Patent Publication Number: US-10329850-B2

Title: Releasable locking mechanism

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to GB Patent Application No. 1613572.5, filed Aug. 8, 2016, which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present disclosure generally relates to a releasable locking mechanism and in particular, to a releasable locking mechanism for use in a cable termination assembly, the cable termination assembly being fixedly coupled to a downhole apparatus and configured for coupling a cable line thereto, and a method for using the same. 
     BACKGROUND 
     Various techniques for implementing artificial lift to increase the flow of liquids from a downhole well are known. Such techniques typically involve inland or offshore wells in which a sub-surface downhole apparatus is deployed in the well using wireline, coiled tubing or umbilical cable arrangements, which connect the downhole apparatus to a well-head. As part of routine maintenance or in order to address fault conditions, the downhole apparatus may need to be retrieved from the well. 
     One of the challenges faced during retrieval is that there is a risk that the downhole apparatus will become stuck in the well, a situation that may arise due to scaling or debris accumulation and other similar factors. The downhole apparatus is typically deployed in a bore with small levels of clearance and hence, relatively small level of scaling or debris accumulation has the potential to affect retrieval. 
     When the tensile load that may be applied through the cable line is insufficient to successfully retrieve the downhole apparatus, the downhole apparatus must still be removed from the downhole well using other means. Various fishing tools are available to retrieve the downhole apparatus. However, before a fishing operation is performed, it is imperative that the cable line attached to the downhole apparatus is removed so that the downhole bore is clear to rig up the fishing equipment. 
     In order to address this situation, it is known to incorporate a weak link design in the coupling between the cable line and the downhole apparatus so that when a tensile load substantially greater than the normal operating load is applied, the cable line may be disengaged from the downhole apparatus. 
     In the past solutions, such as those known from U.S. Pat. Nos. 5,683,115 and 5,109,921, based on mechanical weak link designs using shear pins were proposed. One of the problems with this solution is that the broken pieces of shear pins or bolts, created during the operation of the weak link, further aggravate the problem of debris accumulation over the downhole apparatus. Moreover, such solutions lacked sufficient precision and reliability of operation. Accordingly, other solutions based on electrical and hydraulic actuation systems have been proposed in recent years. While the recent solutions increase the precision, one disadvantage is that in a fault scenario, if the hydraulic or the electrical actuation of the downhole release mechanism becomes inoperable, successfully fishing out the downhole apparatus becomes extremely difficult and cost prohibitive, if not impossible. 
     Reliable operation of the weak link is of paramount importance. In the event that the weak link fails to operate in the intended manner and the cable line breaks in the region near the well-head, the problem is exacerbated because the cable line, which is typically thousands of feet in length, slides into and clogs the bore of the downhole well and itself needs to be fished out before a fishing operation to retrieve the downhole apparatus may be performed. The breakage of the cable line near an offshore platform results in the additional problem of a pile of the cable line over the well-head on the sea bed. 
     In light of the foregoing, there is a need for a reliable releasable locking mechanism to releasably couple a cable line to a downhole apparatus. 
     SUMMARY 
     Accordingly, the present disclosure provides a releasable locking mechanism and a method of using the same. 
     The present disclosure provides a releasable locking mechanism according to claim  1  and a method according to claim  12 . Further embodiments of the present disclosure are addressed in the dependent claims. 
     The present disclosure enables to provide a release collet that latches an extension tube attached to a cable line. In an embodiment, the release collet is biased in a latched state through the combined action of an end cap and a release ring. In an embodiment, the end cap is biased toward a base fixedly secured to a cable termination assembly, which in turn is fixedly coupled to the downhole apparatus. The biasing of the end cap toward the base might be achieved using an extendible retaining means. When the tensile load applied to the cable line exceeds a predefined threshold value, the extendible retaining means permit the end cap to transition from a locking position to a release position. As the end cap transitions from the locking position to the release position, the release collet displaces axially away from the base and expands radially outwards into a recess formed on the inner surface of the release ring. As a result, the extension tube is released. 
     In an embodiment of the present disclosure, a releasable locking mechanism for use with a cable termination assembly is provided. The cable termination assembly might be coupled to a downhole apparatus and configured for coupling a cable line thereto. The releasable locking mechanism comprises an extension tube and a base assembly. The extension tube is adapted to be coupled to the cable line and provided with a first mating profile, and the base assembly is adapted to be coupled to the cable termination assembly and further adapted to engage the extension tube. 
