Abstract:
A seal seals an annulus in a subsea assembly between a wellhead and a casing hanger landed on a shoulder within a bore of the wellhead. The seal includes a casing hanger seal ring disposed within the annulus. The seal ring engaged with an inner diameter surface of the wellhead, and engaged with an outer diameter surface of the casing hanger so that the seal ring prevents flow through the annulus. A lockdown slip ring is secured to a lower end of the seal ring so that, when the seal ring is energized, the lockdown slip ring engages a substantially smooth inner diameter surface portion of the wellhead and a substantially smooth outer diameter surface portion of the casing hanger to limit upwards axial movement of the casing hanger.

Description:
[0001]    This application claims priority to and the benefit of co-pending U.S. Provisional Application No. 61/467,184, filed on Mar. 24, 2011, entitled “Casing Hanger Lockdown Slip Ring,” which application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates in general to wellhead casing hangers and, in particular, to a casing hanger lockdown slip ring that converts axial loads into radial loads. 
         [0004]    2. Brief Description of Related Art 
         [0005]    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 Christmas tree, or a tubing head. A seal or packoff seals between the tubing hanger and the outer wellhead member. Alternately, the inner wellhead member might be a casing hanger located in a wellhead housing and secured to a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. 
         [0006]    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 energizing ring is a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent. 
         [0007]    Thermal growth between the casing or tubing and the wellhead may occur, particularly with wellheads located at the surface, rather than subsea. The well fluid flowing upward through the tubing heats the string of tubing, and to a lesser degree the surrounding casing. The temperature increase may cause the tubing hanger and/or casing hanger to move axially a slight amount relative to the outer wellhead member or each other. During the heat up transient, the tubing hanger and/or casing hanger can also move radially due to temperature differences between components and the different rates of thermal expansion from which the component materials are constructed. If the seal has been set as a result of a wedging action where an axial displacement of energizing rings induces a radial movement of the seal against its mating surfaces, then sealing forces may be reduced if there is movement in the axial direction due to pressure or thermal effects. A reduction in axial force on the energizing ring results in a reduction in the radial inward and outward forces on the inner and outer walls of the seal ring, which may cause the seal to leak. A loss of radial loading between the seal and its mating surfaces due to thermal transients may also cause the seal to leak. 
         [0008]    Prior art apparatuses that attempt to overcome the problems caused by axial movement of the casing hanger or tubing hanger include lockdown seals. Lockdown seals require formation of a groove in the landing sub or wellhead during the manufacturing process. After the wellhead and landing sub are positioned within the wellbore, the lockdown seal is run to the location of the landing sub where a ring of the lockdown seal either expands or contracts into the groove formed into the wellhead or landing sub, respectively. Unfortunately, the groove often fills with debris prior to run-in of the lockdown seal. The debris prevents engagement of the ring and thus, prevents no lockdown benefits of the lockdown seal result. 
         [0009]    Lockdown seals require a significant increase in production costs. This is due in part to increased costs to modify the basic wellhead or landing sub to include the lock ring groove. In addition, the use of these devices necessitate use of specialized tools and other components to properly land and engage the lockdown seal. Furthermore, prior art lockdown seals require some clearance between the landing sub and the lockdown apparatus of the lockdown seal. This clearance allows the lockdown seal to land in the appropriate location relative to the wellhead and landing sub while also providing the necessary space for the lockdown portion of the seal to engage either the wellhead or the landing sub. The clearance also allows the landing sub to shift before the lockdown device properly engages and arrests movement of the landing sub. In such instances, the landing sub may shift axially and cause the seal to fail. Thus, there is a need for a lockdown seal that overcomes the problems in the prior art described above. 
       SUMMARY OF THE INVENTION 
       [0010]    These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention that provide a casing hanger lockdown slip ring, and a method for using the same. 
