WO1992015767A1

WO1992015767A1 - Method for retrieving well casings using an internal gripping device

Info

Publication number: WO1992015767A1
Application number: PCT/GB1992/000408
Authority: WO
Inventors: Hector Mcdonald; Ian Alexander Gillies
Original assignee: Enterra (U.K.) Limited; Pioneer Oil Tools Limited
Priority date: 1991-03-08
Filing date: 1992-03-09
Publication date: 1992-09-17
Also published as: AU1415192A; GB9104912D0

Abstract

A method of retrieving well casing from a cased well bore, said method comprising the steps of deploying a tool assembly which includes a casing grapple and a casing cutter into the bore of the casing in the well substantially to a depth at which the casing is to be cut and thereat actuating said casing cutter to sever the casing, withdrawing said tool assembly until said grapple is adjacent an upper end of the casing and thereat actuating said grapple to grapple the now-severed casing, and utilising said grapple to withdraw the severed casing from the well bore. The hereby used grapple is anchored to the casing in a first step by displacing sleeve (46) by means of a drop ball (58), said sleeve wedging pushrods (52) outwardly to bring a part of the gripping jaws (28) into gripping engagement; in a second step, drill string and thus intermediate grapple body (12) move upwardly thus wedging (34) all of the jaws (28) into gripping engagement.

Description

METHOD FOR RETRIEVING WELL CASINGS USING AN INTERNAL GRIPPING DEVICE

This invention relates to grapples, and relates more particularly but not exclusively to grapples for grappling well casings. The invention also relates to methods of retrieving casings from wells.

When a hydrocarbon well becomes redundant, it is preferable (and may be mandatory) to recover as much of the well casing as is practicable. Once the well-head is removed, prior art casing recovery procedures may involve the lowering of a drill string comprising a casing grapple above a rotary casing cutter. The cutter is rotated, and the severed casing is lifted by means of the deployed string. Prior art casing grapples employed in such operations have externally toothed casing grippers normally standing proud of the grapple and rubbing on the casing bore during downward deployment of the grapple towards the bottom of the casing section to be retrieved, resulting in friction, wear, and delays in deployment. Moreover, such prior art grapples are not readily detachable at will from the casing bore to which they are initially anchored.

In addition, this prior art method of recovering casing is time-consuming and awkward because the drill string extends into the casing along the full length of the severed casing, and the string has to be stripped from the casing at the rotary table after the casing has been raised to the drill floor.

According to a first aspect of the present invention there is provided a method of retrieving well casing from a cased well bore, said method comprising the steps of deploying a tool assembly which includes a casing grapple and a casing cutter into the bore of the casing in the well substantially to a depth at which the casing is to be cut and thereat actuating said casing cutter to sever the casing, withdrawing said tool assembly until said grapple is adjacent an upper end of the casing and thereat actuating said grapple to grapple the now-severed casing, and utilising said grapple to withdraw the severed casing from the well bore.

Said tool assembly is preferably deployed on a string comprising a plurality of mutually detachably coupled string sections, said method further comprising the step of detaching surplus string sections from said string during withdrawal of said tool assembly. In this way the total weight to be lifted during withdrawal of the severed casing from the well bore by use of the grapple may include the weight of only a sufficient plurality of string sections to extend from the drill floor to substantially adjacent the upper end of the severed casing.

Said method preferably includes the step of maintaining said grapple substantially free of the grappling contact with the casing bore during deployment and withdrawal of said tool assembly thereby to minimise friction and wear of the grapple prior to said actuation of the grapple.

According to a second aspect of the present invention there is provided a grapple for selectively grappling the bore of well casing, said grapple comprising a grapple body whose maximum cross-sectional dimensions are less than the dimensions of the casing bore, a plurality of casing-grappling jaws each coupled to said grapple body, said jaws being radially extensible of said body and lying within the maximum cross-sectional dimensions of said grapple body prior to casing-grappling actuation of said grapple, said grapple further comprising primary jaw actuation means and secondary jaw actuation means, said primary jaw actuation means being operable to radially extend at least one of said jaws from said body into initial casing-grappling contact with the casing whereupon operation of said secondary jaw actuation means radially locks said jaws into full casing-grappling contact with said casing to anchor said grapple to said casing.

