Abstract:
A method of completing a well having a casing hanger set in a subsea wellhead housing includes attaching a running tool to a tubular bridging hanger. A metal-to-metal inner seal is attached to a lower exterior portion of the bridging hanger and a metal-to-metal outer seal is located on an upper exterior portion of the bridging hanger. The assembly is lowered into the well and the lower exterior portion of the bridging hanger is inserted into the casing hanger. The inner seal is wedged between the casing hanger and the bridging hanger in response to weight of the running string. The running tool is actuated to set the outer seal between the upper exterior portion of the bridging hanger and the wellhead housing. Then, a tubing hanger is landed and sealed in the interior of the bridging hanger.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application 60/651,284, filed Feb. 9, 2005. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to subsea wellhead equipment and in particular to a metal-to-metal seal for a bridging hanger or tieback connection. 
     BACKGROUND OF THE INVENTION 
     A subsea well assembly includes a wellhead housing that is secured to large diameter conductor pipe extending to a first depth in the well. After drilling to a second depth, a string of casing is lowered into the well and suspended in the wellhead housing by a casing hanger. A packoff seals between an outer diameter portion of the casing hanger and the bore of the wellhead housing. Some wells have two or more strings of casing, each supported by a casing hanger in the wellhead housing. 
     In one type of completion, a string of production tubing is lowered into the last string of casing. A tubing hanger lands and seals in the upper casing hanger. The well is produced through the tubing. Prior to running the tubing, the operator will test the upper casing hanger packoff. On rare occasions, the packoff may be unable to pass the pressure test, possibly due to damage on the interior wall of the wellhead housing. If so, one remedy is to install an emergency or bridging hanger in the wellhead housing. The bridging hanger does not support a string of casing, but has an interior profile that is normally the same as the profile in the upper casing hanger. The operator lands and seals the lower portion of the bridging hanger to the casing hanger. The operator installs a packoff between the upper exterior portion of the bridging hanger and the wellhead housing above the casing hanger. The operator then runs the tubing and lands and seals the tubing hanger in the bridging hanger. 
     In the prior art, the inner seal between the bridging hanger and the casing hanger is normally elastomeric. As the bridging hanger enters the casing hanger, the elastomeric seal deforms to cause the sealing engagement. Metal-to-metal outer seals or packoffs have been used for years because they can withstand higher pressures than elastomeric seals and also do not deteriorate under harsh environments as readily. Metal-to-metal tubing hanger seals are also employed in many wells. A metal-to-metal seal, however, typically requires much more force to set than simply the weight of the running string. Various running tools have been developed to apply the high forces needed. Developing a running tool to set a metal-to-metal inner seal would require an additional trip down the riser with another running tool to set the metal-to-metal outer seal. In offshore wells, particularly in deep water, it is very expensive to run an additional trip. 
     SUMMARY OF THE INVENTION 
     In this invention, a metal-to-metal inner seal is attached to the lower exterior portion of the bridging hanger. The bridging hanger is lowered on a running tool into the wellhead housing and inserted into the casing hanger. The inner seal is set between the interior of the casing hanger and the lower exterior portion of the bridging hanger in response to the weight of the running string. Preferably, the inner seal has a deflectable locking portion to lock the inner seal in the pre-load caused by the weight of the running string. The weight causes the locking portion to defect outward into engagement with a profile in the casing hanger. 
     In one embodiment, while the running tool is still inserted into the wellhead housing, the running tool is actuated to set a metal-to-metal outer seal between the upper exterior portion of the bridging hanger and the wellhead housing. The bridging hanger may be used in place of the casing hanger to support a string of tubing. If so, the tubing hanger lands in and seals to the interior of the bridging hanger 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of a subsea wellhead assembly having a bridging hanger with a metal-to-metal seal in accordance with this invention. 
         FIG. 2  is an enlarged sectional view of the wellhead assembly of  FIG. 1 , showing the bridging hanger being run in prior to energizing the metal-to-metal seal. 
         FIG. 3  is a further enlarged view of a portion of the bridging hanger of  FIG. 2 , showing the metal-to-metal seal prior to being energized. 
         FIG. 4  is a view similar to  FIG. 3 , but showing the seal in the energized position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the subsea wellhead assembly in this embodiment includes an outer or low pressure wellhead housing  11 . A string of conductor pipe  13  is attached to the lower end of low pressure wellhead housing  11  and extends into a first section of the well. A high pressure or outer wellhead housing  15  lands in low pressure wellhead housing  11 . High pressure wellhead housing  15  is secured to a string of casing  17  that extends through conductor pipe  11  to a greater depth in the well. High pressure wellhead housing  15  has an exterior grooved profile  19  for engagement by a drilling riser assembly that extends to a surface vessel. 
