Description

The present invention relates to a process for the separation of a stream comprising various physical phases, said stream consisting of one gas or vapour phase, one or more liquid phases and one solid phase consisting of siliceous sand or having a different composition and/or other suspended solid (for example, clay) , said stream is called multiphase stream in the present text. This stream flows along a pipe and is separated by means of a T-separator or T-junction, which assists the traditional separators. In particular, the present invention relates to a separation process of gas-water streams coming from one or more gas wells, by means of a T-separator or T- junction coupled with traditional separators. In addition, the present invention relates to a process for the separation of gas-oil-water streams coming from one or more oil wells by means of a T- separator or T- junction coupled with traditional separators.

Various technologies are known, which use T- junctions in pipes for monophase and multiphase streams. T-junctions are normally used as flow dividers in numerous industrial applications. In the upstream field, however, various works appeared, in the literature of the seventies' , in which the T-junction had the function of separating the gas phase from the liquid phase, also only partially. It has been noted, in fact, that the division of flows by means of a T- junction normally implies an alteration in the phases according to which the divided flows are enriched in one phase rather than another. A gas-liquid stream, for example, flowing through a T-junction, branches into two flows of which one can be richer in the gas phase and the other can be richer in the liquid phase. In this way, these junctions can function as supporting systems in separation processes of multiphase streams, such as, for example, those exerted in platforms for the optimum exploitation of oil fields. In 1973 and 1978, two articles were published, one on Oil % Gas Journal, 39-44, 1973, "Condensate behaviour in gas pipeline is predictable" by L. Oranje, one on Journal Petr. Tech., 30, 290-296, 1978, "Two phase flow splitting at a pipe tee" di K.C.Hong, in which it was noted that during the winter, the natural gas arrived anhydrous in certain points of the distribution line, whereas in others only condensed wet gas arrived. This behaviour was explained by the tendency of the liquid and gaseous phases to separate due to the T-branchings existing on the lines. A T-branching was proposed as an aid for separating the gas phase from the liquid phase and tests were effected with a laboratory plant with air and water, exploiting the different behaviour of the inertia moment of the two fluids in a branching having different pressure drops along the lines. These junctions have already been proposed since 1983 as real slug catchers in gas production lines as explained in the article published in Oil % Gas Journal, 81, 128- 138, 1983, "Handling two phase gas condensate flow in offshore pipeline systems" . This latter work proposed managing and separating the liquid and gas slug which accumulated in the pipes between the wells and the sorting stations. In order to obtain this effect, the flow was sent to a distributor which conveyed it to a series of side pipes parallel to the main pipe through a series of horizontal and vertical T-junctions. In this way, by constantly exploiting the different inertia moment between liquid and gas, the conveying of a fraction enriched in liquid in the side branches parallel to the main line was obtained, whereas the fraction richer in gaseous phase was separated in the vertical lines.

A method is known from US 4,269,211 for managing vapour flows in steam injection processes in oil fields. The objective is to normalize the quality of the vapour in a series of branches deriving from the main line by means of T-junctions equipped with particular accessories. The system is composed of the main line which runs into a divergent T-junction equipped with a mechanically rotatable perforated plate thanks to a control system which has two sensors on the branch-lines to determine the composition of the flows. The perforated plate can be rotated, positioned or withdrawn from the T-junction, during the passage of the multiphase fluid, in order to manage the distribution of the vapour. From US 4,257,793, a system is known which is capable of producing a gas flow impoverished in water starting from a wet gas. To obtain this, the vertical descending flow of wet gas is partially diverted into a horizontal side piping equipped with suitable deflectors capable of separating, collecting and conveying the droplets of dispersed liquid into a further side pipe. To assist the separation, the diversion is set up on the main pipe so that the flow must re-ascend a section of tilted pipe. Due to the greater inertial and gravitational forces of the liquid phase with respect to the gaseous phase, the water entrained by the gas collides onto the deflector or corrugated collision plane (mist eliminator) . In this way a side gas flow is obtained free of dispersed droplets in the form of mist.

An apparatus is known from US 4,516,986 which is capable of separating a multiphase fluid into two streams, one of which has a certain composition and is prevalently enriched in its liquid component. The patent considers an annular regime situation in which the liquid phase tends to be pushed towards the walls of the main line, so that it can be collected and sent to the side branching. The apparatus can be installed directly inside a pipe without excessive modifications or complications and is essentially based on the insertion of a flow restriction on the main line. This restriction consists of a coaxial pipe having a diameter lower than the main line, assembled by means of supports or spoilers. The central gaseous phase passes through the restriction, contemporaneously- creating along the walls an area in which there is an accumulation of liquid phase in exact coincidence with the mouth of the side line. This quantity can be measured with a flow-rate meter and regulated by means of suitable valves.

