NO20130604A1 - Method and system for collecting and evacuating drainage fluid in an underwater compression system - Google Patents

Abstract

A system and method for collecting and evacuating drainage fluid are described in a subsea compression system by means of a compressor (1), an external drainage tank (10) being arranged in direct flow communication with the compressor sump via a sump evacuation valve (12). The drainage tank can be put under compressor outlet pressure (17, 18) via a pressure setting line (18) for the tank, arranged to connect the drainage tank to the compressor outlet flow (7} upstream of a throttle valve (21), and a tank evacuation line (20) is arranged to connect The drainage tank with the compressor outlet flow (7) downstream of the throttle valve (21) Evacuation of drainage fluid is achieved by adjusting the throttle valve (21) which generates a flow restriction in the compressor outlet flow (7) while the drain tank is placed under the compressor outlet outlet (17). and allowing drainage fluid to flow into a reduced compressor outlet pressure downstream of the throttle valve (21) by opening an evacuation valve (19) in the evacuation line (20).

Description

Method and system for collecting and evacuating drainage fluid in an underwater compression system Technical scope of the invention

The present invention generally relates to a subsea compression system and in particular to a method and a system which is designed to ensure the drainage of a compressor and to control the collection and evacuation of drainage liquid in the compressor which is run for gas compression purposes in subsea gas production.

Background of the invention and prior art

In the production of natural gas from undersea deposits, gas typically forms the main component of a multiphase fluid that contains a low proportion of liquid. Liquid is usually separated from the gas to collect in a scrubber or separator arranged upstream of the compressor, and is then returned to the gas on the discharge side of the compressor. A liquid pump is typically required to increase the pressure in the separated liquid sufficiently to allow injection of the liquid into the gas leaving the compressor at elevated pressure.

However, subsea pumps and compressors must be run for long periods of time without maintenance, and this is equipment that represents significant purchase and operating costs, taking into account that backup equipment may also be required to achieve redundancy during operation. Any attempt to modify the subsea compression system in ways that reduce the complexity of construction and cost will therefore be welcomed by the industry.

An attempt to avoid drainage fluid pumps in a subsea compression system has previously been described in WO 2010/102905.

The drainage liquid discharge system described in this document comprises a drainage storage tank which receives liquid separated in a scrubber arranged upstream of the compressor. Drainage fluid from the compressor housing is delivered to the scrubber and continues into the drainage storage tank. The liquid in the storage tank can be brought under the compressor's discharge pressure via channels and valves. A venturi is mounted in the compressor discharge stream and arranged to generate a pressure lower than the drain storage tank discharge pressure to draw drainage fluid from the pressurized storage tank into the compressor discharge stream. Drainage is carried out in batches using valves. However, it is specified that the venturi tube is designed to be integrated into the compressor's outlet flange or mounted in the compressor's pipework. When the compressor is operating, the compressor discharge flow passes the venturi tube. A pressure difference is therefore constantly generated in the venturi tube's suction intake as a result of the venturi principle.

Summary of the Invention

The present invention seeks to provide an improved method and system for collecting and evacuating drainage fluid in an underwater compression system where discharge of drainage fluid can be achieved without requiring separate drainage fluid pumps.

One purpose of the present invention is to provide a method and system for the collection and evacuation of drainage liquid in a simplified underwater compression system where the collection and evacuation of drainage liquid is carried out without the use of a separate separator or scrubber which is placed upstream of a compressor.

This and other objectives are achieved by a method for collecting and evacuating drainage liquid in a subsea compression system that utilizes a compressor with a compressor inlet, a compressor outlet and a sump for collecting liquid from wet gas, the method comprising arranging the following: an external drain tank in direct flow communication with the compressor sump via a sump evacuation valve, • a tank pressurization line so that the drain tank can be put under compressor discharge pressure using a pressure valve;

a tank pressure relief line which allows the drain tank to be brought below the compressor inlet pressure by use of a relief valve, and

a tank evacuation line that serves to evacuate fluid from the drain tank into the compressor outlet using an evacuation valve,

wherein the tank pressurization line is arranged to connect the drain tank to the compressor's discharge stream upstream of a throttle valve and the tank evacuation line is arranged to connect the drain tank to the compressor's

discharge flow downstream of the throttle valve, and as evacuation of drain fluid from the drain tank is achieved by adjusting the throttle valve to generate a flow restriction in the compressor discharge flow while the drain tank is placed under the compressor discharge pressure by opening the pressure valve and allowing

drain fluid to flow in under a reduced compressor discharge pressure downstream of the throttle valve by opening the evacuation valve.

