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BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to locking mechanisms for a blowout preventer as deployed in the petroleum exploration and recovery industry. More particularly, the invention relates to a roller lock mechanism to prevent the undesirable reversal of a thrust rod for a ram-type blowout preventer.  
         [0003]     2. Background Art  
         [0004]     Wellbores are drilled deep into the earth&#39;s crust to recover oil and gas deposits trapped in the formations below. Typically, these wellbores are drilled by an apparatus that rotates a drill bit at the end of a long collection, or string, of threaded pipes known as a drillstring. Because of the energy and friction involved in such an operation, a drilling fluid, commonly referred to as drilling mud, is used to lubricate and cool the drill bit as it cuts the rock formations below. Furthermore, the drilling mud is capable of performing the secondary function of removing the drill cuttings from the bottom of the wellbore to the surface. Typically, drilling mud is delivered to the drill bit under high pressures through a central bore of the drillstring. From there, nozzles on the drill bit direct the pressurized mud to the cutters on the drill bit where the pressurized mud cleans and cools the bit. As the fluid is delivered downhole through the central bore of the drillstring, the fluid returns to the surface in the annulus formed between the outside of the drillstring components and the cut wellbore. Therefore, a hydrostatic column of drilling mud typically extends from the surface to the bottom of the hole being cut.  
         [0005]     As wellbores are drilled several thousand feet below the surface, the hydrostatic column of drilling mud serves to help prevent blowout of the wellbore as well. Often, hydrocarbons and other fluids trapped in subterranean formations exist under significant pressures. Absent any flow control schemes, fluids from such ruptured formations can blow out of the wellbore like a geyser and spew hydrocarbons and other undesirable fluids into the atmosphere. Several thousand feet of hydraulic head from the drilling mud column helps to prevent the wellbore from blowing out under normal conditions. However, under certain unforeseen circumstances, the drill bit will encounter pockets of pressurized formations and will cause the wellbore to “kick” or experience a rapid increase in pressure. Because formation kicks are unpredictable and would otherwise result in disaster, flow control devices known as blowout preventers (“BOP&#39;s”), are mandatory on most drilling rigs in use today.  
         [0006]     Blowout preventers are devices configured to seal the annular space that surrounds the drillstring. One of the most common types of blowout preventer is known as a ram-type blowout preventer. A ram-type blowout preventer includes a large housing mounted atop the wellhead that includes a large passageway through which the drillstring (and any components connected thereto) is able to pass. The housing also includes two or more rams located in a plane substantially normal to the axis of the drillstring and wellhead that are configured to move between retracted and extended positions. The ends of the rams are configured so that when extended, they provide a complete annular hydraulic seal around the drillstring disposed therethrough.  
         [0007]     Referring initially to  FIG. 1 , a typical ram-type BOP  100  is shown schematically. Ram-type BOP  100  shown includes a main housing  102 , two or more piston ram housings  104 , and top  106  and bottom  108  bolting flanges for connection to other wellhead components (not shown). Ram-type blowout preventer  100  is preferably constructed such that a generally cylindrical through bore  110  allows oilfield tubulars  112  to unobstructedly pass through along a drillstring axis  114 . Oilfield tubulars  112  are typically components of a drillstring and may include, but are not limited to, drill pipe, drill collars, measurement tools, coiled tubing, or wirelines. Under normal conditions, through bore  110  is open and not obstructed such that fluids pass through an annulus  116  formed between the outer profile of tubulars  112  and the inner profile of through bore  110 . When the wellbore below is to be shut off such that fluids below BOP  100  can no longer communicate with the wellbore above BOP  100 , ram assemblies  118  are activated to provide a 360° seal of annulus  116  between bore  110  and tubulars  112 .  
