Abstract:
A method of installing a socket with a socket contact on an underwater plug with a plug contact is provided so as to establish conductive contact between the socket contact and the plug contact. The socket is disconnectably attached to a recoverable fluid exchange unit and the socket engages with the plug to establish the conductive contact between the socket contact and plug contact. The recoverable fluid exchange unit is then operated to substantially replace a first fluid within the socket with a second fluid from the recoverable fluid exchange unit.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to electrical connectors and particularly to electrical connectors for use underwater. 
   A known electrical connector for making underwater connections is described in patent WO89/08934. The connector includes a plug with an electrical contact pin and a socket adapted to receive the contact pin and which contains a socket contact for electrical engagement with the contact pin of the plug. The socket forms part of a socket module which includes a chamber filled with gas at a lower pressure than that of liquid in the socket. When the plug engages the socket valve means permit substantially all of the liquid in the socket to be exchanged for gas from the chamber. The chamber and valve means remain connected to the engaged plug and socket and are accordingly not available for use in establishing a further electrical connection. Furthermore, should any maintenance of the valve means, chamber or any other part of the complex and expensive equipment associated with the socket become necessary, disconnection of the socket from the plug will be necessary in order that such equipment can be returned to the surface for the necessary maintenance. 
   An object of the invention is to overcome at least some of the disadvantages associated with such prior art electrical connectors. 
   SUMMARY OF THE INVENTION 
   Thus according to a first aspect of the present invention there is provided a method of installing a socket with a socket contact on an underwater plug with a plug contact so as to establish conductive contact between the socket contact and the plug contact, characterized by the steps of:
         (a) providing a recoverable fluid exchange unit;   (b) engaging the socket with the plug and establishing the conductive contact between the socket and plug contacts;   (c) operating the fluid exchange unit to substantially replace a first fluid within the socket with a second fluid from the fluid exchange unit, wherein the recoverable fluid exchange unit is connected to the socket before or after step (b); and   (d) disconnecting the fluid exchange unit from the socket and recovering it.       

   It may be desirable for the step of substantially replacing the first fluid within the socket to include discharging the first fluid exteriorly of the fluid exchange unit and the socket. 
   The method may include the step of supplying a flushing fluid to the plug after the first fluid within the socket has been substantially removed therefrom. This allows the socket to be flushed clean whilst underwater. The flushing fluid may be forced from a chamber of the fluid exchange unit into the socket by ambient pressure. The ambient pressure may act on at least a flexible portion of a wall of the flushing fluid chamber. It may be desirable to include the step of substantially replacing the removed first fluid with the second fluid before supplying the flushing fluid to the plug. The flushing fluid may be substantially removed from the socket and subsequently charging the socket with the second fluid. The second fluid may be accommodated in a pressure vessel in the fluid exchange unit. It may be delivered to the socket as a consequence of the first fluid being drawn out of the socket. 
   At least one of the flows of fluid to or from the socket may be effected by at least one positive displacement device of the fluid exchange unit. The or each positive displacement device may comprise a positive displacement pump. There could be simultaneous exchange of fluids between the socket and a fluid storage region of the fluid exchange unit wherein one positive displacement device may be used to force one fluid into the socket and simultaneously draw a second fluid therefrom. At least one of the positive displacement devices could be connected by ducts and valve means so that movement of a displaceable member thereof acts to force a second fluid from one part of the device to the socket and simultaneously draw a first fluid from the socket into a second part thereof. The or each positive displacement device may comprise a piston and cylinder device activated by an actuator. Each actuator could comprise a pump which is selectively connectable to pressurized actuator fluid on a first or second side of an actuator piston slidable in an actuator cylinder. Alternatively, the or each actuator may comprise mechanical and/or electrical means. 
   The step of substantially replacing the first fluid within the socket could include transferring the first fluid from the socket to a fluid storage region of the fluid exchange unit. Thus, if the first fluid is considered harmful to the environment surrounding the fluid exchange unit and socket it would not need to be discharged into it. 
   The step of engaging the socket with the plug may include venting the socket exteriorly of the fluid exchange unit to permit part of the first fluid in the socket displaced by entry of the plug thereinto to be discharged exteriorly of the fluid exchange unit. Alternatively, the socket may be connected to a compensator of the fluid exchange unit into which a part of the first fluid displaced by entry of the plug thereinto flows thus preventing the first fluid being discharged into the environment surrounding the fluid exchange unit and socket. 
   The step of replacing the first fluid in the socket with the second fluid from the fluid exchange unit could include the steps of exchanging the first fluid in the socket with a flushing fluid into the fluid exchange unit; and subsequently exchanging the flushing fluid in the socket with the second fluid from the fluid exchange unit. This allows the socket to be flushed clean whilst underwater. It may be convenient for the step of substantially replacing the first fluid within the socket with the second fluid from the fluid exchange unit to cause the socket to be pressure sealed from the environment surrounding the socket. 
   The flow of fluids between the socket and the fluid exchange unit may be controlled by valve means of the fluid exchange unit. The valve means could comprise a plurality of spool valves. 
   The step of disconnecting the fluid exchange unit from the socket may include disconnecting one or more stab connectors between the fluid exchange unit and the socket each of which has male and female parts which are disengageable by pulling the fluid exchange unit away from the socket. The or each stab connector may comprise at least part of a separable fluid connection interconnecting the fluid exchange unit and the socket. There are preferably two separable fluid connections. The socket portions of the two connections are preferably in fluid communication with an interior chamber of the socket substantially at opposite ends thereof. 
   According to a second aspect of the present invention there is provided a method of retrieving a socket from an underwater plug comprising a reversal of the steps set out in the method described above. The retrieved socket may be reused for connection to another plug. The fluid exchange unit may be reused to replace a first fluid in another socket with a second fluid from the fluid exchange unit. 
   According to a third aspect of the invention there is provided an apparatus including a fluid exchange unit for effecting installation of a socket with a socket contact on an underwater plug with a plug contact so as to establish conductive contact between the socket contact and the plug contact, characterized in that the fluid exchange unit is adapted to be connected to the socket and is recoverable, the fluid exchange unit comprising means to substantially replace a first fluid within the socket with a second fluid from the fluid exchange unit, the fluid exchange unit being connected to the socket before or after the installation of the socket on the plug, and the apparatus comprising means for disconnecting the fluid exchange unit from the socket. 
