Facility and method for underwater disposal of the water produced during underwater production of hydrocarbons at great depths

Abstract

A facility for the subsea disposal of the water produced during deepwater hydrocarbon production, includes a subsea oil/water separation station fed with fluids coming directly from at least one hydrocarbon production well, operating at a pressure independent of and lower than the ambient pressure, and comprising an oil outlet for connecting to a production unit and a water outlet, a flat gravity oil/water separation tank resting on the seabed, continuously fed with water leaving the oil/water separation station, operating at a pressure substantially equal to the ambient pressure, and comprising an oil outlet for connecting to the production unit and a water outlet, and a subsea high-pressure pump connected to the water outlet of the oil/water separation station and to a water inlet of the tank to raise the pressure of the water leaving the oil/water separation station to the ambient pressure before it is admitted into the tank.

Claims

1. A facility for subsea disposal of water produced during deepwater hydrocarbon production, comprising: a subsea oil/water separation station fed with fluids coming directly from at least one hydrocarbon production well, operating at a pressure independent of and lower than the ambient pressure, and comprising an oil outlet for connecting to a production unit and a water outlet; a flat gravity oil/water separation tank resting on the seabed, continuously fed with water leaving the oil/water separation station, operating at a pressure substantially equal to the ambient pressure, and comprising an oil outlet for connecting to the production unit and a water outlet; and a subsea high-pressure pump connected, on the one hand, to the water outlet of the oil/water separation station and, on the other hand, to a water inlet of said tank to raise the pressure of the water leaving the oil/water separation station to the ambient pressure before the water is admitted into said tank.

2. The facility according to claim 1, wherein the flat tank has a geometric configuration that allows a long residence time of the produced water, a low migration path for the oil droplets present in the water and a large oil/water interface area in order to promote oil/water separation by gravity.

3. The facility according to claim 2, wherein the tank further comprises, at the top, an oil droplet collection device opening toward the oil outlet of the tank.

4. The facility according to claim 3, wherein the tank further comprises, at the top, means for periodically discharging the oil present at the level of the collection device.

5. The facility according to claim 4, wherein the means for periodically discharging the oil droplets comprises a network of suction pipes controlled by an on-off valve capable of opening into the collection device.

6. The facility according to claim 1, wherein the tank further comprises, at the bottom, means for collecting and periodically discharging any solid particles deposited on a floor of the tank.

7. The facility according to claim 6, wherein the means for collecting and discharging solid particles comprise a network of suction pipes controlled by a valve, and nozzles for injecting pressurized water toward the floor of the tank.

8. The facility according to claim 1, wherein the oil outlet of the tank is connected to the oil outlet of the oil/water separation station via at least one on-off valve.

9. The facility according to claim 1, wherein the water outlet of the tank opens into the sea via a low-pressure pump or opens into a water injection well via a high-pressure pump.

10. The facility according to claim 1, wherein the tank has a cylindrical shape with a flat floor, an upwardly sloping roof to facilitate the collection of oil droplets, the water inlet which opens into the tank at the center thereof via a cyclonic device to impose an initial rotation speed on the water admitted into the tank, the oil outlet which is positioned at the level of the sloping roof, and the water outlet which comprises a plurality of scoops formed at the periphery of the tank and opening into the tank in tangential directions.

11. The facility according to claim 1, wherein the tank has a plurality of pipes of large diameter and long length arranged parallel to each other, resting on a seabed, the water inlet of the tank being at a same inlet end of each pipe, and the oil outlet and the water outlet of the tank being at a same opposite outlet end of each pipe.

12. The facility according to claim 11, wherein the seabed on which the tank rests has a horizontal slope, the pipes resting on the seabed so that their outlet end is tilted upwards relative to their inlet end.

13. The facility according to claim 11, wherein the seabed on which the tank rests is horizontal, the pipes resting on the seabed via tilting means so that their outlet end is tilted upwards relative to their inlet end.

14. The facility according to claim 13, wherein the tilting means comprises supports giving a tilt to the tank pipes resting thereon.