Said jaws are preferably arranged for conjoint movement axially of said grapple body.

Said primary jaw actuation means preferably comprises a sleeve axially slidable within a hollow axial bore in said grapple body, said sleeve being coupled to said at least one jaw such that movement of said sleeve from a first position towards a second position of said sleeve within said bore causes said radial extension of said at least one jaw from said body into initial casing-grappling contact with said casing. Said primary jaw actuation means is preferably such that hydraulic pressure selectively applied to said sleeve induces said downward movement of said sleeve from said first position towards said second position.

Said sleeve preferably comprises a drop-ball seating thereon whereby the seating of a drop-ball on said seating on said sleeve enables the build-up of hydraulic pressure in hydraulic fluid above the seated drop-ball to induce downward movement of said sleeve. Said drop-ball seating on said sleeve is preferably provided with a hydraulic bypass to cause or permit a relatively slow decay of said built-up hydraulic pressure.

Said sleeve preferably comprises at least one wedging surface thereon, said at least one wedging surface having a lesser radius with respect to the sleeve axis at a lower position on said wedging surface and a greater radius with respect to the sleeve axis at a higher position on said wedging surface, said primary jaw actuation means further comprising at least one pushrod movable radially with respect to said grapple body, said pushrod having a radially inner end coupling a pushrod-contacting zone on said at least one wedging surface to said at least one jaw, whereby said induced downward movement of said sleeve causes said wedging surface to present a pushrod-contacting zone of progressively increasing radius, with respect to the sleeve axis, to the radially inner end of said pushrod and thus move said pushrod radially outwards of said grapple body to cause corresponding radially outward movement of said at least one jaw.

Said secondary jaw actuation means preferably comprises an externally conical wedging surface on and preferably substantially coaxial with said grapple body, said externally conical wedging surface having a progressively increasing radius with respect to the cone axis from an upper end of said externally conical wedging surface towards a lower end thereof, said externally conical wedging surface contacting all of said jaws during actuation of said secondary jaw actuation means by relative movement of said jaws with respect to said body axially of said grapple in a direction which raises said body with respect to said jaws, whereby said initial casing-grappling contact of said at least one jaw with the bore of said casing inhibits movement of said at least one jaw axially of said casing to enable said actuation of said secondary jaw actuation means by lifting said grapple body to induce said relative movement of said jaws with respect to said body axially of said grapple and to cause said externally conical wedging surface to wedge all of said jaws into radially extended casing-grappling contact with the bore of said casing.

Said externally conical wedging surface may be substituted by a plurality of mechanically eguivalent wedging surfaces.

Said plurality of jaws may be formed as a like plurality of externally serrated surfaces on the free end or ends of one or a plurality of fingers cantilevered from a common ring member disposed circu ferentially around said grapple body and axially movable with respect thereto. Said jaws are preferably biased in a direction tending to lift said jaws with respect to said conical wedging surface to tend to relieve the radially outward wedging effects thereof. Said lifting bias is preferably applied to said jaws by a compression spring means compressed between said common ring member and an abutment on said grapple body.

In the context of a downhole tool coupled to a hollow string wherein the tool is hydraulically pressurisable by hydraulic fluid in said string and wherein pressuration of the tool is initiated by means of a drop-ball dropped down said string to land upon and substantially block hydraulic fluid through a drop-ball seating member, there is also provided a method of draining hydraulic fluid from said string after deployment of a drop-ball, said method comprising the step of providing said seating member with a bypass for bypassing hydraulic fluid around the blocked seating member.

Said bypass is preferably dimensioned to permit hydraulic flow therethrough at a relatively low rate compared to the rate at which the hollow string can be filled with hydraulic fluid to cause pressurisation of said tool following deployment of the drop-ball. In one form, said bypass may comprise a simple drilling linking the upstream and downstream sides of said seating member.

