Gas test method for marine liquefied carbon dioxide liquid cargo system

Abstract

The invention belongs to the field of liquefied gas ships, and particularly relates to a gas test method of a liquefied carbon dioxide liquid cargo system for a ship. Comprising the following steps: drying a cargo hold space; liquid cargo tank and pipeline drying; pressurizing a liquid cargo pipeline; liquid cargo tank gas replacement; pressurizing the liquid cargo tank and the ventilation system; a liquid cargo tank cold tank; loading in a liquid cargo tank; testing the liquid cargo pump; unloading the liquid cargo tank; sweeping the liquid cargo tank; discharging the loading pipeline; heating the liquid collecting well; heating and ventilating the liquid cargo tank. According to the invention, the gas test process of the marine liquefied carbon dioxide liquid cargo system is simplified, and the operation efficiency is improved; before the liquid cargo tank gas replacement, pressurization and tank cooling are carried out, all the liquid phase pipelines are pressurized through dry gas, the gas test of the marine liquefied carbon dioxide liquid cargo system is carried out under the semi-cooling and semi-pressure condition, and the efficiency and safety of gas test operation are improved.

Description

Technical Field

[0001] The invention belongs to the field of liquefied gas ships, and in particular relates to a gas testing method for a ship-based liquefied carbon dioxide liquid cargo system. Background Art

[0002] Gas testing is one of the key links in the construction process of liquefied gas carriers. Its purpose is to prove that the liquid cargo system equipment, instruments and other components can safely realize the functions of liquid cargo loading, processing, storage and unloading in accordance with the design requirements.

[0003] Liquefied gas carriers of the prior art, such as LNGC, etc., usually use a fully refrigerated (low temperature and normal pressure) system to store and transport liquid cargoes in consideration of transportation efficiency. Since the cargo it carries is flammable, the gas test procedure requires first drying and inerting the insulation layer and the isolated empty compartments, and then drying, inerting, gas replacement, cold compartments, filling and other operations of the large compartments. Finally, the fuel users such as LNG should be tested for the gas supply system, main engine, generator, boiler, etc. Unlike the above-mentioned media, since the triple point of carbon dioxide - the pressure and temperature when the three phases of solid, liquid and gas coexist in equilibrium - is 5.18barg / -56.6℃, it cannot be liquefied by lowering the temperature under normal pressure conditions. Therefore, it is impossible to use a fully refrigerated method to store and transport liquefied carbon dioxide in engineering; dry ice is easily generated during the gas test of the marine liquefied carbon dioxide liquid cargo system, and the gas test operation is unsafe; at the same time, the gas test method of flammable media such as LNG has many gas test processes and low operating efficiency, and is not entirely suitable for the gas test of liquefied carbon dioxide. Summary of the invention

