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BACKGROUND 
     This present disclosure relates generally to techniques for performing wellsite operations. More specifically, the present disclosure relates to techniques for preventing blowouts involving, for example, a blowout preventer, ram assembly, a door assembly, and/or a lock. 
     Oilfield operations may be performed to locate and gather valuable downhole fluids. Oil rigs are positioned at wellsites, and downhole tools, such as drilling tools, are deployed into the ground to reach subsurface reservoirs. Once the downhole tools form a wellbore to reach a desired reservoir, casings may be cemented into place within the wellbore, and the wellbore completed to initiate production of fluids from the reservoir. Downhole tubular devices may be positioned in the wellbore to enable the passage of subsurface fluids to the surface. 
     Leakage of subsurface fluids may pose an environmental threat if released from the wellbore. Equipment, such as blow out preventers (BOPs), may be positioned about the wellbore to form a seal about a tubular therein to prevent leakage of fluid as it is brought to the surface. BOPs may have selectively actuatable rams or ram bonnets, such as pipe rams or shear rams, that may be activated to seal and/or sever a tubular in a wellbore. Some examples of BOPs are provided in U.S. Patent/Application Nos. 20110000670; U.S. Pat. Nos. 7,814,979; and 7,367,396. 
     It may be desirable to provide BOPs with various features, such as seals, ram blocks, doors and/or lock mechanisms, etc., for use in wellsite operations. Examples of BOPs and/or features may be provided in patent application Ser. Nos. 13/018,217 (not yet published at the time of filing), US2010/0243926, US2011/0012311, U.S. Pat. Nos. 5,897,094, 7,044,430, 7,051,989, 5,575,452, WO 2012/012192, WO 2012/012193, U.S. Pat. Nos. 7,195,224 and 7,798,466. 
     SUMMARY 
     In at least one aspect, the disclosure relates to a lock assembly for a blowout preventer. The blowout preventer includes a housing and a ram assembly. The housing has a bore and a channel therethrough. A tubular is positionable through the bore and engageable therein. The ram assembly is slidably positionable in the channel and engageable with the tubular, and includes a ram block, a ram cylinder, and a ram shaft operatively connecting the ram block to the ram cylinder. The lock assembly includes a lock cylinder and a lock piston. The lock cylinder is operatively connectable to the ram cylinder. The ram shaft has an end slidably extendable into a lock cavity of the lock cylinder. The lock piston is slidably positionable in the lock cylinder and separates the lock cavity into a variable volume lock chamber and a variable volume closing chamber, and is movable between a locked and unlocked position, the lock piston having a ram surface engageable with an end of the ram shaft when the lock piston is in the locked position and has a ram hole therethrough to receive the end of the ram shaft when the lock piston is in the unlocked position whereby the ram block is selectively lockable about the tubular. 
     The lock assembly may also include a guide piston extending into a guide cavity in the lock piston and slidably engageable therewith, and/or a lock ring slidably positionable in the closing chamber about the lock piston. The lock ring separates the closing chamber into a variable volume ring chamber and a variable volume ring fluid chamber, the ring fluid chamber fluidly connected to a ram chamber in the ram cylinder. The lock assembly may also include a ram piston slidably positionable in the ram cylinder and define the ram chamber therein, the ram chamber having a variable volume. The lock assembly may also include a valve fluidly coupled to the ring chamber to selectively vent fluid therefrom, and/or a valve fluidly coupled to the lock chamber to selectively vent fluid therefrom. The lock assembly may also include a ram piston slidably positionable in the ram cylinder and defining a variable volume ram chamber therein, the ram chamber fluidly connectable to the closing chamber. The lock assembly may also include fasteners operatively connecting the lock cylinder to an end of the ram cylinder. 
     In another aspect, the disclosure relates to a blowout preventer for sealing about a tubular extending from a wellbore of a wellsite. The blowout preventer includes a housing having a bore and a channel therethrough (with the tubular positionable through the bore and engageable therein), a plurality of ram assemblies slidably positionable in the channel and engageable with the tubular (each of the plurality of ram assemblies comprising a ram block, a ram cylinder, and a ram shaft operatively connecting the ram block to the ram cylinder), and a plurality of locking assemblies for locking the plurality of ram assemblies. Each locking assembly includes a lock cylinder operatively to connectable the ram cylinder (the ram shaft having an end slidably extendable into a lock cavity of the lock cylinder) and a lock piston. The lock piston is slidably positionable in the lock cylinder and separates the lock cavity into a variable volume lock chamber and a variable volume closing chamber. The lock piston is movable between a locked and unlocked position, and has a ram surface engageable with the end of the ram shaft when the lock piston is in the locked position and having a ram hole therethrough to receive the end of the ram shaft when the lock piston is in the unlocked position whereby the ram block is selectively lockable about the tubular. 
