WO2004033106A1

WO2004033106A1 - Multiphase separator

Info

Publication number: WO2004033106A1
Application number: PCT/NO2003/000230
Authority: WO
Inventors: John Bjørn FOSSDAL; Martin SØRENSEN
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2002-07-04
Filing date: 2003-07-02
Publication date: 2004-04-22
Also published as: AU2003296873A1; NO20023254L; NO20023254D0; NO317899B1

Abstract

A multiphase separator for separation of a fluid flow (30) comprising several fluid components, and disposed in a horizontally located tank (5) in connection with an underwater installation for petroleum production. The tank comprises an inlet for the fluid flow to a preferably horizontally located cyclone (10) with separate outlets for liquid and gas respectively to the tank (5), and where the inlet (13) is located at a first end (11) of the cyclone (10). An outlet (14) for gas that is connected to a centrally located and axially directed tube (16) and an outlet (15) for liquid from an external annulus in the cyclone (10) are both located at the second, opposite end of the cyclone. An additional tube (19) is provided between the gas tube (16) and the cyclone tube (20) and arranged to retard the speed of the liquid without forming turbulence with attendant formation of emulsions.

Description

Multiphase separator

The present invention relates to a multiphase separator for separation of a fluid flow comprising several fluid components, preferably for separating gas and liquid phases. The separator comprises a preferably horizontally located cyclone with separate outlets for liquid and gas to a tank, where the inlet is located at a first end of the cyclone, an outlet for gas is located centrally and axially directed at a second, opposite end of the cyclone, in addition to which an outlet for liquid from an external annulus in the cyclone is located at the second, opposite end of the cyclone and where the liquid outlet leads to an open channel that communicates with a desired admission area in the tank.

The separator tank is preferably employed in connection with an underwater installation for petroleum production.

The multiphase mixture is accelerated in the inlet of a cyclone, thus separating the fluids. The gas phase settles in the middle of the cyclone tube, while the liquid phase(s) are forced out towards the tube wall as a thin layer. This layer of liquid moves at a speed that has components both in the angular and axial direction. A flow path will therefore draw a spiral-shaped pattern along the wall.

A multiphase separator of the above-mentioned type is known from international patent application WO 90/05591. The separator comprises an inlet which is substantially perpendicular to a tubular main duct, whereby the fluid flow can be set in rotating motion. This rotation causes the heavy fraction, i.e. oil and water, to flow in a helical motion along the wall of the tubular main duct, while the light fraction, i.e. gas, will move towards the middle of the duct. In another part of the cyclone the wall slopes outwards and a conical plug is mounted therein. The conical plug has an external surface at the same angle as the outwardly sloping wall and has an internal channel, which leads to an outlet and which is located so that the gas flows through it, while the liquid flows in the annulus between the conical external surface and the inner wall of the cyclone to a second outlet. The conical plug can be moved axially in the cyclone, thus enabling the size of the annulus to be adjusted for optimal separation.

The disadvantage of the invention is that the speed of the fluid that is fed into the tank can be very high, thus making it difficult to attain a good separation of liquid and gas, i.e. gas will be transported down into the tank with the liquid, with the attendant problem of emulsions. In order to achieve a good separation of the gas, it is necessary to cause the gas droplets to collect into larger drops (which escape more easily from the fluid flow). Since the residence time of the fluid in the channel is relatively short, it will be difficult for the gas drops to be collected to an extent that is sufficient to enable them to escape. It has been shown that in order to achieve the shortest possible residence time in the tank with the high degree of separation that is required, it is vital to achieve a satisfactory retardation of the fluid passed down into the tank. The multiphase fluid flows at very high speed into the tank and unless a satisfactory retardation can be achieved, the liquid phase of the fluid (i.e. oil and water) will "break up" the liquid in the tank, resulting in the formation of emulsions and the development of drops. The residence volume in the tank will thereby increase. A solution to this problem is illustrated and described in NO patent publication no. 311.789.

