Abstract:
A tubular connection, an example of which is a subsea wellhead having a primary and secondary seal areas allows the use of a backup or contingency gasket for engagement with the secondary seal area in the wellhead should a failure occur in the primary seal area. In the preferred embodiment, the primary and secondary seal areas are sufficiently separated such that the erosion damage which occurs from leakage with the original gasket adjacent the primary seal area, which can spread below the primary seal area, leaves the secondary seal area unaffected. A backup or contingency gasket can be inserted for sealable contact with the secondary sealing area for further well operations.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of this invention relates in general to tubular joints, particularly subsea wellhead housings and wellhead connectors, and in particular to a seal assembly that provides sealing if the wellhead housing conical sealing surface becomes damaged.  
         BACKGROUND OF THE INVENTION  
         [0002]    A subsea well has a wellhead housing located at the subsea floor. The wellhead housing is a tubular member having a bore. A wellhead connector is lowered from a vessel at the surface over the wellhead housing to connect the subsea well to the surface. The wellhead connector has a connection for connecting to the exterior of the wellhead housing. Thus, a wellhead is one specific type of a tubular joint which is often used in the oilfield.  
           [0003]    The wellhead housing has an upward-facing shoulder on its upper end that is engaged by a downward-facing shoulder on the lower end of the wellhead connector. The wellhead housing has a conical upward-facing shoulder at its upper end. The wellhead connector has a conical downward-facing shoulder. The wellhead connector also has a recess located radially inward from the downward-facing shoulder.  
           [0004]    A metal seal locates between the wellhead connector and the wellhead housing. The metal seal has a conical upper surface that seals against the conical surface of the wellhead connector. The metal seal has a lower conical surface that seals against the conical surface of the wellhead housing. A rib extends radially outward from the two conical surfaces for location in the recess.  
           [0005]    While the metal seal works well, if the conical surface of the wellhead housing becomes damaged, problems occur. The metal seal will not seal against the damaged lower surface. The wellhead housing is cemented in the ground and connected to casing and conductor pipe. It is not possible to pull the wellhead housing from the subsea floor for redressing the conical sealing surface.  
           [0006]    A prior design for addressing this problem are illustrated in U.S. Pat. No. 5,103,915. In this design, the subsea wellhead housing has a secondary sealing surface machined below its conical primary sealing surface during manufacturing. The secondary sealing surface extends downward and is of a greater diameter than the bore. A conventional metal seal locates between the wellhead housing and the wellhead connector. The conventional seal seals against the primary sealing surface of the wellhead housing. The secondary sealing surface is not used so long as the wellhead housing primary sealing surface is in good condition. If the wellhead housing primary sealing surface becomes damaged, then a second seal ring is utilized in lieu of the first seal ring. The second seal ring has a support surface that leads to a secondary surface. The secondary surface is cylindrical and is sized to seal against the secondary surface in the wellhead housing. The support surface on the second seal ring is sized so that it will be spaced by a slight gap from the damaged primary sealing surface of the wellhead housing. This prior art device claims that a good seal between the wellhead housing and the wellhead connector can be maintained without need to redress the wellhead housing primary sealing surface. In another embodiment, the secondary seal surface is disclosed as being conical rather than cylindrical and at a lesser angle relative to vertical than the primary sealing surface. This configuration provides for a primary conical sealing surface at one angle, leading into a secondary conical sealing surface at another angle.  
           [0007]    The different configurations of the design just described are illustrated in FIGS. 2 and 4 of U.S. Pat. No. 5,103,915. The main problem with this design is that the primary sealing surface, when it fails, is usually eroded due to the velocity effects of leaking fluid. These erosive effects attack not only the primary sealing surface but also the adjacent secondary sealing surface which, looking in the direction of the leaking fluid, presents itself first so that the erosive effects wind up damaging not only the primary but the secondary sealing surfaces in the wellhead. Thus, in effect, the design depicted in U.S. Pat. No. 5,103,915 is not serviceable, even with a replacement gasket, since the secondary surface has irregularities from the erosive effects and can no longer create a seal with the gasket against the connector. This phenomenon is illustrated in FIGS.  1 - 3  of the present application which depict a prior design akin to that shown in U.S. Pat. No. 5,103,915. Referring to FIG. 1 of this application, the wellhead  10  is shown having a single sealing surface  12 , which is tapered. Gasket  14  has a matching taper  16  so that it can be squeezed against the sealing surface  12  by the connector  18 . A clamp, generally referred to as  20  and which is of a known design, secures the wellhead  10  to the connector  18  and at the same time, forcing the connector  18  down against the gasket  14  to press the tapered surface  16  of the gasket  14  hard against the sealing surface  12  on the wellhead. In this design, the internal pressure in bore  22  can over time develop a leakpath which begins adjacent the lower end  24  of the gasket  14  in the transition area between bore  22  and tapered surface  16 . As fluid under pressure begins to escape past the gasket  14 , it begins to erode away part of the tapered sealing surface  12  and, in the configuration of FIG. 1, portions of the wall defining bore  22 .  
