EP2799716A2

EP2799716A2 - A method and a system for drain liquid collection and evacution in a subsea compression system

Info

Publication number: EP2799716A2
Application number: EP20140001520
Authority: EP
Inventors: Rune Strand; Francesco Bongini
Original assignee: Vetco Gray Scandinavia AS
Current assignee: Vetco Gray Scandinavia AS
Priority date: 2013-04-30
Filing date: 2014-04-30
Publication date: 2014-11-05
Also published as: EP2799716A3; BR102014008799A8; NO335664B1; BR102014008799A2; AU2014202201A1; US20140318636A1; NO20130604A1

Abstract

A system and a method for drain liquid collection and evacuation in a subsea compression system using a compressor (1) is disclosed, wherein an external drainage tank (10) is arranged in direct flow communication with the compressor sump via a sump evacuation valve (12). The drainage tank can be set under compressor discharge pressure (17, 18) via a tank pressurizing line (18) arranged to connect the drainage tank to a compressor discharge flow (7) upstream of a throttle valve (21), and a tank evacuation line (20) is arranged to connect the drainage tank to the compressor discharge flow (7) downstream of the throttle valve (21). Drain liquid evacuation is accomplished through generation, by adjusting the throttle valve (21), of a flow restriction in the compressor discharge flow (7) while setting the drainage tank under compressor discharge pressure by opening of the pressure valve (17) and permitting drain liquid flow into a reduced compressor discharge pressure downstream the throttle valve (21) by opening an evacuation valve (19) in the evacuation line (20).

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to a subsea compression system and in particular to a method and a system arranged to accomplish drainage of a compressor and to control collection and evacuation of drain liquid in the compressor which is operated for gas compression purposes in subsea gas production.

BACKGROUND AND PRIOR ART

In the production of natural gas from subsea deposits the gas typically constitutes the major component of a multiphase fluid containing a minor fraction of liquid. Conventionally, liquid is separated from the gas to be collected in a scrubber or separator arranged upstream of the compressor, and returned to the gas on the discharge side of the compressor. A liquid pump is typically required to raise the pressure in the separated liquid sufficiently to permit injection of the liquid into the gas which leaves the compressor at elevated pressure.
However, subsea pumps and compressors are required to run for long time periods without maintenance and is equipment that represents considerable acquisition and production costs, considering also that backup equipment may be required in order to accomplish redundancy in operation. Any attempt to modify the subsea compression system in ways that reduces the complexity in design and costs would thus be encouraged by the industry.
An attempt to avoid drain liquid pumps in a subsea compression system is previously disclosed in WO 2010/102905 . The drain liquid relief system disclosed in this document comprises a drainage storage tank that receives liquid which is separated in a scrubber arranged upstream of the compressor. Drain liquid from the compressor casing is delivered to the scrubber and further into the drain storage tank. The liquid in the storage tank can be set under compressor discharge pressure via conduits and valves. A Venturi tube is arranged in the compressor discharge flow and arranged to generate a pressure lower than the discharge pressure in the drain storage tank in order to suck drain liquid from the pressurized storage tank into the compressor discharge flow. Draining is done batch-wise by proper operation of valves. Notably however, the Venturi tube is integrally formed in the compressor discharge nozzle or fitted into the compressor piping. When in operation of the compressor the compressor discharge flow passes the Venturi tube. A differential pressure is thus constantly generated at the Venturi tube suction port in effect of the Venturi principle.

SUMMARY OF THE INVENTION

The present invention aims at providing an improved method and system for drain liquid collection and evacuation in a subsea compression system wherein discharge of the drain liquid can be accomplished without requiring separate drain liquid pumps.
One object of the present invention is to provide a method and system for drain liquid collection and evacuation in a simplified subsea compression system wherein drain liquid collection and evacuation is accomplished separate from a separator or scrubber arranged upstream of a compressor.
This and other objects are met through a method for drain liquid collection and evacuation in a subsea compression system using a compressor having a compressor inlet, a compressor discharge and a sump for collecting liquid from wet gas, the method comprising arranging:

an external drainage tank in direct flow communication with the compressor sump via a sump evacuation valve;
a tank pressurizing line by which the drainage tank can be set under compressor discharge pressure by operation of a pressure valve;
a tank pressure relief line by which the drainage tank can be set under compressor inlet pressure by operation of a relief valve, and
a tank evacuation line by which liquid can be evacuated from the drainage tank into the compressor discharge by operation of an evacuation valve,

