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This application is a division of application Ser. No. 09/089,921 filed on Jun. 3, 1998, now U.S. Pat. No. 6,102,828. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates, in general, to safety systems for the production of oil and gas, and in particular to, an electrohydraulic control unit for operating the hydraulic actuator of, for example, surface safety valves and subsurface safety valves. 
   BACKGROUND OF THE INVENTION 
   Formation fluids, including oil and gas produced at a well head are typically conveyed through flow lines to remote processing equipment. It is conventional practice to use surface safety valves which are responsive to changes in operating conditions to automatically shut off flow in the flow lines at the onset of unusual or unscheduled operating conditions. Such surface safety valve installations are designed to automatically close in response to fluctuations in selected conditions in the flow lines, either above or below predetermined settings, such as high and low liquid levels, high and low temperatures, high and low pressures and the like. 
   Conventional surface safety valves typically include a pneumatic or hydraulic actuator coupled to a gate value for selectively permitting or disallowing flow of production fluids through the flow lines. For example, surface safety valves may be installed as a secondary master valve on a well head tree or as a wing valve directly on the flow line. Surface safety valves typically include a valve body having a central axis aligned with inlet and outlet passages and a space therebetween to receive the gate that may be moved perpendicularly to open and close the valve. In the closed position, the gate surfaces typically seal against sealing rings which surround the fluid passage through the valve body. 
   One type of surface safety valve includes a pneumatic actuator that is operated by a pneumatic supply system that is independent of well fluids and pressures. The pneumatic surface safety valve is designed to be held open by pneumatic control pressure acting on an actuator piston. Loss of pneumatic pressure in the actuator cylinder permits the well or flow line pressure acting on the gate along with the force exerted by a closing spring to drive the gate into a closed position. Such an actuator may be termed “fail safe,” since in the event of an emergency causing loss of pneumatic pressure, the actuator will automatically cause the valve to assume the safe or closed state. 
   It has been found, however, that the use of pneumatic controlled actuators for surface safety valves is limited due to the size requirements of the actuator piston needed to operate gate valves particularly for high pressure and high volume flow lines which may require large bore gate valves. In addition, due to condensation and contamination within the air system utilized for pneumatic actuation, it has been found that venting of the air into the atmosphere is environmentally unsatisfactory. 
   To overcome the size limitation of pneumatic controlled actuators, another type of surface safety valve utilizes a hydraulic actuator that employs a hydraulic circuit to operate the actuator and to open and close the surface safety valve. The hydraulic actuators are typically part of a large hydraulic system that is controlled by a remote hydraulic control panel. As with the pneumatic actuators, the hydraulic actuators typically operate by acting hydraulic control pressure on an actuator piston. It has been found, however, that the gate and actuator piston in a hydraulic system will stroke at a limited speed due to the flow rate of hydraulic fluid and the volume of hydraulic fluid that is typically used in hydraulic systems. 
   Along with the surface safety valve on the well head, it is common for producing wells to include a subsurface safety valve located in the well production tubing several hundred feet below the ground surface. Subsurface safety valves may typically be flapper valves or ball valves which may be carried in a tubing connection or may be installed and set in place by wireline. Subsurface safety valves are typically operated using hydraulic fluid to operate the actuator to an open position. As with hydraulic actuated surface safety valves, when an out of range condition occurs, hydraulic pressure is released and the subsurface safety valve will actuate to the valve closed position. It has been found, however, that as with hydraulic actuated surface safety valves, the volume of hydraulic fluid in the hydraulic system affects the speed and depth at which the subsurface safety valves will operate. 
   Therefore, a need has arisen for a control system for actuating surface safety valves and subsurface safety valves that minimizes the volume of hydraulic fluid necessary to operate a hydraulic actuator between the valve closed position and the valve open position. A need has also arisen for such a control system that eliminates the need for a remote hydraulic control panel through which hydraulic fluid is circulated to multiple hydraulically controllable devices. Further, a need has arisen for such a control system that may be attached to existing surface safety valve actuator and subsurface safety valve actuators. 
   SUMMARY OF THE INVENTION 
   The present invention disclosed herein comprises an electrohydraulic control unit for operating the hydraulic actuators of surface safety valves and subsurface safety valves. Each control unit includes a closed loop reservoir of hydraulic fluid that minimizes the volume of hydraulic fluid necessary to operate the actuators and eliminates the need for a remotely located hydraulic control panel by utilizing a low voltage computer operated electrical system for operating the control units. The control units may be attached to existing surface safety valve actuators and subsurface safety actuators thereby minimizing the cost of operating the system of the present invention. 
