Patent Application
20140096977
This invention relates in general to wellhead assemblies and in particular to a semi-rigid lockdown device that includes a split locking ring that minimizes fretting of the seal surfaces due to movement while the lockdown device is energized.
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 that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. 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. In addition to the seal between the inner and outer wellhead members, another annular seal, or emergency seal, may be located below this seal.
A variety of annulus seals of this nature have been employed. Conventional annulus seals include, for example, elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely 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. One type of metal-to-metal seal has inner and outer walls separated by a conical slot. An energizing ring is pushed into the slot to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The energizing ring is a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
Thermal growth between the casing or tubing and the wellhead may occur. The well fluid flowing upward through the tubing heats the string of tubing, and to a lesser degree the surrounding casing. The temperature increase may cause the tubing hanger and/or casing hanger to move axially a slight amount relative to the outer wellhead member or each other. During the heat up transient, the casing hanger and/or tubing hanger can also move radially due to temperature differences between components and the different rates of thermal expansion from which the component materials are constructed. If the seal has been set as a result of a wedging action where an axial displacement of energizing rings induces a radial movement of the seal against its mating surfaces, then sealing forces may be reduced if there is movement in the axial direction due to pressure or thermal effects. A reduction in axial force on the energizing ring results in a reduction in the radial inward and outward forces on the inner and outer walls of the seal ring, which may cause the seal to leak. A loss of radial loading between the seal and its mating surfaces due to thermal transients may also cause the seal to leak.
A need exists for a technique that addresses the seal leakage problems described above. In particular a need exists for a technique to provide a seal that minimizes fretting of the seal surfaces due to movement while the lockdown device is energized.
In an embodiment of the present technique, a wellhead seal assembly is provided that forms a metal-to-metal seal between inner and outer wellhead members. A seal member has inner and outer seal legs separated by a slot. A locking ring is positioned in the outer seal leg and has wickered sections, where each of the wickered sections protrude through a window in the outer seal leg of the seal member. An energizing ring has an upper tapered surface that is oblique to an axis of the annular energizing ring and extending laterally from the axis over a portion of the locking ring and a lower tapered surface that is oblique to and extending laterally from the axis. As the energizing ring is moved into the slot, the upper tapered surface engages an inner surface of the locking ring, and the lower tapered surface engages an inner annular wall of the slot.
In an example embodiment, the wellhead seal assembly further includes a spring element (e.g., a c-ring, a wave spring, a belleville disk, a crush tab, etc.) positioned underneath at least one of the wickered sections in a corresponding window of the outer seal leg, where the spring element is compacted when the wickered section engages the inner wellhead member. The inner wellhead member may have an inner surface with wickers for contacting the wickered section of the locking ring when the energizing ring is moved into the slot.
In an example embodiment, the locking ring is a c-ring having open ends that are positioned behind solid webbing of the seal member, the solid webbing being between neighboring windows of the outer seal leg. The wickered sections of the locking ring may have a groove pitch in a range of about 3.175 mm to 12.7 mm.
Because multiple shoulders are engaged in the locking profile of the wickers, increased lockdown capacity is advantageously achieved in the bore of wellhead housing. Further, the fitting of the locking ring into the milled windows of the seal ring advantageously decreases the risk of premature actuation or stripping of the locking ring. The relatively fine groove pitch of the locking profile provides a semi-rigid lock that minimizes fretting of the seal ring and locking ring due to movement occurring while seal assembly is energized.
Referring to
Continuing to refer to
In the example embodiment of
Crush washer 32 is positioned below locking ring 33 in the milled windows 34 to ensure that wickers 18 on locking ring 33 can fully engage wickers 12 of wellhead housing 10 radially. In other embodiments, an alternative spring element such as a wave spring, a belleville disk, or a crush tab may be used instead of crush washer 32 to allow for full radial engagement of wickers 12, 18. Wickers 12, 18 may include triangular grooves parallel to each other. In other embodiments, wickers 12, 18 may include helical, concentric grooves, which are triangular in configuration.
Annular energizing ring 41 engages slot 35 on the upper side. As shown, energizing ring 41 has an axis AR that is substantially parallel with an axis (not shown) of the wellhead assembly. Energizing ring 41 is forced downward into slot 35 by a running tool (not shown) connected to grooves 43 on the inner diameter of upper energizing ring 41 during setting. Alternatively, seal assembly 21 and energizing ring 41 may be part of a string that is lowered into bore 11, the weight of which forces energizing ring 41 into slot 35. If retrieval is required, the grooves 43 can be engaged by a retrieving tool (not shown) to pull energizing ring 41 from the set position. Energizing ring 41 can be formed of metal, such as steel.
Still primarily referring to
Seal assembly 21 further includes locking ring 33 formed of a metal such as steel. Locking ring 33 is a locking member having several turned grooves in its outer diameter. The outer perimeter of locking ring 33 includes a set of milled slots to form raised wickered sections along the outer perimeter of locking ring 33. Locking ring 33 is positioned in the seal assembly 21 such that the set of wickered sections protrude through the set of milled windows of outer seal leg 31 and such that the milled slots are positioned behind the sections of solid webbing of outer seal leg 31.