     In an embodiment, the base assembly comprises a base adapted to be coupled to the cable termination assembly, a release ring mounted on the base and provided with a recess, an end cap, an extendible retaining means arranged to bias the end cap toward the base and adapted to permit an axial displacement of the end cap from a locking position to a release position relative to the base when a tensile load in the cable line exceeds a predefined threshold value, and a release collet provided with a second mating profile adapted for mating with the first mating profile. 
     In the embodiment, when the end cap is held in the locking position, the release collet is restrained from displacement in an axial direction away from the base by the end cap and in a radial direction by the release ring such that the first and second mating profiles are engaged, thereby retaining the extension tube, and when the end cap is displaced to the release position, the release collet is permitted to displace in an axial direction away from the base and expand in the radial direction at least partially into the recess of the release ring such that the first and second mating profiles are disengaged, thereby releasing the extension tube. 
     Thus, the present disclosure provides a releasable locking mechanism that relies on mechanical actuation and obviates the need for provision of cumbersome and error-prone hydraulic or electrical actuation systems. The extendible retaining means can be configured with precision to achieve a release at a predefined threshold value of the tensile load applied to the cable line. As the releasable locking mechanism according to an embodiment of the disclosure does not depend on availability and operability of electrical and/or hydraulic supply lines downhole but instead, relies on application of tensile load to the cable line, the releasable locking mechanism is not only much more precise but also highly reliable. While the releasable locking mechanism disclosure is based on a mechanical design, the mechanism advantageously does not rely on shearing or breaking coupling elements such as shear pins in order to release the cable line from the downhole apparatus. Accordingly, the problems associated with broken pieces of coupling means such as shear pins are advantageously avoided. 
     In an embodiment of the disclosure, the first mating profile may comprise a first set of circumferential recessed profiles spaced apart in an axial direction and provided on an outer surface of the extension tube, and the second mating profile comprises a second set of circumferential recessed profiles spaced apart in an axial direction and provided on an inner surface of the release collet. This technical feature does not require specific rotational alignment for achieving precise engagement between the release collet and the extension tube. 
     In an embodiment of the disclosure, the recess in the release ring may be part of a first toothed profile in an axial direction on the inner surface of the release ring, and the release collet may be provided with a second toothed profile adapted for mating with the first toothed profile, and where the release ring and the release collet are arranged such that when the end cap is held in the locking position, the first and second toothed profiles engage in a non-mating manner, whereby the release collet might be biased toward the extension tube, and when the end cap is displaced to the release position and the release collet displaces in the axial direction away from the base, the first and second toothed profiles engage in a mating manner, and thereby, permit radial expansion of the release collet. This feature enables the individual toothed profiles on the release ring and the release collet to be shaped in such a manner that the tensile load in the cable line for releasing the extension tube can be varied during the displacement of the end cap and the release collet away from the base from a relatively high value in the beginning to progressively lower values subsequently. As a result, the time duration for which the cable line is subjected to high tensile load is relatively reduced. 
     In an embodiment of the disclosure, the base assembly may comprise a sealing member disposed at an interface between the extension tube and the base. According to this technical feature, the interface between the extension tube and the base assembly may be sealed so as to prevent ingress of ambient gas and/or liquid into a spatial region delimited by the extension tube and the base assembly, which is in continuum with the inner cavity within the cable termination assembly. 
     In an embodiment of the disclosure, the base assembly comprises a seal compression ring disposed between the sealing member and the release collet such that when the end cap is in the locked position, the seal compression ring compresses the sealing member such that the sealing member is urged toward respective inner surfaces of the base and the extension tube. In this manner, the efficacy of the sealing member might be improved. 
     In an embodiment of the disclosure, the extension tube may be provided with a set of anti-rotation slots, and the end cap may be provided with a set of anti-rotation latches, wherein each anti-rotation latch comprises a sprung key and a casing, and protrudes radially inwards from a corresponding housing slot formed in an inner surface of the end cap, wherein the casing is profiled to prevent a rotational motion of the extension tube about the axial direction but permit a translational motion of the extension tube in the axial direction. Rotation of the extension tube relative to the base assembly can thus be prevented, and thereby undesirable torsional forces on the supply cables coupled to the downhole apparatus through the cable termination assembly might be avoided. 
