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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 61/791,615, filed Mar. 15, 2013, the full disclosure of which is hereby incorporated by reference herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present disclosure relates in general to dampening the opening and closing of hydraulic actuators for mud lift pump valves by providing cavities for hydraulic fluid accumulation in the actuators. 
     2. Description of Prior Art 
     Subsea drilling systems typically employ a vessel at the sea surface, a riser connecting the vessel with a wellhead housing on the seafloor, and a drill string. A drill bit is attached on a lower end of the drill string, and used for excavating a borehole through the formation below the seafloor. The drill string is suspended subsea from the vessel into the riser, and is protected from seawater while inside of the riser. Past the lower end of the riser, the drill string inserts through the wellhead housing just above where it contacts the formation. Generally, a rotary table or top drive is provided on the vessel for rotating the string and bit. Drilling mud is usually pumped under pressure into the drill string, and is discharged from nozzles in the drill bit. The drilling mud, through its density and pressure, controls pressure in the well and cools the bit. The mud also removes formation cuttings from the well as it is circulated back to the vessel. Traditionally, the mud exiting the well is routed through an annulus between the drill string and riser. However, as well control depends at least in part on the column of fluid in the riser, the effects of corrective action in response to a well kick or other anomaly can be delayed. 
     Fluid lift systems have been deployed subsea for pressurizing the drilling mud exiting the wellbore. Piping systems outside of the riser carry the mud pressurized by the subsea lift systems. The lift systems include pumps disposed proximate the wellhead, which reduce the time for well control actions to take effect. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a system for lifting drilling mud from subsea to a drilling vessel that addresses vibratory forces generated by a valve actuator. In an example the system includes mud pumps selectively disposed subsea, a valve in a flow line that contains drilling mud from the wellbore, and an actuator coupled with the valve. The actuator is made up of an actuator body, a cylinder in the body, a piston in the cylinder, and a cavity in the body in unrestricted communication with the cylinder. In an embodiment, the cavity is strategically located in the actuator body so that when the piston reciprocates in the cylinder in response to application of fluid to a high pressure side of the piston, fluid on a low pressure side of the piston flows into the cavity. Optionally, the cavity is disposed proximate an end of the cylinder. Example cavities include a frustoconical chamber that projects axially away from an end of the cylinder and into the actuator body, an annular chamber that circumscribes the cylinder, and the like. The mud pump can include a housing with a bladder disposed inside to define a water space on one side that is in communication with a water supply line and a water discharge line, and a mud space on an opposite side that is in communication with a mud supply line and a mud discharge line, and wherein selectively providing pressurized water in the water supply line pressurizes mud in the mud space. 
     An alternative system for lifting drilling mud from a subsea wellbore includes a mud pump which is made of a housing, a water space in the housing, a mud space in the housing that is in pressure communication with the water space, a bladder mounted in the housing having a side in contact with the water space and an opposing side in contact with the mud space, and that defines a flow barrier between the water and mud space, a mud valve disposed in a line having drilling mud and that is in communication with the mud space, and a hydraulic actuator coupled with the mud valve. The actuator has an actuator body, a cylinder in the actuator body, a piston that reciprocates in the cylinder, and a cavity in the actuator body proximate an end of the cylinder, so that when the piston is at an end of a stroke, hydraulic fluid pools in the cavity to define a cushion that absorbs energy from a deceleration of the piston. The cavity can be an upper cavity that projects away from an end of the cylinder distal from a valve coupled to the hydraulic actuator. The cavity can alternatively be a lower cavity that is defined where an axial portion of the cylinder has an increased radius. Optionally, the system can have a first cavity that is strategically disposed to absorb energy when the piston is at the end of a stroke in a first direction, and a second cavity distal from the first cavity and strategically disposed to absorb energy when the piston is at the end of a stroke in a second direction. The mud valve can be a mud inlet valve that is disposed in the line between mud flowing from the wellbore and to the mud pump. The mud valve can also be a mud outlet valve that is disposed in the line between the mud pump and sea surface. The mud pump can further include a water inlet line having an entrance in selective communication with a source of pressurized water, and an exit in communication with the water space, and a water discharge line having an entrance in communication with the water space, and an exit in selective communication with a water effluent line. 
