NO340348B1

NO340348B1 - Liquid Extraction Unit

Info

Publication number: NO340348B1
Application number: NO20151058A
Authority: NO
Inventors: Lachlan Mckenzie; Stein Følkner; Thomas Krebs
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2015-08-20
Filing date: 2015-08-20
Publication date: 2017-04-10
Also published as: WO2017029318A1; NO20151058A1

Description

Field of the invention

The present invention relates to the field of liquid extraction units (LEU), and more particularly to LEU's for use in a multiphase pumping system.

Background

During operation of a multiphase pump recycling of liquid may be required under certain conditions. Generally, this is to prevent the multiphase pump from operating in the minimum flow regime.

When recycling a multiphase flow to a multiphase pump it is important that the flow being recycled to the inlet of the multiphase pump has the same (at worst) or lower gas volume fraction (GVF) as the flow exiting the pump. If this is not achieved (i.e. a flow with higher GVF is recycled) the GVF at the suction side of the pump will increase resulting (most likely) in moving further out into the minimum flow regime.

In addition, the recycled flow should also have a lower concentration of sand than the flow exiting the pump. This is to prevent an increase of the concentration of sand in the recycled flow which may exceed allowable levels.

In present systems the required recycled flow is commonly obtained by using a tank-based liquid extraction unit (LEU). The tank-based design operates on the principle of gravity separation enriching the liquid content of a fluid that is subsequently recycled. The key disadvantages of this design are that it requires manufacturing as a vessel or tank (large, expensive and heavy) and requires drainage at the base of the liquid accumulation section of the tank to prevent build-up of sand. The requirement for drainage limits the effective operational range of the tank-based LEU because there is a continuous stream of liquid leaving the LEU via drainage holes. Thus, when insufficient liquid is fed into the LEU, meeting the requirement of a fluid with an enriched liquid content in the recycle is impossible (liquid is lost via the drainage holes).

Systems using a simple T-shaped pipe section as an LEU have also been disclosed, ref. US patent no. 6007306. However, a literature review reveals that the performance of such a solution in terms of gas content in the recycle flow is uncertain and dependent on the inlet conditions as to whether the recycle will be enriched in gas or liquid. An additional disadvantages of a simple T-shaped pipe section is the problem of ensuring that sand will not accumulate in the recycle line when it is not in use and/or a potential increased sand concentration in the recycle when it is in use (if'recycle tee' is vertically down from the main flow), ref. Lahey et al, Phase Separation in Impacting Wyes and Tees; International Journal of Multiphase Flow, 15, 1989, 965-975.

The object of the present invention is to provide a liquid extraction unit which avoids at least some of the disadvantages of the prior art liquid extraction units.

Summary of the invention

The present invention provides a liquid extraction unit which is simple, efficient and avoids or minimizes at least some of the disadvantages of the known LEU's. The invention is defined by the appended claims and in the following: The invention provides a liquid extraction unit comprising a main pipe and an extraction pipe, wherein the main pipe is horizontally arranged during use;

- the main pipe comprises a main inlet and a main outlet,

- the extraction pipe comprises an extracted liquid inlet and an extracted liquid outlet, the extracted liquid inlet is arranged inside the main pipe, and the extracted liquid outlet is arranged external to the main pipe;

wherein

- the extracted liquid inlet is arranged above the level of a lower internal surface of the main pipe; and - at least a part of the extraction pipe is a pipe section comprising a lower end and an upper end, wherein the lower end comprises the extracted liquid inlet.

