Abstract:
A dummy undersea hydraulic coupling member protects an opposing undersea hydraulic coupling member when the hydraulic lines are not operating. The dummy undersea hydraulic coupling member has a water displacement expansion chamber with a piston in the chamber that allows trapped water and/or air to move from the receiving chamber into the water displacement expansion chamber during connection of the dummy coupling member to the opposing coupling member. The piston has both a primary seal and a secondary, hydrostatic seal for preventing seawater from entering the portion of the water displacement expansion chamber that is contacted by the primary seal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/726,970, filed on Nov. 15, 2012. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT: Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     This invention relates in general to hydraulic couplings, and specifically to hydraulic couplings used in undersea oil well drilling and production applications. More particularly, this invention involves a dummy undersea hydraulic coupling member that may be used for protecting an opposing coupling member that is stationed subsea. 
     2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98. 
     A variety of subsea hydraulic couplings are known in the art. The couplings generally consist of a male member and a female member with sealed fluid passageways connecting therebetween. The female member generally is a cylindrical body with a relatively large diameter longitudinal bore at one end and relatively small diameter longitudinal bore at the other. The small bore facilitates connections to hydraulic lines, while the large bore seals and slidingly engages the male member of the coupling. The male member includes a cylindrical portion at one having an outer diameter approximately equal to the diameter of the large bore in the female member of the coupling. The male member also includes a connection at its other end to facilitate connection to hydraulic lines. When the cylindrical portion of the male member is inserted into the large bore of the female member, according to various embodiments of the device, fluid flow is established between the male and female members. Several couplings of this type are disclosed in patents owned by National Coupling Company, Inc. of Stafford, Tex. 
     In undersea drilling and production applications, the male member of the coupling may be connected to a manifold plate or other securement at a subsea location at the inside or outside of a well bore. In many cases, the male members are positioned so that the end or leading face of each member points vertically up from the sea floor. The female members, which also may be secured to a manifold plate, are moved into position over the male members and then lowered onto the male members by a diver or subsea vehicle, such as an ROV (remotely operated vehicle). When the female members are positioned on the male members, hydraulic fluid flow is typically from the female member to the male member of each coupling. Typically, one or both of the coupling members have poppet valves. 
     Each poppet valve typically includes a conical valve face which seats, in the closed position, against a valve seat in the coupling member. The poppet valve opens to allow fluid flow, and closes against the valve seat within the bore to arrest the flow. Generally, the poppet valve is spring-biased to the closed position. The valve may include a valve actuator which may be a nose or stem extending from the apex of the valve face along the longitudinal axis of the poppet valve. 
     When the male and female coupling members are disconnected, the male coupling members typically remain subsea, and the female coupling members are retrieved. Frequently, well bores in which the couplings are positioned contain debris. The male member, which remains subsea when the coupling is disconnected, is subject to debris accumulating in exposed flow passages when it is disconnected from the female member. The debris may contaminate the hydraulic fluid or cause wear to the seals and sealing surfaces in the hydraulic couplings and hydraulic system. Additionally, the coupling members that remain subsea are subject to marine growth, sand, silt, and other mechanical impacts unless there is some form of protection. 
     To reduce or prevent damage to the coupling member remaining subsea, dummy coupling members have been used. A dummy coupling member mates with the opposing coupling member, but the dummy is not connected to hydraulic lines and therefore does not function to conduct hydraulic fluid through the system. Instead, the dummy coupling member protects the opposing coupling member when the hydraulic line through that coupling is not in use. One particular dummy hydraulic coupling member of the prior art is described in U.S. Pat. No. 6,631,734. 
     Typically, dummy undersea coupling members are the female coupling member because the male coupling member remains subsea. Dummy female coupling members may have one or more seals in a receiving chamber, and these seals engage the male member when the male member enters the receiving chamber. Dummy female coupling members also may have a bore and/or a vent passage extending between the receiving chamber and an outer surface of the dummy coupling member body. This allows trapped seawater and/or trapped air in the receiving chamber to escape out of the receiving chamber of the dummy coupling member when it engages the opposing coupling member. However, the bore or vent passage also may be subject to ingress of silt, debris, etc., with less effective protection of the opposing coupling member. 
