Method and system for barrier testing

Abstract

Disclosed is a system and method of barrier testing a subsea system (300) for carbon dioxide injection to a subterrain reservoir (400) with a main injection line (200). The system comprises a fluid line (330) between a set of two or more barrier valves (310, 320); at least one pressure monitoring device (380); a main injection line (200) configured for injecting carbon dioxide; and a test line (100) for injecting an inert gas. The method comprises pressurising (610) the inert gas in the test line (100); opening (620) a test valve (110); flushing (630) out any fluid in the fluid line (330); closing (640) the downstream barrier valve (310), the upstream barrier valve (320), and an injection valve (210); opening (650) the test valve (110) and injecting the inert gas to a predetermined pressure in the fluid line (330) using the test line (100); and keeping (660) the predetermined pressure for a predetermined period of time and checking the at least one pressure monitoring device (380) to detect any pressure changes.

Description

[0001] P394008.W0.01

[0002] 1

[0003] METHOD AND SYSTEM FOR BARRIER TESTING

[0004] Technical Field

[0005] The present disclosure relates to a method and a system for barrier testing. More particularly, the present disclosure relates to a method and a system for barrier testing a subsea system for carbon dioxide injection to a subterrain reservoir with a main injection line for carbon dioxide.

[0006] Background

[0007] Barriers, valves, in the oil and gas industry are required to be tested regularly to ensure barrier integrity. Such testing and frequency of testing is regulated and can be as often as once a month. Present systems use, for example, methanol or glycol for barrier testing. In a subsea environment a mixture of carbon dioxide and water result in a corrosive environment making such mixtures with carbon dioxide unsuitable. The use of carbon dioxide is also problematic because various pressures and temperatures in an injection system causes the carbon dioxide to change phase. This results in incorrect barrier testing results.

[0008] A further problem is that barrier testing at low pressures is limited to hydrostatic head of the surrounding seawater. How to provide a system and a method for testing if barrier valves are intact is a problem. How to cost effective make and provide for barrier testing is a problem. How to cost effective provide for arrangements and systems for making barrier testing is a problem. How to minimize the time for making barrier testing is a problem. It is desirable to test barriers, valves, of a Xmas tree injection system, especially for carbon dioxide injection to a subterrain reservoir subsea.

[0009] A further technical problem is that any part of a solution must function without a possibility to fail, fulfil technical and legal requirements, and is easy to use. It is desirable that any solution is simple, not expensive to produce, and is reliable. It is further a technical problem to avoid cumbersome arrangements that are expensive to manufacture or assemble. It is desirable that any part of the solution, or at least some parts of the solution, can be retrieved or interchanged.

[0010] Summary of the Invention

[0011] It is an object of the present invention to provide a method and a system for barrier testing a subsea system for carbon dioxide injection to a subterrain P394008.W0.01

[0012] 2 reservoir with a main injection line. This object can be achieved by the features as defined by the independent claims. Further enhancements are characterized by the dependent claims. The invention is defined by the claims. The method and the system for barrier testing may be used for any other subsea valve configuration that needs barrier testing.

