Abstract:
A high pressure pump for use in the injection of liquid chemicals into subsea oil or gas wells, and intended to be positioned in the subsea environment adjacent to the wellhead, comprises a piezoelectric actuator ( 19 ) for reciprocating a plunger ( 22 ) which acts to compress and expand the effective volume of a pumping chamber ( 29 ) having a valved inlet ( 15 ) connected to a source of the liquid and a valved outlet ( 16 ) to lead the liquid to the well. The device has a minimum of moving parts and in particular avoids the need for any rotating parts and attendant high performance bearings and seals.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a system for injecting liquid chemical into a subsea well and to pumps designed for use in such a system. Although the term “subsea” is used for convenience to indicate the location of wells to which the system relates, this should be understood to include reference to any substantial body of water beneath which a well may be located. Furthermore pumps of the character to be more particularly disclosed herein are not restricted to use in such systems and may also find application in, for example, automotive fuel injection systems, hydraulic actuator systems, or in other areas where high fluid pressures need to be generated by electrically-powered pumps with a minimum of moving parts. 
     (2) Description of the Art 
     It is a well known practice, in order to maintain the efficient operation of a production oil or gas well, to inject certain chemicals in liquid form into the well at selected times and positions, for example corrosion inhibitors to inhibit corrosion of downhole equipment and wax inhibitors to inhibit the formation of waxy substances that block the flow of product. For high pressure, high temperature (HPHT) wells and extremely high pressure, high temperature (XHPHT) wells, pressures typically in the range of 15,000-25,000 PSI (100-170 MPa) need to be generated by the pumps in such systems. In the case of subsea wells it is not always practical to have pumps at the surface platform (or only at the surface platform) due to the cost of running high pressure umbilicals down to the wellheads (which can involve umbilical lengths of some thousands of meters) and the pressure drop across such long umbilicals, meaning that control of the delivery pressures and flow rates at the wellheads can be quite problematic. It is therefore common to employ the pumps (or additional pumps) for such systems underwater in the vicinity of the wellheads. However, a subsea environment presents particularly serious challenges to the reliability of such chemical injection pumps due to the aggressive conditions under which they are required to operate and the difficulty of accessing and effecting any required maintenance or repair of the equipment located underwater. Current systems typically employ hydraulically-actuated pumps, requiring hydraulic control lines to be run down to the sea bed, and regular maintenance, and are therefore both complex and costly to operate. The present invention therefore aims to provide an alternative pumping system for such service, which can be electrically operated, has a minimum of moving parts and in particular avoids the need for any rotating parts and attendant high performance bearings and seals; in other words an essentially “solid state” solution. 
     SUMMARY OF THE INVENTION 
     In one aspect the present invention accordingly resides in a system for injecting liquid chemical into a subsea well comprising: 
     a source of liquid chemical; 
     a pump located in the subsea environment comprising a pumping chamber, an inlet and an outlet opening to said chamber, a reciprocal plunger adapted to compress and expand the effective volume of said chamber, and a piezoelectric actuator for reciprocating said plunger;
 
conduit means for leading liquid chemical from said source to said inlet of said pump; and
 
conduit means for leading liquid chemical from said outlet of said pump to said well.
 
     The invention also resides per se in various features of the pump to be more particularly described and illustrated herein. 
    
    
     