In other words, in the subsea compression system to which the invention is applied, liquid in wet gas supplied to the compressor is collected in a separate drain liquid tank which communicates directly with the compressor's sump, avoiding complex tubing and valves.
The application of a controllable and adjustable throttle valve in the compressor discharge permits drain liquid evacuation to be performed intermittently during production by generation, temporarily, of a pressure drop over the throttle valve while the drainage tank is isolated from the compressor sump. The drainage tank is hereby preferably set under compressor discharge pressure from upstream of the adjustable throttle valve.
In particular, a drain liquid evacuation sequence during production comprises the steps of:

closing the sump evacuation valve;
opening the tank pressure and evacuation valves;
adjusting the throttle valve to generate a pressure drop, and after accomplished evacuation of liquid from the drainage tank:
closing the pressure and evacuation valves;
opening the throttle valve, the relief valve and the sump evacuation valve, followed by
closing the relief valve.

By adjusting the throttle valve a pressure difference, typically in the order of 1 bar and above depending on operational conditions, is generated between inlet and outlet sides of the drainage tank, during the evacuation thereof.
At start-up of a subsea wet gas compressor large volumes of liquid may have accumulated in the compressor housing. Since in this mode of operation there is no compressor generated gas pressure available to drive out the liquid, the present invention foresees that during start-up.of the system liquid evacuation is performed by temporarily pressurizing the drainage tank from a separate pressure source or gas accumulator.
Alternative embodiments of the invention comprises first and second drainage tanks arranged in parallel, each drainage tank individually connected to the compressor sump, the compressor inlet and the compressor discharge flow via separate lines and valves, respectively. A method for operation of the system comprises: alternately evacuating liquid from one of the first and second drainage tanks into the compressor discharge flow while the other drainage tank is operative for collecting liquid from the compressor sump.
Another alternative embodiment of the invention comprises first and second drainage tanks arranged in series one after the other in the flow direction from the compressor sump, the first drainage tank emptying into the second drainage tank via an on/off tank isolation valve. A method for operation of the system comprises: evacuating liquid from the second drainage tank while the same is isolated from the first drainage tank.
A system for drain liquid collection and evacuation in a subsea compression system using a compressor having a compressor inlet, a compressor discharge and a sump for collecting liquid from wet gas comprises:

an external drainage tank arranged in direct flow communication with the compressor sump via a sump evacuation valve;
a tank pressurizing line wherein a pressure valve is operable for communicating compressor discharge pressure to the drainage tank;
a tank pressure relief line wherein a relief valve is operable for communicating compressor inlet pressure to the drainage tank;
a tank evacuation line wherein an evacuation valve is operable for evacuation of liquid from the drainage tank into the compressor discharge,
wherein the tank pressurizing line connects the drainage tank to the compressor discharge flow upstream of a throttle valve and the tank evacuation line connects the drainage tank to the compressor discharge flow downstream of the throttle valve, and wherein the throttle valve is operable for generating, by adjustment of the throttle valve, a flow restriction in the compressor discharge flow while the drainage tank is set under compressor discharge pressure via an open pressure valve and drain liquid flow into a reduced compressor discharge pressure is permitted via an open evacuation valve.

A separate pressure source or gas accumulator is preferably arranged at or in close vicinity to the subsea compression system and connectable to the drainage tank via suitable tubing and valves. In a modification of the system the pressure source may be arranged at land or surface, or arranged subsea and supplied pressure from a land or surface based pressure generator. Another modification,of the system foresees first and second drainage tanks connectable in parallel or in series to the compressor sump.
Further details and advantages achieved by the subject method and system will appear from the following detailed description of embodiments.

SHORT DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be further explained below with reference made to the accompanying schematic drawings. In the drawings,

