WO2018172497A1

WO2018172497A1 - System and method for subsea separation of produced water

Info

Publication number: WO2018172497A1
Application number: PCT/EP2018/057383
Authority: WO
Inventors: Marius BJØRN; Julie LUND; Tom Grimseth; René MIKKELSEN
Original assignee: Nov Subsea Products As
Priority date: 2017-03-24
Filing date: 2018-03-22
Publication date: 2018-09-27

Abstract

Subsea produced water separation system comprising a flexible and collapsible membrane tank (2) with a separation volume, a mechanical support structure (3); and a produced water inlet (4), a hydrocarbon outlet (14), a gas outlet (19), and a water fraction outlet (11) all in fluid communication with the separation volume. As well as a method for subsea separation of produced water.

Description

System and method for subsea separation of produced water

The present invention relates to a method and system for subsea separation of produced water. Especially the present invention relates specifically to a system and method for achieving a water cleanliness of less than 30 ppm oil to provide a water fraction that can be discharged to the surrounding environment or injected to well.

Background

Increasing oil and gas exploration and production, as well as development of more challenging environments require more effective offshore produced water management. Produced water is the aqueous liquid phase that is co-produced from a producing well along with the oil and/or gas phases during normal production operations. The amount of produced water, contaminants and concentrations generally differ significantly over the lifetime of a field. The produced water composition changes, is complex, and site specific because it is a function of (including but not limited to) geological formation, the type of production chemistry and rock/fluid interactions. The produced water concentration may increase with time to tens of times the rate of oil produced. Finding the best treatment solution for produced water technology is important and there is a need to develop a more efficient and suitable system to achieve the required water quality for discharge or well injection. In subsea processing installations a number of process elements are organized to gradually separate the oil or condensate phase from the gas phase and the water phase. Subject invention relates to the part of the subsea process designed to treat the produced water (PW) phase as delivered from upstream separation elements, e.g. such as a first stage separator. PW from such a first stage separator may typically contain oil in a concentration of 200-2000 ppm and oil droplets smaller than 200 microns. The PW will also contain a number of smaller solid particles from the reservoir. The size distribution of such particles varies significantly with a large number of parameters, but a distribution with a mass bases average diameter D50 of between 50 and 150 μιη is suggested as common. The amount of sand per m³ of PW also varies with a large number of parameters. It is common to assume that the dominant part of sand carried with the well stream is deposited in the first stage separator and that only smaller particles pass through to downstream process elements.

In the corresponding process found on platforms it is customary to organize a set of cyclonic devices to separate out the remaining oil content from the main PW stream. Platforms are characterized by high cost of carrying weight and providing space, consequently the compactness of cyclonic devices are preferred. With centripetal forces exceeding 3000 G in practical machines, such cyclonic devices are rational despite the need for maintenance. In contrast, the sea bed offers low cost space and weight carrying capacity, but maintenance is associated with high cost, in some instances to a prohibitive level. It is thus rational to select technologies which demand more space but less

maintenance. Consequently, there is an interest in the industry for low maintenance separation elements.

Prior art

WO2016/202977 discloses a system for subsea separation of produced water wherein a subsea collapsible flexible bag is used for gravitational separation of solids within the bag, wherein the separated solids a removed together with the bag. There is a need for an improved system for subsea separation of hydrocarbons from produced water.

Objectives of the invention

The objective of the present invention is to provide a system and method for subsea separation of produced water. Especially the objective is to provide for separation of oil from the produced water.

A further objective is to provide for a system which provides high quality separation at reduced installation costs compared to methods involving very large retention times.

Another object is to provide for improved oil separation in a subsea arranged collapsible tank.

A further objective is to provide a system that may comprise a set of sub-units arranged in parallel or partly in series.

There is a need for an efficient, reliable, robust, and flexible subsea produced water system as an enabler for subsea reinjection or direct disposal of produced water to sea.

