Abstract:
A method of separating oil and water in a flow-stream through a gravity settling vessel ( 10 ) in which the flow-stream ( 12 ) separates into a lower water layer ( 22 ) and an upper oil layer ( 26 ), includes feeding an off-take stream ( 34 ) of the oil layer and/or an emulsion layer ( 20 ) that forms between the oil layer and the water layer through a compact electrostatic coalescer (CEC) ( 38 ) that has electrically isolated electrodes. The CEC coalesces water droplets in the off-take stream, and the coalesced off-take stream is then returned to the settling vessel. An associated separator apparatus for comprises: a gravity settling vessel ( 10 ); a CEC ( 38 ) including electrically isolated electrodes; a coalescer feed line ( 34 ) configured to provide an off-take stream of an oil phase and/or an emulsion layer from the settling vessel to the CEC; and a return line ( 40 ) from the CEC for returning the off-take stream to the settling vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of, claims priority to and the benefit of, and incorporates herein by reference in its entirety International (PCT) Patent Application No. PCT/IB2005/003263, which was filed on Sep. 9, 2005 and which claimed priority to and the benefit of Great Briton Patent Application No. 0419994.9, filed on Sep. 9, 2004 and also incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for improving the performance of a separator. More particularly the invention relates to a method and apparatus for improving the performance of a separator used for separating crude oil and water in oil production well-streams. 
     The production well-stream from an oil well may contain a mixture of oil and water (and may also include gases, solids and other fluids). The water may arise naturally, or may be a result of water (e.g. seawater) pumped into the well to raise the pressure and force more oil out. Separation of the various constituents is an important part of the processing, which it is desirable to carry out as soon as possible before the crude oil is stored or transported for further processing. Although oil and water are immiscible liquids, separation is particularly problematic when the water is carried in the form of small droplets in a continuous oil phase. In some situations the oil and water form an emulsion of very small water droplets dispersed throughout the oil. 
     Most separation processes include a settling vessel in which oil and water phases separate under gravity, with the heavier water falling to form a water layer below a lighter oil layer. 
     The gravity separation process requires a sufficient residence time in the settling vessel for the water droplets to sink to the water layer. The effectiveness of the separation depends not only on the residence time but also on the nature of the oil and water mixture entering the vessel. In many situations, an emulsion layer forms at the interface between the oil and water layers. The emulsion layer (also known as a rag layer) tends to inhibit the separation and prevent water droplets sinking into the water layer. Furthermore, in these situations the emulsion layer tends to build up over time, further retarding the separation process and reducing the volume of oil and water layers in the settling vessel. 
     It is known to reduce the problem of a build-up of the emulsion layer by removing a portion of this layer through an outlet provided at an appropriate position on the settling vessel. This may be done periodically in a batch operation after the emulsion layer has been allowed to build up for a time. The emulsion removed may be taken away for treatment or disposal. 
     WO 92/19347 and WO 92/19349 describe processes in which a stream is taken from the emulsion layer in a separating vessel and fed into and oil-water hydrocyclone separator before being returned to the separating vessel inlet. A hydrocyclone may be effective in aiding separation of oil droplets from a water-continuous emulsion, but has a problem in that it is only effective over a limited range of oil/water proportions. Also hydrocyclones are not effective in separating water droplets in oil-continuous emulsions. Another problem with using a hydrocyclone is that although some degree of separation and coalesecence of droplets may be achieved, much of this is destroyed at the outlet where the shear forces tend to remix the liquid phases. 
     Even when there is little or no emulsion layer between the oil and water layers, the size of the settling vessel is limited by practical or economic constraints such that the fluids have insufficient residence time in the vessel for the smallest water droplets to drop out of the oil. 
     It is an aim of the present invention to provide a method and apparatus for improving separator performance so as to alleviate the aforementioned problems. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a method of separating oil and water in a flow-stream through a gravity settling vessel in which the flow-stream separates into a lower water layer and an upper oil layer, the method including feeding an off-take stream of said oil layer and/or an emulsion layer that forms between the oil layer and the water layer through a compact electrostatic coalescer (CEC) having electrically isolated electrodes, so as to coalesce water droplets in the off-take stream, and then returning the coalesced off-take stream to the settling vessel. 
