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FIELD OF THE INVENTION 
     This invention relates in general to hydrocarbon well stimulation equipment and, in particular, to a modular ball drop that permits a plurality of frac balls to be simultaneously injected into a stimulation fluid stream that is being pumped into a hydrocarbon well. 
     BACKGROUND OF THE INVENTION 
     Current methods for completing hydrocarbon wells often involve sequentially pumping fracturing fluids into one or more production zones of a well. In order to improve the efficiency of this process, ball-actuated frac sleeves were invented. The ball-actuated frac sleeve has side ports that block fluid access to a production zone with which it is associated until an appropriately sized frac ball is pumped down from the surface to open the sleeve by landing on a frac ball seat through which the frac ball cannot pass. Consequently, when the stimulation of a zone is completed, a frac ball is dropped or injected into a frac fluid stream being pumped down the well. The frac ball lands on the frac ball seat in the ball-actuated frac sleeve and frac fluid pressure on the frac ball forces the side ports in the frac sleeve to open and provide fluid access to that production zone, while blocking access to the zone that was just completed. If many zones are to be stimulated, a large number of size-graduated frac balls are required to stimulate the entire well without interruption. As understood by those skilled in the art, a diameter of the starting frac ball decreases as the required number of frac balls increases. The use of small diameter frac balls has disadvantages. First, all stimulation fluid must be pumped through the frac ball seat orifices, and each seat is at least marginally smaller in diameter than a diameter of the associated frac ball. If the frac ball seat orifice is very small, the rate at which frac fluid can be pumped into the associated zone is affected. Furthermore, small frac balls are more fragile and more likely to get trapped in casing joints or the like on their way down the well casing. 
     In order to overcome the first problem, certain ball actuated frac sleeves have two or more small frac ball seats, each having an orifice through which frac fluid can be pumped. This permits higher stimulation fluid throughput, but requires the simultaneous release of multiple frac balls of the same diameter. In order to overcome the second problem, many operators require the injection of two or more frac balls of the same diameter for each frac ball seat when the required frac ball(s) is less than a predetermined diameter. This likewise requires the simultaneous release of multiple frac balls of the same diameter. 
     Most known ball drops and ball injectors are incapable of, or poorly adapted to, simultaneously drop/inject multiple balls of the same diameter. 
     There therefore exists a need for a modular ball drop that permits multiple balls of the same diameter to be simultaneously injected into a well. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a modular ball drop that permits multiple balls of the same diameter to be simultaneously injected into a well. 
     The invention therefore provides a modular ball drop module, having: a tubular body having a central passage; a ball retainer mechanism movable from a ball retention position in which the ball retainer mechanism retains at least one frac ball in the central passage to a ball released position in which the at least one frac ball is released from the central passage; and an actuator stem lock mechanism that automatically locks the ball retainer mechanism in the ball released position when the ball retainer mechanism is moved from the ball retainer position to the ball released position. 
     The invention further provides a modular ball drop module, having: a top end and a bottom end, the top end including a tubular body having a central passage, the top end adapted to be mounted to another ball drop module or a purge valve; and the bottom end connected to the top end and having a central passage of the same diameter and aligned with the central passage of the top end, the bottom end being adapted to be mounted to any one of: another ball drop module, a frac head and a frac iron; a ball retainer mechanism housed by the tubular body and obstructing the central passage of the tubular body when the ball retainer mechanism is in a ball retention position, the ball retainer mechanism being movable from the ball retention position to a ball released position in which the central passage is unobstructed; and an actuator stem lock mechanism that automatically locks the ball retainer mechanism in the ball released position when the ball retainer mechanism is moved from the ball retainer position to the ball released position. 