   The fluid exchange unit could include a reservoir of flushing fluid and means for flowing the flushing fluid into the socket. 
   The socket may include means for spraying the flushing fluid inside the socket, said spraying means being adapted to spray the flushing fluid over a plug installed in the socket. 
   Mechanical securing means for securing the socket to the fluid exchange unit may be included. Means may be included for remotely selectively engaging and disengaging the mechanical securing means. 
   The fluid exchange unit may include means for remotely actuating valve means for controlling fluid flow to and/or from the socket. 
   The apparatus of the present invention advantageously makes use of non-specialized components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Four embodiments of the present invention will now be described by way of example with reference to the accompanying figures, in which:— 
       FIG. 1  is a detailed schematic view of an electrical connector and plug of a first embodiment of the invention; 
       FIGS. 2(   a ) to  2 ( f ) are schematic views of the first embodiment of the invention showing the installation of the socket of the connector on the plug; 
       FIGS. 3(   a ) to  3 ( g ) are schematic views of the first embodiment of the invention showing the retrieval of the socket from the plug; 
       FIG. 4  is a detailed schematic view of an electrical connector and plug of a second embodiment of the invention; 
       FIGS. 5(   a ) to  5 ( f ) are schematic views of the second embodiment of the invention showing the installation of the socket of the connector on the plug; 
       FIGS. 6(   a ) to  6 ( g ) are schematic views of the second embodiment of the invention showing the retrieval of the socket from the plug; 
       FIG. 7  is a detailed schematic view of an electrical connector and plug of a third embodiment of the invention; 
       FIGS. 8(   a ) to  8 ( g ) are schematic views of the third embodiment of the invention showing the installation of the socket of the connector on the plug; 
       FIGS. 9(   a ) to  9 ( g ) are schematic views of the third embodiment of the invention showing the retrieval of the socket from the plug; 
       FIG. 10  is a detailed schematic view of an electrical connector and plug of a fourth embodiment of the invention; and 
       FIGS. 11(   a ) to  11 ( g ) are schematic views of the fourth embodiment of the invention showing the installation of the socket of the connector on the plug. 
   

   The embodiments are the same except where noted. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , an electrical connector  10  according to a first embodiment of the invention is shown. The connector comprises an external fluid exchange unit  20  with an attached socket  30 . Below the connector is shown a plug  40 . 
   The socket  30  comprises a chamber  31 . A first stab connector  32  connects the top of the chamber to the external fluid exchange unit  20  above and a second stab connector  33  connects the bottom of the chamber to the fluid exchange unit. Inside the chamber are electrical contacts  36  and connected to the socket is at least one electrical cable  35 . The plug  40  can enter into the chamber via an aperture surrounded by an “O” ring  34  at the base of the chamber. 
   The outside of the plug  40  is insulated except for where there are electrical contacts  41  for coupling with the electrical contacts  36  of the socket  30 . 
   Inside the fluid exchange unit  20  is a fluid container  50  that is connected to the socket  30  via first and second hydraulic valves  60 , 70 . There are two ports  52 , 54  on one side of the fluid container with the first port  52  located proximate the top of the container and the second port  54  located proximate the bottom of the container. A first container socket conduit  62  connects the first port to the first stab connector  32  of the socket via the first hydraulic valve  60  and a second container socket conduit  64  connects the second port to the second stab connector  33  of the socket via the second hydraulic valve  70 . An ambient conduit  72  is also connected to the second hydraulic valve. The ambient conduit allows the socket to be connected to the fluid surrounding the fluid exchange unit via the part of the second container socket conduit  64  between the second stab connector  33  and the second hydraulic valve  70 . 
   Above the fluid container  50  is an actuator cylinder  90 . Contained within the fluid container is a container piston  91  and contained within the actuator cylinder is an actuator piston  92 . The container piston and the actuator piston are interconnected by a connecting rod  93 . There are two ports  94 , 95  on one side of the actuator cylinder with the first port  94  located proximate the top of the cylinder and the second port  95  located proximate the bottom of the cylinder. The actuator cylinder  90  is connected to a conventional pump  100  via an actuating valve  110  and a pump valve conduit  102  connecting the pump to the actuating valve. A first valve actuator conduit  96  connects the actuating valve to the first port of the actuator cylinder and a second valve actuator conduit  97  connects the actuating valve to the second port of the actuator cylinder. 
   Referring to  FIGS. 2(   a ) to  2 ( f ), and additionally to  FIG. 1 , the process of installing the socket  30  on the plug  40  in seawater  45  will be described. 
   [ FIG. 2(   a )] The fluid exchange unit  20  and socket  30  is moved towards the plug  40 . Inside the socket is seawater  130  that is at ambient pressure. Inside the container  50 , the container piston  91  is positioned slightly above the second port  54 . Above the container piston is a gas, preferably air  120 , at a pressure of 10 5  Pa (1 bar). The first and second hydraulic valves  60 , 70  are initially configured to close first and second container socket conduits  62 , 64  and thus isolate the container  50 . However, the second valve  70  is also configured to connect the socket to the ambient conduit  72 . The actuating valve  110  is configured to close the valve actuator conduits  96 , 97  and hence isolate the actuator cylinder  90  from the pump  100 . 
   [ FIG. 2(   b )] As the plug  40  enters the socket  30  via the “O” ring  34 , seawater  130  is displaced from inside the socket into the sea surrounding the fluid exchange unit  20  via the part of the second container socket conduit  64  between the second stab connector  33  of the socket and the second hydraulic valve  70  and via the ambient conduit  72 . This maintains the remaining seawater in the socket at ambient pressure. The electrical contacts  41  of the plug become coupled to the electrical contacts  36  of the socket once the plug has been fully inserted into the socket. However, no power is as yet supplied to this connection. 
   [ FIG. 2(   c )] The hydraulic valves  60 , 70  are then reconfigured by conventional means (not shown) to connect the socket to the fluid container  50  via first and second container socket conduits  62 , 64 , the second hydraulic valve  70  closing the ambient conduit  72 . The seawater  130  in the socket  30  is thus connected to the fluid container  50  where air  120  is contained at a pressure of 10 5  Pa (1 bar), hence the seawater is now at a pressure of 10 5  Pa (1 bar). The seawater in the socket is accordingly sealed from the surrounding seawater at ambient pressure as there is a pressure difference across the “O” ring  34 . 
