OVERSHOT FOR DOWN HOLE DRILLING OPERATIONS FIELD OF THE INVENTION

This invention relates to an overshot for bore hole drilling operations. An overshot is a down hole tool used for retrieving, inter alia, an inner tube of a drill string from a bore hole. BACKGROUND ART

A typical prior art overshot consists of a body portion having a swivel at its upper end which is connected to a wireline rope. A jar staff connects the body portion to a head portion which has pivotally mounted lifting dogs that are spring biased to a latching position in which they are adapted to engage the lower end of the inner tube.

The overshot is lowered down the bore hole, inside the drill rods, on a wireline rope until the lifting dogs couple to a spear point on the latch body of the inner tube. The wireline is then retrieved by a winch drum to bring the inner tube to the surface.

In the event of loss of circulation fluid in the hole, a dry lowering sleeve is placed on the head of the overshot above the lifting dogs. The dry lowering sleeve has an elongated slice along and around its periphery to allow access over the wireline rope. When the tube reaches support for its weight, which may be its position in the core barrel or a water level, the shape below the spear point causes the lifting dogs to open on the spear point and thus close on the upper side of the , pivot pin. The dry lowering sleeve descends, holding the upper side closed and conversely the bottom side of the dogs open thus releasing the tube. 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), the dry lowering sleeve is lowered over the wireline, taped up and dropped down the

hole. Sometimes the dry lowering sleeve arrives in place to release the tube and sometimes it doesn't. Even if the dry lowering sleeve arrives in place, it can take a lot of time depending on the hole depth. In the event that the dry lowering sleeve does not arrive in place it presents a difficult retrieval problem. 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 first jar staff engaged with the upper end of the lower end portion, a second jar staff slidably mounted on the first jar staff and adapted to be connected by its upper end to a wireline rope, a lower drive key on the lower end portion, an upper drive key on the second jar staff and a drive sleeve rotatable and slidable along the second jar staff, the drive sleeve having a first array of teeth adapted to engage and be rotated by the lower drive key upon downward movement of the drive sleeve, and a second array of teeth spaced from the first array of teeth adapted to engage and be rotated by the upper drive key upon upward movement of the drive sleeve and wherein the lower end of the drive sleeve has a slot which when aligned with the lower drive key by the rotation of the drive sleeve allows the lower end of the drive sleeve to engage the upper end of the lifting dogs so as to open the lower end of the lifting dogs. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional view of an overshot according to one embodiment of the invention, Fig. 2 is an exploded cross-sectional view of the overshot shown in Fig. 1 ,

Fig. 3 is a cross sectional view of the locking mechanism in the lower part of the overshot, and

Fig. 4 is a front elevational view of the locking mechanism shown in Fig. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The overshot 10 shown in the drawings has a lifting dog body 11 at its lower end which carries a pair of lifting dogs 12 which will be described in detail below. On the outside of the upper end 13 of the lifting dog body 11 there is a lower drive key 14. The overshot 10 also includes an internal jar staff 15, a lower sleeve 16, an external jar staff 17 and an upper sleeve 18. The lower sleeve 16 and upper sleeve 18 constitute a drive sleeve which is rotatable and slidable along the external jar staff 17. The lower end 19 of the internal jar staff 15 has a threaded portion 20 which engages into a correspondingly threaded portion 21 in recess 22 at the upper end 13 of the dog body 11. A lock screw 23 engages the threaded portion 20 to prevent rotation of the internal jar staff 15 with respect to the lifting dog body 11.

At the upper end of the internal jar staff 15 there is an enlarged head 24 which abuts against the internal shoulder 25 of the external jar staff 17 whereby the internal jar staff 15 is held captive by the external jar staff 17. Along the external face of the internal jar staff 15 there is a guide 26 which slides in a guide way 26a in the lower end of the external jar staff 17.

The lower sleeve 16 has an array of teeth 27 around its lower periphery and a slot 28 in between adjacent teeth 27. The teeth 27 sequentially engage the lower drive key 14 and in so doing rotate the lower sleeve 16 one half of the pitch between adjacent teeth 27. The upper end 38 of the lower sleeve 16 has a threaded recess 39 which receives the correspondingly threaded lower

end portion 37 of the upper sleeve 18. The lower and upper sleeves 16 and 18 are secured together by lock nut 29.

Around the lower periphery of the upper sleeve 18 there is an array of teeth 30 which are sequentially engaged by an upper drive key 31 on the external jar staff 17 and in so doing rotate the lower sleeve 16 one half of the pitch between adjacent teeth 30. The pitch of the teeth 27 is the same as the pitch of the teeth 30 so that when the external jar staff 17 is raised and lowered the lower sleeve 16 is rotated by the pitch of the teeth.

When the lower sleeve 16 has been sufficiently rotated, the slot 28 aligns with the lower drive key 14 on the lifting dog body 11 so that the lower sleeve 16 can pass over the upper ends of the dogs 12 to latch open the lower ends of the dogs 12. The number of lifts of the external jar staff 17 leading to the latching of the dogs 12 can be initially set by rotating the lower sleeve 16 so that the drive key 16 is spaced from the slot 28 by the required number of teeth (see the markings on the teeth 27 in Fig. 1) which represents the required number of up and down movements of the external jar staff 17.

Within the lifting dog body 11 there is a locking mechanism 40 which includes a lock pin 41. The locking mechanism 40 is shown in detail in Figs. 3 and 4. The locking pin 41 has a slot 43 for receiving the blade of a screwdriver so that it may be rotated between the horizontal locked position shown in Fig. 4 or the vertical unlocked position shown in dotted outline in Fig. 4. The locking pin 41 has a locking arm 42 located between the upper ends 44 of the dogs 45.

The overshot is lowered into a bore hole in the same way as when lowering a dry lowering sleeve. By working the external jar staff 17 up and down, the required number of times, the overshot unlatches with the tube in place or supported by fluid. When the slot 43 in the push-in pin 41 is in the vertical position the dogs 45 are unlatched. By putting a screwdriver blade into

the slot 43, pushing in against spring 46 and turning 90° and releasing, the locking arm 42 is turned into position to lock the dogs 45 in the hold position thereby ensuring that the tube is not accidentally released.

By turning the arm 42 90° back to the vertical position, the dogs 45 are unlocked and the tube can be released from the dogs.

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