SE2150708A1 - Drill rig, safety arrangement on a drill rig and a method of safely handling a drill string - Google Patents

Abstract

The invention relates to safety arrangement for a hoist system on a drill rig (1). A rod holder (6) for holding a drill rod of the drill string (10) is arranged at a lower portion of the mast (2), the rod holder (6) being operable between an open mode and a closed mode in which it holds the drill rod. The safety arrangement comprises a force meter (8), e.g. configured to detect the tension in the cable (4), and a safety device (12) configured to block opening of the rod holder (6) such that it may not be opened unless the force meter (8) detects that the tension of the cable (4) is at or above a force threshold indicating that the cable (4) is stretched and attached to the drill string (12). The invention also relates to a drill rig and a method of handling drill string.

Description

TECHNICAL FIELD The invention relates to a safety arrangement on a drill rig. The invention also relates to a drill rig including such a safety arrangement. Further, the invention relates to a method of safely handling a drill string at a drill rig. BACKGROUND ln the mining industry, drilling is most often performed over great depths into rocks where the drill string consists of several interconnected drill rods. ln so called surface drilling, where the drill rig performing the drilling operation is located above ground the drilling is generally performed downwards, or at least partly downwards. ln some applications, such as in exploration drilling, the drill hole may be several kilometres deep or long and it is not unusual that the leading drill bit needs to be replaced several times during such an operation. When the drill bit is to be replaced the whole drill string needs to be pulled out from the rock, the drill string needs to be disassembled into individual drill rods, or in sections of e.g. two or three drill rods, in order to access the drill bit and replace the same for a new one. Once the drill bit has been replaced the drill string is re-assembled by connecting drill rods to each other. ln such systems a hoist system may be arranged on a derrick or mast of a drill rig to pull the drill string out of the rock, and to allow the drill string to redescend in a controlled manner into the drilled hole. The hoist system generally includes a winch configured to pull a cable that is attachable to the drill string via a drill string hoisting plug such as a plug. When the drill string is pulled out of the drill hole or released into the drill hole it is often pulled or released in sections of two or three drill rods depending on the height of the derrick or mast.

For each new section of the drill string that is pulled out from or allowed to descend into the drilled hole, the hoisting plug needs to be loosened from the removed section and to be reconnected to the uppermost drill rod of the drill string. Conventionally, problems have occurred where the drill string or parts thereof are dropped into the drilled hole or that hoist system is damaged as result of that the drill string falls uncontrollably before its motion is stopped by the hoist system.

The invention is also relevant for applications where no hoist system is arranged, such as top drive drilling or pull with a head drilling. lt would be advantageous to achieve a system overcoming, or at least alleviating, at least one or some of the drawbacks of the prior art. ln particular, it would be desirable to achieve a safety system for a drill rig which reduces the risk of damaging or losing parts of the drilling equipment during a drilling operation. SUMMARY OF THE INVENTION lt is an object of the invention to provide a device and a method that provides an increased safety in the handling of a drill string during drilling operation, specifically a deep hole drilling operation.

According to a first aspect the invention relates to a safety arrangement on a drill rig, the safety arrangement being arranged at a mast of the drill rig for ensuring safe handling of a drill string, wherein the drill rig comprises: - a rotation unit for holding and rotating the drill string and driving the same during a drilling operation, - a rod holder for holding a drill rod of the drill string and keeping the drill string fixed in between drilling operations, the rod holder being arranged at a lower portion of the mast and being operable between an open mode and a closed mode in which it holds the drill rod of the drill string, characterised in that the safety arrangement comprises at least one force meter for detecting a carried weight of the drill string, and a safety device configured to allow opening of the rod holder only if the force meter has detected that the drill string is not solely carried by the rod holder.

The inventive safety arrangement may be implemented in different ways and on different kinds of drill rigs, with different kinds of arrangements for raising and descending the drill string.

According to specific embodiments the safety arrangement further includes a force meter is configured to detect the weight of the drill string carried by the rod holder, and wherein the safety device allows opening of the rod holder in response to that said force meter detects that the weight of the drill string carried by the rod holder is at or below a force threshold indicating that the drill string is not solely carried by the rod holder.

According to specific embodiments of the safety arrangement a cable is arranged to be attached to the drill string, a winch being arranged for pulling and releasing the cable, and wherein a force meter is configured to detect the weight of the drill string carried by the by the cable, and wherein the safety device allows opening of the rod holder in response to that said force meter detects that the cable carries a weight over a threshold value, thereby indicating that weight of the drill string is at least partly carried by the cable.

According to specific embodiments of the safety arrangement the force meter configured to detect the weight of the drill string carried by the by the cable is configured to detect the tension in the cable, and wherein the safety device allows opening of the rod holder in response to that said force meter detects that the tension in the cable is above a threshold value indicating that weight of the drill string is at least partly carried by the cable.

