NO20150835A1

NO20150835A1 - Separator system and method for breaking down a dispersion band

Info

Publication number: NO20150835A1
Application number: NO20150835A
Authority: NO
Inventors: Per Eivind Gramme; Jon Sigurd Berntsen
Original assignee: Kanfa As
Priority date: 2015-06-25
Filing date: 2015-06-25
Publication date: 2016-12-26
Also published as: WO2016209086A1

Description

The present invention relates to a separator system for separating gas, oil and water. The invention also relates to a method for breaking down a dispersion band between an oil phase and a water phase. It especially relates to a separator system for use on an offshore installation.

The well fluid produced from an oil or gas well will nearly always be a mixture of oil, gas and water. It is a great advantage to perform most of the separation of these three phases before the fluids are transported to the shore. This will ease the handling, reduce risk associated with the handling and reduce the need for transportation of water.

The experience from the technology used today on offshore installations is that the large gas quantities that are fed into the separator, especially large inlet separators, results in large torques and thereby extensive mixing of large quantities of gas into the liquid phases. Even for gas condensate fields with low viscosities and few stabilizing chemical components, it has been shown that the whole liquid column is filled with gas bubbles all the way from the inlet to the outlet of the separator. The inlet arrangements used today, such as vanes or cyclones, or other types that give high shear forces, re-disperse already free gas into the liquid phases. As gas bubbles are dispersed into the liquid phases, the good function of the separator is deteriorated and the separation becomes ineffective. This results in unnecessary large quantities of water in the oil outlet, oil in the water outlet and that oil and/or water will be carried over with the gas that is let out through the gas outlet from the separator.

The experience from the technology used today on offshore installations is also that the inlet arrangement and separator design often deteriorates the separation of oil, gas and water rather than promote and enhance separation.

To achieve a good separation process a holistically view of the separation process is required. It has therefore been suggested to separate out as much as possible of the free gas from the liquid phases before oil and water are separated, i.e. before the inlet to the separator and to introduce a low momentum inlet arrangement that will promote an efficient separation of the phases.

From WO 2006098637 and from US 4760742 are known separator arrangements where a large proportion of the gas is diverted out of the well stream before the well stream enters a separator, i.e. upstream of a pipe separator. In WO 2006098637 this is done by arranging a degassing section upstream of the separator. The degassing section comprises a plurality of substantially vertical degassing pipes connected at a first end to the inlet pipe upstream of the separator. The degassing pipes are connected at a second end to a gas collection pipe. This gas collection pipe connects with the gas outlet pipe from the separator. US 4760742 concerns a device for separating a multiphase petroleum production stream flowing in a sub-sea pipe. Here a gas collection pipe is inclined downwards in the downstream direction, which will allow any liquid carried with the gas to flow downwards together with the gas and thereby pollute the gas with liquid.

US 8864881 shows a slug suppressor apparatus comprising an inlet separator capable of gas-liquid separation of well stream fluid and expanded inclined liquid pipe for dampening slugs. The inlet separator has an inlet arrangement for receiving the well stream fluid, a separated gas outlet in its upper section, and a separated liquid outlet in its lower section. The separated gas outlet and the separated liquid outlet are operationally connected to a gas bypass line and the expanded inclined liquid pipe, respectively. The expanded inclined liquid pipe has means for dampening liquid slugs and is connectable to a 3-phase separator.

The present invention aims to improve the separation efficiency over the above prior art.

Separation in pipes is very efficient and is in the present invention utilized to stratify the gas and liquid phase upstream of a gas removal unit, in order to facilitate gas removal upstream of the separator. The more efficient separation in pipes is also utilized in the present invention to achieve maximum separation in the inlet arrangement prior to the flow entering the main separator volume in the separator. The more efficient separation in pipes is due to the fact that the rate determining step is not the sedimentation of water droplets orfloating of oil droplets to the water-oil interface, but the break-down of the formed dispersion band or dense packed layer of droplets accumulated at the interface. This dense packed layer of droplets is most efficiently broken down when exposed to shear forces acting between the phases.