         [0011]    In accordance with an embodiment of the present invention, a subsea wellhead assembly is disclosed. The subsea wellhead assembly includes a subsea wellhead defining a bore having a shoulder. The subsea wellhead assembly further includes a casing hanger landed on the shoulder within the bore of the subsea wellhead and defining an annulus between the subsea wellhead and the casing hanger. A casing hanger seal ring is disposed within the annulus. The seal ring is engaged with an inner diameter surface of the wellhead and engaged with an outer diameter surface of the casing hanger so that the seal ring prevents flow through the annulus. A lockdown slip ring is secured to a lower end of the seal ring so that, when the seal ring is energized, the lockdown slip ring engages a substantially smooth inner diameter surface portion of the wellhead and a substantially smooth outer diameter surface portion of the casing hanger to limit upwards axial movement of the casing hanger. 
         [0012]    In accordance with another embodiment of the present invention, a seal for sealing an annulus between inner and outer tubular members, wherein the inner tubular member is landed in a bore of the outer tubular member, is disclosed. The seal includes a seal ring adapted to land in the annulus and adapted to expand radially when energized to engage an inner diameter surface of the outer tubular member and an outer diameter surface of the inner tubular member. The seal further includes a lockdown slip ring secured to a lower end of the seal ring so that, when energized, the lockdown slip ring may engage an inner diameter surface of the outer tubular member and an outer diameter surface of the inner tubular member to limit upwards axial movement of the inner tubular member. The lockdown slip ring has a neck on an upper end of the lockdown slip ring, and the neck has a groove on an outer diameter of the neck. The seal ring has a lower leg on a lower end of the seal ring, and the lower leg has a recess on an inner diameter of the lower leg. A retainer ring comprising a split ring is interposed between the neck of the lockdown slip ring and the lower leg of the seal ring so that the retainer ring is partially within the groove and partially within the recess, securing the lockdown slip ring to the seal ring. 
         [0013]    In accordance with yet another embodiment of the present invention, a method for sealing a casing hanger to a wellhead is disclosed. The method begins by landing the casing hanger on a shoulder in the wellhead. Next, the method secures a lockdown slip ring to a lower end of a casing hanger seal and lands the casing hanger seal in an annulus between the casing hanger and the wellhead. The casing hanger seal is then energized by exerting a downward axial force on the casing hanger seal to compress the seal and the lockdown slip ring against a shoulder of the casing hanger. The downward axial force causes the lockdown slip ring to engage a substantially smooth inner diameter surface of the wellhead and a substantially smooth outer diameter surface of the casing hanger to limit upward axial movement of the casing hanger. 
         [0014]    An advantage of a preferred embodiment is that disclosed embodiments provide a lockdown seal that seals a casing hanger to a wellhead without the need for formation of a groove in either the casing hanger or wellhead. In addition, the disclosed embodiments do not require clearance between the casing hanger and the lockdown portion of the seal in order to engage. Thus, the disclosed embodiments may provide lockdown capability that prevents axial motion of the casing hanger caused by high pressures and thermal expansion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained, and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings that form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
           [0016]      FIG. 1  is a schematic representation of a casing hanger lockdown slip ring landed in place within an annulus between a casing hanger and a wellhead. 
           [0017]      FIG. 2  is a schematic representation of an alternative lockdown slip ring. 
           [0018]      FIG. 3  is a schematic representation of the lockdown slip ring of  FIG. 2  energized within an annulus between a wellhead and a casing hanger. 
           [0019]      FIG. 4  is a schematic representation of an alternative lockdown slip ring. 
           [0020]      FIG. 5  is a schematic representation of the lockdown slip ring of  FIG. 4  energized within an annulus between a wellhead and a casing hanger. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied 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 the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments. 
         [0022]    In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Additionally, for the most part, details concerning well drilling, running operations, and the like have been omitted in as much as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the skills of persons skilled in the relevant art. 
         [0023]    Referring to  FIG. 1 , a casing hanger  11  having an axis  14  is shown disposed within a subsea wellhead  13 . Generally, casing hanger  11  will land on a shoulder  12  formed in wellhead  13  to form an annulus  15  between casing hanger  11  and wellhead  13 . In the illustrated embodiment, a portion of an exterior surface of casing hanger  11  contacts a portion of an interior surface of wellhead  13  at a shoulder  12 . A person of ordinary skill in the art will understand that casing hanger  11  and wellhead  13  may be any inner and outer tubular members such that the inner tubular member may fit within a bore of the outer tubular member. 
