised that the percussive drive 112 may be provided separately of the percussive drilling apparatus 114. For example, the percussive drive 112 may be used in combination with any other apparatus or system which requires impact based impulsive forces to be applied.

In the examples provided above the impulsive forces are applied by impact of a hammer on a transmission shaft, with the example percussive drives (10, 110) including an impulsive force assembly in the form of a hammer assembly. However, non-impact based percussive drives may alternatively be used, an example of which will be described with reference to Figures 12 and 13 which illustrate, in cross-section, an alternative percussive drilling assembly 210 in different configurations. The percussive drilling assembly 210 is largely similar to assembly 10 first shown in Figure 1 and as such like features share like reference numerals, incremented by 200. In view of the similarities between assemblies 10, 210 only those features necessary to sufficiently identify the differences will be described.

Referring initially to Figure 12, the assembly 210 includes a percussive drive 212, percussive drilling apparatus 214 and drill bit 216 provided as part of a drilling BHA 217. The percussive drive 212 includes a percussive force assembly 232 in the form of a reciprocating shaft 234 which includes a piston 238 at one end and is integrally formed with a transmission shaft 278 of the drilling apparatus 214 at an opposite end. Although the reciprocation and transmission shafts 2343, 278 are integrated in the present example, these may alternatively be separately provided and connected together. The percussive force assembly 232 is operated to reciprocate in reverse first and second directions 20, 22 via a valve assembly 254 in the same manner as in other

examples above.

The percussive drilling apparatus 214 includes the transmission shaft 278 mounted within a support housing 274, wherein the transmission shaft 278 is connected to the drilling BHA 217 via connector sub 285 (although connector sub 285 may be integrated with or form part of the transmission shaft 278). The drilling apparatus 214 includes a first interface 284 having a first pair of load shoulders 286, 288 respectively on the reciprocating shaft 234/transmission shaft 278 and the support housing 274, and a second interface 290 having a second pair of load shoulders 292, 294 respectively on the support housing 274 and connector sub 285. An adjustment mechanism including an adjustment sleeve 295 is provided to facilitate adjustment within the second interface. The function of the first and second interfaces 284, 290 and the adjustment sleeve 295 is similar to that described in relation to other examples and as such no further description will be given.

The assembly 210 is illustrated in Figure 12 in an initial configuration in which the transmission shaft 278 and connected drill bit 216 are fully retracted (i.e., fully moved in the second direction 22). Rotation applied via a connected drill string (not shown) will reconfigure the valve assembly 254 as shown in Figure 13, in which the piston 238 is exposed to pressure within an internal flow path 219 via the valve assembly 254, thus rapidly accelerating the reciprocating shaft 234 and transmission shaft 278 in the first direction 20, with the permitted movement limited by the first interface 284. As such, the impulsive force is applied directly to the transmission shaft 278 by virtue of the piston 238 being rapidly exposed to pressure.

A further alternative example of a percussive drilling assembly 310 is illustrated in Figure 14. This assembly 310 is similar to assembly 210 first shown in Figure 12 and as such like features share like reference numerals, incremented by 100. In this respect the percussive drilling assembly 310 includes a percussive drive 312, percussive drilling apparatus 314 and drilling BHA 17 which includes a drill bit 316.

The percussive drive 312 includes a percussive force assembly 332 which includes a reciprocating shaft 334 moveable in reverse first and second directions 20, 22 by operation of a valve assembly 354 in the same manner as described in earlier examples. The transmission shaft 234 is integrated with or connected to a transmission shaft 378 of the drilling apparatus 214, wherein the transmission shaft 378 is connected to the drilling BHA 317 via connector sub 385 (although connector sub 385 may be integrated with or form part of the transmission shaft 378). The drilling apparatus 314 includes a first interface 384 between the reciprocating shaft 334/transmission shaft 378 and a support housing 374 of the drilling apparatus 314, and a second interface 390 between the support housing 374 and connector sub 385.

