Abstract:
An actuator for use with a subsea choke or similar apparatus is disclosed. The actuator uses a planetary roller screw mechanism to allow greater control of the torque output while allowing a fast closing of the choke when well conditions require so. A handle is positioned on one end of the actuator housing for manipulation of the actuator housing by a remotely operated vehicle, when required. A pressure responsive axially reciprocable piston is positioned around the driven shaft and controls movement thereof. Other embodiments show the actuator controlling the position of the cage in a cage type choke. A final embodiment shows the actuator configured for use on a subsea tree.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is generally related to an actuator for operating a wellhead choke using a planetary roller screw. More particularly, the invention provides apparatus for operating a choke in a subsea environment that performs the fast close function of a hydraulic motor while giving control similar to that of a stepping actuator in a single compact unit. 
     2. General Background 
     The ongoing search for hydrocarbons in offshore locations to satisfy the world&#39;s increasing need for energy has led to the need to drill for and produce these hydrocarbons in increasingly deeper waters. This has led to the need for production trees suitable for use in a subsea environment. These subsea trees require valves and chokes that can be operated remotely with a minimum of diver intervention. In the case of deep water trees where diver intervention is not possible, the only alternative is to use remotely operated vehicles (ROVs) to provide intervention capabilities. Therefore, it is necessary that any such actuator for a choke or valve be operable by normal hydraulic means while allowing for override by an ROV. 
     Current technology often uses a hydraulic torque motor when it is desired to have a fast closing operation. These hydraulic torque motors suffer from a number of deficiencies. Among these are the motors require a large amount of hydraulic fluid to operate and the amount and type of fluid required may vary according to the motor design and the required output torque for varying loads, Additionally, the running torque is much higher than the starting torque which can cause problems at stall conditions at the end of the travel. The roller screw actuator of the present invention overcomes these deficiencies. 
     2. Description of Related Art 
     U.S. Pat. Nos. 4,741,250; 4,881,419; and 5,027,667 to P. P. Weyer show hydraulically and spring powered actuators using various rollers. 
     A high torque rotating actuator suitable for subsea use is disclosed in U.S. Pat. No. 6,231,027 B1 to G. S. Baker et al. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an actuator for use with a subsea choke or similar apparatus. The actuator uses a planetary roller screw mechanism to allow greater control of the torque output while allowing a fast closing of the choke when well conditions require so. The actuator includes a two part housing secured in abutting relationship by bolts. Centrally disposed in the actuator housing is a driven or output shaft that is supported in support assemblies at each end. These support assemblies include support and thrust bearings that restrain the driven shaft from axial movement while allowing rotation. A handle is positioned on one end of the actuator housing for manipulation of the actuator housing by a remotely operated vehicle, when required. A plurality of anti-rotation keys are positioned on the opposite end of the actuator housing. A pressure responsive axially reciprocable piston is positioned around the driven shaft. The piston has a counterbore and a bore therethrough. 
     A plurality of spindle shafts are disposed circumferentially around the counterbore of the piston. The plurality of spindle shafts are helically threaded and in threaded engagement with a complementary helical thread on the central driven shaft. An anti-rotation means is disposed between the piston and the actuator housing to prevent rotation of the piston as it is reciprocated. The support assemblies positioned on each end of the central driven shaft are coupled to the ends of the central driven shaft by mating splines. The support assembly adjacent the end of the actuator housing supporting the plurality of anti-rotation keys includes an extension extending through a bore in the end of the actuator housing. The extension includes a splined end having a splined adapter thereon having an internal hex on its outer end. 
     The axially reciprocable piston includes a annular flange formed thereon that is positioned in a counterbore in the actuator housing. The annular flange and the counterbore have annular seals positioned thereon to form an annular chamber in which the piston reciprocates. The actuator housing includes pressure ports at either end of the annular chamber to supply pressurized fluid for reciprocation of the piston. Another embodiment showing the actuator controlling the position of a choke cage is shown along with the actuator as used on a subsea tree. 
     A principal object of the present invention is to provide an actuator for a choke that allows fast closing of the choke while giving control of the positioning of flow control element approximating that of a stepping actuator. 
     These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention are set forth below and further made clear by reference to the drawings, wherein: 
     FIG. 1 is a perspective view of a wellhead housing extension with an actuator of the present invention and a subsea choke positioned thereon. 
     FIGS. 2A and 2B are a sectional view of the actuator of the present invention. 
     FIG. 3 is an sectional view, taken along lines  3 — 3  of FIG.  2 . 
     FIGS. 4A and 4B are a sectional view of the actuator of the present invention in combination with a choke. 
     FIGS. 5A,  5 B and  5 C are a sectional view of the actuator of the present invention on a mounting plate for use on a subsea tree. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, and particularly to FIG. 1, a perspective view of an actuator  10  embodying the present invention is shown in combination with subsea choke  12  attached to wellhead housing extension  14 . Alternatively, choke  12  may be replaced by a primary actuator for a choke or valve. Wellhead housing extension  14 , well known to those of ordinary skill in the art, is typically positioned on a subsea tree with production fluid being flowed through choke  12 . 
