Abstract:
A seal assembly with an accumulator ring for protecting a seal from explosive decompression is disclosed. The accumulator ring and the seal are contained within a seal gland. The seal is made of a material that is susceptible to damage from explosive decompression. When exposed to a high-pressure fluid environment, the accumulator ring fills with a quantity of high-pressure fluid, either by permeation if it is solid or by permeation and/or vents and/or channels leading to a void if it is hollow. During an explosive decompression event, the accumulator ring expands to fill the remaining volume of the seal gland. Then, the accumulator ring exerts an axial compressive or supporting force on the seal for a sufficient amount of time to allow the seal to expel fluid contained in it and minimize the effects of explosive decompression thereto.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a seal assembly and method of sealing for use in high pressure application, for example, drilling and production applications. More particularly, the invention involves a seal assembly and method of sealing the junction between the male and female members or face seal members, wherein the seals are susceptible to explosive decompression effects. 
     2. Related Art 
     Ring seals are used in a variety of environments. Of particular interest are ring seals used in high-pressure environments. These include applications such as oil field drilling and production operations, hydraulic couplings, blowout preventer packings, etc. 
     Typical ring seals are often made from semi-permeable elastomeric materials. The sealing rings are intended to seal against a particular medium; if the pressure brought against the sealing rings by the medium becomes too great, the medium begins to penetrate the elastomeric material that constitutes the sealing rings. If a sudden pressure drop occurs in the surroundings of the sealing ring after such a penetration has occurred, the medium that has penetrated into the sealing rings abruptly expands. As a consequence of this expansion, the sealing rings are damaged or even destroyed. This kind of event is called an “explosive decompression.” This is a particular problem when the fluid is a gas at high temperature and pressure. In order to reduce the risk of an explosive decompression occurring, it is known to use seals, for example, o-rings, having a reduced cross-section and, as a result, a reduced exposed surface area. The likelihood of explosive decompression is also decreased by using seals made of materials having very low or very high permeability rates, such as costly and weaker silicone materials. Low permeability rates minimizes the incorporation of fluid in such materials, typically having a high durometer, thereby minimizing the effects of explosive decompression thereto. High permeability rates allows any fluid within such a material to be quickly released, thereby minimizes such effects thereto. Additionally, seal designers attempt to achieve a maximum volume fill of the seal gland partly to try to minimize such effects, but are constrained from doing so due to the thermal coefficient of expansion of the materials placed within the seal gland. Thus, a 100% volume fill cannot be accomplished. At best, about a 90% volume fill is attainable at room temperature. 
     There is a need to protect ring seals and provide a degree of resistance to explosive decompression regardless of the material that constitutes the ring seals. 
     SUMMARY OF THE INVENTION 
     In a broad aspect, there is provided a seal assembly for high pressure applications, said assembly comprising: 
     a first member having a first surface; 
     a second member having a second surface, wherein the first surface and second surface oppose and are spaced from each other when the first and second members are joined; 
     a third surface and a fourth surface, wherein each of the third and fourth surfaces is integral to, attached to or supported by one of the first and second surfaces and wherein the third surface and fourth surface oppose and are spaced from each other, 
     wherein the first, second, third and fourth surfaces define a seal gland therewithin; 
     a ring seal positioned within the seal gland, wherein the ring seal being axially compressible to sealingly engage the first surface and the second surface and the ring seal is susceptible to explosive decompression effects; and 
     an accumulator ring positioned within the seal gland on the side of the ring seal to be exposed to a high pressure fluid, wherein said accumulator ring has an exterior and an interior, wherein the interior has a void fraction defining an accumulator volume and said accumulator ring has at least one channel providing communication between the exterior of the accumulator ring and the accumulator volume, wherein in an installed position exposed to high pressure fluids, the accumulator volume fills with fluid and during an explosive decompression event, the at least one channel allows the fluid in the accumulator ring to escape at a rate sufficient to allow the exterior to compress the ring seal for a sufficient amount of time to minimize the explosive decompression effects on the ring seal. 
     In one embodiment, the first member is a female member and the second member is a male member. In this embodiment, the seal assembly comprises: 
     a female member having a longitudinal bore and an annular shoulder in the bore; 
     a male member insertable into the female member bore, having an annular shoulder on the outer surface thereof, 
     wherein the male and female members are joined and define a seal gland between the shoulders thereof; 
     a ring seal positioned within the seal gland, wherein the ring seal is axially compressible to sealingly engage the male member and the female member bore and the ring seal is susceptible to explosive decompression effects; and 
     an accumulator ring positioned within the seal gland on the side of the ring seal to be exposed to a high pressure fluid, wherein said accumulator ring has an exterior and an interior, wherein the interior has a void fraction defining an accumulator volume and the accumulator ring has at least one channel providing communication between the exterior of the accumulator ring and the accumulator volume, wherein in an installed position exposed to high pressure fluids, the accumulator volume fills with fluid and during an explosive decompression event, the at least one channel allows the fluid in the accumulator ring to escape at a rate sufficient to allow the exterior of the accumulator ring to compress the ring seal for a sufficient amount of time to minimize the explosive decompression effects on the ring seal. This active support of the seal during pressure venting provided by the accumulator ring simulates a 100% volume fill design (or zero void fraction) of the seal gland. 
