Abstract:
A metal seal assembly to seal components in highly corrosive environments, such as a sour well environment. The seal assembly is comprised of a base metal structural component with a softer metal layer applied onto its surface. The purpose of the soft metal layer is to locally deform and, thereby, form a seal against a surface of an opposing component. The base metal structure of the seal may be comprised of a corrosion-resistant alloy. In addition, the soft metal layer may be comprised of a corrosion-resistant alloy, such as a refractory metal like tantalum.

Description:
BACKGROUND 
       [0001]    The invention relates generally to metallic seal assemblies for use in sealing components of oil and gas wells. In particular, the invention relates to a seal assembly for use in a highly-corrosive well environment, such as a well having high levels of hydrogen sulfide, carbon dioxide, water, and chlorides. 
         [0002]    Oil and gas wells may contain many substances that combine together to create a highly-corrosive environment for oil field equipment. A seal that is used in a highly-corrosive environment that is not able to withstand the corrosive effects of the environment will begin to corrode. Eventually, the integrity of the seal will be lost and the seal assembly will fail. 
         [0003]    Wells are generally categorized as being either “sweet” or “sour.” A well is categorized as a sweet well if it is only mildly corrosive. Conversely, a well is categorized as a sour well if it is very corrosive. The presence of several different compounds can make a well a sour well, such as hydrogen sulfide, carbon dioxide, chlorides, and free sulfur. 
         [0004]    In particular, equipment exposed to corrosive well bore fluids must be able to resist stress corrosion cracking (SCC). Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals or tough thermoplastics subjected to a constant tensile stress in a corrosive environment, especially at elevated temperature (in the case of metals). This type of corrosion often progresses rapidly. The corrosive environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating failure. 
         [0005]    Sulfide stress cracking (SSC), or sulfide stress corrosion cracking (SSCC), is a form of stress corrosion cracking. Susceptible alloys, especially steels, react with hydrogen sulfide, forming metal sulfides and elementary atomic hydrogen. Atomic hydrogen, created as a by-product of a cathodic reaction in the presence of H 2 S, diffuses into the metal matrix. Small quantities of hydrogen present inside certain metallic materials make the latter brittle and susceptible to sub-critical crack growth under stress. Some materials may exhibit a marked decrease in their load carrying capacity and fail in a brittle fashion when stressed in an atmosphere containing hydrogen. Both of these processes may be called hydrogen embrittlement. 
         [0006]    As oil and gas wells are drilled in deeper and deeper waters, the demand on the materials used in the wells increases. In addition to being able to withstand the corrosive elements present in a well, the materials used must be able to withstand the greater temperatures and pressure requirements for wells drilled in ever deeper waters. As a result, the materials used within a corrosive well typically are selected based on their corrosion-resistance and strength, as well as cost-effectiveness. 
         [0007]    As a result, there is a need for a seal assembly that has the strength and corrosion-resistance to form and maintain a seal in the highly-corrosive environment of a deepwater oil or gas well. In particular, there is a need for a seal assembly that has the strength and corrosion-resistance to form and maintain a seal in a corrosive deepwater well, especially under high pressure and high temperature conditions. 
       BRIEF DESCRIPTION 
       [0008]    A technique is provided for sealing components located in highly corrosive environments, such as sour wells operating at high temperatures and pressures. A seal assembly is used to form a seal between components. The seal assembly is comprised of a base metal structure with a softer metal layer over the base metal structure. The purpose of the soft metal layer is to deform and, thereby, form a seal against a surface of an opposing component. The material of the base metal structure is chosen to provide structural integrity to the seal. Ideally, both materials should be selected to be compatible with the corrosive fluids. 
         [0009]    Preferably, the base metal structure of the seal is comprised of a corrosion-resistant alloy. Examples of some corrosion resistant alloys commonly used in the oil and gas industry are nickel and cobalt alloys, such as UNS N07718, UNS N07716, UNS N07725, UNS N09925, UNS R30006 and UNS R31233. In addition, preferably, the metal layer also is comprised of a corrosion-resistant alloy or metal, such as titanium, or a refractory metal, such as tungsten, molybdenum, rhenium and more specifically, tantalum. 
