WO2011008103A1

WO2011008103A1 - Subsea drainage system

Info

Publication number: WO2011008103A1
Application number: PCT/NO2010/000259
Authority: WO
Inventors: Erik Baggerud; Leif Arne TØNNESEN; Helge E. Mordt; Anja Haugland; Zabia M. F. Elamin
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2009-07-15
Filing date: 2010-07-02
Publication date: 2011-01-20
Also published as: NO20092680A1; NO331265B1

Abstract

The present invention regards a subsea drainage system, comprising a first inlet (10) for a liquid drained from a subsea unit, leading to a drainage line (7) of the system with an outlet (30) for the drained liquid leading to a dumping space. According to the invention the system further comprises first means (20,21,22,23,24,25,26,27) for providing at least one motive fluid, and second means (11,12,13,14,15,120) being connected to the drainage line (7) between the inlet (10) and the outlet (30) and configured such that the means (11,12,13,14,15,120) provide an interaction between the motive fluid and the drained liquid such that the drained liquid can be transferred to the dumping space.

Description

Subsea drainage system

The present invention regards a subsea drainage system for liquids, the liquids may for instance be liquid drained from a subsea gas compressor.

Compressors are designed to boost the pressure of gas and will not handle liquids very well. Often a gas will contain small amounts of free liquids, e.g. water and such liquids tend to collect in the compressor housing. To avoid build up of liquids in the compressor housing that will, if it is continued, impede on the function of the compressor, it needs from time to time to be drained of the collected liquid. In normal topside installations this is simply done by having a drainage tap that can be opened (either by hand or automatically) to empty the excess liquid into a container or such.

In a subsea compression system a robust drainage philosophy is to drain a subsea compressor back to a scrubber. The gas to be compressed comes from the scrubber. Normally drainage is executed during stand-still utilizing gravity and with pressure equalized. If one would desire to drain during operation there will be a pressure difference between the drainage point and the receiving vessel /scrubber. To overcome this pressure difference an elevation in the order of 3-10 meters above the highest liquid level in the scrubber (HHLL) of the scrubber may be necessary, thus also elevating the compressor in the subsea structure. An aim for subsea equipment is generally to design low structures and elevating a heavy piece of rotating equipment in the structure is not beneficial.

Another issue for a subsea drainage system is the limited places to drain a liquid, compared with onshore solution. The places to drain a liquid subsea are limited firstly because there are limited proper locations in the system to drain to that if drained to do not introduce operational hazards, risk or complexity in the system.

Secondly, due to space and weight limitations, subsea systems do generally not have separate dedicated drain systems as could be available on a platform or land based system.

An aim with the present invention it to design a robust subsea drainage system with larger operational use window, which also provides a low structure design.

These aims are achieved with a system and method as defined in the attached claims.

The present invention regards a subsea drainage system. The drainage system comprises a first inlet for a liquid with a first pressure drained from a subsea unit, where the inlet leads to a drainage line and an outlet for the drained liquid leading to a dumping space. The subsea unit may be a subsea compressor, but it may also be other types of subsea equipment where there is a need for draining of a liquid, as a condensate trap, a lower part of a bend of a pipeline etc. The dumping space may be a scrubber positioned upstream of the compressor. This scrubber may also be the source for the gas to be compressed in the compressor. However the dumping space may be other suitable containments subsea as a suction line to a pump, preferably connected to a line which through equipment will lead to a receiving facility (e.g. platform or land based plant) or down in a well.

According to the invention the system comprises first means for providing at least one motive fluid to the system, and second means connected to the drainage line between the inlet of the drained liquid and the outlet of the system, where these second means are configured such that it together with the motive fluid provides a possibility of draining the drained liquid to a dumping space. The means are configured such that it provides an interaction between the motive fluid and the drained liquid, thereby in one embodiment providing a pressure increase in the drained liquid, making it possible to transfer the drained liquid to the dumping space even if the dumping space has a pressure higher than the first pressure of the drained liquid at the inlet to the drainage system. In another embodiment the first means providing the motive fluid are so configured that the motive fluid has a pressure which is lower than the drained liquid as the motive fluid is interacting with the drained liquid and thereby draws the drained liquid out through to a dumping space.

