Abstract:
A wellhead assembly having an outer tubular, an inner tubular inserted into the outer tubular, an annular space between the inner and outer tubulars, a lock ring in the annular space, and an activation ring that axially strokes between the lock ring and one of the tubulars. The lock ring selectively locks together the inner and outer tubulars when the activation ring slides between the lock ring and the one of the tubulars. The surface of the activation ring that contacts lock ring is contoured so that an interface surface between the activation ring and lock ring when the lock ring is in its locked position, is offset an angle from an axis of the wellhead that is less than an offset between the axis of the wellhead and an interface surface between the activation ring and lock ring when the activation ring is stroking downward.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to a system and method for locking together tubulars in a wellhead assembly. 
         [0003]    2. Description of Prior Art 
         [0004]    Wellheads used in the production of hydrocarbons extracted from subterranean formations typically comprise a wellhead assembly attached at the upper end of a wellbore formed into a hydrocarbon producing formation. Wellhead assemblies usually provide support hangers for suspending production tubing and casing into the wellbore. The casing lines the wellbore, thereby isolating the wellbore from the surrounding formation. The tubing typically lies concentric within the casing and provides a conduit therein for producing the hydrocarbons entrained within the formation. 
         [0005]    Wellhead assemblies also typically include a wellhead housing adjacent where the casing and tubing enter the wellbore, and a production tree atop the wellhead housing. The production tree is commonly used to control and distribute the fluids produced from the wellbore and selectively provide fluid communication or access to the tubing, casing, and/or annuluses between the tubing and casing. Valves assemblies are typically provided within wellhead production trees for controlling fluid flow across a wellhead, such as production flow from the borehole or circulating fluid flow in and out of a wellhead. 
         [0006]    Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a production tree, or a tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member. Alternately, the inner wellhead member might be an isolation sleeve secured to a production tree. A seal or packoff seals between the isolation sleeve and a casing hanger located within the wellhead housing. 
         [0007]    A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal has inner and outer walls separated by a conical slot. An energizing ring is pushed into the slot to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent. Sometimes a lockdown ring is provided in the annular space between the tubulars, which is put into a position that locks the tubulars to one another when the seal is set. 
       SUMMARY OF THE INVENTION 
       [0008]    Provided herein is an example of a wellhead assembly. In one example, a wellhead assembly includes an axis, an outer tubular, an inner tubular inserted into the outer tubular and defining an annular space between the inner tubular and outer tubular, and a lock ring in the annular space that is moveable from an unlocked position into a locked position. The wellhead assembly of this embodiment also includes an activation ring having a profiled surface that is axially slidable against and in contact with the lock ring to define a contact interface that is offset an angle from the axis when the lock ring is in the locked position, and to define a contact interface that is offset an angle from the axis when the lock ring is in the unlocked position, so that the angle when the lock ring is in the locked position is less than the angle when the lock ring is in the unlocked position. Optionally, a portion of the profiled surface is curved that is in contact with the lock ring when the lock ring is in the unlocked position, and a portion of the profiled surface that is in contact with the lock ring when the lock ring is in the locked position is substantially linear. In one example embodiment, the lock ring is set radially inward from the outer tubular when in the unlocked position, and the lock ring comprises a protrusion that engages a depression formed in an inner radial surface of the outer tubular. Alternatively, the lock ring is set radially outward from the inner tubular when in the unlocked position, and the lock ring has a protrusion that engages a depression formed in an outer radial surface of the inner tubular. In an alternate embodiment, the wellhead further includes a seal assembly that transfers a downward axial force to an upper end of the activation ring and that is energized by an energizing force. The force applied to the activation ring that slides the activation ring along the lock ring may urge the lock ring from the unlocked position to the locked position, and the energizing force can be greater than the force applied to the activation ring. In one example, the outer tubular is a wellhead housing and the inner tubular is a tubing hanger. 
         [0009]    Also provided herein is a system for locking together tubulars that are disposed in a wellhead assembly. In this example the system includes a lock ring that axially rests on one of the tubulars and selectively engages an adjacent tubular thereby axially locking together the one of the tubulars and the adjacent tubular. Also included is an activation ring axially moveable to between the lock ring and the one of the tubulars and having a surface in sliding contact with the lock ring that transitions from a curved profile to a linear profile as the activation ring moves to between the lock ring and the one of the tubulars. In an example embodiment of the system, the activation ring contacts the lock ring along an interface that is offset from an axis of the wellhead assembly by an angle up to about 5 degrees when the lock ring is engagement with the adjacent tubular. Optionally, the activation ring contacts the lock ring along an interface that is offset from an axis of the wellhead assembly by an angle that ranges from at least about 5 degrees to about 30 degrees when the curved profile is in contact with the lock ring. The one of the tubulars can be a tubing hanger and the adjacent tubular can be a wellhead housing. In an alternate example, the one of the tubulars can be a wellhead housing and the adjacent tubular can be a tubing hanger. Optionally, an upper end of the activation ring is in axial contact with a seal assembly, where the seal assembly is energized with an axial force that exceeds a force applied to slide the activation ring from a position above the lock ring to a position adjacent the lock ring. 