   The electrohydraulic control unit of the present invention is operably associated with the actuator of a safety valve such that the safety valve may be operated between open and closed positions. The electrohydraulic control unit comprises a housing having a hydraulic fluid reservoir that is in fluid communication with the chamber of the actuator. A piston disposed within the housing regulates the flow of the hydraulic fluid between the hydraulic fluid reservoir and the chamber of the actuator. An electric motor is securably attached to the housing and has a shaft that is selectively rotatable. A converter, such as a planetary gear mechanism, is used to transform the rotational motion of the shaft to translational motion of the piston, thereby shifting the piston of the actuator to operate the safety valve between open and closed positions. 
   The planetary gear mechanism may include a first planetary gear housing coupled to the shaft of the electric motor, one or more gears coupled to the first planetary gear housing, a transmission ring coupled to the gears and a second planetary gear housing coupled to the gears. The second planetary gear housing may be coupled to a circulating ball nut that is coupled to a worm screw. The worm screw is mounted to the piston of the electrohydraulic control unit. 
   The electrohydraulic control unit may include a torque limiter that is selectively engageable with the planetary gear mechanism to selectively permit and prevent the translational motion of the piston. The torque limiter may comprises a solenoid operably supporting a locking member that is selectively engageable with a recess in the transmission ring. One or more centralizers may also engage recesses in the transmission ring. 
   In the method of the present invention, a safety valve is actuated between open and closed positions by operably coupling an electrohydraulic control unit to the actuator of the safety valve to provide a path for fluid communication between a hydraulic fluid reservoir of the electrohydraulic control unit and the actuator. The piston disposed within the hydraulic fluid reservoir is then used to regulate the flow of the hydraulic fluid between the hydraulic fluid reservoir and the actuator. Upon energizing an electric motor to selectively rotate a shaft, the rotational motion of the shaft is transformed into translational motion of the piston to actuate the safety valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, references now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
       FIG. 1  is a side elevation view, partly in section, of an electrohydraulic control unit of the present invention coupled to a hydraulic actuator of a surface safety valve shown in the closed position; 
       FIG. 2  is a side elevation view, partly in section, of an electrohydraulic control unit of the present invention coupled to a hydraulic actuator of a surface safety valve shown in the open position; 
       FIG. 3  is a quarter sectional view of an electrohydraulic control unit of the present invention; 
       FIG. 4  is a top view, partly in section, of an electrohydraulic control unit of the present invention; 
       FIG. 5  is a top view, partly in section, of an electrohydraulic control unit of the present invention; and 
       FIG. 6  is a schematic illustration of a well head installation including two electrohydraulic control units operating surface safety valves and one electrohydraulic control unit operating a subsurface safety valve. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the making and using of various embodiments of the present invention is discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
   Referring now to  FIG. 1 , an electrohydraulic control unit coupled to a hydraulic actuator of a surface safety valve is depicted in generally designated  10 . Electrohydraulic control unit  12  is coupled to hydraulic actuator  14  via hydraulic coupling  16 . Hydraulic actuator  14  is assembled onto surface safety valve  18  which is designed for controlling the flow of fluids through a flow line from a source, such as a well head, to a remote processing or storage location. Surface safety valve  18  is adapted for connection into the flow line adjacent to the well for shutting flow through the flow line at the well head in the event that the pressure in the flow line downstream of surface safety valve  18  exceeds a predetermined maximum value or falls below a predetermined minimum value. Hydraulic actuator  14  is designed to close surface safety valve  18  quickly upon loss of hydraulic pressure from electrohydraulic control unit  12 . 