In the first principle embodiment of the solution a drained liquid may be dumped into another fluid which has a pressure which is higher than the drained liquid originally. Such a solution gives an increased possibility of removing drained liquids subsea. By having such a solution one will also achieve a robust system which no longer is needed to be elevated to the same height as ordinary

gravitational based systems, to build pressure in the drained liquid. A system according to the invention therefore gives the possibility of providing a subsea system which may be built with a relative low structure height.

As according to the second principle embodiment, by providing a motive fluid with a lower pressure than the drained liquid, possibly in a system where there originally is one common fluid separated in a scrubber, and the gas part is compressed in a compressor and the liquid part is guided to a pump, where the drained liquid is a liquid part from the compressor and the motive fluid used in the system is the liquid from the scrubber guided to the pump, one also provides a easy operable solution with no specific need for extra equipment, and thereby a robust system with minimal need for elevation.

According to one aspect the subsea unit is a compressor, and liquid trapped in the compressor during compression is the liquid to be drained. In an embodiment the gas to be compressed is received from a scrubber, separating a well stream in a liquid and gas part, where the liquid part is lead to a pump. The gas from the compressor and the liquid from the pump may in one embodiment be joined downstream of the pump and the compressor to be transported to another location.

According to the invention the first means in the system are providing at least one motive fluid for interaction with the drained liquid. The motive fluid is mixed with the drained liquid. In one embodiment the motive fluid in the system is arranged to come into the system through a second inlet. This second inlet may be connected to a scrubber, a high pressure side of a pump or a compressor.

According to another aspect the first means for providing a motive fluid is connected to a high pressure side of a pump. The pump may be a pump pressurizing a liquid part of a well stream separated by a scrubber, where liquid to be drained is drained from a compressor pressurizing a gas part from the same scrubber. The drained liquid would then normally be added to a fluid line upstream of the pump, for instance in the suction line for the pump or in a scrubber arranged upstream of the pump. The outlet of the system may therefore be adapted to be connected to a scrubber or to a suction line of a pump.

According to another aspect the first means for providing a motive fluid is formed at least in part by a suction line of a pump. The pump may be connected to the same scrubber as the compressor, wherefrom liquid is drained, is connected. According to this aspect the first means comprises in the suction line a restriction to provide the necessary pressure at the entry point for the drained liquid, e.g. the connection point of the drainage line to the suction line of the pump. In an embodiment of this aspect the restriction in the suction line of the pump may form an ejector or a venturi, where the fluid in the suction line forms the motive fluid of the drainage system according to the invention. Alternatively there may be a restriction in the suction line of the pump upstream of the connection of the drainage line to the suction line. This restriction may be a specific restriction or formed by the pipeline itself by the general pressure drop in the line from the scrubber to the connection of the drainage line to the suction line if this pressure drop is sufficient to provide the necessary pressure at the connection point to transfer the drained liquid from the drainage system to the suction line, and thereby to the dumping space. In this solution the motive fluid, the liquid in the suction line is added to the system with a pressure equal to or blow the pressure in the drained liquid and will thereby draw the drained liquid into the suction line of the pump. According to another aspect the first means for providing a motive fluid is connected to a high pressure side of a compressor. This compressor may be the same compressor as the liquid is drained from. The drained liquid may in this embodiment be added to a scrubber arranged upstream of the compressor as the dumping space. So also in this embodiment the outlet of the system may be adapted to be connected to a scrubber. The gas to be compressed by the compressor may be drawn from this scrubber.

According to one aspect of the system the second means may comprise an ejector arranged such that it is driven by the motive fluid to transfer the drained fluid to the dumping space. An ejector will by the ejection of the motive fluid draw the drained liquid with it and compress the drained liquid in the process such that it may be added to a dumping space with a higher pressure than the drained liquid had initially. Alternatively the motive fluid may be used to flush out the drained liquid to the dumping space.

