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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 wellhead assemblies, and in particular to a seal and lock-down ring for use between inner and outer wellhead members. 
         [0003]    2. Description of Prior Art 
         [0004]    Seals are typically provided in an annulus between coaxial wellhead tubular members to isolate internal well pressure. The inner wellhead member is sometimes 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 wellhead housing, a production tree, or tubing head. A packoff or seal typically forms a barrier between the tubing hanger and the outer wellhead member. In other times, the inner wellhead member is a casing hanger landed in a wellhead housing and that has a string of casing that depends down into the well. A seal or packoff usually seals between the casing hanger and the wellhead housing. 
         [0005]    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 seal has inner and outer legs separated by a slot; in which an energizing ring is inserted that deforms the inner and outer legs apart into sealing engagement with the inner and outer wellhead members. The energizing ring is usually a solid member. The seals with inner and outer legs typically plastically deform when pushed into sealing engagement with the inner and outer wellhead members. 
       SUMMARY OF THE INVENTION 
       [0006]    Disclosed herein is a seal and lock-down system for use between downhole inner and outer tubulars. In an example the system includes a seal element having a body and inner and outer annular legs projecting from the body that are spaced radially apart to define a gap between the legs, a lock-down ring between the tubulars, and a nose ring on an end of the body of the seal element distal from the legs. In this example the nose ring is elongate and generally parallel with an axis of the tubulars. The nose ring is selectively changeable to a set configuration that is generally oblique with the axis when inserted between the lock-down ring and inner tubular. In one example, when the nose ring is inserted between the lock-down ring and inner tubular, the nose ring substantially occupies the space between the lock-down ring and inner tubular. In an alternate embodiment, when the nose ring is inserted between the lock-down ring and inner tubular, an outer radial portion of the lock-down ring projects into a profile in the outer tubular and an inner radial portion is disposed in a lock-down groove on the inner tubular thereby axially affixing together the inner and outer tubulars. Slots may be included that extend through sidewalls of the nose ring from an end of the nose ring distal from the seal element; and wherein fingers can be defined between adjacent slots. In an example embodiment, the inner radius of the lock-down ring projects radially inward proximate a side of the lock-down ring distal from the seal element. The outer tubular can be a wellhead housing that is part of a wellhead assembly, and the inner tubular can be a casing hanger. In one example, directing the energizing ring against the seal element with an energizing force that urges the nose ring between the lock-ring and the inner tubular, wherein inserting the energizing ring into the gap between the inner and outer legs with an energizing force to the energizing ring, drives the energizing ring into the gap and urges the legs radially outward into sealing contact with the tubulars, and wherein the energizing force for the seal element is greater than the energizing force for the lock-ring. 
         [0007]    Also disclosed herein is a seal system for sealing between coaxial tubulars, where the tubulars are part of a wellhead assembly. In an example, the seal system includes an annular seal element having radially spaced apart inner and outer legs that define a gap therebetween, and that are in sealing contact with opposing surfaces of the tubulars. The seal system further includes a lock-down ring having opposing radial portions in interfering contact with oppositely facing profiles in the tubulars, so that portions of the tubulars adjacent the seal element are axially static, and a nose ring. In this embodiment, the nose ring has an end coupled with an end of the seal element, and a portion spaced from the seal element is wedged between the lock-down ring and one of the tubulars. When wedged as such, the nose ring projects along a path generally oblique to the seal element, and thereby retaining the lock-down ring in interfering contact with the tubulars. The one of the tubulars can be a casing hanger; in this example the portion of the nose ring projects radially inward. A notch can be scored on a radial surface of the nose ring so the nose ring can be changed from an elongate shape to the oblique shape when inserted between the lock-ring and the one of the tubulars. The nose ring can change from an elongate shape to the oblique shape when inserted between the lock-ring and the one of the tubulars, and wherein a force for inserting the nose ring between the lock-ring and the one of the tubulars is less than a force for energizing the seal element. The tubulars can be made up of an inner tubular and an outer tubular with oppositely facing profiles that include an upward facing pedestal defined by a lower surface of a lock-down groove formed along an outer circumference of the inner tubular. The outer tubular can have a downward facing shoulder defined by a profile formed along its inner circumference. 
         [0008]    Further described herein is a system for sealing between tubulars in a wellhead assembly. In an example the system includes a seal element that is selectively inserted between the tubulars, and a lock-down ring selectively disposed on an profile on an outer surface of a one of the tubulars. The lock-down ring can be selectively urged to a position towards another one of the tubulars and into interfering contact with an oppositely facing profile on the another one of the tubulars. Further included in this example is a nose ring on an end of the seal element that selectively inserts between the lock-down ring and the one of the tubulars into a setting position to urge the lock-down ring into the interfering contact, so that forces resulting from relative axial movement of the tubulars are applied to the lock-down ring and bypass the seal element. In an example, the interfering contact of the lock-down ring maintains the portions of the tubulars adjacent the seal in relative static positions. Optionally, when the nose ring is in the setting position the nose ring substantially occupies the space between the lock-ring and the one of the tubulars. In an example, the one of the tubulars is a casing hanger and the another one of the tubulars is a wellhead housing. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a side sectional view of an example of a seal system being inserted between a pair of downhole tubulars in accordance with an embodiment of the present invention. 
