Abstract:
The present invention provides a hydraulically driven multiphase pump system and methods for pumping a fluidstream from the surface of a well. The hydraulically driven multiphase pump system consists of two vertically disposed plungers. The plungers are hydraulically controlled and actuated to work in alternate directions during a cycle using a closed loop hydraulic system. Each cycle is automatically re-indexed to assure volumetric balance in the circuits. An indexing circuit ensures that each plunger reaches its full extended position prior to the other plunger reaching its preset retracted position. A power saving circuit is used to transfer energy from the extending plunger to the retracting plunger. A trim circuit is used to ensure a proper fluid level in the indexing circuit and the power saving circuit. Additionally, a rapid reversal circuit may be employed to increase the rate of the two vertically disposed plungers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/036,737, filed on Dec. 21, 2001, now U.S. Pat. No. 6,592,334 entitled “Hydraulic Multiphase Pump,” which patent application is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an apparatus and method used to transport hydrocarbons from a wellbore to another location. More particularly, the invention relates to a multiphase pump for transporting hydrocarbons from the surface of a producing well. More particularly still, the invention relates to a pump having two vertically disposed plungers and circuitry providing more efficient operation of the pump. 
   2. Description of the Related Art 
   Oil and gas wells include a wellbore formed in the earth to access hydrocarbon bearing formations. Typically, a borehole is initially formed and thereafter the borehole is lined with steel pipe, or casing in order to prevent cave in and facilitate the isolation of portions of the wellbore. To complete the well, at least one area of the wellbore casing is perforated to form a fluid path for the hydrocarbons that either flow upwards to the surface of the well due to naturally occurring formation pressure or are urged upwards with some form of artificial lift. Regardless of the manner in which the hydrocarbons reach the surface of the well, this flow will arrive as a mixture of oil, gas, dirt and sand which is referred to as a “wellstream” or “fluidstream”. The fluidstream is then transported by a flowline to a predetermined location, such as a separator where it may be separated into gas, liquids, and solids. If the fluidstream cannot flow to the separator, it may be pumped by a multiphase pump. These pumps must be capable of moving volumes of the oil, gas, water or other substances making up the fluidstream. The pumps can be located offshore or onshore and can be connected to a single or multiple wellheads through the use of a manifold. 
   Over the past 20 years, two principle types of rotary pumps have been used as multiphase pumps: the twin screw pump and the helico-axial pump. The twin screw pump is a positive displacement pump constructed basically of two intermeshing screws. The fluidstream enters the pump from the wellhead and is trapped between the screws of the pump. The rotation of two screws forces the fluidstream into the downstream flowline. The helico-axial style pump combines positive displacement with dynamic compression and is basically constructed of turbine blades in combination with a screw drive. This combination imparts energy from turbine blades and the screw drive into the discharged fluids. 
   The rotary style multiphase pumps have been popular due to their long market exposure but have demonstrated deficiencies. Maintenance problems that usually require more than 24 hours to resolve is one deficiency that affects both the twin screw pump and the helico-axial pump. Many of these problems are associated with erosion or heat that damage the mechanical seals. Sand can also erode the screws and liners of the pumps. Excessive amounts of gas can cause a reduction in the dynamic performance occur in the helico-axial pumps and can lead to build up and gas locking in the twin screw pumps. Conversely, excessively long liquid slugs can affect the efficiency of the helico-axial pumps. 
   A horizontal, reciprocating pump has been successfully deployed for low to medium gas volume fraction applications. This pump contains horizontal rams that are moved in and out by a rotating crankshaft. The pump has reasonable tolerance for sand in the well stream. It uses replaceable liners to cover and protect the compression cylinders which can be changed in the field. Even though the horizontal reciprocating pump overcomes some of the deficiencies of a rotary style multiphase pump it may experience dynamic problems if the flow is mainly gas. 
   More recently, a vertical reciprocating pump (the RamPump™) has been used to transport well stream. This pump was introduced to overcome deficiencies of rotary pumps. It operates at a slower pace than the rotary pumps, using larger volume chambers and long strokes to attain the flow rates desired. Due to the slow fluid velocities and vertical plunger design, sand and other impurities from a wellbore have little adverse effect on its moving parts. Because it has no rotating mechanical seals; it can handle a full range of fluid mixtures without requiring liquid trapping or re-circulation to insure seal survival. Preferably driving cylinders are placed in line with their respective plungers. Power fluid supplied from a pressure compensated pump is used to drive one plunger fully down, triggering a sudden pressure increase at the end of the stroke. This pressure spike is used to shift a shuttle valve, causing the swash plate of the compensated pump to reverse angle and to redirect the power fluid to the opposite cylinder. Each power circuit is connected to the piston end of one cylinder and also to the rod end of the other cylinder, thus assuring that the opposite plunger will be driven upward when the first plunger is moving downward. 