         [0024]    A casing hanger seal ring  17  is interposed between casing hanger  11  and wellhead  13 . Casing hanger seal ring  17  substantially fills annulus  15  between casing hanger  11  and wellhead  13 , sealing annulus  15  and setting casing hanger  11  to wellhead  13 . Casing hanger seal ring  17  has an energized and an unenergized position. When in the energized position, as described in more detail with respect to  FIGS. 3 and 5 , casing hanger seal ring  17  will seal the annulus by engaging both the inner diameter surface of wellhead  13 , and the outer diameter surface of casing hanger  11 . When in the unenergized position, as shown in  FIGS. 2 and 4 , casing hanger seal ring  17  may be run into the wellbore to land in annulus  15  between casing hanger  11  and wellhead  13 , or pulled from annulus  15  between casing hanger  11  and wellhead  13 . In the illustrated embodiment, casing hanger seal ring  17  includes an energizing ring  19 , a seal ring  21 , a lockdown slip ring  23 , and a locking ring  25 . 
         [0025]    In the illustrated embodiment, lockdown slip ring  23  couples to a lower end of seal ring  21  and defines an annular protrusion that may include a taper  18  on an outer diameter at a lower end as shown in  FIG. 1 . Lockdown slip ring  23  has a ramped surface  20  on its inner diameter adapted to interface with an exterior tapered surface  16  of casing hanger  11  such that upward axial movement of casing hanger  11  will engage lockdown slip ring  23 . Preferably, the slope of ramped surface  20  may match the slope of exterior tapered surface  16  of casing hanger  11 . In the illustrated embodiment, the slope may comprise an angle of approximately 20 degrees from vertical; however, a person skilled in the art will understand that any suitable angle may be used. In the illustrated embodiment, lockdown slip ring  23  does not move axially relative to seal ring  21 ; however, lockdown slip ring  23  may move radially relative to seal ring  21 . The surfaces of taper  18  and ramped surface  20  may have differing friction factors such that ramped surface  20  is more likely to slip than taper  18 . This may be achieved in any suitable manner such as by employing wickers or teeth on the surface of taper  18 , by using a variety of friction gripping coatings, or the like. A person skilled in the art will understand that both the surface of taper  18  and ramped surfaced  20  may include friction coatings, wickers, or the like. 
         [0026]    As casing hanger  11  moves upward axially relative to wellhead  13 , exterior surface profile  16  of casing hanger  11  will first abut and then exert an upward axial force on casing hanger seal  17  through the adjacent ramped surface  20  of lockdown slip ring  23 . The upward axial force on lockdown slip ring  23  will cause lockdown slip ring  23  to move radially outward, engaging taper  18  with the inner diameter of wellhead  13  in response. Due to the increase frictional gripping force caused by the differing frictional forces between taper  18  and ramped surface  20 , casing hanger seal  17  may not move axially in response to upward axial movement of casing hanger  11 . As a consequence, upward axial movement of casing hanger  11  will be limited due to the engagement of casing hanger seal  17  with casing hanger  11 . 
         [0027]    As shown in  FIG. 2 , an alternate embodiment of lockdown slip ring  23  may comprise two annular rings, a coupling ring  27  and a slip ring  29 . Coupling ring  27  has a protrusion  31  at an upper end that defines a retaining groove or slot  33  in an outer diameter surface of protrusion  31 . Groove  33  may be an annular groove or alternatively, groove  33  may extend only partway around the outer circumference of protrusion  31 . Coupling ring  27  also defines an annular upward facing shoulder  35 . Upward facing shoulder  35  extends from an outer diameter of coupling ring  27  to a base of protrusion  31  such that groove  33  faces an area axially above upward facing shoulder  35 . In the illustrated embodiment, upward facing shoulder  35  has a width that is approximately half the width of a cross section of coupling ring  27 . 