According to specific embodiments of the safety arrangement the winch is driven by a hydraulic motor or an electric motor and wherein one force meter of the at least one force meter is an indirect force meter configured to indirectly detect the weight carried by the cable by detecting a hydraulic pressure of the hydraulic motor or an electric power of the electric motor.

According to specific embodiments of the safety arrangement a force meter is configured to detect the weight of the drill string carried by the rotation unit, and wherein the safety device allows opening of the rod holder in response to that said force meter detects that the force detected by said force meter is above a threshold value indicating that the weight of the drill string is at least partly carried by the rotation unit.

According to specific embodiments of the safety arrangement the holder is hydraulically operated and wherein the safety device involves a control valve, which in a closed mode blocks the provision of hydraulic fluid to the rod holder, thereby blocking two opposed clamps of the rod holder in a closed mode in which the clamps are configured to hold the drill string.

This is a type of fail-safe arrangement that guarantees that the rod holder may not be accidentally opened by malfunction, or by an inaccurate command from the operator.

According to specific embodiments of the safety arrangement a control unit is comprised in the safety arrangement and is configured to control the safety device in response to that a carried weight detected by the at least one force meter is above a threshold indicating that the weight of the drill string is at least partly carried by the cable or by the rotation unit, or below a threshold indicating that the weight of the drill string carried by the rod holder is at or below a force threshold.

According to specific embodiments of the safety arrangement the control unit is configured to the specific threshold during a drilling operation based on a previously detected carried weight, a directly or indirectly measured weight of the drill string and/or an estimated weight based on an expected length of the drill string.

According to specific embodiments of the safety arrangement the control unit is configured to control the rod holder automatically and wherein the rod holder may only be released from the closed mode to the open mode in response to that a carried weight detected by the at least one force meter is above or below the specific threshold.

This is advantageous as it makes it possible not only to avoid accidents, but also to adapt the safety device to only allow opening of the rod holder when an alternative holding arrangement, such as a rotation unit or hoisting arrangement is in tune to carry the correct weight of the drill string or close to the correct weight and to thereby achieve a smooth handover of the weight from the rod holder. According to specific embodiments the force meter includes a load cell.

The safety arrangement may be part of a drill rig or may be a separate part configured to be implemented at a mast of a drill rig.

According to a second aspect the invention relates to a drill rig that comprises a safety arrangement as described above arranged at a mast of the drill rig.

According to a third aspect the invention relates to a method of safely handling a drill string during a drilling operation, the method comprising: - holding a drill string with a rod holder, - before releasing the rod holder, controlling that the drill string is not solely carried by the rod holder by detecting a carried weight of the drill string, - allowing the rod holder to be released only if it has been detected that the drill string is not solely carried by the rod holder.

According to specific embodiments a detection that the drill string is not solely carried by the rod holder includes detecting that the weight of the drill string carried by the rod holder is at or below a force threshold indicating that the drill string is not solely carried by the rod holder.

According to specific embodiments the method further comprises a step of attaching a cable to the drill string, wherein the verifying that the drill string is not solely carried by the rod holder is achieved by detecting a tension in the cable, and/or by detecting a force acting on the rotation unit and releasing the rod holder only in response to that the detected tension in the cable and/or the force acting on the rotation unit is at or above a force threshold.

According to specific embodiments the force threshold is updated during a drilling operation based on a previously detected parameter, a directly or indirectly measured weight of the drill string and/or an estimated weight based on an expected length of the drill string.

Other embodiments and advantages of the invention will be apparent from the detailed description and the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS Below, specific embodiments of the invention will be described with reference to the appended drawings, of which: Fig. 1 shows a drill rig on which the inventive safety arrangement may be implemented, Fig. 2 shows a drill rig on which a drill rod is to be added to the drill string, Fig. 3 shows a drill rig on which the cable holding the drill rig is not stretched, Fig. 4 shows an embodiment of a safety arrangement on a drill rig, Fig. 5 shows a drill rig with a safety arrangement according to an alternative embodiment, Fig. 6 shows a drill rig with a safety arrangement according to another alternative embodiment, and Fig. 7 shows a drill rig without a hoist system and with a safety arrangement according to yet another alternative embodiment.

DETAILED DESCRIPTION OF THE SHOWN El\/lBODll\/IENTS ln fig. 1 a drill rig 1 is shown that comprises a mast 2, along which a drilling machine in the form of a rotation device 7 is arranged to provide an axial and rotational movement to a drill string arranged alongside the mast 2. ln a conventional manner the mast 2 is preferably tiltable to any position between the upright vertical position shown in the drawing and a 45-degree tilted position. Hence, the illustrated drill rig 1 is arranged to perform drilling vertically into the ground or with at an angle of up to approximately 45 degrees with respect to a vertical line.

