GB2185279A - Well fishing tool - Google Patents

Abstract

A well fishing tool has a camming mechanism (34, 72) with a plurality of sequential catch and release positions whereby when the camming mechanism (34, 72) is in a first catch or first release position, longitudinal movement of the drill string causes rotary movement and results in engagement of at least one other catch or release position being obtained prior to moving to a different functional position. The mechanism can be applied to an overshot or to a spear. In a preferred arrangement there are four catch positions (112) and two release positions (114) which the tool can be sequentially cycled through. <IMAGE>

Description

SPECIFICATION Improved well tool The present invention relates to well tools for retrieving objects from wells, bore holes and the like. It will be understood that such tools are generally referred to in the field as "fishing tools" and the objects to be retrieved from the well are generally referred to as "fish".

There are two principle types of fishing tools used to retrieve fish from wells, bore holes and the like. One such tool is called an overshot which includes a pluralityofradiallymovablegrapplefingerswhich may be lowered around the fish, for example, a piece of drill pipe or a wire line sonde, and then the grapple fingers are contracted inwardlyto grip the fish by a surrounding sleeve which has inwardly deflecting surfaces. Anothertype of fishing tool used in the field is called a spear and has similarly the radially movable slips which may be lowered into the fish, for example, a length of casing orotherhollowtubularmemberand then expanded outwardly by an expander also having deflecting surfaces to grip the interior ofthe fish.

Such fishing tools have a camming mechanism which converts longitudinal movement into rotary movement to enable the fishing tool to catch the fish by rotating between a release and a catch position.

Typically, when the tool is lowered into the hote to engage the fish it is in a release position and upon the tool engagingthefish within thetool, in the case of an overshot, the longitudinal movement of the drill string is converted into rotary movementto move the camming mechanism from a release position to a switching position.When the tool is again pulled up a further longitudinal movement is converted by the camming mechanism into a rotary movement to cause a gripping action ofthe grapple fingers around the fish to effectively catch the tool and lock it in place.

US Patent No. 4124245 to Kuenzel discloses such a camming arrangementwherethe fishing tool when engaging the fish, cycles from a release position through a switch position to a catch position. If the tool does not engage it can be recycled through a switching step back to a release step and soon to catch to have anothertryat catching the fish. Although this arrangement is adequate for retrieving a fish from a well it has been found unexpectedly that it is often desirableto use a fishing tool to lower objects into wells either using an overshot in which case the object is retained within the tool, or using a spearwhich has an end which penetrates a hollow end of an object such as a casing.In such a situation it is most desirable thatwhen loweringthetool,shoulda restriction be encountered by the tool, the longitudinal movement ofthetool does not cause the camming mechanism to move from a catch to a release position otherwise the object carried by the fishing tool could be lost prior to the desired depth in the well. With the Kuenzel arrangement th is is not possible because during a descent two or even three serious restrictions may be encountered and, if so, abutment of the object carried may cause longitudinal movementofthetool string to be converted into a rotary movement within the tool and effect premature relese ofthe object.It will be appreciated the object would then have to be retrieved through a fishing sequence before it can be carried on to the desired level.This may happen manytimesand there is no guarantee that the tool will be retrieved.

It is therefore an object of the present invention to obviate or mitigate disadvantages associated with the aforesaidfishingtoolsandcamming mechanism.

Accordingly, in one aspect ofthe present invention there is provided a camming mechanism for use with awelltool,thecamming mechanism having a plurality of sequential catch stages followed by a plurality of sequential release stages whereby when the mechanism is in first catch or release position rotation ofthe camming mechanism results in the engagement of at least one further catch position or release position prior to moving to a differentfunctional position.

Accordingly, in another aspect ofthe present invention, there is provided a well tool having a camming mechanism with a plurality of sequential catch positions and a plurality ofsequential release positions whereby when said camming mechanism is in afirstcatch orafirst release position longitudinal movement of the drill string causing rotary movement ofthe camming mechanism results in engagement of at least one other catch position or release position being obtained priorto moving to a differentfunctional position.

