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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Appin. No. 62/170,446, which was filed on Jun. 3, 2015, the full disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present technology relates generally to oilfield equipment. In particular, the present technology relates to hydraulic accumulators for use in oilfield applications. 
         [0004]    2. Discussion of the Background 
         [0005]    In the drilling industry, hydraulic controls are used for controlling many safety components. In addition, many such components are powered by hydraulic accumulators. To ensure safety, it is desirable to know whether an accumulator will have enough hydraulic fluid to fire a particular safety component. One way to determine the volume of fluid in a hydraulic accumulator is to determine the position of the piston within the accumulator. 
         [0006]    Known methods of determining the amount of hydraulic fluid in an accumulator include the use of cable extension transducers, wherein a cable is physically attached to the piston in the accumulator. Based on the amount of cable pulled into the accumulator, the position of the piston in the accumulator can be determined. Use of cable extension transducers can be problematic because they require physical invasion into the accumulator and multiple mechanical parts working together, each of which leads to decreased reliability of the accumulator, and greater maintenance costs. 
         [0007]    Another method of determining piston position within an accumulator includes use of acoustic signals transmitted into the accumulator. This method avoids physical intrusion into the accumulator by a cable, but has problems of its own. For example, to accurately determine distance through a medium using acoustic waves, certain properties of the medium must be known, such as temperature and pressure. Thus, temperature and pressure gauges need to be installed within the medium in the accumulator to determine these parameters. The introduction of additional sensors decreases reliability of the system, as well as the accuracy of information obtained regarding position of the piston. 
       SUMMARY OF THE INVENTION 
       [0008]    One embodiment of the present technology provides a hydraulic accumulator for use in powering components of a subsea drilling system, including an accumulator housing enclosing a cavity, the accumulator housing defining an aperture in a first end, and having a longitudinal axis, a piston disposed within the cavity and movable relative to the accumulator housing in a direction parallel to the longitudinal axis, and a laser piston position sensor attached to an end of the accumulator housing adjacent the aperture in the first end. The laser piston position sensor includes a sensor housing enclosing a cavity and defining a recess in an outer surface of the sensor housing, a lens positioned in the recess of the sensor housing, and exposed to the cavity in the sensor housing, the lens positioned between the cavity in the sensor housing and the aperture of the accumulator housing, and a laser sensor attached to the sensor housing in the cavity, that emits a laser directed through the lens and aperture in the accumulator housing to the piston. 
         [0009]    Another embodiment of the present technology provides a laser piston position sensor for determining the position of a piston within a hydraulic accumulator, the laser piston position sensor configured to attach to an end of the hydraulic accumulator, and the hydraulic accumulator having an aperture in the end thereof exposing the piston within the hydraulic accumulator to the laser piston position sensor. The laser piston position sensor includes a sensor housing enclosing a cavity containing a low pressure gas, and defining an opening from the cavity toward the hydraulic accumulator, a laser sensor positioned within the cavity for emitting a laser toward the piston of the hydraulic accumulator via the opening in the sensor housing and the aperture in the hydraulic accumulator, and a transparent lens positioned between the laser sensor and the hydraulic accumulator to allow passage of the laser from the laser sensor to the piston, and to separate gases in the hydraulic accumulator from gases in the cavity of the sensor housing. 
