Ship

Abstract

A ship (10) is provided with: cargo tanks (2) that interchangeably store two types of liquefied gases, namely first liquefied gas and second liquefied gas; a first liquefied gas tank (4) that is provided separately from the cargo tanks (2) and that stores the first liquefied gas; a second liquefied gas tank (5) that is provided separately from the cargo tanks (2) and that stores the second liquefied gas; a mixed gas tank (3) that stores a mixed gas in which gaseous first gas of the first liquefied gas and gaseous second gas of the second liquefied gas are mixed; and a plurality of valves (11 to 13) that are provided such that gas discharged from the cargo tanks (2) is transferred to any one of the first liquefied gas tank (4), the second liquefied gas tank (5), and the mixed gas tank (3).

Description

Ship

[0001] The present invention relates to a ship for transporting liquefied gas.

[0002] Generally, ships for transporting LNG (liquefied natural gas) or LPG (liquefied petroleum gas) are known. For example, an LNG carrier for suppressing the release of carbon dioxide into the atmosphere is disclosed (see Patent Document 1).

[0003] However, in order to replace two types of liquefied gases such as LNG and liquefied carbon dioxide in one tank, the gas remaining in the tank before replacement must be released from the tank. Therefore, every time two types of liquefied gases are replaced in the tank, the gas released from the tank will be discarded.

[0004] International Publication No. 2022 / 250078

[0005] An object of an embodiment of the present invention is to provide a ship that efficiently replaces and transports two types of liquefied gases.

[0006] The ship according to the aspect of the present invention includes a cargo tank for replacing and storing two types of liquefied gases, a first liquefied gas tank provided separately from the cargo tank for storing the first liquefied gas, a second liquefied gas tank provided separately from the cargo tank for storing the second liquefied gas, a mixed gas tank for storing a mixed gas in which a gaseous first gas of the first liquefied gas and a gaseous second gas of the second liquefied gas are mixed, and a plurality of valves provided so that the gas discharged from the cargo tank is transferred to any one of the first liquefied gas tank, the second liquefied gas tank, or the mixed gas tank.

[0007] Figure 1 is a configuration diagram showing the configuration of a liquefied gas transport vessel according to an embodiment of the present invention. Figure 2 is a schematic diagram showing the state transition when methane gas is replaced with carbon dioxide gas in a cargo tank according to this embodiment. Figure 3 is a flow diagram showing the procedure for replacing methane gas with carbon dioxide gas in a cargo tank according to this embodiment. Figure 4 is a schematic diagram showing the state transition when carbon dioxide gas is replaced with methane gas in a cargo tank according to this embodiment. Figure 5 is a flow diagram showing the procedure for replacing carbon dioxide gas with methane gas in a cargo tank according to this embodiment.

[0008] (Embodiment) Figure 1 is a configuration diagram showing the structure of a liquefied gas transport vessel 10 according to an embodiment of the present invention. The same parts in the drawing are denoted by the same reference numerals, and redundant explanations are omitted as appropriate.

[0009] The liquefied gas carrier 10 is a vessel that transports two different gases by alternating between them. For example, when departing for its destination, the liquefied gas carrier 10 carries liquefied carbon dioxide. When returning from its destination, the liquefied gas carrier 10 carries LNG. Therefore, at the port of departure, LNG is transferred to liquefied carbon dioxide, and at the destination port, liquefied carbon dioxide is transferred back to LNG.

[0010] Note that this explanation describes the case of swapping LNG with liquefied carbon dioxide, but you can also swap LPG with liquefied carbon dioxide, or any other two different gases.

[0011] The liquefied gas carrier 10 comprises a hull 1, multiple cargo tanks 2, a mixed gas tank 3, an LNG tank 4, a carbon dioxide tank 5, a gas compressor 6, an LNG liquefaction unit 7, a carbon dioxide liquefaction unit 8, and three valves 11, 12, and 13.

[0012] The hull 1 is the skeletal part that forms the outer shape of the ship. A deck DK is provided on the upper surface of the hull 1. In Figure 1, the hull 1 is shown with the bow FR on the right side and the stern AF on the left side, but the hull 1 can have any shape.

[0013] The cargo tank 2 is located inside the hull 1 (below the deck DK). There may be one or more cargo tanks 2. The cargo tank 2 can be of any type or shape as long as it can store two types of liquefied gases (for example, LNG and carbon dioxide) in a suitable condition. For example, the cargo tank 2 may be monolobe-type (cylinder-type) or bilobe-type.