     In an embodiment of the disclosure, the extendible retaining means may comprise at least one tension bolt, each tension bolt comprising at least a head region, a tail region, and a weak neck region, wherein the tension bolts extend in the axial direction, wherein the head region is engaged by the end cap, the tail region is engaged by the base, and the weak neck region extends through the release ring. The tension bolts exhibit precise stress-strain interrelationship and thus, enable precise and reliable characterisation of releasable locking mechanism in respect of required tensile load to release the extension tube. 
     In an embodiment of the disclosure, the extendible retaining means may comprise a set of tension bolts, each tension bolt comprising at least a head region, a tail region, and a weak neck region, wherein the tension bolts are uniformly distributed along a periphery of the end cap, and extend along the axial direction, wherein the head region is engaged by the end cap, the tail region is engaged by the base, and the weak neck region extends through the release ring. The uniform distribution of the tension bolts, as per this technical feature, evenly distributes the tensile load over multiple tension bolts along the periphery of the end cap. 
     The release collet may be a multi-piece collet comprising a plurality of longitudinally extending sub-parts coupled in a pair-wise manner in a circumferential direction. Beneficially, the individual pieces of the release collet are better suited to expand radially outwards into the recess of the release ring as compared to a release collet with a monolithic design. 
     When the extension tube and the base assembly are engaged, a load path traverses through the extension tube, the first mating profile on the extension tube, the second mating profile on the release collet, the release collet, the end cap, the extendible retaining means and the base. Accordingly, when a tensile load is applied to the cable line coupled to the extension tube, the tensile load is transmitted through the load path and the extendible retaining means, among others, is also subjected to the applied tensile load. Additionally, due to the load path, when the downhole apparatus becomes stuck in the well and a tensile load greater than a predefined threshold value is applied to the cable line, the extension tube transmits the tensile load to the release collet, which in turn, forces the end cap in an axial direction away from the base and in this process, the release collet also displaces away from the base. 
     In an embodiment of the disclosure, the base assembly may comprise a fishing profile secured to the base through a retaining ring so as to permit a fishing operation to be carried out to retrieve the downhole apparatus subsequent to release of the extension tube and removal of the cable line from the downhole well. 
     In a second aspect of the present disclosure, a method is provided. A cable line might be coupled to a downhole apparatus through a releasable locking mechanism as provided with the present disclosure. A tensile load might be applied to the cable line equal to or greater than the predefined threshold value such that the cable line is disengaged from the downhole apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure will now be further described with reference to illustrated embodiments shown in the accompanying drawings, in which: 
         FIG. 1  depicts a schematic representation of a downhole well in which various embodiments may be practiced; 
         FIG. 2  depicts a releasable locking mechanism in an engaged state in accordance with some embodiments; 
         FIG. 3  depicts a releasable locking mechanism in a disengaged state in accordance with some embodiments; 
         FIGS. 4A-4B  depict two partially exploded views of a releasable locking mechanism in accordance with some embodiments; 
         FIG. 5  depicts a perspective view of a release collet and a release ring in accordance with some embodiments; 
         FIG. 6  depicts a cross-sectional view of a releasable locking mechanism in accordance with some embodiments; 
         FIGS. 7A-7B  depict cross-sectional views of a releasable locking mechanism in an engaged state in accordance with some embodiments; 
         FIGS. 8A-8B  depict cross-sectional views of a releasable locking mechanism in a disengaged state in accordance with some embodiments; 
         FIG. 9  depicts a perspective view of a sub-part of a multi-piece release collet in accordance with some embodiments; 
         FIGS. 10A-10D  depict cross-sectional views showing changes in relative engagement of a first and a second toothed profiles disposed respectively on a release collet and a release ring in accordance with some embodiments; 
         FIG. 11  depicts a graphical representation of variation of tensile load in a cable line as a function of displacement of an extension tube relative to a base assembly in accordance with some embodiments; and 
         FIG. 12  depicts a method for using a releasable locking mechanism in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details. 
       FIG. 1  depicts a schematic representation of a downhole well  10  in which various embodiments of the present disclosure may be practiced. As shown in the figure, a downhole apparatus  12  is deployed in the downhole well  10 . The downhole apparatus  12  is suspended from the cable line  13  using the cable termination assembly  11 , which is adapted to couple the downhole apparatus  12  to the cable line  13 . 