     The blowout preventer may also include a fluid source operatively connectable to the lock chamber and/or the closing chamber, and selectively providing fluid pressure thereto. The blowout preventer may also include an actuator operatively connectable to one of the blowout preventer, the ram assemblies, and/or the lock assemblies for selective activation thereof. The blowout preventer may also include a door positionable about the ram assembly and the channel, and providing selective access thereto. 
     In another aspect, the disclosure relates to a method of locking a blowout preventer. The blowout preventer includes a housing and a ram assembly. The housing has a bore and a channel therethrough, with the tubular positionable through the bore and engageable therein. The ram assembly is slidably positionable in the channel and engageable with the tubular. The ram assembly includes a ram block, a ram cylinder, and a ram shaft operatively connecting the ram block to the ram cylinder. The method involves providing the lock assembly about the ram assembly and selectively locking the ram block about the tubular by selectively applying pressure to one of the lock chamber and the closing chamber. 
     The lock assembly may also include a guide piston extending into a cavity in the lock piston and slidably engageable therewith and a lock ring slidably positionable in the closing chamber about the lock piston. The method may involve increasing a force of the lock piston by slidably moving the lock ring adjacent the lock piston. The method may also involve selectively venting fluid from the lock chamber and/or the closing chamber and/or moving the ram block by fluidly coupling the closing chamber to a ram chamber in the ram cylinder to selectively drive a ram piston therein. 
     The disclosure may also relate to a blowout preventer for sealing about a tubular. The blowout preventer includes a housing, a ram assembly and a lock assembly. The lock assembly selectively locks the ram assembly in a sealed position and includes a locking cylinder, a locking piston, and a locking ring. The locking piston is positionable in the locking cylinder to define a lock chamber and a closing chamber, and is slidably movable between an unlocked position that permits the ram shaft to move to a retracted position and a locked position that prevents the ram shaft from moving to the retracted position. The locking ring is slidably positionable in the closing chamber between a piston position adjacent the piston and a chamber position a distance therefrom. When in the piston position, the locking ring increases a force required to move the locking piston to the locked position. 
     The disclosure may also relate to a method of sealing a tubular involving providing a blowout preventer including a housing having a bore and a channel therethrough, the tubular positionable through the bore, a ram assembly slidably movable through the channel of the housing (the ram assembly including a ram block having a ram shaft extending therefrom and slidably movable within a ram cylinder, the ram block sealingly engageable with the tubular), and a lock assembly for selectively locking the ram assembly in a sealed position. The lock assembly includes a locking cylinder, a locking piston positionable in the locking cylinder to define a lock chamber and a closing chamber (the locking piston is slidably movable between an unlocked position to permit the ram shaft to move to a retracted position and a locked position to prevent the ram shaft from moving to the retracted position), and a locking ring slidably positionable in the closing chamber between a piston position adjacent the piston and a chamber position a distance therefrom such that, when in the piston position, the locking ring increases a force required to move the locking piston to the locked position. The method may also involve positioning a tubular through the bore of the housing, and selectively locking the ram blocks in a sealed position by selectively pressurizing the lock and the closing chambers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the above recited features and advantages of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments and are, therefore, not to be considered limiting of its scope. The figures are not necessarily to scale and certain features, and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
         FIG. 1  depicts a schematic view of an offshore wellsite having a blowout preventer (BOP) with a ram assembly with a lock assembly. 
         FIG. 2  depicts a vertical cross-sectional view of a BOP with a ram assembly with a lock assembly. 
         FIG. 3  depicts a horizontal cross-sectional view of a portion of a BOP depicting a lock assembly therein. 
         FIGS. 4A-1  through  4 H- 1  are horizontal cross-sectional views of a portion of a BOP and  FIGS. 4A-2  through  4 H- 2  are vertical cross-sectional views of the portion of the BOP, depicting sequential operation of a lock assembly of a ram assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The description that follows includes exemplary apparatus, methods, techniques, and/or instruction sequences that embody techniques of the present subject matter. However, it is understood that the described embodiments may be practiced without these specific details. 