US 4,522,218 and US 4,574,827 disclose a system for sending a multiphase flow having a specific composition and flow-rate from a main line into a secondary line. For this purpose, two secondary lines are inserted on the main line, vertical with respect to the first line and equipped with suitable regulating valves . The two vertical lines are subsequently sent to a diversion line. The quality and quantity of the flow can be controlled in the side branchings by acting on the valves or by varying the diameter of the side lines. In general, by acting on both vertical lines a fluid can be brought into the side branch, having a composition either the same or different from that of the main line. The system is applied to a fluid consisting of at least one gaseous component and one liquid component, such as water and vapour; furthermore it is applied in the case of the transportation of air and water in annular flow or slug regime with different pipe diameters .

A device is disclosed in US 4,708,793, which is capable of separating the gas/liquid slug in oil productions . The apparatus is composed of a pipe having a diameter D connected to a tubing having a diameter at least 2D, equipped with a series of vertical and descending T-branchings which start from the bottom of the same tubing. The breaking of the slug takes place due to the change in the section of the pipe, and therefore the rate, and the flow regime is transformed into one of the stratified type. In this way, the separation of the phases is obtained along the broadened duct: the liquid phase descends along the vertical tubing and is collected in a new tubing destined for transporting the liquid. The gaseous phase is sent to a traditional centrifuge separator of the hydrocyclone type, through a vertical branching from the top of the 2D tubing, where the separation of the gaseous phase is completed. The number of vertical tubings and their distances are important for optimizing the system.

A method is known from US 4,800,921 for collecting portions of a multiphase fluid, in particular, when it is in annular regime. The distribution lines of multiphase fluids are described, such as, for example, vapour/water lines in thermal plants or in steam injection systems.

The invention is based on a system on whose main line a series of T-branchings are inserted, whose position, number and diameter are such as to allow the collection of pure liquid, liquid/gas mixtures or pure gas. The apparatus can be applied in horizontal pipelines. A series of optimum T-divergences are claimed, at different angles with respect to the axial direction of the flow: +45°, -45°, 0°, -90°, -90°. In this method, the thickness of the liquid film in the main fluid is extremely important as this factor locally influences the quantity of liquid sent into the side branching.

An integrated system is known from EP 0331295, for the separation of a gas-liquid flow reaching an offshore oil platform. The invention described and claimed has the objective of providing a better management of slug regimes for which devices of the slug catcher type, which are cumbersome and costly, are normally used. The system, object of the invention, is designed to manage flow regimes which are established during the start-up and shut-down of the production lines, in which the gas and liquid rates are different from those of the production. This system consists of an underwater line (horizontal) which merges with a vertical riser connected to a liquid separator. A second riser equipped with one or more valves is installed on this separator by means of a vertical T. The secondary riser is destined for mainly transporting gaseous streams and is connected to a vertical gas separator. A series of capacitive sensors capable of controlling the liquid - gas interface along the main line through measuring, for example pressure differences, completes the system. In the absence of slugs along the line, the sensors do not reveal pressure increases. In the presence of gas slugs, the pressure tends to increase, causing the intervention of a valve which conveys the gas into the secondary riser. In this way, the main line always receives a fraction enriched in liquid and the secondary line enriched in gaseous phase. The distance between the T and vertical riser is of fundamental importance for managing the different frequency of the arriving slugs. The T can also be installed with angulations different from 90°, according to the production characteristics. It is important for the line joining the T and main riser to be tilted by 2 degrees with respect to the horizontal. The system has the advantages of occupying little space (it is underwater) , it has few moving parts and is simple to construct and maintain. The moving or easily wearable parts are mainly installed on the surface.

US 5,218,985, US 5,250,104 and US 5,551,469 disclose a system capable of avoiding the misdistribution of phases which is generated along the branchings of the pipe in multiphase fluids. The system is based on the use of a T-junction joining horizontally equipped with an apparatus which separates the liquid phase from the gaseous phase before the branching, recombining the phases downstream of the junction and maintaining their composition unvaried or varying it. The tubing upstream of the T has a larger diameter with respect to the previous piping and a length sufficient for allowing the complete separation of the liquid and gaseous phases, thanks to the reduction in the flow-rate and allowing the separation induced by the gravitational forces. The entrance to T consists of a constriction opening which helps the separation of the phases. The liquid component is collected by gravity in the widening on the bottom of the tubing and consequently flows through the siphon immediately flowing downstream of the constriction.

Due to the flow of the gaseous phase leaving the opening, there is a low-pressure area which sucks the liquid phase. This phenomenon allows a phase of finely dispersed droplets to be formed at the outlet of the siphon, which are entrained by the gaseous phase entering the real branching. The flow which is obtained is in the form of a mist and substantially has the same composition in the two branches leaving the T, as the finely dispersed phase must be formed and this must be prevented from being reorganized into drops having larger dimensions, reforming a substantially non- homogeneous liquid. In all three patents, it can be noted that it is sufficient to install a control system of the liquid phase or gaseous phase for altering the composition leaving the T, as desired.

