BACKGROUND TO THE INVENTION Drill rods are lengths of hollow pipe which have a male thread at one end and a female thread at the other end. Drill rods range in size up to about ten metres long and a plurality of drill rods are assembled in forming a drill string by engaging the male thread of an upper drill rod with the female thread of an adjacent lower drill rod or vice versa. The lower end of a drill string typically culminates in a cutting head or the like which drills through the ground when rotated. In operation, the upper end of the drill string is rotated by a drive mechanism mounted on a drilling rig from which the drill string downwardly projects. A drill string may project vertically from a drill rig or at a desired angle to vertical. Drill strings are commonly many hundreds of metres long and can be thousands of metres long.

The disconnecting of drill rods is commonly referred to as breaking and is often performed manually by an operator using a pair of stilsons. Such an operation is inherently dangerous with injuries to operators an ever present possibility and there have been attempts to address these safety issues by use of breakout wrenches.

US patent no. 4194419 teaches a power actuated breakout wrench which has two opposed jaws for clamping a drill rod therebetween. The jaws are pivotally mounted together and the clamping force is provided by rotating one jaw towards the other under action of a hydraulic ram. The clamping force is dependent upon the force exerted by the hydraulic ram and, in breaking the threaded joint, the hydraulic ram maintains the clamping force while both jaws rotate about the longitudinal axis of the drill rod under the action of another hydraulic ram.

US patent no. 5537900 also teaches a power actuated breakout wrench. The breakout wrench has a pair of

opposed jaws which are connected by a toggle linkage which is driven by a hydraulic ram. The toggle linkage acts to multiply the clamping force exerted by the hydraulic ram but, as in the case of US patent no. 4194419, the hydraulic ram maintains the clamping force during breaking of the threaded joint.

The breakout wrenches of US patent nos. 4194419 and 5537900 suffer from the disadvantage that the clamping force is not proportional to the torque required for breaking the threaded joint. Consequently, if the clamping force is insufficient, the jaws will slip about the drill rod and if the clamping force is excessive, the drill rod or the breakout wrench may be damaged. A further disadvantage of the breakout wrenches of US patent nos. 4194419 and 5537900 is that limited rotation (typically only 5° to 100) of the drill rod is possible after the thread is broken. A maximum amount of rotational movement is desirable when drill rods are connected via tapered threads.

SUMMARY OF THE INVENTION The present invention provides an apparatus for disconnecting a drill rod from a drill string, the apparatus being arranged to be mounted to a drilling rig and the apparatus including a body having an opening arranged to loosely receive the drill rod and a plurality of jaws arranged to exert a clamping force on the drill rod, at least one of the jaws being arranged to increase the clamping force exerted on the drill rod as force is exerted on the body to rotate the body substantially about the longitudinal axis of the drill rod.

Preferably, said at least one of the jaws is pivotally mounted to the body adjacent the opening and, more preferably, said at least one of the jaws has a drill rod engaging surface which is gradually curved such that it is cam-like.

Advantageously, the apparatus of the present invention can be smaller than prior art breakout wrenches

with the result that it can be more readily retro-fitted to a greater range of drill rigs.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a plan view of an apparatus according to the preferred embodiment of the present invention mounted to a drilling rig, and Figures 2-9 are views similar to Figure 1 illustrating operation of the apparatus of Figure 1 in moving through various positions.

BEST MODE FOR CARRYING OUT THE INVENTION Referring firstly to Figure 1, a breakout wrench 10 generally comprises a body 11 having a fixed jaw 12 and a movable jaw 14, a breakout wrench support plate 15, a body hydraulic ram 16, a movable jaw hydraulic ram 18 and a positioning hydraulic ram 20.

The body 11 is formed of spaced apart upper and lower plates of substantially the same shape which are positioned on top of one another as viewed in Figure 1, which are joined along their corresponding edges, and which may be internally reinforced for added rigidity.

The body 11 is thus substantially hollow but it is to be noted that it could be formed from solid material. The body 11 has two ends, namely a first end 22 and a second end 24, which are rigidly connected together. The first end 22 of the body 11 has a bush 26 by which the body 11 is connected to hydraulic ram 16 while the second end 24 comprises a fixed jaw 12 which is located adjacent an arcuate opening 28. The opening 28 is sized to receive a drill rod 30. The internal dimensions of the opening 28 are such that grip pads in the form of tungsten carbide inserts 34 which are attached to the internal surface of the opening 28 can contact an external surface 32 of the drill rod 30.