     Unless the trapped seawater or air is allowed to escape from the receiving chamber, it may be very difficult or impossible to fully mate the dummy coupling to the opposing coupling member. Another undesirable consequence that may occur if trapped seawater or trapped air is not allowed to escape, is due to increased pressure that may force the poppet valve of the opposing coupling member open and allow the trapped seawater or trapped air to enter the hydraulic lines. Disconnecting the dummy coupling member from the opposing member also may be difficult due to hydraulic lock. 
     Thus, a dummy undersea hydraulic coupling member is needed to prevent debris and marine growth and other objects from damaging the coupling member remaining subsea, and which allows the dummy coupling member to be engaged and disengaged from the opposing member without resistance due to trapped seawater and/or trapped air. 
     U.S. Pat. No. 5,692,538 to Robert E. Smith III, assigned to National Coupling Company, Inc., does not show a dummy coupling member, but shows an undersea hydraulic coupling member having angled flow passages in the body of the male member to help prevent ingress of debris. When the female member is attached to the male member, hydraulic pressure through the angled flow ports and against the face of the poppet valve urges the poppet valve of the male member open to allow fluid to flow between the coupling members. The poppet valve in combination with the angled flow ports in the male member body help prevent ingress of debris, while allowing trapped hydraulic fluid pressure to bleed off when the coupling members are disconnected. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is embodied in a dummy undersea hydraulic coupling member having a water displacement expansion chamber and a piston for varying the volume of the water displacement expansion chamber in response to the pressures acting on the opposing faces of the piston. When the dummy coupling member is connected to an opposing coupling member subsea, seawater and/or air in the receiving chamber of the dummy is displaced by the opposing coupling member. That seawater and/or air enters the water displacement expansion chamber, and the piston allows the volume of that chamber to increase as a result of pressure from displaced seawater and/or air acting on the front face of the piston until the chamber reaches the volume required for a pressure equilibrium. 
     When the dummy coupling member is disconnected from the opposing coupling member subsea, seawater pressure acting on the back face of the piston tends to urge the piston to decrease the volume of the water displacement expansion chamber, thus allowing the trapped seawater and/or air to prevent a vacuum in the receiving chamber. The piston decreases the size of the water displacement expansion chamber until it reaches the volume required for a pressure equilibrium. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein. 
         FIG. 1  is a section view of a dummy female coupling member according to the prior art. 
         FIG. 2  is a section view of another dummy female coupling member according to the prior art. 
         FIG. 3  is a cross-sectional view of a dummy female coupling member according to a first embodiment of the invention. 
         FIG. 4  is a cross-sectional view of a dummy female coupling member according to a second embodiment of the invention. 
         FIG. 5  is a cross-sectional view of a dummy female coupling member according to a third embodiment of the invention. 
         FIG. 6  is a cross-sectional view of a dummy female coupling member according to a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention may best be understood by reference to the exemplary embodiments illustrated in  FIGS. 3 through 6 . An understanding of the invention will be aided by concurrent reference to the dummy couplings of the prior art illustrated in  FIGS. 1 and 2 . 
     Referring to  FIG. 1 , a dummy coupling member  10  according to the prior art has first end  12  and second end  13 , and receiving chamber  11  for receiving the opposing male coupling member therein. The male member (not shown) typically is positioned on the subsea floor and in some instances faces upwardly from the subsea floor so that the leading face of the male coupling member faces upwardly. The male coupling members are commonly attached to a manifold plate using various means, such as set screws or threads, and the female coupling members often are attached to an opposing manifold plate. Techniques for attaching hydraulic coupling members to manifold plates are well known to those skilled in the art. 
     The receiving chamber of the dummy female coupling member may have one or more seals therein for engaging and sealing with the male member. In the coupling of  FIG. 1 , the dummy female coupling member includes a pressure-energized annular metal seal  17  that is retained on shoulder surface  70  by retainer  15 . The retainer may be a sleeve-shaped body threaded to the female member. Or the retainer may be a two piece retainer with one piece having a sliding interfit with cylindrical wall  28  of the receiving chamber and inserted into the receiving chamber until it abuts internal shoulder  18 , and a second piece threaded to the wall of the receiving chamber. Elastomeric seal  16  may be positioned between the two pieces. The elastomeric seal has a dovetail interfit with the two-piece retainer. The receiving chamber terminates at internal shoulder  70 . 