Preferably also, the camming mechanism has first and second camming portions, wherein in the case of an overshot camming mechanism thefirst camming portion is an upper camming portion and is engaged to determine the catch or release position and the second camming portion is a lower portion which is engaged to cause the tool to rotate between a catch and release positions.

Alternatively, in the case of a speartheupper camming portion determines the rotation ofthe fishing tool between catch and release positions and the lower camming portion determines whetherthe tool is in a catch ora release position.

Preferably also, there are four catch and two release positionsinthefirstcamming portion and there area total of six engagement positions which the tool can be cycled through.

Preferably also, the first camming portion has first and second camming surfaces which are adapted to - be engaged to determine the catch or release position, andthefirstcammingsurfaceisdefined bya downwardly extending fingerwhich is adapted to be received by one ofsaid second camming surfaces, which are defined by one offour deep notches each adapted to receive the finger and define a catch position, or one of two shallow notches each adapted to receive a finger and to define a release position.

Preferably also, the second camming portion has lower first and second camming surfaces for engagementto determine a catch or a release position, said lowerfirst and second camming surfaces having a generally sawtooth configuration and being com plementary, each sawtooth having a recessed notch to prevent bottoming out bythe apex of a tooth ofthe opposite camming surface when said lower fi rst a nd second camming surfaces are engaged.

Preferablyalso,there are six teeth in each sawtooth configuration.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which :- Figs. (a) and 1 (b) are longitudinal cross-sections through an overshot in accordance with a first embodimentofthe invention; Fig. 2 is a developed view ofthe upper cam mechanism of the overshot; Fig. 3 is a developed view ofthe lower cam mechanism ofthe overshot; Fig. 4 is a perspective view ofthe camming mechanism ofthe overshot in the release position; Fig.5 is aviewsimilarto Fig. 4 of the overshot camming mechanism in the first switching position; Fig.6 is a view of the overshotcamming mechanism in the catch position;; Fig. 7 is a perspective view ofthe overshot camming mechanism with the mechanism in the second switching position; and Figs. 8(a) and (b) are longitudinal sectional views of a spear in accordance with an alternative embodiment ofthe arrangementwith the camming mechanism shown in Figs. 2 to 7 inverted.

Reference is first madeto Figs. (a) and 1 (b) ofthe drawings which illustrate a iongtudinal sectional view of an overshotgenerally indicated by reference numeral 20. The overshot 20 comprises firstand second elongate assemblies connected for relative telescopic movement. The first assembly, in this case the innerassembly, comprises awash pipe 22, the lower end of which is connected to a mandrel 24. The mandrel 24 has an upper portion of enlarged diameter and a lower portion 26 of reduced diameter.As shown in Fig. 1 (b) a sub 14 has a large diameter upper portion threadedly connected to the lower end ofthe mandrel 24 and a smaller diameter lower portion 26 threadedly connected to a carrier member 28, the latter also serving as a fish locator. The first assembly also comprises a first cam member 32 surrounding the reduced diameter portion of the mandrel 24 The first cam member comprises a main body portion 34 and a shorter annular portion 36 secured aboutthe lower end of portion 34 by a key 38.

An annularspacer40 is located typically between the upper end of portion 34 and the annular shoulder formed between the large and smaller diameter portions of mandrel 24. The similarspacer42 is fitted between the upper end of sub 28 and the lower ends of cam member portions 34 and 36.