   [ FIG. 2(   d )] The actuating valve  110  is reconfigured in a conventional manner to connect the pump  100  to the actuator cylinder  90  via the pump valve conduit  102  and the second valve actuator conduit  97 . The pump forces a pressurized liquid into the actuator cylinder via the second port  95  that is below the actuator piston  92 , causing the piston to rise towards the top of the actuator cylinder  90 . Liquid in the part of the actuator cylinder above the actuator piston is expelled in a known manner via the first valve actuator conduit  96  and the actuating valve. The movement of the actuator piston  92  causes the container piston  91 , that is connected to the actuator piston by the connecting rod  93 , to rise forcing air  120  stored in the fluid container  50  into the socket  30  via the first container socket conduit  62 . The air enters the socket  30  via the first stab connector  32  at the top of the socket. This forces the seawater  130  out of the socket via the second stab connector  33  which is connected to the base of the socket and into the fluid container  50  via the second container socket conduit  64 . 
   [ FIG. 2(   e )] The actuating valve  110  is reconfigured to isolate the pump  100  from the actuator cylinder  90 . Then the hydraulic valves  60 , 70  are reconfigured to close the container socket conduits  62 , 64  between the socket  30  and the fluid container  50 , isolating the socket from the fluid exchange unit  20 . 
   [ FIG. 2(   f )] The stab connectors  32 , 33  disengage from the container socket conduits  62 , 64  of the fluid exchange unit  20  as the fluid exchange unit is lifted away from the socket  30 . Power can now be applied to the electrical coupling between the plug  40  and socket in a known manner, the air in the socket being at a pressure of 10 5  Pa (1 bar). 
   Referring to  FIGS. 3(   a ) to  3 ( g ), and additionally to  FIG. 1 , the process of retrieving the socket  30  from the plug  40  will be described. 
   [ FIG. 3(   a )] Power is switched off to the electrical coupling between the plug  40  and socket  30 . The fluid exchange unit  20  is moved towards the socket and plug. The air  120  in the socket is at a pressure of 10 5  Pa (1 bar) and the container  50  is substantially filled with water  130  at a pressure of 10 5  Pa (1 bar). 
   [ FIG. 3(   b )] The fluid exchange unit  20  connects with the socket  30  as the first and second container socket conduits  62 , 64  engage first and second stab connectors  32 , 33  respectively. 
   [ FIG. 3(   c )] The hydraulic valves  60 , 70  are reconfigured to connect the socket  30  to the fluid container  50  via first and second container socket conduits  62 , 64 . 
   [ FIG. 3(   d )] The actuating valve  110  is reconfigured to connect the pump  100  to the actuator cylinder  90  via the pump valve conduit  102  and the first valve actuator conduit  96 . The pump forces pressurized liquid into the actuator cylinder via the first port  94 , causing the actuator piston  92  to be pushed towards the base of the actuator cylinder  90 . Liquid in the part of the actuator cylinder below the actuator piston is expelled via the second valve actuator conduit  97  and the actuating valve. The movement of the actuator piston pushes down the connected container piston  91 , forcing seawater  130  stored in the fluid container  50  into the socket  30  via the second container socket conduit  64 . This forces air  120  out of the socket and into the container  50  via the first container socket conduit  62 . 
   [ FIG. 3(   e )] The hydraulic valves  60 , 70  are reconfigured to close the container socket conduits  62 , 64  between the socket  30  and the fluid container  50 , isolating the socket from the fluid exchange unit  20 . The actuating valve  110  is reconfigured to isolate the pump  100  from the actuator cylinder  90 . 
   [ FIG. 3(   f )] The second hydraulic valve  70  is reconfigured to connect the socket  30  to the ambient conduit  72  via the part of the second container socket conduit  64  between the second stab connector  33  of the socket and the second hydraulic valve  70 . This balances the pressure of the seawater  130  in the socket with the surrounding seawater. Thus, the seawater in the socket is now at ambient pressure and is accordingly no longer sealed from the surrounding seawater as there is no longer a pressure difference across the “O” ring  34 . 
   [ FIG. 3(   g )] The fluid exchange unit  20  moves away with the retrieved socket  30  from the plug  40 . The surrounding seawater is drawn into the socket as this occurs. 
   A second embodiment of the invention will now be described with reference to  FIGS. 4 to 6(   g ). Where a part in the first embodiment has a reference numeral and there is a substantially corresponding part in the second embodiment the same reference numeral has been used except that the number two hundred has been added to the latter. Not all such parts will be described in detail. 
   Referring to  FIG. 4 , an electrical connector  210  according to a second embodiment of the invention is shown. The connector comprises an external fluid exchange unit  220  with an attached socket  230 . Below the connector is shown a plug  240 . Both the socket and plug are the same as that described in the first embodiment except that the socket additionally has a hinged flat plate seal  237  situated above the “O” ring seal  234 . The socket and plug will have mateable contacts  236  and  241  respectively. 
   Inside the fluid exchange unit  220  is a fluid container  250  that is connected to the socket  230  via first and second hydraulic valves  260 , 270  and a compensator cylinder  350  that is connected to the socket via the second hydraulic valve  270 . There is a first port  252  on one side of the fluid container located proximate the top of the container. A first container socket conduit  262  connects the port to a first stab connector  232  of the socket via the first hydraulic valve  260  and a second container socket conduit  264  connects the base of the fluid container to a second stab connector  233  of the socket via the second hydraulic valve  270 . 
   The compensator cylinder  350  is open at its top end and has a compensator piston  352  below which liquid can be stored. The piston is free to move in a direction substantially perpendicular to a central axis of the cylinder. Returns  354  at the top of the compensator cylinder retain the piston. The base of the compensator cylinder  350  is connected to the second hydraulic valve  270  via a compensator conduit  356 . 
   Above the fluid container  250  are an actuator cylinder  290 , a pump  300  and an actuating valve  310 . These are the same as those described in the first embodiment and are connected in the same way. 
   Referring to  FIGS. 5(   a ) to  5 ( f ), and additionally to  FIG. 4 , the process of installing the socket  230  on the plug  240  in seawater  245  will be described. 