Fig. 1 illustrates a set up and flow chart of a first preferred embodiment of the subsea compression system;
Fig. 2 shows a first alternative embodiment, and
Fig. 3 shows a second alternative embodiment of the subsea compression system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1, reference number 1 refers to a compressor in a subsea compression system. The compressor 1 comprises a motor 2, a rotor axis 3, and a rotor which is journalled for rotation inside the rotor housing 4. The compressor 1 is configured to process wet gas that is recovered from a hydrocarbon well and delivered to the suction side of the compressor via flow pipeline to a compressor inlet 5 via an on/off valve 6. Processed gas leaves the compressor at elevated pressure via compressor discharge 7 on the discharge side of the compressor.
As used herein, the expression "wet gas" refers to a multiphase fluid containing a mixture of hydrocarbons and non-hydrocarbons in both gaseous and liquid states. The relation between gas and liquid may be in the order of 9:1, i.e. the gas volume fraction in the mixed fluid will normally amount to about 90 % or more, such as 95-97 % gas. However, the gas volume fraction in the mixed gas flow is typically not constant but varies over time.
The compressor 1 is designed for pressure boosting of unprocessed well streams and is for this purpose equipped to handle liquid volume fractions normally in the order of about 5 % and will intermittently tolerate liquid slugs, without mechanical failure or interruption of operation. A flow conditioner 8 of non-complex structure may be arranged upstream of the compressor inlet for slug suppression and homogenization of the flow before entering the compressor 1.
Liquid which enters the compressor in the mixed flow will not completely leave via the compressor discharge. Due to leakage between rotor and rotor housing, e.g., liquid will be separated from the flow through the compressor and accumulate in a sump 9, arranged in the lower end of the compressor 1. In order not to overfill the sump 9 and compressor casing, a system for drain liquid collection and evacuation is installed with the wet gas compressor 1.
In particular, a system for drain liquid collection and evacuation in the subsea compression system using the wet-gas compressor 1 comprises a drain liquid storage tank 10, in the following referred to as drainage tank 10. The drainage tank 10 is in flow communication with the compressor sump 9 from which liquid can be dumped to the drainage tank 10 via a sump evacuation line 11 and sump evacuation valve 12. The sump evacuation valve 12 is an on/off valve which is controlled by a valve control unit 13 and a level sensor S that monitors the liquid level 14 in the drainage tank 10. The valve 12 may be configured to stay normally open, and controlled to close intermittently upon emptying of the drainage tank 10 as will be explained further down in the specification.
The drainage tank 10 is a pressure vessel that communicates with the suction and discharge sides of the compressor 1. In particular, the drainage tank can be set under compressor suction pressure via a valve 15 controlling the flow through a line 16 which connects the drainage tank 10 with the inlet pipeline or compressor inlet 5. Alternatively, the drainage tank 10 can be set under compressor discharge pressure via a valve 17 controlling the flow through a line 18 which connects the drainage tank 10 with the compressor discharge flow 7. Liquid can be evacuated from the drainage tank 10 via a valve 19 controlling the flow through a line 20 which connects a lower end of the drainage tank 10 with the compressor discharge flow 7. The pressure line 18 connects to the compressor discharge upstream of a valve 21, whereas the evaluation line 20 connects to the compressor discharge downstream of the same valve 21.
The valves 12, 15, 17 and 19 are on/off valves which are controllable between fully open and fully closed positions. In contrast, the valve 21 is a throttle valve which is adjustable to set a temporary flow restriction to the compressor discharge flow 7.
The valves 12, 15, 17, 19 and 21 are controllable in response to a detected liquid level 14 in the drainage tank 10. The liquid level is monitored by the sensor S from which the information is transferred to the valve control unit 13 which shifts the valves and the drainage system into drain liquid evacuation mode. All valves and the valve control unit may be electrically and/or hydraulically powered and supplied from top side at surface or land, as indicated schematically in the drawings through the open-ended dotted line which leads to the valve control unit 13.
More precisely, in normal production, the sump evacuation valve 12 is open for dumping liquid from the compressor sump 9 into the drainage tank 10. Valves 15, 17 and 19 are closed, whereas the throttle valve 21 is shifted fully open.
Evacuation mode is initiated as the liquid level monitor S senses that the liquid in the drainage tank 10 reaches a predetermined level. In evacuation mode the sump evacuation valve 12 is closed, whereupon valves 17 and 19 are opened, and the throttle valve 21 shifted to set a temporary restriction to the compressor discharge flow. In result, the pressure in the drainage tank is elevated to compressor discharge pressure via pressure valve 17 in the pressure line 18. Since in response to the flow restriction set in the throttle valve 21 a reduced discharge pressure downstream of the throttle valve is communicated to the lower end of the drainage tank 10, the pressure difference over the drainage tank 10 forces liquid out from the tank to be introduced in the compressor discharge flow, via the evacuation valve 19 and the evacuation line 20.
