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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The present disclosure relates in general to a wellhead assembly having a seal between coaxial members, where wickers are formed on a surface of the seal. 
         [0003]    2. Description of Prior Art 
         [0004]    Seals are typically inserted between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member is generally a hanger for supporting either casing or tubing that extends into the well. Outer wellhead members are usually one of a wellhead housing, or can be a casing hanger when the inner member is a tubing hanger. A variety of seals located between the inner and outer wellhead members are known. Examples of known seals are elastomeric, metal, and combinations thereof and elastomeric rings. 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 metal-to-metal seal has seal body with inner and outer walls separated by a cylindrical slot, forming a “U” shape. An energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members, which may have wickers formed thereon. The energizing ring is typically a solid member with a lower end having a wedge-shaped cross section. The deformation of the seal&#39;s inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent. 
         [0005]    Thermal growth between the casing or tubing and the wellhead may occur. The well fluid flowing upward through the tubing, and annulus fluids, 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. During the heat up transient, annulus pressure may build up as the fluids comprising the volume below the seal try to expand. This annulus pressure build-up and thermal expansion of the casing and/or tubing string combine to exert a large upward axial force, often referred to as a “lockdown force”, against the annulus seal. If this force exceeds the retention capacity of the seal, the pressure controlling barrier between the inner and outer wellhead tubular members can be compromised. Seal leakage can also occur due to a collection of debris on the wickers that interferes with energizing the seal and introduces a leak path across the wickers. 
         [0006]    A large axial load between the seal and its mating surfaces due to thermal transients may also cause the seal to leak. One approach to preventing this type of movement is through the use of lockdown C-rings on the seal. The C-rings engage the outer tubular member and/or the hanger when the seal is set, locking the seal to the hanger, as well as the hanger to the wellhead. Another approach has been to use the sealing element itself as a locking mechanism. In these approaches, lockdown as well as sealing is thus provided by the seal. Further, a lockdown style hanger may be utilized to lock the casing hanger in place. This requires an extra trip to install the lockdown style hanger. 
       SUMMARY OF THE INVENTION 
       [0007]    Disclosed herein is an example of a seal assembly for use in a wellhead assembly which includes an annular seal body which is made up of an elongate inner leg, an elongate outer leg set radially outward from the inner leg, and an elongate slot defined between the inner and outer legs. Wickers are provided on a curved surface of the seal body and selectively engage a tubular surface within the wellhead assembly. The wickers can be on the inner leg, the outer leg, or on both legs. Optionally, the tubular can be a casing hanger, tubing hanger, or wellhead housing. The seal assembly can further include a protective foam layer adhered to the wickers. 
         [0008]    Also disclosed herein is a wellhead assembly which includes inner and outer wellhead tubulars, an annulus between the inner and outer tubulars, and an annular seal in the annulus. In this example the annular seal is made up of an inner leg that is in selective sealing contact with the inner tubular, an outer leg that is in selective sealing contact with the outer tubular, a space between the inner and outer legs, and wickers on a circumference of a curved surface of the seal. In an example, the wickers are on an inner surface of the inner leg and project into an outer surface of the inner tubular. Optionally, the wickers can be on an outer surface of the outer leg and project into an inner surface of the outer tubular. In another example, wickers are on an inner surface of the inner leg and are on an outer surface of the outer leg. An inlay may be set in one of the inner and outer tubulars and strategically located for engagement with the wickers when the legs are in sealing contact with the tubulars. In one example the seal includes nickel alloy. 
         [0009]    A method of sealing an annulus between an inner and outer tubular in a wellhead assembly is disclosed herein. In one example the method includes providing a seal assembly with an annular seal body with inner and outer legs, and with wickers that circumscribe a curved surface on the body, inserting the seal assembly into the annulus, and urging the inner and outer legs radially apart and into respective sealing engagement with an outer surface of the inner tubular and an inner surface of the outer tubular. The wickers can be on an outer surface of the outer leg and embed into the inner surface of the outer tubular during the step of urging the legs apart. In an alternative, the wickers can be on an inner surface of the inner leg and embed into the outer surface of the inner tubular when the legs are urged apart. The method can further involve removing the seal assembly from the wellhead assembly, applying a protective layer onto the wickers, and repeating the steps of inserting and urging apart. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    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: 
           [0011]      FIG. 1  is a side sectional view of an example of a seal assembly in accordance with the present invention. 