   [ FIG. 5(   a )] The fluid exchange unit  220  and socket  230  is moved towards the plug  240 . Oil  340  initially fills the socket to protect the electrical contacts in the chamber  231  and keeps them clean although other fluids could be used. The space below the compensator piston  352  is also filled with oil. The oil is sealed from the seawater surrounding the socket by the hinged flat plate seal  237  being in its closed position over the “O” ring  234 . Inside the container  250 , the container piston  291  is positioned slightly above the base of the container. Above the container piston  291  is a gas, preferably air  320 , at a pressure of 10 5  Pa (1 bar). The first and second hydraulic valves  260 , 270  are initially configured to close first and second container socket conduits  262 , 264  to isolate the socket  230  from the container  250 . The compensator cylinder  350  contains oil below the compensator piston  352 . As the compensator piston  352  is free to move in a direction substantially perpendicular to the axis of the cylinder, the oil below the compensator piston is at ambient pressure. The second valve  270  is also configured to connect the socket to the compensator cylinder via the part of the second container socket conduit between the second stab connector  233  and the second hydraulic valve and via the compensator conduit  356 . Thus the oil inside the socket is pressure balanced with the oil inside the compensator cylinder and so is also at ambient pressure. The actuating valve  310  is reconfigured to close the valve actuator conduits  296 , 297  and hence isolate the actuator cylinder  290  from the pump  300 . 
   [ FIG. 5(   b )] As the plug  240  enters the socket  230  via the “O” ring  234 , the seal plate  237  is pushed open and leans on the inserted plug. Oil  340  is displaced from inside the socket  230  by the plug and into the compensator cylinder  350  via the part of the second container socket conduit  264  between the second stab connector  233  of the socket and the second hydraulic valve  270  and via the compensator conduit  356 . This increases the amount of oil beneath the compensator piston  352  in the compensator cylinder, raising the piston and thus maintaining the oil remaining in the socket  230  at ambient pressure. The electrical contacts  241  of the plug  240  become coupled to the electrical contacts  236  of the socket  230  once the plug has been fully inserted into the socket. However, no power is as yet supplied to this connection. 
   [ FIG. 5(   c )] The hydraulic valves  260 , 270  are then reconfigured by conventional means (not shown) to connect the socket  230  to the fluid container  250  via first and second container socket conduits  262 , 264 , the second hydraulic valve  270  closing the compensator conduit  356 . The oil  340  in the socket is thus connected to the fluid container  250  where air  320  is contained at a pressure of 10 5  Pa (1 bar) above the container piston  291 . Hence the oil in the socket is now at a pressure of 105 Pa (1 bar). The oil in the socket is accordingly sealed from the surrounding seawater at ambient pressure as there is a pressure difference across the “O” ring  234 . 
   [ FIG. 5(   d )] The actuating valve  310  is reconfigured in a conventional manner to connect the pump  300  to the actuator cylinder  290  via the pump valve conduit  302  and the second valve actuator conduit  297 . The pump forces a pressurized liquid into the actuator cylinder via the second port  295 , that is below the actuator piston  292 , causing the actuator piston to rise towards the top of the actuator cylinder  290 . Liquid in the part of the actuator cylinder above the actuator piston is expelled in a known manner via the first valve actuator conduit  296  and the actuating valve. The movement of the actuator piston causes the container piston  291 , that is connected to the actuator piston by the connecting rod  293 , to rise, forcing air  320  stored in the fluid container  250  into the socket  230  via the first container socket conduit  262 . The air enters the socket  230  via the first stab connector  232  at the top of the socket. This forces the oil  340  out of the socket via the second stab connector  233  at the base of the socket and into the fluid container via the second container socket conduit  264 . 
   [ FIG. 5(   e )] The actuating valve  310  is reconfigured to isolate the pump  300  from the actuator cylinder  290 . The hydraulic valves  260 , 270  are reconfigured to close the container socket conduits  262 , 264  and isolate the socket  230  from the fluid container  250  and the compensator cylinder  350  thus totally isolating the socket  230  from the fluid exchange unit  220 . 
   [ FIG. 5(   f )] The stab connectors  232 , 233  disengage from the container socket conduits  262 , 264  of the fluid exchange unit  220  and the fluid exchange unit moves away from the socket  230 . Power can now be applied to the electrical coupling between the plug  240  and socket  230  in a known manner, the air in the socket being at a pressure of 105 Pa (1 bar). 
   Referring to  FIGS. 6(   a ) to  6 ( g ), and additionally to  FIG. 4 , the process of retrieving the socket  230  from the plug  240  will be described. 
   [ FIG. 6(   a )] Power is switched off to the electrical coupling between the plug  240  and socket  230 . The fluid exchange unit  220  is moved towards the engaged socket  230  and plug  240 . The air  320  in the socket is at a pressure of 10 5  Pa (1 bar). 
   [ FIG. 6(   b )] The fluid exchange unit  220  connects with the socket  230  as the first and second container socket conduits  262 , 264  engage first and second stab connectors  232 , 233  respectively. 
   [ FIG. 6(   c )] The hydraulic valves  260 , 270  are reconfigured to connect the socket  230  to the fluid container  250  via first and second container socket conduits  262 , 264 . The air  320  in the socket  230  is connected to the fluid container  250  where oil  340  is contained at a pressure of 10 5  Pa (1 bar) below the container piston  291 . 
   [ FIG. 6(   d )] The actuating valve  310  is reconfigured to connect the pump  300  to the actuator cylinder  290  via the pump valve conduit  302  and the first valve actuator conduit  296 . The pump forces pressurized liquid into the actuator cylinder via the first port  294 , causing the actuator piston  292  to be pushed towards the base of the actuator cylinder  290 . Liquid in the part of the actuator cylinder below the actuator piston is expelled via the second valve actuator conduit  297  and the actuating valve  310 . The movement of the actuator piston pushes down the connected container piston  291 , forcing oil  340  stored in the fluid container  250  into the socket  230  via the second container socket conduit  264 . This forces air  320  out of the socket and into the container  250  via the first container socket conduit  262 . 
   [ FIG. 6(   e )] The actuating valve  310  is reconfigured to isolate the pump  300  from the actuator cylinder  290 . The hydraulic valves  260 , 270  are reconfigured to close the container socket conduits  262 , 264  isolating the socket  230  from the fluid container  250  and the compensator cylinder  350  thus totally isolating the socket  230  from the fluid exchange unit  220 . 
