Abstract:
An actuation system is usable for a choke. The choke can be moved between open and closed positions by a variety of devices, including a stepping actuator. A rapid shutdown feature involving a hydraulically driven override allows the stem of the choke to be moved rapidly to close the choke. The actuator resumes its old position when the hydraulic shutdown mode is overridden, thus allowing the choke to return to its prior position before it was rapidly closed. Shutdown of the choke is rapid, while a return to its former position is gradual.

Description:
FIELD OF THE INVENTION 
     The field of this invention relates to actuators with incremental movement capabilities such as by a stepper motor, and more particularly to applications involving the use of such actuators for chokes where fast closure is desirable. 
     BACKGROUND OF THE INVENTION 
     Various types of valves used in the oil and gas industry are operated by actuators. Chokes are a particular type of valve and are generally used as throttling devices to reduce downstream pressures or flow rates or regulate them to a predetermined set point or range of pressures or flow rates. Normally, other types of valves are in the system in which the choke is installed. Occasions can arise where it is necessary to obtain rapid closure of the particular system in which the choke is installed. Chokes have traditionally been operated by stepping actuators which allow for fine adjustment on the degree of opening of the chokes for control of the flowing liquid or gas stream. When a rapid shutdown has been required in the past, the stepping actuator was actuated in reverse. The problem with that mode of operation is that it took too long to close the choke, thus allowing the possibility of erosion damage to valves further downstream, which could see the effects of increased velocity until the choke could be closed fully, using the stepping actuator. 
     Yet other techniques would be to use hydraulically actuated motors and disable the stepping actuator drive mechanism and drive the stem under power of a hydraulic motor. The problem with these designs has been that the motor torque output of a hydraulic motor has to be carefully controlled or else it would apply potentially significant torques which could twist the stem completely, making the choke inoperable for further operations. Another shortcoming of using the hydraulic motor to actuate the stem for a rapid shutdown of the choke would be that hydraulic motors require high pressures and volume in the hydraulic pressure source. This entails the adaptation of a complex hydraulic system in order to make such a design functional for each individual actuator-operated choke. 
     One of the objectives of the present invention is to provide a simple mechanism which can rapidly close the choke when the situation warrants, yet still have the positional accuracy afforded by a stepping actuator. Another objective is to allow the choke to resume its old position at the time it was actuated for rapid closure after the situation requiring the closure has been corrected. This would, in turn, allow the system to stabilize because the old settings could be easily resumed. Yet another objective of the present invention is to provide the rapid closure function with a hydraulic system that employs high pressure but low volume, thus giving the potential for using seawater for the fluid medium for accomplishing rapid closure. Yet another objective of the present invention is to allow for rapid actuation of the actuator to close the choke, using the hydraulically operated assist, while at the same time allowing the choke to reopen at a more gradual rate to prevent damage to downstream equipment. These and other features of the present invention will be more readily appreciated by those skilled in the art from a review of the preferred and an alternative embodiment described in detail below. 
     SUMMARY OF THE INVENTION 
     An actuation system is usable for a choke. The choke can be moved between open and closed positions by a variety of devices, including a stepping actuator. A rapid shutdown feature involving a hydraulically driven override allows the stem of the choke to be moved rapidly to close the choke. The actuator resumes its old position when the hydraulic shutdown mode is overridden, thus allowing the choke to return to its prior position before it was rapidly closed. Shutdown of the choke is rapid, while a return to its former position is gradual. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevational view of the preferred embodiment, shown in a position where it is stepped fully closed. 
     FIG. 2 is the view of FIG. 1, with the choke stepped fully open. 
     FIG. 3 is the view of FIG. 2, with the choke fully closed after it has been stepped half-closed. 
     FIG. 4 is the view of FIG. 3, with the choke closed after being stepped fully open. 
     FIG. 5 is an alternative embodiment of the actuator for the choke, showing the choke stepped fully closed. 
     FIG. 6 is the view of FIG. 5, showing the choke stepped half-closed. 
     FIG. 7 is the view of FIG. 6, with the choke shown stepped fully open. 
     FIG. 8 is the view of FIG. 7, with the choke being quickly closed after having been stepped fully open. 