15. The facility according to claim 13, wherein the tilting means comprises systems for anchoring to the seabed the inlet end of the pipes and float systems connected to the outlet end of the pipes to tilt them upwards relative to the inlet ends.

16. The facility according to claim 1, wherein the tank has a pipe of large diameter and very long length, the water inlet of the tank being at an inlet end of the pipe which is anchored to the seabed, and the oil outlet and the water outlet of the tank being at the opposite outlet end of the pipe which is connected to floats to give the pipe a catenary shape.

17. The facility according to claim 11, wherein each pipe has a loop, serpentine or spiral shape.

18. The facility according to claim 1, wherein the tank comprises a single pipe forming a helix, the water inlet of the tank being at one inlet end of the pipe, and the oil outlet and the water outlet of the tank being at the opposite outlet end of the pipe.

19. The facility according to claim 1, wherein the tank comprises a plurality of channels of parallelepipedal cross-section which are arranged in a helix, the water inlet of the tank being at a same inlet end of each channel, and the oil outlet and the water outlet of the tank being at a same opposite outlet end of each channel.

20. The facility according to claim 1, wherein the tank comprises a plurality of identical tiles each in the form of a dihedral, inclined at 30° to the horizontal, and arranged in several series of tiles.

21. The facility according to claim 1, wherein the high-pressure pump is a multi-stage coalescing pump.

22. The facility according to claim 1, wherein the tank comprises a rigid shell.

23. A process for subsea disposal of water produced during deepwater hydrocarbon production, comprising feeding fluids coming directly from at least one hydrocarbon production well to a subsea oil/water separation station operating at a pressure independent of and lower than the ambient pressure and continuously feeding a flat gravity oil/water separation tank resting on the seabed with produced water leaving the oil/water separation station, the produced water feeding the tank having first been pressurized to reach a pressure substantially equal to the ambient pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a deepwater hydrocarbon production facility to which the invention applies;

(2) FIG. 2 shows a subsea produced water disposal facility according to an embodiment of the invention;

(3) FIG. 3 is a side view of the tank of the facility of FIG. 2;

(4) FIG. 4 is a cross-sectional view of the tank of FIG. 2;

(5) FIG. 5 shows in detail an example of collecting and discharging solid particles from the tank of FIG. 2;

(6) FIG. 6 shows in detail another example of collecting and discharging solid particles from the tank of FIG. 2;

(7) FIG. 7 shows a subsea produced water disposal facility according to another embodiment of the invention;

(8) FIG. 8 is an alternative embodiment of the tank of FIG. 7;

(9) FIG. 9 is another alternative embodiment of the tank of FIG. 7;

(10) FIG. 10 shows a tank of a subsea produced water disposal facility according to yet another embodiment of the invention;

(11) FIG. 11 shows a tank of a subsea produced water disposal facility according to an alternative embodiment of FIG. 10;

(12) FIG. 12 shows a tank of a subsea produced water disposal facility according to yet another embodiment of the invention;

(13) FIG. 13 shows a tank of a subsea produced water disposal facility according to yet another embodiment of the invention;

(14) FIG. 14 shows a tank of a subsea produced water disposal facility according to yet another embodiment of the invention;

(15) FIG. 15 shows in detail one of the channels of the tank of FIG. 14; and

(16) FIG. 16 shows a tank of a subsea produced water disposal facility according to yet another embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

(17) The invention applies to deepwater (i.e., greater than 700 meters) production and processing of hydrocarbons, in particular oil and gas, from oil fields with a growing increase in water content production. More precisely, it relates to the disposal (or treatment) of water content from the produced oil.

(18) Increasing the water content of the produced oil has become very common in most oil production fields, in particular during the latter part of the operating life, when the aquifer tends to reach the production wells.

(19) FIG. 1 shows an example of a deepwater hydrocarbon production facility 2 from an oil production field 4.

(20) Typically, an oil production field 4 is operated at a water depth p comprised between 1000 m and 3000 m. It consists of a plurality of hydrocarbon production wells 6 for collecting oil and gas from an oil reservoir 8 in the reservoir rock, and optionally at least one produced water injection well 7 in the aquifer 9.