   [ FIG. 6(   f )] The second hydraulic valve  270  is reconfigured to connect the socket  230  to the compensator cylinder  350  via the part of the second container socket conduit  264  between the second stab connector  233  of the socket and the second hydraulic valve  270  and via the compensator conduit  356 . This balances the pressure of the oil  340  in the socket with the oil in the compensator cylinder  350 . Thus, the oil in the socket  230  is now at ambient pressure and there is no longer a pressure difference across the “O” ring  234 . However, the inserted plug prevents oil escaping into the surrounding seawater. 
   [ FIG. 6(   g )] The fluid exchange unit  220  moves away with the retrieved socket  230  from the plug  240  and oil is drawn into the socket from the compensator cylinder  350 , lowering the compensator piston  352 . The hinged flat plate seal  237 , which had been leaning on the inserted plug, is closed by force of gravity as the plug is withdrawn. 
   A third embodiment of the invention will now be described with reference to  FIGS. 7 to 9(   g ). Where a part in the first embodiment has a reference numeral and there is a substantially corresponding part in the third embodiment the same reference numeral has been used except that the number four hundred has been added to the latter. Not all such parts will be described in detail. 
   Referring to  FIG. 7 , an electrical connector  410  according to a third embodiment of the invention is shown. The connector comprises an external fluid exchange unit  420  with an attached socket  430 . Below the socket  430  is shown a plug  440  having electrical contacts  436  and  441  respectively. Both the socket and plug are the same as that described in the first embodiment. 
   Inside the fluid exchange unit  420  are first and second fluid containers  450 , 650  that are connected to the socket  430  via first, second and third hydraulic valves  460 , 470 , 480 . There are two ports  452 , 454  on one side of the first fluid container  450  with the first port  452  located proximate the top of the container and the second port  454  located proximate the bottom of the container. There are two ports  652 , 654  similarly located on the second fluid container  650 . A first container first valve conduit  456  connects the first port  452  of the first fluid container  450  to the first hydraulic valve  460  and a first container second valve conduit  458  connects the second port  454  of the first fluid container  450  to the second hydraulic valve  470 . An ambient conduit  600  is connected by a junction  602  to the first container first valve conduit  456 . The ambient conduit  600  provides a connection to the fluid surrounding the fluid exchange unit. A second container second valve conduit  656  connects the first port  652  of the second fluid container  650  to the second hydraulic valve  470  and a second container first valve conduit  658  connects the second port  654  of the second fluid container  650  to the first hydraulic valve  460 . The first hydraulic valve  460  is connected to the third hydraulic valve  480  by a first valve third valve conduit  462  and the second hydraulic valve  470  is connected to the third hydraulic valve  480  by a second valve third valve conduit  472 . The third hydraulic valve is connected to the first and second stab connectors  432 , 433  of the socket by first and second stab connector conduits  482 , 484  respectively. 
   Above the fluid containers  450 , 650  are first and second actuator cylinders  490 , 690 . Contained within the first fluid container  450  is a first container piston  491  and contained within the first actuator cylinder is a first actuator piston  492 . The first container piston  491  and the first actuator piston  492  are interconnected by a first connecting rod  493 . There are two ports  494 , 495  on one side of the first actuator cylinder  490  with the first port  494  located proximate the top of the first actuator cylinder and the second port  495  located proximate the bottom of the first actuator cylinder. Similarly, the second fluid container  650  contains a second container piston  691  and the second actuator cylinder  690  contains a second actuator piston  692  with these pistons being interconnected by a second connecting rod  693 . There are also two ports  694 , 695  on one side of the second actuator cylinder  690  with the first port  694  located proximate the top of the second actuator cylinder and the second port  695  located proximate the bottom of the second actuator cylinder. The first and second actuator cylinders  490 , 690  are connected to a conventional pump  500  via first and second actuating valves  510 , 710  respectively. The pump has an exhaust outlet  508 . A pump junction conduit  502  connects the pump to a junction  504 . The junction is connected to the first and second actuating valves by first and second actuating valve conduits  506 , 706  respectively. A first valve first port conduit  496  connects the first actuating valve  510  to the first port  494  of the first actuator cylinder  490  and a first valve second port conduit  497  connects the first actuating valve  510  to the second port  495  of the first actuator cylinder  490 . Similarly, a second valve first port conduit  696  connects the second actuating valve  710  to the first port  694  of the second actuator cylinder  690  and a second valve second port conduit  697  connects the second actuating valve  710  to the second port  695  of the second actuator cylinder  690 . Each actuating valve  510 , 710  has a respective exhaust outlet  512 , 712 . 
   Referring to  FIGS. 8(   a ) to  8 ( g ), and additionally to  FIG. 7 , the process of installing the socket  430  on the plug  440  in seawater  445  will be described. 
   [ FIG. 8(   a )] The fluid exchange unit  420  and socket  430  is moved towards the plug  440 . Inside the socket is seawater  530  that is at ambient pressure. Inside the first fluid container  450 , the first container piston  491  is positioned slightly below the first port  452 . Below the first container piston  491  is freshwater  570  that will be used for flushing purposes, although other fluids could be used. Inside the second fluid container  650 , the second container piston  691  is positioned slightly below the first port  652 . Below the second container piston  691  is a gas, preferably air  520 , at a pressure of 10 5  Pa (1 bar). The hydraulic valves  460 , 470 , 480  are initially configured to isolate the socket from the first and second fluid containers  450 , 650  by the first valve  460  closing the second container first valve conduit  658 , the second valve  470  closing second container second valve conduit  656  and the third valve  480  closing the second valve third valve conduit  472 . However, the first and third hydraulic valves  460 , 480  are also configured to connect the socket to the ambient conduit  600  via the part of the first container first valve conduit  456  between the junction  602  and the first valve  460 , the first valve third valve conduit  462  and the first stab connector conduit  482 , thus connecting the socket with the seawater surrounding the fluid exchange unit. The position of the first container piston  491  isolates the freshwater  570  in the first fluid container  450  from seawater from the socket or the ambient conduit  600 . The first and second actuating valves  510 , 710  are configured to connect the pump  500  to the second ports  495 , 695  at the bottom of the first and second actuator cylinders  490 , 690  respectively. Thus, the pump cannot push down either the first or second actuator pistons  492 , 692  as any pressurized liquid pumped into either cylinder will only try to force the respective piston further up. 