     FIG. 9 is the view of FIG. 6, with the choke being rapidly closed after being stepped half-closed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a choke body  10  has an inlet  12  and an outlet  14 . A cage  16  has a plurality of openings  18 . Movable sleeve  20  is shown covering openings  18  in FIG. 1, representing the fully closed position which prevents all fluid communication between inlet  12  and outlet  14 . Stationary sleeve  22  has openings  24 . Openings  24  can communicate with openings  18  when the movable sleeve  20  is upwardly retracted, as shown in FIG.  2 . Sleeve  22  can be removed out of body  10  in a manner known in the art by releasing clamp  26  and pulling the actuator  28 . Actuator  28  is a stepper-type actuator used in choke service in the past and constitutes a design that is well-known in the art. The actuator  28  is attached to the body  10  through a sleeve  30 . Clamp  26  holds sleeve  30  to body  10 . Within sleeve  30  is an actuator rod  32  which is affixed to sliding sleeve  20  at one end. Actuator rod  32  extends through an opening  34  in hydraulic housing  36 . Hydraulic housing  36  has passages  38  and  40  for selective application of fluid pressure for actuation of actuator rod  32 . Abutting the actuator rod  32  is a stepper shaft  42 , which extends through an opening  44  in sleeve  30 . Plate  46  seals off opening  44  by virtue of O-ring seals  48  and  50 . O-ring seal  52  seals between actuator rod  32  and stepper shaft  42 . O-ring seals  54  and  56  seal between the actuator rod  32  and the hydraulic housing  36 . As better shown in FIG. 2, O-ring seals  54  and  56  define an annular cavity  58 , which is in fluid communication with passage  40  for selective application of hydraulic or other fluid as will be described below. Similarly, O-ring  56 , in conjunction with O-rings  48  and  50 , defines another annular cavity  60 . Those skilled in the art will appreciate that pressure in cavity  58  acts on surface  62  while pressure in cavity  60  acts on surface  64 . Surfaces  62  and  64  oppose each other, thus defining how movement of actuator rod  32  will occur for quick movements of sleeve  20 . 
     The actuator  28  has an output gear  66  which is meshed with drive gear  68 . Drive gear  68  has a bore  70  therethrough, within which is a driving thread  72 . Driving thread  72  meshes with a similar thread  74  such that rotation of gear  68  by output gear  66  results in translation of stepper shaft  42 . Stepper shaft  42  is keyed to hydraulic housing  36  by virtue of its extension into splines  76  in hydraulic housing  36 . Accordingly, those skilled in the art will appreciate that the normal stepping operation between fully closed in FIG.  1  and fully open in FIG. 2 occurs by rotation of the gear  66  in opposed directions. As shown in FIGS. 1 and 2, in stepping the sleeve  20  from the full open to full closed positions, the stepper shaft  42  drives the actuator rod  32 , overcoming hydraulic pressure which is continuously applied during normal operations to passage  40 . During reverse stepping motion, where gear  66  drives gear  68 , making stepper shaft  42  move in an upward direction out of body  10 , the constant application of hydraulic pressure to passage  40  forces the actuator rod  32  to follow the movement of the stepper shaft  42 . 
     In order to allow the above-described movements, annular passage  78  is vented through passage  80  through passage  82  in stepper shaft  42  to an accumulator  84  within the actuator  28 . Thus, when movement with stepping action between the full open position of FIG. 2 to the full closed position of FIG. 1, the volume of annular chamber  78  is reduced and fluid is displaced through passages  80  and  82  to the accumulator  84 . The reverse flow occurs when moving from the closed to the open position shown, respectively, in FIGS. 1 and 2. 
     It should be noted that the actuator rod  32  does not rotate but merely translates with the stepper shaft  42 , as described previously. Thus, O-ring seal  38  between them experiences no relative rotation. 