(21) The extracted hydrocarbons are typically sent to a surface production unit 10, for example a floating production storage and offloading (FPSO) unit, via subsea pipes and risers 16.

(22) Before being sent to the production unit 10, the hydrocarbons extracted from the oil reservoir 8 are treated at the seabed 12 to, in particular, separate the produced oil from the water contained therein.

(23) To this end, the subsea hydrocarbon production facility 2 comprises a subsea oil/water separation station 14 which is fed with fluids from the hydrocarbon production wells 6.

(24) According to the invention, this oil/water separation station 14 operates at a pressure independent of and lower than the ambient pressure. This operating pressure is defined by the operator during crude oil flow and reservoir rock simulation studies. It is optimal for improving oil recovery. By way of example, for a water depth p of about 2000 meters, the operating pressure of the oil/water separation station can be of the order of 80 bars (the ambient pressure at this depth being 200 bars).

(25) Different types of subsea oil/water separation station may be used. In particular, as shown in more detail in FIG. 2, a modular liquid/liquid gravity separation device may be used consisting of a plurality of pipes without internal elements forming parallel loops 14a (called coils) which can be installed on the seabed due to the fact that their reduced diameter provides a high resistance to external/internal differential pressure.

(26) An example of such a modular liquid/liquid gravity separation device is described in the publication WO 2011/161343 on behalf of the Applicant. The long length and the circular cross-section of the pipes of this separation device allow for high differential pressure resistance in a relatively low weight configuration. Moreover, its modular configuration allows flexible installation conditions, even for high-capacity systems (high inlet flow). Thus, this device allows the primary separation of bulk raw water at any desired pressure, in particular at low pressure, independent of the ambient pressure (water depth).

(27) As shown in FIG. 2, the subsea oil/water separation station 14 further comprises a hydrocarbon inlet 14b, and an oil (and gas) outlet 14c which is connected to the production unit 10 at the surface via a riser 16 (and optionally by means of a pump 18).

(28) The subsea oil/water separation station 14 further comprises a water outlet 14d which is for connecting to a flat gravity oil/water separation tank 20-1 resting on the seabed 12.

(29) More precisely, the feed of water from the subsea oil/water separation station 14 is provided by means of a subsea high-pressure pump 22 which is connected, on the one hand, to the water outlet 14d of the oil/water separation station 14 and, on the other hand, to a water inlet 24 of said tank.

(30) The low, flat tank 20-1 is a produced water treatment system located downstream of the oil/water separation station 14. This system, which is described in detail below, operates at a pressure substantially equal to the ambient pressure (i.e., the external hydrostatic pressure due to the sea water column), i.e., at a pressure greater than the operating pressure of the oil/water separation station.

(31) To this end, the function of the high-pressure pump 22 is to raise the pressure of the water leaving the oil/water separation station until it reaches the ambient operating pressure of the tank 20-1 before it is admitted into the latter. For example, for a water depth p of about 2000 meters (i.e., an ambient pressure of 200 bars), the operating pressure of the oil/water separation station being of the order of 80 bars, the differential pressure required by the high-pressure pump 22 is 120 bars.

(32) Preferably, the high-pressure pump 22 is a multi-stage centrifugal coalescing pump to avoid shearing of the dispersed oil droplets by the action of the centrifugal pump impellers.

(33) The tank 20-1 according to the invention is sized to be installed and operated on the seabed, all the time, while in pressure equilibrium with the external hydrostatic pressure (due to the sea water column). This allows for much reduced loads on the tank leading to thin walls, which facilitates both the manufacturing and installation process of the tank.

(34) The produced water effluent from the subsea oil/water separation station 14 enters the tank 20-1 and moves through it at a very slow rate and remains inside it for a significant period of time (at least two orders of magnitude longer than in separation equipment normally used for this function). This means that the tank has a very large volume to allow its use in line with field production, in a continuous operation mode, with a long residence time (typically a minimum of 1 day).

