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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates in general to a method and apparatus to form a high pressure seal between two wellbore members, and in particular to wickers having an increased lockdown capability. 
         [0003]    2. Description of the Related Art 
         [0004]    Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger that supports a string of casing extending into the well for the flow of production fluid. The casing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a casing head. A packoff (or other seal assembly) seals the annulus between the casing hanger and the outer wellhead member. Alternately, the inner wellhead member can be a tubing hanger located in a wellhead housing and secured to a string of tubing extending into the well. A pack off (or other seal assembly) seals the annulus between the tubing hanger and the wellhead housing. In another alternative design, the inner wellhead member may be an isolation sleeve, such as might be used to isolate high pressure, abrasive fracturing fluids from certain portions of the wellhead. A packoff (or other seal assembly) seals the annulus between the isolation sleeve and the outer wellhead member. 
         [0005]    There are many types of annulus seals, including rubber, rubber combined with metal, and metal-to-metal. One metal-to-metal seal in use has a U-shape, having inner and outer walls or legs separated from each other by an annular clearance. An energizing ring, which has smooth inner and outer diameters, is pressed into this clearance to force the legs apart to seal in engagement with the inner surface of the outer wellhead member and with the exterior of the inner wellhead member. 
         [0006]    Some annular seals utilize wickers. Wickers may be located on the exterior of the inner wellhead member, in the bore of the outer wellhead member, or both. The outer leg of the seal embeds into the wickers of the outer wellhead member while the inner leg of the seal embeds into the wickers of the inner wellhead member. This provides the function of both locking the annulus seal in place, providing axial restraint to the inner wellhead member, as well as forming a seal. Lockdown is the term used for the capacity and capability of the inner wellhead member and seal assembly to stay in place vertically in the wellhead when a pressure or other force is applied from below. This force may be, for example, the result of annulus pressure build-up or from thermal growth of the casing attached on bottom of the hanger or a combination of both. A sufficient lockdown capacity is needed to ensure that the seal integrity is maintained and the inner wellhead member and seal remains static. 
         [0007]    The sealing wickers are machined directly into the bore of the outer wellhead member or landing subs and the neck of the inner wellhead member. The annulus seal is made of a sufficiently deformable metal to allow it to deform against the wickers. The deformation occurs as the wickers “bite” into the annulus seal. In order to cause the seal to deform without damaging the wickers, the annulus seal is made of a metal that is softer than the steel used for the inner and outer wellhead members. The wicker bite resists the lockdown force from the inner wellhead member as a shear resistance. The higher the wicker bite, the higher the lockdown capacity. The lower the wicker bite, the lower the lockdown capacity. 
         [0008]    The future of oil and gas exploration lies in deep waters and greater depth (for pay-zone) under the seabed, which renders the subsea equipment to harsh conditions like high pressure and high temperatures (HPHT), sour fluid etc. This calls for designing the subsea equipment and tools for this widened HPHT envelope. The all-metal seal, in discussion, is a very critical component of the wellhead system that performs annulus sealing as well as acts as a lockdown device. This HPHT condition causes a greater hanger movement due to higher thermal growth and annulus pressure build-up. Usually, this movement of the hanger is constrained by the seal. The invention caters to the above problem of greater hanger movement at the same time maintaining the field-proven design. The proposed all-metal seal rated to a higher lockdown capacity would allow higher reliability. 
         [0009]    An improved system that provides a more robust and reliable lockdown capacity is sought. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the foregoing, various embodiments of the present invention advantageously provide seal assemblies to address shortfalls of the prior art. Embodiments of the present application provide enhanced lockdown capacity which widens the application in which the seal can be used, thus aiding hydrocarbon producers in being able to pursue wells in more extreme environments. Embodiment of the current application provide increased sealing capability and reliability, can use existing tools for installation. 