     In one embodiment, the accumulator ring and the ring seal fill the seal gland in an installed position. 
     In an alternate embodiment, a seal retainer is placed in the seal gland. The seal retainer has a first end and a second end. The first end is configured to contact the annular shoulder of the male member and the second end configured to contact the ring seal to compress the ring seal axially. A locking means may be added to limit axial movement of the seal retainer. The locking means may, for example, be a clip engageable with the female member bore to hold the retainer in the bore. 
     In one embodiment, the accumulator ring is an o-ring made of a flexible but relatively strong and preferably semi-resilient material, for example, metal and plastics (such as Kevlar), where the o-ring is provided with at least one vent hole between the exterior and interior of the o-ring. The o-ring is pressure-energized to engage the ring seal and the shoulder abutting accumulator ring in response to an explosive decompression event in the seal assembly, thereby supporting the seal. After the pressure venting process has terminated or during any application of pressure that has equalized, the accumulator ring, being made of a strong yet resilient material, eventually restores itself to its original shape. 
     In another embodiment, the accumulator ring is made of a material permeable by the high-pressure fluid. In this case, the extent of permeation of the high-pressure fluid is the void fraction. The material releases the permeated fluid therefrom during an explosive decompression event at a slower rate than the ring seal. This allows the accumulator ring to expand and place an axial compressive load the ring seal within the seal gland to minimize the explosive decompression effects on the ring seal. The accumulator ring may also be partly hollow providing an additional volume for containing the high-pressure fluid that has permeated through the material and thus increases the void fraction. In either configuration, when the rate of pressure change is near zero, the accumulator ring will eventually restore itself to its original shape. 
     In another embodiment, the accumulator ring has at least a first toroidal ring and a second toroidal ring. Each of the first and second toroidal rings has an axial cross section that defines a planar figure. The planar figure partially encloses an area having an open side and a closed side. The open side of the first toroidal ring is placed within the open side of the second toroidal ring with the closed sides of the first and second toroidal rings opposing each other. The thus positioned first and second toroidal rings define an enclosed volume therewithin and an exterior surface. 
     The planar figure formed by the axial cross section of the first and second toroidal rings is preferably U-shaped. Therefore, the first and second toroidal rings are each a U-shaped toroidal ring having an open side and a closed side. 
     Preferably, at least one of the first and second toroidal rings has at least one vent to allow high-pressure fluid to enter or exit the enclosed volume. During an explosive decompression event, the fluid in the enclosed volume expands and exits through the at least one vent. The number and size of the vent(s) controls the exit flow rate of the fluid and allows the expanding fluid remaining within the enclosed volume to urge the first and second toroidal rings to expand radially against each other and the male member. The first and second toroidal rings are also urged apart axially against the ring seal and the shoulder abutting the accumulator ring to axially compress or support the ring seal for a sufficient amount of time during an explosive decompression event to minimize the explosive decompression effects on the ring seal. 
     Alternatively or in addition, the material of at least one of the first and second toroidal rings may be permeable to the high-pressure fluid. If the rate of permeation is sufficient, vent(s) may not be necessary. 
     Each sealing element of the present invention is fabricated in the shape of a toroidal ring, which is defined herein as the body formed by the rotation of a planar figure about a line or axis of rotation which lies in the same plane as the planar figure but does not intersect it. The axis of rotation is the axis of the toroidal ring. The axial cross section of a toroidal ring therefore is defined by the intersection of the ring with a plane, wherein the axis of the toroidal ring lies entirely in the plane. As an illustration, the rotation of a disc about an axis of rotation which lies in the same plane as the disc forms a toroidal ring typically known as an o-ring, and the axial cross section of the o-ring forms a disc. 
     Each sealing element has an axial cross section forming any planar shape or figure that defines an area having an open side and a closed side. The open side of the figure formed by the cross section is oriented in a generally axial direction relative to the toroidal ring, and the closed side is oriented in a generally opposite direction from the open side. Likewise, the toroidal ring defines a general circumferential volume enclosed or defined by an open side and a closed side. Typically, the open side of the toroidal ring is oriented in a generally axial direction and the closed side is oriented in a generally opposite axial direction from the open side. The circumferential volume typically is oriented in a generally axial direction. 
     As an example, one of the preferred sealing elements of the invention has an axial cross section defining a U-shaped body in which the open end of the U is oriented in the axial direction and the closed end of the U is oriented in the opposite axial direction. When used in a seal assembly as described below, two U-shaped rings are used with the open side of the inner U inserted into the open side of the outer U-shaped ring. This defines an enclosed volume within this double U assembly. Vents in one or both of the inner and outer U-shaped rings and gaps between the inner and outer U-shaped rings allows fluid to enter the enclosed volume. The inner and outer U-shaped rings are each a toroidal ring defined by the rotation of the U-shaped body about the axis. 