US 5,415,195 discloses a system which ensures a uniform distribution in diverted vapour lines for industrial use. The system is proposed for both joining T-junctions and divergent junctions, and is particularly suitable for situations in which the flow regime is annular. ith respect to the state of the art, the system is based on the variation of the section close to the junction in relation to the ratio of the surface rates of the multiphase fluid in the outgoing branches. This section can be varied in order to obtain a certain distribution of the mass of multiphase fluid in the side branches, in the first instance dividing it into two streams having the same composition. The multiphase fluid encounters a series of static mixers in order to obtain a homogeneous mixture regardless of the ingoing fluid, the mixture is sent to a movable deflector having a certain geometry. Said deflector consists of a plane having an increasing width starting from the mixer as far as the bend, where it becomes equal to the internal diameter of the tubing. It consists of a semi-circumference having the circular edge facing the mouth of the side T-piping. The whole unit can be moved or rotated from the outside so that a certain fraction of mass of the multiphase mixture can be conveyed into the side branch with the same composition as that which is entering. When the static mixer section undergoes pressure drops, a modified system is adopted again for annular regimes. This system comprises a circular crown which hinders the trajectory of the liquid phase on the walls, diverting it to the centre of the piping through the reduced opening. Further semi-cylindrical deflectors are inserted around the moveable part facilitating the channelling of the flow. The gaseous phase which is moving in the centre of the pipe therefore transports and disperses the liquid flow towards the movable wall.

A solution is known from US 5,437,299 for preventing the phase misdistribution in a divergent T in which the side branch has a much smaller diameter than the main line. A configuration is therefore proposed where the liquid phase is forcedly formed and collected and sent by means of a bypass to the side line. The insertion of the liquid line is beyond a constriction specifically created for causing a pressure drop. The preparation of a side branching is therefore claimed, equipped in turn with a connection which draws from a cavity situated on the bottom of the main pipe. The cavity allows the liquid phase to be accumulated. If the flow regime is stratified or annular, this cavity become useless and the liquid is collected directly from the bottom of the pipe. A further constriction is inserted on the side branching. The system is designed so that the pressure drop of the passage of a liquid is equal to the pressure drop of a gas passing through the constriction, once a certain extraction factor of the vapour of the main lines has been defined and selected. In this way, it is possible to distribute a multiphase fluid in a side pipe maintaining the composition unvaried. The system is applied to fluids such as water and vapour. With this system, it is possible to vary the quantity of liquid to be sent to the divergence by modifying the diameter of the tubings or inserting suitable dosage valves of the liquid phase. A system is therefore generated, which is capable of extracting a multiphase fluid in the desired quantity and quality. US 5,478,504 discloses a device for managing slugs of a certain entity and dangerousness (severe slug) in multiphase underwater lines by means of a T-branching of the main line.

A secondary line is inserted along the main line, which connects the horizontal branch to the upward vertical branch of the same main line. The distance at which this line initiates and the height at which this line is re-connected are important elements for the efficiency of the system. The secondary line is destined for the control and transportation of a fraction enriched in gas due to the pressure difference existing between the two collection and insertion points. The system is completed by sensors capable of determining the pressure difference between various points along the tubing connected to a control system which manages the opening of the valve . Upon the arrival of a gas slug of a certain entity, the sensors convey this phase along the secondary line, segregating and fractionating the gaseous phase. This phase is again admitted into the reascending line, diluting the gas in the liquid and reforming a more manageable flow.

EP 0823679 and US 6,116,259 disclose a method applied to vapour/water distribution lines, in particular those lines in which there is a flow regime of the annular or stratified type. The invention exploits the typicality of these flows for obtaining and controlling a liquid flow-rate in a side branching to the main line. Two T-branchings are inserted on the main line, a secondary line and a collector on the bottom connected to the previous branching by means of a specific pipe. This collector separates a fraction of liquid by gravity, whereas the line is capable of collecting a fraction of gas or fraction enriched in gas. The diverted lines are equipped with control systems of the flow-rates, connected to a control system. A specific gas/liquid phase ratio in the side branching can be collected by measuring, for example, the differential pressure or liquid flow-rate, and regulating the valve.

US 6,250,131 discloses a system capable of controlling, measuring and distributing a multiphase mixture in the side branches using a T-junction at the beginning of the same system. The junction can be horizontal or vertical depending on the performances to be obtained. According to the invention, the T-junction joining vertically allows the multiphase flow to be divided into two sub-flows, one flow enriched in the gaseous phase, the other in the liquid phase. By playing on the pressure difference which is created, varying the diameter of the pipes or inserting suitable calibrated restrictions, flows having certain flow- rates and compositions can be conveyed along the side branchings. The system can also be equipped with specific flow meters for gases and liquids in particular sections of piping, which are capable of measuring the overall flow-rate of an ingoing multiphase stream. This system can be used independently of the flow regime in arrival as it is strong enough to be able to manage various gas and liquid flow-rate ranges.

