Abstract:
A wellhead seal assembly that forms a metal-to-metal seal between inner and outer wellhead members. A metal seal ring has inner and outer walls separated by a slot. An elastomeric seal is located below the seal ring and has a bottom portion that contacts an upward facing shoulder of a hanger. An energizing ring with a tapered nose is moved into the slot. The tapered nose has a compound angle that determines how much the nose travels into the slot when a force is applied to the energizing ring. Once the elastomeric seal is compressed to a desired level, the load on the energizing ring has increased to the point that the tapered nose of the energizing ring will further enters the slot and force the outer and inner walls of the metal seal into sealing engagement with the inner and outer wellhead members.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to wellhead assemblies and in particular to an energizing ring nose profile that allows increased compression of a seal before a U-seal is locked down. 
     BACKGROUND OF THE INVENTION 
     Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. Alternatively, the inner wellhead member could 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 packoff or seal seals between the tubing hanger and the outer wellhead member. In addition to the seal between the inner and outer wellhead members, another annular seal, or emergency seal, may be located below this seal. 
     A variety of seals located between the inner and outer wellhead members have been employed in the prior art.  FIG. 1  shows a portion of a seal assembly in the prior art within a wellhead housing  10 . Housing  10  is typically located at an upper end of a well and serves as an outer wellhead member. An energizing ring  2  is typically forced downward by a running tool or the weight of a string to force it into a slot  3  defined by a U-type metal seal ring  4 . This deforms inner and outer walls of the seal ring  4  apart into respective sealing engagement with inner and outer wellhead members  15 ,  10 . The energizing ring is typically a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring  4 , making the deformation permanent. Prior art seals may also 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. Located below the seal ring  4  is an emergency seal  5 , in case seal ring  4  fails, that rests on a shoulder  6  formed on an inner wellhead member, such as a hanger  15 . The emergency seal  5  may be fabricated from metallic, non-metallic, or elastomeric materials, or a combination thereof. The emergency seal  5  may be compressed when downward force from the string is applied to the energizing ring  2  to thereby cause emergency seal  5  to bulge outwards to contact the inner and outer wellhead members  15 ,  10  at a point below the seal ring  4  above. However, the energizing ring  2  also deforms the metal seal ring  4  against the outer wellhead member  10  and the inner wellhead member  15 . If the metal seal ring  4  is deformed against the inner and outer wellhead members  15  and  10  before the emergency seal  5  is compressed sufficiently to bulge outwards against the outer wellhead member  10 , then the emergency seal  5  may not be able to perform its function as an emergency seal and pressure integrity may diminish. 
     A need exists for a technique that addresses the seal leakage problems described above. In particular a need exists for a technique to compress an emergency seal a desired amount prior to deformation of the walls of the metal-to-metal seal. The following technique may solve these problems. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present technique, a seal assembly is provided that forms a metal-to-metal seal and has features that enhance sealability in the seal assembly. The seal assembly also includes features that enhance emergency or backup sealing capabilities. The seal ring has inner and outer walls separated by a slot and an elastomeric seal is located below the seal ring and has a bottom portion that contacts an upward facing shoulder of a hanger. A metal energizing ring has a tapered nose that may be pushed into the slot during installation to deform the inner and outer walls into sealing engagement with inner and outer wellhead members having wickers formed thereon. A radial gap exists between the outer wall of the seal and the inner wall of the mating housing. Such gap is required for installation in the field and is sufficiently large to require plastic deformation of the seal body, but not the energizer ring. 
     In an illustrated embodiment, the nose of the energizing ring has a compound angle configuration that can be tuned to allow a predetermined amount of force to be transmitted to the emergency seal below the seal ring. The compound angle also determines how much the nose travels into the slot when a force is applied to the energizing ring. This force and the accompanying reaction force from the shoulder of the hanger compresses the elastomeric seal to cause it to bulge outwards. The outward bulging of the elastomeric seal creates a seal between the inner surfaces of the inner and outer wellhead members. Once the elastomeric seal is compressed to a desired level, the load on the energizing ring has increased to the point that the tapered nose of the energizing ring will further enter the slot and force the outer and inner walls of the metal seal into sealing engagement with the inner and outer wellhead members. At this point, no additional compression of the elastomeric seal is possible. 
     In an example embodiment, the seal assembly also comprises the energizing ring that engages the slot. The retainer ring rests in a machined pocket on the outer surface of the energizing ring. The outer leg of the seal ring is machined with a taper that engages a taper formed on the retainer ring. The engagement ensures that the seal assembly remains intact as one solid structure during landing, setting, and retrieval operations. The retainer ring can alternatively rest in a machined pocket on the inner surface of the energizing ring to lock the seal onto the hanger. 
