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[0001]    The following invention relates to an actuator for a valve in a subsea installation. 
       BACKGROUND OF THE INVENTION 
       [0002]    The production from a subsea well is controlled by a number of valves that are assembled into a Christmas tree. The actuation of the valves is normally dependent upon hydraulic fluid to operate hydraulic actuators for the valves and is therefore entirely dependent upon an external source for the supply of the hydraulic fluid. Hydraulic fluid is normally supplied through an umbilical running from a station located on a vessel on the surface or, less commonly, from a land based station. Usually the actuators are controlled by pilot valves housed in a control module located at or near the subsea installation. The pilot valves direct the supply of fluid to each actuator, as dictated by the need for operation. The pilot valves may be operated by electric means, and such a system is therefore called an electrohydraulic system. 
         [0003]    The design of actuators and valves for subsea wells is dictated by stringent demands on the standard and function for these valves because of the dangers of uncontrolled release of hydrocarbons. A typical demand is that these valves must be failsafe closed, meaning that they must close upon loss of power or control. The only practical means today in subsea environments is to use springs that are held in the compressed state by the hydraulic pressure, keeping the valve open, and are released in the event of loss of hydraulic pressure, thus closing the valve. The spring force needed to close a valve is dependent on both the well pressure and the ambient pressure, with larger ambient pressure demanding larger springs. 
         [0004]    For the control of subsea wells, a connection between the well and a monitoring and control station must be established. This station can either be located in a floating vessel near the subsea installation or in a land station a long distance away. Communication between the control station and the subsea installation is normally provided by installing an umbilical between the two points. The umbilical contains lines for the supply of hydraulic fluid to the various actuators in or by the well, electric lines for the supply of electric power and signals to various monitoring and control devices, and lines for signals to pass to and from the well. This umbilical is a very complicated and expensive item, costing several thousand dollars per meter. 
         [0005]    It would therefore be very cost-saving to be able to eliminate the umbilical. In the invention, the standard hydraulic lines can be eliminated while maintaining the standard hydraulic spring-operated failsafe system. 
         [0006]    European Patent application No. 1209394 discloses an electro-hydraulic control unit with a piston/cylinder arrangement in which the piston divides the cylinder into two chambers, a fluid connection between the two chambers, and a valve to configure the fluid flow such that pressurized hydraulic fluid may only flow in one direction and not in two directions. 
         [0007]    U.S. Pat. No. 6,269,874 discloses an electro-hydraulic surface controlled subsurface safety valve actuator that comprises an electrically actuated pressure pump and a dump valve that is normally open so that if power fails, the pressure is released and the safety valve closes. 
         [0008]    An electro-hydraulic actuator similar to the present invention is also known that comprises a pump unit that supplies hydraulic fluid under pressure to a piston so as to move the valve to its open position. The actuator is intended to be fitted to an existing valve. In this disclosed actuator, the failsafe function is provided by a two-way valve that is located in a bypass fluid line connecting one side of the piston with a drain accumulator. The two-way valve is fitted with a solenoid that closes the bypass line and a spring that biases the two-way valve towards a position where fluid can flow through the bypass line. Upon loss of power to the solenoid, the spring will force the two-way valve to open the bypass passage. The solenoid must therefore be strong enough to withstand the force of the spring. Also, the spring must be strong enough to force the valve open against the pressure in the line. This pressure can be very high, especially in systems located at great depths. 
         [0009]    The object of the present invention is to provide a smaller and more compact actuator that can be releasably fitted to the valve and which overcomes the disadvantages related to the prior art. 
         [0010]    A further objective of the invention is to provide an actuator that overcomes the shortcomings identified above. 
         [0011]    The invention is set forth in the independent claim  1 , while the dependent claims define other characteristics of the invention. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention relates to an actuator comprising a bypass control unit providing a failsafe function to a valve, wherein a fluid in a hydraulic line acts on the movable element in the bypass control unit, forcing the control unit towards opening a passage to drain the fluid in the actuator and thereby enable a valve spring to close the valve. 
         [0013]    An actuator for a valve is provided which is suitable for use in a subsea installation, the valve having a valve element that is movable between a first position and a second position and a spring that biases the valve element to the first position, the actuator comprising an exchangeable actuator module comprising a hydraulic cylinder with a movable piston, the piston being mechanically connectable to said valve element, a fluid pump operatively connected to said hydraulic cylinder, said fluid pump being adapted to supply fluid through a first hydraulic line which is operatively connected between said hydraulic cylinder and said fluid pump, said first hydraulic line supplying a sufficient pressure to move the piston to cause the valve element to move to the second position, a reservoir for storing a quantity of hydraulic supply fluid, said fluid in said hydraulic fluid supply reservoir having a pressure that is less than said sufficient pressure, a second hydraulic line providing a fluid passage between the first hydraulic line and said hydraulic fluid supply reservoir, a bypass control unit located in said second hydraulic line and having a movable element operating to open or close said fluid passage and provide a failsafe function to the valve, first and second force generating means, both of which bias the movable element to close the fluid passage, and a third force generating means which biases the movable element to open the fluid passage. 
