Abstract:
A valve system comprising a closure member that is linearly translatable within a valve body. A fail safe assembly is connected to the valve body and a first rod member that is connected to the closure member. A linear actuator is movably connected to the fail safe assembly and is operable to move the first rod member. A mechanical override system is connected to the linear actuator and is operable to move the linear actuator relative to the valve body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND 
     The present invention relates generally to valve actuators. More specifically, the present invention relates to override and backup systems for subsea valve actuators. Still more specifically, the present invention relates to override systems for subsea valve applications. 
     Increasing performance demands for subsea hydrocarbon production systems have led to a demand for high performance control systems to operate subsea pressure control equipment, such as valves and chokes. Hydraulic actuators are used to operate many of the pressure control equipment used subsea. Pressurized hydraulic fluid may be supplied to the hydraulic actuators by a direct hydraulic control system or an electrohydraulic control system. Direct hydraulic control systems provides pressurized hydraulic fluid directly from the surface to the subsea valve actuators. Electrohydraulic control systems utilize electrical signals transmitted to an electrically actuated valve manifold that controls the flow of hydraulic fluid to the hydraulic actuators of the pressure control equipment. 
     The performance of both direct hydraulic and electrohydraulic control systems is affected by a number of factors, including the water depth in which the components operate, the distance from the platform controlling the operation, and a variety of other constraints. Thus, as water depth and field size increases, the limits of hydraulic control systems become an increasing issue. Further, even when the use of a hydraulic control system is technically feasible, the cost of the system may preclude its use in a smaller or marginal field. 
     In order to provide an alternative to hydraulic control systems, full electrical control systems, including electric actuators, have been developed. Instead of relying on pressurized hydraulic fluid to actuate the pressure control components, electrical actuators are supplied with an electric current. The reliance on electric current can allow for improved response times, especially over long distances and in deep water. 
     Even with electrical control systems and actuators, many operators still desire some sort of system that allows for operation of the actuators in the case of failure of the electric actuator or interruption in the supply of electrical current. In certain applications, an operator may want to be able to override the electrical control system and operate a valve, or some other components, via remote operation or direct intervention, such as with a remotely operated vehicle (ROV). 
     Thus, there remains a need to develop methods and apparatus for allowing operation of subsea actuators that overcome some of the foregoing difficulties while providing more advantageous overall results. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention are directed toward methods and apparatus for a valve system comprising a closure member that is linearly translatable within a valve body. A fail safe assembly is connected to the valve body and a first rod member that is connected to the closure member. A linear actuator is movably connected to the fail safe assembly and is operable to move the first rod member by moving the linear actuator relative to the valve body. A mechanical override system is connected to the linear actuator and is operable to move the linear actuator relative to the valve body. 
     Thus, the present invention comprises a combination of features and advantages that enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more detailed description of the preferred embodiment of the present invention, reference will now be made to the accompanying drawings, wherein: 
         FIG. 1  is a partial sectional view of a valve actuator with an override system constructed in accordance with embodiments of the invention; 
         FIG. 2  is a partial sectional view of an override system constructed in accordance with embodiments of the invention; 
         FIG. 3  is a partial sectional view of a valve actuator with an override system constructed in accordance with embodiments of the invention; 
         FIG. 4  is a partial sectional view of a valve actuator with an override system constructed in accordance with embodiments of the invention; and 
         FIG. 5  is a partial sectional view of a valve actuator with an override system constructed in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , valve system  10  comprises valve body  12 , closure member  14 , linear actuator  16 , fail safe assembly  18 , and mechanical override system  32 . First end  20  of fail safe assembly  18  is connected to valve body  12 . Rod  22  of fail safe assembly  18  is coupled to closure member  14 . Linear actuator  16  is connected to second end  24  of fail safe assembly  18  by a mechanical override system  32 . Referring now to  FIG. 2 , fail safe assembly  18  comprises cylindrical body  26 , piston  28 , and spring  30 . Piston  28  forms receptacle  40  and closely engages the inner surface of body  26 . Rod  22  is connected to one end of piston  28 . Spring  30  is disposed between first end  20  and piston  28  so as to bias the piston toward second end  24 . 