In this example the reciprocating shaft 334 is telescopic via an adjustable connector 106, such that the effective length of the reciprocating shaft 334 may be varied. Such variation in the effective length of the transmission shaft 334 may function to establish a maximum gap as within the first interface 384, thus establishing a maximum permitted movement of the transmission shaft 378 and connected drill bit 316 in the first direction 20.

The effective length of the reciprocating shaft 334 may be adjusted, for example as shown in Figure 15, by manipulation of the adjustable connector 106 to provide an alternative maximum gap as within the first interface 384, thus adjusting the maximum permitted movement of the transmission shaft 378 and connected drill bit 316 in the first direction 20.

In an alternative example the transmission shaft 378 may be connected to the reciprocating shaft 334 via an adjustable (e.g., telescoping) connector which provides the adjustment function described above.

It should be understood that the examples provided herein are indeed exemplary and that various modifications may be made thereto. Further, although different assemblies have been described individual components of the assemblies may be provided in isolation, for example used in combination with or as part of alternative systems. For example, the various drilling apparatus (14, 114, 214, 314) may be used in combination with any other percussive drive. Similarly, the various example percussive drives (12, 112, 212, 312) may be used in combination with any other apparatus, assembly or system which requires the application of impulsive forces.

Claims

CLAIMS: 1. A wellbore percussive drilling apparatus, comprising: a support; a transmission shaft axially moveable relative to the support in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with a percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a depth of cut control mechanism for providing a predetermined limit of permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.
2. The wellbore percussive drilling apparatus of claim 1, wherein the depth of cut control mechanism is settable to define the predetermined limit.
3. The wellbore percussive drilling apparatus of any preceding claim, wherein the depth of cut control mechanism is a variable depth of cut control mechanism configured to allow the predetermined limit of permitted movement of the transmission shaft in the first direction to be varied.
4. The wellbore percussive drilling apparatus of claim 3, wherein the variable depth of cut control mechanism is configured to permit a user to set the predetermined limit.
5. The wellbore percussive drilling apparatus of claim 3 or 4, wherein the variable depth of cut control mechanism comprises an adjustable interface between the support and the transmission shaft.
6. The wellbore percussive drilling apparatus of any one of claims 3 to 5, wherein the variable depth of cut control mechanism comprises an adjustment mechanism, wherein adjustment of the adjustment mechanism provides for variability of the depth of cut control mechanism.
7. The wellbore percussive drilling apparatus of claim 6, wherein the adjustment mechanism comprises a telescoping arrangement.
8. The wellbore percussive drilling apparatus of claim 6 or 7, wherein the adjustment mechanism is mounted on or relative to one or both of the support and transmission shaft.
9. The wellbore percussive drilling apparatus of any one of claims 3 to 8, wherein the variable depth of cut control mechanism comprises a plurality of limit elements configured to be selectively mounted on the apparatus.
10. The wellbore percussive drilling apparatus of any one of claims 3 to 9, wherein the variable depth of cut control mechanism allows variation of a maximum permitted stroke length of the transmission shaft in the first direction.
11. The wellbore percussive drilling apparatus of any preceding claim, comprising a first interface between the support and the transmission shaft, the first interface including a first pair of cooperating features provided respectively on or associated with the support and the transmission shaft.
12. The wellbore percussive drilling apparatus of claim 11, wherein: the first interface is configured to be registered when the transmission shaft is axially moved relative to the support in the first direction in response to an applied impulsive force, to thus define an end limit of movement of the transmission shaft in the first direction; and the first interface is configured to be deregistered when the transmission shaft is axially moved relative to the support in the second direction.
13. The wellbore percussive drilling apparatus of claim 11 or 12, comprising a second interface between the support and the transmission shaft, the second interface comprising a second pair of cooperating features respectively provided on or associated with the support and the transmission shaft.
14. The wellbore percussive drilling apparatus of claim 13, wherein: the second interface is configured to be registered when the transmission shaft is axially moved relative to the support in the second direction to thus define and end limit of movement of the transmission shaft in the second direction; and the second interface is configured to be deregistered when the transmission shaft is axially moved relative to the support in the first direction.