[0013] According to one embodiment, a method of barrier testing a subsea system 300 for carbon dioxide injection to a subterrain reservoir 400 with a main injection line 200 is disclosed. The system comprises a fluid line 330, at least one pressure monitoring device 380, a main injection line 200, and a test line 100. The fluid line 330 is between a set of two or more barrier valves 310, 320, one of the barrier valves of the set of two barrier valves being a first barrier valve 310 closer to an environment 20 and the other barrier valve being a second barrier valve 320 closer to the reservoir 400. The at least one pressure monitoring device 380 is for monitoring pressure in the fluid line 330. The main injection line 200 is for injecting carbon dioxide to a first connection point 332 on the fluid line 330 between the first barrier valve 310 and the second barrier valve 320. The main injection line 200 comprises an injection valve 210. The test line 100 is for injecting an inert gas to the first connection point 332. The test line 100 comprises a pressurised inert gas in the test line 100. The test line 100 comprises a test valve 110 for allowing or preventing the inert gas to reach the fluid line 330. This system is used for realising the method of barrier testing the subsea system 300 for carbon dioxide injection to the subterrain reservoir 400 with the main injection line 200. The method comprises the following steps in any order: pressurising 610 the inert gas in the test line 100; opening 620 the test valve 110; flushing 630 out any fluid in the fluid line 330; closing 640 the downstream barrier valve 310, the upstream barrier valve 320, and the injection valve 210; opening 650 the test valve 110 and injecting the inert gas to a predetermined pressure in the fluid line 330 between the first barrier valve 310 and the second barrier valve 320 using the test line 100; and monitoring 660 the predetermined pressure for a predetermined period of time and checking the at least one pressure monitoring device 380 to detect any pressure changes. During the monitoring, other valves like test valve 110 and a vent vale 120 may be closed. P394008.W0.01

[0014] 3

[0015] According to one embodiment, the system is configured to provide the inert gas at a predetermined pressure to the fluid line 330 between the first barrier valve 310, the second barrier valve 320, and the injection valve 210. The test line 100 may further comprise a vent valve 120 on the test line 100, between the test valve 110 and the first connection point 332, for venting out to the environment 20. The system may further comprise a vent valve 120 on the test line 100, between the test valve 110 and the first connection point 332, for venting out to the environment 20. The method may further comprise venting the inert gas to an environment 40 at atmospheric pressure 1 .01 kPa, 1 atm.

[0016] As described herein the method allows for, and the system can be used for, barrier testing above and below the pressure that surrounds the subsea system 300. By using an inert gas venting can be done without any problems to the environment and the barrier testing can be done reliably since the inert gas does not change phase or react. The vent line 140 and the vent valve 120 provides reliable and controlled venting. The prefilled test line 100 with the inert gas at a predetermined test pressure provides fast and reliable barrier testing.

[0017] At least one of the above embodiments provides a method and a system for barrier testing a subsea system for carbon dioxide injection to a subterrain reservoir with a main injection line, and one or more solutions to the problems and disadvantages with the background art. Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following description and claims. Various embodiments of the present application obtain only a subset of the advantages set forth. No one advantage is critical to the embodiments. Any claimed embodiment may be technically combined with any other claimed embodiment or embodiments.

[0018] Brief Description of the Drawing

[0019] The accompanying drawing illustrate presently exemplary embodiments of the disclosure and serve to explain, by way of example, the principles of the disclosure.

[0020] Fig 1 is a diagrammatic illustration, a flow chart, of a system for barrier testing according to an exemplary embodiment of the disclosure; and

[0021] Fig 2 is a diagrammatic illustration, a flow chart, of a method for barrier testing with the system according to an exemplary embodiment of the disclosure. P394008.W0.01

[0022] 4

[0023] Detailed Description

[0024] Figures 1 and 2 are diagrammatic illustrations of embodiments of a method and a system for barrier testing a subsea system 300 for carbon dioxide injection to a subterrain reservoir 400 with a main injection line 200. A seabed 10 is illustrated between the reservoir 400 for carbon dioxide and the system 300 in figure 1 . The system 300 is in an environment 20, the sea water 20, between the seabed 10 and the sea surface 30 in figure 1 . At and above the sea surface 30 is the upper environment 40 at atmospheric pressure 1.01 kPa, 1 atm. The environment 20 may stretch from the seabed 10 and up and to, including, the sea surface 30 and the upper environment 40. In other words, the pressure of the environment 20 may be anything from the hydrostatic pressure subsea where the system 300 is and up to 1.01 kPa, 1 atm. The subterrain reservoir 400 may be a storage for carbon dioxide, for example an old empty production oil or gas well converted for storing carbon dioxide, or other geological trap, for example a saline aquifer.

