OA19947A - Modular installation and process for liquid / gas separation, in particular of the liquid and gas phases of crude oil. - Google Patents

Abstract

The present invention relates to a modular installation for liquid/gas separation of a multiphase fluid such as crude oil comprising a plurality of unitary separation devices consisting of an enclosure (1) cooperating at its base with a coaxial connection part (6) connected or intended to be connected to a support structure (10) resting on the seabed (20) capable of receiving a plurality of said unitary separation devices (1) in a reversible manner so as to be able to ensure disconnections at will, in particular to increase the separation capacities of the installation and/or ensure maintenance of the unitary separation device on the surface.

Description

MODULAR INSTALLATION AND LIQUID / GAS SEPARATION PROCESS,

IN PARTICULAR LIQUID AND GASEOUS PHASES OF CRUDE OIL.

The present invention relates to a modular installation for liquid / gas separation of vertical type as well as a process for the separation of multiphase fluid, in particular oil-water and gas contained in crude oil.

The technical sector of the invention is more particularly the field of oil production, and more particularly the field of offshore oil fields at great depth.

The production of oil in the deep sea is generally carried out from a floating support anchored near the oil wells located at the level of the seabed, that is to say at varying depths of 1000 to 2500m, see more. The floating support generally includes anchoring means to remain in position despite the effects of currents, winds and swells. It also generally comprises means for storing and treating the oil as well as means for unloading it towards stripping oil tankers, the latter appearing at regular intervals to carry out the removal of the production. The common name of these floating supports is the Anglo-Saxon term

Floating Production Storage Offloading (meaning floating means of storage, production and unloading) of which the abbreviated term FPSO is used throughout the following description.

The wellheads are generally connected to said FPSO by underwater pipes either of the SCR type, that is to say pipes suspended in the configuration of a chain, or of the hybrid tower type comprising:

a vertical riser, the lower end of which is anchored to the seabed and connected to a said pipe resting on the seabed, and the upper end is stretched by a float submerged in the subsurface 30 to which it is connected, and

- a connecting pipe, in general a flexible connecting pipe, between the upper end of said riser and a support floating on the surface, said flexible connecting pipe taking, where appropriate, by its own weight the shape of a curve in a plunging chain, that is to say descending well below the float to then go up to said floating support.

The entire crude oil production is thus generally brought back on board the FPSO to be treated there with a view to separating the oil itself, water, gas, and possible sandy components. The oil, once separated, is thus stored on board, the gas is washed, then sent to the gas turbines for the production of electricity and the heat required on board, then the surplus is reinjected into the field reservoir. tanker so as to pressurize said reservoir. The water, after having been released from the sand in suspension, is finally either discharged into the sea after extensive extraction of any oil particles, or also reinjected into the reservoir, with additional seawater taken from the sub-surface. , generally coming in addition, to achieve the necessary flow rate of water injection into the tank. The extracted sand, which represents only minimal quantities, is finally washed and then discharged into the sea.

We know the method of separating gas, water and crude oil commonly used on land fixed installations and on FPSOs, which consists of using pressure-resistant enclosures, of very large volumes, generally of elongated cylindrical shape, the crude oil entering at one end and traveling along said enclosure for a period of the order of 3-10 minutes during which the various phases separate naturally by gravity to reach the second end. The gas is then recovered in the upper part of said enclosure, water and sand in the lower part, and petroleum (oil) in the intermediate part. There is a very wide variety of separators of this type which generally incorporate additional internal devices, such as horizontal, vertical or oblique screens, the object of which is to facilitate the separation of the phases and prevent them from being remixed in a single phase. later stage.

These separators operate at low pressure, for example 3-10 bars, sometimes even under vacuum, so as to optimize the degassing of the crude oil. If it is desired to install this type of separator at the bottom of the sea, said enclosure must be able to withstand crushing under the effect of the pressure which is substantially 100 bars, or substantially lOMPa per section of 1000m of water, as well as to the internal pressure which depends on that of the oil reservoir and can reach in some cases 1400 bars. Thus the transposition of such an enclosure for its use in great and very great depth would require wall thicknesses of 100 to 300 mm to resist implosion and such boilerwork elements would be very delicate and very expensive to produce and install. at the bottom of the sea at great depth.

In FR 2 915 403 in the name of the applicant, an underwater gas-liquid separation device is known, but it comprises an enclosure containing a set of enclosures arranged vertically fixedly connected to each other of large masses and volumes, this assembly proving, in certain cases, difficult to set up at the bottom of the sea and requiring installation vessels of significant offshore lifting capacity, in particular requiring the use of installation vessels of more than 200m in length . In addition, internal elements of the enclosure such as the sensors risk breaking down and / or becoming clogged, possibly with a potential risk of clogging of the liquid discharge outlets. These incidents require maintenance which cannot be carried out by disconnecting and recovering the enclosure individually by the usual maintenance vessels (known under the name “IMR vessel”) which are generally of relatively small size and lifting capacity.

In another field, in FR 2 961 712 in the name of the applicant, a modular installation for subsea liquid-liquid separation has been proposed consisting of a plurality of pipes slightly inclined with respect to the horizontal in parallel which can be connected and disconnected. individually at will and which can therefore be installed at the bottom and therefore handled, thus limiting the mass of the elements to be handled from the surface.

An object of the present invention is to provide an improved gas / liquid separation device capable of being installed and operating at the bottom of the sea at great depth, in particular at least 1000 m, which is simpler and less expensive to produce, install and put. implemented at the bottom of the sea, and more particularly, an installation which can be installed and retrieved with a vessel of reduced size and lifting capacity and whose maintenance can be carried out using a vessel of usual size and lifting capacity for offshore installation maintenance and assistance.

To do this, the present invention provides a modular underwater installation for the liquid / gas separation of two respectively liquid and gaseous phases of a fluid, in particular the liquid and gaseous phases of a crude oil, comprising:

(a) a main support structure resting and / or anchored to the seabed on which are connected or able to be connected in a reversible manner a plurality of unitary liquid / gas separation device, and (b) at least one unitary device for liquid / gas separation fixed on said main support structure, each said unitary liquid / gas separation device comprising a single enclosure comprising a sealed wall resistant to the pressure at the bottom of the sea and to the internal pressure of said fluid arranged along an axis of revolution vertical, said enclosure containing:

bl) a first internal pipe for supplying the multiphase fluid going up within said enclosure from its lower end comprising or cooperating with a first orifice passing through a wall part called the bottom wall of the enclosure, to its end upper open opening into the upper part of the enclosure, and b2) a second internal gas discharge duct extending within said enclosure from its open upper end, preferably arranged higher than said upper opening of the first duct , up to its lower end comprising or cooperating with a second orifice passing through the bottom wall of the enclosure, and b3) a third orifice passing through said bottom wall of the enclosure around said first and second orifices of the bottom wall of the enclosure, and

c) a plurality of 3-way connection pieces fixed to said support structure, including at least one said connection piece connected to a said enclosure, each said connection piece comprising:

c) a first connection duct, an upper end of which is connected or adapted to be connected to a said first port in the bottom wall of said enclosure, said first connection duct comprising a first outlet port being connected or adapted to be connected to an external pipe for supplying said multiphase fluid to the seabed or a pipe element connected to the end of an external supply pipe for said multiphase fluid to the seabed, and c2) a second connection duct, an upper end of which is connected or able to be connected to a said second orifice in the bottom wall of said enclosure, said second connection duct comprising a second outlet orifice connected or able to be connected to an external duct d 'gas discharge, and c3) a third connection duct arranged around said first and second connection ducts at its upper end connected or able to be connected to a n said third orifice in the bottom wall of said enclosure, said third connection duct comprising a third outlet orifice connected or capable of being connected to an external liquid evacuation duct.