     Blowout preventers (BOPs) may be positioned about a tubular and provide a seal therewith, for example, during a blowout. The BOPs may be provided with ram blocks for sealing with and/or severing the tubular. The ram blocks may be reciprocated by extending and retracting a ram shaft in a ram cylinder. The ram assembly may be selectively locked in a sealed position with a lock assembly. The lock assembly may include a locking piston slidably positionable in a locking chamber and provided with a locking ring to control movement thereof. The lock assembly may be selectively pressurized to control the locking of the ram assembly. 
       FIG. 1  depicts an offshore wellsite  100  having a subsea system  104  and a surface system  102 . The surface system  102  may be used to facilitate the oilfield operations at the offshore wellsite  100 . The surface system  102  may include a rig  105 , a platform  106  (or vessel) and a controller  108 . 
     The subsea system  104  includes a conduit (e.g., riser)  110  extending from the platform  106  to a sea floor  112 . The subsea system further includes a wellhead  114  with a tubular  116  extending into a wellbore  118 , a BOP  120  and a controller  108 . The BOP  120  has various BOP components, such as a ram assembly  124  for shearing the tubular  116  and sealing the wellbore  118 . The ram assembly  124  is engageable with the tubular  116  to form a seal about the tubular and/or to sever or cut the tubular  116 . The ram assembly  124  has a lock assembly  125  for selectively locking the ram assembly  124  to prevent movement thereof, for example, to maintain the ram assembly  124  in a sealed position about the tubular  116 . 
     The surface system  102  and subsea system  104  may be provided with one or more controllers  108  located at various locations to control the surface system  102  and/or the subsea system  104 . Communication links  126  may be provided for communication between the controllers  108  and various parts of the wellsite  100 . 
     Although the BOP  120  is described herein as being used in subsea operations, it will be appreciated that the wellsite  100  may be land or water based, and the BOP  120  and/or lock assembly  125  may be used in any wellsite environment. The BOP  120  may sever and/or seal a tubular device, such as tubular  116 . ;Tubular devices&#39; as used herein refers to tubular members or devices, such as pipes, certain downhole tools, casings, drill pipe, liner, coiled tubing, production tubing, wireline, slickline, or other tubular members positioned in the wellbore, and associated components, such as drill collars, tool joints, drill bits, logging tools, packers, and the like (referred to as ‘tubulars’ or ‘tubular strings’). 
       FIG. 2  depicts a longitudinal cross-sectional view of a BOP  220  in an unsealed position.  FIG. 2  depicts the ram assembly and lock assembly in greater details. The ram assembly is movable in the BOP  220  to a sealed position (not shown). The BOP  220  includes a housing  228  with a bore  230  and a channel  232  therethrough. The BOP  220  also includes a ram assembly  224  and a lock assembly  225 . The ram assembly  224  includes a ram block  234 , a ram shaft  236  a ram cylinder  238  and an actuator  240 . The ram shaft  236  extends through a passageway  237  through the ram assembly  224 . The lock assembly  225  is also provided to lock the ram assembly  224  about tubular  216  in position as will be described further herein. 
     The BOP of  FIG. 2  is depicted with two opposing ram assemblies  224 , but any number may be provided. The ram assemblies  224  may be activated to converge about the tubular  216  to seal and/or sever therewith as indicated by the arrows. The ram block  234  may seal with and/or sever a tubular  216 . An example of a ram assembly usable with the BOP are described in U.S. Pat. No. 5,735,502. A door assembly  227  may also optionally be included with a door  242  to provide access to the channel  232  and the components of the BOP  220 . The ram shaft  236  extends through the door  242  with the ram cylinder  238  external thereto. 
       FIG. 3  depicts a horizontal cross-sectional view of a portion of a ram assembly of a BOP, such as the BOP  220  of  FIGS. 2A and 2B , depicting the lock assembly  225  in greater detail. The lock assembly  225  includes a locking cylinder  340  with a locking piston  342  slidably positioned therein. The lock assembly  225  may be secured in position with fasteners, such as bolts  341 . The locking piston  342  has a lock chamber  344  and a closing chamber  346  on either side thereof. Each of these chambers is of a variable dimension that changes as the piston  342  reciprocates within the locking cylinder  340 . A ram chamber  345  is positioned in the ram cylinder  238  outside the lock assembly  225 , and is in fluid communication with the closing chamber  346 . 
     The locking piston  342  is movable between a locked position adjacent a locked end  348  and an unlocked position adjacent an unlocked end  350 . When moved to the locked position, the piston  342  blocks the passageway  237 , and prevents the ram shaft  236  from moving to a retracted position away from the tubular  216  (see, e.g.,  FIG. 2 ). In the locked position, the piston  342  may also press against the ram shaft  236  to maintain the ram block  234  in a sealed position about the tubular  216  (see, e.g.,  FIG. 2 ). The locking piston  342  may be provided with an inclined contact surface  343  for slidingly engaging an end of the ram shaft  236 . 