In the present application, the term "a lower internal surface" is meant to define a surface on which a liquid enriched flow may accumulate to a desired level during use. A lower internal surface is commonly a horizontal surface, for instance formed by part of the main pipe håving a mainly horizontal flow direction, such as the internal surface of the lower half or lower quarter of a horizontally arranged main pipe. The lower internal surface may in some instances also be inclined in the flow direction of the main pipe. Although the main pipe may have different cross sections, such as circular, oval or square, the lower internal surface may in most instances form a curved surface when seen in a cross section of the pipe, but form a mainly linear surface, either substantially horizontal or inclined, when seen in a longitudinal cross section of the main pipe. The lower internal surface is preferably arranged below the level of the centerline of the main pipe. The lower internal surface of the main pipe may optionally be the lowest or lowermost internal surface of the main pipe.

By håving the extracted liquid inlet arranged inside the main pipe, it should be understood that the inlet is positioned within the main pipe and does not form part of the wall of the main pipe as such. In other words, the lower end comprising the extracted liquid inlet is arranged at a distance from the inner wall of the main pipe.

In an embodiment the extracted liquid inlet of the extraction pipe is positioned at a distance from the inner wall of the main pipe. Preferably, the extraction pipe (and/or the pipe section) extends through the wall of the main pipe at a position arranged at or above the lowest level of the extracted liquid inlet. The extraction pipe (and/or the pipe section) extends from the extracted liquid inlet in a horizontal or upward direction, wherein the upward direction is inclined with respect to the horizontal or is vertical, and then out through the wall of the main pipe to the extraction liquid outlet.

The extraction pipe (or pipe section) may also be defined as being arranged such that a liquid enriched flow must move against gravity to exit the extracted liquid outlet during use. The term "move against gravity" is intended to define that to obtain a liquid enriched flow out of the extracted liquid outlet, the liquid enriched flow must be sucked out of the main pipe by håving a lower pressure at the extracted liquid outlet compared to the pressure at the extracted liquid inlet, irrespective of the liquid level in the main pipe.

In an embodiment of the invention, the lower internal surface is unobstructed by the liquid extraction pipe, such that at least parts of a solid and/or at least parts of a liquid enriched flow may pass underneath the extracted liquid inlet during use.

The term "underneath" is intended to mean that at least parts of the solids and/or at least parts of the liquid enriched flow may pass directly under, or directly beneath, the extracted liquid inlet. By håving the lower internal surface unobstructed by the liquid extraction pipe, the liquid extraction pipe will not prevent solids from flowing along the lower internal surface towards the main outlet.

In an embodiment of the invention, the extracted liquid inlet comprises an opening facing the lower internal surface, an opening facing the main outlet and/or at least one radial opening arranged in the extraction pipe, or various combinations of these alternatives. The extracted liquid inlet should not face upwards, and a main part of the liquid inlet should not face the main inlet.

In an embodiment of the invention, the extracted liquid inlet is arranged at a level of, or below, the centerline of the main pipe. This position/level will depend on the ratio of liquid to gas and solid expected in the specific use of the invention In a further embodiment of the invention, the liquid extraction unit comprises a liquid obstruction element arranged downstream of the extracted liquid inlet and upstream of the main outlet, such that a liquid may accumulate at the lower internal surface of the main pipe during use. This will increase and ensure accumulation of liquid underneath the extracted liquid inlet. The liquid obstruction element may be designed such that the liquid may accumulate to any desired level within the main pipe.

In yet an embodiment of the invention, the liquid obstruction element is arranged such that the liquid may accumulate to at least the level of the extracted liquid inlet. Alternatively, the liquid obstruction element is arranged such that the liquid may accumulate to the lowermost level of the main outlet, or to an upper edge of the liquid obstruction element.

In yet an embodiment of the invention, the liquid obstruction element comprises at least parts of a restricted inner circumference of the main pipe.

In yet an embodiment of the invention, the restricted inner circumference provides the main pipe with a section of an internal surface which is inclined upwards in a direction towards the main outlet from the level of the lower internal surface to the level of the extracted liquid inlet. Such a solution would provide an obstruction element for the liquid part of the fluid flow, but still provide a flow path for particles that normally would flow in the lowermost part of the pipe. The flow path is not abruptly altered or changed, so that the particles (sand) are less likely to accumulate within the liquid extraction unit.

