This application is a divisional of U.S. application Ser. No. 12/605,872 filed Oct. 26, 2009. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to subsurface, or downhole pumps, such as are used to pump oil and other fluids and bases from wells. 
     BACKGROUND OF THE INVENTION 
     When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure that is sufficient for the well to produce on its own. In other words, the oil rises to the surface without any assistance. In many oil wells, and particularly those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. Subsurface pumps are located in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, beam pump unit or other devices. A prime mover, such as a gasoline or diesel engine, an electric motor or a gas engine, on the surface, causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing. 
     The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is reciprocated inside of a barrel by the sucker rods. The barrel has a standing one way valve, while the plunger has a traveling one way valve, or in some pumps the plunger has a standing one way valve, while the barrel has a traveling one way valve. Reciprocation charges a chamber between the valves with fluid and then lifts the fluid up the tubing towards the surface 
     In many instances, the well is sandy. A sandy environment causes more wear on a pump and its components as the sand is abrasive. A worn pump does not operate particularly well if at all. 
     To fix the pump, it is pulled from the well, inspected, and the worn components replaced. 
     When a tubing pump is repaired, its various components are pulled from the well. For example, pulling the sucker rod string pulls the plunger to the surface. The plunger components can be replaced. Likewise, the standing valve can be pulled and replaced. 
     The barrel may be worn and require replacement. If the pump is an insert pump, then the barrel can be pulled from the well by the sucker rod string. However, if the pump is a tubing pump, the pump is in-line with, and forms a part of, the tubing. Consequently, the entire string of tubing must be pulled to access and replace the barrel. Pulling the entire tubing string is time consuming and costly. Yet, tubing pumps offer some advantages over insert pumps. For example, tubing pumps provide more fluid capacity that insert pumps. 
     It is desirable to minimize the cost of repairing and replacing tubing pumps which have barrel damage. 
     SUMMARY OF THE INVENTION 
     The present invention provides a downhole pumping assembly that comprises a tubing string with plural barrels. The barrels are arranged in-line with the tubing. Each of the barrels has an inside diameter. The tubing string has a seating location adjacent to each barrel. A standing valve arrangement comprises a standing valve and a seating device. The standing valve is located adjacent to a lower end of one of the barrels and coupled to the seating device. The seating device is coupled to the tubing string at the respective seating location. A plunger is provided with a traveling valve. The plunger is located for reciprocation within the one barrel. The plunger has a clearance with each of the barrels so as to form a fluid seal. 
     In accordance with one aspect of the present invention, the barrels are located adjacent to each other in the tubing string. 
     In accordance with still another aspect of the present invention, the barrels are coupled to the tubing string by barrel couplings, which barrel couplings form the seating locations. 
     In accordance with still another aspect of the present invention, the barrels are separated from each other in the tubing string by tubing. 
     In accordance with still another aspect of the present invention, the seating device is removably coupled to the tubing string. 
     In accordance with still another aspect of the present invention, the seating device comprises a lock mandrel. 
     In accordance with still another aspect of the present invention, the lock mandrel comprises a collar lock mandrel that is locked into a recess formed by a barrel coupling. 
     In accordance with still another aspect of the present invention, the lock mandrel removably engages a seating nipple in the tubing string. 
     The present invention also provides a method of operating a sucker rod pump assembly in a well that extends from a surface of the earth into the ground. Plural barrels are incorporated into a tubing string in the well. A plunger is located in the first one of the barrels. The plunger is reciprocated within the first one of the barrels and produces fluid from the well to the surface. The reciprocation of the plunger is suspended. The plunger is relocated to a second one of the barrels while maintaining the tubing string in the well. The plunger is reciprocated within the second one of the barrels and produces fluid from the well to the surface. 
     In accordance with one aspect of the present invention, the pump assembly comprises a standing valve. The standing valve is located in the first one of the barrels before the plunger is reciprocated within the first one of the barrels. The standing valve is relocated to the second one of the barrels before the step of reciprocating the plunger within the second one of the barrels. 
     The step of relocating the plunger to a second one of the barrels further comprises relocating the plunger to a second one of the barrels that is closer to the surface than the first one of the barrels. 
