US20150345246A1

US20150345246A1 - Wellhead Component with Seal Surface and Method of Forming the Seal

Info

Publication number: US20150345246A1
Application number: US14/821,355
Authority: US
Inventors: Jeffrey Allen Raynal; Daniel Caleb Benson; David Lawrence Ford
Original assignee: Vetco Gray LLC
Current assignee: Vetco Gray LLC
Priority date: 2011-12-02
Filing date: 2015-08-07
Publication date: 2015-12-03

Abstract

A subset wellhead component has a metal seal surface having voids. The voids are initially filled with an elastomeric substrate having hollow beads. When lowered into the subsea assembly, the beads have sufficient strength to resist collapsing under a hydrostatic pressure of liquid within the subsea wellhead assembly to prevent shrinking of a volume of the substrate. Exerting a setting force on the metal seal surface to form the sealing engagement causes the beads to collapse and reduce the volume of the substrate within the voids. The seal surface may be folds of a bellows-type nose ring of a seal assembly. The seal surface may alternately be wickers formed in the bore of the wellhead housing.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 13/310,172, filed Dec. 2, 2011.

BACKGROUND

1. Field of Invention
The invention relates generally to a subsea wellhead assembly. More specifically, the invention relates to a seal surface having voids filled with an elastomeric substrate having imbedded hollow beads that collapse under a setting force to shrink a volume of the substrate.
2. Description of Prior Art
Seals are typically inserted between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member is generally a hanger for supporting either casing or tubing that extends into the well. Outer wellhead members are wellhead housings, or they can be a casing hanger when the inner member is a tubing hanger. A variety of seals located between the inner and outer wellhead members are known. Examples of known seals are elastomeric, metal, and combinations thereof. 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 seal body with inner and outer walls separated by a cylindrical slot, forming a “U” shape. An energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead member. The energizing ring is typically a solid wedge-shaped member. During setting, the deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
The seal surfaces on the inner and outer wellhead members may have wickers. Wickers comprise a set of annular parallel grooves formed in the seal surface. Typically, the wickers have saw tooth shapes, defining valleys and crests. The setting tool forces the seal surfaces of the seal assembly against the wickers, causing the crests to embed into the seal surfaces.
The seal assembly may have a secondary or emergency sealing portion below an upper sealing portion. The emergency sealing portion may comprise nose bellows having metal folds with corners or crests between each fold that form a metal to metal seal between the inner and outer wellhead members.
The subsea wellhead housing may be filled with drilling fluid or mud during the running and setting of an annulus seal assembly. The drilling mud contains ingredients and debris that can enter the voids of the sealing surfaces before the seal assembly sets. The voids may be between the folds of the nose ring bellows, if one is employed. The voids may be in the valleys of the wickers, if they are employed. The drilling fluid debris in the voids, can form a hydraulic lock during the setting operation, preventing the full setting of the seal assembly.

SUMMARY OF THE INVENTION

A method of forming a sealing engagement in a subsea wellhead assembly having an axis, includes providing a subsea wellhead component with a metal seal surface having voids. The voids are filled with an elastomeric substrate having hollow beads embedded therein. The wellhead component is then lowered into the sea and installed in the subsea wellhead assembly. The beads have sufficient strength to resist collapsing under a hydrostatic pressure of liquid within the subsea wellhead assembly to prevent shrinking of a volume of the substrate. Then, a setting force is exerted on the metal seal surface to form the sealing engagement. The setting force is at a level sufficient to collapse the beads and reduce the volume of the substrate within the voids.
The beads are filled with a compressible fluid, such as a gas. In the preferred embodiment, the beads comprise glass microspheres. The elastomeric substrate is substantially incompressible so as to avoid shrinking of the volume until the beads collapse.
The seal surface may comprise a set of wickers formed on a side wall of a bore of a wellhead housing, the wickers comprising parallel grooves having valleys and crests. The voids filled with the substrate comprise the valleys of the wickers. The method includes lowering a metal seal member into the bore of the wellhead housing. The setting force is exerted by radially moving the seal member against the wickers to embed the crests into the seal member and form the sealing engagement.
Alternately or in addition, the seal surface may comprises a seal body with folds defining crests and valleys, the voids comprising the valleys. In that instance, the setting force is exerted axially on the seal body, causing the folds to move toward each other and the crests to sealingly engage a portion of the wellhead assembly, to form the sealing engagement.
The beads are capable of withstanding collapsing up to a pressure in a selected range at least from 10,000 to 20,000 psi. The beads preferably collapse when in a range from 25,000 to 30,000 psi.