[0004] The present invention aims to solve the problems that the liquefied gas transport ship in the prior art cannot liquefy carbon dioxide by lowering the temperature under normal pressure conditions, and thus cannot use a fully refrigerated method to store and transport liquefied carbon dioxide in engineering; the liquefied gas transport ship in the prior art has many gas test processes and low operating efficiency; dry ice is easily generated during the gas test, and the gas test operation is unsafe. A gas test method for a marine liquefied carbon dioxide liquid cargo system is provided, comprising the following steps: S1: Dry cargo space; The dry air is passed through the piping system to replace the air in the cargo hold space, reaching the humidity and pressure values ​​set for drying the cargo hold space; S2: Drying of cargo tanks and pipelines; First, use dry air to pressurize the cargo tank, and then use the pressure difference between the cargo tank and the pipeline system to inject dry air into the pipeline system connected to the cargo tank to replace the wet air therein until the dew point temperature setting value of the gas in the cargo tank and the pipeline is reached; S3: Liquid cargo pipeline pressurization; Close the gas dome valve of the liquid cargo tank and the connecting valves of the liquid phase pipeline and the gas phase pipeline, open the valve of the liquid phase pipeline in the liquid cargo system, and fill the liquid phase pipeline with dry nitrogen from the shore station through the deck nitrogen pipeline until the pressure setting value of the pressurized gas in the liquid cargo pipeline is reached; S4: cargo tank gas replacement; Replace the gas in the cargo tank with carbon dioxide; When using liquid carbon dioxide, it needs to be evaporated into gas in the cargo evaporator and then injected into the liquid cargo tank; when using gaseous carbon dioxide, it can be directly injected into the liquid cargo tank until the carbon dioxide concentration reaches the set value; S5: Pressurization of cargo tanks and venting systems; Stop returning gas to the shore station and continue to inject carbon dioxide vapor into the cargo tank until the cargo tank pressure increases to the set value; use the nitrogen system to pressurize part of the venting system pipeline connected to the cargo system to reach the set venting system pressure; S6: cargo tank cold tank; Receive supercooled liquefied carbon dioxide from the shore station, cool the cargo tank to the set temperature at a safe cooling rate according to the cargo tank material, and ensure that the cargo tank pressure is maintained within the cargo tank cold tank set value range; S7: Cargo tank loading; Receive the liquefied carbon dioxide required for gas testing from the shore station and inject it into the cargo tank. At the same time, send the carbon dioxide vapor replaced in the cargo tank back to the shore station. Maintain the cargo tank pressure within the cargo tank loading pressure setting range by controlling the return gas volume. S8: Liquid cargo pump test; The functionality of the cargo pumps is tested using liquefied carbon dioxide loaded in the cargo tanks; S9: cargo tank unloading; Use liquid cargo pumps to pump liquefied carbon dioxide in the cargo tank to the shore station, and receive carbon dioxide vapor returned from the shore station. Maintain the pressure in the cargo tank within the set value range for unloading by controlling the return gas volume; S10: cargo tank stripping; By utilizing the pressure difference between the receiving end and the liquefied carbon dioxide in the cargo tank, the remaining liquefied carbon dioxide in the cargo tank is sent to the shore station through the stripping pipeline, and the pressure in the cargo tank is maintained within the set value range by controlling the input of carbon dioxide vapor or the discharge of liquefied carbon dioxide; S11: Loading pipeline discharge; The pressure difference between the liquid cargo loading pipeline and the liquid cargo tank is used to discharge the residual liquefied carbon dioxide in the loading pipeline and the cargo evaporator back to the liquid cargo tank; S12: Liquid collecting well heating; The cargo compressor is used to generate carbon dioxide vapor, which is then injected back into the liquid collecting well of the cargo tank through the tank stripping pipeline to vaporize the residual liquefied carbon dioxide therein, and the pressure in the cargo tank is maintained within the set value range of the heating pressure of the liquid collecting well by controlling the supply of carbon dioxide vapor; S13: Cargo tank heating and ventilation; Carbon dioxide vapor is generated by the cargo compressor and injected back into the liquid cargo tank through the liquid cargo loading pipeline, so that the temperature of the liquid cargo tank is gradually increased to the set temperature for liquid cargo tank heating; air is injected into the liquid cargo tank using a nitrogen generator to replace the carbon dioxide vapor therein until the oxygen content set value for liquid cargo tank heating and ventilation is reached.

[0005] According to the above-mentioned gas test method for a marine liquefied carbon dioxide cargo system, it is characterized in that steps S1 to S3 are all carried out at very low pressure and temperature.

[0006] According to the above-mentioned gas test method for the marine liquefied carbon dioxide liquid cargo system, it is characterized in that: in step S1, when the cargo hold space pressure is 20~40mbarg, the air dew point temperature is ≤-10°C, and the gas supply is stopped.

[0007] According to the gas test method for the marine liquefied carbon dioxide liquid cargo system described above, it is characterized in that: in step S2, the liquid cargo tank and pipeline are dried and replaced with nitrogen; the pressure of the liquid cargo tank and pipeline is reduced to 0.5 barg, and the dew point temperature setting value of the gas in the liquid cargo tank and pipeline is ≤-40°C.

[0008] According to the gas test method for the marine liquefied carbon dioxide liquid cargo system described above, it is characterized in that: in step S3, the pressure setting value of the pressurized gas in the liquid cargo pipeline is 6.5~7.0 barg.

[0009] According to the above-mentioned gas test method for the marine liquefied carbon dioxide cargo system, it is characterized in that: in step S4: the carbon dioxide concentration setting value is ≥95%.