     FIGS. 2A and 2B show in greater detail the construction of actuator  10 . Actuator  10  is a generally cylindrical member with upper actuator housing  16  secured to lower actuator housing  18  by suitable securing means as bolts  20 . Suitable sealing means as elastomeric seal  22  seals therebetween. Upper actuator housing  16  has a closed end with ROV handle  24  positioned thereon for purposes to be explained hereinafter. The interior of upper actuator housing  16  has counterbore  26  machined therein. Keyway  28  is milled in counterbore  26  and receives anti-rotation key  30  therein. 
     Lower actuator housing  18  has shoulder  32  formed on its interior, adjacent upper actuator housing  16 . Shoulder  32  has seal retainer  34  resting thereon and axially restrained by upper actuator housing  16  when upper actuator housing  16  is secured to lower actuator housing  18 . Seal retainer has seal rings  36  and  38  on its exterior and interior, respectively, to form a pressure chamber in a manner to be explained hereinafter. 
     Lower actuator housing  18  has reduced bore  40  formed at its middle to provide stop shoulder  42 . Seal ring  44  is positioned in reduced bore  40 . Lower actuator housing  18  terminates with stepped bores  46  and  48  adjacent reduced bore  40 . Driven member  50  is centrally disposed in actuator  10  and includes central driven shaft  52  that is supported by support assemblies  54  and  56  at either end. Driving member  58  is positioned in the annulus between driven member  50  and actuator housings  16  and  18 . Support assembly  54  is positioned in upper actuator housing  16  and includes thrust bearings  60  and support bearings  62  to axially restrain and allow rotation of central driven shaft  52 . Splines  64  on central support shaft  52  and splines  66  in support assembly  54  transmit rotation therebetween. In a similar manner, support assembly  56  is positioned in lower actuator housing  18  and includes thrust bearings  68  and support bearings  70  to axially restrain and allow rotation of central driven shaft  52 . Splines  72  on central support shaft  52  and splines  74  in support assembly  56  transmit rotation therebetween. 
     Driving member  58  is a generally cylindrical member with counterbore  76  and bore  78  therethrough. Keyway  80  is milled in counterbore  76  and receives anti-rotation key  82  therein. Anti-rotation key  82  mates with carrier housing  84  to prevent rotation between driving member  58  and carrier housing  84 . Carrier housing  84  seats on shoulder  86  of driving member  58 . Driving member  58  has annular flange  88  formed on its exterior with seal ring  90  positioned thereon that seals against bore  92  of lower actuator housing  18 . Pressure ports  94  and  96  are formed in the wall of lower actuator housing  18  to allow introduction of pressurized fluid. Thus, pressure chamber  98  is formed between seal rings  36  and  38  on seal retainer  34  and seal ring  44  sealing against driving member  58 . Since seals  38  and  44  seal on the same diameter and thereby having the same pressure area, no volume compensation apparatus is required. Annular flange  88  acts as a reciprocable piston when pressurized fluid is introduced into pressure chamber  98  through pressure ports  94  and  96 . 
     Carrier housing  84  has a plurality of spindle shafts  100  disposed circumferentially around its periphery. Carrier housing  84  is retained within counterbore  76  of driving member  58  by retainer nut  102  that engages threads  104  on the interior of driving member  58 . Snap ring  106  engages groove  108  of driving member  58  to ensure retainer nut  102  does not disengage. Spindle shafts  100  have helical thread  110  formed on their exterior and engage complementary helical thread  112  formed on the exterior of driven shaft  52  and complementary interior helical thread  113  on the interior of carrier housing  84  simultaneously. Lubrication port  114  is formed in lower actuator housing  18  adjacent support assembly  56  to inject lubricant. Seal ring  116  is positioned in bore  48  to retain this lubricant. 
     The outer end of support assembly  56  extends through bore  48  and has splines  118  formed thereon to engage complementary splines  120  of drive adapter  122 . Drive adapter  122  has set screw  124  radially disposed therein to engage the outer end of support assembly  56  and ensure splines  118  and  120  remain engaged. The outer end of drive adapter  122  has internal hex  126  formed therein to engage complementary hex  128  of choke stem  130  (partially shown). Although items  126  and  128  are shown as hex shaped, other suitable shapes such as a square could be used for torque transmission therebetween without departing from the scope of the present invention. Adjacent to drive adapter  122  on the exterior of lower actuator housing  18 , slots  132  are milled and receive mating drive keys  134 . Drive keys  134  are secured within slots  132  by cap screws  136 . 