Wickers 12, 18 are located on an inner wall surface of wellhead housing 10 and an outer wall surface of locking ring 33, respectively. In this example, the profiles of each set of wickers 12, 18 are shown as continuous profiles; however, wickers 12, 18 may be configured in other arrangements. Wickers 12, 18 of each of the wall surfaces engage each other as the seal assembly 21 is energized. Wickers 12, 18 enhance the grip to aid in the prevention of axial movement of seal ring 23 once set.
Seal ring 23 further includes inner seal leg 27 shown spaced laterally from outer seal leg 31 and above lower extension 100. Outer seal leg 31 and inner seal leg 27 are substantially perpendicular to the axis of the wellhead assembly. The casing hanger 15 has conical portion 42 that is engaged by a lower portion of inner seal leg 27.
Referring now to
In the example embodiment of
Energizing ring 41 may have upper tapered surface resting on an upper end of locking ring 33 and lower tapered surface at the lower end of nose 61, which may also have a vent to prevent hydraulic locking. The inner surface of locking ring 33 has an upward facing, tapered shoulder at its upper end, and the inner and outer legs 27, 31 of the seal ring 23 have tapered, upward facing shoulders at their upper ends that are proximate to the opening of the slot 35. When locking ring 33 and seal ring 23 are in unset positions, the shoulder of locking ring 33 forms a corresponding surface on which upper tapered surface of the energizing ring 41 rests, and the shoulders of the inner and outer legs 27, 31 form a corresponding surface on which lower tapered surface of the nose 61 rests.
As force is applied to energizing ring 41, upper tapered surface of the energizing ring 41 engages the shoulder of locking ring 33 to thereby drive locking ring 33 radially outward against the inner wall surface of wellhead housing 10. Specifically, wickers 18 of locking ring 33 are driven into wickers 12 on the inner wall surface of wellhead housing 10. Crush washer 32 is positioned below locking ring 33 in the milled windows 34 to ensure that the wicker 18 on the locking ring 33 can fully engage wickers 12 of wellhead housing 10 radially. In other embodiments, a wavespring, a belleville disk, or a crush tab may be used instead of crush washer 32 to allow for full radial engagement of wickers 12, 18. If the tips of wickers 18 of locking ring 33 strike the downward facing tapered surfaces of wickers 12 on wellhead housing 10, crush washer 32 collapses axially to thereby allow the locking ring 33 to fully expand into the inner surface of wellhead housing 10. Otherwise, if the tips of wickers 18 of locking ring 33 strike the upward facing tapered surfaces of wickers 12 on wellhead housing 10 or if wickers 12, 18 strike tip-to-tip, the locking ring 33 may displace upward from the crush washer 32 as wickers 18 of locking ring 33 fully engage wickers 12 of wellhead housing 10 radially. Additionally, nose 61 enters slot 35 and thereby deforms the legs 27, 31 of seal ring 23 against the walls of housing 10 and casing hanger 15 as downward axial force is applied to energizing ring 41.
Referring to
In an example operation of the embodiment shown in
As nose 61 of energizing ring 41 engages slot 35, inner wall 25 of inner seal leg 27 engages the outer wall of casing hanger 15, and outer wall 29 of outer seal leg 31 engages the inner wall of wellhead housing 10. The inner and outer walls 25, 29 form a seal in the annular space between the casing hanger 15 and the wellhead housing 10.
Because multiple shoulders are engaged in the locking profile of the wickers 12, 18, increased lockdown capacity is achieved in the bore 11 of wellhead housing 10. Further, the fitting of locking ring 33 in milled windows 34 of seal ring 23 decreases the risk of premature actuation or stripping of locking ring 33. In this example, the locking profile of wickers 12, 18 is shown with a 3.175 mm groove pitch. The relatively fine groove pitch of the locking profile provides a semi-rigid lock that minimizes fretting of the seal ring 23 and locking ring 33 due to movement occurring while seal assembly 23 is energized, which improves seal life. In some embodiments, a wider range of axial uncertainty can be accommodated for by using a larger groove pitch for the locking profile (e.g., 12.7 mm).
Subsequently, during production, hot well fluids may cause the casing to grow axially due to thermal growth. If so, casing hanger 15 may move upward relative to wellhead housing 10. Inner seal leg 27 will move upward with casing hanger 15 and relative to outer seal leg 31. Wickers 12, 18 will maintain locking engagement between the inner surface of wellhead housing 10 and the outer surface of locking ring 33.
In the event that seal assembly 21 is to be removed from bore 11, a running tool is connected to threads 43 on upper energizing ring 41. An upward axial force is applied to upper energizing ring 41, causing it to withdraw from slot 35. In this case, once annular nut 44 is engaged by the energizing ring 41, the upward axial force withdraws seal assembly 21 from bore 11.
In an additional embodiment (not shown), wellhead housing 10 can be a tubing spool or a Christmas tree. Furthermore, casing hanger 15 can instead be a lockdown hanger, tubing hanger, plug, safety valve or other device.
Referring to
While a semi-rigid lockdown device 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. For example, while the embodiments above are described with respect to a housing and casing hanger, the seal may be configured to be used in various annular spaces (e.g., between nested casing hangers) of a wellhead assembly.
