GB2318599A

GB2318599A - Dual metal seal for wellhead housing

Info

Publication number: GB2318599A
Application number: GB9727093A
Authority: GB
Inventors: Donald B Wafer
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1995-10-06
Filing date: 1996-10-03
Publication date: 1998-04-29
Anticipated expiration: 2016-10-03
Also published as: GB9727094D0; GB2318600A; GB9727093D0; GB2318599B; GB2318600B

Abstract

A system for sealing a wellhead comprises: a wellhead housing 11 for connecting to a well string casing, the wellhead housing being annularly disposed about a central axis 26 and having a primary sealing surface 12 and a secondary sealing surface 16 or 18; a wellhead connector 21 annularly disposed about the central axis and having a connector sealing surface 36; and a seal ring 15 which is annularly disposed about the central axis 26 and has a ring seal surface 15e for sealing contact with the connector sealing surface 36, a first seal surface 15c for sealing with the primary seal surface 12 and an extension 15b extending below the ring seal surface and having disposed thereon a sealing land 15a for sealing contact with the wellhead housing 18 (or 16). Vertical compressive forces between the housing 11 and connector 21 generate a twisting moment in the ring 15 which energizes the sealing land 15a. The wellhead 11 and connector 21 may also accommodate a standard seal ring (not shown) which seals with the primary sealing surface 12.

Description

DUAL METAL SEAL FOR WELLHEAD HOUSING This invention relates generally to wellhead housings and connectors, and in particular to seal assemblies that provide sealing even if the wellhead housing primary sealing surface becomes damaged.

Metal-to-metal sealing surfaces have been proposed to seal junctions between tubular members such as: blowout preventers, production trees, side outlet valves and wellhead housings. Metal-to-metal seals must be precisely manufactured to enable a seal upon contact of the polished sealing surfaces.

Metal-to-metal seals will not effectively seal if one of the sealing surfaces becomes damaged. This is particularly problematic for inaccessible applications, such as subsea wellheads, where the seal surface cannot be easily removed to refinish the damaged seal surface. Thus, secondary sealing surfaces have been developed for use when a primary surface becomes damaged, for example, see U. S. Patent No. 5,039,140 and U. S. Patent No. 5,103,915.

However, such proposals are difficult to implement due to the difficulty in maintaining a tolerance in locating the secondary seal surface or in energization of the secondary seal. Further, no prior secondary seal for such an application has provided for a dual metal-to-metal seal, which is desirable for safety reasons.

According to the present invention, a system for sealing a wellhead comprises: a wellhead housing annularly disposed about a central axis and having primary and secondary sealing surfaces disposed therein; a wellhead connector annularly disposed about the central axis and having disposed therein a connector sealing surface; and a seal ring annularly disposed about the central axis and comprising: a ring seal surface for sealing contact with the connector sealing surface; a first seal surface for sealing contact with the primary sealing surface; and a second seal surface for sealing contact with the secondary sealing surface.

Our co-pending application no. 9620597.6, publication no.

GB 2305951 relates to a method for making a wellhead housing is provided, the method comprising: forming a wellhead housing; machining a reference surface; machining a primary sealing surface using the reference surface as a guide; machining a secondary sealing surface using the reference surface as a guide.

That application further relates to:1) a wellhead housing produced by: forming a wellhead housing; machining a reference surface; machining a primary sealing surface using the reference surface as a guide; machining a secondary sealing surface using the reference surface as a guide; and 2) a system for sealing a wellhead, the system comprising: a wellhead housing annularly disposed about a central axis and comprising: a primary sealing surface disposed within the wellhead housing; a reference surface extending below the primary sealing surface; and a secondary sealing surface extending below the reference surface; the system further comprising a wellhead connector annularly disposed about the central axis and having disposed therein a connector sealing surface; and a seal ring annularly disposed about the central axis and comprising: a ring seal surface for sealing contact with the connector sealing surface; and a housing seal surface for sealing contact with the wellhead housing.

Our co-pending application no.

publication no. GB , related to a system for sealing a wellhead comprising: a wellhead housing annularly disposed about a central axis and having primary and secondary sealing surfaces disposed therein; a wellhead connector annularly disposed about the central axis and having disposed therein a connector sealing surface; and a seal ring annularly disposed about the central axis and comprising: a ring seal surface for sealing contact with the connector sealing surface; a wellhead sealing surface for sealing contact with the wellhead housing; wherein compressive forces imposed by the wellhead housing and the wellhead connector generate a resultant twisting moment which energises the wellhead sealing surface.