     The locking piston  342  has a passage  352  therethrough for receiving an end of the ram shaft  236 . When moved to the unlocked position, the passage  352  is aligned with the ram shaft  236  to permit passage of the ram shaft  236  therethrough. The ram shaft  236  may move a distance through the locking piston  342 . In the unlocked position, the piston  342  may move to a retracted (or unsealed) position such that the ram block  234  is a distance from the tubular  216  (see, e.g.,  FIG. 2A ). 
     The lock assembly  225  may also be provided with a guide piston  354 . The locking piston  342  has a guide cavity  356  therein shaped to slidingly receive the guide piston  354 . The guide piston  354  may be used to guide the travel of the locking piston  342  through the locking cylinder  340 . 
     The lock assembly  225  may also be provided with a locking ring  357  having an aperture  359  therethrough for receiving and slidingly engaging the guide piston  354 . The locking ring  357  is slidably positionable along the guide piston  354  between a piston position adjacent locking piston  342  and a distance therefrom adjacent lock end  348 . The locking ring  357  may be selectively moved by manipulating the pressures in the locking chamber  344 , closing chamber  346  and positioning locking piston  342 . 
     The position of the locking ring  357  may be used to selectively resist movement of the locking piston  342 . For example, when the locking ring  357  is positioned adjacent the locking piston  342 , the locking ring  357  increases the force and/or the pressure in lock chamber  344  required to move the locking piston  342 . In another example, when the locking ring  357  is positioned a distance from the locking piston, the force required to move the locking piston  342  does not increase. In this manner, the locking ring  357  may be used to adjust the movement of the locking piston  342 . 
     The actuator  240  may include a pressure source for selectively applying pressure to the locking chamber  344 , closing chamber  346  and/or the ram chamber  345 . The actuator  240  may also include a controller, sensor or other electronics. Optionally, the lock assembly  225  may be operated without an external pressure source. Valve  360  is provided to selectively vent ram chamber  345 . Other fluid control devices, such as an external pressure source, may optionally be provided to manipulate the pressure differentials and control operation of the lock assembly  225 . 
       FIGS. 4A-1  and  4 A- 2  through  4 H- 1  and  4 H- 2  depict a sequential operation of the ram assembly  224  and the lock assembly  225 .  FIGS. 4A-1  through  4 H- 1  depict horizontal cross-sectional views of the ram assembly  224  and lock assembly  225 .  FIGS. 4A-2  through  4 H- 2  depict vertical cross-sectional views of the ram assembly  224  and lock assembly  225 . Each of these figures depicts the ram shaft  236  moving through passageway  237  between a sealed and unsealed (or retracted) position about tubular  216  (see, e.g.,  FIG. 2 ), and the locking piston  342  moving between a locked and unlocked position in locking cylinder  340 .  FIGS. 4A-1  through  4 H- 1  also show a ram piston  462  slidably positionable in the ram cylinder  238  for selectively changing a volume of the ram chamber  345 . A second valve  466  is also visible in  FIGS. 4A-2  through  4 H- 2  for selectively releasing or inputting pressure into the locking chamber  344 . 
     Starting with  FIGS. 4A-1  and  4 A- 2 , the lock assembly  225  is in a closed position. In these figures, the ram shaft  236  is moved to a retracted position extending through passage  352  of locking piston  342 . The locking piston  342  is moved to the unlocked position adjacent an unlocked end  348 . The locking ring  357  is a distance from the locking piston  342  and adjacent the lock end  350 . Valve  360  is open to vent closing chamber  346  as indicated by the dashed arrow, and valve  466  is in a closed position. 
     At  FIG. 4B-1  and  4 B- 2 , the ram shaft  236  begins to move to the sealed position as indicated by the arrow. Ram piston  462  begins to moves to an expanded position in ram cylinder  238 . The ram shaft  236  moves with the ram piston  462  and is advanced towards the tubular  216 . The locking ring  357  moves to the piston position adjacent locking piston  342 . Fluid fills chamber  470  between piston  462  and the lock assembly  225 , and chamber  346 . 
     At  FIG. 4C-1  and  4 C- 2 , the ram shaft  236  and ram piston  462  continue to move to the sealed position as indicated by the arrow. Valve  466  is then moved to an open position and begins to vent the locking chamber  344 . 