         [0010]    Yet another embodiment of the present technology provides a method of determining the position of a piston within a hydraulic accumulator. The method includes the steps of emitting a laser from a laser piston position sensor attached to the hydraulic accumulator, directing the laser through an aperture in the hydraulic accumulator to the piston, and receiving reflected light from the piston. The method further includes determining the amount of time between emission of the laser and receipt of the reflected light, then, based on such time, determining the distance between the laser piston position sensor and the piston, and determining a volume of hydraulic fluid within the hydraulic accumulator based on the position of the piston within the hydraulic accumulator and dimensional characteristics of the hydraulic accumulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present technology can be better understood on reading the following detailed description of nonlimiting embodiments thereof, and on examining the accompanying drawings, in which: 
           [0012]      FIG. 1  is perspective view of a lower blow out preventer (BOP) stack, including accumulators and sensors according to an embodiment of the present technology; 
           [0013]      FIG. 2A  is a perspective view of an accumulator according to an embodiment of the present technology, including a laser piston position sensor mounted thereon; 
           [0014]      FIG. 2B  is an enlarged perspective view of the laser piston position sensor of  FIG. 2A ; 
           [0015]      FIG. 3  is a side cross-sectional view of an upper portion of a hydraulic accumulator and a laser piston position sensor according to an embodiment of the present invention; 
           [0016]      FIG. 4  is an enlarged side cross-sectional view of the laser piston position sensor of  FIG. 3 ; 
           [0017]      FIG. 5  is an alternate view of the laser piston position sensor of  FIG. 4 , rotated 180 degrees around its longitudinal axis. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    The foregoing aspects, features, and advantages of the present technology can be further appreciated when considered with reference to the following description of preferred embodiments and accompanying drawings, wherein like reference numerals represent like elements. The following is directed to various exemplary embodiments of the disclosure. The embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, those having ordinary skill in the art can appreciate that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0019]      FIG. 1  shows a lower stack  10  of a subsea drilling system. The lower stack  10  includes a frame  12  which supports numerous components, including blow out preventer (BOP) rams  14 , cable trays  16 , a control pod  18 , and accumulators  20 . In operation, a drill pipe or production pipe (not shown) passes parallel to the axis  22  of the BOP, through the BOP from the top of the lower stack  10 , and through to the wellhead (not shown), located below the lower stack  10 . The accumulators  20  are hydraulically attached via hydraulic lines  23  to functions on the lower stack  10 , such as, for example the BOP rams  14 . One purpose of the accumulators is to provide a force to close the BOP rams  14  if desired, or to fire other functions on the lower stack  10  or lower marine riser package (not shown). The structure of the accumulators that enables this functionality is described in detail below. Each of the accumulators  20  (or each laser piston position sensor  26 , shown in  FIGS. 2A through 5  and described below) can be attached via cables  25  to a programmable logic controller (PLC)  25  at a remote location, such as, for example, the control pod  18 , or on a platform or vessel on the sea surface. In addition, the PLC  25  can be attached to a human machine interface (HMI) display  27 , to allow an operator to monitor the accumulator and sensors. By using the multiple accumulators  20  in a system, the total accumulator volume can be increased. 
         [0020]      FIG. 2A  depicts an enlarged perspective view of an accumulator  20 , according to an embodiment of the present technology. Specifically, there is shown an exterior view of the accumulator housing  24 , and an accumulator volume detector in the form of a laser piston position sensor  26 . The laser piston position sensor  26 , shown in greater detail in  FIG. 2B , sits atop the accumulator  20  and directs a laser downwardly into the accumulator toward the accumulator piston (shown in  FIG. 3 ). The laser piston position sensor  26  is attached to the accumulator, such as through the use of common flanges and fittings. As described below, the laser can detect the position of the piston inside the accumulator  20 , and can therefor determine the amount of hydraulic fluid in the accumulator  20 . Data about the amount of hydraulic fluid in the accumulator  20  in turn can be transmitted to an operator to help the operator to know whether there is sufficient hydraulic fluid in the accumulator  20  to for a particular function to fire. 