     According to a second aspect of the present invention there is provided a separator apparatus for separating oil and water in a flow-stream, comprising: 
     a gravity settling vessel; 
     a compact electrostatic coalescer (CEC) including electrically isolated electrodes; 
     a coalescer feed line configured to provide an off-take stream of an oil phase and/or an emulsion layer from the settling vessel to the CEC; and 
     a return line from the CEC for returning the off-take stream to the settling vessel. 
     The coalescer feed line and/or the return line may be provided to/from respective end locations within the settling vessel. Preferably, the coalescer feed line and/or the return line are provided, at their respective end locations, with one or more orifices at a level inside the settling vessel where fluid treatment is required. 
     A CEC is described in European Patent No. 1082168. The CEC provides significant advantages leading to an improvement in the separation performance. A CEC is effective in coalescing droplets in emulsions for a wide range of oil/water proportions. These include the oil/water proportions that are often found to be the most difficult to separate effectively using gravity separation. When the off-take is taken from the oil phase, particularly the oil phase at or near an oil outlet of the settling vessel, the water droplets are very small. The CEC coalesces such small droplets, and allows a stream containing much larger coalesced droplets to be fed back into the settling vessel. The larger droplets settle quickly into the water phase and thereby enhance the separation process. When the off-take includes a large proportion of emulsion taken from the emulsion layer, the water content may be very high (e.g. 50% or more). Other known types of electrostatic coalescers cannot be used with such high water contents because the water conducts electricity to short-circuit the electrodes or to cause electric arcing. The CEC does not suffer from these problems because it uses electrically isolated electrodes. The compact design of the CEC makes it particularly suitable in off-shore or sub-sea processing due to the relatively small size and weight of the equipment. 
     In one embodiment, the settling vessel has an oil outlet for providing a flow of separated crude oil from the settling vessel and the off-take is taken from the oil outlet. Preferably, the off-take stream, after being fed through the CEC is returned to the settling vessel at or close to a flow-stream inlet of the settling vessel. More preferably, the off-take is returned into the oil phase in the settling vessel. Alternatively, an off-take outlet may be provided on the settling vessel whereby the off-take stream is taken from the oil phase in the settling vessel. 
     In another embodiment, an off-take outlet is provided on the settling vessel whereby the off-take stream is taken from an emulsion layer. Preferably, the off-take stream, after passing through the CEC, is returned to the settling vessel into either the emulsion layer or into the oil phase. The off-take outlet may be provided at a position close to a flow-stream inlet of the settling vessel. The off-take is preferably returned to the settling vessel at a location close to, but a short distance downstream of the off-take outlet. 
     As stated above, an advantage of the CEC when compared with other known electrostatic coalescers is that it can operate to coalesce water droplets even when the water content is high. Consequently, the water droplets in the emulsion layer can be coalesced into larger droplets. This assists both in breaking down the emulsion and in separating the water from the oil phase. Also, by returning the off-take stream with the coalesced water droplets back to the settling vessel at a position that is only a short distance downstream of the off-take outlet, there is a minimal effect on the overall residence time of the flow stream in the settling vessel (when compared, for example, with systems that recycle the off-take back to the inlet of the settling vessel where the effect of recycling is to reduce the residence time). If the emulsion contains a relatively small water fraction, then the CEC may be effective in completely breaking it down. In that case, the off-take stream can be returned to the oil phase in the settling vessel so that the coalesced water droplets can sink to the water phase. On the other hand, if the emulsion layer cannot be completely broken down by the CEC, it is preferable to return the off-take stream back into the emulsion layer in the settling vessel. The coalesced water droplets will sink out of the emulsion into the water phase and as a consequence the thickness of the emulsion layer is reduced. 
     In another alternative embodiment, a blockage plate is provided in the settling vessel to provide a blockage to part of the flow stream, the off-take stream being taken from one side, preferably upstream, of the blockage plate and returned to the other side. Preferably, the blockage plate provides a blockage to flow of the emulsion layer through the settling vessel. It is an advantage that by blocking the flow of the emulsion layer part of the way through the vessel the emulsion layer may be eliminated altogether beyond the blockage. This reduces the total volume of emulsion inside the vessel, and leaves more of the vessel volume available for gravity separation of water from the oil layer. 
