WO2004007908A1

WO2004007908A1 - Apparatus for separating water from oil

Info

Publication number: WO2004007908A1
Application number: PCT/IB2003/003644
Authority: WO
Inventors: Arne Myrvang Gulbraar; Ole Okland
Original assignee: Kvaerner Process Systems A.S.; Statoil Asa
Priority date: 2002-07-12
Filing date: 2003-07-14
Publication date: 2004-01-22
Also published as: AU2003255925A8; AU2003255925A1; GB0216368D0

Abstract

An oil/water separation arrangement comprising a compact electrostatic coalesces (CEC) (11) having an inlet (12) for receiving a well production stream including, under normal operating conditions, an oil continuous phase containing water droplets, a processing chamber in which the stream passes through an electrical field between charged electrodes which promotes coalescence of the water droplets, and an outlet (13) for well production stream which has been subjected to electrostatic coalescence, the oil/water separation arrangement further including a compact in-line oil/water separator (CILOWS) (14) receiving production stream from the outlet (13) of said CEC, said CILOWS having a separation vessel diameter D in the range d to 3d, where d is in the range 0.08 metres (3 inches) to 1.07 metres (42 inches), and a length of at least 10D.

Description

APPARATUS FOR SEPARATING WATER FROM OIL

Technical Field

This invention relates to apparatus for use in separating water from oil in the production stream of a hydrocarbon well.

Background Art

It has long been recognised that a production stream which is oil continuous, that is to say which has a continuous oil phase containing discreet water droplets, will separate under gravity in a gravity separation vessel provided that the oil continuous stream is allowed sufficient residence time in the vessel for the water droplets to sink to the bottom of the vessel and to coalesce into a water layer with the oil continuous layer floating thereon. Such separation techniques give rise to large and expensive separation vessels with a relatively low flow rate through the vessel thereby providing a long residence time.

Smaller water droplets take longer to migrate through the oil layer and to coalesce into the water layer and thus require a larger residence time than is the case for larger water droplets.

Processing of the production stream upstream of the gravity separator vessel, for example the use of pressure reduction valves and de-gassing apparatus tends to shear the water droplets to produce a larger quantity of smaller droplets and thus tends to increase the residence time necessary to achieve separation. In order to try to increase the flow through the system, without increasing the size of the gravity separation vessel further, it has been proposed to use an electrostatic coalescer upstream of the gravity separation vessel, it being recognised that an electrostatic coalescer will promote the coalescence of small water droplets into larger water droplets, and thus speed the migration of the water droplets into the water layer in the gravity separation vessel. Nevertheless, known combinations of electrostatic coalescer with gravity separator tend to be extremely large, bulky and expensive. For example, a conventional gravity separator may be 6 metres in diameter and 30 metres in length although typically a conventional gravity separator vessel will be between 1.2 and 4 metres in diameter and between 3 and 24 metres in length.

In an attempt to reduce the overall size of the package compact electrostatic coalescers (CEC) have been developed, a good example being illustrated in United States patent 6136174. Conventional large scale electrostatic coalescers use relatively widely spaced electrically charged electrodes through which the production flow passes relatively slowly so that the flow is non-turbulent or substantially non-turbulent and in addition to droplet coalescence gravity separation of the oil and water occurs as the liquid passes through the electrostatic coalescer. By comparison, the compact electrostatic coalescer has been designed to accommodate a high flow rate (potentially turbulent flow) of the production stream through the coalescer vessel which substantially precludes gravity separation during coalescence in the vessel of the coalescer. The compact electrostatic coalescer has much more closely spaced electrodes conveniently in the form of coaxial cylinders, permitting a much more compact coalescer construction without a reduction in its flow capacity. Nevertheless the resultant package is, by virtue of the large gravity separator vessel downstream of the CEC, still bulky, expensive and inconvenient particularly for use on a skid or platform where space is restricted. Relatively recently a smaller diameter gravity separator has been proposed for use "downhole". The second species of separator is referred to as an "inline separator" and for convenience herein will be referred to as a CILOWS (compact in-line oil/ water separator). The separator has a sufficiently small diameter to be accommodated below ground in the well bore. The necessary residence time to achieve separation of the water droplets from the oil is afforded by significantly increasing the length of the separator such that the separator has a relatively small external diameter and a relatively large overall length. Moreover, as the CILOWS is arranged to be positioned in the well bore little or no processing of the well stream will have taken place prior to the well stream entering the CILOWS and the well stream will thus have a much higher proportion of un-sheared large water droplets such that separation under gravity can be achieved with a shorter residence time than would be the case for a higher proportion of smaller water droplets.