The purpose of subject invention is to provide a large volume tank (separator) but with a design which offer cost effective installation and retrieval. For the case of a rigid tank there is a choice of installing the tank water filled or gas filled. Partial filled during installation (mixture of gas and water) is inherently associated with stability issues during lifting. For the case of an air-filled tank installation is in principle impossible as any closed tank of reasonable weight will float. For the case of a water filled tank the weight of the mechanical part plus the weight of the water will need to be lifted from the deck of the installation vessel. For the tank considered as part of subject invention the difference between an air-filled tank and a water filled tank could be as 50 tons to 250 tons. The difference in cost of surface support required for installation is considerable. The purpose of subject invention is thus to provide a large volume separator which is installed (and retrieved) without the bulk of the water which normally occupies its volume.

To obtain these and possibly other objectives the present invention provides the solutions according the claims.

In a first aspect the present invention provides a subsea produced water separation system comprising

- a flexible and collapsible membrane tank with a separation volume,

- a mechanical support structure; - a produced water inlet, a hydrocarbon outlet and a water fraction outlet all in fluid communication with the separation volume, and a gas outlet in fluid communication with the separation volume, wherein the gas outlet is arranged in the upper section of the separation volume.

The hydrocarbon outlet may be identical to the gas outlet, such that a separation of gas from liquid hydrocarbons can be performed in a separate unit. The hydrocarbon outlet is arranged to be in fluid communication with the separated hydrocarbon phase and may be arranged in the upper section of the separation volume. The arrangement of hydrocarbon outlet is adapted to the level of the oil/water interface that the separator is designed for. In one embodiment of the first aspect of the subsea produced water separation system the gas outlet is connected to a gas circulation system such that at least a part of the gas removed true the outlet is returned to the produced water before or when the produced water enters the subsea produced water separation system. The subsea produced water separation system of this embodiment may comprise a gas loop from the gas outlet to the produced water inlet. The subsea produced water separation system of this embodiment may further comprise a water recycle loop from the water fraction outlet to the produced water inlet, wherein the water recycle loop comprises a pump and a compressor, and the gas loop is combined with the water loop in the compressor. In the first aspect of the subsea produced water separation system, the subsea produced water separation system may further comprise one or more inductive level sensors for measuring the position of a produced water - oil - gas interfaces.

Subject invention is a gravitational separation device, based on a large volume and with a low cost and convenience of installation and retrieval, such that what little maintenance that may be required may be carried out onshore. Whereas it is true that the sea bed offers low cost for industrial sites per m³ space and ton weight, the cost of transportation from the shore and installation of large structures on to the sea bed tend to be significant and is a natural limitation in terms of cost. This purpose of providing a large volume separator which is installed (and retrieved) without the bulk of the water is achieved by introducing a flexible and collapsible membrane tank/bag, made typically from such materials as nitrile butadiene rubber (NBR) or Hydrogenated Nitrile Butadiene Rubber (HNBR), to be collapsed during installation and inflated during operation. It is protected by a rigid external tank, made typically from glass reinforced plastic (GRP) or steel or similarly suitable materials commonly found in subsea equipment. The rigid tank is floodable by virtue of a number of gratings which offer good passage for water into (installation) and out of (retrieval) the tank during the transit through the splash zone, with the membrane tank being essentially deflated. In its basic form the membrane tank is thus a single barrier between the PW and the ambient the grating/tank only supports the membrane mechanically.

In an alternative design the rigid tank is equipped with movable hatches, open during installation and closed after the rigid tank is filled with water. This would provide a secondary barrier between the PW and the ambient sea. None of these barriers are planned for large differential pressures, accidental internal

overpressures are handled by safety devices (pressure safety devices, PSD) to protect the mechanical integrity of the barriers. The single barrier concept can only be pursued for very low oil content in the separator. A concentration of 1000 ppm of oil in a 200 m³ separator volume amounts to 200 litres of oil. An inherently low probability of barrier failure combined with a very low consequence in terms of accidental release of hydrocarbons will be acceptable to most operators and

Statutory Authorities.

