Method and system for cleaning a gas compressor

Abstract

A multiphase fluid processing system and a method of charging a liquid cleaning agent injection apparatus in situ in a multiphase fluid processing system. The system 100 comprises a gas / liquid separat

Description

TECHNICAL FIELD The present invention relates to methods and systems for cleaning a gas compressor, and in particular a method for charging a liquid cleaning agent injection apparatus in situ in a multiphase fluid processing system. BACKGROUND In the oil and gas industry, gas compressors are used during the processing of well fluids to compress gas, thereby helping transport the well fluid from one location to the next. It can be necessary to use gas compressors to achieve a sufficiently high rate of production from the well. In multiphase fluid processing, it is common to remove as much liquid as possible from the gas before the gas is passed through the compressor and compressed. This is because liquid passing through the compressor can cause damage or fouling of the compressor. To achieve this, processing components are commonly provided upstream of the compressor to try to reduce or minimise the liquid content in the gas before it reaches the compressor. For example, a multiphase flow may be separated into gas and liquid in a separator upstream of the compressor. Preparation of the gas upstream of the compressor may be imperfect, such that the gas entering the compressor may contain liquid or moisture in very small quantities. High temperatures inside the compressor can cause the liquid entrained in the gas to vaporize away resulting in solid materials such as scale being deposited on surfaces inside the compressor. Such deposits can detrimentally affect compressor performance and reduce the life time of the compressor. “Online” cleaning solutions have been proposed in which a liquid cleaning agent (e.g. liquid hydrocarbons, condensed hydrocarbon gas, a glycol, an alcohol, water, or an acid) is passed through the compressor, during operation of the compressor, to clean deposited solid material from a fouled portion of the compressor. It has been found that adding a relatively small quantity of liquid to the gas stream can effectively remove deposited solid material that has accumulated within the compressor without damaging the compressor. The cleaning can be primarily mechanical, due to the impact of the liquid against the deposited solid material which acts to knock the deposits away from the internal surface of the compressor. Certain liquid cleaning agents may also have a chemical effect to aid removal of any deposited solid material. In order to inject the liquid cleaning agent into the gas stream, it must be supplied at a pressure that is higher than the pressure of the gas stream. It is common to utilise one or more pumps to pump the liquid cleaning agent to a sufficiently high pressure so that it can be mixed with the gas stream. It is desirable to provide an online cleaning solution for a multiphase fluid processing system that is less reliant on the use of pumps to pump the cleaning agent to a sufficiently high pressure, so that it can be mixed with the gas stream. SUMMARY Viewed from a first aspect of the present invention, there is provided a method of charging a liquid cleaning agent injection apparatus in situ in a multiphase fluid processing system. The multiphase fluid processing system comprises: a gas / liquid separator arranged to produce a gas stream and a liquid stream from a multiphase fluid, and a gas compressor arranged to compress the gas stream, wherein the liquid cleaning agent injection apparatus is arranged such that a charged gas within the liquid cleaning agent injection apparatus will drive a liquid cleaning agent from the liquid cleaning agent injection apparatus into the gas stream upstream of the gas compressor during washing of the gas compressor. The method comprises: supplying the liquid cleaning agent injection apparatus with liquid from the liquid stream as at least part of the liquid cleaning agent; and setting a pressure of the charged gas within the liquid cleaning agent injection apparatus, using the pressure of the gas stream, such that that the pressure of the charged gas is greater than a suction pressure of the gas compressor, when in operation at a normal operational speed. By setting the pressure of the charged gas within the liquid cleaning agent injection apparatus using the pressure of the gas stream, the liquid cleaning agent injection apparatus can be passively pressurised in situ in the multiphase fluid processing system. Accordingly, it is not necessary to use additional pumps or infrastructure to drive the liquid cleaning agent from the liquid cleaning agent injection apparatus. Since the method of the first aspect also comprises supplying the liquid cleaning agent with liquid from the liquid stream as at least part of the liquid cleaning agent, it is also not necessary to use additional infrastructure for supplying a liquid cleaning agent to the liquid cleaning agent injection apparatus. Since the method also comprises using a charged gas present within the liquid cleaning agent injection apparatus to drive the liquid therefrom, it is not necessary for any pumps or infrastructure to supply the liquid cleaning agent to the compressor at any point during its operation. Accordingly, the method of charging a liquid cleaning agent injection apparatus in situ in a multiphase fluid processing system does not necessarily require the use of additional pumps or infrastructure to supply the liquid cleaning agent to the compressor at any point during its operation. That is, setting the pressure of the charged gas can avoid the need for additional pumps or infrastructure that would otherwise detract from the benefits of systems which would not otherwise rely on such additional pumps or infrastructure. The liquid cleaning agent injection apparatus may comprise a supply tank for storing the liquid cleaning agent and a fluid line arranged to supply the liquid cleaning agent from the supply tank to the gas stream on the suction side of the gas compressor. The supply tank may comprise a lower part for storing liquid cleaning agent and an upper part for storing charged gas. The fluid line may comprise a controllable valve for controlling a flow of liquid cleaning agent from the liquid cleaning agent injection apparatus into the gas stream upstream of the gas compressor during washing of the gas compressor. The gas / liquid separator may be arranged to supply gas from the gas stream, and liquid from the liquid stream, to the liquid cleaning agent injection apparatus. The normal operational speed of the gas compressor will be understood to be the operational speed of the gas compressor in use, i.e. when receiving fluid at the suction side, compressing the gas therein, and exhausting it at the discharge side of the compressor. The normal operational speed of the gas compressor can be understood to be the desired operational speed of the gas compressor when processing the gas stream of the multiphase fluid. The normal operational speed of the compressor may be suitable for compressing gas to a pressure of greater than 500 kPa, greater than 600 kPa, greater than 700 kPa, greater than 800 kPa, greater than 900 kPa, or greater than 1000 kPa. The method may comprise fluidly communicating the liquid cleaning agent injection apparatus with the liquid stream, and optionally with the gas stream, to supply the liquid from the liquid stream; and after supplying the liquid from the liquid stream to the liquid cleaning agent injection apparatus, isolating the liquid cleaning agent injection apparatus from the liquid stream and the gas stream. The method may comprise opening a first valve such that the gas stream is in fluid communication with the liquid cleaning agent injection apparatus; and isolating the liquid cleaning agent injection apparatus from the gas stream by shutting the first valve. The first valve may be a controllable shut-off valve. The first valve may be located in a first conduit coupling the gas stream to the liquid cleaning agent injection apparatus, and more preferably an upper part thereof. The first valve may be arranged to supply gas from the gas / liquid separator to the liquid cleaning agent injection apparatus. The first conduit may couple the gas / liquid separator to the liquid cleaning agent injection apparatus. The method may comprise opening a second valve such that the liquid stream is in fluid communication with the liquid cleaning agent injection apparatus; and isolating the liquid cleaning agent injection apparatus from the liquid stream by shutting the second valve. The second valve may be a controllable shut-off valve or a one-way valve. The second valve may be located in a second conduit coupling the liquid stream to the liquid cleaning agent injection apparatus, and more preferably a lower part thereof. The second valve may be arranged to provide liquid from the gas / liquid separator to the liquid cleaning agent injection apparatus. The second conduit may couple the gas / liquid separator to the liquid cleaning agent injection apparatus. The method may comprise: reducing a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure; fluidly communicating the liquid cleaning agent injection apparatus with at least one of the liquid stream and with the gas stream to set the pressure of the charged gas at the increased pressure; and isolating the liquid cleaning agent injection apparatus at the increased pressure from the liquid stream and the gas stream. When the compressor is running, i.e. when its working element is spinning, the pressure of the gas stream on the suction side of the gas compressor is less than a pressure of the compressed gas stream output on a discharge side of the compressor. However, if the speed of the compressor reduces, e.g. partially or completely, the pressure of the gas stream can increase. Under these circumstances, the increased pressure of the gas stream on the suction side can be suitable for setting the pressure of the charged gas within the liquid cleaning agent injection apparatus such that the pressure of the charged gas is greater than a suction pressure of the gas compressor, when in operation at a normal operational speed. When the liquid cleaning agent injection apparatus is put in fluid communication with only the gas stream, e.g. by opening only the first valve, gas from the gas stream can be directly used to set the pressure of the charged gas within the liquid cleaning agent injection apparatus. This may be useful for adding additional pressure when the liquid agent injection apparatus already contains liquid cleaning agent, but does not comprise sufficient pressure to discharge the liquid cleaning agent. When the liquid cleaning agent injection apparatus is put in fluid communication with only the liquid stream, e.g. by opening only the second valve, gas from the gas stream is indirectly used to set the pressure of the charged gas within the liquid cleaning agent injection apparatus, since liquid from the liquid stream will compress charged gas already present within the liquid cleaning agent injection apparatus. When the liquid cleaning agent injection apparatus is put in fluid communication with both the gas stream and the liquid stream, e.g. by opening the first valve and the second valve, the liquid cleaning agent injection apparatus may both be filled with liquid from the liquid stream and the charged gas within set at the pressure of the gas stream directly by gas from the gas stream. As mentioned above, the gas / liquid separator may be arranged to supply gas from the gas stream, and liquid from the liquid stream, to the liquid cleaning agent injection apparatus. Providing gas from the gas stream, directly or indirectly, to set the pressure of the charged gas within the liquid cleaning agent injection apparatus and liquid from the liquid stream from the gas / liquid separator may provide a compact and simple arrangement by which the liquid cleaning agent injection apparatus can be charged in situ. In some embodiments, liquid from the liquid stream is supplied to the liquid cleaning agent injection apparatus at the same time as setting the pressure of the charged gas within the liquid cleaning agent injection apparatus. That is, both operations may be efficiently performed simultaneously, or at least as part of the same operational procedure. Thus, the method may comprise: fluidly communicating the liquid cleaning agent injection apparatus with the liquid stream, and optionally with the gas stream, to supply the liquid from the liquid stream, and before isolating the liquid cleaning agent injection apparatus from the liquid stream and the gas stream: reducing a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure; wherein fluidly communicating the liquid cleaning agent injection apparatus with the liquid stream, and optionally with the gas stream, additionally sets the pressure of the charged gas at the increased pressure; and wherein the liquid cleaning agent injection apparatus is at the increased pressure when isolated from the liquid stream and the gas stream. The speed of the gas compressor may reduce due to the compressor shutting down. The shutdown may be planned (i.e. stopping the compressor) or unplanned. The speed of the compressor can completely reduce, i.e. fall to a zero speed, when the compressor is shut down. The method may comprise determining that the compressor is in shutdown (e.g. the shutdown may be unplanned). In this situation, the step of reducing a speed of the gas compressor may comprise waiting for the speed of the gas compressor to reduce or slow down, e.g. taking no particular action, in response to the unplanned shutdown. The step of reducing a speed of the gas compressor may comprise shutting down the gas compressor. This step may be performed actively (i.e. the shutdown may be planned). The method may comprise waiting for a pressure of the gas stream to reach a settle-out pressure of the gas compressor before isolating the liquid cleaning agent injection apparatus from the liquid stream and the gas stream. The settle-out pressure is the increased pressure, in this situation. This step may be performed when the shutdown is planned or unplanned. The settle-out pressure may be between 60 and 80 bar (600 and 800 kPa), or more preferably between 65 and 75 bar (650 and 750 kPa). The settle-out pressure may be about 70 bar (700 kPa). The method may comprise measuring the pressure of the gas stream, for example using a pressure sensor. The method may comprise determining that the pressure of the gas stream has reached the settle-out pressure based on the measured pressure. Alternatively, the method may comprise determined that the pressure of the gas stream has reached the settle-out pressure once a predetermined period of time has elapsed. The predetermined period of time may correspond to a known period of time for allowing the pressure of the gas stream to the reach the settle-out pressure. The method may comprise supplying compressed gas from the gas stream at a discharge side of the gas compressor to the liquid cleaning agent injection apparatus to set the pressure of the charged gas at a discharge pressure of the gas compressor. During normal operation of the gas compressor, i.e. when the gas compressor is running at preferably its normal operational speed, the pressure on the discharge side of the gas compressor will be greater than the pressure on the suction side of the gas compressor. Accordingly, the pressure of the charged gas within the liquid cleaning agent injection apparatus can be appropriately set using gas at the discharge pressure of the gas compressor. Using gas from the discharge side of the gas compressor to set the pressure of the charged gas within the liquid cleaning agent injection apparatus may avoid the need to use additional pumps or other apparatus to charge the liquid cleaning agent injection apparatus. Using gas from the discharge side of the gas compressor to set the pressure of the charged gas within the liquid cleaning agent injection apparatus may also avoid the need to reduce the speed of the compressor, such that the compressor can continue to run as desired whilst still charging the liquid cleaning agent injection apparatus in situ. The method may comprise opening a third valve such that liquid cleaning agent injection apparatus is in fluid communication with the gas stream downstream of the compressor (i.e. with the gas stream located on a discharge side of the compressor). The third valve may be a controllable shut-off valve or a one-way valve. The third valve may be located in a third conduit coupling the gas stream downstream of the compressor to the liquid cleaning agent injection apparatus, and more preferably an upper part thereof. The method may comprise isolating the liquid cleaning agent injection apparatus from the gas stream downstream of the compressor by shutting the third valve. This step may be performed responsive to a detected pressure of the charged gas within the liquid cleaning agent injection apparatus reaching or exceeding a predetermined threshold. The method may comprise supplying a liquid cleaning solution to the liquid cleaning agent injection apparatus. The method may comprise fluidly communicating the liquid cleaning agent injection apparatus with a reservoir of liquid cleaning solution. The liquid cleaning solution may be supplied under action of a liquid pump. Supplying the liquid cleaning solution to the liquid cleaning agent injection apparatus may compress the charged gas already within the liquid cleaning agent injection apparatus, additionally setting the pressure of the charged gas within the liquid cleaning agent injection apparatus. The liquid cleaning agent may be a liquid hydrocarbon, a condensed hydrocarbon gas, a glycol, an alcohol, water, or an acid. Viewed from a second aspect of the present invention, there is provided a method ofcleaning a gas compressor in situ in a multiphase fluid processing system. The method comprises: charging a liquid cleaning agent injection apparatus in situ in the multiphase fluid processing system according to the method of the first aspect; operating the gas compressor at the normal operational speed; and supplying liquid cleaning agent from the liquid cleaning injection apparatus into the gas stream on the suction side of the gas compressor. The above description of the method of the first aspect is equally applicable to the method of the first aspect, and the method of the first aspect may have one or more or all of the features (including optional features) of the aforementioned first aspect. The method may comprise increasing the speed of the gas compressor, e.g. to return the gas compressor to its normal operational speed. The method may comprise opening a fourth valve such that the liquid cleaning agent injection apparatus is in fluid communication with the gas stream. The fourth valve may be a controllable shut-off valve. The fourth valve may be the controllable valve of the liquid cleaning agent injection apparatus. In each of the first aspect and the second aspect, the multiphase fluid processing system can be for processing a multiphase fluid from a hydrocarbon well. Viewed from a third aspect of the present invention, there is provided a multiphase fluid processing system. The multiphase fluid processing system comprises: a gas / liquid separator arranged to produce a gas stream and a liquid stream from a multiphase fluid; a gas compressor for compressing the gas stream; a gas line arranged to supply the gas stream from the gas / liquid separator to the gas compressor; a liquid cleaning agent injection apparatus for containing a charged gas and a liquid cleaning agent, the liquid cleaning agent injection apparatus being arranged such that the charged gas will drive the liquid cleaning agent into the gas stream in the gas line during a cleaning operation; a first conduit including a first valve, the first conduit coupling the gas stream to an upper part of the liquid cleaning agent injection apparatus; a second conduit including a second valve, the second conduit coupling the liquid stream to a lower part of the liquid cleaning agent injection apparatus; and a controller in communication with the first valve and the second valve, wherein the controller is configured to: control at least the second valve to supply the liquid cleaning agent injection apparatus with liquid from the liquid stream as at least part of the liquid cleaning agent; and control at least one valve to set a pressure of the charged gas within the liquid cleaning agent injection apparatus, using the pressure of the gas stream, such that that the pressure of the charged gas is greater than a suction pressure of the gas compressor when in operation at a normal operational speed. By controlling at least one valve to set a pressure of the charged gas within the liquid cleaning agent injection apparatus using the pressure of the gas stream, the liquid cleaning agent injection apparatus can be passively pressurised in situ in the multiphase fluid processing system. Accordingly, it is not necessary to use additional pumps or infrastructure to drive the liquid cleaning agent from the liquid cleaning agent injection apparatus. Since the system of the third aspect also controls the supply of liquid from the liquid stream as at least part of the liquid cleaning agent using the second valve, it is also not necessary to use additional infrastructure for supplying a liquid cleaning agent to the liquid cleaning agent injection apparatus. Since the system is also arranged such that a charged gas present within the liquid cleaning agent injection apparatus will drive the liquid therefrom for cleaning of the compressor, it is not necessary for any pumps or infrastructure to supply the liquid cleaning agent to the compressor at any point during its operation. Accordingly, the multiphase fluid processing system can avoid the need for additional pumps or infrastructure does not necessarily require the use of additional pumps or infrastructure to supply the liquid cleaning agent to the compressor at any point during its operation. That is, controlling at least one valve to set a pressure of the charged gas within the liquid cleaning agent injection apparatus using the pressure of the gas stream can avoid the need for additional pumps or infrastructure that would otherwise detract from the benefits of systems which would not otherwise rely on such additional pumps or infrastructure. The multiphase fluid processing system may have one or more or all features corresponding to features or steps of the methods of the first and second aspects. Therefore, above description of the methods of the first and second aspects, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the multiphase fluid processing system of the third aspect. The first valve and / or the second valve may be a controllable shut-off valve. The controller may be configured to close at least the second valve after supplying the liquid cleaning agent injection apparatus with the liquid. The controller may further be in communication with the gas compressor, the controller being configured to: reduce a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure; open at least the first valve or the second valve to pressurise the liquid cleaning agent injection apparatus to the increased pressure; and close the first valve and / or the second valve to isolate, from the gas stream, the charged gas in the liquid cleaning agent injection apparatus at the increased pressure. When the compressor is running, i.e. when its working element is spinning, the pressure of the gas stream on the suction side of the gas compressor is less than a pressure of the compressed gas stream output on a discharge side of the compressor. However, if the speed of the compressor reduces, e.g. partially or completely, the pressure of the gas stream can increase. Under these circumstances, the increased pressure of the gas stream on the suction side can be suitable for setting the pressure of the charged gas within the liquid cleaning agent injection apparatus such that the pressure of the charged gas is greater than a suction pressure of the gas compressor, when in operation at a normal operational speed. In some embodiments, liquid from the liquid stream is supplied to the liquid cleaning agent injection apparatus at the same time as setting the pressure of the charged gas within the liquid cleaning agent injection apparatus. That is, the controller may be configured to: open at least the second valve to supply the liquid cleaning agent injection apparatus with liquid from the liquid stream as at least part of the liquid cleaning agent; and reduce a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure, at the same time or as part of the same operational procedure. After setting the pressure of the charged gas within the liquid cleaning agent injection apparatus, the controller may then be configured to close the second valve, and optionally the first valve if also open. The controller may be configured to: shut down the gas compressor. The controller may be configured to generate a signal for shutting down the gas compressor, or operate a switch configured to shut down the gas compressor. The controller may be similarly configured to: turn the compressor on and / or operate the gas compressor at a normal operational speed. The controller may be configured to: wait for the pressure of the gas stream to reach a settle-out pressure of the compressor before closing the first valve and / or the second valve to isolate the liquid cleaning agent injection apparatus from the liquid stream and the gas stream. The settle-out pressure may be between 60 and 80 bar (600 and 800 kPa), or more preferably between 65 and 75 bar (650 and 750 kPa). The settle-out pressure may be about 70 bar (700 kPa). The controller may be configured to receive a measurement of the pressure of the gas stream from a pressure sensor. The controller may be configured to determine that the pressure of the gas stream has reached the settle-out pressure based on the measured pressure. The controller may be configured to determine that the pressure of the gas stream has reached the settle-out period once a predetermined period of time has elapsed. The predetermined period of time may correspond to a known period of time for allowing the pressure of the gas stream to the reach the settle-out pressure. The controller may be configured to: increase a speed of the gas compressor. The multiphase fluid processing system may comprise: a third conduit including a third valve, the third conduit coupling a discharge side of the gas compressor to the liquid cleaning agent injection apparatus. The controller can be in communication with the third valve, the controller being configured to: open the third valve to pressurise the liquid cleaning agent injection apparatus to a discharge pressure of the gas compressor. During normal operation of the gas compressor, i.e. when the gas compressor is running at preferably its normal operational speed, the pressure on the discharge side of the gas compressor will be greater than the pressure on the suction side of the gas compressor. Accordingly, the pressure of the charged gas within the liquid cleaning agent injection apparatus can be appropriately set using gas at the discharge pressure of the gas compressor. Using gas from the discharge side of the gas compressor to set the pressure of the charged gas within the liquid cleaning agent injection apparatus may avoid the need to use additional pumps or other apparatus to charge the liquid cleaning agent injection apparatus. Using gas from the discharge side of the gas compressor to set the pressure of the charged gas within the liquid cleaning agent injection apparatus may also avoid the need to reduce the speed of the compressor, such that the compressor can continue to run as desired whilst still charging the liquid cleaning agent injection apparatus in situ. The third valve may be a controllable shut-off valve or a one-way valve. The controller may be configured to shut the third valve to isolate the liquid cleaning agent injection apparatus from the gas stream downstream of the compressor. The controller may be configured to detect a pressure of the charged gas within the liquid cleaning agent injection apparatus, and may be configured to shut the third valve when the pressure reaches a predetermined threshold. The controller may be configured to: open the first valve and the second valve to supply the liquid cleaning agent injection apparatus with liquid from the liquid stream for use as at least part of the liquid cleaning agent; and close the first valve and the second valve to isolate the liquid cleaning agent injection apparatus from the liquid stream and the gas stream. This operation may be used to supply the liquid cleaning agent injection apparatus with the liquid without requiring the liquid in the liquid stream to be under pressure (e.g. by reducing the speed of the gas compressor, or otherwise). The liquid cleaning agent injection apparatus may comprise a supply tank for storing the liquid cleaning agent, and a cleaning fluid line arranged to supply the liquid cleaning agent from the supply tank to the gas stream on the suction side of the gas compressor. The cleaning fluid line may comprise a fourth valve. The fourth valve may be a controllable shut-off valve. The controller may be configured to control the fourth valve to supply liquid cleaning agent from the supply tank to the gas stream on the suction side of the gas compressor. The cleaning fluid line may be regarded as a fourth conduit. The supply tank may comprise the lower part for storing liquid cleaning agent and the upper part for storing charged gas. The gas / liquid separator may be arranged to supply gas from the gas stream, and liquid from the liquid stream, to the liquid cleaning agent injection apparatus. The gas / liquid separator may be fluidly connected to an upper part of the liquid cleaning agent injection apparatus by the first conduit; and the gas / liquid separator may be fluidly connected to a lower part of the liquid cleaning agent injection apparatus by the second conduit. With this configuration, the gas / liquid separator may supply gas from the gas stream and liquid from the liquid stream to the liquid cleaning agent injection apparatus. This may result in a simple and compact arrangement for the multiphase fluid processing system. The multiphase fluid processing system may comprise: a fifth conduit including a fifth valve, the fifth conduit coupling the liquid cleaning agent injection apparatus to a reservoir containing liquid cleaning solution. The controller can be in communication with the fifth valve, the controller being configured to: control a fifth valve to supply a liquid cleaning solution to the liquid cleaning agent injection apparatus. The controller may be in communication with a liquid pump associated with the reservoir. The controller may be configured to generate a signal to activate the pump. The pump may be configured to supply the liquid cleaning solution to the liquid cleaning agent injection apparatus under pressure. The multiphase fluid processing system may comprise the reservoir. The liquid cleaning agent may be a liquid hydrocarbon, a condensed hydrocarbon gas, a glycol, an alcohol, water, or an acid. The multiphase fluid processing system can be for processing a multiphase fluid from a hydrocarbon well. BRIEF DESCRIPTION OF THE DRAWINGS Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a schematic diagram of a system for cleaning a compressor according to a first embodiment; Figure 2 illustrates a schematic diagram of a system for cleaning a compressor according to a second embodiment; Figure 3 illustrates a schematic diagram of a system for cleaning a compressor according to a third embodiment; and Figure 4 illustrates a schematic diagram of a system for cleaning a compressor according to a fourth embodiment. DETAILED DESCRIPTION Figure 1 shows, schematically, a multiphase fluid processing system 100 for processing a flow of multiphase fluid from a hydrocarbon well according to a first embodiment. The multiphase fluid processing system 100 includes a gas / liquid separator 1, a gas compressor 2 and a liquid cleaning agent injection apparatus 10. The separator 1 is configured to separate a multiphase fluid into a gas stream and a liquid stream. The compressor 2 is arranged to receive the gas stream at a suction side of the compressor 2, pressurise the gas stream, and output the pressurised gas stream from a discharge side of the compressor 2. The liquid cleaning agent injection apparatus 10 is configured to supply a liquid cleaning agent to the gas stream for washing the compressor 2. The separator 1 is configured to receive multiphase fluid from the hydrocarbon well via an inlet 11, separate the multiphase fluid into a gas stream and a liquid stream, and to provide the gas stream to the compressor 2. The gas stream is communicated from the separator 1 to the compressor 2 via a first fluid line L1. In the present embodiment, the separator 1 is a gravity separator. An upper portion 1a of the separator 1 is thus a gas-filled portion, and a lower portion 1b of the separator 1 is thus a liquid-filled portion. The separator 1 comprises a first outlet 01 located at or towards the top of the upper section 1a of the separator 1 for providing the gas stream to the compressor 2 via the first fluid line L1, and comprises a second outlet 02 located at or towards the bottom of the lower section 1b of the separator 1 for providing the liquid stream to one or more components downstream of the separator 1. The compressor 2 is operable to pressurise or compress the gas stream to provide a compressed gas stream. The gas stream is at a first pressure P1 at a suction side of the compressor 2, i.e. upstream of the compressor 2, and the compressed gas stream is at a second pressure P2 at a discharge side of the compressor 2, i.e. downstream of the compressor. In the present embodiment, the compressor 2 is operable to increase the pressure of the gas stream from a first pressure P1 of 50 bar (500 kPa) to a second pressure P2 of 100 bar (1000 kPa). During operation of the compressor 2, deposits of solid materials can form on interior surfaces of the compressor 2. Accordingly, the multiphase fluid processing system 100 includes the liquid cleaning agent injection apparatus 10 for washing the compressor 2. The liquid cleaning agent injection apparatus 10 is suitably arranged for online cleaning of the compressor 2, i.e. for washing the compressor 2 during normal operation of the compressor 2 or when the compressor 2 is running. The liquid cleaning agent injection apparatus 10 comprises a supply tank 11 for storing liquid cleaning agent, and a second fluid line L10 for supplying the liquid cleaning agent to the gas stream. The liquid cleaning agent is suitable for washing the solid deposits from the internal surfaces of the compressor 2. The liquid cleaning agent injection apparatus 10 is arranged to supply the liquid cleaning agent into the gas stream via the second fluid line L10. The liquid cleaning agent mixes with the gas stream upstream of the compressor 2 to be drawn into the compressor 2. A third valve V3, such as a controllable shut-off valve, is provided in the second fluid line L10 such that injection of the liquid cleaning agent into the gas stream can be controlled. In order to inject the liquid cleaning agent into the gas stream, it must be supplied at a pressure that is higher than the first pressure P1 of the gas stream. In operation, the liquid cleaning agent is therefore driven by a charged gas at a third pressure P3, wherein the third pressure P3 of the charged gas is greater than the first pressure P1 of the gas stream. That is, a pressure differential between the third pressure P3 of the charged gas and the first pressure P1 of the gas stream is used to drive injection of the liquid cleaning agent into the gas stream. The supply tank 11 of the liquid cleaning agent injection apparatus 10 can therefore be considered to function as a hydraulic accumulator, wherein injection of the liquid cleaning agent into the gas stream is driven by the charged gas, acting as a source of mechanical energy, since the charged gas is pressurised relative to the gas stream. The liquid cleaning agent injection apparatus 10 does not require an external source of energy, such as electricity to drive a pump, to drive the liquid cleaning agent. This mitigates the need for further infrastructure associated with supplying the liquid cleaning agent to the compressor 2. To clean the gas compressor 2, a washing operation is performed by opening the third valve V3 arranged in the second fluid line L10 leading from the supply tank 11 of the liquid cleaning agent injection apparatus 10. The third valve V3 can be opened intermittently or periodically, as desired. The third valve V3 can be left open for a predetermined period of time, or until all the remaining liquid cleaning agent is injected into the gas stream. A degree of opening of the third valve V3 can be selected to regulate the rate that the liquid cleaning agent is supplied to the gas stream. The liquid cleaning agent can be driven from the supply tank 11 by the charged gas provided the third pressure P3 of the charged gas is greater than the first pressure P1 of the gas stream. Over time, as the liquid cleaning agent is driven from the supply tank 11, the third pressure P3 of the charged gas will decrease. Once the third pressure P3 of the charged gas reaches the first pressure P1 of the gas stream, the liquid cleaning agent will no longer be driven from the supply tank 11. A method for recharging the liquid cleaning agent injection apparatus 10 is provided, such that further washing operations can be performed. The recharging comprises supplying further liquid cleaning agent to the supply tank 11 and either re-pressurising the charged gas the supply tank 11 or supplying additional pressurised charged gas to the supply tank 11. As will be described in further detail, the liquid cleaning agent injection apparatus 10 is arranged to be passively pressurised in situ in the multiphase fluid processing system 100. That is, the pressure of the charged gas within the liquid cleaning agent injection apparatus 10 is arranged to be set using the pressure of the gas stream of the multiphase fluid processing system 100. Therefore, the liquid cleaning agent injection apparatus 10 does not require the use of additional pumps or infrastructure to supply the liquid cleaning agent to the compressor 2 at any point during its operation. When the compressor 2 is running, i.e. when its working element is spinning, the first pressure P1 of the gas stream is less than the second pressure P2 of the pressurised gas exhausted from the compressor 2. However, if the compressor 2 slows down, or is shut off, the first pressure P1 of the gas stream can increase. For example, if the compressor 2 slows down then the rate at which gas is drawn through the compressor 2 decreases. This causes a backpressure to form on the suction side of the compressor 2, thereby increasing the first pressure P1 of the gas stream. If the compressor 2 is shut off or shuts down, then the first pressure P1 of the gas stream on the suction side may balance out or equilibrate with the second pressure P2 on the discharge side of the compressor 2. Under these circumstances, the first pressure P1 of the gas stream can increase such that the first pressure P1 of the gas stream is greater than the third pressure P3 of the charged gas in the liquid cleaning agent injection apparatus 10. When this occurs, gas from the gas stream is suitable for use as the charged gas for driving the liquid cleaning agent from the liquid cleaning agent injection apparatus 10. The liquid cleaning agent injection apparatus 10 takes advantage of this available source of energy by setting the pressure of the charged gas therein using pressurised gas from the gas stream. To charge the liquid cleaning agent injection apparatus 10, the gas from the gas stream is supplied to the supply tank 11 and then the supply tank 11 is fluidly isolated from the gas stream, such that the charged gas in the supply tank 11 is maintained at the increased third pressure P3. Once the liquid cleaning agent injection apparatus 10 is charged with pressurised gas as the charged gas, the compressor 2 can return to its normal operation by increasing its speed or by turning it on. The compressor 2 accordingly returns to pumping gas from the gas stream at a greater rate, such that the first pressure P1 of the gas stream on the suction side of the compressor 2 decreases and the second pressure P2 on the discharge side of the compressor 2 increases. Since the charged gas in the liquid cleaning agent injection apparatus 10 is isolated from the gas stream, it remains at the increased third pressure P3 and is therefore available as a passive source of pneumatic energy for driving the liquid cleaning agent into the gas stream during a washing operation of the compressor 2. By taking advantage of passive increases in the first pressure P1 of the gas stream as described above, the liquid cleaning agent injection apparatus 10 can be charged in situ in the multiphase fluid processing system 100 and without the need for any additional pumps or pumping apparatus to recharge or otherwise pressurise the liquid cleaning agent injection apparatus 10 to enable supply of the liquid cleaning agent to the gas stream. In the present embodiment, the supply tank 11 of the liquid cleaning agent injection apparatus 10 is in fluid communication with the separator 1 via two supply lines or conduits. A first valve V1 is located in the first conduit, and is operable to supply gas from the gas stream to the liquid cleaning agent injection apparatus 10. The first conduit connects to the upper portion 1a of the separator 1. The first valve V1 may be a controllable shut-off valve. A second valve V2 is located in the second conduit, and is operable to supply liquid from the liquid stream. The second valve V2 may be a controllable shut-off valve, or a one-way valve. The liquid from the liquid stream is thus the liquid cleaning agent, and the gas / liquid separator 1 acts as a reservoir of liquid cleaning agent for the liquid cleaning agent injection apparatus 10. The second conduit connects to the lower portion 1b of the separator 1. In this arrangement, the supply tank 11 can be filled with liquid cleaning agent from the separator 1 at the same time as the supply tank 10 is filled with gas from the separator 1, if desired. Using the separator 1 to provide both gas and liquid cleaning agent to the liquid cleaning agent injection apparatus 10 provides a compact and simple arrangement for the multiphase fluid processing system 100. Using the separator 1 to provide liquid cleaning agent also reduces the need for additional infrastructure associated with supplying the liquid cleaning agent from a separate or standalone reservoir. In other embodiments, the liquid cleaning agent injection apparatus 10 can be supplied with liquid cleaning agent from a separate liquid cleaning agent reservoir. In such arrangements, the liquid cleaning agent can be a different cleaning solution to the liquid of the liquid stream. In these and other embodiments, the supply tank 11 can be arranged to receive gas from the gas stream directly from the first fluid line L1, rather than via the separator 1. As described above, the first pressure P1 of the gas stream can be increased by reducing the speed of the compressor 2. The greater the reduction in the speed of the compressor 2, the greater the increase in first pressure P1 of the gas stream will be. Shutting the compressor 2 down altogether may result in the greatest increase in first pressure P1 of the gas stream. A greater increase of the first pressure P1 of the gas stream results in the liquid cleaning agent injection apparatus 10 being charged with gas at a greater pressure. However, reducing the speed of the compressor 2, or shutting it down altogether reduces the production output of the compressor 2. Accordingly, the reduction of the speed of the compressor 2 can be controlled to balance the need for recharging the liquid cleaning agent injection apparatus 10 with pressurised gas and the need for maintaining the production output of the compressor 2 at a desirable level. The multiphase fluid processing system 100 includes a controller 20 comprising a processor and a memory. The controller 20 is in communication, e.g. wired or wireless communication, with the gas compressor 20 and each of the valves V1,V2, V3. The memory stores instructions which, when executed by the processor, are operable to cause the multiphase fluid processing system 100 to perform a charging operation or a cleaning operation. The multiphase fluid processing system 100 also includes a number of sensors (not shown) that are configured to monitor the speed of the compressor 2, the third pressure P3 of the gas stored in the supply tank 11, a level of liquid cleaning agent stored in the supply tank 11, and the first pressure P1 of the gas stream. The controller 20 is in communication with each of these sensors, and can control one or more operations of the multiphase fluid processing system 100 in response to readings made by each of these sensors. Operation of the multiphase fluid processing system 100 will now be described in the situation where the compressor 2 is shut down, or switched off, and the liquid cleaning agent injection apparatus 10 is recharged whilst the compressor 2 is shut down. It will be appreciated that a similar method of control may also performed where the speed of the compressor 2 is merely reduced, compared to the compressor 2 being shut down altogether. The controller 20 is adapted to perform each of the operations discussed below, or execute instructions resulting in the operations discussed below occurring. In normal operation, the compressor 2 receives the gas stream at a first pressure P1 and increases the pressure of the gas stream to provide a compressed gas stream at an increased second pressure P2. In the present embodiment, the compressor 2 increases the pressure from a first pressure P1 of 50 bar (500 kPa) to a second pressure P2 of 100 bar (1000 kPa). When the compressor 2 shuts down, either as a result of actively shutting down the compressor 2 or as a result of an unplanned shutdown, the speed of the compressor 2 decreases until it stops spinning altogether. Once the speed of the compressor 2 reaches a zero value, the pressure across the compressor 2 equilibrates and reaches a settle-out pressure provided that the pressure from the multiphase fluid processing system 100 is not bled. That is, the first pressure P1 of the gas stream on the suction side of the compressor 2 increases and the second pressure P2 of the compressed gas stream on the discharge side of the compressor 2 decreases until they are balanced. In the present embodiment, the settle-out pressure is 70 bar (700 kPa). Since the settle-out pressure is greater than the first pressure P1 of the gas stream under normal operation of the compressor 2, the liquid cleaning agent injection apparatus 10 can be charged by the gas stream as its first pressure P1 approaches or is at the settle-out pressure. Prior to or during shutdown of the compressor 2, the first valve V1 is opened such that the supply tank 11 is in fluid communication with the gas stream provided to the compressor 2. The supply tank 11 thus equilibrates with the gas stream via the first conduit, and is charged with gas at a pressure at or approaching the settle-out pressure. Once the supply tank 11 is charged with charged gas, the first valve V1 located in the first conduit is closed. Closing the first valve V1 results in the supply tank 11 being fluidly isolated from the gas stream. The third pressure P3 of the charged gas is thus locked in at or approaching the settle-out pressure. At the same time as the supply tank 11 is charged with charged gas, the second valve V2 is opened such that the supply tank 11 is in fluid communication with the liquid stream. This results in the supply tank 11 refilling with liquid cleaning agent. The separator 1 and the supply tank 11 of the liquid cleaning agent injection apparatus 10 are positioned relative to each other such that the liquid in the lower portion 1b of the separator 1 refills the liquid cleaning agent in the supply tank 11 to the same level as a gas / liquid level within the separator 1. The third valve V3 in the second fluid line L10 is shut, or remains shut, during refilling of the supply tank 11 with liquid cleaning agent, such that liquid cleaning agent is not uncontrollably discharged from the liquid cleaning agent injection apparatus 10. Once the liquid cleaning agent injection apparatus 10 is recharged with charged gas and cleaning liquid, normal operation of the compressor 2 is resumed. The compressor 2 is switched back on, and the pressure across the compressor 2 returns to its initial level. That is, the first pressure P1 of the gas stream at the discharge side of the compressor 2 returns to 50 bar (500 kPa) and the second pressure P2 of the compressed gas stream at the suction side of the compressor 2 returns to 100 bar (1000 kPa). The liquid cleaning agent injection apparatus 10 remains fluidly isolated from the rest of the multiphase fluid processing system 100 as the compressor 2 returns to normal operation. Accordingly, the third pressure P3 of the charged gas remains compressed, i.e. its pressure is locked in, relative to the first pressure P1 of the gas stream. To clean or wash the compressor 2, the third valve V3 in the second fluid line L10 of the liquid cleaning agent injection apparatus 10 is opened, and liquid cleaning agent is driven from the supply tank 11 by the charged gas. The liquid cleaning agent is mixed into the gas stream, and is supplied to the compressor 2. The multiphase fluid processing system 100 is particularly advantageous for use in remote locations. For example, the multiphase fluid processing system 100 may be located subsea, such as on a seabed and / or at a depth of at least 100m or at least 500m below sea level, or on a normally-unmanned offshore platform. Figure 2 shows, schematically, a multiphase fluid processing system 200 for processing a flow of multiphase fluid from a hydrocarbon well according to a second embodiment. The multiphase fluid processing system 200 according to the second embodiment is a modified arrangement of the multiphase fluid processing system 100 according to the first embodiment as described above, and therefore any discussion of identical features, structures, functions and operations is not repeated. The multiphase fluid processing system 200 comprises a second inlet I2 in fluid communication with the liquid cleaning agent injection apparatus 10. The second inlet I2 is arranged to receive a liquid cleaning solution at a pressure P4, and a third conduit is provided to supply the liquid cleaning solution to the liquid cleaning agent injection apparatus 10. The pressure P4 of the liquid cleaning solution provided via the second inlet I2 is greater than the pressure P3 in the liquid cleaning agent injection apparatus 10. In this particular arrangement, the third conduit is tied into second conduit in which the second valve V2 is located, and downstream of the valve V2. In other embodiments the third conduit may tie in elsewhere to the liquid cleaning agent injection apparatus 10. A fourth valve V4 is located in the third conduit and is in communication with the controller 20. The fourth valve V4 is operable to control supply of a liquid cleaning solution from the second inlet I2 to the liquid cleaning agent injection apparatus 10. In the present embodiment, the liquid cleaning solution is an additive, and the fourth valve V4 is operable to supply the additive to the liquid cleaning agent injection apparatus 10. The additive is an acid in the present embodiment, and the valve V4 is thereby used to control the pH of the liquid cleaning agent present in the liquid cleaning agent injection apparatus 10. In another embodiment, the external source is an external source of pressurised hydraulic fluid for use with subsea Christmas trees and subsea valves. The hydraulic fluid may be used as the liquid cleaning agent, rather than as an additive. Figure 3 shows, schematically, a multiphase fluid processing system 300 for processing a flow of multiphase fluid from a hydrocarbon well according to a third embodiment. The multiphase fluid processing system 300 according to the third embodiment is a modified arrangement of the multiphase fluid processing system 100 according to the first embodiment as described above, and therefore any discussion of identical features, structures, functions and operations is not repeated. The multiphase fluid processing system 300 comprises a third fluid line L20 connecting a discharge side of the compressor 2 to the supply tank 11. In the present embodiment, the third fluid line L20 is connected to the first conduit, downstream of the first valve V1. A fifth valve V5 is disposed in the third fluid line L20, and is in communication with the controller 20. The fifth valve V5 can be used to set the pressure of the charged gas within the liquid cleaning agent injection apparatus 10. The fifth valve V5 can be opened to supply pressurised gas at a pressure P2 from the discharge side of the compressor 2 to the supply tank 11. During normal operation of the compressor 2, i.e. when the compressor 2 is running, the pressure P2 on the discharge side of the compressor 2 will be greater than the pressure P1 on the suction side of the compressor 2. Accordingly, when the fifth valve V5 is opened, the charged gas within the supply tank 11 can be set at a pressure P4 that is greater than the pressure P1 on the suction side of the compressor 2. The gas within the liquid cleaning agent injection apparatus 10 will hence be set a pressure P4 capable of driving the liquid cleaning agent from the supply tank 11. In the third embodiment, the fifth valve V5 can be opened, and hence used to set the pressure within the liquid cleaning agent injection apparatus 10, without requiring the speed of the compressor 2 to be reduced or requiring the compressor 2 to shut down altogether. The fifth valve V5 can therefore be used to fill the supply tank 11 with charged gas, where the pressure within the supply tank 11 has decreased through operation of the liquid cleaning agent injection apparatus 10 but the compressor 2 is desired to not stop running at its desired speed. This mode of operation may be used when there is still liquid cleaning agent within the liquid cleaning agent injection apparatus 10, but the pressure of the charged gas has dropped such that it is insufficient to discharge the liquid cleaning agent into the gas stream upstream of the compressor 2. In another operation, the fifth valve V5 is used to facilitate the liquid cleaning agent injection apparatus 10 being recharged, without the compressor 2 needing to be slowed down or shut down. In this operation, the first valve V1 and the second valve V2 are both opened such that the supply tank 11 refills with liquid cleaning agent. This may be done without slowing or stopping the compressor 2. The first valve V1 and the second valve V2 are then closed so that the supply tank 11 is fluidly isolated from the liquid stream and the gas stream provided by the gas / liquid separator 1. Having opened the valves V1 and V2, the pressure P3 in the supply tank 11 will equalise with the pressure of the gas stream provided by the gas / liquid separator 1. To set the pressure in the supply tank 11 to a desired level, the fifth valve V5 is subsequently opened so that the supply tank 11 is charged with gas from the discharge side of the compressor 2. Through the use of the fifth valve V5, the supply tank 11 is filled with liquid cleaning agent and charged gas without requiring the compressor 2 to slow down or shut down. In the third embodiment, the controller 20 of the multiphase fluid processing system 300 is configured to control at least the valves V1, V2, V5 so as to set the pressure P3 of the charged gas within the supply tank 11 to that of the pressure P2 on the discharge side of the compressor 2 when the compressor 2 is running. Optionally, the controller 20 of the multiphase fluid processing system 300 may be further configured to control the speed of the compressor 2, as in the first embodiment, so as to set the pressure P3 of the charged gas within the supply tank 11 to that of the pressure P1 when the compressor 2 has slowed down or shut down. Through control of the valves V1, V2, V5, and optionally the speed of the compressor 2, the controller 20 can set the pressure P3 of the charged gas in the supply tank 11 passively, i.e. without the use of any additional pumping apparatus or the like. In other embodiments, the controller 20 may be configured to set the pressure P3 of the charged gas using only the fifth valve V5 as described above. Figure 4 shows, schematically, a multiphase system 400 for processing a flow of multiphase fluid from a hydrocarbon well according to a fourth embodiment. The multiphase fluid processing system 400 according to the fourth embodiment combines the structure and functionality of the multiphase fluid processing system 200 of the second embodiment with the structure and functionality of the multiphase fluid processing system 300 of the third embodiment. Therefore, any discussion of identical features, structures, functions and operations is not repeated.