The present inventor was faced with the problem of producing a compact, relatively low cost oil/water separator for use on a production platform or a sub-sea skid and has recognised that, notwithstanding the length of the device, a CILOWS type separator can be accommodated relatively easily in a production platform or sub-sea skid by recognising that the small diameter, elongate, vessel does not need to be rectilinear, and can be provided with bends to follow a convenient path on the platform or skid. However, the inventor has also recognised that by the time the production stream reaches the platform or skid it will have been subjected to shear in, inter alia, gas separators and/or pressure reducing valves and in order to avoid an unacceptable lengthening of the CILOWS to provide increased residence time, the inventor has combined with the CILOWS, upstream thereof, a compact electrostatic coalescer, (a CEC), to restore the water droplet size in the production stream in advance of the CILOWS.

Disclosure of the Invention

In accordance with the present invention therefore there is provided an oil/water separation arrangement comprising a compact electrostatic coalescer (CEC) having an inlet for receiving a well production stream including, under normal operating conditions, an oil continuous phase containing water droplets, a processing chamber in which the stream passes through an electrical field between charged electrodes which promotes coalescence of the water droplets, and an outlet for well production stream which has been subjected to electrostatic coalescence, the oil/water separation arrangement further including a compact in-line oil/water separator (CILOWS) receiving production stream from the outlet of said CEC, said CILOWS having a diameter D in the range d to 3d, where d is in the range 0.08 metres (3 inches) to 1.07 metres (42 inches), and a length of at least 10D.

The CEC is an electrostatic coalescer in which, by comparison with a conventional electrostatic coalescer, the charged electrodes are relatively closely spaced, the flow of production stream through the coalescer is rapid, and gravity separation within the coalescer is minimal.

The range of the variable 'd' , used in the dimensional definitions of the CILOWS, is the range of usual pipe diameters utilised in well production stream processing facilities assuming maximum flow velocity in a liquid filled pipeline of 2-6m/sec (Norsok standard) with a usual velocity of less than 3m/sec. This range of pipe "sizes" is well understood in the oil industry and in a processing facility the size 'd' in the range is chosen in accordance with operating parameters such as production flow rate, pressure loss, noise, vibration and erosion. Generally 'd' is a pipe external diameter but it will be recognised that where small sizes are concerned the industry refers to the pipe internal diameter when specifying pipe sizes.

Preferably d is in the range 0.16m (6 inches) to 0.86m (34 inches).

Preferably the length of the CILOWS does not exceed 400D.

Desirably the CEC is arranged such that the flow therethrough is substantially vertical in use. More desirably the flow enters the CEC adjacent the uppermost end of the CEC and flows downwardly therein. Conveniently the CEC has an outlet for treated mixture adjacent its lowermost end.

Alternatively the CEC has inlet and outlet means adjacent its lower end and the mixture flows upwardly then downwardly along the length of the CEC.

Preferably the CEC incorporates a degasser device for removing gas from the mixture being treated. More preferably said device is a cyclone separator.

Desirably the CILOWS inlet end is directly connected to the CEC outlet.

Conveniently the CEC is disposed downstream of a three phase gravity separator which pretreats an oil/water mixture, the CEC being coupled to the gravity separator to receive the oil continuous phase therefrom in use, the gas and water continuous phases of the gravity separator being processed elsewhere. Alternatively the CEC is disposed downstream of a two phase gravity separator which pretreats an oil/ water mixture, the CEC being coupled to the gravity separator to receive the oil continuous phase therefrom in use, the water continuous phase of the gravity separator being processed elsewhere.