     Adjacent the receiving chamber is the first section  14  and second section  27  of the water displacement expansion chamber. Although the water displacement expansion chamber in the coupling shown in  FIG. 1  has a first section with a smaller diameter and a second section with a larger diameter, the chamber may have a single uniform diameter, or more than two diameters. In the coupling of  FIG. 1 , the first section of the water displacement expansion chamber has inclined shoulder  19 . 
     In  FIG. 1 , piston  20  is positioned in the second section of the water displacement expansion chamber, and abuts internal shoulder  29  absent significantly higher water or air pressure acting on front face  21  of the piston. Spring  24  urges the piston against shoulder  29 . Spring  24  is anchored by collar  25  which may be threaded or otherwise engaged to the second section of the water displacement expansion chamber adjacent second end  13  of the dummy female coupling member. The collar is a sleeve-shaped member having internal bore  26  therethrough to allow entry of water into the second section of the water displacement expansion chamber and exert pressure against rear face  22  of the piston. The piston which slides longitudinally in the water displacement expansion chamber should provide a sliding seal with the walls of the chamber, and in the coupling of  FIG. 1 , annular seal  23  is shown to provide such a seal. 
     When the dummy coupling female coupling member is connected to an opposing coupling member subsea, water and/or air in receiving chamber  11  is forced from the receiving chamber into first section  14  of the water displacement expansion chamber. The additional water and/or air pressure acting on front face  21  of piston  20  will urge the piston towards second end  13 , expanding the water displacement expansion chamber until it has sufficient volume. The piston continues to expand the water displacement expansion chamber until the pressure of water and/or air acting on the front face of the piston is balanced to the pressure of seawater and spring  24  acting on rear face  22 . In one particular coupling, the diameter of the piston is the same or substantially the same as the diameter of the male member entering the receiving chamber, to balance the pressure during connection and disconnection of the male member with the dummy female coupling member. 
     When the dummy coupling member is disconnected from the opposing coupling member, a vacuum is created in the receiving chamber and the water and/or air in the water displacement expansion chamber will fill that void and avoid hydraulic lock. This results from seawater pressure acting on the rear face of the piston exceeding the pressure acting on its front face. The piston then moves toward the first end of the dummy coupling member, until the pressure is equalized. 
     Now referring to  FIG. 2 , a second coupling of the prior art is shown with dummy female coupling member  40  having first end  42 , second end  43  and receiving chamber  41  for connection to a male coupling member. Retainer  45  is engaged to the dummy female coupling member adjacent the first end for retaining annular metal seal  47  on shoulder  80 . Retainer  45  may be a one-piece or two piece retainer having a first piece in sliding engagement with receiving chamber wall  58  until it abuts shoulder  48 . Elastomeric seal  46  is held between the two pieces of the retainer. The receiving chamber terminates at shoulder  81 . 
     In the coupling of  FIG. 2 , poppet valve  60  is provided in first section  44  of the water displacement expansion chamber. The poppet valve is urged by valve spring  61  into the closed position against inclined shoulder  49 . The valve spring is held by collar  62  and collar clip  63  engages the wall of the first section of the water displacement expansion chamber. In this coupling, when the dummy female coupling member is connected to the male member, water and/or air in the receiving chamber exerts pressure against poppet valve  60  to urge the poppet valve open, entering the water displacement expansion chamber. 
     Still referring to  FIG. 2 , piston  50  slides in second section  57  of the water displacement expansion chamber. Spring  54  urges front face  51  of the piston towards shoulder  59 . The piston moves in sliding relationship with cylindrical wall  57  and has annular seal  53  to seal with the wall. The spring is anchored by collar  55  adjacent second end  43  of the dummy coupling member. Seawater entering through bore  56  in collar  55  acts against rear face  52  of the piston, until the pressure on each side of the piston is balanced. 
     An improved dummy female hydraulic coupling according to one embodiment of the invention is shown in  FIG. 3 . Receiving chamber  314  has crown-type probe seal  310  contained in seal cartridge  308 . A crown-type probe seal of the type illustrated is disclosed in U.S. Pat. No. 6,575,430. A seal cartridge of the type illustrated at  308  is disclosed in U.S. Pat. No. 7,163,190. Seal cartridge  308  is comprised of shell  322  and retainer  320  which have a sliding interference fit at  324 . Retainer  320  is externally threaded for engaging an internally threaded portion of central axial bore  388  proximate first end  370  of coupling body  340 . Retainer  320  may have spanner holes  328  for engagement with a tool for inserting and removing seal cartridge  308 . 