           [0011]      FIG. 2  is a side sectional view of the seal system of  FIG. 1  being set and energized into a sealing and lock-down configuration in accordance with an embodiment of the present invention. 
           [0012]      FIG. 3  is a side perspective view of an example of nose ring from the seal system of  FIG. 1  in accordance with an embodiment of the present invention. 
           [0013]      FIG. 4  is a side partial sectional view of an embodiment of the seal system and tubulars of  FIG. 1  in a wellhead assembly in accordance with an embodiment of the present invention. 
           [0014]      FIG. 5  is a plan view of an example of a lock-down ring from the seal system of  FIG. 1  in accordance with an embodiment of 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. 
         [0018]    Shown in a sectional view  FIG. 1  is one example of a portion of a wellhead assembly  10  that includes a pair of coaxial tubulars  12 ,  14 . In the example, tubular  14  is an inner tubular and proximate an axis A x  of the wellhead assembly  10 . Further in the example, tubular  12  is an outer tubular which circumscribes tubular  14 . Examples exist, where tubular  12  is a wellhead housing, and tubular  14  is a casing hanger. Optionally, tubular  14  may also be a tubing hanger, wherein tubular  12  may be a casing hanger. A seal assembly  16  is shown being inserted into an annulus  18  formed between the tubulars  12 ,  14 . The seal assembly  16  includes a seal element  19  shown having an elongate outer leg  20  oriented substantially parallel with axis A x . Seal element  19  further includes an inner leg  24 , which like outer leg  20  is elongate and projects along a path generally parallel with axis A x . Between legs  20 ,  24  an annular gap  26  is defined that is elongate in an axial direction. Optional wickers  30 ,  31  are formed respectively on portions of the outer and inner surfaces  28 ,  22 . Seal element  19  further includes a body  31  on which the legs  20 ,  24  mount; and the body  31  defines a bottom of the gap  26 . 
         [0019]    Shown threadingly mounted to an end of the body  31  opposite from legs  20 ,  24  is an annular nose ring  32  that is elongate in an axial direction and depends from body  31  deeper into the annulus  18 . Other means for mounting the nose ring  32  to the body  31  may be employed, such as a C-ring (not shown) and/or threaded fasteners. A lock-down groove  34  is illustrated circumscribing the inner tubular  14  formed into the outer surface  28 , and spaced downward from nose ring  32 . A wall of the lock-down groove  34  that is distal from an opening of the annulus  18 , projects radially outward to define a pedestal  36 . In the example of  FIG. 1 , the pedestal  36  provides a support ledge on the tubular  14  shown supporting a lock-down ring  38 . An example embodiment of the lock-down ring  38  extends substantially the length of the lock-down groove  34 , such as a “C” ring. In the example of  FIG. 1 , the radial section of the lock-down ring  38  has an outer surface substantially parallel with axis A x . While a portion of the inner surface of the lock-down ring  38  proximate pedestal  36  is substantially parallel with axis A x , the inner surface tapers radially outward with distance away from pedestal  36 . The angle of the taper changes to define a transition  39 , where angle of the taper between the transition  39  and the pedestal  36  is more oblique to axis A x  than the angle of the taper between transition  39  and the end of the lock-down ring  38  distal from pedestal  36 . Optionally, the lock-down ring  38  can fully circumscribe lock-down groove  34 . Further illustrated in  FIG. 1  is a profile on the inner surface  22  of tubular  12  that projects radially inward to define a shoulder  40 , wherein shoulder  40  is opposite from and faces pedestal  36 . 