   [ FIG. 8(   b )] As the plug  440  enters the socket  430  via the “O” ring  434 , seawater  530  is displaced from inside the socket into the sea surrounding the fluid exchange unit  420  via the first stab connector conduit  482 , the first valve third valve conduit  462 , the part of the first container first valve conduit  456  between the junction  602  and the first hydraulic valve  460 , and the ambient conduit  600 . This maintains the remaining seawater in the socket at ambient pressure. The electrical contacts  441  of the plug  440  become coupled to the electrical contacts  436  of the socket  430  once the plug has been fully inserted into the socket. However, no power is as yet supplied to this connection. 
   [ FIG. 8(   c )] The hydraulic valves  460 , 470 , 480  are then reconfigured by conventional means (not shown) to connect the first stab connector  432  of the socket  430  to the first port  452  of the first fluid container  450  via the first stab connector conduit  482 , the first valve third valve conduit  462 , and the first container first valve conduit  456 , and to connect the second stab connector  433  to the second port  454  of the first fluid container  450  via the second stab connector conduit  484 , the second valve third valve conduit  472 , and the first container second valve conduit  458 . The first actuating valve  510  is reconfigured in a conventional manner to connect the pump  500  to the first actuator cylinder  490 . The pump forces a pressurized liquid into the first port  494  of the first actuator cylinder  490  that is above the first actuator piston  492  via the pump junction conduit  502 , the first actuating valve conduit  506  and the first valve first port conduit  496 . This pushes the first actuator piston  492  towards the base of the first actuator cylinder  490 . Liquid in the first actuator cylinder below the first actuator piston is expelled in a known manner via the first valve second port conduit  497 , the first actuating valve  510  and its associated exhaust outlet  512 . The movement of the first actuator piston  492  pushes down the first container piston  491 , connected to the first actuator piston  492  by the first connecting rod  493 , forcing the freshwater  570  stored in the first fluid container  450  into the socket via the first container second valve conduit  458 , the second valve third valve conduit  472  and the second stab connector conduit  484 . The freshwater  570  enters the socket via the second stab connector  433  at the base of the socket. This forces the seawater  530  out of the socket via the first stab connector  432  at the top of the socket and into the first fluid container  450  via the first stab connector conduit  482 , the first valve third valve conduit  462  and the first container first valve conduit  456 . 
   [ FIG. 8(   d )] The first and second hydraulic valves  460 , 470  are then reconfigured to connect the first stab connector  432  of the socket  430  to the second port  654  of the second fluid container  650  via the first stab connector conduit  482 , the first valve third valve conduit  462 , and the second container first valve conduit  658 , and to connect the second stab connector  433  to the first port  652  of the second fluid container  650  via the second stab connector conduit  484 , the second valve third valve conduit  472 , and the second container second valve conduit  656 . The freshwater  570  in the socket is thus connected to the second fluid container where air  520  is contained at a pressure of 10 5  Pa (1 bar), hence the freshwater is now at a pressure of 10 5  Pa (1 bar). The freshwater in the socket is accordingly sealed from the surrounding seawater at ambient pressure as there is a pressure difference across the “O” ring  434 . 
   [ FIG. 8(   e )] The second actuating valve  710  is reconfigured in a conventional manner to connect the pump  500  to the second actuator cylinder  690 . The pump forces pressurized liquid into the first port  694  of the second actuator cylinder above the second actuator piston  692  via the pump junction conduit  502 , the second actuating valve conduit  706  and the second valve first port conduit  696 . This pushes the second actuator piston  692  towards the base of the second actuator cylinder  690 . Liquid in the second actuator cylinder below the second actuator piston is expelled in a known manner via the second valve second port conduit  697 , the second actuating valve  710  and its associated exhaust outlet  712 . The movement of the second actuator piston  692  pushes down the second container piston  691 , connected to the second actuator piston by the second connecting rod  693 , forcing the air  520  stored in the second fluid container  650  into the socket via the second container first valve conduit  658 , the first valve third valve conduit  462  and the first stab connector conduit  482 . The air enters the socket via the first stab connector  432  at the top of the socket. This forces the freshwater  570  out of the socket via the second stab connector  433  via the base of the socket and into the second fluid container  650  via the second stab connector conduit  484 , the second valve third valve conduit  472  and the second container second valve conduit  656 . 
   [ FIG. 8(   f )] The third hydraulic valve  480  is reconfigured to isolate the socket from the fluid exchange unit  420 . The first and second hydraulic valves  460 ,  470  are already configured to isolate the first fluid container  450  closing the first container first valve conduit  456  and the first container second valve conduit  458 . The third hydraulic valve  480  closes the second valve third valve conduit  472  isolating the first port  652  of the second fluid container  650 . The second container piston  652  is positioned at the base of the second fluid container thus sealing the second port  654  of the second fluid container. 
   [ FIG. 8(   g )] The stab connectors  432 , 433  disengage from the stab connector conduits  482 , 484  of the fluid exchange unit  420  and the fluid exchange unit moves away from the socket  430 . Power can now be applied to the electrical coupling between the plug  440  and socket in a known manner, the air in the socket being at a pressure of 10 5  Pa (1 bar). 
   Referring to  FIGS. 9(   a ) to  9 ( g ), and additionally to  FIG. 7 , the process of retrieving the socket  430  from the plug  440  will be described. 
   [ FIG. 9(   a )] Power is switched off to the electrical coupling between the plug  440  and socket  430 . The fluid exchange unit  420  is moved towards the socket and plug. The air  520  in the socket is at a pressure of 10 5  Pa (1 bar). 
   [ FIG. 9(   b )] The fluid exchange unit  420  connects with the socket  430  as the first and second stab connector conduits  482 , 484  engage first and second stab connectors  432 , 433  respectively. 