     A typical method of use for the embodiment of FIG. 2 would be as follows. Actuator  10  would be positioned adjacent a valve or choke to be actuated by a ROV utilizing ROV handle  24 . Drive adapter  122  would engage a mating hex on the choke or valve. Pressurized fluid would be supplied through a control system, well known to those of ordinary skill in the art, to either pressure port  94  or  96 , depending on the direction of rotation desired. Assuming that pressurized fluid is supplied to port  96  for the configuration shown in FIG. 2, the resulting pressure will act on annular flange  88  between seals  44  and  90  and urge annular flange  88  of driving member  58  toward seal retainer  34 . As driving member  58  thus moves, carrier housing  84  is moved also along with spindle shafts  100 . As spindle shafts  100  engage central driven shaft  52  through helical threads  110  and  112 , driving member  58  is anti-rotated with respect to upper actuator housing  16  (and thus actuator  10 ) by key  30  and driving member  58  is anti-rotated also with respect to carrier housing  84  by key  82 . With driving member  58  thus anti-rotated, as carrier housing  84  translates, the rotation of spindle shafts  100  is transmitted to central shaft  52  thereby “driving” central shaft  52 . The rotation of central shaft  52  is transmitted through drive adapter  122  to choke or valve stem  130 . If reversal of the direction of rotation is desired, pressurized fluid is applied to port  94  to reverse the direction of driving member  58 . It is important to note that movement of driving member  58  and thus the output rotation of driven central shaft  52  can be controlled with considerable precision by controlling the volume of pressurized fluid supplied to ports  94  or  96 . This volume can be controlled with considerable precision through the control system discussed previously. Additionally, driving member  58  can be driven to any position along its travel, stopped, and the direction of travel reversed simply by controlling the volume of pressurized fluid supplied and to which port. Furthermore, it becomes necessary to rapidly close a choke or valve, a large volume of pressurized fluid may be supplied to the appropriate port and the closing is accomplished rapidly as driving member  58  is essentially a positive displacement pump with no fluid lost. It is this ability to control driving member  58  and driven central shaft  52  with such precision that allows actuator  10  to be used as shown in the second embodiment of FIG.  4 . 
     A second embodiment of the present invention showing its use to operate a cage style choke is shown in FIGS. 4A and 4B. Such cage style chokes are well known in the oilfield industry and a typical example is shown in U.S. Pat. No. 5,086,808 to R. G. Pettus. Those items that are the same as in the first embodiment retain the same numerical designation. Actuator  200  is secured to choke  202  by clamp  204  (shown partially). As in the first embodiment, actuator  200  includes upper actuator housing  206  and lower actuator housing  208 . ROV bucket  210  is secured to upper actuator housing  206  to provide a manual override, when it is necessary. Central driven shaft  52  is carried in support assemblies  212  and  214 . Support assemblies  212  and  214  function as in the previous embodiment, supporting and permitting rotation of driven shaft  52 . Support assembly  212  has internal thread  216  at its outer end that engages mating external thread  218  of choke stem  220 . Splines  222  are formed on the middle portion of choke stem  220  and engage mating splines  224  on adapter sleeve  226 . Adapter sleeve  226  is anti-rotated with respect to choke  202  by dowel pins  228 . Choke stem  220  is connected to cage  230  and is moved linearly and will thereby control well fluid flow between inlet port  232  and outlet port  234 . 
     When it is desired to operate choke  202  by positioning cage  230 , actuator  200  is supplied by supplying pressurized fluid to either port  94  or  96 , depending on the direction cage  230  is moved. When pressurized fluid is supplied thusly, rotation of driven shaft  52  occurs as in the first embodiment. In this embodiment, however, when driven shaft  52  rotates support assembly  214 , threads  216  and  218  are engaged and therefore try to impart rotation to choke stem  220 . As choke stem  220  is anti-rotated by the interaction of splines  222  and  224 , adapter sleeve  226  and dowel pins  228 , choke stem  220  is forced to translate linearly and thereby allow positioning of cage  230 . As in the first embodiment, actuator  200  allows positioning of cage  230  with considerable precision while retaining the ability to rapidly move cage  230  its entire range of travel. 
     A third embodiment of the present invention showing its use on a mounting plate for use on a subsea tree is shown in FIGS. 5A,  5 B and  5 C. Those items that are the same as in the first embodiment retain the same numerical designation. Actuator  300  is secured to mounting plate  302  by suitable means as bolts  304 . Actuator  300  includes ROV bucket  306  as in the second embodiment. Secured to mounting plate  302  is guide pin  308  used to orient actuator  300  when it is deployed onto a subsea tree in a manner well known to those of ordinary skill in the art. Delay valve  310  is used to control fast close operation of actuator  300  in conjunction with a primary actuator such as item  12 . In all other respects, actuator  300  functions as in the first embodiment. 
     The construction of our roller screw actuator for a subsea choke will be readily understood from the foregoing description and it will be seen that we have provided an actuator that performs the fast close function of a hydraulic motor while giving control similar to that of a stepping actuator in a single compact unit. Furthermore, while the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the appended claims.