Figs. 1-3 are schematical views of a drill rig 1 in different phases of a drilling operation. The drill rig 1 comprises a winch 3 arranged on a first non-operational side of the mast 2. The winch 3 is part of a hoist system arranged at the mast 2 of the drill rig and is configured to control a drill string 10 via a cable 4. The cable 4 may e.g. be a steel cable or a nylon cable. The winch is driven by a motor 13, which may be a hydraulic motor or an electric motor.

A threaded connector known as a hoisting plug 5 is configured to connect the cable 4 to the drill string 10. A swivel 20 is preferably arranged to allow the hoisting plug 5 to rotate with respect to the cable 4. Namely, the cable 4 is configured to be connected to the drill string 10 during unthreading and threading of a section is removed from or added to the drill string 10.

During a drilling operation, when a rotation unit 7 is driving the drill string into the rock/ground whilst rotating the same, a water conduit is conventionally connected to the drill string 10, to provide flushing water via the inside the drill string 10, which is hollow, to the drill bit. During the drilling operation, the cable 4 may be connected to the drill string 10 via a water swivel. The water swivel involves an inlet connected to the water conduit and an output that may be threadedly connected to the drill string 10. The water swivel allows the drill string 10 to be rotated without rotating the water conduit. The water swivel is configured to bear the weight of a full drill string, such that the cable 4 may carry the drill string, via the water swivel, when the water swivel is connected to the drill string.

Hence, as a safety measure, the cable 4 may be configured to be connected to the drill string 10 during the drilling operation when a rock drilling machine 11 is driving the drill string into the rock/ground whilst rotating the same, whereby the water swivel allows the drill string 10 to be rotated without rotating the cable 4. During drilling, the cable 4 is not tense, but includes a slack allowing the drill sting 10 to penetrate further into the ground.

Between drilling operations, when the drill string 10 is pulled out from the drill hole or allowed to re-enter the drill hole, the cable 4 may instead be connected to the drill string via the hoisting plug 5 and the water swivel may be disconnected. The cable 4 may e.g. be a steel cable or a nylon cable strong enough to carry the full weight of the several hundred metres long drill string, which may even be up to several thousand metres long. ln this operation, a hoisting arrangement is arranged to pull and release the drill string, via the cable 4, which is part of the hoisting arrangement.

A rod holder 6 for holding a drill rod of the drill string 10 is arranged at a lower portion of the mast 2, on the active side of the mast, i.e. the side where the drill string 10 is arranged. The rod holder 6 is configured to hold the drill string 10 when no drilling is performed. The rod holder 6 is hence operable between an open mode and a closed mode in which it holds a drill rod of the drill string 10. The rod holder 6 may be electrically or hydraulically operated. ln accordance with the invention a safety arrangement is arranged, which may include at least one force meter 8. ln the embodiment shown in Figs. 1-3 a force meter 8 is arranged along the inactive side of the mast 2 to detect the tension of the cable 4 between the winch 3 and a sheave wheel 9 arranged at the top of the mast 2. The force meter 8 is arranged to detect that the cable carries at least a part of the weight of the drill string 10, and hence that the rod holder 6 does not solely carry the weight of the drill string 10. ln this specification the feature indicating that the force meter 8 detects a tension or a force signifies that the force meter monitors or measures a tension or a force, for example by means of a load cell included in the force meter 8. The force meter may continuously register the weight or may be configured to detect the weight, tension or force at specific occasion, such as when the weight surpasses a specific threshold value.

The force meter 8 of the embodiment shown in Figs. 1-3 is configured to detect a lateral component of the tension in the cable 4. Specifically, the force meter 8 preferably comprises a sheave or guide wheel in contact with the cable 4 and a load cell configured to detect the force acting on the guide wheel, which force is proportional to the tension in the cable, such that said tension may be calculated. ln Fig. 1 the cable 4 is stretched and in condition to carry the drill string 10. Hence, in the state shown in Fig. 1 the force meter 8 has detected that the cable carries at least a part of the weight of the drill string 10, and hence that the rod holder 6 does not solely carry the weight of the drill string 10. ln addition to the force meter 8, the safety arrangement comprises a safety device 12 configured to block the rod holder 6 such that it may not be opened unless it has been detected that the dri|| string 10 is not solely carried by the rod holder 6.

