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TECHNICAL FIELD 
     The present invention relates generally to the field of subsea drilling, processing and production equipment, and more particularly to an improved subsea actuation system for such equipment. 
     BACKGROUND ART 
     In subsea oil and gas exploration, the drilling system or wellhead may be located many thousands of feet below the sea surface. Specialized equipment is therefore used to drill, produce and process oil and gas on the sea floor, such as subsea trees, processing systems, separators, high integrity pipeline protection systems, drills, manifolds, tie-in systems and production and distribution systems. Such equipment is commonly controlled by a number of types of valves, including blow-out preventers to stop the unintended discharge of hydrocarbons into the sea. 
     With existing systems, such valves are typically operated hydraulically by providing pressurized hydraulic fluid from a surface vessel down to the wellhead. Large hydraulic power lines from vessels or rigs on the ocean surface feed the ocean floor drilling, production and processing equipment, and the many subsystems having valves and actuators. However, such lines are expensive to install and maintain and in some cases may not be feasible, such as at depths over 10,000 feet or under the arctic circle ice caps. 
     Accordingly, it would be desirable to provide an actuator that would not require such an umbilical connection from the surface and that would still operate with the desired force and functionality. 
     BRIEF SUMMARY OF THE INVENTION 
     With parenthetical reference to corresponding parts, portions or surfaces of the disclosed embodiment, merely for the purposes of illustration and not by way of limitation, the present invention provides a subsea drilling, production or processing actuation system comprising a variable speed electric motor ( 10 ) adapted to be supplied with a current, a reversible hydraulic pump ( 8 ,  28 ) driven by the motor, a hydraulic piston assembly ( 92 ,  101 ,  111 ,  121 ,  131 ) connected to the pump and comprising a first chamber ( 2 ), a second chamber ( 3 ) and a piston ( 4 ) separating the first and second chambers and configured to actuate a valve ( 91 ) in a subsea system, a fluid reservoir ( 14 ) connected to the pump and the hydraulic piston assembly, the pump, hydraulic piston assembly and reservoir connected in a substantially closed hydraulic system, and a pressure compensator ( 13 ,  65 ) configured to normalize pressure differences between outside the hydraulic system and inside the hydraulic system. 
     The subsea system may further comprise a failsafe mechanism ( 98 ). The fail-safe mechanism may comprise a spring element ( 36 ) biasing the piston in a first direction. The fail-safe mechanism may comprise a fail-safe valve ( 35 ) between the first chamber and the second chamber or between the second chamber and the reservoir and the fail-safe valve may be arranged to open in the event of a power failure allowing equalization of fluid pressure in the first and second chamber on each side of the piston. The fail-safe mechanism may comprise a two-stage actuator. 
     The subsea system may further comprise a filter between the pump and the hydraulic piston assembly. 
     The electric motor may comprise a brushless DC motor, or may be selected from a group consisting of a stepper motor, brush motor and induction motor. The hydraulic pump may be selected from a group consisting of a fixed displacement pump, a variable displacement pump, a two-port pump, and a three-port pump. The pump may comprise a two-port pump ( 8 ) or a three-port pump ( 28 ). The piston may comprise a first surface area exposed to the first chamber and a second surface area exposed to the second chamber. The first surface area ( 4   c ) may be substantially equal to the second surface area ( 4   b ). The first surface area ( 4   a ) may be substantially different from the second surface area ( 4   b ). 
     The hydraulic piston assembly may comprise a cylinder ( 1 ) having an first end wall ( 1   b ) with the piston disposed in the cylinder for sealed sliding movement therealong, and a first actuator rod ( 5 ) connected to the piston for movement therewith and having a portion sealingly penetrating the first end wall. The cylinder may have a second end wall ( 1   a ) and the hydraulic piston assembly may comprise a second actuator rod ( 5   a ) connected to the piston for movement therewith and having a portion sealingly penetrating the second end wall. 
     The valve may comprise a stop valve in a subsea blow-out preventor, and the stop valve may comprise a shearing ram. The valve may comprise a control valve in a subsea production or processing system. 
