AU5995899A

AU5995899A - Separation of a mixture of liquid hydrocarbons and water

Info

Publication number: AU5995899A
Application number: AU59958/99A
Authority: AU
Inventors: Bjorn Christiansen; Jan Hoydahl; Gunnar Vangen
Original assignee: Den Norske Stats Oljeselskap AS; Kvaerner Process Systems AS
Current assignee: Fjords Processing AS; Equinor ASA
Priority date: 1998-10-23
Filing date: 1999-10-20
Publication date: 2000-05-15
Also published as: NO20011944L; EP1128882A1; NO20011944D0; WO2000024493A1; BR9914760A; GB9823136D0; CA2348790A1

Description

WO 00/24493 PCT/IB99/01714 1 SEPARATION OF A MIXTURE OF LIQUID HYDROCARBONS AND WATER This invention relates to apparatus for, and a method of, separating a mixture of hydrocarbon liquid and aqueous liquid into discrete hydrocarbon liquid, and aqueous liquid phases. The invention is primarily concerned with the three phase separation of a production flow from an oil well, and although strictly speaking the mixed phases within such a production flow are gas, hydrocarbon liquid, and aqueous liquid, it is conventional in the industry to refer to the phases as gas, oil and water. It must also be recognised that there are oil wells which have a production flow free, or substantially free, of gas, either because no gas is present or because gas is removed as a first operation, and thus the invention disclosed herein encompasses the processing of gas free oil and water mixtures. It is known to supply a gas, oil and water mixture to a three phase gravity separator wherein the three phases separate, under gravity, into discrete layers within a separator vessel. It is also known to utilize a gas/liquid separator as an inlet component of the gravity separator to perform a preliminary separation of the inlet mixture into a gas phase and a liquid phase, a cyclone separator being convenient for this purpose. It is well understood in the art that although a large majority of the gaseous phase will separate quickly from the liquid phase of the mixture, the oil and water phases take much longer to separate, and thus a particular residence time within the separation vessel is needed to achieve adequate separation of the oil and water phases. The residence time, together with the capacity of the vessel are primary factors in determining the volume of mixture which WO 00/24493 PCT/IB99/01714 2 can be processed by the gravity separator. It is an object of the present invention to augment the separation of the oil and water phases taking place in the vessel so as to provide an increased separation capacity. In accordance with the present invention there is provided a separation apparatus comprising a gravity separator vessel having, an inlet through which mixture to be separated enters the vessel, water and oil outlets spaced from the inlet of the vessel so as to afford a predetermined residence time within the vessel, a further liquid outlet from the vessel, and a further separator receiving liquid from the gravity separation vessel by way of said further liquid outlet, said further separator separating said liquid into oil rich and water rich flows. Desirably said further separator is a centrifugal separator. Preferably said further separator is a hydrocyclone. Preferably the apparatus further includes a monitoring and control arrangement for controlling the flow rate of liquid to said further separator in accordance with the nature of the liquid flowing to the further separator. Conveniently said monitoring and control arrangement is responsive to whether said liquid is an oil continuous mixture or a water continuous mixture. Preferably said monitoring and control arrangement is responsive to the oil/water ratio of said liquid.

WO 00/24493 PCT/IB99/01714 3 Preferably said monitoring and control arrangement includes a watercut meter. Conveniently there is provided a second further liquid outlet from said vessel and a second further separator receiving liquid from the vessel by way of said second further liquid outlet. Alternatively there is provided a second further liquid outlet from the vessel and said second further liquid outlet communicates with the first mentioned further outlet upstream of said further separator which thus, in use, receives a mixture of the liquid mixtures extracted from the vessel by way of the first and second further liquid outlets. Preferably each of said first and second further liquid outlets has its own monitoring and control arrangement for controlling the flow rate of liquid extracted by way of that outlet. The invention further resides in a method of separating an oil and water mixture comprising the steps of supplying the mixture into the vessel of a gravity separator, the vessel having an inlet through which mixture to be separated enters the vessel, water and oil outlets spaced from the inlet of the vessel so as to afford a predetermined residence time within the vessel, and a further liquid outlet from the vessel, and supplying liquid from said vessel by way of said further liquid outlet to a further separator which separates said liquid into oil rich and water rich flows. Preferably the method includes the further step of extracting further liquid from the vessel by way of a second further outlet.