         [0011]    In one embodiment of the current application, a wellhead assembly comprises an outer tubular wellhead member with a bore having an axis and a concentric inner seal surface, an inner tubular wellhead member for receiving an annular seal landed within the outer tubular wellhead member defining a seal pocket between them, the inner tubular member having a concentric outer seal surface, and a plurality of circumferentially extending, parallel wickers formed in at least one of the seal surfaces of the outer tubular and on the outer surface of the inner tubular. A profile of at least one of the wickers differs from a profile of at least some of the other wicker. 
         [0012]    In an alternative embodiment, the assembly further comprises an annular seal that is adapted to be disposed within the seal pocket and to be urged against the wickers on the outer tubular and the inner tubular to create a seal and to resist upward motion of the inner tubular relative to the outer tubular and wherein the plurality of wickers are adapted to deform a surface of the annular sealing ring. 
         [0013]    In yet another embodiment, the wickers on at least one of the seal surfaces are located in a bottom region of the seal pocket, a middle region of the seal pocket which is above the bottom region, and a top region of the seal pocket which is above the middle region, and wherein the profiles of the wickers in each of the regions differs from the profiles in each of the other regions. The profile of the wickers in the bottom region may be substantially similar to each other, the profile of the wickers in the middle region may be substantially similar to each other, and the profile of the wickers in the top region may be substantially similar to each other. 
         [0014]    In an alternative embodiment, the profile of the wickers in the bottom region are different than the profile of the wickers in the middle region and the top region, and the profile of the wickers in the middle region are different than the profile of the wickers in the top region. 
         [0015]    In another alternative embodiment, the wickers in the bottom region have more material on the top flank of each wicker than on the bottom flank of each wicker, the profile of the wickers in the middle region are symmetrical about a mid plane of each wicker, the wickers in the top region have more material on the bottom flank of each wicker than on the top flank of each wicker. 
         [0016]    In yet another alternative embodiment, the wickers in the bottom region have a tilted mid plane symmetrically dividing each wicker in half, the tilted mid plane being inclined relative to a normal plane perpendicular to the axis, resulting in more material below the normal plane in each wicker, and the wickers in the top region have a tilted mid plane symmetrically dividing each wicker in half, the tilted mid plane being inclined relative to a normal plane perpendicular to the axis, resulting in more material above the normal plane in each wicker. 
         [0017]    In other alternative embodiments, the wickers may have a mid-plane passing through a crest of each wicker that is normal to the axis, and the wickers in the top region and in the bottom region may be asymmetrical about their respective mid-plane. Alternatively, the wickers may have a mid-plane passing through a crest of each wicker that is normal to the axis, and some of the wickers may be asymmetrical about their respective mid-plane. In some embodiments, the wickers may have an aspect ratio of less than 1.0. 
         [0018]    In other embodiments of the current application, a wellhead assembly comprises an outer tubular wellhead member with a bore having an axis and a concentric inner seal surface, an inner tubular wellhead member for receiving an annular seal landed within the outer tubular wellhead member defining a seal pocket between them, the inner tubular member having a concentric outer seal surface, and a plurality of circumferentially extending, parallel wickers formed of a material in at least one of the seal surfaces of the outer tubular and on the outer surface of the inner tubular. The wickers may have a mid plane passing through a crest of each wicker dividing each wicker substantially in half, and a normal plane passing through the crest and perpendicular to the axis. The material above the normal plane of at least one of the wickers may not equal the material below the normal plane of such at least one wicker. 
         [0019]    In other embodiments, at least one wicker may have a mid plane inclined relative to the normal plane, resulting in more material below the normal plane in each wicker than above. Alternatively, at least one wicker may have a mid plane inclined relative to the normal plane, resulting in more material above the normal plane in each wicker than below. The mid plane and normal plane of each wicker are parallel to each other and a profile of at least one wicker is asymmetrical about its mid plane. 
         [0020]    In alternative embodiments, an upper flank and a lower flank of at least one wicker are at the same angle relative to the axis, the mid plane bisects a base of such wicker equidistant from a top and a bottom of the base of such wicker, and there is more material above the mid plane than below. Alternatively, an upper flank and a lower flank of at least one wicker may be at the same angle relative to the axis, the mid plane may bisect a base of such wicker equidistant from a top and a bottom of the base of such wicker, and there may be more material below the mid plane than above. 
