WO2020256565A1

WO2020256565A1 - Removing deposits from a subsea separator

Info

Publication number: WO2020256565A1
Application number: PCT/NO2020/050166
Authority: WO
Inventors: Cecilie Gotaas Johnsen; Knut Sveberg
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2019-06-18
Filing date: 2020-06-17
Publication date: 2020-12-24
Anticipated expiration: 2021-12-18
Also published as: GB201908736D0; GB2584998A

Abstract

A system for removing deposits from the interior of a separator (1), the system comprising: a separator (1) for separating a constituent fluid from a combined fluid; a heat source (15a); (15b) arranged to receive and heat the separated constituent fluid; and a conduit (17a); (17b) arranged to convey the heated constituent fluid back into the interior of the separator (1) for use in removing deposits from the interior of the separator.

Description

REMOVING DEPOSITS FROM A SUBSEA SEPARATOR

The present invention relates to the removal of deposits from a fluid separator. It is particularly, but not exclusively, applicable for removing deposits of wax from subsea separators used to separate hydrocarbon-continuing fluid produced subsea.

Subsea hydrocarbon wells produce fluid that may contain various combinations of oil, water and gas, along with contaminants, sand etc. These constituents may be separated, typically using a gravity separator. These comprise a vessel into which the produced fluid is supplied by a pipe of substantially smaller diameter than the vessel. As a result, the flow velocity of the fluid reduces to a point that allows the different fluids to separate into“layers” as a result of their differing densities. In a two-phase separator, the liquid phase is then removed from an outlet arranged towards the bottom of the vessel and the gas is removed from one at or near the top. In a“three-phase” separator, the (liquid) oil and water components are regarded as separate“phases”, with separate outlets being provided for each. This relies upon the water being denser than the oil, though in both cases the respective outlets are provided towards the bottom of the vessel. There may be provided a boundary in the form of a weir over which the oil may flow in order to separate it from the water.

It is possible to locate such separators at the sea bed, rather than at the surface, to reduce the need to transport fluids from the sea bed to a platform, particularly as they must generally then be returned to the sea-bed, either to a transport pipeline or (in the case of water or gas) to an injection wellhead for reinjection in the context of enhanced oil recovery. Water may also be purified at the seabed for release into the sea.

Although these are significant factors in favour of locating the separator at the sea bed, such a location does result in drawbacks in terms of increased complexity for maintenance. One particular problem is the need to control and remove deposits that may build up within a separator and thus reduce its efficiency. As will be explained below, these deposits typically comprise waxes, but particulates such as sand are also an issue. The likelihood of waxes being formed depends on the composition, temperature and pressure of the fluids.

Subsea separators can operate at different pressures, which can be both below, similar to, or above the surrounding pressure at the seafloor. The subsea separator operating pressure is normally optimised in order to maximise production from the reservoir. The operating temperature is normally determined by that of the wellstream fluids, but will be cooled down by the surrounding seawater. Whilst the operating pressures and temperatures of a subsea reservoir can therefore fall within a wide range, a typical subsea separator may operate in a temperature range of between 10 and 25°C and a pressure range of 10 to 70 bar.

A single separator can be connected to a number of producing wellheads. Each wellhead may produce a hydrocarbon-containing fluid with different pressure, temperature and ratios of oil, gas and water (amongst other constituents). When the fluid from each reservoir is combined, these parameters determine the wax appearance temperature (WAT) of the combined fluid.

The WAT is the temperature below which wax precipitates begin to form in the fluid. Such wax precipitates can cling to the walls of pipelines and other components and can lead to many issues, including a reduced flow, performance, or even a blocked pipeline or separator.

Thus, if the operating temperature of a subsea separator is below the WAT of the fluid produced by the reservoir, wax precipitates will begin to form in the separator, particularly on the interior walls that are cooled by the surrounding seawater. Over time, this build-up up of wax precipitates adversely affects the performance and reduces the output of the separator. It is therefore important to slow the build-up of wax precipitates in a separator and occasionally remove wax precipitates that have built up over time.

In addition to this, other substances contained within the fluid produced by the subsea reservoir, such as sediments (e.g. sand particulates), have a tendency to collect in the subsea separator (typically at its base). This collection of particulates further reduces the performance of the separator, and it is therefore important to also reduce the build-up of particulates and occasionally remove them from the separator altogether.