WO 00/24493 PCT/IB99/01714 4 Conveniently said liquid extracted by way of said second further outlet is processed in a second further separator. Alternatively liquid extracted by way of said second further outlet is mixed with the liquid extracted by way of the first mentioned further outlet for processing by the further separator. In the accompanying drawings: Figure 1 is a diagrammatic representation of a basic separation apparatus in accordance with a first example of the present invention; and Figures 2, 3, 4, 5 and 6 are, respectively, diagrammatic representations of separation apparatus in accordance with second, third, fourth fifth and sixth examples respectively of the present invention. Referring first to Figure 1 of the drawings the separation apparatus includes a generally conventional three phase gravity separator 11 including a horizontally elongate separation vessel 12. The vessel 12 has an inlet assembly 13 at one end thereof by way of which a production flow comprising a mixture of hydrocarbon gas, hydrocarbon liquid and aqueous liquid (conventionally referred to as gas, oil and water) enters the vessel 12. In the example illustrated in Figure 1 the inlet assembly 13 of the gravity separator 11 includes a gas/liquid cyclone separator 14 through which the whole of the inlet flow passes. The cyclone separator 14 separates the inlet flow into a gas phase and a liquid phase, and discharges the gas phase into a head space 15 of the vessel 12. The head space 15 has an outlet 16 by way of which separated gas can flow from the vessel 12 for collection and/or further processing.

WO 00/24493 PCT/IB99/01714 5 The liquid phase discharged from the underflow outlet of the separator 14 occupies the volume of the vessel 12 below the head space 15 and immediately commences to separate under gravity into an oil phase floating on a water phase. However, there is an intermediate phase comprising an oil/water dispersion the depth of which diminishes with distance from the inlet assembly 13 in accordance with the residence time of the liquid in the vessel 12. It will be understood that the majority of the gas in the inlet mixture is separated therefrom by the separator 14, although further dissolved gas may be evolved from the liquid phase while it remains resident in the vessel, this further evolved gas collecting in the head space. A perforated plate 17 is positioned within the vessel 12 adjacent the inlet assembly 13, 14 and divides at least the liquid containing part of the vessel 12 into an inlet section and a settling section, the plate 17 serving to minimise the effect on the liquid in the settling section of any turbulence which may arise in the region of the inlet assembly. Adjacent its end remote from the inlet the vessel 12 has a vertically extending weir plate 18 occupying the full width of the vessel and extending upwardly from the base of the vessel to a height in excess of the maximum intended height of the top of the water and oil / water dispersion layers in that end region of the vessel. The oil layer in use overflows the weir plate 18 so that the far end region of the vessel 12, downstream of the weir plate, defines an oil outlet region 19 communicating with an oil outlet line 21 containing a control valve 21a. The oil flow from the vessel 12 by way of the line 21 is controlled by the valve 21a, and, as will be understood by experts in the art, the oil in the line 21 may still contain a small proportion of water, and dependent upon the quantity of retained water in the oil in the line 21 the oil may be classed as "export quality" in which case it may be transported from the production site for further processing elsewhere, or alternatively may be subject to a further water removal process, WO 00/24493 PCT/IB99/01714 6 for example in a coalescer before being classed as "export quality". It follows therefore that the final destination of the line 21 will be determined by the quality of the oil in the line 21, but this is not of significance to the present invention. Upstream of the weir plate 18, that is to say on the side of the weir plate presented to the inlet assembly 13, 14, the vessel 12 has a water outlet line 22. The water outlet line communicates with a low point in the vessel 12 adjacent the weir plate 18, so that the water extracted through the outlet line 22 is the purest water stratum within the vessel. However it is recognised that the water extracted from the line 22 will still contain some oil and so may require further processing to minimise its oil content before the produced water can be discharged into the environment. The apparatus so far described in relation to Figure 1 is conventional and it will be understood by those skilled in the art that the processing capacity of the gravity separator 12, in relation to a given production flow composition entering the inlet 13, is determined by the dimensions of the vessel 12 and the desired residence time during which the oil and water phases separate under gravity. It will also be recognised that the apparatus described above could be modified by the substitution of a convenient form of external gas/liquid separator upstream of the inlet assembly 13, and external of the vessel 12, in place of the internal cyclone assembly 14, so that the vessel 12 is solely concerned with the separation of the oil and water phases of the production flow, as would be the situation in the event of the well production flow being naturally gas free. Moreover it will be understood the form of the vessel 12 and the construction of its oil outlet arrangement may vary widely from that shown in Figure 1.