A signal may be issued to the safety device 12 in response to that the force meter 8 detects that the tension of the cable 4 is at or above a force threshold, indicating that the cable 4 is stretched and attached to the dri|| rod. The safety device 12 will be more closely described below. ln both Figs. 2 and 3 the dri|| rig is illustrated in a condition where the tension in the cable is low, and hence below the required force threshold such that no signal that verifies that the dri|| string 10 is not solely carried by the rod holder 6 is issued based on the illustrated force meter 8, and as a consequence the rod holder 6 is not allowed to be opened. lt should however be noted, that in specific embodiments a force meter may be configured in the rod holder 6, which could detect the weight acting on the rod holder 6 such a force meter could indicate that that rod holder only carries part of the weight of the dri|| string, and hence that the dri|| string 10 is not solely carried by the rod holder 6 and as a consequence the rod holder 6 may be allowed to be opened. ln the embodiments of fig. 2 and 3, this would imply that the dri|| string 10, if not carried by the cable 4, is also carried by the rotation unit 7. Also, the rotation unit 7 may be provided with a force meter to verify that it carries at least a part of the weight of the dri|| string 10. As a complement, in specific embodiments, a verification may be issued when the chucks of the rotation unit 7 are activated and confirmed to be in engagement with a portion of the dri|| string. Such an arrangement should however preferably be complemented with a control system making sure that the dri|| rod held by the rotation unit is indeed threaded so as to form part of the dri|| string 10. ln the condition shown in Fig. 2, an additional dri|| rod 22 is about to be added to the dri|| string 10, which in this moment is held by the rod holder 6. From this position the dri|| rod 22 will be pulled up by the winch 3 and guided into connection with the free end of the dri|| string 10. The rotation unit 7 may be utilised to rotate the dri|| rod 22 so as to make the thread thereof and attach the dri|| rod 22 to the dri|| string 10. During this operation the cable 4 is preferably attached to the dri|| string, via a water swivel if it is during an ongoing drilling operation, or directly via a hoisting plug 5 if it is during a relowering of the dri|| string, e.g. after a concluded exchange of dri|| bits on the dri|| string. ln Fig. 3, the drill rod has been connected the drill string 10 via a joint 21 and is in condition to be drilled into the ground by means of the rotation unit 7, which is slidably arranged on the mast 2 so as to move the drill string 10 axially at the same time as it provides a rotational movement to the drill string 10. The rotation unit 7 may be moved along the mast 2 by means of a double acting hydraulic cylinder.

During a dri||ing operation the cable 4 is normally attached to the drill string via a water swivel, although the drill string 10 is held by the rotation unit 7 during dri||ing and a certain slack in the cable 4 is therefore required not to obstruct the lowering of the drill string 10. The drill string may hence be carried by the rotation unit 7, by the cable 4, either via the hoisting plug or via a water swivel, or by the rod holder 6, e.g. when a rod section is added or removed.

Between dri||ing operations, the drill string 10 may be both raised and lowered by means of the winch 3 acting on the cable 4. Firstly, the winch 3 is utilised to raise the drill string 10 after a concluded dri||ing operation. ln this part of the operation the drill string 10 is normally raised section by section where each section may include the length of more than one drill rod, typically two or three drill rods. Conventionally, the length of a drill rod is about three metres, and the height of the mast is often about six or nine metres such that the drill string may be raised in sections of the length of for example two or three drill rods at a time. ln addition to that the drill string is pulled out of the drill hole 10 after a concluded dri||ing operation, it may also be necessary to pull out the drill string from a drill hole and re-enter the drill string in order to exchange the drill bit, which may be worn out before the dri||ing operation is concluded. Namely, in inter alia the application of exploration dri||ing, the drill hole may be several kilometres deep and normally a drill bit is worn out and needs to be replaced at even intervals, typically every 100-300 metres of dri||ing. Hence, in such operations, the drill string needs to be raised from and lowered into the drill hole every couple of hundred metres, which implies that this is repeated several times during the dri||ing of a deep hole.

After the raising of a section comprised of tvvo or three drill rods, the drill rods may be removed individually from the drill string or in lengths corresponding to the sections of two or three drill rods, provided that there is space available to store such lengths at the operation area surrounding the drill rig.

As a section of drill rods is unscrewed from the drill string 10 the rod holder 6 is activated to hold the uppermost drill rod of the drill string. After removal of the section of drill rods the hoisting plug 5 of the cable 4 is fastened into a screw opening of the uppermost drill rod of the drill string 10, whereby a next section of drill rods may be raised by means of the winch 3.