   Hydraulic actuator  14  is energized by a closed loop, environmentally sound hydraulic system contained within electrohydraulic control unit  12  and hydraulic actuator  14 . Surface safety valve  18  is initially opened by transmitting hydraulic fluid from the hydraulic reservoir located in chamber  20  of electrohydraulic control unit  12  to chamber  22  of hydraulic actuator  14 . Hydraulic fluid is transferred from electrohydraulic control unit  12  to hydraulic actuator  14  by operation of piston  24  of electrohydraulic control unit  12  against the hydraulic fluid within chamber  20 . The hydraulic fluid is forced out of chamber  20  through hydraulic coupling  16  and into chamber  22  of hydraulic actuator  14 . The hydraulic fluid acts on piston  26  to drive surface safety valve  18  to the fully opened position, as best seen in FIG.  2 . Loss of hydraulic pressure within chamber  22  of hydraulic actuator  14  allows the pressure within the flow line to act on gate stem  28  in combination with a bias force exerted by closing spring  30  to detract piston  26  and move valve gate  32  to a position that blocks the flow through bore  34  of surface safety valve  18 , as best seen in FIG.  1 . Bore  34  of surface safety valve  18  provides a longitudinal flow passage for connection into a production flow line. Surface safety valve  18  includes a body portion  36  through which bore  34  is formed and in which a gate cavity  38  is formed. Valve body portion  36  may be securely coupled to the flow line by connector bolts or studs which are extended through aligned apertures  40  in flange  42  and flange  44 . 
   Seat recesses  46 ,  48  are formed internally of valve body portion  36  and are adapted to receive valve seat elements  50 ,  52  respectively. Valve seat elements  50 ,  52  have annular grooves formed on their facing surfaces for receiving annular face seal rings  54 , while the opposite side surfaces of each valve seat element  50 ,  52  receives rear sealing elements  56 . 
   Connection between valve gate  32  and gate stem  28  is accomplished by threaded coupling  58  which is secured against release by a connection pin  60 . In the open position, as seen in  FIG. 2 , valve gate  32  is slidably receive within gate cavity  62 . In this configuration, aperture  64  of valve gate  32  is aligned with bore  34  of surface safety valve  18  such that fluid flow may proceed through the flow line. 
   Surface safety valve  18  will remain in the open position as long as hydraulic pressure is maintained within chamber  22  of hydraulic actuator  14 . In the event hydraulic control pressure is interrupted, for example in response to operator control or in response to the detection of an unscheduled operating condition such as a sudden decrease or increase in flow line pressure, closing spring  30  will drive piston  26  upwardly thus moving valve stem  28  and gate  32  toward the closed position, as seen in FIG.  1 . In this configuration, valve gate  32  engages annular sealing ring  54  for blocking flow through bore  34  of surface safety valve  18 . 
   Hydraulic actuator  14  includes a bonnet  66  for coupling hydraulic actuator  14  onto surface safety valve  18 . Body portion  36  of surface safety valve  18  has a coupling collar  68  for engaging bonnet  66 . Bonnet  66  is provided with an annular flange  70  which is secured onto coupling collar  68  by threaded bolt connectors  72 . 
   Gate stem  28  extends through the center of bonnet  66  and has a sealing engagement against static annular packing seal  74 . Packing seal  74  is retained within bonnet  66  by a threaded packing collar  76 . The engagement of packing seal  74  against gate stem  28  produces a fluid seal to prevent exposure of the internal components of hydraulic actuator  14  to flow line fluids. 
   The upper end of hydraulic actuator  14  includes a top plate  78 . Top plate  78  includes a bore  80 . Gate stem  28  includes an indicator stem  82  that projects through bore  80  for providing an indication of the operating mode of surface safety valve  18 . In the valve open mode, indicator stem  82  is barely visible. When surface safety valve  18  has been actuated to the valve closed position, however, indicator stem  82  will project substantially above top plate  78  to indicate and verify the closed position of valve gate  32 . A fluid seal is provided between indicator stem  82  and bore  80  by annual packing assembly  84 . 
   The operation of surface safety valve  18  and hydraulic actuator  14  is controlled by electrohydraulic control unit  12 . Electrohydraulic control unit  12  may be interchangeably attached to a variety of hydraulic actuators  14  which operate surface safety valves  18  of a variety of sizes and pressure ratings. Electrohydraulic control unit  12  is light weight and requires minimum space for installation and operation. Electrohydraulic control unit  12  is suitable for use in high pressure and high temperature service. Electrohydraulic control unit  12  includes an electric motor  86  which is flange mounted via threaded bolt connectors  88  to housing  90  of electrohydraulic control unit  12 . Electric motor  86  may be operated by a computer controlled electrical system which may operate at a low voltage, such as 24 volts. 