     
    
     
       DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  is a cross-sectional view of a seal disposed between a wellhead and a wellhead connector, in accordance with an exemplary embodiment of the present technique; 
           [0012]      FIG. 1A  is a detailed cross-sectional view taken generally along line  1 A- 1 A of  FIG. 1 , in accordance with an exemplary embodiment of the present technique; 
           [0013]      FIG. 2  is a cross-sectional view of a seal disposed between a casing hanger and a wellhead, in accordance with an exemplary embodiment of the present technique; 
           [0014]      FIG. 3  is a cross-sectional view of the seal of  FIG. 2  activated to form a seal between the casing hanger and the wellhead, in accordance with an exemplary embodiment of the present technique; 
           [0015]      FIG. 3A  is a detailed cross-sectional view taken generally along line  3 A- 3 A of  FIG. 3 , in accordance with an exemplary embodiment of the present technique; 
           [0016]      FIG. 4  is a cross-sectional view of a seal assembly for a swivel device, in accordance with an exemplary embodiment of the present technique; and 
           [0017]      FIG. 4A  is a detailed cross-sectional view taken generally along line  4 A- 4 A of  FIG. 4 , in accordance with an exemplary embodiment of the present technique. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring generally to  FIGS. 1 and 1A , the present invention will be described as it might be applied in conjunction with an exemplary technique, in this case a subsea wellhead assembly  20  comprising a high pressure wellhead  22  and a wellhead connector  24 . The wellhead connector  24  is used to connect an object, such as a subsurface tree, to the high pressure wellhead  22 . The wellhead connector  24  has a lower portion (not shown) that is disposed over the exterior of the wellhead  22 . The wellhead connector  24  has a locking member, such as dogs (not shown) that are moved into engagement with grooves (not shown) formed on the exterior of the wellhead  22 . The high pressure wellhead has an inner bore  26  that is coaxial with an inner bore  28  of the wellhead connector  24  when the wellhead connector  24  is secured to the wellhead  22 . 
         [0019]    A gasket or seal ring  30  is disposed between the high pressure wellhead  22  and wellhead connector  24  to seal the inner bore  26  of the wellhead  22  to the inner bore  28  of the wellhead connector  28 . Seal ring  30  is generally T-shaped and has an upper leg  32  and a lower leg  34 . In this embodiment, the upper leg  32  and lower leg  34  are symmetrical. Alternatively, the upper leg  32  and lower leg  34  may be asymmetrical. In addition, in this embodiment, each leg has a first seal band  36  and a second seal band  38 . In addition, the seal ring  30  is formed so that the first and second seal bands  36 ,  38  have a conical shape in this embodiment. This enables the seal ring  30  to form a seal against a conical sealing surface  40  of the wellhead connector  24  and a conical sealing surface  42  of the high pressure wellhead  22 . The seal ring  30  has a rib  44  that is received into a recess  46  of the wellhead connector  24 . The recess  46  forms a pocket between the wellhead connector  24  and a shoulder  48  of the wellhead  22 . When the wellhead connector  24  is secured to the wellhead  22 , the rib  44  of the seal ring  30  is captured in the recess  46  between the wellhead  22  and the wellhead connector  24 . 