The provision of a bypass not only enables the automatic gradual depressurisation of the tool, but also draining of the string through the bypass. This reduces the weight to be lifted when retrieving the tool to the surface since the column of fluid filling the string above an unbypassed drop-ball would have a substantial weight. A further advantage in draining of the string consists in obviation or minimisation of fluid spillage when the retrieved string is being dismantled at the drill floor by uncoupling of the sections of which it is composed.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings wherein:-

Fig. 1 is a half-sectioned longitudinal elevation of a first embodiment of well casing grapple in accordance with the present invention; Fig. 2 is a transverse cross-section of the first embodiment, taken in the line II - II in Fig l; Fig. 3 is a transverse cross-section of the first embodiment, taken on the line III - III in Fig. 1; Fig. 4 is a transverse cross-section of the first embodiment, taken on the line IV - IV in Fig. 1; and Fig. 5 is a longitudinal sectional elevation of the right half of a second embodiment of well casing grapple in accordance with the present invention.

Referring first of Fig. 1, a well casing grapple 10 comprises a generally cylindrical grapple body 12 having a hollow upper body section 14, a hollow intermediate body section 16, and a hollow lower body section 18. The upper body section 14 is joined to the top of the intermediate body section 16 by means of a cylindrical thread connection 20 secured by a cotter assembly 22 (also shown in Fig. 2). The lower body section 18 is jointed to the bottom of the intermediate body section 16 by means of a taper thread connection 24.

The intermediate body section 16 carries a generally cylindrical jaw assembly 26 comprising six externally serrated arcuate jaws 28 (also shown in Fig. 4) equi-angularly spaced around the assembly 26. Each jaw 28 is individually mounted on the free end of a pair of axially extending fingers 30 (also shown in Fig. 3) cantilevered from a common ring member 32 encircling the upper end of the jaw assembly 26. The material of which the jaw assembly 26 is formed is such that the fingers 30 are resilient to an extent allowing a certain radial outward displacement of each jaw 28, as will be detailed below. The jaw assembly 26 is preferably formed as an initially tubular article, turned or ground to requisite radial dimensions, and longitudinally slotted to separate individual jaws and fingers.

The radially inner surfaces of the jaws 28 are shaped to define a generally conical surface overlying and substantially dimensionally matching an externally conical wedging surface 34 formed near the lower end of the intermediate body section 16. The wedging cone formed by the surface 34 points upwards within the grapple body 12. The conical wedging surface 34 is circumferentially interrupted by six radially projecting buttresses 36 (also shown in Fig. 4) which are equi-angularly spaced around the longitudinal axis of the grapple body 12. The radially outer edges 38 of of the buttresses 26 define the maximum diameter of the grapple 10. With the jaw assembly 26 in its uppermost position on the grapple body 12 (as shown in Fig. 1), the jaws 28, and in particular their external serrations, lie radially beneath the radially outer edges 38 of the buttresses 36. At all times, the jaws 28 are individually disposed between circumferentially adjacent pairs of the buttresses 36 (as most clearly shown in Fig. 4), such that while the buttresses 36 leave the jaws 28 radially and axially unconstrained, the buttresses 36 constrain circumferential motion of the jaws 28 around the grapple body 12, and so inhibit twisting or skewing that might damage the jaw-mounting fingers 30.

At their lower end the jaws 28 bear against a shoulder provided on stop keys 39 secured to the intermediate body section 16. Three stop keys 39 are provided, equispaced around the tool. At the upper end of the jaw assembly 26, the common ring member 32 overlies a flanged ring 40 to which it is secured by circumferentially-distributed radially extending cap screws 42 (only one being shown). The radially inner surface of the ring 40 supports the upper end of the jaw assembly 26 for sliding movement axially along the intermediate body section 16. A coiled compression spring 44 (omitted from Fig. 3) is compressed between the underside of the ring 40 and a thrust ring 43 which bears against the upper ends of the jaws 28. The engaging faces of the thrust ring 43 and the jaws 28 are inclined to resist radially outward movement of the jaws 28. The lower end of the thrust ring 43 is castellated to provide notches (not visible) such that the thrust ring 43 normally rests over the upper ends of the buttresses 36 with a small axial clearance therefrom. However, if the jaws 28 are prematurely dragged downwards over the conical wedging surface 34 (for example by debris between the jaws 28 and the inner surface of the casing) , the thrust ring 43 descends slightly into contact with the upper ends of the buttresses 36 since this downward movement of the jaws 28 is transmitted to the thrust ring 43 by the spring 44. The resultant compression of the spring 44 tends to restore the jaws 28 to their pre-deploy ent axially upwards positions and thus inhibits premature engagement of the jaws 28 with the casing due to such downward drag.