WO 00/24493 PCT/IB99/01714 7 For example the oil outlet arrangement could incorporate a known "bucket and pipe" weir arrangement rather than the weir plate 18 illustrated The arrangement shown in Figure 1 differs from known arrangements in that its processing capacity is augmented by the addition of a liquid extraction line 26 through which part of the liquid phase in the vessel 12 is extracted for parallel processing. The liquid phase in the vessel 12 will have been substantially degassed as it enters vessel 12, either externally or by an arrangement such as the gas/liquid cyclone 14. Thus the liquid phase extracted through the line 26 is a degassed liquid and so is suitable for further separation into the individual liquid phases by means of one or more hydrocyclones. The line 26 is connected through a watercut meter 27 to the inlet of a pre deoiler hydrocyclone 28 which separates the mixture flowing into it from the line 26 into an oil rich phase flowing from the overflow outlet of the hydrocyclone 28 into a line 29, and a water rich phase leaving the underflow of the hydrocyclone 28 into a line 31. In the specific example illustrated in Figure 1 the line 26 communicates with the vessel 12 upstream of the perforated baffle plate 17 and so is closely proximate the discharge of the separator 14. Furthermore, the line 26 opens into the interior of the vessel close to the low point thereof so as to take liquid which has undergone partial separation, and so is a water continuous, oil / water mixture. However the point at which the line 26 extracts liquid from the vessel can be selected to achieve optimum augmentation of the operation of the vessel 12. For example it is recognised that a pre-deoiler hydrocyclone is more effective when operating on a mixture of constant oil/water ratio and extraction from the less turbulent region downstream of the plate 17 may be preferred in some arrangements. Control valves 29a, 31 a are positioned in the lines 29, WO 00/24493 PCT/IB99/01714 8 31 respectively, and as mentioned previously, the ultimate destination of the various output lines is not of significance to the present invention. The watercut meter 27 is preferably a microwave meter which can output a control signal representative of the oil-to-water ratio of the liquid mixture in the line 26 and this control signal can be used to control the settings of the valve 31 a to control the rate of flow of liquid through the hydrocyclone 28, and thus the rate at which liquid is being extracted from the vessel 12 through the line 26. When the meter 27 detects that the oil content of the flow extracted through line 26 is above a predetermined value, indicating that extraction of the water/oil dispersion layer is commencing, the valve 31a is adjusted towards its closed condition to reduce the flow through the line 26. Conversely when the meter detects that the watercut of the mixture is too high the valve will be opened to increase the flow through line 26. In one example the nature of the oil/water mixture is such that its oil/water inversion point (the point at which the mixture becomes water continuous) occurs at a watercut of 70%, and thus 70% watercut would be an ideal setting for the meter 27. However in practice it is necessary to allow an operating tolerance and so a setting in the range 75 % - 85 %, conveniently 80%, is selected so that the valve 31a_is caused to open should the meter 27 detect that the watercut of the liquid in the line 26 exceeds 80%, and the valve 31a is caused to close when the detected watercut falls below 80%. In a modification the signal from the meter 27 is used in conjunction with differential pressure monitors sensing respectively the pressure differential Dpl between the input of the cyclone 28 and its overflow output into the line 29, and the pressure differential Dp2 