[0010] According to the above-mentioned gas test method for the marine liquefied carbon dioxide liquid cargo system, it is characterized in that: in step S5: the liquid cargo tank is pressurized to 11.5~12 barg; the ventilation system is pressurized to 6.5~7.0 barg.

[0011] According to the above-mentioned gas test method for the marine liquefied carbon dioxide liquid cargo system, it is characterized in that: in step S6: the average temperature in the liquid cargo tank finally reaches -24~-30℃.

[0012] According to the gas test method of the marine liquefied carbon dioxide cargo system described above, it is characterized in that: in step S12: the temperature rise rate of the liquid collecting well does not exceed 10°C / h.

[0013] According to the gas test method of the marine liquefied carbon dioxide liquid cargo system described above, it is characterized in that: in step S13: the temperature rise rate of the liquid cargo tank does not exceed 10°C / h; until the temperature of the liquid cargo tank rises to above 15°C, the heating operation is stopped.

[0014] The beneficial effects of the present invention are as follows: 1. After the cargo hold space is dried, the liquid cargo tank and the pipeline are dried, there is no need to inert the cargo hold space; the gas test process of the marine liquefied carbon dioxide liquid cargo system is simplified, and the gas test efficiency is improved.

[0015] 2. The present invention can be used not only for medium-pressure carbon dioxide liquid cargo systems, but also for low-pressure carbon dioxide liquid cargo systems. It is only necessary to adjust some parameter values ​​accordingly according to the system design pressure and temperature. The gas test method has high versatility.

[0016] 3. Before the gas replacement, pressurization and cooling of the cargo tank, the present invention first uses dry gas to pressurize all liquid phase pipelines to prevent the carbon dioxide from undergoing a phase change due to a sudden pressure drop during transmission, thereby achieving a gas test of the marine liquefied carbon dioxide cargo system under semi-cold and semi-pressure conditions and ensuring the safety of the gas test process.

[0017] 4. During the gas test, the present invention monitors the air humidity, pressure and temperature of the cargo hold space and controls the speed of change of pressure and temperature, which can effectively prevent the generation of dry ice due to drastic changes in pressure and temperature during the carbon dioxide gas test, thereby improving the safety of the operation.

[0018] 5. The present invention utilizes pressure difference to realize drying of liquid cargo tanks and connecting pipelines, gas replacement, tank sweeping, discharge and other operations, thereby improving operation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Flow chart of the gas testing method of the marine liquefied carbon dioxide cargo system of the present invention.

[0020] Figure 2 Schematic diagram of the ship's liquefied carbon dioxide cargo system.

[0021] In the figure: 1- cargo hold space, 2- nitrogen generator, 3- cargo hold space air pipeline, 4- liquid cargo tank, 5- liquid cargo tank degassing pipeline, 6- deck nitrogen pipeline, 7- liquid phase pipeline, 8- gas phase pipeline, 9- cargo evaporator, 10- cargo compressor, 11- liquid cargo pump, 12- small-diameter spray pipe in liquid cargo tank, 13- degassing pipe in liquid cargo tank, 14- loading pipe in liquid cargo tank, 15- metering system, 16- tank sweeping pipeline, 17- liquid collecting well, 18- seawater pipeline, 19- liquid cargo return pipe, 20- shore station. DETAILED DESCRIPTION

[0022] Preferred Embodiments The following is a detailed description of only one possible embodiment.

[0023] like Figure 1-2 As shown: The gas test method and steps of the marine liquefied carbon dioxide cargo system are as follows: Step 1: Drying of cargo space 1.

[0024] S1.1: Start the shipboard nitrogen generator 2, select the dry air mode, and inject dry air into the cargo hold through the cargo hold air pipeline 3 to replace the original air.

[0025] S1.2: Monitor the air humidity and pressure of cargo hold space 1. When the air dew point temperature reaches below -10℃ and the pressure reaches 20~40mbarg, stop the air supply. The slight positive pressure can prevent the external humid air from entering the cargo hold space. At the same time, considering the pressure bearing capacity of the hull structure, which is usually 76mbarg, the pressure is set at 20~40mbarg.