It is important to achieve as uniform and laminar a flow motion as possible as this will make it easier for residue gas to escape. Turbulence in the liquid will cause the gas bubbles to remain relatively small and accompany the liquid down into the tank. The phases must be separated from each other without the formation of foam and emulsions. In principle, this is accomplished by avoiding sudden changes in speed (accelerations) in the flow. According to the present invention it is proposed that the gas outlet should be connected to an internal tube arranged concentrically in the cyclone tube over a part thereof and that a second tube is arranged between the gas tube and the cyclone tube. This will cause the flow rate of the fluid flow to be reduced, thus achieving a controlled collapse of the liquid flow. The object is achieved with a multiphase separator where the characterising features will be apparent in the attached claims.

In an advantageous embodiment of the invention, the internal tube comprises a first part which is at an angle to the cyclone's axis and a second part which extends parallel to and laterally offset from the cyclone axis. Any residue gas will migrate towards the inner wall of the channel and escape through a centrally located tube.

The invention will now be described in greater detail with reference to the accompanying drawings, in which fig. 1 illustrates a section through a separator tank, wherein the phase separator according to the invention is sketched.

Fig. 2 illustrates a section through a cyclone tube according to the invention.

Fig. 3 is a section along A-A in fig. 2.

Figure 1 illustrates a multiphase separator 1 according to the invention disposed in a tank 5, for example a separation tank located on a base on the seabed. In the tank there is provided a preferably horizontal cyclone 10 in the form of a tube 20 with a first end 11, where an inlet 13 is provided for supply of a multiphase fluid 30, and a second, opposite end 12 with separate outlets 14 and 15, where the outlet 15 communicates with a desired admission area in the tank 5. The cyclone 10 can be divided into three main sections, with a first section at the first end 11, a second, middle section and a third end section where the cyclone comprises an internal gas tube 16 concentrically disposed in the cyclone tube 20, to form an annulus 17.

The first section of the cyclone is arranged to set the fluid flow 30 in rotating motion along the inner wall of the tube 20. The inlet 13 may be axially and/or tangentially arranged relative to the direction of flow in the cyclone 10. In this embodiment guide vanes are provided in the first section in order to induce rotating motion of the fluid flo 30.

In a preferred embodiment the inlet is mounted so that it is perpendicular to the axis of the cyclone 10 and in a substantially tangential manner so that the fluid flow 30 is set in rotating motion in the cyclone 10. The inlet may also be cochleate in form, which will reinforce the rotating motion in the first section of the cyclone 10 and thereby permit a greater degree of separation of liquid 32 and gas 31.

The central section of the cyclone 10 is arranged to keep the fluid flow 30 in rotating motion and induce a separation of the three fractions, oil, water and gas, in the cyclone. The central section may advantageously have a narrowed diameter in relation to the first and third sections. When the diameter is increased again in the third section, the rate of fluid flow 30 will be reduced, thereby reducing turbulence and shear forces in the fluid flow. When the liquid film with its spiral-shaped flow pattern enters this section, the centripetal acceleration is given a substantial component along the tube wall. This assists in accelerating the liquid layer in the axial direction and consequently reducing the spin angle. With the larger tube diameter in the third section, the centripetal acceleration is also reduced, thus facilitating the collapse of the fluid flow and enabling it to collect as a stable layer in the bottom of the tube. A collapse of this kind will provide the desired situation, but the collapse in itself may also result in the formation of undesirable foam and emulsions. In order to achieve this, therefore, the cyclone is equipped with an additional tube 19 disposed in the third section, as illustrated in fig. 2. A tube with a diameter approximately equal to the diameter of the central section is inserted a short distance into the third section. A first part 21 of the tube is located with its axis at a small angle X relative to the cyclone's centre axis and in such a manner that the end of the inlet (on the left side of figure 2) is coaxial with the inner tube 16. The angle X is advantageously less than 20 degrees. The second part 22 of the tube extends with its centre axis parallel to, but laterally offset from, the cyclone's centre axis. The tube thus remains located at a short distance from the outer wall 20 on the top from approximately halfway along the third section all the way to the end of the cyclone.

When the spiral-shaped fluid flow enters this part of the cyclone tube, it is thereby forced through a cross section that becomes increasingly narrower the longer downstream it comes. Flow with strong momentum will come past and down into the bottom of the outlet tube. Flow with less momentum will be subjected to increasing resistance and will finally either be forced through the narrow gap at the top or it will change direction of rotation. In this manner a controlled collapse is obtained which has been shown to treat the liquid gently and to collect the liquid efficiently in the bottom of the outlet tube.