This is achieved, according to the present invention, by ensuring that the two liquid phases flow with different velocity. This difference in velocities is best achieved in pipes. It has been realized that there is a great potential in utilizing the separator inlet arrangement to facilitate separation. It is also an important aspect of the present invention to avoid making the separation more difficult by re-dispersing the phases. Therefore, steps need to be tåken to achieve low torques and shear forces before, i.e. upstream of, the separator and through the inlet of the separator and to facilitate stratification and separation of the phases, while in the inlet arrangement of the separator, it is ensured that the two phases have substantially different velocities.

Surprisingly the gas removal device according to WO 2006098637 has proven

to be efficient also for topside process facilities in combination with conventional gravity separators as will be described in connection with the present invention.

The present invention therefore has as a first aim to improve the separation in a perse conventional separator by removing as large a portion of the free gas as possible. Thereby the torque through the inlet of the separator and the entrainment of the free gas into the liquid phases are reduced.

A second aim of the present invention is to provide an inlet arrangement to a perse conventional separator, which imposes low torque and low shear force to the fluids as such. Thereby is achieved reduced re-dispersing of free gas, a flow regime that facilitates an improved freeing of gas from the liquid phases and a flow regime that facilitates oil and gas separation.

A third aim of the present invention is to provide an inlet arrangement to a per se conventional separator with dimensions facilitating the formation of stratified liquid flow. Thereby is achieved improved separation of oil and water as well as more efficient gas release.

A fourth and most important aim of the present invention is to provide an inlet arrangement that will hold back the water phase and let the oil phase flow with a higher velocity than the water phase, thereby creating shear forces between the phases that will break down the droplets of oil or water that have accumulated at the interface between the water and oil.

This results in a more efficient separation of oil, water and gas, which in turn results in advantages such as reduced size of the separator, reduced number of separation stages for oil and water separation and more efficient separation of water from oils that are difficult to separate water from.

The above aims of the invention are achieved by a separator system for separating gas, oil and water, comprising an inlet pipe, a degassing section and a gravity separator; said degassing section comprising a plurality of substantially vertical degassing pipes connected at a first end to the inlet pipe upstream of the separator, the degassing pipes being connected at a second end to a gas collection pipe, said gravity separator comprising a separation receptacle and an inlet arrangement that includes a substantially horizontal inlet arrangement pipe, with perforations in its upper part, arranged within said separation receptacle, wherein the pipe has a downstream end section comprising a threshold at its lower part, said threshold retaining water up to a specific level in the inlet arrangement pipe, the upper part of the end section being open to allow oil to flow freely over the threshold.

Thereby is created a situation where the water will flow at zero or a very low velocity, while the oil will flow on top of the water at a somewhat higher velocity. Shear forces are created between the phases that will break up the dispersion band, i.e. droplets of water in the oil phase and droplets of oil in the water phase, between the phases. The droplets will coalesce into larger droplets that will have a tendency to coalesce further with other droplets or to the continuous phase at the interface. The larger droplets that have not coalesced to a continuous phase will also be much easier to separate through gravity separation.

Various embodiments of the threshold can serve this purpose, such as an upward inclination in the downstream direction of the inlet arrangement pipe or an end wall of said inlet arrangement pipe, said end wall closing a lower part of the pipe.

It is an advantage if the threshold does not end abruptly. If an excess amount of water enters the inlet arrangement pipe, the level will rise and water will flow over the threshold. If the threshold is configured so that it gradually allows an increasing amount of water to flow out, the velocity of the water may still be kept lower than the velocity of the oil. This may be achieved in several ways, such as: the upward inclination comprising perforations, where the perforations may have increasing size in the downstream direction and the perforations may be equipped with downwardly extending tubes; or the end wall has a notch extending from the upper edge of the end wall and downwards at least a part of the end wall height, where the notch has a decreasing width in the downward direction; the end wall has at least one opening below its upper edge and preferably a plurality of openings below its upper edge, said openings being of a smaller size with the distance downward from the upper edge.

If the end wall has a notch, the notch is preferably generally V-shaped.

The end wall may be inclined upwards in the downstream direction.

To ensure proper function of the gas removal unit upstream of the separator the inlet pipe has an upwardly extending portion between the degassing section and the gravity separator. Thereby is ensured that the portion of the inlet pipe downstream of the degassing section always contains liquid. This liquid will prevent large gas amounts from entering the separator.

The length of the inlet pipe upstream of said degassing section is adapted to the well fluid to be separated and the diameter of the inlet pipe, and it has been found that the following ranges are suitable, depending on the properties of the fluid:

- between 30 and 90 times the diameter of the inlet pipe.

