This invention relates in general to wellhead assemblies and in particular to a seal for sealing between inner and outer wellhead members.
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. The casing hanger could also be the outer wellhead member, with an isolation sleeve as the inner wellhead member. 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.
A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely or partially 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.
If the bore or surface of the outer wellhead member is damaged, a seal would struggle to maintain a seal. The elastomeric portion can provide additional robustness to the seal to help maintain a seal. In addition, a softer metal on the outer surface of a seal can also be used to fill scratches and surface imperfections on the surfaces of the wellhead members.
A need exists for a technique that addresses the seal leakage as described above. The following technique may solve these problems.
In an embodiment of the present technique, a bi-metallic seal assembly for use in subsea oil and gas applications is provided that comprises a metallic U-shaped seal that forms a metal-to-metal seal and has features that increase the reliability of the seal assembly in the event surface degradation or defects in a bore of a wellhead member increases the difficulty of maintaining a seal. The seal assembly also has a softer, lower yield metal at regions on the seal assembly where sealing occurs. The U-shaped seal incorporates tapered faces on its internal slot or pocket and is set (conditioned to seal at low pressure) by a test pressure applied to the seal assembly via an interim or bulk seal coupled to a wedge element that drives the legs of the U-shaped seal apart. The softer, low yield metal on the outer portions of the legs is forced against the surfaces of the wellhead members, causing localized yielding of the low yield metal to fill defects on wellhead member surfaces.
The bulk seal is on the primary pressure side and the taper of the legs is acute enough to prevent friction lock to allow seal retrieval. The wedge may be vented to allow fluid to flow as the wedge is forced into the seal pocket and thus avoid hydraulic lock. An additional compressible element may be fitted into the pocket of the U-shaped seal to avoid hydraulic lock. The compressible element could either be in the pocket or in the annulus formed between the interim seal and the metal seal. Axial loads required to push the seal assembly into its annular space between the wellhead members are minimal as only a small amount of radial squeeze, i.e. interference fit, is needed to maintain a sealing contact at low pressure. This also ensures that if the wedging mechanism fails, a seal can be obtained at least on surfaces without defects. Further, two U-shaped seals may be mounted back to back to allow sealing in two directions.
The seal assembly is preferably pre-assembled onto an inner wellhead member, such as an isolation sleeve or tubing hanger. The inner wellhead member and seal assembly may then be lowered into an outer wellhead member, such as a wellhead housing, in the same run and the seal set by applying pressure to the bulk seal.
In the event of bulk seal failure, the U-shaped seal is self-energizing and when pressurized is capable of sealing and filling against damaged annular surfaces of wellhead members. The pocket formed by the legs of each of the U-shaped seals may allow well pressure to act on the inner side of the legs, pushing the legs outward against the outer and inner wellhead members.
The seal assembly can rest on a shoulder formed on the wellhead housing and can be set by pressurizing the annular space between the outer and inner wellhead members to push the seal assembly into place. The combination of the lower yield metal on the exterior of the seal legs, as well as the bulk seal coupled to the wedge, improves sealing in wellhead members having surface degradations.
Referring to
The seal assembly 10 is shown in the unset position and comprises a U-shaped metal seal 14 having legs 15 that form a U-shaped slot 19. In this embodiment, the metal seals 14 may be bi-metallic, with the body formed out of a higher yield strength metal and a lower yield metal seal bands 17 forming the areas of sealing contact, such as the tips 18 of the legs 15.
Continuing to refer to
Referring to
The radial force applied by the energizing ring 30 to the lower yield strength metal bands 17 causes them to deform outward against the surfaces of, for example, the casing hanger 11 and tubing hanger 13, causing localized yielding in the bands 17. Extensive material yielding of the bands 17 thus occurs during energization. The lower yield strength metal bands 17 are soft and malleable enough to flow into defects and degradations on the surfaces of the casing hanger 11 and tubing hanger 13. This improves the metal-to-metal seal with the bore of the casing hanger 11 and the outer surface of the tubing hanger 13 when set.
In the event of bulk seal 32 failure, the U-shaped seal 14 is self-energizing and when pressurized is capable of sealing and filling against damaged annular surfaces of wellhead members with the low yield metal 17. The slot 19 formed by the legs 15 of the U-shaped seals 14 may allow pressure to act on the inner sides of the legs 15, pushing the legs 15 outward against the outer and inner wellhead members 11, 13.
The axial loads required to push the seal assembly 10 into its annular space between the wellhead members 11, 13 are minimal as only a small amount of radial squeeze, i.e. interference fit, is needed to maintain a sealing contact at low pressure.
In another embodiment illustrated in
In yet another embodiment illustrated in
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.
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Number | Date | Country | |
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20110180275 A1 | Jul 2011 | US |