[0025] Figures 1 and 2 disclose a method of barrier testing a subsea system 300 for carbon dioxide injection to a subterrain reservoir 400 with a main injection line 200. The system comprises a fluid line 330; at least one pressure monitoring device 380; a main injection line 200; and a test line 100.

[0026] The fluid line 330 is between a set of two or more barrier valves 310, 320, one of the barrier valves of the set of two barrier valves being a first barrier valve 310 closer to an environment 20 and the other barrier valve being a second barrier valve 320 closer to the reservoir 400. The two barrier valves may be part of an electric Xmas tree for the reservoir 400. The fluid line 330 may closed off by further valves like an injection valve 210, a test valve 110, and a vent valve 120.

[0027] The at least one pressure monitoring device 380 is for monitoring pressure in the fluid line 330. There may be a plurality of pressure monitoring device, for example one close to each barrier valve. There may be one or more other pressure monitoring devices 340 for monitoring the barrier valves 310, 320 on an opposite side of the fluid line 330.

[0028] The main injection line 200 is configured for injecting carbon dioxide to a first connection point 332 on the fluid line 330 between the first barrier valve 310 P394008.W0.01

[0029] 5 and the second barrier valve 320. The main injection line 200 comprising an injection valve 210. The injection valve may be configured to be opened and closed to allow carbon dioxide to enter or to exit the reservoir 400. In this way carbon dioxide can be supplied to or from the reservoir via the fluid line 330 and the main injection line 200. The main injection line 200 may extend from the subsea system 300 to the top, for example a platform 500 above the sea surface 30.

[0030] The test line 100 is for injecting an inert gas to the first connection point 332. The test line 100 comprises the inert gas in the test line 100. The test line 100 may for example be prefilled with inert gas, preferably prefilled with pressurised inert gas. Barrier testing may be achieved quickly and accurately if the test line 100 is prefilled with the inert gas at a pressure that is equal to a predetermined test pressure for the barrier valves. The test line 100 comprises a test valve 110 for allowing or preventing the inert gas to reach the fluid line 330. The test line 100 may connect to the fluid line 330 or to the main injection line 200. The test valve 110 may be in the proximity of the subsea system 300, for example part of the subsea system 300.

[0031] This system is used for realising the method of barrier testing the subsea system 300 for carbon dioxide injection to the subterrain reservoir 400 with the main injection line 200. The method comprises the following steps in any order: pressurising 610 the inert gas in the test line 100; opening 620 the test valve 110; flushing 630 out any fluid in the fluid line 330; closing 640 the downstream barrier valve 310, the upstream barrier valve 320, and the injection valve 210; opening 650 the test valve 110 and injecting the inert gas to a predetermined pressure in the fluid line 330 between the first barrier valve 310 and the second barrier valve 320 using the test line 100; and monitoring 660 the predetermined pressure for a predetermined period of time and checking the at least one pressure monitoring device 380 to detect any pressure changes.

[0032] At pressurising 610 the inert gas in the test line 100, the test line may be filled with, comprise, the inert gas. The inert gas may be pressurised to a P394008.W0.01

[0033] 6 predetermined pressure that is substantially equal to the required pressure to test the barrier valves 310, 320. This may be a pressure higher, or lower, than a water pressure surrounding the subsea system 300.

[0034] The inert gas may be for example nitrogen. However, any inert gas may be used as long as the inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The inert gas may not necessarily be elemental and may be a compound gas. The non-reactivity may be at least due to the valence, the outermost electron shell, being complete in all the inert gases.

[0035] At opening 620 the test valve 110, the inert gas is allowed to enter into the fluid line 330. The injection valve 210 may be closed before opening the test valve 110. If there is a vent valve 120, then the vent valve 120 may also be closed before opening the test valve 110. One or both of the barrier valves 310, 320 may be opened.

[0036] At flushing 630 out any fluid in the fluid line 330, the inert gas enters the fluid line 330 and flushes out the fluid line 330. For example, any carbon dioxide in the fluid line 330 may be flushed down into the reservoir 400 via an open second barrier valve 320, if the second barrier valve 320 is open and the first barrier valve 310 is closed. For example, fluid may be flushed out via the vent line 140. The injection valve 210 may be open, but preferably closed, while flushing.