In other words, in the subsea compression system where the invention is used, liquid in wet gas delivered to the compressor is collected in a separate drainage liquid tank that communicates directly with the sump in the compressor, thus avoiding complicated piping and valves.

The use of a controllable and adjustable throttle valve in the compressor outlet allows evacuation of drain fluid to be carried out intermittently during production by temporarily generating a pressure drop across the throttle valve while the drain tank is isolated from the compressor sump. The drainage tank is thereby preferably placed under compressor discharge pressure upstream of the adjustable throttle valve.

In particular, an evacuation sequence for drainage fluid during production includes the following steps:

closing the sump evacuation valve,

opening the valves for tank pressure and evacuation, adjusting the throttle valve to generate a pressure drop,

and after evacuation of liquid from the drainage tank: closing of pressure and evacuation valves,

opening of the throttle valve, relief valve and

the sump evacuation valve, followed by

closing the relief valve.

By adjusting the throttle valve, a pressure difference, typically of the order of 1 bar and above, depending on operating conditions, is generated between the inlet and outlet sides of the drainage tank during its evacuation.

When starting up a subsea wet gas compressor, large quantities of liquid may have accumulated in the compressor housing. Because in this operating mode there is no compressor-generated gas pressure available to drive out the liquid, the present invention ensures that the evacuation of liquid is carried out during start-up of the system, by temporarily pressurizing the drainage tank from a separate pressure source or gas accumulator.

Alternative embodiments of the invention include a first and second drainage tank arranged in parallel, where each drainage tank is individually connected to the compressor sump, the compressor's inlet flow and outlet flow via separate lines and valves. A method of operating the system includes the following: evacuation of liquid from alternately first and second drainage tanks into the compressor discharge stream while the second drainage tank is in operation to collect 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 direction of flow from the compressor sump, the first drainage tank being emptied into the second drainage tank via an on/off tank isolation valve. A method of operating the system includes: evacuating liquid from the second drain tank while isolated from the first drain tank.

A system for collecting and evacuating drainage liquid in a subsea compression system using a compressor with a compressor inlet, a compressor outlet and a sump for collecting liquid from wet gas comprises the following: an external drainage tank arranged in direct flow communication with the compressor sump via a sump evacuation valve, a tank pressure line where a pressure valve can be operated to connect compressor outlet pressure to the drain tank, a tank pressure relief line where a relief valve can be operated to connect the compressor inlet pressure to the drain tank, • a tank evacuation line where an evacuation valve can be operated to evacuate liquid from the drain tank into the compressor outlet,

the tank pressure line connecting the drain tank to the compressor discharge stream upstream of a throttle valve and the tank vacuum line connecting the drain tank to the compressor discharge stream downstream of the throttle valve, and the throttle valve being operable to, by adjustment of the throttle valve, generate a flow restriction in the compressor discharge stream while the drain tank is placed under compressor discharge pressure via an open pressure valve and flow of drain fluid into a reduced compressor discharge pressure is permitted via an open evacuation valve.

A separate pressure source or gas accumulator is preferably arranged on or near the subsea compression system and can be connected to the drainage tank via suitable piping and valves. In a modification of the system, the pressure source can be arranged on land or on the surface, or arranged underwater and supplied with pressure from a pressure generator on land or on the surface. Another modification of the system is based on the fact that the first and second drainage tank can be connected in parallel or in series to the compressor sump.

Further details and advantages that are achieved by the present method and system will appear from the following detailed description of embodiments.

Brief description of the drawing figures

Preferred embodiments of the invention will be explained in more detail below with reference to the attached drawings. The drawing figures show as follows: Fig. 1 illustrates an assembly and flow diagram 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 underwater compression system according to the present invention.

Detailed description of preferred designs

In Figure 1, reference number 1 shows a compressor in a subsea compression system. The compressor 1 comprises a motor 2, a rotor shaft 3, and a rotor which is stored for rotation inside the rotor housing 4. The compressor 1 is configured to process wet gas recovered from a hydrocarbon well and delivered to the suction side of the compressor via the flow line to a compressor inlet 5 via an on/off valve 6. Processed gas leaves the compressor at elevated pressure via compressor outlet 7 on the outlet side of the compressor.

As used herein, the term "wet gas" refers to a multiphase fluid containing a mixture of hydrocarbons and non-hydrocarbons in both gaseous and liquid form. The ratio between gas and liquid can be of the order of 9:1, i.e. the volume fraction of gas in the mixture will normally be around 90% or more, such as 95-97% gas. However, the gas volume fraction in the mixed gas stream is typically not constant but varies over time.