[0014] An upper sensor 21 is provided in the upper part of the interior of the cargo tank 2. A lower sensor 22 is provided in the lower part of the interior of the cargo tank 2. For example, the upper part of the interior of the cargo tank 2 is the uppermost part above half or one-third of the interior height of the cargo tank 2. The lower part of the interior of the cargo tank 2 is the lowermost part below half or one-third of the interior height of the cargo tank 2. Note that any number of sensors 21 and 22 may be provided in the cargo tank 2.

[0015] For example, sensors 21 and 22 detect the type and concentration of a gas. Each sensor 21 and 22 may be a combination of multiple sensors that detect different types of gases separately. The types of gases to be detected are methane or carbon dioxide. Assuming that no gases other than methane and carbon dioxide exist inside the cargo tank 2, measuring the concentration of either methane or carbon dioxide will allow us to determine the concentration of the other gas. Therefore, each sensor 21 and 22 only needs to be able to detect at least one of the two types of gases.

[0016] The cargo tank 2 is provided with an injection port for injecting gas into the interior and a discharge port for discharging gas from the interior to the exterior. The cargo tank 2 may have any number of injection ports and discharge ports. Furthermore, the injection port and discharge port may be separate, or there may be a single port that serves both injection and discharge purposes. Figure 1 shows an upper port P1 located at the top of the interior of the cargo tank 2 and a lower port P2 located at the bottom of the interior of the cargo tank 2. The upper port P1 and lower port P2 represent either an injection port or a discharge port, or both.

[0017] The mixed gas tank 3 is a tank for storing the mixed gas generated when the liquefied gas in the cargo tank 2 is replaced. For example, when the contents of cargo tank 2 are replaced with LNG and liquefied carbon dioxide, a mixed gas of methane and carbon dioxide is generated. Therefore, the mixed gas tank 3 stores this mixed gas of methane and carbon dioxide.

[0018] LNG tank 4 is a tank for storing LNG. When the contents of cargo tank 2 are replaced from LNG to liquefied carbon dioxide, methane gas is discharged from cargo tank 2. The discharged methane is liquefied into LNG and stored in LNG tank 4. If the liquefied gas transport ship 10 is equipped with a main engine that uses LNG as fuel, LNG tank 4 may also be a fuel tank. If one of the liquefied gases transported in cargo tank 2 is not LNG but another liquefied gas (such as LPG), a tank for storing that liquefied gas will be provided instead of LNG tank 4.

[0019] The carbon dioxide tank 5 is a tank for storing carbon dioxide. When the contents of the cargo tank 2 are replaced from liquefied carbon dioxide to LNG, carbon dioxide (gaseous carbon dioxide) is discharged from the cargo tank 2. The discharged carbon dioxide is liquefied and stored in the carbon dioxide tank 5. For example, the inside of the carbon dioxide tank 5 is maintained at 8 atmospheres, and the liquefied carbon dioxide is managed under these conditions. Any carbon dioxide generated on the liquefied gas transport ship 10 may be liquefied and stored in the carbon dioxide tank 5. The carbon dioxide tank 5 may be configured to be removable from the ship's hull 1 so that it can be unloaded. If one of the liquefied gases transported in the cargo tank 2 is not carbon dioxide (liquefied carbon dioxide) but another liquefied gas, a tank for storing that liquefied gas will be provided instead of the carbon dioxide tank 5.

[0020] The mixed gas tank 3, LNG tank 4, and carbon dioxide tank 5 are self-supporting tanks, each mounted on a deck DK or structure above the hull 1. Any number of these three tanks 3-5 can be provided, as long as there is at least one of each. Furthermore, some or all of these three tanks 3-5 may be located anywhere on the liquefied gas carrier 10, including inside the hull 1.

[0021] The gas compressor 6 is a device that compresses the gas discharged from the cargo tank 2. A separate gas compressor 6 may be provided for each type of gas to be compressed. Furthermore, the gas compressor 6 may be omitted or provided as part of other equipment.

[0022] The LNG liquefaction unit 7 is a device that liquefies methane gas into LNG in order to store it in the LNG tank 4. The LNG liquefaction unit 7 cools and liquefies the methane gas compressed by the gas compressor 6. The LNG liquefaction unit 7 may also be equipped with a gas compression function equivalent to that of the gas compressor 6. The liquefaction of methane gas by the LNG liquefaction unit 7 may be performed not only when the liquefied gas carrier 10 is anchored in a port, etc., but also while it is underway.