Desirably the downstream end of the CILOWS is bifurcated to diverge into upper and lower limbs terminating in oil and water outlets respectively.

Alternatively the downstream end of the CILOWS has upper and lower outlets for oil and water respectively.

Brief Description of the Drawings

One example of the invention is illustrated in the accompanying drawings wherein: -

Figure 1 is a diagrammatic representation of a separator assembly;

Figure 2 is a view similar to Figure 1 of a modification of the assembly of Figure 1 to include de-gassing;

Figure 3 is a view similar to Figure 1 illustrating the assembly used downstream a conventional three-phase separator;

Figure 4 is a view similar to Figure 3 but showing a conventional two- phase separator upstream of the assembly;

Figure 5 is a view similar to Figure 1 but illustrating a hydrocyclone separator upstream of the assembly; Figure 6 illustrates a modification of the assembly of Figure 1 and further illustrates two alternative outlet arrangements;

Figure 7 is a view similar to Figure 6 of a further modification; and

Figures 8, 9 and 10 are diagrammatic views respectively of three alternative forms of compact electrostatic coalescer for use in the assembly; and

Figure 11 is a table and diagram of dimensions.

Preferred Modes for Performing the Invention

Referring first to Figure 1 the separator assembly is arranged to separate oil and water from an oil continuous well production stream. The assembly is intended to be used on a production platform or a sub-sea skid, where space constraints are important. Prior to being supplied to the assembly the well stream will have been subjected to pre-processing in any one of a number of different ways dependent upon the composition, pressure, temperature, flow rate, and other parameters of the production stream issuing from the well head. Many of these procedures will to some extent involve a shearing action, if only by passage through pressure reduction valves, and thus large water droplets in the oil continuous stream will have been broken into a plurality of smaller droplets.

As shown in Figure 1 the assembly includes a compact electrostatic coalescer (CEC) 11 having an inlet 12 through which the well stream enters the coalescer and an outlet 13 through which the treated well stream exits the coalescer. Within the coalescer the well stream passes through an electrical field between electrically charged electrodes which promote the coalescence of small water droplets to produce large water droplets. In a conventional electrostatic coalescer the flow through the coalescer is a quiescent (substantially non-turbulent) flow and the electrically charged electrodes of the coalescer are relatively widely spaced. However, a CEC is, as its name suggests, a smaller and more compact apparatus than a conventional electrostatic coalescer for a given capacity. In the CEC the electrically charged electrodes are significantly closer together, conveniently in the form of coaxial spaced cylinders, and the flow rate through the CEC is higher, to the extent that a turbulent flow through the coalescer can be accommodated.

A compact electrostatic coalescer (CEC) is disclosed in United States patent 6136174 the whole content of which is imported herein by this reference. Exact details of the construction of the CEC are not of importance to the present invention.

The treated well stream issuing from the outlet 13 of the coalescer 11 enters a "Compact In-Line Oil/Water Separator" (CILOWS). A CILOWS is an elongate, relatively small diameter, gravity separator and a CILOWS for downhole use is disclosed in US patent 6277286. It is important to note that the CILOWS of the present invention is not intended for downhole use, and so it is not possible to import the teaching of US patent 6277286 directly into this invention. The design of the CILOWS for use in the present application is widely variable dependent upon the nature of the well stream, and the nature of the skid or platform environment into which the CILOWS is to be incorporated. However, it will be understood that a CILOWS, being a relatively small diameter component, which often can be made from standard lengths of standard diameter pipe, is, contrary to intial expectation, admirably suited to use on a skid or platform when it is recognised that it is not essential for the CILOWS to be rectilinear, and that bends can be incorporated in the pipe work of the CILOWS so that the CILOWS can follow a convenient, horizontal, path on the skid or platform. It will be recognised that for a given flow capacity a conventional gravity separator vessel will be of relatively large diameter and relatively short length by comparison with the CILOWS. For example, whereas in a conventional gravity separator the diameter will be large, and the length will be typically in the range of three to five times the diameter, in a CILOWS the diameter will be close to the piping diameter which is normally required for liquid transport on a skid or platform, and typically the length of the CILOWS would be ten to one hundred times the diameter. Thus whereas the bulky nature of the vessel of a conventional gravity separator may preclude its use on a skid or platform through insufficient available space on the skid or platform, a CILOWS may be readily accommodated by virtue of its pipe-like configuration and its ability to accept bends along its length. In practice the CILOWS may be received in the conventional pipe racking of the skid or platform so as to follow a convoluted path on the skid or platform making use of the pipe run space available.