A system is known from WO 03/067146 for managing the slug of multiphase fluids in underwater pipes . For this purpose, the primary pipe is integrated with a series of side T-branchings capable of functioning as slug catcher. The branchings are in fact produced so as to have pressure drops which allow the segregation of the liquid slugs. To obtain this, the outgoing lines have T-divergences , preferably vertical, with sections having a different inclination from the main line or equipped with suitable regulation valves . Alternatively, pipes with a different diameter than that of the main line can be used, or the rate of the fluid in the primary pipe can be decreased by increasing its section, in this way allowing the accumulation of a liquid slug to be diverted onto the secondary line. The two lines, main and secondary, arrive at two different terminals, suitably equipped and dimensioned for managing the different flows.

One of the problems of managing oil wells relates to situations characterized by streams in slug regime which can be common during the start-up and shut-down of the production lines, i.e. situations in which the gas and liquid rates are significantly different from the production rates. To overcome these problems devices of the slug-catcher type are necessary, which however are cumbersome and costly. The life of a well, moreover, is progressively characterized by an increase in the flows of entrained water, which, with time, make extraction processes unmanageable, with an underdimensioning of the whole separation system.

Finally, the advantages of compactness and separation efficacy of the gas from the liquid in a T- separator can be thwarted by the management of the T itself in particular during the transient conditions which characterize any field exploitation.

The use of a T-separator in systems in which it is the gas which entrains the liquid has not yet been developed on an industrial scale as, if there is an increase in the liquid phase in the pipe, said separator may lack precision and even be ineffective.

The field application of the separation technology of gas-oil-water streams creates a problem of stability of the emulsion which is formed between its components. Numerous parameters influence this problem. These parameters can be divided into two categories:

• parameters deriving from problems of mixing and dimensioning of separators,

• parameters which cause physico-chemical alterations of the water-oil system and which depend on chemical modifications of the environment, as happens at times for the addition or modification of the products used as anticorrosion agents or products for water treatment .

The first point is technically solved with a good calculation of the dimensioning of the separator, which is generally of the gravitational type, and whose calculation is based on very precise physico-chemical methods for each application. The management of these separation systems requires the application of control algorithms of the liquid volumes accumulated and temperature of these for preventing load variations of the separator and optimizing its cost parameters. At times, the liquid volumes are defined a priori and an overfall mechanically defines the liquid contained in the separator, it is therefore the value of the flow- rates fed to the separator which defines the gas-liquid separation times. For facing the second type of parameters, there is no one single solution, but it is possible to avail of analyses based on a comparison of the conditions at the battery limits of the extraction and separation processes which leads to the definition of those parameters which act on the stability of the emulsions. It is in any case fundamental to effect a first physical separation of the gases coming from the wells, which can also be entrained by the liquids, in order to avoid the formation of foams which would encumber the oil-water separation process, and which would increase the residence times of the mixture in the volumes established for this separation with an increase in the relative energy consumptions .

All of these technical problems benefit from an optimum management of the separation conditions of the multiphase flow by means of the process, object of this patent .

An objective of the present invention is to define and optimize a separation process of j multiphase streams, with j varying from 1 to n, wherein n is an integer, and preferably gas-water or gas-water-oil streams, by means of j T-junctions coupled with at least j+1 accumulation volumes and/or traditional separators and equipped with at least j+1 automatic control systems of the level of the liquids for gas fields or equipped with flow meters situated on the evacuation line of the liquid for oil fields. The traditional separators can also be of the gravitational type and in particular can be gravimetric separators .

In an embodiment, the present invention relates to a separation process for j multiphase streams which flow into j pipes, with j varying from 1 to n, where n is an integer number, said multiphase streams comprising a gas or vapour phase, one or more liquid phases and a solid phase, said process characterized in that it comprises the following steps:

• transporting j multiphase streams through j pipes towards j T junctions situated along said pipes,-

• separating, by means of said j T junctions, each of j streams into two fluxes so that one flux has a higher volumetric gas composition and the other has a higher volumetric liquid composition;

• completing the phase separation in the 2j fluxes leaving the T junctions by means of at least j+1 traditional separators, of which at least j separators substantially separate fluxes which are characterized by a higher volumetric gas composition, and at least one separator separates fluxes characterized by a higher volumetric liquid composition;

• controlling the velocity of the two fluxes leaving each j T junction by means of a control system, in order to regulate the volumetric ratio of the fluxes leaving each T junction and to ensure the critical entrainment of the gas phase in each flux having a higher volumetric gas composition;

• storing the liquid phases separated by means of traditional separators in an accumulation vessel and conveying the gas phases separated by means of traditional separators having higher volumetric composition of gas, to an evacuating line.