Preferably, said second orifice in the bottom wall of the enclosure is a second annular orifice arranged coaxially around said first tubular orifice in the bottom wall of the enclosure, and said third orifice in the bottom wall of the enclosure is a third annular orifice 30 arranged coaxially around said first and second orifices of the bottom wall of the enclosure, and said three-way connection piece is a coaxial three-way connection piece fixed vertically on said support structure comprising:

c) a first connection duct arranged vertically and of which said first outlet orifice is formed by the axial lower end of said first connection duct, and c2) a second connection duct, arranged coaxially and around said first duct at its end upper, connected or be able to be connected vertically to a said second annular orifice of said bottom wall of the enclosure, comprising a said second lateral outlet orifice, and c3) a third connection duct arranged coaxially around said first and second connection ducts at its upper end, connected or capable of being connected vertically to a said third annular orifice of said bottom wall of the enclosure, comprising a said third lateral outlet orifice.

More particularly, said first and second lateral outlet orifices are oriented in two diametrically opposed directions on each side of the connection piece and in a direction substantially perpendicular to said vertical axial direction.

The term “reversible” connection is understood to mean that the unitary separation devices after connection to the support structure can be disconnected in order to be brought up to the surface and / or undergo maintenance before being reconnected.

Advantageously, said support structure supports valves cooperating with the external fluid supply pipe, external gas discharge pipe and respectively external liquid discharge pipe to allow the closure of said external pipes before connection and disconnection of. a so-called unitary separation device.

It is also understood that said first connection duct is open at its lower end in axial continuity with its upper opening connected or connectable to said first central tubular orifice of the enclosure, and, on the other hand, said second and third connection ducts. the 3-way connection piece are closed at their axial lower ends.

The ability to implement a plurality of said unit devices makes it possible to use enclosures of relatively low size and weight, and in particular less than 100 T and preferably less than 50 T, so as to be able to lower them from the surface to to a so-called support structure at the bottom of the sea, in particular at great depths of more than 1000 m or even more than 3000 m, with maintenance boats of lifting capacity usual for assistance boats and maintenance of offshore installations. Thus, it is possible to increase or reduce the number of unit devices for treating a required quantity of fluid to be separated or for maintaining it on the surface, at will as needed.

The coaxial arrangement according to the invention of said first, second and third orifices passing through the bottom wall of the enclosure and of said first, second and respectively third conduits of said connection piece is particularly advantageous in several respects.

First of all, this coaxial arrangement allows that after connection of the connection piece with said first, second and third orifices on the underside of the bottom wall of said enclosure, a first internal seal between said first duct of the connection piece and the lower end of said first tubular orifice of the internal fluid supply pipe, is substantially in equal pressure, being exposed on one side to the pressure Pl inside said first internal pipe of fluid supply, and on the other side substantially the same pressure P1 in said second internal gas discharge pipe. Likewise, a second internal seal between said second duct of the connection piece and the lower end of said second annular orifice of the internal gas discharge duct is exposed on one side substantially to the pressure Pl in said second internal pipe and on the other side within said enclosure outside said second internal gas discharge pipe also substantially at the same pressure P1.

This equal pressure of the two internal seals is particularly advantageous because the fluid leak rate in the event of failure of said seals is relatively negligible. In addition, the risk of seawater leakage in the gas line is extremely low, as the simultaneous failure of the 3 concentric seals would be necessary for this critical event for the gas line to occur.

The coaxial arrangement of the connection piece with said first, second and third orifices on the underside of the bottom wall of said enclosure is also advantageous for more easily making the connection of the enclosure with said connection piece because it allows angular rotations. relative of the enclosure along the axial vertical axis of the enclosure and of said connection piece during the lowering and docking of the enclosure on the connection piece, these angular variations frequently occurring in such circumstances and being difficult to prevent. Thus, the docking guidance is simplified, requiring no precise angular indexing to achieve this docking.

Said second internal duct and said second connection duct may be arranged coaxially inside said first internal duct and respectively said first connection duct, at the level of said first and second orifices of the enclosure. However, preferably, said second internal duct and said second connection duct are arranged coaxially around said first internal duct and respectively said first connection duct at said first and second orifices of the enclosure. In fact, the central axial arrangement of the opening at the lower end of said first central duct of said connection piece is advantageous because it also allows said external supply pipe for said multiphase fluid at the bottom of the sea or said pipe element connected to the end of an external pipe for supplying said multiphase fluid to the seabed, has a sufficiently large radius of curvature (without making a T-bend) at its junction with the part of connection with respect to the axial direction ZZ 'of flow of the fluid within said connecting piece then said first axial internal pipe so as not to disturb this flow and therefore to avoid shearing the fluid to avoid the splitting of gas bubbles and / or liquid droplets making the subsequent gas-liquid separation more difficult, before the phase separation within the enclosure at the upper outlet of the first pipe.

Preferably, said support structure resting on the seabed supports at least one multiphase fluid division and distribution device having a single main tubular axial feed orifice preferably arranged vertically, the open lower end of which is connected or connectable to the end of an external pipe for supplying said multiphase fluid to the seabed, and the open axial upper end of which communicates with a plurality of secondary tubular outlet orifices inclined with respect to said axial direction, likewise diameter and inclined at the same angle (beta) and disposed symmetrically and regularly with respect to and around said axial direction, said secondary tubular outlet orifices being connected or able to be connected to a plurality of fluid transfer pipe elements power supply connected or able to be connected respectively to a plurality of said first di your connection pieces connected or able to be connected to the lower ends of a plurality of said enclosures.

Preferably, said connection pieces and said enclosures are arranged relative to said dividing device, preferably above and around said dividing device, so that the pressure drops between said dividing device and said enclosures are substantially identical between them. they.

More preferably, said connecting pieces and said enclosures are arranged symmetrically with respect to said dividing device above and around said dividing device.

It is understood that said secondary tubular outlet orifices have a diameter smaller than that of said main tubular axial feed orifice. Thus, the flow of multiphase fluid supply is divided into a plurality of streams of the same flow rate, the flow rate of each divided flow thus becoming less than the main flow arriving through the main external supply pipe.