   Even though the vertical RamPump™ overcomes many of the deficiencies in the prior pumps, problems still exist with the use of vertical plungers in a hydraulically driven multiphase pump. For example, if a deficit of hydraulic fluid occurs, the pump will pause, and go to neutral, and may need intervention to restart. In another example, pressure spikes created during the operation of the hydraulically driven pump can cause premature failures in relief valves and hoses at the end fittings. These pressure spikes occur when one of the plungers reaches its preset retracted position and thereby causing the fluid to be further compressed in the hose without any way of escape. This increase pressure is utilized in the system to cause the swash plate in the pressure compensated pump to reverse angle thereby redirecting the flow of hydraulic fluid to the opposite cylinder. Since the swash plate does not change direction instantaneously, the pressure continues to increase in the hoses thereby causing a very high pressure spike resulting in failure of hydraulic components. In yet another example, when an inlet pressure is insufficient to raise the ascending plunger ahead of the descending plunger the pump begins to short stroke on subsequent cycles and ultimately stop pumping. The combination of these problems greatly reduced the functionality of hydraulically driven multiphase pump. 
   In view of the deficiencies of currently available hydraulically driven multiphase pump a need exists for a hydraulically driven pump that operates effectively and efficiently in pumping multiphase liquids and does not systematically pause during a pumping cycle. There is a further need for a hydraulically driven multiphase pump that is not subject to premature failure of hydraulic components and hoses. There is yet a further need for a hydraulically driven multiphase pump that does not short stroke while operating in various pressure conditions. 
   SUMMARY OF THE INVENTION 
   The present invention provides a hydraulically driven multiphase pump system with improved efficiency due to elimination of pressure spikes and priming problems of the plunger moving toward the extended position. The hydraulically driven multiphase pump system consists of two vertical disposed plungers. The plungers are hydraulically controlled and actuated to work in alternate directions during a stroking cycle using a closed loop hydraulic system. Each cycle is automatically re-indexed to assure volumetric balance in the circuits. An indexing circuit ensures that each plunger reaches its full extended position prior to the other plunger reaching its preset retracted position. The multiphase pump system is capable of operating in 100% gas and 100% liquids without requiring auxiliary liquid circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a schematic view of a complete hydraulically driven multiphase pump system. 
       FIG. 2  is a schematic view showing a closed loop circuit in the hydraulically driven multiphase pump system. 
       FIG. 3  is a schematic view showing an indexing circuit in the hydraulically driven multiphase pump system. 
       FIG. 4  is a schematic view showing a charging circuit in the hydraulically driven multiphase pump system. 
       FIG. 5  illustrates a power saving circuit in the hydraulically driven multiphase pump system. 
       FIG. 6  illustrates a trim circuit in the hydraulically driven multiphase pump system. 
       FIG. 7  illustrates a rapid reversal circuit in the hydraulically driven multiphase pump system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a schematic view of a complete hydraulically driven multiphase pump system  100 . For ease of explanation the invention will be first be described generally with respect to  FIG. 1 , thereafter more specifically with  FIGS. 2–7 . The system  100  contains a first  310  and second  315  plunger, each movable between an extended position and a retracted position. The first plunger  310  is moveable by a first and a second hydraulic cylinders  222 . The second plunger  315  is movable by a first and a second hydraulic cylinders  224 . When the first plunger  310  is moving toward the extended position, a suction is created by the plunger  310 , urging the fluidstream from the wellbore to enter the system  100  through an inlet  110  and fill a first plunger cavity  311 . Simultaneously, the second plunger  315  is moving in an opposite direction toward a preset retracted position, thereby expelling the fluidstream in a second plunger cavity  316  to a discharge  120 . As the first plunger  310  reaches its full extended position, the second plunger  315  then reaches its preset retracted position, thereby completing a cycle. The first plunger  310  then moves toward the preset retracted position expelling the fluidstream into the discharge  120 , as the second plunger  315  moves toward the extended position creating a suction and urging the fluidstream to enter the inlet  110 . In this manner, the plungers operate as a pair of substantially counter synchronous fluid pumps. While the described embodiment includes plungers acting in a counter-synchronous manner, it will be understood that so long as they move in a predetermined way relative to one another, a predetermined phase relationship, the plungers can assume any position as they operate. 
