FIELD OF THE DISCLOSURE

The present disclosure relates to percussive drilling, and more particularly to a drill assembly for DTH (down-the-hole) drilling. The present disclosure further concerns a foot valve used in such a drill assembly.

BACKGROUND OF THE DISCLOSURE

In percussive drilling, percussive impacts transmitted by the drill bit are used to break the ground at the bottom of the borehole. In so-called DTH (down-the-hole) applications, the impact device is arranged within the borehole in connection with the drill bit, and is powered by a pressurized fluid. Typically, this pressurized fluid is also used to flush cuttings from the bottom of the borehole.

In conventional arrangements the pressurized fluid is selectively conducted between an uphole facing surface of the percussion device, driving an impact piston towards a downhole direction a foot valve, and a downhole facing surface, thereby driving the piston towards an uphole direction. In an intermediate period in between driving the impact piston in a downhole direction and driving the piston in a uphole direction, the pressurized fluid is conducted to a central flushing channel of an associated drill bit, so as to be further conducted to the bottom of the borehole for flushing cuttings therefrom.

In order to ensure sufficient pressure build-up required to drive the impact piston in the uphole direction, the opening of the drill bit’s central flushing channel has conventionally been equipped with a foot valve extending in an uphole direction therefrom, through which foot valve the pressurized fluid is conducted to central flushing channel for the drill bit. To prevent pressurized fluid from flowing to the central flushing channel after the impact piston has reached its lowermost position (i.e., when pressurized fluid is required to drive the impact piston towards the uphole direction), the foot valve has been configured to be received within a cavity of the impact piston, thereby blocking flow to the central flushing channel.

It has been considered a general desire to improve the durability of the foot valve, as failure thereof results in the percussion device becoming inoperable interrupting the drilling operation to a standstill until the foot valve is replaced. Furthermore, replacing the foot valve requires removing the whole drill string from the borehole in addition to disassembling the percussion device, which is very time consuming. As a result, the financial costs associated with a foot valve failure increase as the borehole becomes deeper.

Typical failure mechanisms of the foot valve are thought to include dislodging of the foot valve caused by a worn or loose interference fit, cracking of the foot valve caused by impacts from drilling debris, water between an impact surface of the drill bit and the impact piston shooting against the foot valve, the drill bit’s impact surface peening over towards the foot valve, and damaged piston bore or impact surface.

Moreover, the general trend towards increased drilling pressure (the pressure at wich the impact device is operated) would appear to increase the risk of foot valve failure.

In order to overcome some of the shortcoming, DTH percussion devices without foot valves have been developed However, such attempts arrangements have generally resulted in excessively complicated structures and the efficiency of such valveless percussion devices have not achieved the efficiency of the devices with foot valve.

BRIEF DESCRIPTION OF THE DISCLOSURE

An object of the present disclosure is to provide a drill assembly for percussive drilling, in which the durability of the foot valve is improved.

The object of the disclosure is achieved by the drill assembly which is characterized by what is stated in the independent claim. The preferred embodiments of the disclosure are disclosed in the dependent claims.

The disclosure is based on the idea of providing the foot valve with an internal stiffener thereby improving the capability of the foot valve to withstand the strain exerted thereon.

An advantage of the disclosure is that more reliable operation of the impact device is achieved.

According to the disclosure, a drill assembly for percussive drilling is provided. The drill assembly comprises a drill bit, a percussion device and a foot valve.

The drill bit extends along a longitudinal direction of the drill assembly, and comprises a distal portion having an end surface equipped with inserts for transferring percussive impact to points of contact with a borehole. The distal portion of the drill bit is at a downhole end of the drill assembly, i.e. at the bottom of the borehole, when in use.

The drill bit further comprises a shank portion extending from the distal portion in an uphole direction. The shank portion is equipped with a male coupling configuration and, on a longitudinal uphole end of the shank portion, a first impact surface. Preferably, but not necessarily, the male coupling configuration may be provided as a male splined surface extending longitudinally along the shank portion.

The drill bit further comprises a first central flushing channel for conducting a flushing fluid flow to the end surface of the distal portion. The first central flushing channel extends through at least the shank portion and is open towards the longitudinal uphole end of the shank portion, such that the first impact surface is formed annularly around the central flushing channel.

The percussion device extends along a longitudinal direction and comprises, on downhole side end thereof, a female coupling configuration for coupling with the male coupling configuration of the drill bit. Preferably, but not necessarily, the female coupling configuration may be provided as a female splined surface arranged to cooperate with a male splined surface of the drill bit.

The percussion device further comprises an impact piston arranged within a housing and configured to reciprocate in the longitudinal direction when a flushing fluid flow is conducted through the percussion device. The impact piston has a central recess, and a second impact surface annularly surrounding the central recess for impacting against the first impact surface of the drill bit when the impact piston is reciprocated. For example, the central recess of the impact piston may be provided as a second central flushing channel for conducting a flushing fluid flow through the impact piston

The drill assembly further comprises a hollow cylindrical foot valve attached to the drill bit. The foot valve is partially received within the first central flushing channel and extends therefrom in the uphole direction.

