Patent Publication Number: US-8534368-B2

Title: Downhole tool with slip releasing mechanism

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority of U.S. Provisional Application No. 60/859,140 filed on Nov. 15, 2006, the disclosure of which is incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to downhole tools for anchoring a portion of the tool in a well. More particularly, the present invention relates to a downhole tool such as a liner hanger assembly for hanging a liner in a well, and to an improved slip releasing mechanism within the downhole tool. 
     BACKGROUND OF THE INVENTION 
     Various types of downhole tools employ slips which expand radially outward to engage the interior of a downhole tubular, thereby securing the tool in the well. Various types of slip mechanisms have been used for this purpose, with slips conventionally having outer teeth which bite into the inner surface of the downhole tubular to secure the tool in the well. 
     One of the significant problems with downhole tools having slip mechanisms is that the slip mechanism may be inadvertently actuated before actuation is intended, in which case the slip or slips may radially expand and engage the tubular. It may then be difficult or practically impossible in some instances to break the connection between the downhole tool and the tubular, and accordingly the tool cannot be easily retrieved to the surface, repaired, and then again reinserted into the well. In other instances, the slip may be expanded when the tool is at a desired downhole position, but the running tool/set tool connection cannot be reliably released, so that the retrievable portion of the tool cannot be returned to the surface. 
     U.S. Pat. Nos. 3,920,057 and 4,281,711 disclose a liner hanger assembly for hanging a liner in a well. A liner hanger assembly is also disclosed in U.S. Pat. No. 6,739,398. The &#39;398 patent discloses a tool that relies upon shear pins to keep the slips from prematurely releasing while the tool is run in the hole. While pushing on the running tool may not release the slip accidentally, if debris were to build up around the tieback or other components of the liner hanger and the operator then pushed or pulled on the tool, the shear pins may shear and thereby release the slip. Tools that allow axial forces to be transmitted to the tool through the running string to shear pins and release the slip may thus inadvertently be actuated when running the tool to the desired depth in the well. 
     U.S. Pat. Nos. 4,712,614, 4,603,743, 4,311,194 and 4,287,949 also rely on shear pins to keep from releasing the slips. U.S. Pat. No. 5,318,131 is another example of the downhole tool using shear pins to prevent release of slips. Downhole tools with shear pins to keep the slip in the retracted position while running the tool in the hole generally have exposed hydraulic actuation ports. If debris builds up around the tool while tripping into the hole, the dragging action could shear the pins, in which case the slip will set and thereby prevent the tool from being positioned at its desired location in the well. Increased hydraulic pressure may also cause the pins to prematurely shear. 
     The disadvantages of the prior art are overcome by the present invention, and a liner hanger with an improved slip releasing mechanism is hereinafter disclosed. 
     SUMMARY OF THE INVENTION 
     According to one embodiment, a running tool for releasing a slip and preventing premature activation of the slip includes a tool mandrel supported on a running string, and a blocking member for closing off a port in the mandrel when the tool is run in the well and for selectively opening the port to set the slip. A locking member prevents premature setting of the slip, with the locking member being axially securable to the mandrel when the running tool is run in the well. An actuating piston is moveable with respect to the tool mandrel when the port is open to move the locking member and release the slip to set in the well. 
     The running tool may lock the slip in a reduced diameter position when running the tool into the wellbore and selectively releases the slip to move radially outward to a set position for suspending a downhole tool in a tubular. The running tool includes a mandrel having a through passage, and a locking device supported on the mandrel for retaining the slip in the reduced diameter position when running the tool into the well. A hydraulic port in the mandrel is closed to mandrel internal pressure as the downhole tool is run into the wellbore, and is selectively open when desired. A piston is moveable in response to mandrel internal pressure applied through the hydraulic port to unlock the locking device. Further movement of the piston may release the slip from the reduced diameter position to the set position. Still further movement of the piston may disengage the running tool from the downhole tool, so that the running tool may be retrieved to the surface while the downhole tool is set in the well. 
     These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1G  illustrate sequentially the primary components of a suitable liner hanger running tool. 
         FIG. 2  illustrates in greater detail the cross-sectional view of a slip releasing mechanism generally shown in  FIG. 1B . 
         FIG. 3  illustrates the slip releasing mechanism shown in  FIG. 2  after a collar is lowered to open a hydraulic port and raise a piston to disengage a locking collar. 