     The invention yet further provides a modular ball drop with at least two ball drop modules, the respective ball drop modules having: a central passage that stores frac balls to be dropped by a ball retainer mechanism of the ball drop module, the ball retainer mechanism comprising a retainer ball that obstructs the central passage in a ball retention position and in a ball released position opens the passage to let the frac balls drop through the central passage of the ball drop module; an actuator stem connected to the ball retainer mechanism, the actuator stem being adapted to rotate the ball retainer mechanism from the ball retention position to the ball released position; and an actuator stem lock mechanism that is constantly urged to lock the actuator stem in the ball released position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, in which: 
         FIG. 1  is an isometric diagram of one embodiment of a ball drop module of the modular ball drop in accordance with the invention; 
         FIG. 2  is a front elevational diagram of the embodiment of the ball drop module shown in  FIG. 1 ; 
         FIG. 3  is right elevational view of the embodiment of the ball drop module shown in  FIG. 1 ; 
         FIG. 4  is a rear elevational diagram of the ball drop module shown in  FIG. 1 ; 
         FIG. 5  is a top plan view of the of the ball drop module shown in  FIG. 1 ; 
         FIG. 6  is a schematic cross-sectional diagram, taken along lines  6 - 6  shown in  FIG. 2 , of the ball drop module in a ball retention position; 
         FIG. 7  is a schematic cross-sectional diagram, taken along lines  6 - 6  shown in  FIG. 2 , of the ball drop module in a ball released position; 
         FIG. 8  is a schematic cross-sectional diagram, taken along lines  8 - 8  shown in  FIG. 1 , of the ball drop module in the ball retention position; 
         FIG. 9  is a schematic cross-sectional diagram, taken along lines  8 - 8  shown in  FIG. 1 , of the ball drop module in the ball released position; 
         FIG. 10  is a schematic diagram of an exemplary configuration of the modular ball drop in accordance with the invention; and 
         FIG. 11  is a schematic diagram of an exemplary configuration of the modular ball drop in accordance with the invention incorporated in an exemplary frac stack. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a modular ball drop that permits a group of frac balls of the same diameter to be simultaneously injected into a well. Any required number of ball drop modules can be vertically stacked to permit a required number of groups of frac balls to be sequentially injected into the well. The modular ball drop may also be used to inject only one ball at a time, or any combination of single and/or multiple balls, into the well. A positive lock engages when a module is moved from the ball retention to the ball released position to prevent obstruction of subsequent ball drops from the modular ball drop. 
       FIG. 1  is an isometric view of one embodiment of a ball drop module  20  in accordance with the invention. The ball drop module  20 , hereinafter referred to as module  20 , includes a tubular body  22 . This embodiment of the module  20  is provisioned with quick-disconnect threaded unions described in assignee&#39;s U.S. Pat. No. 7,484,776 which issued Feb. 3, 2009, the specification of which is incorporated herein by reference. A male component  24  of the threaded union is machined on a top end of the tubular body  22 . The male component  24  is used to mount another module  20  or a purge valve to top of the module  20 , as will be explained below with reference to  FIGS. 10 and 11 . A female component  26  of the threaded union is connected to a bottom end of the tubular body  22 . The female component  26  supports a hammer nut  28 , as explained in the assignee&#39;s above-referenced patent. The female component  26  and the hammer nut  28  are used to connect the module  20  to another module  20  as will be explained below with reference to  FIG. 10 , or to a frac head, a high pressure line or a frac stack, as will be explained below with reference to  FIG. 11 . 
     In this embodiment, the module  20  is operated using an actuator shown in  FIGS. 10 and 11  that is mounted to the tubular body  22  by a mounting bracket  30 . The mounting bracket  30  is secured to the tubular body  22  by a plurality of fasteners (not shown) received in threaded bores  32  in the tubular body  22 . An actuator stem  34  is connected to a ball retainer mechanism of the module  20 , as will be explained below with reference to  FIGS. 6 and 7 . The actuator stem  34  is turned 90° by the actuator to move the ball retainer mechanism from the ball retention position to the ball released position, as will also be explained below with reference to  FIGS. 6 and 7 . The mounting bracket  30  also supports an actuator stem lock mechanism  36 , which will be explained in detail with reference to  FIGS. 8 and 9 . The actuator stem lock mechanism  36  is connected to the mounting bracket  30  by a plurality of threaded fasteners  38 . The actuator stem lock mechanism  36  automatically locks the actuator stem  34  and the ball retainer mechanism in the ball released position when the actuator moves the ball retainer mechanism to the ball released position. This ensures that the ball retainer mechanism cannot interfere with any subsequent ball drops from other modules, as will be explained below in more detail. 
       FIG. 2  is a front elevational diagram of the module  20  shown in  FIG. 1 . 
       FIG. 3  is a right side view of the module  20  shown in  FIG. 1 . 
       FIG. 4  is a rear elevational view of the module  20  shown in  FIG. 1 . A port  40  in a rear side of the control body supports a pressure balance stem  42  of the ball retainer mechanism, which will be explained below in more detail with reference to  FIGS. 6 and 7 . 
       FIG. 5  is a top plan view of the module  20  showing the actuator stem in the ball retention position. A lock bore  44  in the actuator stem  34  receives a lock bolt to lock the actuator stem  34  in the ball released position, as will be explained below with reference to  FIGS. 8 and 9 . 