Reverting to Figure 1, it can be seen that the CILOWS 14 receives the flow from the CEC by way of a relatively short length of conventional piping 15. At the opposite end of the CILOWS 14 there are discreet oil and water outlets 16, 17 through which the separated oil and water exit the CILOWS. Various control regimes can be provided, and in Figure 1 it is shown that there is a liquid level monitoring arrangement 18 which monitors the level of liquid upstream of the CEC 11 and supplies a control signal to control equipment, conveniently in the form of a valve 19 in the oil outlet line, to control the flow through the assembly to maintain a predetermined flow of liquid through the CEC. Similarly, a level sensing arrangement 21 monitors the water level adjacent the outlet end of the CILOWS and supplies a control signal to control equipment 22 in the form of a valve in the water outlet line 17 to maintain the water level at the outlet end of the CILOWS well below the oil outlet so that there is no danger of separated water contaminating the oil flow through the outlet 16. Other control environments will of course be within the competence of the skilled man and will be adapted to suit the particular application taking account of factors such as the pressure, temperature, flow rate and composition of the well stream entering the CEC.

As mentioned above the structure of the CILOWS is widely variable dependent upon the application. However, the CILOWS will be constructed from metal pipe, conventionally metal pipe of standard and easily available diameters and lengths, and will have a diameter D in the range d to 3d where d is in the range 0.08m (3 inches) to 1.07m (42 inches). The length of the CILOWS is more difficult to define. However, while the maximum length of the CILOWS is, in theory, unlimited other than by the practical considerations of accommodating the CILOWS, and the law of diminishing returns, the minimum practical length of the CILOWS can be taken to be 10D. It is probable that a practical maximum length of the CILOWS is 400D. It will be recognised that a primary requirement of a gravity separator is to provide sufficient residence time of the liquid in the vessel for the majority of the water droplets to coalesce into a water layer. For a given flow rate, then the longer the vessel the greater is the residence time, and the greater is the proportion of water droplets which can coalesce. However, there is a law of dimmishing returns and, for example, it may not be worth doubling the length of the vessel to permit only a tiny remaining fraction of the water droplets to coalesce.

Figure 11 shows ranges of CILOWS dimensions for use in the present invention together with similar ranges of dimensions for conventional gravity separators of equivalent processing capacity.

In a typical CILOWS the diameter D of the CILOWS will be 2d and the length of the CILOWS will be 100D.

The pipe size (d) range for well production processing will be 0.08m (3 inches) to 1.07m (42 inches). It will be recognised therefore that a small CILOWS may have a diameter in the range 0.08m (3 inches) to 0.24m (9.5 inches) and a length in the range 0.8m (30 inches) to 96m (315 feet). A typical small system therefore might have a diameter of 0.16m (6.3 inches) and a length of 16m (52.5 feet). At the opposite end of the range a large CILOWS might have a diameter in the range 1.07m (42 inches) to 3.21m (10.5 feet) with a typical diameter of 2.14m (7 feet), and a length in the range of 10.7m (35 feet) to 1284m (4213 feet) with a typical length being 214m (702 feet). For comparison a standard conventional two- or three- phase gravity separator will have a vessel diameter Y in the range 6d to 20d (using the same values for d given above) and will have a length in the range 2.5Y to 6Y. A CILOWS comparable in capacity to a conventional gravity separator 0.48m (1.58 feet) in diameter and 1.2m (3.94 feet) in length would be 0.08m (3 inches) diameter and 8m (26.25 feet) in length.