     The cable line  13  may be a wireline cable, coiled tubing, or an umbilical cable. In an example, the downhole apparatus  12  is suspended using a wireline cable that terminates in a rope socket. Various supply cables, such as electrical, instrumentation and hydraulic are run along the wireline cable and are attached thereto for mechanical support. In contrast, when using a coiled tubing or an umbilical cable, the supply cables may reside within a tubing cable that is coupled to the downhole apparatus  12  using the cable termination assembly  11 . 
     The concepts described herein are applicable to any possible variations in the manner in which the downhole apparatus  12  is suspended in the downhole well  10 . In particular, the term “cable line” as used herein is intended to encompass, within its meaning, wireline, coiled tubing, umbilical cable and any other analogous or equivalent implementations. 
     The releasable locking mechanism of the present disclosure is suitable for use in any coupling assembly intermediate to the downhole apparatus  12  and the cable line  13  such that when the need arises, the releasable locking mechanism may be actuated to decouple the cable line  13  from the downhole apparatus  12 . 
     Referring now to  FIGS. 2 and 3 , an embodiment of a releasable locking mechanism  20  of the disclosure is depicted in two distinct states. In particular,  FIG. 2  depicts the releasable locking mechanism  20  in an engaged state while  FIG. 3  depicts the releasable locking mechanism  20  in a disengaged state. 
     As shown in the examples of  FIGS. 2 and 3 , the releasable locking mechanism  20  includes an extension tube  21  and a base assembly  23 . The extension tube  21  is adapted to be coupled to the cable line  13 . The extension tube  21  may be coupled to cable line  13  in any suitable manner. In one embodiment, the extension tube  21  is a hollow elongated tubular member such that the supply cables such as electrical wires, instrumentation cables, and hydraulic cables are permitted to pass there through and enter into the cable termination assembly  11  coupled to the downhole apparatus  12 . The extension tube  21  is provided with a first mating profile  22 . The base assembly  23  is adapted to be coupled to the cable termination assembly  11  and is further adapted to engage the extension tube  21 . In particular, the base assembly  23  is provided with a release collet that has a second mating profile suitable for mating with the first mating profile  22  so as to removably engage the extension tube  21 . The release collet and the corresponding features will be described in more detail below. 
     Various individual components constituting the base assembly  23  will now be described in more detail in conjunction with  FIGS. 4A, 4B, 5 and 6 . 
       FIGS. 4A and 4B  depict two partially exploded views of a releasable locking mechanism and  FIG. 5  depicts a perspective view of a release collet  29  and a release ring  25  in accordance with embodiments of the disclosure.  FIG. 6  depicts a cross-sectional view of a releasable locking mechanism  20 . 
     The base assembly  23  includes a base  24 , a release ring  25 , an end cap  27 , an extendible retaining means  28  and a release collet  29 . The base  24  is adapted to be coupled to the cable termination assembly  11 . The base  24  may be coupled to the cable termination assembly  11  in any suitable manner. In one example, a threaded engagement is used. The base  24  is provided with threads on the outer surface in the end region to be engaged with the cable termination assembly  11 . The cable termination assembly  11  is provided with matching threads on the inner surface in the end region thereof. Thus, the base  24  is engaged to the cable termination assembly  11  in a threaded manner. Various alternative threaded engagements based on sleeves and/or collar type engagements may alternatively be used. In another implementation, the base  24  may be provided with a radially outwards extending terminal flange which is attached to the cable termination assembly  11  using bolts. In various embodiments, the base  24  has a through hole such as to permit passage of supply cables there through. 
     The release ring  25  is mounted on the base  24  using any suitable mounting means. It should be noted that in an embodiment, instead of being formed as individual components, the base  24  and the release ring  25  is formed as an integral component. 
     The release ring  25  is provided with a recess  26 . The recess  26  is formed on an inner surface of the release ring  25  and is adapted according to the structure of the release collet  29 . The release ring  25  resides in between the base  24  and the end cap  27  and thus, the end cap  27  is held in a spaced relationship relative to the base  24 . The extendible retaining means  28  is arranged to bias the end cap  27  toward the base  24 . The extendible retaining means  28  is adapted to permit an axial displacement of the end cap  27  from a locking position (L) to a release position (R) relative to the base  24  when a tensile load in the cable line  13  exceeds a predefined threshold value. The release collet  29  is provided with a second mating profile  30  that is adapted for mating with the first mating profile  22 . 