Fish gripping means intheform of a grapple is also carried bythefirst elongate assembly. The grapple includes an upper sleeve 44 which generally surrounds the lower portion of sub 28 and carrier member 30 and includes a plurality of grapple fingers 46 which are integral with the sleeve 44 andwhich depend downwardly therefrom. An annular space 48 is formed between the grapple sleeve 44 and the carrier member 30 ofthe lower portion ofthe sub 28 on the other hand, and opposed axially facing shoulders 50 and 52 areformedon the grapple sleeve 44 and carrier30 respectively at opposite ends of annular space 48.A compression spring 54 is disposed in the annular space 48 and is confined between shoulders 50 and 52 to urge the grapple upwardly with respectto the carrier30. Such upward movement is limited by engagementofthe upper end 56 ofthe grapple sleeve 54 with the external annular shoulder 58 formed between the large and small diameter portions of sub 28. Thus it will be appreciated that the grapple is carried by the first assembly at its lower end while limited upward movement ofthe first assembly with respecttothe grapple or downward movement of the grapplewith respectto the first assembly is permitted by compression of spring 54.This latter movement is, however, limited by engagement of a shoulder 60 formed by the upper edge ofthe carrier 30 with the shoulder.

The second, or outer, elongate assembly of the tool 20 comprises an upperconnectorsub 62which has a threaded top portion 64 which maybethreadedly connected into a string of drill pipe andwhich slidingly receives the wash pipe 22 ofthefirst assembly. A sleeve 68 is threadedly connected to the sub 62 and extendsdownwardlytherefrom in a surrounding relationship to the upper parts ofthe first assembly.

Acompression spring 70 is received inthe annular space between the wash pipe 22 and sleeve 68 and bears against the lower end ofthe sub 62 and the upper end of sub 28to biasthefirstelongate assemblies in a first directional mode of telescopic movement. It will be understood that in the embodimenu shown in Figs. (a) and 1 (b) this first mode of telescopic movement includes downward movement ofthe first or inner assembly with respect to the outer or second assembly, and/or upward movement of the outer or second assembly with respect to the inner or first assembly. However, as later described the first mode may be reversed by suitable rearrangement of parts of the fishing tool.

Asecond cam member 72 is threadedly connected to the lower end of the sleeve 68 and generally telescopically receives the main body portion 34 ofthe first cam member 32. It will be understood that the first cam member 32 and cam member 72 comprise the camming mechanism of the fishing tool 20. A second sleeve 74 is threadedly connected to the lower end of cam member 72 and extends downwardlytherefrom.

The lower portion 76 of the sleeve 74 forms a "bowl" which serves as the deflecting means forthe grapple fingers 46.

It will be seen that each of the grapple fingers 46 has a plurality ofexternal downwardly and inwardly tapered surfaces 82 along its length separated by axially upwardly facing shoulders 84. The lower sleeve portion 76 has a plurality of matching internal downwardly and inwardly tapered surfaces 86 along its length separated by axially downwardlyfacing shoulders 88. The grapple fingers 46 are formed of spring metal and are biassed radially outwardly. Itwill be seen that upon downward movement of the lower portion 76 engagement of shoulders 84 and 88 will cause the grapple to move downwardly also with the lower portion 76.However, upon upward movement ofthe lower portion 76 ofthe grapple is retained in a fixed longitudinal position, the tapered surfaces 86 will, by their engagement with the matching surfaces 82,urgethegrapplefingers46radialyinwardly. Itwill also be seen thatthe inner surfaces 90 ofthe grapple fingers 46 are serrated to enable them to grip a fish surrounded by the fingers when they are urged inwardly as will be later described in more detail.

Theoperationofthecamming mechanism will now be described in more detail with reference to Figs. 2 to 7 as well as Figs. 1(a) and 1(b). Firstly, it will be understood that the second cam member 72 is a generally sleeve-like member having lower end surfaces machined to form a plurality of contiguous teeth 92 which are disposed circumferentially aboutthe second cam member. As best seen in Fig. 3, each tooth 92 has a longitudinally extending side 94 and a circumferentially extending side 96. The longitudinally extending side 94 and circumferentially extending side 96 defined recess notches 98 which are generally rectangular in shape.It will also be seen that portion 36 ofthefirstcam member32 has its upper end machined to form a series of contiguous teeth 100 similarly disposed circumferentially round the first cam member. Each tooth 100 has a longitudinally extending portion 102 and a circumferentially extending portion 104whichtogetherdefinea generally rectangular shaped notch 106 between adjacent teeth.