     An optional system for lifting drilling mud from a subsea wellbore includes a mud pump selectively disposed subsea that connects with a mud supply line that contains mud from the wellbore, and that connects to a discharge line having drilling mud discharged from the pump and that terminates above sea surface, a selectively openable and closeable mud inlet valve in the mud supply line, and an actuator. In this example the actuator has a body, a cylinder formed in the body, a piston reciprocatingly disposed in the cylinder, a stem connected between the piston and a valve member in the mud inlet valve, and a cavity in the body having an interface surface that borders a portion of an outer periphery of the cylinder. The cavity can project axially away from an end of the cylinder and the interface surface is substantially planar, or alternatively can project radially outward from an outer circumference of the cylinder and the interface surface is curved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side sectional view of an example of a subsea drilling system having a lift pump assembly and in accordance with the present invention. 
         FIGS. 2 and 3  are partial side sectional views of an example of a subsea pump for use with the drilling system of  FIG. 1  in different pumping modes and in accordance with the present invention. 
         FIG. 4  is a side sectional view of a valve used with the pump of  FIGS. 2 and 3  and in accordance with the present invention. 
         FIG. 4A  is an enlarged side sectional view of a portion of the valve of  FIG. 4  in accordance with the present invention. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
     Shown in  FIG. 1  is a side partial sectional view of an example embodiment of a drilling system  10  for forming a wellbore  12  subsea. The wellbore  12  intersects a formation  14  that lies beneath the sea floor  16 . The wellbore  12  is formed by a rotating bit  18  coupled on an end of a drill string  20  shown extending subsea from a vessel  22  floating on the sea surface  24 . The drill string  20  is isolated from seawater by an annular riser  26 ; whose upper end connects to the vessel  22  and lower end attaches onto a blowout preventer (BOP)  28 . The BOP  28  mounts onto a wellhead housing  30  that is set into the sea floor  16  over the wellbore  12 . A mud return line  32  is shown having an end connected to the riser  26  above BOP  28 , which routes drilling mud exiting the wellbore  12  to a lift pump assembly  34  schematically illustrated subsea. Within the lift pump assembly  34 , drilling mud is pressurized for delivery back to the vessel  22  via mud return line  36 . 
       FIG. 2  includes a side sectional view of an example of a pump  38  for use with lift pump assembly  34  ( FIG. 1 ). Pump  38  includes a generally hollow and elliptically shaped pump housing  40 . Other shapes for the housing  40  include circular and rectangular, to name a few. An embodiment of a flexible bladder  42  is shown within the housing  40 ; which partitions the space within the housing  40  to define a mud space  44  on one side of the bladder  42 , and a water space  46  on an opposing side of bladder  42 . As will be described in more detail below, bladder  42  provides a sealing barrier between mud space  44  and water space  46 . In the example of  FIG. 2 , bladder  42  has a generally elliptical shape and an upper open space  48  formed through a side wall. Upper open space  48  is shown coaxially registered with an opening  50  formed through a side wall of pump housing  40 . A disk-like cap  52  bolts onto opening  50 , where cap  52  has an axially downward depending lip  53  that coaxially inserts within opening  50  and upper open space  48 . A portion of the bladder  42  adjacent its upper open space  48  is wedged between lip  53  and opening  50  to form a sealing surface between bladder  42  and pump housing  40 . 
     A lower open space  54  is formed on a lower end of bladder  42  distal from upper open space  48 , which in the example of  FIG. 2  is coaxial with upper open space  48 . An elliptical bumper  56  is shown coaxially set in the lower open space  54 . The bumper  56  includes upper and lower segments  58 ,  60  coupled together in a clam shell like arrangement, and that respectively seal against upper and lower radial surfaces on the lower open space  54 . The combination of sealing engagement of cap  52  and bumper  56  with upper and lower open spaces  42 ,  54  of bladder  42 , effectively define a flow barrier across the opposing surfaces of bladder  42 . Further shown in the example of  FIG. 2  is an axial rod  62  that attaches coaxially to upper segment  56  and extends axially away from lower segment  58  and through opening  50 . 