     As best seen in  FIG. 5 , in an embodiment, the release collet  29  is a multi-piece collet formed using a plurality of longitudinally extending sub-parts  29 - 1  to  29 - 6  coupled in a pair-wise manner in a circumferential direction. 
     In the embodiment depicted in  FIG. 5 , six individual pieces are coupled to form the release collet  29 . It should be noted that the release collet  29  may have fewer or more pieces. For example, the release collet  29  may be formed using only the pieces  29 - 1 ,  29 - 3  and  29 - 5 . Thus, in this implementation, the release collet  29  spans over the circumference of the extension tube  21  in a discontinuous manner. The first mating profile  22  and the first toothed profile  31  may be adapted accordingly. Alternatively, the first mating profile  22  and the first toothed profile  31  may remain unchanged to maintain rotational symmetry. 
     Further, in the embodiment depicted in  FIGS. 4A and 4B , the end cap  27  has a split design, which is to say, it is in a two part form, as best seen in  FIG. 4B . Such a design of the end cap  27  facilitates on-site assembly of the releasable locking mechanism  20  and advantageously results in an overall more compact design of the releasable locking mechanism  20  and in particular, that of the extension tube  21 . 
     Referring particularly to  FIG. 6 , a cross-sectional view of a releasable locking mechanism  20  is depicted. 
     As described above, the base  24  is coupled to the cable termination assembly  11 . The release ring  25  is mounted on the base  24 . The end cap  27  is coupled to the base  24  using the extendible retaining means  28 . 
     The extension tube  21  is engaged or disengaged based on relative state of mating between the first mating profile  22  provided on an outer surface of the extension tube  21  and the second mating profile  30  provided on an inner surface of the release collet  29 . In  FIG. 6 , the first mating profile  22  and the second mating profile  30  are shown in a mated state. 
     In an embodiment, the first mating profile  22  is formed using a first set of circumferential recessed profiles  22 - 1 ,  22 - 2 ,  22 - 3  spaced apart in the axial direction (Z) and provided on an outer surface of the extension tube  21  and the second mating profile  30  is formed using a second set of circumferential recessed profiles  30 - 1 ,  30 - 2  spaced apart in the axial direction (Z) and provided on an inner surface of the release collet  29 . 
     The release ring  25  is provided with a first toothed profile  31  in the axial direction (Z) on the inner surface of the release ring  25  such that the recess  26  is a part thereof. In other words, multiple recesses  26  might be provided on the release ring  25  by way of the first toothed profile  31 . The release collet  29  is provided with a second toothed profile  32  adapted for mating with the first toothed profile  31 . 
     The release ring  25  and the release collet  29  are arranged within the releasable locking mechanism  20  in such manner that when the end cap  27  is held in the locking position (L), the first and second toothed profiles  31 ,  32  engage in a non-mating manner, whereby the release collet  29  is biased toward the extension tube  21 . The assembly is shown in this state in  FIG. 6 . 
     As will be explained in more detail below in conjunction with  FIGS. 7A-7B and 8A-8B , when the end cap  27  is displaced to the release position (R) and the release collet  29  displaces in the axial direction (Z) away from the base  24 , the first and second toothed profiles  31 ,  32  engage in a mating manner, and thereby, permit radial expansion of the release collet  29 , whereby the extension tube  21  is disengaged from the base assembly  23 . 
     The base assembly  23  may also include a sealing member  33  disposed at an interface between the extension tube  21  and the base  24 . In an example, as depicted in  FIG. 6 , a metal ring seal with a circular segment shaped cross-section, arranged such the flat face interfaces with the extension tube  21 , is used as the sealing member  33 . The base  24  and the extension tube  21  are cylindrical tubular members with the outer diameter of the extension tube  21  being marginally less than the inner diameter of the base  24  such that the extension tube  21  may be snugly inserted into the base  24 . The inner surface at a terminal region of the base  24  diverges outwards to increase the cross-sectional area of the cavity bounded by the base  24  so as to permit the sealing member  33  to be disposed between the base  24  and the extension tube  21 , as shown in  FIG. 6 . 
     The base assembly  23  may also include a seal compression ring  34  disposed between the sealing member  33  and the release collet  29  such that when the end cap  27  is in the locking position (L), the seal compression ring  34  compresses the sealing member  33  such that the sealing member  33  is urged toward respective inner surfaces of the base  24  and the extension tube  21 . 