       FIG. 6  is a schematic cross-sectional diagram of the module  20  taken along lines  6 - 6  shown in  FIG. 2  with the ball retainer mechanism  58  in the ball retention position. The tubular body  22  is cylindrical and has a sidewall  46  having a yield strength adequate to withstand frac fluid pressures, e.g. up to at least 15,000 psi. A central passage  48  of the tubular body  22  and the female component  26  is larger than a diameter of a largest frac ball to be dropped into a well. A ball retainer mechanism bore  50  in the tubular body  22  receives a circular upper ball seat  52 , a spherical retainer ball  54 , and a circular lower ball seat  56  of the ball retainer mechanism  58 . The upper ball seat  52 , the retainer ball  54 , and the lower ball seat  56  are locked in the ball retainer mechanism bore  50  by an inner end  62  of the female component  26 , which in this embodiment threadedly engages a box thread  60  in a bottom end of the tubular body  22 . The inner end  62  of the female component  26  is received in a seal bore  64  in the bottom end of the tubular body  22 . O-ring grooves  66   a ,  66   b  in the seal bore  64  respectively retain fluid seals that provide a high pressure fluid seal around the inner end  62  of the female connector  26 . 
     The retainer ball  54  is supported by the lower ball seat and the upper ball seat  52  and is rotated from the ball retention position to the ball released position by a retainer ball stem  43 . The retainer ball stem  43  and the pressure balance stem  42  are T-shaped with respective inner ends  42   a ,  43   a  that are rectangular in end view and have a truncated pyramid shape in side view, as can be seen in  FIG.7 . The inner ends  43   a ,  42   a  of the retainer ball stem  43  and the pressure balance stem  42  are received in respective grooves  55  machined in opposed sides of the retainer ball  54 . The respective grooves  55  have inwardly inclined planar side edges as seen in  FIG. 7 . This permits the retainer ball  54  to float between the lower ball seat  56  and the upper ball seat  52 . 
     The retainer ball stem  43  is connected to the actuator stem  34  by a hex head on an outer end  47  of the retainer ball stem  43 . This decouples the retainer ball stem  43  from the actuator stem  34  so that the retainer ball stem  43  can move in a radial direction with respect to the central passage  48  in response to pressure changes in the central passage  48  without stressing the mounting bracket  30  or the actuator stem  34 . The outer end  47  of the retainer ball stem  43  has the same cross-sectional area as an outer end of the pressure balance stem  42 . A pressure balance bore  49  in the actuator stem  34  exposes the outer end  47  of the retainer ball stem  43  to atmospheric pressure. This ensures that the retainer ball  54  is not exposed to any uneven outward force applied by the retainer ball stem  43  and the pressure balance stem  42 . The retainer ball  54  therefore remains balanced and centered between the upper ball seat  52  and the lower ball seat  56  regardless of a frac fluid pressure in the central passage  48 . 
     The retainer ball  54  has a ball release bore  68  with a diameter at least as large as the central passage  48 . The retainer ball  54  also has through bores  69   a - 69   d  on opposite sides of the ball release bore  68 . The through bores  69   a - 69   d  provide fluid communication between an interior of the central passage  48  in the tubular body  22  and the central passage  48  in the female component  26 . This ensures that another module  20  or a purge valve mounted to a top of the module  20  is exposed to frac fluid pressure, and further ensures that the retainer ball  54  is free to rotate within the lower ball seat  56  and the upper ball seat  52  since it is pressure balanced on all sides. 
       FIG. 7  is the schematic cross-sectional diagram of the module  20  taken along lines  6 - 6  shown in  FIG. 2  with the ball retainer mechanism  58  in the ball released position. In this position the ball retainer mechanism  58  has been rotated 90° by the actuator so that the ball release bore  68  in the retainer ball  54  is aligned with the central passage  48 . In the ball released position, any ball(s) held above the retainer ball  54  are released and drop through the central passage  48 . As will be explained below with reference to  FIGS. 8 and 9 , when the ball retainer mechanism  58  is rotated to the ball released position the ball retainer mechanism  58  is automatically locked in that position and cannot be moved without a manual reset. 
       FIG. 8  is a schematic cross-sectional diagram, taken along lines  8 - 8  shown in  FIG. 1 , of the module  20  with the ball retainer mechanism  58  in the ball retention position shown in  FIG. 6 . The actuator stem lock mechanism  36  is shown in cross-section. In this embodiment, the actuator stem lock mechanism  36  is a fluid cylinder  70  having a flange  72  that receives the threaded fasteners  38  to connect the actuator stem lock mechanism  36  to the mounting bracket  30 , as explained above with reference to  FIG. 1 . The fluid cylinder  70  has an end cap  74  that is threadedly secured to the fluid cylinder  70  in a manner well known in the art. A piston  76  has a fluid seal  78  that retains fluid (pneumatic or hydraulic) within a fluid chamber  80  of the fluid cylinder  70 . A port  82  supports the connection of a fluid supply line (not shown) to the cylinder  70 . A rod  84  connected to a fluid end of the piston  76  has a piston position indicator  86  that reciprocates through a fluid seal  88  in the end cap  74 . The position indicator  86  provides a visual indication of the position of a lock bolt  90  connected to an opposite side of the piston  76 . In operation the lock bolt  90  is constantly urged through a circular port  92  in the inner end of the cylinder  70  by fluid pressure in the fluid chamber  80 . 