[0037] At closing 640 the downstream barrier valve 310, the upstream barrier valve 320, and the injection valve 210, the test area, the volume to be filled by the inert gas is created. If there is a vent valve 120, then the vent valve 120 may be closed or opened, this is further explained herein.

[0038] With the opening 650 of the test valve 110, the inert gas is injected to a predetermined pressure in the fluid line 330 between the first barrier valve 310 and the second barrier valve 320. The inert gas provided by the test line 100 does not react or change phase and therefore provides a reliable and quick predetermined test pressure. The pressure may be read by the at least one pressure monitoring device 380. The test valve 110 may be closed to maintain the predetermined pressure in the fluid line 330. P394008.W0.01

[0039] 7

[0040] At monitoring 660 the predetermined pressure for a predetermined period of time, the inert gas is kept pressurised in the fluid line 330 and one checks the at least one pressure monitoring device 380 to detect any pressure changes. If there is a pressure change, then one or more valves, for example the barrier valves 310, 320, are leaking or not functioning properly. One may further determine which valve is leaking with further pressure monitoring devices 340. The valves 110, 120, 210, 310, and 320 may be closed to keep the fluid line 330 pressurised during the predetermined period of time.

[0041] The system may further comprise a vent valve 120 on the test line 100. The vent valve 120 may be between the test valve 110 and the first connection point 332. The method may further comprises venting the inert gas to the environment 20, 40. The vent valve 120 may be for venting out to the environment 20, for example venting out to the environment proximate to the subsea system, or venting out to the environment 40 at 1.01 kPa, 1 atm, the atmospheric pressure above the sea surface 30, or venting to any pressure therebetween.

[0042] The system may further comprise a vent line 140 for venting the inert gas to the environment 20. The method further comprises venting the inert gas through the vent line 140. Venting out to the environment as mentioned in this disclosure may be venting in the proximity of the subsea system 300, i.e. at the water pressure that surrounds the subsea system 300. Venting out to the environment as mentioned in this disclosure may be venting out to the environment at 1.01 kPa, 1 atm, the atmospheric pressure above the sea surface 30. Venting out to the environment as mentioned in this disclosure may be venting to any pressure therebetween. For example, the vent line 140 may extend all the way up to the sea surface 30 and the venting would then be at 1.01 kPa. The vent line 140 may end in the proximity of the subsea system 300 and the venting would then be at the pressure that the water has surrounding the subsea system 300. The inert gas may be vented trough the vent line 140. Since it is an inert gas that is vented, the venting may be done without any problem into the environment.

[0043] According to one embodiment that may be combined with any other embodiment mentioned herein, the test line 100 may comprise sufficient inert gas P394008.W0.01

[0044] 8 to fill, at the predetermined pressure, the fluid line 330 between the first barrier valve 310, the second barrier valve 320, and the injection valve 210 of the injection line 200. The test line 100 may already comprising the inert gas before the test procedure starts. This provides a fast and controlled barrier test. The test line 100 may go all the way up to the sea surface 30, and preferably onto an offshore platform. Inert gas may be provided from a storage on the platform to the test line 100. The inert gas may be in the test line 100 before the test starts, for example the inert gas in the test line 100 may be pressurised at the predetermined pressure for making the barrier test and there may be sufficient inert gas in the test line 100 for filling the fluid line 330 at the predetermined pressure. The required volume may be fixed by the fluid line 330 between the first barrier valve 310, the second barrier valve 320, and the injection valve 210 of the injection line 200. The required volume may be fixed by the fluid line 330 between the first barrier valve 310, the second barrier valve 320, the injection valve 210 of the injection line 200, the test valve 110, and the vent valve 120.