     As another example, one of the preferred sealing elements of the invention has an axial cross section defining an E-shaped body in which the open end of the E is oriented in the axial direction and the closed end of the E is oriented in the opposite axial direction. When used in a seal assembly as described below, the E-shaped ring is placed with the open side in contact with the ring seal. This defines an enclosed or internal volume within the E-shaped body, since the open end is now closed by the ring seal. Vents in E-shaped ring or the permeability of the material the E-shaped ring is made of, together with gaps between the open end thereof and the ring seal, allows fluid to enter the enclosed volume. The E-shaped ring is a toroidal ring defined by the rotation of the E-shaped body about the axis with the open end oriented in the axial direction. 
     Other axial cross sections of the sealing element or toroidal ring are possible which form planar figures having other general shapes, some of which can be described schematically by the letters C, E, H, V, W, X, and Y. Other more complex shapes can be envisioned which have similar mechanical properties. The common characteristic of these planar figures is that the toroidal ring defined by each figure has a circumferential volume which is oriented in a generally axial direction and allows one to be placed within another in a similar fashion as the double U assembly to in turn define an internal volume. 
     During pressure venting, including an explosive decompression event, the pressurized fluid in the internal volume expands and urges the compliant arms or members of the ring to expand against both each other (as in the double U-shaped ring embodiments) and axial surfaces of a seal gland (as in both the double U-shaped ring embodiments and the E-shaped ring embodiment) to temporarily retain and controllably release or vent the pressurized fluid contained in the internal volume. Also, as the pressurized fluid in the internal volume expands, the inner and outer toroidal rings in the double U-shaped ring embodiments are urged axially apart, thereby exerting a compressive force against the seal being protected due to such fluid expansion. In the E-shaped ring embodiment, the expanding pressurized fluid within the internal volume exerts the compressive force against the seal being protected. This allows any fluid that may have permeated therein to escape at a rate that prevents or at least minimizes the deleterious effects of such an event. The fluid within the internal volume is allowed to escape at a controlled rate via permeation or a suitable number and sized vents in one or both of the toroidal rings to allow a sufficient amount of time for fluid to escape from the protected seal. 
     There is also provided well sealing assembly for sealing between the interior surface of a housing and the exterior of a tubular body. The tubular body having a first exterior surface, a second exterior surface that has a larger diameter than the first exterior surface and a tapered surface between said first and second surfaces. The well sealing assembly comprises a seal ring assembly, an accumulator ring, and means for moving said seal ring assembly axially from its position surrounding said first surface of said tubular member, over said tapered surface and into its set position surrounding said second surface. 
     The seal ring assembly has a resilient ring with upper and lower flat surfaces, an interior convex surface and an exterior flat surface and annular metal end caps. The metal end caps have flat portions bonded on the upper and lower flat surfaces of the resilient ring, inner legs tapering with the convex interior surface of the resilient ring and outer legs on the flat exterior surface of the resilient ring. The interior surface of the resilient ring is in close spaced relationship to the first surface of the tubular member when said seal ring assembly surrounds the first surface. 
     The accumulator ring axially positioned adjacent to the seal ring assembly on the side of the seal ring assembly to be exposed to a high-pressure fluid. The accumulator ring has an exterior and an interior. The interior has a void fraction defining an accumulator volume. The accumulator ring has at least one channel providing communication between the exterior of the accumulator ring and the accumulator volume. The channel may be a vent or present in the material due to molecular spacing allowing permeation of a fluid therethrough. In an installed position exposed to high-pressure fluids, the accumulator volume fills with fluid. During pressure venting, for example, an explosive decompression event, the at least one channel allows the fluid in the accumulator ring to escape at a rate sufficient to allow the exterior to compress or support the seal ring assembly for a sufficient amount of time to minimize the explosive decompression effects on the material of the resilient ring of the seal ring assembly. 
     The resilient ring has sufficient resiliency and volume to expand outward when moved to the set position to seal against the interior housing surface and to move the exterior legs of the end caps into metal-to-metal sealing engagement with the interior housing surface and to seal against the second surface with the interior legs in metal-to-metal sealing engagement with the second surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 ,  1 A and  1 B are section views of various seal glands formed between a first member and a second member (prior art). 
         FIG. 2  is a section view showing a metal end cap seal and a hollow, vented o-ring according to a first embodiment of the present invention. 
         FIG. 2A  is an enlarged section view of circle  2 A in  FIG. 2  showing the metal end cap seal and the hollow, vented o-ring according to a first embodiment of the present invention. 
         FIG. 2B  is a partial section view of a hollow o-ring containing a spring according to another embodiment of the present invention. 
         FIG. 3  is a section view showing a seal retainer, a metal end cap seal, and a hollow, vented o-ring according to a second embodiment of the present invention. 
         FIG. 4  is a section view showing a metal end cap seal and a permeable, solid o-ring according to a third embodiment of the present invention. 
         FIG. 5  is a section view showing a metal end cap seal and a hollow, permeable o-ring according to a fourth embodiment of the present invention. 
         FIG. 6  is a section view showing a metal end cap seal and a slip U-assembly according to a fifth embodiment of the present invention. 
         FIG. 6A  is an enlarged section view of circle  6 A in  FIG. 6  showing the metal end cap seal and the slip U-assembly according to a fifth embodiment of the present invention. 
         FIGS. 6B and 6C  are section view showing a metal end cap seal and an interlocking U-assembly according to a sixth embodiment of the present invention. 