It will be seen that portion 36 is radially upsetfrom portion 34. Teeth 92 and 100 are generally com plimentaryand as seen in Figs. 3,4 and 6 are inclined inwardly from left to right four meshing in a manners shown in Fig. will be will be appreciated that although the term complementary used herein is meantto mean inclined in the same general direction, one ofthe complementary surfaces could be curved and the other straight or one could be shorter than the other.

The upper end of cam member portion 34 has a radially outwardly extending finger 108 as best seen in Fig. 4which has a tapered lower end 110 defined by converging surfaces 112 and 114. The upper end surface ofthe cam member72ismachinedtoform four deep notches 112 of which one is clearly shown in Figs. 4,5 and 6 and two shallow notches 114 as best seen in Fig. 2. It will be seen that the four deep notches 112 are disposed adjacent each other around part of the circumference of the member 72 and separated by thetwo shallow notches 114which are generally convergentsurfaces 116,1 18todefine notchesattheir apex indicated by reference numeral 120.

The shallow notches 114 are proportioned to receive the lower end V-shaped portion 110 of finger 108 when in a desired engaging position. As will be later described as the first member 34 is rotated and longitudinally moved the notch 108 will engage with a respective deep notch 112 or shallow notch 114to define either a respective catch or release position.

Referring now to Figs. 2 and 3 in conjunction with Fig. 4, it will be seen that the notch 108 is engaged in shallow notch 1 14to define a release position. In this release position the fishing tool may be lowered into the well bore until the lower end of the carrier member 30 strikes or locates the fish to be retrieved. This abutment preventsfurtherdownward movement of the first or inner assembly, thus upon exertion of further downward force in the tool the second or outer assembly moved downwardly with respectto the first or inner assembly. Such movement is defined as the second directional mode of telescopic movement.

Upon such second directional mode of telescopic movement, the cam member 72 moves downwardly with respect to cam member 32 with the effect that projections 110 are brought out of shallow notches 114 and the apex of teeth 100 abut surface 96 causing the inner assembly to rotate clockwise until teeth 192 are interengaged as shown in Fig. SIt will be seen in this position that the finger 108 is offset from the notch 114. Rotation is, of course, arrested when the sides 1 or2 and 98 abut. The notches 98 and 106 preventthe bottoming ofthe teeth when in this engaged position called a first switching position.

Similarly, the grapple 46 has also been caused to move downwardlywith the second assembly by virtue of abutment of shoulders 84 and 88 such movement being permitted by spring 54which isthus compressed and the upper end sleeve 44 of the grapple is separatedfrom shoulder58. Although not illustrated, it will be appreciated in this position that the grapple fingers are moved inwardly because of engagement between the axiallyfacing shoulders 84 and88to engage afish which is located within the lower sleeve portion 76.In this regard it will be appreciated that the serrated inner surfaces 90 of the grapplefingers46areconfiguredtogripasmooth wall pipe orsimilartype sonde, withoutthe necessity fora collar or other abutment on the pipe for engagement with the grapple fingers.

With the tool in the first switching position, an upward pull is now exerted on the second or outer assembly. The inner orfirst assembly is prevented from moving upwardly because of its own weight as well as by the force of spring 70. The first and second assemblies are now movable in theirfirsttelescopic mode. During such movement teeth 92 are raised out of engagement with the teeth 100 and convergent surface 114 engages with the apex 122 of upstanding finger portion to guide the finger 108to the notch 112 and simultaneously causing the inner memberto rotate so that a configuration ofthe camming mechanism as shown in Fig. 6 is achieved. The force of spring 70 also assists in carrying the finger 108 into the notch 112. This position is the catch position.