     The combination of stored energy provided for by the energizing ring, the compound angle configuration of the energizing ring nose, and the compressible elastomeric seal below the seal ring, advantageously provide enhanced emergency sealing if the metal-to-metal seal fails. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a seal assembly of the prior art with an energizing ring locked to the seal, but unset, and with an emergency seal decompressed; 
         FIG. 2  is a sectional view of a seal assembly being lowered between outer and inner wellhead members, in accordance with an embodiment of the invention; 
         FIG. 3  is a sectional view of the seal assembly of  FIG. 2  landed between outer and inner wellhead members in an unset position and with compression of an emergency seal, in accordance with an embodiment of the invention; 
         FIG. 4  is a sectional view of the seal assembly of  FIG. 2  landed between outer and inner wellhead members in a set position, in accordance with an embodiment of the invention; 
         FIG. 5  is a sectional view of the nose of an energizing ring before entering the slot of a seal ring, in accordance with an embodiment of the invention; 
         FIG. 6  is a sectional view of the nose of an energizing ring after entering a slot of a seal ring and deforming walls of the seal ring, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 , an embodiment of the invention shows a portion of a wellhead assembly that includes a high pressure wellhead housing  10 . In this example, the housing  10  is located at an upper end of a well and serves as an outer wellhead member of the wellhead assembly. Housing  10  has a bore  11  located therein. In this example, an inner wellhead member is a casing hanger  15 , which is shown partially in  FIG. 2  within bore  11 . Alternately, wellhead housing  10  could be a tubing spool or a Christmas tree, and casing hanger  15  could instead be a tubing hanger, plug, safety valve, or other device. Casing hanger  15  has an exterior annular recess radially spaced inward from bore  11  to define a seal pocket  17 . Wickers  12  are located on a portion of the wellhead bore  11  and wickers  18  are located on a portion of the cylindrical wall of seal pocket  17 . In this example, the profiles of each set of wickers  12 ,  18  are shown as continuous profiles on the bore  11  and seal pocket  17 . However, the wickers  12 ,  18  may be configured in other arrangements. 
     Continuing to refer to  FIG. 2 , a metal-to-metal seal assembly  21  is lowered between the housing  10  and casing hanger  15  and located in seal pocket  17 . Seal assembly  21  includes a seal ring  23  formed of a metal such as steel. Seal ring  23  has an inner wall  25  that is an inner seal leg  27  for sealing against the cylindrical wall of casing hanger  15 . Seal ring  23  has an outer wall surface  29  comprised of outer seal leg  31  that seals against wellhead housing bore  11 . Each wall surface  25 ,  29  is cylindrical and smooth and engages the wickers  12 ,  18  when deformed against the bore  11  of the housing  10  and seal pocket  17  of the casing hanger  15 . The wickers  12 ,  18  enhance the grip to aid in the prevention of axial movement of the seal assembly once set. 
     In the example  FIG. 2 , seal ring  23  is uni-directional, having an upper section only; however, a seal ring that is bi-directional may optimally be used. The upper section has a slot  35 . The inner and outer surfaces forming slot  35  comprise generally cylindrical surfaces, that when viewed in an axial cross-section are generally parallel and each follow a straight line. 
     An annular energizing ring  41  engages slot  35  on the upper side. As shown, the energizing ring  41  has an axis A R  that is substantially parallel with an axis (not shown) of the wellhead assembly. Energizing ring  41  is forced downward into slot  35  by a running tool (not shown) connected to grooves  43  on the inner diameter of upper energizing ring  41  during setting. Alternatively, seal assembly  21  and energizing ring  41  may be part of a string that is lowered into bore  11 , the weight of which forces energizing ring  41  into slot  35 . If retrieval is required, the grooves  43  can be engaged by a retrieving tool (not shown) to pull energizing ring  41  from set position. Energizing ring  41  can be formed of metal, such as steel. The mating surfaces of energizing ring  41  and outer seal leg  31  may be formed at a locking taper. 
     In an embodiment of the invention, an outwardly biased retainer ring  44  is carried in a pocket  45  on the outer surface of upper energizing ring  41 . Ring  44  has grooves  47  on its outer surface and an edge that forms an upward facing shoulder  49 . On the upper end of the outer seal leg  31  and on its inner surface, is a downward facing shoulder  51  that abuts against shoulder  49  of retainer ring  44 , preventing energizing ring  41  from pulling out of seal ring  23  once the two are engaged. 