An experienced operator knows that best practice includes tightening the cable 4 before the rod holder 6 is released. ln view of the length of the drill string 10 it may be very heavy at times such that the cable and the hoist system, including the winch 3 and the sheave wheel 9 will need to withstand correspondingly high forces. The hoist system is however well adapted to handle such forces in a state of equilibrium. Problems may however arise if the cable is slacking or if the hoisting plug is not properly connected to the drill string. ln a worst-case scenario, if the hoisting plug 5 is not properly connected to the drill string 10 when the rod holder 6 is opened, the drill string 10 may fall uncontrollably into the drill hole. Depending on the relation between the depth of the drill hole and the length of the drill string it may not be possible to retrieve the drill string from the drill hole. ln the case where the hoisting plug is properly connected to the drill string 10, but the cable 4 has a slack when the rod holder 6 is opened, the drill string 10 will fall as far down as the slack allows before it will be restrained by the cable and the hoist system. The damages caused by such a fall is of course dependent on the length of the slack and of the weight of the drill string 10 which will result in a momentum that will provoke an impulse on the hoist system when the cable reaches a fully stretched position. For a long drill string the weight is well over a ton and may be up to 18 tons and above, which will induce a high momentum on even a short slack. lfthe momentum is too big, the winch may not be able to withstand this momentum, which may hence result in material damages of the winch or other parts of the hoist system and of other parts of the drill rig 1, such as the drill string , the rotation unit 7 and the mast 2. ln Figs. 4 and 5 two embodiments of a safety arrangement is shown. ln accordance with the invention, the safety device 12 is configured to make sure that the rod holder 6 may not be prematurely released during the handling of the drill string 10. ln both embodiments shown in Figs. 4 and 5 the cable 4 is in a stretched condition such that the force meter 8 detects that the tension of the cable 4 is at or above a force threshold and such that a safety device allows the rod holder 6 to be opened. ln the shown embodiments the safety device 12, typically in the form of a hydraulic control valve, is configured to obstruct the possibility to accidentally open the rod holder 6, and/or 11 to make sure that the cable 4 is in a prescribed stretched condition when the rod holder 6 is opened. ln the embodiment shown in Fig. 4, a first control line 18a is provided from the force meter 8 to a control unit 11, the force meter 8 being arranged on the mast 2 between the winch 3 and the sheave wheel 9 arranged at the top of the mast 2. A second control line 18b is arranged from the control unit 11 to the control valve that constitutes the safety device 12 in this embodiment. ln accordance with this embodiment a first signal indicating the detected tension in the cable is sent in the first control line 18a to the control unit 11, wherein the detected tension is compared to a threshold tension or a threshold force. lf the detected tension is at or above a threshold tension or force, a signal verifying that the drill string 10 is not solely carried by the rod holder 6 may be issued to the safety device 12, i.e. the control valve, which is then positioned in the shown open position such that the hydraulic line 17 is open towards a rod holder arrangement 24. However, the opening of the safety device 12 does not automatically open the rod holder 6. ln addition, a command to open the rod holder 6 needs to be issued, e.g. by the operator, and received at the rod holder 6. ln an alternative embodiment the rod holder 6 could be electrically operated, wherein the safety device 12 could include a circuit breaker preventing the rod holder 6 to be opened when the circuit breaker is in a non-connected position.

For illustrative reasons, the rod holder arrangement 24 is shown separate from the holder 6, although the rod holder 6 of course is part of the rod holder arrangement 24. Clamps 14 are included in the rod holder arrangement 24 and should be construed as active parts of the holder 6 and are configured to clamp a drill rod of the drill string 10. The clamps 14 of the illustrated embodiment of the rod holder arrangement 24 are activated by means of a pressure chamber 16 which is connected to a pressure source (not shown) via a hydraulic line 17. The safety device 12 is arranged on the hydraulic line 17, between the safety device 12 and the pressure source. A spring 15 is configured to force the clamps 14 into a closed position as long as the pressure chamber 16 is not pressurised. ln order to open the rod holder 6, i.e. to force the clamps 14 apart, pressurised hydraulic fluid needs to be provided into the pressure chamber 16 so as to overcome the action of the spring 15. The spring 15 is preferably an air spring but may also be a mechanical spring.

Hence, when the safety device 12 is in the shown open position the clamps 14 may be forced apart by activation of a controller 23, e.g. on a joystick or the like in an operator's 12 cab (not shown), so as to allow pressurised hydraulic fluid to enter the pressure Chamber 16. As may be understood from the drawing, it will not be possible to pressurise the pressure chamber 16 unless the safety device 12 is in the open position. As a consequence, in specific embodiments, the rod holder 6 may only be opened when the force meter has detected a force indicating that the drill string is not solely carried by the rod holder, for example when the cable 4 is stretched such that its tension is above a threshold value. This is advantageous as it makes sure that the rod holder 6 may not be accidentally opened due to an operator error, and specifically that the cable 4 is always correctly stretched when the rod holder 6 is opened to achieve a soft hand over of the weight of the drill string 10 to the cable 4, from the rod holder 6 to the hoist system, including the cable 2, the sheave wheel 9, and the winch 3, or alternatively to the rotation unit 7.