- between 60 and 180 times the diameter of the inlet pipe.

- between 100 and 300 times the diameter of the inlet pipe.

- between 150 and 450 times the diameter of the inlet pipe.

- between 180 and 550 times the diameter of the inlet pipe.

Preferably, the inlet pipe has a downward inclination in the downstream direction at least in the degassing section. This will promote the separation of gas in the degassing section.

It has been found that this inclination should have an angle of 1 ° - 20°, preferably 3° -15° and more preferably 5° -12°.

It has been found that the substantially vertical degassing pipes should have a diameter that is substantially equal to the diameter of the inlet pipe upstream of the degassing section and a length of between 2 and 20 times the diameter of the inlet pipe upstream of said degassing section. This will promote the separation of gas.

Preferably, the gas collection pipe has an upward inclination in the downstream direction, said inclination håving an angle of 1° - 20°, preferably 3° -15° and more preferably 5° -12°. Thereby gas will flow towards the gas outlet and any liquid that has been carried with the gas, will have a tendency to flow back to the degassing section and back to the inlet pipe.

Preferably, the inlet pipe is substantially horizontal between the degassing section and the gravity separator or has an upward inclination in the downstream direction. This will ensure that the pipe is filled with liquid hindering free gas from entering the separator.

Preferably, the diameter of the inlet pipe between the degassing section and the gravity separator is at least equal to the diameter of the inlet pipe upstream of the degassing section. This promotes stratified flow of the liquids.

In one embodiment, the upwardly extending portion of the inlet pipe between the degassing section and the gravity separator is a pipe section with a substantially steeper inclination than the inlet pipe between the degassing section and the gravity separator as such.

The steeper inclination may be a part of an inverted u-shaped pipe section, i.e. a gooseneck.

Preferably, the inlet pipe between the degassing section and the gravity separator has a constant diameter, to promote stratified flow.

The perforated inlet arrangement pipe within said separation receptacle is preferably substantially horizontal and has a diameter that is larger than the inlet pipe upstream of the degassing section and is at least equal to the diameter of the inlet pipe between the degassing section and the gravity separator. This will reduce the velocity of the flow and promote stratified flow of the phases as such. It will also ensure that the water phase will settle in the lower part of the pipe.

The invention also defines a method for breaking down a dispersion band between an oil phase and a water phase, wherein the water is retained at and allowed to establish at a certain level, so that the water flows at zero or low velocity, while the oil is allowed to flow freely on top of the water phase at a higher velocity. This method promotes a difference in velocities between the water phase and the oil phase. Thereby, shear forces will act between the phases and break up the dispersion band that has formed between the phases.

Preferably, a major part of the free gas is removed from the oil and water upstream of the breaking down of the dispersion band. Thereby the flow will primarily consist of oil and water and gas will not interfere with the process of establishing stratified flow in the separator inlet pipe. A stratified flow regime is ensured upstream of the degassing section by keeping the superficial liquid velocity:

- below 9 m/s for superficial gas velocities below 7 m/s or above 14 m/s and

- below 3 m/s for superficial gas velocities between 7 and 14 m/s.

The invention will now be explained in detail, referring to the examples shown in the accompanying figures, where: Figure 1 shows a schematic general outline of the separator system of the present invention, Figure 2 shows a first embodiment of a liquid trap in inlet pipe in the form of a gooseneck, Figure 3 shows a second embodiment of a liquid trap in the inlet pipe in the form of a step-up, Figure 4 shows a section of the inlet of the separator in a first embodiment, Figure 5 shows a section of the inlet of the separator in a second embodiment, Figure 6 shows a section of the inlet arrangement of the separator in a first embodiment, Figure 7 shows a section of the inlet arrangement of the separator in a second embodiment, Figure 8 shows a section of the inlet arrangement of the separator in a third embodiment, Figure 9 shows a section of the inlet arrangement of the separator in a fourth embodiment, Figure 10 shows a section of the inlet arrangement of the separator in a fifth embodiment, Figure 11 shows a section of the inlet arrangement of the separator in a sixth embodiment, Figure 12 shows a section of the inlet arrangement of the separator in a seventh embodiment, Figure 13 shows a section of the inlet arrangement of the separator in an eight embodiment, Figure 14 shows a section of the inlet arrangement of the separator in a ninth embodiment, Figure 15 shows a section of the inlet arrangement of the separator in a tenth embodiment,

It should be appreciated that the person of skill in the art would realize that any combination of the embodiments explained below would fall within the scope of the present invention.

