Improvements in or relating to rotary positive displacement fluid machines

Abstract

 A positive displacement pump is described which is particularly suitable for pumping a multiphase fluid mixture, for example, production oil, water, gas or solids in suspension from offshore or onshore oil fields. The pump has a casing (l) within which is disposed an outer rotor (5) having internal helical lobes (8) and an internal rotor (ll) disposed within the outer rotor (5) and having external helical lobes which co-operate to define cavities (l7) therebetween. The outer rotor (5) and inner rotor (ll) are both rotatable relative to each other about respective fixed axes (l5,l6) which are parallel but spaced from each other so that a working medium can be axially conveyed, via the cavities, through the machine. In a preferred arrangement the inner (ll) and outer rotor (5) have portions of opposed hand whereby the working medium is conveyed in opposed directions between each end of the machine and an intermediate location. The pump can be connected in series or parallel with like pumps to form multi-pump arrangements.

Description

[0001] This invention relates to rotary, positive dis­placement fluid machines and particularly, but not exclusively, to fluid pumps for pumping multi-phase fluid mixtures, e.g. oil, water, gas or solids in suspension.

[0002] In the production of oil from offshore wells, it has until recently been the practice to locate a floating or gravity production platform directly above the wellheads.

[0003] An important function of this platform is to separate suspended solids, gas and water from the multiphase production fluid produced by each well, so that the crude oil export pumps on the platform, which discharge the oil to tankers or to the shore via subsea pipelines, handle oil only with very little free gas.

[0004] A recent development offshore has been the installation of subsea completion systems and wellheads for some oil wells, flowing the multiphase fluid produced by such wells through pipelines on the seabed to a production platform some distance from the wells. Such completions have, to date, been limited to comparatively short distances between the wellheads and the associated production platforms, because unstable flow conditions can and do occur in long horizontal pipelines handling multi­phase fluids. This limitation is a serious one, since if subsea well completions could be installed at greater distances from surface production facilities, the capital costs of the development of many offshore oilfields could be substantially reduced.

[0005] The problem of flowing the multiphase production fluid from the wellhead through seabed pipelines can be overcome if a pump capable of handling multiphase flow at its suction is installed on the seabed at the wellhead, the pump being designed to raise the fluid pressure to a level where all or most of the free gas is re-absorbed into solution, since this will result in virtually single phase flow downstream in the seabed pipeline, with very little free gas, providing much more stable flow conditions. It is a principal objective of this invention to provide such a pump.

[0006] For the pumping of abrasive liquids, one form of pump which is suitable is the well known progressive cavity internal screw type pump. Such a pump incorporates a helical rotor, the helical surface of the rotor containing n lobes, which co-operates with a fixed stationary outer cylindrical stator, generally lined with resilient material, having an internal helical profile formed with (n+l) lobes, where n is any integer. On rotation of the rotor relative to the stator, the rotor rotates and precesses simultaneously, engaging the stator at spaced intervals to define cavities within which the medium being pumped is contained and moved axially and spirally through the pump. Such pumps operate satisfactorily but have dis­advantages in that the rotor is subject to axial and radial pressure loads which cause high friction forces between the rotor and stator. They are therefore subject to wear particularly when used for pumping fluids containing abrasive particles. Furthermore, the precession of the rotor results in centrifugal acceleration loads and these loads also cause wear between the rotor and the stator unless the rotor rotational speed is kept comparatively slow. In subsea installations, the necessity to shut down an installation for repair, maintenance or replacement of a pump is a very expensive operation and consequently the longer such repair or maintenance can be deferred the better.

[0007] An object of the present invention is to provide a positive displacement pump suitable for pumping multi­phase mixtures including gas, liquids or solids in which the foregoing disadvantages of the progressive cavity type pump are obviated or mitigated. It is a further objective of this invention that this pump should be suitable for submerged operation on the seabed.

[0008] According to the present invention, there is provided a positive displacement fluid machine comprising a casing; a first member having internal helical lobes mounted within said casing; and a second member having external helical lobes mounted within said casing for rotation relative to said first member whereby co-operating helical lobes on the first and second members define cavities therebetween wherein, on relative rotation of the first and second members, a working medium can be conveyed axially through the machine characterised in that both first and second helically-lobed members are rotatable.