[0045] The test line 100 may comprise an accumulator 130 with the inert gas to fill, at the predetermined pressure, the fluid line 330 between the first barrier valve 310, the second barrier valve 320, and the injection valve 210 of the injection line 200. As described in the previous paragraph, the accumulator 130 may have sufficient inert gas to fill the required volume of the fluid line 330 at the predetermined pressure.

[0046] The subsea system 300 may be a carbon dioxide injection tree. The subterrain reservoir 400 may be a carbon dioxide storage facility reservoir.

[0047] The subsea system 300 may be a Xmas tree injection system 300. The Xmas tree injection system 300 may be connected to a reservoir 400, for example to a well or to a tubing hanger. The injection line 200 may extend from the main injection supply to the Xmas tree injection system 300. The system may be a carbon storage injection system, for example for injection of carbon dioxide into a subsea well for storage. The Xmas tree injection system 300 may be an electric system, for example with no hydraulics.

[0048] According to one embodiment, the method and system may include controlling, for example opening and closing, the valves mentioned herein remotely and electrically. The method and system may include detecting, for example reading, the pressure monitoring devices mentioned herein remotely and electrically. This P394008.W0.01

[0049] 9 may provide an efficient method and system. A battery providing electric power may be installed at the subsea system 300.

[0050] The method as described herein may be used for testing with a predetermined low test pressure. The low test pressure may be higher than a pressure of the subsea environment 20 around the subsea system 300. Following the method as described above, the method may further comprise, after injecting 630 the inert gas to a predetermined pressure in the fluid line 330 between the first barrier valve 310 and the second barrier valve 320 using the test line 100, venting to the environment 20 the inert gas to the predetermined low test pressure, and monitoring 640 the predetermined low test pressure for a predetermined period of time and checking the at least one pressure monitoring device 380 to detect any pressure changes.

[0051] The method as described herein may be used for testing with a predetermined low test pressure. The low test pressure may be lower than a pressure of the subsea environment 20 around the subsea system 300. Following the method as described above, the method may further comprise after injecting 630 the inert gas to a predetermined pressure in the fluid line 330 between the first barrier valve 310 and the second barrier valve 320 using the test line 100, venting the inert gas to the predetermined low test pressure via the vent line 140 to an environmental 20 pressure that is lower than the predetermined low test pressure; and monitoring 640 the predetermined low test pressure for a predetermined period of time and checking the at least one pressure monitoring device 380 to detect any pressure changes. For example, by venting the inert gas to an environment at 1.01 kPa via the vent line 140 the method and system may perform barrier testing at pressures that is lower than the pressure surrounding the subsea system 300. The vent line 140 may go to a top surface at atmospheric pressure. For example, where the vent line 140 goes all the way up to a platform 500 at the sea surface 30 for venting to the environment. The platform 500 may be an onshore facility 500. The method may further comprises venting the inert gas and / or an injection gas through the vent line 140. Carbon dioxide could be flushed out via the vent line 140 to a storage on the platform 500 for subsequent injection into the reservoir 400. P394008.W0.01

[0052] 10

[0053] According to one embodiment, a system for barrier testing a subsea system 300 for carbon dioxide injection to a subterrain reservoir 400 with a main injection line 200 is disclosed. The system may best be taken from figure 1. The system comprises a fluid line 330, at least one pressure monitoring device 380, a main injection line 200, and a test line 100. The fluid line 330 is between a set of two or more barrier valves 310, 320, one of the barrier valves of the set of two barrier valves being a first barrier valve 310 closer to an environment 20 and the other barrier valve being a second barrier valve 320 closer to the reservoir 400. The at least one pressure monitoring device 380 is for monitoring pressure in the fluid line 330. The main injection line 200 is for injecting carbon dioxide to a first connection point 332 on the fluid line 330 between the first barrier valve 310 and the second barrier valve 320. The main injection line 200 comprises an injection valve 210. The test line 100 is for injecting an inert gas to the first connection point 332. The test line 100 comprises a pressurised inert gas in the test line 100. The test line 100 comprises a test valve 110 for allowing or preventing the inert gas to reach the fluid line 330. The system is configured to provide the inert gas at a predetermined pressure to the fluid line 330 between the first barrier valve 310, the second barrier valve 320, and the injection valve 210. As described herein the system can be used for barrier testing above and below the pressure that surrounds the subsea system 300. By using an inert gas venting can be done without any problems to the environment and the barrier testing can be done reliably since the inert gas does not change phase or react.