According to an aspect of the invention the system may comprise a drainage vessel connected to the drainage line downstream of the first inlet. This drainage vessel may be a separate vessel, e.g. separate equipment, or it may be at least partly incorporated in the subsea unit where from the liquid should be drained, e.g. a compressor. In one possible embodiment the drainage vessel also at least partly may form an ejector in the system. The ejector in such a system may then be formed by the shape of the drainage vessel, or at least a part of the ejector may be formed by the shape of the drainage vessel. Alternatively in another embodiment the ejector may be a separate element.

According to an aspect first means for providing a motive fluid in the system may be connected to a gas part of a scrubber, with a valve in the line between the scrubber and a drainage vessel, the outlet may be connected to the same scrubber with a second valve in the line between the scrubber and the drainage vessel. The system may further comprise a valve in the drainage line between the first inlet and the drainage vessel. The drainage vessel will be connected to both the first inlet and the first means and the outlet. The second means of one embodiment of the system will in this embodiment comprise the three valves and the drainage vessel. The first means will comprise the fluid line and the valve in this line. The operation of the system will then be to close the valve between the drainage vessel and the subsea unit, then open the valve for the motive fluid. This will provide a pressure in the drainage vessel which is equal or similar to the pressure in the scrubber. The valve at the outlet of the system may then be opened and the liquid in the drainage vessel may be transferred into the scrubber. A liquid level in the scrubber may also be lowered by operating a pump downstream of the scrubber and thereby transfer a further amount of liquid in the drainage vessel.

According to a further aspect the system may comprise a regulation device, e.g. a control valve, choke valve, flow orifice etc, between the source of motive fluid and the ejector or the drainage vessel, for regulation of the amount of motive fluid through the ejector or the drainage vessel. By such a regulation device the drained liquid may be transferred continuously or periodically, or when a given amount of liquid is drained into the system according to the invention, the regulation device will form part of the first means to provide the motive fluid in the system.

According to another aspect the system may comprise a passive device controlling the amount of motive fluid allowed in the system, where the passive device is configured to open and close as a response to the amount of drained liquid in the drainage vessel. In an embodiment the drainage vessel in the system may at least partly form the passive device, where a valve body and valve housing of the passive device is configured such that the valve open an close an outlet from the drainage vessel as a response to the amount of drained liquid in the drainage vessel. The device according to the invention may comprise some or all of the above mentioned aspects. In one embodiment there may be two different motive fluids in the system, a first motive fluid from the scrubber to the drainage vessel, and a second motive fluid from the high pressure side of the pump linked to an ejector, whereto the outlet of the drainage vessel leads.

The invention will now be explained with non-limiting embodiments with reference to the enclosed drawing where;

Fig. 1 depicts a prior known solution for drainage of liquid from a compressor,

Fig. 2 depicts a first embodiment of the invention,

Fig. 3 depicts a second embodiment,

Fig. 4 depicts a third embodiment,

Fig. 5 depicts a fourth embodiment,

Fig. 6 depicts a fifth embodiment,

Fig. 7 depicts a sixth embodiment,

Fig. 8 depicts a seventh embodiment and

Fig. 9 depicts an alternative to a detail of the seventh embodiment.

In fig. 1 there is shown a prior known system for drainage of compressors used subsea. In this system a well stream 1 is lead into a scrubber 2, to separate the well stream through gravity into a gas part and a liquid part within the scrubber 2. There will within the scrubber 2 be a highest liquid level, HHLL, indicated by the dotted line 3. From the scrubber 2 there is in top part of the scrubber connected a gas line 4, leading gas from the scrubber to a compressor 6, for pressurizing the gas part of the well stream. In the bottom part of the scrubber 2 there is a liquid line 5, leading to a pump 9 for pressurizing of the liquid part of the well stream. When pressurizing the gas part of the well stream there may be some fall out of liquid which needs to be drained from the compressor. Due to gravity liquid will be collected in a bottom part of the compressor 6 and there is a drain line 7 connected to the compressor at a lower or lowest point of the compressor 6. This drain line 7 leads to the liquid part of the scrubber 2, and there is positioned a control valve 8 in this drain line 7. As explained before, drainage of a compressor in such a system will normally be executed during stand still utilizing gravity and pressure equalized. If one would desire to drain during operation there will be a pressure difference between the drainage point and the receiving vessel /scrubber. To overcome this pressure difference an elevation in the order of 3-10 meters above the highest liquid level in the scrubber (HHLL) of the scrubber may be necessary, thus also elevating the compressor in the subsea structure.