[0009] Preferably, the first and second members are rotatable about respective fixed axes which are parallel to but spaced from each other.

[0010] Preferably also, the helical lobes of both first and second members have two portions of opposed hand whereby the working medium can be conveyed in opposed directions between end of the machine and an intermediate location.

[0011] An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. l is a vertical sectional view through a rotary positive displacement pump in accordance with the present invention; and

Fig. 2 is a diagrammatic illustration of a cross-­section of the pump rotors showing their relative dimensions.

[0012] Referring to the drawings, a rotary positive dis­placement pump comprises a cylindrical outer casing l in which is provided a pair of fluid inlets 2,3 and a fluid outlet 4.

[0013] Concentrically disposed within the casing l is a first cylindrical rotor 5 constituted by an outer sleeve 6 provided with a resilient liner 7, e.g. of rubber. The function of the resilient liner 7 is to inhibit wear due to abrasion, erosion or corrosion. The liner 7 has, as shown in the drawing, a left-hand portion 7a and a right-­hand portion 7b, the two portions 7a and 7b being identical with each other in that they each have "two-start" internal helical lobes 8 of the same profile, except that the left-hand portion 7a has a right-hand helix and the right-hand portion 7b has a left-hand helix. By this means, fluid passing through the rotor is directed to central radial ports 9 for passage out of the pump through outlet 4.

[0014] The rotor 5 is rotatably mounted in radial bearings 10 disposed between the rotor sleeve 6 and the pump casing l. Mounted in operative relationship within the first rotor 5 is a single, rigid, second rotor ll integral with a driven shaft l2. The second rotor ll is of "single start" external helical profile and extends through both portions 7a and 7b of the first rotor 5, the left and right-hand portions of the rotor ll having right and left-­hand helices respectively to co-operate with their respective helices on rotor 5. The rotor shaft l2 is mounted for rotation within sleeve bearings l3,l4 about an axis l5 parallel to the axis l6 of rotation of first rotor 5 but spaced therefrom so that the driven rotor ll rotates eccentrically relative to rotor 5 and in mesh therewith so that the first rotor 5 is drivingly rotated by the second rotor ll without precession and at half the rotational speed of the latter due to said meshing engagement between the helical lobes of the rotors 5,ll. In use, the interengaging surface of the rotors 5,ll are preferably separated by a thin lubricating fluid film.

[0015] As will be seen from Fig. l of the drawings, the first and second rotors 5,ll define between them a series of helical cavities l7 wherein a medium can be transported axially through pump from one of the inlets 2,3 to the outlet 4.

[0016] The second rotor ll is of any suitable material, e.g. metal, provided with a surface coating to inhibit abrasion, erosion or corrosion.

[0017] The axial lead of the helical lobes 8 is constant along the rotor length on both the first and second rotors 5,ll, so that no reduction or increase in the volume occupied by the pumped or working fluid, and therefore no compression or expansion, takes place as the fluid passes axially through the pump. In progressive cavity pumps of this general type the second rotor ll may have n external helical lobes (where n is at least l) and the meshing first rotor 5 may have n+l internal helical lobes, the ratio of rotational speeds then being

.

[0018] In the particular example shown in Fig. 2, the second rotor ll has one lobe only of constant circular cross-section along its length having a diameter d and the axis l5 of rotation of the second rotor ll is spaced from the axis l6 of rotation of first rotor 5 by a distance equal or very nearly equal to distance e which is the radial eccentricity of each lobe of rotor ll. The radial cross-section of the internal bore of first rotor 5 is defined by two opposed semi- circular recesses each of diameter d+2c (where c is the mean radial clearance between the two rotors 5,ll in a radial plane), said two semicircles being joined by two parallel straight lines each of a length substantially equal to 4e and each line being tangential to the ends of the two semicircles. (note that c may be negative i.e. rotor ll may rotate with interference inside the resiliently lined rotor 5.

