Patent Application
20090322029
This invention relates in general to subsea oil and gas well production, and in particular to a metal-to-metal seal for use in a tieback connector.
Metal-to-metal scaling is commonly used in subsea hydrocarbon production assemblies. For example, subsea wells typically have a subsea wellhead assembly at the seafloor with a subsea production tree mounted on the wellhead assembly. The tree has valves connected to flowlines for controlling flow from the well. In another type of installation, a string of tieback conduit extends from the subsea wellhead assembly to a platform at the surface. A surface tree is mounted on the upper end of the tieback conduit. Some riser systems have inner and outer tieback conduits, each of which is run separately and connected by a tieback connector. The inner and outer tieback conduits make up the tieback riser in that type of system.
The inner tieback conduit is installed by connecting a tieback connector to the lower end of the conduit and lowering it into the bore of the subsea wellhead housing assembly. The tieback connector has a locking member that locks to the subsea wellhead housing or to the tapered stress joint at the bottom of the outer tieback conduit. The inner tieback connector also includes a seal between where the tieback connector lands onto the subsea wellhead assembly. The seal is preferably a metal-to-metal seal, and it seals to an internal component of the subsea wellhead housing assembly. Metal-to-metal seals have a variety of configurations. While many work well, improvements are desired.
The seal assembly includes an annular metal seal body having oppositely extending legs. The legs extend from the seal mid-portion and have a sealing area on the ends of the legs. The sealing area is in sealing contact with a seal surface. Each leg includes at least one support between the leg end and the mid-section. Curved recesses are located above and below each support area, effectively reducing the thickness of each seal leg. The sealing force between the seal and the sealing surface is largely absorbed by the sealing area and the support. The support dimensions and location are adjustable, thus controlling the force applied to the sealing area.
The seat assembly can also include a web member extending from the mid-portion and perpendicular to the seal axis. The web member has a top and bottom surface aligned oblique to one another. The seal assembly can be used in tubular wellhead assemblies, including subsea assemblies. In one example, the seal assembly is used in conjunction with a junction between a tie-back connector and a subsea wellhead assembly.
Referring to
The tieback connector 20 further includes a seal assembly 50 affixed to the lower end of the mandrel 22.
Mandrel 22 preferably has a lower cylindrical sealing surface 29a, a central cylindrical or slightly conical support surface 29b and an upper conical sealing surface 29c. Lower cylindrical sealing surface 29a has a diameter larger than central cylindrical support surface 29b and is separate by a conical transition area. Upper conical sealing surface 29c has a maximum diameter at its junction with support surface 29b.
Seal 50 is an annular member circumscribing a longitudinal axis of mandrel 22. An upper leg seal groove 72 is optionally formed on a cylindrical lower portion of the upper leg outer surface proximate to the web 54. An elastomeric seal 74, located in seal groove 72, is shown in cross-section and elastically deformed when pressed against lower cylindrical sealing portion 29a of the opposing sealing surface 29 of the mandrel 22. An undercut or recess 68 is formed into the upper leg outer surface beginning a short distance above groove 74. Recess 68 begins at a portion of seal 50 that may be considered to be part of a base of first leg 56. The undercut 68 preferably has a generally circular profile terminating at a support area 66; the support area 66 may be cylindrical and has a finite axially dimension or thickness. Support area 66 may contact but does not necessarily seal to mandrel central support surface 29b. Support area 66 exerts less force against upper tubular member 22 than sealing area 62. The support area 66 axial thickness is less than the axial length of the undercut 68.
A second upper leg undercut or recess 64 is formed in the upper leg 56. Undercut 64 begins at support area 66 and terminates at an upper leg outer sealing area 62. The sealing area 62 is formed proximate the tip of the upper leg 56, and as will be described below, provides a primary point of sealing contact between the seal assembly 50 and the mandrel 22 upper conical sealing area 29c. Undercut 64 is shown as having a larger radius and axial length than undercut 68, but variations are possible.
In the embodiment of
Upper leg 56 deflects elastically when being installed on mandrel 22, with sealing area 62 deflecting radially inward slightly to form a metal-to-metal seal with sealing surface 29c, Support area 66 preferably deflects a very small amount compared to the deflection of sealing area 62. Prior to being installed on mandrel 22, a line 69 tangent to seal area 62 and support area 66 would be a small positive angle Ta relative to a vertical line 70 parallel with the axis of mandrel 22. After installation the taper angle Ta between lines 69 and 70 increases slightly. Prior to installation, taper angle Ta may be from about 8° to about 15′.