         FIG. 6D  is a section view showing a metal end cap seal and a slip U-assembly according to a seventh embodiment of the present invention, similar to that shown in  FIG. 6  with the exception that grooves in a pair of legs of one of the U-shaped members rather than vent holes in each of the two U-shaped members of the fifth embodiment. 
         FIGS. 6E and 6F  are section view showing a metal end cap seal and an E-shaped ring according to an eighth embodiment of the present invention, with  FIG. 6G  depicting a section view of the E-shaped ring alone. 
         FIG. 7  is a partial sectional view of a well housing with the improved sealing assembly of the present invention shown in half section and with its running tool. 
         FIG. 8  is a partial sectional view of the unset or running position of the sealing assembly. 
         FIG. 9  is a view similar to  FIG. 8  illustrating the set position of the sealing assembly. 
         FIG. 10  is an enlarged partial sectional view of the set position of the seal ring assembly. 
         FIG. 11  is a section view showing a metal end cap seal and a hollow, permeable o-ring on either side of the metal end cap seal according to a ninth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a section view of a first member  10  and second member  12 . The first member  10  has a first surface  13  and a shoulder  14 , which in this embodiment is the third surface. The second member  12  has a second surface  15  and a shoulder  16 , which in this embodiment is the fourth surface. A space or void known as a seal gland  18  is defined by the first surface  13 , second surface  15 , and shoulders  14  and  16 , the third and fourth surfaces respectively. 
       FIG. 1A  is a section view of a first member  10   a  and second member  12   a . The first member  10   a  has a first surface  13   a  and a shoulder  14   a  and shoulder  16   a , which in this embodiment are the third and fourth surfaces. The second member  12   a  has a second surface  15   a . A space or void known as a seal gland  18   a  is defined by the first surface  13   a , second surface  15   a , and shoulders  14   a  and  16   a , the third and fourth surfaces respectively. 
       FIG. 1B  is a section view of a first member  10   b  and second member  12   b . The first member  10   b  has a first surface  13   b , a threaded portion  17  and a shoulder  14   b , which in this embodiment is the third surface. The first surface  13   b  is located between the threaded portion  17  and the shoulder  14   b . The second member  12   b  has a second surface  15   b . A threaded member  19  has a bottom surface  16   b , which in this embodiment is the fourth surface. The threaded member  19  in an installed position threadedly engages the threaded portion  17 . A space or void known as a seal gland  18   b  is defined by the first surface  13   b , second surface  15   b , bottom surface  16   b  and shoulder  14   b , the latter two being the third and fourth surfaces respectively. 
     Other seal gland embodiments are shown in FIG.  3  and FIG.  9 . 
     In a first embodiment of the present invention shown in  FIGS. 2 and 2A , the seal gland  18  of seal assembly  1  contains a metal end cap seal  20  in the set position and a hollow o-ring  24 . Though seal gland  18  is depicted herein, any other seal gland may be used which is defined by four surfaces, for example, those shown in  FIGS. 1A ,  1 B,  3  and  9 . The hollow o-ring  24  has vents  26 . In the set position, the metal end cap seal  20  seals against surfaces  13  and  15 . The hollow o-ring  24  is made of a material that substantially retains its shape during the setting of the metal end cap seal  20 . The o-ring  24  may be a hollow metal o-ring or of an elastomeric material. In another embodiment of the o-ring  24 , a spring  33 , for example, a helical or wave spring, as shown in  FIG. 2B  can be contained within the hollow o-ring in this embodiment referred to a  24   a  to aid in retaining the shape of the o-ring  24   a  during the setting operation. 
     During high-pressure operations, the fluid follows path F through gap  28 . The seal made by the metal end cap seal  20  against surfaces  13  and  15  prevents the fluid from getting passed this point. The high-pressure fluid, typically a gas, enters the interior of the o-ring  24  through vents  26 . 
     During pressure venting, including an explosive decompression event, some of the gas may have permeated the non-metal portions of the metal end cap seal  20  behind metal end cap  22  on the high-pressure side. The rapid decompression causes the gas that has permeated into the metal end cap seal  20  to expand rapidly. To prevent or at least minimize the deleterious effects of rapid pressure venting or explosive decompression on the non-metallic portions of the metal end cap seal  20 , the vents  26  and the interior of the o-ring  24  are sized to gradually release the rapidly expanding gas contained therein. This causes the o-ring to exert a compressive force against the metal end cap seal  20 . This compressive force allows the gas trapped within the metal end cap seal  20  to be released at a rate that prevents or minimizes these deleterious effects on the seal  20 . The compressive load applied to the metal end caps seal  20  by the accumulator ring during the venting process simply supports the elastomer of the metal end cap seal  20  and therefore prevents its elastomeric material from axially elongating too far and rupturing. 
     If a spring  33  is contained within the o-ring  24   a , it can also provide continuous compressive loading against the seal  20 . In this case, the compressive loading provided by the spring  33  is a backup for the compressive or support force provided by the expanding gases. Further, with the spring, the sizing of the interior of the o-ring  24   a  and the vents  26  is less critical. If desired, the spring  33  can be sized to provide the required compressive load without relying on the compressive force generated by the expanding gas within the o-ring  24   a . However, since it is believed that the axial compressive forces developed by the o-ring on the seal at high pressures is so much greater than could be provided with the spring, the spring&#39;s role would in many cases be an expander or restorer for the o-ring  24   a.    