In yet an embodiment of the invention, the liquid obstruction element is a shaped element arranged at the lower internal surface such that a fluid flow may pass over the shaped element. The shaped element may for instance be a box or plate element arranged relative the lower internal surface such that a fluid flow may pass over the element at a level above the lower internal surface. Preferably, the element extends a distance away from the lower internal surface and forms a partial blocking for the fluid in the lower half of the main pipe. The shaped element may be a solid element or it may be an element allowing a partial flow through the element, such as for instance a grid. The shaped element is a distinct element not forming an integral part of the main pipe.

In yet an embodiment of the invention, the shaped element is provided within the main pipe such that there is a gap between a lower end of the shaped element, for instance a plate element, and the lower internal surface. This would provide a benefit in relation to allowing particles to pass the liquid obstruction element together with a liquid flow in the lowermost part of the liquid extraction unit and not accumulate within the unit. The liquid obstruction element may in addition provide additional gaps on the sides or through the element itself.

In yet an embodiment of the invention, the shaped element comprises an upper edge arranged at or above the level of the extracted liquid inlet.

In yet an embodiment of the invention, a fluid flow passage is provided between the shaped element and a part of the main pipe below the shaped element. The part of the main pipe below the shaped element is preferably the lower internal surface of the main pipe.

In yet an embodiment of the invention, the liquid obstruction element or the shaped element is provided with at least one flow passage through the liquid obstruction element. The at least one flow passage is arranged in the longitudinal direction of the main pipe.

In yet an embodiment of the invention, the extracted liquid inlet is arranged at, or downstream of, the halfway point between the main inlet and the main outlet. This feature is to ensure a partial separation within the liquid extraction unit, thereby providing a majority of the liquid within the flow in the lower half of the unit.

In yet an embodiment of the invention, the liquid extraction unit comprises a creep flow preventer arranged upstream of the extracted liquid inlet, the creep flow preventer comprises a rib arranged at the circumference of at least part of an upper half of the internal surface of the main pipe such that any liquid flowing at the upper half of the internal surface is directed towards the lower internal surface of the main pipe.

In yet an embodiment of the invention, at least a section of the main pipe has an inner circumference larger than the inner circumference of the main inlet and/or the main outlet. The section may advantageously extend from a position upstream of the extracted liquid inlet to a position downstream of the extracted liquid inlet.

In another embodiment of the invention, the section of the main pipe håving a larger inner circumference is arranged non-centrically in relation to the main inlet. In one embodiment the lowermost point of the main inlet and the lowermost point of the section with a larger inner circumference may be flush or at the same level, thereby providing a relatively continuous unchanged flow pattern in the lowermost half of the main pipe.

In yet another embodiment of the invention, the main inlet is provided with a lowermost point positioned relatively lower than a lowermost point of the main outlet. This will provide an increased accumulation of the heavier fluids within the main pipe in the direction of the main outlet.

In another aspect, the present invention also provides for the use of a liquid extraction unit as defined above for providing a recycle flow to a multiphase pump.

In yet another aspect, the present invention provides a multiphase pump system comprising a liquid extraction unit as defined above, wherein the main inlet is arranged downstream a multiphase pump and the extracted liquid outlet is fluidly connected upstream of the multiphase pump. The extracted liquid outlet is preferably fluidly connected upstream of the multiphase pump by a fluid conduit or a pipe.

Since the liquid extraction unit is for providing a recycle flow to a multiphase pump, it may also be termed a multiphase pump recycle flow extraction unit. As discussed above, certain requirements apply to a liquid extraction unit to be suitable for use in a multiphase pump system.

Short description of the drawings

The invention is described in detail by reference to the following drawings:

Fig. 1 is a cross-sectional side view of a first embodiment of the invention.

Fig. 2 is a cross-sectional side view of a second embodiment of the invention.