(35) Various materials and components may be used in the construction of the tank 20-1, namely, for example, steel or aluminum sheets (with or without internal or external reinforcing structure), polymer or rigid composite sheets (with or without internal or external reinforcing structure or internal or external steel reinforcing structure), etc. The tank comprises a rigid shell.

(36) The detailed configuration of the tank 20-1 may vary (it may be flat parallelepipedic in shape, tubular or cylindrical in shape, or multi-cylindrical, with rigid or flexible walls, or in the shape of a very long tube, etc.), as long as the main geometrical characteristics are maintained, i.e., with a low height and a large footprint, forming a flat configuration. The configuration to be adopted will be selected according to the project data, aiming to implement an economical technical solution.

(37) Whatever its configuration, the tank has a geometrical configuration allowing a long residence time of the water, a low migration path for the oil droplets present in the water and a large oil/water interface area in order to promote the oil/water separation by gravity.

(38) Furthermore, the installation of the tank from the surface to the seabed will be carried out by filling it with a liquid less dense than sea water (fresh water, sea water, oil, diester, alcohol, etc.) or with sea water, an operation which may or may not be assisted by the use of floats to give the assembly positive buoyancy. The internal liquid and the floats will be recovered after installation on the seabed. Alternatively or additionally, solid floating balls inside the tank may also be used for the same purpose, i.e., they will be designed to ensure proper density management during installation to facilitate deployment at significant water depths, without the need for high-capacity lifting vessels.

(39) Injection of solid floating balls can also be used as well as injection of liquid less dense than sea water to recover the tank during the dismantling phase.

(40) The geometrical configuration of a tank according to a first embodiment of the invention will now be described in connection with FIGS. 3 to 6.

(41) In this first embodiment, the tank 20-1 has a cylindrical shape with a flat floor 26, an upwardly sloping roof 28 to facilitate collection of oil droplets, a water inlet 24 that opens into the tank, an oil outlet 30 that is positioned at the sloping roof 28, and a water outlet 32.

(42) By way of example, for 100 000 barrels per day of water to be disposed of (about 16 000 m.sup.3/day) and a residence time of one day, the dimensions of the tank could be a diameter ϕ of 90 m, a minimum height H.sub.m of 2 m (at the periphery) and a maximum height H.sub.M (at the center) of 4 m (the tank has the shape of a cylinder 2 m high assembled to a cone 90 m in diameter and 2 m in height).

(43) More precisely, the water inlet 24 opens into the center of the tank via a cyclonic device 34 for imposing an initial rotation speed on the water admitted into the tank. Such a cyclonic device 34 is known per se and will therefore not be described in detail. It promotes an initial separation of the oil and the dispersed sediments. The initial rotation speed inside the tank is also intended to avoid certain dead volumes (due to short fluid circuits) inside the tank.

(44) As shown in FIG. 4, the water is discharged by means of a plurality of scoops 36 formed at the periphery of the tank, which open into the tank in tangential directions, and which open toward the water outlet 32. In particular, these scoops 36 have tangential directions which allow the rotational movement imposed on the water by the cyclonic device 34 to be maintained.

(45) Furthermore, the tank 20-1 also comprises, at the top (i.e., at the highest part of its roof 28), an oil droplet collection device 38 (hereinafter referred to as a skimmer) which opens toward the oil outlet 30 of the tank.

(46) It should be noted that no internal tank structure is necessary to allow the oil droplets to reach the skimmer 38 positioned at the top point of the tank. Indeed, the oil droplets rise by gravity toward the roof 28 of the tank and the slope of the latter allows them to be directed toward the skimmer.

(47) The skimmer 38 is advantageously coupled to means for periodically discharging the skimmed oil. More precisely, oil accumulated in the skimmer is periodically (and automatically) removed therefrom via a network of suction pipes 40 which open into the skimmer.

(48) It will be noted that no active device is required for this discharge of the skimmed oil. Only an “on-off” valve 42 upstream of the connection of the suction pipe network 40 to the multiphase flow line on the side heading toward the surface is necessary to allow this periodic suction operation of the skimmed oil volume. In particular, the oil entrainment force is achieved by the pressure difference between the pressure inside the tank (corresponding to the subsea hydrostatic pressure on the seabed) and the operating pressure of the multiphase flow line which is lower to allow for increased oil recovery from the tank.