     In a second embodiment of the present invention shown in  FIG. 3 , seal assembly  2  is similar to seal assembly  1  shown in  FIG. 2 , with the exception that the seal gland  18  of seal assembly  2  further contains a seal retainer  30 . 
     In a third embodiment of the present invention shown in  FIG. 4 , seal assembly  3  is similar to seal assembly  1  shown in  FIG. 2 , with the exception that the hollow o-ring  24  has been replaced by a solid o-ring  32 . O-ring  32  is made of a material that is permeable by the high-pressure gas. The o-ring  32  is sized to allow a sufficient amount of gas to permeate and accumulate within it; and, during pressure venting including an explosive decompression event, o-ring  32  expands as a result of the gas rapidly expanding therein trying to escape. This expansion exerts a compressive force against seal  20  to prevent or at least minimize the explosive decompression effects on seal  20 . The rate of gas escape from the o-ring  32  is less than that of the non-metallic material of the seal  20 . 
     In a fourth embodiment of the present invention shown in  FIG. 5 , seal assembly  4  is similar to seal assembly  1  shown in  FIGS. 2 and 2A , with the exception that the hollow o-ring  24  has been replaced by a hollow o-ring  34 . O-ring  34  is made of a material that is permeable by the high-pressure gas. The interior void  36  of o-ring  34  is sized to allow a sufficient amount of gas to permeate and accumulate within it and during an explosive decompression event expands as a result of the gas rapidly expanding therein trying to escape. This expansion exerts a compressive force against seal  20  to prevent or at least minimize the explosive decompression effects on seal  20 . The rate of gas escape from the o-ring  34  is less than that of the non-metallic material of the seal  20 . 
     In a fifth embodiment of the present invention shown in  FIGS. 6 and 6A , seal assembly  5  is similar to seal assembly  1  shown in  FIG. 2 , with the exception that the hollow o-ring  24  has been replaced by a slip double U assembly  38 . The slip double U assembly has an inner U-shaped ring  40  and an outer U-shaped ring  54 . The inner U-shaped ring  40  has a back  42 , a vent or vents  52  in the back  42 , legs  44  and  46  axially extending substantially perpendicular from the back  42 , and outer U leg stops  48  and  50  radially extending from the back  42  in opposite directions. The outer U-shaped ring  54  has a back  56 , a vent or vents  62 , and legs  58  and  60  axially extending substantially perpendicular from the back  56 . The radial spacing between the outside surfaces of legs  44  and  46  is less than the radial spacing of the inner surfaces of legs  58  and  60 . The inner U-shaped ring  40  is inserted into the outer U-shaped ring  54  with the legs  58  and  60  placed adjacent to the outer U leg stops  48  and  50 , respectively. This placement of the inner and outer U-shaped rings  40  and  54 , respectively, defines an internal volume or void  64 . The interior volume  64  of the slip double U assembly  38  is sized to allow a sufficient amount of gas to accumulate within it to allow the assembly  38  to exert a compressive force against the seal  20  during an explosive decompression event for a sufficient amount of time to prevent or at least minimize the deleterious effects of the explosive decompression event to seal  20 . 
     During pressure venting, including an explosive decompression event, the fluid within the volume  64  rapidly expands. The fluid may initially be a liquid that converts to a gas or a pressurized gas that rapidly expands during such an event. The vents  52  and  62  are of a size and number that allows the rapidly expanding gas in volume  64  to exert an outward force against the interior surfaces of the double U assembly  38 . This gas expansion causes legs  44  and  46  to radially diverge and place an outward load against legs  58  and  60  creating a seal to prevent gas escape therebetween. Therefore, the only gas escape route is via vents  52  and  62 . The gas expansion also causes the inner and outer U-shaped rings to axially diverge from each other with legs  44  and  46  and legs  58  and  60 , respectively, slipping over each other as the gas continues to expand. This gas expansion places a compressive load against the seal  20  to prevent or at least minimize the explosive decompression effects on seal  20 . The rate of gas escape from the slip double U assembly  38  is controlled by the size and number of vents  52  and  62  in the inner and outer U-shaped rings  40  and  54 , respectively. 
