Patent Publication Number: US-7581596-B2

Title: Downhole tool with C-ring closure seat and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. Patent Application Ser. No. 60/785,653, filed Mar. 24, 2006 for a DOWNHOLE TOOL WITH C-RING CLOSURE SEAT, which is incorporated herein in its entirety for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates to downhole tools adapted for receiving a ball or other closure member to provide for the increase in fluid pressure above the seated closure within the tool, thereby actuating components of the tool. More particularly, the present invention relates to a liner hanger assembly for hanging a liner in a well, and to a relatively simple and highly reliable closure seat which allows a ball to reliably pass by the seat after desired tool operations are complete. 
   BACKGROUND OF THE INVENTION 
   Various types of downhole tools are adapted for utilizing an increase in fluid pressure to actuate components of the tool. Packer setting tools, multilateral tools and liner hangers are plus exemplary of downhole tools which rely upon an increase in fluid pressure above a seated closure to actuate the tool. 
   Some tools utilize collet fingers as a ball seat, so that the collet fingers are shifted from the contracted position to an expanded position to allow the ball to drop through the expanded ball seat. Various problems with this design may occur when the collet fingers fail to properly seal and do not allow for pressure to build up so that the collet fingers can move downward and let the ball drop through the seat. Another problem with this type of expandable ball seat is that wellbore fluids pass by the collet fingers, thereby eroding the fingers and tending to cause the ball seat to fail. A ball seat design with collet fingers may also fail to seal properly and not allow for the pressure to build up so that the collets release to pass the ball through the seat. U.S. Pat. Nos. 4,828,037, 4,923,938, and 5,244,044 are examples of patents disclosing expandable ball seats. 
   U.S. Pat. No. 5,553,672 discloses another design for setting a ball on a seat. This design relies upon a rotating ball valve, so that in one position there is a small hole in the valve which acts as the ball seat. A small ball lands on the small hole, and pressure is applied to the tool. Pressure is applied to rotate the ball, allowing the small ball to drop. This design is complicated with many parts and components that may cause failure. 
   U.S. Pat. No. 6,681,860 discloses a yieldable ball seat. Quality control for the expandable area may be difficult, and the expandable ball seat may not yield when intended. Material control is also important since the expandable areas expand at a certain pressures. Expandable ball seats thus do not always reliably release the ball at a preselected pressure. In some situations, pressure used to release the ball from the upper seat may generate a full force sufficient to pass the ball through the lower seat, which then makes it impractical to further operate the tool. High pressure applied to the ball releasing system may also damage the tool or damage the skin of the downhole formation. 
   U.S. Pat. No. 6,866,100 discloses a mechanically expanding ball seat which utilizes pipe manipulation of a drill string after the liner hanger is set to open the seat and release the ball. This system releases the ball mechanically rather than using fluid pressure. The design as disclosed in this patent is complicated, and one has to equalize the pressure across the ball seat before mechanically manipulating the drill string to release the ball. 
   The disadvantages of the prior art are overcome by the present invention and an improved downhole tool with a C-ring closure seat for receiving a ball or other closure member is hereinafter disclosed. 
   SUMMARY OF THE INVENTION 
   According to one embodiment, a liner hanger assembly includes a tool mandrel supported from a running string, a slip assembly for setting slips to engage the casing and support the liner hanger from the casing, and a releasing mechanism for releasing the set liner hanger from portions of the tool returned to the surface. The liner hanger assembly further comprises an expandable C-ring seat positioned about a central flow path in the tool for seating the closure member. The C-ring is initially retained in an upper position by a radially outward retainer. A seal is provided above the C-ring for sealing with the ball or other closure member when seated on the C-ring. A release member, such as a shear pin, releases the C-ring for axial movement in response to a predetermined fluid pressure above the ball. An enlarged C-ring receiving cavity is provided for receiving an expanded C-ring when released by the releasing member, thereby releasing the closure member from the C-ring. The desired liner hanger operations may be performed with increasing fluid pressure controlled by the operator at the surface. The ball or other closure member may be released upon completion of the desired tool operations. In another embodiment, the C-ring seat and the releasing member may be provided in other downhole tools, including a production packer, a downhole setting tool, or a multilateral tool. 
   In another embodiment, the liner hanger assembly as discussed above is provided with an expandable C-ring and a seal for sealing with the closure member when positioned on the C-ring. A shear pin release member need not be provided, and instead the operator may selectively pick up the work string, thereby lifting a sleeve-shaped retainer which holds in pins which serve as stops to hold the C-ring in an axially intermediate position. Upward movement of the retainer thus allows the C-ring to expand to its expanded position within an enlarged lower diameter cavity, thereby releasing the ball. A similar assembly may be used in other downhole tools to activate tool components in response to a varying pressure level within the tool, including one or more production packers, a downhole setting tool, or a multilateral tool. 