between the input of the cyclone 28 and its underflow output into the line 31. There is a known (determined from WO 00/24493 PCT/IB99/01714 9 experiments) unique relationship between the measured pressure ratio PDR, defined as PDR=Dpl/Dp2, and the flow split F across the hydrocyclone, defined as F=QverflWI,/Qip.et. In other words, from the measured PDR the flow split F can be determined. F is regulated to maintain the following relation: F = (1 - WCmeasured + DF), where WC is the watercut of the liquid extracted from the vessel and DF is a safety margin determined by the operator (typically 5-10%), the margin being needed since in order to get a "clean" water-phase one has to reject more fluid than the incoming oil fraction (oil fraction = 1-WC). Example: WC-me=0.85 (i.e. 85%water 15% oil) and DF=0.05 (5%) giving a desired split ratio of F=(1 - 0.85) + 0.05 = 0.20 (20%). The required PDR giving this split ratio of 20% is known from experimental data and valve 29a is then regulated to achieve this PDR value. Optionally, one can use the watercut set value instead of the measured watercut. The PDR ratio would then be fixed as for a de-oiling cyclone regulating routine. F is regulated to maintain the following relation: (F=(1 WC set value) + DF). Example WC set value = 0.8 (80%) and DF=0.05 (5%) giving a desired split ratio of F=(1 - 0.8) + 0.05 = 0.25 (25%). PDR is then known and valve 29a is then regulated to achieve this value. In the example described above with reference to Figure 1 the line 26 is positioned to receive a water continuous mixture resulting from partial separation of the inlet mixture in the inlet zone of the vessel 12. However, it is to be understood that in some applications the line 26 could be positioned to receive an oil continuous mixture with the hydrocyclone processing of the WO 00/24493 PCT/IB99/01714 10 flow along the line 26 being arranged in a number of hydrocyclone stages to de-water oil continuous flows. Furthermore the gravity separation may be augmented by extraction of both water continuous and oil continuous phases separately from the vessel 12. Where a hydrocyclone processes an oil continuous phase extracted from the vessel 12 the oil rich overflow of the hydrocyclone may be pumped back into the vessel 12 while the underflow is routed for further processing before the water is discharged. Alternatively it will be recognised that if there is one or more lower pressure separation vessels downstream of the vessel 12, as is often the case in gravity separation systems, then the overflow can be fed, without the need to pump, into the oil layer in a downstream vessel. Similarly the underflow can be fed to a pre deoiler or deoiler hydrocyclone, or to the water layer of a downstream vessel dependent upon its oil/water ratio. The extraction of an oil continuous mixture could be of particular advantage where the vessel is receiving the production flow from a well producing a mixture of very high oil content. It is further to be understood that in some gravity separator applications the perforated baffle plate 17 is omitted, and thus there is not a readily defined inlet zone with which the line 26 communicates. Ideally the line 26 takes liquid from the vessel 12 proximate the inlet, but advantages can be obtained by extracting liquid phase anywhere along the vessel upstream of the weir plate 18 or its equivalent, but desirably between the inlet assembly and midway between the inlet assembly and the weir plate 18. Moreover there could be two or more spaced perforated baffle plates 17 with the line 26 extracting liquid between a pair of plates 17. The positioning both longitudinally of the vessel, and vertically within the vessel, of the intake end of the line 26 will be determined by experience in relation to the geometric configuration of the vessel 12 itself, and the WO 