     Piston  28  operates in a pressure-balanced mode where the hydraulic fluid moves across the piston through annular gap  34  between the piston and body  26 . In certain embodiments, piston  28  may also comprise additional fluid passageways  35  that allow fluid to flow through the piston. Annular gap  34  and fluid passageways  35  may be sized so as to restrict the flow of fluid across piston  28  and thus limit the speed at which the piston may travel. 
     Referring now to  FIG. 3 , receptacle  40  receives a portion of linear actuator  16  that is connected to fail safe assembly  18  by mechanical override system  32 .  FIG. 3  shows valve assembly  10  in a retracted position where piston  28  is positioned toward second end  24  and spring  30  is expanded. To shift closure member  14 , linear actuator  16  is activated and rod  46  extends from the actuator, as shown in  FIG. 4 . 
       FIG. 4  shows valve assembly  10  in an extended position where piston  28  is positioned toward first end  20  and spring  30  is collapsed. Piston  28  is moved toward first end  20  by the operation of actuator  16 . Actuator  16  extends rod  46  that bears against rod  22  that is connected to piston  28 . The movement of piston  28  toward first end  20  compresses spring  30 . As actuator  16  retracts rod  46 , spring  30  pushes piston  28  toward second end  24  and the initial position as shown in  FIG. 3 . 
     Thus, piston  28  and spring  30  operates as a fail-safe device where spring  30  pushes piston  28  toward second end  24  unless rod  46  is extended from linear actuator  16 . Rod  46  of actuator  16  may also be coupled to piston  28  such that the piston can be used to control the speed at which rod  46  retracts. 
     Mechanical override system  32  maintains the position of actuator  16  relative to valve body  12  so that the extension of rod  46  places closure member  14  in the proper position. Mechanical override system  32  also allows actuator  16  to be moved relative to valve body  12  so as to move closure member  14  when rod  46  can not be extended due to component malfunction or other failure. Mechanical override system  32  may comprise a gear system, screw drive, or other mechanically activated translation mechanism that can move actuator  16  with sufficient force to compress spring  30  and shift closure member  14  within valve body  12  Mechanical override system  32  may also comprise an ROV interface that allows the mechanical override system to be operated by an ROV. 
     Referring now to  FIG. 5 , one embodiment of mechanical override system  32  comprises retainer  50 , split ring  52 , base  54 , gear assembly  56 , threaded rod  58 , guide rod  60 , and drive rod  62 . Base  54  is fixably coupled to the end of body  26 . Split ring  52  engages linear actuator  16  and is held in place by retainer  50  that is slidably mounted to threaded rod  58  and guide rod  60 . Gear assembly comprises drive gear  64  that is mounted to drive rod  60  and traveling gear  66  that engages threaded rod  58 . Drive rod  62  has an ROV interface  68  that allows an ROV to rotate the rod and operate the mechanical override system. 
     As drive rod  62  is rotated, drive gear  64  rotates traveling gear  66 . The rotation of traveling gear  66  causes it to move along threaded rod  58 , pushing retainer  50  toward base  54 . As retainer  50  moves, linear actuator  16  is moved toward valve body  12 , compressing spring  30  and moving closure member  14  within the valve body to the fully actuated position of  FIG. 5 . Gear assembly  56  may preferably be a self-locking system that will maintain the position of linear actuator  16  until drive rod  62  is rotated in the opposite direction. 
     Thus, valve system  10  can be actuated in a first mode (as shown in  FIG. 4 ), where linear actuator  16  extends rod  46  so as to move piston  28  toward first end  20  of fail safe assembly  18 . In the first mode, the position of linear actuator  16  relative to valve body  12  is maintained by override system  32 . Valve system  10  can also be actuated in a second mode (as shown in  FIG. 5 ), where linear actuator  16  is moved relative to valve body  12  by mechanical override system  32 . The movement of linear actuator  16  moves piston  28  toward first end  20  of fail safe assembly  18 . 
     Mechanical override system may utilize any of a number of mechanical systems to move the linear actuator and shift the position of the closure member. For example, a mechanical override system may use a geared or threaded system that transforms rotational motion into linear translation of the actuator. Other mechanical override systems may comprise external hydraulic rams or other type devices to push the linear actuator. 
     While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied, so long as the mechanical override apparatus retain the advantages discussed herein. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.