           [0008]    An alternative known prior art design is illustrated in U.S. Pat. No. 5,103,915 and shown in FIGS. 2 and 3 of this application. In FIG. 2, the original gasket  26  is shown with its tapered surface  28  firmly against the tapered sealing surface  30  on the wellhead  32 . As before, the connector  34  is clamped by clamp  36  to hold tapered surface  28  against the sealing surface  30  of wellhead  32 . Sealing surface  30  is set to be the primary sealing surface, while an adjacent surface  38 , which can be cylindrical or tapered, extends immediately below the primary sealing surface  30 . During normal operations with an effective seal being formed between surfaces  28  and  30 , the gasket  26  is not in contact with the secondary sealing surface  38 . The intention of this design is to make use of secondary sealing surface  38  should leakage occur past sealing surface  30 . The problem occurs when erosion damage, which is shown in FIG. 3, begins near the lower end  40  of the primary sealing surface  30 . As indicated by the cross-hatched area  42  in FIG. 3, the erosive effects spread to a significant portion of the secondary sealing surface  38 . Thus, when an oversized replacement gasket, which extends further downwardly with the intent of sealing against the secondary surface  38  is installed in the wellhead  32 , the result is unsatisfactory as the hoped for sealing surface  38  has been damaged by the fluid velocity leaking past gasket  26  at surface  30 . Thus, the problem with the design shown in FIGS. 2 and 3 of this application is that the secondary sealing surface  38  is configured so that it is in harm&#39;s way when the erosive effects of a leak begin. It, therefore, is not available as a smooth surface necessary to get reliable sealing with a replacement gasket made to bridge the damaged primary sealing surface  30  and further designed to seal up against the secondary sealing surface  38  which, at this time, is not serviceable.  
           [0009]    Accordingly, it is an object of the present invention to configure a tubular connection, one example of which could be a wellhead, internally, so that in the event leakage past a gasket occurs, the secondary sealing surface is available for use in a serviceable condition, thereby allowing the leak to be repaired, despite the damage to the primary sealing area. By virtue of the proper configuration between the secondary and primary sealing surfaces, the configuration of the present invention allows for reliable use of a secondary or backup sealing surface in conjunction with a backup or contingency gasket configured to reach the secondary sealing surface. The conforming shape of the contingency gasket to the wellhead configuration is also one of the novel inventions disclosed.  
           [0010]    Other related wellhead designs of the prior art are disclosed in U.S. Pat. Nos. 5,687,794; 5,039,140; 4,709,933; 4,563,025; 4,474,381; 4,214,763; 3,749,426; 3,556,568; and 3,507,506.  
           [0011]    Those skilled in the art will better appreciate the scope of the present invention from a review of the description of the preferred embodiment below.  
         SUMMARY OF THE INVENTION  
         [0012]    A tubular connection, an example of which is a subsea wellhead having a primary and secondary seal areas allows the use of a backup or contingency gasket for engagement with the secondary seal area in the wellhead should a failure occur in the primary seal area. In the preferred embodiment, the primary and secondary seal areas are sufficiently separated such that the erosion damage which occurs from leakage with the original gasket adjacent the primary seal area, which can spread below the primary seal area, leaves the secondary seal area unaffected. A backup or contingency gasket can be inserted for sealable contact with the secondary sealing area for further well operations.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a sectional elevational view of a prior art design, indicating a primary seal area in the wellhead with no secondary seal area.  
         [0014]    [0014]FIG. 2 a sectional elevational view of an alternative prior art design, showing the use of adjacent primary and secondary seal areas operating with the original gasket.  
         [0015]    [0015]FIG. 3 is the view of FIG. 2, showing the erosive effects of a leak and damage to the secondary seal area.  
         [0016]    [0016]FIG. 4 is a sectional elevational view of the present invention, using a wellhead as the preferred embodiment, illustrating the juxtaposition of the primary and secondary seal areas, with the original gasket installed.  
         [0017]    [0017]FIG. 5 is the view of FIG. 4, showing that erosion due to a leak has eradicated the primary sealing area and has spread to the transition zone between the primary and secondary sealing areas.  
         [0018]    [0018]FIG. 6 is the view of FIG. 5, showing the backup or contingency gasket installed and sealingly disposed against the secondary sealing area which is unaffected by the erosion damage.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring to FIG. 4, the wellhead  44  has a primary sealing surface  46  which is tapered with respect to the longitudinal axis  48  of bore  50 , a portion of which is shown in FIG. 4. The connector  52  is mounted above the wellhead  44  and secures the initial gasket  54  to the wellhead  44 . Gasket  54  has a tapered surface  56  which conforms to the primary sealing surface  46  to an area just above transition surface  58 . Located below transition surface  58  is tapered secondary sealing surface  60 . Arrow  62  illustrates how a leakpath begins between primary sealing surface  46  and the conforming tapered surface  56  on gasket  54 . As seen in FIG. 5, hatched area  64  illustrates the ravages of erosion as the metal disappears due to high velocity fluid flow past the primary sealing surface  46 . The band of material lost expands at its lower end to encompass a significant portion of the transition surface  58 . However, as shown in FIG. 5, the tapered secondary sealing surface  60  is unaffected. As further shown in FIG. 6, a contingency gasket  54 ′ can be inserted between the wellhead  44  and the connector  52 , which is longer than the original gasket  54  such that it contains tapered surfaces  56 ′ and  66 , of which surface  66  conforms to the secondary tapered sealing surface  60 . In between is surface  65 , which can be radial or sloped and preferably is parallel to surface  58  on the wellhead  44  or the tubular connection on which the invention is used. Thus, the contingency gasket  54 ′ has two sealing surfaces  56 ′ and  66 , separated longitudinally by a transition surface  65 . If the surface  58  is still intact, then gasket surface  65  has an opportunity to seal against it in conjunction with gasket surface  66  on surface  60 . The gasket  54 ′ can have a mirror image of surfaces  56 ,  66  and  65  at an opposite end, in the preferred embodiment, to allow for a similar sealing effect to, for example, a connector  52 .  