         [0010]    Yet further provided herein is a method of locking together tubulars in a wellhead assembly. One example embodiment of the method includes providing a lock ring on a lateral surface of one of the tubulars, applying a force onto the lock ring in a direction oblique with an axis of the wellhead assembly to radially urge the lock ring towards an adjacent tubular, changing the direction of the force to be substantially perpendicular to the axis and engaging the lock ring with the adjacent tubular, and retaining the lock ring in engagement with the adjacent tubular by continuing to apply the force in a direction substantially perpendicular to the axis. The method may further include providing an activation ring having a contact surface that transitions from a curved portion to a linear portion. In one example, the step of applying a force onto the lock ring in a direction oblique with an axis of the wellhead assembly involves axially urging the activation ring so the curved portion slides against a side of the lock ring. In an optional embodiment, the step of applying a force onto the lock ring in a direction perpendicular to an axis of the wellhead assembly includes axially urging the activation ring so the linear portion slides against a side of the lock ring. In an example, the step of continuing to apply the force in a direction substantially perpendicular to the axis includes retaining the activation ring adjacent the lock ring so the linear portion is in contact with a side of the lock ring. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIGS. 1A and 1B  are side sectional views of an example of coupling together tubulars in a wellhead assembly in accordance with the present invention. 
           [0013]      FIGS. 2A-2C  are side sectional detailed views of operation of a locking mechanism of  FIGS. 1A and 1B  in accordance with the present invention. 
           [0014]      FIG. 3  is a side partial sectional view of an example embodiment of the wellhead assembly of  FIGS. 1A and 1B  set over a wellbore in accordance with the present invention. 
       
    
    
       [0015]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0016]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
         [0017]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the improvements herein described are therefore to be limited only by the scope of the appended claims. 
         [0018]      FIG. 1A  is a side sectional view of one example embodiment of a wellhead assembly  10  that includes a portion of a wellhead housing  12  adjacent a tubing hanger  14 . The wellhead housing  12  and tubing hanger  14  are generally tubular members that are spaced apart. An example of a seal assembly  16  is illustrated set within an annulus  18  that is between the tubing hanger  14  and wellhead housing  12 . The embodiment of the seal assembly  16  includes an annular seal ring  20  insertable into the annulus  18 . The seal ring  20  of  FIG. 1A  has an inner leg  22 , which as provided in the sectional view is an elongate member that extends generally parallel with an axis A X  of the wellhead assembly  10 . The example of the seal ring  20  also includes an outer leg  24 ; which also extends substantially parallel with the axis A X  and is longer than the inner leg  22 . The outer leg  24  is set radially outward from the inner leg  22  to define an annular space  26  set between the inner and outer legs  22 ,  24 . The inner and outer legs  22 ,  24  are connected to one another on their respective lower ends by a cross piece that also defines a lower surface of the space  26 . An energizing ring  28  is shown having a lower end inserted into an upper end of the space  26 . In an example, axially urging the seal assembly  20  into annulus  18  then inserting energizing ring  28  into space  26  urges legs  22 ,  24  radially outward from one another thereby creating a pressure barrier in the annulus  18 . A ring-like collar  30  is shown circumscribing a portion of the energizing ring  28  and threadingly engaged with an upper end of the outer leg  24 . 
         [0019]    Further illustrated in the example of the wellhead assembly  10  of  FIG. 1A  is an annular activation ring  32 , which is set in the annulus  18  below the seal assembly  20 . In the example of  FIG. 1A , the lower end of the seal assembly  20  rests on an upper end of the activation ring  32 . Below the activation ring  32  is a lock ring  34  for axially locking together the wellhead housing  12  and tubing hanger  14 . Referring now to  FIG. 2A , illustrated in detailed sectional view is an example of the activation ring  32  and lock ring  34 . In the example of  FIGS. 1A and 2A , the lock ring  34  is in an unlocked configuration, thus the wellhead housing  12  and tubing hanger  14  may move axially with respect to one another. As shown in  FIG. 2A , a surface of the activation ring  32  facing axis A X  defines an inner surface  36  that is shown having a transition  38  where a radius of the surface  36  changes. Opposite the inner surface  36  is an outer surface  40  shown having a lower portion  42  that transitions into an upper portion  44 . The transition  46  at the upper end of upper portion  44  defines where a change in length of radius of the outer surface  40  takes place. 
         [0020]    Profiles  48  are shown formed on the outer surface  40  and above transition  46  to define a handle  50  for raising and lowering the activation ring  32  within the annulus  18  ( FIG. 1A ). In the example of  FIG. 2A , a line L 1  is shown extending tangentially across upper portion  44  illustrating in the example of  FIG. 2A  that the outer surface  40  is generally linear along upper portion in the axial direction. In contrast, outer surface  40  along lower portion  42  is curved and in some portions thereof maintains a consistent radius. A line L 2  is shown tangentially across a portion of an inner surface of the lock ring  34 . In one example embodiment, the line L 2  is at about 20 degrees to about 35 degrees offset from axis A X . 