         [0021]    In yet other embodiments, at least one wicker has a mid plane inclined relative to the normal plane, resulting in more material below the normal plane in each wicker than above, and at least one wicker has a mid plane inclined relative to the normal plane, resulting in more material above the normal plane in each wicker than below. The wickers may be symmetrical about the mid plane and asymmetrical about normal plane. 
         [0022]    In other embodiments of the current application, a wellhead assembly comprises an outer tubular wellhead member with a bore having an axis and a concentric inner seal surface, an inner tubular wellhead member for receiving an annular seal landed within the outer tubular wellhead member defining a seal pocket between them, the inner tubular member having a concentric outer seal surface, and a plurality of circumferentially extending, parallel wickers formed in at least one of the seal surfaces of the outer tubular and on the outer surface of the inner tubular. Each adjacent wicker being separated by a valley and having a wicker radial width measured from the base of the valley to a crest, and an axial length dimension measured as the distance between the base of the valley on either side of such wicker. The wicker radial width of at least one wicker is equal to no more than the axial length dimension. In some embodiments, the valley between the wickers comprises a curved surface having a radius of at least 0.04 inches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    So that the manner in which the features and advantages of the invention, as well as others which will become apparent, may be understood in more detail, a 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, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it may include other effective embodiments as well. 
           [0024]      FIG. 1  is a sectional view of portions of a wellhead assembly providing a annulus seal 
           [0025]      FIG. 2   a  is a sectional view of a prior art wicker profile. 
           [0026]      FIG. 2   b  is a sectional view of a wicker profile of an embodiment of the current application. 
           [0027]      FIG. 2   c  is a sectional view of a wicker profile of another embodiment of the current application. 
           [0028]      FIG. 2   d  is a sectional view of a wicker profile of another embodiment of the current application. 
           [0029]      FIG. 3   a  is a sectional view of a wicker profile of another embodiment of the current application. 
           [0030]      FIG. 3   b  is a sectional view of a wicker profile of another embodiment of the current application. 
           [0031]      FIG. 4   a  is a sectional view of a prior art sectional view of wickers. 
           [0032]      FIG. 4   b  is a sectional view of a section of wickers of another embodiment of the current application. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    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. Prime notation, if used, indicates similar elements in alternative embodiments. 
         [0034]    Referring to  FIG. 1 , an outer wellhead member  10  is presented. In the illustrated embodiment, the wellhead member  10  is a conventional high pressure housing for a subsea well. It is a large tubular member located at the upper end of a well, such as a subsea well. Wellhead housing  10  has an axial bore  12  extending through it. An inner wellhead member  14  is located within axial bore  12 . In the illustrated embodiment, the inner wellhead member is a casing hanger  14 , which lands in the wellhead housing  10 . Casing hanger  14  is a tubular conduit secured to the upper end of a string of casing (not shown). Casing hanger  14  has an upward facing shoulder  16  on its exterior. The exterior wall  18  of casing hanger  14  is parallel to the wall of bore  12  but spaced inwardly. This results in an annular pocket or clearance between casing hanger exterior wall  18  and bore  12 . A set of wickers  20  is located on the exterior surface  18  of casing hanger  14 . A similar set of wickers  22  may also be located radially across on an inner surface bore  12 . Wickers  20 ,  22  are circumferential grooves defined by parallel circumferential ridges and valleys. They are not threads. Wickers  20 ,  22  may be configured as one of the embodiments of  FIG. 2   b ,  2   c ,  2   d ,  3   a ,  3   b  or  4   b.    