         [0020]    In the illustrated embodiment, the seal ring  30  is manufactured to be resistant to sulfide stress cracking (SSC) and stress corrosion cracking (SCC). In particular, the seal ring  30  is manufactured to satisfy the requirements for “HH-Sour Service” as set forth in ANSI/API (Approved American National Standard/American Petroleum Institute) Specification 6A, “Specification for Wellhead and Christmas Tree Equipment.” According to Table 3 of ANSI/API Specification 6A, a material satisfies the requirements for “HH-Sour Service” if it is a CRA (Corrosion Resistant Alloy) in compliance with NACE (National Association of Corrosion Engineers) standard: “MR 0175.” Section 3.1.30 of ANSI/API Specification 6A defines a Corrosion Resistant Alloy (CRA) as a “nonferrous-based alloy in which any one or the sum of the specified amount of the elements titanium, nickel, cobalt, chromium, and molybdenum exceeds 50% (mass fraction).” NACE MR 0175 is entitled: “Petroleum and natural gas industries-Materials for use in H 2 S-containing environments in oil and gas production.” Section 3.6 of Part 1 of NACE MR 0175 defines a corrosion-resistant alloy (CRA) as an “alloy intended to be resistant to general and localized corrosion of oilfield environments that are corrosive to carbon steels.” Here, a corrosion-resistant alloy (CRA) is defined as a material that is “an alloy intended to be resistant to general and localized corrosion of oilfield environments that are corrosive to carbon steels” and/or “a nonferrous-based alloy in which any one or the sum of the specified amount of the elements titanium, nickel, cobalt, chromium, and molybdenum exceeds 50% (mass fraction).” 
         [0021]    In the illustrated embodiment, the seal ring  30  is comprised of a metal body  50  that is covered with a metal layer  52 . In the illustrated embodiment, the metal body  50  comprises a corrosion-resistant alloy (CRA). Corrosion resistant alloys are well suited for service in extreme environments. These alloys form a thick and stable oxide layer on their surface protecting the alloy from the corrosive environment. However, the metal body  50  may be comprised of a metal other than a CRA. 
         [0022]    Examples of corrosion resistant alloys that may be used for the metal body  50  are nickel and cobalt alloys such as UNS N07718, UNS N07725, UNS N09925, UNS R30006 and UNS R31233. UNS N07718, UNS N07716, UNS N07725 and UNS N09925 are generally classified as precipitation-hardenable nickel alloys. UNS R30006 and UNS R31233 are generally categorized as cobalt based alloys. These nickel and cobalt alloys and others (not listed) are intentionally alloyed and heat treated to provide the corrosion resistance and strength. The combination of elements makes the alloy resistant to hydrogen embrittlement and stress-corrosion cracking. These alloys are resistance to general corrosion, pitting, crevice corrosion, and stress-corrosion cracking in many aqueous environments, including sulfides and chlorides. However, an alloy other than the aforementioned alloys may be used. 
         [0023]    Alloys UNS N07718, UNS N07716, UNS N07725, UNS N09925, and UNS R31233 are listed in Annex A of Part 3 of NACE MR 0175 as CRAs. Part 3 of NACE MR 0175 is entitled: “Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.” Annex A is entitled: “Environmental cracking-resistant CRAs and other alloys.” Precipitation-hardened nickel-based alloys that are CRAs and their environmental and material limits are listed in Section A.9 of Annex A by their UNS number. UNS N07718, N09925 are listed in Tables A.31 and A.32, while UNS N07725 is listed in Table A.33 and UNS R31233 in Table A.38 of Annex A. Other CRAs not listed in these industry standards have been successfully and extensively used in oil and gas production fluids containing hydrogen sulfide, such as UNS R30006. 
         [0024]    As noted above, the metal body  50  in the illustrated embodiment is covered with a metal layer  52 . In the illustrated embodiment, the metal layer  52  comprises an alloy, preferably a metal such as a refractory metal. Refractory metals are a class of metals extraordinarily resistant to heat, wear, and corrosion. The five refractory metals are: Tungsten (W), Molybdenum (Mo), Niobium (Nb), Tantalum (Ta), and Rhenium (Re). Preferably, the metal layer  52  is comprised of tantalum. Tantalum is one of the most corrosion resistant substances available. However, a different refractory metal may be used. In the illustrated embodiment, the metal layer  52  has a greater ductility than the metal body  50 . The metal layer  52  is provided to form a seal against an opposing seal surface and the metal body  50  is provided to supply structural integrity and strength for the metal layer  52 . In addition, the metal layer  52  is disposed over the entire surface of the seal ring  30  in the illustrated embodiment. However, the metal layer  52  may be disposed over less than the entire surface of the seal ring  30 . For example, in an alternative embodiment, the metal layer  52  may be disposed only over a sealing surface or sealing surfaces. 