     As shown in  FIGS. 2 ,  3 , and  4 , a recess  53  is formed below shoulder  51  on the inner surface of outer seal leg  31 . Grooves  55  are formed on the inner surface of outer seal leg  31  just below recess  53 . Referring now to  FIG. 4 , the energizing ring  41  is put in a set position by downwardly ratcheting the ring  41  to align grooves  47  with grooves  55 . When energizing ring  41  is set, as in  FIG. 4 , retainer ring  44  will move radially from pocket  45 , and grooves  47  on the outer surface of retainer ring  44  will engage and ratchet by grooves  55  on the inner surface of outer seal leg  31 , locking energizing ring  41  to seal ring  23 . Retainer ring  44  can move downward relative to grooves  55 , but not upward. 
     Energizing ring  41  has a nose  61  or engaging portion that engages slot  35 . Energizing ring  41  has an inner surface  63  and an outer surface  65  for engaging the opposite inner sidewalls of slot  35  in seal ring  23 . Inner and outer surfaces  63 ,  65  may be straight surfaces as shown, or optimally curved surfaces. Key features of the nose  61  of the energizing ring  41  are discussed in more detail in the description of  FIGS. 5 and 6 . 
     In the example embodiment of  FIG. 2 , a lower extension  100  secures by threads to the lower portion of seal ring  23 . The lower extension  100  extends down and connects to an upper metal ring  102 . The upper metal ring  102  may be bonded, soldered, welded, or fastened to the lower extension  100 . In this example, the upper metal ring  102  together with a lower metal ring  104 , hold an emergency or backup seal  106  in between. The emergency seal  106  may be bonded to both metal rings  102 ,  104  and may be fabricated from elastomeric, metallic, or non-metallic materials, or a combination thereof. In this example, a landing nose  108  is connected to the a back end of the lower metal ring  104  to facilitate landing on an upward facing shoulder  110  formed on the interior of the casing hanger  15 . The shoulder  110  provides a reaction point during setting operations. 
     Referring to  FIGS. 5 and 6 , an enlarged sectional view of the nose  61  of the energizing ring  63  is shown in the unset and set positions, respectively. The nose  61  may have a vent  70  to prevent hydraulic locking and may have a first tapered surface or portion  72  that tapers downwards at an angle  74  and have a second tapered surface or portion  80 . The inner and outer legs  27 ,  31  of the seal ring  23  have tapered, upward facing shoulders  76 ,  82  at their upper ends and proximate the opening of the slot  35 . The shoulders  76 ,  82  form a corresponding surface on which the second tapered surface  80  of the nose  61  rests when in the unset position. The taper of the first and second tapered surfaces  72 ,  80  form a compound angle that may be varied to achieve a delay in the entry of the energizing ring  63  into the slot  35  of the seal ring  23 . For example, if less taper is provided to the second tapered surface  80  such that it is flatter, more force will be required to be applied to the energizing ring  41  ( FIG. 2 ) to force the nose  61  into the slot  35  and consequently the emergency seal  106  will be compressed more than if a lesser force were applied. The second tapered surface  80  may vary in taper from 0 degrees (flat), which provides the most resistance, up to 90 degrees. The first tapered surface  72  may have a taper angle  74  that varies between 0 and 30 degrees. Various combinations of angles for both tapered surfaces  72 ,  80  may be used depending on the applications and may be affected by the material and construction of the emergency seal  106 . 
     By delaying the entry of the energizing ring nose  61  into the slot  35  as force is applied to the energizing ring  41  ( FIG. 2 ), setting of the legs  27 ,  31  of the seal ring  23  is delayed and the force is thereby transmitted to the shoulder  110  ( FIG. 3 ) on the hanger  15 , which acts as a reaction point. The force on the energizing ring  41  and the reacting force from the shoulder  110  ( FIG. 3 ) thereby compress the emergency seal  106  ( FIGS. 2-4 ) to cause it to bulge outwards until it forms a seal against the bore  11  of the housing  10  ( FIG. 3 ). Once the emergency seal  106  is compressed sufficiently to bulge outwards against the outer wellhead member  10 , the surface force between the second tapered surface  80  of the nose  61  and the upward facing shoulder  76  may be overcome by the force applied to the energizing ring  41  ( FIG. 4 ) to thereby initiate the entry of the nose  61  into the slot  35 . In an example embodiment, the first tapered surface  72  of the nose  61  is significantly more tapered than that of the second tapered surface  80  to facilitate entry of the nose  61  into the slot  35  and thereby deform the legs  27 ,  31  of the seal ring  23  against the wickers  12 ,  18  of the housing  10  and hanger  15 . Once the legs  27 ,  31  are set, generally the elastomeric seal  106  ( FIG. 4 ) cannot be compressed further. Control of the amount of compression in the elastomeric seal  106  ( FIG. 4 ) can also be tuned by varying the surface area between the contacting surface of the second tapered surface  80  and the upward facing shoulder  76 . A larger surface area at this contact surface may aid the delay of entry of the nose  61  into the slot  35 . 
