Abstract:
A wellhead system comprising an outer wellhead housing, an inner wellhead member, and an annulus therebetween. The inner wellhead member, such as a casing hanger, is adapted to land in the outer wellhead housing. The outer wellhead housing may comprise a wickers formed in a hardened metal inlay. Alternatively, or in addition to the wickers formed in the outer wellhead housing, wickers may be formed in a hardened metal inlay in the inner wellhead member. An annular metal seal may be disposed in the annulus and driven into the wickers to seal the annulus between the inner wellhead member and the outer wellhead housing and to axially restrain the seal within the annulus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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 and an annular sealing ring having an increased rated working pressure. 
     2. Brief Description of Related Art 
     In hydrocarbon production wells, a wellhead housing is located at the upper end of the well. The wellhead housing is a large tubular member having an axial bore extending through it. Casing will extend into the well and will be cemented in place. A casing hanger, which is on the upper end of the casing, will land within the wellhead housing. The exterior of the casing hanger is spaced from the bore of the wellhead housing by an annular clearance which provides a pocket for receiving an annulus seal. 
     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 bore and with the exterior of the casing hanger. 
     Some annular seals utilize wickers. Wickers may be located on the exterior of the casing hanger, in the bore of the wellhead housing, or both. The outer leg of the seal embeds into the wickers of the bore while the inner leg of the seal embeds into the wickers of the casing hanger. This locks the annulus seal in place, providing axial restraint, as well as forming a seal. 
     The sealing wickers are machined directly into the bore of the high pressure housing and landing subs or the neck of the casing hangers. The annulus seal is made of a sufficiently deformable metal to allow it to deform against the wickers of the casing hanger. 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 casing hangers. 
     SUMMARY OF THE INVENTION 
     Various embodiments of this invention provide a seal between a wellhead housing and a casing hanger, or between other wellbore members such as a landing sub, wherein the seal is formed between wickers having a higher yield strength than the underlying material, and, in some embodiments, an annular sealing ring also having a higher yield strength than a conventional annular sealing ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a sectional view of a casing hanger, wellhead housing, seal, and energizing ring. 
         FIG. 2  is a sectional view showing an exemplary embodiment of a casing hanger with a hardened wicker inlay and a seal. 
         FIG. 3  is a detail view of the casing hanger and seal of  FIG. 2  with the seal energized. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     Referring to  FIG. 1 , a wellhead housing  10  is presented. In the illustrated embodiment, the wellhead housing  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. A casing hanger  14  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 wall  18  of casing hanger  14 . A similar set of wickers  22  is located radially across on bore  12 . Wickers  20 ,  22  are grooves defined by parallel circumferential ridges and valleys. They are not threads. 
     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. 
     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 wickers  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  42  that will frictionally engage the inner diameter of outer leg  30 . Energizing ring  40  has an inner diameter  44  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 . 
     Referring to  FIG. 2 , an embodiment of a wellhead system  48  utilizing high strength wickers  50 ,  52  to seal and secure wellhead members is presented. In the illustrated embodiment, the high strength wickers  50 ,  52  are located on the casing hanger  56  and high pressure housing  54 , respectively. However, the high strength wickers may be located on other components, or on only one of these components. Casing hanger  56  can include exterior wall  57  and upward facing shoulder  58 . 
     In the illustrated embodiment, the high strength wickers  50 ,  52  are formed in an inlay material deposited on casing hanger  56  and the high pressure housing  54 , respectively. In this embodiment, an elongated groove  60  is formed on the bore of high pressure housing  54 . Elongated groove  60  may have an axial length that is longer than the axial length of outer leg  62  of seal member  64 . In an exemplary embodiment, elongated groove  60  has an axial length of roughly 3.5 inches. However, the axial length may be longer or shorter. In the illustrated embodiment, elongated groove  60  is filled with inlay  68 , which is made of a material having a yield strength and a hardness greater than the yield strength and hardness of high pressure housing  54 . In this embodiment, the yield strength and hardness of inlay  68  are also greater than the yield strength and the hardness of seal member  64 . Similarly, an elongated groove  66  is formed on an outer diameter of casing hanger  56 . Elongated groove  66  is filled with inlay  72 , which is made of a material having a yield strength and a hardness greater than the yield strength and hardness of casing hanger  56 . In this embodiment, the yield strength and hardness of inlay  72  are also greater than the yield strength and hardness of seal member  64 . Casing hanger elongated groove  66  may have an axial length that is longer than the axial length of inner leg  70  of seal member  64 . In an exemplary embodiment, casing hanger elongated groove  66  has an axial length of roughly 3.5 inches. However, the axial length may be longer or shorter. 