(49) The tank 20-1 further comprises, at the bottom, means for collecting and periodically discharging solid particles 44 which may possibly be deposited on the floor 26 of the tank.

(50) These solid particles 44 are typically solid residual sediment (namely sand) that is settled inside the tank due to the long residence time of the water. These particles are small in quantity and very small in size (most of the sand in the produced water having been treated upstream of the tank).

(51) As shown in FIG. 5, the means for collecting and periodically discharging solid particles may include a network of suction pipes 46 which are controlled by a valve 48, and water injection nozzles 50 which inject pressurized water toward the floor 26 of the tank.

(52) Specifically, the suction pipes 46 are distributed over the entire surface of the floor 26 of the tank (see FIGS. 2 and 3) and the water injection nozzles 50 are fed with water from the high-pressure pump 22 via a valve 52 and are suitably distributed a short distance from the tank floor.

(53) Moreover, similar to the above-mentioned suction relating to the accumulated oil disposal procedure, no active device is required to remove solid particles 44. Only an “on-off” valve 48 upstream of the connection of the suction pipe network 46 to a branch line 54 connected to the multiphase flow line (riser 16) on the surface side is required to enable the periodic suction operation, the entrainment force being the pressure difference between the pressure inside the tank (corresponding to the subsea hydrostatic pressure on the seabed) and the lower operating pressure of the multiphase flow line.

(54) Furthermore, it may be provided to inject pressurized water into the multiphase flow line 54 upstream of the valve 48 prior to its opening, which pressurized water also comes from the outlet of the high-pressure pump 22 via a valve 56.

(55) In an alternative embodiment shown in FIG. 6, the suction pipes 46′ of the periodic solid particle collection and discharge means 44 are external to the tank, i.e., they are located under the floor 26 of the tank outside the tank.

(56) In this alternative embodiment, the operation of the solid particle discharge remains identical to that described in connection with FIG. 5.

(57) The geometrical configuration of a tank according to a second embodiment of the invention will now be described in connection with FIGS. 7 to 8.

(58) In this second embodiment, the tank 20-2 has an assembly of a plurality of juxtaposed pipes 58 (or pipe sections) of large diameter (typically greater than 3 m) and long length (typically a few hundred meters) that are arranged parallel to each other.

(59) The size and number of pipes 58 are determined according to the flow rate of the produced water to be treated for a specific application. By way of example, for 100 000 barrels per day of water to be disposed of (about 16 000 m.sup.3/day) and a residence time of one day, there may be 10 juxtaposed pipes, each with a diameter of 4 m, and a length of 135 m.

(60) The water inlet 24 of the tank 20-2 is located at one inlet end of each pipe 58, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of the pipes (called the outlet end).

(61) As in the first embodiment previously described the oil outlet 30 is at the top of the pipes 58 at the same end thereof through an oil droplet collection device 38 (or skimmer) and means for periodically discharging the skimmed oil (not shown in the figure).

(62) Similarly, the tank 20-2 may further comprise means for collecting and periodically discharging solid particles that may have settled at the bottom of the pipes 58 of the tank.

(63) In this second embodiment, such collection means (not shown) are in the form of the means described in connection with FIG. 6 (i.e., with the suction pipes located outside the tank pipes).

(64) Furthermore, the seabed on which the tank 20-2 rests advantageously has a slight horizontal slope so that the pipes 58 can be made to rest so that their outlet end (that provided with the oil outlet 30 and water outlet 32) is inclined upwards with respect to their inlet end (that provided with the water inlet 24).

(65) In other words, it is advantageous to exploit the slope of the seabed to slightly tilt the tank pipes upwards in the direction of water flow. This upward slope thus facilitates the collection of oil at the outlet end of the pipes.