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This fluid flow dividing device is advantageous because it makes it possible to distribute in equal parts and homogenize the fluid sent within the various unit separation devices in terms of composition and in terms of pressure drops undergone by the fluid within the units. said transfer control elements.

More particularly, said enclosure is integral with first guide elements of the male or preferably female type, arranged outside said enclosure, and said support structure comprises second guide elements, of the female or respectively male type, cooperating or capable of in cooperating with said first complementary guide elements to achieve vertical docking of said unitary device in front of said connection part.

It will be understood that these first and second guide elements make it easier to make the coaxial connections of said upper ends of said first, second and third conduits of the connection piece with said first, second and respectively third orifices passing through the bottom wall of the enclosure descended from the surface.

More particularly, said enclosure comprises a cylindrical side wall of circular section, the upper end of which is closed by a domed cap, preferably spherical and the bottom wall of which at its lower end is in the form of a funnel tapering downwards. to form a vertical tubular bottom wall delimiting said third annular liquid discharge port. The funnel shape facilitates the evacuation of solid sedimentations, in particular sand.

More particularly, said first internal pipe for supplying the multiphase fluid extends vertically, preferably substantially in the axis of said enclosure, inside said second internal pipe below an upper part of the first internal pipe, which part upper first internal duct forms a turning loop, outside said second internal duct, in particular a loop with multiple curvatures in the form of a gooseneck, terminating in a fluid outlet nozzle preferably with a flared wall defining a said upper opening of said first internal pipe, also preferably with a flattened cross section, so that said upper opening of the first pipe opens out near the side wall of the enclosure and that the fluid flows therefrom parallel to said cylindrical side wall and oriented in a downward sloping direction, and said second internal discharge duct of gas extends axially coaxially with said first internal duct below said upper portion thereof.

The flattened shape of the cross section of said opening of said nozzle causes laminar flow of the fluid pressed against the wall of the enclosure. On the other hand, the widening flared shape of the wall of said nozzle slows down the flow speed of the fluid at the outlet of the fluid supply pipe. And this slowing down of the fluid as well as its tangential flow to the lateral wall of the enclosure of the multiphase fluid at the outlet of said nozzle by the centrifugal force which it undergoes, prevents the impact of the fluid against the wall and promotes separation in two phases of gas and liquid of said fluid while minimizing the fractionation of gas bubbles, as well as the projection of drops of liquid towards the suction port of said internal gas discharge pipe.

Preferably, to minimize the fractionation of gas bubbles, said outlet nozzle at the upper end of said first internal duct is further oriented inclined downwardly at an angle substantially identical to that of the helical discharge gutter. liquid described below.

In a preferred embodiment, said first internal pipe and second internal pipe have coaxial portions of larger diameters than their respective lower parts. Said first internal pipe in fact comprises an intermediate portion of larger diameter in the middle part so as to contribute to a slowing down of the flow rate of the fluid within said first pipe, the second coaxial internal pipe also undergoing the same widening. of diameter opposite said first internal pipe, to maintain an annular space between said first and second internal pipes substantially constant.

More particularly, the enclosure comprises a helical gutter fixed near and / or against the internal surface of said side wall of the enclosure and the upper end of which is against the bottom of the upper opening of the first internal duct and the lower end arrives above said third annular orifice.

In one embodiment, said helical gutter is attached to said second internal pipe.

Said helical gutter makes it possible to promote the liquid-gas separation by increasing the residence time of the partially degassed fluid to descend to the bottom of the chamber and to channel the discharge of said liquid separated from said gas at the outlet of said nozzle towards said third peripheral annular orifice . These characteristics, combined with the implementation of a flared outlet nozzle as described above, also allow the speed of the particles of partially degassed fluid flowing downward inside said enclosure, to be reduced with respect to the velocity of the raw fluid particles entering said first internal duct.

More specifically, the enclosure. contains in its upper part a device for filtering liquid droplets, preferably a coalescing device, on the gas path between the upper opening of the first internal pipe and the upper opening of the second internal pipe. In this way, this type of coalescing device is of the sieve type (also referred to as a “demister” or “demister”) and filters the liquid droplets which can be entrained by the gas.

More particularly, the enclosure contains pressure detection sensors and liquid level sensor within the enclosure, preferably a level control device which is a radar or sonar type probe or a nucleonic level sensor consisting of a ray-emitting bar and one or more measuring bars making it possible to measure the density profile of the fluid over the entire height of the enclosure. Such a sensor, known to those skilled in the art, here makes it possible to check that the liquid level arrives above the lower opening of said third orifice of the first internal pipe and does not protrude above the upper opening of the second internal gas discharge pipe, that is to say sufficiently distant from said upper openings of said first and second internal pipes.

More particularly, the enclosure is formed of two enclosure parts fixed to one another comprising:

- an upper wall part comprising at its upper end a domed, preferably spherical cap, and

- a lower wall part comprising a said bottom wall.

This feature makes it possible to facilitate the maintenance of the elements inside the enclosure, such as the coalescing device, the sensors, said first and second internal conduits, the helical gutter, by accessing them by separating the two wall parts of the 'enclosure, and if necessary by raising to the surface only the upper part of the enclosure and the equipment attached to it, to carry out their maintenance.

More particularly, said support structure comprises a plurality of said connection pieces and said enclosures which are connected thereto, preferably at least 4 said connection pieces, preferably arranged symmetrically.

According to other particular characteristics:

- Said enclosure is of elongated shape with a circular section, the upper end of which is in the form of a partial spherical cap, of the cigar type, arranged vertically; the shape of said enclosure gives it good resistance to hydrostatic pressure at the bottom of the sea, making it possible to use walls only 10 to 300mm thick;

- The length L1 of said enclosure is greater than or equal to 10 times its diameter D1, preferably 15 to 30 times its diameter DI; L1 being more preferably from 5 to 50m and its diameter Dl from 0.5 to 5m; this dimensioning of the diameter and wall thickness ensures optimum resistance to compression at large bottom for the walls of said pressure-resistant enclosure;

- The opening at the upper end of said second internal pipe is located at a height h relative to the upper opening of said first internal pipe of at least 1 m.

A subject of the present invention is also a unitary device for the liquid / gas separation of two respectively liquid and gaseous phases of a fluid, useful in a modular installation as defined above.

A subject of the present invention is also a method for producing a modular installation according to the invention, characterized in that the following successive steps are carried out in which:

a) a so-called unitary device suspended from a link is lowered from a vessel on the surface, and

b) preferably, its approach to docking is guided with a said connection piece fixed to said support structure, using first guide elements integral with said enclosure and said second guide means integral with said support structure , said second guide elements cooperating with said first guide elements, and

c) said enclosure is connected to said connection piece, in a disconnectable manner, by means of a fixing flange and coaxial seals 20 between said first, second and third orifices passing through the bottom wall of the 'enclosure and said first, second and respectively third connection conduits of said connection piece.