         [0021]    Still referring to  FIG. 2A , an outer surface  54  of the lock ring  34  is shown having protrusions  56  that extend radially outward and away from inner surface  52 . Referring back to  FIG. 1A , the protrusions  56  are profiled to correspond to depressions  58  shown formed along an inner surface  60  of the wellhead housing  12  and adjacent lock ring  34 . Further illustrated in  FIG. 1A  are wickers  62 ,  64  shown respectively formed on the inner surface  60  of the wellhead housing  12  and an outer surface  66  of the tubing hanger  14 . In an embodiment, the wickers  62 ,  64  are ridge-like members formed in the surfaces  60 ,  66 , so that when the seal  20  is set in the annulus  18 , the wickers  62 ,  64  deform respective outer surfaces of the inner leg  22  and outer leg for enhancing the sealing function of the seal assembly  20 . 
         [0022]      FIG. 1B  illustrates in side sectional view an example of the lock ring  34  set in a locked position and in engagement with the depressions  58  on the wellhead housing  12 . Further illustrated in  FIG. 1B  is that a lower end of the lock ring  34  rests on a shoulder  67  defined where the outer surface  66  of the tubing hanger  14  juts radially outward and away from axis A X . Engaging the protrusions  56  with the depressions  58  axially retains the lock ring  34  in place. Also, by contacting the shoulder  67  with lower end of the block ring  34 , the tubing hanger  14  is prevented from moving axially upward with respect to the wellhead housing  12  by the axially static lock ring  34 . Further in the example of  FIG. 1B , the lock ring  34  engages the profiles  58  by being moved radially outward from axis A X  by downward axial movement of the activation ring  32 . Thus, retaining the activation ring  32  in the position of  FIG. 1B , the tubing hanger  14  is axially constrained to the wellhead housing  12 . 
         [0023]    A detailed example of interaction between the activation ring  32  and lock ring  34  in the locked position is illustrated in side sectional view in  FIGS. 2B and 2C . In  FIG. 2B , shown is an example of the activation ring  32  having a force F applied to its upper end thereby slidingly urging the activation ring  32  to a position adjacent the lock ring  34  ( FIG. 2C ). In one example, the activation ring  32  and lock ring  34  are substantially coaxial when the lock ring  34  is in the locked position. In the example of  FIG. 2B , the lower curved portion  42  of the activation ring  32  is in contact with the inner surface  52  of the lock ring  34 . As such, a resultant force F R  is exerted against the lock ring  34  and shown being in a direction generally oblique to the axis A X . With further downward movement of the activation ring  32 , the direction of resultant force F R  rotates from its oblique orientation and to one that is close to being substantially perpendicular to axis A X  ( FIG. 2 ). In the example of  FIG. 2B , a portion of line L 3  extends along a contact interface between the activation ring  32  and lock ring  34 . Similarly, line L 4  in  FIG. 2C  is drawn along a contact interface between the activation ring  32  and lock ring  34  when the activation ring  32  is substantially adjacent lock ring  34 . As shown, line L 4  is at an offset angle from axis A X  that is less than an offset angle between L 3  and axis A X  ( FIG. 2B ). 
         [0024]    An advantage of the curved lower surface  42  is that the lock ring  34  may be urged radially outward into its locked configuration with the wellhead housing  12  by a stroke distance of the activation ring  32  that is shorter than a corresponding stroke distance in instances where the lower portion  42  is linear. Moreover, by transitioning the outer surface  40  of the activation ring  32  from a curved lower portion  44  to a linear upper portion  44 , the resultant force F R  has a reduced axial component exerted from the lock ring  34  onto the activation ring  32 . As such, more force from the lockdown system may be distributed towards retaining the tubing hanger  14  rather than maintaining the lock ring  34  in its locked position. 
         [0025]      FIG. 3  is a side partial sectional view of one example of the wellhead assembly  10  shown set over a wellbore  68 , where the wellbore  68  extends through a formation  70 . In the example of  FIG. 3 , an example of a lockdown assembly  71  is schematically illustrated for locking the tubing hanger  14  to the wellhead housing  12 . In the example of  FIG. 3 , the lockdown assembly  71  includes examples of the activation ring and lockdown ring as described above. Further illustrated in  FIG. 3  is a string of tubing  72  that depends downward into the bore hole  68  from the tubing hanger  14 . 
         [0026]    An additional advantage of the lockdown assembly illustrated herein is that by transitioning the outer surface of the retaining ring  32 , axial forces required for retaining the lockdown ring  34  in its locked position are reduced that in turn allows for higher preloads on a seal assembly  20  ( FIG. 1A ). Thus, the lock ring  34  can be set at an axial force below that which may initiate energizing of a seal set in the annulus  18 . 
         [0027]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.