Abstract:
A chemical injection pump for injecting chemicals into subsea system at depths up to 10,000 feet is described which uses a minimum of moving parts by employing an actuator, for instance a solenoid, to power a double acting actuator rod and plungers thereon. The pump would generate low pressures and low fluid volumes, but be more durable and reliable than conventional rotating pumps operating under subsea conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/194,433 filed Apr. 4, 2000. 
    
    
     FIELD OF THE INVENTION 
     The instant invention relates to relatively low volume chemical injection pumps, and more particularly relates, in one embodiment, to low volume chemical injection pumps for use in subsea applications. 
     BACKGROUND OF THE INVENTION 
     In the art and science of recovering hydrocarbons from reservoirs beneath water, such as through off shore drilling platforms and other subsea operations, it is necessary to inject treatment chemicals into the well or wellbore, the drilling fluid therein, or in hydrocarbon transmission pipelines, etc. Such treatment chemicals may include, but are not necessarily limited to, corrosion inhibitors, scale inhibitors, paraffin inhibitors, hydrate inhibitors, demulsifiers, and the like, and mixtures thereof. 
     The injection of treatment chemicals into these systems requires generally only low flow rates. When delivering low flow rates using positive displacement-type pumps in an atmospheric system, net positive suction head (NPSH) is often a problem. A good design for a subsea pump should try to inherently eliminate NPSH problems. Further, a major problem with positive displacement pumps, especially at high pressure, is that the check valve seats and piston/plunger packing can be inherently leaky, and cause fluid to leak through the pump, back to the suction side or back into the suction piping. Another problem with small volume, positive displacement diaphragm or plunger pumps is that they can vapor or air lock very easily. Small bubbles in the pump chamber can expand and contract with plunger movement and cavitate and stall the pump. 
     Further, because the location of such chemical injection pumps is by definition at the bottom of the ocean or sea, they are subjected to severe conditions and are difficult to service due to their remote location. Thus, subsea chemical injection pumps should be strong, durable, and if possible, reparable at a distance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus for injecting chemical into a system that is underwater or subsea. 
     Another object of the present invention is to provide a subsea chemical injection pump that has a minimum of moving parts. 
     It is yet another object of the invention is to provide a subsea chemical injection pump which can be repaired from a remote distance and/or which may continue to operate if partially disabled. 
     In carrying out these and other objects of the invention, there is provided, in one form, a subsea chemical injection pump having a housing comprising opposing chambers, one on either side of a central enclosure. Each chamber has parallel walls and a cross section, and the opposing chambers extend from the central enclosure on opposite sides thereof. That is, opposing chambers are lined up across the central enclosure, although the opposing chambers are not necessarily coaxial with one another. There is present in the central enclosure at least one actuator (e.g. solenoid coil), where the actuator drives an actuator rod. The actuator rod has two ends, one each extending into an opposing chamber, and a first and second plunger, one on each end of the actuator rod, where first plunger has a circumference adapted to fill and mate with the cross section of its chamber, and where second plunger has a circumference adapted to fill and mate with the cross section of its chamber. The actuator rod and plungers on either end move back and forth between maximum travel points in the opposing chambers under the influence of the actuator, alternately decreasing and increasing the volumes of the opposing chambers, respectively. A seal is preferably present on the circumference of each plunger to inhibit fluid from entering the central enclosure from the opposing chambers. An inert coolant and lubrication fluid is present in the central enclosure between the plungers. Finally, each opposing chamber contains a suction check valve and a discharge check valve therein, in a region beyond the maximum travel point of the plunger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a schematic, cross-sectional illustration of a subsea chemical injection pump of this invention, in one embodiment. It will be appreciated that the FIGURE is not to scale and that many features are not shown in actual or optimum proportion so that the invention may be clearly illustrated. For instance, the plungers may actually be thinner relative to the actuator rod from what is shown. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that a double-acting solenoid pump, in one non-limiting embodiment, meets many, if not all of the requirements of a subsea chemical injection pump. Such a pump would be relatively low volume, for example delivering from about 2 to about 250 gallons per day, and produce high pressures, unique to this design up to 15,000 psi differential pressure. 
