Abstract:
An actuator for use with valves in a subsea wellhead assembly. The actuator includes a shaft with an alternating polarity along its length and windings spaced around the shaft that create a magnetic field around the shaft. The shaft is connectable to a valve stem so that flowing current through the windings creates a magnetic field for linearly moving the shaft and operating the valve. A fail safe mechanism can be included that pushes the shaft back to its original position thereby also moving the valve into its original, or fail safe, position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/056,680, filed May 28, 2008, the full disclosure of which is hereby incorporated by reference herein. 
     
    
     1. FIELD OF INVENTION 
       [0002]    This invention relates in general to production of oil and gas wells, and in particular to an actuator comprising a linear motor for use with a subsea wellhead. 
       2. DESCRIPTION OF RELATED ART 
       [0003]    Systems for producing oil and gas from subsea wellbores typically include a subsea wellhead assembly that includes a wellhead housing attached at a wellbore opening, where the wellbore extends through one or more hydrocarbon producing formations. Casing and tubing hangers are landed within the housing for supporting casing and production tubing inserted into the wellbore. The casing lines the wellbore, thereby isolating the wellbore from the surrounding formation. Tubing typically lies concentric within the casing and provides a conduit for producing the hydrocarbons entrained within the formation. 
         [0004]    Wellhead assemblies also typically include a production tree connecting to the upper end of the wellhead housing. The production tree controls and distributes the fluids produced from the wellbore. Valves assemblies are typically provided within wellhead production trees for controlling the flow of oil or gas from a wellhead and/or for controlling circulating fluid flow in and out of a wellhead. Gate valves and other sliding stem-type valves have a valve member or disc and operate by selectively moving the stem to insert/remove the valve member into/from the flow of fluid to stop/allow the flow when desired. 
       SUMMARY OF INVENTION 
       [0005]    Disclosed herein is a valve for a subsea wellhead assembly that includes a valve body having a fluid flow passage, a gate selectively moveable to open and close the fluid flow passage, magnetized segments stacked together and coupled to the gate for movement therewith, and a winding array mounted proximate the magnetized segments, so that flowing electrical current through the winding array applies a magnetic field to linearly move the magnetized segments and cause the gate to move relative to the winding array. The winding array can include stator rods arranged around the magnetized segments, each stator rod having a winding. A multi-phase electrical supply can be included that is in communication with the winding of each stator rod, wherein adjacent windings the stator rods receive current at a different phase from each other. The phase of the current delivered to adjacent windings on the stator rods can vary by about 120°. A resilient member, such as a spring, can be engaged with the magnetized segments, so that movement of the magnetized segments from a first position compresses the resilient member and when the magnetic field is removed, the resilient member expands and moves the magnetized segments and gate back to the first position. The winding array can alternatively have a plurality of stator rods, each having a separate winding, the stator rods being spaced in a circular array around and parallel to the segments. The magnetized segments can be disk like members stacked together to form a shaft, and adjacent segments have opposite polarity. 
         [0006]    Also disclosed herein is an actuator for use with a subsea valve. In one example, the actuator includes a valve having body, a passage in the body, and a gate that selectively is disposed into the passage. The actuator can be formed from a housing, an elongated shaft in the housing with magnetic segments along its length of alternating polarity, the shaft being non-rotatable relative to the housing and axially moveable relative to the housing, a winding array arranged around and spaced radially outward from the shaft, the winding array being fixed relative to the housing so that applying an alternating current to the winding array causes the shaft to move axially, and a coupling on the shaft that is attachable to the gate for moving the gate in unison with the shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a side view of a subsea production tree having a valve actuator in accordance with the present disclosure. 
           [0008]      FIG. 2  is a perspective view of an example of a linear motor for use with the valve actuator of  FIG. 1 . 
           [0009]      FIG. 3  is a side cutaway view of an example of the subsea actuators of  FIG. 1  having a linear motor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]      FIG. 1  is a side schematic view of a wellhead assembly  10  that includes a wellhead housing  12  seated on the seafloor  8  and connected to a hydrocarbon producing wellbore  5 . Coupled on the upper end of the wellhead housing  12  is a production tree  14  having a tree bore  36  axially formed through the tree  14 . The wellhead assembly  10  and production bore  5  can be accessed through the tree bore  36 ; such as for production, workover, fluid injection, or bypass operations. A production passage  18  is shown laterally extending from the tree bore  36  to outside the production tree  14 . A wing valve  16  provided in the production passage  18  can be used to selectively regulate production fluid flow through the production passage  18 . A wing valve actuator  20  is schematically illustrated and is coupled to the wing valve  16  for its selective operation. 
         [0011]    Additional fluid flow paths are shown bored within the wellhead assembly  10  regulated with in line valves; in one example a fluid flow path is a bypass passage  24  illustrated in dashed outline in the wellhead assembly  10 . The bypass passage  24  can provide fluid communication from the outside of the wellhead assembly  10  and into a tubing annulus (not shown) formed between production tubing and casing in the borehole  5 . Flow through the bypass passage  24  can be regulated with an inline bypass valve  22  that in turn can be controlled with a bypass valve actuator  26  shown on the production tree  14  housing. Also shown in dashed outline in the wellhead assembly  10  is a workover passage  30  for conveying workover fluids to a portion of the wellbore  5  where workover operations are being undertaken. Fluid through the workover passage  30  is controlled with an inline workover valve  28  shown with an associated workover valve actuator  32  for powering the valve  28  open, closed, or in a throttling position. Thus workover and/or bypass operations through the wellhead assembly  10  can be performed by flowing fluid respectively through the bypass passage  24  and/or workover passage  30 . A swab valve  34  is disposed within the tree bore  36 , a swab valve actuator  38  attached to the housing bore for providing an opening and closing means for the swab valve  34 . 
