TECHNICAL FIELD

The present invention relates to a pressurized liquid driven downhole drilling machine with a device for soft start from a flush position according to the preamble of patent claim 1. The invention further relates to an impact piston making part of such a downhole drilling machine according to claim 10.

BACKGROUND

The invention relates to a pressurized liquid driven downhole drilling machine of the kind provided with a slide tube making part of a valve housing and which in a slide control interface protrudes into and has a sliding fit against a flush channel section in a rear end portion of an impact piston and in which slide control ports of the slide control interface are acting to control a valve slide received in the valve housing. Such a downhole drilling machine is known for example from SE 526 252.

Such a downhole drilling machine includes an impact mechanism with a machine housing in which the impact piston is accommodated and provided so as to strike on the rear end or a neck of a drill bit which is movably attached in a drill attachment (drill chuck) at a front end of the downhole drilling machine. The reciprocating movement of the impact piston is trigged by the valve slide received in a valve housing. The valve slide reverses a force exerted by the pressurized liquid on the impact piston so that the impact piston alternately moves backwards in a direction from the neck of the drill bit respectively is accelerated forwards to strike the neck of the drill bit. The impact piston has a differential pressure area balanced rear holding position in the machine housing and is constantly affected by a small hydraulic force that drives the impact piston after each stroke to move backwards from the drill bit, towards the valve housing. The force on the impact piston is changed by alternately pressurizing respectively depressurizing. Conversely, the movements of the impact piston trigger the movements of the valve slide so that the valve slide alternately resets the pressure acting on the driving surface of the impact piston. The valve slide is correspondingly differential pressure area balanced and is constantly driven with a small force towards a forward holding position in the valve body. The hydraulic force on the valve slide is changed by alternately pressurize respectively depressurize a control pressure surface on the valve slide via said ports in the slide control interface between impact piston and slide tube.

With this type of a downhole drilling machine a so-called flush position may occur, in which the drill bit of the rock drilling machine suddenly stops during the drilling operation. The drill bit is here in its most forward end position and is relieved from feeding force against a rock face in front of the drill bit. In this case, the impact piston rests with full hydraulic force forwards against the neck of the stationary drill bit at the same time as the pressurized liquid (water) is flushed from the front surface through bore holes in the drill bit. In order to be able to resume the drilling work again, the stroke mechanism of the downhole drilling machine must be restarted, which is done through a so-called on-cut, wherein the drill bit is applied to the rock face with such a force that it causes the impact piston to follow with an activation distance somewhat backwards in the rock drilling machine until the impact piston and said slide control ports end up in a start position in the machine housing. In the start position, the impact piston is in a position where a slide control port in the slide control interface communicates with a slide control channel that makes part of the slide housing, wherein the drive area of the valve slide is pressurized, the valve slide is reset and the stroke mechanism of the downhole drilling machine restarts.

The activation distance required to reach the start position again from the flush position and perform a restart position of the stroke mechanism can be defined as the distance, along the central or longitudinal axis (machine direction) of the downhole drilling machine, by which the drill bit (impact piston) must be pushed back from the flush position into the machine in order for the slide control ports in the slide control interface to open and the resulting pressure flow resets the valve slide from holding position to start position in the valve housing. A problem when restarting from flush mode with said downhole drilling machines is not only that the activation distance to reach the restart position is relatively long (usually approx. 10 mm or more) without the drill bit having also to be applied with a relatively large force against the oncoming rock face. When drilling with downhole drilling machines, the above-described on-cuts with a restart from the flush position entail an undesirable and cumbersome interruption that reduces the efficiency of the drilling. In addition, there is an inconvenience with known downhole drilling machines going into and out of flush mode uncontrollably, which during drilling work in broken or fissured rock due to the intermittent drilling resistance that the drill bit normally encounters during the drilling in such rocks. It should be understood that constantly occurring flush position/on-cut and the resulting setting time caused thereby reduces the efficiency of productivity of the downhole drilling machines when drilling.