Figure 1 shows schematically the general arrangement of the separator system of the present invention. It comprises an inlet pipe 1 that is coupled to the well via appropriate valves and other equipment that is well known in the field. The inlet pipe is coupled to the inlet of a gravity separator 4, as will be explained in detail below.

Upstream of the separator 4 is a degassing section 2. The degassing section 2 is in principle known from the above-mentioned references WO 2006098637 and US 4760742, but will nevertheless be explained in the following for completeness.

The degassing section 2 comprises a plurality of substantially vertical pipes 7- 11 that are connected to the inlet pipe 1 at their lower ends. The inlet pipe 1 has a downward inclination in the downstream direction in the degassing section, starting upstream of the upstream vertical pipe 8 and ending at or downstream of the downstream degassing pipe 11, denoted by 12. This inlet pipe section is denoted 13.

At their upper ends, the vertical pipes 7-11 are connected to a gas collection pipe 14. The gas collection pipe joins the gas outlet 15 from the separator 4. From here the gas is exported to further handling, such as compression or burning in a flare. The gas collection pipe 14 has two sections, a first section 16 that extends across the upper ends of the vertical pipes 7-11 and a second section 17 that extends from the first section to connect to the gas outlet 15.

From the degassing section 2 the inlet pipe 1 extends through a further section 3 to the inlet 18 of the gravity separator 4. The gravity separator 4 has an internal inlet arrangement 19, which is connected to the inlet 18. The inlet arrangement will be explained in further detail below. The separator 4 also has a baffle 21 and an oil outlet 22 and a water outlet 23.

Some key design factors will now be explained referring to figure 1.

A prerequisite for efficient removal of gas in the degassing section is that a proper flow regime is achieved in the inlet pipe 1. Efficient free gas removal typically takes place when the flow is either slug flow or stratified flow. The flow regime achieved depends on a multiple of factors, such as the chemical properties of the oil, water and gas, i.e. viscosity and density of the three phases of the well fluid, interface tensions between the three phases, coalescence properties for droplets of one phase in another, etc. It also depends on water cut, phase inversion point, production rates, the pressure of the well fluid, the roughness of the inside of the inlet pipe 1, etc.

It has been found that the superficial velocity of the liquid phases and the gas phases are determining how much mixing of the phases there will be when the well fluid flows through the inlet pipe. It has also been found that the desired flow regime, i.e. slug flow or stratified flow, is achieved when the inlet pipe is substantially horizontal until the degassing section 2. It has also been found that there is a correlation between the superficial gas velocity and the superficial liquid velocity and that a desired flow regime is achieved when the relationship between the superficial velocities are as follows: - a superficial liquid velocity below 9 m/s for superficial gas velocities below 7 m/s or above 14 m/s and - a superficial liquid velocity below 3 m/s for superficial gas velocities between 7 and 14 m/s.

From this, it has been found that the diameter of the inlet pipe should be within the range of 6" to 36" (152 mm to 915 mm), preferably 6" to 24" (152 mm to 610 mm), and more preferred 6" to 16" (152 mm to 407 mm). A more accurate choice of diameter will have to be partly based on experiments where the actual well fluid is used.

The length of the inlet pipe 1 is also a key factor. It has been found that the length upstream of the degassing section 2 should be as long as possible. This will ensure that a desired flow regime is established. However, for practical reasons there are limitation on how long the inlet pipe 1 can be. It has therefore been found that the required length L1 has to be chosen depending on the diameter D1. A typical length L1 for a well fluid that is extremely difficult to separate will be in the range of 180 - 550 times the diameter D1. For a well fluid that is slow or difficult to separate, the length L1 should be within the range of 150 - 450 times the diameter D1. For a well fluid that is moderately difficult to separate, the length should be within the range of 100 - 300 times the diameter D1. For a well fluid that is moderately easy to separate, the length should be within the range of 60 -180 times the diameter D1. Finally, for a well fluid that is easy to separate, the length should be within the range of 30 - 90 times the diameter D1. The separability of a fluid is best determined by experimental separation studies of the live fluids, i.e. well fluid samples tåken from the relevant well, at actual system conditions.