         FIG. 4  shows the slip releasing mechanism piston further raised to release a tieback pickup ring. 
         FIG. 5  shows the slip releasing piston fully stroked to release the downhole portion of the tool anchored to the tieback from the retrievable portion of the tool. 
         FIG. 6  is a cross-sectional view showing the connection between the adjustable nut sleeve and the tieback pickup ring which causes the tieback pickup ring to move upward during further movement of the piston. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1 , which consists of  FIGS. 1A-1G , illustrates one embodiment of a liner hanger running tool  100  including a running tool tieback locking mechanism  80  ( FIG. 1A ), a slip release assembly ( FIG. 1B ) operatively responsive to the upper C-ring seat assembly  110 , packer setting lugs  180  ( FIG. 1C ), a liner hanger release assembly  170  operatively responsive to the lower C-ring seat assembly ( FIG. 1D ), a cementing bushing  130  ( FIG. 1E ), and a ball diverter  140  and plug release assembly  150  ( FIG. 1G ).  FIG. 1E  illustrates the packer  122  and  FIG. 1F  illustrates the slip assembly  120 , which are not part of the running tool retrieved to the surface, and remain downhole with the set liner. 
     To hang off a liner, the running tool  100  is initially attached to the lower end of a work string and releasably connected to the liner hanger, from which the liner is suspended for lowering into the bore hole within the previously set casing or liner C. 
     A tieback receptacle  102  as shown in  FIG. 1A  is supported about the running tool  100 . The upper end of the tieback receptacle  102 , upon removal of the running tool, provides for a casing tieback (not shown) to subsequently extend from its upper end to the surface. The tool  100  includes a central mandrel  104 , which may comprise multiple connected sections, with a central bore  106  in the mandrel. The lower end of the tieback receptacle  102  is connected to the packer element pusher sleeve  121 , as shown in  FIG. 1E , whose function will be described in connection with the setting of the packer element  122  about an upper cone  124 , as well as setting of the slips  126  about a lower cone  128  (see  FIG. 1F ). 
     By incorporating an axially movable slick joint  132  (which may functionally be an extension of the mandrel  104 ), the running tool may be axially moved without breaking the seal provided by the cementing bushing  130  (see  FIG. 1E ). The running tool  100  also includes a ball diverter  140  (see  FIG. 1G ) at the lower end of the running tool. The cementing bushing  130  provides a retrievable and re-stabbable seal between the running tool  100  and the liner hanger assembly for fluid circulation purposes. 
       FIG. 1A  also illustrates a tieback locking and releasing mechanism, which locks the tieback  102  to the running tool mandrel  104  to prevent premature actuation of the liner hanger or downhole portion of the tool as it is run in the well. The locking mechanism unlocks the tieback  102  to allow the slips  126  to be set and the retrievable portion of the tool to be returned to the surface. 
     The slip release subassembly  210  as shown in  FIG. 1B , and more particularly in  FIG. 2 , is used to release the liner hanger slip for setting, and includes a sleeve  112  disposed within and axially moveable relative to the running tool mandrel  104 . A C-ring ball seat  116  is supported on the sleeve  112 . A seal  115  is provided for sealing with the seated ball. A ball  118  may thus be dropped from the surface into the running tool bore  106  and onto the seat  116 . An increase in fluid pressure within the mandrel  104  above the seated ball will shear the pins  114  and lower the ball seat  116  and sleeve  112  to a lower position in the bore of the running tool, e.g., against the stop shoulder  108 . Once the sleeve  112  is lowered, fluid pressure may subsequently pass through ports  166  to stroke a piston and thereby release the slips for subsequent setting. 
     Piston sleeve  216  is disposed about and is axially moveable relative to mandrel  104 . An upper sealing ring  162  is disposed about a smaller O.D. of the running tool mandrel than is the lower sealing ring  164  to form an annular pressure chamber between them for lifting the tieback receptacle  102  from the position shown in  FIG. 1B  to an upper position for setting the slip or slip segments  126 . Ports  166  formed in the running tool mandrel  104  connect the running tool bore with the surrounding pressure chamber once the seat  116  and sleeve  112  are lowered. An increase in pressure through the ports  166  will raise the piston sleeve  216 . Upward movement of the piston sleeve  216  causes its upper end to raise the tieback receptacle  102 , and also raise the slips  126 . 