         [0035]    A seal assembly  26  lands in the pocket between casing hanger exterior wall  18  and bore wall  12 . Seal assembly  26  may be made up entirely of metal components. These components may include a generally U-shaped seal member  28 . Seal member  28  has an outer wall or leg  30  and a parallel inner wall or leg  32 , the legs  30 ,  32  being connected together at the bottom by a base and open at the top. The inner diameter of outer leg  30  is radially spaced outward from the outer diameter of inner leg  32 . This results in an annular clearance  36  between legs  30 ,  32 . The inner diameter and the outer diameter are smooth cylindrical surfaces parallel with each other. Similarly, the inner diameter of inner leg  32  and the outer diameter of outer leg  30  are smooth, cylindrical, parallel surfaces. 
         [0036]    An energizing ring  40  is employed to force legs  30 ,  32  radially apart from each other and into sealing engagement with wickers  20 ,  22 . The wickets  20 ,  22  bite into the inner leg  30  and outer leg  32 , respectively, of the seal assembly  26  as the energizing ring  40  forces the legs  30 ,  32  against the wickers  20 ,  22 . Energizing ring  40  has an outer diameter that will frictionally engage the inner diameter of outer leg  30 . Energizing ring  40  has an inner diameter that will frictionally engage the outer diameter of inner leg  32 . The radial thickness of energizing ring  40  is greater than the initial radial dimension of the clearance  36 . 
         [0037]    During the seal setting process, wickets  20  at the bottom region  42  of the housing bore are deformed at a top flank  44  ( FIG. 2   a ) of the wicker. The magnitude of this top flank  44  deformation keeps reducing with every subsequent wicker along the length towards a middle region  46 , such that the farther away a wicker  20  is located below wickers  20  in the middle region  46 , the greater the deformation of top flank  44 . The profile of the wickets in the middle region  46  during the sealing process remain unchanged. Wickers at the top region  48  of the housing bore are deformed at a bottom flank  50 . The magnitude of this bottom flank  50  ( FIG. 2   a ) deformation keeps reducing with every subsequent wicker along the length towards middle region  46 , such that the farther above a wicker  20  is located from middle region  46 , the greater the deformation of bottom flank  50 . These local plastic deformations follow this consistent pattern and reduce the effectiveness of the wickers. Embodiments of the current invention modify the prior art wicker flank such that local modifications are made in a pattern to counter the deformation and keep the “v” shaped wicker profile intact after seal setting. This provides for increased lockdown capabilities. 
         [0038]    The prior art wicker  52  of  FIG. 2   a , has a uniform profile in all regions horizontally, of the outer wellhead member, with a symmetrical profile about wicker mid plane  54 . Wicker mid-mid plane  54  bisects the profile of each wicker and crest  60  such that an equal volume of the wicker is above and below mid plane  54  from the base  55  of the wicker to at least the midpoint of its radial width  57 . Mid plane  54  bisects crest  60  equidistant along an axial length of crest  60 . Flanks  44 ,  50  intersect each other at a fairly sharp crest  60 . 
         [0039]    Embodiments of the current application are illustrated in  FIGS. 2   b - 2   d.  In the bottom region  42  ( FIG. 1 ), wickers  56  of  FIG. 2   b  are used. The profile of wicker  56  is such that top flank  44  comprises more material  58  near crest  60  than does bottom flank  50 . Before the seal setting process, the profile of wicker  56  is not symmetrical about wicker mid plane  54 . The excess material  58  is sacrificial in nature in that it is expected to undergo deformation during the seal setting process. Because of this expected deformation of excess material  58 , at the end of the seal setting process, wicker  56  is expected to have a substantially symmetrical “v” shaped profile about wicker mid plane  54 . In this embodiment, flank  44  may be the same angle relative to axis  23  as the prior art wicket  52 , which may be 55 degrees as an example. Excess material  58  has a flat upper surface intersected by flank  44 . Crest  60  has a greater axial length than prior art wicker  52 . 
         [0040]    The wickers in the middle region  46  will comprise wickers  62  of  FIG. 2   c . Wicker  62  has a profile that is symmetrical about wicker mid plane  54 . Because there is no deformation of the wickers  20  in middle region  46  during the seal setting process, wickers  62  will substantially maintain their symmetrical “v” shaped profile about wicker mid plane  54 . Wickers  62  may be the same as prior art wickers  52 . 