     In accordance with still another aspect of the present invention, the step of relocating the plunger to a second one of the barrels further comprises relocating the plunger to a second one of the barrels that is further from the surface than the first one of the barrels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a well, shown with pumping equipment. 
         FIG. 2  is a schematic cross-sectional view of the pump assembly of the present invention, in accordance with a preferred embodiment, showing the pump in a first configuration. 
         FIG. 3  is a cross-sectional view of the standing valve assembly and collar lock mandrel. 
         FIG. 4  is a cross-sectional view of the standing valve assembly and landing nipple. 
         FIG. 5  is a schematic cross-sectional view of the pump assembly of  FIG. 2 , shown in a second configuration. 
         FIG. 6  is a schematic cross-sectional view of the pump assembly of  FIG. 2 , shown in a third configuration. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a low cost way to repair tubing pumps which have barrel damage, which damage is typically due to sand and other abrasive agents. The tubing pump assembly of the present invention incorporated plural barrels and a single plunger. At any given time during pump operation, the plunger reciprocates inside one of the barrels. The other barrels are not used. Instead, the unused barrels simply stand by and await future use. As the plunger reciprocates over time, the barrel in use exhibits wear, particularly in a sandy environment. When the barrel is ready to be replaced, the plunger is relocated to another, unused, barrel in the well. Surprisingly, the barrels that are not in use do not exhibit wear during pump operation. In this manner, a worn barrel can be replaced with another barrel without the need to pull the tubing string from the well. This, replacing a worn barrel in a tubing pump can occur in a shorter time. 
     The plunger can be replaced by pulling the sucker rods and plunger; the tubing need not be pulled to replace the plunger. 
     In  FIG. 1 , there is shown a schematic diagram of a producing oil well  11 . The well has a borehole that extends from the surface  13  into the earth, past an oil bearing formation  15 . 
     The borehole has been completed and therefore has casing  17  which is perforated at the formation. A packer or other method (not shown) optionally isolates the formation  15  from the rest of the borehole. Tubing  19  extends inside of the casing from the formation  15  to the surface  13 . 
     A subsurface pump  21  is located in the tubing  19  at or near the formation  15 . A string of sucker rods  23  extends from the pump  21  up inside of the tubing  19  to a polished rod and a stuffing box  25  on the surface  13 . The sucker rod string  23  is connected to a pump jack unit  24  which reciprocates up and down due to a prime mover  26 , such as an electric motor, a gasoline or diesel engine, or a gas engine. 
     The pump  21  described herein is a tubing pump (as opposed to an insert pump). A tubing pump has its barrel incorporated into the string of tubing  19 . An insert pump inserts a barrel into the tubing, which barrel is secure with a hold down and a seating nipple. The pump described herein has a fixed barrel and a reciprocating plunger. 
     The pump assembly  21  will now be described, with reference to  FIG. 2 . The pump assembly has plural barrels  31 , a plunger  33  and associated valves. The valves typically include a standing valve  35  and a traveling valve  37 . The plural barrels  31  are located in the tubing string  19 . Only one of the barrels is used at any given time to pump oil and other fluids. In the embodiment shown, there are three barrels  31  contained in the tubing string, namely a top barrel  31 A, an intermediate barrel  31 B and a bottom barrel  31 C (referring to the orientation of  FIG. 2 ). The barrels  31  are joined into the tubing string  19  with tubing collars. 
     The tubing string  19  contains lengths of tubing  43 , typically 30 feet long. Shorter lengths of tubing can be used, which are referred to as tubing subs  45 . The tubing lengths  43  are joined together by tubing collars. A tubing collar has two sets of female threads, which threads engage and couple the male threads on the ends of the tubing  43 . Almost all tubing threads are tapered. 
     A barrel, like a length of tubing, also has male threads on each end. These threads are typically non-tapered, or straight. As discussed below, barrel couplings  39  are used to couple the barrels into the tubing string  19 . A recess  41  is formed in the interior of the tubing string by each barrel coupling  39 . 
     A barrel  31  may come in various lengths, depending on the particular application, such as four feet to thirty feet long. The inside of a barrel has a smooth finish and a precise size, in order to accommodate the plunger and provide a seal with the plunger. The tolerance on the inside diameter of a barrel is typically 0.002 inches, while the tolerance on tubing is typically 0.032 inches. Furthermore, the tubing inside lacks the necessary concentricity and is eccentric. In fact, if a plunger is used in a piece of tubing, a fluid seal would not be formed between the plunger and the tubing. A barrel  31  is typically smaller in inside diameter than is tubing  43  of corresponding size. 