The bore of the hollow intermediate body section 16 is counter-bored near its lower end (immediately above the connection 24) to accommodate a sleeve 46 which is coaxially slidable within the intermediate body section 16 between an upper position as shown in Fig. 1 and a lower position (not illustrated) which is defined by abutment of the lower end of the sleeve 46 with the upper end of the lower body section 18. A coiled compression spring 48 is fitted within a counter-bore in the upper end of the bore of the hollow lower body section 18 to bias the sleeve 46 to its upper position (as shown in Fig. 1).

A central part of the peripheral surface of the sleeve 46 is formed as a conical wedging surface 50 having a lesser radius with respect to the sleeve axis towards its lower end and a greater radius towards its upper end. Three of the six jaws 28 are coupled to the conical wedging surface 50 of the sleeve 46 by means of a respective pushrod 52 (see particularly Fig. 4) each of which is held within a respective radial bore 54 in the intermediate body section 16 for purely radial sliding movement therein. Each pushrod is provided adjacent a respective one of the stop keys 39. The inner end of each pushrod 52 bears against a respective pushrod-contacting zone on the conical wedging surface 50, while the outer end of each pushrod 52 bears against the inner side of the respective one of the three jaws 28.

The upper rim of the axial bore of the sleeve 46 is chamfered to form a drop-ball seating 56 to accommodate a flow-blocking drop-ball 58 (shown in chain-dash outline in Fig. 1). An angled drilling 60 extending between radially outer and inner surfaces of the sleeve 46 hydraulically bypasses the drop-ball seating 56. The peripheral surface of the sleeve 46 is slidingly sealed to the sleeve-mounting counterbore in the lower end of the intermediate body section 16 by means of a circumferential seal 62 located between the bypass drilling 60 and the wedging surface 50, and also by means of a further circumferential seal 64 below the surface 50.

The upper end of the upper body section 14 is formed as a standard tapered female thread connector (or "box") 66 for connection of the grapple 10 to the lower end of a hollow drill string (not shown). The lower end of the lower body section 18 is formed as a standard tapered male thread connector (or "pin") 68 for connection of the grapple 10 either to the upper end of a further section of hollow drill string (not shown) or directly to the upper end of a casing cutter (not shown) .

The hollow through bores in the body sections 14, 16, and 18, together with the hollow through bore in the sleeve 46 provide a continuous and relatively low flow resistance passage to hydraulic fluid (eg drilling mud) flowing down through the grapple body 12 between the top connector 66 and the bottom connector 68, in the X ..

absence of a drop-ball on the seating 56.

Operation of the grapple 10 to retrieve well casing from a well will now be described.

As a preliminary stage of operation, a suitably-sized tool assembly is made up of the grapple 10 (Figs 1 - 4) coupled by means of the bottom connector 68 to a known form rotary casing cutter tool (not shown), the coupling being either direct or through the intermediary of a short length of hollow drill string (not shown) .

This made-up tool assembly is then coupled by means of the top connector 66 to the lower end of a hollow drill string (not shown) and deployed down the bore of the well casing in the well, with requisite further sections of hollow drill string being coupled to the top of the already-deployed string during lowering of the tool assembly, until the cutter reaches the depth at which the casing is intended to be cut for retrieval of the casing above the cut. During lowering, both the cutter and the grapple 10 are maintained in inactive status. Since the radially outer edges 38 of the buttresses 36 define the maximum diameter of the grapple body 12 (selected to be a clearance fit in the casing bore) and since the serrated outer faces of the jaws 28 lie radially beneath these radially outer edges 38 while the grapple 10 is inactive (ie in non-grappling configuration) , the presence of the grapple 10 in the tool assembly does not impede lowering of the tool assembly by rubbing contact of grapple jaws with the casing bore (as in the prior art) , and the absence of such rubbing contact precludes the premature jaw wear of the prior art grapples. When the tool assembly has been lowered to the selected depth within the well casing, cutter blades are radially extended, and the string is then rotated to cause the extended cutter blades totally to sever the well casing.