   As best seen in  FIG. 3 , electrohydraulic control unit  12  utilizes a planetary gear and worm screw to transform the rotary motion of electric motor  86  into translation motion of piston  24 . This design requires minimum maintenance due to the low stresses in the gear reduction system. Extending outwardly from electric motor  86  is motor drive shaft  92  including key  94 . Motor drive shaft  92  extends into housing  90  and is coupled to top planetary gear housing  96 . A spacer  98  supports top planetary gear housing  96  such that top planetary gear housing  96  remains engaged with key  94 . Spacer  98  may typically be constructed of a packing material. A bearing  100  is disposed between spacer  98  and top planetary gear housing  96  to provide further support to top planetary gear housing  96  and to allow top planetary gear housing  96  to rotate about the axis of electrohydraulic control unit  12 . 
   Top planetary gear housing  96  is coupled with gears  102 , only one of which is depicted. Gears  102  rotate about gear retaining pins  104 . Gears  102  are supported by top load bearing  106  and bottom load bearing  108 . Gears  102  engage transmission ring  110 . The rotation of transmission ring  110  is controlled by solenoid  112  which selectively engages a locking member such as, drive pin  114  and ball  116 , with transmission ring  110  as will be more fully explained with reference to FIG.  4 . Solenoid  24  may be operated by a computer controlled electrical system that may operate at a low voltage, such as 24 volts. 
   Gears  102  engage bottom planetary gear housing  118 . Bottom planetary gear housing  118  is supported by spacer  120 . Disposed between spacer  120  and bottom planetary gear housing  118  is side load bearing  122  that allows bottom planetary gear housing  118  to rotate about the axis of electrohydraulic control unit  12 . Bottom planetary gear housing  118  engages circulating ball nut  124 . Circulating ball nut  124  includes a plurality of balls  126  that engage the threads of worm screw  128  which is coupled to piston  24 . 
   In operation, when electric motor  86  and solenoid  112  are energized by the electrical system, drive pin  114  engages ball  116  with transmission ring  110  and drive shaft  92  rotates. The rotary motion of drive shaft  86  is then transformed into translation motion of piston  24 . Specifically, drive shaft  92  imparts rotation to top planetary gear housing  96  via key  94 . Rotary motion of top planetary gear housing  96  rotates gears  102  within the stationary transmission ring  110  which is fixed due to the engagement of ball  116  which is controlled by solenoid  112 . The rotation of gears  102  within transmission ring  110  allows transmission of torque to bottom planetary gear housing  118 . The rotary motion of bottom planetary gear housing  118  about the axis of electrohydraulic control unit  12  causes balls  126  to circulate within circulating ball nut  124  which imparts linear motion to worm screw  128 . As worm screw  128  translates, piston  24  is driven downwardly, thereby forcing hydraulic fluid from chamber  20  of electrohydraulic control unit  12  into chamber  22  of hydraulic actuator  14  which actuates surface safety valve  18  as discussed with reference to  FIGS. 1 and 2 . Once surface safety valve  18  is actuated to the open position, a position indicator (not shown) within surface safety valve  18  may signal the electrical system to switch off electric motor  86 . Solenoid  112 , however, remains energized such that ball  116  is engaged with transmission ring  110 . In this configuration, piston  24  is held in place as rotation of circulating ball nut  124  is prevented. 
   In the event that electrical power is interrupted to solenoid  112 , for example in response to the emergency shut down system, operator control, failure in the electrical system or in response to the detection of an unscheduled operating condition such as a sudden decrease or increase in flow line pressure, drive pin  114  is retracted into solenoid  112  such that ball  116  disengages transmission ring  110 . Once transmission  110  is free to rotate, gears  102 , bottom planetary gear housing  118  and circulating ball nut  124  are free to operate such that the hydraulic pressure exerted against piston  24  causes worm screw  128  to translate without the transmission of torque through to top planetary gear housing  96  or electric motor  86 . As worm screw  128  translates, hydraulic fluid is returned to chamber  20  of electrohydraulic control unit  12  from chamber  22  of hydraulic actuator  14  via hydraulic coupling  16 . As the hydraulic pressure holding piston  26  of hydraulic actuator  114  is removed and closing spring  30  upwardly shifts piston  26 , surface safety valve  18  returns to the closed position as described above with reference to  FIGS. 1 and 2 . In order to further protect electric motor  86  from reverse drive during the closing of surface safety valve  18 , a sprag clutch  130  may be position between drive shaft  92  and housing  90  of electrohydraulic control unit  12 . 