[0026] S1.3: To prevent moisture from remaining, measure the air humidity in cargo hold space 1 again after 1 hour. If the dew point temperature still meets the requirement of ≤-10°C, step S1 is completed.

[0027] Step 2: Drying of cargo tank 4 and pipelines.

[0028] S2.1: Start the shipboard nitrogen generator 2, select the nitrogen mode, first complete the drying of the small-diameter spray pipe 12 in the liquid cargo tank through the liquid cargo tank degassing pipeline 5 to prevent the small-diameter spray pipe 12 in the liquid cargo tank from freezing during the drying process of the liquid cargo tank 4; the return gas returns to the shore station 20 through the gas phase pipeline 8.

[0029] S2.2: Dry nitrogen is injected into the cargo tank 4 through the cargo tank degassing pipe 13 at the top of the cargo tank 4, and the return gas returns to the shore station 20 through the gas phase pipeline 8; until the dew point temperature of the gas in the cargo tank 4 reaches below -40°C and the pressure reaches 0.5 barg.

[0030] S2.3: Close the return air and continue to inject dry nitrogen into the cargo tank 4 until the pressure in the tank reaches 2.0~2.5barg. During the process, a pressure sensor is used to monitor the pressure in the cargo tank 4, and the dry nitrogen supply is adjusted to prevent overpressure in the cargo tank 4.

[0031] S2.4: Turn off the nitrogen supply, connect the pipeline that needs to be dried, and use the pressure difference between the liquid cargo tank 4 and the pipeline to fill the pipeline with dry nitrogen until the pressure of the liquid cargo tank 4 drops to about 0.5 barg.

[0032] S2.5: Repeat the above operation until the dew point of the gas in the pipeline reaches below -40°C.

[0033] S2.6: Switch to the next set of pipelines and repeat the above operation until all pipelines connected to the liquid cargo tank 4 have completed one drying operation.

[0034] S2.7: Repeat the above steps S2.3 to S2.6 at least 4 times to ensure that the cargo tank 4 and the pipeline are completely dry.

[0035] Step 3: Pressurize the liquid pipeline.

[0036] Since liquefied carbon dioxide needs to be stored under semi-cold and semi-pressure conditions, all liquid phase pipelines need to be pressurized with dry gas before gas replacement to prevent the carbon dioxide from undergoing phase change due to a sudden drop in pressure during transmission; Close the gas dome valve of the liquid cargo tank 4 and the connecting valves of the liquid phase pipeline 7 and the gas phase pipeline 8, open the valve of the liquid phase pipeline 7, and fill the liquid phase pipeline 7 with dry nitrogen from the shore station 20 through the deck nitrogen pipeline 6 until the gas pressure in the pipeline reaches 6.5~7.0barg.

[0037] Step 4: Gas replacement in cargo tank 4.

[0038] S4.1: Confirm that the seawater heating system is operating normally and adjust the seawater flow rate of cargo evaporator 9 to not less than 800 m 3 / h.

[0039] S4.2: Receive saturated liquefied carbon dioxide from the shore terminal 20 through the liquid pipeline 7 at a pressure of 12~25barg. When the pressure of the liquid pipeline 7 reaches 5.5~6.0barg, slowly open the inlet and outlet valves of the cargo evaporator 9 to gradually increase the temperature and pressure of the liquefied carbon dioxide flowing therethrough until the pressure reaches 15.5barg.

[0040] S4.3: Slowly open the valve of the loading pipe 14 in the cargo tank, inject carbon dioxide vapor into the cargo tank 4, and return the displaced nitrogen to the shore station 20 through the gas phase pipeline 8 until the carbon dioxide content in the cargo tank 4 reaches more than 95%. During this process, monitor the pressure in the cargo tank 4 to maintain it at 0.5~1.0barg.

[0041] Step 5: Pressurize cargo tanks and venting system.

[0042] S5.1: Stop returning gas to the shore station 20 and continue to receive liquefied carbon dioxide from the shore station 20, inject it into the liquid cargo tank 4 after passing through the cargo evaporator 9, until the pressure in the liquid cargo tank 4 reaches 11.5~12barg.