     After drilling the well through high pressure wellhead housing  15  to a greater depth, a next section of casing  23  is run on a casing hanger  21 . Casing hanger  21  lands in high pressure wellhead housing  15 . A packoff or casing hanger seal  25  seals the annulus around casing hanger  21  to high pressure wellhead  15 . Some wells may have only one casing hanger such as casing hanger  21 . In this example, an additional casing hanger  27  is shown, casing hanger  27  being attached to a string of production casings  29  that extends to a final depth in the well. A casing hanger seal  31  seals between the outer diameter of the upper casing hanger  27  and the bore of wellhead housing  15 . 
     A bridging hanger  33  is shown landed on production casing hanger  27 . Bridging hanger  33  would be employed in the event that upper casing hanger seal  31  could not be installed or if it leaked. Bridging hanger  33  has an interior or bore substantially identical to bore  39  of production casing hanger  27  in this example. Bridging hanger  33  has a structure similar to casing hanger  27 , except there is no provision for securing casing to its lower end. A casing hanger outer seal  35 , which may be identical to casing hanger seals  25  and  31 , seals the annulus around bridging hanger  33  to wellhead housing  15 . Bridging hanger  33  has an interior grooved profile  37  that is engaged by a conventional casing hanger running tool  38 , illustrated in  FIG. 2  by dotted lines. Running tool  38  carries outer seal  35 , and after bridging hanger  33  lands, moves outer seal  35  downward and sets it. Bridging hanger  33  may subsequently support a conventional tubing hanger  40 , shown by dotted lines in  FIG. 1 . Tubing hanger  40  has a seal  42  that sealingly engages a sealing surface in bridging hanger  33  below profile  37 . 
     Referring to  FIG. 3 , bridging hanger  33  is shown being lowered into bore  39  of production casing hanger  27 , but running tool  38  ( FIG. 2 ) is not shown for clarity. Bridging hanger  33  has a retainer ring  41  on its lower end that is secured by threads  43 . Preferably, retainer ring  41  has a backup elastomeric seal  45  that seals against a portion of production casing hanger bore  39 . Retainer ring  41  also has an inner seal  47  that seals to an exterior portion of bridging hanger  33  above threads  43 . The body of bridging hanger  33  has a guide portion  49  on its outer diameter that is cylindrical and has an outer diameter less than the outer diameter of retainer ring  41 . A seal surface  51  is formed on the outer diameter of the body of bridging hanger  33  above guide portion  49 . Seal surface  51  is finished to a desired metal-to-metal surface finish, such as 32 RMS. Seal surface  51  has an outer diameter that is slightly greater than the outer diameter of guide portion  49 , but less than the outer diameter of retainer ring  41 . A transition shoulder  53  is located between seal surface  51  and guide portion  49 . 
     A bridging hanger seal ring  55  is carried on the outer diameter of bridging hanger  33  above retainer ring  41 . Seal ring  55  has a metal seal  57  on its lower end. In this embodiment, metal seal  57  comprises a welded inlay of a conventional type of material suitable for forming metal-to-metal seals. For example, the inlay may be a nickel-base alloy. Metal seal  57  has a cylindrical surface on its inner diameter and a downward facing tapered surface on its outer diameter. The tapered surface mates with a tapered seal surface  58  formed in bore  39  of production casing hanger  27 . Seal surface  58  is prepared for metal-to-metal sealing, having a finish substantially the same as seal surface  51  on bridging hanger  33 . In this embodiment, tapered seal surface  58  is formed at taper angle, such as 20 degrees, that is considerably larger than a locking taper, which is typically about 7½ degrees. Seal surface  51  on the exterior portion of bridging hanger  33  is cylindrical in this example. 
     Seal ring  55  has a plurality of vertical slots (not shown) spaced circumferentially apart from each other, defining a collet section with collet fingers  59 . The slots extend through the upper end of seal ring  55 , thus collet fingers  59  are not connected to each other at their upper ends. The individual fingers  59  with free upper ends enable the upper portion of seal ring  55  to plastically deflect outwardly from a cylindrical configuration to a conical configuration, as shown in  FIG. 4 . In the running-in position shown in  FIG. 3 , fingers  59  are located around seal surface  51  of the body of bridging hanger  33 . Seal ring  55  may be made of any suitable metal, such as Inconel 718. 
     A drive or cam ring  61  is secured to bridging hanger  33  above seal ring  55 . Drive ring  61  has an upper end that abuts a downward facing shoulder  65  on the outer diameter of bridging hanger  33 . Preferably, a plurality of fasteners  63  may be used to secure ring  61  and prevent it from sliding downward. Fasteners  63  insert into oversized holes  64  in bridging hanger  33  in the preferred embodiment. Thermal changes that cause axial cyclic deflections will not be transferred through fasteners  63  due to a clearance provided between fasteners  63  and holes  64 . Up and down movement between casing hanger  27  and bridging hanger  33  will not loosen drive ring  61 . 