     Retainer  320  and shell  322  have angled shoulders on their inner, cylindrical surfaces configured to engage corresponding surfaces on the ends of ring-shaped probe seal  310 . The outer circumference of probe seal  310  may have one or more grooves for holding O-ring seals  326  which may act to seal between probe seal  310  and shell  322 . 
     Seal cartridge  308  retains metal C-seal  312  on shoulder  332  within bore  388  of coupling member  300 . A metal C-seal of the type illustrated is disclosed in U.S. Pat. No. 6,962,347. O-ring  318  in a groove on shoulder  330  provides a seal between seal cartridge  308  and body  340  of coupling member  300 . 
     Piston  350  is provided in a portion of bore  388  (which may act as a cylinder within which piston  350  slides). Piston  350  has a first end  352  oriented towards first end  370  of coupling body  340  and an opposing second end  354  oriented towards second end  372  of coupling body  340 . 
     First end  352  of piston  350  may have a frusto-conical surface  336  which bears against a corresponding frusto-conical surface  334  on the inner surface of bore  388  to provide a metal-to-metal seal. 
     Second end  354  of piston  350  comprises a portion of reduced outer diameter  356  which forms shoulder  348  in the body of piston  350 . Frusto-conical surface  360  may be provided in portion  356  proximate second end  354 . 
     Compression spring  358  bears against shoulder  348  at one end thereof and against spring seat  303  at a second end thereof. Spring seat  303  may be externally threaded and engage an internally-threaded portion of bore  388  proximate second end  372 . A groove  366  in the external, cylindrical surface of spring seat  303  may contain O-ring seal  368 . 
     Spring seat  303  has central axial bore  362  which has frusto-conical surface  316  at one end and hexagonal wrench receiver  364  at an opposing end thereof. Frusto-conical surface  316  may be sized and configured to bear against surface  360  on piston  350  when it is at one extreme of its travel (downward in  FIG. 3 ) thereby providing a metal-to-metal seal between piston  350  and spring seat  303 . 
     In the absence of differential fluid pressure, spring  358  acting between spring seat  303  and shoulder  348  on piston  350  urges piston  350  against surface  334 . 
     Annular groove  342  in the outer circumference of piston  350  contains the primary seal which may comprise O-ring  306  flanked by a pair of back up rings  304 . 
     Annular groove  344  in the outer circumference of piston  350  contains the secondary seal which may comprise O-ring  346 . Secondary seal  346  acts to prevent seawater from contacting the portion of bore  388  indicated at  302  thereby providing an uncontaminated surface for primary seal  306  to seal against. It will be appreciated by those skilled in the art that the axial distance between primary seal  306  and secondary seal  346  should not be less than the full stroke of piston  350  in order to ensure that primary seal  306  contacts only protected portion  302  of bore  388  during a full stroke of piston  350 . 
     In the embodiment illustrated in  FIG. 3 , the piston is moveable within the bore of the coupling member. Frusto-conical surfaces provide metal-to-metal seals, as indicated, at opposing ends of the piston. The spring ( 358 ) urges the piston upward in  FIG. 3 . Under the influence of hydraulic fluid pressure from the probe of a male coupling member (not shown) inserted in receiving chamber  314 , the piston may move downward in  FIG. 3  and establish a metal-to-metal seal at frusto-conical seat  316  on the bottom seat. However, fluid is also retained by the main seal which, in the illustrated embodiment, is comprised of O-ring seal  306  flanked by a pair of back up rings  304 . 
     In the dummy couplings of the prior art, the portion of the bore indicated at  27  in  FIG. 1  and at  57  in  FIG. 2  is exposed to seawater when the coupling is used subsea. It has been found that calciferous deposits may form on this surface which deposits may compromise the sealing ability of the main seal. This effect may occur especially when the main seal slides along the inner bore when the piston moves in response to hydraulic pressure within the coupling. In the improved dummy coupling of the present invention, the hydrostatic pressure seal ( 346 ) keeps seawater from entering the portion of the bore indicated at  302  thereby avoiding the build-up of mineral deposits and marine growth. In the illustrated embodiment of  FIG. 3 , this hydrostatic pressure seal is an O-ring seal ( 346 ) retained in a circumferential groove ( 344 ) on the outer circumference of the piston. 