[0043] S5.2: Cut off the supply of liquefied carbon dioxide and evaporate or discharge the residual liquefied carbon dioxide in the cargo evaporator 9 and the liquid phase pipeline 7.

[0044] S5.3: Stop the seawater supply, drain the seawater in the seawater pipeline 18, and fill it with nitrogen after flushing with fresh water to prevent the seawater pipeline from rusting.

[0045] S5.4: Start the nitrogen system and pressurize the vent line connected to the liquid cargo return pipe 19 connected to the liquid cargo system to 6.5~7.0 barg.

[0046] S5.5: Use the carbon dioxide vapor pressure in the cargo tank 4 to complete the gas replacement of the cargo compressor 10 system pipeline so that its carbon dioxide concentration reaches more than 95%.

[0047] Step 6: Cargo tank cooling.

[0048] S6.1: Slowly receive the subcooled liquefied carbon dioxide from the shore station 20 through the liquid phase pipeline 7: the pressure is 25.0~26.0barg, the temperature is -18~-25℃, and inject it into the liquid cargo tank 4 to avoid damage to the liquid cargo pipeline and the liquid cargo tank 4. The return gas returns to the shore station 20 through the gas phase pipeline 8 of the header area.

[0049] S6.2: The pressure and temperature of the liquid carbon dioxide are controlled by controlling the flow rate of the liquid carbon dioxide entering the liquid cargo tank 4. The pressure is to be maintained at 11.5~12.0 barg, the temperature drop rate is not to exceed 10℃ / h, and the average temperature in the liquid cargo tank 4 is to reach -24~-30℃.

[0050] S6.3: Manually rotate the rotor of liquid cargo pump 11 2 turns for every 3°C drop in temperature, or manually rotate the rotor of liquid cargo pump 11 1 turn every hour to prevent the formation of dry ice.

[0051] Step 7: Cargo tank loading.

[0052] S7.1: The liquefied carbon dioxide required for the gas test is received from the shore station 20 through the liquid phase pipeline 7. The valves should be opened slowly in the order from the shore station 20 to the gas dome of the liquid cargo tank.

[0053] S7.2: Maintain the pressure of cargo tank 4 between 11.5 and 18.0 barg by controlling the return air volume.

[0054] S7.3: Monitor the amount of liquefied carbon dioxide received through the metering system 15 until it reaches a preset value of 350~385m³.

[0055] Step 8: Cargo pump test.

[0056] S8.1: Manually turn the rotor of liquid cargo pump 11 before starting.

[0057] S8.2: Use the liquid cargo pump 11 to transfer liquefied carbon dioxide between liquid cargo tanks 4 and confirm that the system pipelines and equipment are operating normally.

[0058] S8.3: Test the performance of the cargo pump in accordance with the cargo pump manufacturer’s requirements.

[0059] Step 9: Unloading of cargo tanks.

[0060] S9.1: Manually turn the rotor of liquid cargo pump 11 before starting.

[0061] S9.2: Start the liquid cargo pump 11 to transfer the liquefied carbon dioxide in the liquid cargo tank 4 to the shore station 20 through the liquid phase pipeline 7 until the metering system 15 shows that the liquid level height in the liquid cargo tank is 0m.

[0062] S9.3: Maintain the pressure of cargo tank 4 between 11.5 and 18.0 barg by controlling the return air volume.

[0063] Step 10: Cargo tank stripping.

[0064] S10.1: When the pressure in the cargo tank 4 is higher than the pressure at the receiving end of the shore station 20 by more than 5.0 barg, the pressure difference can be directly used to send the residual liquefied carbon dioxide in the cargo tank 4 to the shore station 20 via the stripping pipeline 16.

[0065] S10.2: When the pressure in the cargo tank 4 is less than 5.0 barg higher than the pressure at the receiving end of the shore station 20, it is necessary to use the cargo compressor 10 to extract carbon dioxide vapor from the cargo tank 4 and pressurize it, and then inject it back into the cargo tank 4 to increase the pressure of the cargo tank 4.

[0066] S10.3: Maintain the pressure in cargo tank 4 between 11.5 and 18.0 barg by controlling the input of carbon dioxide vapor or the discharge of liquefied carbon dioxide.