In a further embodiment, the present invention relates to the process previously described wherein the control system actuates a control procedure of the multiphase flow rates coming from gas fields, said control procedure comprising the following phases:

• when the differential pressure of the gas decreases on the evacuating line and/or there is an increase in the differential liquid head in the separator, in which a flow rich in gas has accumulated, and in case the high liquid level in the separator in which a flow rich in gas has accumulated has been reached:

• taking and transmitting to the controller the low pressure signal and/or the high liquid level signal; • if necessary taking the volumetric flow rates of the fluxes leaving each T junction;

• partially or totally closing the modulator valve situated on the line having a higher volumetric composition of gas;

• if necessary driving the opening of a bottom valve according to an ON/OFF procedure draining, in the time limit between its opening and subsequent closing, the control volume of said separator;

· partially or totally opening the modulator valve on the line rich in liquid phase in order to regulate the pressure on the evacuating line of the gas phase.

In a further embodiment, the present invention relates to the process as previously described wherein the control system actuates a control procedure of the multiphase velocities coming from oil wells, said control procedure comprising the following phases:

• when the differential pressure of the gas decreases on the evacuating line and/or the volumetric rate of the liquid phase out flowing the separator, under level control, wherein a flux rich in gas is stored, increases :

a. detecting and transmitting to the controller the low differential pressure signal and/or high liquid volumetric rate signal;

b. detecting the variations in the volumetric rates of the fluxes leaving the T-junctions;

c. partially or totally closing the modulator valve downstream of the T-junction on the line rich in gas phase;

d. if necessary driving the opening of a bottom valve by means of a continuous modulation in order to extract the surplus liquids inside the separator control volume;

e. partially or totally opening the modulator valve on the line rich in liquid phase in order to regulate the pressure on the evacuating line of the gas phase .

When the T-junctions are coupled with traditional separation systems, they have the following technical advantages :

• improving the reliability of the accumulation volumes and/or traditional separation systems, preferably of the gravitational type, wherein a multiphase stream coming from one or more extraction wells is separated by gravitational sedimentation, i.e. said phase separation is caused by a velocity drop due to the sudden broadening of the fluid transporting section and/or takes place due to the coalescence of liquid drops dispersed in the gas;

• defining optimum separation modes for feeding streams in slug flow, churn flow, bubble flow, annular or stratified regimes;

· favouring the debottlenecking, with a low surface impact on the platform, of particular gas-liquid separation situations characterized by high liquid flow-rates in the case of oil fields;

• a better management of slug flows, i.e. large volumes of liquids which can occasionally and chaotically accompany the exploitation of a gas field;

• reducing the formation of foams with a significant technical improvement in the oil-water separation process of oil fields and with a consequent reduction in the residence times of the mixture in the volumes relating to this separation, and finally a reduction in the relative energy consumptions;

• a better management of the start-up and shut-down transients of the well;

• reducing the operative costs linked to the wear of the outflow valve of the liquids at the bottom of the separator in the case of gas field separators.

Although the quantification of the economical impact of these improvements is not significant without reference to specific contexts, it is potentially extremely important, in particular in the following cases: the start-up of offshore fields which have a limited availability of space in surface plants, the substitution of highly encumbering separators (such as slug-catchers for example) , optimization of the separation process with respect to the yield to liquid hydrocarbons, elimination of accumulation phenomena of solid deposits in the internal volumes of the separators (as in the case of gravitational separators) , simplification in maintenance processes for underwater systems.

Further objectives and advantages of the present invention will appear more evident from the following description and enclosed drawings, provided for purely illustrative and non- limiting purposes, wherein;

Figure 1 is the joining (1) and divergent (3) T assembly configuration in which the opening angle of T (2) is indicated;

Figure 2 is the horizontal (4) and vertical (5) T assembly configuration;

Figure 3 is a plant scheme with a single feeding

(6) where a T-junction (7) is installed, wherein SI and S2 are traditional separators, PV1 and PV2 are modulator valves situated downstream of the T-junction

(7) , said valves can also be situated downstream of the separators on the outlet line of the phase rich in gas (even if this option is not indicated in figure 3) , LI is a liquid level indicator, LC2 and LC3 are liquid level controllers, PCI is a pressure controller and RC (RATIO CONTROL) is a controller which regulates the volumetric ratio between the flows leaving the T- j unction (7) ;

Figure 4 is a general plant scheme which uses j feeding streams, with j varying from 1 to n and wherein n is an integer ranging from 1 to 10,000, j T-junctions (7), j+1 traditional separators S _j , 2j modulator valves PV _j situated downstream of each T, said valves can also be situated downstream of each separator on the line rich in gas (even if this option is not indicated in figure 4) , j RC (RATIO CONTROL) controllers which each regulate the volumetric ratio between the flows leaving each T-junction, j liquid level indicators Ll _j , j+1 liquid level controllers LC _j , 1 pressure controller PC;

Figure 5 is a separation test plant by means of T- junctions ;

Figure 6 is a control test of the separation test plant by means of T-junctions.