As the tool moves into the catch position the grapple fingers46moveupwardlywithinthe lowersleeve portion 46 allowing the spring 54to expand until the uppersurface56ofthegrappleabutstheshoulder58 whereupon further upward movement of the grapple is prevented so that upon further upward movement ofthe lower sleeve portion 76the grapple fingers 46 are urged radially inwardly into gripping engagement with the fish (not shown) by the interengagementof surfaces 84 and 88. The grapplefingers arethus thoroughly wedged between the fish and the lower sleeve portion 76and this wedging engagement when sufficientlytight, preventsfurthertelescopic move ment ofthe first and second assemblies in the first directional mode. Thus, continued upward pull on the second outer assembly causes the entire tool as well as the engaged fish to be raised o ut of th e well. twill be seen in Fig. 6thatthere is clearance between the bottom of the finger and the bottom ofthe notch 112 and thus bottoming out does not occur between the fingers and the notches under any normal circumst ances and confers no limitation on the tightness with which the grapple fingers can be wedged against the fish. What has been previously described is move mentfrom a release to a catch position. However, this is only necessa ry in the situation where the fish ing tool is used to retrieve a tool i - st in the well.

Itwill be understood thatthe fishing tool hereinbefore described with the camming mechanism can be used for lowering tools to specific locations within the well.This is the reason whyfourdeep notches are provided for catch positions and two shallow notches are provided for release positions. When lowering the fishing tool into the well with a probe attached thereto the tool will be in the catch position as shown in Fig. 6.

During descent, should the tool encounter an obstruction then the relative movement between the inner and outer assemblies will cause relative longitudinal movement which will be converted to rotary movement by the lower camming portion so that a second switching stage such as shown in Fig. 7 will occur wherethefingerhas been moved outofthe notch 112 and effectively rotated clockwise around the upper camming surface as shown in Fig. 2.It is highly desirablethatshould such an obstruction cause movementthen the tool will instantly be re-engaged and accordinglywhen afurtherupward pull on the toolinexertedthefingerl08isforcedtoengagethe next deep notch 112 in the clockwise direction around the circumference ofthe camming surface 72 thus the tool remains in the catch position during the descent.

Four deep notches are provided so that should at least three serious obstructions be encounteredthetool will remain in a catch position at the desired location.

In orderto releasethe tool the drill string is pushed down sothatthe camming mechanism enters the second switching stagesimilarto that shown in Fig. 5 and the tool is then pulled up so that the finger 108 engages shallow notch 114 as best seen in Fig. 4. In this position the grapple fingers are at their maximum radial diameterand the drill string can be pulled up to leave the object at the desired location in the well.

Should release not be completely effectual the proce dure can be repeated becausethe next notch is a further release position to ensurethattheobjectis clearly freed at a desired location. Should it be desired to retrieve the objectforany reason then this can simply be done by re-engaging the fishing tool with the object and fishing as in the normal sequence because the next position will be the catch position in which case the object can be retrieved if so desired.

Turning nowto Figs. (a) and 8 (b),these illustrate a spear in accordance with another embodiment ofthe invention. This type oftool is used to grip a large diameter conduit such as a piece of tubular casing and retrieve it from the well. Alternatively, the spear can be used to lower a large diameter tube into the well at a desired location. In the spear, the first of the elongate assemblies is the outer assembly and comprises an upper sleeve 130 rigidly connectedto a first cam member 132 which issubstantially identical to the cam member72 ofthe overshot except that its position is inverted so that its teeth 134 extend upwardly and that shallow and deep notches 136,138 respectively open inwardly.A second sleeve 140 is threadedly connected to the lower end of the cam member 132 and extends downwardlytherefrom.The gripping means is threadedly connected to the lower end of sleeve 140 and consists of an annular connector 142 having a largediameterupperportion and a reduced diameter lower portion 144 defining an external downwardly directed shoulder 48 for engaging the upper surface ofthetubular members such as casing. A plurality of slips 150 extend downwardly from theconnector 144 and the slips 150 are biassed radially inwardly but can be radially extended which will be explained in detail later.