The present invention also relates to a process for separating the two liquid and gaseous phases of a crude oil using a modular installation according to the invention, characterized in that the following steps are carried out, in which:

1) the crude oil is sent, via an inlet pipe, to said first orifice of said bottom wall of the enclosure 30 via and through said first pipe, a said connection piece, to a reduced pressure PI, lower than the static pressure on the seabed P2, and

2) the crude oil rises within said first internal pipe enclosure, then flows at the upper outlet of said first internal pipe in flow tangentially to the side wall with a circular section of the chamber, which causes its separation into a liquid phase and a gas phase, then

3) the partially degassed liquid or fluid phase descends back to said third orifice in the bottom wall at the lower end of the enclosure, and preferably fills said enclosure to a height level below the upper opening of the enclosure. said first internal pipe, and is collected at an external liquid discharge pipe connected to said third outlet port of said connecting piece, and

4) the gas separated from said oil is sucked in said second internal pipe towards said second orifice in the bottom wall at the lower end of the enclosure, and it is recovered, at the level of an external discharge pipe gas connected to said second outlet 15 of said connection piece.

Other features and advantages of the present invention will become apparent in the light of the detailed description of the embodiments which follows, with reference to Figures 1 to 4:

- Figure 1 shows a view of a lowering of a unitary separation device 1 for docking and connection to a support structure 10 resting on the seabed and on which two unit separation devices 1 are already connected , the various connection pieces 6 being connected to a flow dividing device 12 supplied by a crude oil supply line 13 connected to a wellhead 18, and

- Figure IA shows a top view of a modular installation comprising a support structure 10 resting on the seabed to which are connected eight unit separation devices 1 and,

- Figure IB shows a schematic view of an underwater liquid-gas separation installation according to the invention showing a single unit separation device including said first internal pipe 2 for supplying multiphase fluid and second internal pipe 3 of gas evacuation which are schematized side by side and not coaxially as according to the invention, in order to better differentiate them and to show the differentiated outlets of the external multiphase fluid supply pipe 13 on the one hand, and the external pipe of liquid discharge 15 and external gas discharge pipe 16 cooperating with valves 11a, 15b and 16b respectively, and

- Figure 2 shows a longitudinal sectional view of a unitary gas-liquid separation device 1 according to the present invention comprising said first internal pipe 2 for supplying phasic fluid and said second internal pipe 3 for discharging gas 10 arranged coaxially and connected to their bases to a connection piece 6, and

- Figure 2A is a view showing the outlet and the winding of the upper part 2b of said first internal pipe to the outside and around said second internal pipe, and

- Figure 2B is a schematic horizontal sectional view of Figure 2A in top view above the fluid outlet nozzle 2c at the upper end 2b of the first fluid supply pipe 2, and

- Figure 3 shows a view in vertical axial section of the part 20 of three-way coaxial connection 6 according to the invention connected via annular joints 7-1, 7-2, 7-3 to the tubular lower part lc-2 of the bottom wall le of the enclosure 1 of FIG. 2, and

- Figure 4 shows a perspective view of a multiphase fluid flow dividing device 12 comprising six upper outlet orifices 12bl to 12b6, and

- Figure 4A shows a sectional view along a vertical median axial plane of the dividing part 12 of Figure 4.

The main support structure 10 rests on the seabed 20 by being fixed to a base resting on the seabed 20 or a suction anchor 14 driven into the seabed, this main support structure 10 consists of a frame formed of lattice girders.

The main support structure 10 is arranged near an oil well 18 equipped with a well head 18a connected by a crude oil inlet pipe 13 conveying the crude oil from the well head to said support structure. 10.

Said support structure 10 supports:

- a plurality of vertical guide rods 10a forming said second guide elements, and

- a plurality of three-way coaxial connection pieces 6, and

- at least one flow division device 12, and

- valves for closing / opening said following external conduits: a valve 11a controlling the arrival of multiphase fluid upstream of said connection part 6, a valve 15b controlling the discharge of the liquid within the external discharge pipe of liquid 15, and a valve 16b controlling the evacuation of gas within the external gas evacuation pipe 16, and

- an export pump 17 for the evacuation of liquid via the external liquid evacuation pipe 15.

Said main support structure 10 also advantageously supports portions of pipes connected or able to be connected to the ends of said external pipe for the arrival of multiphase fluid 13, external pipe for discharging degassed liquid 15, and external pipe for discharging gas. 16, said pipe portions being connected directly or indirectly to the lateral tubular orifices 6bl and 6cl and the central lower tubular orifice 6al of the connection piece 6 described below.

The unitary separation device 1 as shown in the figures comprises an enclosure 1a, the wall of which comprises a cylindrical main part of circular section surmounted by a domed bottom, such as for example a spherical cap, and ends at its lower end with a bottom wall the comprising an upper part in the form of a funnel here and a lower cylindrical part lc-2 of smaller diameter delimiting or extending by a part forming said first, second and third coaxial tubular orifices 5a, 5b, 5c described herein -after.

The cylindrical wall of the enclosure 1a is equipped on the outside with a frame 8 supporting said first guide elements 8a defining orifices delimited in the lower part by flared funnel walls to facilitate their passage through the guide rods 10a of the main support structure 10 for guiding the enclosure 1a when it is lowered from the ship 24 to the surface 25 using a cable 19 and possibly an ROV 26 (submarine robot operated by remote control distance).

The frame 8 comprises feet 8b making it possible to mechanically stabilize the unitary device 1 resting on the main support structure 10 and / or to fix it on the main support structure 10 after docking and connection to the connection piece 6 as described below. after. The complementary guide elements 8a and 10a can also be used for fixing the unitary device 1 after docking on the main support structure 10 and fixing to the connection piece 6.

Said first guide elements 10a are positioned relative to the connection pieces 6 fixed to the support structure 10 such that when the enclosure 1 is lowered to be docked on the main support structure 10 by causing said first and second complementary means to cooperate guide 8a and 10a, the tubular lower part lc-2 of the bottom wall le and / or the part containing said first, second and third coaxial orifices 5a, 5b and 5c, described below, these arrive at the vertical and in coincidence with the upper ends of the first, second and third conduits 6a, 6b and 6c respectively of the connection part 6 described below.

In FIG. 1A, there is shown eight enclosures 1a arranged vertically parallel, arranged in a square with three enclosures per side, a flow dividing part 12 being arranged in the central axis of the square below.

In Figure 4, there is shown a flow division part 12 consisting of:

- a single lower main supply tubular orifice 12a arranged vertically when it is fixed to the main support structure 10, the open lower end 12a of which is connected to the end of an external pipe for supplying multiphase fluid 13 , and

- A plurality of upper secondary tubular outlet orifices 12b arranged in an inclined manner, here comprises six orifices 12b1 to 12b6, capable of supplying only six enclosures la, but they could contain eight inclined tubular secondary outlet ports 12b to feed eight enclosures as shown in Figure IA.