One known method of reducing the build-up of sediments over time is to cycle part of the separated fluid back through the separator in order to flush it. For example, a separator may separate produced fluid from the subsea reservoirs into oil, water and gas. Some of the oil and/or water may then be diverted from a point downstream of the separator, back to the interior of the separator, typically via nozzles directed at the base where sediments collect. This“flushes” the separator, and the fluid that is cycled back through the separator can carry sediments and wax precipitates out of the separator. They can then be removed from the fluid by further processing down the line. A drawback with this method is that it is ineffective at removing wax precipitates that cling to the interior walls of the separator, where they are likely to form.

Other methods for reducing the build-up of wax precipitates in separators involve the use of chemicals known as wax inhibitors. These substances are injected directly into the fluid to be separated anywhere between the well and processing equipment but this approach requires storage and supply lines for the provision of such substances. This complicates the installation and maintenance of subsea systems.

Alternative methods for removing the build-up of wax precipitates from a separator require the operation of the separator to be temporarily suspended and, if the separator is a subsea separator, require it to be lifted topside. The build-up of wax precipitates is then removed mechanically, via a scraping method or similar. Such a method is typically carried out when the performance of the separator no longer meets the required output. Whilst this method is effective at removing the wax precipitates, it is clearly disadvantageous to suspend the operation of the separator (which means that another separator must be used in order to meet the required output of the wells, or else production from the wells must also be suspended or reduced). It is also a difficult and time consuming process to lift a separator topside for such a method to be performed.

There therefore exists a need in the art for an effective method for reducing and/or removing the build-up of wax precipitates in a separator, particularly a subsea separator, preferably without shutting down the operation of the separator.

According to a first aspect of the invention, there is provided a method for removing deposits from the interior of a separator, the method comprising:

separating a constituent fluid from a combined fluid in the separator; removing the separated constituent fluid from the separator; heating at least part of the separated constituent fluid to a temperature higher than the temperature of the combined fluid in the separator; conveying the heated constituent fluid back into the interior of the separator; and using the heated constituent fluid to remove deposits from the interior or the separator.

Thus, by means of the invention, fluid separated by the separator may be heated and then used to remove deposits from within the separator. In particular, the fluid is used to assist in the removal of deposits, such as wax, which may be softened and/or melted and/or rendered more soluble by heat. Accordingly, the invention is particularly useful where the combined fluid is produced hydrocarbon- containing fluid received from a wellhead (in which wax may form) and thus the constituent fluid may be water or oil.

Once returned to the separator, the heated constituent fluid is preferably directed towards at least one interior surface of the separator because this is where deposits tend to occur. In particular, this is an effective means of treating deposits of wax which tend to cling to the walls. The heated constituent fluid may be directed at the walls of the separator. In particular, the heated constituent fluid may be directed at interior walls where wax precipitates are most likely to form, and is preferably directed at the side walls of the separator which are normally oil-wetted.

These oil-wetted regions are located between the oil and water boundary and the oil and gas boundary in the separator, and so are exposed to oil during the separation process. Thus, the heated fluid is preferably directed at the separator’s walls between these boundaries. These regions of the sidewall are typically where a large amount of wax precipitates from the oil forms and clings to the sidewalls as they are cooled by the surrounding seawater. By directing heated constituent fluid directly at these regions that are most affected by wax appearance, wax removal can be improved.

Furthermore, when heated constituent fluid is directed at a region of the sidewall in the oil-wetted region, the heated constituent fluid will flow down the side walls as it cools. Provided it initially is at a sufficiently high temperature, as is preferred, it will remain at a higher temperature than the other fluids in the separator as it does so. If the constituent fluid is water, it will then enter the water-wetted region before exiting out of the separator. In this way, residual heat in the constituent fluid can be used to clear wax precipitates from other areas of the sidewall, in particular those below the oil-wetted region at which the constituent fluid is initially directed.

If the constituent fluid is oil, then it will remain in the oil section of the separator, whereas water would have to be subsequently separated from the oil by settling into the water region.

The fluid is preferably directed by means of openings which may be in the form of nozzles. These may be designed to provide jets of fluid, whereby the heated constituent fluid is impinged onto the interior surfaces of the separator. This may provide a mechanical scouring effect in addition to the softening/melting/dissolving effect provided by the elevated temperature. Particularly in combination, the effects of heat and mechanical scouring are effective on deposits such as wax and/or sand.