   The plungers  310 ,  315  move in the opposite directions causing continuous flow of fluid from the inlet  110  to the discharge  120 . A first biasing member  325  is disposed at the lower end of the first plunger  310 , to facilitate the movement of the first plunger  310  toward the extended position. A second biasing member  327  is disposed at the lower end of the second plunger  315  to facilitate the movement of the second plunger  315  toward the extended position. The hydraulic cylinders  222 ,  224  are shown on the side of the plungers  310 ,  315 , which is a preferred embodiment. However, this invention is not limited to orientation of the hydraulic cylinders  222 ,  224  as shown on  FIG. 1 . For instance, depending on space requirement the plungers can be disposed in any orientation that is necessary and effective. 
   The system  100  includes a power fluid circuit which is referred to as a closed loop circuit  200  for supply of hydraulic fluid from a pressure compensated pump  230  to a rod end  221  of the first and the second hydraulic cylinders  222  of the first plunger  310  and to a rod end  223  of the first and the second hydraulic cylinders  224  of the second plunger  315 . The system  100  also includes an indexing circuit  300  providing hydraulic fluid to and from a blind end  227  of the first and the second hydraulic cylinders  222  of the first plunger  310  and to a blind end  229  of the first and the second hydraulic cylinders  224  of the second plunger  315 . The indexing circuit  300  ensures that one plunger reaches its full extended position prior to the other plunger reaching its preset retracted position. Additionally, the system  100  further includes a power saving circuit  500  to transfer energy between the first  310  and the second  315  plunger. The system  100  further includes a charge circuit  400  for providing hydraulic fluid to the closed loop circuit  200 , the indexing circuit  300  and the power saving circuit  500 . 
     FIG. 2  is a schematic view showing the closed loop circuit  200  in the hydraulically driven multiphase pump system  100 . In the circuit  200 , the rod end  221  of the first and the second hydraulic cylinders  222  of the first plunger  310  and to the rod end  223  of the first and the second hydraulic cylinders  224  of the second plunger  315  is connected to the pressure compensated hydraulic pump  230 . The pump  230  is energized by an external power source  265  such as an electric motor or an engine. The circuit  200  further includes a first  330  and a second  335  limit switch to commence the reversal of fluid flow by the pressure compensated hydraulic pump  230 . During a cycle, the pump  230  directs hydraulic fluid towards the first and the second hydraulic cylinders  222  of the first plunger  310  thereby causing the plunger  310  to move towards the retracted position. Once the plunger  310  reaches the preset retracted position, the limit switch  330  is triggered. The first  330  and the second  335  limit switches are arranged and constructed to trigger a signal to box  340 . The box  340  is connected to a control valve  270  which causes the pressure compensated pump  230  to redirect the flow of fluid in the closed loop circuit  200 . When redirected, the pump  230  draws the fluid from the rod end  221  the first and the second hydraulic cylinders  222  of the first plunger  310  in the retracted position and sends the fluid to the rod end  223  of the first and the second hydraulic cylinders  224  of the second plunger  315  in the extended position, thereby completing a cycle. The first  330  and the second  335  limit switches are movable to adjust the first  310  and the second  315  plunger preset retracted positions in order to optimize the pump cycle. The pump system is optimized when the volume of well stream pumped over time is increased. 
   In the event the circuit  200  experiences leakage through a loop flushing valve  245  or through normal leakage from the compensated pump  230  to a drain  260 , a replenishment flow of fluid can be introduced into the closed loop circuit  200  by means of the charge circuit  400 . The charge circuit  400  includes an accumulator  255  that stores fluid under pressure. A valve  250  between the accumulator  255  and the closed loop circuit  200  permits fluid introduction to the closed loop circuit  200  in the event that fluid pressure in the circuit  200  falls below a preset valve. 