           [0012]      FIG. 1A  is a side sectional and enlarged view of a portion of the seal assembly of  FIG. 1 . 
           [0013]      FIG. 2  is a side sectional view of an alternate embodiment of the seal assembly of  FIG. 1  and in accordance with the present invention. 
           [0014]      FIG. 3  is a side sectional view of the seal assembly of  FIG. 1  installed in a wellhead assembly. 
       
    
    
       [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. 
         [0018]    Shown in  FIG. 1  in side sectional view is an example embodiment of a wellhead assembly  10  in which a production casing hanger  12  is shown landed onto an intermediate casing hanger  14 . A downward facing shoulder  15  on production casing hanger  12  contacts an upward facing shoulder  16  on an upper terminal surface of casing hanger  14 . An annular wellhead housing  18  circumscribes both the production hanger  12  and intermediate hanger  14 . An annulus  20  extends axially between the intermediate hanger  12 , and wellhead housing  18 . In the example of  FIG. 1 , an upper end of annulus  20  is defined by the sealing interfaces created between the outer seal leg  30  and wellhead housing  18  as well as the inner seal leg  28  and intermediate hanger  14 . 
         [0019]    Inserted within annulus  20  is a seal assembly  22  shown urged into sealing contact with the respective outer and inner surfaces of intermediate casing hanger  14  and wellhead housing  18 . An annular retaining ring  24  threadingly engages an upper end of the seal assembly  22  and annular energizing ring  26  is shown inserted within the seal assembly  22 . Energizing Ring  26  urges inner leg  28  of the seal assembly  22  radially inward and against the intermediate casing hanger  14 . Energizing ring  26  also urges outer leg  30  of seal assembly  22  radially outward against casing hanger  18 . The seal assembly  22  is an annular member having a curved surface along its outer radius and inner radius. The legs  28 ,  30  are elongate members whose elongate sides extend in a direction substantially parallel with an axis A X  of the seal assembly  22 . Outer leg  30 , which is shown into sealing contact with an inner surface of wellhead housing  18 , further includes wickers  34  on its outer radial surface, and which extend an axial length on the outer radial surface. 
         [0020]    When the seal assembly  22  is energized and outer leg  30  is urged radially outward, wickers  34  project radially outward and into the wellhead housing  18 ; a sealing surface is formed along the interface between the outer leg  30  and wellhead housing  18 . The material of the seal assembly  22  is harder than material of the wellhead housing  18  so that the wickers  34  can penetrate and plastically deform the wellhead housing  18 . An optional inlay  36  may be strategically located on the inner circumference of the wellhead housing, so the wickers  34  engage the inlay  36  when seal assembly  22  is energized. In this example, inlay  36  can be formed from a metal softer than wellhead housing  18  and the metal making p seal assembly  22 . Example materials for the seal assembly include nickel-based alloys, such as Inconel® 718. In addition to providing a sealing interface between the seal assembly  22  and wellhead housing  18 , engaging wickers  34  with wellhead housing  18  can also create a coupling which opposes axial respective movement between seal assembly  22  and wellhead housing that may be introduced by thermal expansion and other similar occurrences. 
         [0021]    Referring now to  FIG. 1A , an enlarged view of a portion of the wellhead assembly  10  is shown in side sectional view. In this example, wickers  34  are generally triangular elements with a tip  37  with sharp pointed ends. The sharp ends reduce the force required to embed the wickers  34  into wellhead housing  18  and plastically deform the material of the wellhead housing  18 . Although the wickers  34  are shown as adjoining, spaces may be included between adjacent wickers  34 , or optionally spaces may exist between adjacent groups of wickers  34 . In one example, the distance between adjacent tips  37  is about 0.125 inches. Additionally, the material of what is being deformed by the wickers  34  (e.g. the casing  14 , housing  18 , or inlays  36 ,  40 ), can have a hardness that ranges from about 45% to about 55%, or more, of the hardness of the material of the seal assembly  22 . 