The assembly illustrated in Figure 2 is arranged to accommodate a well stream where hydrocarbon gas forms part of the fluid mixture entering the CEC. Thus the CEC 11 incorporates, immediately downstream of the inlet 12, a gas separator device 23 which can take a number of known forms, but which conveniently is a gas/liquid cyclone separator. Thus immediately the well stream passes through the inlet 12 it is subjected to gas separation at the separator 23 and a de-gassed oil continuous oil/water mixture passes to the electrostatic coalescing part of the CEC. The separated gas issues from an upper gas outlet 24 of the CEC into a gas discharge line and additional to the controls described above with reference to Figure 1 gas pressure in the CEC is controlled by control equipment, conveniently a valve 26 in the gas output line, the valve 26 receiving control signals from a pressure monitor 25 monitoring the gas pressure in the CEC.

Figure 3 illustrates that the assembly of a CEC 11 and CILOWS 14 can be used downstream of a conventional three-phase separator. It will be understood that electrostatic coalescers function satisfactorily only where the oil/water mixtures to be treated are oil continuous. If the mixtures were water contmuous then electrical shorting of the charged electrodes of the coalescer would occur. Thus where the production stream from the well head is not oil continuous some of the water must be removed to produce an oil continuous feed to supply to the CEC. In Figure 3 the production stream enters a conventional three-phase gravity separator which separates the production stream into a gas component, a water continuous component, and an oil continuous component. It can be seen that the gravity separator vessel 27 has a well stream inlet 28 adjacent one end, and gas, water and oil outlets 29, 31, 32 adjacent the opposite end of the vessel 27.

The gas pressure within the vessel 27 is controlled by a valve 34 in the gas outlet line in combination with a pressure sensor 33 monitoring the pressure within the vessel 27. The water continuous component from the vessel 27 flows through a water discharge line coupled to the outlet 31 , the water discharge line containing a valve 36 controlled by a level sensor 35 in the vessel 27 monitoring the height of the water layer in the vessel 27 to ensure that the water layer does not overflow an internal weir and contaminate the oil layer. Water discharged from the vessel 27 may well still contain oil, and in this case may be subjected to further processing prior to discharge into the environment.

The oil continuous component from the vessel 27 flows from the outlet 32 through a short section of conventional piping to the inlet 12 of the CEC 11. It is recognised that further gas may be evolved from the oil continuous component in the CEC, and thus the CEC incorporates a gas bleed line 37 for periodically or continuously bleeding any collected gas from the CEC. The gas can be recombined with the oil exiting the CILOWS, although if pressure conditions permit it the gas could be bled into the main gas discharge line from the outlet 29 of the vessel 27.

It can be seen that the water outlet line 17 from the CILOWS communicates with the main water discharge line from the vessel 27 although if desired the water from the CILOWS could be discharged separately. The control arrangement differs from the control arrangement of Figure 1 in that the valve 19 in the oil outlet line 16 of the CILOWS receives signals from an oil level control sensor 38 of the vessel 27 to reduce the flow through the system in the event that the oil level falls below a predetermined level in the vessel 27. Figure 4 illustrates an arrangement similar to the arrangement shown in Figure 3 but utilising a conventional two-phase gravity separator upstream of the CEC. Such an arrangement may be used where the well stream is oil continuous naturally, or has been treated elsewhere to produce an oil continuous stream. As the oil continuous stream enters the vessel 39 of the two-phase separator by way of an inlet 41 and gas is discharged as described above in relation to Figure 3. The de-gassed oil continuous oil/water mixture is fed to the inlet 12 of the CEC 11, and the arrangement operates as described with reference to Figure 3 with the exception that there is no water continuous phase exiting the vessel 39.