Such nozzles are preferably arranged to direct the heated fluid in such a way that at least the worst- affected areas of the interior side and bottom walls are impinged with the fluid flow. However, the number and arrangement may be varied as required for a particular application. Pressure drop across the nozzles will vary with spread angle and flow rate. A high volume of flow and high pressure drop is preferred as this will result in the most effective wax removal.

Although the invention may further comprise its own heat source for this purpose, for example, an electrical heater or a heat-exchanger using a separate source of heated fluid as a heat source, the constituent fluid is preferably heated as part of a treatment process downstream of the separator. Such processing may involve further separation and/or purification stages. Thus, it will be appreciated that the inventors have realised that it is possible to use residual heat that is a result of such further fluid processing, to more effectively remove deposits such as wax precipitates from the interior of a separator. In this way, conventional treatment systems can be used to heat the constituent fluid and existing subsea separator arrangements can be modified for use in the invention.

For example, water separated from produced fluid from a hydrocarbon reservoir is usually treated for any number of uses including releasing it into the sea or re-injection into the reservoir in order to support the reservoir pressure.

Accordingly, the processing apparatus may treat the separated water to standard that it can be released into the sea, or to such a standard that it can be re-injected into the subsea reservoir in order to support reservoir pressure.

This treatment of the separated water typically occurs at temperatures of between 30 and 60 degrees Celsius. In order to achieve this temperature, the separated water from the subsea separator (at temperatures of around 13 degrees Celsius, for example) must be heated and will retain much of this heat after the treatment process is completed.

The treatment temperatures of around 30 to 50 degrees Celsius are normally higher than the WAT of fluid produced by subsea reservoirs, and the operating temperatures of separators. The inventors have realised that this warmer treated water can be diverted back into the separator in order to remove wax precipitates from the separator more effectively. The higher temperatures can at least partially melt the wax precipitates, causing them to detach from interior surfaces of the separator and subsequently be removed from the separator.

Oil separated from the produced fluid must typically also undergo further processing comprising heating downstream.

Where the separator is located sub-sea, it is preferable for the source of heat also to be located sub-sea. However, it is within the scope of the invention for remote heat sources to be used, for example, when treatment occurs topside. If such treated water is then to be re-injected into the subsea reservoir, it must be passed down to the sea bed via a riser. Such re-injection is typically carried out via injection points on the sea bed and the heated separated water will be conveyed through flow lines on the sea bed from the base of the riser prior to reaching these injection points. If processed oil from the topside is to be stored or transported via pipelines subsea, this must also be passed down to the sea bed via a riser

Therefore, if the separator is a subsea separator, part of the heated water and/or oil can be diverted relatively easily from the flow lines on the sea bed leading to the subsea injection points to instead be directed back to the subsea separator. In this way, existing subsea separator arrangements can be modified for use in the invention.

Even where heating is provided by means of such downstream processing, in some cases it may be necessary or expedient to provide additional heating after such treatment in order to achieve the desired temperature. This may be provided by an electric heater or heat exchanger, for example, as mentioned above.

Alternatively, a dedicated heater or heat exchanger could be used to raise the temperature of part of the constituent fluid before treatment, and this constituent fluid could be diverted back to the separator before any treatment occurs.

The particular temperature of the returned constituent fluid may be selected as required for a particular application. However, in a case where wax formation is a particular concern, it may be heated to above the wax appearance temperature of the combined fluid in the separator. Accordingly, the method may further comprise determining the wax appearance temperature of the combined fluid in the separator.

Although the invention may be performed during shut-down of the separator, a particular advantage is that it may be carried out during normal operation, i.e. whilst the separator continues to separate fluids. The method may further comprise monitoring the performance of the separator, and the step of conveying the heated constituent fluid back into the separator may then be carried out only when the performance of the separator is below a predetermined level. Monitoring of performance may comprise measuring the output (e.g. the flow rate and/or composition) from the separator or measuring the build-up of deposits, e.g. the thickness of wax precipitates on the walls of the separator. Monitoring and control may be carried out by means of a controller, such as a microprocessor-based controller. It may receive inputs from sensors such as temperature and flow sensors and control the flow of heated fluid by means of pumps and/or valves, as will be discussed further below.