The proper downward inclination a of the inlet pipe section 13 in the downstream direction relative to the horizontal has also been found to depend on the diameter D1 of the inlet pipe. An interface between gas and liquid will form in the area of the downstream vertical degassing pipe 11. Typically, the inclination a of the inlet pipe section 13 will be in the range of 1 - 20°, preferably 3-15°, and more preferred 5-12°.

The number of vertical pipes 7-11 in the degassing section 2 can vary, but is typical from 4 to 6 pipes, depending on flow rates and size of slugs. The diameter of the vertical pipes 7-11 is preferably the same as the diameter D1 of the inlet pipe 1. The length of each of the vertical pipes 7-11 may vary, but will typically be in the range of 2 to 20 times the diameter of the pipe.

The first section 16 of the gas collection pipe 14 may have the same inclination a as the inlet pipe section 13 but in an upward direction in the downstream direction. Thereby any liquid that has been drawn out with the gas will have a tendency to run back to the degassing section 2 and down the vertical pipes 7-11.

The second section 17 of the gas collection pipe 14 can be either horizontal or have a slight upward inclination in the downstream direction to facilitate any liquid running back to the degassing section 2, especially during shutdown of the separation system.

The further section 3 of the inlet pipe 1 has at least the same diameter D3 as the upstream inlet pipe 1,13, but may also have a larger diameter, depending on the desired flow regime in this part of the separation system. The length L3 of this further section 3 may be fairly short, but a longer length may be appropriate if it proves necessary for the establishment of an oil-water interface in the pipe before the fluid enters the separator 4.

The transition between the inlet pipe section 13 covering the degassing section 2 and the further inlet pipe section 3 is denoted 12. This transition should be situated at or downstream of the last, i.e. furthest downstream, vertical pipe 11. The further section 3 of the inlet pipe 1 has a gooseneck, such as shown in figure 2, or an inclination upward in the downstream direction, such as shown in figure 3. The gooseneck or the upward inclination should have a height H that is at least the same as the diameter D3 of the pipe section 3. This gooseneck or inclination ensures that the pipe section 3 upstream of the gooseneck or inclination will be filled with liquid. Thereby, gas pockets are prevented from finding their way into the separator 4.

Inside the separator 4 there is an inlet arrangement 19 that is in connection with the inlet 18. The inlet arrangement 19 preferably comprises a pipe 20 with a diameter D4 that is at least the same as the diameter D3 of the inlet pipe section 3. The diameter D4 is larger than the diameter D1 of the inlet pipe 1 upstream of the degassing section 2. Thereby, the diameter will be at least the same for the fluids from the inlet pipe 1 upstream of the degassing section 2 through to the inlet arrangement 19. However, it is preferred that the diameter is increasing in the downstream direction along these pipe sections.

A transition from a smaller diameter pipe section 3 to a larger diameter pipe 20 is preferably shaped so that the lower limitation of the pipe section 3 and the pipe 20 are on the same level, as shown in figures 4 and 5. Thereby the liquids in the flow to the separator, which have a tendency to seek to the lower limitation of the pipes, will be disturbed as little as possible. The upper limitation may have any of many suitable forms and may especially be a short incline 24 in a substantially straight line, as shown in figure 4, or a curved line, as shown in figure 5.

The pipe 20 of the inlet arrangement 19 is situated in the upper part of the gravity separator 4. The pipe 20 is perforated along its upper limitation. Gas in the fluid entering the inlet arrangement 19 is allowed to escape through these perforations 25, and the gas can escape through the gas outlet 15 from the separator 4 to join the gas from the degassing section 2. The perforations may be slits or other holes distributed along the length of the pipe 20.

The inlet arrangement, with its preferred increased diameter D4 compared to the diameter D1, is designed to facilitate improved separation of the gas, oil and water phases. The increased diameter will lower the liquid velocity, leading to increased gas release and facilitating liquid-liquid stratification along the pipe 20. At the downstream end of the pipe 20 there is an opening 6 allowing the liquid to escape into the separator 4 volume. The downstream end of the pipe 20 has a threshold 5 that will ensure that the water phase, which due to the stratified flow will be in the lower portion of the inlet pipe 20, is held back, i.e. the velocity of the water phase is reduced. This formation of and hold-up, i.e. retaining, of the water phase after stratification in the pipe 20 will happen regardless of incoming amounts of water in the oil-water-gas mixture (water cut). Only when the amount of water exceeds a certain level, the water will be allowed to flow out of the inlet pipe 20. Due to this construction of the downstream end of the pipe 20 the water phase and the oil phase is flowing at different velocities, i.e. the water at a low velocity and the oil on top at a higher velocity, and thereby shear forces are created at the interface. These shear forces aid the separation by breaking down the dispersion band, i.e. bubbles or droplets of oil or water at the interface. The downstream end of the pipe 20 can be designed in various ways to achieve the desired functionality.