[0054] According to one embodiment, the vent line 140 may be used to pre-fill with a predetermined pressure of an inert gas for barrier testing. Pressure in the vent line 140, preferably when going to the surface 30, may be set to a second predetermined pressure for barrier testing, independent of the predetermined pressure set in the test line 100. This would allow as quick change of pressures used for barrier testing. For example, the predetermined pressure set in the test line 140 may be first used for barrier testing as described herein and the predetermined pressure set in the vent line 140 may then be used for barrier testing as described herein. For example, the test line 140 may be first used for flushing and the predetermined pressure set in the vent line 140 may then be used for barrier testing as described herein. P394008.W0.01

[0055] 11

[0056] According to one embodiment, the system may comprise a surface controlled subsea safety valve, SCSSV, 410, and the method may comprise barrier testing the SCSSV 410. This may be done as described herein and with the fluid line 330 being limited by barrier valve 310, injection valve 210, and test valve 110 and any vent valve 120. Any barrier valve 320 are then open and the SCSSV 410 is closed. For example, the fluid line 330 may be pressurised as described herein, or the fluid line 330 may be vented via the vent line 140. This allows for monitoring 640 any pressure changes, for example, via the pressure monitoring device 380, to verify the correct functionality of the SCSSV 410.

[0057] The test line 100 may further comprise a vent valve 120 on the test line 100, between the test valve 110 and the first connection point 332, for venting out to the environment 20. The system may further comprise a vent line 140 for venting the inert gas to the environment 20. The vent line 140 may be configured for venting to the environment 20, 40, or the sea surface 30, at atmospheric pressure. The test line 100 may extend from the subsea system 300 up to a platform or onshore facility 500. The test line 100 may comprise a pressure increasing unit 130 or an accumulator 130, or a combination of the two. The system may comprise a storage 150 for an inert gas. These embodiments may realise at least one embodiment of the methods as described herein.

[0058] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using the system and performing the methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements within substantial differences from the literal languages of the claims.