BRIEF DESCRIPTION OF DRAWINGS

Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side partial sectional view of an example embodiment of a seal assembly between downhole tubulars in accordance with the present invention.

FIG. 2 is a detailed portion of the embodiment of FIG. 1.

FIG. 3 is a sectional view of an embodiment of the seal assembly of FIG. 1 in a set configuration in accordance with the present invention.

FIG. 4 is a side partial sectional view of a wellhead assembly having an embodiment of a seal assembly in accordance with the present invention.

FIG. 5 is a sectional view of an alternate embodiment of the present invention.

FIG. 6 is an enlarged, sectional view of a wicker portion of the wellhead housing before setting of the seal.

FIG. 7 is a sectional view of the wicker portion of FIG. 6 after setting of the seal.

While the invention will be described in connection with preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF INVENTION

The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout.
It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalent will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the improvements herein described are therefore to be limited only by the scope of the appended claims.
An example embodiment of a subsea wellhead assembly 20 is shown in a side partial sectional view in FIG. 1. In the example of FIG. 1, the wellhead assembly 20 is shown having an annular hanger 22 coaxially disposed within an annular wellhead housing 24. The hanger 22 may be one of a casing hanger or a tubing hanger, wherein the housing 24 can be one of a high pressure housing or casing hanger. A seal assembly 26 is shown set within an annular space between the concentric hanger 22 and housing 24. The seal assembly 26 includes a seal member 28 on its upper end. In the example of FIG. 1, part of the seal member 28 is a ring-like outer leg that runs substantially in an axial direction and adjacent an inner surface of the housing 24. An inner leg is included with the seal member 28 that is set radially inward from the outer leg to define a space or slot between. The inner leg is connected to the outer leg by a lower end that projects radially inward from the outer leg and then axially upward to the inner leg.
A nose ring assembly 30 is shown mounted on a lower end of the seal member 28 and beneath the inner leg. Partially coaxially within the seal member 28 is an annular energizing ring 32, having a lower wedge-shaped extension shown inserted in the slot between the outer and inner legs of the seal member 28. In the example of FIG. 1, ridge-like wickers 34 are shown formed respectively on the outer surface of the hanger 22 and inner surface of the housing 24. Wickers 34 comprise small, parallel grooves that are in planes perpendicular to the axis. In this embodiment, wickers 34 are saw tooth in shape. An annular retainer ring 37 is shown threadingly secured to an upper end of the seal member 28 that extends radially inward from the seal member 28. As described in more detail below, by axially urging the energizing ring 32 downward, so its lower end inserts into the space between the inner and outer legs of the seal member 28, sealing may take place within the annular space between the hanger 22 and housing 24. Moreover, retainer ring 37 extends radially inward from the seal member 28 and interferes with a ledge on the energizing ring 32 when the energizing ring 32 is pulled upward to also pull an the seal member 28.
Referring now to FIG. 2, a portion of the embodiment of FIG. 1 is shown in a side partial sectional view and in greater detail. More specifically, FIG. 2 depicts that a bellows-like nose ring body 38, as an emergency seal, is included with the nose ring assembly 30 along with a lower base 40 on the lower end of the body 38. In the example of FIG. 2, the lower base 40 is a generally ring-like annular member and configured to land on a ledge 41 (FIG. 1) provided on the hanger 22. An upper base 42 is shown mounted on an upper end of the nose ring body 38 and is also a generally ring-like annular member. A portion of the upper base 42 is threaded for attachment to the seal member 28. In the example of FIG. 2, the nose ring body 38 is made up of a series of V-shaped folds 44 shown arranged generally oblique to each adjacent fold 44. Crests or edges 45 of folds 44 are defined on the body 38 where the adjacent folds 44 join. On the outer surface of the edges 45 are optional metal inlays 46 that as an example are formed from a metal that is softer (i.e., having a lower yield strength) than that of the hanger 22 and housing 24. In one example embodiment, the metal inlay 46 includes tin indium. In the embodiment of FIG. 2, a mid portion of the edges 45 is recessed for receiving the inlays 46, the recesses define points 47 on the upper and lower ends of the edges 45 that penetrate into and grip the hanger 22 and housing 24 when the body 38 axially compresses.