     Alternatively, or additionally, the blockage plate may provide a blockage to flow of the oil phase. Conveniently, the height of the blockage plate is adjustable so that the blockage to the emulsion layer can be maintained even if the level of the emulsion layer changes. 
     The blockage plate may be provided as a blockage to the oil layer, the off-take stream being taken from the oil layer upstream of the blockage plate and returned to the oil layer downstream of the blockage plate. The oil layer may have an upper surface level in the settling vessel that is higher at the upstream side of the blockage and lower at the downstream side. Advantageously, the difference in the oil layer surface levels provides a hydraulic head that assists in forcing the oil layer off-take stream through the CEC. 
     Alternatively, the blockage plate may be an integral part of a distributor plate, extending partly or completely across the cross sectional area of the settling vessel. 
     Where the off-take stream is taken from the settling vessel, the off-take outlet may comprise a height-adjustable orifice so that the level at which the off-take stream is taken from the settling vessel can be altered. Additionally, where the off-take stream is returned directly to the settling vessel, a height-adjustable inlet may be provided so that the level at which the off-take stream is returned to the settling vessel can be altered. 
     It is an advantage that the provision of a fixed or height-adjustable blockage plate, inlet or outlet requires relatively few modifications to the design of an existing settling vessel. Consequently, the apparatus is particularly suitable for retrofitting to an existing separator. 
     Another advantage arises when apparatus according to the present invention is used in a subsea installation. In such installations the settling vessel should not be provided with internal equipment that requires maintenance. However, periodically it is necessary to recover equipment such as a CEC for maintenance. This can be done by simply closing the off-take stream (e.g. by way of isolation valves) and isolating the CEC without interrupting the flow stream from the well. The settling vessel can operate in a conventional manner without the off-take (albeit with a reduced performance) while the CEC is being overhauled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the following drawings. 
         FIG. 1  is a sectional illustration of a known arrangement of settling vessel. 
         FIG. 2  shows the settling vessel of  FIG. 1  incorporating an improvement according to a first embodiment. 
         FIG. 3  shows the settling vessel of  FIG. 1  incorporating an improvement according to a second embodiment. 
         FIG. 4  shows the settling vessel of  FIG. 1  incorporating an improvement according to a third embodiment. 
         FIGS. 5   a  and  5   b  show the settling vessel of  FIG. 1  incorporating two alternative arrangements of an improvement according to a fourth embodiment. 
         FIG. 6  shows a settling vessel similar to that shown in  FIG. 1  incorporating an improvement according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a gravity settling vessel  10  has an inlet  12  for receiving a well-stream. The well stream includes crude oil, water and gas. The well-stream is fed to an inlet cyclone  14  which separates the gas from the other constituents. The gas enters a space  16  at the top of the settling vessel  10  and leaves the vessel by way of a gas outlet  18 . The oil and water leave the inlet cyclone  14  as a mixture  20 . The mixture  20  most often consists of water in the form of droplets carried by a continuous oil phase. Provided the water droplets are large enough they fall under gravity to the bottom the settling vessel  10  to form a water layer  22 . The mixture  20  forms a region or layer between the water layer  22  and an oil layer  26 . The oil layer  26  is predominantly crude oil, but contains small droplets of water. 
     The water layer  22 , oil layer  26 , and the mixture  22  move away from the inlet cyclone  14 . A distributor plate  23  helps to distribute the flow over the whole of the cross-sectional area of the settling vessel  10 . The water layer  22  moves towards a water outlet  24  at or near the bottom of the settling vessel  10 . The oil layer  26  moves towards an oil outlet  28 , also located at or near the bottom of the vessel. A baffle  30  extends upwardly from the bottom of the settling vessel  10  between the water outlet  24  and the oil outlet  28 . The baffle  30  extends high enough to completely block any further flow of the water layer  22 , or the mixture  20 , but does not extend high enough to prevent the oil layer  26  flowing past the top of the baffle  30 . 
     The settling vessel will continue to perform effectively as a gravity separator as long as the consistency of the mixture  20  is such that the water droplets are large enough to fall down to the water layer  22 , and the settling vessel  10  is large enough for the mixture  20  to have sufficient residence time in the vessel for this to occur. In these conditions the mixture  20  gradually separates so that the region or layer of the mixture becomes thinner as the fluids flow through the settling vessel  10 . Note that  FIG. 1  depicts the mixture  20  as a distinct layer having a definite interface between the mixture  20  and the oil layer  26 . In reality there is unlikely to be such a definite interface. Instead there is a more gradual change in the spatial density and size of water droplets moving down from the oil layer  26  to the mixture  20 . 