     A second embodiment of the invention is illustrated in  FIG. 4 . Dummy female hydraulic coupling member  400  comprises generally cylindrical body  440  having central axial bore  488 . The outer diameter of body  440  may be stepped, forming external shoulder  490 . Portion  484  of the external surface of tail section  486  of coupling body  440  may be threaded for attachment to a manifold plate or the like (not shown). Wrench flat  482  may be provided in order to engage a tool to draw shoulder  490  of coupling body  440  tightly against such a mounting device. 
     Coupling body  440  may have an internally threaded section of bore  488  proximate first end  470 . Coupling  400  may comprise a seal cartridge in receiving chamber  414  of the type described above in connection with coupling  300 . 
     Piston  450  is provided in a portion of bore  488  (which may act as a cylinder within which piston  450  slides). Piston  450  has a first end  452  oriented towards first end  470  of coupling body  440  and an opposing second end  454  oriented towards second end  472  of coupling body  440 . 
     First end  452  of piston  450  may have a ring-shaped edge  416  which bears against frusto-conical surface  434  on the inner surface of bore  488  to provide a metal-to-metal seal. 
     Circumferential groove  442  is provided on the exterior surface of piston  450  for containing primary seal  408  (which may be an O-ring seal) and back-up rings  410 . Other kinds of bi-directional soft seals may be used. 
     Second end  454  of piston  450  comprises a portion of reduced outer diameter  456  which forms shoulder  448  on the body of piston  450 . Circumferential recess  444  is provided on the exterior of piston  450  adjacent shoulder  448 . Washer  480  rests on shoulder  448  and together with recess  444  forms a circumferential groove for holding secondary seal  404 . Secondary seal  404  may provide a hydrostatic seal. 
     Compression spring  458  bears against washer  480  at one end thereof and against spring seat  403  at a second end thereof. Spring seat  403  has a shoulder  474  on one end thereof and is retained in bore  488  by a retainer ring  476  in groove  402  in the wall of bore  488 . The retainer ring may be a circlip, C-clip, snap ring, or the like. Spring seat  403  has central axial bore  462  for the passage of fluid into and out of the portion of bore  488  between secondary seal  404  and spring seat  403 . 
     In the absence of differential fluid pressure, spring  458  acting between spring seat  403  and washer  480  on shoulder  448  of piston  450  urges piston  450  against frusto-conical surface  434  forming a metal-to-metal seal. 
     Recess  444  in the outer circumference of piston  450  contains the secondary seal which may comprise scrapper seal  404 . Secondary seal  404  acts to prevent seawater from contacting the portion of bore  488  between primary seal  408  and secondary seal  404  thereby providing an uncontaminated surface for primary seal  408  to seal against. It will be appreciated by those skilled in the art that the axial distance between primary seal  408  and secondary seal  404  should not be less than the full stroke of piston  450  in order to ensure that primary seal  408  contacts only the protected portion of bore  488  during a full stroke of piston  450 . 
     A portion of reduced outside diameter  478  may be provided in piston body  450  between primary seal  408  and secondary seal  404 . 
     In the embodiment of  FIG. 4 , the hydrostatic pressure seal is a “scraper seal” comprised of a notched elastomer body  404  with a V-shaped metal insert  406  which biases the legs of the elastomer seal in opposing radial directions to enhance the sealing effectiveness of the scraper seal. The V-shaped notch in seal  404  may also act to provide a pressure-energized seal which responds to hydrostatic pressure when the seal is used subsea. 
     A third embodiment of the invention is shown in  FIG. 5 . This embodiment is generally similar to that shown as coupling  400  in  FIG. 4 . 
     Dummy female hydraulic coupling member  500  comprises generally cylindrical body  540  having central axial bore  588 . The outer diameter of body  540  may be stepped, forming external shoulder  590 . Portion  584  of the external surface of tail section  586  of coupling body  540  may be threaded for attachment to a manifold plate or the like (not shown). Wrench flat  582  may be provided in order to engage a tool to draw shoulder  590  of coupling body  540  tightly against such a mounting device. 
     Coupling body  540  may have an internally threaded section of bore  588  proximate first end  570 . Coupling  500  may comprise a seal cartridge in receiving chamber  514  of the type described above in connection with coupling  300 . 