The compressor 1 is designed for pressure increase in untreated well streams and for this purpose is equipped to handle liquid volume fractions usually of the order of about 5%, and will intermittently tolerate liquid plugs without mechanical damage or interruption of operation. A flow conditioner (flow conditioner) 8 of simple structure can be arranged upstream of the compressor inlet to suppress plugs and to homogenize the flow before it enters the compressor 1.

Liquid flowing into the compressor in the mixed flow will not fully pass out via the compressor outlet. For example, due to leakage between the rotor and the rotor housing, liquid will be separated from the flow through the compressor and accumulate in a sump 9 which is arranged at the lower end of the compressor 1. In order not to overfill the sump 9 and the compressor housing, a system for collection and evacuation of drainage liquid installed at the wet gas compressor 1.

In particular, a system for collecting and evacuating drainage liquid in the subsea compression system by means of the wet gas compressor 1 comprises a drainage liquid storage tank 10, hereinafter called drainage tank 10. Drainage tank 10 is in flow connection with the compressor sump 9, from which liquid can be emptied into 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 that 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 can be configured to normally be open and be controlled to to close intermittently during emptying of the drainage tank 10, as will be explained further down in the description.

The drainage tank 10 is a pressure vessel that communicates with the suction side and the discharge side of the compressor 1. In particular, the drainage tank can be put under suction pressure from the compressor via a valve 15 which regulates the flow through a line 16 that connects the drainage tank 10 to the inlet pipeline or the compressor inlet 5. Alternatively, the drainage tank 10 can be set below the compressor's discharge pressure via a valve 17 which regulates the flow through a line 18 which connects the drainage tank 10 with the compressor's discharge stream 7. Liquid can be evacuated from the drainage tank 10 via a valve 19 which regulates the flow through a line 20 which connects a lower end of the drainage tank 10 with the compressor outlet flow 7. Pressure line 18 is connected to the compressor outlet upstream of a valve 21, while evacuation line 20 is connected to the compressor outlet downstream of this valve 21.

Valves 12, 15, 17 and 19 are on/off valves that can be adjusted between fully open and fully closed positions. In contrast, valve 21 is a throttle valve which is adjustable to set a temporary flow limitation in the discharge flow from the compressor 7.

The valves 12, 15, 17, 19 and 21 are adjustable in response to a detected liquid level 14 in the drainage tank 10. The liquid level is monitored by the sensor S, from which information is transmitted to the valve control unit 13 which sets the valves and the drainage system to evacuation mode for drainage liquid. All valves as well as the valve control unit can be driven electrically and/or hydraulically and supplied from the surface or land, as indicated schematically in the drawings with the dashed line leading to the valve control unit 13.

More precisely, sump evacuation valve 12 is open for emptying liquid from compressor sump 9 to drainage tank 10. Valves 15, 17 and 19 are closed, while throttle valve 21 is set to fully open.

Evacuation mode is initiated as the liquid level monitor S registers that the liquid in the drainage tank 10 reaches a predetermined level. In the evacuation mode, the sump evacuation valve 12 is closed, after which the valves 17 and 19 are opened, and the throttle valve 21 is adjusted to set a temporary limitation of the compressor discharge flow. As a result, the pressure in the drain tank increases to the compressor outlet pressure via pressure valve 17 in the pressure line 18. In response to the flow restriction set in the throttle valve 21, a reduced outlet pressure downstream of the throttle valve is communicated to the lower end of the drain tank 10, and the pressure differential across the drain tank 10 forces liquid out from the tank so that it is introduced into the compressor's discharge flow via the evacuation valve 19 and the evacuation line 20.

During evacuation of drainage liquid from the drainage tank 10, compressor 1 can be temporarily accelerated to maintain constant production also in evacuation mode. When returning to position after evacuation mode, the valves 17 and 19 are first set in the closed position and the throttle valve 21 is again set in the fully open position. The elevated pressure in the drainage tank 10 is then vented to the suction side of the compressor via the pressure relief valve 15 and the relief line 16. The sump evacuation valve 12 is then opened again and the relief valve 15 is closed.

When starting a compression system with a liquid-filled compressor, there will be no discharge pressure from the compressor available to evacuate liquid from the compressor via the compressor sump 9 and the drainage tank 10. For this situation, an additional pressure source or gas accumulator 22 can be arranged in communication with the drainage tank 10 with the purpose of blowing clean the tank via a valve 23 arranged in a line 24. In the case of a gas accumulator 22, this can be supplied with pressure from a surface or land-based pressure source, and the valve 23 can be controlled from a platform on the surface or another device, as indicated by arrows and free-end lines 25, 26 in figure 1. Obviously, to avoid pressure loss, the gas accumulator 22 must be placed underwater at a short distance from the compression system and the drainage tank 10. In fact, a gas accumulator for cleaning the drainage tank by start-up of the compression system can alternatively be mobile, such as transported by an ROV.

