UNDERGROUND UNLATCH OVERSHOT FIELD OF THE INVENTION

This invention relates to an underground unlatch overshot for use in the drilling of upward, horizontal, or flat angled boreholes, predominantly in underground mines. BACKGROUND ART

Typically, in these non-downward bore holes a normal surface overshot will not function due to the pull of gravity.

Therefore underground overshots are pumped into location to latch onto the back end of an article such as an inner tube. The overshot and inner tube are retrieved via a wireline rope.

In the event of a tube becoming stuck in the core barrel for whatever reason (and there are a lot of possible ways for that to happen), it would be advantageous to be able to unlatch the overshot and retrieve it from the rod string without breaking or cutting the wireline. It is an object of this invention to provide such an underground overshot. SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided an overshot tool for bore hole operations comprising a lower end portion including lifting dogs for engaging an article such as a tube in a bore hole, a swivel shaft adapted to be connected by its upper end to a wireline rope, a pressure seat shaft located between the lower end portion and the swivel shaft, a lower drive key on the lower end portion, an upper drive key on the pressure seat shaft, a lower rotator rotatable and slidable along the lower end portion, the lower rotator having a first array of teeth adapted to engage and be rotated by the lower drive key upon movement of the lower rotator, and an upper rotator having a second array of teeth adapted to engage and be rotated by the upper

drive key upon movement of the upper rotator and wherein the lower end of the lower rotator has a slot which when aligned with the lower drive key by the rotation of the lower rotator allows the lower end of the bottom rotator to engage the upper end of the lifting dogs so as to open the lower end of the lifting dogs to release the article.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an exploded cross-sectional view of an overshot according to one embodiment of the invention.

The overshot 10 shown in the drawings has a locator body 11 at its lower end which carries a pair of latch dogs 12. The locator body 11 has a boss 14 which receives a latch locator 15 that carries a bottom drive key 16 which, in this instance, has three teeth 17 adapted to engage the teeth 18 at the lower end of the lower rotator 19.

A key connecting shaft 20 passes through the lower rotator 19 and is screwed into the upper end of the stem 21 of the locator body 11. A large spring 22 is located within the upper end of the lower locator 19 and seats on shoulder 23. The key connecting shaft 20 has a shoulder 24 against which seats small spring 25 that is located on the stem 26 of the key connecting shaft 20. The upper end of the stem 26 is threaded into the lower end of the pressure seat shaft 27. A centre drive key 28 mounted on the lower end of the pressure seat shaft 27 has teeth 29 adapted to engage the teeth 30 on the lower end of the upper rotator 31.

A spring locating bush 32 located on the top of the small spring 35 seats within the threaded cap 32 that is located within the lower end of the centre housing 34. Above the upper rotator 31 there is a first washer 35, seal 36, a second washer 37 and an adjusting nut 38. A water flow shaft 39 has an upper

stem 40 that receives a connector 41 and swivel shaft 42 rotatabie within the thrust bearing 43 that is held captive by the iock nut 44.

The lower end of the lower rotator 19 has a slot 19a which is adapted to slide over the drive key 17 when the slot is aligned thereto. In essence, therefore, the invention provides an overshot tool 10 for bore hole operations has a swivel shaft 42 at its upper end for connecting the overshot 10 to a wireline rope, a locator body 11 at its lower end which has a pair of lifting dogs 12 for engaging an article in the bore hole, a pressure seat shaft 27 between the locator body 11 and the swivel shaft 42, a lower rotator 19 slidable and rotatabie with respect to the locator body 11 having a slot 19a, an upper rotator 31 movable with the lower rotator 19 which has teeth 18 which engage a lower drive key 16 on the locator body 11.

The upper rotator 31 has teeth 30 which engage an upper drive key 28 on the pressure seat shaft 27. The lower rotator 19 and the upper rotator 31 form a sub-assembly which is rotated by the drive keys 16 and 28 upon upward and downward movement of the rotators 19 and 31 until the slot 19a is aligned with the lower drive key 16 which allows the lower end of the lower rotator to engage the upper end of the lifting dogs so as to open the lower end of the lifting dogs to release the article.

The underground unlatch overshot of the invention has been designed to allow the drill rig operator to unlatch the overshot from an inner tube should the inner tube become stuck.

By quickly and efficiently unlatching the overshot when the inner tube is stuck in the outer tube, the operator can drill another inch or so to try and free up the inner tube, or in the worst case, pull the rods without having to cut the wire line.

The up and down movement of the overshot is translated into rotational movement whereby the lower rotator 19 is rotated so that the slot 19a can slide over the lower drive key 16 putting pressure on the top of the lifting dogs 12 to unlatch the overshot 10 from the tube. The pump down features of the overshot are maintained and the adjustments and other action of the overshot all remain as before. The major change is the ability to unlatch at will by dialling up the number of strokes chosen to unlatch the unit. This is done by selecting the number of strokes desired after cocking the unit as per usual. The number of strokes is translated into the number of teeth 18 around the lower end of the lower rotator 19.

Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention.