NO20131744A1 - Submarine fluid processing system - Google Patents

Description

Subsea fluid processing system

The present invention relates to a fluid processing system which comprises a subsea pump and/or compressor which is driven and which serves to transport production fluid from a subsea hydrocarbon well to a receiving station on the surface or on land.

Background to the invention and prior art

Pumps or compressors that work above sea level will normally work with constant suction pressure, and therefore have no problems staying within the operating requirements for the pump or compressor during transitional operations, such as start and stop sequences. Compressors and pumps operating below sea level, on the other hand, are exposed to far more dynamic conditions, especially if they are installed immediately downstream of a production well. Some of the dynamic conditions for which subsea pumps and compressors must be designed are explained below.

For example, the suction pressure at the start of a subsea pump or compressor will be equal to the shut-in pressure for the well, due to a zero-flow condition. This pressure could be higher than the discharge pressure from a compressor at a certain, nominal flow. This suction pressure will drop as soon as well fluid begins to flow. Shutdown of a pump or compressor is the opposite of a start sequence and includes an increase in the suction pressure, possibly so that the operating point moves outside the specifications of the pump or compressor.

For pumps for example, it is important to stay within the recommended operating range (at high rpm), and a current higher than 1.5 times the point of best efficiency is mostly undesirable. At higher currents, the risk of cavitation and vibration increases. Vibration is caused by poor adaptation of flow angles in the impeller inlet. A solution to this could be to run the pump at a lower speed, but this is not always possible, because the engine requires a minimum speed for the bearings to support the rotor, etc.

When operating pumps or compressors in subsea fluid processing systems, it is thus a problem to maintain the operation of the pump or compressor within the operating requirements at all times.

Summary of the Invention

The present invention aims to provide a solution that is technically uncomplicated and economically reasonable, where a pump and/or compressor can be kept within specified operating conditions in a subsea fluid processing system.

The purpose is fulfilled by a subsea fluid processing system which comprises a system inlet that can be connected to an import line for a fluid flow that is produced upstream of the system and a system outlet that can be connected to an export line for further transport of the fluid flow downstream of the system. A bypass line is arranged and can be set for connecting and disconnecting the system from the fluid flow, and in addition a flow path is arranged which connects the system inlet with the system outlet. A pump and/or compressor is installed in the flow path, and the pump or compressor is arranged to receive fluid via the system inlet and can ensure the discharge of fluid via the system outlet. The subsea fluid processing system further comprises a set of valves on the supply side and/or on the discharge side of the pump or compressor respectively, and the valves can be controlled to put the pump or compressor in communication with the fluid flow. Each set of valves comprises a main valve and a secondary valve arranged in parallel in the flow path, where the main valve is sized for a normal process flow and the secondary valve is sized for a partial process flow, the main valve and the secondary valve being individually adjustable for individual or combined flows across the valves.

The secondary valve is therefore smaller in size than the main valve, which is sized for full production flow, and the secondary valves are sized for a partial flow and engage during fluctuating pressure conditions such as during start-up or shutdown procedures. In other words, the main valve can have an opening with a first diameter and the secondary valve an opening with a second diameter which is smaller than the first diameter. In particular, the secondary valve may have an opening with a second, fixed diameter smaller than the first diameter.

The secondary valve can be considerably smaller than the main valve, so that a pressure drop is formed across the valve. More precisely, the diameter of the second opening can be dimensioned to generate a pressure drop across the secondary valve of the order of 1-5 bar higher than the pressure drop generated by the diameter of the first opening in the main valve.

The sets with main valve and secondary valve in parallel will in this way isolate and protect the pump and/or compressor from pressure variations in the fluid flow on the inlet side and/or outlet side of the pump or compressor. In this way, the secondary valves will ensure a pressure drop which ensures that the rotating subsea equipment at all times works within the specified operating conditions, also during start-up or shutdown.

For example, during the start-up procedure, the valve in the main inlet and/or outlet of the pump or compressor is closed while the secondary valve is open. Because it takes time for the reservoir pressure to stabilize, production fluid is initially processed only through the secondary valve, and when the pressure is stabilized, the main valve is opened for full production flow. During the shutdown procedure, the secondary valves are opened before the main valves are closed, so that a smooth transition from full production flow to zero is formed. Consequently, in another aspect of the invention, a method of sequential operation of a subsea fluid processing system during startup or shutdown is provided, a pump and/or compressor being arranged in a flow path connecting a system inlet with a system outlet in the subsea fluid processing system, where the method comprises: • installation of a main valve dimensioned for a normal process flow, and a secondary valve dimensioned for a partial process flow, in parallel in the flow path on the supply side and/or discharge side of the pump or compressor respectively,

opening of the secondary valve before opening of the main valve in the start-up phase, and opening of the secondary valve before closing of the main valve during the shutdown phase.

The invention can be implemented in subsea systems that process liquid, gas, wet gas or multiphase fluid. In a compression system for gas or wet gas according to the invention, a liquid/gas separator is arranged in the flow path on the supply side of a compressor. A liquid line connects the separator to the system outlet, and a pump in the liquid line serves to discharge liquid from the separator. A set of valves is arranged in the liquid line on the discharge side of the pump, where the valve set comprises a main valve and a secondary valve arranged in parallel in the liquid line, the main valve is designed for a normal flow and the secondary valve is designed for a partial flow, the main valve and secondary valve being individual adjustable for individual or combined flow through the valves.

Further details of the invention will be discussed below.

Brief description of the drawing figures

Embodiments of the invention will be explained below with reference to the attached schematic drawings which show: Figure 1 shows the layout of an undersea compression station that makes use of the present invention, Figure 2 shows the layout of an undersea wet gas compression station that makes use of the present invention, and Figure 3 shows the layout of a subsea pump or multiphase pumping station that makes use of the present invention.

Detailed description of designs

With reference to figure 1, a subsea compression station 1 comprises a compressor 2 installed in a fluid flow path which connects a station inlet 3 with a station outlet 4. The inlet 3 puts the compressor 2 in fluid flow communication with an import line 5 for a fluid flow produced upstream of the compression station 1. The outlet 4 puts the compressor 2 in fluid flow communication with an export line 6 for further transport of the fluid flow downstream of the compression station 1. A gas/liquid separator or scrubber 7 which performs the separation of gas and liquid phases in the fluid flow that is received via the station inlet 3, can be installed in the flow the path upstream of the compressor 2. A cooler 8 can be installed in the flow path further upstream of the separator/scrubber 7. In a separate branch of the flow path through the compression station 1, a pump 9 is installed and serves to return the separated liquid phase from the separator/scrubber to the produced the fluid flow via u the flow 4. The compression station 1 can be completely isolated from the produced fluid flow by controlling a bypass valve 10 which regulates the flow through a bypass line 11 which connects the import line 5 with the export line 6 when the valve 10 is in the open position. The discharge side of the compressor can be put in fluid communication with the suction side upstream of the compressor when an anti-surge valve 12, installed in an anti-surge loop 13, is in the open position. Similarly, liquid can be recycled from pump 9 to the separator/scrubber by controlling a pump recirculation valve 14 which is installed in a pump recirculation loop 15.

The flow of production fluid through the compression station 1 is achieved via a first set of valves or inlet valves 16, 17 arranged in parallel in the flow path upstream of the compressor 2 and a second set of valves or outlet valves 18, 19 arranged in parallel in the flow path downstream of the compressor 2 Each set of valves comprises a main valve, respectively 16 and 18, and a secondary valve, respectively 17 and 19. The main valve and the secondary valve in each set of valves are individually adjustable between open and closed conditions. Accordingly, the inlet valves 16 and 17 can be set for individual or combined flows supplied to the compressor 2, and the outlet valves 18 and 19 can correspondingly be set for individual or combined flows led to the outlet into the export line 6.

To illustrate the invention, the flow path between station inlet 3 and station outlet 4 is represented by the valves 16 and/or 17, the compressor 2, and the valves 18 and/or 19.

In each set of inlet valves and outlet valves, both main valves and secondary valves can be on/off valves and can be set to a fully closed or fully open state. In particular, at least the secondary valves 17 and 19 are valves with a fixed diameter and a fixed opening diameter which leads to a noticeable pressure drop across the valve. The pressure drop over the secondary valves 17 and 19 can be in the order of 1-5 bar higher than any pressure drop formed over the main valves 16, 18. The pressure drop formed over the main valves 16 and 18 will typically be negligible because the main valves are normally sized so that they allow full production flow through the main valves when the secondary valves are closed.

In a similar way, the pump 9 can be connected to a set of main valves and secondary valves 20 and 21 arranged in parallel in the pump outlet line between the pump outlet and the outlet from the compression station 4.

On the suction side and on the pressure side of the pump or compressor, the main valve and secondary valve in all designs can be designed as on/off valves, such as e.g. gate valve or ball valve.

Another subsea fluid processing system is illustrated in figure 2, which schematically shows a subsea wet gas compression station 100. The wet gas compression station 100 has several components in common with the preceding subsea compression station 1, and for these components the same reference number will be used in the remainder of the description.

Accordingly, wet gas compression station 100 comprises a compressor 2 installed in a fluid flow path connecting a station inlet 3 with a station outlet 4. The inlet 3 puts the compressor 2 in fluid flow communication with an import line 5 for a fluid flow produced upstream of the gas compression station 100. The outlet 4 puts the compressor 2 in fluid flow communication with an export line 6 for further transport of the fluid flow downstream of the compression station 100. Installed in the flow path upstream of compressor 2 is a flow mixer (flow conditioner) 22 which performs homogenization by mixing gas and liquid phases in the fluid flow received via the station inlet 3. A cooler 8 may be installed in the flow path further upstream of the flow mixer 22.

The wet gas compression station 100 can be completely cut off from the produced fluid flow by means of a bypass valve 10 which controls the flow through a bypass line 11 which connects the import line 5 with the export line 6 when the valve 10 is in the open position. The discharge side of the compressor can be set in fluid communication with the suction side upstream of the compressor when an anti-surge valve 12 installed in an anti-surge loop 13 is in the open position.

Flow of production fluid through the wet gas compression station 100 is achieved via a first set of valves or inlet valves 16, 17 arranged in parallel in the flow path upstream of the compressor 2, and a second set of valves or outlet valves 18, 19 arranged in parallel in the flow path downstream for compressor 2. Each set of valves comprises a main valve, respectively 16 and 18, and a secondary valve, respectively 17 and 19. The main valve and secondary valve in each set of valves can be set individually between open and closed conditions. Accordingly, the inlet valves 16 and 17 are adjustable for individual or combined flow supply to the compressor 2, and the outlet valves 18 and 19 are likewise adjustable for individual or combined flow outlet from the compressor 2.

For the purpose of illustrating the invention, the flow path between the station inlet 3 and the station outlet 4 is thus represented by the valves 16 and/or 17, the compressor 2 and the valves 18 and/or 19.

In each set of inlet valves and outlet valves, both the main valve and the secondary valve can be on/off valves that can alternate between a fully closed or fully open state. In particular, at least the secondary valves 17 and 19 are fixed diameter valves having a fixed opening diameter which leads to a noticeable pressure drop across the valve. The pressure drop across the secondary valves 17 and 19 can be in the order of 1-5 bar higher than the pressure drop which is then generated across the main valves 16, 18. The pressure drop which is generated across the main valves 16 and 18 will typically be negligible because the main valves are normally dimensioned so that they allow full production flow through the main valves when the secondary valves are closed.

In both cases, the optional route via the secondary valves 17, 19 which have a smaller opening diameter leads to a pressure drop both on the suction side and the discharge side of the compressor, and in this way compensates for pressure fluctuations in the produced fluid flow which is received from a well upstream of the wet gas compression station 100 .

Yet another subsea fluid processing system is illustrated in Figure 3, which schematically shows a subsea pumping/multiphase pumping station 200. The pumping station 200 has several components in common with the preceding subsea compression station 1 and the wet gas compression station 100, and for these components the same reference number will be used in the rest of the description .

Consequently, the subsea pumping station 200 comprises a pump 9 installed in a fluid flow path connecting a station inlet 3 with a station outlet 4. The inlet 3 puts the pump 9 in fluid flow communication with an import line 5 for a fluid flow produced upstream of the pumping station 200. The outlets 4 put pump 9 in fluid flow communication with an export line 6 for further transport of the fluid flow downstream of pump station 200. Installed in the flow path upstream of pump 9 is a mixing or recycling tank 23 which ensures homogenization of the fluid flow received via the station inlet 3. A cooler 8 can be installed in the flow path further upstream of the mixing/recirculation tank 23.

The pumping station 200 can be completely isolated from the produced fluid flow by means of a bypass valve 10 which controls the flow through a bypass line 11 which connects the import line 5 with the export line 6 when the valve 10 is open. Liquid can be recirculated from the pump 9 to the mixing/recirculation tank 23 using a pump recirculation valve 14 installed in a pump recirculation loop 15.

Flow of production fluid through the pump station 200 is carried out via a first set of valves or inlet valves 16, 17 arranged in parallel in the flow path upstream of pump 9, and a second set of valves or outlet valves 20, 21 arranged in parallel in the flow path downstream of pump 9. Each set of valves comprises a main valve, respectively 16 and 20, and a secondary valve, respectively 17 and 21. The main valve and the secondary valve in each set of valves are individually adjustable between open and closed state. Accordingly, the inlet valves 16 and 17 are adjustable for individual or combined flow supply to pump 9, and the outlet valves 20 and 21 are likewise adjustable for individual or combined flow from the pump outlet.

For the purpose of illustrating the invention, the flow path between the station inlet 3 and the station outlet 4 is therefore represented by the valves 16 and/or 17, the pump 9 and the valves 20 and/or 21.

In each set of inlet and outlet valves, both the main valve and the secondary valve can be on/off valves and adjustable between fully closed and fully open conditions. In particular, at least the secondary valves 17 and 21 are fixed diameter valves having a fixed opening diameter which provides a noticeable pressure drop across the valve. The pressure drop across the secondary valves 17 and 21 can be in the order of 1-5 bar higher than the pressure drop which can be generated across the main valves 16, 20. Typically, the generated pressure drop across the main valves 16 and 20 will be negligible, because the main valves are normally dimensioned to allow full production flow through the main valves when the secondary valves are closed.

In both cases, the optional route, via the secondary valves 17, 21 with a smaller hole diameter, leads to a pressure drop both on the suction side and the discharge side of the pump, and in this way compensates for fluctuations in the pressure in the produced fluid flow received from a well upstream for pumping station 200.

By using a small, optionally connected valve with a fixed opening size arranged in parallel with the normally dimensioned full production valve as described, an increased pressure drop can be achieved in relation to normal production with the same flow. In this way, the operating point can be maintained within desired operating conditions also under dynamic pressure conditions, so that damage to the underwater rotating equipment is avoided, and at a price far lower than the cost of control valves that are suitable for underwater use.

Claims (5)

1. Subsea fluid processing system (1; 100; 200) comprising: a system inlet (3) connectable to an import line (5) for a fluid stream produced upstream of the system, a system outlet (4) connectable to an export line ( 6) for further transport of the fluid flow downstream of the system, a bypass line (11) adjustable to connect and disconnect the system from the fluid flow, a flow path connecting the system inlet to the system outlet, a pump (9) and/or compressor (2) in the flow path, where pump or compressor is designed to receive fluid via the system inlet and can ensure the discharge of fluid via the system outlet, characterized in that the system comprises a set of valves on the supply side and/or on the outlet side of the pump or compressor, where the valves serve to put the pump or compressor in communication with the fluid flow, each set of valves comprising a main valve (16; 18; 20) and a secondary valve (17; 19; 21) arranged in parallel in the flow path, and the main valve is dimensioned for a normal process flow and the secondary valve is dimensioned for a partial process flow, the main valve and secondary valve being individually adjustable for individual or combined flow through the valves. 2. The system according to claim 1, characterized in that the main valve (16; 18; 20) has an opening with a first diameter and the secondary valve (17; 19; 21) has an opening of a second determined diameter which is smaller than the first diameter. 3. The system according to claim 2, characterized in that the second opening diameter is dimensioned to generate a pressure drop across the secondary valve in the order of 1-5 bar higher than the pressure drop generated by the first opening diameter in the main valve. 4. The system according to any preceding claim, further comprising: a fluid/gas separator (7) arranged in the flow path on the supply side of the compressor (2), a fluid line connecting the separator to the system outlet, a pump (9) in the fluid line that serves to discharge fluid from the separator, characterized in that a set of valves (21, 20) is arranged in the fluid line on the discharge side of the pump, where the set of valves comprises a main valve (20) and a secondary valve (21) arranged in parallel in the fluid line, where the main valve is dimensioned for a normal flow and the secondary valve is dimensioned for a partial flow, the main valve and secondary valve being individually adjustable for individual or combined flow through the valves. 5. Method of sequential operation of a subsea fluid processing system during startup or shutdown, characterized in that a pump and/or compressor is arranged in a flow path that connects a system inlet with a system outlet in the subsea fluid processing system, which includes: arrangement of a main valve dimensioned for a normal process flow, and a secondary valve dimensioned for a partial process flow, in parallel in the flow path on the supply side and/or discharge side of the pump or compressor respectively, opening the secondary valve before opening the main valve in the start-up phase, and opening the secondary valve before closing the main valve during the shutdown phase.