   Referring now to  FIG. 4 , one embodiment of a transmission ring retention system is depicted. Transmission ring  110 , gears  102 , bottom planetary gear housing  118  and worm screw  128  are housed within housing  90  of electrohydraulic control unit  12 . Transmission ring  110  includes four recesses  130  for receiving ball  116  when solenoid  112  is energized. Recesses  130  are designed to prevent rotation, in either direction, of transmission ring  110  within predetermined torque conditions. For example, each recess  130  may include a steep sloping surface  132  and a gradual sloping surface  134  depending upon the desired maximum allowable torque for rotation in a particular direction. The torque limitation feature of the present invention may be used to prevent over torque of electric motor  86  or to allow surface safety valve  18  to close if the hydraulic pressure exerted on piston  24  exceeds a predetermined level. It should be noted by one skilled in the art that the exact profile of recesses  130  will be selected based upon factors including the capacity of electric motor  86 , the size of electrohydraulic control unit  12  as well as the production rate and pressure of fluids being produced through the flow lines. 
   Even though the locking member has been described as solenoid  24  operating pin  114  and ball  116 , it should be understood by one skilled in the art that other locking members having alternate configuration may be used and that fall within the scope of the present invention. For example, the solenoid may have a direct engagement with transmission ring  110 . Likewise, alternative designs may be desirable depending upon the profile of recesses  130 . For example, a cylindrical or barrel shaped member may be used instead of ball  116  to engage transmission ring  110 . 
   Referring now to  FIG. 5 , an alternate embodiment of a transmission ring retention system is depicted. In this embodiment, in addition to ball  116  associated with solenoid  112 , additional balls  116  are mounted within centralizers  136  to engage transmission ring  110 . Centralizers  136  are spaced around transmission ring  110  at ninety degree increments. Centralizers  136  are used to equalize the forced distribution about transmission ring  110  which would otherwise be transmitted to rotating components within electrohydraulic control unit  12 . Centralizers  136  help to stabilize transmission ring  110  and may provide additional torque control over transmission ring  110  if, for example, suitably high bias springs within centralizers  136  are used to engage balls  116  with transmission ring  110 . 
   Even though electrohydraulic control unit  12  has been described with reference to actuating valve gate  32  of surface safety valve  18 , it should be understood by one skilled in the art that electrohydraulic control unit  12  of the present invention is well suited for hydraulically actuating other devices when it is desirable to have a closed loop hydraulic system including, but not limited to, ball valves and choke valves. For example, as depicted in  FIG. 6 , three electrohydraulic control units are operating at a well head installation that is generally designated  140 . Electrohydraulic control units  142 ,  144  are respectively coupled to hydraulic actuators  146 ,  148 . Hydraulic actuator  146  may be used to operate a crown valve while hydraulic actuator  148  may be used to operate a surface safety valve such as that discussed with reference to  FIGS. 1 and 2  above. Electrohydraulic control unit  150 , however, is being used to control the actuation of subsurface safety valve  152 . Subsurface safety valve  152  is disposed in well production tubing  154  and may be several hundred feet below the ground surface. Subsurface safety valve  152  may utilize a flapper valve or a ball valve to shut-in production through tubing  154 . The operation of subsurface safety valve  152  is controlled by electrohydraulic control unit  150  via hydraulic control line  156 . Together, electrohydraulic control unit  150 , hydraulic control line  156  and the actuation device of subsurface safety valve  152  provide a closed loop hydraulic system which is suitably controlled by electrohydraulic control unit  150  in the manner described above with reference to FIG.  3 . 
   While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
An electrohydraulic control unit for operating an actuator of a hydraulically controllable device is disclosed. The electrohydraulic control unit comprises a housing including a chamber for hydraulic fluid. The housing is coupled to the actuator such that a path for fluid communication between the chamber and the actuator is created. Disposed within the chamber is a piston for regulating the flow of the hydraulic fluid between the chamber and the actuator. An electric motor is securably attached to the housing. The electric motor includes a selectively rotatable shaft. The electrohydraulic control unit also comprises a converter configured to transform the rotational motion of the shaft to translational motion of the piston, thereby controlling the actuation of the hydraulically controllable device.