     The pressure compensator may comprise a membrane ( 15 ) in the fluid reservoir ( 13 ). The pressure compensator may comprise a piston ( 67 ) in a cylindrical housing ( 66 ). 
     The valve may be in an assembly selected from a group consisting of a subsea blow-out preventer, a subsea production tree or wellhead system, a subsea processing or separation system, a subsea tie-in system, a subsea chock, a subsea flow module or a subsea distribution system. The subsea system may further comprise blocking valves operatively arranged to selectively isolate the pump from the first and second chambers. The subsea system may further comprise a position sensor ( 40 ) configured to sense the position of the piston. The subsea system may further comprise a pressure sensor ( 41 ,  42 ) configured to sense pressure in the first or second chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a component view of a fail-safe embodiment of the subsea actuation system operating a valve in a subsea oil processing line. 
         FIG. 2  is a detailed schematic view of a first embodiment of the subsea actuation system shown in  FIG. 1 , this view showing an unequal piston area with anti-cavitation form. 
         FIG. 3  is a detailed schematic view of a second embodiment of the subsea actuation system shown in  FIG. 1 , this view showing a spring fail-safe form. 
         FIG. 4  is a detailed schematic view of a third embodiment of the subsea actuation system shown in  FIG. 1 , this view showing an equal piston area and dual rod form. 
         FIG. 5  is a detailed schematic view of a fourth embodiment of the subsea actuation system shown in  FIG. 1 , this view showing a three-port pump form. 
         FIG. 6  is a cross-sectional view of the piston assembly shown in  FIG. 2 . 
         FIG. 7  is a cross-sectional view of the bi-directional pump shown in  FIG. 2 . 
         FIG. 8  is a cross-sectional view of the electric variable-speed servo-motor shown in  FIG. 2 . 
         FIG. 9  is a cross-sectional view of the reservoir and compensator shown in  FIG. 2 . 
         FIG. 10  is a cross-sectional view of an alternate embodiment of the reservoir and compensator shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
     Referring now to the drawings, and more particularly to  FIG. 1  thereof, the present invention broadly provides a subsea actuation system for a subsea valve, of which an embodiment is indicated at  90 . As shown in  FIG. 1 , assembly  90  is adapted to actuate a subsea process valve  91  or other type of valve or similar component in a subsea environment.  FIG. 1  shows the control valve architecture with a pressure compensated canister that protects the spring assembly. In this embodiment, subsea fluid such as oil or gas is metered by process valve  91  and the forces required to meter valve  91  are created by subsea actuator system  90 , which includes piston actuator assembly  92 , integrated bidirectional pump  8 , variable speed bidirectional electric servomotor  10 , electronic motor controller  95 , fluid logic elements/check valves  96 , reservoir/compensator  13 , and spring failsafe assembly  98 . Spring failsafe assembly  98 , depending on the design requirements, will drive process valve  91  in a failed close or a failed open condition when power is lost. Motor controller  95  includes drive electronics to commutate motor  10  and receives feedback from sensors in the system and controls motor  10  accordingly. 
       FIG. 2  shows an embodiment  100  of the subsea actuation system. As indicated, system  100  includes variable speed electric motor  10 , bi-directional or reversible pump  8  driven by motor  10 , hydraulic piston assembly  101 , system pressure compensated reservoir  13  with system fluid tank  14 , pressure transducers  41  and  42  that feed back to motor  10  controller  95 , and position transducer  40  that feeds back to motor controller  95 . Pump  8 , piston assembly  101  and tank  14  are connected by a plurality of hydraulic flow lines  6 ,  7 ,  12 ,  17 ,  19  and  20  to form a closed fluid system. 