Each adjacent fold 44 defines a wedge-shaped void, valley, or space in which is disposed a volume compensating material 48. The compensating material 48 provides a barrier to debris and other undesirable material that may otherwise enter into the space between the folds 44. In one example embodiment, the compensating material 48 is shown having a series of hollow beads or microspheres 50 that are dispersed within a substrate 52. Examples of microspheres 50 include a compressible fluid, such as a gas, encapsulated inside of a load-bearing non-porous housing, such as a glass or ceramic. The shape of microspheres 50 is preferably spherical, but it could differ. The substrate 52 may include elastomeric materials, such as rubber, composites, and the like. Normally, substrate 52, apart from microspheres 50, will be incompressible but deformable. Microspheres 50 for this use may be in a variety of diameters, typically 0.001 to 002 inches.
Microspheres 50 must have the ability to withstand the fluid pressure imposed on seal assembly 26 before setting. The fluid pressures will depend on the type of liquid within wellhead housing 24, the depth of the sea, as well as test pressures that may occur. Wellhead housing 24 may be located thousands of feet below the sea surface, resulting in a high hydrostatic fluid pressure. Also, higher pressures can be exerted on the liquid 59 (FIG. 2) in wellhead housing 24 during various well completion operations, such as testing. Liquid 59 may comprise drilling fluid or mud, which is typically heavier than sea water, as well as containing solids and debris. Drilling fluid 59 may also be present in a drilling riser extending from wellhead housing 24 to a drilling platform at the sea surface. If compensating material 48 is not employed, the solids and debris in the drilling fluid 59 can otherwise enter the voids between folds 44 and prevent them from moving toward each other during setting. For the purposes herein, microspheres 50 are configured to withstand collapsing in a range at least from 10,000 to 20,000 psi. During the setting procedure, microspheres 50 must collapse in order to allow folds 44 to move toward each other. Microspheres 50 may collapse when under pressures in excess of 20,000 psi, and preferably will collapse or fracture under pressures in the range from 25,000 to 30,000 psi, which occur during setting. The total volume of microspheres 50 relative to substrate 52 between each fold 44 may vary, but typically is more than half of the substrate 52 volume.
Substrate 52, along with the dispersed microspheres 50, will be installed between folds 44 while seal assembly 26 is at the surface. Substrate 52 may be initially in a liquid state that cures. Then seal assembly 26 is lowered down the drilling riser (not shown) into subsea wellhead 24. The shape of folds 44 remains constant during the running procedure because compensating material 48 will not shrink in volume as the hydrostatic fluid pressure increase. Prior to setting, the volume of compensating material 48 remains the same as at the surface, preventing debris in drilling fluid 59 from entering the voids between folds 44. After landing seal assembly 26 will be set or energized.
In the example embodiment of FIG. 3 the seal assembly 26 is in an energized and sealing configuration. As shown, the energizing ring 32 is axially inserted within the legs of the seal member 28 thereby projecting the legs of the seal member 28 radially apart from one another and into contact with the housing 24 and also the hanger 22. In a typical installation, the wickers 34, 36 in the housing 24 and the hanger 22 form corresponding grooves 54, 56 on the inner and outer legs of the seal member 28 after setting. Additionally, the downward urging of energizing ring 32 transmits a setting force through the seal member 28 and onto the nose ring assembly 30. While the bellows portion of the nose ring body 38 axially deforms, sufficient axial stresses remain within the nose ring assembly 30 to maintain the lockdown force between the hanger 22 and housing 24.
When the seal assembly 26 is in the set position of FIG. 3, the axial deformation of the nose ring body 38 allows the edges 45 to move radially inward and outward into contact with the respective outer and inner surfaces of the hanger 22 and housing 24. With sufficient inward and outward radial force, the softer metal inlays 46 on the edges 45 deform and create sealing contact between the nose ring body 38 and surfaces of the hanger 22 and housing 24. Discontinuities 58, such as scratches, scores or other damage, may be present on one of the sealing surfaces of the hanger 22 or housing 24. The deforming metal inlays 46 can fill the space of the discontinuities 58, thereby ensuring a sealing surface within the annulus between the hanger 22 and housing 24.