         [0028]    A lower end of coupling ring  27  has an approximately triangular shaped cross section having a substantially vertical surface forming the outer diameter of coupling ring  27  extending from the lower end to upward facing shoulder  35 , and a ramped surface  37  extending from the lower end of coupling ring  27  to a downward facing shoulder  39  axially beneath protrusion  31 . A lower end of the exterior diameter surface of coupling ring  27  may include wickers  73  that are adapted to engage the inner diameter surface of wellhead  13  as shown in  FIG. 3 . Wickers  73  may comprise gripping teeth or the like. Referring to  FIG. 2 , downward facing shoulder  39  extends from an inner diameter of coupling ring  27  to a base of ramped surface  37 . A slip ring limiter  41  may protrude from a portion of ramped surface  37  to define annular upper and lower coupling ring channels  43 ,  45 , respectively. In the illustrated embodiment, slip ring limiter  41  is positioned approximately halfway between a lower end of coupling ring  27  and downward facing shoulder  39 . 
         [0029]    Slip ring  29  comprises a substantially wedged shaped object having an inner diameter that is substantially vertical, and an outer diameter comprising a ramped surface  47  adapted to mate with ramped surface  37  of coupling ring  27 . A lower end of the inner diameter surface may include wickers  71  adapted to engage an exterior diameter surface of casing hanger  11  or, alternatively exterior surface profile  16  of casing hanger  11 . Wickers  71  may comprise gripping teeth or the like. A slip ring recess  49  is formed in ramped surface  47  and extends inward from ramped surface  47 . Slip ring recess  49  is an annular recess adapted to receive slip ring limiter  41 . In the illustrated embodiment, a height of slip ring recess  49  is greater than a height of slip limiter  41 , allowing slip ring limiter  41  to move axially within slip ring recess  49 . As shown, slip ring  29  may slide axially relative to coupling ring  27  through slip ring recess  49 . Slip limiter  41  will limit axial movement of slip ring  29  through contact with an upward facing shoulder  51  of slip ring recess  49  and a downward facing shoulder  53  of slip ring recess  49 . Slip ring  29  may secure to coupling ring  27  with a shear element, such as shear retaining pin  55 . Shear retaining pin  55  will prevent axial movement of slip ring  29  relative to coupling ring  27  during running of casing hanger  17 . 
         [0030]    In the illustrated embodiment, seal ring  21  comprises an annular member having an approximately U-shaped cross section  57  with seal ring legs  59 ,  61  and a lower leg  63 . Lower leg  63  extends past protrusion  31  of coupling ring  29  proximate to upward facing shoulder  35  of coupling ring  27 . In the illustrated embodiment, lower leg  63  defines a retainer recess  65  proximate to and facing groove  33 . A retainer ring  67  may be interposed between lower leg  63  of seal ring  21  and protrusion  31  of coupling ring  27  such that retainer ring  67  substantially fills groove  33 . A portion of retainer ring  67  will extend into retainer recess  65 , causing coupling ring  27  to move axially in response to axial movement of seal ring  21 . When thus positioned, the width of the combined protrusion  31  of coupling ring  27  and lower leg  63  of seal ring  21  is approximately equivalent to a width of seal ring  21  across the base of U-shaped cross section  57 . Retainer ring  67  may be any suitable ring such as a split ring or the like. 
         [0031]    Energizing ring  19  comprises a ring having an axially lower end slightly larger than the slot defined between seal ring legs  59 ,  61  of seal ring  21 . Energizing ring  19  has an upper end adapted to be releasably coupled to a running tool so that the running tool may run casing hanger seal  17  to the location shown in  FIG. 1 , and then operate energizing ring  19  to energize casing hanger seal  17 . 