         [0020]    In the preferred embodiment, the transition surface  58  is cylindrical, but it can have a slight taper and still be within the scope of the invention.  
         [0021]    It is the positioning of the secondary sealing surface  60  out of the flow-path of the fast-moving fluid which is escaping through a leak between primary sealing surface  46  and tapered surface  56  of gasket  54  which, in part, protects the secondary sealing surface  60  from the erosive effects of the fast-moving fluid. That physical juxtaposition, coupled with the separation of the primary sealing surface  46  from the secondary sealing surface  60 , ensures that, even in the event of failure of the primary seal at surface  46 , erosion will not damage the secondary sealing surface  60  so that the contingency gasket  54 ′ can be installed with the knowledge that it will perfect the seal between the wellhead  44  and the connector  52 .  
         [0022]    Recent developments in the oilfield have dictated that the seal between the wellhead  44  and connector  52  be of metallic construction as opposed to being a resilient seal. One of the reasons for this requirement is that some wells operate at temperatures in excess of 350° F. and at pressures in excess of 12,000 psi. In these conditions, well operators require metal seals. In view of this, many solutions used in the past to repair leaks between the wellhead  44  and the connector  52 , which involve resilient seals, cannot be used in these operating conditions.  
         [0023]    The interface between the gasket and the sealing area can be damaged in several ways. One way is debris that lands on the sealing area whereupon the connector  52  is locked down on the wellhead  44  through a connection of known design, thus impregnating the sealing surface with debris or leaving a multitude of small dents in the sealing surface. This manifests itself as a slight leak in the first BOP test and has generally in the past been fixed with the use of a resilient gasket between the wellhead  44  and connector  52 . Erosion damage of the sealing surface caused by extended flow through a minor leakpath can also damage the sealing surface severely and can erode through the entire hub area of the wellhead  44 . When this occurs, a resilient gasket has not been effective to solve the problem. Instead, a bore seal and spacer spool are run into the bore  50  of the wellhead  44  to provide a replacement sealing area for the gasket between the wellhead  44  and the connector  52 .  
         [0024]    If damage to the primary sealing area, which can be caused by debris or remote-operated vehicle impact or improper wellhead handling, is noticed on the rig, it can be buffed out or the actual wellhead housing replaced. On the other hand, if such a problem is discovered subsea, a resilient seal gasket has been used in the past with some success. It should be noted that the gaskets themselves, if not properly designed, or if the connector  52  is not properly locked to the wellhead  44 , or if for some reason the primary sealing surface has been mechanically altered, conditions supporting a leak will be present. In view of the temperature and pressure requirements of well operators and the need to use metal-to-metal seals in those conditions, many of the solutions tried in the past can no longer be used in most installations. It thus becomes more important to be able to configure the sealing areas, both primary  46  and secondary  60 , in a configuration where the secondary sealing area will not be damaged due to erosive effects of a leak of the primary sealing area  46 .  
         [0025]    It should be noted that the configuration shown in FIGS.  4 - 6  does not require a reduction in the bore size of bore  50 , which would be undesirable. Instead, the pressure rating of the wellhead  44  is retained and the secondary sealing area  60  is spaced apart from the primary sealing area  46  and set back so that erosion damage due to a leak, as shown in FIG. 5, will at most damage only the transition area  58  between the primary sealing area  46  and the secondary sealing area  60 . The secondary sealing area  60  can be tapered or cylindrical and the taper angle can be less than, equal to, or greater than the taper angle for the primary sealing area  46 . Transition area  58  can be cylindrical or tapered. The three distinct areas  46 ,  58 , and  60  can all be tapered, with the transitional area  58  having a different taper angle than area  46 . This difference sets back area  60  from exposure to harmful erosive effects of high-velocity fluids if a leak occurs at area  46 . The further away that area  60  is placed from area  46 , the less likely is area  60  to be damaged by erosion. Stated differently, the longer the separation distance as measured in the longitudinal direction between areas  46  and  60  within limits of the contingency gasket  54 ′ to reach surface  60  and seal effectively, the less likely is surface  60  to be damaged.  
         [0026]    The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.