[0019] The driven rotor shaft l2 extends from or is attached to, rotor ll to form the shaft of a turbine l8 of known form. The turbine l8 is preferably but not necessarily driven by means of high pressure sea water which is supplied to the turbine blades through sea water inlet l9 in order to rotate shaft l2. The sea water leaves the turbine through sea water outlets 20, from where it may either flow direct to the sea, or be piped to the wellheads of sea water injection wells.

[0020] Between the tubine l8 and the pump there is provided in casing l, a cavity 2l within which is located a hydrostatic balance or sealing disc 22. The disc 22 is movable axially to seal against a face of cavity 2l when the turbine and pump are inoperative in order to prevent leakage of production oil or gas from the pump. When the turbine l8 is supplied with high pressure sea water, the disc 22 is displaced from its sealing position due to the sea water pressure acting on the disc 22 through access passages 23 in order to balance the axial thrust on the turbine when running.

[0021] Suitable bearings (e.g. 24) are mounted in casing l to carry the turbine rotor. These bearings can be of the hydrostatic or hydrodynamic type.

[0022] The axial and radial thrust bearings of the pump of the invention are lubricated with a lubricating fluid from an external source, e.g. sea water, which is compatible with the pumped fluid, the lubricant fluid mixing with the pumped fluid when discharged from the bearings. Alternatively, the bearings can be lubricated with the working fluid itself.

[0023] In use of the pump as described above, sea water at high pressure is fed to turbine l8 in order to rotate shaft l2 and associated rotor ll. Rotation of rotor ll causes simultaneous rotation of rotor 5 and fluid to be pumped is drawn into the pump through inlets 2,3, and fed in opposed streams through left-hand and right-hand portions of the pump to common central outlet 4. Such opposed flow serves to balance the axial thrusts on the rotors 5,ll which arises from pressure rise or fall in the fluid machine. The interfacial loads between the co-operating helical surfaces of the helical rotors 5,ll are small, being mainly restricted to the relatively small load required to rotate the first rotor 5 against bearing friction torque only. Consequently, the pump can operate with much less wear and consequent need for maintenance than previously proposed types of pump for this purpose.

[0024] Pump units as described above can be used as a single unit. Alternatively, a plurality of pump units can be connected together by suitable interconnecting ducting, in series or in parallel, the shafts l2 of each unit being mechanically coupled together. The shafts l2 of such an interconnected plurality of units can be disposed so that they rotate about the same longitudinal axis. The turbines of such a plurality of units can be supplied by high pressure sea water which passses from one turbine to another.

[0025] For the pumping of fluids containing free gas, a preferred arrangement is a plurality of pump units mounted on separate shafts, each pump unit being mechanically coupled to its own turbine. In such an arrangement, each pump would serve as one stage of a multi­stage pump system, the reduction in volumetric flow resulting from compression of the fluid with increased pressure being accommodated by turbine speed changes without the need for external controls.

[0026] Although the embodiment described above relates to a pump, it will be readily appreciated by those skilled in the art, that the machine described can equally well function as a motor.

[0027] In the above-described embodiment as shown in the drawings, an arrangement is disclosed in which the axial lead of the helical lobes of the rotors 5,ll is constant along the rotor length so that no compression or expansion of the working fluid takes place within the pump. In an alternative embodiment the axial lead of the helical lobes of the rotors 5,ll varies either progressively or in a series of steps along the axes of the two rotors so that the volume of the cavities l7 is reduced or increased as the fluid passes axially through the pump. Thus the fluid is compressed or expanded which permits the machine when acting as a pump on compressible fluids to operate with internal compression, and when acting as a fluid motor on compressible fluids to act as an expander with internal expansion. Such an expander is particularly suitable for comparatively low volumetric flow or high pressure ratio duties and wet gas conditions (i.e. with droplets of condensate and/or water in suspension). A gas expander of this type would, for example, be suitable for low cost energy recovery, particularly offshore gas produced under pressure from oil/gas wells.

