Abstract:
A slip assembly ( 60 ) for securing a tool in a well includes an upper and a lower c-ring slip body ( 12, 14 ) each including an outer gripping surface ( 16, 18 ) and inner gripping surface ( 20, 22 ). An actuator member ( 68 ) is axially movable relative to both slip bodies and includes a camming surface ( 70 ) for engagement with the upper and lower slip bodies. The slip assembly is reliably able to withstand high axial loads, and forces are equally distributed between the slip bodies.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to downhole slip assemblies of the type used to secure a tool within a downhole tubular in a well. More particularly, this invention relates to a downhole slip assembly having a c-ring slip construction capable of reliably withstanding high axial loads. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various types of slip assemblies have been devised for securing a tool at a desired depth within a downhole tubular. Many such devices include multiple slips, slip arms, cages, and cones. Slip assemblies with dozens of downhole components are inherently a reliability concern. For example, slip segments may fall off a respective slip arm, causing failure of the downhole tool. Slip assemblies including numerous components may also cause local overstressing of the downhole casing or other tubular due to tolerance variation buildup, thereby causing casing failure due to the non-uniformity of distributing stresses over all the slip segments. 
         [0003]    Reducing overstressing of a liner hanger body or a casing from a slip assembly in high axial load applications conventionally requires a sufficient slip area to handle the demanding loads. Increased loads may be the result of the longer and heavier liners, and their corresponding increased test pressures. To achieve additional slip area, additional slips and cones may be used, or the slip taper length may be made longer to achieve more slip area without adding system components. 
         [0004]    U.S. Pat. No. 1,066,000 discloses slips for anchoring in a well. The well packer disclosed in U.S. Pat. Nos. 4,512,399 and 4,582,134 include slips and an expander with tapered expansion surfaces. U.S. Pat. No. 5,413,180 discloses a gravel packing service tool with slips. U.S. Pat. No. 5,906,240 discloses a c-ring slip having a passageway for installation of lines therethrough. U.S. Pat. No. 6,655,456 discloses a liner hanger assembly, and U.S. Pat. No. 6,761,221 discloses a liner hanger assembly with a c-ring slip body as shown in  FIGS. 2A and 5 . U.S. Pat. No. 6,739,39 8  discloses a linger hanger running tool with c-ring slips, as shown in  FIGS. 1G ,  2 B,  8 E, and  9 A. 
         [0005]    The disadvantages of the prior art are overcome by the present invention, and an improved slip assembly for securing a tool within a downhole tubular in a well is hereinafter disclosed. 
       SUMMARY OF THE INVENTION 
       [0006]    In one embodiment, a slip assembly is provided for securing a tool or tubular within another downhole tubular in a well. An upper c-ring slip body and a lower c-ring slip body each include a plurality of circumferentially spaced outer gripping surfaces. An actuator member is axially movable relative to both the upper and lower c-ring slip bodies, and has camming surfaces for engagement with each slip body, which preferably is biased radially outward for engagement with the downhole tubular. A plurality of circumferentially spaced and axially extending slats interconnect the upper and lower slip bodies, with each slat having a outer slat surface spaced radially inward of the upper and lower gripping surfaces. 
         [0007]    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 
         [0008]      FIG. 1  is an isometric view of the c-ring slip assembly according to the present invention. 
           [0009]      FIG. 2  is an isometric view of the c-ring slip assembly after a machining operation and before heat treating. 
           [0010]      FIG. 3  is a side view of a portion of a c-ring slip assembly in the reduced diameter or run-in configuration, and portions of the slip assembly are shown in greater detail in  FIGS. 4 and 5 . 
           [0011]      FIG. 6  is a side view of a portion of a c-ring slip assembly in the engaged or enlarged diameter position, and portions of the slip assembly are shown in greater detail shown in  FIGS. 7 and 8 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0012]      FIG. 1  depicts a c-ring slip assembly  60  which includes an upper c-ring slip body  12  and a lower c-ring slip body  14  each including a plurality of circumferentially spaced outer gripping surfaces  18  and  16 , respectively and also preferably including a plurality of circumferentially spaced inner gripping surfaces  20 ,  22 , which also serve as cam engaging surfaces, as discussed below. Each c-ring slip body thus includes a plurality of circumferentially spaced gripping members  24 , with connecting sections  26  on each slip body not including outer gripping surfaces thereon, but instead circumferentially and fixedly interconnecting the circumferentially spaced gripping members  24 . Each c-ring slip body preferably has a circumferential span of at least 250°, and preferably at least 300°. 