   [ FIG. 9(   c )] The third hydraulic valve  480  is configured to connect the first stab connector  432  of the socket  430  to the second port  654  of the second fluid container  650  via the first stab connector conduit  482 , the first valve third valve conduit  462 , and the second container first valve conduit  658 , and to connect the second stab connector  433  to the first port  652  of the second fluid container via the second stab connector conduit  484 , the second valve third valve conduit  472 , and the second container second valve conduit  656 . The second actuating valve  710  is reconfigured to connect the pump  500  to the second actuator cylinder  690 . The pump forces pressurized liquid into the second port  695  of the second actuator cylinder  690  below the second actuator piston  692  via the pump junction conduit  502 , the second actuating valve conduit  706  and the second valve second port conduit  697 . This forces the second actuator piston  692  up towards the top of the second actuator cylinder  690 . Liquid in the second actuator cylinder above the second actuator piston is expelled via the second valve first port conduit  696 , the second actuating valve and its associated exhaust outlet  712 . The movement of the second actuator piston pulls the connected second container piston  691  upwards, forcing the freshwater  570  stored in the second fluid container  650  into the socket via the second container second valve conduit  656 , the second valve third valve conduit  472  and the second stab connector conduit  484 . This forces the air  520  out of the socket and into the second fluid container via the first stab connector conduit  482 , the first valve third valve conduit  462  and the second container first valve conduit  656 . 
   [ FIG. 9(   d )] The hydraulic valves  460 , 470 , 480  are then reconfigured to connect the first stab connector  432  of the socket  430  to the second port  454  of the first fluid container  450  via the first stab connector conduit  482 , the second valve third valve conduit  472  and the first container second valve conduit  458 , and to connect the second stab connector  433  to the first port  452  of the first fluid container via the second stab connector conduit  484 , the first valve third valve conduit  462 , and the first container first valve conduit  456 . The second stab connector  433  is also connected to the ambient conduit  600  via the junction  602 . Hence, the socket is connected with the seawater at ambient pressure outside the fluid exchange unit  420 . Thus, the freshwater  570  in the socket is also now at ambient pressure. There is no longer any pressure difference across the “O” ring  434 . 
   [ FIG. 9(   e )] The first actuating valve  510  is reconfigured to connect the pump  500  to the first actuator cylinder  490 . The pump forces pressurized liquid into the second port  495  of the first actuator cylinder  490  below the first actuator piston  492  via the pump junction conduit  502 , the first actuating valve conduit  506  and the first valve second port conduit  497 . This forces the first actuator piston up towards the top of the first actuator cylinder. Liquid in the first actuator cylinder  490  above the first actuator piston  492  is expelled via the first valve first port conduit  496 , the first actuating valve  510  and its associated exhaust outlet  512 . The movement of the first actuator piston  492  pulls the connected first container piston  491  upwards, forcing the seawater  530  stored in the first fluid container  450  into the socket  430  via the first container first valve conduit  456 , the first valve third valve conduit  462  and the second stab connector conduit  484 . This forces the freshwater  570  out of the socket  430  and into the first fluid container  450  via the first stab connector conduit  482 , the second valve third valve conduit  472  and the first container second valve conduit  458 . 
   [ FIG. 9(   f )] The third hydraulic valve  480  is reconfigured to isolate the socket  430  from the fluid containers  450 , 650 . The first and second hydraulic valves  460 , 470  are already configured to isolate the second fluid container  650  having closed the second container first valve conduit  458  and the second container second valve conduit  458 . The third hydraulic valve  480  closes the second valve third valve conduit  472  isolating the second port  454  of the first fluid container  450 . The first container piston  491  is positioned at the top of the first fluid container  450  thus sealing the first port  452  of the first container  450 . 
   [ FIG. 9(   g )] The fluid exchange unit  420  moves away with the retrieved socket  430  from the plug  440  with the surrounding seawater being drawn into the socket. 
   The flushing action of the freshwater in the third embodiment removes seawater and any residue from the socket. 
   A fourth embodiment of the invention will now be described with reference to  FIGS. 10 to 11(   g ). Where a part in the first embodiment has a reference numeral and there is a substantially corresponding part in the fourth embodiment the same reference numeral has been used except that the number eight hundred has been added to the latter. Not all such parts will be described in detail. 
   Referring to  FIG. 10 , an electrical connector  810  according to a fourth embodiment of the invention is shown. The connector comprises an external fluid exchange unit  820  with an associated separate socket  830 , the unit and socket being adapted to be connected to each other by first and second stab connectors  832   a , 832   b ; 833   a , 833   b . The fluid exchange unit  820  has first portions  832   a , 833   a  of the first and second stab connectors and the socket  830  has second complementary portions  832   a , 833   a  of the first and second stab connectors. The first and second portions  832   a , 832   b ; 833   a , 833   b  of the stab connectors isolate the inside of the fluid exchange unit  820  and the socket  830  until they engage each other. Below the socket  830  is shown a plug  840 . Both the socket and plug are substantially the same as that described in the first embodiment. 
   Inside the fluid exchange unit  820  is a flushing fluid device  940  comprising a chamber or reservoir, such as a storage bladder, with a thin flexible wall  972 , the flushing fluid device being connected to the first portion  832   a  of the first stab connector via a hydraulic valve  860 . A pressure vessel  942  is also connected to the connector first portion  832   a  via the hydraulic valve  860 , there being a pressure regulator  944  between the pressure vessel and the hydraulic valve. In addition, the fluid exchange unit  820  has a positive displacement pump  946  connected to the first portion  833   a  of the second stab connector. 
   A device-valve conduit  948  connects the flushing fluid device  940  to the hydraulic valve  860  and a vessel-valve conduit  950  connects the pressure vessel  942  to the hydraulic valve  860  via the pressure regulator  944 . A valve-stab connector conduit  952  connects the hydraulic valve  860  to the first portion  832   a  of the first stab connector and the first portion  833   a  of the second stab connector is connected to an outlet  954  to the fluid (e.g. the sea) surrounding the fluid exchange unit  820  by a fluid discharge line  956  in which the positive displacement pump  946  is connected. The pump  946  prevents backflow from the outlet  954  to the first portion  833   a  of the second stab connector. 
   Referring to  FIGS. 11(   a ) to  11 ( g ), and additionally to  FIG. 10 , the process of installing the socket  830  on the plug  840  in seawater  960  will be described. 
   [ FIG. 11(   a )] The socket  830  forms part of a module (not shown) lowered towards the plug  840  by a vessel at sea level. Inside the socket is seawater  930  that is at ambient pressure. As the plug  840  enters the socket  830  via the “O” ring  834 , seawater  930  is compressed inside the socket. However, means, such as a one way valve, may be provided to enable seawater to be displaced from inside the socket into the surrounding sea when the plug is inserted. 