Another embodiment of the safety arrangement is shown in Fig. 5, in which a force meter is comprised in the winch 3 and/or at the sheave wheel 9 of the hoist system. ln this embodiment a first control line 19a is provided from the force meter, which may be integrated in the winch 3, to the control unit 11, and/or an alternative first control line 19b is provided from a force meter, which may be arranged at the sheave wheel 9, to the control unit 11. A second control line 19c is arranged from the control unit 11 to the control valve that constitutes the safety device 12. Hence, except for the force meters, this embodiment is similar to the embodiment of Fig. 4, and in both illustrations the cable 4 is sufficiently stretched for the safety device 12 to be in its open position, allowing the rod holder 6 to be opened.

An alternative embodiment is shown in Fig. 6, in which no control unit is arranged. lnstead, a control line 25 is arranged directly from the force meter. The control line 25 may either be an electric control line or a hydraulic control line providing a control pressure to the safety device 12. ln the embodiment shown in Fig. 6 the cable 4 is not stretched and the safety device 12 is in its closed position. Specially, the hoisting plug 5 for connecting the cable 4 to the drill string 10 is disconnected and about to be connected to a drill rod 22 to be added to the drill string.

Preferably, in the closed position, the safety device 12 includes a non-return valve allowing the pressure member to empty but blocking pressurised hydraulic fluid from the hydraulic pressure line.

An alternative embodiment of the drill rig 1 is shown in Fig. 7, in which no hoist system is arranged. Hence no cable is arranged to be connected to the drill string 10, lnstead, the 13 drill string is either held by the rotation unit 7 or by the rod holder 6. This embodiment typically corresponds to a drill rig with where the drill string 10 is driven by so called top- drive, or of the type know in the art as pull with a head. ln the shown embodiment the rotation unit 7 is positioned near the top of the mast 2, ready to drive the drill string 10 further down the drill hole by a simultaneous rotation and axial driving of the rotation unit 7 downwards alongside the mast 2. ln specific embodiments, the rotation unit 7 may be complemented by a percussion unit to provide a striking action to the drill string 10 when drilling is performed. ln this embodiment the weight acting on the rod holder 6 may not be evaluated or detected by means of a detected tension in the cable, because no cable is arranged. Hence, the confirmation that that the drill string 10 is not solely carried by the rod holder 6 is to be provided in an alternative manner. ln the shown embodiment a force meter 8 may be arranged at or in the rod holder 6 to detect the weight of the drill string 10 acting on the rod holder 6 directly. When, and only when, the detected weight is at or below a force threshold that indicates that the drill string 10 is also carried by another entity and hence not solely by the rod holder 6 will it be possible to open the rod holder 6.

Of course, in an embodiment where no hoist system or alternative carrying arrangement is arranged, the drill rod 10 is carried by the rotation unit 7 when it is not held by the rod holder. Typically, the handover of the weight of the drill string 10, which may be up to 18 tons and more, from the rod holder 6 to the rotation unit 7 is actualised at two different occasions. Firstly, as is illustrated in Fig. 7, after the addition of a section of one or more drill rods, the rod holder 6 will be opened to allow the rotation unit 7 to drive the drill string further down into the ground.

Secondly, when the drill string 10 is retrieved from the drill hole, it is firstly pulled up to the position shown in Fig. 7. Subsequently, a section of one or more drill rods is unthreaded from the drill string 10 by means of the rotation unit 7 as the remainder of the drill string is held by the rod holder 6. The section is then removed and the rotation unit 7 is moved downwards to a position close to the rod holder 6 where a new firm grip is engaged between the rotation unit 7 and the top of the drill string 10. ln that position, when the rotation unit 7 firmly grips the top of the drill string 10, the rod holder 6 may be opened to allow the rotation unit 7 to pull the drill string 10 further upwards from the drill hole, to release a new section of drill rods. 14 ln accordance with the invention the rod holder 6 may not be opened unless it has been detected that the drill string 10 is carried not only by the rod holder 6. ln this case this implies that the drill string 10 should also be carried by the rotation unit 7 before the rod holder may be released. A verification may be produced in response to a detected weight on the rotation unit 7, a reduced Weight acting on the rod holder 6, or a verification confirming that the rotation unit 7 is firmly engaged with a drill rod of the drill string 10 to carry the weight thereof. Typically, the rotation unit 7 includes chucks that may be moved in and out of engagement with a drill rod of the drill string 10. However, in a top drive application the rotation unit is arranged at the top of the drill string 10 and includes a connector part configured to engage the uppermost thread of the drill string. ln embodiments of the invention, it may be possible to detect the Weight carried by the rotation unit 7 in order to verify that the drill string 10 is carried not only by the rod holder 6. This may e.g. be achieved by slightly lifting the rotation unit 7 after accomplished engagement with the drill string 10, e.g. by a threaded engagement at the top of the drill string or by activation of the chucks of the rotation unit 7, and by a subsequent detecting of the weight carried by the rotation unit 7. When a hydraulic cylinder is utilised to move the rotation unit 7 the weight may be indirectly detected by a measurement of the pressure of the hydraulic fluid in the pressure cylinder. However, it is also possible to provide a load cell to detect and/or measure the carried weight.