Turning now to the example illustrated in Figure 5 the combined CEC and CILOWS assembly is used downstream of a de-oiling hydrocyclone, to de- water the oil reject from the hydrocyclone. It will be recognised that while the hydrocyclone 42 in Figure 5 is shown as a single component it is most probable that the de-oiling hydrocyclone will be a plurality of hydrocyclone liners in one or more hydrocyclone packages and the oil overflow or reject of each line will discharge an oil continuous mixture of oil and water into an outlet line 43 connected to the CEC inlet 12. The water continuous underflow of the hydrocyclone 42 flows through a water discharge line 44 containing a control valve 36. The separated water at the outlet end of the CILOWS flows through the line 17 and the control valve 22 into the line 44 downstream of the valve 36. Again it is recognised that further gas may be evolved from the oil continuous mixture entering the CEC 11 , thus the CEC is provided with a gas bleed line 37 whereby evolved gas can be recombined with the separated oil downstream of the valve 19 in the line 16 from the CILOWS 14. It is probable that a water-cut meter will monitor the composition of the water continuous discharge from the hydrocyclone underflow, and will control the setting of the valve 36 accordingly.

As an alternative to removing gas in or upstream of the CEC it is anticipated that gas could be permitted to pass with the liquid into the CILOWS and for the CILOWS to incorporate a gas outlet whereby the CILOWS acts as a three-phase separator rather than an oil/ water separator. As a still further alternative it may be possible for the gas and oil to exit the CILOWS together for separation downstream of the CILOWS. Sand and other sediments will normally be flushed through the CILOWS with the water layer.

Figure 6 illustrates a combination of CEC 11 and CILOWS 14 where the vertically arranged CEC 11 has a concentric bottom spool providing the inlet 12 and the outlet of the CEC. Thus oil continuous oil/water mixture flows through the inlet 12 vertically within the CEC to a high point of the CEC and then flows downwardly through concentric electrically charged cylindrical plates of the CEC to the outlet of the CEC which is concentric with the inlet assembly 12. Furthermore, Figure 6 shows that the CILOWS 14 is of conventional, standard, pipe diameter, and preferably is formed from conventional pipe segments bolted together. The inlet end of the CILOWS is bolted directly to the outlet port of the CEC so there is no intervening separate pipework.

Figure 6 also shows two alternative CILOWS outlet assemblies. The nature of the outlet assembly of the CILOWS is not crucial to the present invention and in the main part of Figure 6 it can be seen that the oil and water outlets 16, 17 are generally normal to the length of the CILOWS, and thus receive oil and water from the upper and lower periphery of the end region of the CILOWS tube respectively. In Figure 6a the outlet end region of the CILOWS tube is bifurcated and internally partitioned so that outlet pipes 16, 17 for oil and water respectively diverge from the end of the CILOWS 14. International patent application WO 01/78860 discloses a convenient CILOWS outlet arrangement. Although the CILOWS of WO 01/78860 is a downhole device the outlet arrangement described therein could be utilised in the CEC/ CILOWS assembly of the present invention.

Figure 7 shows an arrangement similar to Figure 6 with the exception that the outlet port of the concentric bottom spool of the CEC is coupled to the inlet end of the CILOWS 14 through a short length of conventional piping 15 of smaller diameter than the diameter of the CILOWS 14. Although gravity separation of the flow from the coalescer commences immediately, and may actually commence in the pipe 15 so that a stratified flow enters the larger diameter region of the CILOWS 14, it is to be understood that irrespective of the use or absence of a concentric bottom spool of the CEC, the increased CILOWS diameter permits better level control of the separated oil and water phases adjacent the outlet end of the CILOWS.

Figures 8 and 9 are enlarged views of two alternative CEC inlet arrangements described above. In Figure 8 the inlet to the CEC is at the upper end of the CEC enclosure and the outlet is at the lower end, the oil continuous oil/water mixture flowing downwardly between the plates of the electrostatic coalescer. In Figure 9, the concentric bottom spool arrangement is utilised so that both inlet and outlet connections are made at the lower end of the CEC, the inlet mixture flowing from the bottom, to the top of the CEC enclosure by means of an internal, centrally disposed pipe. Figure 10 illustrates a CEC of the form shown in Figure 9, but incorporating a gas/liquid cyclone or other form of de-gassing device 23 receiving the oil continuous flow from the lower inlet 12 by way of the internal, centrally disposed feed pipe mentioned above. The CEC housing also has a gas outlet 24 for removal of the gas separated from the oil continuous mixture by the device 23.