Fig. 3 is a cross-sectional side view of a third embodiment of the invention.

Fig. 4 and 5a-b depicts the embodiment of fig. 3 combined with two advantageous internal features. Fig. 6 is a schematic view of a multiphase pump system comprising the second embodiment of the invention.

Detailed description of an embodiment of the invention

An embodiment of a liquid extraction unit (LEU) according to the present invention is shown in fig. 1. The LEU comprises a main pipe 1 håving an inlet 3 (main inlet) and an outlet 4 (main outlet). The outlet and the inlet are connectable to further equipment or pipes by flanges 10, 11 arranged at the respective outlet/inlet. When in use the main pipe is arranged in a horizontal position (as shown in fig. 1) such that it comprises a lower internal surface 7. An inlet 5 (i.e. extracted liquid inlet) of an extraction pipe 2 is arranged inside the main pipe and situated above the lower internal surface 7. In this embodiment, the extraction pipe is made up of a pipe section 17 comprising a lower end 15 and an upper end 16. The lower end comprises the inlet 5. The outlet 6 (i.e. extracted liquid outlet) is arranged outside the main pipe. The outlet 6 is connectable to further equipment or pipes by a flange 18. In use, the outlet 6 is fluidly connected upstream of a multiphase pump inlet.

The inner circumference of the main pipe 1 is restricted at a section 9 of internal surface (i.e. a liquid obstruction element) arranged between the inlet 5 of the extraction pipe 2 and the outlet 4 of the main pipe 1. This restriction ensures that a liquid phase may accumulate at the lower internal surface of the main pipe. In this embodiment, the section 9 of internal surface is inclined from the level of the lower internal surface 7 to the lowest level of the outlet 4. In other embodiments, the section 9 may be inclined to the lowest level of the most restricted inner circumference (which in this particular embodiment is equal to the lowest level of the outlet 4). During use, the minimum upper level of a liquid flowing through the LEU is decided by the design of the restriction. In this embodiment, the minimum upper level will be equal to the lowest level of the outlet 4. The inclination of the section 9 is preferably such that a liquid flow will be able to push solids (usually sand) present on the lower internal surface 7 towards and out of the outlet of the main pipe during normal flow conditions providing a significant flow underneath (i.e. underflow) the inlet 5 of the extraction pipe. The inlet 5 of the extraction pipe 2 is arranged at the same level as the minimum upper level of a liquid, i.e. at the same level as the lowest level of the outlet 4 or the lowest level of the most restricted inner circumference of the main pipe. In other embodiments the inlet 5 may be at a higher or lower level, but will always be at a level above the level of the lower internal surface 7 of the main pipe. In the disclosed embodiments, the inlet 5 of the extraction pipe is arranged between the halfway point of the main pipe and the outlet of the main pipe, and upstream of the inclined section 9 (or the restricted inner circumference). This arrangement of the inlet 5 ensures that at least a part of the solids present in the flow entering the LEU will gravitate towards the lower internal surface and not be part of the recycle flow. Alternatively, the inlet 5 may be arranged closer to the inlet 3 of the main pipe (i.e. closer than the halfway point of the main pipe) provided the main pipe is long enough to ensure that a liquid enriched flow may be extracted through the inlet 5. In cases where the underflow is not sufficient (high flow splits and/or low liquid inlet rates), the solids (or sand) may accumulate in the lower half of the LEU. However, the solids will be flushed out into the downstream piping once normal operation is resumed (i.e. significant production to topside). Since the LEU does not have any liquid drain (as in tank-based designs), no lower limit on the performance envelope is anticipated.