The embodiment of the lower leg 58 of
The lower leg 58 optionally includes a cylindrical portion on its outer surface with a base portion having a seal groove 90 and an elastomeric O-ring seal 92 that seals to upper cylindrical surface 39c. The lower leg 58 also includes third and fourth recesses or undercuts 82, 86 separated by a support area 84. Recess 86 begins at what may be considered to be part of the base of second leg 58. Support area 84 may contact but does not necessarily seal to lower cylindrical surface 39a. Support area 84 exerts less force against casing hanger 38 than second sealing area 80. In this example, the lower undercut 82 and the upper undercut 84 have about the same axial lengths and radii, but the lower undercut 82 is shallower. Sealing area 80 on the lower leg 58 is proximate to the lower leg tip and optionally may have a rounded cross-section. Sealing area 80 sealingly engages casing hanger cylindrical surface 39a. The axial length of each undercut 82, 84 is greater than the axial thickness of support section 84 and the axial length of the contacting portion of sealing area 80.
One of the differences between the lower leg 58 and the upper leg 56 is the difference between the initial upper taper angle Ta and a lower taper angle Tb. Lower taper angle Tb is the initial angle, prior to installation, between a tangent line 81 and vertical line 70, which is parallel with the axis of casing hanger 38. The tangent line 81 is tangent to support area 84 and lower sealing area 80. Lower taper angle Tb is a reverse taper relative to vertical line 70 from upper taper angle Ta. Sealing area 80 has an outer diameter slightly greater than the diameter of casing hanger cylindrical surface 39a. When sealing area 80 is forced against casing hanger cylindrical surface 39a, lower leg 58 elastically deflects inward, thereby decreasing taper angle Tb. Angle Tb decreases during installation, and prior to installation is preferably no greater than about 2 degrees.
The web 54 has a generally frusto-conical cross section, its width decreasing from the body 52 mid section to the web 54 crown 55. The crown 55 outer surface is profiled to form a ridge 57 along the crown's 55 outer circumference in this example. In this embodiment, the web 54 upper surface 67 and lower surface 65 are not parallel. These surfaces 65, 67 may have the same angle with respect to the axis of seal 50, or can have different angles as shown. In another embodiment, the surfaces 65, 67 may be generally parallel with each other, resulting in a near uniform thickness of rib 54. A retainer assembly 59 engages the ridge 57 for retaining the seal assembly 50 on the lower end of the mandrel 22. The retainer assembly 59 comprises a clip 61 having an elongated body with an inwardly protruding lip 71 on its lower end. The lip 71 mates with the ridge 57 and supports the seal assembly 50 on the mandrel 22. At this stage the seal assembly 50 is only partially in sealing engagement with the mandrel 22, thus a gap 31 remains between the mandrel 22 lower terminal surface 27 and the web 54 upper surface 67. The upper end of the clip 61 includes a base 53 wedged into a channel 25 formed on the mandrel 22 outer radial surface. An annular ring 63 circumscribes the outer portion of the base 53 for slidingly retaining it within the channel 25. Other retainers are feasible.
With reference now to
An enlarged view of the seal assembly 50 wedged between the mandrel 22 and the casing hangar 38 is provided in a cross-sectional view in
Internal pressure acts against seal 50 to apply an internal force on seal areas 62 and 80, which is reacted by casing hanger seal surface 39a and mandrel sealing surface 29c. Internal pressure may also cause support areas 66 and 84 to contact mandrel surface 29b and casing surface 39a, respectively. This contact is not necessarily a sealing contact, however, and is less than the forces imposed by the sealing areas. Strategically positioning the support area 66 on the sealing surface, in combination with the curved undercuts 64, 68, provides a means for controlling the sealing stress value between the sealing area 62 and the sealing surface 29c when energizing the seal assembly 50. A significant increase in sealing stress is achievable using the control means, wherein the maximum sealing stress is maintained below the yield point of the respective materials of the mandrel 22 and the seal 50. Additionally, controlling the stress at the sealing area 62 also insures other high stress points in the seal 50 will not exceed their respective yield values. The support areas 66 and 84 provide stiffening of legs 56, 68 against internal pressure loads. It is well within the capabilities of those skilled in the art to form seal legs having appropriately dimensioned undercuts, supports, and seal areas to achieve the desired results described herein.
The angled upper and lower surfaces 67, 65 of the web 54 comprise an additional feature of the seal assembly 50. The corresponding lower terminal end 27 of the mandrel 22 and the upper terminal end 41 of the casing hangar 38 are correspondingly angled to match the upper and lower surface contours. A lateral force exerted to the exterior of either the mandrel 22 or the casing hangar 38 is transferred to the other annular member via the wedge shaped web 54. This force transfer effectively couples the members together, thereby resisting lateral movement of one member with respect to the other.
While the invention has been shown in a single form, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention. The upper lower seal leg could be mounted to a seal ring having a considerably different upper seal leg than shown, and vice-versa. For example, in the embodiment shown, the upper seal leg is configured to allow rotation between the mandrel and the upper seal leg prior to full setting, but this not need be the case. A seal with only a single seal leg and no rib or web is also feasible, particularly if the seal is formed as a lip on a sleeve. The clip for retaining the seal member during deployment could be replaced with a threaded fastener.