     FIG. 3  is a schematic view showing the indexing circuit  300  in the hydraulically driven multiphase pump system  100 . The indexing circuit  300  ensures that each plunger reaches its full extended position prior to the other plunger reaching its preset retracted position. Circuit  300  connects the blind end  227  of the first and the second hydraulic cylinders  222  of the first plunger  310  to the blind end  229  of the first and the second hydraulic cylinders  224  of the second plunger  315 . In a low inlet pressure scenario, the extending plunger has less external force urging it toward the extended position. To compensate, the pressure increases in the indexing circuit  300  thereby preventing fluid introduction by the charge circuit  400 . One feature to address this problem is the use of an acceleration valve  350  for selective communication with the closed loop circuit  200  and the indexing circuit  300 . As the pump system  100  completes a cycle and one of the plungers moves from the extended position to the retracted position, the acceleration valve  350  briefly provides a small volume of fluid from the closed loop circuit  200  to the indexing circuit  300 . This fluid entering the indexing circuit  300  accelerates the movement of the plunger towards its extended position, thereby assuring that the plunger will reach its full extended position prior to the time the other plunger reaches its preset retracted position. A second feature in the preferred embodiment for low inlet pressures is the use of the first  325  and the second  327  biasing member for biasing at least one of the plungers as the plunger moves from the retracted position. The first biasing member  325  propels the first plunger  310  towards the extended position, thereby temporarily lowering pressure in the indexing circuit  300  below the pressure in the charge circuit  400 . A first pressure sensing member  415  in the charge circuit  400  opens and introduces fluid to the indexing circuit  300 . This fluid further ensures that the plunger moving toward the extended position will arrive prior to the time the other plunger reaches its preset retracted position. Likewise, upon reversal of pump  230 , the second biasing  327  member propels the second plunger  315  toward the extended position thereby following the same sequence of events as described. 
   The indexing circuit  300  further includes a first  320  and a second  322  check valve for selective communication from the indexing circuit  300  to the close loop circuit  200 . The first  320  and second  322  check valves are arranged to allow fluid to enter the suction line of pressure compensated pump  230  in the closed loop circuit  200  as one plunger reaches its full extended position while the other plunger proceeds to its preset retracted position thereby maintaining volumetric balance in the system  100 . 
     FIG. 4  is a schematic view showing the charging circuit  400  in the hydraulically driven multiphase pump system  100 . This circuit  400  picks up hydraulic fluid from a reservoir  450  and pumps it throughout the circuit  400  to re-supply the closed loop circuit  200 , the indexing circuit  300  and the power saving circuit  500  with hydraulic fluid. The charge circuit  400  has a predetermined pressure that is maintained by a charging pump  410 . The circuit also includes first  415  and a second  420  pressure sensing member. If the closed loop circuit pressure falls below the predetermined charge circuit pressure the first pressure sensing member  420  causes the introduction of hydraulic fluid into the close loop circuit  200  to replenish its supply of fluid. If the indexing circuit pressure falls below the predetermined charge circuit pressure the second pressure sensing member  415  causes the introduction of hydraulic fluid to flow into the indexing circuit  300  to replenish its supply of fluid. A hand operated valve  365  allows selective fluid communication from the charge circuit  400  to the indexing circuit  300 . Any fluid not needed by the system  100  is surplus, and is returned to the reservoir  450 . 
     FIG. 5  illustrates the power saving circuit  500  in the hydraulically driven multiphase pump system  100 . Circuit  500  will transfer energy between the plungers,  310 ,  315  as they move in opposite directions. The power saving circuit  500  includes a first and second power saving hydraulic cylinders  510  disposed adjacent to the first plunger  310  connected to a first and second power saving hydraulic cylinders  515  disposed adjacent to the second plunger  315 . In high inlet pressure scenarios, the plunger moving toward the extended position is urged upwards by the inlet pressure of the fluidstream resulting in useful energy. This energy is transferred from the plunger moving toward its extended position to the plunger moving toward its preset retracted position by the power saving hydraulic cylinders  510 ,  515 . Therefore, the amount of work needed from the pressure compensated pump  230  in the closed loop circuit  200  directed to the plunger moving toward the preset retracted position is substantially reduced. In low inlet pressure scenarios, the power saving circuit  500  in same manner as previously described may be economically applied where the plunger diameter is large thereby having a large surface area to act upon. Any excess fluid in the circuit  500  may be relieved to the reservoir  450  through valve  520 . While the described embodiment in  FIG. 5  includes hydraulic cylinders  510 ,  515 , it will be understood that any mechanism that facilitates the transfer of energy such as sheaves, chains, or hydraulic cylinders could be used. Additionally, this invention is not limited to the orientation of the hydraulic cylinders as shown on  FIG. 5  but rather may be disposed in any orientation that is necessary and effective. 