During use a multiphase flow will enter the inlet 3 of the main pipe, and a part of the liquid of the multiphase flow will form a liquid enriched phase which at least accumulates to the lowest level of the outlet 4 of the main pipe. In some cases, when the multiphase flow entering the main pipe has a very high GVF, the liquid enriched phase may for instance be enriched by håving a higher concentration of liquid droplets relative to said multiphase flow. Some of the solids present in the multiphase flow will also accumulate at the internal lower surface 7. During normal flow conditions, the solids will be pushed along with the liquid flow through the outlet of the main pipe, preventing accumulation of any large amounts of solid.

A second embodiment of an LEU according to the invention is shown in fig. 2. The main difference compared to the LEU of fig.l is that the restriction of the inner circumference of the main pipe is asymmetric. The restricted inner circumference upstream of the inlet 5, in the LEU of fig. 2, does not provide a threshold at a level above the lowest internal surface of the main pipe. However, in any conceivable flow condition, the liquid level will still be high enough to provide a reduced gas volume fraction (GVF) in the recycle flow (compared to the GVF of the flow entering the main inlet 3) exiting the outlet 6 of the extraction pipe 2. If required, the main pipe may alternatively be arranged at a slight inclination in relation to the horizontal, such that the main inlet is at a higher level than the main outlet. In connection with the second embodiment, such an inclination would provide a reduced GVF in the recycle flow even during flow conditions providing very low GVF in the multiphase flow entering the main inlet.

The present invention also provides for a third LEU embodiment as shown in fig. 3. The third embodiment does not comprise a liquid obstruction element 9 arranged between the inlet 5 of the extraction pipe 2 and the outlet 4 of the main pipe 1. However, in most flow conditions the LEU of fig. 3 will still provide a reduced gas volume fraction (GVF) in the recycle flow despite missing a liquid obstruction element, as shown in fig. 1 and 2, since liquid in the flow entering the main inlet will migrate towards the lower internal surface due to gravity. To compensate for the missing liquid obstruction element, the level of the inlet 5 of the extraction pipe may in some cases be arranged lower in the LEU of fig. 3 compared to the inlet 5 of the LEU's in figs. 1 and 2. In all the embodiments of the invention, the level (between the centerline and the lower internal surface) at which the inlet 5 is arranged may vary depending on the expected properties of the produced flow, i.e. the GVF and solids content.

The LEU of fig. 3 may further be modified by håving a blocking plate 12 as a liquid obstruction element. The blocking plate provides the same effect as obtained by the restricted inner circumference at a section 9 of internal surface of the LEU's in figs.

1 and 2.

To further improve the flow condition in the LEU's shown in figs. 1-4, the main pipe may advantageously comprise a rib 14 (or a creep flow preventer) arranged at the circumference of at least an upper half of the internal surface of the main pipe 1. The rib will guide any liquid flowing at the upper half of the internal surface in the direction of the lower internal surface of the main pipe. The liquid/gas ratio will increase in the lower half of the main pipe, and the GVF of a recycle flow will thus decrease.

A common feature of all the embodiments of the present invention is the extraction pipe 2 which has an inlet 5 arranged inside the main pipe 1 at a level below the centerline C of the main pipe and above the level of a lower internal surface 7 of the main pipe, i.e. there is a gap between the opening of the inlet 5 and the lower internal surface. This feature provides two advantageous effects; firstly, at least some of the solids gravitating to the lower internal surface will follow the flow of liquid underneath the inlet 5 and pass through the main outlet 4, thus providing a recycle flow håving a solids concentration equal to or lower than the solids concentration of the flow entering the main inlet, and secondly, since the liquid/gas ratio is higher in the lower half of the main pipe, the recycle flow will have a GVF being equal to or lower than the GVF of the flow entering the main inlet.