     As shown in further detail in  FIG. 8 , in this embodiment motor  10  is a brushless D.C. variable-speed servo-motor that is supplied with a current. Motor  10  has an inner rotor  50  with permanent magnets and a fixed non-rotating stator  51  with coil windings. When current is appropriately applied through the coils of stator  51 , a magnetic field is induced. The magnetic field interaction between stator  51  and rotor  50  generates torque which may rotate output shaft  52 . There are no mechanical brushes that commutate the stator fields in this embodiment of the motor. Drive electronics, based on resolver  53  angular position feedback, generate and commutate the stator fields to vary the speed and direction of motor  10 . Accordingly, motor  10  will selectively apply a torque on shaft  52  in one direction about axis x-x at varying speeds and will apply a torque on shaft  52  in the opposite direction about axis x-x at varying speeds. Other motors may be used as alternatives. For example, a variable speed stepper motor, brush motor or induction motor may be used. 
     As shown in further detail in  FIG. 7 , in this embodiment pump  8  is a fixed displacement bi-directional internal two-port gear pump. The pumping elements, namely gears  55  and  56 , are capable of rotating in either direction, thereby allowing hydraulic fluid to flow in either direction  47  or  48 . This allows for oil to be added into and out of the system as the system controller closes the control loop of position or pressure. The shaft of gear  55  is connected to output shaft  52  of motor  10 , with the other pump gear  56  following. Fluid is directed to flow to the outside of gears  55  and  56 , between the outer gear teeth of gears  55  and  56  and housing  57 , respectively. Thus, rotation of gear  55  in clockwise direction  46  causes fluid flow in one direction  48 , from port  8   a  out port  8   b . Rotation of gear  55  in counterclockwise direction  45  cause fluid flow in opposite direction  47 , from port  8   b  out port  8   a . Thus, the direction of flow of pump  8  depends on the direction of rotation of rotor  50  and output shaft  52  about axis x-x. In addition, the speed and output of pump  8  is variable with variations in the speed of motor  10 . Other bi-directional pumps may be used as alternatives. For example, a variable displacement pump may be used. 
     As shown in further detail in  FIG. 9 , in this embodiment reservoir  13  includes a bladder type pressure compensator for the fluid system. As shown, reservoir  13  is separated into two variable volume chambers  14  and  16  by an elastomeric bladder or diaphragm  15 . Chamber  16  is open to sea water via port  60 , and chamber  14  operates as the hydraulic reservoir, through port  61 , for system fluid and is sealed and pressure balanced from the outside environment  16  by bladder  15 . As the system fluid is displaced, bladder  15  will move and displace water in chamber  16  on the other side. Bladder  15  is easy to move and ensures that the fluid inside is substantially equal to the ambient water pressure outside the system. 
       FIG. 10  shows an alternative piston type pressure compensator for reservoir  14 . As shown, it functions generally the same as the bladder type, with the exception that the barrier between the system fluid in chamber  14  and the water in chamber  16  is piston  67 , which is slidably disposed within cylindrical housing  66 . As the system fluid is displaced, piston  67  will move and displace water in chamber  16  on the other side. Piston  67  moves in housing  66  to ensure that the fluid inside is substantially equal to the ambient water pressure outside the system. 
     As shown in  FIG. 2  and  FIG. 6 , piston assembly  101  includes piston  4  slidably disposed within cylindrical housing  1 . Motor  10 , pump  8 , the valves and lines, and compensator  13  are typically integrated in housing  1 . Rod  5  is mounted to piston  4  for movement with piston  4  and extends to the right and sealably penetrates right end wall  1   b  of housing  1 . Piston  4  is slidably disposed within cylinder  1 , and sealingly separates left chamber  2  from right chamber  3 . In this embodiment, almost all of leftwardly-facing circular vertical end surface  4   a  of piston  4  faces into left chamber  2 . However, only annular rightwardly-facing vertical end surface  4   b  of piston  4  faces rightwardly into right chamber  3  due to the addition of rod  5  through chamber  3  and outside housing  1 . This creates an unequal piston area configuration, with the surface area of face  4   a  being greater than the surface area of face  4   b.    