The second or inner assembly ofthetool comprises an upper sub 152 forconnectingthe tool to a pipe string orthe like. The sub 154 having a reduced diameter upper portion and an enlarged lower portion 156.Asecondcam memberhaving a main body portion 158 and a short annular portion 160 secured to portion l58bykeyl62surroundsthe reduced diameter portion ofthe sub 154. An annular spacer 164 is disposed between the upper end of the cam member 132 and the sub 152, and a similarspacer is disposed between the lower end ofthe cam member 132 and a shoulder 164formed between the small and large diameter portions ofthe sub 154.It will be appreciated thatthe cam member 158 and other parts are substantially identical to the cam member 72 described in the first embodiment exceptforthe orientation. For example, it includes inwardly directed teeth 170 substantially identical to teeth 134 and configured to mesh therewith and upwardly extend ingfinger 172 which is proportioned and configured to be received in shallow notches 174 or deep notches 176. The shallow notches define a release position and the deep notches define with the finger notches 172 a catch position.

An expandercarrierl82 is threadedly connected to the lower end of sub 154 and extends downwardly between the slips 150. An annular space 184 is defined between the carrier 182 andthesleeve 140,and the compression spring 186 is disposed in this space to bear against the lower end ofthe sub 154 ofthe second assembly andthe upperend ofthe annularconnector 144 of a grouping means (which is connected to the first assembly) to urge the two assemblies towards a first mode of telescopic movement which includes upward movementofthe innerorsecond assembly and/or downward movement of the outer orfirst assembly. An expander 188 itthreadedly connected to the lower end of the carrier 146 and has an external frustoconical surface 190 which tapers upwardly and radially inwardly. Each ofthe slips 150 has a matching internal surface 192 tapered upwardly and radially inwardly and the slips also have serrated outer surfaces 194 for biting into the casing to be retrieved in orderto gripthe same.

Itwill be understood that Figs.8 (a ) and 8 (b} illustratethetool in running in or release position after the shouIder 148 has just come into abutmentwith an upper end of a piece ofcasing 196 to be retrieved. Slips 150 and expander 188 are thus disposed within the casing and a continued downward force exerted at this point causes a second or inner assembly to live downwardly, that is in a second mode oftelescopic movement, with respect to the first assembly. The first assembly cannot move downwardly due to its abutmentwith the casing 196. The first cam surfaces ofthe two assemblies defined by teeth 134 and 170 engage and cause the outer assemblyto rotate bringing the tool to its first switching position.

An upward force exerted on the inner assembly then causes the tool to move in its first telescopic mode augmented bythespring 186whereupon the second cam surfaces of the first assembly and the fingerl72furtherrotatestheouterassemblyguiding the fingers 172 into deep notches 138 which is the catch position. The expander 188 is thus committed to move upwardly with respect to the slips 150 wedging the slips between the expander and the casing, which can then be retrieved. Should the casing be lost, the tool can be passed to a second switching position to return to the release position and repeat the gripping process.

It will be understood that the camming mechanism shown in this embodiment hasfourcatch and two release positions as in thefirst embodiment of the overshot. This is becausethe spearcan beusedto lower pieces of casing at a desired location and should obstructions be encountered then the spear will move from a catch to another catch position so as to retain the casing thus three obstructions can be encountered and overcome before the tool is inadvertently released. Similarly, should the tool be in the release position readyto release the casing art a desired location andforsome reason itis not released then the tool can be cycled to a subsequent release position to facilitate proper release ofthe casing.It will be understood that operation and cycling ofthe camming mechanism is similarto that described with regard to the preferred embodiment exceptthatthe camming mechanism is inverted in the spear.