A gap 36 in a portion of the side wall of the second end 24 of the body 11 enables the body 11 to receive the

movable jaw 14 which is located by a pin 38. The pin 38 passes through aligned holes in the movable jaw 14 and the upper and lower plates of the body 11.

The breakout wrench 10 is fixed with respect to a drill rig 42 via the breakout wrench support plate 15 and hydraulic ram 16. Attachment of the hydraulic ram 16 to the drill rig 42 is provided by body hydraulic ram gusset plates 44 which are attached to the drill rig 42 and which have holes for receiving corresponding pins 46 which are attached to the hydraulic ram 16.

Attachment of the movable end 48 of the hydraulic ram 16 to the first end 22 of the body 11 is achieved by way of an assembly of the bush 26 located between an upper plate 52 and a lower plate (not shown). The plates 52 and (not shown) extend from and are integrally formed with the movable end 48 of the hydraulic ram 16. A pin 54 passes through holes in the plates 52 and (not shown) and locates the bush 26 relative to the plates 52 and (not shown).

The movable end 56 and fixed end 58 of the positioning hydraulic ram 20 are formed with bushes 60 and 62 respectively for attachment to the drill rig 42 and body hydraulic ram 16 respectively. An upper gusset plate 64 and a lower gusset plate (not shown) extend from the drill rig and receive the bush 60 therebetween with a pin 66 locating the bush 60 relative to holes in the upper gusset plate 64 and lower gusset plate (not shown).

Attachment of the bush 62 to the body hydraulic ram 16 is similarly achieved with the bush 62 inserted between an upper gusset plate 68 and a lower gusset plate (not shown) and held relative thereto by a pin 70 which passes through holes in the upper gusset plate 68 and lower gusset plate (not shown).

Contraction of the ram 20 results in movement of the movable end 48 of the hydraulic ram 16 from the position as shown in Figure 1 through the position shown in Figure 2 to that shown in Figure 3.

The lower plate of the body 11 has a downwardly projecting lug 72 which projects into a slot 80 in the breakout wrench support plate 15. Movement of the body 11 from the position illustrated in Figure 1 through that illustrated in Figures 2 and 3 is guided by interaction between the lug 72 and the slot 80. In moving between the position illustrated in Figure 1 where the body 11 is disengaged from the drill rod 30 to that illustrated in Figure 3 where the body 11 is engaged with the drill rod 30, the body 11 moves through the position illustrated in Figure 2 in which a tip 74 of fixed jaw 12 meets the drill rod 30. The size and shape of the opening 28 is such that the body 11 is self-guided into engagement with the drill rod 30. That is to say that movement of the body 11 between the positions illustrated in Figures 1 and 3 need not be guided exclusively by interaction between the lug 72 and slot 80.

Clamping of the drill rod 30 between the fixed jaw 12 and movable jaw 14 is achieved by moving the movable jaw 14 from the unclamped position as shown in Figures 1- 3 to the clamped positioned as shown in Figure 4.

Movement of the movable jaw between the unclamped and clamped positions is effected by the hydraulic ram 18.

The fixed end 82 of the hydraulic ram 18 is attached to the body 11 while the movable end 84 of the hydraulic ram 18 is attached to the movable jaw 14. Upper gusset plate 86 and lower gusset plate (not shown) extend from the body 11 such that a bush (not shown) which is attached to the fixed end 82 of the hydraulic ram 18 is received therebetween. The upper gusset plate 86 and the lower gusset plate (not shown) have holes which are aligned with the bush (not shown) and are located relative to the bush (not shown) by a pin 88.

The movable jaw 14 has a scalloped-out portion 90 for receiving the movable end 84 of the hydraulic ram 18 which is located relative to the movable jaw 14 by a bush 91 and a pin 92 which passes through the bush 91 and a hole in the movable jaw 14.

Contraction of the hydraulic ram 18 results in the rotation of the movable jaw 14 about the pin 38 from the position as shown in Figures 1 to 3 to that shown in Figure 4 where the drill rod 30 is clamped between the tungsten carbide inserts 34 of the fixed jaw 12 and a tungsten carbide insert 93 attached to the movable jaw 14.