         [0032]    As described in more detail below, a running tool will apply an axial force to energizing ring  19 , forcing energizing ring  19  axially into seal ring  21 , providing an interference fit that will press seal ring legs  61 ,  59  of seal ring  21  into adjacent wickers  67  and  69  ( FIG. 1  and  FIG. 3 ). This will seal annulus  15  between casing hanger  11  and wellhead  13  at seal ring  21 . A person skilled in the art will understand that the energizing ring  19  may be energized by a running tool or the like. 
         [0033]    Referring now to  FIG. 3 , casing hanger seal  17  is run to land and set as shown in a typical running operation. While running into annulus  15 , the elements of casing hanger seal  17  are as illustrated in  FIG. 2 . An axial force is then applied to energizing ring  19 , such as with a running tool. Energizing ring  19  moves downward axially in response such that an end of energizing ring  19  applies a corresponding downward axial force to upper surfaces of seal ring legs  59 ,  61 . Continued application of downward axial force to energizing ring  19  pushes a lower end of slip ring  29  into contact with exterior surface profile  16  of casing hanger  11  near location  12  of  FIG. 1 . Referring to  FIG. 3 , lockdown slip ring  23  is then compressed between seal ring  21  and the exterior surface of casing hanger  11  at upward facing shoulder  16  by energizing ring  19 , causing shear pin  55  to shear. Coupling ring  27  will then move axially downward through slip recess  49 . Eventually, a lower surface of slip retainer  41  may land against upward facing shoulder  51  of slip ring  29  as shown in  FIG. 3 . 
         [0034]    Downward movement of coupling ring  27  through slip recess  49  causes slip ring  29  to move radially into engagement with casing hanger  11  in response. As slip ring  29  moves radially into casing hanger  11 , wickers  71  will grip the surface of casing hanger  11 , holding slip ring  29  in engagement with casing hanger  11 . Similarly, wickers  73  will engage an inner diameter surface of wellhead  13 , holding coupling ring  27  in engagement with wellhead  13 . 
         [0035]    Further downward axial movement of energizing ring  19  causes an end of energizing ring  19  to insert into the slot formed by seal ring legs  59 ,  61 . As the end of energizing ring  19  inserts into the slot, seal ring legs  59 ,  61  will deform radially into engagement with wickers  67 ,  69 , respectively. The inner diameter surface of seal ring leg  59  will then be deformed by wickers  67  of casing hanger  11 , and the outer diameter surface of seal ring leg  61  will be deformed by wickers  69  of wellhead  13 , forming a seal of annulus  15 . 
         [0036]    During subsea operation of wellhead  13 , thermal expansion of casing suspended from casing hanger  11 , or fluid pressure within annulus  15  beneath casing hanger seal  17  may place an upward axial load on casing hanger  11 . As casing hanger  11  attempts to move axially upward relative to wellhead housing  13  in response to such a load, casing hanger seal  17  will counteract this movement in the following manner. As casing hanger seal  11  moves upward, slip ring  29  will move axially upward as a result of the gripping engagement of wickers  71  with the exterior surface of casing hanger  11 . This will cause slip ring  29  to slide further up the mating ramped surfaces  47  and  37  relative to coupling ring  27 . As shown in  FIG. 3 , this movement will cause slip ring  29  to move radially inward resulting in an increase of the width of casing hanger seal  17  at slip ring  29  and coupling ring  27 . Slip ring  29  and coupling ring  27  will now be radially adjacent within annulus  15  as shown in  FIG. 3 . This radial movement will more tightly grip casing hanger  11  to wellhead  13  through casing hanger seal  17 . Continued upward movement of slip ring  29  is prevented when upward facing shoulder  51  of slip ring  29  lands on slip limiter  41 , thereby preventing further upward axial movement of casing hanger  11  and increasing the strength of the seal within annulus  15 . 
         [0037]    Referring now to  FIG. 4 , there is shown another alternative casing hanger seal  17 ′. Casing hanger seal  17 ′ includes the components of casing hanger seal  17  of  FIG. 2 , modified as described below. As shown in  FIG. 4 , casing hanger seal  17 ′ includes energizing ring  19 , seal ring  21 , and locking ring  25  of  FIG. 2 . Energizing ring  19 , seal ring  21 , and locking ring  25  of  FIG. 4  are positioned and operate as described above with respect to  FIG. 2  and  FIG. 3 . 