     The subsea chemical injection pump of this invention is schematically shown in the Figure generally at  10 , which has a housing  12  of three main sections, opposing chambers, first chamber  14  and a second chamber  18  on either side of a central enclosure  16 . Opposing chambers  14  and  18  each have parallel walls and a cross-section. Parallel walls are defined as walls a plunger of constant circumference and shape can travel along while the plunger circumference is in constant contact with the walls. In one preferred embodiment of the invention, opposing chambers  14  and  18  are cylinders and their cross-sections are circles, for ease of manufacture, but this is not a requirement. Indeed, in one preferred, but non-limiting embodiment, entire housing  12  generally, and central enclosure  16  may also be cylinders. In the case where opposing chambers  14  and  18  are cylinders, it can be appreciated that the parallel walls are a continuous, curved wall. While it is expected that opposing chambers  14  and  18  would be of equal volumes in most instances, this is not required. Furthermore, while opposing chambers  14  and  18  extend from the central enclosure  16  on opposite sides thereof, it will be appreciated that the chambers  14  and  18  may not be exactly 180° apart, but could be at a lesser angle with respect to each other. Further, it is anticipated that in some embodiments, there may be more than two opposing chambers  14  and  18 . 
     Central enclosure  16  contains at least one actuator  20  that is connected to and/or drives an actuator rod  22 . In one non-limiting embodiment of the invention the actuator  20  is a solenoid surrounding actuator rod  22 . Other suitable devices for driving the actuator rod  22  may be used. Actuator rod  22  is oriented in the same direction as opposing chambers  14  and  18 , and the actuator rod  22  has two opposite ends, first end  24  and second end  26 . 
     In a preferred embodiment, opposing chambers  14  and  18  have the same direction in the sense that they are generally aligned with each other, but they are not necessarily coaxial. That is, the chambers  14  and  18  are aligned such that actuator rod  22  within solenoid coil  20  is parallel to, but not necessarily coaxial with the chambers. In one preferred embodiment, actuator rod  22  is straight. In another preferred embodiment of the invention, opposing chambers  14  and  18  may actually be coaxial with actuator rod  16  and each other. Alternatively, there could be two actuator rods  20  which could be in line with each other (at a 180° angle) or at an angle less than 180° as long as opposing chambers were at the same angle. One rod  22  would then bear first plunger  30  and the other rod  22  would bear second plunger  32 . 
     Actuator rod  22  has a first plunger  30  and second plunger  32 , on the first end  24  and second end  26 , respectively, thereof. First plunger  30  has a circumference adapted to fill and mate with the cross-section of its chamber, here first chamber  14 . Since plunger  30  is seen edge-on in the Figure the entire circumference is not seen. However, if first opposing chamber  14  is a cylinder with a circular cross-section, the circumference of first plunger  30  would be circular in shape. Similarly, second plunger  32  has a circumference adapted to fill and mate with the cross-section of its chamber, here second chamber  18 . Actuator rod  22  and plungers  30  and  32  on either end move back and forth between maximum travel point A in chamber  14  and maximum travel point B in chamber  18  under the influence of actuator or solenoid coil  20 . This action alternately decreases and increases the working volumes of the opposing chambers  14  and  18 . That is, the volume of opposing chamber  14  which may contain treating chemical is decreased the same amount that the volume of opposing chamber  18  which also may contain the same or different treating chemical is increased, respectively, and vice versa. 
     There should be at least one seal  34  present on the circumference of each plunger  30  and  32  to inhibit fluid, such as the treatment chemical from entering the central enclosure  16  from the opposing chambers  14  and  18 . Tolerances of seals  34  with respect to the cross-sections of the chambers  14  and  18  should be sufficiently tight to accomplish the sealing function, but not so tight as to undesirably interfere with the movement of plungers  30  and  32 , respectively. Within central enclosure  16  and between the plungers  30  and  32 , and surrounding the solenoid coil  20  and actuator rod  22  there is present an inert coolant and lubrication fluid  36 . 
     In a preferred embodiment, the central solenoid enclosure  16  is pressurized with inert, lubricating fluid  36  that serves several purposes, including, but not necessarily limited to, 1) lubricating the actuator rod  22  and piston seals  34 ; 2) providing resistance or “damping” of the actuator rod  22  movement (slightly slowing down actuator rod  22  so that it does not snap or slam back and forth); and 3) allowing the pump  10  to be pressurized at the surface, so that pressure equalizes as it descends to the sea floor for placement. These multiple functions are anticipated to increase pump life under expected heavy loading. In another non-limiting embodiment of the invention, the pump  10  may be pressurized such that equalization occurs approximately half-way to the bottom so that the design thicknesses of the housing  12  only needs to be half that of the pressure the pump  10  will be subjected to at the total water depth. This will keep a positive pressure in the central enclosure  16  and help prevent chemical or sea water from penetrating the central enclosure  16 . 