During evacuation of drain liquid from the drainage tank 10, the compressor 1 may be accelerated temporarily in order to maintain constant production also in evacuation mode. Upon return from evacuation mode the valves 17 and 19 are first set in closed position and the throttle valve 21 is again set to fully open position. The elevated pressure in the drainage tank 10 is then vented to the compressor suction side via pressure relief valve 15 and relief line 16. Next, the sump evacuation valve 12 is again opened and the relief valve 15 is closed.
At start-up of a compression system with a liquid filled compressor there will be no compressor discharge pressure available to evacuate liquid from the compressor via the compressor sump 9 and drainage tank 10. For this situation an additional pressure source or gas accumulator 22 may be arranged in communication with the drainage tank 10 for the purpose of blowing the tank via a valve 23 arranged in a line 24. In the case of a gas accumulator 22, the same may be supplied pressure from a surface or land based pressure source and the valve 23 may be controlled from a surface platform or other host, as indicated by arrows and open-ended lines 25, 26 in Fig. 1. Obviously, in order to avoid pressure loss, the gas accumulator 22 should be located subsea in close vicinity to the compression system and the drainage tank 10. In fact, a gas accumulator for blowing the drainage tank at start-up of the compression system may alternatively be mobile, such as carried on an ROV, e.g.
The subsea compression system of Fig. 2 differs from the embodiment of Fig. 1 with respect to the implementation of first and second drainage tanks 10a and 10b, connectable in series to the compressor sump 9. More precisely, a complementary drainage tank 10a is interconnected between the compressor sump 9 and the evacuated drainage tank 10b. During production mode liquid is dumped from the compressor sump 9 into the first drainage tank 10a via on/off valve 12a, and further from the first drainage tank 10a into the second drainage tank 10b via complementary on/off valve 12b. The second drainage tank 10b is installed in the system similarly to the drainage tank 10 of the previous embodiment. Thus, in evacuation mode the valve 12b is closed the second drainage tank 10b pressurized via pressure valve 17 and pressure line 18 and liquid is evacuated from the second drainage tank 10b into the compressor discharge 7. During evacuation of liquid from the second drainage tank 10b, liquid may continuously be collected in the first drainage tank 10a via open valve 12a dumping liquid from the compressor sump 9. Each drainage tank may be associated with a separate liquid level sensor S 1 and S2 which transfers the information to a valve control unit 13. Each drainage tank 10a, 10b is individually connectable to the compressor suction side via separate relief valves 15a, 15b. Similar to the embodiment of Fig. 1, the second drainage tank 10b may be associated with a supplementary pressure source or gas accumulator 22 to aid in start-up of the compressor 1 with a liquid-filled compressor housing.
The subsea compression system of Fig.3 differs from the aforementioned embodiments with respect to the implementation of first and second drainage tanks 10a and 10b connectable in parallel to the compressor sump 9. In particular, each drainage tank 10a, 10b communicates individually with the compressor sump via separate sump evacuation valves 12a and 12b. In similar way, each drainage tank is individually pressurized via separate pressure valves 17a, 17b in pressure lines 18a, 18b, respectively. Each drainage tank is likewise individually evacuated via separate evacuation valves 19a and 19b, arranged in the associated evacuation lines 20a and 20b, respectively. Similar to the second embodiment, each drainage tank 10a, 10b is individually connectable to the compressor suction side via separate relief valves 15a, 15b arranged in separate relief lines 16a and 16b, respectively. In the embodiment of Fig. 3 the liquid level in each of the first and second drainage tanks arranged in parallel needs to be individually monitored and detected, to which purpose separate sensors S 1 and S2 are arranged to transfer the information to a common valve control unit 13. Further in the embodiment of Fig. 3, one or two supplementary pressure sources or gas accumulators (not shown) may be arranged as disclosed above to serve both drainage tanks in common or each drainage tank individually upon start-up of the compressor 1 with a liquid-filled compressor housing,
It will be understood that in both embodiments of Figs. 2 and 3, evacuation of drain liquid from one drainage tank is possible while simultaneously collecting drain liquid from the compressor sump in the other drainage tank.
It will also be understood that according to the drain liquid collection and evacuation method and system taught and disclosed herein, the compressor discharge is only temporarily manipulated during evacuation, whereas between evacuation modes the discharge flow is unaffected and the compressor capacity fully reserved for processing the gas through the compressor.
The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