A typical method of operating a drill rig 1 on which a safety arrangement in accordance with the invention is arranged will now be described. ln an initial step of the method the rod holder 6 holds a drill string 10.

Before releasing the rod holder 6, it is verified that the drill string 10 is not solely carried by the rod holder 6. ln embodiments where a cable 4 is used it may be verified that the cable is attached to a drill string 10 orthat the drill string is carried by the rotation unit 7. This may be achieved by detecting that the tension in the cable 4, or the weight acting on the rotation unit 7, respectively, is at or above a force threshold. The rod holder 6 is prevented from being released unless a signal verifying that the drill string 10 is not solely carried by the rod holder 6 has been received at the safety device 1. For a cable embodiment the rod holder 6 is prevented from being released unless the tension in the cable 4 is at or above the force threshold.

The rod holder 6 is released (opened) if a verification that the drill string 10 is not solely carried by the rod holder 6 has been received at the safety device, and a command to release the rod holder 6 is issued by an operator.

The signal verifying that the drill string 10 is not solely carried by the rod holder 6 is issued in response to that a detected parameter is above and/or below threshold. The threshold is of course adapted to the parameter that is to be detected. The parameter may e.g. be one of the following: a force acting on the cable, if a cable is arranged, the weight (force) acting on the rod holder 6, the weight (force) acting on the rotation unit 7.

Also, the relevant parameter may be directly or indirectly detected.

The threshold may be an upper and/or a lower limit a limit depending on nature of the parameter. The object of the parameter is to confirm/verify that the drill string 10 is not solely carried by the rod holder 6. This may hence be verified by verifying that the weight acting on the rod holder is below a certain threshold, or that the weight carried by a unit other than the rod holder is above a certain threshold. Such a unit may e.g. be a cable or a rotation unit.

A relevant limit of the acceptable threshold may be updated during a drilling operation based on a previously detected parameter, a directly or indirectly measured weight of the drill string 10 and/or an estimated weight based on an expected length of the drill string 10. For instance, if more and more sections of drill rods are added to the drill string it will be expected that the weight of the drill string will increase, and if drill rods are removed from the drill string it will be expected that the weight of the drill string will decrease, which may hence be reflected by an increased or decreased limit value. ln embodiments with a hoist arrangement a step of attaching a cable to the drill string 10 may be comprised, wherein the verifying that the drill string 10 is not solely carried by the rod holder 6 may achieved by detecting a tension in the cable and releasing the rod holder only in response to that the detected tension in the cable is at or above a force threshold that represents a lower limit of the acceptable range.

As is illustrated above this may be achieved with a force meter 8 arranged in different manners. ln addition to the above a force meter in the form of an indirect force meter may be arranged to indirectly detect the weight carried by the cable 4. Such an indirect force meter may be comprised in the motor 13 as illustrated in Fig. 1, wherein the indirect force may be detected by monitoring a hydraulic pressure if the motor 13 is a hydraulic motor, or if the motor 13 is an electric motor, by measuring the output power of the electric motor. 16 A control unit 11 may be arranged to receive signals representing the detected tension and to compare the detected tension with the force threshold that the tension in the cable should be at or above to allow the rod holder to be released.

Also, the force threshold may be updated during a drilling operation so as to correspond to an expected weight of the drill string attached to the cable.

The force threshold may preferably be updated based on a previously detected tension in the cable, a measured weight of the drill string attached to the cable and/or an estimated weight based on the length of the drill string. Also, the control unit may be configured to compensate force threshold based on the drilling angle. lf the drilling is close to 45 degrees or more the force may be slightly less than the corresponding weight based on that part of the weight may rest on the walls of the inclined bore.

The safety arrangement is hence adapted to releasing the rod holder in response to an activation order, e.g. from an operator, if, and only if it is verified that the drill string 10 is not solely carried by the rod holder 6, e.g. in that a detected force is at or above a force threshold, or that a force acting on the rod holder 6 is measured to be below a force threshold.

Above, the invention has been described with reference to specific embodiments. The invention is however not limited to these embodiments. lt is obvious to a person skilled in the art that other embodiments are possible within the scope of the following claims.