In a simple implementation, the deposit-removal process may be controlled in a binary fashion (i.e.“on” or“off”). However, in alternative implementations, the amount of water redirected back to separator may be varied, depending on performance - i.e. the flow may be increased in cases of high deposit build-up. If necessary, the flow may be continuous, but more typically the process will be activated as required.

In another alternative, the passing of the heated constituent fluid back into the separator is performed periodically at predetermined time intervals. This may be on a simple a“need it or not” basis, with the time intervals estimated conservatively to ensure that there is only an acceptable level of deposit build-up between cycles. However, the time intervals may be determined based on any one or a combination of: wax appearance temperature of fluid in separator; operating temperature of separator; performance of separator; and wax build up in separator, in order to optimise operation.

The invention also extends to a corresponding system for performing it and therefore, viewed from another aspect, there is provided a system for removing deposits from the interior of a separator, the system comprising: a separator for separating a constituent fluid from a combined fluid; a heat source arranged to receive and heat the separated constituent fluid; and a conduit arranged to convey the heated constituent fluid back into the interior of the separator for use in removing deposits from the interior of the separator.

As previously discussed, a plurality of openings, such as nozzles, may be provided by the conduit for the directing the heated constituent fluid towards at least one interior surface of the separator. The heated constituent fluid is preferably directed at the side walls of the separator in oil-wetted zones, as discussed previously. The method and system of the present invention may be employed in any type of separator known in the art, for example gravity separators or centrifugal separators, and the nozzle arrangement may be configured for use with any such separator.

In order to control the system, it may further comprise a sensor for measuring the operating temperature of the separator; and a controller for controlling the heating of the separate constituent fluid such that it is heated to a temperature greater than the operating temperature of the separator measured by the sensor. In addition, there may also be provided a sensor for measuring the temperature of heated constituent fluid. This may be used to determine the amount of heat that must be supplied to the constituent fluid by the heat source and hence to control, for example, the flow of constituent fluid or the temperature of the source in order to ensure that the desired degree of heating is obtained.

The controller may be arranged to control the heating of the separated constituent fluid such that it is heated to a temperature greater than the wax appearance temperature of the combined fluid in the separator. Accordingly, it may also comprise a means for determining the wax appearance temperature of the combined fluid in the separator, based on the parameters discussed above.

The system may also comprise one or more sensors for monitoring the performance of the separator, wherein the heated constituent fluid is conveyed back into the interior of the separator only when the performance of the separator is below a predetermined level. Such sensors may measure the flow and composition of fluid from the separator (on the basis that a build-up of deposits will tend to block parts of the separator and hence reduce the quality of the products out of the separator). Alternatively or additionally, a sensor may directly measure the depth of deposit build-up on walls of the separator (e.g. optically or mechanically).

The system may provide suitable valves to control the flow of heated fluid. For example, a valve may be provided to control the flow of heated fluid from a location downstream of the separator. The valve, which may be a metered valve may be under the control of the controller and may provide feedback thereto. Thus, if the performance of the separator is above a predetermined level, the controller may close the valve so that no heated constituent fluid is diverted back to the separator unnecessarily. The controller can then open this metered valve if the performance drops below the predetermined level in order to remove wax precipitates from the separator. Additionally, the valve may be variable such that the controller can alter the amount of constituent fluid diverted back to the separator depending on its performance level. For example, if only a small impact on performance is determined (and thus a minimal amount of wax precipitates are present in the separator) then a smaller amount of constituent fluid can be diverted back to the separator by partially closing the valve. Again, this prevents unnecessary amounts of constituent fluid from being diverted back to the separator, thus optimising the efficiency of the separator system.

The controller can also control the metered valve such that it is opened at predetermined time intervals, so that heated constituent fluid is diverted back to the separator periodically. These periods can be a predetermined cycle or may be based on any of the measurements discussed herein.

The controller can also determine the wax appearance temperature of the produced fluid in the separator. This may done by testing the produced fluid, or by measuring relevant parameters such as pressure, temperature, and the ratio of constituents of the produced fluid using additional sensors. The controller can then ensure that the constituent fluid is heated to a temperature greater than the wax appearance temperature of produced fluid in the separator.