         [0021]      FIG. 3  depicts a cross-sectional side view of an upper portion of the accumulator  20 , with the laser piston position sensor  26  attached to the upper end thereof. This view shows additional accumulator  20  components as well, such as, for example, the accumulator housing  24  and the accumulator piston  28 . In the embodiment of  FIG. 3 , the accumulator housing  24  defines an aperture  29  through an end of the housing. In addition, the accumulator housing  24  is hollow and the accumulator piston  28  spans the inside of the accumulator housing  24  to divide the hollow interior into a first accumulator cavity  30  and a second accumulator cavity  32 . The accumulator piston  28  is sealingly engaged with the walls of the accumulator housing  24  so that fluids and gases inside the accumulator housing  24  cannot flow between the first accumulator cavity  30  and the second accumulator cavity  32 . Typically, such sealed engagement is accomplished using seals  33 , which can be elastomeric seals. In the embodiment shown, the first accumulator cavity  30  can contain a gas, such as, for example, nitrogen or a similarly inert gas. The gas is expandable and compressible, and so can expand and compress as the piston  28  moves up and down relative to the accumulator housing  24  along the longitudinal axis  36  of the accumulator housing  24 . The second accumulator cavity  32  is filled with fluid, such as hydraulic fluid. 
         [0022]    In operation, the second accumulator cavity is fluidly attached to a function, such as a BOP ram. In its fully charged condition, the second accumulator cavity is filled with hydraulic fluid until the piston  28  is positioned near the top of the accumulator housing  24  (as shown in  FIG. 3 ). Such positioning of the piston  28  reduces the volume of the first accumulator cavity  30 , and compresses the gas in the first accumulator cavity  30 , thereby increasing the pressure of the gas. In some embodiments, additional gas can be added via a separate gas line (shown in  FIG. 5 ) to further increase pressure in the first accumulator cavity  30 . 
         [0023]    When an operator desires to use the accumulator  20  to fire a function, such as to close the BOP rams  14 , a valve  38  (shown in  FIG. 1 ) can be opened in the hydraulic line  23  between the accumulator  20  and the function. With the valve  38  open, the pressurized gas in the first accumulator cavity  30  expands and pushes the piston  28  downward in the accumulator housing  24 . As the piston  28  moves downward, it pushes the hydraulic fluid in the second accumulator cavity  32  out of the accumulator  20 , through the hydraulic line  23  connecting the accumulator  20  to the function, and into the function to help fire the function. In some embodiments, the accumulator  20  can be recharged by refilling the second accumulator cavity  32  with hydraulic fluid, thereby pushing the piston  28  toward the top of the accumulator  20 , and compressing the gas in the first accumulator cavity  30 . 
         [0024]    In order for successful firing of a function, it is necessary that the accumulator  20  contain sufficient hydraulic fluid to flow out of the accumulator  20  to the function when the accumulator fires. One way to determine the volume of hydraulic fluid in the accumulator  20  is to determine the position of the accumulator piston  28  within the accumulator housing  28 . This is because the second accumulator cavity  32  is filled with hydraulic fluid, which is substantially incompressible, so that the accumulator piston  28  will rise and fall within the accumulator housing  24  according to changes in the volume of hydraulic fluid in the second accumulator cavity  32 . Accordingly, certain embodiment of the present technology include the laser piston position sensor  26 , mounted at an upper end of the accumulator  20 , to determine the position of the accumulator piston  28  within the accumulator  24 , as described below. 
         [0025]      FIG. 4  shows an enlarged cross-section view of the laser piston position sensor  26  according to one embodiment of the present technology. The laser piston position sensor  26  includes an electronics connector  40  including electronic contacts  42 . The electronics connector can have threads  44  that serve to threadedly connect the laser piston position sensor  26  to external subsea cable. A cable  25  (shown in  FIG. 1 ) can connect to the electronic contacts  42 , and may provide power to the laser piston position sensor  26 . The cable  25  may also serve to relay data from the laser piston position sensor  26  to other parts of the lower stack  10 , such as the control pod  18 , or even directly to an operator at the sea surface. Such cable  25  can be, for example, a pressure balanced oilfield cable  25 , or a molded cable  25 , and can carry about 4-20 mA of current. Internal wiring  43  can connect the electronic contacts  42  to the laser sensor  70  (discussed below). 