         [0008]     Each ram assembly  118  includes a sealing ram  120  having a leading edge  122  connected to a hydraulic piston  124  through a thrust rod  126 . Leading edge  122  is preferably contoured such that it corresponds with an outer profile of oilfield tubular  112  so that a tight seal can be formed therebetween. Connected to sealing rams  120  through thrust rods  126 , hydraulic pistons are activated by an external hydraulic source (not shown) to engage leading edges  122  against tubular  112  and seal off annulus  116 . While it may be typical for there to be two ram assemblies  118 , each with a corresponding semi-circular profile for leading edges  122 , it should be understood that a ram-type BOP  100  may employ three or more ram assemblies  118  with corresponding circular portions for leading edges  122  to shut off and seal annulus  116 .  
         [0009]     To seal off annulus  116 , pressurized fluids are applied to hydraulic port  128  in communication with a reservoir  130  of ram housing  104 . Increases in pressure in reservoir  130  urge back face  132  of piston  124 , causing piston  124 , rod,  126 , and ram  120  to be thrust toward tubular  112 . The higher the pressure in communication with reservoir  130 , the higher the loads transmitted through rod  126  to ram  120 . To retract rams  120  out of annulus  116 , pressure to port  128  is reduced and piston  124  is able to retract into reservoir  130 . Retraction of pistons  124  may be assisted through the use of retraction springs (not shown), hydraulic retraction, or through any other means known in the art. Particularly, a retraction port  129  can be used to provide hydraulic access to a retraction reservoir  131 , such that an increase in hydraulic pressure displaces piston  124  to retract ram  120 .  
         [0010]     With rams  120  extended and leading edges  122  engaging oilfield tubular  112 , a strong hydraulic seal prevents fluid from escaping the wellbore through annulus  116 . As long as hydraulic pressure is maintained in reservoir  130 , rams  120  will continue to seal annulus  116 . However, there are circumstances where it is desired to maintain the annular seal regardless of the operational abilities of ram-type BOP  100 . For example, it is desirable to maintain seal integrity in the event of a power failure at the rigsite. Furthermore, if wellbore is to be shut-in for an extended period of time, maintaining hydraulic pressure over that time period is not always reliable or prudent. Therefore, systems and mechanisms to “lock” hydraulic rams  120  in place once activated are highly desirable in the oilfield.  
         [0011]     One former method to lock hydraulic rams  120  in place involves the tightening of mechanical screws to lock pistons  124  in place once displaced in reservoir  130 . Such screws were tightened either manually or through power devices and would effectively lock rams  120 , thrust rods  126 , and pistons  124  in place. However, accessibility concerns make such a solution less than optimal. Particularly, in deep-sea installations, these locking screws must be activated by remotely operated vehicles or through electrical actuators. As such, their reliability is suspect in depths of several hundred feet or more. Furthermore, on land-based rigs, the BOP  100  is typically located beneath the rig floor. As such, engaging and disengaging the locking screws takes considerable time, time that is not always available in the event of an emergency.  
         [0012]     Additional solutions to lock hydraulic rams are available to lock thrust rods  126  in place. Formerly, ratchet profiles (e.g. U.S. Pat. No. 3,941,141 to Robert, hereby incorporated by reference herein) have been used upon the outer profiles of thrust rods  126  in conjunction with matching locking members to retain thrust rods  126  in place. Furthermore, various internal threaded mechanisms (e.g U.S. Pat. No. 4,052,995 to Ellison and U.S. Pat. No. 4,076,208 to Olson, both hereby incorporated by reference herein) have been employed to secure thrust rods  126  in place using whereby thrust rods  126  are threaded and corresponding jam-nut devices lock thrust rods  126  in place. Finally, various wedging solutions (e.g. U.S. Pat. Nos. 4,305,565 to Abbe and U.S. Pat. No. 4,969,390 to Williams, both hereby incorporated by reference herein) have been proposed to lock thrust rods  126  in place. While promising, each of these solutions are considered by many to be less than optimal in that they exhibit a slight amount of slip or “play” in the reverse direction known as backlash before they engage and lock the thrust rods  126  in place. In the event of a high pressure “kick” to a wellbore, even infinitesimal displacements in rams  120  connected to thrust rods  126  can result in a catastrophic release of wellbore fluids.  