The open upper ends of said secondary tubular outlet orifices 12b1 to 12b6 are respectively connected to a plurality of transfer duct elements 11, the latter being connected at their other end to the lower end of the first central ducts 6a of the different parts. connection 6 of the various unit fluid separation devices 1 fixed to the main support structure 10.

In figure 1B, the pipe 13 is shown as being bent to connect in the vertical axial direction of the lower tubular supply port 12a of the flow dividing part 12 shown in figure 4, in order to facilitate its handling. representation. However, in reality, one avoids forming an elbow in the external multiphase fluid supply pipe 13 supplying the dividing part 12.

The flow dividing part 12 is in fact arranged in height relative to the seabed 20 so that the end of the external multiphase fluid supply pipe 13 connected to the lower end 12al of the part 12 has a sufficient radius of curvature to avoid the fractionation of the phases of the multiphase fluid which it conveys.

Likewise, the inclination of an angle β = 30 ° of said upper secondary tubular outlet orifices 12bl to 12b6 as well as the length and possibly double-curved shape with a mid-point of inflection of the transfer pipe elements 11 contribute to allow the radii of curvature to be observed in the transfer pipe elements 11 to again avoid the fractionation of the liquid and gaseous phases of the multiphase fluid.

The diameters of the various upper secondary tubular outlet orifices 12bl a 12b6 are smaller than that of the lower main orifice 12a, but the sum of the diameters of the various secondary orifices 12b is equal to that of the lower main orifice 12a, so that the flow velocities at the inlet and outlet of the distributor are all identical.

In Figure 2, there is shown a unitary separation device according to the present invention whose enclosure takes a wall 1a is formed of 2 parts, namely:

- an upper part la-1 consisting of the cylindrical main part with circular section surmounted by a spherical cap lb, and 10 - a lower part la-2 whose upper face forms a fixing flange using screws la- 3 with the lower face of the upper part la-1, fixed together using screws la-3.

The lower part la-2 of the wall of the enclosure defined at its lower end a bottom wall lc in the form of a funnel lc-1 extending at its lower end by a cylindrical part lc-2 of diameter D2 reduced by relative to the diameter DI of the cylindrical part of the enclosure 1a, D2 being for example less than 1/4 of Dl, and DI being for example from 0.5 to 3 meter (s) for an enclosure length L1 of 5 to 30 meters.

The cylindrical wall lc-2 delimits an annular orifice for evacuating the liquid contained inside said chamber, called the third annular orifice 5c.

A first internal pipe 2 is arranged, in its running part, with the exception of its angled upper part 2b described below, vertically and axially along the axis ZZ 'at the center of said enclosure. Its lower end ends or is extended by a first central orifice 5a passing coaxially through the cylindrical lower part lc-2 of the bottom wall.

At approximately mid-height of the enclosure, the first internal duct 2 30 comprises a flare 2a 'then a section of the first duct of larger diameter 2' terminating at its upper end in a turning coil or loop with multiple curvatures in gooseneck shape 2b such that the outlet nozzle 2c at the open upper end 2a of the first internal duct 2 arrives near and parallel to the internal surface of the cylindrical wall 1a as described below. The nozzle 2c has a flared side wall so that its opening 2a is larger than the diameter of the end of the loop of the upper part 2b of the first internal duct to which said nozzle 2c is fitted.

The function of the flaring 2a 'and of the widening of the section 2' of the first pipe with respect to the lower portion 2 of the inner pipe of smaller diameter is to reduce the velocity of the multiphase fluid in the first inner pipe. The flaring of the side wall 10 of the nozzle 2c also has the effect of reducing the speed of the fluid leaving the pipe 2.

In FIGS. 2A and 2B there is shown the curved profile in horizontal section of the upper part 2b and of the nozzle 2c at the end of the upper part 2b of the bent portion of the first internal pipe 15 2 on the outside and around the upper axial part 3b of the second pipe 3 ending in the opening 3a.

This curved profile with a profile parallel to that of the cylindrical wall 1a of the enclosure, combined with a vertical rectilinear nozzle of the upper opening 2a with a flattened section at the end of the nozzle 2c has the effect of producing a flow of the fluid at the outlet of the first pipe 2 in the form of a blade tangential to the internal wall of the cylindrical wall la of the enclosure and thus to minimize the destructuring of the multiphase fluid to avoid the fractionation of the gas bubbles which is detrimental to the separation as well than the projection of drops of liquids which could be entrained in the gas outlet opening. Thus the multiphase fluid is placed at the level of the cylindrical internal wall of the enclosure at the outlet of the nozzle 2c, by the centrifugal forces which result from the swirling flow against the cylindrical wall of the enclosure.

Then, the liquid part of the multiphase fluid will descend by gravity in the intermediate part 1-2 of the enclosure to the lower part 1-3 which will fill with liquid to a certain height hl, before and / or during its flow. through the third annular liquid discharge port 5c.

The gaseous part of the multiphase fluid, lighter, will rise towards the upper part 1-1 of the enclosure, at the outlet of the nozzle 2c to be discharged through the upper end 3a of a second internal gas discharge pipe 3. described below.

For the evacuation of the liquid, in a preferred embodiment, shown in FIG. 2, the enclosure comprises a helical gutter 4 fixed against the internal surface of the cylindrical wall of the enclosure 1a, the upper end of which 4a arrives at. against the bottom of the nozzle outlet 2c and the lower end 4b of which arrives just above the lower funnel-shaped part 1c-1 of the bottom wall of the enclosure.

The pitch of the helix of the helical gutter 4 is such that the latter is inclined at an angle α with respect to the horizontal substantially identical to the angle of inclination downwards of the direction YY ′ of the flow of fluid coming out of the opening 2a of the nozzle 2c.

A second internal pipe 3 serving for the evacuation of the gas extends vertically and axially in the center of the enclosure la arranged in its lower part coaxially around the middle part 2 'of larger diameter and the lower part 2 of smaller diameter of the first inner pipe 2. The second inner pipe 3 therefore comprises a middle part of larger diameter 3 ′ so as to follow the widening of the middle part 2 ′ of the first pipe 2, so that the width of l 'annular space between the first internal pipe 2 and the second internal pipe 3 remains substantially identical.

The lower end of the second internal pipe 3 defined or 25 is extended by a second tubular orifice called the second annular gas discharge orifice 5b coaxially and around the first central orifice 5a at the lower end of said second internal pipe 2.

Said third annular orifice 5c at the lower end of the cylindrical lower part 1c-2 of the bottom wall delimits an annular space around the second internal pipe 3 for the discharge of liquid from the lower part 1-3 of. the enclosure.

The upper end 3a of the second internal pipe 3 arrives near the cover of the enclosure formed by the spherical cap lb at a height h of 1 to 2 meter (s) above the opening 2a of the nozzle 2c at the upper end of the first internal duct 2.