[0067] S10.4: When the sound emitted by the liquefied carbon dioxide flowing through the stripping pipeline 16 changes, it means that carbon dioxide vapor has been mixed in and the stripping operation can be stopped.

[0068] Step 11: Loading pipeline release.

[0069] S11.1: Start the cargo compressor 10, extract carbon dioxide vapor from the liquid cargo tank 4 and pressurize it to 3 barg higher than the pressure in the liquid cargo tank 4.

[0070] S11.2: Inject pressurized carbon dioxide vapor into the loading pipe 14 and the cargo evaporator 9 in the liquid cargo tank respectively.

[0071] S11.3: Utilize the pressure difference between the pipeline and the liquid cargo tank 4 to discharge the liquefied carbon dioxide remaining in the pipeline back into the liquid cargo tank via the stripping pipeline 16.

[0072] S11.4 The above steps shall be repeated at least twice to ensure that the loading pipe 14 in the cargo tank is completely discharged.

[0073] Step 12: Heating the liquid collecting well.

[0074] S12.1: Start the cargo compressor 10, extract carbon dioxide vapor from the liquid cargo tank 4, and inject it into the liquid collecting well 17 of the liquid cargo tank 4 through the tank stripping pipeline 16 after pressurization. During the process, the pressure of the liquid cargo tank is controlled to be maintained at 11.5~18.0barg.

[0075] S12.2: Control the steam supply to ensure that the temperature rise rate of the liquid collecting well 17 does not exceed 10°C / h. The operation can be stopped until the temperature of the liquid collecting well 17 rises rapidly in a short time.

[0076] Step 13: Cargo tank heating and ventilation.

[0077] S13.1: Start the cargo compressor 10, extract carbon dioxide vapor from the liquid cargo tank, and inject it into the liquid cargo tank 4 through the loading pipeline 14 in the liquid cargo tank after pressurization. During the process, the pressure of the liquid cargo tank 4 is controlled to be maintained at 11.5~18.0barg.

[0078] S13.2: Control the steam supply to ensure that the temperature rise rate of the cargo tank 4 does not exceed 10°C / h; stop the heating operation until the temperature of the cargo tank 4 rises to above 15°C.

[0079] S13.3: Release the pressure in cargo tank 4 to 0.5~1.5 barg.

[0080] S13.4: Start the nitrogen generator 2 and inject air into the cargo tank 4 to replace the carbon dioxide vapor therein until the oxygen content reaches more than 20%, allowing personnel to enter.

[0081] The present invention can be used not only for the above-mentioned medium-pressure carbon dioxide liquid cargo system with a pressure of 19 barg and a temperature of -35°C, but also for the low-pressure carbon dioxide liquid cargo system with a pressure of 8 barg and a temperature of -55°C. It is only necessary to adjust some parameter values ​​accordingly according to the system design pressure and temperature.