15. The wellbore percussive drilling apparatus according to claim 14, wherein the depth of cut control mechanism is variable by virtue of adjusting one or both of the first and second limit mechanisms.
16. The wellbore percussive drilling apparatus of any preceding claim, wherein the transmission shaft is configured to be rotatably coupled to a drill string or assembly.
17. The wellbore percussive drilling apparatus of any preceding claim, wherein the percussive drive comprises a hammer and the transmission shaft is configured to be directly or indirectly repeatedly struck by the hammer in the first direction.
18. The wellbore percussive drilling apparatus of claim 17, wherein the percussive drive comprises an adjustable hammer system in which the distance over which the hammer travels is adjustable.
19. The wellbore percussive drilling apparatus of claim 17 or 18, comprising an impact control mechanism for providing control of the effect of a hammer impact on the transmission shaft.
20. The wellbore percussive drilling apparatus of claim 19, wherein the impact control mechanism damps each hammer impact.
21. The wellbore percussive drilling apparatus of any one of claims 1 to 16, wherein the percussive drive comprises a reciprocating drive shaft and the transmission shaft is connected to the reciprocating drive shaft.
22. The wellbore percussive drilling apparatus of any preceding claim, wherein the percussive drive comprises: a housing and a mandrel, wherein the mandrel and the housing are configurable to be rotated relative to each other; a reciprocating piston mounted within a piston housing to define a piston chamber, wherein the piston is moveable in the first and second axial directions; a rotary valve assembly for facilitating the reverse movement of the reciprocating piston, the rotary valve assembly comprising a valve inlet for communicating with a pressure region and a valve exhaust for communicating with an exhaust region, the rotary valve assembly being operated by relative rotation between the mandrel and the housing to be cyclically reconfigured between: a pressure configuration in which the piston chamber is in pressure communication with the valve inlet and isolated from the valve exhaust to permit the piston to move in the first axial direction in accordance with the piston chamber being pressurised via the valve inlet; and an exhaust configuration in which the piston chamber is isolated from the valve inlet and in pressure communication with the valve exhaust to permit the piston chamber to be depressurised and the piston to move in the second axial direction, wherein movement of the piston in the first axial direction generates or facilitates the generation of an impulsive force.
23. The wellbore percussive drilling apparatus of claim 22, comprising a biasing arrangement configured to bias the reciprocating piston in the second axial direction, wherein when the rotary valve assembly is in its pressure configuration pressure applied within the piston chamber will act to bias or move the reciprocating piston in the first axial direction against the bias of the biasing arrangement, and when the rotary valve assembly is in its exhaust configuration the biasing arrangement will act to bias or move the reciprocating piston in the second axial direction.
24. The wellbore percussive drilling apparatus of claim 23, wherein the biasing arrangement defines a pre-activation pressure range, in which pressure within this pre-activation pressure range may be applied within the piston chamber without causing operation of the reciprocating piston
25. A method for performing percussive drilling of a bore hole, comprising: mounting a drill bit to a transmission shaft of a drilling apparatus, the transmission shaft being axially moveable in reverse first and second directions, the first direction corresponding to a direction of drilling; setting a depth of cut control mechanism to define a predetermine limit of permitted axial movement of the transmission shaft in the first direction; engaging the drill bit with a rock to be drilled; and operating a percussive drive to repeatedly apply axial impulsive forces on the transmission shaft in the first direction.
26. The method of claim 25, comprising rotating the drill bit while the percussive drive is operating to provide rotary-percussive drilling.
27. A wellbore percussive drilling apparatus, comprising: a percussive drive; a support; a transmission shaft axially moveable relative to the support in reverse first and second directions (i.e., the second direction is opposite to the first direction), wherein the first direction corresponds to a direction of drilling, the transmission shaft configured to be connected to a drill bit and be engaged with the percussive drive such that the percussive drive can repeatedly apply axial impulsive forces on the transmission shaft in the first direction; and a depth of cut control mechanism for providing a predetermined limit of permitted axial movement of the transmission shaft relative to the support in the first direction in response to each applied axial impulsive force.