[0023] The carbon dioxide liquefaction unit 8 is a device that liquefies carbon dioxide in order to store it in the carbon dioxide tank 5. The carbon dioxide liquefaction unit 8 cools and liquefies the carbon dioxide compressed by the gas compressor 6. The carbon dioxide liquefaction unit 8 may also be equipped with a gas compression function equivalent to that of the gas compressor 6. The carbon dioxide liquefaction unit 8 may also be used in conjunction with the LNG liquefaction unit 7. The liquefaction of carbon dioxide by the carbon dioxide liquefaction unit 8 may be performed not only when the liquefied gas carrier 10 is anchored in a port or the like, but also while it is underway.

[0024] Valves 11-13 are installed in the path (piping, etc.) through which gas is sent from the cargo tank 2 to the mixed gas tank 3, LNG tank 4, and carbon dioxide tank 5. Valves 11-13 are opened and closed according to the type of gas discharged from the cargo tank 2. When the first valve 11 is opened, the gas discharged from the cargo tank 2 is transferred to the mixed gas tank 3. When the second valve 12 is opened, the gas discharged from the cargo tank 2 is transferred to the LNG tank 4. When the third valve 13 is opened, the gas discharged from the cargo tank 2 is transferred to the carbon dioxide tank 5.

[0025] Furthermore, the valves 11-13 may be equipped with an interlocking function so that only one valve is opened and the other two valves are closed, so that the gas discharged from cargo tank 2 is transferred to only one of the three tanks 3-5. In addition, the valves 11-13 may be control valves whose opening and closing operations are controlled by a computer, or they may be opened and closed manually by an operator. As long as the system is configured so that the gas discharged from cargo tank 2 is transferred to one of the three tanks 3-5, there may be any number of valves and piping and other equipment that form the gas path, and they may be configured in any way.

[0026] The liquefied gas transport vessel 10 may be equipped with a control device that controls any equipment installed on the liquefied gas transport vessel 10 using a computer. The control device is not limited to one device, but may consist of multiple devices. Hereafter, the control device will be interpreted similarly.

[0027] Figure 2 is a schematic diagram showing the transitions between states ST11, ST12, ST13, and ST14 in the cargo tank 2 according to this embodiment, where methane gas is replaced with carbon dioxide gas. Figure 3 is a flowchart showing the procedure for replacing methane gas with carbon dioxide gas in the cargo tank 2 according to this embodiment. Note that in Figure 2, the shape of the cargo tank 2 is simply represented by a circle, but the actual shape of the cargo tank 2 can be any shape.

[0028] Referring to Figures 2 and 3, we will now describe the process of replacing the contents of cargo tank 2 from LNG to liquefied carbon dioxide. Here, all operations are described as being performed by a control device, but some or all of the operations may be performed manually by workers.

[0029] First, when all the LNG is discharged from cargo tank 2, methane gas remains in cargo tank 2. Carbon dioxide gas is injected into cargo tank 2, which is filled with methane gas, to replace the methane gas with carbon dioxide gas. Once cargo tank 2 is filled with carbon dioxide gas, the injection of liquefied carbon dioxide gas into cargo tank 2 is started. The carbon dioxide gas to be injected may be supplied from outside the liquefied gas transport ship 10, or it may be carbon dioxide gas that has been forcibly vaporized in the liquefied carbon dioxide gas tank 5 of the liquefied gas transport ship 10.

[0030] Next, the procedure for changing the cargo tank 2 from a state ST11 where it is filled with methane gas to a state ST14 where it is filled with carbon dioxide gas will be explained. Before carrying out this procedure, the valve 12 provided in the route (piping, etc.) from cargo tank 2 to LNG tank 4 is opened, and the valve 11 provided in the route (piping, etc.) from cargo tank 2 to mixed gas tank 3 and the valve 13 provided in the route (piping, etc.) from cargo tank 2 to carbon dioxide tank 5 are closed. That is, the gas discharged from cargo tank 2 is transferred to LNG tank 4 via the LNG liquefaction unit 7.