         [0013]     Therefore, there is a long-felt need in the industry for an apparatus to quickly, positively, and solidly lock ram-type blowout preventers in an engaged position with minimal operator interaction and with minimal backlash of the rams before locking occurs.  
       SUMMARY OF INVENTION  
       [0014]     In one embodiment, a blowout preventer includes a housing configured to be positioned above a wellhead and to surround a drillstring, a plurality of rams positioned perpendicular to an axis of the drillstring, and a roller lock positioned about a thrust rod of each ram. The rams may be configured to engage the drillstring and hydraulically isolate an annulus between the wellbore and the drillstring from components located above the housing when the rams are in a closed position. The roller locks may be configured to maintain the rams in the closed position.  
         [0015]     In one aspect, the present invention related to a locking apparatus to be used with a blowout preventer. In one embodiment, the locking apparatus includes a plurality of spherical locking elements to engage and restrict movement of thrust rods connected to rams of the blowout preventer. The locking apparatus may also include a plurality of receptacles for the spherical locking elements, wherein each receptacle includes an inclined surface configured to thrust the spherical locking elements into compressive contact with the thrust rods when the operating rams are urged open. Furthermore, the locking apparatus may also include a release cage to retract and retain the spherical locking elements into the receptacles and to direct them out of compressive contact with the thrust rods when the operating rams are to be opened.  
         [0016]     In one aspect, the present invention relates to a method to lock rams of a blowout preventer. The method may include positioning spherical locking elements inside receptacles located adjacent to thrust rods of the rams, wherein the receptacles include inclined surfaces configured to engage the spherical locking elements into the thrust rods when the rams are urged open. The method may further include locking the thrust rods with compressive engagement of the spherical locking elements. The method may further include retracting the spherical locking elements into their respective receptacles with a release cage to unlock the rams.  
         [0017]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0018]      FIG. 1  is a schematic section-view drawing of a ram-type blowout preventer.  
         [0019]      FIG. 2  is a schematic drawing of a thrust rod retainer in accordance with an embodiment of the present invention.  
         [0020]      FIG. 3  is a schematic drawing of a thrust rod retainer in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     Referring now to  FIG. 2 , a schematic representation of a thrust rod lock  250  in accordance with an embodiment of the present invention is shown. In this embodiment, thrust rod lock  250  is preferably located within a piston ram housing (e.g.  104  of  FIG. 1 ) in a fixed position surrounding a thrust rod  226 . Thrust rod lock  250  is desirably configured to allow displacement of thrust rod  226  in a locking direction  252 , while disallowing displacement of thrust rod  226  in an unlocking direction  254 . Thrust rod lock  250  includes a main body  256 , into which one or more roller receptacles  258  are formed. Roller receptacle  258  is shown including a deep section  260  that extends to a shallow section  262  through an inclined surface  264 .  
         [0022]     As such, each roller receptacle  258  is configured to retain and deploy a roller  266  within and from receptacle  258  when locking of thrust rod  226  is selectively desired. Particularly, when roller  266  is located within deep section  260 , substantially no contact occurs between roller and thrust rod  226 , but as roller  266  travels down inclined surface  264 , thrust rod  226  is increasingly locked into a bind by roller  266  and surface  264 . Furthermore, once so locked, further increases in load upon thrust rod  226  in unlocking direction  254  tend to cause roller  266  in contact with inclined surface  264  to compress against thrust rod  226  even tighter, resulting in an even stronger locked position. Displacements of thrust rod  226  in locking direction  252  tend to roll roller  266  up inclined surface  264  toward deep section  260  of main body  256 , such that rod  226  is free to move in locking direction  252 .  