The present invention may be better understood by reading the following description of nonlimitative embodiments with reference to the attached drawings, wherein like parts in each of the several figures are identified by the same reference character, which are briefly described as follows: FIG. 1 is a cross-sectional view of a portion of a wellhead housing; FIG. 2 is a cross-sectional view of a portion of a wellhead connector; FIG. 3 is a cross-sectional view of a portion of a seal ring; FIG. 4 is a cross-sectional view of a wellhead housing, a wellhead connector, and a secondary seal ring wherein they are assembled; this assembly not forming an embodiment of the present invention; FIG. 5 is a cross-sectional view of a second embodiment of a wellhead housing, a wellhead connector, and a secondary seal ring wherein they are assembled; FIG. 6 is a cross-sectional view of a third embodiment of a wellhead housing, a wellhead connector, and a secondary seal ring wherein they are assembled, this assembly not forming an embodiment of the present invention; and FIG. 7 is a cross-sectional view of a fourth embodiment of a wellhead housing, a wellhead connector, and a secondary seal ring wherein they are assembled.

It is to be noted, however, that Figs. 1-3, 5 and 7 of the appended drawings illustrate only typical embodiments of the invention and are therefore not to be considered as limitative, the invention including other embodiments falling within the scope of the claims.

Referring to Figure 1, a cross-sectional view of a portion of a wellhead housing (11) is depicted. The wellhead housing (11) is an annular structure having a central axis (26). There is an upward facing shoulder (10) at the top. A primary sealing surface (12) extends downward and inward from the upward facing shoulder (10), whereby it forms a conical surface. Below the primary sealing surface (12) there is a cylindrical reference surface (14) which is used during production of the wellhead housing as a reference for machining the sealing surfaces. Further below, there is a conical surface (16) that extends downward and radially inward from the cylindrical reference surface (14). Next, there is a cylindrical surface (18) that extends downwardly from the conical surface (16) . Finally, transition surface (20) connects cylindrical surface (18) to the wellhead bore (defined by the wellhead bore central axis (26)).

The cylindrical reference surface (14) provides a reference point from which to machine both the primary sealing surface (12) and a secondary sealing surface. In separate embodiments, the secondary sealing surface is either conical (16) or cylindrical (18).

Referring to Figure 2, there is shown a cross-sectional view of a portion of a wellhead connector (21). Like the wellhead housing (11), the wellhead connector (21) is an annular structure having a central axis (26). The wellhead connector (21) has a downward facing shoulder (32) for engagement with the upward facing shoulder (10) of the wellhead housing (11) (see Fig. 1) . Also, there is a connector sealing surface (36) that extends upward and radially inward. A recess (34) is positioned between the downward facing shoulder (32) and the connector sealing surface (36).

Referring to Figure 3, there is depicted a crosssectional view of a portion of a secondary seal ring (15).

The secondary seal ring (15) is also an annular structure having a central axis (26). It has a flange (15d) on its outer periphery that is insertable into the recess (34) of the wellhead connector (21) (see Fig. 2). A ring seal surface (15e) extends upward and radially inward from the flange (15d). A surface (15c) extends downward and radially inward from the flange (15d). Below the surface (15c) there is an extension (15b) that extends downward. At the end of the extension (15b) there is a sealing land (15a) that extends downward from the extension (1sub). The interior surface (15f) of the secondary seal ring (15) faces the central axis (26).

Referring to Figure 4, there is depicted a crosssectional view of the wellhead housing (11), the wellhead connector (21), and the secondary seal ring (15) wherein they are assembled. In this arrangement which does not embody the present invention, upward facing shoulder (10) and downward facing shoulder (32) abut against each other. The flange (15d) of the secondary seal ring (15) is inserted into the recess (34) of the wellhead connector (21). Connector sealing surface (36) forms a metal-to-metal seal with ring seal surface (15e) and sealing land (15a) forms a metal-tometal seal with cylindrical sealing surface (18).

Also, in this arrangement, surface (15c) does not contact primary sealing surface (12). Extension (15b) is spaced near the reference surface (14) and conical surface (16) such that the extension (15b) does not contact any of these surfaces.

Referring to Figure 5, there is shown a cross-sectional view of an embodiment of the present invention wherein the wellhead housing (11), the wellhead connector (21), and the secondary seal ring (15) are assembled. As before, upward facing shoulder (10) and downward facing shoulder (32) abut against each other. The flange (15d) of the secondary seal ring (15) is inserted into the recess (34) of the wellhead connector (21). The connector sealing surface (36) forms a metal-to-metal seal with ring seal surface (15e) and sealing land (15a) forms a metal-to-metal seal with cylindrical sealing surface (18).

However, in this embodiment, surface (15c) contacts primary sealing surface (12). If the primary sealing surface (12) has not become previously damaged, then an effective metal-to-metal seal may be obtained. Thus, there may be two metal-to-metal seals between the secondary seal ring (15) and the wellhead housing (11).