         [0014]    In an embodiment of the invention, during use of the actuator the third force generating means is a pressurized fluid. 
         [0015]    According to an aspect of the invention, the first force generating means is a solenoid, optionally in combination with a lever. 
         [0016]    According to an aspect of the invention, the second force generating means is a spring. 
         [0017]    In an aspect of the invention, the movable element is a piston arranged in a casing. 
         [0018]    In an embodiment of the invention, a seal is arranged at a circumference of the piston when the fluid passage is closed, and a retainer element is arranged within the casing to retain the seal in place when the fluid passage is open. The retainer element may be arranged to follow the movements of the piston, e.g., by being fixed to the piston or being pre-tensioned by a spring or similar means. 
         [0019]    In an embodiment of the invention, the retainer element abuts the piston when the fluid passage is closed and follows the movements of the piston until a stop element on the retainer element comes into contact with a stop surface in the casing. In this embodiment, the fluid passage comprises an axial gap formed between the retainer element and the piston, and the fluid passage extends through the axial gap to an outlet in the casing. 
         [0020]    In an aspect of the invention, the retainer element abuts the piston when the fluid passage is closed and is adapted to follow the movements of the piston, and the retainer element is formed with at least one radial through-going opening forming a part of the fluid passage when the fluid passage is open. 
         [0021]    The second force generating means may in an aspect of the invention be configured to apply a force on the movable element which is greater than the force applied by the first force generating means on the movable element. 
         [0022]    A third hydraulic line may be connected to the second hydraulic line and act on the movable element to force the bypass control unit towards opening said fluid passage. 
         [0023]    The control unit may comprise a permanent magnet which is adapted to apply a force that overrides the force of the first and second force generating means, thereby forcing the fluid passage open. 
         [0024]    More specifically, the present invention relates to an actuator for a valve in a subsea installation, the valve having a valve element that is movable between a first position and a second position and a spring biasing that biases the valve element to the first position, the actuator comprising an exchangeable actuator module comprising a cylinder with a movable piston, the piston being mechanically connectable to said valve element, a fluid pump operatively connected to said hydraulic cylinder, said fluid pump being adapted to supply fluid through a first hydraulic line which is operatively connected between said hydraulic cylinder and said fluid pump, said first hydraulic line supplying a sufficient pressure to move the piston to cause the valve element to move to the second position, a reservoir for storing a quantity of hydraulic supply fluid, said fluid in said hydraulic fluid supply reservoir having a pressure that is less than said sufficient pressure, a second hydraulic line providing a fluid passage between the first hydraulic line and said hydraulic fluid supply reservoir, a bypass control unit located in said second hydraulic line and having a movable element operating to open or close said fluid passage and provide a failsafe function to the valve, wherein a third hydraulic line is connected to the second hydraulic line and acts on the movable element, forcing the bypass control unit towards opening said fluid passage. 
         [0025]    In an aspect of the invention the bypass control unit may be a two-way valve. The two-way valve may comprise a spring which is adapted to move the movable element to close the fluid passage. The force of the spring is preferably less than the force of fluid acting on the movable element. 
         [0026]    The bypass control unit may in an aspect of the invention comprise a solenoid for holding the moveable element in a position to close the fluid passage. The control unit, in the form of the two-way valve comprising the movable element, may have pressurized hydraulic fluid acting on one side of the movable element. The spring, the solenoid, and/or a combination of the spring and the solenoid may act on the other side of the movable element. The spring may be calibrated to have a force that is a little less than the force of the fluid acting on the other side. The solenoid can therefore be made such that its holding power may be small. 
         [0027]    In an embodiment of the invention the bypass control unit may comprise a lever that on one side is hinged to a block containing the fluid passage and on the other side is held by an electro-magnet. By positioning the lever such that the movable element acts on a fulcrum system the holding force of the magnet can again be very small. Further, in a first position, the lever acts on the movable element to close the fluid passage and, in a second position, the lever enables the pressure in the third hydraulic line to move the movable element to open the fluid passage. 