     The slip  120  shown in  FIG. 1F  is made up of arcuate slip segments  126  received within circumferentially spaced recesses in slip body sleeve about the lower end of the liner hanger and adjacent the lower cone  128 . Each slip segment  126  includes a relatively long tapered arcuate slip having teeth  127  on its outer side and an arcuate cone surface  125  mounted on its inner side for sliding engagement with lower cone  128 . When multiple circumferentially spaced slip segments are used, each of the multiple recesses may include a slot in each side. Alternatively, a one piece C-slip may be used to replace the slip segments. The teeth  127  are adapted to bite into the casing C as the liner weight is applied to the slip. The slips  126  are thus movable vertically between a lower retracted position, wherein their outer teeth  127  are spaced from the casing C, and an upper position, wherein the slips  126  have moved vertically over the cone  128  and into engagement with the casing C. 
       FIGS. 1E and 1F  show the relationship of both the packer element  122  and the circumferentially spaced slips  126  about the upper  124  and lower  128  cones, respectively. The annular packer element  122  is disposed about a downwardly-enlarged upper cone  124  beneath the pusher sleeve  121 . The packer element  122  is originally of a circumference in which its O.D. is reduced and thus spaced from the casing C. However, the packer element  122  is expandable as it is pushed downwardly over the cone  124  to seal against the casing. 
       FIG. 1E  generally illustrates the cementing bushing  130 . The cementing bushing provides a retrievable and re-stabbable seal between the running tool and the liner hanger for fluid circulation purposes prior to cementing, and also for the cementing operation. The cementing bushing  130  cooperates with the slick joint  132  to allow axial movement of the running tool without breaking the seal provided by the cementing bushing. The mandrel  104  of the released running tool can be used to raise the cementing bushing  130  to cause the lugs  133  to move in and unlock from the liner hanger. The liner hanger  70  is shown with an annular groove  72  for receiving the lugs  133 . The cementing bushing  130  seals between a radially outward liner running adapter of the liner hanger and a radially inward running tool mandrel. 
     Ratchet ring  136  is also shown in  FIG. 1E . This ratchet ring allows the packer element  122  to be pushed downward over the upper cone  124 , then locks the packer element in its set position. 
     The packer element  122  may be set by using spring-biased pusher C-ring  180  (see  FIG. 1C ) which, when moved upwardly out of the tieback receptacle  102 , will be forced to an expanded position to engage the top of the tieback receptacle. The released running tool may be picked up until the packer setting subassembly is removed from the top of a tieback receptacle, so that the pusher C-ring  180  is raised to a position above the top of the tieback receptacle and expanded outward. When the packer setting assembly is in this expanded position, weight may be slacked off by engaging the pusher C-ring  180  to the top of the tieback  102 , which then causes the packer element  122  to begin its downward sealing sequence. When weight is set down, the expanded pusher C-ring  180  transmits this downward force through the tieback receptacle  102  to the pusher sleeve  121 , and then the packer element  122  (see  FIG. 1E ). A sealing ring  182  is shown in  FIG. 1C  between the packer setting assembly and the tieback receptacle to aid in setting the packer element with annulus pressure assist. The lower portion of  FIG. 1C  illustrates the upper portion of a clutch  185  splined to the OD of the running tool mandrel  104  to transmit torque while allowing axial movement between the clutch and the mandrel to disengage or reengage the clutch from the hanger. The central portion of the clutch  185  is shown in  FIG. 1D , and may move in response to biasing spring  184 . 
     The first time the packer setting assembly is moved out of the polished bore receptacle, a trip ring may snap to a radially outward position. When the packer setting assembly is subsequently reinserted into the polished bore receptacle, the trip ring will engage the top of the polished bore receptacle, and the packer setting C-ring is positioned within the polished bore receptacle. When set down force is applied, the trip ring will move radially inward due to camming action. The entire packer setting assembly may thus be lowered to bottom out on a lower portion of the running adapter prior to initiating the cementing operation. The next time the packer setting assembly is raised out of the polished bore receptacle, the radially outward biasing force of the C-ring will cause the C-ring to engage the top of the tieback. Further details regarding the packer seating assembly are disclosed in U.S. Pat. No. 6,739,398, hereby incorporated by reference. 
     The packer element  122  may be of a construction as described in U.S. Pat. No. 6,666,276, hereby incorporated by reference, comprising an inner metal body for sliding over the cone and annular flanges or ribs which extend outwardly from the body to engage the casing. Rings of resilient sealing material may be mounted between such ribs. The seal bodies may be formed of a material having substantial elasticity to span the annulus between the liner hanger and the casing C. 