In the upper part 1-1 of the interior of the enclosure on the gas path, between the upper opening 2a of the first 2a of the first internal pipe 2 and the upper opening 3a of the second internal pipe 3, there is a device 9 for coalescing liquid droplets entrained by the gas which makes it possible, in combination with the upper arrangement of the opening of the upper end 3a of the second internal pipe 3, to prevent fine droplets of liquid are entrained with the gas phase evacuated through the second internal pipe 3.

The cleaning of this coalescing device 9 is made possible by the possibility of disconnecting the two parts la-1 and la-2 of the enclosure when the latter has been raised for surface maintenance with the equipment attached to it.

Said enclosure is also traversed by pressure sensors and liquid level sensor 20-21, 22. One can more particularly implement nucleic acid level sensors consisting of a beam emitting bar and one or more bars. measurements arranged vertically within the enclosure, the known function of which is to determine the density profile of the fluids over the entire height of the enclosure in order to be able to deduce therefrom the level of the gas-liquid interface 1-4 within the 'pregnant. These bars are housed in sealed and pressure-resistant sheaths passing through the top of the enclosure. Watertight plugs 22a operable by an underwater robot 26 close the sheath tubes of said detection bars and can be opened with a view to recovering said detection bars for surface maintenance. In another embodiment, the sheaths of the various sensor bars 21, 22 pass through the bottom wall of the enclosure and are not linked to the upper part of the enclosure so that the recovery of the detection bars takes place. then done by disconnecting and recovering the complete enclosure 1.

The separation enclosure 1 comprises lifting points preferably in the upper part which may be lifting lugs ld integrated into the wall of the enclosure or recesses or bosses made in the wall of the enclosure to serve as a bearing for a lifting clamp covering the upper part of the enclosure wall. It is also possible to place the waterproof enclosure in a metal frame which will support the lifting points.

In Figure 3, there is shown the lower part lc-2 of the bottom wall which ends in an intermediate piece 5 welded to the lower end of the lower cylindrical part lc-2 of the bottom wall lc. This part 5 of the lower end of the cylindrical part lc-2 forms and delimits a first central tubular orifice 5a for supplying fluid to the first internal pipe 2, a second annular orifice 5b coaxial defining an annular space around the first orifice tubular 5a as an extension of the lower end of the second internal pipe 3, annular space 5b through which the gas coming from the upper end 3a of the second internal pipe 3 is evacuated outside the enclosure 1, and a third annular tubular orifice 5c also coaxial defining an annular space around the second annular tubular orifice 5b, said third annular orifice 5c being delimited by the outer wall of the part 5 or of the lower part of the cylindrical part lc-2.

The lower end of this lower part 5 forms a fixing flange 5-1 capable of being fixed in a reversible manner by fixing jaws 7 (not shown in FIG. 3) with the upper end 6-1 of the part of connection 6. Coaxial metal gaskets 7-1, 7-2, 7-3, are interposed between the lower end 5-1 of the part 5 and the upper end 6-1 of the connecting part 6 .

The connection piece 6 is made in three parts 6-1, 6-2 and 6b2 welded to one another. An internal part 6-2 of the part 6 comprises a first central duct 6a arranged axially, vertically. This first central duct 6a is an extension and continuity of said first tubular orifice 5a after connection of the part 6 by the fixing jaws 7 to the lower end of the part 5.

The lower end 6a-1 of the first central duct 6a of the connection piece 6 is connected to the end of a transfer duct element 11 or directly to the end of an external multiphase fluid supply duct. 13.

Around the first central duct 6a, the internal part 6-2 of the connection part 6 forms a second annular duct 6b coaxial with the first duct 6a, and the upper end of which communicates with the second annular orifice 5b for evacuating gas from part 5. The second annular duct 6b is closed at its lower end 6b-2 and comprises a first lateral gas outlet orifice, with an axis XiXi perpendicular to the axis ZZ 'of the connection part 6, allowing connection at the end of an external gas discharge pipe 16.

Around the second annular duct 6b, the external part 6-1 of the connection part 6 defines and forms with the internal part 6-2, a third annular liquid discharge duct 6c, the upper end of which communicates with the third orifice annular 5c of part 5 to allow liquid to escape. The third evacuation duct 6c of the connection piece 6 is closed at its lower end 6c-2 and comprises a second lateral outlet for liquid 6c-l, of axis / 2X2 'perpendicular to the axis ZZ', and connected to the end of an external liquid discharge pipe 15.

The funds of the annular spaces 6b and 6c can be of helical shape 6d in order to collect and force the flow of solid particles of sand which can be conveyed with the liquid part of the multiphase fluid to thus avoid any accumulation of solid particles (sand ...) which could clog the pipes.

The crude oil arrives under the well head 18a at high pressure, for example 100 to 200 bars. The well head 18a can be equipped with a pressure reduction device and an automated flow control valve piloted from the surface and connected to the external oil supply line 13 which arrives at reduced pressure, for example. bars, at the level of the flow division part 12. The fluid upstream of the separator is slowed down to avoid excessively violent flows entering the enclosure and thus to avoid disturbing the separation.

The crude oil enters the enclosure via the first internal pipe 2 and pours out and separates its liquid and gas phases at the upper end 2a of the first pipe The pressure drop occurs at the wellhead, the pressure is at roughly constant in the separation station. The liquid part fills the bottom of the enclosure 10 to a level 1-4, the height of which is controlled by the level sensors 21-22 and regulated by the export pump 17 for evacuating the liquid via the externel5 pipe. .

The open upper end 3a of the second internal duct 3 is arranged at a height h of 1 to 2 meter (s) sufficient above the end of the opening 2a of the second duct 2, so that projections plugs of petroleum or liquid parts of petroleum caused by the sudden arrival of large pockets of gas at the outlet 2a of the pipe 2 does not reach the upper end 3a of the second pipe 3.

The degassed liquid petroleum, recovered in the lower part of the enclosure is directed via the outlet orifice 6-1 of the connection part 6 to the external liquid discharge pipe 15 can be sent to a floating support on the surface or a secondary water / oil separator at the bottom of the sea so that the oil is sent to the surface, the water being either reinjected into a well similar to well 18, or simply released back to sea, as long as it has an acceptable cleanliness, that is to say a sufficiently low quantity of residual particles of crude oil. The liquid discharge can be controlled using an export pump 17.

Likewise, the gas evacuated by the external gas evacuation pipe 16 can be sent to the surface or re-compressed at the level of the seabed and then reinjected into a well similar to the well 18.

More particularly, said crude oil is sent to the lower end of said first internal pipe, preferably such that the pressure differential ΔΡ = P1-P0, PO being the inlet pressure of the gas line to the FPSO at the surface, or greater than the pressure drops in the external gas discharge pipe from said first lateral orifice to the surface. This makes it possible to bring the gas to the surface without additional equipment and / or without providing additional external energy, in particular without the use of a compressor.