     It is not practicable to provide a long enough residence time for gravitational forces to separate very small droplets from the oil. Consequently the oil layer  26  will still contain very small water droplets, which are carried in the oil leaving through the oil outlet  28 . 
     Referring to  FIG. 2 , the settling vessel  10  is provided with an off-take outlet  32  for taking an off-take stream from the oil layer  26 . A pump  36  pumps the off-take stream through a feed line  34  to a compact electrostatic coalescer (CEC)  38 . The off-take stream passes through the CEC  38  and is returned to the settling vessel  10  by way of a return line  40  and a return inlet  42 . The return inlet  42  is situated upstream of the off-take outlet  32 . 
     The CEC  38  is an electrostatic coalescer having particular characteristics. As its name implies, the CEC  38  is a compact device with small overall dimensions, making it particularly suitable for installation where space is at a premium (such as on off-shore oil platforms). Another feature of the CEC is that the electrodes are electrically isolated. This means that a mixture having a high water content can be fed through a CEC, which will continue to operate to coalesce the water droplets without the risk of short-circuiting the electrodes. The CEC can even be fed with pure water without short-circuiting. 
     In use, the oil layer  26  at the position of the off-take outlet  32  will contain very small water droplets (as discussed above). The CEC  38  provides an effective means for coalescing even these very small droplets so that the water droplets in the return line  40  are much larger. The oil containing these coalesced droplets is returned to the oil layer  26  at the return inlet  42 . Because the droplets are much larger than those taken out in the off-take stream, they will settle much more quickly. In this way the separation of water from the oil can be enhanced. 
     Referring to  FIG. 3 , an alternative arrangement to that shown in  FIG. 2 , instead of taking the off-take stream from the oil layer  26  in the settling vessel  10 , an off-take line  44  is taken from the oil outlet  28 . An off-take stream is pumped through the off-take line  44  to the CEC  38 . This ensures that only the very smallest droplets, which would otherwise be carried with the oil from the settling vessel  10 , are fed through the CEC  38 . 
     A further problem with gravity separators, as depicted in  FIG. 1 , arises when the composition of the well-stream fluids is such that the water and the oil in the mixture  20  form a stable emulsion. An emulsion inhibits the gravity separation process so that the mixture  20  tends to become thicker instead of thinner as it moves through the settling vessel  10 . The emulsion layer builds up over time, with the result that less oil and water are separated, and less of the volume of the settling vessel  10  is taken up by the oil and water layers. This results in deterioration of separator performance (i.e. more water is carried over with the oil, or the throughput has to be reduced to give the required quality of separation). 
     Coalescing of the water droplets in the emulsion is an effective way of helping to break it down and improve separation. The water content in an oil and water emulsion is considerably higher than, for example, that of the oil layer  26  in the settling vessel  10  of  FIG. 1 . In most known designs of electrostatic coalescer the high water content would cause short-circuiting of the electrodes, resulting in failure or damage to the coalescer. However, this problem does not arise with the electrically isolated electrodes used in the CEC. Thus one possibility is to use an arrangement similar to that shown in  FIG. 2 , but with the off-take outlet  32  situated at a level on the settling vessel to extract emulsion from the mixture layer  20 . The off-take could be returned to the settling vessel either into the oil layer  26  (the position shown in  FIG. 2 ), or into the mixture layer  20 . 
     Referring to  FIG. 4 , in another alternative arrangement, an off-take stream is removed from the settling vessel  10  by means of a pump  46  extracting emulsion from the mixture layer  20  through an off-take outlet  48 . The off-take stream is fed through an off-take line  50  to a CEC  52  and returned via a return line  54  to the settling vessel  10  at an off-take return inlet  56 . The off-take return inlet  56  is located a short distance downstream of the off-take outlet  48 . As shown in  FIG. 4 , the off-take stream is returned to the oil layer  26 . However, depending on the extent to which the emulsion is broken down in the CEC  52 , it may be more effective for the off-take to be returned to the mixture layer  20 . 