Detailed description

A possible application of the process, object of the present invention, is the separation of multiphase streams coming from oil wells, whether they be oil fields or gas fields, in any oil plant. This separation is effected by means of T-junctions coupled with accumulation volumes and/or traditional separators . Said traditional separators are preferably of the gravitational type. Said accumulation volumes are preferably gravimetric separators . Multiphase stream refers to a mixture comprising a gas or vapour phase, one or more liquid phases, in which the liquid phase can be water and/or different hydrocarbons separable by temperature and pressure, and a solid phase consisting of silica sand or having a different composition and/or other solid suspended such as clay for example. This process preferably separates gas-water streams or gas- oil-water streams. The T separation area can be set up according to two preferred configurations: joining T (1) or divergent T (2) according to Figure 1. The feeding stream can have a component which is preferably horizontal (4) or vertical (5) according to Figure 2. The opening angle (2 in Figure 1) of the T-junction can also be greater than or less than 90°. With reference to Figure 4, the present invention describes a separation process of j multiphase streams, with j varying from 1 to n, wherein n is an integer which depends on at least the morphological and geological characteristics of the well, and ranges from 1 to 10,000, and preferably from 1 to 5,000, and even more preferably from 1 to 3,000. These streams come from j wells and are conveyed along j pipes, on which j T-junctions (7) are installed. Each T-junction, in order to determine inlet conditions, divides each stream into two flows, in this text indicated as Fj(l) and F _j (2) with j varying from 1 to n, wherein n is an integer, and contemporaneously effects a first separation of the phases in each flow so that Fj(l) has a greater gaseous volumetric composition and Fj(2) a greater liquid volumetric composition. The richest stream in liquid phase by definition is the stream in which the volumetric flow-rate of the liquid phase in F _j (2) is higher with respect to the liquid volumetric flow-rate in F _j (l), the latter is consequently defined as a stream richer in gas phase.

All the flows separated are subsequently sent into at least j+1 accumulation volumes and/or traditional separators and in particular, the flow F _j (1) is conveyed towards the traditional separator Sj with j = [1; n] wherein n is an integer, whereas F _j (2) is sent towards the traditional separator S _n+1 . The accumulation volumes and/or traditional separators are preferably in a number equal to n+1 as indicated in Figure 4. In j+1 accumulation volumes and/or separators, the streams are separated in a substantial mode. A stream at the head with a volumetric composition rich in gas phase (Gl, Gn, Gn+1) and a stream at the bottom with a volumetric composition rich in liquid phase (LI, Ln, Ln+1) leaves each of said accumulation volumes and/or separators. Entrainments of the solid phase present in the feeding stream are possible in this phase. A single separator S _n+ i treats all the flows with a higher liquid volumetric fraction and the liquid phase thus obtained can be collected in a storage tank, not indicated in Figure 4.

The separation tank as illustrated in Figure 4 is controlled by a velocity control system of the 2j flows leaving the j T-junctions. For this purpose, each of the at least j+1 separators S _j must be equipped with the instrumentation normally installed on these apparatuses: at least j+1 pressure sensors, at least j+1 temperature sensors, at least j+1 liquid level indicators Ll _j , at least j+1 liquid level controllers LC _j with a high and low level alarm, and at least j+1 draining valves of the bottom liquid (10) as a safety requirement. The application of the invention requires, in addition to the instrumentation normally used as previously described, at the most 2j modulator valves PV _j with j = [1; n] , also known as actuation servomechanisms , installed in pairs on j lines downstream of the j T-junctions or on the lines of the gaseous streams leaving the separators downstream of this; a pressure controller PC installed on the evacuating line of the phase rich in gas (Gl) ; at the most j controllers RC which regulate the volumetric flow-rates of the two streams leaving each T-junction. Said controllers RC are preferably RATIO CONTROL. The RATIO CONTROL controller operates a control of the multivariate type on the volumetric flow-rates of the two streams leaving each T-junction. The control system of the velocities of the flows leaving each T-junction regulates the volumetric ratio of said flows so that the critical entrainment condition of the gas phase is respected in each flow with a larger gaseous volumetric composition. The controller is continuously searching for the minimum liquid flow and maximum gas flow condition in each stream with a larger gaseous volumetric composition, regulating this ratio. The critical entrainment rate is the maximum velocity at which a phase can run countercurrent with respect to another phase. If the velocity of the gas is lower than the entrainment velocity or critical entrainment rate, then the gas phase is not able to entrap and entrain the liquid present (even when in the form of droplets) and this therefore allows the separation of the phases . With higher rates than the entrainment velocity or critical entrainment rate, the gas transports the liquid present preventing it from refluxing. The question is treated and modelled for example in Wallis G. "Flooding velocities for air and water in vertical tubes" UKAEA Report AEEW - R123. The correct volumetric ratio F _j (l)/F _j (2) was determined by studying the separation of the T-junctions, and this study ranged from feedings characterized by streams under flow conditions varying from slug flow regime to annular flow regime, on the basis of the Aziz definition.

The currents circulating in the process often entrain sands or contain for example clays as suspended solids. This condition is the cause of erosion of the modulator valves and for this reason these valves are preferably positioned downstream of each separator S _j on the outlet line of the phase rich in gas. In this way, the erosion of the valves is reduced, increasing their average life.