         [0013]    As shown in  FIG. 1 , the upper and lower c-rings  12 ,  14  are spaced axially apart, with this fixed axial spacing being provided by a plurality of circumferentially spaced and axially extending slats  28  which rigidly interconnect the upper c-ring slip body  12  and the lower c-ring slip  14 . Each slat has an outer slat surface  30  which is spaced radially inward of both the adjacent upper and lower outer gripping surfaces on the upper and lower slip bodies, so that the slats  28  do not interfere with the slips gripping an outer tubular body, and allow fluid to flow axially in the gap between a slat  28  and the tubular to be gripped by the slip assembly. In a preferred embodiment, the c-ring slip assembly as shown in  FIG. 1  is biased radially outward, and may be retained in an inward position until released, at which time the outward bias will cause the slip bodies to engage the downhole outer tubular. An actuator hanger body  68  as discussed below may force the outer gripping surfaces  18 ,  16  into secured engagement with an outer tubular and may force inner gripping surfaces  20 ,  22  into secured engagement with an exterior surface of a tool component, such as actuator hanger body  68 . A portion of sections  26  may be cut away to reduce the axial length of each connecting section, as shown in  FIG. 1 , thereby providing for axially extending cutouts  34 ,  32  which result in an enhanced flow past the set c-ring slip assembly. 
         [0014]      FIG. 2  depicts a c-ring slip assembly after machining and prior to stress relieving and hardening. If the assembly were manufactured as shown in  FIG. 1  and then heat treated, the assembly would be subjected to high stress points, and likely would significantly deform as a result of the heat treating process. Since warping of a slip body may undesirably cause nonuniform loading of the slip body on the casing being gripped by the slip assembly, the unit as shown in  FIG. 2  may be initially machined with an lowermost complete ring  40  and an uppermost complete ring  42 , and with circumferential bars  43 ,  44 ,  45  and  47 ,  48 ,  49  interconnecting the ends of each c-ring. Each of these bars is removed by machining after stress relief and hardening operations, and removal of these components along with large slat  52  ultimately creates a circumferential gap  54  as shown in  FIG. 1  between the ends of each of the upper and lower c-rings. The connection sections  26  each between a pair of circumferentially spaced gripping members  24  as shown in  FIG. 1  may be formed by removing portions of bars  49  and  45  after stress relief and hardening operations. The circumferential bars  45  and  49  preferably extend between ends of adjacent gripping members  24  to maintain the desired shape of the slip assembly during heat treating.  FIG. 2  also depicts a plurality of circumferential bars  56 ,  58  which act between circumferentially spaced slats  28 . These additional axially spaced bars also maintain the desired geometry of the slip assembly during heat treating, and subsequently are removed by machining for creating a sizable gap between adjacent slats  28 . The circumferential width of each slat  28  preferably is less than the circumferential width of each gap  32 ,  34  between adjacent gripping members  24 . 
         [0015]    The configuration of the slip assembly as shown in  FIG. 2  thus maintains the desired configuration and close tolerances between components for the c-ring slip assembly during machining, stress relief, and tooth hardening operations. Machining is also performed with radiused corners to reduce stress concentration points. 
         [0016]    Referring again to  FIG. 1 , the reduced diameter of the circumferentially spaced connecting sections  26  forms a circumferential and radial gap between each connecting section and the radially outward tubular to be gripped, thereby forming a fluid flow channel when the slip assembly is set. Moreover, each of the plurality of upper and lower gripping members  24  include axially extending slip portions  15 , as shown in  FIG. 1 , which extend axially beyond the adjacent connecting sections  26  to form a circumferential gap  32 ,  34  between adjacent gripping members, so that the axial length of connection section  26  is less than the axial length of the adjacent gripping members  24 . 
         [0017]    Each C-ring body  12 ,  14  preferably includes a solid C-shaped ring, as shown in  FIG. 1 . The upper C-ring slip body, the lower C-ring slip body, and the plurality of slats are preferably formed as a unitary structure, and preferably a monolithic structure. The entire slip assembly as shown in  FIG. 1  may thus first be machined from a single tubular piece to form the shape as shown in  FIG. 2 , then the slip assembly subjected to stress relief and slip tooth hardening operations, and subsequently machined to form the final shape as shown in  FIG. 1 . 