[0031] The control device starts injecting carbon dioxide into the cargo tank 2, which is filled with methane gas (step S101). The carbon dioxide is injected through the lower hole P2 of the cargo tank 2. Since carbon dioxide is denser than methane gas, it accumulates at the bottom of the cargo tank 2. By injecting carbon dioxide from the bottom of the cargo tank 2, the methane gas and carbon dioxide can be kept separated vertically inside the cargo tank 2. As a result, when the gas is discharged from the upper hole P1 of the cargo tank 2, only methane gas is discharged without any carbon dioxide mixing in.

[0032] The methane gas discharged from cargo tank 2 is converted into LNG by the LNG liquefaction unit 7 and transferred to LNG tank 4 (step S102). As a result, the methane gas in cargo tank 2 is stored in LNG tank 4 as LNG. This state continues until carbon dioxide is detected at the top of cargo tank 2 (NO, state ST12 of step S103).

[0033] As carbon dioxide is continuously injected into cargo tank 2, methane gas is discharged from the top, and carbon dioxide accumulates from the bottom of cargo tank 2. As a result, carbon dioxide gradually mixes with the methane gas at the top of cargo tank 2. When carbon dioxide is detected by the upper sensor 21 located at the top of cargo tank 2, the system switches from valve 12 to valve 11 (YES in step S103, step S104). For example, the control device closes valve 12 and opens valve 11. Whether or not carbon dioxide has been detected may also be determined by the concentration of carbon dioxide. For example, if the concentration of carbon dioxide exceeds a threshold, the control device determines that carbon dioxide has been detected.

[0034] After switching valves 11 and 12, the gas discharged from cargo tank 2 is treated as a mixture of methane and carbon dioxide. The mixed gas discharged from cargo tank 2 is transferred to the mixed gas tank 3 via valve 11 (step S105). As a result, the concentration of methane gas inside cargo tank 2 gradually decreases. This state continues until the concentration of methane gas at the top of cargo tank 2 falls below a predetermined value (NO. of step S106, state ST13).

[0035] When the methane gas concentration detected by the upper sensor 21 falls below a predetermined value, the injection of carbon dioxide into the cargo tank 2 is stopped (YES in step S106, step S107, state ST14). The state in which the methane gas concentration falls below a predetermined value is a state in which all the methane gas has been discharged from inside the cargo tank 2 and the inside of the cargo tank 2 can be considered to be filled with carbon dioxide. For example, the predetermined value is 0.5%, 1%, or 2%. The predetermined value may be a set value that can be arbitrarily changed by the worker. Note that the stopping of carbon dioxide injection may also be determined by the carbon dioxide concentration.

[0036] In this way, the cargo tank 2 is filled with carbon dioxide gas, ST14, and the injection of liquefied carbon dioxide gas into the cargo tank 2 is started.

[0037] Figure 4 is a schematic diagram showing the transitions between states ST21, ST22, ST23, and ST24 in which carbon dioxide is replaced with methane gas in the cargo tank 2 according to this embodiment. Figure 5 is a flowchart showing the procedure for replacing carbon dioxide with methane gas in the cargo tank 2 according to this embodiment. Note that in Figure 4, the shape of the cargo tank 2 is simply represented by a circle, but the actual shape of the cargo tank 2 can be any shape.

[0038] Referring to Figures 4 and 5, we will now describe the process of replacing the contents of cargo tank 2 from liquefied carbon dioxide to LNG. Here, all operations are described as being performed by a control device, but some or all of the operations may be performed manually by workers.

[0039] First, all the liquefied carbon dioxide is discharged from cargo tank 2, leaving carbon dioxide in the cargo tank 2. Methane gas is then injected into cargo tank 2, which is now filled with carbon dioxide, to replace the carbon dioxide with methane gas. Once cargo tank 2 is filled with methane gas, the injection of LNG into cargo tank 2 is started. The methane gas to be injected may be supplied from outside the liquefied gas carrier 10, or it may be obtained by forcibly vaporizing the LNG in the LNG tank 4 of the liquefied gas carrier 10.

[0040] Next, the procedure for changing the cargo tank 2 from a state ST21 where it is filled with carbon dioxide to a state ST24 where it is filled with methane gas will be explained. Before carrying out this procedure, the valve 13 provided in the route (piping, etc.) from the cargo tank 2 to the carbon dioxide tank 5 is opened, and the valve 11 provided in the route (piping, etc.) from the cargo tank 2 to the mixed gas tank 3 and the valve 12 provided in the route (piping, etc.) from the cargo tank 2 to the LNG tank 4 are closed. That is, the gas discharged from the cargo tank 2 is transferred to the carbon dioxide tank 5 via the carbon dioxide liquefaction device 8.