A method for drain liquid collection and evacuation in a subsea compression system using a compressor (1) having a compressor inlet (5), a compressor discharge (7) and a sump (9) for collecting liquid from wet gas, the method comprising:
- arranging an external drainage tank (10) in direct flow communication with the compressor sump via a sump evacuation valve (12);

- arranging a tank pressurizing line (18) by which the drainage tank can be set under compressor discharge pressure by operation of a pressure valve (17);

- arranging a tank pressure relief line (16) by which the drainage tank can be set under compressor inlet pressure by operation of a relief valve (15);

- arranging a tank evacuation line (20) by which liquid can be evacuated from the drainage tank into the compressor discharge by operation of an evacuation valve (19),

- wherein the tank pressurizing line (18) is arranged to connect the drainage tank to the compressor discharge flow (7) upstream of a throttle valve (21) and the tank evacuation line (20) is arranged to connect the drainage tank to the compressor discharge flow (7) downstream of the throttle valve (21), and wherein evacuation of drain liquid from the drainage tank (10) is accomplished through generation, by adjusting the throttle valve (21), of a flow restriction in the compressor discharge flow (7) while setting the drainage tank under compressor discharge pressure by opening of the pressure valve (17) and permitting drain liquid flow into a reduced compressor discharge pressure downstream the throttle valve (21) by opening of the evacuation valve (19).
The method of claim 1, wherein drain liquid evacuation is performed intermittently during production by generation, temporarily, of a pressure drop over the throttle valve (21) while the drainage tank (10) is isolated from the compressor sump (9).
The method of claim 2, wherein a drainage sequence during production comprises the steps of:
- closing the sump evacuation valve (12);

- opening the tank pressure and evacuation valves (17; 19);

- adjusting the throttle valve (21) to generate a pressure drop in the compressor discharge flow (7), and after accomplished evacuation of liquid from the drainage tank:

- closing the pressure and evacuation valves (17; 19);

- opening the throttle valve (21), the relief valve (15) and the sump evacuation valve (12), followed by

- closing the relief valve (15).
The method of any previous claim, wherein by adjusting the throttle valve (21) a pressure difference is generated between inlet (17, 18) and outlet (19, 20) sides of the drainage tank, during evacuation thereof.
The method of any previous claim, wherein drain liquid evacuation is performed during start-up of the system by temporarily pressurizing the drainage tank from separate pressure source or gas accumulator (22).
The method of any previous claim, wherein first and second drainage tanks (10a, 10b) are arranged in parallel, each drainage tank individually connected to the compressor sump (9), the compressor inlet (5) and the compressor discharge flow (7) via separate lines and valves, respectively, and evacuation of drain liquid comprises:
- alternately evacuating liquid from one of the first and second drainage tanks into the compressor discharge flow while the other drainage tank is operative for collecting liquid from the compressor sump.
The method of any of claims 1-6, wherein first and second drainage tanks (10a, 10b) are arranged in series one after the other in the flow direction from the compressor sump (9), the first drainage tank (10a) emptying into the second drainage tank (10b) via an on/off tank isolation valve (12b), and evacuation of drain liquid comprises:
- evacuating liquid from the second drainage tank (10b) while being isolated from the first drainage tank (10a).
A subsea compression system using a compressor (1) having a compressor inlet (5), a compressor discharge (7) and a sump (9) for collecting liquid from wet gas, the system comprising:
- an external drainage tank (10) arranged in direct flow communication with the compressor sump (9) via a sump evacuation valve (12);

- a tank pressurizing line (18) wherein a pressure valve (17) is operable for communicating compressor discharge pressure to the drainage tank;

- a tank pressure relief line (16) wherein a relief valve (15) is operable for communicating compressor inlet pressure to the drainage tank;

- a tank evacuation line (20) wherein an evacuation valve (19) is operable for evacuation of liquid from the drainage tank into the compressor discharge,

- wherein the tank pressurizing line (18) connects the drainage tank (10) to the compressor discharge flow (7) upstream of a throttle valve (21) and the tank evacuation line (20) connects the drainage tank (10) to the compressor discharge flow (7) downstream of the throttle valve (21), and wherein the throttle valve (21) is operable for generating, by adjustment of the throttle valve, a flow restriction in the compressor discharge flow while the drainage tank is set under compressor discharge pressure via an open pressure valve (17) and liquid flow into a reduced compressor discharge pressure is permitted via an open evacuation valve (19).
The subsea compression system of claim 8, further comprising a separate pressure source or gas accumulator (22) connectable to the drainage tank.
The subsea compression system of any of claims 8 or 9, further comprising first and second drainage tanks (10a, 10b) connectable in parallel or in series to the compressor sump (9).