In all of the examples described above the CEC supplies treated oil continuous mixture to a single CILOWS unit. It is to be understood that in some applications it may be possible for a single CEC to supply mixture to two or more CILOWS units operating in parallel. Moreover all of the CECs described above are operated in a vertical orientation with a downward flow. Horizontal orientation of the CEC is possible but undesirable as water might tend to pool (coalesce into large drops) between adjacent charged plates and risk short circuiting. Vertical orientation of the CEC with upward flow through the CEC is a possibility as are skew orientations of the CEC between vertical and horizontal.

Claims

CLAIMS :-

1. An oil/water separation arrangement characterised by comprising a compact electrostatic coalescer (CEC) (11) having an inlet (12) for receiving a well production stream including, under normal operating conditions, an oil continuous phase containing water droplets, a processing chamber in which the stream passes through an electrical field between charged electrodes which promotes coalescence of the water droplets, and an outlet (13) for well production stream which has been subjected to electrostatic coalescence, the oil/water separation arrangement further including a compact in-line oil/water separator (CILOWS) (14) receiving production stream from the outlet (13) of said CEC, said CILOWS having a separation vessel diameter D in the range d to 3d, where d is in the range 0.08 metres (3 inches) to 1.07 metres (42 inches), and a length of at least 10D.

2. An oil/water separation arrangement as claimed in claim 1 characterised in that said CEC is an electrostatic coalescer in which the charged electrodes are relatively closely spaced, the flow of production stream through the coalescer is rapid, and gravity separation within the coalescer is minimal.

3. An oil/water separation arrangement as claimed in claim 1 or claim 2 characterised in that d is in the range 0.16m (6 inches) to 0.86m (34 inches).

4. An oil/water separation arrangement as claimed in any one of claims 1 to 3 characterised in that the length of the CILOWS does not exceed 400D.

5. An oil/water separation arrangement as claimed in any one of claims 1 to 4 characterised in that the CEC is arranged such that the flow therethrough is substantially vertical in use.

6. An oil/ water separation arrangement as claimed in claim 5 characterised in that the flow enters the CEC adjacent the uppermost end of the CEC and flows downwardly therein.

7. An oil/water separation arrangement as claimed in claim 6 characterised in that the CEC has an outlet for treated mixture adjacent its lowermost end.

8. An oil/ water separation arrangement as claimed in claim 5 characterised in that the CEC has inlet and outlet means adjacent its lower end and the mixture flows upwardly then downwardly along the length of the CEC.

9. An oil/water separation arrangement as claimed in any one of the preceding claims characterised in that the CEC incorporates a degasser device for removing gas from the mixture being treated.

10. An oil/water separation arrangement as claimed in claim 9 characterised in that said device is a cyclone separator.

11. An oil/water separation arrangement as claimed in any one of the preceding claims characterised in that the CILOWS inlet end is directly connected to the CEC outlet.

12. An oil/ water separation arrangement as claimed in any one of the preceding claims characterised in that the CEC is disposed downstream of a three phase gravity separator which pretreats an oil/ water mixture, the CEC being coupled to the gravity separator to receive the oil continuous phase therefrom in use, the gas and water continuous phases of the gravity separator being processed elsewhere.

13. An oil/ water separation arrangement as claimed in any one of the preceding claims 1 to 11 , characterised in that the CEC is disposed downstream of a two phase gravity separator which pretreats an oil/water mixture, the CEC being coupled to the gravity separator to receive the oil continuous phase therefrom in use, the water continuous phase of the gravity separator being processed elsewhere.

14. An oil/ water separation arrangement as claimed in any one of the preceding claims characterised in that the downstream end of the CILOWS is bifurcated to diverge into upper and lower limbs terminating in oil and water outlets respectively.

15. An oil/ water separation arrangement as claimed in any one of the preceding claims 1 to 13, characterised in that that the downstream end of the CILOWS has upper and lower outlets for oil and water respectively.