     The provision of sealing member  33  and the seal compression ring  34  facilitate sealing the interface between the extension tube  21  and the base assembly  23  such as to prevent ingress of ambient gas and/or liquid into a spatial region bound by the extension tube  21  and the base assembly  23 , which is in continuum with the inner cavity within the cable termination assembly  11 . 
     In an embodiment, the extension tube  21  is provided with a set of anti-rotation slots  35 , and the end cap  27  is provided with a set of anti-rotation latches  36 , wherein each anti-rotation latch  36  includes a sprung key  37  and a casing  38 , and protrudes radially inwards from a corresponding housing slot  39  formed in an inner surface of the end cap  27 , wherein the casing  38  is shaped to prevent a rotational motion of the extension tube  21  about the axial direction (Z) but permit a translational motion of the extension tube  21  in the axial direction (Z). In one example, the casing  38  has a wedge-shaped profile, wherein the upright faces are parallel to the axial direction (Z) while the slanting faces extend parallel to an axis orthogonal to the axial direction (Z). This technical feature may prevent rotational motion of the extension tube  21  relative to the base assembly  23  after being engaged thereto but permits the extension tube  21  to displace axially away from the base  24  while the releasable locking mechanism  20  transitions from the engaged state to the disengaged state. 
     In one embodiment, the extendible retaining means  28  includes a set of tension bolts, each tension bolt comprising at least a head region  28 - 1 , a tail region  28 - 2 , and a weak neck region  28 - 3 . In this example, multiple tension bolts  28  might be used and these tension bolts  28  might be uniformly distributed along a periphery of the end cap  27 , and extend in the axial direction (Z). The head region  28 - 1  is engaged by the end cap  27 , the tail region  28 - 2  is engaged by the base  24 , and the weak neck region  28 - 3  extends through the release ring  25 . In an example, where the end cap  27  is split in section, dowel tubes  28 - 5  are employed to form a continuous passage for the tension bolts  28  to prevent a direct interface between the length of the tension bolts  28  and the release ring  25 . The dowels  28 - 5  are inserted in through holes formed along an axial length of the release ring  25  and define a passage for the tension bolts  28  through the release ring  25 . The use of tension bolts  28  is advantageous in that the tension bolts  28  can be adapted with high degree of precision in relation to applied stress and resulting strain. Thus, the displacement of the end cap  27  from the locking position (L) to the release position (R) can be precisely configured as a function of the tensile load in the cable line  13 . 
     In various embodiments, the tension bolts  28  may be engaged to the base  24  in any suitable manner. In one example, tension bolts  28  are provided with threads in the tail region  28 - 2  to engage the base  24 , which is provided with corresponding threaded bore holes to receive and engage the tail region  28 - 2  of the tension bolts  28 . As can be seen in  FIG. 6 , a bolt collar  28 - 4  positioned under the bolt heads is also provided to prevent bolt disengagement due to vibrations in the base assembly  23  during operation. The bolt collar  28 - 4  might thus enable to provide vibration resistance and prevent the tension bolts from unwinding from their threads and loosening their pre-loaded tension. In one example, the bolt collar  28 - 4  is a cam locking washer. 
     In one implementation, a single tension bolt is used. In other implementations, the extendible retaining means  28  may be implemented using other means such as tension bar, compression spring, and so on. 
     In various embodiments of the present disclosure, when the extension tube  21  and the base assembly  23  are engaged in the manner shown in  FIG. 6 , a load path traverses through the extension tube  21 , the first mating profile  22  on the extension tube  21 , the second mating profile  30  on the release collet  29 , the release collet  29 , the end cap  27 , the extendible retaining means  28  and the base  24 . 
     In one implementation, the normal operating load is estimated to be up to about 30 klbs, the dimensions, including overall length, length of weak neck region  28 - 3 , and the material of the tension bolts  28  is selected such that a load of about 45+/−5 klbs results in a strain value of 0.1, which in turn results in displacement of the end cap  27  from the locking position (L) to the release position (R). 
     In an embodiment, the base assembly  23  includes a fishing profile  40  secured to the base  24  through a retaining ring  41 . This technical feature may enable the downhole apparatus  12  to be retrieved using appropriate fishing tools subsequent to release and removal of the cable line  13  through operating the releasable locking mechanism  20 . 