     Piston  550  is provided in a portion of bore  588  (which may act as a cylinder within which piston  550  slides). Piston  550  has a first end  552  oriented towards first end  570  of coupling body  540  and an opposing second end  554  oriented towards second end  572  of coupling body  540 . 
     First end  552  of piston  550  may have a ring-shaped edge  516  which bears against frusto-conical surface  534  on the inner surface of bore  588  to provide a metal-to-metal seal. 
     Annular groove  542  in the outer circumference of piston  550  contains the primary seal. The primary seal in this embodiment comprises T-seal  508  flanked by a pair of backup rings  510 . Backup rings  510  may be fabricated of metal or an engineering plastic such as PEEK, DELRIN® acetal, TEFLON® polytetrafluoroethylene, or the like. A metal-to-metal seal may be provided at  516 . 
     Second end  554  of piston  550  comprises a portion of reduced outer diameter  556  which forms shoulder  548  on the body of piston  550 . Circumferential recess  544  is provided on the exterior of piston  550  adjacent shoulder  548 . Washer  580  rests on shoulder  548  and together with recess  544  forms a circumferential groove for holding secondary seal  504 . Secondary seal  504  may be a hydrostatic seal. 
     Compression spring  558  bears against washer  580  at one end thereof and against spring seat  503  at a second end thereof. Spring seat  503  has a shoulder  574  on one end thereof and is retained in bore  588  by a retainer ring  576  in groove  502  in the wall of bore  588 . The retainer ring may be a circlip, C-clip, snap ring, or the like. Spring seat  503  has central axial bore  562  for the passage of fluid into and out of the portion of bore  588  between secondary seal  504  and spring seat  503 . 
     In the absence of differential fluid pressure, spring  558  acting between spring seat  503  and washer  580  on shoulder  548  of piston  550  urges piston  550  against frusto-conical surface  534  forming a metal-to-metal seal. 
     Recess  544  in the outer circumference of piston  550  contains the secondary seal which may comprise scrapper seal  504 . Secondary seal  504  acts to prevent seawater from contacting the portion of bore  588  between primary seal  508  and secondary seal  504  thereby providing an uncontaminated surface for primary seal  508  to seal against. It will be appreciated by those skilled in the art that the axial distance between primary seal  508  and secondary seal  504  should not be less than the full stroke of piston  550  in order to ensure that primary seal  508  contacts only the protected portion of bore  588  during a full stroke of piston  550 . 
     A portion of reduced outside diameter  578  may be provided in piston body  550  between primary seal  508  and secondary seal  504 . 
     In the embodiment of  FIG. 5 , the hydrostatic pressure seal is a “scraper seal” comprised of a notched elastomer body  504  with a V-shaped metal insert  506  which biases the legs of the elastomer seal in opposing radial directions to enhance the sealing effectiveness of the scraper seal. The V-shaped notch in seal  504  may also act to provide a pressure-energized seal which responds to hydrostatic pressure when the seal is used subsea. 
     A fourth embodiment of the invention is shown in  FIG. 6 . This embodiment has the same general configuration, seal cartridge, primary seal and hydrostatic pressure seal as the embodiment illustrated in  FIG. 3 . However, in this embodiment, the metal-to-metal seals at either end of the piston are replaced with poppet-type seals. The poppet seals are comprised of a polymer ring  603 ,  699  retained by a metal insert  605 ,  695 . Poppet-type seals of different sizes may be provided on either end of the piston. As shown in  FIG. 6 , the poppet seal on the lower end of the piston (as oriented in  FIG. 6 ) may seal against frusto-conical surface  616  on the spring seat. The poppet seals may provide enhanced sealing of fluid pressure within the system. 
     An improved dummy female hydraulic coupling  600  according to this fourth embodiment of the invention is shown in  FIG. 6 . Receiving chamber  614  has crown-type probe seal  610  contained in seal cartridge  608  which may be the same as seal cartridge  308  described above. 
     Seal cartridge  608  retains metal C-seal  612  on a shoulder within bore  688  of coupling body  640 . A metal C-seal of the type illustrated is disclosed in U.S. Pat. No. 6,962,347. O-ring  618  in an adjacent groove on this shoulder provides a seal between seal cartridge  608  and body  640  of coupling member  600 . 