Preferably, the raw fluid inlet flow rate upstream of said connection piece is controlled by a flow control valve and / or the discharge flow rate of the degassed fluid downstream of said connection piece 6 is controlled by a flow control valve and / or by the speed of said export pump 17 as a function of the measurements of at least one said device for controlling the level of the fluid within said enclosure so that the level of liquid at bottom of the enclosure remains sufficiently far from the separator inlets / outlets.

More particularly, said support structure is installed at the bottom of the sea, at a depth of 100 to 4000 m and a pressure PI of 10 to 50 bars (10 x 10 ⁵ to ^{50 x 10 5} Pa), preferably 20 bars ( 20 × 10 ⁵ Pa) within said enclosure.

Preferably, said enclosure is thermally insulated. This embodiment makes it possible to maintain the crude oil fluid at the high temperature of 40 to 100 ° C, or even more, at which it leaves the wellhead, and thus facilitate the rise to the surface of the fluid, while avoiding solidification. paraffins or the formation of gas hydrates by cooling the crude oil below 30-35 ° C, which can create plugs and blockages within the internal pipe 2. In the event of production stoppage, the thermal insulation of the enclosure also makes it possible to maintain the internal temperature of the enclosure for as long as possible above the critical temperature for the formation of gas hydrates, thus giving the operator time to carry out the preservation operations necessary to to mitigate this risk.

The possibility of depositing and connecting additional unitary separation devices 1 on the main support structure 10 resting on the seabed from a vessel 24 at the surface 25, makes it possible to adapt the separation capacities of crude oil on the seabed to the over time and also advantageously makes it possible to ensure surface maintenance by disconnecting and lifting the unitary separation devices 1 from a vessel 24 at the surface 25.

Taking into account the possibility of implementing enclosures of relatively smaller sizes given their greater number, the handling of the enclosures 1a from the surface can be carried out by standard-size boats used for assistance and maintenance. offshore oil installations, without resorting to excessive lifting capacities.

An essential advantageous characteristic of the invention consisting in the coaxial arrangement of the fluid feeds and gas phase and liquid phase outlets at the lower end of the enclosure 1a via a three-way coaxial connection piece 6 facilitates the installation. place and connection of the unitary separation devices 1 at the level of the main support structure 10 at the seabed, as well as the maintenance of the unitary device 1, as explained previously.

Indeed, the coaxial arrangement of the various internal conduits for supplying fluid and separate evacuation of the gas and liquid phases 20 allows the seals 7-1, 7-2, 7-3 to be sealed, in this case metal joints, between the lower end of the openings 5a, 5b, 5c of the bottom wall of the enclosure 1 and the various conduits 6a, 6b, 6c of the connection piece 6 at its upper end, remains in equal pressure s 'acting on the two internal seals 7-1 and 7-2 thus avoiding damage to said seals in the event of pressurization or depressurization of the enclosure during these various disconnections or maintenances, as explained previously.

Claims (4)