         [0026]    The laser piston position sensor  26  also includes a sensor housing  46 . The sensor housing defines a low pressure cavity  48  containing gas (such as inert gas). In some embodiments the gas within the sensor housing can be maintained at a pressure of about 1 atmosphere. As shown, the housing can be composed of multiple housing pieces  46   a,    46   b,    46   c,  which can be assembled and fastened together with bolts  50  or other appropriate fastening means. Seals  52  can be positioned between adjacent housing pieces  46   a,    46   b,    46   c  to prevent ambient fluid from entering the cavity  48 . Seals can also be positioned between the housing  46  and the electronics connector  40 . In some embodiments, the seals  52  can be dynamic seals composed of an elastomer or other appropriate material. Although the housing  46  of  FIG. 4  is shown with multiple housing pieces  46   a,    46   b,    46   c,  alternate embodiments of the present technology may include integral housings without multiple pieces, or may include fewer or more than the three housing pieces depicted in the drawings. 
         [0027]    Referring still to  FIG. 4 , there is shown a hollow connector  54  attached to the sensor housing  46  and, as shown in  FIG. 3 , oriented toward the first accumulator cavity  30  and piston  28  of the accumulator  20 . When the laser piston position sensor  26  is attached to the accumulator housing  24 , the hollow connector  54  aligns with the aperture  29  in the end of the accumulator housing  24 , so that a laser (described in greater detail below) can pass from the cavity  48  in the sensor housing  46 , through the hollow connector  54 , and into the first accumulator cavity  30 . 
         [0028]    The laser piston position sensor  26  also includes a lens assembly  56  positioned between the cavity  48  in the sensor housing  46  and the hollow connector  54 . The lens assembly includes a lens  58  that is at least partially transparent, a lens retainer  60  and lens seals  62 . The lens assembly  56  is positioned in a recess  64  in the sensor housing  46 . During assembly and installation of the laser piston position sensor  26  and hydraulic accumulator  20 , one function of the lens retainer  60  is to maintain the position of the lens  58  relative to the sensor housing  46  while the accumulator  20  and laser piston position sensor  26  is brought to the surface, in the scenario that gas from the accumulator has leaked into the cavity  48  in the sensor housing  46 . In addition, another function of the lens holder  60  is to hold the lens  58  in place in the recess  64  during installation of the accumulator. To accomplish this, the lens  58  can be placed in the recess  64 , with lens seals  62  sealing the interface between the lens  58  and the sensor housing  46 . During operation of the laser piston position sensor  46 , the lens seals  62  prevent liquid or gas gas typically located in the hollow connector  54  (which is in fluid communication with the first accumulator cavity  30  via aperture  29 ) from entering the low pressure sensor cavity  48 . Thus the lens  58  can act as a barrier between nitrogen or other gas in the first accumulator cavity  30  and the low pressure gas in the cavity  48  of the sensor housing  46 . The lens retainer  60  can be attached to the sensor housing  46  with fasteners  66 , or by any other appropriate means. An aperture  68  in the sensor housing  46  exposes at least a portion of the lens  58  to the cavity  48  in the sensor housing  46 . 
         [0029]    Within the cavity  48  of the sensor housing  46  there is positioned a laser sensor  70 . The laser sensor  70  performs multiple functions. For example, the laser sensor generates and directs a laser  72  through the aperture  68  in the sensor housing  46 , the lens  58 , the hollow connector  54  and the aperture  29  in the accumulator housing  24 , and into the first accumulator cavity  30  to the piston  28 . The laser sensor  70  also receives reflected light returning to the sensor by the same path. 