(66) In the absence of a seabed with a slight horizontal slope (the case of a seabed on which the tank rests which is horizontal), it is possible to make the tank pipes rest on the seabed by means of tilting means so that their outlet end is inclined upwards with respect to their inlet end.

(67) The alternative embodiment of FIG. 8 is an example of the implementation of tilting means.

(68) In this alternative embodiment, the tank 20-2 is identical to that described in connection with FIG. 7 with its juxtaposed pipes 58 of large diameter and long length.

(69) Compared with the embodiment described in connection with FIG. 7, the tank pipes 58 here rest on a rigid support structure 60 giving an incline to the pipes in the direction of water flow.

(70) It will be noted that the pipes 58 of the tanks 20-2 described in connection with FIGS. 7 and 8 may have a circular cross-section as shown in those figures. Alternatively, in an alternative embodiment not shown in the figures, the tank pipes have a square or rectangular cross-section.

(71) In connection with FIG. 9, an alternative embodiment of this second embodiment of the tank according to the invention will now be described.

(72) In this alternative embodiment, the tank 20-2 still has an assembly of a plurality of juxtaposed pipes 58 of large diameter and long length that are arranged parallel to each other (the other elements of the tank are not shown in FIG. 9).

(73) In this alternative embodiment, the pipes 58 are held inclined in the direction of water flow by systems for anchoring 62 to the seabed 12 at their inlet end, and by float systems 64 connected to their outlet end to incline them upwards relative to the inlet ends.

(74) Here, in other words, the pipes 58 in the tank are given a catenary shape by the use of floats and anchoring systems.

(75) In connection with FIG. 10, the geometrical configuration of a tank 20-3 according to a third embodiment of the invention will now be described.

(76) In this third embodiment, the tank 20-3 comprises a single pipe 66 of large diameter (typically greater than 3 m) and very long length (typically several hundred meters to several kilometers).

(77) The size of the pipe 66 is determined according to the flow rate of produced water to be treated for a specific application. By way of example, for 100 000 barrels per day of water to be disposed of (about 16 000 m.sup.3/day) and a residence time of one day, the pipe 66 may have a diameter of 4 m and a length of 1350 m.

(78) The water inlet 24 of the tank 20-3 is located at an inlet end of the pipe 66 which is anchored to the seabed 12 by mooring systems 68, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of the pipe (outlet end) which is connected to floats 70 to give the pipe a catenary shape.

(79) In connection with FIG. 11, the geometrical configuration of a variant of the tank according to the third embodiment of the invention will now be described.

(80) In this alternative embodiment, the tank 20-3 further comprises a single pipe 66′ which does not extend in a single direction but has a serpentine shape. The water inlet 24 of the tank 20-3 is always located at an inlet end of the pipe 66′, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of the pipe (outlet end).

(81) The footprint of the facility can thus be reduced.

(82) In connection with FIG. 12, the geometrical configuration of a tank according to a fourth embodiment of the invention will now be described.

(83) In this fourth embodiment, the tank 20-4 has an assembly of a plurality of juxtaposed pipes 72 (or pipe sections) of large diameter (typically greater than 3 m) and long length (typically a few hundred meters) that are arranged in loops.

(84) The size and number of pipes 72 are determined according to the flow rate of the produced water to be treated for a specific application.

(85) Furthermore, the water inlet 24 of the tank 20-4 is located at one inlet end of each pipe 72, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of each pipe.

(86) More precisely, depending on the tilt of the pipe loops 72, the oil outlet 30 may be at the level of the pipe loop (case of FIG. 12—tilt toward the top of the tank) or at the level of the bottom of the loops (case of a tilt toward the bottom of the tank—not shown in the figures).

(87) In an alternative of FIGS. 11 and 12 not shown in the figures, the pipe(s) may be spiral shaped.

(88) In connection with FIG. 13, the geometrical configuration of a tank according to a fifth embodiment of the invention will now be described.

(89) In this fifth embodiment, the tank 20-5 comprises a single helix-shaped pipe 82. The diameter and length of this helix-shaped pipe depends on the flow rate of water to be treated. For example, the pipe 82 may have a diameter of 3.5 m and be wound on a cylinder 46 m in diameter and 42 m in height, which represents a flow rate of about 100 000 barrels per day of water to be disposed of (about 16 000 m.sup.3/day).