     At  FIG. 4D-1  and  4 D- 2 , because the piston area of the locking piston  342  adjacent the locking chamber  344  is greater than the piston area of the locking piston  342  adjacent to the closing chamber  346 , the locking piston  342  begins to move to the unlock end  348  as indicated by the arrow, and the locking chamber  344  fills with fluid. The locking ring  357  is carried by the locking piston  342  as it travels. The contact surface  343  slidingly engages the ram shaft  236  and urges the ram shaft  236  toward the sealed position. The volume of the locking chamber  344  is increasing in size as the volume of the closing chamber  346  reduces. The volume of the guide cavity  356  is also increased as the locking piston  342  slidingly receives the guide piston  354 . Valves  360  and  466  remain open. Valve  466  continues to vent the locking chamber as indicated by the dashed arrow. 
     At  FIG. 4E-1  and  4 E- 2 , shows the lock assembly  225  in a locked and vented position with no opening or closing pressure applied thereto. With everything vented, a spring force moves valve  360  to the closed position. At  FIG. 4F-1  and  4 F- 2 , the locking chamber  344 , closing chamber  346  and ram chamber  345  are vented. The locking piston  342  moves toward the lock end  350  and the locking ring  357  moves toward the unlock end  348 . Pressure in the closing chamber  346  and the ram chamber  345  apply pressure P to the ram shaft  236  to remain in the sealed position. The pressure may be, for example, about 3000 psi (210.97 kg/cm). 
     At  FIG. 4G-1  and  4 G- 2 , locking ring  357  moves adjacent to the unlock end  348  and the locking piston  342  moves adjacent the lock end  350 . The locking piston  342  presses against valve  360  and moves it to an open position as indicated by the arrows to release pressure from closing chamber  346 . Valve  466  is under pressure and remains open to vent the locking chamber  344 . 
     At  FIG. 4H-1  and  4 H- 2 , the ram shaft  236  begins to pass through the passage  352  in the locking piston  342  and moves to the retracted position as indicated by the arrow. Ram piston  462  also moves toward locking piston  342 . Valve  466  remains open to vent from locking chamber  344 . 
     The process may be varied by manipulating pressures in the locking, unlocking and closing chambers. The actuator may be provided with electronics, such as a controller, processor, data storage, etc. to selectively operate the BOP, lock assembly and/or ram assembly. 
     It will be appreciated by those skilled in the art that the techniques disclosed herein can be implemented for automated/autonomous applications via software configured with algorithms to perform the desired functions. These aspects can be implemented by programming one or more suitable general-purpose computers having appropriate hardware. The programming may be accomplished through the use of one or more program storage devices readable by the processor(s) and encoding one or more programs of instructions executable by the computer for performing the operations described herein. The program storage device may take the form of, e.g., one or more floppy disks; a CD ROM or other optical disk; a read-only memory chip (ROM); and other forms of the kind well known in the art or subsequently developed. The program of instructions may be “object code,” i.e., in binary form that is executable more-or-less directly by the computer; in “source code” that requires compilation or interpretation before execution; or in some intermediate form such as partially compiled code. The precise forms of the program storage device and of the encoding of instructions are immaterial here. Aspects of the disclosure may also be configured to perform the described functions (via appropriate hardware/software) solely on site and/or remotely controlled via an extended communication (e.g., wireless, internet, satellite, etc.) network. 
     While the embodiments are described with reference to various implementations and exploitations, it will be understood that these embodiments are illustrative and that the scope of the inventive subject matter is not limited to them. Many variations, modifications, additions and improvements are possible. For example, one or more ram assemblies may be provided with a lock assembly with various fluid activated pistons in various configurations for selectively locking the ram assembly in the sealed and unseal position. The lock assemblies may be positioned about the ram assembly, and be made integral therewith or independent therefrom. 
     Plural instances may be provided for components, operations or structures described herein as a single instance. In general, structures and functionality presented as separate components in the exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the inventive subject matter.

Summary:
A lock assembly of a blowout preventer is provided. The blowout preventer includes a ram assembly slidably positionable in the channel and engageable with a tubular. The ram assembly includes a ram block, a ram cylinder, and a ram shaft. The lock assembly includes a lock cylinder and a lock piston. The lock cylinder is operatively connectable to the ram cylinder. The ram shaft has an end slidably extendable into a lock cavity of the lock cylinder. The lock piston is slidably positionable in the lock cylinder and separates the lock cavity into a lock and a closing chamber. The lock piston has a ram surface engageable with the end when the lock piston is in a locked position, and a ram hole therethrough to receive the end when the lock piston is in an unlocked position whereby the ram block is selectively lockable about the tubular.