The system preferably operates in accordance with the method previously described, and in particular, the preferred features thereof.

The controller may be microprocessor-based and have inputs from the various sensor discussed above and provide output signals to the valve(s), heat source and optionally other components such as pumps. It may operate under the instructions of suitable software. Accordingly, viewed from a still further aspect, there is provided a computer program product for providing instructions to cause such a processor to perform the method(s) described above.

Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic elevation of a prior art three phase separator that is flushed in a manner known in the art;

Figure 2 shows a schematic elevation of a prior art two phase separator that is flushed in a manner known in the art;

Figure 3 shows a cross-section along the line A-A of the three phase separator of Figure 1 ;

Figure 4 shows a schematic elevation of a three phase separator according to an embodiment of the present invention; Figure 5 shows a cross-section along the line A-A of the three phase separator of Figure 4; and

Figure 6 is a schematic diagram of the control system of the embodiments.

Figure 1 shows an example of a three phase subsea separator 1 known in the art, with an inlet 2 for receiving produced fluid 3 from a subsea reservoir (not shown). The produced fluid comprises oil, gas, water and sediments. The separator has walls 4, an internal divide 5, an outlet 6 for gas, an outlet 7 for water and an outlet 8 for oil. The separator 1 is located at the seabed.

The separator shown in Figure 1 operates on the known principle of gravity separation. The density difference between the oil, water and gas causes them to settle into“layers” and separate due to their differing densities and the effect of gravity. Figure 1 shows an example boundary 9 between gas G and oil O and a boundary 10 between oil O and water W. The configuration of internal divides and the outlets makes it possible to remove the layers of oil, water and gas from the separator individually. It will be noted that water collects behind divide 5 and that oil may flow over the divide, which acts as a weir, into the region where outlet 8 is provided. Any sediment will typically collect at the bottom of the separator.

At the base of the separator 1 a pump 11 is provided for pumping separated oil out of the separator from the outlet 8 to a conduit 13. Conduit 13 leads to further processing apparatus for processing the oil in the conventional manner. Likewise, second pump 12 is provided for pumping water out of the separator through outlet 7 to conduit 14. Conduit 14 leads via further conduit 18 to further processing apparatus for treating the separated water.

There is a metered valve 16 provided downstream of the outlet 7 and pump 12 so that the separated water (or a portion of it) may be diverted through it from conduit 14. The remainder of the separated water continues along conduit 18 to further processing apparatus, re-injection apparatus or to be released into the sea.

Metered valve 16 leads to conduit 17, which in turn leads back to the separator at inlet 19.

In Figure 1 , it is assumed that the pressure in both the oil and water conduits 13, 14 downstream of the separator is higher than the separator pressure, and consequently pumps 11 , 12 are required to maintain the flow of oil and water out of the separator. However, it will be appreciated that if the pressure in the oil or water conduits downstream of the separator is lower than the pressure in the separator, pumps 11 , 12 are not necessarily be required and may be exchanged for metered valves, and metered valve 16 may be exchanged for a pump.

The purpose and effect of the returned separated water will now be described, with reference to Figure 3. Here, the boundary 9 between gas G and oil O and boundary 10 between oil O and water W ean be seen. Conduit 19 leads, via inlet 19, to the interior of the separator where it divides into many branches, two of which 21 , 22 are shown. These each have a plurality of openings 23.

After passing through the inlet, the separated water in conduit 17 is directed through branches 21 , 22 and out of openings 23. The openings 23 are in the form of nozzles directed at bottom 31 of the separator such that sediment and wax that has collected here is flushed out by the water through outlet 7 and removed further downstream.

Figure 2 shows a two phase separator 20 known in the art for separating gas G from liquid L, i.e. there is no separate oil-water separation. However, it is otherwise generally similar to the first separator described above. Thus, a boundary 28 can be seen between gas G and liquid L phases. Also similar to separator 1 , this two phase separator has an inlet 2 for produced fluid 3 and an outlet 6 for gas. However, this two-phase separator has only one outlet for liquid, outlet 29. This separator does not feature the internal divide of the three-phase separator (as this is unnecessary in a two-phase separator).

The flushing system is similar to that described above in relation to Figure 1 in that separated water is supplied via a conduit 17 and inlet 19 to nozzles 23 arranged within the separator 20 to remove collected sediment and wax.