   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 a top view of the C-ring seat subassembly shown in  FIG. 1B . 
       FIG. 3  is a cross-sectional enlarged view of the C-ring seat subassembly shown in  FIG. 1B . 
       FIG. 3A  is an exploded pictorial view of the c-ring seat subassembly shown in  FIG. 3 . 
       FIG. 4  shows the C-ring seat shifted downward, allowing the C-ring to expand and release the ball. 
       FIG. 5  shows another C-ring seat subassembly within the liner hanger assembly shown in  FIG. 1D . 
       FIG. 6  illustrates a ball landed on the seat shown in  FIG. 5 , and the seat shifted downward to an intermediate position in response to fluid pressure above the ball. 
       FIG. 7  illustrates a portion of the running tool shifted upward to remove a retainer which prevented the plurality of pins from moving radially outward, thereby lowering the C-ring to an expanded position to release the ball. 
       FIG. 8  discloses an alternate technique for releasing the ball from the ball seat. 
   

   DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     FIG. 1 , which consists of  FIGS. 1A-1G , illustrates one embodiment of a liner hanger tool  100  with two C-ring seat subassemblies each for seating with a closure member in a liner hanger application. An upper C-ring seat subassembly  110  is shown in  FIG. 1B , and a lower C-ring seat subassembly  170  is shown in  FIG. 1D . Other than components associated with seating and releasing the closure member, the primary components of the liner hanger running tool  100  as shown in  FIG. 1  include a running tool tieback locking mechanism  80  ( FIG. 1A ), a slip release assembly operatively responsive to the upper C-ring seat assembly  110 , packer setting lugs  180  ( FIG. 1C ), a liner hanger release assembly 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. The C-ring seat subassemblies disclosed more fully below are used in the liner hanger running tool to activate the slip assembly using an upper C-ring ball seat, and to separately activate a liner hanger releasing assembly using a lower C-ring ball seat. The function served by each C-ring ball seat will thus vary with the tool functions being activated, and the pressure levels and sequencing of the tool. 
   To hang off a liner, the running tool  100  is initially be 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 beneath 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 ). 
   The running tool  100  also includes a cementing bushing  130  (see  FIG. 1E ), and 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. 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 relative to components to remain in the well without breaking the seal provided by the cementing bushing  130 . 
     FIG. 1A  also illustrates a tieback locking mechanism  80 . A split ring  82  locks the tieback  102  to the running tool mandrel  104 . The tieback locking mechanism prevents premature actuation of the tool as it is run in the well. The locking mechanism  80  unlocks the tieback  102  to allow the slips  126  to be set. More particularly the slips  126  are kept from prematurely setting as the tool  100  is run into the wellbore by the tieback locking mechanism  80 , which grippingly engages the upper end of the tieback  102  to prevent its upward movement prior to setting the slips. 
   The tool actuator subassembly  110  as shown in  FIG. 1B  is used to release the liner hanger slips for setting, and includes a sleeve  112  disposed within and axially movable relative to the running tool mandrel  104 . The sleeve  112  is held in its upper position by shear pins  114 . 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 subassembly is lowered, fluid pressure may pass through ports  166  to stroke a piston and thereby release the slips for setting. 
   Piston sleeve  160  is disposed about and is axially movable 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 slips 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  160 . Upward movement of the piston sleeve  160  causes its upper end to raise the tieback receptacle  102 , and also raise the slips  126 . 
   The slip assembly  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 three circumferentially spaced slip segments are used, each of three 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  also 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. The cementing bushing  130  cooperates with the slick joint  132  to allow axial movement 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  132  to move in and unlock from the liner hanger. The liner hanger  70  is shown with an annular groove  72  for receiving the lugs  132 . 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 ). This weight also activates a sealing ring  182  (see  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. 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 running tool, 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, and 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. 
