Abstract:
A metallic sealing element comprises a generally tubular body having first and second opposed ends and a center section which is located between the first and second ends. In use the sealing element is compressible to bring the first and second ends towards one another to cause a center section to bulge transversely and sealingly engage a co-operating sealing surface.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to metallic seals and seal components which are energized in a novel manner, which are damage resistant and which can be made smaller than conventional metal-to-metal seals. Although for illustrative purposes the invention is discussed below largely in the context of completions for oil and gas wells, it is applicable to metallic seals in general. 
     There is a trend towards subsea completions incorporating increasingly large bores. Current subsea xmas tree system configurations (both parallel and concentric) can be inefficient in terms of space usage within the tubing hanger assembly. For large completion bore systems it would be advantageous to reconfigure the subsea xmas tree system whilst maintaining a large number of down-hole lines through the tubing hanger. A solution for releasing additional radial space to facilitate larger completion bores would be to reduce the size of the mechanism for sealing off the annulus void. 
     The design of large bore subsea xmas trees and completions is constrained due to requirements of utilizing existing standard BOP configurations. Therefore, in order to run larger completion tubing, space must be saved elsewhere to permit using existing BOP&#39;s. Additionally, particularly in the case of deepwater developments, significant cost savings can be achieved by using smaller standard BOP and casing programs while still maintaining—or increasing—the radial space available for the completion tubing. In this way vessel selection is made easier, and hence costs decreased, due to smaller handling requirements associated with the smaller BOP size. 
     The problematic situation of a drive toward larger bore completions coupled with potentially utilizing smaller BOP stacks makes the radial space taken within the well system for annular packoffs of prime importance. Any space saved here can have a direct impact on the size of the completion tubing that can be accommodated. 
     Essentially, the sealing requirement for a slick bore tubing hanger is to seal the annulus between the tubing hanger and spool (wellhead, xmas tree or tubing spool), maintaining a clearance while running in the hanger, and once the hanger is in position, energizing the seal to a set (sealed) condition. In the particular case of horizontal production outlet tubing hangers, it is usual to seal the annulus above and below the horizontal outlet. In the case of conventional tubing hangers (or casing hangers), only one seal barrier is required to seal off the annulus. 
     The prior art is replete with descriptions of seal systems involving a metal sealing element that bears against a metallic surface to establish a metal-to-metal sealing interface preventing the passage of corrosive or non-corrosive pressurized fluid throughout a wide temperature and pressure range. Packoffs providing metal-to-metal seals are disclosed for example in U.S. Pat. Nos. 4,900,041 and 5,174,376. Both patents disclose annular metal seal elements having a generally U-shaped cross-section. An energizing mandrel is driven downwardly between the two legs of the U-shaped seal profiles thereby deflecting the outer leg outwardly. However the energizing mandrel and its associated actuating mechanism is often relatively large, leading to a bulky packoff assembly. Furthermore, the range of movement imparted to the seal element outer leg by the mandrel is restricted, meaning that the sealing surface or area within the wellhead housing or other spool bore with which the seal element outer leg co-operates must closely surround the seal element in its relaxed or non-energized state. Such seal areas/surfaces therefore cannot be recessed for protection during drilling operations, so remaining exposed and vulnerable to damage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a seal assembly comprises an annular sealing surface and a metallic sealing element comprising in an unenergized state a thin-walled, substantially cylindrical tube or sleeve having first and second opposed ends, the sealing element in use being compressed so as to move the opposed ends towards one another, such movement causing a center section of the sealing element between the ends to bulge transversely of their direction of relative movement and sealingly engage the sealing surface. By this means relatively large deflections of the element center section can be obtained. 
     Such a sealing element has a wide variety of applications. It may be used to seal between a bore and a member, such as a shaft, received within the bore to isolate pressure in either direction across the sealing element. 
     To permit the necessary kinking and bulging, the length to wall thickness ratio of the tube is preferably greater than about 4. However, to avoid unstable or asymmetric buckling, the length to diameter ratio of the tube should not be too great, preferably no larger than about 2.5. A preferred length to wall thickness ratio is about 15. The particular dimensions most appropriate to a given sealing application will vary from case to case and can be determined experimentally. The sealing element may also include geometrical features that initiate the kinking and bulging in the required manner. For example, in the case of a generally tubular sealing element, the end faces may be shaped so as to produce higher compressive loading near the bore than towards the circumference of the sealing element. Additionally or alternatively, the bore may be widened towards the center section of the sealing element. 
     The invention also provides a metallic sealing element comprising a hollow center section whose interior is filled with a relatively easily deformable, relatively incompressible substance, the center section comprising walls of V-shaped cross-section or of concertina or bellows-shaped cross-section; the sealing element comprising first and second opposed ends which in use are compressed so as to move towards one another, such movement causing the V-shaped or concertina or bellows-shaped center section walls to bulge transversely of the direction of relative movement of the ends and sealingly engage a co-operating sealing surface. 
     A seal assembly including the sealing element, a bore and a member received in the bore, may be formed in which the member and/or the bore comprise(s) a pair of bodies having respective faces against which the respective ends of the sealing element are received, the bodies being movable towards each other to cause the bulging of the sealing element center section. Such movement may be generated by any of the means used to actuate conventional metal-to-metal seals into the energized or set condition, for example weight applied to one of the bodies, adjustment nuts or similar wedging means, or hydraulic piston and chamber actuators. 
     The member or bore preferably comprises a recessed sealing surface with which the sealing element center section makes sealing contact. Because it is recessed, the sealing surface is protected against mechanical damage and erosion. 