00/24493 PCT/IB99/01714 11 constituents of the production flow entering the inlet 13 and their gravity separation characteristics. The intake end of the line 26 can be vertical or horizontal and can be located in any of the water, oil, and oil/water dispersion zones. It is to be recognised that other forms of inlet assembly 13, 14 can be utilized in a gravity separator. It is, for example, not essential that the inlet assembly includes a gas/liquid separator in the form of a cyclone separator. A well known form of gravity separation vessel has an inlet assembly in which a nozzle directs the inlet flow onto a splash plate which deflects the flow downwardly into one end region of the vessel. The effect of the splash plate is to promote evolution of gas from solution, and so acts to de-gas the inlet flow. However, the turbulence generated by the splash plate also can emulsify the oil and liquid mixture rendering it more difficult to separate under gravity. Nevertheless it is to be understood that the invention can be utilized in relation to gravity separators having splash plate type inlet assemblies or other types of inlet distributor. Referring now to Figure 2 it can be seen that equipment ancillary to the vessel 12 and hydrocyclone 28 is shown. The assembly of Figure 2 represents a convenient separation system. The line 22 feeds the inlet of a deoiler hydrocyclone 23. As is conventional the hydrocyclone 23 has an overflow outlet and an underflow outlet, the overflow outlet communicating with a line 24 through a control valve 24a and the underflow outlet communicating with a line 25 through a control valve 25a. The function of the deoiler hydrocyclone 23 is to separate substantially the whole of the oil content of its inlet flow and thus the liquid flow from the underflow outlet of the hydrocyclone 23 into the line 25 is clean water, the purity of which is such that it is suitable for discharge into the environment if so desired. The WO 00/24493 PCT/IB99/01714 12 liquid flowing from the overflow outlet of the hydrocyclone 23 into the line 24 is a mixture of water and substantially the whole of the oil separated from the inlet flow to the hydrocyclone 23, and this passes along the line 24 for further treatment to recover the oil phase, and to clean the water phase to a point at which it can be discharged to the environment if desired. Again the destination of the lines 24, 25 is not of significance to the present invention and will be well understood by those skilled in the art. The line 31 carrying the underflow of the hydrocyclone 28 supplies the inlet of a deoiler hydrocyclone 32. The output of the deoiler hydrocyclone 32 is similar to the output of the deoiler cyclone 23, the oil leaving the hydrocyclone 32 by way of the overflow outlet and a line 33 as an oil and water mixture, and the clean water leaving the underflow outlet of the hydrocyclone 32 by way of a line 34. The lines 33 and 34 contain control valves 33a, 34a and the line 33 communicates with the overflow line 24 of the hydrocyclone 23 while the line 34 communicates with the underflow line 25 of the hydrocyclone 23. It will be noted that the valve in the line 31 is omitted, its control function under the action of the meter 27, being performed by the valve 34a. The remaining control valves in the output lines of the hydrocyclones 23, 32, and in the line 21 will be controlled in known manner in accordance with other parameters of the system, such for example as fluid levels in the vessel 12, to achieve given operating characteristics of the system. It is however within the ambit of the present invention to provide a control system which provides control signals for all of the control valves, or to have all of the valves manually controlled in accordance with analysis in use of the liquid mixtures in the various output lines.