         [0038]    As shown in  FIG. 4 , casing hanger seal  17 ′ also includes lockdown slip ring  23 ′. Lockdown slip ring  23 ′ couples to a lower end of seal ring  21 . Lockdown slip ring  23 ′ may comprise two annular rings, a coupling ring  27 ′ and a slip ring  29 ′. Coupling ring  27 ′ has a protrusion  31 ′ at an upper end that defines a retaining groove or slot  33 ′ in an outer diameter surface of protrusion  31 ′. Groove  33 ′ may be an annular groove or alternatively, groove  33 ′ may extend only partway around the outer circumference of protrusion  31 ′. Coupling ring  27 ′ also defines an annular upward facing shoulder  35 ′. Upward facing shoulder  35 ′ extends from an outer diameter of coupling ring  27 ′ to a base of protrusion  31 ′. In the illustrated embodiment, upward facing shoulder  35 ′ has a width that is approximately half the width of a cross section of coupling ring  27 ′. 
         [0039]    A lower end of coupling ring  27 ′ has an approximately triangular shaped cross section having a substantially vertical surface forming the inner diameter of coupling ring  27 ′. The substantially vertical surface extends from the lower end to a top of protrusion  31 ′. The lower end of coupling ring  27 ′ has a ramped surface  37 ′ extending from the lower end of coupling ring  27 ′ to a downward facing shoulder  39 ′ axially beneath upward facing shoulder  35 ′. A lower end of the exterior diameter surface of coupling ring  27 ′ may include wickers  73 ′ that are adapted to engage the inner diameter surface of wellhead  13  as shown in  FIG. 5 . Wickers  73 ′ may comprise gripping teeth or the like. Referring to  FIG. 4 , downward facing shoulder  39 ′ extends from an outer diameter of coupling ring  27 ′ to a base of ramped surface  37 ′. A slip ring limiter  41 ′ may protrude from a portion of ramped surface  37 ′ to define upper and lower coupling ring channels  43 ′,  45 ′, respectively. In the illustrated embodiment, slip ring limiter  41 ′ is positioned approximately halfway between a lower end of coupling ring  27 ′ and downward facing shoulder  39 ′. 
         [0040]    Slip ring  29 ′ comprises a substantially wedged shaped object having an outer diameter that is substantially vertical, and an inner diameter comprising a ramped surface  47 ′ adapted to mate with ramped surface  37 ′ of coupling ring  27 ′. A lower end of the inner diameter surface may include wickers  71 ′ adapted to engage an exterior diameter surface of casing hanger  11  or, alternatively an upward facing shoulder  16  of casing hanger  11 . Wickers  71 ′ may comprise gripping teeth or the like. A slip ring recess  49 ′ is formed in ramped surface  47 ′ and extends inward from ramped surface  47 ′. Slip ring recess  49 ′ is an annular recess adapted to receive slip ring limiter  41 ′. As shown, slip ring  29 ′ may slide axially relative to coupling ring  27 ′ through slip ring recess  49 ′. Slip limiter  41 ′ will limit axial movement of slip ring  29 ′ through contact with upward facing shoulder  51 ′ of slip ring recess  49 ′ and downward facing shoulder  53 ′ of slip ring recess  49 ′. Slip ring  29 ′ may secure to coupling ring  27 ′ with a shear element, such as shear retaining pin  55 ′. Shear retaining pin  55 ′ will prevent axial movement of slip ring  29 ′ relative to coupling ring  27 ′ during running of casing hanger  17 ′. 
         [0041]    In the illustrated embodiment, lower leg  63  of seal ring  21  extends past protrusion  31 ′ of coupling ring  29 ′ proximate to upward facing shoulder  35 ′ of coupling ring  27 ′. As described above, lower leg  63  defines an retainer recess  65  proximate to and facing groove  33 ′. A retainer ring  67  may be interposed between lower leg  63  of seal ring  21  and protrusion  31 ′ of coupling ring  27 ′ such that retainer ring  67  substantially fills groove  33 ′. A portion of retainer ring  67  will extend into retainer recess  65  causing coupling ring  27  to move axially in response to axial movement of seal ring  21 . 