         [0041]    In the top region  48 , wickers  64  of  FIG. 2   d  are used. The profile of wicker  64  is such that bottom flank  50  comprises more material  66  near crest  60  than does top flank  44 . Before the seal setting process, the profile of wicker  64  is not symmetrical about wicker mid plane  54 . The excess material  66  is sacrificial in nature in that it is expected to undergo deformation during the seal setting process. Because of this expected deformation of excess material  66 , at the end of the seal setting process, wicker  64  is expected to have a substantially symmetrical “v” shaped profile about wicker mid plane  54 . In this embodiment, flank  50  may be the same angle relative to axis  23  as the prior art wicket  52 , which may be  55  degrees as an example. Excess material  66  has a flat lower surface intersected by flank  50 . Crest  60  has a greater axial length than prior art wicker  52 . 
         [0042]    The use of varying wicker profiles in top region  48 , middle region  46  and bottom region  42  will provide for stronger wickers after the seal setting process. This in turn allows the seal assembly  26  to resist a higher upward force of hanger  14  and gives the wickers  20  a greater lockdown capacity. 
         [0043]    In addition to providing local modifications to wicker profiles, the ability of the wickers  22  to resist an upward force from hanger  14  alternatively may be improved by designing the profile such that wicker axes  54  is not perpendicular to the central axis  23  of the wellbore. The prior art wicker  52  of  FIG. 2   a  has a wicker mid plane  54  that is normal or perpendicular to a central axis  23  of the bores  12 ,  18 . In  FIGS. 3   a  and  3   b , wicker  72  has a wicker mid plane  54  that is not perpendicular to axis  23  as seen by comparing mid plane  54  to normal plane  70  that is perpendicular to axis  23 . Instead crest  60  of wicker  72  is tilted downward, while maintaining the “v” shaped profile of wicker  72 . This results in wicker mid plane  54  being at an acute angle  74  with normal plane  70 . Angle  74  may be for example, in the range of 2 to 5 degrees. Upper flank  44  has a lesser angle relative to axis  23  than lower flank  50 . For example, upper flank  44  may be  50  degrees relative to axis  23  and lower flank  50  may be 65 degrees relative to axis  23 . The lengths of flanks  44 ,  50  may be the same. Crest  60  has a same height as in  FIG. 2   a . The lower flanks  50  of wicker  72  are inclined more than the lower flanks  50  of prior art wickers  52 . Them material in wicker  72  above normal plane  70  exceeds the material below normal plane  70 . Wickers  72  may be employed in upper region  48 . 
         [0044]    A higher upward force is required to deform the wickers  72 , which are tilted downward, as compared to the amount of force required to deform horizontally aligned wickers  52  of the prior art. Therefore use of wickers  72  results in the ability of the seal assembly  26  to resist a higher upward force of hanger  14  and hence provides a higher lockdown capacity than using prior art wickers  52 . 
         [0045]    Similarly, in lower region  42 , wickers upward tilting wickers  68  of  FIG. 3   b  may be employed. Wicker  68  has a wicker mid plane  54  that is not perpendicular to axis  23  as seen by comparing mid plane  54  to normal plane  70  that is perpendicular to axis  23 . Instead crest  60  of wicker  68  is tilted upward, while maintaining the “v” shaped profile of wicker  68 . This results in wicker mid plane  54  being at an acute angle  75  with normal plane  70 . Angle  75  may be for example, in the range of 2 to 5 degrees. Upper flank  44  has a greater angle relative to axis  23  than lower flank  50 . For example, upper flank  44  may be 65 degrees relative to axis  23  and lower flank  50  may be  50  degrees relative to axis  23 . The lengths of flanks  44 ,  50  may be the same. Crest  60  has a same height as in  FIG. 2   a . The lower flanks  50  of wicker  68  are inclined more than the lower flanks  50  of prior art wickers  52 . The material in wicker  68  below normal plane  70  exceeds the material above normal plane  70 . 