The floodability of the rigid, supporting tank structure is closely associated with the sea state during installation, suggesting a higher floodability for oil provinces known for rough sea states (e.g. Norwegian continental shelf, NCS) and less in calmer waters (e.g. Australia). Floodability is a compromise between structural stability of the rigid tank and the maximum sea state acceptable for installation. It can and should be optimized for each installation.

As considerations of installation cost impose certain constraints on size of subsea modules it is still interesting to reduce separator module size by improvements in the separation efficiency. One method for such improvements involves installation of traditional separator internals, such as various devices which provide a coalescing effect and a weir plate to assist definition of the oil phase.

Through special adaptation of the flexible and collapsible membrane tank, adapted versions of such internals may be included in the membrane tank during deflation and inflation, as an integrated part of the membrane tank.

In a further embodiment of the first aspect of the invention a gas phase is included in the top of the separator and gas is circulated through the PW phase to effect flotation of oil droplets to the PW/oil interface. A simple water booster and an eductor may be installed to circulate gas through the PW phase even when the breakout of a gas phase is insufficient to provide required gas for floatation.

Although gas floatation per see is well known from topside separation processes, the subsea use is new and requires considerable adaptation to the different environment. The size of the gas phase may be from 2-15 % of the volume of the total volume of the membrane tank, preferably 5-12 % by volume, typically 10 % by volume.

The presence of a gas phase will result in a buoyancy force on the membrane, transmitted into the supporting rigid structure. As an example, a 200 m³ tank with 10% volume gas will typically experience a buoyancy for of 20 tons. This may suggest use of more steel and less GRP in the rigid, supporting structure than what would be the case without the free gas phase. The added weight also assists module penetration of the splash zone without risk of slack slings.

The present invention provides in a second aspect a subsea produced water separation system comprising a flexible and collapsible membrane tank with a separation volume, a mechanical support structure; and a produced water inlet, a hydrocarbon outlet and a water fraction outlet all in fluid communication with the separation volume.

The mechanical support structure can be a rigid frame structure, where the rigid frame structure provides mechanical support during installation, use and retrieval and has opening that allows free circulation of surrounding water in and out of the frame structure.

Alternatively, in the subsea produced water separation system according to the second aspect, the mechanical support structure is a rigid external tank surrounding the flexible and collapsible membrane tank, wherein the rigid external tank comprises one or more hatches or one or more gratings allowing the rigid external tank to be flooded. In one embodiment according to the second aspect of the subsea produced water separation system the flexible and collapsible membrane tank has a horizontal elongated configuration when arranged on the sea bed. The flexible and collapsible membrane tank may in this embodiment have a substantially rectangular cross section in the vertical direction. The term "substantially rectangular" is used here to refer to rectangular as well as rectangular with curved corners.

In a third aspect of the present invention the subsea produced water separation system, the flexible and collapsible membrane tank comprises one or more internals selected from baffle plates, wire plates, and blocking plates. Preferably, the internals are secured to an inner surface of the flexible and collapsible membrane tank. In one embodiment of the third aspect the flexible and collapsible membrane tank has a horizontal elongated configuration when arranged on the sea bed and has a rectangular cross section in the vertical direction. The internals are generally more rigid than the flexible membrane. In this embodiment the internals are preferably secured to the internal wall of the membrane tank at selected vertical positions so that when the membrane tank is deflated the one or more internals will be able to tilt into an almost horizontal position only possibly limited by any bag material arranged between the internal and the horizontal sea floor.

In a fourth aspect the subsea produced water separation system may further comprise a control unit receiving signals from one or more pressure sensors measuring pressure within the collapsible tank and or pressure difference between the collapsible tank and outside the collapsible tank, and wherein the control unit provides control signals to control the internal pressure in the collapsible tank, the feed rate of produced water to the system and the rate of the water fraction leaving the system.