     In a sixth embodiment of the present invention shown in  FIGS. 6B and 6C , seal assembly  7  is similar to seal assembly  5  shown in  FIGS. 6 and 6A , with the exception that the slip double U assembly  38  has been replaced by an interlocking double U assembly  138 . The interlocking double U assembly has an inner U-shaped ring  140  and an outer U-shaped ring  154 . The inner U-shaped ring  140  has a back  142 , a vent or vents  152  in the back  142 , legs  144  and  146  axially extending substantially perpendicular from the back  142 , outer U leg stops  148  and  150  radially extending from the back  142  in opposite directions, and grooves  149  and  151 , for example, adjacent the outer U leg stops  148  and  150 . The outer U-shaped ring  154  has a back  156 , a vent or vents  162 , legs  158  and  160  axially extending substantially perpendicular from the back  156 , and ridges  159  and  161 . Ridges  159  and  161  are adapted to matingly engage grooves  149  and  151 , preferably with the grooves  149  and  151  sized to be axially wider than the corresponding ridges  159  and  161 . As seen in  FIG. 6C , this difference in axial width shown as gaps  163  and  165  allows the respective U-shaped rings  140  and  154  to expand and contract in the axial direction. The radial spacing between the outside surfaces of legs  144  and  146  is less than the radial spacing of the inner surfaces of legs  158  and  160 . The inner U-shaped ring  140  is inserted into the outer U-shaped ring  154  with the legs  158  and  160  placed adjacent to the outer U leg stops  148  and  150 , respectively. This placement of the inner and outer U-shaped rings  140  and  154 , respectively, defines an internal volume or void  164 . The interior volume  164  of the interlocking double U assembly  138  is sized to allow a sufficient amount of gas to accumulate within it to allow the assembly  138  to exert a compressive or supporting force against the seal  20  due to gas expansion within the internal volume  164  during an explosive decompression event for a sufficient amount of time to prevent or at least minimize the deleterious effects of the rapid pressure venting or explosive decompression event to seal  20 . In  FIGS. 6B and 6C , a seal gland  18   b  is used as depicted in  FIG. 1B , though any of the other seal glands can be used. 
     During pressure venting, including an explosive decompression event, the fluid within the volume  164  rapidly expands. The fluid may initially be a liquid that converts to a gas or a pressurized gas that rapidly expands during such an event. During such an event, the fluid within the interior volume  164  expands faster than it can escape therefrom. Specifically, in this embodiment, the vents  152  and  162  are of a size and number that allows the rapidly expanding gas in volume  164  to exert an outward force against the interior surfaces of the double U assembly  138  (see black arrows in FIG.  6 C). This expansion causes legs  144  and  146  to radially diverge and place an outward load against legs  158  and  160  creating a seal to prevent gas escape therebetween. Therefore, the only escape route for the gas contained in the internal volume  164  is via vents  152  and  162 . The expansion also causes the inner and outer U-shaped rings  140  and  154  to axially diverge from each other. This axial divergence is initially limited by the difference in width between the width of the grooves  149  and  151  and the width of the ridges  159  and  161  with legs  144  and  146  and legs  158  and  160 , respectively, slipping over each other as the gas continues to expand producing the gaps  163  and  165  shown in FIG.  6 C. If the differential pressure is sufficiently great between the interior and exterior of the volume  164 , the ridges  159  and  161  will ride up on the grooves  149  and  151 . The U-shaped rings  140  and  154  will try to disengage from each other as the U-shaped rings  140  and  154  continue to axially move apart until there is no additional room left in the seal gland to do so. This expansion places a compressive load against the seal  20  to prevent or at least minimize the explosive decompression effects on seal  20  (see white arrows showing direction of force exerted on back  156  acting on seal  20 ). The rate of gas escape from the interlocking double U assembly  138  is controlled by the size and number of vents  152  and  162  in the inner and outer U-shaped rings  140  and  154 , respectively. 
     The slip double U assembly  38  and the interlocking double U assembly  138  are made of materials that have the properties and capabilities described above. One example of such materials are high performance engineering plastics. One such plastic is polyether ether ketone, also known as PEEK, which is a strong polymer that would flex slightly during venting, withstand the stresses placed on it in operation, but would still provide an adequate seal between the legs of the inner and outer U-shaped rings during venting to minimize fluid escape therebetween. This would force the expanding fluid to exit the respective assembly by flowing through the vent holes provided. 
     In a seventh embodiment of the present invention shown in  FIG. 6D , seal assembly  5 A is similar to seal assembly  5  shown in  FIGS. 6 and 6A . However, rather than having vent holes extend through one or both of the U-shaped rings, seal assembly  5 A has the vent holes formed by having axial grooves  21  and  23  provided on at least one pair of legs corresponding to one of the U-shaped rings. When the two U-shaped rings are engaged, the grooves  21  and  23  on one pair of legs together with the adjacent legs of the other U-shaped ring define the venting holes. 
     In an eighth embodiment of the present invention shown in  FIGS. 6E ,  6 F and  6 G, seal assembly  8  shown in  FIGS. 6E and 6F  is similar to seal assembly  5  shown in  FIGS. 6 and 6A , with the exception that the slip double U assembly  38  has been replaced by a single E-shaped toroidal ring  254 . The seal gland is like that shown in  FIGS. 6B and 6C . Ring  254  depending on perspective can also be described as M-shaped or W-shaped. 
     As shown more clearly in  FIG. 6G , the cross-section of ring  254  reveals that it has a back  256 , a vent or vents  262 , a lateral channel  257  intersecting the vent  262 , outer legs  258  and  260  having ends  270  and  271 , respectively, a central leg  261  having end  272 , an equilibration vent  263 , ribs or ridges  273  and  274 , and internal volumes or voids  264  and  265 . Though optional, the central leg  261  is preferably present to provide ring  254  with increased load support capability. Ribs  273  and  274  are also optional, but preferred so as to provide a better seal of leg  258  and  260  against the adjacent surfaces  275  and  276  of members  10   b  and  12   b , respectively, during pressure venting. Vent or vents  262  are also optional if ring  254  is made of a permeable material that has a suitable permeability rate. 