         [0028]    In an aspect of the invention, the lever may be held in the first position by an electro-magnet. 
         [0029]    In an aspect of the invention, the fluid pump may be driven by an electric motor. 
         [0030]    In an aspect of the invention, the piston may be releasably connected to the valve element. 
         [0031]    The actuator may further comprise a one-way valve arranged in said first hydraulic line to enable fluid to flow in only one direction from the fluid pump to the cylinder. 
         [0032]    In yet another aspect of the invention the actuator may further comprise a fourth fluid line connecting said hydraulic fluid supply reservoir to the pump. 
         [0033]    In an embodiment of the invention, the actuator is a module that can releasably connected to the valve. 
         [0034]    In an aspect of the invention, the actuator comprises a fluid line connecting said fluid supply reservoir with the pump. 
         [0035]    It is obvious for the person skilled in the art that the different features in the different aspects or embodiments may be combined in any way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention will now be described in a non-limiting embodiment, with reference to the accompanying drawings, wherein: 
           [0037]      FIG. 1  is a drawing of the valve and actuator module of the present invention; 
           [0038]      FIG. 2  is a drawing showing the detachable actuator module in the first position; 
           [0039]      FIG. 3  is a drawing showing the detachable actuator module in the second position; 
           [0040]      FIG. 4  is a schematic fluid diagram illustrating the fail-safe function of the actuator; 
           [0041]      FIG. 5  is a drawing of an embodiment of a fail-safe mechanism in the second position; 
           [0042]      FIG. 6  is a drawing of the embodiment of the fail-safe mechanism of  FIG. 5  in the first position; and 
           [0043]      FIG. 7  is a drawing of a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    In  FIG. 1  is shown a valve  100  to be operated by the actuator  10  of one embodiment of the present invention. The valve  100  has a valve element  102  which is connected to a valve stem  104  that extends into and through a spring housing  106 . A spring  108  is located in the spring housing  106  between an end plate  103  and a movable spring actuating flange  110 . The valve stem  104  extends into the spring housing  106  through the end plate  103  and is rigidly attached to the spring actuating flange  110 . At this attachment point the spring actuating flange  110  has a recess  105  for the actuator  10 . As can be understood from  FIG. 1 , when the spring actuating flange  110  is moved to the right, the spring  108  will be compressed and at the same time the movement of the valve stem  104  will cause the valve element  102  to move to its open position. 
         [0045]    The actuator  10  is shown in  FIGS. 2 and 3 . The actuator  10  comprises a housing  12  which includes a portion that defines a cylinder  12   a . A piston  14  is axially movable in the cylinder  12   a  between a first position and a second position. A stem  16  is attached to the piston  14  and extends through the housing  12 . The stem  16  has a first end  17  that connects into the recess  105  and a second end  18  with a linear override tool  20  which is used for manually moving the stem  16 , e.g. using an ROV tool. The whole actuator  10  can be locked into the valve housing. 
         [0046]    The piston  14  includes seals to seal the piston  14  against the cylinder  12   a , as is well known in the art. The piston  14  defines first  13  ( FIG. 3 ) and second  15  ( FIG. 2 ) chambers in the housing. The housing  12  further contains a pump unit comprising a pump  47  and a motor  31 . Two pump units may be located inside the housing  12  to allow for redundancy in case one unit fails. 
         [0047]    The various parts of the actuator  10  are in communication with an electronics module (not shown). The electronics module may be in communication with a remote station (not shown), to receive power and communication signals therefrom. 
         [0048]    In the preferred embodiment, the motor  31  is a brushless DC motor. Also, in the preferred embodiment the electronics module includes a battery to provide primary power to the motor and the solenoid. The battery is trickle-charged from a local power source or from the surface. Because of this, only a small cable is needed to provide power to the battery. Alternatively, primary power can be supplied from the remote station. 
         [0049]    The hydraulic circuit is shown schematically in  FIG. 4 . A fluid line  42  connects the second chamber  15  with a variable volume fluid accumulator  44 . A fluid line  45  connects the accumulator  44  with the intake side of a pump  47 . A fluid line  36  connects the first chamber  13  with the pressure side of the pump  47 . A one-way valve  34  is installed in fluid line  36 , allowing fluid to only flow towards chamber  13 . 
         [0050]    To move the valve element  102  to its open position, the motor  31  is operated to drive the pump  47 . The pump  47  will pump fluid from the accumulator  44  (through line  45 ) to the first chamber  13  of the actuator  10  to displace the piston  14  from its first position ( FIG. 2 ) to its second position ( FIG. 3 ). Because the actuator stem  16  abuts recess  105  in spring retaining plate, i.e., the spring actuating flange  110 , it will move the spring retaining plate, and with it the valve stem  104 , to open the valve. 