         [0022]    Referring back to  FIG. 1 , inner leg  28  may also include wickers  38  that are shown engaging with an outer radial surface of casing hanger  16  and plastically deforming that portion to create a sealing interface between inner leg  28  and casing hanger  16 . Optionally, an inlay  40  may be provided on the outer surface of casing hanger  16  and strategically located so that inlay  40  is engaged by wickers  38 . Similar to inlay  36 , inlay  40  may be formed from a material softer than the material used for forming production casing hanger  12  and/or inner leg  28 . Optional embodiments exist, wherein no inlays are provided on either of the casing hanger  16  or wellhead housing  18 . Further optionally, wickers  34  may be provided on a single one of the legs  28 ,  30 ; in this example the other leg  28 ,  30  would be substantially smooth. 
         [0023]      FIG. 2  illustrates an alternate embodiment of the seal assembly  22 A in an uninstalled configuration wherein the energizing ring  26  is set upward from its energizing position of  FIG. 1 . In this example, an upward facing shoulder on the outer radial surface of the energizing ring  26  is in contact with a lower terminal end of the retaining ring  24 . Further in the example of  FIG. 2 , foam  42  is provided on the wickers  34  on the outer leg  30 . Also in the example of  FIG. 2 , the inner radial surface of inner leg  28  is substantially smooth and having no wickers  34  formed thereon as mentioned above. In this example, the foam  42  fills the space between adjacent wickers  34 . In an example, the foam includes gas filled beads suspended in a matrix, such as a binding agent or resin. The beads (not shown) can be glass. By axially urging energizing ring  26  downward so that its lower nose enters space  32  between legs  28 ,  30 , wickers  34  are urged radially outward. In an example, the resulting forces from energizing the seal assembly  22 ,  22 A crushes the foam  42  and releases a volume of gas trapped inside the glass beads which reduces the hydraulic pressure that builds between the wicker tips  37  as the seal assembly  22 ,  22 A is energized in a submerged environment into particles that do not interfere with formation of the sealing interface between wickers and the associated tubular in which the wickers are embedded. Advantages of the embodiments described herein are that the seal element  22  can be constructed from a much higher yield strength than the wellhead housing  18  or the casing hangers  12 ,  14 , which in turn allows much greater lockdown capacities to be achieved. While at surface, the seal assembly  22 ,  22 A can either be replaced or repaired, and a fresh amount of foam  42  reapplied before inserting the replacement or repaired seal assembly  22 ,  22 A back into the wellhead assembly  10 . Current known embodiments of seal assemblies with wickers are provided on tubulars, which are usually not pulled when replacing a seal, and as such wickers will no longer benefit from the original volume of foam during a reinstallation of a seal assembly because the foam is crushed and consumed during an initial seal assembly installation. 
         [0024]    The wellhead assembly  10  is shown in  FIG. 3  and which includes a production tree  44  mounted on wellhead housing  18 . In this example embodiment, production casing hanger  12  and intermediate casing hanger  14  are circumscribed by wellhead housing  18  and seal assembly  22  is shown set in the annulus between tubing and casing hangers  12 ,  14  and wellhead housing  18 . Further shown in  FIG. 3  is the wellbore  46  over which wellhead assembly  10  is mounted. Depending into wellbore  46  is a casing string  48  whose upper end is attached to intermediate casing hanger  14 , and which circumscribes a tubing string  50  which is supported on production casing hanger  12 . In the example of  FIG. 3 , wellhead assembly  10  can be subsea or on surface and used for controlling fluids produced from within wellbore  46 . 
         [0025]    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.

Summary:
An annular seal for use in a wellhead assembly has inner and outer legs that each extend in a direction that is generally parallel with an axis of the seal to define an annular space therebetween. Wickers are provided on an outer surface of the seal, so that when the seal is energized and the legs are urged radially apart from one another, the wickers engage with a mating surface of a downhole tubular. Embedding the wickers into the tubular creates a flow barrier across the interface between the seal and the tubular. The wickers deform the surface of the tubular, which creates a lock down force that opposes relative axial movement of the tubular.