Following the clamping of the drill rod 30, extension of the hydraulic ram 16 (with hydraulic ram 20 floating) results in the rotation of the first end 22 of the body 11 substantially about the longitudinal axis of the drill rod 30 from the position shown in Figure 4 to that shown in Figure 5 such that the drill rod 30 becomes threadingly disengaged from an adjacent lower drill rod (not shown) to which it was threadingly attached. That is to say that the joint between the drill rods has been broken or, put another way, the thread has been cracked.

A portion 95 (see Figures 4 and 5) of the movable jaw 14 is gradually curved such that a clamping surface of the movable jaw 14 is cam-like. Counter-clockwise rotation (as viewed in Figures 4 and 5) of the body 11 results in the cam-like surface tightening against the drill rod 30. Counter-clockwise rotation (as viewed in Figures 4 and 5) of the movable jaw 14 about its central pivot pin 38 continues only until the drill rod 30 is clamped between the grip pads 34 and 93 of fixed jaw 12 and movable jaw 14 respectively with sufficient force to threadingly disengage the drill rod 30 from the adjacent lower drill rod (not shown). As such, the clamping force provided by the movable jaw 14 increases until the resulting friction between the tungsten carbide inserts 34 and 93 and outside surface 32 of the drill rod 30 is sufficient to transmit the necessary torque to threadingly disengage the drill rod 30.

It is to be noted that hydraulic ram 18 does not act to provide a clamping force between fixed jaw 12 and movable jaw 14 sufficient to hold drill rod 30 during cracking of the thread. Rather, hydraulic ram 18 moves

movable jaw 14 to a position where the drill rod 30 is loosely clamped between fixed jaw 12 and movable jaw 14 with the clamping force then increasing as body 11 is rotated. The function of hydraulic ram 18 is thus merely to move movable jaw 14 into contact with drill rod 30 and hydraulic ram 18 could be replaced by, for example, a spring.

The slot 80 is arranged to enable the lug 72 to move relative to the support plate 15 during movement of the body 11 between the position illustrated in Figures 4 and 5. However, interaction between the lug 72 and the slot 80 in no way guides movement of the body 11 between the positions illustrated in Figures 4 and 5. That is to say that the lug 72 does not contact an edge of slot 80 at any time during movement between the positions illustrated in Figures 4 and 5.

Extension, contraction, remaining stationary and floating of the hydraulic rams 16, 18 and 20 are controlled by logic hydraulic sequence cartridges such that the necessary hydraulic rams are actuated in the appropriate sequence to effect a required movement and such that the hydraulic rams are never working against one another.

A full cycle of operation of the breakout wrench 10 involves movement: (A) from the disengaged, unclamped, rod threaded position of Figure 1 to the engaged, unclamped, rod threaded position of Figure 3; (B) from the engaged, unclamped, rod threaded position of Figure 3 to the engaged, clamped, rod threaded position of Figure 4; (C) from the engaged, clamped, rod threaded position of Figure 4 to the engaged, clamped, thread cracked position of Figure 5; (D) from the engaged, clamped, thread cracked position of Figure 5 to the engaged, unclamped, thread cracked position of Figure 6;

(E) from the engaged, unclamped, thread cracked position of Figure 6 to the engaged, unclamped, thread cracked, and readied for disengagement position of Figure 7; (F) from the engaged, unclamped, thread cracked, and readied for disengagement of Figure 7 to the disengaged, unclamped, thread cracked position of Figure 9.

To provide the aforementioned movements (A-F), the hydraulic rams 16, 18 and 20 are either contracting (c), extending (e), floating (f) or stationary (s) as shown in Table 1. A | B C D | E F 16 s s e s c s 18 s c s e S S 20 c s f f f e Table 1: Operation of Rams 16, 18 and 20 for Movement A-F It is to be noted that Figures 7, 8 and 9 are identical to Figures 3, 2 and 1 respectively except that in Figures 1, 2 and 3 the drill rod 30 is threaded to the adjacent lower drill rod (not shown); whereas, in Figures 7, 8 and 9 the thread between the drill rod 30 and the adjacent lower drill rod (not shown) has been cracked.