Moreover, the spheres 50 within the compensating material 48 will fracture or collapse and the encapsulated gas in the spheres 50 will compress in response to the setting forces (and pressures) encountered while energizing the seal assembly 26. The collapsing spheres 50 and compression of the gas allows axial compression of the nose ring body 38, with folds 44 moving axially toward each other. Thus, should any liquid 59, such as drilling fluid in wellhead housing 24, enter the space between adjacent folds 44, the compensating material 48 compresses to make up for the presence of the liquid 59 so that movement of the folds 44 is not restricted when the seal assembly 26 is being energized.
FIG. 4 illustrates a side partial sectional view of one example of a subsea wellhead assembly 20 that is mounted over a subsea well 60. In the example of FIG. 4, the wellhead assembly includes a subsea production tree 62 mounted on an outer wellhead housing 64. Casing 66 depends downward from the wellhead assembly 20 and into the wellhead 60. The casing 66 is shown concentrically around a string of production tubing 68 that also depends into well 60. In the example of FIG. 4, the seal assembly 26 (FIG. 1) is inserted between the casing hanger, on which the casing 66 is supported, and a high pressure wellhead housing 24.
In one example of use, the seal assembly 26 described herein may be used in an emergency situation wherein an initial seal has failed. In such an example, deformations, such as the discontinuities 58 of FIG. 3, may be present within one or more of the tubulars in the wellhead. Moreover, the wickers 34, 36 (FIG. 1) may have also been damaged, thereby compromising the sealing ability of a seal such as seal member 28. In this example, implementing a nose ring as described herein can provide sealing capabilities to overcome the sealing problems encountered. An advantage of the nose ring described herein is that it can be integrated with the standard seal member 28 and installed using steps typically undertaken to install the seal member 28.
FIG. 5 represents a second embodiment. A subsea wellhead assembly includes a wellhead housing 70 that is installed at or near a sea floor at an upper end of a subsea well. Wellhead housing 70 is a tubular member having a bore 72 with a set of wickers 74 on its side wall. As illustrated in FIG. 6, wickers 74 comprise parallel annular grooves, each having a crest 76 separated by flanks forming a valley 78. In this embodiment, crests 76 are sharp V-shaped edges, each in a plane perpendicular to the axis of bore 72. Wickers 74 are not threads, rather each groove is separated from the one above and the one below. Wickers 74 typically have a saw tooth configuration and may have a radial depth from crest 76 to the base of valley 78 of about 1/16 inch and an axial dimension between crests 76 that is about ⅛ inch. Other dimensions are feasible.
Before lowering wellhead housing 70 into the sea, a substrate 80 will be applied to wickers 74. Substrate 80 is preferably an elastomeric material, such as silicone rubber, that is deformable, but not compressible. That, is the volume of substrate 80 does not shrink under pressure, although the shape of substrate 80 may deform. Normally, substrate 80 is applied to wickers 74 as a liquid, then allowed to cure so as to bond to the metal wickers 74. Substrate 80 preferably completely fills the voids create by wicker valleys 78. Once cured, the inner diameter of substrate 80 is cylindrical and flush with wicker crests 76.
Hollow beads 82 are dispersed throughout substrate 80 before it is applied to wickers 74. Beads 82 preferably are hollow microspheres, but the shapes may vary. Beads 82 are formed of a rigid, load supporting material such as glass. The hollow, sealed interior of each bead 82 is filled with a compressible fluid, namely a gas such as air. Beads 82 will fracture or collapse when under a selected pressure.