     In an installed position, the ends  270 ,  271  and  272  contact the metal cap  22 . The cooperation of the ring  254  and the metal cap  22  closes the open end of ring  254  allowing pressurized fluid to fill the internal volumes  264  and  265 . During pressure venting, such as an explosive decompression event, the pressurized fluid in the internal volumes  264  and  265  exerts forces in the directions of the arrows shown in FIG.  6 F. In one aspect, such forces push outward in a radial direction on legs  258  and  260  forcing ribs  273  and  274  into sealing engagement with surfaces  275  and  276 , respectively. In another aspect, the back  256  is pushed against surface  278 , thereby filling the seal gland volume. Further, as the pressurized fluid contained within the internal volumes  264  and  265  expands, this expanding fluid also exerts a supporting force on the metal cap  22  of ring seal  20 . Vent  256  and channel  263  with gap  280  cooperate to controllably release the pressurized fluid temporarily retained in the internal volumes  264  and  265  at a rate sufficient to allow any fluid contained within ring seal  20  to escape at a rate that will minimize the effects of rapid pressure venting thereto. 
     The o-rings  24 ,  32 , and  34  may be made of any suitable semi-permeable elastomeric material that have the properties and capabilities described above that would flex during venting, withstand the stresses placed on it in operation, and return to its original pre-venting shape after the venting event has stabilized or a represurization has occurred and re-stabilized. One example of such materials are materials having a durometer of at most that of the elastomeric portion of the seal  20 . One example of such a material is nitrile rubber, preferably having a durometer of at least 70. Note unless otherwise specified, a durometer value specified herein is based on the Shore A scale, which is determined pursuant to ASTM D-1415. 
     The accumulator ring, in any of the foregoing embodiments, preferably provides at least 30 minutes of protection to the seal  20  during an explosive decompression or venting event to avoid or minimize the adverse effects of such an event on the seal  20 . 
     One particular application for the embodiments of the present invention is in the drilling and production area, for example, in the seal assembly disclosed in U.S. Pat. No. 4,496,162, which is hereby incorporated by reference. The seal assembly thereof is modified to include an accumulator ring  66  of the present invention. Accumulator ring  66  may be any of those disclosed previously in the first through fifth embodiments as the hollow, vented o-ring  24  ( FIGS. 2 ,  2 A and  3 ), solid o-ring  32  (FIG.  4 ), hollow permeable o-ring  34  (FIG.  5 ), slip double U assembly  38  (FIGS.  6  and  6 A), interlocking double U assembly  138  (FIGS.  6 B and  6 C), and E-shaped ring  254  ( FIGS. 6E ,  6 F and  6 G). Reference is now made to  FIGS. 7 through 10 . 
     Improved sealing assembly  6  of the present invention is deployed to, for example, a subsea well housing  72  on running tool  74 . Sealing assembly  6  is landed on and connected to hanger  76  supported in housing  72  in a known manner. Tubular body  78  of sealing assembly  6  engages hanger  76  through the ratchet (spring loaded, threaded segments) connection R and has its external downwardly facing shoulder  80  engaged on upper surface  82  of hanger  76 . 
     Sealing assembly  6  includes tubular body  78 , having upwardly facing external shoulder  84 , outer surface  86 , inwardly and upwardly tapering surface  88 , and upper surface  90  which is smaller in diameter than surface  86 , back-up ring  92  surrounding and releasably connected to surface  86  by shear pin  94 , seal ring assembly  96  and accumulator ring  66  above ring  92  and setting sleeve  98  above seal ring assembly  96 . In the unset or running position as shown schematically in  FIG. 8 , running tool  74  being removed for clarity, back-up ring  92  is positioned around surface  86  and extends upward to the bottom of tapered surface  88  into abutting relationship with seal ring assembly  96 , which in turn is in abutting relationship with the accumulator ring  66 . Setting sleeve  98  engages the upper end of seal ring assembly and surrounds surface  90 . Tapered split ring  100  is positioned in groove  102  in sleeve  98  and in groove  104  in body  78 . Split ring  100  and shear pin  94  retains sleeve  98 , sealing assembly  96 , accumulator ring  66  and back-up ring  92  in position during running. If desired, a pin or pins in back-up ring sliding in a slot in the exterior of body  78  can be used to assist to maintain the elements in position on body  78  during running. Sleeve  98  has internal groove  106  above groove  102  for the purposes hereinafter set forth. 
     Seal ring assembly  96  includes resilient ring  108  having metal end caps  110  and  112  on its upper and lower ends as shown and preferably bonded thereto. End caps  110  and  112  have a central portion  114  with legs  116  (including inner legs  116   b  and outer legs  116   a ) extending upwardly and downwardly, respectively, in a direction toward the mid-point of resilient ring  108 . The inner central portion  118  of resilient ring  108  is convex and extends inward into light engagement or close spaced relationship with surface  90  in its unset position. It is preferred that resilient ring  108  be made of an elastomeric material, such as a nitrile rubber as sold by B. F. Goodrich Company under the trademark HYCAR, and metal end caps are a thin type  316  stainless steel. The metal end caps and the elastomeric portion of the seal ring assembly  96  can be coated with Teflon material, for example, to prevent the sticking of the seal ring assembly  96  to the first and second surfaces of the seal gland. 