The control system of the process in the case of gas fields is based on the at least j+1 liquid level controllers and alarm LC _j , with an alarm for the minimum and maximum level, and on the pressure controller PC. It also includes a motorized control valve of the flow of drained liquid which can be preceded by a flow blockage valve functioning with an ON/OFF procedure (10) . With reference to Figure 4, the method for controlling the multiphase flow rate coming from gas fields comprises the phases described hereunder. The control sensor PC reveals a reduction in the differential pressure of the gas on the evacuating line of the flows rich in gas (Gl) . The control sensors LC _j detect an increase in the differential liquid head in each separator which is reached by the flows rich in gas (Sj with j ranging from 1 to n) . These signals can also be alternative. The control sensors LC _j if necessary detect a signal indicating that the high liquid level limit has been reached in the separators in which the flows rich in gas are treated. If necessary suitable flow meters (not indicated in Figure 4) reveal the volumetric flow-rates of the flows leaving each T-junction. These signals are then transmitted to each controller RC which commands the partial or total closure of the modulator valves PVj situated on the line having the largest volumetric composition of gases. These controllers RC optionally command the opening of the ON/OFF blockage valves (10) so as to evacuate the control volume of said accumulation volumes and/or separators. Finally, said controllers command the partial or total opening of the modulator valves PV _j positioned on each line rich in liquid phase (in volumetric terms) so as to regulate the pressure on the evacuating line of the gas phase.

In the case of a low pressure alarm of the gas on the evacuating line and/or for a minimum level in the separator in which a flow rich in gas has accumulated, the control system actuates a safety procedure and operates a blockage valve of the ON/OFF type (10) to close the liquid flow leaving all the accumulation volumes and/or separators (LI, Ln, Ln+1) .

The use of a second level sensor for each traditional separator, although not strictly necessary, allows the reliability level of the procedure to be increased .

Reference is now made to Figure 3. In an operative situation characterized by a liquid slug which reaches a T-junction (7) , the controller RC reads an increase in the liquid level in SI, or another slug monitoring measurement on the feeding pipe to the T, and drives the valve PV1 to be in a minimum closing position with the flow almost totally re -directed on S2 and commands PV2 to be brought to the control value of PCI . The liquid control level LC3 on S2 evacuates 200 or 300 litres per minute (for traditional separators normally used) or for the time which lapses between the opening and complete closing of the bottom valve of S2. The controller LC2 , on the other hand, operates under the reaching of high level conditions in the separator SI. The outflow conditions of the liquids are those normally used on traditional separators on site. These conditions are sufficient for evacuating liquid flows characterized by a surface rate of up to 20 m/s in the feeding to the separator. This policy of use of the T- j unction completes and compensates the operative difficulties of its use under normal conditions and makes the structure of use stable and reliable.

The control system of the process in the case of oil fields is formulated in a slightly different way with respect to that applied for streams coming from gas fields. In this embodiment of the invention, in traditional separators it may be necessary to maintain a liquid head which is almost constant, even during the outflow phase of the liquids, to ensure the release of the dispersed or dissolved gas. The liquids can also be stratified and evacuated separately by specific level control systems. The liquid head can be maintained by means of an overfall or by a level control with a high and low level alarm. The latter is preferably a Proportional Integral Derivative controller. In some cases, it can also be a liquid draining valve based on an ON/OFF logic. In the process, flow meters must be present, each positioned on each draining line of the stream rich in liquid. A liquid level indicator is also present on each accumulation volume and/or phase separator, which however does not have any function in the control method of the multiphase flow rates. With reference to Figure 4, the method for controlling the multiphase flow rates coming from oil fields comprises the phases described hereunder.

The control sensor PC reveals a reduction in the differential pressure of the gas on the evacuating line of the flows rich in gas. The flow meters, not indicated in Figure 4 and each positioned on each draining line of the stream rich in liquid, detect an increase in the flow-rate of the liquid extracted from each accumulation volume and/or separator reached by the flows rich in gas. These signals can also be alternative. Specific flow meters not indicated in Figure 4) reveal the volumetric flow-rates of the flows leaving each T-junction. These signals are then transmitted to each controller RC which commands the partial or total closing of the modulator valves PV _j positioned on the line with the greatest volumetric composition of gas. Said controllers RC optionally command the opening of the bottom valves, continuously modulating them so as to extract the excess liquids within the control volume of each separator. Finally, said controllers command the partial or total opening of the modulator valves PV _j positioned on each line rich in liquid phase (in volumetric terms) so as to regulate the pressure on the evacuating line of the gas phase (Gl) .

In the case of low pressure alarm of the gas on the gas evacuating line or minimum level in the separator in which a flow rich in gas has accumulated, the control system actuates a safety procedure and operates a blockage valve to close the liquid flow leaving each accumulation volume and/or phase separator.