Claims

1. A gas test method for a marine liquefied carbon dioxide cargo system, characterized in that: The following steps are involved: S1: Dry cargo space; The dry air is passed through the piping system to replace the air in the cargo hold space, reaching the humidity and pressure values ​​set for drying the cargo hold space; S2: Drying of cargo tanks and pipelines; First, use dry air to pressurize the cargo tank, and then use the pressure difference between the cargo tank and the pipeline system to inject dry air into the pipeline system connected to the cargo tank to replace the wet air therein until the dew point temperature setting value of the gas in the cargo tank and the pipeline is reached; S3: Liquid cargo pipeline pressurization; Close the gas dome valve of the liquid cargo tank and the connecting valves of the liquid phase pipeline and the gas phase pipeline, open the valve of the liquid phase pipeline in the liquid cargo system, and fill the liquid phase pipeline with dry nitrogen from the shore station through the deck nitrogen pipeline until the pressure setting value of the pressurized gas in the liquid cargo pipeline is reached; S4: cargo tank gas replacement; Replace the gas in the cargo tank with carbon dioxide; When using liquid carbon dioxide, it needs to be evaporated into gas in the cargo evaporator and then injected into the liquid cargo tank; when using gaseous carbon dioxide, it can be directly injected into the liquid cargo tank until the carbon dioxide concentration reaches the set value; S5: Pressurization of cargo tanks and venting systems; Stop returning gas to the shore station and continue to inject carbon dioxide vapor into the cargo tank until the cargo tank pressure increases to the set value; use the nitrogen system to pressurize part of the venting system pipeline connected to the cargo system to reach the set venting system pressure; S6: cargo tank cold tank; Receive supercooled liquefied carbon dioxide from the shore station, cool the cargo tank to the set temperature at a safe cooling rate according to the cargo tank material, and ensure that the cargo tank pressure is maintained within the cargo tank cold tank set value range; S7: Cargo tank loading; Receive the liquefied carbon dioxide required for gas testing from the shore station and inject it into the cargo tank. At the same time, send the carbon dioxide vapor replaced in the cargo tank back to the shore station. Maintain the cargo tank pressure within the cargo tank loading pressure setting range by controlling the return gas volume. S8: Liquid cargo pump test; The functionality of the cargo pumps is tested using liquefied carbon dioxide loaded in the cargo tanks; S9: cargo tank unloading; Use liquid cargo pumps to pump liquefied carbon dioxide in the cargo tank to the shore station, and receive carbon dioxide vapor returned from the shore station. Maintain the pressure in the cargo tank within the set value range for unloading by controlling the return gas volume; S10: cargo tank stripping; By utilizing the pressure difference between the receiving end and the liquefied carbon dioxide in the cargo tank, the remaining liquefied carbon dioxide in the cargo tank is sent to the shore station through the stripping pipeline, and the pressure in the cargo tank is maintained within the set value range by controlling the input of carbon dioxide vapor or the discharge of liquefied carbon dioxide; S11: Loading pipeline discharge; The pressure difference between the liquid cargo loading pipeline and the liquid cargo tank is used to discharge the residual liquefied carbon dioxide in the loading pipeline and the cargo evaporator back to the liquid cargo tank; S12: Liquid collecting well heating; Generate carbon dioxide using cargo compressors Generate carbon dioxide vapor using cargo compressors, and inject it back into the liquid collecting well of the cargo tank through the tank stripping pipeline to vaporize the residual liquefied carbon dioxide therein, and maintain the pressure in the cargo tank within the set value range of the heating pressure of the liquid collecting well by controlling the supply of carbon dioxide vapor; S13: Cargo tank heating and ventilation; Carbon dioxide vapor is generated by the cargo compressor and injected back into the liquid cargo tank through the liquid cargo loading pipeline, so that the temperature of the liquid cargo tank is gradually increased to the set temperature for liquid cargo tank heating; air is injected into the liquid cargo tank using the shipboard air compressor to replace the carbon dioxide vapor therein until the oxygen content set value for liquid cargo tank heating and ventilation is reached.

2. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 1, characterized in that: Steps S1 to S3 are all performed under very low pressure and temperature.

3. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 2, characterized in that: In step S1, when the cargo hold space pressure is 20-40mbarg and the air dew point temperature is ≤-10°C, the air supply is stopped.

4. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 3, characterized in that: In step S2, the cargo tank and pipeline are dried and replaced with nitrogen; the pressure of the cargo tank and pipeline is reduced to 0.5 barg, and the dew point temperature of the gas in the cargo tank and pipeline is set to ≤-40°C.

5. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 4, characterized in that: In step S3, the pressure setting value of the boosting gas in the liquid cargo pipeline is 6.5-7.0 barg.

6. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 1 or 5, characterized in that: In step S4: the carbon dioxide concentration is set to ≥95%.

7. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 6, characterized in that: In step S5: the cargo tank is pressurized to 11.5-12 barg; the ventilation system is pressurized to 6.5-7.0 barg.

8. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 7, characterized in that: In step S6: the average temperature in the cargo tank eventually reaches -24~-30℃.

9. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 8, characterized in that: In step S12: the temperature rise rate of the liquid collection well does not exceed 10°C / h.

10. The gas test method for a marine liquefied carbon dioxide cargo system according to claim 9, characterized in that: In step S13: the temperature rise rate of the liquid cargo tank does not exceed 10°C / h; the heating operation is stopped until the temperature of the liquid cargo tank rises to above 15°C.

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