         [0023]     Furthermore, a retainer cage  268  of thrust rod lock  250  retains roller  266  within receptacle  258  and is configured to retrieve roller  266  into deep section  260  when thrust rod  226  is to be released. Retainer cage  268  preferably includes a slot (not shown) adjacent to roller  266  to allow retainer cage  268  to displace roller  266  without interfering with the roller&#39;s engagement with thrust rod  226 . In the case where roller  266  is a spherical roller, slot can be a longitudinal slot or a spherical section to match the outer profile of roller  266 . Alternatively, in the case where roller  266  is cylindrical, the slot can be a transverse slot approximately the same width as roller  266 . Regardless of configuration, when thrust rod  226  is to be displaced in unlocking direction  254 , retainer cage  268  is displaced in direction  270  to retrieve roller  266  up inclined surface  264  and into deep section  260 , away from the engagement with outer surface of thrust rod  226 . Retainer cage  268  may be biased so that roller  266  is biased in the direction opposite  270  and into thrust rod  226 . Alternatively, cage  268  may be unbiased allowing movement of thrust rod  226  to be the sole force in causing roller  266  to be engaged therewith.  
         [0024]     It should be understood that any means to displace or bias retainer cage known in the art may be employed, including, but not limited to, hydraulic lines, springs, and tension cables. Particularly, retainer cage can be constructed to be displaced when hydraulic pressure is applied to a hydraulic actuator attached thereto. Furthermore, if a hydraulic device is employed to retract thrust rod  226  into unlocking direction  254 , a control system (not shown) can be used to direct such hydraulic pressure to either the retainer cage actuator, the thrust rod retractor, or both. Alternatively, a hydraulic system to release thrust rod lock  250  can be distinct from a hydraulic system to displace thrust rod  226  in unlocking direction  254 .  
         [0025]     Furthermore, it should be understood that inclined surface  264  can be any of various types known in the art. Particularly, surface  264  can be a mere planar surface or can be profiled to fit the contours of spherical rollers  266 . Furthermore, while rollers  266  are described generically, it should be understood that they can be constructed as spherical or cylindrical devices and can be constructed with various hardness and friction values to facilitate contact and engagement between thrust rod  226  and inclined surface  264 .  
         [0026]     Referring now to  FIG. 3 , a schematic of a thrust rod lock  350  in accordance with an embodiment of the present invention is shown. Like thrust rod lock  250  of  FIG. 2 , thrust rod lock  350  of  FIG. 3  is preferably located within a piston ram housing  304  in a fixed position surrounding a thrust rod  326 . While a single thrust rod lock  350  is shown, it should be understood that a plurality of rod locks  350  can surround thrust rod  326  radially and axially.  
         [0027]     Thrust rod lock  350  is desirably configured to allow displacement of a thrust rod  326  in a locking direction  352 , while resisting displacement of thrust rod  326  in an unlocking direction  354 . Thrust rod lock  350  includes a main body  356 , into which two series of roller receptacles  358 A,  358 B are formed. Like receptacles  258  of  FIG. 2 , inner roller receptacle  358 A is profiled to urge a roller  366 A into engagement with thrust rod  326  when thrust rod  326  is displaced in unlocking direction  354 . At the same time, outer roller receptacle  358 B is profiled to urge a roller  366 B into engagement with an outer wear plate  380  when main body  356  is displaced in unlocking direction  354 . Therefore, for each location about and along thrust rod  326 , one roller  366 A bites with thrust rod  326  and another roller  366 B bites with wear plate  380  to resist displacement of thrust rod  326  in unlocking direction  354 . As such, for each incremental displacement in unlocking direction  354 , thrust rod  326  of  FIG. 3  will experience double the radial compression as would be experienced by a thrust rod in a single roller configuration (e.g.  FIG. 2 ).  