     The tubing string  19  thus has tubing extending from the surface down to the formation of interest. Attached to the tubing  43  is a first, or top, barrel  31 A, then a tubing sub  45 , a second or intermediate barrel  31 B, another tubing sub  45 , and a third, or lower, barrel  31 C. Other types of tubing can be used in lieu of tubing subs to separate the barrels from one another in the tubing string. The barrels are attached to the tubing by way of barrel couplings  39 . Recesses  41  are created between the ends of the barrels and tubing subs and between the ends of the barrels and the tubing. 
     The tubing string has perforations  34  therein. Thus, fluid can flow from the formation, through casing perforations  36  and through the tubing perforations  34  into the tubing string. 
     A standing valve  35  is provided for the barrels. The standing valve is located in a releasable seating assembly  49 . There can be various types of releasable sealing assemblies. Illustrated herein are lock mandrels. Some lock mandrels  51  can engage the coupling recess  41  (see  FIG. 3 ), while other lock mandrels  61  require a landing nipple  63  to be incorporated into the tubing string (see  FIG. 4 ), which landing nipple provides the necessary recess  67 . 
     Referring to  FIG. 3 , the collar lock mandrel  51  is shown engaged. The collar lock mandrel is conventional and commercially available. The collar lock mandrel has an inside flow passage  53  (shown by dashed lines) that extends between the ends of the mandrel. The mandrel has dogs  55  (the mandrel is partially broken to show the dogs  55 ) that move between stowed and extended positions. In the stowed position, the dogs  55  are retracted, allowing the mandrel to be run into the tubing  43  and barrels  31 . The mandrel  51  is located below the desired coupling recess  41 . In order to set the mandrel, the mandrel is picked up, wherein the dogs  55  move to the extended position (as shown in  FIG. 3 ) and engage the coupling recess  41 . Once the dogs are engaged, the collar lock mandrel is locked in place. The collar lock mandrel also has a resilient element  57  on the outside, below or above the dogs. Upward jarring of the collar lock mandrel results in the outward expansion of the resilient element  57 , which causes a seal to be made against the tubing string  19 . 
     Referring to  FIG. 4 , the lock mandrel  61  is shown, together with the landing nipple  63 . The landing nipple  63 , which receives and secures the lock mandrel  61 , is part of the tubing string  19 . Below each barrel  31  is a landing nipple. The landing nipple can be spaced below the respective barrel by way of a tubing sub, or the landing nipple can be located directly beneath, and coupled to, the respective barrel. The lock mandrel  61  has locking keys or dogs  65  that deployed to engage a groove  67  or a recess in the landing nipple  63 . The lock mandrel also has resilient sealing elements  69  that provide a seal around the outside diameter of the mandrel. The lock mandrel  61  operates in a similar manner to the collar lock mandrel  51 . The lock mandrel is lowered below the landing nipple  63 , and then picked up to deploy the dogs  65 . The dogs  65  catch in the recess or groove  67 . After the dogs catch in the groove, further pulling up expands the resilient element  69 . 
     The standing valve  35  is located on the top end of the collar lock mandrel ( FIG. 3 ) or lock mandrel ( FIG. 4 ). The standing valve  35  has a cage  71 , a ball  73 , a seat  75  and a seat coupler  77 . The seat coupler  77  couples the standing valve to the respective mandrel. The outside of the cage  71  has a fishing neck  79 . 
     The plunger  33  (see  FIG. 2 ) is a tube having an outside diameter sized to the inside diameter of the barrel  31 . Plungers and barrels are made with tight tolerances. In the preferred embodiment, a clearance  81  of 0.002-0.008 inches is provided between the plunger  33  and the barrel  31  so as to form a liquid or fluid seal. If the clearance  81  is too big, then a fluid seal is not formed and the pump will experience leakage past the plunger. Barrels and plungers must be concentric to assure proper clearance. A non-concentric diameter will result in either a gap (lack of fluid seal) between the plunger and barrel or contact between the plunger and barrel, which produces high wear between the contacting components. 