     During running, sealing assembly  6  is supported on running tool  74  by pins  120  and  122 . Upon landing of assembly  6  running tool  74  is rotated to tighten connection R and then it is lowered to cause pin  124  to engage sleeve  98  and move it downward to the position shown in FIG.  9 . This downward movement shears pin  94  and moves seal ring assembly  96  and accumulator ring  66  downward onto outer surface  86 . This downward movement of seal ring assembly  96  moves it radially outward on tapered surface  88  and onto larger diameter surface  86 . Thus, this downward movement provides the radial energization of seal ring assembly  96  to move it to its sealing position between surface  86  and the inner surface of housing  72 . In this position there is metal-to-metal seals of the legs  116   a  of end caps  110  and  112  with the sealing surfaces of housing  72  and body  78  because the inner central portion  118  of ring  108  is compressed radially outward which creates an internal force on the outer legs of the end caps outward, toward their related sealing surface. This force ensures that the inner legs  116   b  also are held in sealing engagement with surface  86 . While this seal is energized by axial movement, which can be a weight set as shown, a screw set or other actuation, it creates the radial energizing of resilient ring  108  which ensures sealing and requires no axial load to maintain sealing after having been energized. During this setting movement of setting sleeve  98  the taper on groove  102  moves split ring  100  into groove  104  until sleeve  98  moves down to cause groove  106  to align with groove  104  at which position snap ring  100  moves into groove  106  and locks sleeve  98  against upward movement to thereby retain sealing assembly  6  in its set position. In this set position, resilient ring  96  is free to expand axially but is restricted from radial movement by surface  86  and the inner surface of housing  78 . 
     The improved sealing assembly provides a long life well annulus seal which is suitable for use in high pressure and high temperature environments and is radially energized so that a thread or weight setting load are not necessary to maintain the seal. Further, this assembly does not require the application and maintenance of a fluid under pressure to maintain the seal. There is no extrusion of the resilient ring by well pressure as it is completely encased at its ends by the end caps which provide the metal-to-metal annulus seal. Further, if it becomes necessary to retrieve the seal assembly from the well bore, this can be easily done by lifting the assembly upward in which case the metal end caps ensure that subsequent upward travel through the well bore does not pull off a section of the seal and possibly hang the seal assembly or drop a segment of the seal into the well. 
     Additionally, the accumulator ring protects the seal ring assembly from explosive decompression by placing the seal ring assembly under compression in such an event thereby avoiding or minimizing the deleterious effects of such an event on the seal ring assembly, which is also a ring seal. 
     In a ninth embodiment of the present invention as shown in  FIG. 11 , the seal gland  18   b  as shown in  FIG. 1B  is depicted with seal assembly  1   d  which contains a metal end cap seal  20  in the set position and two hollow o-rings  24  and  25  with one on each side of the seal  20 . Though seal gland  18   b  is depicted herein, any other seal gland may be used which is defined by four surfaces, for example, those shown in  FIGS. 1A ,  1 B,  3  and  9 . Further, though the accumulator rings shown herein are hollow o-rings  24  and  25 , the accumulator rings may be any of those disclosed previously in the first through seventh embodiments as the hollow, vented o-ring  24  ( FIGS. 2 ,  2 A and  3 ; used in FIG.  11 ), solid o-ring  32  (FIG.  4 ), hollow permeable o-ring  34  (FIG.  5 ), slip double U assembly  38  ( FIGS. 6 and 6A  and FIG.  6 D), interlocking double U assembly  138  (FIGS.  6 B and  6 C), and an E-shaped ring  254  ( FIGS. 6E ,  6 F and  6 G). Additionally, the two accumulator rings can be the same or different. 
     A variation of the present invention is the attachment of at least a portion of the accumulator ring to at least a portion of the ring seal. In one aspect, this will facilitate installation. For example, vented o-ring  24  ( FIGS. 2 ,  2 A and  3 ), solid o-ring  32  (FIG.  4 ), and hollow o-ring  34  ( FIG. 5 ) can be attached to the ring seal, for example, at the point of contact thereof with the metal end cap  22  of ring seal  20 . Similarly, the ends  270 ,  271  and optionally  272  of legs  258 ,  260  and  263  of E-shaped ring  254  ( FIGS. 6E ,  6 F and  6 G) can be attached to the ring seal, for example, at the point of contact thereof with the metal end cap  22  of the ring seal  20 . Relative to the double U-shaped rings  38  ( FIGS. 6 ,  6 A and  6 D) and  138  (FIGS.  6 B and  6 C), the back  42  of ring  40  and the back  156  of ring  158  can be attached or even integral to, for example, the metal end cap  22  of ring seal  20 . 
     Although variations in the embodiment of the present invention may not each realize all the advantages of the invention, certain features may become more important than others in various applications of the device. Therefore, the invention should be understood to be limited only by the scope of the appended claims.