   [ FIG. 11(   b )] The electrical contacts  841  (only one shown) of the plug become coupled to the electrical contacts  836  of the socket once the plug has been fully inserted into the socket. However, no power is as yet supplied to this connection. The external fluid exchange module  820  is lowered towards the socket  830  by a remotely operated vehicle (ROV). The pressure inside the fluid exchange module is substantially ambient. The flushing fluid device  940  contains glycol, water or other flushing medium  970  and the pressure vessel  942  is full of pressurized gas  920  such as air, nitrogen or sulphur hexafluoride (SF 6 ). Nitrogen or SF 6  may also be used in any of the other three embodiments described. The hydraulic valve  860  is initially configured to connect the pressure vessel  942  to the first portion  832   a  of the first stab connector. 
   [ FIG. 11(   c )] When the first portions  832   a , 833   a  of the first and second stab connectors engage the complementary second portions  832   b , 833   b , the pressure vessel is connected to the chamber  831  of the socket and the fluid discharge line  956  is connected to the chamber  831 . The seawater  930  in the socket  830  is thus connected to the pressure regulator  944  where gas  920  from the pressure vessel  942  is regulated to a pressure of 10 5  Pa (1 bar), hence the seawater is now at a pressure of 10 5  Pa (1 bar). If nitrogen or SF 6  is used, the pressure regulator  944  would regulate gas from the pressure vessel to about 2×10 5  Pa to 3×10 5  Pa (2 to 3 bar). The seawater in the socket is accordingly sealed from the surrounding seawater at ambient pressure as there is a pressure difference across the “0” ring  834 . 
   [ FIG. 11(   d )] The positive displacement pump  946  is actuated to remove the seawater  930  from the chamber  831  of the socket  830  into the seawater surrounding the fluid exchange unit  820  via the second stab connector  833   a,b  and the fluid discharge line  956  enabling gas  920  from the pressure vessel  942  to enter the chamber  831  via the vessel-valve conduit  950 , the hydraulic valve  860 , the valve-stab connector conduit  952  and the first stab connector  833   a,b , the gas being at the pressure set by the pressure regulator  944 . 
   [ FIG. 11(   e )] The hydraulic valve  860  is reconfigured in a conventional manner to connect the flushing fluid device  940  to the socket  830 , enabling flushing fluid  970  to flow from the device into the chamber  831  via the valve-stab connector conduit  952 . The ambient pressure inside the fluid exchange device  820  acts on the flexible wall  972  of the flushing fluid device forcing the flushing fluid to be sprayed onto the plug  840  via a nozzle  958  or other suitable spraying means cleaning the plug insulation. This removes salt and/or dirt/contaminants from the surface of the plug which could otherwise build up to form an electrical path that could short circuit the electrical connection of the engaged plug  840  and socket  830 . The sprayed flushing fluid  970  collects at the bottom of the chamber  831 . 
   [ FIG. 11(   f )] The first hydraulic valve  860  is reconfigured to its initial position and the positive displacement pump  946  is activated to remove the sprayed flushing fluid from the chamber  831  and into the seawater surrounding the fluid exchange unit  820  via the fluid discharge line  956  enabling further gas  920  from the pressure vessel  942  to enter the chamber. However, a small amount of flushing fluid  970  may remain in the chamber  831  once the pumping has been finished. Thus, the chamber  831  is now substantially filled with gas  920  at the pressure set by the pressure regulator  944 . 
   If insufficient dirt/salt is removed from the plug  840  then the process as described above for  FIGS. 11(   e ) and  11 ( f ) can be repeated until the required electrical isolation is achieved. 
   [ FIG. 11(   g )] The fluid exchange unit is lifted away from the socket  830  causing the first and second portions  832   a , 833   a ; 832   b , 833   b  of the stab connectors to disengage, sealing the insides of the socket  830  and the exchange unit  820  against the ingress of seawater. Power can now be applied to the electrical coupling between the plug  840  and socket  830  in a known manner, the air in the socket being at a pressure of 10 5  Pa (1 bar). If nitrogen or SF 6  is used, the gas in the socket would be about 2×10 Pa to 3×10 5  Pa (2 to 3 bar). 
   To remove the socket  840 , the module containing the socket can be simply retrieved by a vessel at sea level and the socket can be used on other plugs. When the socket is used on another plug, the external fluid exchange unit  820  can be lowered to remove salt/dirt from the plug  830  in the way as just described above. The fluid exchange unit  820  is accordingly not left connected to the socket  830  and can be used to install other sockets and can be retrieved for maintenance and/or replenishment of the flushing device and pressure vessel. 
   In the described embodiments the fluid exchange unit can be manoeuvred underwater in a variety of ways such as by ROVs, by divers, or by a holding frame or crane. The fluid exchange unit may be a remotely operated tool (ROT). The pump and valves can be activated remotely or automatically in a conventional manner. 
   The pressures given in the above embodiments are approximations. 
   An advantage of the connector described over known connectors is that the connector makes use of standard pieces of equipment such as hydraulic valves, pumps, containers and compensators, thus easing manufacture and reducing costs. 
   Furthermore a single fluid exchange unit can be used to install or retrieve many sockets since it does not have to be left on the sea-bed connected to a socket that it has installed. A further advantage of the separable nature of the fluid exchange unit and socket is that the fluid exchange unit can easily be recovered to the sea surface thus permitting any maintenance to be easily effected. 
   Whilst particular embodiments have been described above it will be understood that various modifications may be made without departing from the scope of the invention. For example, the air used may be replaced by a fluid such as an inert gas, and the pressure of fluid that is not at ambient pressure does not necessarily have to be at 10 5  Pa (1 bar). The or each hydraulic actuator, which is connected to a fluid container, may alternatively be replaced by a mechanical and/or an electrical actuator. The mechanical actuator may include a driven screw thread which moves the container piston within the fluid container. Suitable alternative hydraulic actuators may also be used. As an alternative to using a liquid as an actuating medium for operating the device described above, a fluid, such as air or another gas or gaseous mixture, could be used. 
   In the fourth embodiment, the fluid discharge line may be replaced with a shuttle valve and empty bladder to retain the flushing liquid if liquid considered harmful to the environment is used. The pressure of the gas  970  from the pressure vessel  942  may, at least, partially force fluid from the socket  830 . The flushing fluid device may comprise a reverse osmosis system and storage chamber for producing flushing fluid from seawater.