       DESCRIPTION OF THE DRAWINGS The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a subsea chemical injection system according to the invention; 
         FIG. 2  is a longitudinal section through one embodiment of a pump according to the invention for use in the system of  FIG. 1 ; 
         FIG. 3  shows the plunger and head portion of the pump of  FIG. 2 , to an enlarged scale; 
         FIG. 4  is a scrap section showing the sealing arrangement of the plunger to the head in the pump of  FIG. 2 , to a further enlarged scale; 
         FIG. 5  illustrates schematically a control system for the pump of  FIG. 2 ; 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , this illustrates schematically one example of a system according to the invention. There is shown an oil or gas wellbore  1  extending down from the sea floor and equipped with a wellhead  2  from which product flows through tubing  3  to a production platform  4  at the surface. Although the platform  4  is shown as a floating (off-shore) platform in the Figure, depending on the topography of the oil or gas field it could alternatively be a land-based platform serving the subsea well  1 / 2 . Adjacent to the wellhead there is a unit  5  housing one or more—and in practice most likely to be a multiplicity acting in series and/or parallel—of pumps of the kind described below, for use in injecting liquid chemical into the well. The chemical or chemicals to be injected are stored on the platform  4  and supplied to the unit  5 , partially pre-pressurised if required, through an umbilical  6  which also carries electrical power and any required data and/or control signals to the pumping unit. Tubing  7  conveys the chemical for injection from unit  5  to the wellhead whence it is distributed as required. 
       FIGS. 2 and 3  illustrate the structure of one embodiment of a pump  10  for use in the unit  5 . It has a barrel-like body part  11  typically of stainless steel, closed by a monolithic head  12  typically of a nickel-based alloy such as Hastelloy® for resistance to the chemicals which will be handled by the pump. The head  12  is attached to the body part  11  through mating fine pitched screw threads  13  and secured in place by a set of, say, six clamping bolts  14 A pressing on a ring  14 B on top of the body part  11 , as will be more particularly explained hereafter. The head  12  has inlet and outlet fittings  15  and  16  for the chemical to be pumped, fitted with respective micro non-return valves  17 ,  18  and leading to/from the pumping chamber referred to below. 
     Within body part  11  is mounted an elongate piezoelectric actuator  19 , being fixed at its base by a screw  20 . In this respect the actuator  19  sits in a cradle  21  at its base equipped with flats to prevent rotation of the actuator as the screw  20  is tightened. This actuator comprises a stack of piezoelectric ceramic discs (not individually shown) within a housing, preloaded by an internal spring (also not shown), which when energized expand in the longitudinal direction of the stack with a maximum strain rate of around 0.1% of the length of the stack, and return to their unstrained condition, with assistance from the spring, when the energising voltage is removed. By applying voltage pulses to the actuator, therefore, its free end (upper end as viewed in the Figures) can be caused to reciprocate at the frequency of the pulses. Leads carrying the energising voltage to the actuator are routed through a radial bore in the body part  11  (not shown). Actuators of this kind are commercially available and typically used for generating mechanical vibrations at sonic frequencies e.g. for sonar equipment. 
     Rigidly screwed to the free end of the actuator  19  is a plunger  22 , typically of Hastelloy®, which consequently also reciprocates in use in accordance with the energisation of the actuator. The plunger  22  is formed at its upper and lower ends with narrower and wider cylindrical surfaces  23  and  24 , joined by a frustoconical surface  25 . The surfaces  23  and  24  are a close sliding fit in correspondingly bored portions  26  and  27  of the head  12  and the bores  26  and  27  are joined by an internal frustoconical surface with clearance around the surface  25  of the plunger to define a small space  28  and accommodate the reciprocation of the plunger. A small pumping chamber  29  is defined between the topmost surface of the plunger  22  and the facing surface of the head  12 , through which ports  30  and  31  open from the valves  17  and  18 . As the plunger is reciprocated by energisation of the actuator  19 , therefore, its upper end acts as a piston to alternately compress and expand the volume of the chamber  29 . More particularly movement of the plunger to the top of its stroke compresses the volume of the chamber  29 , causing the valve  18  to open and expelling the contents of the chamber towards the outlet  16 . As the plunger  22  returns to the bottom of its stroke the volume of the chamber  29  is expanded so that the valve  18  closes, the valve  17  opens and a fresh quantity of chemical enters the pumping chamber from the inlet  15 . 
     In this respect the upper end (piston) of the plunger  22  is sealed against the bore  26  of the head  12  as shown in  FIG. 4  (from which the ports  30  and  31  are omitted for simplicity). That is to say the plunger surface  23  is formed with a groove in which is located an “O” ring  32  e.g. of Viton® which is slightly compressed in the radial direction when fitted in the head  12  and forms a sliding seal against the bore  26  as the plunger reciprocates. This ring is supported on each side by a PTFE back up ring  33 ,  34  of substantially the same effective radial thickness as the compressed “O” ring  32  so there is no danger of the “O” ring becoming damaged by extrusion against any sharp edges in use. The fit of the plunger surface  24  ( FIGS. 2 and 3 ) in the bore  27  of the head  12  ensures that the piston portion of the plunger remains centralised in the bore  26  and further assures that the piston is evenly sealed around the head as it reciprocates. The head  12  is itself machined from a monolithic block and provides no leakage path for liquid from the pumping chamber  29 . 