         [0012]    An example of a linear motor  40  is shown in a prospective side view in  FIG. 2 . The linear motor  40  comprises a stator array  42  that includes a series of stator rods  44  arranged parallel to each other in a generally circular pattern. A shaft  48  is disposed within the array  42  circle and oriented substantially parallel to the individual stator rods  44 . The stator rods  44  include windings  46  disposed on the surface of the rods  44  facing the shaft  48 . As shown in  FIG. 2 , the windings  46  include loops  47  of electrically conductive material lying in a plane substantially perpendicular to each rod  44 . Each stator rod  44  can include multiple groups of windings  47 . The windings  46  may each be connected to a three phase alternating current power source (not shown) where the three phases differ by 120°. Similarly, the power supplied to adjacent windings of the stator rods  44  may be sequentially 120° apart. In one embodiment, the number of stator rods  44  comprising the array  42  is a multiple of 3. 
         [0013]    The shaft  48  is a cylindrical member comprising magnetized segments  50  stacked along a common axis, which also is the axis of stator array  42 . The segments  50  may comprise a permanent magnet or electromagnet, or can be temporarily magnetized. In the embodiment shown, each segment  50  has a polarity different than adjacent segments  50 . Applying magnetic fields at strategic locations on the shaft  48  exerts a magnetic moment that can move the shaft  48  in a direction along its axis A X . For example, flowing current through the coils or windings  46  of the array  42  as described above can form magnetic fields that act upon the segments  50  to move the shaft  48 . The direction of shaft  48  movement depends on the direction of electrical current flow through the individual coils  46 . 
         [0014]    A side cross sectional view of an embodiment of an actuator  52  including a linear motor  40  is shown in  FIG. 3 . The actuator  52  comprises an elongated and substantially hollow outer housing  54 . An end cover  55  covers an end of the housing  54  and a valve bonnet  64  is seated in an end of the housing  54  opposite the end cover  55 . A valve stem  58  is shown inserted through an axial bore  65  in the bonnet  64  and projecting into the housing  54 , where a coupling  56  connects the valve stem  58  to the linear motor shaft  48 . A valve gate  60 , attached to the free end of the valve stem  58 , an opening  62  formed through the gate  60 . Shown in dashed outline is an example valve body  63  having an associated fluid flow passage  67  through the body  63 . The valve actuator  52  can be used with any of the valves mentioned in  FIG. 1  on the wellhead assembly  10 . However use of the actuator  52  is not limited to the valves of  FIG. 1  but can include any valve associated with subsea production or any valve located subsea. 
         [0015]    As described above, energizing the coils  46  within the stator rods  44  linearly moves the shaft  48 . When the linear motor  40  is coupled to a valve stem  65 , the stem  65  moves with the shaft  48  to selectively open and/or close the associated valve (not shown). In one embodiment of use, selective passage or blockage of flow can be accomplished with the valve gate  60  by energizing the coils  46  to extend the shaft  48  from the linear motor  40 . In the embodiment of  FIG. 3 , moving the shaft  48  outward in turn pushes the valve gate  60  from its bonnet  64 , aligning the opening  62  with an the passage  67  to open the valve. Reversing current flow through the coils  46  pulls the shaft  48  and valve stem  58  in a direction towards the end cap  55  to position the valve gate  60  as shown in  FIG. 3 . Optionally, the opening  62  can be located on the far end of the gate  60  so that the valve is closed by pushing the valve stem  58  and opened by pulling the valve stem  58 . 
         [0016]    A fail safe plunger  68  is shown coaxially disposed within the housing  54  and circumscribing an end of the motor  40 . In the example of  FIG. 3 , the plunger  68  includes a hollow cylindrical housing  71  with a flange  70  around its outer surface. The housing  71  is shown further having a closed end  69  and an open end  73 . The plunger  68  is oriented so the closed end  69  is proximate to the valve bonnet  64  and the open end  73  is proximate the end cover  55 . A bore  72  is provided in the closed end  69  to accommodate passage of the valve stem  58 . A spring  66  is shown coaxial within the housing  54  and depending from an end adjacent the valve bonnet  64  into the annular space between the plunger housing  71 . The coupling  56  includes a cross pin  57  shown engaging the closed end  69  so that the plunger  68  moves when the coupling  56  is moved towards the valve bonnet  64 . 
         [0017]    The radial flange  70  is dimensioned to contact and compress the spring  66  as it is moved towards the valve bonnet  64 . If power is lost to the coils  46 , potential energy stored in the compressed spring  66  moves the valve stem  58 , aligning the valve gate  60  into a fail safe position. It should be pointed out however, that the actuator  52  can be configured such that energizing the motor  40  and extending the valve stem  58  produces selective blockage of a corresponding valve and wherein the retraction of the shaft  48  and the valve stem allows flow through the associated valve. 
         [0018]    Optionally, the actuator  52  may be configured to fail open. This can be accomplished with the present spring  66  and flange  70  arrangement by reversing the opening  62  location on the valve gate  60  to its lower end. Alternatively, the actuator  52  is also configurable to remain “as is” upon a failure. The actuator  52  is usable with a subsea manifold, a vertical wellhead, a horizontal wellhead, and any other type of subsea application. 
         [0019]    The present system and method described herein, therefore, is well adapted to carry out and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment 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.