In fig. 2 there is shown a first embodiment of the present invention, which is a two stage gravitational drain system. Similar elements are given the same numbering as in the prior known solution, and we here refer to the explanation of fig. 1. In this embodiment the system also comprises a scrubber 2 for separation of a well stream 1 into a liquid part and a gas part, where the liquid through a liquid line 5 is lead to a pump 9 for increasing the pressure of the liquid and the gas part is through a gas line 4 to a compressor 6 for increasing the pressure in the gas. Liquid formed in the compressor 6 is guided to a drainage system according to the invention as indicated with the dotted line numbered 100 in the figure, which comprises a first inlet 10, leading to a drain line 7 which leads to an outlet 30 of the drainage system, which in this first embodiment is connected to the liquid part of the scrubber 2, e.g. below a lower liquid level, LLL, in the scrubber 2. In the drain line 7 after the inlet 10 there is arranged a first drain valve 11, a control valve before a drainage vessel 12. The drain line 7 continues from an outlet of the drainage vessel 12, where in this part of the drain line 7 there is arranged a second drain valve 13, a control valve before the outlet 30 of the drainage system 100. There is in addition in the drainage system the first means providing a motive fluid to the system comprising a second inlet 20 for the motive fluid, which second inlet 20 through a motive fluid line 21 with a motive fluid valve 22, leads a motive fluid to the drainage vessel 12. The motive fluid in this embodiment is the gas part of the separated well stream in the scrubber 2. There are sensors in the system and control means operating the valves. The system can be operated such that normally the motive fluid valve 22 and second drain valve 13 are closed while first drain valve 11 is open. In such way liquid collected in the compressor 6 can drain continuously to the drainage vessel 12 by gravity. When the drainage vessel 12 is full (level indicators (LT) should be included) the first drain valve 11 should be closed. The motive fluid valve 22 should open to allow pressure equalization between the drain vessel 12 and the scrubber 2. Thereafter the second drainage valve 13 can be opened to drain the liquid from the drain vessel 12 to the scrubber 2. The compressor 6 should therefore have a small capacity for

accumulation of liquid during the emptying of the drain vessel. The benefits of this solution compared to the solution in Figure 1 is that the compressor can be lowered since the draining is executed with the pressure balanced with the motive fluid through the motive fluid line 21 through motive fluid valve 22. This reduces the pressure difference that must be overcome by elevation of the drain point in the compressor 6. Utilizing the pump 9 to lower the liquid level of the scrubber 2 to the low liquid level (LLL), the elevation of the compressor 6 should be Y m above the LLL. The reduced elevation of the compressor 6 (compared to the gravity solution in figure 1 ) is quantified in the difference in X vs Y ( Y<X) and the additional distance LLL is below the HHLL which is the reference for X. Figure 3 shows a second embodiment of the invention which is a direct ejector drainage system. In this embodiment the first inlet 10 leads to the drainage line 7 and a first drainage valve 11 arranged before the drainage line 7 leads into an ejector 14 which lead to the outlet 30 of the drainage system. The first means for providing a motive fluid comprises a second inlet 20 for the motive fluid connected to a high pressure side of the pump 9, leading high pressure liquid, as the motive fluid, through a motive fluid line 21 with a motive fluid valve 22, to the ejector 14. Thereby the ejector 14 driven by liquid extracted from the high pressure side of the pump 9 is utilized to directly transport or evacuate liquid from the compressor 6. In figure 3 the ejector 14 is evacuating fluids continuously from the compressor sump 6 if the motive fluid valve 22 is omitted in the system. This will however lead to some recirculation of gas which implies somewhat reduced efficiency of the compressor 6. The motive fluid valve 22 is optional but can be utilized to adjust the operation of the ejector 14. The pump 9 provides the motive fluid to the ejector 14. The pressure drop in the ejector 14 draws fluid from the compressor 6 and pushes it back into the scrubber 2. This solution has potential for lowering the compressor 6 even further compared to the solution in figure 2.