It will be understood that various modifications can be made to the embodiments hereinbefore described without departing from the scope of the invention. For example,the shape of the camming surface may be varied consistent with providing functions of four catch and two release positions and other modifications may involve changes in the structure oroperation ofthe gripping means. In addition, it will be understood that the requirement offour catch and two release positions are exemplary only and a different numberof positions may be used depending on the requirements, for examplefive catch and three release positions may be used.

Itshouldalso be noted that camming mechanism perse may be advantageously used in numerous othertypes of devices in which it is necessary to convert longitudinal movement to rotary movement in discreet steps. For example, the mechanism may be used in gas release valves.

It will also be appreciated that various sizes of tool maybe retrieved and lowered byvaryingthediameter ofthe lower sleeve portion 76 by simply screwing of and replacing it with a more appropriate size or, in the case ofthe spear, varying the size of expanderand radius ofthe slips.

Advantages ofthe structure hereinbefore described are that a conventional fishing tool is readily modified to be used to lower objects into a well as well as retrievethemandthearrangementoffourcatch and two release positions permits obstructions to be encountered in the hole and overcome without causing premature release ofthe object. In addition, the design is such thatthefishing tool can readily be modified to carry objects of different diameters and finally can be simply used as a conventional fishing tool if so desired. It will be appreciated that minimal modifications required to prior art structures are envisaged so that familiarity with the operation of the tool is not limited to skilled technical personnel and it can readily be used with minimal training time because ofthis.

Claims (11)

1. Acamming mechanism for use with a well tool, the camming mechanism having a plurality of sequential catch stages followed by a plurality of sequential release stageswherebywhen the mechanism is in first catch or release position rotation ofthe camming mechanism results in the engagement of at least one further catch position or release position prior to moving to a different functional position.

2. Acamming mechanism as claimed in claim 1 wherein there are 4sequential catch stages and 2 sequential release stages.

3. Awell tool having a camming mechanism with a plurality of sequential catch positions and a plurality of sequential release positions whereby when said camming mechanism is in a first catch or a first release position longitudinal movement ofthe drill string causing rotary movement of the camming mechanism results in engagement of at least one other catch position or release position being obtained prior to moving to a different functional position.

4. Swell tool as claimed in claim 3whereinthe camming mechanism has first and second camming portions, wherein in the case of an overshotcamming mechanismthefirstcamming portion is an upper camming portion and is engaged to determine the catch or release position and the second camming portion is a lower portion which is engaged to cause the tool to rotate between a catch and release position.

5. Awelltool as claimed in claim 3wherein in the caseofaspeartheuppercamming portion determinesthe rotation ofthefishing tool between catch and release positions and the lower camming portion determineswhetherthetool is in a catch ora release position.

6. Awell tool as claimed in anyone of claims 5 5 wherein there are four catch and two release positions in thefirstcamming portion and there are a total of six engagement positions which the tool can by cycled through.

7. Awell tool as claimed in claim 6wherein the first camming portion has first and second camming surfaces which are adapted to be engaged to determine the catch or release position, and the first camming surface is defined by a downwardly extending fingerwhich is adapted to be received by one of said second camming surfaces,which are defined by one of four deep notches each adapted to receive the fingeranddefineacatch position, or one of two shallow notches each adapted to receive a finger and to define a release position.

8. A well tool as claimed in any one of claims 3 to 7 wherein the second camming portion has lowerfirst and second camming surfacesforengagementto determine a catch ora release position, said lowerfirst and second camming surfaces having a generally sawtooth configuration and being complementary, each sawtooth having a recessed notch to prevent bottoming out by the apex of a tooth ofthe opposite camming surface when said lower first and second camming surfaces are engaged.

9. Awelltool as claimed in claim 8 wherein there aresixteeth in each sawtooth configuration.

10. Acamming mechanism substantially as he- reinbefore described with reference to the accompanying drawings.

11. Awell tool substantially as hereinbefore described with reference to the accompanying drawings.