For the purposes herein, the collapsing pressure required must be high enough to withstand the hydrostatic pressure of the liquid in bore 72 of wellhead housing 70 once wellhead housing 70 is installed. The pressures will depend on the depth of the sea above wellhead housing 70, the type of liquid contained in bore 72, and whether any test pressures are imposed. Before heads 82 are collapsed, a drilling riser (not shown) will be attached to wellhead housing 70. At times during drilling of the well, the riser and wellhead housing bore 72 will be filled with drilling fluid or mud, which is normally heaver in weight than sea water. Wellhead housing 70 may be thousands of feet below the sea level in deep water subsea wells, thus the hydrostatic pressure can be thousands of pounds per square inch. In certain operations, the pressure of the drilling fluid within wellhead housing 70 may be higher than hydrostatic pressure, such as test pressures. To withstand typical fluid pressures, beads 82 should not collapse until reaching pressures of about 10,000 to 20,000 psi. However, beads 82 are configured to collapse during seal setting operations, thus may collapse above 20,000 psi and preferably collapse in a range from 25,000 to 30,000 psi.
The diameters of beads 82 may vary, but are quite small in order to locate within valleys 78. The percentage volume of heads 82 within substrate 80 before collapsing can vary, however, the percentage is preferably greater than 50 percent. The total volume of beads 82 should be high enough to cause substrate 80 to shrink significantly when beads 82 undergo collapsing. For example, the volume of substrate 80 including beads 82 should shrink sufficiently once beads 82 collapse to expose crests 76 to a desired radial depth, such as about 0.020 inch. The volume decrease once beads 82 collapse thus could be about 20 to 40 percent, for example, of the initial volume. As the elastomeric substrate 80 will not shrink under pressure, the shrinkage occurs entirely due to the collapsing of beads 82 and the compression of the gas contained in beads 82. The collapsing of each bead 82 greatly reduces the volume that each bead previously occupied within substrate 80.
Referring again to FIG. 5, a casing hanger 84 lands in wellhead housing 70 and supports a string of casing (not shown) that extends into and is cemented in the well. Optionally, casing hanger 84 may have a set of wickers 86 on a seal surface on an outer diameter. If employed, at least part of wickers 86 will be axially aligned with wickers 74. Wickers 86 may be identical to wickers 74 and be filled with the same material as substrate 80 and beads 82.
A seal assembly 88 will be lowered into the pocket between wickers 74 and 86, then set to form a sealing engagement between casing banger 84 and wellhead housing 70. Seal assembly 88 may vary in configuration. In this example, seal assembly 88 has a metal seal body 90, preferably steel, with inner and outer legs separated by a slot 92. An energizing ring 94 retained by a retaining ring 96 initially locates at the upper end of slot 92. Energizing ring 94 has a radial width greater than the radial dimension of slot 92.
In the operation of the embodiment of FIGS. 5-7, before lowering wellhead housing 70 into the sea, substrate 80 containing dispersed beads 82 will be applied to wickers 74 and 86, then cured. As wellhead housing 70 is being deployed and after deployment, substrate 80 will be exposed to the pressure of the liquid within wellhead housing bore 72. That liquid at times will be drilling fluid. The collapsing strength of beads 82 is greater than the liquid pressures encountered. Consequently, wicker valleys 78 remain filled with substrate 80 as illustrated in FIG. 6, preventing the entry of debris from the drilling mud into wicker valleys 78.
One or more casing hangers 84 will be set within wellhead housing 70. Seal assembly 88 will be lowered into the pocket between casing hanger 84 and wellhead housing wickers 74. Drilling mud or another liquid would be within bore 72 as seal assembly 88 lands on an external shoulder of casing banger 84. A downward axial force is then applied to energizing ring 94 to moves the outer seal surface of seal body 90 into sealing engagement with wickers 74 and the inner seal surface of seal body 90 into sealing engagement with wickers 86. The setting force causes crests 76 of wickers 74 to embed into the outer sealing surface of seal body 90 as well as the crests of wickers 86 to embed into the inner sealing surface of seal body 90. The outward radial movement of the outer surface of seal body 90 applies a pressure to substrate 80 that is high enough to fracture or collapse beads 82. The fracturing of beads 82 allows the volume of substrate 80 within each valley 78 to reduce sufficiently to expose a desired radial dimension of crests 76. The drilling mud previously between seal body 90 and wickers 74 will be squeezed above and or below wickers 74 are the outer surface of seal body 90 contacts wicker crests 76. The same shrinkage will also occur to the substrate within casing hanger wickers 86, if employed.
The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims. For example, the substrate and beads of FIGS. 5-7 could be employed with the wickers shown in FIGS. 1 and 3.