     The hydraulic control system, which is connected to passages  40  and  38 , is illustrated in FIG. 8 which, although describing an alternative internal embodiment, employs the exact same control system. A high-pressure, low-volume source  86 , which can be hydraulic fluid from an existing system or surrounding seawater pumped to the requisite pressure for the selective actuation of actuator rod  32 , is connected to a diverter valve  88 . Valve  88  has lines  90 ,  92  and  94  connected to it. Line  94  is connected to a low-pressure supply for the hydraulic system (not shown). Ultimately, a pump (not shown) builds the pressure up and delivers it as the hydraulic pressure source  86 . The two positions of the diverter valve  88  are shown in FIGS. 5 and 8. A signal, represented by line  96 , can be used to move the diverter valve between the positions shown in FIGS. 5 and 8. In FIG. 8, the high-pressure hydraulic source crosses over to line  92 , while the return to the sump (not shown) is connected to line  90 . In the FIG. 5 position, the hydraulic pressure source  86  goes directly to line  90 , while line  92  is connected to the low-pressure sump (not shown) through line  94 . Line  92  further contains a restriction orifice  98  and a check valve  100  piped in parallel to it. As shown in FIGS. 1-4, line  90  is connected to passage  40 , while line  92  is connected to passage  38 . During normal operations, there is always pressure in line  86 . Thus, referring to FIGS. 1 and 2, the pressure from line  86  communicates through line  90  into passage  40 . Thus, when the stepper shaft  42  is moved out of body  10 , as shown by comparing the FIG. 1 position to the FIG. 2 position, the pressure from hydraulic pressure source  86  communicates through line  90  into passage  40 , pressurizing cavity  58  and putting a force on surface  62  to make the actuator rod  32  follow the outward movement of the stepper shaft  42 . 
     Regardless of the position of the stepper shaft  42 , the hydraulic circuit illustrated in FIGS. 5 and 8 and applicable to the preferred embodiment shown in FIGS. 1-4, can be used to independently operate the actuator rod  32 , independently from the stepper shaft  42 . By the appropriate signal  96  to the diverter valve  88 , the position in FIG. 8 is assumed. When that happens, the hydraulic pressure source  86  is connected to line  92 . While some flow goes through orifice  98 , most of the flow goes through the check valve  100 , which allows flow in the direction away from diverter valve  88 . Thus, pressure is communicated to cavity  64  and a force results on surface  62 . In order to allow the actuator rod  32  to move downwardly, cavity  58  is connected through passage  40  and line  90  back to the low-pressure sump (not shown) through line  94 . Accordingly, there is a quick stroking action imparted to actuator rod  32  when a signal  96  shifts the diverter valve  88  to the position shown in FIG.  8 . This, in effect, pressurizes cavity  60  while depressurizing cavity  58 , resulting in a stroking of the actuator rod  32  toward the left, moving the sleeve  20  over the openings  18 , closing off the choke body  10 . 
     After an emergency closure, should the choke need to be put back into service, signal  96  is removed from the diverter valve  88  allowing the spring  89  to put it back in the position shown in FIG.  5 . Pressure from the source  86  is then applied to line  90 . However, the check valve  100  does not permit flow back from cavity  60  toward the diverter valve  88 . Hence, the only flow out of chamber  60  through passage  38  can occur through restrictor  98  which acts as a speed control device on movement of rod  32 . This slows down the return movement of the actuator rod  32 . Accordingly, the actuator rod  32  resumes its former position when it is fully in contact with the stepper shaft  42 . It should be noted that the stepper shaft does not move when the signal  96  changes the position of diverter valve  88  from that shown in FIG. 5 to that shown in FIG.  8 . Accordingly, when the signal is again given to the diverter valve  88  to go back to the position of FIG. 5, the actuator rod  32  resumes its former position against the stepper shaft  42 , which in the interim during the emergency shutdown has not moved. FIG. 3 illustrates that the stepper shaft  42  is in the half-closed position, putting it about midway between the positions shown in FIGS. 1 and 2. It retains that position when the diverter valve  88  is switched to the position in FIG. 8 due to a signal  96 . Thus, when an increase in pressure in passage  38  strokes the actuator rod  32  with respect to the stationary stepper shaft  42 , the choke body  10  is closed between inlet  12  and outlet  14 . The stroking of the actuator rod  32  displaces fluid from cavity or chamber  78  through passage  80  in the actuator rod  32  and through passage  82  in the stepper shaft  42 , and eventually to the accumulator  84 . In the FIG. 3 position, the actuator rod  32  will go back fully against the stepper shaft  42  when signal  96  again puts the diverter valve  88  in the position shown in FIG.  5 . 
     FIG. 4 shows the stepper shaft  42  in the full open position at the time a quick shutdown of the choke body  10  is required between inlet  12  and outlet  14 . As a result, the stepper shaft  42  retains the same position it had in FIG. 2, while the hydraulic system, through a signal  96  and movement of the diverter valve  88 , fully strokes the actuator rod  32  to isolate openings  18  with sleeve  20 . Upon reversal of the position of the diverter valve  88  and again referring to FIG. 4, the actuator rod will move out of body  10  until contact with the stepper shaft  42 . 