     Still referring to  FIG. 2 , a mud line  64  is shown having an inlet end connected to mud return line  32 , and an exit end connected with mud return line  36 . A mud inlet valve  66  in mud line  64  provides selective fluid communication from mud return line  32  to a mud lead line  68  shown branching from mud line  64 . Lead line  68  attaches to an annular connector  70 , which in the illustrated example is bolted onto housing  40 . Connector  70  mounts coaxially over an opening  72  shown formed through a sidewall of housing  40  and allows communication between mud space  44  and mud line  64  through lead line  68 . A mud exit valve  74  is shown in mud line  64  and provides selective communication between mud line  64  and mud return line  36 . 
     Water may be selectively delivered into water space  46  via a water supply line  76  shown depending from vessel  22  and connecting to lift pump assembly  34  ( FIG. 1 ). Referring back to  FIG. 2 , a water inlet lead line  78  has an end coupled with water supply line  76  and an opposing end attached with a manifold assembly  80  that mounts onto cap  52 . The embodiment of the manifold assembly  80  of  FIG. 2  includes a connector  82 , mounted onto a free end of a tubular manifold inlet  84 , an annular body  86 , and a tubular manifold outlet  88 , where the inlet and outlet  84 ,  88  mount on opposing lateral sides of the body  86  and are in fluid communication with body  86 . Connector  82  provides a connection point for an end of water inlet lead line  78  to manifold inlet  84  so that lead line  78  is in communication with body  76 . A lower end of manifold body  86  couples onto cap  52 ; the annulus of the manifold body  86  is in fluid communication with water space  46  through a hole in the cap  52  that registers with opening  50 . An outlet connector  90  is provided on an end of manifold outlet  88  distal from manifold body  86 , which has an end opposite its connection to manifold outlet  88  that is attached to a water outlet lead line  92 . On an end opposite from connector  90 , water outlet lead line  92  attaches to a water discharge line  94 ; that as shown in  FIG. 1 , may optionally provide a flow path directly subsea. 
     A water inlet valve  96  shown in water inlet lead line  78  provides selective water communication from vessel  22  ( FIG. 1 ) to water space  46  via water inlet lead line  78  and manifold assembly  80 . A water outlet valve  98  shown in water outlet lead line  92  selectively provides communication between water space  46  and water discharge line  94  through manifold assembly  80  and water outlet lead line  92 . 
     In one example of operation of pump  38  of  FIG. 2  mud inlet valve  66  is in an open configuration, so that mud in mud return line  32  communicates into mud line  64  and mud lead line  68  as indicated by arrow A Mi . Further in this example, mud exit valve  74  is in a closed position thereby diverting mud flow into connector  70 , through opening  72 , and into mud space  44 . As illustrated by arrow A U , bladder  42  is urged in a direction away from opening  72  by the influx of mud, thereby imparting a force against water within water space  46 . In the example, water outlet valve  98  is in an open position, so that water forced from water space  46  by bladder  42  can flow through manifold body  86  and manifold outlet  88  as illustrated by arrow A Wo . After exiting manifold outlet  88 , water is routed through water outlet lead line  92  and into water discharge line  94 . 
     An example of pressurizing mud within mud space  44  is illustrated in  FIG. 3 , wherein valves  66 ,  98  are in a closed position and valves  96 ,  74  are in an open position. In this example, pressurized water from water supply line  76  is free to enter manifold assembly  80  where as illustrated by arrow A Wi , the water is diverted through opening  50  and into water space  46 . Introducing pressurized water into water space  46  urges bladder  42  in a direction shown by arrow A D . Pressurized water in the water space  46  urges bladder  42  against the mud, which pressurizes mud in mud space  44  and directs it through opening  72 . After exiting opening  72 , the pressurized mud flows into lead  68 , where it is diverted to mud return line  36  through open mud exit valve  74  as illustrated by arrow A Mo . Thus, providing water at a designated pressure into water supply line  76  can sufficiently pressurize mud within mud return line  36  to force mud to flow back to vessel  22  ( FIG. 1 ). 