[0028] It will be apparent that other modifications can be made to the embodiments described above without departing from the scope of the present invention. For example, rather than driving the rotors by means of a fluid turbine, the rotors could be driven by any other suitable means such as an electric motor or an internal combustion engine. Also instead of the rotor 5 being driven by rotor ll through meshing engagement, the angular positions of the rotors 5,ll can be phased relative to each other by timing gears or the like which prevent meshing contact between the two rotors.

[0029] It will also be apparent that the parallel axes of the rotors and casing can be arranged either horizontally, vertically or at any angle between horizontal and vertical.

[0030] Pumps in accordance with the present invention are particularly suitable for the pumping of production oil, water and gas from onshore or offshore oilfields, either as a single phase or as a multiphase fluid mixture, e.g. gas/liquid or liquid/solid suspensions. Such pumps can be located on the seabed and connected to associated pipelines by subsea high pressure pipe connectors or may be located within the wells themselves.

Claims

1. A positive displacement fluid machine comprising a casing; a first member having internal helical lobes mounted within said casing; and a second member having external helical lobes mounted within said casing for rotation relative to said first member whereby co-operating helical lobes on the first and second members define cavities therebetween wherein, on relative rotation of the first and second members, a working medium can be conveyed axially through the machine characterised in that both first and second helically-lobed members are rotatable.

2. A positive displacement fluid machine as claimed in claim l, characterised in that the first and second members are rotatable about respective fixed axes which are parallel to but spaced from each other.

3. A positive displacement fluid machine as claimed in claim l or 2, in which rotation of the helical lobes of the first member is effected on rotation of the second helically-lobed member due to meshing engagement between the lobes of said first and second members.

4. A positive displacement fluid machine as claimed in any of claims l to 3, characterised in that the helical lobes of both first and second members have two portions of opposed hand whereby the working medium can be conveyed in opposed directions between each end of the machine and an intermediate location.

5. A positive displacement fluid machine as claimed in any preceding claim characterised in that said helical lobes have an axial lead which is constant along the length on both said first and said second members.

6. A positive displacement fluid machine as claimed in any of claims l to 4 characterised in that said helical lobes have an axial lead which varies progressively or in a series of steps along the length on both said first and said second member.

7. A positive displacement fluid machine as claimed in any preceding claim characterised in that said first member has n+l internal helical lobes and said second member has n external helical lobes.

8. A positive displacement fluid machine as claimed in any preceding claim characterised in that said second member comprises a drive means for rotating said second member.

9. A positive displacement fluid machine as claimed in claim 8 characterised in that the drive means is a rotary drive unit having a drive shaft, said drive shaft being rotatable in response to fluid flow through the rotary drive unit in order to rotate said second member.

l0. A positive displacement fluid machine as claimed in claim 9 characterised in that said casing and said second member define a cavity within which is disposed a hydrostatic sealing means, said sealing means being movable axially to seal against a face of said cavity when said rotary drive and said pump is inoperative to prevent leakage of said working medium from said machine.

11. A positive displacement fluid machine as claimed in any preceding claim characterised in that a plurality of said machines are connected in series or in parallel with respective second members being mechanically coupled together.

12. A positive displacement fluid machine as claimed in claim ll characterised in that when said machines are connected in series, said second members rotate about the same longitudinal axis.

13. A positive displacement fluid machine as claimed in claim ll characterised in that when said machines are connected in parallel, each first member is coupled to a separate shaft and rotary drive.

14. A positive displacement pump as claimed in any one of claims 8 to l3 characterised in that said drive means is a fluid turbine.

15. A positive displacement pump as claimed in claims 8 to l3 characterised in that said rotary drive is an electric motor or internal combustion engine.

16. A positive displacement pump as claimed in any preceding claim characterised in that the angular position of said first and said second members can be phased relative to each other to prevent meshing contact between the first and second members.

17. A positive displacement pump as claimed in any one of claims 2 to l6 characterised in that said parallel axes of said first and second members and said casing can be arranged horizontally, vertically or at an angle between horizontal and vertical.

18. A method of transporting a multi-phase mixture comprising utilising a positive displacement fluid machine as claimed in any preceding claim.

Drawing