The downstream end of the pipe 20 has a threshold, which in figures 6 and 7 is a gooseneck 26. This will retain water in the pipe 20 so that a water level W is established. The water level W will be retained at the same height as the height Hw of the bottom of the gooseneck as shown in figure 6. The gooseneck 26 may have perforations 27 at its upward slope a distance above the lower limitation of the pipe 20, as shown in figure 7 to drain water from the pipe 20. In that case, the water level W will establish at the height of the perforations 27. The end of the pipe 20 is in both cases open, so that the oil will run on top of the water and escape into the interior of the separator 4.

Figures 8-10 show other embodiments of the end of the pipe 20. These have a steep inclination 28 instead of a gooseneck 26. In figure 8, the inclined pipe section 28 ends abrupt at a height Hw. In figure 9, the inclined pipe section 28 has perforations 29 at the lower end of the inclination to allow water to escape when it exceeds the level Hw. In figure 10, the inclined pipe section 28 is equipped with narrow vertical pipes 30, which have increasing diameters in the downstream direction. The cross section of the perforations or narrow pipes are selected so that liquid is retained in the pipe 20 at substantially the level W.

Further alternative shapes of the downstream end of the pipe 20 are shown in figures 11-15. In these embodiments, the pipe 20 has an end wall 31 covering the lower half of the cross section. The end wall may have openings in the form of a notch 32, a single hole 33 or a multiple of openings 34 with increasing diameter in the upward direction. The end wall may also be inclined in the downstream and upward direction as shown in figure 15.

It is preferred that the water is allowed to escape gradually when it exceeds the level W, to prevent a sudden increase in water velocity when water spills over the end of the pipe 20. Therefore, the end of the pipe 20 is configured so that the cross section of the opening gradually increases with the height. A preferred embodiment is shown in figure 12, where a notch 32 defines the water level W. When the amount of water increases, the water will first flow out through the narrow bottom of the notch 32. If the water amount continues to increase, the water level W will rise further up the notch and the amount of water escaping through the notch 32 will increase. If a large enough amount of water is supplied to the inlet pipe 20, the water level W may increase to the ledge 35. Now, the velocity of the oil and water will be substantially the same. However, such a situation will last only for a short periods during production, normal production ischaracterized byconstant rate differences. Thus, the water level W will only increase beyond the ledge 35 for short periods of time.

When the oil and smaller amount of water leaves the inlet pipe 20 through the opening at the downstream end, the dispersion band has been substantially broken down and the oil and water exist in larger droplets or as a laminarflow.

This liquid of pre-separated oil and water will collect at the lower side of the separator 4. As the oil and water is largely separated, the gravity separation will now be very effective. Due to gravity, the oil and water will separate. Water is heavier than oil and will collect at the bottom and be drained through the water outlet 23. A baffle 21 is arranged between the water outlet 23 and the oil outlet 22. The interface between the oil and water is monitored and the water outlet 23 is adjusted so that the interface is kept below the top of the baffle 21. Thereby oil is allowed to flow over the top of the baffle and out the oil outlet 22.

Claims

1. A separator system for separating gas, oil and water, comprising an inlet pipe (1), a degassing section (2) and a gravity separator (4); said degassing section comprising a plurality of substantially vertical degassing pipes connected at a first end to the inlet pipe upstream of the separator, the degassing pipes being connected at a second end to a gas collection pipe, said gravity separator comprising a separation receptacle and an inlet arrangement that includes a substantially horizontal inlet arrangement pipe, with perforations in its upper part, arranged within said separation receptacle,characterised inthat the pipe has a downstream end section comprising a threshold at its lower part, said threshold retaining water up to a specific level in the inlet arrangement pipe, the upper part of the end section being open to allow oil to flow freely over the threshold.