     Piston  650  is provided in a portion of bore  688  (which may act as a cylinder within which piston  650  slides). Piston  650  has a first end  652  oriented towards first end  670  of coupling body  640  and an opposing second end  654  oriented towards second end  672  of coupling body  640 . 
     First end  652  of piston  650  may have an internally threaded recess for engaging externally threaded stem  692  on poppet seal retainer  605 . Poppet seal retainer  605  has shoulder  694  for retaining polymer ring seal  603  against first end  652  of piston  650 . Seal  603  bears against frusto-conical surface  634  on the inner surface of bore  688 . Hexagonal recess  693  may be provided for engaging a tool (e.g., an Allen wrench) for installing and/or removing seal retainer  605 . 
     Second end  654  of piston  650  comprises a portion of reduced outer diameter  656  which forms shoulder  648  in the body of piston  650 . Frusto-conical surface  660  may be provided in portion  656  proximate second end  654 . 
     A second poppet-type seal may be provided on the second end  654  of piston  650 . Second end  654  of piston  650  may have an internally threaded recess for engaging externally threaded stem  696  on poppet seal retainer  695 . Poppet seal retainer  695  has shoulder  698  for retaining polymer ring seal  699  against second end  654  of piston  650 . Seal  699  bears against frusto-conical surface  616  on the inner surface of bore  662  of spring seat  603 . Hexagonal recess  697  may be provided for engaging a tool (e.g., an Allen wrench) for installing and/or removing seal retainer  695 . 
     As shown in  FIG. 6 , seal retainer  695  may be of a different size than seal retainer  605 . 
     Compression spring  658  bears against shoulder  648  at one end thereof and against spring seat  603  at a second end thereof. Spring seat  603  may be externally threaded and engage an internally-threaded portion of bore  688  proximate second end  672 . A groove  666  in the external, cylindrical surface of spring seat  603  may contain O-ring seal  668 . 
     Spring seat  603  has central axial bore  662  which has frusto-conical surface  616  at one end and hexagonal wrench receiver  664  at an opposing end thereof. Frusto-conical surface  616  may be sized and configured to bear against poppet seal  699  on piston  650  when it is at one extreme of its travel (downward in  FIG. 6 ) thereby providing a seal between piston  650  and spring seat  603 . 
     In the absence of differential fluid pressure, spring  658  acting between spring seat  603  and shoulder  648  on piston  650  urges piston  650  against surface  634 . 
     Annular groove  642  in the outer circumference of piston  650  contains the primary seal which may comprise O-ring  606  flanked by a pair of back up rings  604 . 
     Annular groove  644  in the outer circumference of piston  650  contains the secondary seal which may comprise O-ring  646 . Secondary seal  646  acts to prevent seawater from contacting the portion of bore  688  indicated at  602  thereby providing an uncontaminated surface for primary seal  606  to seal against. It will be appreciated by those skilled in the art that the axial distance between primary seal  606  and secondary seal  646  should not be less than the full stroke of piston  650  in order to ensure that primary seal  606  contacts only protected portion  602  of bore  688  during a full stroke of piston  650 . 
     Under the influence of hydraulic fluid pressure from the probe of a male coupling member (not shown) inserted in receiving chamber  614 , the piston may move downward in  FIG. 6  and establish a seal at frusto-conical seat  616  on the bottom seat. However, fluid displaced from receiving chamber  614  is retained by the main seal which, in the illustrated embodiment, is comprised of O-ring seal  606  flanked by a pair of back up rings  604 . 
     In the dummy couplings of the prior art, the portion of the bore indicated at  27  in  FIG. 1  and at  57  in  FIG. 2  is exposed to seawater when the coupling is used subsea. It has been found that calciferous deposits may form on this surface which deposits may compromise the sealing ability of the main seal. This effect may occur especially when the main seal slides along the inner bore when the piston moves in response to hydraulic pressure within the coupling. In an improved dummy coupling according to the present invention, the hydrostatic pressure seal keeps seawater from entering the portion of the bore indicated at  602  thereby avoiding the build-up of mineral deposits and marine growth. In the illustrated embodiment of  FIG. 6 , this hydrostatic pressure seal is an O-ring seal ( 646 ) retained in a circumferential groove ( 644 ) on the outer circumference of the piston. 
     Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.