     An alternative embodiment is illustrated in FIGS. 5-9. A choke body  102  has an inlet  104  and an outlet  106 . An actuator  108  turns a gear  110  with a stepper motor in a manner known in the art. A stepper shaft  112  has the cage  114  connected at one end. A key  116  ensures that the stepper shaft  112  can only translate and not rotate. Key  116  is in groove  118  of body  120 . The stepper shaft  112  extends through an actuator rod  122 . Actuator rod  122  has a bore  124 , through which extends end  126  of stepper shaft  112 . Rotating with gear  110  is actuator rod  122 . Since stepper shaft  112  is keyed for longitudinal movement by key  116 , rotation of gear  110  with actuator rod  122  translates the stepper shaft  112 . An engaging thread  128  between the actuator rod  122  and the stepper shaft  112  assures translation of stepper shaft  112  upon rotation of gear  110  with actuator rod  122 . FIG. 5 shows the fully closed position, showing the stepper shaft  112  advanced most fully into body  102 . In FIG. 6, the stepper shaft  112  has been moved somewhat out of body  102 , while in FIG. 7, the full open position is illustrated. These movements are accomplished by stepwise actuation of gear  110  which rotates actuator rod  122  which, in turn, due to the presence of threads  128  translates the stepper shaft  112  which is keyed against rotation by key  116 . 
     A rapid closure of the choke can be accomplished to put the cage  114  in the position shown in FIG. 8 by virtue of the hydraulic system previously described and shown in each of the FIGS. 5-9. In this situation, the actuator rod  122  has a surface  130  which, when pressure is exerted upon it and vented from cavity  132 , creates the end result of movement of the actuator rod  122  toward the body  102 . When this occurs, the actuator rod  122  is translated toward body  102  and takes with it the stepper shaft  112 . Thus, comparing FIGS. 7 and 8, one can see that in FIG. 7, the stepper shaft  112  is in the fully open position, while in FIG. 8, with application of pressure into line  92  and onto surface  130 , the net result has been that the actuator rod  122  has fully stroked, without rotation, taking with it the stepper shaft  112  to move the cage  114  from the fully open position shown in FIG. 7 to the fully closed position shown in FIG.  8 . This is to be contrasted with the view in FIG. 9 where the cage  114  had previously been in the stepped half-closed position at the time pressure is applied in line  92  onto surface  130 . Since the stepper shaft  112  was already in a half-closed position, the stroke of actuator rod  122  is shorter to close the choke in FIG. 9 than the stroke of the actuator rod in FIG. 8, where the choke was stepped full open prior to the rapid shutdown. Those skilled in the art will appreciate that upon reversal of the positions of the diverter valve  88  in the embodiment and position shown in FIG. 9, the stepper shaft  112  will simply move back, but at a slower rate, in tandem with the actuator rod  122  until the actuator rod can no longer move. Thus, the position of FIG. 6 will result after operation of the diverter valve  88  in progressing from the position of FIG. 9 back to the position of FIG.  6 . 
     One of the disadvantages of the embodiment shown in FIGS. 5-9 is that the seals  134  and  138  mounted to the actuator rod  122  rotate with respect to housing  140 . This could cause wear on the seals which will require maintenance. Additionally, assembly is more difficult in the design of FIGS. 5-9 because alignment is required among key  116  on stepper shaft  112 , threads  128  between stepper shaft  112  and actuator rod  122 , and a key  142  which secures the actuator rod  122  to sleeve  144  which, through a gear  146  is meshed to gear  110 . Thus, the design of FIGS. 5-9 is more difficult to assemble and may require incrementally more maintenance than the design in FIGS. 1-4. 
     In all other respects, the actuator  108  comprises a design that is known in the art, having such well-known features as a position indicating mechanism  148 . 
     Those skilled in the art will appreciate that the advantage of the present invention allows for rapid closure of the choke and a resumption of the original position of the cage after the rapid shutdown. The actuation mechanism can be used for devices other than chokes without departing from the spirit of the invention. Thus, other styles of control valves, used both in surface systems and downhole, can employ the rapid shutdown feature described. Although a stepper motor operation has been described as being incorporated into the actuator, other types of normal movement can be employed without departing from the spirit of the invention. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.