     As shown in  FIG. 2 , one side or port  8   a  of pump  8  communicates with left chamber  2  via fluid line  6 , and the opposite side or port  8   b  of pump  8  communicates with right chamber  3  via fluid line  7 . One side  8   a  of pump  8  communicates with tank  14  via fluid line  12  and the opposite side  8   b  of pump  8  communicates with tank  14  via fluid line  17 . Chamber  3  communicates with tank  13  via lines  7  and  17 , and chamber  2  communicates with tank  13  via lines  6  and  12 . 
     Piston  4  will extend or move to the right when bidirectional motor  10  is rotated in a first direction, thereby rotating bidirectional pump  8  (namely driven gear  55 ) in first direction  46  and drawing fluid through port  8   b  from line  7  and chamber  3 . Pilot operated check valve  11  is opened by the pressure built up in line  20  due to the output of pump  8  into line  6 , which allows additional drawing of fluid from line  12  and reservoir  14 . Bidirectional pump  8  also outputs fluid through port  8   a  into line  6 , closing check valve  9  and thereby isolating line  6  from reservoir  14 . The fluid in line  6  flows into chamber  2  of assembly  101 , thereby creating a differential pressure on piston  4  and causing it to extend rod  5  to the right. 
     Piston  4  will retract rod  5  or move to the left when bidirectional motor  10  is rotated in the other direction, thereby rotating bidirectional pump  8  in direction  45  and drawing fluid through port  8   a  from line  6  and chamber  2 . Pilot operated check valve  9  is opened by the pressure built up in line  19  due to the output of pump  8  into line  7 , which allows additional fluid from line  6  to flow into system pressure compensated reservoir  14 . Bidirectional pump  8  also outputs fluid from port  8   b  into line  7 , closing check valve  11  and thereby isolating line  7  from reservoir  14 . The fluid in line  7  flows into chamber  3  of assembly  101 , thereby creating a differential pressure on piston  4  and causing it to retract rod  5 . 
     The function of this anti-cavitation configuration is to address the volumetric differences between opposed chambers  2  and  3 . For example, when piston  4  moves leftwardly within cylinder  1 , the volume of fluid removed from collapsing left chamber  2  will be greater than the volume of fluid supplied to expanding right chamber  3 . 
     Controller  95  controls the current to motor  10  at the appropriate magnitude and direction. The position of rod  5  is monitored via position transducer  40 , and the position signals are then fed back to motor controller  95 . In addition or alternatively, the pressure in lines  6  and  7  to chambers  2  and  3  are monitored with pressure transducers  41  and  42 , respectively, and the pressure signals are fed back to motor controller  95 . Variable speed bidirectional motor  10  and pump  8  control the speed and force of piston  4 , and in turn rod  5 , by changing the flow and pressure acting on piston  4 . This is accomplished by looking at the feedback of position transducer  40  and/or pressure transducers  41  and  42  and then closing the control loop by adjusting the motor  10  speed and direction accordingly. While position sensor  40  is shown as a magnetostrictive linear position sensor, other position sensor may be used. For example, an LVDT position sensor may be used as an alternative. 
     Another embodiment  110  is shown in  FIG. 3 . This embodiment includes fail-safe mechanism  98 , shown in  FIG. 1 , for when it becomes necessary to close valve  91 , such as in an emergency situation. In this embodiment, springs  36  are provided to bias rod  5  towards an extended position. One side or port  8   a  of pump  8  communicates with left chamber  2  via fluid line  6 , and the opposite side or port  8   b  of pump  8  communicates with right chamber  3  via fluid line  7 . One side  8   a  of pump  8  communicates with tank  14  via fluid line  22  and the opposite side  8   b  of pump  8  does not include a fluid line to tank  14 . Bypass fluid line  21  connects lines  6  and  7 , and therefor chambers  1  anti  3 , and solenoid-operated valve  35  is provided in line  21 . Pump  8 , piston assembly  111  and tank  14  are connected by a plurality of hydraulic flow lines  6 ,  7 ,  21  and  22  to form a closed fluid system. When in regular operation, valve  35  is energized so the state of valve  35  is blocked port, thereby blocking flow between chambers  2  and  3  through line  21 . However, the solenoid valve is biased by a spring to move valve  35  to an open position. 