In an embodiment of the subsea produced water separation system according to the fourth aspect, the system further comprises on or more oil-in-water sensors connected to the control unit, such that the control unit can provide signals to control the rate of the hydrocarbons leaving the system through the hydrocarbon outlet.

The present invention also provides a method for subsea separation of produced water comprising

- feeding produced water comprising 400-2000 ppm oil to a subsea produced water separation system according to any one of the aspects of the invention,

- maintaining the produced water in the collapsible tank for a retention time;

- maintaining an internal differential pressure in the collapsible tank; - removing oil through the hydrocarbon outlet,

- removing water fraction through the water fraction outlet.

In one embodiment of the method, the produced water is fed from one or more initial separators to the subsea produced water separation system, wherein in the initial separators may be selected from traditional topside separators, sand accumulator, coalescer.

In another embodiment of the method the internal differential pressure is between 0.01 and 2.0 bar.

The retention time is such that the water fraction has an oil content of less than 50 ppm, preferably less than 30 ppm.

In a further embodiment the method comprises providing a gas content in the produced water, separating the produced water in a gas fraction, an oil fraction and a water fraction, and removing the gas fraction through the gas outlet.

The method may comprise recycling at least part of the gas fraction into the produced water feed into the separator.

Especially the method may comprise mixing the gas fraction into a part of the water fraction, increasing the pressure of the obtained mixture and injecting the

pressurised mixture into the produced water feed stream.

In one embodiment the method further comprises measuring and controlling the position of a produced water - oil - gas interfaces.

In yet another embodiment the method comprises measuring the oil-in-water content of the produced water and the water fraction.

Additionally, the method may further comprise feeding the water fraction to a degasser to enhance separation of hydrocarbons and reduce retention time. In one embodiment the invention is based on using a large separator volume

(essentially a long, slender, horizontal separator) to achieve a long retention time without the need for a costly installation of a large, rigid structure. The separator volume is defined by a collapsible membrane contained in a floodable

container/structure. The main advantage of a horizontal design is that the vertical distance an oil droplet is required to travel to get to the oil reject is relatively short and the probability of getting to the oil reject stream is favourable.

When the membrane is deflated (collapsed) it contains practically no water, thus lifting it over the freeboard of a vessel for installation requires only lifting of the mechanical components. A rigid separator would have required also lifting the water mass contained in the module (for a 200 m³ volume this represents 200 tonnes of water), alternatively, if a closed, rigid container is air filled, it will not sink and cannot penetrate the surface. The combination of the floodable structure

(supporting) and the deflateable membrane reduces the mass to be lifted from 250 tonnes to 50 tonnes. The difference in cost of support vessel/crane is substantial.

With a rational and moderately costly installation and a similarly credible retrieval, it is economical to base maintenance of the separator (such as removal of sand) by intervention rather by remote controlled equipment.

The separator is designed to operate at ambient pressure. Only a slight internal overpressure is maintained and controlled to keep the membrane fully inflated at all times in operation. Sensors (measuring pressure, oil-in-water, interphase levels) providing signals to a control unit and the control unit sending signals to flow regulators will control and maintain the required pressure under normal operation conditions. Safety measures, such as a burst disc or similar, may be included in the collapsible tank to secure a controlled depressurisation in case of increased pressure due to failure in the normal regulation.

The flexible membrane internals might be included to enhance separation efficiency. Internals that might be included are: baffle plate and weir plate. An inlet device can be integrated in the inlet pipe of the flexible separator.

The method according to the invention may additionally be combined with other separation technologies such as dissolved gas flotation using higher velocity of small particles, tilted plates, coalescer (including Mares Tail technology), Pect-F (using fibre materials to increases oil droplet size) or combinations thereof, as separate units or as a part of the flexible separator tank. The internals may be arranged within and fastened to the membrane through vulcanization of membrane material to edges of the internals.