     FIG. 6  illustrates a trim circuit  600  in the hydraulically driven multiphase pump system  100 . Generally, the trim circuit  600  provides fluid to the indexing circuit  300  and the power saving circuit  500 . The trim circuit  600  includes a pump  605 , such as a gear pump, that is operatively attached to the pump  230 . The pump  605  supplies fluid to the trim circuit  600 . The trim circuit  600  further includes a directional control valve  610  for controlling the fluid through the circuit  600 . In the normal position (as illustrated), the control valve  610  restricts fluid flow to the indexing circuit  300  and the power saving circuit  500  causing fluid to accumulate in an accumulator  620  and eventually flow through a relief valve  615 . After the fluid in the accumulator  620  reaches a predetermined pressure, the valve  610  may be opened to allow fluid to flow into indexing circuit  300  and the power saving circuit  500 . The trim circuit  600  further includes a first limit switch  625  and a second limit switch  630 . The limit switches  625 ,  630  are generally used to selectively trigger the valve  610  to direct fluid into the indexing circuit  300  or into the power saving circuit  500 . More specifically, after the first limit switch  625  is triggered by a predetermined control such as a PLC (not shown), the valve  610  allows fluid to enter into the power saving circuit  500  which has the effect of shortening or adjusting the maximum stroke between the two plungers  310 ,  315 . On the other hand, after the second limit switch  630  is triggered by a predetermined control such as the PLC, the valve  610  allows fluid to enter into the indexing circuit  300  which assures the ascending plunger will reach its full stroke and maintain the counter-synchronous relationship between the plungers  310 ,  315 . The trim circuit  600  further includes relief valves  635  and  640  to limit the maximum pressure in the power saving circuit  500  and the indexing circuit  300 , respectfully. The trim circuit  600  also includes a needle valve  640  to drain the circuit or adjust the frequency of adding fluid into the power saving circuit  500 . 
     FIG. 7  illustrates a rapid reversal circuit  700  in the hydraulically driven multiphase pump system  100 . Generally, the rapid reversal circuit  700  provides a means for rapidly changing the direction of the plungers  310 ,  315 . In other words, instead of relying on the pump  230  to reverse its flow and subsequently the direction of the plungers  310 ,  315 , the rapid reversal circuit  700  uses a plurality of poppet valves  705 ,  715 ,  725 ,  735  to change the direction of the plungers  310 ,  315 . Each poppet valve  705 ,  715 ,  725 ,  735  includes a respective control valve  710 ,  720 ,  730 ,  740  to selectively control the flow of fluid into and out of the poppet valve. More specifically, when each control valve  710 ,  720 ,  730 ,  740  is energized fluid enters the poppet valve and when each control valve  710 ,  720 ,  730 ,  740  is de-energized fluid exits the poppet valve and subsequently drains to the tank. 
   In operation, the rapid reversal circuit  700  controls the direction of the plungers  310 ,  315  by selectively energizing each control valve  710 ,  720 ,  730 ,  740  after a limit switch (not shown) is triggered. For instance, as plunger  315  has descended, it will cause pilot pressure to flow into poppet valves  715 ,  735  and allow pressure to exit out of poppet valves  705 ,  725 . Preferably, the poppet valve is closed when pilot pressure is introduced therein and closed when relieved from pilot pressure. Therefore, as poppet valve  715  opens the high pressure from the pressure compensated pump  230  flows into the cylinders  222  of plunger  310 , thereby causing the plunger  310  to descend. At the same time, plunger  315  will ascend and cause fluid to flow through the poppet valve  735  back to the inlet of the pressure compensated pump  230 . Subsequently, plunger  310  triggers its limit switch thereby causing the control valves  710 ,  720 ,  730 ,  740  to reset and allow the fluid flow from the pressure compensated pump  230  to be directed through the poppet valve  725  while flow back from plunger  315  returns through poppet  705 . Preferably, a PLC control (not shown) controls the opening and closing sequence and uses the throttling settings on each poppet valve  705 ,  715 ,  725  to control the rate that the poppet valve moves. These control settings determine the rate the plungers  310 ,  315  reverse direction. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.