Embodiments of the present invention will now be described with reference to Figures 4 to 6, which use the same reference numbers as Figures 1 to 3 for corresponding parts.

Figure 4 shows a three-phase separator 1 generally similar to the separator described in relation to Figure 1 , except the separator 1 is additionally provided with a system for controlling and removing the build-up of wax and particulates using heated water and/or oil that has been output from the separator.

Again, separator 1 is provided with an inlet 2 for receiving produced fluid 3 from a subsea reservoir (not shown). The produced fluid comprises oil, gas, water and sediments. The separator has walls 4, an internal divide 5, an outlet 6 for gas, an outlet 7 for water and an outlet 8 for oil. It is also located at the seabed. Again, as in Figure 1 , at the base of the separator 1 a pump 11 is provided for pumping separated oil out of the separator from the outlet 8 to a conduit 13. Conduit 13 leads to further processing apparatus for processing the oil in the conventional manner. Likewise, second pump 12 is provided for pumping water out of the separator through outlet 7 to conduit 14. Conduit 14 leads via further conduit 18 to further processing apparatus for treating the separated water.

The features associated with the separated water flow will now be discussed more detail. As in Figure 1 , a metered valve 16a is provided downstream of pump 12 so that the separated water (or a portion of it) may be diverted through it from conduit 14 to conduit 17a.

Conduit 17a branches so that a portion of this water flows to inlet 19 from which it flows via conduits 21 and 22 to openings 23 at the base of the separator (see Figure 5) as previously described. However, the remainder flows via a heat source (shown indicatively at 15a), which heats the water before it enters the separator at heated water inlet 19a for use in removing built up wax as will be described below.

It is advantageous for the heat source 15a to be located downstream of valve 16a and the branch in conduit 17a (as shown), so that only the portion of the separated water necessary for removing the build-up of wax is heated. However, the heat source could be located upstream of valve 16a, for example in conduit 14 or downstream of valve 16a but upstream of the branch in conduit 17a.

Heat source 15a may be part of the conventional water processing apparatus or a separate heat source, as will be described further below.

The features associated with the separated oil flow are somewhat similar to the features relating to the separated water described above. A metered valve 16b is provided downstream of pump 11 so that the separated oil (or a portion of it) may be diverted through it from conduit 13 to conduit 17b.

Downstream of valve 16b a heat source (shown indicatively at 15b) is provided for heating some or all of the oil flowing through conduit 17b. This may be part of the conventional oil processing apparatus or a separate heat source. Again, this could be alternatively located upstream of valve 16b.

Conduit 17b is insulated and may have additional heat exchangers incorporated into it for heating of the separated oil. The heated oil is then returned to the separator at inlet 19b. Again, similar to the system described in relation to Figure 1 , it is assumed that the pressure in both the oil and water conduits 13, 14 downstream of the separator is higher than the separator pressure, and consequently pumps are required to maintain the flow of oil and water out of the separator. However, it will be appreciated that if the pressure in the oil or water conduits downstream of the separator is lower than the pressure in the separator, pumps 11 , 12 are not necessarily required and may be exchanged for metered valves, and metered valves 16a, 16b may be exchanged for pumps.

The purpose and effect of the returned heated water will now be described, with reference to Figure 5. Whilst the effect of the heated water will be described, it should be appreciated that the heated oil is utilised in the same manner as described with corresponding features.

In Figure 5, as in prior art Figure 2, the boundary 9 between gas G and oil O and boundary 10 between oil O and water W ean be seen. In addition, the flow path for water via conduits 21 and 22 (fed via inlet 19) to openings 23 which direct water at the base 31 of the separator are also as described with reference to that figure.

Conduit 17a leads, via inlet 19a, to the interior of the separator where it divides into many branches, two of which 21a, 22a are shown; these each have a plurality of openings 23a.

Briefly turning back to Figure 4, opening 23a, along with a corresponding arrangement for the heated oil terminating in a plurality of openings 23b, can be seen. Whilst separate inlets 19, 19a and 19b are shown for each of the conduits 17, 17a and 17b in Figure 4, it will be appreciated that conduits 17, 17a and 17b could be configured to all enter the interior of the separator through the same inlet point (e.g. by means of a concentric arrangement at the location of inlet 19).