The foot valve 9 is also partially received within the central recess, at least when the impact piston is in a lowermost position of its reciprocation cycle, i.e., when the impact piston abuts the drill bit. On the other hand, the foot valve is no longer received within the central recess, at least when the impact piston is in an uppermost position of its reciprocation cycle. Preferably, but not necessarily, the foot valve is withdrawn from and inserted to the central recess at a position of the impact piston between the uppermost and lowermost positions of its reciprocation cycle.

Particularly, the foot valve comprises an internal stiffener extending radially inwardly from at least two circumferential positions of an inner surface of the foot valve. Preferably, but not necessarily, the internal stiffener may extend between at least two circumferential positions of an inner surface of the foot valve. In an embodiment according to the present disclosure, the internal stiffener extends along the longitudinal length of the foot valve.

It should be noted, however, that the internal stiffener does not need to extend for the whole length of the foot valve.

In an embodiment according to the present disclosure, the internal stiffener is formed as one or more internal walls of the foot valve.

Preferably, but not necessarily, the internal stiffener is formed as a plurality of internal walls of the foot valve such that at least two of the internal walls coincide with each other, as seen along the cross-sectional profile of the foot valve perpendicular to the longitudinal direction thereof.

Preferably, but not necessarily, the internal stiffener is formed as a plurality of internal walls of the foot valve such that at least two of the internal walls intersect with each other, as seen along the cross-sectional profile of the foot valve perpendicular to the longitudinal direction thereof.

In an embodiment according to the present disclosure, the internal stiffener is formed as an integral part of the foot valve. This may be achieved, for example, by forming the foot valve by moulding, such as injection moulding. Alternatively, the foot valve may be machined from a suitable solid piece or profile piece, or it could be constructed as an assembly of separate parts.

Suitable materials for the foot valve include, for example, polymers such as polyoxymethylene (POM or fibre reinforced plastics (FRP). Alternatively, if the foot valve is constructed as an assembly of separate parts, a combination of polymer and metal materials may be used.

In an embodiment according to the present disclosure, the internal stiffener extends longitudinally for a first distance from an uphole side end of the foot valve, the first distance being less than or equal to a distance for which the foot valve projects from the drill bit.

In this kind of an arrangement, the stiffener helps the foot valve maintain its form against forces caused by the impact piston, while insertion of the foot valve into the first flushing channel is facilitated, as the foot valve may more easily deform at a portion thereof at which the stiffener is not exhibited.

It should be noted, however, that the stiffener 10 does not need to extend from the uphole side end of the foot valve, but may be longitudinally distanced therefrom. In an embodiment according to the present disclosure, the internal stiffener extends longitudinally for a first distance from an uphole side end of the foot valve, the first distance being more than or equal to a distance for which the foot valve projects from the drill bit.

In this kind of an arrangement, the stiffener helps the foot valve maintain its form against forces caused by the impact piston, while secure attachment of the foot valve within the first flushing channel is facilitated, as the foot valve may exhibits increased rigidity also within the flushing channel.

It should be noted, however, that the stiffener 10 does not need to extend from the uphole side end of the foot valve, but may be longitudinally distanced therefrom.

In an embodiment according to the present disclosure, a cut-out extending radially through foot valve is provided extending axially for a second distance from a downhole side end of the foot valve, wherein the second distance being less than or equal to a length at which the foot valve is received within the drill bit.

That is, a cut-out extending laterally through the outer wall of foot valve may be provided on the foot valve at a portion thereof received within the drill bit.

Such an arrangement facilitates insertion of the foot into the first flushing channel as it enables the respective portion of the foot valve to yield more easily, thereby conforming into the shape of the first flushing channel

Preferably, but not necessarily, a projection of the cut-out on a plane parallel with the longitudinal direction is a differentiable curvature opening towards a downhole side end of the foot valve. In other words, the outline of the cut-out is smooth and continuous, thereby eliminating excessive peaks of internal stresses caused by a discontinuous geometry.

In an embodiment according to the present disclosure, the foot valve is provided with a section of increased outer diameter at a portion of the foot valve received within the drill bit. This may be carried out to ensure sufficient friction between the outer wall of the foot valve and the inner wall of the first flushing channel caused to maintain the foot valve in place during operation.

It should be noted that the present disclosure encompasses any combination of two or more embodiments, or variants thereof, as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which Fig. 1 and Fig. 2 illustrate longitudinal cut view of a drill assembly according to the present disclosure.