     In the illustrated embodiment, high pressure housing  54  and casing hanger  56  are comprised of 8630-modified low alloy steel. The 8630-modified low alloy steel has a yield strength of, approximately, 80 ksi. The standard for materials used in corrosive environments in oil and gas production is NACE (National Association of Corrosion Engineers) standard “MR 0175”, entitled: “Petroleum and natural gas industries-Materials for use in H 2 S-containing environments in oil and gas production.” For corrosion protection, NACE standard MR 0175 limits the hardness of 8630-modified low alloy steel for use in corrosive environments in oil and gas production to a hardness of 22 Rockwell C (“HRC”). 
     Inlays  68 ,  72  may be made from a high strength alloy, such as a nickel alloy. In some embodiments, inlays  72  and  68  are made from an austenitic nickel-chromium-based alloy such as nickel alloy 725 (UNS N07725). In an exemplary embodiment, the high strength alloy used for inlays  72  and  68  has a yield strength of 120-130 ksi. The hardness of the inlay varies depending on the type of inlay material and the subsequent treatments such as heat treating. The hardness can be between roughly less than 20 HRC to greater than roughly 37 HRC. Preferably, the hardness is at least approximately 22 HRC. In some embodiments, the inlay hardness may be roughly 27-29 HRC. The greater hardness of the wickers  50 ,  52  formed in inlays  68 ,  72  enables them to bite into the seal to a greater degree than similar wickers made of 8630-modified low alloy steel. Thus, producing a better seal. The higher yield strength of the wickers  50 ,  52  formed in inlays  68 ,  72  enables them to restrain axial movement of the seal to a greater degree than similar wickers made of 8630-modified low alloy steel. 
     Inlay  72 ,  68  may be formed by a variety of manufacturing techniques. In an exemplary embodiment, inlays  72 ,  68  are formed by welding the inlay material onto the surface of elongated grooves  66 ,  60 . A welder may, for example, make multiple passes to fill grooves  66 ,  60  with a weld bead. Other forms of deposition may be used. The radial thickness of inlays  72 ,  68  may be any thickness including, for example, roughly 0.125 inches to 0.5 inches. 
     After inlay  72 ,  68  is created, each inlay surface is machined to form wickers  50 ,  52 . Wickers  50  are a series of parallel grooves on the surface of inlay  72 . Wickers  52  are a series of parallel grooves on the surface of inlay  68 . Each groove is defined by a valley having two sides, the sides of two adjacent valleys forming a ridge. The sides of an individual valley may have the same pitch or may have different pitches. 
     After depositing inlay  72 ,  68  material and/or after machining wickers  50 ,  52 , the inlay material may be heat treated. Heat treating may be used to relieve residual stress present in the inlay as a result of the heating and cooling process that occurs during the inlay deposition process. In some embodiments, stress-relief heat treatments are used to relieve stress in the inlay but not to substantially alter the as-deposited hardness of the inlay. In these exemplary embodiments, the inlay material is left in its “soft,” or annealed, state, which still has a greater hardness than the hardness of 33 ksi plain carbon steel. Some nickel alloys become harder as a result of heat treatment at temperatures and for durations beyond stress-relief heat treatment. Additional heat treating of inlays  72  and  68  may be used to harden, or “age,” the inlay material to a higher hardness than the “soft” state. The increased hardness may cause increased brittleness in the bond between the inlays  72 ,  68  and the surface of the elongated grooves  66 ,  60 . In an exemplary embodiment, inlay  72 ,  68  are heat treated for approximately four hours to provide stress relief after wickers  50 ,  52  are machined into inlay  72 ,  68 . 