   The packer element  122  may be of a construction as described in U.S. Pat. No. 4,757,860, 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 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  when released from the upper seat will land onto the lower seat  176 . Once the ball is seated, the 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 . Higher fluid pressure may then be applied to cause the piston sleeve  177  to move upward and thereby disengage the running tool from the set liner hanger. Assembly  170  releases the remainder of the tool to be retrieved to the surface from 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 tool 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 . More particularly, piston  175  may be pumped upward at substantially atmospheric pressure prior to running the tool in the well. As the tool is lowered in the well and hydrostatic pressure increases, the increased pressure above the piston  173  will be balanced by a substantially identical pressure below piston  173 , and thus is the pressure in the cavity between piston  173  and sleeve  174 , resulting in some downward movement of piston  173  to equalize pressure. 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  illustrates split ring  178  for gripping the liner hanger  70 . The split 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. 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. 
   After activating the lower C-ring seat subassembly  170 , 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 pump down plug follows the cement and the liner wiper plug to form a barrier to the previously displaced cement and the displacement fluid. 
   Referring now to  FIG. 3 , the upper C-ring seat subassembly which serves as a tool actuator for setting the slips is shown in greater detail. Upper sleeve portion  112  includes an annular slot or one or more circumferentially spaced slots  113  as shown in  FIG. 3  each for receiving a respective shear pin  114 , as shown in  FIG. 1B . Upper sleeve portion  112  is threaded at  118  to lower body  111 . One or more external seals  115  on the upper sleeve portion  112  are provided for sealing engagement with the interior wall of the mandrel  104 . A seal  117  is provided on the interior of the upper sleeve portion  112  for sealing with the ball or other closure member when seated on the C-ring  116 . A seal alternatively may be supported on the closure itself, or on another component. The body may be made in two parts, which are connected by threads  118 . The upper sleeve portion  112  is not shown in  FIG. 2 , which is a top view of C-ring  116  and sleeve body  111 . 
   Once the ball has landed on the C-ring  116 , it is sealed with the upper sleeve portion  112  by seal  117 . The operator may then increase fluid pressure in the bore above the seated ball, until the shear pin  114  releases the subassembly to move in a manner of a piston until the lower end of the body engages the stop shoulder  108 , as shown in  FIG. 1B . When in this position, the C-ring  116 , which had been retained in its compressed position by the inner surface of the mandrel which acts as a C-ring retainer, is released to a lower expanded position when entering the larger diameter bore  107  above the stop surface  108 . Releasing the C-ring  116  to its normally relaxed and expanded position thus allows the ball to drop through the C-ring.  FIG. 4  shows the subassembly in the lower position wherein the C-ring has been expanded to release the ball. 
   The C-ring  116  as shown in  FIG. 2  has a plurality of radially outward projections  119  that each pass through circumferentially spaced slots  117  in the lower sleeve portion  111 . The outer surface of the projections  119  engage the inner wall of the mandrel  104  to retain the C-ring in its compressed position prior to shearing the pins  114 . To maintain proper alignment of the C-ring within the bore of the mandrel, the C-ring may be split at the location of one of these projections  119 , so that each end of the C-ring, as well as intermediate portions between these ends, has a projection to engage the bore of the mandrel. 
     FIG. 3  is a cross-sectional view of the C-ring seat subassembly shown in  FIGS. 1 and 2 .  FIG. 3A  is an exploded view and more clearly depicts how the upper sleeve portion  112  may be threaded to the lower sleeve portion  111 , with the C-ring  116  having radially outwardly projecting tabs  119  which each fit within a respective slot  121  in lower body portion  111 . 
   A significant advantage of the C-ring seat mechanisms as shown in  FIGS. 1B and 1D  is that any desired fluid pressure, e.g., from several hundred to several thousand psi, may be used to reliably perform one or more tool operations, e.g., releasing the slips for setting, or releasing the set liner hanger from the running tool. In many cases, high fluid pressures are desired for some tool operations to increase their effectiveness, or to ensure activation at pressures above other tool operation activation pressures. Once these operations are complete, a relatively low fluid pressure may be used to pass the ball through the expanded C-ring seat. Since the ball release operation is performed at a low pressure, and optionally a pressure less than, and in many cases significantly less than, the one or more tool operation pressures, there is less likelihood of damaging the skin of one or more downhole formations during the ball releasing operation. 
     FIG. 5  shows in greater detail the C-ring seat  176  generally shown in  FIG. 1D  mounted within the bore of the running tool mandrel  104 . The lower C-ring seat subassembly  170  serves as a tool actuator for releasing the tool from the set liner, as explained above. Sleeve  174  includes a pair of elastomeric seals similar to the seals  115  shown in  FIG. 3 . In this application, the sleeve  174  has an axially extended lower portion  154 , with its lower end connected to end piece  158 . A radially outer sleeve  155  includes an annular radially outward projection  156  thereon. A plurality of circumferentially spaced pins  157  are mounted in apertures provided in the mandrel  104 , and are radially moving with respect thereto. 