         [0018]    Referring now to  FIG. 3 , the slip assembly  60  on the downhole tool is positioned within an outer tubular OT at the desired depth in the well. The tool includes a mandrel  62  having a central bore  64  for passage of fluid. An annulus  65  thus exists between the OD of mandrel  62  and the ID of actuator member  68 . The slip assembly as shown in  FIG. 3  is thus in the reduced diameter or run-in position with the gripping surface  16 ,  18  as shown in  FIG. 1  being out of engagement with the inner surface of the outer tubular OT. An actuator member, such as liner hanger actuator body  68 , is provided radially within the slip body and includes upper and lower tapered camming surfaces  70  (see  FIG. 6 ) for sliding engagement with the interior surfaces  20 ,  22  of the slip body (see  FIG. 1 ). Actuator member  68  is thus moved axially with respect to the slip body to set the slips, and may be powered by various mechanisms, including hydraulically actuated pistons and/or set down weight. Various types of seals  78  may be used to seal between the actuator  68  and the packer sleeve  80  shown in  FIG. 3 , including O-ring or Chevron- type packing in one or more grooves. 
         [0019]      FIG. 4  is an enlarged view of an upper portion of the slip assembly shown in  FIG. 3  and illustrates in greater detail tie bars  74  extending downward to the slip body and having a catch member  76  at the lower end thereof for fitting within the respective groove in the slip body to hold the slip body in the run-in position. Each of the upper and lower slip bodies may be biased radially outward, and includes a retainer  71  as shown in  FIG. 4  for preventing the slip body from moving radially outward into engagement with the outer tubular OT until the cone packer sleeve  80  (see  FIG. 3 ) and the tie bars  14  move axially to release the slip body.  FIG. 5  is a detailed view of a lower portion of the slip assembly shown in  FIG. 3 , and illustrates the radially outer teeth  16  and the inner teeth  22  shown in  FIG. 1 . 
         [0020]    In  FIG. 6 , the actuator member  68  has moved downward, so that the inner camming surfaces on the actuator member slidably engage the inner surfaces on the slip body, thereby forcing the slip body into gripping engagement with the outer tubular OT. It should be understood that the upper and lower gripping surfaces on the slip body may be formed by axially spaced gripping teeth, although various other forms for grippingly engaging the tubular may be provided, including grit surfaces. Teeth  16 ,  18  on the outside of each gripping member  24 , as shown more clearly in  FIG. 7 , may have downwardly angled teeth, so that the teeth dig into the outer tubular OT and prevent the slip assemblies from sliding downward with respect to the outer tubular. The inner surface  20 ,  22  on the slip bodies  12  and  14  may have upwardly projecting teeth, so that these inner teeth engage the slip and force the slip to bite into the exterior tapered surface of the actuator member when the actuator member moves downward, thereby axially locking the position of the actuator member with respect to the outer tubular OT. 
         [0021]    By providing a slip body as disclosed herein including an upper C-ring slip body and a lower C-ring slip body, substantial axial forces may be transferred from a slip assembly to the tubular being gripped. It is significant that the desired high gripping forces are uniformly applied to each gripping surface on the slip body, and this objective is obtained by providing a unitary and preferably a monolithic slip body, as disclosed herein. This unitary slip body thus cooperates with an actuator member which is also unitary from at least that part of the camming surface on the actuator member which engages the interior surfaces of the upper C-ring slip members  12  and the actuator member camming surfaces which engage the lower C-ring slip members  14 . Significant advantages are obtained by greatly reducing the number of slip assembly components compared to prior art assemblies. Moreover, dimensional stability is achieved between camming surfaces on the actuator member, the interior surfaces of the slip bodies engaged by the actuator member, and the outer gripping surfaces of the slip body which engage the tubular being gripped. Manufacturing tolerances may thus ensure that each of the upper C-ring slip body and lower C-ring slip body are released together and move simultaneously outward to uniformly engage the tubular. Moreover, the upper and lower slips are axially “fixed” or spaced apart relate to the actuator camming surfaces so that the actuator exerts substantially the same radial force on each slip, which in turn exerts substantially the same force on the tubular being gripped. The taper on the camming surfaces of the actuator member and the slip body may be controlled to accommodate the desired load. In some applications, three or more integral slip bodies may be provided each moving in response to a single actuator. 
         [0022]    Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.