     In use the pump  11  will be immersed in a bath of hydraulic fluid and bores (not shown) through the body part  11  convey this fluid to the space  35  around the piezoelectric stack  19  for cooling the same. Circulation of this fluid to enhance cooling may occur through natural convective flow or an additional small conventional circulating pump (not shown) may be provided for this purpose. Bores (not shown) through the head  12  also convey this fluid to the space  28  around the plunger  22  for lubricating the movement of the plunger, the seal  32  also serving to keep this fluid out of the pumping chamber  29 . 
     It will be appreciated that by virtue of the limited stroke length of the actuator  19  and corresponding size of the pumping chamber  29  only a small volume of liquid will be pumped in each cycle, although the total flow rate is of course a function of the actuation frequency. By way of example, a single pump substantially as illustrated, with an actuator length of 200 mm and stroke of 0.2 mm, has been found to be capable of pumping liquid at a rate of up to 5 liters per hour at an outlet pressure of up to 20,000 PSI (140 MPa) from an inlet pressure of up to 10,000 PSI (70 MPa) when actuated at between 30 and 70 Hz, and substantially higher rates and/or pressures should be achievable by ganging a plurality of such pumps together. The ratio of the swept volume of the pumping chamber  29  to its total volume (including the volume of the ports  30 ,  31  and any “dead” space between the valves  17 ,  18 ) will be at least 1:7. 
     A typical control system for the pump  10  within a unit  5  is illustrated in  FIG. 5 . The pump is shown connected to the chemical supply line (umbilical)  6  through an inline filter system  36  for removing any debris that may accumulate from the long umbilical, and to the chemical output line  7 . The pump is energised from an electrical power supply  37  via a driver unit  38  under the control of a driver control unit  39  which is itself linked by a two way data and control line  40  to a topside control unit  41  using any standard serial communication technique (e.g. RS422/RS485). Transducers  42  and  43  monitor the pressures in the supply and output lines, from which the flow rate can also be computed. The control unit  39  controls the driver  38  to energise the pump  10  to inject the chemical as demanded by the topside controller, to achieve a desired flow rate by control of the applied voltage amplitude, duty cycle and/or frequency. 
     The assembly of the pump shown in  FIGS. 2-4  is achieved as follows. First the plunger  22  is fitted to the actuator  19 , the actuator is slid into the cradle  21  in the body part  11 , with its leads routed as required, and the bolt  20  is loosely fitted Next the “O” ring  32  and back up rings  33 ,  34  are fitted to the plunger  22  and the clamping ring  14 B is placed on the body part  11 . The inside surfaces of the head  12  are then lubricated and the head is screwed onto the body part  11  ensuring that it is correctly located over the plunger  12  but not screwed all the way down. The bolt  20  is then tightened and the head  12  is screwed further until it abuts the top surface of the plunger  22 . The clamping bolts  14 A are fitted into the head  12  and turned to engage loosely in respective cups  44  formed in the ring  14 B. The head  12  is then backed off from the top of the plunger by turning it in the reverse direction through a specified arc to define the required depth of the pumping chamber  29 —to facilitate which the clamping ring  14 B (which now turns on the body part  11  with the head  12  by virtue of its engagement with the bolts  14 A) is provided with a series of markings around its periphery which can be related to an index mark on the body part  11 . Finally the bolts  14 A are tightened to take up any play in the screw threads  13  and to clamp the head  12  against the body part  11  in the relative rotational position to which it has been set. This process ensures that the volume of the pumping chamber  29  is consistent from pump to pump notwithstanding any variations which may exist in the axial lengths of the actuators  19  or other engineering tolerances on the plunger and head profiles. 
     A feature of the pump  10  described and illustrated herein is that the plunger  22  is connected directly to the actuator  19  and avoids the use of any lever or the like force —or movement-amplifying means. In the described chemical injection system the pump also acts directly on the liquid to convey it towards the injection point(s) in the well as distinct from a system where, say, a piezoelectric pump is used to pressurise a hydraulic fluid for operation of a ram or the like. 
     The pump  10 , being a positive displacement pump, can also usefully function as a metering unit by controlling the frequency or other characteristic of operation of the piezoelectric actuator, meaning that separate orifice plates or the like devices need not be employed for this purpose. Indeed such a pump can be used as a metering unit even in the case where it is not required to provide, or boost, the pressure of the system, then simply controlling the rate of flow of fluid though it under a separately-generated pressure differential.