Claims (1)

1.A safety arrangement on a drill rig (1), the safety arrangement being arranged at a mast (2) of the drill rig (1) for ensuring safe handling of a drill string (10), wherein the drill rig comprises: - a rotation unit (7) for holding and rotating the drill string (10) and driving the same during a dri||ing operation, - a rod holder (6) for holding a drill rod of the drill string (10) and keeping the drill string (10) fixed in between dri||ing operations, the rod holder (6) being arranged at a lower portion of the mast (2) and being operable between an open mode and a closed mode in which it holds the drill rod of the drill string (10), characterised in that the safety arrangement comprises at least one force meter (8) for detecting a carried weight of the drill string (10), and a safety device (12) configured to allow opening of the rod holder (6) only if the force meter (8) has detected that the drill string (10) is not solely carried by the rod holder (6). The safety arrangement according to claim 1, wherein a force meter (8) is configured to detect the weight of the drill string (10) carried by the rod holder (6), and wherein the safety device (12) allows opening of the rod holder (6) in response to that said force meter (8) detects that the weight of the drill string (10) carried by the rod holder (6) is at or below a force threshold indicating that the drill string (10) is not solely carried by the rod holder (6). The safety arrangement according to claim 1 or 2, wherein a cable (4) is arranged to be attached to the drill string (10), a winch (3) being arranged for pulling and releasing the cable (4), and wherein a force meter (8) is configured to detect the weight of the drill string (10) carried by the by the cable (4), and wherein the safety device (12) allows opening of the rod holder (6) in response to that said force meter (8) detects that the cable (4) carries a weight over a threshold value, thereby indicating that weight of the drill string (10) is at least partly carried by the cable (4). The safety arrangement according to claim 3, wherein the force meter (8) configured to detect the weight of the drill string (10) carried by the by the cable (4) is configured to detect the tension in the cable (4), and wherein the safety device (12) allows opening of the rod holder (6) in response to that said force meter (8) detects that the tension in the cable (4) is above a threshold value indicating that weight of the drill string (10) is at least partly carried by the cable (4). The safety arrangement according to claim 3 or 4, wherein the winch (3) is driven by a hydraulic motor or an electric motor and wherein one force meter (8) of the at least one force meter (8) is an indirect force meter configured to indirectly detect the weight carried by the cable (4) by detecting a hydraulic pressure of the hydraulic motor or an electric power of the electric motor. The safety arrangement according to anyone of the preceding claims, wherein a force meter (8) is configured to detect the weight of the drill string (10) carried by the rotation unit (7), and wherein the safety device (12) allows opening of the rod holder (6) in response to that said force meter (8) detects that the force detected by said force meter (8) is above a threshold value indicating that the weight of the drill string (10) is at least partly carried by the rotation unit (7). The safety arrangement according to anyone of the preceding claims, wherein the holder (6) is hydraulically operated and wherein the safety device (12) involves a control valve, which in a closed mode blocks the provision of hydraulic fluid to the rod holder (6), thereby blocking two opposed clamps (14) of the rod holder (6) in a closed mode in which the clamps (14) are configured to hold the drill string (10). The safety arrangement according to anyone of the preceding claims, wherein a control unit (11) is comprised in the safety arrangement and is configured to control the safety device (12) in response to that a carried weight detected by the at least one force meter (8) is above a threshold indicating that the weight of the drill string (10) is at least partly carried by the cable (4) or by the rotation unit (7), or below a threshold indicating that the weight of the drill string (10) carried by the rod holder (6) is at or below a force threshold. The safety arrangement according to claim 8, wherein the control unit (11) is configured to the specific threshold during a drilling operation based on a previously detected carried weight, a directly or indirectly measured weight of the drill string (10) and/or an estimated weight based on an expected length of the drill string (10). The safety arrangement according to claim 8 or 9, wherein the control unit (11) is configured to control the rod holder (6) automatically and wherein the rod holder (6) may only be released from the closed mode to the open mode in response to that a carried weight detected by the at least one force meter (8) is above or below the specific threshold. A drill rig (1) characterised in that it comprises a safety arrangement according to anyone of the preceding claims arranged at a mast (2) of the drill rig (1 ). A method of safely handling a drill string during a drilling operation, the method comprising: - holding a drill string (10) with a rod holder (6), - before releasing the rod holder, controlling that the drill string (10) is not solely carried by the rod holder (6) by detecting a carried weight of the drill string (10), - allowing the rod holder (6) to be released only if it has been detected that the drill string (10) is not solely carried by the rod holder (6). The method according to claim 12, wherein a detection that the drill string (10) is not solely carried by the rod holder (6) includes detecting that the weight of the drill string (10) carried by the rod holder (6) is at or below a force threshold indicating that the drill string (10) is not solely carried by the rod holder (6). The method according to claim 12 or 13, further comprising a step of attaching a cable (4) to the drill string (10), wherein the verifying that the drill string (10) is not solely carried by the rod holder (6) is achieved by detecting a tension in the cable (4), and/or by detecting a force acting on the rotation unit (7) and releasing the rod holder (6) only in response to that the detected tension in the cable (4) and/or the force acting on the rotation unit (7) is at or above a force threshold. The method according to claim 13 or 14, wherein the force threshold is updated during a drilling operation based on a previously detected parameter, a directly or indirectly measured weight of the drill string (10) and/or an estimated weight based on an expected length of the drill string (10).