(90) In this embodiment, the water inlet 24 of the tank 20-5 is always located at an inlet end of the pipe 82, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of the pipe (outlet end).

(91) In the embodiments described in connection with FIGS. 7 to 13, the pipe(s) is/are made from polymeric plastic materials, such as Weholite® brand high-density polyethylene or the like.

(92) In this case, the inner wall of the pipe(s) could be coated with a material suitable for contact with the solvents contained in the oil (mainly aromatic cuts), such as Teflon®.

(93) In connection with FIGS. 14 and 15, the geometrical configuration of a tank according to a sixth embodiment of the invention will now be described.

(94) In this sixth embodiment, the tank 20-6 comprises a plurality of channels 84 of parallelepipedal cross-section that are arranged in a helix. The dimensions of the channels and of the helix that they form depend on the flow rate of water to be treated. By way of example, the channels 84 may each have a height of 0.5 m and a width of 4 m and the helix they form may be arranged on a cylinder with an internal diameter of 8 m and an external diameter of 16 m, a height of 28 m and having an angle α with the horizontal plane of 30° (see FIG. 15), which represents a total volume of about 4256 m.sup.3.

(95) FIG. 15 shows in more detail one of the channels 84 of such a tank. In particular, in this embodiment, the water inlet 24 of the tank is located at one inlet end of each channel 84, and the oil outlet 30 and water outlet 32 of the tank are located at the opposite end of each channel.

(96) Moreover, the means for collecting and periodically discharging the solid particles 44 are here realized for each channel 84 by a suction pipe 46 located at the lower part of the inlet end of the channel.

(97) Such a tank has a number of advantages. In particular, the slope of the helix prevents the accumulation of solid particles and the formation of oil pockets. Moreover, the path of an oil drop between the inlet and outlet of the tank is minimized, which makes it possible to drastically reduce the volume of the tank for a given performance. Furthermore, because the path of the oil drops is very short, the tank can be flat and of low height.

(98) In connection with FIG. 16, the geometrical configuration of a tank according to a seventh embodiment of the invention will now be described.

(99) In this seventh embodiment, the tank 20-7 comprises a plurality of identical tiles 86 each in the form of a dihedral, inclined at 30° to the horizontal, and arranged in four series. These tiles 86 define as many zigzag channels within which the oil droplets tend to rise and be guided by the angle of the dihedral, while the solid particles tend to fall from the edge of the dihedral faces.

(100) By way of example, this tank 20-7 can be inscribed in a rectangular parallelepiped of 10 m in width by 44 m in length and 23 m in height, in which the tiles each have a passage section of 0.75 m in height by 11 m in width, for a developed length of 40 m. This represents a volume of 6600 m.sup.3 for a volume of the parallelepiped of 10120 m.sup.3.

(101) In this embodiment, the water inlet 24 of the tank 20-7 is located at an inlet end at the bottom of the parallelepiped, and the oil outlet 30 and water outlet 32 of the tank are located at opposite ends at the top of the parallelepiped.

(102) Moreover, the means for collecting and periodically discharging solid particles are here realized at the bottom by suction pipes 46.

(103) Whatever the configuration of the tank, it should be noted that the water outlet of the tank may lead directly into the sea by means of a low-pressure pump 74 optionally coupled with a filtration system 76.

(104) In this case, the filtering elements of this filtration system 76 will present a low clogging load during the operation of the facility according to the invention, due to the very low concentration of residual oil expected in the removed water, allowing a long operation campaign, before requiring a filtering element replacement. Thus, the filtration system must be designed to allow the exchange of these elements by ROV (with a tooling developed for the need), without requiring the recovery of the entire module, in order to respect the high robustness, reliability and availability of the facility according to the invention.

(105) Alternatively, the water outlet of the tank may lead to a water injection well 7 via a high-pressure pump 80 (see FIG. 2 in particular).