The term "oil" as used herein refers to the liquid hydrocarbons present in the produced water.

The term "subsea" is used here to refer to a position below the surface of the sea, normally on the seabed, or partly imbedded in the seafloor.

A person skilled in the art will appreciate that the different aspects and different embodiments of the present invention may be freely combined.

Brief description of the drawings Embodiments of the present invention will be descried in further detail with reference to the enclosed figures, wherein

Figure 1 shows a schematic cross-sectional view of a subsea separation system

Figure 2 illustrates an embodiment of the separation system with a section of the outer tank removed to show the flexible tank arranged therein.

Figure 3 illustrates an embodiment with internals arranged in the flexible tank.

Figure 4 illustrates an embodiment adapted for containing a gas volume.

Figure 5 schematically illustrates a separation system including gas recycling.

A person skilled in the art will appreciate that the features illustrated on the different figures may be freely combined to provide additional embodiments of the present invention.

Principal description of the invention

The Separator System according to the present invention works as a single unit to achieve target water cleanliness. The PW Separator system is a gravity based, remote storage unit that employs the concept of a flexible membrane protected by a protection structure for produced water storage on the sea floor. It can be placed at any water depth and is adjustable in size depending on operator needs during the development and expansion of the field. The seawater outside of the protection structure surrounds the flexible membrane through free-flow seawater openings in the protection structure, such that the hydrostatic pressure acts directly on the stored fluid.

More than one separator system may be arranged in parallel to provide the required separation capacity and allow for continued separation during maintenance of one unit.

Another embodiment of the concept is the opportunity to implement a separator with a rectangular cross-section rather than the cylindrically shaped known from the prior art. This shape provides for effective separation of the oil from the water phase. In one embodiment the flexible and collapsible membrane tank comprises one or more internals selected from baffle plates, wire plates and blocking plates. The internals are secured to an inner surface of the flexible and collapsible membrane tank. In the inner surface is made of a material that may be vulcanized then the internals could be secured vulcanization of said inner surface. In a further embodiment the system comprises an inlet device in the inlet pipe. The inlet device is for preliminary phase separation. Typical examples of inlets include: Flat impact plates, Dished-head plates, Half-open pipes, Vane-type inlet and Cyclone-cluster inlet. The inlet device could be integrated in the inlet pipe or in the flexible membrane.

In one embodiment the flexible separator may also contain a small controlled (<10 volume % of the collapsible tank) gas phase at the top of the separator. Gas is circulated in a loop from top of the separator to the bottom for floatation of the oil droplets to the gas/water interface. The system according to the invention provides a subsea separator for oil/produced water separation, typically handling PW of 400 -2000 ppm concentration of oil, with long retention time compared to conventional size limited topside equipment.

A flexible membrane receiving the PW fed from an upstream first stage separator, providing a large separation volume and operating at a marginal internal differential pressure (0.01 to 2.0 bars). Membrane supported by a rigid structure taking up the forces of the internal pressure. The membrane is collapsible/deflateable to facilitate installation from a small surface vessel. The supporting structure is floodable for easy transit in through the splash zone. The membrane is fully inflated by the operating differential pressure. The oil phase/liquid hydrocarbon fraction may be removed continuously or batch wise regulated by signal(s) from the oil-in- water sensor(s). The hydrocarbon outlet may be arranged in a section downstream a weir plate.

The enclosed figures illustrate embodiments of the present invention. The same reference number is used to refer to similar elements. Figure 1 is a cross sectional view in the longitudinal vertical direction through a subsea separation system 1. The system comprises a flexible and collapsible membrane tank 2 arranged inside a mechanical support structure 3, which in this embodiment is tank-like structure with opening to the surroundings, see the two-way arrows. The support structure further comprises a bottom plate 15 arranged for resting on the sea bed. Additional mechanical support is provided by reinforcements 16.