1. Modular underwater installation for the liquid / gas separation of two respectively liquid and gaseous phases of a fluid, in particular the liquid and gaseous phases of crude oil, comprising: (a) a main support structure (10) resting and / or anchored to the seabed (20) to which are connected or capable of being reversibly connected a plurality of unitary liquid / gas separation devices (1), and (b) at least one unitary liquid / gas separation device (1) fixed to said main support structure (10), each said unitary liquid / gas separation device (1) comprising a single enclosure comprising a resistant sealed wall (la) to the pressure of the seabed and to the internal pressure of said fluid arranged along a vertical axis of revolution (ZZ '), said enclosure containing: bl) a first internal pipe (2) for supplying the multiphase fluid going up inside said enclosure from its lower end comprising or cooperating with a first orifice (5a) passing through a part of wall called the bottom wall (le) of the enclosure, up to its open upper end (2a) opening into the upper part (1-1) of the enclosure, and b2) a second internal gas discharge pipe (3) extending within said enclosure from its open upper end (3a), preferably disposed higher than said upper opening (2a) of the first pipe, up to its lower end comprising or cooperating with a second orifice (5b) passing through the bottom wall (le) of the enclosure, and b3) a third orifice (5c) passing through said bottom wall of the enclosure around said first and second openings (5a, 5b) of the bottom wall of the enclosure, and c) a plurality of 3-way connection pieces (6) fixed to said support structure (10), including at least one said connection piece connected to a said enclosure (1), each said connection piece comprising: 5 cl) a first connection duct (6a), an upper end of which is connected or capable of being connected to a said first orifice (5a) of the bottom wall of said enclosure, said first connection duct comprising a first outlet orifice (6a1) connected or able to be connected to an external supply pipe of said multiphase fluid (13) at the seabed or a pipe element (11) connected to the end of an external supply pipe in said multiphase fluid (13) at the bottom of the sea (20), and c2) a second connection duct (6b), an upper end of which is connected or able to be connected to a said second orifice (5b) of the wall bottom of said enclosure, said second connection duct (6b) comprising a second outlet port (6bl) connected or able to be connected to an external gas discharge duct (16), and c3) a third connection duct (6c) disposed around 20 of said first and second connecting conduits ion (6a, 6b) at its upper end connected or able to be connected to a said third orifice (5c) of the bottom wall of said enclosure, said third connection duct (6c) comprising a third outlet orifice (6cl) connected or capable of being connected to an external liquid discharge pipe 25 (15). 2. Modular installation according to claim 1, characterized in that said second orifice (5b) of the bottom wall of the enclosure is a second annular orifice disposed coaxially around said first orifice (5a) tubular of the bottom wall of the enclosure, and said third port (5c) in the bottom wall of the enclosure is a third annular port disposed coaxially around said first and second port (5a, 5b) in the bottom wall of the enclosure, and said three-way connection part (6) is a coaxial three-way connection part fixed vertically on said support structure (10) comprising: c) a first connection duct (6a) arranged vertically and of which said first outlet port (6a) is formed by the axial lower end of said first connection duct (6a), and c2) a second connection duct (6b) , arranged coaxially and around said first duct at its upper end, connected or being able to be connected vertically to a said second annular orifice (5b) of said bottom wall of the enclosure, comprising a said second outlet orifice (6bl) lateral, and c3) a third connection duct (6c) disposed coaxially around said first and second connection ducts (6a, 6b) at its upper end, connected or able to be connected vertically to a said third annular orifice (5c) of said bottom wall of the enclosure, comprising a said third lateral outlet port (6cl). 3. Modular installation according to claim 2, characterized in that said support structure (10) resting on the seabed supports at least one device for dividing and distributing (12) the multiphase fluid having a single main tubular orifice for axial supply. (12a) preferably arranged vertically, the open lower end of which (12a) is connected or connectable to the end of an external pipe for supplying said multiphase fluid (13) to the seabed (20), and the open axial upper end of which (12a2) communicates with a plurality of secondary tubular outlet orifices (12b, 12bl to 12b6) inclined with respect to said axial direction, of the same diameter and inclined at the same angle (β) and arranged symmetrically and regularly with respect to and around said axial direction, said secondary tubular outlet orifices (12bl to 12b6) being connected or able to be connected to a plurality of working pipe elements feed fluid feed (11) connected or able to be connected respectively to a plurality of said first conduits (6a) of said connection pieces (6) connected or able to be connected to the lower ends of a plurality of said enclosures (1). 4. Modular installation according to claim 3, characterized in that said connecting pieces (6) and said enclosures (1) are arranged relative to said dividing device (12), preferably above and around said dividing device (12). ), so that the pressure drops between said dividing device (12) and said enclosures (1) are substantially identical to each other, preferably, said connection pieces (6) and said enclosures (1) being arranged symmetrically with respect to said dividing device (12) above and around said dividing device (12). 5. Modular installation according to one of claims 1 to 4, characterized in that said enclosure is integral with first guide elements (8a) of the male or preferably female type, arranged outside said enclosure, and said support structure (10) comprises second guide elements (10a), of the female or respectively male type, cooperating or able to cooperate with said first complementary guide elements to achieve vertical docking 20 of said unitary device opposite to a said connection piece (6). 6. Modular installation according to one of claims 1 to 5, characterized in that said enclosure comprises a side wall (la) cylindrical with circular section, the upper end of which is closed by a domed cap, preferably spherical (lb) and the bottom wall (Ic) of which at its lower end is in the form of a funnel (Here) tapering downwards to form a vertical tubular lower wall (lc2) delimiting said third annular liquid discharge port (5c) . 7. Modular installation according to one of claims 1 to 6, characterized in that said first internal pipe (2) for supplying the multiphase fluid extends vertically, preferably substantially in the axis of said enclosure, to the 'inside of said second inner pipe (3) below an upper part (2b) of said first inner pipe, which upper part (2b) of said first inner pipe forms a turning loop, outside of said second inner pipe, preferably terminating in a flared-walled fluid outlet nozzle (2c) defining a said upper opening (2a) of said first internal pipe, more preferably with a flattened section, so that said upper opening (2a) of the first pipe opens near the side wall (la) of the enclosure so that the fluid flows therefrom parallel to said cylindrical side wall and oriented in a direction (YY ') inclined (a) towards the bottom, and said second internal gas discharge pipe (3) extends axially coaxially with said first internal pipe below said upper part (2b) thereof, preferably said first internal pipe (2) and second internal pipe (3) having coaxial portions (2 ', 3') of larger diameters than their respective lower parts (2, 3). 8. Modular installation according to one of claims 1 to 7, characterized in that the enclosure comprises a helical gutter (4) fixed near and / or against the internal surface of said side wall of the enclosure and of which the The upper end is against the bottom of the upper opening of the first internal pipe and the lower end arrives above said third annular orifice (5c). 9. Modular installation according to one of claims 1 to 8, characterized in that the enclosure contains in its upper part (11) a device for filtering liquid droplets, preferably a coalescing device (9), on the gas path between the upper opening (2a) of the first internal pipe and the upper opening (3a) of the second internal pipe. 10. Modular installation according to one of claims 1 to 9, characterized in that it contains pressure detection sensors and liquid level sensor (21,22) within the enclosure, preferably a device for level control which is a radar or sonar type probe or a nucleonic level sensor consisting of a ray-emitting bar (21) and one or more measuring bars (22) making it possible to measure the density profile of the fluid over any the height of the enclosure. 11. Modular installation according to one of claims 1 to 10, characterized in that the enclosure is formed of two enclosure parts fixed (la-3) to one another comprising: - an upper wall part (la-1) comprising at its upper end a domed cap, preferably spherical (lb), and - A lower wall part (la-2) comprising a said bottom wall (le). 12. Modular installation according to one of claims 1 to 11, characterized in that said support structure (10) comprises a plurality of said connection parts and said enclosures (la) which are connected thereto, preferably at least 4 said connecting pieces, preferably arranged symmetrically. 13. Unit device for liquid / gas separation of two respectively liquid and gaseous phases of a fluid, useful in a modular installation as defined in one of claims 1 to 12, characterized in that said unitary device (1) of liquid / gas separation comprises a single enclosure comprising a sealed wall (la) resistant to the pressure of the seabed and to the internal pressure of said fluid arranged along a vertical axis of revolution (ZZ '), said enclosure containing: 1) a first internal pipe (2) for supplying the multiphase fluid up inside said enclosure from its lower end comprising or cooperating with a first orifice (5a) passing through a part of wall called the bottom wall (lc) of the enclosure, up to its open upper end (2a) opening into the upper part (1-1) of the enclosure, and 2) a second internal gas discharge pipe (3) extending within said enclosure from its open upper end (3a) preferably arranged higher than said upper opening (2a) of the first pipe, up to at its lower end comprising or cooperating with a second orifice (5b) passing through the bottom wall (le) of the enclosure, and 3) a third orifice (5c) passing through said bottom wall of the enclosure coaxially around said first and second orifices (5a, 5b) of said bottom wall of the enclosure. 14. A method of producing a modular installation according to one of claims 1 to 12, characterized in that the following successive steps are carried out in which: a) a said unitary device suspended from a link (19) is lowered from a ship (24) on the surface (25), and b) preferably, its approach to docking is guided with a said connection piece (6) fixed on said support structure (10), using first guide elements (8a) integral with said enclosure (1) and said second guide means (10a) integral with said support structure (10), said second guide elements cooperating with said first guide elements, and c) said enclosure is connected to said connection piece, in a disconnectable manner, using a fixing flange (7) and coaxial seals (7-1, 7-2 and 7-3) between said first, second and third orifices (5a, 5b, 5c) passing through the bottom wall (le) of the enclosure and said first, second and respectively third connection ducts (6a, 6b, 6c) of said connection piece . 15. A method of separating the two liquid and gaseous phases of a crude oil using an installation according to one of claims 1 to 12, characterized in that the following steps are carried out, in which: 1) the crude oil is sent, via an inlet pipe (13), to said first orifice (5a) of said bottom wall of the enclosure via and through said first pipe (6a) a said connection piece (6), at a reduced pressure PI, less than the static pressure on the seabed P2, and 2) the crude oil rises within said first internal pipe (2) of the enclosure, then flows at the upper outlet of said first internal duct in flow tangentially to the side wall with circular section of the enclosure, which causes its separation into a liquid phase and a gas phase, then 3) the partially degassed liquid or fluid phase descends towards said third orifice (5c) of the bottom wall at the lower end of the enclosure, and preferably fills said enclosure to a height level below the opening upper (2a) of said first internal pipe (2), and it is recovered, at the level of an external liquid discharge pipe (15) connected to said third outlet port (6cl) of said connection piece , and 4) the gas separated from said oil is sucked in said second internal pipe towards said second orifice (5b) of the bottom wall at the lower end of the enclosure, and it is recovered, at the level of an external pipe gas outlet (16) connected to said second outlet (6bl) of said connection piece.