         [0051]    Consequently, this will move valve stem  104 , causing the valve element  102  to open. A pressure sensor may be arranged in fluid line  36  to shut off power to the motor  31  when the pressure has reached a predetermined level sufficient to drive the valve stem  106  against the power of spring  108 . The one-way valve  34  ensures that the valve element  102  is held in the open position. Alternatively, a position sensor may be located in the actuator to stop the pump when the piston  14  has reached a predetermined position. 
         [0052]    A control valve  39  (i.e., a bypass control unit) is located in an additional fluid line  48  (i.e., a fluid passage) that interconnects pressure line  36  (i.e. the first hydraulic line) with the accumulator  44 . The control valve  39  is movable between a closed position which closes off fluid flow through line  48 , and an open position which allows fluid flow through line  48  to thereby drain fluid from the first chamber  13  to the accumulator  44 . A first force generating means, which in this embodiment is a solenoid  38 , is arranged to move the control valve  39  between the open and closed positions. 
         [0053]    When it becomes necessary to close the valve element  102 , the solenoid  38  is operated to open control valve  39 . This opens the fluid communication path from the first chamber  13  through lines  36  and  48  and to the accumulator  44 . Since the pressure now is equalized on each side of the piston  14 , the spring  108  will force the piston stem  16  backwards to its first position and the valve  100  will close as fluid is transferred to the second chamber  15  from the first chamber  13 . 
         [0054]    In the invention, means are arranged to minimize the forces needed to operate the control valve and to ensure a failsafe operation of the valve element  102 . In a first embodiment of the invention a fluid line  49  connects the fluid line  36  with one side of control valve  39 . When the valve element  102  is open, the pressure in fluid line  49  (which acts as a third force generating means) will act on one side, i.e. the first side, of the control valve  39  to push it towards its open position. On the other side of the control valve  39  is an adjusting spring  40  (which acts as a second force generating means) that acts on the other side, i.e. the second side, of the control valve  39  to push it towards its closed position. The adjusting spring  40  is adjusted so that the force acting on the second side is smaller than the fluid pressure from line  49  acting on the first side. The solenoid  38  will when energized also act on the second side of control valve  39 , i.e. on the same side as the adjusting spring  40 . In this way, the control valve  39  may be held in its closed position against the fluid pressure in line  49 . If current to the solenoid is removed, as for example if the power supply fails, the pressure in line  49  will push the control valve  39  to its open position against the force of the adjusting spring  40  to drain the first chamber  13  of the actuator and close the valve element  102 , as explained above. The use of the adjusting spring  40  will ensure that the difference in forces acting on the solenoid can be made very small and that therefore very little holding power will be required of the solenoid. 
         [0055]    In an embodiment of the invention shown in  FIGS. 5 and 6 , the failsafe function is based on a holding electromagnet, i.e. a solenoid (which acts as the first force generating means). In this embodiment the control valve includes a housing (or casing)  50  having a first bore  52 . One side of the first bore  52  is connected to a fluid line  49   b . In the first bore  52  is arranged a movable piston  54 . A second bore  56  intersects the first bore  52  and is connected to fluid line  48  ( FIG. 6 ). A lever or fulcrum  60  is at one end connected to the housing  50  with a hinge  61 . The piston  54  abuts the lever at a point  65  near hinge  61 . The piston  54  is biased by the spring  40  (i.e., the second force generating means) towards the closed position. As can be seen in  FIG. 5 , when lever  60  is in its “down” position, the movable piston  54  covers the second bore  56  and thus shuts off flow through fluid line  48 . In this position the lever  60  is held by an electromagnet  62 . If current to the electromagnet is shut off, pressure in line  49   b  (i.e., the third force generating means) will force the movable piston  54  upwards and uncover bore  56 , thus opening flow through line  48  to accumulator  44 . As described above, this will open the valve element  102 . 
         [0056]    Positioning the movable piston  54  near the hinge point of the lever  60  ensures that the holding force needed by the electromagnet  62  to hold the lever  60  in its “down” position can be made very small. Typically, the ratio between the two lengths may be somewhere between one in five (1:5) and one in ten (1:10), depending on the pressure rating of the pump and accumulator. This again depends on the well pressure (when the main valve is a valve on a Christmas tree) and, when this is a subsea accumulator, the depth of the seabed. 