During severing of the casing by means of the rotary cutter, the grapple 10 is still maintained inactive and free of grappling contact with the casing. This avoids impeding rotation of the casing cutter, and avoids the need for providing a rotary connection passing through the grapple (as in the prior art).

Centrifugal force tending to move the jaws 28 radially outwardly during operation of the cutters is resisted by engagement of the inclined faces of jaws 28 and the thrust ring 43.

Still without actuating the grapple 10, the tool assembly is lifted up the bore of the now-severed casing until the grapple 10 is near the upper end of the casing, eg about 30 feet (about 9.2 meters) or one standard length of casing section below the top of the casing. Surplus sections of the deployed drill string will have been detached in stages during the lifting operation, such that the length and hence the weight of suspended drill string is minimal at the end of the lifting operation (in comparison to the length and hence to the weight of drill string that had to be suspended during cutting of the casing) .

During lifting, the action of debris within the casing tending to move the jaw assembly 26 axially downwards relative to the wedging surface 34, which would cause premature actuation of the jaws 26, is prevented by the abutment of the jaws 26 with the stop keys 39.

When the tool assembly lifting has proceeded to the aforementioned point at which the grapple 10 is near the upper end of the casing, lifting is halted and the grapple 10 is actuated in the manner now to be detailed.

A drop-ball of a small enough diameter to pass down the bore of the remaining sections of drill string and through the bore of the upper and intermediate body sections 14 and 16, but of a large enough diameter to seat securely on the drop-ball seating 56, ie a diameter about that of the drop-ball 58 shown in chain-dash outline in Fig. 1, is deployed down the bore of the drill string to land on the seating 56. The procedure blocks hydraulic flow down through the grapple body 12 except for the limited flow continuing to be permitted by the bypass 60. The drill string bore is now hydraulically pressurised by pumping hydraulic mud (or any other suitable hydraulic fluid) into the drill string to pressurise the interior of the grapple body 12 above the seated drop-ball 58.

Such pressurisation drives the sleeve 46 (carrying the seated drop-ball 58) downwards within the grapple body 12 against the upward bias of the spring 48. This downward movement of the sleeve 46 from its upper position (as shown in Fig. 1) presents pushrod-contacting zones on the conical wedging surface 50 of progressively increasing radius with respect to the axis of the sleeve 46, and so forces the pushrods 52 to move progressively radially outwards from the central axis of the grapple body 12. The radially-slidable pushrods 52 transfer this outwards movement to the three of the six jaws 28 whose inner surfaces are contacted by a respective one of the outer ends of the pushrods 52. The resilient jaw-mounting fingers 30 in the jaw assembly 26 allow these three jaws 28 to move radially outwards of the grapple body 12, past the outer edges 38 of the buttresses 36, and the stop keys 39 and into firm contact with the bore of the casing encircling the grapple 10. The jaw assembly 26 is now in an initial stage of casing-grappling contact with the basing bore, and the jaw assembly 26 is significantly restrained from axial movement relative to the casing.

Following this primary actuation of the jaw assembly 26, casing-grappling contact with the casing bore is completed by a secondary jaw actuation, undertaken by applying a lifting force to the grapple body 12 through the intermediary of the remaining length of drill string. This lifting force causes the grapple body 12 to move axially upwards with respect to the jaw assembly 26 which remains immobile with respect to the casing following the preceding primary jaw actuation. The conical wedging surface 34 on the intermediate body section 16 is thereby moved axially upwards beneath each of the jaws 28, and so each of the six jaws 28 is moved radially outwards of the grapple body 12 (to the extent that they have not been so moved by the primary actuation) and into full casing-grappling contact with the bore of the casing.

The casing is now fully grappled, and in normal operation, pressurisation is now terminated while maintaining lift on the drill string. This provides a check on proper grappling of the casing. (If the grapple is not securely gripping the casing, then the X D

grappling procedure is repeated) . The casing can now be lifted to bring the first casing joint above the rig workdeck, whereupon lifting is temporarily halted, the casing is scotched beneath the joint by applying slips, and the top casing section is removed in the conventional manner. By releasing the grapple 10 (by depressurisation (if not already depressurised as would be the case in normal operation) and dropping the entire grapple to allow the springs 44 and 48 to reset the jaw assembly 26) and regrappling the severed casing about one casing section length further down, and by repeating the associated casing recovery steps, the severed casing can be progressively raised to the rig and dismantled section by section.