WO 00/24493 PCT/IB99/01714 13 Figures 1 and 2 illustrate the line 26 extending through the lower wall region of the vessel 12 at the inlet end of the vessel. It is to be understood however that there will be occasions in which it is desirable to retrofit the line 26 and its associated equipment to an existing gravity separator vessel 12. In such situations it may be desirable for the line 26 to penetrate the wall adjacent the lines 21, 22 in which case the line 26 will extend within the vessel 12 to the point at which it is desired to extract liquid from the vessel. For example, in one embodiment the line 26 extends into the vessel 12 through the union defining the line 22, the line 26 extending physically, coaxially within the line 22 through a common union into the interior of the vessel 12. Within the vessel 12 the line 26 extends axially to the point at which it is desired to extract liquid whereas of course the line 22 extracts water from a point adjacent the weir plate 18. Externally of the vessel 12 the line 26 is led through the wall of the line 22 so that the line 22 feeds the hydrocyclone 23 while the line 26 feeds the hydrocyclone 28 through the intermediary of the watercut meter 27. It will be understood that other physical arrangements of the line 26, and the manner in which it penetrates the wall of the vessel 12 are possible. Figure 3 illustrates an embodiment of the invention in which the vessel 12 has an inlet assembly 13, 14 disposed midway along the length of the vessel 12 with the liquid discharged from the inlet assembly 13, 14 flowing outwardly in opposite directions to duplicated oil and water outlet lines 21, 22 at opposite ends respectively of the vessel. In effect the gravity separator system of Figure 3 is a pair of gravity separators of the kind shown in Figure 1 positioned back-to-back and sharing a common inlet assembly 13, 14. In addition of course there is only a single additional liquid extraction line 26. It is to be understood that the arrangement illustrated in Figure 3, and indeed WO 00/24493 PCT/IB99/01714 14 the arrangement illustrated in Figures 1 and 2 if desired, could have more than one liquid extraction line 26 feeding respective hydrocyclones. Figure 4 illustrates an adaptation of the gravity separator construction of Figure 3 in which the vessel 12 is divided horizontally into upper and lower chambers which communicate with one another at opposite axial ends of the vessel. Parts common the arrangement in Figure 3 carry the same reference numerals, and it can be seen that the horizontal dividing wall 36 has a centrally disposed well 37 into which the inlet assembly extends. The line 26 extends upwardly through the bottom wall of the vessel 12 and through the dividing wall 36 to extract liquid from the lower region of the well 37 beneath the inlet assembly 13, 14. The dividing wall 36 in effect extends the flow path of liquid between the inlet and outlets, liquid flowing outwardly above the wall from the inlet assembly 14 towards the opposite axial ends of the vessel, and then subsequently flowing beneath the wall 36 inwardly from the axial ends towards the central region. The two weir plates 18 are positioned adjacent the central section of the vessel 12 and the vessel can therefore have a single oil outlet line 22 disposed beneath the well 37 of the plate 36. Variants described above in relation to Figures 1 and 2 can generally be utilised in the embodiments illustrated in Figures 3 and 4. It is mentioned above that in each of the described embodiments an additional line 26a can be provided so that there are two (or more) extraction lines. It is suggested that the additional line or lines 26a supply respective hydrocyclones, but in a modification (Figure 5) the flow in the lines 26, 26a is mixed for supply to a single hydrocylone or chain of hydrocylones. Each line 26, 26a could be provided with a respective watercut meter 27 or the like and an associated control valve arrangement 27a for controlling the flow in the respective line 26 in relation to the composition of that flow as described WO 00/24493 PCT/IB99/01714 15 above. Moreover given that the lines 26, 26a will extract liquid from different regions of the vessel then they will carry flows of different composition and it is within the scope of this invention to control the flows in the lines 26 in relation to one another so as to achieve a desired mixed flow composition at the hydrocylone inlet. Figure 6 shows how the control arrangement of lines 26 and 26a of Figure 5 may be changed to facilitate control to achieve a desired mixture composition at the hydrocyclone inlet. A single watercut meter 27 is interposed between the point at which the lines 26, 26a merge and the inlet of the hydrocyclone 28 to monitor the composition of the inlet mixture, and controls the flow in the lines 26, 26a, by controlling valves 27a in the lines 26,26a. Where the flows in two or more lines 26 are to be mixed and supplied to a common hydrocylone the lines 26 will be positioned to extract liquid such that the mixture supplied to the hydrocylone is water continuous. For example it could be advantageous to mix liquid with a high water content extracted from near the bottom of the vessel adjacent the plate 17 with oil/water dispersion extracted from the dispersion layer adjacent the weir plate 18. The resultant mixture from the two lines 26 could be processed by the hydrocylone 28 and the risk of stable dispersion escaping over the weir plate 18 with the separated oil would be correspondingly reduced. In all of the above described examples of the invention and variants thereof the additional liquid removed from the vessel is monitored by a watercut meter and is further separated in a hydrocyclone. A watercut meter is a preferred way of monitoring the nature of the liquid extracted from the vessel since it can proved an accurate guide to the actual oil/water ratio of the liquid. However other less accurate forms of monitoring and control could be utilised in some applications, for example a device which determines WO 00/24493 PCT/IB99/01714 16 simply whether the liquid mixture is water continuous or oil continuous could be used to control the flow in the line 26. Moreover it is not essential that the further separation takes place in a hydrocyclone, other separation devices being capable of use in some applications. Desirably the further separator will be chosen to operate on a centrifugal separation principle, and particularly where the additional extraction from the vessel 12 is an oil continuous mixture a centrifuge may be utilized in place of the hydrocyclone 28.