         [0030]    Referring back to  FIG. 3 , the mode of operation of the laser piston position sensor  26  will now be described. To determine the position of the piston  28  within the accumulator housing  24 , the laser sensor  70  generates a laser  72  and directs the laser  72  through the aperture  68  in the sensor housing  46 , the lens  58 , the hollow connector  54 , the aperture  29  in the accumulator housing  24 , and the first accumulator cavity  30  to the piston  28 . When the laser  72  reaches the piston  28 , light from the laser  72  is reflected back along the same path (i.e., through the second accumulator cavity  30 , the aperture  29  in the accumulator housing, the hollow connector  54 , the lens  58 , and the aperture  68  through the sensor housing  46 ) to the laser sensor  70 . Based on the time between emission of the laser  72 , and return receipt of the reflected light, the laser sensor  70  can either calculate the distance from the laser sensor  70  to the piston  28 , or transmit the required data to a processor at a remote location to calculate such distance. The distance between the laser sensor  70  and the piston  28 , in turn, is used to calculate the position of the piston  28  within the accumulator housing  24 . The piston  28  position, along with known accumulator dimensional properties, can be used to determine the volume of gas in the first accumulator cavity  30 , and the difference between that volume and the total known volume of the accumulator can be used to determine the volume of hydraulic fluid in the second accumulator cavity  32 . 
         [0031]    In the embodiment shown in  FIGS. 3 and 4 , the lens  58  is shown angled relative to the laser sensor  70 . Such angled orientation serves to prevent or minimize reflection of the laser  72  off the lens  58  during operation of the laser piston position sensor  26 . Such reflection could lead to false position readings of the piston  28  in the accumulator housing  24 . In the embodiment shown, the lens  58  is angled about  15  degrees relative to the bottom  74  of the cavity  48  in the sensor housing  46 , although the lens  58  can alternately be positioned at any appropriate angle. 
         [0032]    Referring now to  FIG. 5 , there is shown an alternate view of the laser piston position sensor  26 . In the view of  FIG. 5 , the laser piston position sensor  26  is rotated about its longitudinal axis  36  180 degrees from the view shown in  FIG. 4 . As shown in  FIG. 5 , the laser piston position indicator  26  can include a gas fill line  76  with a fill valve  78 , a seal sub  80 , and a vent plug  84 . The gas fill line provides a means of fluid communication between the fill valve  78  and the hollow connector  54 , which is in communication with the first accumulator cavity  30  via aperture  29  in the accumulator housing. Thus, the gas fill line  76  can be used to add or take away gas from the first accumulator cavity. The seal sub  80  serves to seal the interface between sensor housing pieces  46   a,    46   b  at the place where the gas line  76  traverses the interface between the pieces  46   a,    46   b.  As shown, the seal sub  80  is surrounded by seals  82  to prevent fluid or gas from passing between the seal sub  80  and the housing pieces  46   a,    46   b.  The fill valve  78  can be a Schrader valve, or any other appropriate type of valve. The vent plug  84  can be opened to release gas from the accumulator to the ambient environment if needed or desired by an operator. 
         [0033]    Use of the laser piston position sensor of the present technology provides numerous advantages over the prior art. For example, the laser is requires no physical contact with the piston to measure the position of the piston, is non-invasive to the accumulator, and has no moving parts. These features greatly enhance the reliability of the sensor. Furthermore, the laser piston position sensor of the present technology is adaptable to a wide range of piston accumulators used in many industries and for many different applications. 
         [0034]    While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, can appreciate that other embodiments may be devised which do not depart from the scope of the disclosure as described herein. Accordingly, the scope of the disclosure should be limited only by the attached claims.

Summary:
A laser piston position sensor for determining the position of a piston within an accumulator, the laser piston position sensor configured to attach to an end of the accumulator, and the accumulator having an aperture in the end thereof exposing the piston within the accumulator to the laser piston position sensor. The laser piston position sensor including a sensor housing enclosing a cavity containing a low pressure gas, and defining an opening from the cavity toward the accumulator, a laser sensor positioned within the cavity for emitting a laser toward the piston of the accumulator via the opening in the sensor housing and the aperture in the accumulator, and a transparent lens positioned between the laser sensor and the accumulator to allow passage of the laser from the laser sensor to the piston, and to separate gases in the accumulator from gases in the cavity of the sensor housing.