   When in the upper position as shown in  FIG. 5 , the shear pins  172  maintain the entire subassembly in the upward position. Once the ball lands on the seat  176  and pressure increases above the seated ball, the increased fluid pressure will shear the pins  172 , moving the subassembly downward to an intermediate position as shown in  FIG. 6 , wherein the circumferential projection  156  engage the pins  157 , which act as stops to prevent further downward movement of the subassembly. 
   With the sleeve shifted to the intermediate position as shown in  FIG. 6 , apertures  173  in the mandrel  104  adjacent the shear pins  172  allow fluid to flow radially outward of the mandrel  104 , and to an operating piston  177 . Once the tool is activated, piston  177  is raised, raising slotted retainer  159 , which is connected to the lower end of piston  177 . This allows the C-ring  178  to collapse radially inward to release the running tool from the set liner, and prior to setting the packer  122 . The tool may then be lifted upward to ensure that it is disengaged from the set liner hanger. 
   Assuming the function served by lifting piston  177  is the last tool function to be performed, the ball may be dislodged from the tool as follows. The I.D. of top of the liner hanger  70  acts as a retainer to hold the pins  157  radially inward in the  FIG. 6  position. For this embodiment, the retainer is thus part of the liner hanger. The running tool and lower outer sleeve  168  are then pulled upward to a position to allow the pins  157  to be above and free of the retainer, so the pins can move out and release the ball sleeve  174 , thereby resulting in the release of the ball.  FIG. 7  shows lifting the entire tool upward with respect to the set liner hanger. The pins  157  will move radially outward and release the projections  156  to pass below the pins  157 . This action also moves the C-ring  176  to a lower position within the enlarged diameter bore  166  in the mandrel  104 , so that the C-ring  176  may be expanded to pass the ball by the C-ring, as shown in  FIG. 7 . 
   Various components other than pins may be used for moving radially outward and thereby releasing the closure seat to move within the enlarged diameter bore  166  and thus expand outward to release the ball. Radially movable lugs or buttons alternatively could be used, or this function may be served by a C-ring. A portion of the liner hanger  70  may thus act as a retainer to hold the pins  157  radially inward, as shown in  FIG. 6 , so that the running tool may be pulled upward to raise the pins above the upper end of the liner hanger. Other embodiments of a suitable retainer may include slots or windows to allow the pins to move radially outward. Also, this axial movement between the pins and the retainer may be accomplished at the surface by either raising or lowering the running tool. For other applications, a downhole actuator may be provided, such as a hydraulic actuator, to controllably stroke one component axially relative to another to allow the pins to move radially outward. The axial movement of the pins  157  relative to the retainer thus allows the closure seat to release the ball. 
     FIG. 8  discloses an alternative mechanism that will allow for the discharge of the ball from the running tool to regain circulation in the event that the operator cannot release the running tool from the liner hanger. If the running tool release mechanism does not function, the  FIG. 8  mechanism allows the ball to be discharged by increasing fluid pressure above the set ball to shear pins  195  and  196 , thereby releasing the sleeve  174  to move downward and discharge the ball as the C-ring expands into larger bore  166 . The tool and the liner hanger may then be retrieved from the well. 
   The  FIG. 8  operating mechanism also allows tool operation if the pins  157  are prevented from moving radially outward. For example, debris in various passageways in the running tool could prevent the pins from moving outward. In this case, an alternative operating mechanism for releasing the ball from the seat  176  includes the use of a shear member, such as pins  195  and  196  as shown in  FIG. 7 , which interconnect the lower portion  154  of the sleeve  174  and the sleeve  155  radially outward of sleeve  154 . As shown in  FIG. 8 , an increase in fluid pressure above the set ball causes the pins  195  and  196  to shear, dropping the sleeve  154 , and allowing the C-ring  176  to expand into the larger diameter cavity. 
   In order to reduce the likelihood of a ball discharged by an upper seat assembly will land on and inadvertently pass through a lower seat assembly, the lower seat assembly may include two or more sets of axially spaced shear pins  195 ,  196  between the seat sleeve  154  and the sleeve  155  with the radially outward projection  156 . The lower shear pins  195  may each be tightly positioned within a hole provided in the sleeve  174 , while the upper shear pins  196  are positioned within a vertical slot  197  within the same sleeve. A ball landed on the seat  176  while positioned as shown in  FIG. 6  may first cause shearing of the lower shear pins  195 . Limited downward movement of the sleeve  174  relative to sleeve  155  may occur until the upper shear pins hit the upper end of the respective slot  197 . Due to the energy absorbed by shearing the lower shear pins, the upper shear pins are not sheared when the lower pins are sheared, which prevents the tool from improperly actuating or passing the ball through the lower seat. The upper shear pins may have substantially the same pressure rating as the lower shear pins, and will shear at the desired pressure level. 