Fig. 1A shows schematically in a longitudinal section view a part of a said downhole drilling machine 1 which is in a start position, wherein a first and a second slide port C respectively D are in a mutually open position. The downhole drilling machine 1 comprises a machine housing 2 in which a differential pressure air balanced impact piston 20 with a rear holding position in the machine housing is axially reciprocally movably received, a valve housing 4 in which a differential pressure air balanced valve slide 6 with a front holding position in the valve housing 4 is axially reciprocally movably accommodated in an annular slide chamber 4b, slide tube 4a comprised in the valve housing and which in a slide control interface 17 protrudes into and have a sliding fit against a diametrically widened flush channel section 15a in a rear end portion of the impact piston 20, where said first and second slide control port C, D are operated and with which the axial end positions of the valve slide 6 in the slide chamber 5 are controlled by pressurization or pressure relief of a control pressure surface A2 of the valve slide 6.

As can be seen from the figure, said control pressure surface A2 is located inside an annular slide control chamber 4b to which a control pressure flow is led via a slide control channel 4c defined in the slide tube 4a. The slide control channel 4c is a commutation channel which alternately, in one direction leads pressurized liquid to the annular slide control chamber 4b for pressurization of the control pressure surface A2 of the valve slide 6, which drives the valve slide backwards from its forward holding position in the valve housing 4 and vice versa, in a second direction, via the slide control chamber 4b being drained on pressure flow to the central flush channel 15 via the second slide control port D. Draining of pressure flow in the slide control chamber is carried out via a radial drainage groove 16 in the inner surface located at the rear of the flush channel section 15 of the impact piston 20.

In Fig. IB, a part of the downhole drilling machine in Fig. 1A is shown schematically in a longitudinal section view in flush position, wherein the first slide control port C is open, wherein pressure flow flows into the flush channel 15, while the second slide control port D to the inlet of the slide control channel 4c is closed. In this position, the drill bit (not shown) and the impact piston 20 have been pushed out about 30 mm from the start position (compare the total axel length of the drive chamber 30 of 55 mm in the start position in Fig. 1A and its axial total length of 85 mm in the flush position). In the flush position, the impact piston 20 is kept extended with full pressure on the drive area Al in the annular drive chamber 30. To return from flush position to start position, the drill bit and impact piston 20 must be pushed in an activation distance Z of at least approx. 10 mm with full force in order for the first and second slide control ports C, D to occupy a mutually open position (cf. the start position in Fig. Al). This means that for restarting, the impact piston 20 must be driven so far back in the machine housing 2 that said first and second control slide ports C, D open to pressurize slide control channel 4c and thus the control pressure surface A2 of the valve slide 6 so that the valve slide 6 is trigged to change from its front holding position in the valve housing 4 to its rear position and thus restarting the stroke mechanism of the downhole drilling machine.

SUMMARY OF THE INVENTION

A first object of the present invention is thus a desire to provide a downhole drilling machine that can be restarted in a smoother and more efficient way than hitherto known downhole drilling machines. A second object of the invention is to provide a downhole drilling machine that shows improved operating features when drilling in a broken, fissured rock.

These objects of the invention are achieved through a downhole drilling machine which has been provided with the features and characteristics stated in claim 1. Further characteristics and technical advantages of the invention appear in the subclaims.

According to another aspect, the invention also relates to an impact piston intended to be used in a downhole drilling machine according to the invention.

Thanks to the invention, the activation distance Z' for the impact piston backwards in the machine housing becomes rather short (in practice approx. 1 mm), which significantly contributes to achieve a softer and more efficient on-cut when restarting the downhole drilling machine.

According to an embodiment, the slide control interface of a downhole drilling machine comprises a pressure relief port E which is provided so as to open, in flush position, a pressure relief channel between a drive chamber provided for the impact piston and slide control channel, which pressure relief port E is further configured to allow, under progressively increasing throttling action, the impact piston to be returned in the direction of its holding position in the machine housing by diversion of pressure flow from the impact piston's drive chamber to the control pressure surface of the valve slide via the slide control channel and thereby triggering the valve slide to move away from its holding position until a balance point being achieved at which the pressure relief port E is closed.