As mentioned earlier, the end section of the cyclone 10 has an internal gas tube 16. The gas fraction 31 that flows along the middle of the cyclone is fed into the internal gas tube 16 and to the outlet 14 which is located centrally and axially directed at the second, opposite end 12 of the cyclone 10. The gas fraction 31 is introduced into the tank above the liquid level 3.

The liquid fraction that flows along the inner wall of the tube 20 is fed into the annulus 17 in the end section. In other words, the gas outlet 14 and the liquid outlet

15 are disposed at the same end of the cyclone.

The liquid outlet 15 may be connected with devices for further separation and/or retardation of the liquid. For example, the liquid outlet may be connected with baffle plates as illustrated in NO patent publication no. 311.789 or a spiral-shaped housing as described in our simultaneous patent application NO 20023253.

When in use, supply of a multiphase flow, such as from a producing hydrocarbon well on the seabed, will be fed into the tank 5 through the cyclone 10. The inlet 13 of the cyclone is designed so as to produce a rotating motion of the incoming fluid, thereby exploiting the centrifugal force generated by the rotating motion to separate the three fractions of fluid from one another. During residence in the cyclone, the light phase, i.e. the gas, will move towards the middle of the tube 20, i.e. towards the centre axis of the cyclone, but the heaviest phase, the water, will flow along the inner wall of the tube 20, with the oil on the inside of the water. The liquid will spin like a film along the wall of the cyclone tube and be conveyed towards the annulus 17 in the third section of the cyclone. The gas will be fed into the internal gas tube

16 and out into the tank 5.

From the above it will be understood that the object of the cyclone is primarily to separate gas from liquid and even though a segregation of the liquid will take place in the cyclone, they will not be further separated from each other. Separation of the liquid, i.e. into oil and water, will therefore essentially take place in the tank. The liquid fraction that runs through the annulus is conveyed through the tube 19 and over into the outlet 15. During this process the liquid will be further retarded before being discharged into the tank 5. This retardation of the liquid will result in the liquid being mixed as homogeneously as possible with the liquid already present in the tank without causing the development of drops or the formation of emulsions.

Claims

PATENT CLAIMS

1. A multiphase separator device for separation of a fluid flow (30) comprising several fluid components, preferably for separating gas and liquid phases, where the device comprises a preferably horizontally located cyclone (10) comprising a cyclone tube (20) with a first end (11) with an inlet (13) and a second end (12) with two separate outlets (14,15), and in the longitudinal direction of a portion of the cyclone tube (20) there is located an internal gas tube (16) concentrically inside the cyclone, thus forming at least one annulus (17) between the internal gas tube (16) and the cyclone tube (20), where the outlet (14) for gas is located centrally and axially directed at the second end (12) and in connection with the internal gas tube (16), and the outlet (15) for the liquid extends from the annulus (17) to a channel that is connected to a desired admission area in the tank (5), characterised in that inside the cyclone tube (20) there is further provided a second internal tube (19) enclosing the internal gas tube (16).

2. A device according to claim 1, characterised in that the cyclone tube (20) has three sections, a first section at the first end (11), a second middle section and a third section at the cyclone tube's second end (12), where the second middle section has a narrowed internal diameter in relation to the first and third sections.

3. A device according to one of the above-mentioned claims, characterised in that the internal gas tube (16) has a length substantially corresponding to the length of the third section of the cyclone tube (20).

4. A device according to one of the above-mentioned claims, characterised in that the second internal tube (19) has a length that is less than or equal to the internal gas tube (16), and that the second internal tube's (19) inlet opening is concentrically located in the cyclone tube (20).

5. A device according to one of the above-mentioned claims, characterised in that the second internal tube's (19) centre axis from the inlet opening in a first portion (21) of the second internal tube (19) extends at an angle to the cyclone tube's (20) centre axis, and that a following second portion (22) of the second internal tube (19) has a centre axis that is parallel to and laterally offset from the cyclone tube's centre axis.

6. A device according to one of the above-mentioned claims, characterised in that the inlet (14) is substantially tangentially arranged at the cyclone's first end and/or that the inlet is cochleate in form.

7. A device according to one of the above-mentioned claims, characterised in that the outlet (15) for the liquid is connected with a device for further retardation of the liquid.