     Piston  4  will move to extend rod  5  when bidirectional motor  10  is rotated in a first direction, thereby rotating bidirectional pump  8  in first direction  45  and drawing fluid through port  8   b  from line  7  and chamber  3 . Bidirectional pump  8  also outputs fluid into line  6  and tank  14 . Since chamber  2  is always connected to tank  14 , springs  36  force piston  4  to the right to extend rod  5 . 
     Piston  4  will move left to retract rod  5  when bidirectional motor  10  is rotated the other direction, thereby rotating bidirectional pump  8  in other direction  46  and drawing fluid through port  8   a  from line  6 . Bidirectional pump  8  also outputs fluid into line  7  and chamber  3 . Since chamber  2  is always connected to reservoir  14 , the differential piston force between the pressure from chamber  3  and springs  36  causes piston  4  to move to the left and retract rod  5 . 
     Again, variable speed bidirectional motor  10  and pump  8  control the speed and force of piston  4  by changing the flow and pressure acting on piston  4  using feedback from position transducer  40  and/or pressure transducers  41  and  42  and then closing the control loop by adjusting the speed and direction of motor  10  accordingly. 
     When valve  35  is de-energized, such as in an emergency power loss, the spring of solenoid valve  35  will return it to an open position. In this state, chamber  3  is connected through line  21  to chamber  2  and to reservoir  14 , thereby equalizing pressure in chambers  2  and  3 . Since the fluid pressure is now equalized on each side of piston  4 , springs  36  will extend rod  5 , and valve  91  will close as fluid is transferred from chamber  3 . Thus, regardless of pump  8  output, springs  36  will extend rod  5  and close valve  91 . If desired, the system could be similarly arranged to provide a failsafe in the piston retracted position. 
     Another embodiment  120  is shown in  FIG. 4 . This embodiment is similar to the embodiment shown in  FIG. 2 , but with dual rod and equal area piston assembly  121 . As shown, piston  4  includes opposed rods  5   a  and  5   b  mounted to piston  4  for movement with piston  4 . Rod  5   b  extends to the right and penetrates the right end wall  1   b  of housing  1 . Rod  5   a  extends to the left and penetrates the left end wall  1   a  of housing  1 . In this embodiment, leftwardly-facing annular vertical end surface  4   c  of piston  4  faces into left chamber  2  due to the addition of rod  5   a  through chamber  2 , and rightwardly-facing annular vertical end surface  4   b  of piston  4  faces into right chamber  3  due to rod  5   b  extending through chamber  3  and outside housing  1 . With rods  5   a  and  5   b  being of an equal diameter, this creates an equal piston area configuration, with the surface area of face  4   c  being substantially the same as the surface area of face  4   b . Pump  8 , piston assembly  121  and tank  14  are connected by a plurality of hydraulic flow lines  6 ,  7 ,  12  and  17  to form a closed fluid system. 
     Piston  4  will move right to extend rod  5   b  and retract rod  5   a  when motor  10  is rotated in a first direction, thereby rotating bidirectional pump  8  in first direction  45  and drawing fluid through port  8   b  from line  7  and chamber  3 . Pump  8  also outputs fluid into line  6  and chamber  2 , creating a differential pressure on piston  4  and causing it to extend rod  5   b  and retract rod  5   a.    
     Piston  4  will move to the left to retract rod  5   b  and extend rod  5   a  when bidirectional motor  10  is rotated the other direction, thereby rotating bidirectional pump  8  in direction  46  and drawing fluid through port  8   a  from line  6  and chamber  2 . Bidirectional pump  8  also outputs fluid into line  7  and chamber  3 , creating a differential pressure on piston  4  and causing it to retract rod  5   b  and extend rod  5   a.    