DESCRIPTION OF THE FIGURES

In the following, the invention is described more in detail with the help of an example of embodiment shown in the enclosed figures on which :

Fig. 1A schematically shows a longitudinal sectional view of a stroke mechanism part comprised in the downhole drilling machine in a known embodiment which is in a start position, wherein a first and a second control slide port C, D are in a mutually open position for pressurization of a slide control channel and switching a valve slide from a front holding position in a valve housing to a start position.

Fig. IB schematically shows a longitudinal sectional view of a stroke mechanism part of the downhole drilling machine in Fig. 1A when it is in the flush position, wherein a first slide control port C is open while a second control slide port D is closed and the impact piston is held out with full liquid pressure in its drive chamber.

Fig. 2 schematically shows a longitudinal sectional view of a liquid-driven downhole drilling machine according to the present invention in which downhole drilling machine the part to which the invention is located has been circled with a dotted- dashed contour line.

Fig. 3 schematically shows a longitudinal sectional view of the downhole drilling machine when it is in a flush position and, in accordance with the invention, the drive chamber of the impact piston, via a pressure relief port E, has been provided in a pressure flow communication with a slide control channel included in a slide control interface and pressurization of control pressure surface of a valve slide for switching the valve slide from the holding position to the start position thereof.

Fig. 4 schematically shows a longitudinal sectional view of the stroke mechanism part according to Fig. 3 in a flush position, but in a position where the impact piston under influence of a prevailing differential area pressure, in accordance with the invention, has been returned to a new and at some distance from the flush position retracted activation position in the machine housing of the downhole drilling machine, and set to a predetermined balance point achieved in connection with the pressure relief port E being closed.

Referring to Fig. 2, a liquid driven downhole drilling machine is generally designated by 1 and comprises a machine housing 2 which is defined by a cylindrical tube which in a front part accommodates a hammer housing 3 and in a rear part a valve housing 4 which, in a slide control chamber 4b, accommodates an axially movable valve slide 6. A drill bit 7 is fixed against rotation in a chuck 8 at a front end of the machine housing 2. The drill bit 7 has a neck 9 at its rear end. The neck 9 refers to the rear shaft-shaped part of the drill bit which receives impact energy. At the rear end of a machine housing 2, a back piece 10 is screwed in. The back piece 10 is intended to be connected to a drill support in form of a drill string (not shown). The back piece 10 thus forms a so-called drill string adapter which is screw attached onto the housing 2 by means of a threading 11 operating between the parts. 12 designates a filter or a strainer.

A pressurized driving liquid (pressurized liquid) in the form of, e.g., water under a pressure of, e.g., 18 MPa is intended to be led from the power source in the form of a drilling string connected to the other end of the drill string via a central channel in the drill string. The drill bit 7 has a central flush channel 15 which in its front terminates in flush outlets 15c which open into the front surface of the drill bit 7. Spent pressurized liquid that is led through the flush channel 15 forms hereby flush liquid by which drill cuttings can be removed from an area in front of the drill bit 7. The machine housing 2 accommodates a stroke mechanism generally designated 16 which is located between the rear part 10 and the drill bit 7 (also see the area circled with dotted-dashed contour line).

With reference also to Fig. 3 and 4, it is shown more in detail that the stroke mechanism 16 essentially comprises two parts, more precisely an impact piston 20 located in the front part of a machine housing 2, which is slidingly controlled and received in a hammer housing 3 to perform a movement between a front and respectively a rear end position, and a valve located in the rear part of the machine housing 2 in which said valve housing 4, slide control chamber4b and axially movable valve slide 6 are included. The valve slide 6 is sleeve-shaped and slidingly controlled in the slide control chamber 4b to perform a reciprocating movement. The valve housing 4 includes an inlet port respectively an outlet port (not shown) which can be put into and out of connection with the annular drive chamber 30 of the impact piston 20. Since the function of the valve slide 6 as such is to control pressure flow to and from the drive chamber 30 of the impact piston 20 and the associated drive area Al, this part of the downhole drilling machine will not be described more in detail. 17 generally designates a slide control interface with a first and second slide control port C, D, respectively, which alternately control the valve slide 6 to move between its frontal holding position and the rear end position respectively, in the slide control chamber 5 of the valve housing 4.