     Each opposing chamber  14  and  18  is provided with at least one “one-way” suction check valve  40  and one “one-way” discharge check valve  42 . These valves  40  and  42  may be of any conventional design or future design which permits fluid to enter chambers  14  and  18  and be discharged therefrom, respectively, in one direction. Valves  40  and  42  must be positioned within their respective chambers at points beyond the maximum travel points (A and B) of the plunger to avoid leaking of the fluid into the central enclosure  16 . 
     Check valves  40  and  42  could be integral to the housing  12 , but in a preferred embodiment they would be independent, discrete parts assembled into the pump housing  12 . In another non-limiting embodiment of the invention, the pump  10  design may incorporate a plurality of suction check valves  40  arranged sequentially in a magazine (not shown) so that the valves  40  may be remotely replaced. In one embodiment, the check valve magazines are operated remotely in a sequential or serial fashion to replace nonfunctioning valves. Such a design that permits changing the valve and seat without having to retrieve the pump  10  if a check valve were to fail would be advantageous. The same could be true of the discharge check valves  42 . 
     Central enclosure  16  may be provided with a leak detector  44  in the interior thereof to determine if any fluid from the opposing chambers  14  and  18  has leaked into the central enclosure  16  and inert coolant and lubrication fluid  36 . Leak detector  44  may be a pressure switch or conductivity probe or other device on the inert fluid side  16  to detect a leak past the dynamic piston seals  34 . Leak detector  44  need not be located in the center of central enclosure  16  as shown in the Figure. For instance, there may be one leak detector  44  on either end of the interior of the central enclosure  16  near to where actuator rod  22  exits solenoid  20 . 
     The subsea chemical injection pump  10  is designed to be electrically actuated via a double-acting solenoid, or two separate, single-acting solenoids, in different, non-limiting embodiments. By “double-acting”, it is meant that the solenoid is of the type that can move the actuator rod  22  alternately in either direction; “single-acting” refers to a solenoid that would move the actuator rod  22  in only one direction; it would have to be paired with a second single-acting solenoid with reverse polarity to move actuator rod  22  back in the other direction. It is expected that the use of one or more solenoids will make the pump  10  precisely controllable. 
     The pump  10  is intended to sit on the sea floor (up to 10,000 ft of water depth) adjacent to the subsea tree or manifold. The pump  10  may be controlled by alternating current polarity in order to change direction of the plungers  30  and  32 , in one non-limiting embodiment. Alternatively, if two different solenoids are employed, the pump may be controlled by current to the two solenoids alternately. 
     Power would be provided by the subsea manifold. Controlling and monitoring of the pump may be conducted via RS-485 communications through a fiber optic line that provides telemetry to and from the subsea manifold, in one embodiment. Monitoring could include, but not necessarily be limited to, determination of pump function such as speed or force, whether the pump is leaking in any chamber or enclosure, whether the valves are operating properly, etc. Control may include, but not necessarily be limited to, controlling pump operation and speed, causing replacement of faulty valves, switching from one chamber to another, performing repair operations, etc. Control operations could be performed manually or automatically in response to the outcome of monitoring. 
     In one embodiment of the invention, the inert coolant and lubrication fluid  36  is selected from fluids including, but not limited to, silicone-based fluids, generally available hydrocarbon-based lubricating fluids, and the like and may have a viscosity between about 10 and about 50 cP. The construction materials must, of course, be strong and durable to withstand the pressures, brines and other conditions of the harsh environment in which they are expected to operate. 
     A purpose of the solenoid design of the pump  10  of the invention is to minimize the number of moving parts and thus eliminate failure modes associated with rotating equipment, such as is the design of many conventional pumps. Workovers on subsea equipment such as this are tremendously expensive, and minimizing economic loss is of primary concern. Thus, it is preferred to reduce complexity, be able to tightly control pump operation and build in redundancy, where possible. 
     A further advantage of the subsea chemical injection pump of this invention is that flow is relatively continuous. That is, one side can be always discharging into the system. Further, the pump in one sense can be understood to be “sealless”, in that a plunger seal leak will only diffuse into the central inert fluid enclosure and not into the environment. 
     The subsea chemical injection pump of this invention would be located adjacent a chemical storage tank on the sea floor, or within the storage tank itself. In one embodiment of the invention, the tank, bladder system and pump could be one integral unit. In a preferred embodiment, the subsea chemical injection pump is integral to coiled tubing or could be retrievable via wireline from the tank. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, it will be evident that various modifications and changes can be made thereto without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, specific proportions, materials, features and operating ranges, falling within the claimed parameters, but not specifically identified or tried in a particular subsea injection pump or in the operation of such a pump, are anticipated to be within the scope of this invention.