     The invention is useful not only in sealing the tubing annulus of a well completion, but also annuli at other well tubulars, such as casing hangers. The member may therefore be a tubing or casing hanger carrying the sealing element, with the bore formed in a spool in which the hanger is landed. The faces referred to above may both be provided on the hanger, or alternatively one face may be provided on the hanger and the other on the spool. 
     The hanger may have a lockdown mechanism that is also arranged to cause the relative movement between the faces for energizing the sealing element. No separate sealing element energizing actuator is therefore necessary, leading to a particularly compact sealing assembly. 
     The sealing element center section, or the sealing surface with which it cooperates, may comprise an insert or coating of relatively soft metal, adapted to form a better seal. 
     In its preferred embodiments, the invention fulfils one or more of the following objectives: 
     1. It provides reliability under cyclical loading, with good protection of co-operating seal surfaces against erosion and accidental damage. 
     2. It can be remotely operated using simple tooling or simply set by tubular string weight and/or a hanger lockdown mechanism. 
     3. It accommodates 10,000 psi (69 MNm −2 ) nominal maximum working pressure as a typical base case. However, a family of such sealing assemblies may be produced, also including, for example, members for 5,000 psi (35 MNm −2 ), 15,000 psi (104 MNm −2 ) and other applications as required. 
     4. It is effective over a temperature range of at least 0° F. to 250° F. (−17.8° C. to 121° C.) and preferably beyond at either end. 
     5. It provides a compact seal assembly. 
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a first embodiment of the invention, featuring a sealing element used to seal the annulus between a hanger and a surrounding spool; 
     FIG. 2 is a diagram of a second embodiment of the invention, featuring a collapsible tubing hanger shown in two different states; and 
     FIG. 3 is a diagram of a third embodiment of the invention, featuring a hollow-walled sealing element of concertina cross-sectional form shown in two different states. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For xmas trees it is necessary to isolate the annuli between the respective casing strings and the tubing hanger. For slick bore applications radial space is at a premium whilst at the same time sealing surfaces require protection during the drilling and completion process. The preferred embodiments facilitate the sealing of the annuli against a recessed surface in a spool bore, e.g., in a wellhead housing or tubing spool, with the recess feature affording the seal bore some protection during drilling and completion. In the case of horizontal xmas trees, the recessed bore with the sealing surface for the tubing hanger will be in the tree. This design may therefore afford the tree drill-through capability. 
     The primary use of the sealing element shown in FIG. 1 may be between a tubing hanger and a spool. However it may be used anywhere in which an annulus between a bore and a member, such as a shaft received in the bore, needs to be isolated to contain pressure in either direction. The sealing element consists of a seal sleeve  10  which prior to being energized comprises a generally tubular body (dotted lines) of sufficiently small outside diameter to drift clear of the spool bore  12  during running. The upper end of the seal sleeve  10  as shown is sealingly engaged about a tubing or casing hanger  14  and butts up against a downwardly directed face  16  on the hanger  14 . This seal may be achieved, for example, by an interference fit between the sleeve  10  and the hanger  14 , by O-rings, packings or a sealing compound between these components, by a circumferential weld, or by a combination of such means. The seal sleeve  10  is energized by the application of an axial displacement. Landing the hanger  14  may cause that displacement (i.e., the sleeve  10  may be energized by tubing weight on the hanger landing shoulder  18 , as shown), by mechanical means or by the application of hydraulic pressure to a chamber. A combination of the casing/tubing weight and a downward force exerted on the hanger  14  by a lockdown mechanism (not shown) may also be used to energize the sleeve  10 . A central section  20  of the energized sleeve  10  forms a kink that is caused to bulge outwardly into sealing contact with a recessed sealing surface or area  22  provided in the spool bore  12 . Additionally, soft metal inserts as indicated at  24  or  26  may be included to help effect a seal. 
     The embodiment of FIG. 2 operates in similar manner to that of FIG.  1 . However in the FIG. 2 embodiment the tubing hanger has two interlinked parts  14   a ,  14   b  that are allowed to collapse telescopically from state A (shown on the left hand side of FIG. 2) to state B (shown on the right hand side of FIG.  2 ), causing the seal sleeve  10  center section to “bell-out” or bulge outwardly and seal against the recessed bore surface  22 . Downward movement of the part  14   a  relative to the part  14   b  may be driven mechanically or hydraulically as before, including by means of the hanger lockdown mechanism and/or by the weight of the tubing or casing string if hung off the part  14   a . A straight pull of the tubing hanger would straighten the sleeve section under the string weight if hung off the part  14   b , restoring state A. 
     The two hanger parts are keyed together for limited relative axial movement by bolts or the like  28 . The sliding joint so formed is provided with an inner cover  30 . One or both of the ends of the sleeve  10  may be sealed to its respective hanger part  14   a  or  14   b  in a similar manner to the sleeve upper end of FIG.  1 . 
     FIG. 3 shows a radial cross section through one side of a hollow walled sealing element  10  in a relaxed or un-energized state A (shown on the left hand side of FIG. 3) and an energized state B (shown on the right hand side of FIG.  3 ). The sealing element includes inner and outer walls that each comprise a concertina or bellows-type profile in cross section. An energizing mandrel  32  is used to crush the sealing element from the relaxed state A to the energized state B, causing the tips or ridges  34  of the concertina profiles to engage and seal against the hanger  14  and the recessed sealing surface  22 . Alternatively, the energizing mandrel  32  may be replaced by a landing shoulder in the spool  36 . Tubing/casing weight and hanger lockdown forces can then be used to crush the seal element  10 , which expands to seal the annulus. A relatively easily deformable, relatively incompressible filler, such as a fluid, elastomer or soft metal, may be used within the hollow interior of the seal element  10  to provide support to the seal element  10  when subjected to pressure. 
     It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.