         [0046]    A higher upward force is required to deform the wickers  68 , which are tilted upward, as compared to the amount of force required to deform horizontally aligned wickers  52  of the prior art. Therefore use of wickers  68  results in the ability of the seal assembly  26  to resist a higher upward force of hanger  14  and hence provides a higher lockdown capacity than using prior art wickers  52 . 
         [0047]    In alternative embodiments of the present application, the aspect ratio of wicker  20 , 22  profiles is also optimized.  FIG. 4   a  illustrates prior art wicker section  76  which comprises individual wickers  78 . Each individual wicker  78  is generally “v” shaped in profile. Generally “v” shaped fillets or valleys  80  separate each wicker  78 . The radial width  82  of each wicker divided by the axial length  84  of each wicker defines the aspect ratio of a wicker  78 . The radial width  82  of each wicker is generally measured from the bottom of the valley  80  to the crest  60  of such ridge. The axial length  84  is generally measured as the distance between the bottom of the valleys  80  on either side of such crest  60 . 
         [0048]      FIG. 4   b  illustrates an improved wicker aspect ratio of the present application. Wicker section  86  comprises a plurality of individual wickers  88 . Wickers  88  are generally “v” shaped in profile. Generally “u” shaped valleys  90  separate each wicker  88 . The radial width  92  of each wicker divided by the axial length  94  of the base of each wicker defines the aspect ratio of a wicker  88 . Wickers  88  have a lower aspect ratio than prior art wickers  78  ( FIG. 4   a ). In the prior art of  FIG. 4   a , the aspect ration is instead in the range of 2.5 to 3.0. In the embodiment of  FIG. 4   b , the aspect ratio is less than 1.0. That is, the radial width  92  of the wicker is not greater than the axial length  94  of the base of such wicker. In one embodiment, the aspect ration be for example, approximately 0.7 or in another embodiment, 0.68. Flanks  44  and  52  of an embodiment of the current application, as show in  FIG. 4   b  may incline at the same angle relative to axis  23  as in the prior art of  FIG. 4   a.    
         [0049]    With a lower aspect ratio, the valleys  90  may comprise a larger radius curve. In the prior art, the radius of each valley  80  was 0.01 to 0.02 inches. In an embodiment of the current application, the radius of each valley  90  is in the range of 0.04 to 0.05 inches, and may be, for example, 0.045 inches. The radius of the valley is, however, limited by the maximum penetration depth of the wicker for obtaining the desired seal penetration while still maintaining enough clearance to avoid hydro locking. 
         [0050]    A lower aspect ratio is achieved by decreasing the depth of valleys  90 , creating a shorter radial width  82  of each wicker  88 . Alternatively, a lower aspect ratio can be achieved by increasing the axial length  94  of each wicker. A combination of decreasing the depth of valleys  90  and increasing the axial length  94  may also be used to decrease the aspect ratio. Both the increased axial length  94  and shorter radial width  92  allow an individual wicker  88  to better resist deformation during upward hanger  14  movement. This results in a stronger wicker with greater lockdown capacity. 
         [0051]    In the drawings and specification, there has been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification. For example, although primarily illustrated in the context of a casing hanger landed within a modified high-pressure wellhead housing, one of ordinary skill in the art will recognize that the featured seal assembly and methods can be readily employed with respect to tubing within modified casing or other tubing.

Summary:
A wellhead assembly comprises an outer tubular wellhead member with a bore having an axis and a concentric inner seal surface, an inner tubular wellhead member for receiving an annular seal landed within the outer tubular wellhead member defining a seal pocket between them, the inner tubular member having a concentric outer seal surface, and a plurality of circumferentially extending, parallel wickers formed in at least one of the seal surfaces of the outer tubular and on the outer surface of the inner tubular. A profile of at least one of the wickers differs from a profile of at least some of the other wicker.