In figure 4 there is shown a third embodiment of the invention where the system comprises a two stage ejector drainage. In this system the first inlet 10 lead to the drainage line 7 with a first drainage valve 11 arranged upstream of a drainage vessel 12. From an outlet of the drainage vessel 12 the drainage line 7 continues to an ejector 14, with a second drainage valve 13 arranged between the ejector 14 and the drainage vessel 12. The first means for providing a motive fluid comprises a second opening 20 connected to a high pressure side of the pump 9, and leads through a motive fluid line 21 with a motive fluid valve 22 to the ejector 14. An outlet from the ejector leads to the outlet 30 of the drainage system, which outlet is connected to the liquid part of the scrubber 2. There is in this embodiment a third inlet 25 to the drainage system, connected to a gas part of the scrubber 2, this third inlet 25 leads through a second motive fluid line 26 with a second motive fluid valve 27 to the drainage vessel 12, forming a second part of the first means for providing a second motive fluid into the drainage system according to the invention. This second part may be omitted.

This embodiment shows that to avoid generating a "short circuit" by continuously draining the compressor 6 with the ejector 14 the drainage can be from the compressor 6 through first drainage valve 11 to an intermediate drain vessel 12 as a buffer volume. As a batchwise operation based on a reading from the level transmitter (LT) indicating filling of the drain vessel 12, the drain vessel 12 may be emptied (through a second drainage valve 13) utilizing an ejector 14 as explained in the previous embodiment (see figure 3). The operation could be executed with first drainage valve 11 open resembling the continuous drain situation described above. If it is beneficial to close first drainage valve 11, second motive fluid valve 26 may be required to avoid generating vacuum in the drain vessel 12. The latter operation is a combination of the operation of the systems shown in figure 2 and 3. This solution will allow for a lower elevation compressor being lower in the system compared to the solution in figure 2. In addition it can generate an extra barrier compared to the solution in figure 3, with respect for potential liquid back-flow form the scrubber to the compressor through the drain vessel.