     Any suitable material may be used to form the components of the releasable locking mechanism  20 . In one example, the tension bolts  28  may be formed using Inconel® 625, the release ring  25 , the release collet  29 , the anti-rotation latch  36  including the sprung key  37  and the casing  38 , the seal compression ring  34 , and the extension tube  21  are formed using Inconel® 718. Inconel® alloys are high resistant to oxidation and corrosion and accordingly, are well suited for downhole applications due to extreme ambient temperature and pressure conditions. The sealing member  33  is formed using Hastelloy® C276. Hastelloy® C276 is highly corrosion resistant like Inconel® alloys and at the same time, exhibits relatively less hardness as compared to Inconel® alloys and hence, is suitable material for providing a metal seal. The metal seal can additionally be coated with a soft coating material such as silver to aid sealing performance by providing a compliant layer with the mating housing material. 
       FIGS. 7A and 7B  depict an axial cross-sectional view and a transverse cross-sectional view respectively of a releasable locking mechanism  20  in an engaged state. 
     The axial cross-sectional view provided in this figure is same as that in  FIG. 6 . Additional indications, indication  42  and indication  43 , have been provided to indicate the locking position (L) of the end cap  27  and the corresponding position of the extension tube  21  respectively. 
     When the end cap  27  is held in the locking position (L), the release collet  29  is restrained from displacement in the axial direction (Z) away from the base  24  by the end cap  27 . As mentioned above, the load path traverses through the extension tube  21 , the first mating profile  22  on the extension tube  21 , the second mating profile  30  on the release collet  29 , the release collet  29 , the end cap  27 , the extendible retaining means  28  and the base  24 . When the downhole apparatus  12  is suspended in the downhole well  10  using the cable line  13 , under normal working conditions, the release collet  29  is prevented from axial movement relative to the release ring  25  in the axial direction (Z) away from the base  24  due to the combined action of the end cap  27  and the extendible retaining means  28 . Additionally, the release collet  29  is restrained from expansion in the radial direction by the release ring  25  such that the first mating profile  22  and the second mating profile  30  are engaged, thereby retaining the extension tube  21 . 
     As the tensile load in the cable line  13  and accordingly, across the load path through the releasable locking mechanism  20  increases beyond the normal operating load, the extendible retaining means  28  start extending, thereby permitting the end cap  27  to displace in the axial direction (Z) away from the base  24 . Accordingly, the release collet  29  also starts to displace along with the end cap  27 . As the tensile load continues to increase, the end cap  27  and the release collet  29  continue to displace away from the base  24 . As described above, the release ring  25  is mounted on the base  24  and does not undergo axial movement relative to the base  24 . When the tensile load reaches or exceeds a predefined threshold value, the end cap  27  reaches a release position (R). The state and the relative positions of the individual components of the releasable locking mechanism  20 , when the end cap  27  is in the release position (R) is as depicted in  FIGS. 8A and 8B . 
     Referring now to  FIGS. 8A and 8B , an axial cross-sectional view and a transverse cross-sectional view respectively of a releasable locking mechanism  20  in a disengaged state are depicted. 
     When the end cap  27  is displaced to the release position (R), the release collet  29  is permitted to displace in the axial direction (Z) away from the base  24  and expand in the radial direction at least partially into the recess  26  of the release ring  25  such that the first mating profile  22  and the second mating profile  30  are disengaged, thereby releasing the extension tube  21 . The relative positions of the end cap  27  and the extension tube  21  are indicated using indication  44  and indication  45 . 
     In an embodiment, the release ring  25  may be provided with a recess on the inner surface such that the axial length of the recess  26  matches the axial length of the release collet  29 . In this example, the release collet  29  slides one full length and expands into the recess formed in the release ring  25 . 
     In another embodiment, as described above, the release ring  25  might be provided with a first toothed profile  31 , such that multiple recesses  26  are formed therein and the release collet  29  is provided with a second profile such that when the end cap  27  is in the locking position (L), the first toothed profile  31  and the second toothed profile  32  engage in a non-mating manner and when the end cap  27  transitions to the release position (R), the first toothed profile  31  and the second toothed profile  32  engage in a mating manner, thereby permitting the release collet  29  to expand radially outwards, as best seen in  FIG. 8B . In this embodiment, the first mating profile  22 , the second mating profile  30 , the first toothed profile  31  and the second toothed profile  32  might be dimensioned in such manner that when the first toothed profile  31  and the second toothed profile  32  engage in a non-mating manner, the first mating profile  22  and the second mating profile  30  engage in a mating manner and vice versa. 