         [0020]      FIG. 2  illustrates a side sectional view of the seal assembly  16  being inserted deeper within the annulus  18  and wherein outer and inner radial surfaces of the legs  20 ,  24  are in respective sealing engagement with the inner and outer surfaces  22 ,  28 . Further, an energizing ring  42  which is inserted into the gap  26  provides a radial force for sealingly engaging legs  20 ,  24  with inner and outer surfaces  20 ,  28 . An axial force F applied to energizing ring  42  further downwardly urges the seal element  19  and nose ring  32  so that nose ring  32  is in contact with lock-down ring  38 . In this example, nose ring  32  is shown having a flexible portion that deforms when wedged between lock-down ring  38  and inner groove  34  in inner tubular  14 . When deformed, nose ring  32  is in a configuration generally oblique to the axis A x , which is in contrast to the elongate configuration of  FIG. 1  that is generally parallel with axis A x . Lock-down ring  38  is shown being urged radially outward at least partially out of lock-down groove  34  and into interfering contact with tubular  12  while remaining in interfering contact with tubular  14 . More specifically, a surface of lock-down ring  38  distal from seal element  19  rests on and is in contact with the pedestal  36  of tubular  14 . Urging the lock-down ring  38  radially outward in the example of  FIG. 2 , further positions a surface of lock-down ring  38  proximate seal element  19  into engaging contact with shoulder  40 . As such, relative axial movement between tubulars  12 ,  14  is arrested by the presence of the interfering lock-down ring  38 . Additionally, substantially all axial forces resulting from respective axial movements of the tubulars  12 ,  14  are transferred through the lock-down ring  38 . Thus, forces on the seal element  19  that result from forces that transfer between the tubulars  12 ,  14 , can be minimized. The compound angle created by the transition  39  on the lock-down ring  38  also reduces relative movement between the seal assembly  16  and the inner tubular  14 . The more oblique surface between the transition  39  and pedestal  36  urges the lower terminal portion of the nose ring  32  radially inward, where it is wedged between the lock-down ring  38  and outer surface  28  of inner tubular  14 . Strategically profiling the inner surface of the lock-down ring  38  and outer surface  28 , in combination with the flexible nose ring  32 , directs forces from the lock-ring  38  to the nose ring  32  in a direction oblique to the axis A x , instead of parallel to the axis A x . Obliquely directing forces from the lock-ring  38  to the nose ring  32 , rather than directing the forces axially, creates a force coupling the nose ring  32 , and attached seal assembly  16 , to the inner tubular  14 , As such, during episodes of thermal expansion of the casing or casing hanger, seal integrity may be maintained between tubulars  12 ,  14  by bypassing the resulting axial forces through lock-down ring  38 . Bending of the nose ring  32  may be facilitated by scoring an inner radial surface of lock-down ring  38  with a notch  43 , wherein notch  43  may extend along an entire circumference of nose ring  32  or along a portion thereof. 
         [0021]    Referring now to  FIG. 3 , shown in perspective view is an alternate embodiment of nose ring  32 A, that includes axial slots  44  that extend from an end of the nose ring  32 A distal from its attachment with seal element  19  into a mid-portion of the body of nose ring  32 A. The slots  44  can each have the same length, or as have different lengths as shown. Positioning of the slots  44  define elongate fingers  46  between adjacent slots  44 , where the absence of material due to slots  44  reduces the force required for deforming sidewalls of the nose ring  32 A, thereby facilitating its deformed setting position as illustrated in  FIG. 2 . In an example, the axial force required for positioning the nose ring  32 ,  32 A into the setting position illustrated in  FIG. 2  is less than the axial force required for energizing the seal element  19 . In this example, the nose ring  32  would be in the set position of  FIG. 2  and between the lock-down ring  38  and inner tubular  14  before the energizing ring  42  would set the legs  20 ,  24  into sealing contact with the inner and outer tubulars  14 ,  12 . 
         [0022]      FIG. 4  provides a side partial sectional view one example of the seal assembly  16  set between tubulars  12 ,  14 . The tubulars  12 ,  14  are part of the wellhead assembly  10 , which is shown mounted on a surface  48  of a formation through which a wellbore  50  is formed. Casing  52  depends downward from tubular  14 , and a production tree  54  is shown mounted on tubular  12 . A main bore  56  extends through wellhead assembly  10  and into communication with wellbore  50 , wherein a swab valve  58  is disposed in main bore  56  for controlling access into the wellbore  50 . Also, wing valves  60  are shown set in lines that mount to the production tree  54 . 
         [0023]    Shown in a plan view in  FIG. 5  is an alternate embodiment of lock-down ring  38 A and shown having slots  62  formed axially from an outer terminal radius of lock-down ring  38 A approximately to a mid-portion of the body of the lock-down ring  38 A. In this example, slots  64  are formed axially through lock-down ring  38 A from its inner diameter that extend radially outward approximately to a mid-portion of the body of lock-down ring  38 A. In the example of  FIG. 5 , slots  62  are offset from slots  64 , however, alternate embodiments exist where slots  62 ,  64  are aligned or spaced apart at different angular locations than as shown. 
         [0024]    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:
A seal system selectively set between coaxial downhole tubulars seals between the tubulars; the system also locks the tubulars together to resist relative axial movement from thermal expansion. The seal system includes a seal element with a nose ring that couples a lock-down ring to both the inner and outer tubulars. Before inserting the seal system between the tubulars, the lock-down ring is disposed in a groove on the inner tubular. Setting the seal system drives a lower tip of the nose ring between the lock-down ring and inner tubular, thereby urging the lock-down ring radially outward. A portion of the lock-down ring remains in the groove, while an outer radial portion of the lock-down ring inserts into a profile on the outer tubular. Axial movement of a tubular transfers force to the other tubular through the lock-down ring, while a minimal amount of force transfers through the seal system.