     The plunger  33  has a traveling valve  37 , which valve communicates with a compression chamber  82 . The compression chamber is between the traveling and standing valves  37 ,  35 . Typically, a traveling valve is located at the bottom of the plunger, although it may be located intermediate along the plunger or even at the upper end of the plunger. The length of the plunger is selected so as to operate with the barrel in the desired configuration, in accordance with conventional practice. The upper end of the plunger  33  has openings  83  to allow fluids to pass out of the plunger interior into the tubing  19 . The sucker rod string  23  is coupled to the plunger either directly or by way of valve rods. 
     To install the pump assembly, the tubing string  19  is assembled piece by piece and lowered into the well. The tubing string  19  includes two or more barrels  31 , with a recess  41  below each barrel for receiving the standing valve assembly. Although the tubing string can be equipped with a standing valve assembly as the surface, the more common practice is to lower the tubing string into the well and then lower the standing valve assembly. 
     A wireline can be used to lower the standing valve assembly. The standing valve assembly is lowered through the desired barrel. For example, as shown in  FIG. 2 , the intermediate barrel  31 B is to be used in pumping operations. The standing valve assembly is thus lowered through the upper barrel  31 A and through the intermediate barrel  31 B. The standing valve assembly is positioned just below the appropriate recess. The standing valve assembly is then picked up to locate and lock the mandrel  51 ,  61  in the recess  41 ,  67 . Once the mandrel is locked, additional upward jarring is applied to expand the resilient element  57 ,  69  and make the seal. The standing valve assembly is now in position, with the standing valve  35  preferably located within the lower end of the barrel  31 . 
     The plunger  33  is lowered into the tubing by the sucker rod string  23 . The plunger is positioned within the intermediate barrel  31 B. The pump is now ready to operate. 
     During pump operations, the sucker rod string reciprocates the plunger inside of the intermediate barrel  31 B. The pump operates normally, lifting fluid to the surface through the tubing  19 . 
     The pump plunger  33  and barrel  31  will experience wear, particularly in a sandy well. To replace the plunger  31 , the sucker rod string  23  and plunger are pulled from the well. The sucker rod string is then run back into the well with the replacement plunger and associated components. 
     If the barrel  31 B needs to be changed, then the sucker rod string and plunger are pulled from the well. A wireline, slick line, etc. is run into the well with a fishing tool, which fishing tool couples to the releasable seating assembly  49 . The mandrel is freed from the tubing (for example by pushing down on the mandrel with a weight or using a jarring action). Once freed, the releasable seating assembly can be retrieved to the surface so as to inspect, and if necessary, replace the standing valve parts. After inspection and/or replacement, the releasable seating assembly is lowered to a position just below the next barrel. In the alternative, the releasable seating assembly is not retrieved to the surface; it is moved to the next barrel and set as described above. The next barrel could be above (see  FIG. 5 ) or below (see  FIG. 6 ) the worn barrel  31 B. 
     Once the releasable seating assembly is repositioned, a plunger is run into the well, positioned in the barrel and pumping operations can resume. 
     Thus, the present invention allows a tubing pump barrel to be changed while minimizing the need for pulling the tubing string. In most instances of barrel replacements, the tubing string stays intact and inside the well while the pump components are relocated to an unused barrel. After all of the barrels become worn, the tubing string is pulled and the barrels are replaced. The tubing string, with new barrels, is then lowered back into the well. The plunger is located inside of a selected one of the barrels, wherein pumping operations can resume once more. 
     The tubing string can be equipped with two or more barrels. The spacing between the barrels is determined by various factors such as the length of the barrels and the length of the standing valve assemblies. 
     The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.
A downhole pumping assembly has a tubing string with plural barrels which are arranged in-line with the tubing. The tubing string has a seating location adjacent to each barrel. A standing valve arrangement, which includes a standing valve and a seating device, is located with the standing valve adjacent to the lower end of one of the barrels. A plunger has a traveling valve that is located for reciprocation within the one barrel. The plunger has a clearance with each of the barrels so as to form a fluid seal. The plunger reciprocates inside the barrel so as to lift fluid to the surface. Over time, the barrel will exhibit wear. Instead of pulling the entire tubing string to replace the barrel, the standing valve and the plunger are relocated to one of the other barrels located in the tubing string and reciprocation is resumed to resume pumping.