Fig. 3a - Fig. 8c illustrate various embodiments of a foot valve according to the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Fig. 1 illustrates a longitudinal cut view of a drill assembly 1 according to the present disclosure. The drill assembly 1 comprises a drill bit 2 having a distal portion (3) at the downhole end side of the drill bit. The distal portion 3 is equipped, at an end surface thereof, with inserts 3a for cutting and/or crushing the ground at the bottom of the borehole. A shank portion 4 extends from the distal portion towards an uphole direction, and has a first impact surface 4a on a longitudinal uphole end thereof. A first central flushing channel 5 extends at least through the shank portion 4 so as to conduct a flushing fluid flow to the end surface of the distal portion 3. Particularly the first impact surface 4a is formed annularly around the first central flushing channel 5.

The drill assembly further comprises a percussion device 6, generally known as a DTH- hammer. The percussion device is coupled to the shank portion of the drill bit with mutual engaging surfaces, such as a male and female splined surfaces on the shank portion and on the downhole side end of the percussion device, respectively.

Particularly, the percussion device 6 has an impact piston arranged within a housing, which is configured to reciprocate in the longitudinal direction, when a flushing fluid flow is conducted through the percussion device. The impact piston 7 has a central recess 8 provided as a second central flushing channel for conducting a flushing fluid flow through the impact piston 7. A second impact surface 7a of the impact piston annularly surrounds the second central flushing channel 8.

Fig. 1 also illustrates a hollow cylindrical foot valve 9 attached to the drill bit 2 and partially received within the first central flushing channel 5 and extending therefrom in the uphole direction.

Particularly, Fig. 1 illustrates the drill assembly in situation in which the impact piston 7 is in an uppermost positions of its reciprocation cycle, wherein the second impact surface 7a is distanced from the first impact surface 4a and the foot valve 9 is not received within the second central flushing channel 8

Fig. 2, in turn, illustrates the drill assembly of Fig. 1 when the impact piston is in a lowermost position of its reciprocation cycle, wherein the first and second impact surfaces 4a, 7a abut each other, and the foot valve 9 partially received also within the second central flushing channel.

Fig. 3a - Fig. 3c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan vie, longitudinally cut view and a side view, respectively. Particularly, the stiffener 10 has been provided as a single linear internal wall 10a extending between opposing point of internal surface of the foot valve 9, through a cross- sectional centre point thereof.

As best illustrated in Fig. 3a, the material thickness of the internal wall 10a is increased where the internal wall 10a adjoins the internal surface of the foot valve 9. Moreover, the internal wall 10a extends for distance from the uphole side ned of the foot valve 9 such that it does not extend up to the opposing downhole side end of the foot valve 9. Additionally, the foot valve 9 is provided with a section of increased outer diameter 9b at a distance from the downhole side end of the foot valve 9. Notably, the section of increased outer diameter 9b is provided at a portion of the foot valve 9 where the stiffener 10 is not exhibited. In use, the section of increased diameter 9b resides within the first central flushing channel 5.

Fig. 4a - Fig. 4c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan view, longitudinally cut view and a side view, respectively. The embodiment of Figs 4a - 4c is similar to that illustrated in Fig. 3a - Fig. 3c with the exception that the stiffener 10 is provided as curved internal wall 10a which does not run through the cross-sectional centre point of the foot valve 9, and extends along the whole length of the foot valve 9.

Fig. 5a- Fig. 5c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan view, longitudinally cut view and a side view, respectively. The embodiment of Figs 5a - 5c is similar to that illustrated in Fig. 3a - Fig. 3c with the exception that the stiffener 10 is provided as two intersecting internal walls 10a forming a cross-like cross-sectional profile, in which the internal walls coincide at the cross-sectional centre point of the foot valve 9.

Fig. 6a- Fig. 6c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan view, longitudinally cut view and a side view, respectively. The embodiment of Figs 6a - 6c is similar to that illustrated in Fig. 3a - Fig. 3c with the exception that the stiffener 10 is provided as a plurality of coinciding internal walls 10a forming a central tubular cross-sectional profile, from which radially extending spokes extend between the central tubular portion and the internal wall of the foot valve 9. Fig. 7a - Fig. 7c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan view, longitudinally cut view and a side view, respectively. The embodiment of Figs 7a - 7c is similar to that illustrated in Fig. 3a - Fig. 3c with the exception that the stiffener 10 is provided as a plurality of intersecting internal walls 10a forming cross-sectional rectangular grid-like pattern, in which rectangular cavities are formed between the internal walls 10a, and the stiffener 10 extends along the whole length of the foot valve 9.

Fig. 8a - Fig. 8c illustrate an embodiment of the foot valve 9 according to the present disclosure as seen as plan view, longitudinally cut view and a side view, respectively. The embodiment of Figs 8a - 8c is similar to that illustrated in Fig. 3a - Fig. 3c with the exception that the stiffener 10 is provided as a plurality of intersecting internal walls 10a forming cross-sectional tubular grid-like pattern, in which tubular cavities are formed between the internal walls 10a.

It should be noted that the appended drawings illustrate exemplary embodiments of the present disclosure, and that other, non-illustrated embodiments are also encompassed by the present disclosure. For example, the stiffener may alternatively be provided as internal splines, i.e. axially extending radial bulges, at which the material thickness of the foot valve is increased.