In fig. 5 there is shown a fourth embodiment of the invention a batchwise gas flushing solution. The first inlet 10 is connected to the drainage line 7 which comprises a first drainage valve 11 arranged before a drainage vessel 12, where an outlet from the drainage vessel continues the drainage line 7 through a second drainage valve 13 to the outlet 30 of the drainage system, connected to the liquid part of the scrubber 2. The first means for providing a motive fluid comprises a second inlet 20 for the motive fluid connected to a high pressure side of the compressor 6, and will through a motive fluid line 21 with a motive fluid valve 22 lead the motive fluid to the drainage vessel 12. In this embodiment the system would be utilizing the high pressure gas at the compressor 6 discharge. As for the batchwise draining above, the compressor 6 is continuously drained through first drainage valve 11 into a buffer volume, the drainage vessel 12. In a specific operation mode first drainage valve 11 is closed, second drainage valve 13 is opened and high pressure gas as the motive fluid is supplied through motive fluid valve 22 to flush the drain vessel 12. The operation may be initiated by readings from the level transmitter (LT) in the drain vessel 12. The solution will require check valves at proper locations, but will provide a system with only little elevation of the compressor. In fig. 6 there is shown a fifth embodiment of the present invention a batchwise automatic gas flushing solution. In this embodiment the first inlet 10 connected to the compressor leads to the drainage line 7, with a first drainage valve 11 arranged between the first inlet 10 and a drainage vessel 12. From an outlet of the drainage vessel 12 the drainage line 7 continues through a second drainage valve 13 to the outlet 30 of the system connected to the scrubber 2. The first means for providing a motive fluid comprising a second inlet 20 leading to the motive fluid line 21, which is connected to the high pressure side of the compressor 6. The motive fluid line 21 is through a motive fluid valve 22, operated by a regulation device 23, connected to the drainage vessel 12. The regulation device 23 is operated passively in response to the level of drainage liquid in the drainage vessel 12. The regulation device may in one embodiment be a flotation device which moves with the liquid level in the drainage vessel 12, and as a response to this opens and closes the motive fluid valve 22. As for the other batchwise solutions the liquid is drained through first drainage valve continuously to the drain vessel 12 (buffer volume). A flotation devise 23 in the buffer volume 12 is connected to a motive fluid valve 22, possibly a check valve, in the motive fluid line 21, e.g. gas piping from the discharge of the compressor 6. As the level of liquid in the vessel 12 rises it will come to a point where the flotation device 23 will open the motive fluid valve 22 and high pressure gas, as the motive fluid, should flush liquid in the drain vessel 12 back to the scrubber 2. This requires careful design of the flotation device 23 and check valves will be needed at different locations in the system. This method will not require a particular operation to empty the drain vessel. In fig. 7 there is shown a sixth embodiment of the invention, a batchwise drainage to pump inlet solution. In this embodiment the inlet 10 is leading to the drainage line 7 leading through a first drainage valve 11 to a drainage vessel 12. The drainage line 7 continues from an outlet of the drainage vessel 12, through a second drainage valve 13 to a connection point to a liquid line, forming a suction line for a pump, in the shown embodiment a liquid line extending between the scrubber 2 and the pump 9. The first means for providing a motive fluid will in the drainage system according to this embodiment be formed by the suction line for the pump, e.g. the liquid line between the scrubber 2 and the pump 9 upstream of the connection point with the drainage line 7. The motive fluid in this embodiment is the liquid in the liquid line between the scrubber 2 and the pump 9, and the line for this liquid will also form the motive fluid line 21 of the drainage system in this embodiment. The motive fluid line 21 comprises upstream of the connection point of the drainage line 7 to the motive fluid line 21 a restriction device 24. This restriction device forms part of the first means for providing a motive fluid and it is generating a pressure drop in the motive fluid, so that the motive fluid has a pressure level at the entry point of the drainage line 7at a level mainly equal to or below the pressure in the drainage vessel 12. The restriction device 24 may be an orifice, a valve, a length of the pipeline creating the necessary pressure drop in the system. The outlet 30 of the drainage system is in the liquid line between the scrubber and the pump downstream of the connection point between this motive fluid line 21 in the form of the liquid line and the drainage line 7. This embodiment will also give a possibility to drain the compressor during operation, by draining the compressor 6 through first drainage valve 11 to an intermediate drain vessel 12 while second drainage valve 13 is closed. When drain vessel 12 is full, read from the level transmitter (LT) in the vessel 12, first drainage valve 11 is closed and second drainage valve 13 is opened. The drain vessel 12 will empty given that the pressure drop in the liquid suction line, forming the motive fluid line 21 of this embodiment, to the pump 9 is sufficient to generate a lower or equal pressure downstream second drainage valve 13 than in the drain vessel 12. This can be overcome by a restriction 24 in the liquid suction line which is the motive fluid line 21, either static (orifice, venturi) or dynamic (control valve). The pressure drop to be generated shall compensate the pressure difference between scrubber 2 and compressor suction plus static liquid height from scrubber to the pump suction. When drain vessel 12 is emptied, second drainage valve 13 is closed and first drainage valve 11 is opened. Due to the pressure drop in the motive fluid line 21 which is formed by the liquid supply line to the pump 9, this solution may require a gas tolerant pump and potentially a hybrid or multiphase pump.

In fig. 8 there is shown a seventh embodiment of the invention. By exchanging the drain vessel 12 in figure 7 by a carefully designed drain volume 120 (Dl) in figure 8 the drainage can be made self controlled, i.e. without need for instrumentation and valves to operate. The drain volume 120 contains a flotation device 121 (ball) that opens the discharge 121 from the drain volume 120 to the pump suction line, which in this embodiment as well acts and works as the motive fluid line 21 with the liquid as the motive fluid, upon filling of liquid in the drain volume and closes the discharge 121 when the liquid is drained out. The discharge 121 is connected to a venture 15 in the motive fluid line 21 generating a pressure drop in the motive fluid sufficient to allow for drainage from the drain volume 120. As for the solution in figure 7 this may also require a gas tolerant pump.