     An advantage of the arrangement shown in  FIG. 4  is that there is no effect on the residence time in the settling vessel  10 . With a recycled off-take stream, such as that shown in  FIGS. 2 and 3 , the residence time in the vessel is reduced (i.e. the velocity of the flow of oil and water through the settling vessel  10  is increased). This is not a problem if the water droplets are coalesced by the CEC so that they are large enough to fall out of the oil layer in the reduced residence time. However, especially where the CEC is used to help break down an emulsion, there may still be a need to provide a long residence time for the water droplets to fall out of the oil. 
     Referring to  FIG. 5   a , a blockage plate  60  is provided in the settling vessel  10 . An off-take stream is taken from upstream of the blockage plate  60  from an off-take outlet  62   a  in the oil layer  26 . An additional off-take outlet  62   b  may be positioned at a lower level so as to take a proportion of the off-take from the mixture layer  20 . The off-take stream is fed through an off-take line  64  to a CEC (not shown) and returned through a return line  66  to a return inlet  68  downstream of the blockage plate  60 . Where the mixture layer  20  is an emulsion layer, the CEC acts to break down the emulsion by coalescing the water into large droplets. After the off-take is returned to the settling vessel  10  downstream of the blockage plate  60  there is no longer an emulsion layer to inhibit the separation of water droplets from the oil. The blockage plate  60  therefore acts as a stop to limit the horizontal extent of the emulsion layer. 
     In the arrangement shown in  FIG. 5   a  the blockage plate  60  extends to above the oil layer  26 . This means that all of the oil that has separated into the oil layer upstream of the blockage plate  60  is taken through the CEC. The arrangement shown in  FIG. 5   b  is the same as  FIG. 5   a , except that the blockage plate  60  does not extend above the oil layer  26 . This means that oil that has separated upstream can continue to flow past the blockage plate  60  and only emulsion from the mixture layer  20  is taken through the CEC. 
     Referring to  FIG. 6 , a blockage plate  70  extends downwards from the top of the settling vessel  10  to a level below the oil layer  26  and the mixture layer  20 . As in the embodiment shown in  FIG. 5   a , an off-take stream is taken from outlets  62   a ,  62   b  upstream of the blockage plate  70 , fed through a CEC (not shown) and returned to the settling vessel  10  downstream of the blockage plate  70 . In this case the upper surface level  72   a  of the oil is higher on the upstream side of the blockage plate  70  with the upper surface level  72   b  being lower on the downstream side. The difference in the levels provides a hydraulic head, which assists in forcing the oil off-take stream through the CEC. 
     In the embodiments shown in  FIGS. 2 to 6 , the only modifications required to be made to the settling vessel  10  of  FIG. 1  are the provision of one or more off-take outlets, a return inlet and, in some embodiments, a blockage plate. These are relatively straightforward modifications that can be retro-fitted to existing settling vessels. In addition, because the constituents of the well-stream can change over time, this may place different demands on the manner in which the separation process is operated. For example, the extent to which the oil and water form an emulsion may change over time. To allow for this the off-take outlets, return inlet and blockage plates may be configured to be adjustable so that the off-take stream can be taken from or returned to a different level in the vessel when conditions demand. The off-take outlets and/or the return inlet may utilise a height-adjustable orifice for this purpose. 
     Periodically it is necessary to carry out maintenance on the CEC and ancillary equipment such as pumps etc. Because these items of equipment are installed in an off-take stream, it is possible to isolate them by closing valves situated in the off-take and return lines. Isolating the CEC and ancillary equipment in this way enables maintenance to be carried out without interrupting the flow stream from the well. The settling vessel can operate in a conventional manner without the off-take (albeit with a reduced performance) while the CEC is being overhauled. Routine maintenance includes cleaning the CEC to flush away accumulated hydrocarbons. Because the CEC is compact this takes much less time than for other comparable equipment. 
     The method and apparatus described above enable existing separators to operate more effectively, and in some cases will enable them to process an increased throughput (which can be useful, for example, when a new production stream is being tied in to existing processing equipment). Where new separators are being installed, these can be designed with smaller settling vessels (for equivalent throughput) and have a wider operating range so that they are effective for most, or all of the life of a well.