Referring to Figure 6, there is depicted a crosssectional view of the wellhead housing (11), the wellhead connector (21), and the secondary seal ring (15) wherein they are assembled. In this arrangement which does not embody the present invention, upward facing shoulder (10) and downward facing shoulder (32) abut against each other. The flange (15d) of the secondary seal ring (15) is inserted into the recess (34) of the wellhead connector (21). Connector sealing surface (36) forms a metal-to-metal seal with ring seal surface (15e). In this arrangement, surface (15c) does not contact primary sealing surface (12). Also, conical surface (16) is machined and forms a metal-to-metal seal with the sealing land (15a).

Referring to Figure 7, there is depicted a crosssectional view of a wellhead housing (11), a wellhead connector (21), and a secondary seal ring (15) wherein they are assembled. In this embodiment of the invention, upward facing shoulder (10) and downward facing shoulder (32) abut against each other. The flange (15d) of the secondary seal (15) is inserted into the recess (34) of the wellhead connector (21). Connector sealing surface (36) forms a metalto-metal seal with ring seal surface (15e) and sealing land (15a) forms a metal-to-metal seal with conical surface (16).

In this embodiment, surface (15c) contacts conical primary sealing surface (12) . If the primary sealing surface (12) has not become previously damaged, then an effective metal-tometal seal may be obtained. Thus, there may be two metal-tometal seals between the secondary seal ring (15) and the wellhead housing (11). However, even if a seal is not obtained, the contact still energizes the secondary seal surface.

Another feature of the illustrated housings, seal rings and connectors is the generation of a moment in the secondary seal ring (15). The moment is generated by vertical forces exerted by the wellhead connector (21) and the wellhead housing (11). There is a downward vertical force component in the contact between the connector sealing surface (36) and the ring seal surface (15e). Similarly, there is an upward vertical force component in the contact between the upward facing shoulder (10) and the flange (15d) or between the primary sealing surface (12) and the surface (15c).

Where the downward vertical force component is closer to the central axis (26) than the upward vertical force component, a moment is generated in the secondary seal ring (15) . The moment is offset by a force in an inward radial direction re-lative to the central axis (26) . This radial force is exerted by the wellhead housing wherever it is contacted by the sealing land (15a) and energizes the metal-to-metal seal between the sealing land (15a) and either the conical sealing surface (16) or the cylindrical sealing surface (18). The particular placement of the contact points between the wellhead connector (21), the wellhead housing (11) and the secondary seal ring (15) is not important so long as the sum of the forces exerted on the secondary seal ring (15) is sufficient to generate a moment. Thus, even if the conical primary sealing surface (12) has become damaged, it may still serve as a lever point to generate a moment in the secondary seal ring (15).

As shown in Figs. 4 and 5 the generated moment may energize the sealing land substantially in the radially outward direction. However by varying the angle of the secondary sealing surface the direction of energization of the sealing land may be varied through to vertical (i.e. parallel to the central axis) or any desired combination of vertical and horizontal (see Figs. 6 and 7).

It will be understood that the vertical compressive forces are not necessarily concentrated at respective points at fixed distances from the central axis (26) to give circumferential contact lines, but may instead be stresses applied over wide surface portions of the seal ring (15).

Such applied stresses may nevertheless be considered as equivalent to resultant vertical forces applied at specified radial distances from the central axis (26).

The wellhead system is also useful in accommodating conventional seal rings designed only for co-operation with the wellhead primary sealing surface. Such primary seal rings may be installed for example when the wellhead is originally installed and prior to any damage to the primary sealing surface. Secondary seal rings which seal with the secondary sealing surface may be utilised where the primary sealing surface is damaged or, in those cases where the secondary seal ring has such capability, in order to obtain dual metal-to-metal seals with secondary and undamaged primary sealing surfaces.

Claims

CLAIMS:

1. A system for sealing a wellhead, the system comprising: a wellhead housing annularly disposed about a central axis and having primary and secondary sealing surfaces disposed therein; a wellhead connector annularly disposed about the central axis and having disposed therein a connector sealing surface; and a seal ring annularly disposed about the central axis and comprising: a ring seal surface for sealing contact with the connector sealing surface; a first seal surface for sealing contact with the primary sealing surface; and a second seal surface for sealing contact with the secondary sealing surface.

2. A system as defined in claim 1 wherein the secondary sealing surface is conical.

3. A system as defined in claim 1 wherein the secondary sealing surface is cylindrical.

4. A system as defined in any of claims 1-3, wherein compressive forces . imposed by the wellhead housing and the wellhead connector generate a moment which energizes the second seal surface.

5. A system as defined in claim 4, wherein the resultant vertical force acting between the ring seal surface and the connector sealing surface is positioned closer to the central axis than the resultant vertical force acting between the wellhead housing and the seal ring.