     An upper backup seal  67  is optionally located above drive ring  61 . Upper backup seal  67  is positioned to engage an upper portion of bore  39  of production casing hanger  27 . Production casing hanger  27  has a grooved profile  69  formed in an upper portion of bore  39  above the cylindrical portion that normally is prepared for sealing engagement with tubing hanger seal  42  ( FIG. 1 ). Profile  69  may take a variety of shapes and is typically used for engagement with running tool  38  ( FIG. 2 ) to run casing hanger  27 . Also, profile  69  may be used for securing a lock member of a tieback assembly (not shown) when tubing hanger  40  ( FIG. 1 ) is not utilized. Profile  69  has a downward and inward facing conical reaction shoulder  71  at its upper edge or end. Tapered seal surface  58  defines the lower edge of profile  69 . 
     Collet fingers  59  of seal ring  55  have mating conical upper ends  72  that engages shoulder  71  when deflected outward as shown in  FIG. 4 . Drive ring  61  has an outer tapered surface  73  that engages the inner diameter of fingers  59  of seal ring  55  to cause collet fingers  59  to deflect outward when drive ring  61  moves downward relative to seal ring  55 . The amount of taper is selected to provide a locking taper to resist upward movement of drive ring  61  relative to seal ring  55  once engaged. 
     In operation, the operator connects running tool  38  ( FIG. 2 ) to bridging hanger  33  and lowers it through a drilling riser into high pressure wellhead housing  15 . Initially, retainer ring  41  will slide into bore  39  of production casing hanger  27 , as shown in  FIG. 3 . Metal seal  57  of seal ring  55  will land on tapered seal surface  58  in bore  39  of production casing hanger  27 . At this point, the inner diameter of seal ring  55  at collet fingers  59  remains cylindrical. 
     Then, continued weight is applied to bridging hanger  33  from the running string, causing bridging hanger  33  to move downward. As shown in  FIG. 4 , metal seal  57  remains in the same axial position while the body of bridging hanger  33  moves downward. Bridging hanger seal surface  51  slides into contact with the inner diameter of metal seal  57 . Drive ring  61  slides between the inner surfaces of collet fingers  59  and the outer diameter of bridging hanger  33 . Tapered surface  73  of drive ring  61  pushes collet fingers  59  outward. Drive ring  61  and collet fingers  59  lock at taper  73 . Tapered upper ends  72  of fingers  59  slide into engagement with reaction shoulder  71  and lock at this point, also. The locking engagement of fingers  59  pre-loads seal ring  57  at seal surface  58 . Any axial motion thereafter must be transmitted through collet fingers  59 . 
     Metal-to-metal sealing engagement occurs on both sides of metal seal  57 . Elastomeric seals  45 ,  47  and  67  provide a secondary backup. The sealing engagement is prevented from movement because of the engagement of tapered upper ends  72  of fingers  59  with reaction shoulder  71 . Subsequently and on the same trip, running tool  38  ( FIG. 2 ) conventionally installs bridging hanger seal  35  ( FIG. 1 ), sealing the annulus around bridging hanger  33 . 
     If the operator wishes to retrieve bridging hanger  33 , he reengages running tool  38  with profile  37  ( FIG. 1 ) and pulls upward. This causes drive ring  61  to move above seal ring  55  as shown in  FIG. 3 . The upper end of retainer ring  41  pushes upward on metal seal  57 , causing upper ends  72  of fingers  59  to slide out of engagement with reaction shoulder  71  for retrieval. 
     After the installation shown in  FIG. 1 , bridging hanger  33  can serve in place of production hanger  27  for receiving tubing hanger  40  ( FIG. 1 ). Alternately, bridging hanger  33  could receive an isolation sleeve, which forms part of a tubing hanger assembly. The tubing hanger could thus be supported in a tubing spool (not shown) mounted on high pressure wellhead housing  15 . Further, bridging hanger  33  could receive an isolation tube suspended from a Christmas tree of a type where the tubing hanger is located within the tree. In that instance, the isolation tube would be considered to be part of the tubing hanger assembly. 
     Alternately, bridging hanger  33  could form the lower end of a tieback connector (not shown), which stabs and locks into production casing hanger  27  and is located at the lower end of a string of conduit extending to the surface. If bridging hanger  33  is part of a tieback connector, it typically would not need an outer annulus seal such as seal  35 . The conduit extending upward from such a tieback connector would extend to a surface vessel for receiving a production tree. 
     The invention has significant advantages. The bridging hanger utilizes a metal-to-metal inner seal, while is longer lasting than elastomeric seals and better able to withstand high pressures. The inner and outer seals are run on the same trip. A special purpose running tool for the inner seal is not required. 
     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.