Claims

1. A method of charging a liquid cleaning agent injection apparatus in situ in a multiphase fluid processing system, wherein the multiphase fluid processing system comprises a gas / liquid separator arranged to produce a gas stream and a liquid stream from a multiphase fluid, and a gas compressor arranged to compress the gas stream, and wherein the liquid cleaning agent injection apparatus is arranged such that a charged gas within the liquid cleaning agent injection apparatus will drive a liquid cleaning agent from the liquid cleaning agent injection apparatus into the gas stream upstream of the gas compressor during washing of the gas compressor, the method comprising:supplying the liquid cleaning agent injection apparatus with liquid from the liquid stream as at least part of the liquid cleaning agent; andsetting a pressure of the charged gas within the liquid cleaning agent injection apparatus, using the pressure of the gas stream, such that that the pressure of the charged gas is greater than a suction pressure of the gas compressor, when in operation at a normal operational speed.

2. A method as claimed in claim 1, comprising:fluidly communicating the liquid cleaning agent injection apparatus with the liquid stream, and optionally with the gas stream, to supply the liquid from the liquid stream; andafter supplying the liquid from the liquid stream to the liquid cleaning agent injection apparatus, isolating the liquid cleaning agent injection apparatus from the liquid stream and the gas stream.

3. A method as claimed in claim 1 or 2, comprising: reducing a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure;fluidly communicating the liquid cleaning agent injection apparatus with at least one of the liquid stream and the gas stream to set the pressure of the charged gas at the increased pressure; andisolating the liquid cleaning agent injection apparatus at the increased pressure from the liquid stream and the gas stream.

4. A method as claimed in claim 3, wherein the step of reducing a speed of the gas compressor comprises shutting down the gas compressor.

5. A method as claimed in claim 4, comprising:waiting for a pressure of the gas stream to reach a settle-out pressure of the gas compressor before isolating the liquid cleaning agent injection apparatus from the liquid stream and the gas stream.

6. A method as claimed in any preceding claim, further comprising: supplying compressed gas from the gas stream at a discharge side of the gas compressor to the liquid cleaning agent injection apparatus to set the pressure of the charged gas at a discharge pressure of the gas compressor.

7. A method of cleaning a gas compressor in situ in a multiphase fluid processing system, the method comprising:charging a liquid cleaning agent injection apparatus in situ in the multiphase fluid processing system according to a method as claimed in any preceding claim;operating the gas compressor at the normal operational speed; and supplying liquid cleaning agent from the liquid cleaning injection apparatus into the gas stream on the suction side of the gas compressor.

8. A method as claimed in any preceding claim, wherein the multiphase fluid processing system is for processing a multiphase fluid from a hydrocarbon well.

9. A multiphase fluid processing system comprising:a gas / liquid separator arranged to produce a gas stream and a liquid stream from a multiphase fluid;a gas compressor for compressing the gas stream;a gas line arranged to supply the gas stream from the gas / liquid separator to the gas compressor;a liquid cleaning agent injection apparatus for containing a charged gas and a liquid cleaning agent, the liquid cleaning agent injection apparatus being arranged such that the charged gas will drive the liquid cleaning agent into the gas stream in the gas line during a cleaning operation;a first conduit including a first valve, the first conduit coupling the gas stream to an upper part of the liquid cleaning agent injection apparatus;a second conduit including a second valve, the second conduit coupling the liquid stream to a lower part of the liquid cleaning agent injection apparatus; anda controller in communication with the first valve and the second valve, wherein the controller is configured to:control at least the second valve to supply the liquid cleaning agent injection apparatus with liquid from the liquid stream as at least part of the liquid cleaning agent; andcontrol at least one valve to set a pressure of the charged gas within the liquid cleaning agent injection apparatus, using the pressure of the gas stream, such that that the pressure of the charged gas is greater than a suction pressure of the gas compressor when in operation at a normal operational speed.

10. A multiphase fluid processing system as claimed in claim 9, wherein the controller is configured to:close at least the second valve after supplying the liquid cleaning agent injection apparatus with the liquid.

11. A multiphase fluid processing system as claimed in claim 9 or 10, wherein the controller is further in communication with the gas compressor, the controller being configured to:reduce a speed of the gas compressor such that a pressure of the gas stream increases on a suction side of the gas compressor to an increased pressure;open at least the first valve or the second valve to pressurise the liquid cleaning agent injection apparatus to the increased pressure; andclose the first valve and / or the second valve to isolate, from the gas stream, the charged gas in the liquid cleaning agent injection apparatus at the increased pressure.

12. A multiphase fluid processing system as claimed in claim 9 to 11, comprising:a third conduit including a third valve, the third conduit coupling a discharge side of the gas compressor to the liquid cleaning agent injection apparatus;wherein the controller is in communication with the third valve, the controller being configured to:open the third valve to pressurise the liquid cleaning agent injection apparatus to a discharge pressure of the gas compressor.

13. A multiphase fluid processing system as claimed in any of claims 9 to 12, wherein the liquid cleaning agent injection apparatus comprises a supply tank for storing the liquid cleaning agent, and a cleaning fluid line arranged to supply the liquid cleaning agent from the supply tank to the gas stream on the suction side of the gas compressor.

14. A multiphase fluid processing system as claimed in any of claims 9 to 13, wherein the gas / liquid separator is fluidly connected to an upper part of the liquid cleaning agent injection apparatus by the first conduit; andwherein the gas / liquid separator is fluidly connected to a lower part of the liquid cleaning agent injection apparatus by the second conduit.

15. A multiphase fluid processing system as claimed in any of claims 9 to 14, wherein the multiphase fluid processing system is for processing a multiphase fluid from a hydrocarbon well.

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