The length of the system 1 can be adapted to the intended retention time and the illustrated length is selected to provide for illustration of the whole system in one figure. A produced water inlet 4 is provided at an inlet end. The inlet passes through the support structure 3 and is connected to the membrane 2 at the inlet connection 5 in fluid communication with the internal separation space of the membrane. At the opposite outlet end the water fraction outlet 1 1 is connected to the membrane with outlet connection 12 and past through the support structure 3. A hydrocarbon outlet 14 is arranged in the top section and connect with connection 13 to the membrane 2. In this embodiment the hydrocarbon outlet 14 is also the gas outlet.

The system further comprises a number of sensors 6, 7, 9 and 10. The sensors 6, 7 and 9 can be arranged to measure pressure in the membrane tank or differential pressure between the membrane tank and the external environment. Sensors 6 and 9 may also be arranged to measure the oil-in-water content of the produced water feed and the water fraction respectively. Sensors 10 illustrated the optional arrangement of sensors that be sending out and retrieving back reflections of signals. This could be a rudimentary sensor system for monitoring of solid accumulation by use of load cells in an array for measuring the increase in mass of the fluid/sand volumes in said modules. Alternatively, or additionally this could be an acoustic sensor system that can provide information on the difference in density of the materials reflecting the signals which can be transformed into information as to the position of the interphases between solids and water, water and oil, and between oil and gas. Figure 2 illustrates a similar embodiment to figure 1 but seen in perspective with a section of the support structure 3 remove to illustrate the membrane 2 arrange therein.

Figure 3 illustrates a separation system according to figure 1 but additionally comprising internals in the form of baffle plates 17 and a wire plate 18. Figure 4 illustrates an embodiment of the system which is further adapted to include a gas phase in the upper section and therefore includes a separate gas outlet 19. The illustration is schematic and the outlets 14 and 19 are arranged so that the intended phase can be removed from the tank by securing that the outlets are in fluid communication with the intended phase. Further, the gas and hydrocarbons may be removed batch wise or continuously, which would provide for adjusting the level of the interfaces.

Figure 5 schematically illustrates a separation system comprising a gas recirculation loop. Produced water enters the system 1 through produced water inlet 4. Upstream the inlet 4 produced water comprising gas can via valve 20 be mixed into the produced water. When the produced water comprising gas enters the membrane tank 2, the gas will, due to its lower density, be separated out and travel in an upwards direction opposite the force of gravity. This will enhance the separation of oil from the produced water as the gas provides for flotation of the oil. The gas will collect in the upper section and is removed through gas outlet 19. The water fraction leaves the system 1 in the opposite end through outlet 1 1 after having travelled in the longitudinally direction through the membrane tank 2. A part of the water fraction is used to introduce the gas into the produced water. The fraction is past through pump 22 and combined with the removed gas 23 in compressor 21 from where it, via valve 20, is mixed into the fed of produced water. The level of the gas - oil interface and the oil -water interface is preferably measured using one or more inductive level sensors. If to much gas is accumulated in the separator part of the gas is removed via a separate line or together with the oil phase.

The arrangement of the inlets and outlets in the figures are just one illustration of a possible arrangement and a person skilled in the art will appreciate that that the position and arrangement may be adapted to the situation at hand.

Claims

Subsea produced water separation system comprising

- a flexible and collapsible membrane tank with a separation volume,

- a mechanical support structure;

- a produced water inlet, a hydrocarbon outlet and a water fraction outlet all in fluid communication with the separation volume, and a gas outlet in fluid communication with the separation volume, wherein the gas outlet is arranged in the upper section of the separation volume.

Subsea produced water separation system according to claim 1 , wherein the gas outlet is connected to a gas circulation system such that at least a part of the gas removed through the outlet is returned to the produced water before or when the produced water enters the subsea produced water separation system.