Returning to Figure 5, after passing through the inlet 19a, the heated water in conduit 17a is directed through branches 21a, 22a leading to a plurality of openings 23a. The openings 23a are directed at interior surfaces of the separator, in this embodiment the openings 23a are directed at the side walls 32 of the separator in the oil-containing region. The plurality of openings 23a comprise nozzles arranged to direct the heated water and impinge it on the interior surfaces of the separator.

The walls 4 of a subsea separator, and in particular the region 32 of the walls in the region where the oil O is found, are typically where wax precipitates collect because the separator is surrounded by cold seawater. This seawater cools the walls to a temperature slightly below the operating temperature of the rest of the separator, thereby inducing the formation of precipitates.

When the heated water/oil impinges on the interior walls of the separator, the wax removing effect is two-fold. First, the pressure of the water/oil directed through the nozzles and impinging on the surface can detach wax precipitates from the interior walls of the separator by mechanical action. Secondly, the heat from the heated water/oil can at least partially melt or soften some of the accumulated wax, thus making it easier to detach it off from the walls.

Once the wax has been removed from the walls, it will typically be carried out of the separator, along with the flow of oil and water, through outlets 7 and 8 respectively.

The plurality of openings 23a may direct the heated water at the side walls of the separator and this may be at a different pressure to that from the openings 23 for the unheated separated water located towards the bottom of the separator. For example, a higher pressure may be used if wax precipitates are more likely to be found on the side walls of the particular separator, or a lower pressure may be used order to limit the mixing of water into the separated oil.

Heated water from nozzles 23a impinged on the sidewalls will typically remain close to the interior wall of the separator, and gradually flow down it due to the effects of the gravity separation. In this way, the entirety of the side walls of the separator from the plurality of openings 23a downwards can be partly heated by the heated water. This can help to remove wax precipitates from a greater interior surface area of the separator in this variant embodiment.

The use of the heated water is also effective for removing sediments from the separator because sediments typically collect in lower regions as the produced fluid in the separator begins to settle, and the heated water will flow down the side walls of the separator into the water region W, before exiting through outlet; in doing so it can remove sediments in this region.

During this process, the separator is operating normally. As such, the reintroduced water is eventually separated out again via the normal separation process and no shutdown of the separator is required in order to carry out the removal of wax precipitates.

Similarly to the heated water, heated oil from nozzles 23b impinged on the sidewalls will typically remain close to the interior wall of the separator, and gradually flow down it due to the effects of the gravity separation. However, heated oil will remain in the oil region O, preventing the need for further separation.

A further distinction from the separator of Figure 1 is that the separator comprises a number sensors for use in controlling the build-up control system described above.

With reference to Figures 4 and 6, the operating temperature of the separator is measured by a sensor S1 in the interior of the separator and/or at the outlets for water and oil 7, 8 (S2). The system also comprises sensors S3 for measuring the temperature of the separated water and oil respectively after heating. In addition, sensors S4 are provided for measuring the performance of the separator (i.e. outlet the flow rate), and hence indicating whether there is a significant build-up of wax precipitates.

A controller 30 (see Figure 6) is provided which is connected to the sensors S1-S4, heat sources 15a and 15b and metered valves 16a and 16b. Controller 30 is a microprocessor-based control system.

Based on the input from sensors S4, the controller determines when the performance of the separator drops below a predetermined level, indicating a build up of wax precipitates. When this occurs it activates the build-up control system, whereby heated water and/or oil is returned to the separator for use in removing the wax.

Based on the inputs from sensors S1 , S2, the controller determines the temperature of water and/or oil leaving the separator and provides control signals to the heat sources 15a and/or 15b to raise the temperature to the desired level. Feedback control of the heated water and/or oil temperature is then provided by sensors S3. Once the water and/or oil is sufficiently heated, metered valves 16a and/or 16b, which are also connected to the controller, are opened. The flow rate through the valves may also be adjusted as part of the control of temperature (a lower flow rate resulting in a higher temperature from a given amount of heat applied to a smaller volume of water/oil). Accordingly, the controller is able to control the flow and temperature of heated water/oil via the metered valves as required based on measurements of the operating temperature of the reservoir and the measured temperature of the heated water/oil. In particular, the controller can ensure that the separated water/oil is heated to a temperature significantly higher than the operating temperature of the separator. The controller may also vary the amount of water/oil supplied depending upon the degree of wax build-up detected. The wax control system described above may also be employed in a two phase separator like that shown in Figure 2. In this case, heated constituent fluid(s) (typically oil) would be supplied via a conduit 17 and inlet 19 to nozzles arranged within the separator 20 and configured to direct the heated fluid at regions of the separator where wax is likely to appear. The heated constituent fluid would be provided and its supply controlled by means of a controller 30 in a manner similar to that previously described.