A multiphase pump system featuring an LEU (the embodiment of fig. 2) is shown in fig. 6. The system comprises three main components; a buffer tank, a multiphase pump and a liquid extraction unit. The outlet of the pump is fluidly connected to the main inlet 3 of the LEU for providing a multiphase flow thereto. The multiphase flow comprises a gas phase and a liquid phase (in addition to any solids present). In the LEU the liquid phase and any solids will gravitate towards the lower part of the LEU and the liquid/gas ratio of the flow will consequently be higher in the lower half of the LEU. To prevent the multiphase pump from operating in the minimum flow regime, recycling of the multiphase flow may be required under certain conditions. A recycle flow is then extracted from the LEU via the inlet 5 of the extraction pipe 2 and through the outlet 6. The outlet 6 of the LEU is fluidly connected to the buffer tank (i.e. upstream of the multiphase pump) by a conduit. The design of the LEU according to the invention ensures that the GVF and the solids content of the recycled flow is equal to or lower than in the multiphase flow entering the LEU.

Claims

1. A liquid extraction unit comprising a main pipe (1) and an extraction pipe (2), wherein the main pipe (1) is horizontally arranged during use; - the main pipe (1) comprises a main inlet (3) and a main outlet (4), - the extraction pipe (2) comprises an extracted liquid inlet (5) and an extracted liquid outlet (6), the extracted liquid inlet (5) is arranged inside the main pipe (1), and the extracted liquid outlet (6) is arranged external to the main pipe (1); characterized in that- the extracted liquid inlet (5) is arranged above the level of a lower internal surface (7) of the main pipe; and - at least a part of the extraction pipe is a pipe section (17) comprising a lower end (15) and an upper end (16), wherein the lower end comprises the extracted liquid inlet (5).

2. A liquid extraction unit according to claim 1, wherein the lower internal surface (7) is unobstructed by the liquid extraction pipe (2), such that a fluid flow may pass underneath the extracted liquid inlet (5).

3. A liquid extraction unit according to any of the preceding claims, wherein the extracted liquid inlet (5) comprises an opening facing the lower internal surface, an opening facing the main outlet and/or at least one radial opening arranged in the extraction pipe (2).

4. A liquid extraction unit according to any of the preceding claims, wherein the extracted liquid inlet (5) is arranged at a level of, or below, the centerline (C) of the main pipe.

5. A liquid extraction unit according to any of the preceding claims, comprising a liquid obstruction element (9, 12) arranged downstream of the extracted liquid inlet (5) and upstream of the main outlet (4), such that a liquid may accumulate at the lower internal surface (7) of the main pipe during use.

6. A liquid extraction unit according to claim 4, wherein the liquid obstruction element is arranged such that the liquid may accumulate to at least the level of the extracted liquid inlet (5).

7. A liquid extraction unit according to claim 5 or 6, wherein the liquid obstruction element (9) comprises parts of a restricted inner circumference of the main pipe (1).

8. A liquid extraction unit according to claim 6 or 7, wherein the restricted inner circumference provides the main pipe with a section (9) of an internal surface which is inclined upwards in a direction towards the main outlet (4) from the level of the lower internal surface (7) to at least the level of the extracted liquid inlet (5).

9. A liquid extraction unit according to claim 5 or 6, wherein the liquid obstruction element is a shaped element (12) arranged relative to the lower internal surface (7) such that a fluid flow may pass over the shaped element at a level above the lower internal surface.

10. A liquid extraction unit according to any of the preceding claims, wherein the extracted liquid inlet (5) is arranged at, or downstream of, the halfway point between the main inlet and the main outlet.

11. A liquid extraction unit according to any of the preceding claims, wherein at least a section of the main pipe (1) has an inner circumference which is larger than the inner circumference of the main inlet (3) and/or the main outlet (4), the section preferably extending from a position upstream of the extracted liquid inlet (5) to a position downstream of the extracted liquid inlet.

12. Use of a liquid extraction unit according to any of the preceding claims for providing a recycle flow to a multiphase pump.

13. A multiphase pump system comprising a liquid extraction unit according to any of claims 1-11, wherein the main inlet (3) is arranged downstream a multiphase pump and the extracted liquid outlet (6) is fluidly connected upstream of the multiphase pump.