A substantial advantage of this casing retrieval method of the present invention, in comparison to prior art casing retrieval methods, lies in the step of grappling the severed casing adjacent its upper end rather than at its lower end, such that a substantially lesser weight of drill string has to be lifted and the drill string can be dismantled at the drill floor before the casing is lifted. This enables very great casing lengths to be retrieved in one trip, as compared to typical prior art methods.

The first embodiment of grapple (Figs 1-4) is proportioned for grappling well casing of a relatively small internal diameter (specifically 9 5/8 inches or about 244 millimetres), whereas the second embodiment (Fig. 5) is proportioned for grappling well casing of a larger internal diameter (specifically 13 3/8 inches or about 340 millimetres) . The second embodiment is structually and functionally equivalent to the first embodiment, and differs only in certain dimensions and other relatively minor details. Accordingly the following description will concentrate on those parts of the second embodiment which differ significantly from identical or equivalent parts of the first embodiment. For a description of any part of the second embodiment not given below, reference should be made to the foregoing description of the corresponding part of the first embodiment. In Fig. 5, which is a longitudinal section of the right half of the second embodiment and corresponds to the right half of Fig. 1, reference numerals applied to components or parts thereof which are identical or equivalent to like parts in the first embodiment are given the same reference numerals as are applied in Figs 1-4 but preceded by a "1" (ie the second embodiment reference numerals are the first embodiment reference numerals plus 100).

Referring now to Fig. 5, the second embodiment of grapple 110 has a maximum diameter of its body 112 , defined by the radially outer edges 138 of the buttresses 136, which is appropriately larger than the equivalent dimension in Figs 1 and 4. The jaws 128 of the jaw assembly 126 also lie on an appropriately larger diameter, and are made radially thicker for strain resistance appropriate to their greater circumferential width. The pushrods 152 each have a length which is greater than the length of the pushrods 52 in the first embodiment by the increase in grapple radius, but in all other essential respects the primary and secondary jaw actuating arrangements of the second embodiment are identical to those of the first embodiment.

In the form of the second embodiment shown in Fig. 5, parts equivalent to the stop keys 39 and the thrust ring 43 of the first embodiment are omitted, but could be included if desired.

In one other respect the second embodiment differs from the first embodiment in more than simple dimensional increases, and this concerns the upper end of the jaw assembly 126. In the first embodiment (Fig. 1) the upper end of the jaw assembly 26 was in two parts (the common ring member 32 and the ring 40) in order to allow the spring 44 to be fitted. However, because in the second embodiment (Fig. 5) the radially innermost surfaces of the jaws 128 (corresponding to the conical wedging surface 134) have a minimum radius substantially exceeding the maximum radius of the spring 144 (identical to the spring 44 and overlying a portion of the intermediate body section 116 having an external diameter substantially identical to the external diameter of the equivalent portion of the intermediate body section 16), the spring 144 can be assembled into the grapple 110 without requiring the overlying part of the top of the jaw assembly 126 to be detachable, and accordingly in the second embodiment, the common ring member 132 has a radial thickness to be slidable directly on the intermediate body section 116.

Operation of the grapple 110 is identical to the operation of the grapple 10 as previously described. Some alternatives to and modifications of the above-described arrangements are possible without affecting the principles of the invention. For example, a number of casing-grappling jaws other than six could be employed, and/or a number of pushrods other than three (so long as at least one jaw could provide a sufficient initial grip on the casing bore upon primary jaw actuation) One or more of the various conical wedging surfaces could be replaced by a respective plurality of individual wedging surfaces each working on a respective one of the jaws or pushrods, as the case may be. A downhole tool assembly could be utilised in which the casing grapple was under the casing cutter, provided the cutter could be actuated to sever the casing at a selected location without simultaneously actuating the associated grapple.

Other modifications and variations are possible without departing from the scope of the invention.