     As can be seen in  FIG. 7A , the anti-rotation latches  36  protrude outwards from the corresponding housing slot  39  to engage the extension tube  21  during the engaged state of the releasable locking mechanism  20 . As the extension tube  21  begins to displace in the axial direction (Z) away from the base  24 , the anti-rotation latches  36  begin to retract inside the housing slot  39 , until the anti-rotation slots  35  are fully disengaged, as can be seen in  FIG. 8A . 
     Referring now to  FIGS. 9 to 11 , changes in relative engagement of the first toothed profile  31  and the second toothed profile  32  as the end cap  27  is displaced from the locking position (L) to the release position (R) are shown along with the resulting variation in the tensile load in the releasable locking mechanism  20 . 
     In particular,  FIG. 9  depicts a perspective view of a sub-part  29 - 1  of a multi-piece release collet  29  in accordance with an embodiment. The cross-sectional profile of the second toothed profile  32  is asymmetric in that the cross-sectional profile includes a flat face  46  facing in a direction away from the base  24 , a flat top  47 , a sloping face  48 , and a flat face  49 , as best seen in  FIG. 10 . 
       FIGS. 10A to 10D  depict cross-sectional views showing changes in relative engagement of the first toothed profile  31  and the second toothed profile  32  disposed respectively on the release collet  29  and the release ring  25 . The second toothed profile  32  includes a series of flat faces  51  and  52 . 
     As shown in  FIG. 10A , when the end cap  27  is in the locking position (L), the first toothed profile  31  and the second toothed profile  32  engage in a non-mating manner wherein the flat face  47  interfaces with the flat face  51  (stage I, between points A and B). As the release collet  29  begins to displace away from the base  24 , at first, the flat face  47  crosses the boundary between the flat faces  51 ,  52  (stage II, between points B and C). As the release collet  29  displaces further away from the base  24 , the flat face  47  moves out of contact with the flat face  51 , which is then engaged by the sloping face  48  (stage III, between points C and D). As the release collet  29  continues to displace further away from the base  24 , the release collet  29  begins expanding radially outwards into the recess  26  formed between the flat faces  51 . Eventually, the release collet  29  expands such that the flat face  50  comes in contact with the flat face  51 , at which point, the first mating profile  22  and the second mating profile  30  are disengaged (stage IV, between points D and E). 
       FIG. 11  depicts a graphical representation of variation of tensile load in a cable line as a function of displacement of the extension tube  21  relative to a base assembly  23  when the first toothed profile  31  and the second toothed profile  32  are implemented in the manner shown in  FIGS. 10A through 10D . The tensile load varies over different stages of relative engagement between the first toothed profile  31  and the second toothed profile  32 , as explained above. The relative engagement between the first toothed profile  31  and the second toothed profile  32  is in turn, is a function of the displacement of the extension tube  21 . 
     During a release operation, the tensile load continues to increase and reaches the maximum value until the flat face  47  is in contact with the flat face  51 . Once the displacement enters the region indicated as stage II (that is, between points B and C on the graphical representation), the residual energy in the cable line  13  causes the mechanism to gain momentum quickly and hence, sustained application of tensile load is not necessary throughout the entire duration of the displacement of the release collet  29  relative to the release ring  25 . 
     Referring now to  FIG. 12 , an example of a method for using a releasable locking mechanism of the disclosure is depicted. The method is suitable for releasably locking a cable line to a downhole apparatus. The method includes coupling ( 61 ) a cable line to a downhole apparatus using a releasable locking mechanism of the disclosure and applying ( 62 ), a tensile load to the cable line equal to or greater than the predefined threshold value such that the cable line is disengaged from the downhole apparatus. Step  62  may be performed when it is desired to retrieve the downhole apparatus, which is stuck in the downhole well. 
     As will now be understood based on the above description, the present disclosure comprises mechanical actuation and obviates the need for provision of cumbersome and error-prone hydraulic or electrical actuation systems. While the releasable locking mechanism of the present disclosure is based on a mechanical design, the mechanism advantageously does not rely on shearing or breaking coupling elements such as shear pins in order to release the cable line from the downhole apparatus. Accordingly, the problems associated with broken pieces of coupling means such as shear pins are advantageously avoided. Further, the extendible retaining means can be configured with precision to achieve a release at a predefined threshold value of the tensile load applied to the cable line. Thus, the releasable locking mechanism according to the present disclosure is not only reliable but also much more precise.