Subsea produced water separation system according to claim 2, wherein the system comprises a gas loop from the gas outlet to the produced water inlet.

Subsea produced water separation system according to claim 3, wherein the system comprises a water recycle loop from the water fraction outlet to the produced water inlet, wherein the water recycle loop comprises a pump and a compressor, and the gas loop is combined with the water loop in the

compressor.

Subsea produced water separation system according to any one of the claims 1-

4, wherein the mechanical support structure is rigid frame structure.

Subsea produced water separation system according to any one of the claims 1-

5, wherein the mechanical support structure is a rigid external tank surrounding the flexible and collapsible membrane tank, wherein the rigid external tank comprises one or more hatches or one or more gratings allowing the rigid external tank to be flooded.

Subsea produced water separation system according to any one of the previous claims, wherein the flexible and collapsible membrane tank has a horizontal elongated configuration when arranged on the sea bed.

Subsea produced water separation system according to claim 7, wherein the flexible and collapsible membrane tank has a substantially rectangular cross section in the vertical direction.

9. Subsea produced water separation system according to any one of the previous claims, wherein the flexible and collapsible membrane tank comprises one or more internals selected from baffle plates, wire plates, and blocking plates.

10. Subsea produced water separation system according to claim 9, wherein the internals are secured to an inner surface of the flexible and collapsible membrane tank.

1 1. Subsea produced water separation system according to any one of the claims 1- 10, wherein the system further comprises one or more inductive level sensors for measuring the position of a produced water - oil - gas interfaces. 12. Subsea produced water separation system according to any one of the previous claims, wherein the system further comprises a control unit receiving signals from one or more pressure sensors measuring pressure within the collapsible tank and or pressure difference between the collapsible tank and outside the collapsible tank, and wherein the control unit provides control signals to control the internal pressure in the collapsible tank, the feed rate of produced water to the system and the rate of the water fraction leaving the system.

13. Subsea produced water separation system according to claim 12, wherein the system further comprises on or more oil-in-water sensors connected to the control unit, such that the control unit can provide signals to control the rate of the hydrocarbons leaving the system through the hydrocarbon outlet.

14. Method for subsea separation of produced water comprising

- feeding produced water comprising 400-2000 ppm oil to a subsea produced water separation system according to any one of the claims 1-13,

- maintaining the produced water in the collapsible tank for a retention time; - maintaining an internal differential pressure in the collapsible tank;

- removing oil through the hydrocarbon outlet,

- removing water fraction through the water fraction outlet.

15. Method according to claim 14, wherein the produced water is fed from one or more initial separators to the subsea produced water separation system, wherein in the initial separators may be selected from traditional topside separators, sand accumulator, coalescer.

16. Method according to any one of the claims 14-15, wherein the internal

differential pressure is between 0.01 and 2.0 bar.

17. Method according to any one of the claims 14-16, wherein the retention time is such that the water fraction has an oil content of less than 50 ppm, preferably less than 30 ppm.

18. Method according to any one of the claims 14-17, wherein the method

comprises providing a gas content in the produced water, separating the produced water in a gas fraction, an oil fraction and a water fraction, and removing the gas fraction through the gas outlet.

19. Method according to any one of the claims 14-18, wherein the method

comprises recycling at least part of the gas fraction into the produced water feed into the separator.

20. Method according to claim 19, wherein the method comprises mixing the gas fraction into a part of the water fraction, increasing the pressure of the obtained mixture and injecting the pressurized mixture into the produced water feed stream.

21. Method according to any one of the claims 14-20, wherein the method

comprises measuring and controlling the position of a produced water - oil - gas interfaces.

22. Method according to any one of the claims 14-21 , wherein the method

comprises measuring the oil-in-water content of the produced water and the water fraction.

23. Method according to any one of the claims 14-22, wherein the method further comprises feeding the water fraction to a degasser to enhance separation of hydrocarbons and reduce retention time.