     In the illustrated embodiment, seal member  64  is formed from a material having a lower yield strength than the yield strength of wickers  50 ,  52 . By using a high yield strength material for wickers  50 ,  52 , it is possible to use a second material having a high yield strength for seal member  64 , provided that the seal member  64  yield strength is lower than that of wickers  50 ,  52 . Once energized, a seal having a higher yield strength than conventional seal member  26  would have a greater ability to resist axial movement of the seal. Seal member  64  could, for example, be made of low carbon steel having a 45 ksi minimum yield strength. Seal member  64  may, however, be made of steel having a minimum yield strength of 15 ksi. 
     The seal assembly comprises a generally U-shaped seal member  64 . Seal member  64  has an outer wall or leg  62  and a parallel inner wall or leg  70 , the legs  62 ,  70  being connected together at the bottom by a base and open at the top. The inner diameter  76  of outer leg  62  is radially spaced outward from the outer diameter  78  of inner leg  70 . This results in an annular clearance  80  between legs  62 ,  70 . The inner diameter  76  and the outer diameter  78  are smooth cylindrical surfaces parallel with each other. Similarly, the inner diameter of inner leg  82  and the outer diameter of outer leg  84  are smooth, cylindrical, parallel surfaces. 
     Referring to  FIG. 3 , wickers  52  are best able to form a seal when wickers  52  are able to “bite” into the surface  84  of the annular seal leg  62 . As seal leg  62  is expanded into wickers  52 , the surface  84  of seal leg  62  flows around wickers  52  as plastic deformation of seal leg  62  occurs. In an exemplary embodiment, the tips of the wickers  52  achieve a depth of approximately 0.030″ below the surface  84  of the annular seal member  62 . If the seal member  62  is made from a material that is too hard in relation to the wickers  52 , the wickers  52  may deform rather than biting approximately 0.030″ into the seal member  62 . High strength wickers  52 , such as wickers formed from nickel alloy 725, are able to bite into a high-hardness seal member  62  without deformation. Outer leg  62  is shown for illustrative purposes in  FIG. 3 , but the same principles apply to inner leg  70 . 
     Referring again to  FIG. 2 , energizing ring  88  applies force to press the legs  62 ,  70  of the seal apart, causing seal legs  62 ,  70  engage the wickers  52 ,  50 . Energizing ring  88  may have a wider cross-section than a conventional energizing ring  40  ( FIG. 1 ) to create more interference with seal legs  62 ,  70  and thus cause increased radial contact force between the seal legs  62 ,  70  and the wicker sealing surface  52 ,  50 . The increased compressive force between the seal surfaces  82 ,  84  and the wickers  50 ,  52  creates a tighter seal against wellbore pressure. If the force applied by the energizing ring  88  is too high in relation to the yield strength of the wicker material, the tips of the wickers  50 ,  52  may fold in response to the compressive force from the seal  64 . The compressive force that causes high yield strength wickers to fail is significantly higher than the compressive force that causes conventional wickers to fail. Some embodiments use a conventional seal  28  with high strength wickers  50 ,  52 . 
     A seal assembly that utilizes high strength wickers  50 ,  52  provides several advantages over conventional wickers. For example, a conventional seal  28  and wicker  20 ,  22  combination may be able to withstand a wellbore pressure of 15,000 psi. However, a high strength seal, pressed against high strength wickers with great force, may achieve a tighter seal and thus withstand a wellbore pressure of 20,000 psi, or more. In addition, objects such as a drill bit or a spinning drill string could cause damage to the sealing surfaces and wickers  50 ,  52 . Damage to the sealing surfaces and wickers  50 ,  52 , even minor damage, may result in an imperfect seal. A scratch may serve as a pathway for high pressure fluids and gasses to pass between the annular seal and the sealing surface. However, the high strength wickers  50 ,  52  are more resistant to scratches, dents, and other damage than conventional strength wickers. A material with a high yield strength, such as a 120 ksi minimum yield strength, is less likely to deform when impacted by another object such as a drill string. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.