The impact piston 20 is differential pressure area balanced and thus normally strives to move with a predetermined primary first force fl towards a rear holding position in close proximity to the valve housing 4, which is illustrated with an arrow fl in Fig. 2. In the slide control interface 17 the slide tube 4a included in the valve housing 4 protrudes into and has a liquid proofing sliding fit with its outer surface against an inner surface of a diametrically widened flush channel section 15a in a rear part of the flush channel 15 of the impact piston 20. The valve slide 6 is a differential pressure area balanced and is dedicated to a front holding position in the valve housing 4 with a predetermined primary second force f2, which is illustrated by the arrow f2 in Fig. 2. When pressurization a control pressure surface A2 included in the valve slide 6 which makes part of the annular slide control chamber 4b (see Fig. 1A), and when slide control ports C, D are open, a backward secondary force is attained on the valve slide 6 designated F2 in Fig.2, i.e., a power supplement which overcomes the primary second force f2 and which, by prevailing differential pressure area action, drives the valve slide 6 into its front holding position in the valve housing 4. The slide tube 4a forms hereby a commutation channel 4c which in an alternative manner, in one direction, leads pressurized liquid to the annular slide control chamber 4b in order to drive the valve slide 6 backwards from the holding position, by the influence on slide control pressure surface A2 and conversely, in a second direction, drains pressurized liquid from the angular slide control chamber 4b to the central flush channel 15 via the radial drainage groove 16 in the inner surface of the rear located flush channel section 15a of the impact piston 20.

As the backward oriented secondary force F2 of the valve slide 6, when pressurization of the control pressure surface A2 of the valve slide 6, is greater than the differential pressure area-based primary force f2 forwards on the valve slide, i.e. F2>f2, the valve slide 6 will remain in its rear position in the valve housing 4 wherein the inlet port to the impact piston 20 opens and pressurized liquid flows in and pressurizes the impact piston driver Al wherein the impact piston is accelerated towards the drill bit 7, which is illustrated by the arrow Fl in Fig. 2.

All the described above essentially constitutes already known technology.

Referring to Fig. 3 and 4, in accordance with the invention, a pressure relief port E is provided in the slide control interface 17 and in which a pressure relief port is controlled by a slide part 4a :1 which consists of the outer surface of the slide tube 4a and the second slide control port D, i.e. in practice, the orifice opening that leads into slide control channel 4c of the slide tube 4a is opened and closed depending on the position of the pressure relief port E in relation to the other slide control port D.

The pressure relief port E is configured so that when the downhole drilling machine 1 is in flush position (see Fig. 3), a pressure relief channel provided in the impact piston 20 generally designated 40 (see detailed magnification) which is flow transfer effective between the drive chamber 30 of the impact piston 20 and the slide control channel 4c, wherein the pressure relief port E, when it is located in the middle of the second slide control port D, under progressively increasing throttling action, allows the impact piston 20 to be returned by the prevailing differential area pressure fl in the direction of its rear holding position in the machine housing 2 until it reaches a new and at some distance from the flush position retracted activation position by the pressure flow diverted from the drive chamber 30 of the impact piston 20 to the slide control channel 4c. The new retracted activation position also results in a new substantially shorter activation distance Z' to reach a start position and the restart of the stroke mechanism of the rock drilling machine.