[0041] The control device starts injecting methane gas into the cargo tank 2 filled with carbon dioxide gas (step S201). The methane gas is injected from the upper hole P1 of the cargo tank 2. Since the specific gravity of methane gas is lighter than that of carbon dioxide gas, the methane gas accumulates at the upper part of the cargo tank 2. By injecting methane gas from the upper part of the cargo tank 2, a state where the methane gas and the carbon dioxide gas are separated vertically inside the cargo tank 2 can be maintained. Thereby, by discharging the gas from the lower hole P2 of the cargo tank 2, only the carbon dioxide gas is discharged without the methane gas being mixed in.

[0042] The carbon dioxide gas discharged from the cargo tank 2 is liquefied into liquefied carbon dioxide gas by the carbon dioxide liquefaction device 8 and transferred to the carbon dioxide tank 5 (step S202). Thereby, the carbon dioxide gas in the cargo tank 2 is liquefied and stored in the carbon dioxide tank 5. This state is continued until methane gas is detected at the lower part of the cargo tank 2 (NO in step S203, state ST22).

[0043] If the injection of methane gas into the cargo tank 2 continues, the carbon dioxide gas is discharged from the lower part, and the methane gas accumulates from the upper side of the cargo tank 2. Thereby, the methane gas is gradually mixed into the carbon dioxide gas at the lower part of the cargo tank 2. When methane gas is detected by the lower side sensor 22 provided at the lower part of the cargo tank 2, the valves are switched from valve 13 to valve 11 (YES in step S203, step S204). For example, the control device closes valve 13 and opens valve 11. Whether methane gas is detected or not may be determined by the concentration of methane gas. For example, when the concentration of methane gas becomes greater than the threshold value, the control device determines that methane gas has been detected.

[0044] After the switching of valves 11 and 13, the gas discharged from the cargo tank 2 is treated as a mixed gas of methane gas and carbon dioxide gas. The mixed gas discharged from the cargo tank 2 is transferred to the mixed gas tank 3 through valve 11 (step S205). Thereby, the concentration of the carbon dioxide gas inside the cargo tank 2 gradually decreases. This state is continued until the concentration of the carbon dioxide gas becomes less than the predetermined value at the lower part of the cargo tank 2 (NO in step S206, state ST23).

[0045] When the concentration of carbon dioxide gas detected by the lower sensor 22 becomes less than a predetermined value, the injection of methane gas into the cargo tank 2 is stopped (YES in step S206, step S207, state ST24). The state where the concentration of carbon dioxide gas becomes less than the predetermined value is a state where all the carbon dioxide gas has been discharged from the inside of the cargo tank 2 and the inside of the cargo tank 2 can be regarded as being filled with methane gas. For example, the predetermined value is 0.5%, 1%, or 2%. The predetermined value may be a set value that can be arbitrarily changed by an operator. Note that the stop of the injection of methane gas may be determined based on the concentration of methane gas.

[0046] In this way, the inside of the cargo tank 2 is filled with methane gas to the state ST24, and the injection of LNG into the cargo tank 2 is started.

[0047] Next, the mixed gas tank 3, the LNG tank 4, and the carbon dioxide tank 5 will be further described.

[0048] The capacity of each of the tanks 3-5 may be determined in any way, and here, an example will be described. The capacity of each of the tanks 3-5 is determined based on the total capacity of all the cargo tanks 2. Each of the tanks 3-5 needs to have a capacity necessary for the gas replacement of the cargo tank 2 described above. Also, when the LNG tank 4 is used as a fuel tank, the capacity of the LNG tank 4 is determined based on the specifications (horsepower, etc.) of the main engine of the liquefied gas carrier 10. When storing the carbon dioxide generated by the main engine in the carbon dioxide tank 5, the capacity of the carbon dioxide tank 5 is also determined based on the specifications (horsepower, etc.) of the main engine of the liquefied gas carrier 10. Considering these, in the order of the carbon dioxide tank 5, the LNG tank 4, and the mixed gas tank 3, the capacity often becomes larger.

[0049] The mixed gas stored in the mixed gas tank 3 separates into methane gas and carbon dioxide gas inside the mixed gas tank 3 according to the specific gravity. Therefore, either one of the methane gas or the carbon dioxide gas can be easily extracted from the mixed gas tank 3. The method of extracting either one of the gases from the mixed gas tank 3 may be performed in any way.