     The packer setting assembly thus allows the C-ring to be locked in a collapsed position by a locking mechanism to prevent the C-ring from moving to its expanded position. As discussed above, this allows the packer setting assembly to be pulled out of the tie back receptacle one time without releasing the C-ring, and allows the lockout mechanism to engage the top of the tie back receptacle for weight set down. The next time the packer assembly is pulled out of the tie back receptacle, the C-ring is allowed to expand radially outward for engagement with the top of the tie back receptacle. 
     The C-ring seat subassembly  170  as shown in  FIG. 1D  may be disposed beneath the upper C-ring seat subassembly  110  shown in  FIG. 1B . The lower C-ring seat subassembly  170  is secured within the running tool bore by shear pins  172 . Sleeve  174  thus supports seat  176 . The ball  118  may first land on the upper seat  116  as shown in  FIG. 1B , and once the ball is released from the upper seat it will land on the lower seat  176 , as shown in  FIG. 1D . Once the ball is seated on the lower seat, a predetermined pressure may be applied to shear pins  172  and move the ball seat  176  and the sleeve  174  downward to uncover the ports  173 . This increased pressure may move the piston sleeve  177  to release the slip. Higher fluid pressure may then be applied to cause the piston sleeve  177  to move further upward and thereby disengage the running tool from the set liner hanger. Assembly  170  releases the retrievable portion of the tool to be returned to the surface from the downhole portion of the tool and the set liner. Upon raising of the inner piston  177 , the running tool may be raised from the set liner hanger, but prior to setting of the packer, thus releasing the ball and permitting circulation of cement downwardly through the tool and upwardly within the annulus between the liner and casing. 
       FIG. 1D  also illustrates a hydrostatic balance piston  175  for balancing fluid pressure across the seal  193  to increase high reliability for the operation of sleeve  174 . Seals  193  above and below port  173  are thus subjected to substantially the same fluid pressure on both sides of the seals, thereby enhancing operation of the sleeve  174 .  FIG. 1D  also illustrates C-ring  178  for gripping the liner hanger  70 . The C-ring may be moved radially to position so that it may contract radially inward, thereby releasing the running tool from the liner hanger. 
       FIG. 1G  illustrates a lower portion of the tool, including a ball diverter  140  and a liner wiper plug release assembly  150 . The assembly  150  replaces the need for shear screws to secure the liner wiper plug to the running tool. The plug holder shown in  FIG. 1G  is functionally similar to the plug release assembly disclosed in U.S. Pat. No. 6,712,152, hereby incorporated by reference. Tool components and operations not detailed herein may be functionally similar to the components and operations discussed in U.S. Pat. No. 6,681,860 or U.S. Pat. No. 6,739,398, each hereby incorporated by reference. 
     After activating the lower C-ring seat subassembly  170  (see  FIG. 1D ), the operator may lift up the tool to pass the ball through seat  176 . A drop in pressure will indicate that the ball has passed through the ball seat, allowing circulation through the running string to continue, and the ball to be pumped downwardly into the ball diverter. Fluids are then circulated through the tool awaiting cement displacement. Cement is then injected through the running tool, and a pump down plug (PDP) follows the cement. The PDP enters the liner wiper plug and forms a barrier to the previously displaced cement and the displacement fluid. 
     The slip or slips are kept from prematurely setting as the running tool and slip are run into the wellbore by a locking member.  FIG. 2  shows the slip releasing mechanism  210  in the run in position. The ports  166  are isolated by the ball sleeve  112 , so that fluid pressure within the string and thus within the interior of the mandrel  104  cannot move the slip releasing piston  216  axially upward to release the slip. The locking sleeve or collar  220  with collet heads  215  functions as a slip locking device and is designed to keep the piston  216  and the tieback receptacle  102  from being able to move up while running the tool into the hole, thereby keeping the slip from prematurely releasing and setting. More particularly, the circumferentially spaced fingers or slats  214  extending downward from the upper collar body each have a collet head  215  which fits within groove  213  in mandrel  104 , and is prevented from moving axially by the upper sleeve portion of piston  216 . The tieback pickup ring  212  also keeps the tieback from moving down when the liner is picked up for any reason, since if the tieback were to move down it could set the packer. As long as the slip releasing piston  216  does not move upward to unlock the locking collar  220 , the slip  126  cannot release.  FIG. 2  also shows a debris cover  222  to prevent relatively large objects and debris from traveling down the hole and preventing the reliable operation of the slip releasing mechanism. 