The thereby prevailing pressure in the slide control channel 4c applies pressure on the control pressure surface A2 of the valve slide 6, wherein the valve slide 6 is triggered to move away (backwards) from its front holding position in the slide control chamber 4b of the valve housing 4 and in the direction of its rear end position in the slide control chamber 4b, which results in that the drain valve (not shown) controlled by the valve slide opens and the annular drive chamber 30 of the impact piston 20 with the drive area Al is pressure relieved. In the here described example of an embodiment, the pressure relief port E comprises a groove-shaped radially directed recess 41 in an inner surface of the impact piston's 20 widened flush channel section 15a in the slide control interface 17 in the rear part of the impact piston 20 and one or a plurality of elongated cavities 42 which run parallel to the central axis of the downhole drilling machine 1 can be formed of one, or along a circumference evenly distributed boreholes or circular arcshaped subspaces that open in the driving surface Al of the impact piston 20 in the driving chamber 30. By selecting the axial position of the groove-shaped recess 41 of the inner surface of the impact piston 20 in the slide control interface 17, one can easily configure a desired activation distance Z' in relation to a nominal activation distance Z (the nominal distance between flush position and start position) to achieve a soft and efficient restart of the downhole drilling machine.

As it is best shown by the two detail magnifications in Fig. 3 and 4, the pressure relief port E is controlled by a sliding portion 4a:l of the slide tube 4a cuts in the grooveshaped recess 41 in the inner surface of the impact piston 20 in the slide control interface 17.

As best shown in Fig. 4, the pressure relief port E remains open until a balance point is reached at which the impact piston 20, by the influence of a differential pressure area action, has moved so far backwards in the slide control interface 17 that the pressure relief port E closes, wherein the impact piston 20 is positioned in a new activation position retracted at some distance from the flush position. In the example of embodiment here described, the relative position of the pressure relief port E in relation to the second slide control port D has been chosen in such a way that the activation distance Z' of the drill bit 7 as well of the impact piston 20 backwards in the rock drilling machine until the impact piston 20 has positioned itself in its new retracted activation position in the machine housing amounts to only approx. 1 mm, i.e., the activation distance Z' = 1 mm. Thanks to the impact piston 20 being positioned in said new retracted activation position, a reduced activation distance Z' is obtained, i.e., the required activation distance Z' to reset the impact piston to the start position with open slide control ports C, D becomes significantly shorter than with previously known technology, which significantly facilitates restarting and minimizes the risk of unwanted production stoppages of the downhole drilling machine.

The distance with which the impact piston 20 is returned from the flush position to the balance point is, in contrast to hitherto known rock drillings machines, thus less than the nominal distance between the flush position of the impact piston and the start position along the central axis of the downhole drilling machine 1.

The ratio between the new reduced activation distance Z' for restarting the downhole drilling machine from the retracted activation position in which the balance point is reached and the nominal activation distance between the flush position and the start position is suitably in the range of 1:2 to 1:10. Practical experiments have shown that the most preferred ratio is in range of 1:5 to 1:10, wherein the latter relationship has been shown to give particular good results.

The downhole drilling machine 1 according to the invention works in the following way :

With reference to Fig. 3, the downhole drilling machine 1 is shown in a flush position, with the pressure relief port E in the open position. The slide control channel 4c is pressurized from the drive chamber 30 of the impact piston, which resets the valve slide 5 by pressurizing the pressure area A2 of the valve slide 6. The impact piston 20 cannot then remain as far outwards or forwards in the machine housing 2 in the flush position, but moves inwards (backwards in the machine housing), until the pressure relief port E closes.

The impact piston 20 therefore moves backwards in the machine housing until it has reached a new activation position withdrawn from the flush position by the pressure flow that is diverted from the drive chamber 30 of the impact piston 20 to the slide control channel 4c. The downhole drilling machine has hereby achieved a new reduced activation distance Z' for restarting the downhole drilling machine which is shorter than a nominal activation distance which, according to known technology, consists of the nominal distance between flush position and start position with regard to the movement of the impact piston 20 along the central axis.

The balance point for the impact piston 20 in the flush position is determined by the axially selected position of the pressure relief port E in the slide control interface 17 with respect to the central axis of the downhole drilling machine. Through an appropriately chosen configuration, it has proven suitable to have an activation distance Z' which amounts to only approx. 1 mm impression of the impact piston 20, for the stroke mechanism of the downhole drilling machine to restart, which means a smooth and easy starting of the downhole drilling machine from the flush position. The resulting softer flush position provides a simpler and more controllable on-cut procedure and offers thereby a possibility to perform more efficient drilling work, also in broken or fissured rock.