         [0028]     Furthermore, inner and outer retainer cages  368 A,  368 B retain rollers  366 A,  366 B inside receptacles  358 A,  358 B and allow for rod lock  350  to be released once retraction of thrust rod  326  is desired. As with retainer cage  268  of  FIG. 2 , retainer cages  368 A,  368 B allow rollers  366 A,  366 B to contact thrust rod  326  and wear place  380  through slots or other forms of apertures (not shown) therethrough. As before, such apertures in retainer cages  368 A,  368 B can take the form of longitudinal slots or spherical sections in the case where rollers  366 A,  366 B are spherical or can be transverse slots if rollers  366 A,  366 B are cylindrical. A pair of bias springs  382 ,  384  is shown working in conjunction with main body  356  and retainer cages  368 A,  368 B to thrust rollers  366 A,  366 B into locking engagement with thrust rod  326  and wear plate  380  by default. As inner retainer cage  368 A is fixed relative to housing  304 , spring  382  urges main body in the direction of arrow  352  such that inclined surface  364 A urges roller  366 A into contact with thrust rod  326 . Likewise, spring  384  between main body  356  and outer retainer cage  368 B urges cage  368 B and roller  366 B in the direction of arrow  352  such that inclined surface  364 B urges roller  366 B into contact with wear plate  380 .  
         [0029]     To release rollers  366 A,  366 B from their engagement with thrust rod  326  and wear plate  380 , an unlocking mechanism  386  is employed. Unlocking mechanism  386  can be constructed any number of ways, but is preferably configured to retract rollers  366 A,  366 B into their respective receptacles  358 A,  358 B so that thrust rod  326  can be retracted in unlocking direction  354 . Unlocking mechanism  386  of  FIG. 3  is shown within a recess  388  of housing  304 . A hydraulic seal  390  surrounding mechanism  386  ensures that when hydraulic pressure is increased to a hydraulic port  392 , mechanism  386  is displaced in the direction of  354  such that a thrust face  394  engages a corresponding load shoulder  396  of outer retainer cage  368 B. When pressure to port  392  is sufficiently elevated, springs  384  and  382  are compressed such that rollers  366 A,  366 B are retained within recesses  358 A,  358 B to enable thrust rod  326  to be retracted. Once thrust rod  326  is retracted, pressure to port  392  can be released so that thrust rod  326  can be quickly engaged and held in place without any secondary locking step necessary. In its ordinary, equilibrium state, rod lock assembly  350  resists disengagement of thrust rod  326  without any supplemental steps, even in the event of total loss of hydraulic power.  
         [0030]     Rod lock assemblies  250  and  350  of the present invention exhibit many advantages over locking mechanisms currently available. In particular, rod lock assemblies  250 ,  350  are capable of securing thrust rods  226 ,  326  almost instantaneously and with little or no backlash or slippage. Furthermore, rod lock assemblies  250 ,  350  are disclosed as “fail safe” devices, in that they lock by default. No affirmative steps are necessary to lock thrust rods  226 ,  326  in place once they are extended. Rod lock assemblies  250 ,  350  automatically engage and resist disengagement of thrust rods  226 ,  326 . As no external power source is necessary to engage rod locks  250 ,  350 , their effectiveness is not compromised by power failures. In contrast, hydraulic (or other) power is only necessary to disengage rod locks  250 ,  350 . Finally, as rod locks  250 ,  350  are configured to engage smooth outer profiles of thrust rods  226 ,  326 , no obstructive features are necessary on thrust rods  226 ,  326 . Former solutions required special profiles that could obstruct thrust rod  226 ,  326  operation and engagement.  
         [0031]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A blowout preventer to hydraulically seal a wellbore includes a housing configured to be positioned above a wellhead and to surround a drillstring, a plurality of rams positioned perpendicular to an axis of the drillstring to engage the drillstring and hydraulically isolate an annulus between the wellbore and the drillstring when in a closed position, and a roller lock positioned about a thrust rod of each ram configured to maintain the rams in the closed position.