     The ball seat  116  and sleeve  112  move axially down after a ball lands on the seat  116 , thereby exposing hydraulic ports  166  to the slip releasing piston  216 . The slip releasing piston  216  thus is moved up in response to pressure within the mandrel, letting the collar  220  and collet heads unlock and move up to allow the slip to move up and release from the mandrel. Once the collet heads  215  enter the groove  228  in the slip releasing piston  216  (see  FIG. 3 ), the locking collar  220  is unlocked and may continue to move upward with the slip releasing piston.  FIG. 4  illustrates the slip releasing piston  216  moved upward to engage an end surface of the collar  220  and pull up on the slip  126 , thereby allowing the slip to expand into engagement with the casing or other downhole tubular. 
     The locking collar  220  thus keeps the tieback  102  from moving up and prematurely releasing the slip  126 . The tieback  102  may be threaded to a pusher sleeve which is attached to the cone  128 , which in turn is attached to the tie bars  117  which pull on and release the slip from the pocket  119 , so that the slip may thereafter move radially outward and set. As the slip releasing piston  216  moves up after engaging collar  220 , it pushes the tieback pickup ring  212  up, since C-ring  212  is supported on a lower end of tieback pickup ring adjustment nut  224 , which is threaded at  225  to the collar  220 . As the slip releasing piston  216  moves up with the C-ring  212 , the pickup ring moves into a groove  230  in the releasing sleeve  226 , releasing from the tieback  102 , thus releasing the retrievable portion of the tool from the anchored portion of the tool. During movement of the piston  216 , the tieback pickup ring releasing sleeve  226  remains stationary with the mandrel  104 . The tieback pickup ring  212 , which acts as a releasing device, in turn pushes up the tieback  102  to release the slip. Once the locking collar  220  is disengaged, the tieback  102  may move axially to release the slip. For these applications, the slip or slips, once released, may be set by applying a substantial weight to the tool to set the slip. 
     A releasing ring adjustment nut  231  (see  FIG. 5 ) is threaded to the mandrel  104 , so that the releasing sleeve is axially moveable at the surface then made stationary with the mandrel. Tieback pickup ring adjustment nut  224  is similarly threaded to attach to the collar  220 , which is axially moveable with the piston  216  once released. This allows the collar and the tieback pickup ring adjustment nut move up in response to movement of the slip releasing piston  216  to release the slip. 
       FIG. 5  shows the tieback pickup ring  212  released, which is accomplished by further upward movement of the slip releasing piston  216 , as explained above. As the slip releasing piston continues to move up, it pushes on the collar  220  and the adjustment nut  224 . The adjustment nut  224  engages the tieback releasing ring  226 , so that as the tieback pickup ring moves up, it will snap into a groove  230  on the tieback pickup ring releasing sleeve  226 . Releasing sleeve  226  includes downward extending and circumferentially spaced slats or fingers  232 .  FIG. 5  shows the slip releasing piston fully stroked, and the tieback pickup ring fully disengaged. 
       FIG. 4  illustrates the adjustment nut  224 , which includes axial extending slots  233  for receiving one of the circumferentially spaced slats or fingers  232  of the releasing sleeve  226  therein, and a plurality of circumferentially spaced grooves  230  for receiving the C-shaped releasing ring  212  therein. During run in of the tool, C-ring  212  is prevented from collapsing due to engagement with the exterior surface of the lower end of the releasing sleeve slats  232 . When the tieback and the tieback pickup ring are moved upward with the adjustment nut by the piston  216 , normally collapsed C-ring  212  automatically retracts inward to release the tieback  102 , and thereby release the anchored portion of the running tool from the portion to be retrieved to the surface. 
     Those skilled in the art will appreciate that the running tool incorporates a locking device to keep the slip from releasing and prematurely setting while tripping into the wellbore. Fluid pressure within the drill string, rotation of the drill string, or axial forces exerted on the drill string will not inadvertently release the slip. The locking collar  220  and C-ring  212  keep the slip  126  locked in its run in position. Only after the ball is dropped and lands on the ball seat so that the hydraulic port  166  is opened can fluid pressure be applied to push the slip releasing piston up. As the piston moves up, it unlocks a locking collar  220 . Further upward movement of the piston  216  releases the slip  126  and still further movement disengages the tieback pickup ring  212 , which unlocks the retrievable running portion of the tool from the slip release piston and related components. 