     In an example of operation of the embodiment shown in  FIGS. 2-6 , a running tool or string (not shown) is attached to seal assembly  21  ( FIG. 1 ) and lowered into the seal pocket  17  Seal assembly  21  may be pre-assembled with energizing ring  41 , retainer ring  44 , seal ring  23 , extension  100 , and emergency seal  106 , all connected as shown in  FIG. 2 . The running tool or string (not shown) can be attached to grooves  43  on energizing ring  41 . The outer wall  29  of outer seal leg  31  will be closely spaced to wickers  12  on the wellhead bore  11 . The inner wall  25  of inner seal leg  27  will be closely spaced to the wickers  18  on the cylindrical wall of seal pocket  17 . By pushing the energizing ring  41  downward (such as by the running tool) with sufficient force such that the second tapered surface  80  at the nose  61  of the energizing ring  41  transmit force via the upward facing tapered shoulders  76 , down through the seal ring  23  to the emergency seal  106 , to thereby compress the seal  106  as shown in  FIG. 3 . Compression of the emergency seal  106  causes it to bulge radially outwards and sealingly engage the bore  11  of the housing  10 . After the seal  106  is compressed sufficiently to cause it to bulge outwards against the outer wellhead member  10 , continued force is applied to the energizing ring  41  to overcome the surface forces between the second tapered surfaces  80  of the nose  61  and the tapered shoulders  76  of the seal ring  23 , to insert the nose  61  in the slot  35 . Urging the nose  61  into the slot  35  is facilitated by the first tapered surfaces  72  of the nose  61  because they have significantly more taper and thus less resistance than the second tapered surfaces  80 . Further, engagement of nose  61  with the slot  35  causes the inner and outer seal legs  27 ,  29  to move radially apart from each other as shown in  FIGS. 4 and 6 . The inner wall  25  of inner seal leg  27  will embed into wickers  18  in sealing engagement while the outer wall  29  of outer seal leg  31  will embed into wickers  12  in sealing engagement. Once the inner and outer seal legs  27 ,  31  seal against the wickers  12 ,  18  of the wellhead members  10 ,  15 , the emergency seal  106  can no longer be compressed. 
     During the downward movement of the energizing ring  41  relative to the seal assembly  21 , the outwardly biased retainer ring  44  rides against recess  53 . As shown in  FIG. 4 , as the wedge member  61  of the energizing ring  41  advances into slot  35 , the retainer ring  44  and grooves  55  engage and ratchet by grooves  55  on the inner surface of seal leg  31 . As a result, retainer ring  44  locks energizing ring  41  to seal ring  23  as shown in  FIG. 4 , preventing retainer ring  44  from working its way out of the seal ring  23 . Vent passages or penetration holes  70  ( FIG. 5 ) may be incorporated across wedge member  61  and through upper energizing ring  41  so that a hydraulic lock condition does not prevent axial make-up of the energizer and seal system. 
     Subsequently, during production, hot well fluids may cause the casing to grow axially due to thermal growth. If so, the casing hanger  15  may move upward relative to the wellhead housing  10 . The inner seal leg  27  will move upward with the casing hanger  15  and relative to the outer seal leg  31 . The retainer ring  44  will grip the grooves  55  to resist any upward movement of energizing ring  41  relative to outer seal leg  31 . The wickers  12 ,  18  will maintain sealing engagement with the inner wall  25  of inner seal leg  27  and the outer wall  29  of outer seal leg  31 . 
     If the seal formed by the wickers  12 ,  18  and the inner and outer seal legs  27 ,  31  is compromised due to excessive thermal growth cycles or higher operating pressures, then the emergency seal  106  can maintain seal integrity between the outer and inner wellhead members  10 ,  15 . 
     In the event that seal assembly  21  is to be removed from bore  11 , a running tool is connected to threads  43  on upper energizing ring  41 . An upward axial force is applied to upper energizing ring  41 , causing it to withdraw from slot  35  and retainer ring  44  to disengage grooves  55  on seal leg  31 . However, due to retaining shoulders  49 ,  51 , energizing ring  41  will remain engaged with seal ring  23 , preventing the two from fully separating ( FIG. 2 ). 
     In an additional embodiment (not shown), the wellhead housing  10  could be a tubing spool or a Christmas tree. Furthermore, the casing hanger  15  could instead be a lockdown hanger, tubing hanger, plug, safety valve or other device. 
     While the invention has been shown in only one 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. For example, the seal could be configured for withstanding pressure in two directions, if desired, having two energizing rings. In addition, each energizing ring could be flexible, rather than solid.