Claims

P394008.W0.0112Claims1 Method of barrier testing a subsea system (300) for carbon dioxide injection to a subterrain reservoir (400) with a main injection line (200), the system comprising: a fluid line (330) between a set of two or more barrier valves (310, 320), one of the barrier valves of the set of two barrier valves being a first barrier valve (310) closer to an environment (20) and the other barrier valve being a second barrier valve (320) closer to the reservoir (400); at least one pressure monitoring device (380) for monitoring pressure in the fluid line (330); the main injection line (200) configured for injecting carbon dioxide to a first connection point (332) on the fluid line (330) between the first barrier valve (310) and the second barrier valve (320), the main injection line (200) comprising an injection valve (210); and a test line (100) for injecting an inert gas to the first connection point (332), the test line (100) comprising the inert gas in the test line (100), the test line (100) comprising a test valve (110) for allowing or preventing the inert gas to reach the fluid line (330); the method comprising the following steps in any order: pressurising (610) the inert gas in the test line (100); opening (620) the test valve (110); flushing (630) out any fluid in the fluid line (330); closing (640) the downstream barrier valve (310), the upstream barrier valve (320), and the injection valve (210); opening (650) the test valve (110) and injecting the inert gas to a predetermined pressure in the fluid line (330) between the first barrier valve (310) and the second barrier valve (320) using the test line (100); and monitoring (660) the predetermined pressure for a predetermined period of time and checking the at least one pressure monitoring device (380) to detect any pressure changes.P394008.W0.01132 The method according to claim 1 , wherein the system further comprises a vent valve (120) on the test line (100), between the test valve (110) and the first connection point (332), for venting out to the environment (20); and the method further comprises venting the inert gas to the environment (20).3 The method according to claim 1 or 2, wherein the system further comprises a vent line (140) for venting the inert gas to the environment (20); and the method further comprises venting the inert gas through the vent line (140).4 The method according to claim 1 or 2, wherein the system further comprises a vent line (140) for venting to the environment (20), or the sea surface (30), at atmospheric pressure; and the method further comprises venting the inert gas through the vent line (140).5 The method according to any one of the preceding claims, wherein the test line (100) comprises sufficient inert gas to fill, at the predetermined pressure, the fluid line (330) between the first barrier valve (310), the second barrier valve (320), and the injection valve (210) of the injection line (200).6 The method according to any one of the preceding claims, wherein the test line (100) comprises an accumulator (130) with the inert gas to fill, at the predetermined pressure, the fluid line (330) between the first barrier valve (310), the second barrier valve (320), and the injection valve (210) of the injection line (200).7 The method according to any one of the preceding claims, wherein the subsea system (300) is a carbon dioxide injection tree, and the subterrain reservoir (400) is a carbon dioxide storage facility reservoir.8 The method according to any one of the preceding claims, for testing with a predetermined low test pressure higher than a pressure of the subsea environment (20) around the subsea system (300); after injecting (630) the inert gas to a predetermined pressure in the fluid line (330) between the first barrierP394008.W0.0114 valve (310) and the second barrier valve (320) using the test line (100), venting to the environment (20) the inert gas to the predetermined low test pressure; and monitoring (640) the predetermined low test pressure for a predetermined period of time and checking the at least one pressure monitoring device (380) to detect any pressure changes.9 The method according to any one of the preceding claims 1 to 7, for testing with a predetermined low test pressure lower than a pressure of the subsea environment (20) around the subsea system (300); after injecting (630) the inert gas to a predetermined pressure in the fluid line (330) between the first barrier valve (310) and the second barrier valve (320) using the test line (100), venting the inert gas to the predetermined low test pressure via the vent line (140) to an environmental (20) pressure that is lower than the predetermined low test pressure; and monitoring (640) the predetermined low test pressure for a predetermined period of time and checking the at least one pressure monitoring device (380) to detect any pressure changes.10 A system for barrier testing a subsea system (300) for carbon dioxide injection to a subterrain reservoir (400) with a main injection line (200), the system comprising a fluid line (330) between a set of two or more barrier valves (310, 320), one of the barrier valves of the set of two barrier valves being a first barrier valve (310) closer to an environment (20) and the other barrier valve being a second barrier valve (320) closer to the reservoir (400); at least one pressure monitoring device (380) for monitoring pressure in the fluid line (330); a main injection line (200) for injecting carbon dioxide to a first connection point (332) on the fluid line (330) between the first barrier valve (310) and the second barrier valve (320), the main injection line (200) comprising an injection valve (210); and a test line (100) for injecting an inert gas to the first connection point (332), the test line (100) comprising a pressurised inert gas in the test line (100),P394008.W0.0115 the test line (100) comprising a test valve (110) for allowing or preventing the inert gas to reach the fluid line (330); wherein the system is configured to provide the inert gas at a predetermined pressure to the fluid line (330) between the first barrier valve (310), the second barrier valve (320), and the injection valve (210).11 The system according to claim 10, wherein the test line (100) further comprises a vent valve (120) on the test line (100), between the test valve (110) and the first connection point (332), for venting out to the environment (20).12 The system according to claim 10 or 11 , further comprising a vent line (140) for venting the inert gas to the environment (20).13 The system according to claim 12, wherein the vent line (140) is configured for venting to the environment (20), or the sea surface (30), at atmospheric pressure.14 The system according to any one of the preceding claims 10 to 13, wherein the test line (100) comprising a pressure increasing unit (130) and / or an accumulator (130).15 The system according to any one of the preceding claims 10 to 14, further comprising a storage (150) for an inert gas.

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