       FIG. 9  is a schematic cross-sectional diagram, taken along lines  8 - 8  shown in  FIG. 1 , of the module  20  with the ball retainer mechanism  58  in the ball released position shown in  FIG. 7 . When the actuator moves the ball retainer mechanism  58  to the ball released position, fluid pressure in the fluid chamber  80  of the cylinder  70  drives the lock bolt  90  through the lock bore  44  in the actuator stem  34 , locking the actuator stem  34  and the ball retainer mechanism  58  in the ball released position. In this embodiment, a manual reset is required to return the ball retainer mechanism  58  to the ball retention position shown in  FIG. 6 . Although the actuator stem lock mechanism  36  is shown to be a pneumatic or hydraulic cylinder, it should be understood that an electric solenoid could also be used for the same purpose. 
       FIG. 10  is a schematic diagram of an exemplary configuration of a modular ball drop  100  in accordance with the invention. A plurality of modules  20   a - 20   c  is vertically stacked to accommodate a plurality of frac balls or groups of frac balls. Each module is preloaded with the number of balls required to be simultaneously dropped before the next module  20  is added to the vertical stack. Alternatively, the vertical stack is built and the ball retainer mechanism  58  of the respective modules  20  is manually moved to the ball retention position after the module  20  below it is loaded with the required number of frac balls. If the capacity of the central passage  48  above the retainer ball  54  is not large enough to accommodate the required balls, a pup joint (not shown) can be added between the modules  20  using appropriate adapter(s) well known in the art. As can be seen, the hammer nut  28   b  connects module  20   b  to module  20   a , and hammer nut  28   c  connects module  20   c  to module  20   b , etc. Each module  20   a - 20   c  is equipped with an actuator  102   a - 102   c . The actuators  102   a - 102   c  can be any control mechanism, including a pneumatic actuator; a hydraulic actuator; a stepper motor; a hydraulic motor; or any other power source capable of reliably moving the ball retainer mechanism  58  from the ball retention position shown in  FIG. 6  to the ball released position shown in  FIG. 7 . 
     A purge valve  104  is connected to a top of the modular ball drop  100  using a high pressure coupling or a high pressure adapter, each of which are known in the art. In one embodiment, the purge valve  104  is a remote controlled hydraulic valve. The purge valve is used to purge the modular ball drop  100  of air after the modular ball drop  100  is directly or indirectly connected to a frac head or a frac iron, for example. 
       FIG. 11  is a schematic diagram of an exemplary configuration of the modular ball drop  100  incorporated in an exemplary frac stack  200 . This frac stack  200  is mounted to a wellhead  202 . The frac stack  200  includes a cross-flow tee  204 , a high pressure valve  206 , an adapter  208 , and a frac head  210  to which a plurality of frac irons (not shown) are connected in a manner well known in the art. An adapter  212 , a Bowen union for example, is used to connect a ball drop wellhead control apparatus  214  to the top of the frac head  210 , as described in Assignee&#39;s co-pending U.S. Pat. No. 9,010,412 which issued Apr. 21, 2015. In this exemplary configuration, a ball drop or a ball injector  216  is mounted to a top of the ball drop wellhead control apparatus  214 . The ball drop or ball injector  216  may be any one of the frac ball drops or frac ball injectors known in the art. The modular ball drop  100  is mounted to a side port of the ball drop wellhead control apparatus  214  using, for example, a frac iron tee  218 . A frac iron  220  is connected to the frac iron tee  218 . A high pressure valve (not shown) controls fluid flow through the frac iron  220  as described in Assignee&#39;s above-referenced U.S. Pat. No. 9,010,412, the specification of which is incorporated herein by reference. 
     As explained above, in use a ball or group of balls is dropped from a module  20  of the modular ball drop  100  at an appropriate time during a well stimulation procedure. Once the frac ball or group of frac balls is dropped by the modular ball drop  100 , the module  20  that dropped the ball or group of balls is locked in the ball released position and cannot be returned to the ball retention position. In the configuration shown in  FIG. 11 , the modular ball drop  100  is used to drop the smallest balls required for a well stimulation operation, or to supplement small balls dropped by the ball drop or ball injector  216 . 
     Although the modules  20  of the modular ball drop  100  have been described as having quick-disconnect threaded unions, it should be understood that the modules  20  could likewise be equipped with API flanges, Graylock® connectors, or any other type of high pressure connector known in the art. 
     The scope of this invention is therefore intended to be limited solely by the scope of the appended claims.

Summary:
A modular ball drop made up of two or more identical ball drop modules that are vertically stacked in a desired number. Each ball drop module can drop one or more frac balls into a fluid stream being pumped into a well.