     In the examples of  FIGS. 2 and 3 , included is a controller  100  shown in communication with actuators  102 ,  104 ,  106 ,  108  respectively coupled with the valves  66 ,  74 ,  78 ,  98  and that provide means for opening and closing valves  66 ,  74 ,  78 ,  98 . In one example embodiment, controller  100  communicates commands to the actuators to selectively open and/or close valves  66 ,  74 ,  78 ,  98 . In an embodiment, controller  100  includes an information handling system (IHS) that receives or contains instructions to selectively operate valves  66 ,  74 ,  78 ,  98 . 
       FIG. 4  is a side sectional view of an example of actuators  102 ,  104  used with mud inlet and exit valves  66 ,  74 . Actuators  102 ,  104  include an elongate body  110  having a cylinder  112  generally coaxial within body  110 . A piston  114  is set in the cylinder  112  and reciprocates therein for opening and closing valves  66 ,  74 . Hydraulic lines  116 ,  118  connect respectively to ports  120 ,  122  shown formed laterally through a sidewall of the body  110  to the cylinder  112 . Hydraulic fluid in hydraulic lines  116 ,  118  selectively flows into cylinder  112  via ports  120 ,  122  for urging the piston  114  axially within the cylinder  112 . A valve stem  124  is shown having one end connected to an end of piston  114  proximate where actuator body  110  mounts onto a valve body  126 . An end of stem  124  opposite its connection to piston  114  connects to a valve gate  128  that reciprocates within a cavity of the valve body  126  to selectively open and close valve  66 ,  74 . 
       FIG. 4A  is a side sectional enlarged view of a portion of actuator  102 ,  104  of  FIG. 4  and illustrates an upper cavity  130  formed into actuator body  110  distal from valve body  126 . More specifically in the example of  FIG. 4A , upper cavity  130  has a frusto-conical shape, is generally coaxial with cylinder  112 , and projects axially away from an upper end of cylinder  112 . Embodiments exist where the upper cavity  130  is formed in the sidewalls of cylinder  112 , such as by a localized increase in a radius of the cylinder  112 , or by grooves (not shown) that circumscribe the cylinder  112  or run axially to the cylinder  112 . As such, when piston  114  reaches an end of its stroke to open valve  66 ,  74  and is proximate a closed end of cylinder  112 , fluid flows into upper cavity  130  to prevent forces from being generated by trapping fluid in an enclosed space. The upper cavity  130  can also absorb and/or attenuate impulse forces generated by the piston  114  that might otherwise be transferred to the surrounding structure. The trapped fluid thereby reduces noise and vibration during operation of the actuator  102 ,  104 . 
     Body  110  includes a lower cavity  132  is shown formed that is axial distal from cavity  130 , and provides dampening when piston  114  is at the end of its down stroke and is closing valve  66 ,  74 . Lower cavity  132  is defined where a radius of the cylinder  112  is increased along a discrete axial length of the body  110  proximate port  122 . Similar to upper cavity  130 , lower cavity  132  provides a space where a volume of hydraulic fluid can collect and absorb impulse forces that occur at the end of the stroke of piston  114 . In the example of  FIG. 4A , upper cavity  130  absorbs a volume of fluid to prevent impulse forces from being generated at an end of an upstroke of piston  114 , and lower cavity  132  absorbs a volume of fluid to prevent impulse forces from being generated at an end of a downstroke of piston  114 . In the example of  FIG. 4A , the upper and lower cavities  130 ,  132  both have a surface that directly and wholly contacts an outer peripheral surface of cylinder  112 . Thus fluid in the cylinder  112  can flow unrestricted into the cavities  130 ,  132 . 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Summary:
Drilling mud is lifted subsea to a drilling vessel with a mud pump having an internal bladder. Applying pressurized water to one side of the bladder urges it against a quantity of the mud to impart a lifting force onto the mud. Mud flow to and from the pump is controlled by valves driven by actuators. The actuators include a piston in a cylinder, a stem that connects the piston to a valve member, and ports for supplying fluid to opposing ends of the piston for selectively reciprocating the piston. Cavities are strategically location in the cylinder for absorbing vibrational forces generated when the piston reaches an end of its stroke.