Claims

1. A method of retrieving well casing from a cased well bore, said method comprising the steps of deploying a tool assembly which includes a casing grapple and a casing cutter into the bore of the casing in the well substantially to a depth at which the casing is to be cut and thereat actuating said casing cutter to sever the casing, withdrawing said tool assembly until said grapple is adjacent an upper end of the casing and thereat actuating said grapple to grapple the now-severed casing, and utilising said grapple to withdraw the severed casing from the well bore.

2. A method according to Claim 1, wherein said tool assembly is deployed on a string comprising a plurality of mutually detachably coupled string sections, said method further comprising the step of detaching surplus string sections from said string during withdrawal of said tool assembly.

3. A grapple for selectively grappling the bore of well casing, said grapple comprising a grapple body whose maximum cross-sectional dimensions are less than the dimensions of the casing bore, a plurality of casing-grappling jaws each coupled to said grapple body, said jaws being radially extensible of said body and lying within the maximum cross-sectional dimensions of said grapple body prior to casing-grappling actuation of said grapple, said grapple further comprising primary jaw actuation means and secondary jaw actuation means, said primary jaw actuation means being operable to radially extend at least one of said jaws from said body into initial casing-grappling contact with the casing whereupon operation of said secondary jaw actuation means radially locks said jaws into full casing-grappling contact with said casing to anchor said grapple to said casing.

4. A grapple according to Claim 3, wherein said grapple actuation means comprises a sleeve axially slidable within a hollow axial bore in said grapple body, said sleeve being coupled to said at least one jaw such that movement of said sleeve from a first position towards a second position of said sleeve within said bore causes said radial extension of said at least one jaw from said body into initial casing-grappling contact with said casing.

5. A grapple according to Claim 4 wherein said primary jaw actuation means is such that hydraulic pressure selectively applied to said sleeve induces said downward movement of said sleeve from said first position towards said second position.

6. A grapple according to Claim 4 or 5, wherein said sleeve comprises a drop-ball seating thereon whereby the seating of a drop-ball on said seating on said sleeve enables the build-up of hydraulic pressure in hydraulic fluid above the seated drop-ball to induce downward movement of said sleeve.

7. A grapple according to Claim 6, wherein said drop-ball seating is provided with a hydraulic bypass to cause or permit a relatively slow decay of said built-up hydraulic pressure.

8. A grapple according to Claim 5, 6 or 7, wherein said sleeve comprises at least one wedging surface thereon, said at least one wedging surface having a lesser radius with respect to the sleeve axis at a lower position on said wedging surface and a greater radius with respect to the sleeve axis at a higher position on said wedging surface, said primary jaw actuation means further comprising at least one pushrod movable radially with respect to said grapple body, said pushrod having a radially inner end coupling a pushrod-contacting zone on said at least one wedging surface to said at least one jaw, whereby said induced downward movement of said sleeve causes said wedging surface to present a pushrod-contacting zone of progressively increasing radius, with respect to the sleeve axis, to the radially inner end of said pushrod and thus move said pushrod radially outwards of said grapple body to cause corresponding radially outward movement of said at least one jaw.

9. A grapple according to any one of Claims 3 to 8, wherein said secondary jaw actuation means comprises an externally conical wedging surface on and preferably substantially coaxial with said grapple body, said externally conical wedging surface having a progressively increasing radius with respect to the cone axis from an upper end of said externally conical wedging surface towards a lower end thereof, said externally conical wedging surface contacting all of said jaws during actuation of said secondary jaw actuation means by relative movement of said jaws with respect to said body axially of said grapple in a direction which raises said body with respect to said jaws, whereby said initial casing-grappling contact of said at least one jaw with the bore of said casing inhibits movement of said at least one jaw axially of said casing to enable said actuation of said secondary jaw actuation means by lifting said grapple body to induce said relative movement of said jaws with respect to said body axially of said grapple and to cause said externally conical wedging surface to wedge all of said jaws into radially extended casing-grappling contact with the bore of said casing.

10. A grapple according to any one of Claims 3 to 9, wherein said jaws are provided as a plurality of externally serrated surfaces on the free end or ends of one or a plurality of fingers cantilevered from a common ring member disposed circumferentially around said grapple body and axially movable with respect thereto.