2. The separator system according to claim 1,characterised in thatsaid threshold is an upward inclination in the downstream direction of the inlet arrangement pipe.

3. The separator system according to claim 2,characterised in thatsaid upward inclination comprises perforations.

4. The separator system according to claim 3,characterised in thatsaid perforations are equipped with downwardly extending tubes.

5. The separator system according to claim 1,characterised in thatsaid threshold is an end wall of said inlet arrangement pipe, said end wall closing a lower part of the pipe.

6. The separator system according to claim 5,characterised in thatend wall has a notch extending from the upper edge of the end wall and downwards at least a part of the end wall height, said notch being generally V-shaped.

7. The separator system according to claim 6,characterised in thatsaid end wall has at least one opening below its upper edge.

8. The separator system according to any of the claims 6 - 7,characterised in thatsaid end wall is inclined upwards in the downstream direction.

9. The separator system according to any of the preceding claims,characterised in thatsaid inlet pipe has an upwardly extending portion between the degassing section and the gravity separator.

10. The separator system according to any of the preceding claims,characterised in thatthe inlet pipe upstream of said degassing section has a length of between 30 and 90 times the diameter (D1) of the inlet pipe.

11. The separator system according to any of the claims 1 - 9,characterised in thatthe inlet pipe upstream of said degassing section has a length of between 60 and 180 times the diameter (D1) of the inlet pipe.

12. The separator system according to any of the claims 1 - 9,characterised in thatthe inlet pipe upstream of said degassing section has a length of between 100 and 300 times the diameter (D1) of the inlet pipe.

13. The separator system according to any of the claims 1 - 9,characterised in thatthe inlet pipe upstream of said degassing section has a length of between 150 and 450 times the diameter (D1) of the inlet pipe.

14. The separator system according to any of the claims 1 - 9,characterised in thatthe inlet pipe upstream of said degassing section has a length of between 180 and 550 times the diameter (D1) of the inlet pipe.

15. The separator system according to any of the preceding claims,characterised in thatthe inlet pipe has a downward inclination in the downstream direction at least in the degassing section (2).

16. The separator system according to claim 15,characterised in thatsaid inclination has an angle of 1° - 20°, preferably 3° -15° and more preferably 5° -12°.

17. The separator system according to any of the preceding claims,characterised in thatthe substantially vertical degassing pipes has a diameter that is substantially equal to the diameter (D1) of the inlet pipe upstream of the degassing section and a length of between 2 and 20 times the diameter of the inlet pipe upstream of said degassing section.

18. The separator system according to any of the preceding claims,characterised in thatsaid gas collection pipe has an upward inclination in the downstream direction, said inclination håving an angle of 1° - 20°, preferably 3° -15° and more preferably 5° -12°.

19. The separator system according to any of the preceding claims,characterised in thatthe inlet pipe is substantially horizontal between the degassing section (2) and the gravity separator (4) or has an upward inclination in the downstream direction.

20. The separator system according to any of the preceding claims,characterised in thatthe diameter of the inlet pipe between the degassing section and the gravity separator is at least equal to the diameter of the inlet pipe upstream of the degassing section.

21. The separator system according to claim 13,characterised in thatthe steeper inclination is a part of an inverted u-shaped pipe section.

22. The separator system according to any of the preceding claims,characterised in thatthe inlet pipe between the degassing section and the gravity separator has a constant diameter.

23. The separator system according to any of the preceding claims,characterised in thatthe perforated pipe within said separation receptacle is substantially horizontal and has a diameter that is larger than the inlet pipe upstream of the degassing section and is at least equal to the diameter of the inlet pipe between the degassing section and the gravity separator.

24. Method for breaking down a dispersion band between an oil phase and a water phase,characterised in thatthe water is retained at and allowed to establish at a certain level, so that the water flows at zero or low velocity, while the oil is allowed to flow freely on top of the water phase at a higher velocity.

25. Method according to claim 24,characterised in thatfree gas is removed from the oil and water upstream of the breaking down of the dispersion band.

26. Method according to claim 24 or 25,characterised in thata stratified flow regime is ensured upstream of by keeping the superficial liquid velocity: - below 9 m/s for superficial gas velocities below 7 m/s or above 14 m/s and - below 3 m/s for superficial gas velocities between 7 and 14 m/s.