The subsea compression system of Figure 2 differs from the embodiment of Figure 1 in the implementation of first and second drainage tanks 10a and 10b, which can be connected in series with the compressor sump 9. More precisely, a complementary drainage tank 10a is connected between the compressor sump 9 and the evacuated the drainage tank 10b. In 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 a complementary on/off valve 12b. The second drainage tank 10b is installed in the system in the same way as the drainage tank 10 in the previous embodiment. Thus, in evacuation mode, valve 12b will be closed, the second drainage tank 10b will be pressurized via pressure valve 17 and pressure line 18, and liquid will be evacuated from the second drainage tank 10b into the compressor outlet 7. During evacuation of liquid from the second drainage tank 10b, liquid can be continuously collected in the first drain tank 10a via open valve 12a which dumps liquid from the compressor sump 9. Each drain tank can be connected to a separate sensor Sl and S2 for liquid level which transmits the information to a valve control unit 13. Each drain tank 10a, 10b can be connected individually to suction side of the compressor via separate relief valves 15a, 15b. Corresponding to the embodiment in Figure 1, the second drainage tank 10b can be connected to a supplementary pressure source or gas accumulator 22 to help during start-up of the compressor 1 with a liquid-filled compressor housing.

The submarine compression system in Figure 3 differs from the above-mentioned designs in terms of the implementation of first and second drainage tanks 10a and 10b which can be connected in parallel in front of the compressor sump 9. In particular, each of the drainage tanks 10a, 10b will communicate individually with the compressor sump via separate sump evacuation valves 12a and 12b. Similarly, 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. Corresponding to the second embodiment, each drain tank 10a, 10b can be individually connected to the suction side of the compressor via separate unloading valves 15a, 15b arranged in separate unloading lines 16a and 16b respectively. In the embodiment in figure 3, the liquid level in each of the first and second drainage tanks which are connected in parallel must be individually monitored and detected, in the sense that separate sensors Sl and S2 are arranged to transmit the information to a common valve control unit 13. Furthermore, applies to the embodiment in figure 3 that one or two additional pressure sources or gas accumulators (not shown) can be arranged as described above, to operate both drainage tanks together or each drainage tank separately when starting compressor 1 with a liquid-filled compressor housing.

It is specified that in both embodiments in figures 2 and 3 it is possible to evacuate drainage liquid from one drainage tank while at the same time collection of drainage liquid from the compressor sump in the other drainage tank is taking place.

It is also specified that in accordance with what is preferred and described here regarding the method and system for collecting and evacuating drainage liquid, the compressor outlet is only temporarily manipulated during evacuation, while the outlet flow between the evacuation intervals is unaffected and the compressor capacity is fully reserved for processing the gas through the compressor.

The invention is of course not limited in any way to the embodiments described above. On the contrary, many possible modifications will be obvious to a person with knowledge in the field, without deviating from the basic idea of the invention as defined in the attached patent claims.

Claims (2)

1. Method for collecting drainage liquid and evacuation in a subsea compression system using a compressor (1) having a compressor inlet (5), a compressor outlet (7) and a sump (9) for collecting liquid from wet gas, where the method includes the following: - arrangement of an external drainage tank (10) in direct flow connection with the compressor sump via a sump evacuation valve (12), - arrangement of a pressurization line (18) for the tank which can place the drainage tank under the compressor's outlet pressure using a pressure valve (17), - arrangement of a tank pressure relief line (16) which can put the drainage tank below the compressor's inlet pressure using a relief valve (15), - arrangement of a tank evacuation line (20) which can evacuate liquid from the drainage tank into the compressor outlet using an evacuation valve (19) ), as the pressurization line (18) for the tank is designed to connect the drainage tank with the compressor's outlet flow (7) op upstream of a throttle valve (21) and the tank evacuation line (20) is arranged to connect the drain tank with the compressor discharge flow (7) downstream of the throttle valve (21), and evacuation of drain fluid from the drain tank (10) is achieved by adjusting the throttle valve (21) as that a flow restriction is generated in the compressor discharge flow (7) while the drain tank is put under compressor discharge pressure by opening the pressure valve (17) and allowing drainage fluid to flow in under a reduced compressor discharge pressure downstream of the throttle valve (21) by opening the evacuation valve (19). 2. Method according to claim 1, in that drainage liquid evacuation is carried out intermittently during production by temporarily generating a pressure drop across the throttle valve (21) while the drainage tank (10) is isolated from the compressor sump (9).