         [0057]      FIG. 7  discloses a second embodiment of the invention. Similar to the first embodiment described above, a motor is operated to drive the pump  47 . The valve element  102  is operated between its open and closed position by different fluid pressure in hydraulic line  36  created by the pump  47 . The pump  47  will pump fluid from the accumulator  44  to the first chamber  13  of the actuator  10  through fluid line  36  (i.e., a first hydraulic line) to displace the piston  14  from its first position (ref.  FIG. 2 ) to its second position (ref.  FIG. 3 ). In this second embodiment, the failsafe function is based on a solenoid  81 , a lever  77  and a spring  71  operating the movable element, which in this case is a piston  70  movably positioned in a casing  75 , e.g. a cylinder. The movable element  70  is held in its closed position by a first force generating means, i.e. the solenoid  81  amplified by the lever  77 , and a second force generating means (i.e., the spring  71 ). Both the first (solenoid  81  and lever  77 ) and second (spring  71 ) force generating means bias the piston  70  to close the fluid passage  48 . A third force generating means (i.e., pressurized fluid from the hydraulic line  36 ,  48 ) biases the movable element  70  to open the fluid passage  48 . 
         [0058]    In situations where it is desirable to hold or keep the fluid passage  48  open, one may apply a force on the lever  77  operating in an opposite direction of the solenoid  81  by the use of a permanent magnet  76 . The permanent magnet  76  is adapted to apply a force large enough to override the force of the first and second force generating means. 
         [0059]    When the fluid passage  48  is closed, the spring  71  and solenoid  81  and the lever mechanism  77  provide a greater force on the piston  70  than the force provided from the pressurized fluid on the other end of the piston  70 . The piston  70  (and piston stem) is configured to float in the cylinder, e.g. the casing  75 . 
         [0060]    In use, a seal  72 , e.g. an O-ring, is arranged at a circumference of the piston  70  when the fluid passage  48  is closed. The piston secures the seal  72  in place as the seal  72  provides a sealing function around the piston  70 . A retainer element  73  is arranged inside the same casing  75  as the piston  70  in an abutting relationship relative the piston  70  and follows the movements of the piston  70  within the casing  75 . In  FIG. 7 , a pre-tensioned spring  74  acts on the retainer element  73  and moves the retainer element  73  towards the piston  70 . In this closed position of the fluid passage  48 , the piston  70  closes off the fluid passage  48  through the control valve  82  to an outlet  79 . However, when the fluid passage  48  is open, i.e. when the force from the third force generating means (i.e., pressurized fluid in line  36 ) is greater than the combined total force from the first and second force generating means (spring  71  and solenoid  81  and lever  77 ), the piston  70  is moved to an end position in the casing  75  (i.e. downwardly in  FIG. 7 ), allowing fluid to exit the control valve  82  through the outlet  79 . The function of the retainer element  73  is to hold the seal  72  in place when the passage is open (i.e. when the piston  70  is no longer holding the seal  72 ). The retainer element  73  may be a sleeve, axial extending fingers, or any other means performing the function of holding the seal  72  in place when the piston  70  is moved to the end position. 
         [0061]    A compensator device  83  can be arranged in the hydraulic line  36  to maintain a constant pressure in the hydraulic line  36  and to mitigate any leaks. 
         [0062]    The actuator can be made very small and compact. It may be releasable connected to the valve  100 , making it easy to retrieve and replace for repairs or maintenance. The modularized arrangement also makes it possible to exchange this actuator with an all-electric actuator. 
         [0063]    It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention as laid out in the attached claims. For example, the invention may be used with a pressure intensifier instead of a pump.

Summary:
The following invention relates to an actuator for controlling a valve ( 100 ) in a subsea installation, the valve having a spring ( 108 ) to return the valve to a failsafe condition upon loss of pressure. The actuator comprises an actuator module ( 10 ) that is exchangeably connected to the valve and comprising a cylinder with a movable piston ( 14 ), the piston being mechanically connected to said valve element, a fluid pump ( 47 ) operatively connected to said hydraulic cylinder, said pump adapted to supply fluid through a first hydraulic line ( 36 ) operatively connected between said hydraulic cylinder and said pump, said hydraulic line supplying a sufficient pressure to move the piston to said second position to cause the valve element to move to its second position, and a second hydraulic line ( 48 ) providing a fluid passage between the first hydraulic line and a reservoir ( 44 ) whereby a bypass control unit ( 39 ) is located in said second hydraulic line and comprising a movable element operating to open or close said fluid passage through the second hydraulic line providing a failsafe function to the valve. According to the invention a third hydraulic line ( 49 ) is connected to the second hydraulic line and acting on the movable element, forcing the control unit towards opening said passage.