         [0042]    Referring now to  FIG. 5 , casing hanger seal  17 ′ is run to land and set as shown. While running into annulus  15 , the elements of casing hanger seal  17 ′ are as illustrated in  FIG. 4 . An axial force is then applied to energizing ring  19 , such as with a running tool. Energizing ring  19  moves downward axially in response such that an end of energizing ring  19  contacts upper surfaces of seal ring legs  59 ,  61 . Continued application of downward axial force to energizing ring  19  pushes a lower end of slip ring  29 ′ into contact with exterior surface profile  16  of casing hanger  11  near shoulder  12  of  FIG. 1 . As shown in  FIG. 3 , lockdown slip ring  23 ′ is then compressed between seal ring  21  and exterior surface profile  16  of casing hanger  11  by energizing ring  19  causing shear pin  55 ′ to shear. Coupling ring  27 ′ will then move axially downward through slip recess  49 ′ in response, eventually landing at a lower surface of slip retainer  41 ′ against upward facing shoulder  51 ′ of slip ring  29 ′. 
         [0043]    Downward movement of coupling ring  27 ′ through slip recess  49 ′ causes slip ring  29 ′ to move radially into engagement with wellhead  13  in response. As slip ring  29 ′ moves radially into wellhead  13 , wickers  71 ′ will grip the surface of wellhead  13 , holding slip ring  29 ′ in engagement with wellhead  13 . Similarly, wickers  73 ′ will engage an outer diameter surface of casing hanger  11 , holding coupling ring  29 ′ in engagement with wellhead  13 . 
         [0044]    Further downward axial movement of energizing ring  19  causes an end of energizing ring  19  to insert into the slot formed by seal ring legs  59 ,  61 . As the end of energizing ring  19  inserts into the slot, seal ring legs  59 ,  61  will deform radially outward into engagement with wickers  67 ,  69 , respectively. The inner diameter surface of seal ring leg  59  will then be deformed by wickers  67  of casing hanger  11 , and the outer diameter surface of seal ring leg  61  will be deformed by wickers  69  of wellhead  13 , forming a seal of annulus  15 . 
         [0045]    During subsea operation of wellhead  13 , thermal expansion of casing suspended from casing hanger  11 , or fluid pressure within annulus  15  beneath casing hanger seal  17 ′ may place an upward axial load on casing hanger  11 . However, the increased radial width of casing hanger seal  17 ′ caused by the movement of slip ring  29 ′ along the ramped surface  47 ′ will act as a type of friction lock, exerting a radial force on casing hanger  11 . As a result, as casing hanger  11  attempts to move axially upward relative to wellhead housing  13  in response to such a load, the friction lock caused by the radial expansion of casing hanger seal  17 ′ will counteract this movement, preventing movement of casing hanger  11 . The additional width of lockdown slip ring  23 ′, caused by the movement of slip ring  29 ′ along ramped surface  47 ′ during setting, increases the radial force exerted between wellhead  13  and casing hanger  11 . This will then prevent upward axial movement of casing hanger  11 . 
         [0046]    Accordingly, the disclosed embodiments provide a metal seal that can land and seal an annulus between a casing hanger and a wellhead without the need of a landing shoulder or dog recess machined within the wellhead. Thus, there is no concern that debris may have landed on the shoulder or filled the dog recess that would prevent setting of the seal. In addition, the disclosed embodiments provide a metal seal that increases in strength as pressure loading within the annulus beneath the seal increases. Furthermore, the metal seal disclosed herein eliminates the need for the seal to tolerate some axial shift before sealing; instead the seal preloads against its own load shoulder and prevents displacement of the casing hanger found in some cyclic loading, allowing the seal to operate for more cycles than in prior art designs. 
         [0047]    It is understood that the present invention may take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or scope of the invention. Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.