Claims

1. A separation apparatus for separating a mixture of hydrocarbon liquid and aqueous liquid into discrete hydrocarbon liquid, and aqueous liquid phases comprising, a gravity separator vessel (12) having, an inlet (13) through which mixture to be separated enters the vessel, and, water and oil outlets (22, 21) spaced from the inlet (13) of the vessel so as to afford a predetermined residence time within the vessel, the apparatus being characterised by a further liquid outlet (26) from the vessel, and a further separator (28) receiving liquid from the vessel by way of said further liquid outlet (26), said further separator in use separating said liquid mixture into oil rich and water rich flows.

2. A separation apparatus as claimed in Claim 1, characterised in that said further separator (28) is a centrifugal separator.

3. A separation apparatus as claimed in Claim 2, characterised in that said further separator (28) is a hydrocyclone.

4. A separation apparatus as claimed in any one of Claims 1 to 3, characterised by a monitoring and control arrangement (27) for controlling the flow rate of liquid to said further separator (28) in accordance with the nature of the liquid flowing to the further separator.

5. A separation apparatus as claimed in Claim 4, characterised in that said monitoring and control arrangement (27) is responsive to whether said liquid is an oil continuous mixture or a water continuous mixture. WO 00/24493 PCT/IB99/01714 18

6. A separation apparatus as claimed in Claim 4 or Claim 5, characterised in that said monitoring and control arrangement is responsive to the oil/water ratio of said liquid.

7. A separation apparatus as claimed in any one of Claims 4 to 6, characterised in that said monitoring and control arrangement includes a watercut meter (27).

8. A separation apparatus as claimed in any one of the preceding claims, characterised by a second further liquid outlet (26a) from the vessel (12), and a second further separator receiving liquid from the vessel by way of said second further liquid outlet (26a).

9. A separation apparatus as claimed in any one of the preceding Claims 1 to 7, characterised by a second further liquid outlet (26a) from the vessel (12), said second further outlet (26a) communicating with the first further outlet (26) upstream of said further separator (28) which thus, in use, receives a mixture of the liquid mixtures extracted from said vessel by way of the first and second further liquid outlets (26, 26a).

10. A separation apparatus as claimed in Claim 8 or Claim 9, characterised by a monitoring and control arrangement (27a) for controlling the flow rate of liquid in said second further liquid outlet (26a).

11. A separation apparatus as claimed in any one of the preceding Claims 1 to 3, characterised by a second further liquid outlet (26a) from the vessel (12), said second further outlet (26a) communicating with the first further outlet (26) upstream of said further separator (28) which thus, in use, receives a mixture of the liquid mixtures extracted from said vessel by way of WO 00/24493 PCT/IB99/01714 19 the first and second further liquid outlets (26, 26a), and a monitoring and control arrangement (27, 27a) for monitoring the composition of the mixed flow supplied to the further separator (28) and controlling the flow through the first and second further liquid outlets (26, 26a) to achieve a desired composition of the mixed flow supplied to the further separator (28).

12. A method of separating an oil and water mixture comprising the steps of supplying the mixture into the vessel (12) of a gravity separator, the vessel having an inlet (13) through which mixture to be separated enters the vessel, water and oil outlets (22, 21) spaced from the inlet (13) of the vessel so as to afford a predetermined residence time within the vessel, characterised by supplying liquid from said vessel by way of by further liquid outlet (26) to a further separator (28) which separates said liquid into oil rich and water rich flows.

13. A method as claimed in Claim 12, characterised by extracting further liquid from the vessel by way of a second further outlet (26a).

14. A method as claimed in Claim 13, characterised by supplying the liquid from said second further outlet to a second further separator.

15. A method as claimed in Claim 13, characterised by mixing the liquid from said second further outlet with the liquid from the first further outlet and supplying the resultant mixture to said further separator. WO 00/24493 PCT/IB99/01714 20

16. A method as claimed in Claim 15, characterised by monitoring the composition of the mixed flow supplied to the further separator (28) and controlling the flow through the first and second further liquid outlets (26, 26a) to achieve a desired composition of the mixed flow supplied to the further separator (28).