Claims

1 A method of forming a sealing engagement in a subsea wellhead assembly having an axis, comprising:

providing a subsea wellhead component with a metal seal surface having voids;

filling the voids with an elastomeric substrate having hollow beads embedded therein;

then, lowering the wellhead component into the sea and installing the wellhead component in the subsea wellhead assembly, the beads having sufficient strength to resist collapsing under a hydrostatic pressure of liquid within the subsea wellhead assembly to prevent shrinking of a volume of the substrate; and

then, exerting a setting force on the metal seal surface to form the sealing engagement, the setting force being at a level sufficient to collapse the beads and reduce the volume of the substrate within the voids.

2. The method according to claim 1, wherein the beads are filled with a compressible fluid.

3. The method according to claim 1, wherein the beads encapsulate a gas.

4. The method according to claim 1, wherein the beads comprise glass microspheres.

5. The method according to claim 1, wherein the elastomeric substrate is substantially incompressible.

6. The method according to claim 1, wherein:

the seal surface comprises a set of wickers formed on a side wall of a bore of a wellhead housing, the wickers comprising parallel grooves having valleys and crests;

the voids comprise the valleys of the wickers;

the method further comprises lowering a metal seal member into the bore of the wellhead housing; and

the setting force is exerted by radially moving the seal member against the wickers to embed the crests into the seal member and form the sealing engagement.

7. The method according to claim 1, wherein:

the seal surface comprises a seal body with folds defining crests and valleys, the voids comprising the valleys; and

the setting force is exerted axially on the seal body, causing the folds to move toward each other, and the crests to sealingly engage a portion of the wellhead assembly to form the sealing engagement.

8. The method according to claim 1, wherein the beads are capable of withstanding collapsing to a pressure in a selected range from 10,000 to 20,000 psi.

9. A method of forming a sealing engagement in a subsea wellhead assembly, comprising:

providing a wellhead housing with a bore having an axis and a side wall containing a set of wickers comprising crests separated by valleys;

filling the valleys with an elastomeric substrate having hollow glass microspheres embedded therein;

then lowering the wellhead housing into the sea and installing the wellhead housing in the subsea wellhead assembly, the microspheres having sufficient strength to resist collapsing due to a hydrostatic pressure of liquid within the wellhead housing, so as to prevent a volume of the substrate horn decreasing;

then, lowering a metal seal member having a metal seal surface into the wellhead housing; then

energizing the seal member to cause the seal surface to radially move into engagement with the crests of the wickers with sufficient force for the crests to embed into the seal surface and the microspheres to collapse to allow the volume of the substrate in the valleys to decrease.

10. The method according to claim 9, wherein the microspheres are filled with a compressible fluid.

11. The method according to claim 9, wherein the microspheres encapsulate a gas.

12. The method according to claim 9, wherein the elastomeric substrate is substantially incompressible.

13. The method according to claim 9, wherein the microspheres are configured to collapse only after reaching a pressure in a selected range from 25,000 to 30,000 psi.

14. A subsea wellhead assembly having an axis and comprising:

a subsea wellhead component having a metal seal surface with voids;

the voids being filled with an elastomeric substrate having hollow beads embedded therein, the beads having sufficient strength to resist collapsing under a hydrostatic pressure of the sea to prevent shrinking of a volume of the substrate; and

energizing means for exerting a setting force on the metal seal surface to form a sealing engagement, the setting force being at a level sufficient to collapse the beads and reduce the volume of the substrate within the voids.

15. The assembly according to claim 14, wherein the beads are filled with a compressible fluid.

16. The assembly according to claim 14, wherein the beads comprise glass microspheres.

17. The assembly according to claim 14, wherein the elastomeric substrate is substantially incompressible.

18. The assembly according to claim 14, wherein:

the wellhead component comprises a subsea wellhead housing having a bore with an axis;

the seal surface comprises a set of wickers formed on a side wall of the bore, the wickers comprising annular parallel grooves having valleys and crests;

the voids comprise the valleys of the wickers;

the assembly further comprises a metal seal member; and

the energizing means comprises an energizing ring that when moved axially, radially moves the seal member against the wickers at a sufficient setting force to embed the crests into the seal member and form the sealing engagement.

19. The apparatus according to claim 14, further comprising:

a subsea wellhead housing having a bore with an axis and a metal seal member deployed in the bore; wherein:

the seal surface comprises folds formed on the seal member that define crests and valleys, the voids comprising the valleys; and

the energizing means comprises an energizing ring that when moved axially, exerts the setting force axially on the seal member, causing the folds to move toward each other, and the crests to sealingly engage a side wall of the bore of the wellhead housing.

20. The assembly according to claim 14, wherein:

the beads are capable of withstanding collapsing up to a pressure in a selected range from 10,000 to 20,000 psi, and are configured to collapse when subjected to a pressure of 25,000 to 30,000 psi.