         [0025]    In this embodiment, the metal layer  52  is a tantalum alloy, such as the tantalum alloy corresponding to UNS No. R05200. Tantalum alloy R05200 is listed in Table A.42 of Annex A of NACE MR 0175 as a CRA. The environmental and material limits for alloy R05200 are provided in Table A.42, as well. As illustrated in  FIG. 5 , Table D.12 from Annex D of Part 3 of NACE MR 0175 provides the chemical composition of alloy R05200. The alloy is comprised of small amounts of carbon, cobalt, iron, silicon, molybdenum, tungsten, nickel, and titanium, and other elements with the remainder tantalum. However, unalloyed tantalum or another tantalum alloy may be used, such as an alloy corresponding to UNS No. R05210. 
         [0026]    Referring generally to  FIGS. 2 ,  3 , and  3 A, another portion of the wellhead assembly  20  is presented. In this portion of the wellhead assembly, a seal assembly  54  is provided to seal an annulus  56  between the wellhead  22  and a casing hanger  58 . The casing hanger  58  is used to support a string of casing (not shown) from the wellhead  22 . 
         [0027]    The illustrated embodiment of the seal assembly  54  comprises a seal ring  60  and an energizing ring  62 . The seal ring  60  is provided to form a seal with the high pressure wellhead  22  on one side and the casing hanger  58  on the other side, thereby sealing the annulus  56  between the wellhead  22  and the casing hanger  58 . Once the casing hanger  58  and the seal assembly  54  are in position within the high pressure wellhead  22 , the energizing ring  62  is used to activate the seal ring  60 . The seal ring  60  has an inner leg  64  and an outer leg  66  with a slot  68  between them. When the energizing ring  62  is driven into the slot  68  of the seal ring  60 , the inner leg  64  is driven against the casing hanger  58  and the outer leg  66  is driven against the high pressure wellhead  22 . 
         [0028]    The seal ring  60  has a metal body  70  with a metal layer  72  disposed over the surface of the metal body  70  in the illustrated embodiment. In the illustrated embodiment, the metal layer  72  comprises tantalum. The energizing ring  62  may also be comprised of a metal body with a metal layer disposed over the surface. 
         [0029]    As with the seal assembly  30  above, the metal layer  72  is used to form a seal and the metal body  70  is provided to support the metal layer  72 . When the inner leg  64  is driven against the casing hanger  58  and the outer leg  66  is driven against the wellhead  22 , the metal layer  72  forms a seal with the wellhead  22  and with the casing hanger  58 . In the illustrated embodiment, the high pressure wellhead  22  and the casing hanger  58  have wickers  74 ,  76 , respectively, formed therein. The metal layer  72  is softer than the metal body  70  and is deformed into the wickers  74 ,  76  forming a seal. In addition, the metal body  70  of the illustrated seal assembly  54  is formed of a corrosion-resistant alloy (CRA), such as a nickel or cobalt alloy. The energizing ring  62  may also comprise a corrosion-resistant alloy (CRA). 
         [0030]    Referring generally to  FIGS. 4 and 4A , a swivel seal assembly is presented and represented generally by reference numeral  78 . The swivel seal  78  is provided to seal the annulus  80  between an inner member  82  and an outer member  84 . In the illustrated embodiment, the inner member  82  and outer member  84  have several seal pads  86  that are used to form seals with the swivel seal assembly  78 . The seal assembly  78  has seal arms  88  that have seal surfaces  90  that are configured to form a seal against the seal pads  86 . In this embodiment, the seal surfaces  90  have a metal layer  92  that is used to form the seal with the seal pads  86 . However, the metal layer  92  may be located on the seal pads  86 , rather than the seal surfaces  90  of the seal assembly  78 . In the illustrated embodiment, the seal arms  88  are comprised of a corrosion-resistant material, such as a nickel or cobalt alloy. In addition, the metal layer  92  also is comprised of a corrosion-resistant material. In particular, the illustrated embodiment of the metal layer  92  can be comprised of tantalum. 
         [0031]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.