     Again, variable speed bidirectional motor  10  and pump  8  control the speed and force of piston  4  by changing the flow and pressure acting on piston  4  using feedback from position transducer  40  and/or pressure transducers  41  and  42  and then closing the control loop by adjusting the motor  10  speed and direction accordingly. 
     Another embodiment  130  is shown in  FIG. 5 . This embodiment is similar to the embodiment shown in  FIG. 2 , but with a three port pump  28 . In this embodiment, three-port pump  28 , rather than two-port pump  8 , is used and the  3  port input to output configuration ratio is matched to the piston area  4   a / 4   b  ratio. Third port  28   c  of pump  28  is connected by line  18  to tank  14 . Pump  8 , piston assembly  131  and tank  14  are connected by a plurality of hydraulic flow lines  6 ,  7 ,  12 ,  17  and  18  to form a closed fluid system. 
     Piston  4  will move right to extend rod  5  when bidirectional motor  10  is rotated in a first direction, thereby rotating bidirectional pump  28  in first direction  45  and drawing fluid through port  28   b  from line  7  and chamber  3  and through port  28   c  from line  18  and reservoir  14 . Bidirectional pump  28  also outputs fluid from port  28   a  into line  6 , closing check valve  9  and thereby isolating line  6  from reservoir  14 . The fluid in line  6  flows into chamber  2 , creating a differential pressure on piston  4  and causing it to extend rod  5 . 
     Piston  4  will move left to retract rod  5  when bidirectional motor  10  is rotated the other direction, thereby rotating bidirectional pump  28  in the other direction  46  and drawing fluid through port  28   a  from line  6  and chamber  2 . Bidirectional pump  28  outputs fluid from port  28   c  into lines  18  and  12  and reservoir  14  and also outputs fluid from port  28   b  into line  7 , closing check valve  11  and thereby isolating line  7  from reservoir  14 . The fluid in line  7  flows into chamber  3 , creating a differential pressure on piston  4  and causing it to retract rod  5 . 
     Again, variable speed bidirectional motor  10  and pump  8  control the speed and force of piston  4  by changing the flow  47  or  48  and pressure acting on piston  4  using feedback from position transducer  40  and/or pressure transducers  41  and  42  and then closing the control loop by adjusting the motor  10  speed and direction accordingly. 
     Check valves  9  and  11  will open to compensate for system fluid changes caused by actuator leakage to the outside environment or system fluid volume changes due to significant thermal changes. Although not shown, a filter unit may be installed in the fluid lines between pump  8  and chambers  2  and  3 . 
     Actuation system  100  provides a number of benefits. Unexpectedly, system  100  provides actuating forces that are high enough to meet the rigorous demands of a subsea environment and subsea systems that require stringent standards and levels of functionality because of the dangers of an uncontrolled release of oil and gas. System  100  allows for variable speed actuation and full control of the location of the actuator within its range of motion. System  100  operates independently of a hydraulic system linked to the ocean surface and is a closed system with self-contained hydraulic supply and return porting and limited fluid contamination and leakage concerns. Power is not required when the system is not in use, which improves efficiency. System  100  also allows for fail safe features which have minimal impact on cost, weight or reliability. 
     The present invention contemplates that many changes and modifications may be made. Therefore, while an embodiment of the improved subsea actuation system has been shown and described, and a number of alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims.

Summary:
A subsea drilling, production or processing actuation system comprising a variable speed electric motor ( 10 ) adapted to be supplied with a current, a reversible hydraulic pump ( 8, 28 ) driven by the motor, a hydraulic piston assembly ( 92, 101, 111, 121, 131 ) connected to the pump and comprising a first chamber ( 2 ), a second chamber ( 3 ) and a piston ( 4 ) separating the first and second chambers and configured to actuate a valve ( 91 ) in a subsea system, a fluid reservoir ( 14 ) connected to the pump and the hydraulic piston assembly, the pump, hydraulic piston assembly and reservoir connected in a substantially closed hydraulic system, and a pressure compensator ( 13, 65 ) configured to normalize pressure differences between outside the hydraulic system and inside the hydraulic system.