A possible configuration of the invention envisages a multiphase stream coming from a gas or oil well which flows in one or more pipes contained in or downstream of said oil well. According to this configuration, the T-junction can be one or more side branchings of the multiphase stream, whereas the accumulation volumes and/or separators are flow spaces formed by two or more pipes, said pipes having the same axis or adjacent axes, horizontal or vertical or having an inclination, and having a different or the same section. Each pipe is intercepted downstream of the T-junction by a control valve which regulates the velocity of the flows leaving the T-junctions in order to respect the critical entrainment condition of the gas phase, and in which one or more extraction pipes of the liquids separated are contained.

Example

As an example of the invention, a separation condition is described, obtained in a plant which uses a T-junction for separating streams consisting of air and water.

The plant was managed as a prototype of situations relating to a phase separation for a gas field and/or for an oil field. Figure 5 represents the plant scheme which is composed of three main sections:

a) a section for the generation of a controlled flow, equipped with a centrifugal pump (PI) , suitable compressed air connections from the supply line (101) and a mixing section

(MIXER) ;

b) a separation section with a T-junction (T) , consisting of a divergent junction in which the flow distribution takes place, and equipped with a series of suitable pressure meters (PI1, PI2, PI3 , PI4 , PI5, PI6, PI7) and flow meters (FI2, FI3) ;

c) an outgoing flow management section, consisting of two traditional separators (SE1, SE2) to allow the measurement of the fluids leaving the T-junction and the possible recycling of the liquid phase to the first section.

The T separation area is characterized by the possibility of being flexibly assembled in various geometrical configurations, in order to be able to evaluate various types of T-junctions: joining or divergent T (see Figure 1) . The plant is equipped so that both vertical flow and horizontal flow types can be evaluated (see Figure 2) .

Figure 5 shows the block scheme of a test plant with T-junctions, wherein SI is the feeding tank of the water liquid phase (100) , PI is the centrifugal pump for liquid feeding, FI1 and FI2 are liquid flow-rate meters, FICl is an air flow-rate regulator (101) . Furthermore, PI1, PI2, PI3 , PI4 , PI5, PI6, PI7 are differential pressure gauges, PCV1 is an air pressure reducer, Til is a temperature indicator, FI3 is an air flow-rate meter, SE1 and SE2 are phase separators. Finally VI, V2 , V3 , V4 , V5 , V6 , V7 , V8 , V9 , V10 and Vll are valves . The plant can run at room temperature and at a pressure lower than 5 bar, with flow-rates of air from the supply line of up to 500 Nm3/hour and of well water of up to 50 m ³ /hour recycled from a primary tank by means of a centrifugal pump. Various experimentation sets were tested with a horizontal and vertical T within the range of velocity values of gas wells and with oil/water/gas systems.

Tests were developed for:

• gas surface velocity ranges from 0.1 m/s to 100 m/s, preferably from 1.83 m/s to 37.47 m/s, liquid surface velocity ranges from 0.01 m/s to 1 m/s, and preferably from 0.012 m/s to 0.163 m/s, with a T- junction arranged vertically with horizontal sampling;

• gas surface velocity ranges from 0.1 m/s to 100 m/sec and preferably from 6.54 m/s to 37.2 m/s and liquid surface velocity ranges from 0.01 m/s to 1 m/s, and preferably with velocities of 0.001 to 0.13 m/s of the liquid, with a T-junction arranged horizontally with vertical sampling.

Separation conditions were verified in which the flow-rates leaving the T-junction are for gas only. Among these conditions, there are many which make this separation quantitatively effective in cases of operative interest.

A T-separator, fed horizontally with a biphasic stream characterized by surface velocities of the gas phase below 30 m/s, preferably below 10 m/s, and surface velocities of the liquid phase below 0.1 m/s, preferably with velocities lower than or equal to 0.01 m/s, for certain separation ratios of the streams, provides two streams:

• stream 1: which contains all the liquid in the feeding and a gas flow-rate, at the most, equal to about 10% by weight of that fed,

• stream 2: gaseous stream with about 90% by weight in the feeding (plus the possible liquid withheld) .

The plant management, under dynamic conditions, was not problematical. Figure 6 represents a situation of complete separation of the liquid fed with a flow-rate from the pump PI equal to 7.2 m3/h and three different gaseous flow-rate levels equal to 30 Nm3/h, 50 Nm3/h and 75 Nm3/h. In each situation, the combined action on the valves V7 and V9 allows all the liquid to be diverted onto the separator SEl .

The reading of the liquid level of SE2, not indicated in Figure 6, and the flow-rate FT2 of the liquid, leaving SEl, allows it to be verified whether the separation objective under the three feeding conditions of the gas has been reached. For each of the three feeding conditions to the T-junction, there is a combination of ratios between the streams leaving the T-junction which allow the separation of a stream of gas alone and, as is evident from Figure 6, without significant transients on the results of the separation astride of the changes in the gas flow-rates operated in the plant .