In fig. 9 there is shown a second embodiment of a drain volume 120, where the flotation device 121 is in the form a piston, which moves with the liquid level and opens or closes a discharge 121 of the drain volume 120.

The invention has now been explained with non-limiting embodiments. A skilled person will understand that there may be made alternations and modifications to the explained embodiment which are within the scope of the protection as defined in the claims. Different elements from the different embodiments may also be combined different than what is explained in the embodiments. Instead of an ejector there may be used a venture solution, the drainage vessel may be omitted, etc.

Claims

1. A subsea drainage system, comprising a first inlet (10) for a liquid drained from a subsea unit, leading to a drainage line (7) of the system with an outlet (30) for the drained liquid leading to a dumping space, characterized in that the system further comprises first means (20,21,22,23,24,25,26,27) for providing at least one motive fluid, and second means (11,12,13,14,15,120) being connected to the drainage line (7) between the inlet (10) and the outlet (30) and configured such that the means (11,12,13,14,15,120) provide an interaction between the motive fluid and the drained liquid such that the drained liquid can be transferred to the dumping space.

2. System according to claim 1 , characterized in that the subsea unit is a compressor (6) having one outlet for compressed gas and one drainage outlet for liquid separated from the gas in the compressor, which drainage outlet is connected to the first inlet (10).

3. System according to claim 1 or 2, characterized in that the first means (20,21,22,23,25,27) for providing the motive fluid is providing a motive fluid with a pressure higher than a first pressure of the drained liquid.

4. System according to claim 3, characterized in that the first means (20,21,22) for providing the motive fluid is connected to a high pressure side of a pump.

5. System according to claim 3, characterized in that the first means

(20,21,22,23) for providing the motive fluid is connected to a high pressure side of a compressor.

6. System according to one of the claims 3 to 5, characterized in that the second means comprises an ejector (14) arranged such that it is driven by the motive fluid to evacuate the drained fluid through the outlet (30) to the dumping space.

7. System according to one of the preceding claims, characterized in that the outlet (30) is connected to a scrubber.

8. System according to claim 1 or 2, characterized in that the first means (20,21,24) for providing the motive fluid is providing a motive fluid with a pressure lower than a first pressure of the drained liquid.

9. System according to claim 8, characterized in that the motive fluid is a fluid in a suction line for a pump, and the first means further comprises a restriction (24) in the suction line for the pump, and the drainage line (7) is connected to the suction line downstream of the restriction (24).

10. System according to one of the preceding claims, characterized in that the second means comprises a drainage vessel (12,120).

11. System according to claim 10, characterized in that first means for providing a motive fluid is connected to a gas part of a scrubber and comprises a valve (22,27) in a line between the scrubber and the drainage vessel (12), the outlet (30) is connected to the same scrubber with a second valve (13) in the drainage line (7) between the scrubber and the drainage vessel (12), and the system further comprises a valve (11) upstream of the drainage vessel (12) in the drainage line (7) downstream the subsea unit wherefrom the liquid is drained.

12. System according to one of the claims 6,7,10 or 11, characterized in that the first means comprises a regulation device (22,23) upstream of the ejector (14) or drainage vessel (12), for regulation of the amount of motive fluid through the ejector (14) or drainage vessel (12).

13. System according to one of the claims 6,7,10 to 12, characterized in that it comprises a passive device (22,23; 120) controlling the amount of motive fluid allowed in the system, where the passive device (22,23; 120) is configured to open and close as a response to the amount of drained liquid in the drainage vessel.

14. System according to one of the claims 10 to 13, characterized in that the drainage vessel (120) at least partly forms a passive device, where the passive device opens and closes an outlet from the drainage vessel as a response to the amount of drained liquid in the drainage vessel.