     Travel of the piston  216  will push up on the tieback pickup ring  212 , which picks up the tieback, which in turn picks up the tie bars which pick up the slip  126  to release the slip. Continued upward movement of a slip releasing piston will unlock the pickup ring  212  from the tieback  102 , thereby unlocking the tieback from the running tool. The slip is thereby released and set, and the slip releasing assembly is disengaged from the liner hanger. 
     If the slips are circumferentially spaced, the reaction of the slip moving up the cone creates hoop loading to cause lower and more uniform stress in the casing and liner hanger. The loads are transferred circumferentially, rather than radially inwardly, thereby preventing hanger collapse and burst of the casing. The upper end of each slip may be connected to the lower end of a tie bar which extends slidably through the downwardly and inwardly tapered cone for the slip. 
     In an alternative slip assembly, the slip assembly may include a ring disposed about the slip cone in which there is a recess beneath the cone taper. The recess receives and retains the lower end of the slip when in its contracted position. However, as the slip is pulled upwardly by raising of the tie bar or slat, the lower end of the slip is pulled out of the recess and the slip is permitted to expand outwardly against the casing. 
     If the slip is a C-shaped slip, it has the ability to contract and expand between a contracted run-in position, and its extended or maximum expansion position. This maximum expansion position preferably is the as-fabricated or as-machined position for the slip. Thus, the slip may be designed so as to approach this expanded position as the slips expand outwardly into engagement with the casing. 
     Those skilled in the art will appreciate that the slip releasing mechanism as disclosed herein may release a single slip which is set to anchor the tool in the downhole tubular, or may release a plurality of axially spaced and/or circumferentially spaced slips to similarly set the tool in the downhole tubular. Also, the releasing mechanism could be employed to release other components of the downhole tool which are moved radially outward to engage the downhole tubular, such as a packer, although premature setting of a downhole tool slip is a more significant problem than premature setting of a packer, since a prematurely set packer may not prevent axial movement of the downhole tool. 
     The techniques of the present invention are particularly well suited for preventing the premature release of slips and the desired reliable setting of slips for a downhole liner hanger, but may also be used to prevent premature setting or releasing of downhole packers, downhole valves, multilateral tools, and other downhole tools. 
     As disclosed herein, the hydraulic ports are opened in response to a ball landing on a seat, which then shifts the ball seat downward to expose the ports. Various types of closure devices other than balls may be used for this purpose, including plugs. If used, the ball may be either deformable or the seat may be radially expanded to allow the ball to ultimately pass through the seat. Hydraulic pressure above the closed seat may be used to move the releasing piston upward, but in other embodiments the downhole tool components could be arranged so that the high pressure above the closed seat passes through the hydraulic port to push the piston downward, which then causes the release of the lock. In still other embodiments, a pair of axially spaced plugs or valves may be positioned along the bore of the mandrel, and pressurized gas released within the bore to pass through the hydraulic port and actuate the piston. 
     In a preferred embodiment, the same piston is used to move axially to unlock the locking member, and continues to move axially to move the releasing ring and the tieback axially, and finally releases the releasing ring from the tieback when the slip is set. In other embodiments, more than one piston could be used for achieving these purposes. It should be understood that the increase in pressure above the seated ball may accomplish each of these tasks in a successive and fairly short timeframe. Alternatively, pressure levels could be increased above the seated ball so that, for example, a first pressure is used to unlock the locking member, and a second higher pressure is then used to move the releasing ring and tieback upward, then if desired, a still higher pressure used to mechanically separate the tieback from the retrievable portion of the tool. 
     As disclosed herein, a collet mechanism is used to lock and subsequently release of the slips once the piston has moved axially upward in the disclosed embodiment. In other embodiments, the function served by the collet mechanism to unlock the slips could be accomplished with a C-ring, which similarly fits within a groove to lock the locking collar to the mandrel until the groove in the piston is aligned with the C-ring to release the collar from the mandrel. Also, a C-ring preferably is used to release the tieback from the portion of the running tool to be retrieved, although a collet mechanism could be used rather than a C-ring for this purpose. 
     While preferred embodiments of the present invention have been illustrated in detail, it is apparent that modification and adaptations of the preferred embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention as set forth in the following claims.