In all of the embodiments, as described, heat is added from a heat source 15a, 15b which is part of the conventional treatment process of the water or oil. This raises the temperature of the separated water/oil to above the operating

temperature of the separator. The heat added to the water or oil can also raise the temperature of the separated water/oil to above the wax appearance temperature of the produced fluid in the separator. Heat may also be added to the separated water/oil via a heat source which is separate from (and/or additional to) the conventional processing means. For example, the separated water/oil could be heated by a heat exchanger prior to or after the treatment of separated water.

Claims

1. A method for removing deposits from the interior of a separator, the method comprising:

separating a constituent fluid from a combined fluid in the separator; removing the separated constituent fluid from the separator;

heating at least part of the separated constituent fluid to a temperature higher than the temperature of the combined fluid in the separator;

conveying the heated constituent fluid back into the interior of the separator; and

using the heated constituent fluid to remove deposits from the interior of the separator.

2. The method of claim 1 , wherein the combined fluid is produced

hydrocarbon-containing fluid received from a wellhead.

3. The method of claim 2, wherein the constituent fluid is water or oil.

4. The method of any previous claim, wherein the heated constituent fluid is directed towards at least one interior surface of the separator, and wherein the interior surface is preferably a surface where deposits are likely to form/collect.

5. The method of any claim 4, wherein the heated constituent fluid is directed at the side walls of the separator, wherein the heated constituent fluid is preferably directed at side walls in an oil region of the separator.

6. The method of any preceding claim, wherein the deposits comprise wax.

7. The method of any preceding claim, wherein the separated constituent fluid is heated to above the wax appearance temperature of the combined fluid in the separator.

8. The method of claim 7, further comprising determining the wax appearance temperature of the combined fluid in the separator.

9. The method of any previous claim, wherein the method is carried out while the separator separates fluids.

10. The method of any previous claim, wherein the method further comprises monitoring the performance of the separator, and wherein the step of conveying the heated constituent fluid back into the separator is carried out only when the performance of the separator is below a predetermined level.

11. The method of any previous claim wherein the passing of the heated

constituent fluid back into the separator is performed periodically at predetermined time intervals.

12. The method of any preceding claim, wherein the constituent fluid is heated as part of a treatment process downstream of the separator.

13. The method of any previous claim wherein the heated constituent fluid is impinged onto the interior surfaces of the separator via a plurality of outlets, such as nozzles.

14. A system for removing deposits from the interior of a separator, the system comprising:

a separator for separating a constituent fluid from a combined fluid; a heat source arranged to receive and heat the separated constituent fluid; and

a conduit arranged to convey the heated constituent fluid back into the interior of the separator for use in removing deposits from the interior of the separator.

15. The system of claim 14, wherein the conduit comprises a plurality of

openings, such as nozzles, for the directing the heated constituent fluid towards at least one interior surface of the separator, and wherein the interior surface is preferably a surface where deposits are likely to form/collect.

16. The system of claim 15, wherein the heated constituent fluid is directed at the side walls of the separator and wherein the heated constituent fluid is preferably directed at side walls in an oil region of the separator.

17. The system of any of claims 14 to 16, wherein the system further comprises: a sensor for measuring the operating temperature of the separator; and a controller for controlling the heating of the separated constituent fluid such that it is heated to a temperature greater than the operating temperature of the separator measured by the sensor.

18. The system of claim 17, wherein the controller is arranged to control the heating of the separated constituent fluid such that it is heated to a temperature greater than the wax appearance temperature of the combined fluid in the separator.

19. The system of any of claims 14 to 18, further comprising a sensor for

monitoring the performance of the separator, wherein the heated constituent fluid is conveyed back into the interior of the separator only when the performance of the separator is below a predetermined level.

20. The system of any of claims 14 to 19, configured for use in the method of any of claims 1 to 13.