   While in the  FIG. 6  position, fluid pressure may thus be increased above the seated ball so that the pins  195 ,  196  shear, thereby releasing the ring  176  and sleeve  174  to move downward relative to sleeve  155  and mandrel  104 . This then allows the subassembly to drop to its lowest position as shown in  FIG. 8 , so that the ball is released from the seat  176 . 
   Those skilled in the art should appreciate that the upper C-ring seat subassembly  110  as shown in  FIG. 1B  may be used in a liner hanger running tool to set the slips, and that the lower C-ring seat subassembly as shown in  FIG. 1D  may be used to release the running tool from the set liner hanger, with both C-ring assemblies cooperating with a single ball. In one alternative embodiment, the upper C-ring seat assembly alone, or only the lower C-ring subassembly alone, may be used to operate the liner hanger tool, either because the slips are otherwise set or the assembly is otherwise released from the liner hanger, or because a single C-ring ball seat subassembly may be used to both set the slips and thereafter release the tool from the set liner. In the former case, the slips may be set by an alternative mechanism which does not utilize increased pressure in the bore of the tool to actuate the tool, and the C-ring seat subassembly may be used to release the running tool from the set assembly. In another alternative, the running tool may be released from the set liner hanger by a mechanism that does not involve an increase in fluid pressure in the tool, and thus the C-ring seat subassembly may be used to only set slips. In a second alternative embodiment, both operations may be performed by the same C-ring seat subassembly. A wide range of fluid pressures are thus available to safely and reliably perform different operations at different fluid pressures. A single mechanism may be provided since relatively low pressures may be used to set the slips and then reliably move the C-ring to a position where it may expand within the running tool mandrel and thereby release the ball. For example, a fluid pressure of 1000 psi may be used to set the slips, while a fluid pressure of 2000 psi may be used to release the running tool from the set liner hanger then release the ball. Two or more piston may thus be actuated to perform the desired operations on the tool, and different fluid pressure levels and porting to the different pistons may be used to perform dual or multiple operations with a tool. Providing a comparatively low ball releasing pressure reduces the likelihood of high formation pressure damaging the skin of the formation, thereby enhancing hydrocarbon recovery. 
   Although a suitable location for the upper C-ring seat subassembly and the lower C-ring seat subassembly are shown in  FIG. 1 , the subassemblies may be positioned differently in another liner hanger running tool, including one with primary components of the assembly. If a single C-ring seat subassembly is used in a liner hanger, the assembly may be positioned for porting to two different pistons which actuate the tool, e.g., the slip setting assembly and the liner hanger releasing assembly. The C-ring seat subassembly may be positioned at any location in the tool which provides a central bore through the tool and porting to the pistons. 
   In other applications, the C-ring seat subassembly may be used for performing downhole operations other than those involving a liner hanger, including tools involved in packer setting operations or multilateral operations, tubing/casing hanger running tools, subsea disconnect tools, downhole surge valves, ball releasing subs, hydraulic disconnect tools, and various types of downhole setting tools. In each of these applications, the tool may be reliably operated at relatively low pressures to release the ball or other closure compared to prior art tools due to the use of the C-ring seat mechanism. 
   In the above discussion, the closure member which is used to seat with the C-ring seat mechanism and thereby increase fluid pressure is discussed as a ball, which is commonly used for this purpose in various applications. In other applications, other types of closure members may be used for seating with the C-ring assembly and reliably sealing with the seal above the C-ring. Darts, plugs, and other closure members may thus be used for this purpose. The tools disclosed herein are relatively simple, particularly with respect to the components which seat with the ball and subsequently release the ball from the seating surface, thereby providing high reliability and lower costs compared to prior art tools. 
   A C-ring closure seat is shown in the drawings for seating with the ball or other closure. In other embodiments, multiple dogs, lugs, pins or buttons could be used to form the closure seat. Each of these components could then move radially outward to release the ball when positioned within a large diameter bore of the tool. Also, a dog, lug, pin or button may move radially outward into the slot or groove provided in the tool body, in which case there may be no change in the diameter of the bore in the tool when the closure seat moves from a retaining position to a releasing position. 
   While preferred embodiments of the present invention have been illustrated in detail, it is apparent that modifications 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.