[0050] The carbon dioxide in the mixed gas tank 3 may be liquefied during navigation to lower the internal pressure of the mixed gas tank 3. The carbon dioxide extracted from the mixed gas tank 3 may be liquefied by a carbon dioxide liquefaction device 8 and stored in the carbon dioxide tank 5. The methane gas extracted from the mixed gas tank 3 may be liquefied by an LNG liquefaction device 7 and stored in the LNG tank 4, or it may be used as fuel by the main engine or the like.

[0051] To liquefy either methane gas or carbon dioxide gas in the mixed gas tank 3, at least one liquefaction device from the LNG liquefaction device 7 or the carbon dioxide liquefaction device 8 may be used, or a separate liquefaction device may be provided in addition to the LNG liquefaction device 7 and the carbon dioxide liquefaction device 8.

[0052] The LNG stored in the LNG tank 4 may be used as fuel for any equipment such as the main engine in the liquefied gas carrier 10, or it may be used to cool the inside of the cargo tank 2, or it may be used as methane gas to discharge carbon dioxide for the aforementioned gas exchange in the cargo tank 2. Also, when the contents of the cargo tank 2 are LNG, the LNG stored in the LNG tank 4 may be transferred to the cargo tank 2.

[0053] The liquefied carbon dioxide stored in the carbon dioxide tank 5 may be unloaded together with the carbon dioxide tank 5 and disposed of, or it may be used as carbon dioxide to discharge methane gas for the aforementioned gas exchange in the cargo tank 2. Alternatively, when the cargo tank 2 contains liquefied carbon dioxide, the liquefied carbon dioxide stored in the carbon dioxide tank 5 may be transferred to the cargo tank 2.

[0054] According to this embodiment, in a liquefied gas transport ship 10 that transports two types of liquefied gases, in addition to the cargo tank 2, tanks 4 and 5 for storing the two types of liquefied gases separately and a mixed gas tank 3 for storing a mixed gas of the two types of gases are provided, thereby enabling efficient exchange and transport of the two types of liquefied gases.

[0055] For example, if one of the liquefied gases being transported is liquefied carbon dioxide, the release of carbon dioxide into the atmosphere can be reduced. If one of the liquefied gases being transported is a fuel gas such as LNG or LPG, the amount of fuel gas that is disposed of can be reduced.

[0056] Furthermore, additional advantages and modifications may readily arise for those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific detailed and representative embodiments described herein. Accordingly, various modifications can be made without departing from the spirit or scope of the general concept of the invention as defined by the appended claims and their equivalents.

Claims

1. A ship characterized by comprising: a cargo tank for exchanging and storing two types of liquefied gases, a first liquefied gas and a second liquefied gas; a first liquefied gas tank provided separately from the cargo tank for storing the first liquefied gas; a second liquefied gas tank provided separately from the cargo tank for storing the second liquefied gas; a mixed gas tank for storing a mixed gas obtained by mixing the gaseous first gas of the first liquefied gas and the gaseous second gas of the second liquefied gas; and a plurality of valves provided to transfer the gas discharged from the cargo tank to any one of the first liquefied gas tank, the second liquefied gas tank, or the mixed gas tank.

2. The vessel according to claim 1, characterized in that the mixed gas tank stores the mixed gas generated when the first gas is replaced with the second gas and when the second gas is replaced with the first gas.

3. The vessel according to claim 1, further comprising: an upper sensor provided in the upper part of the interior of the cargo tank for detecting at least one of the first gas or the second gas; and a lower sensor provided in the lower part of the interior of the cargo tank for detecting at least one of the first gas or the second gas.

4. The vessel according to claim 3, further comprising a control device that controls the opening and closing of the plurality of valves based on the upper sensor or the lower sensor to transfer the gas discharged from the cargo tank to the mixed gas tank.

5. The vessel according to claim 1, characterized in that it is equipped with a liquefaction device for liquefying at least one of the first gas or the second gas discharged from the cargo tank.

6. The vessel according to claim 1, characterized in that the cargo tank is located inside the hull, and the first liquefied gas tank, the second liquefied gas tank, and the mixed gas tank are self-supporting tanks located above the hull.

7. The ship according to claim 1, characterized in that the first liquefied gas stored in the first liquefied gas tank is used as fuel for the main engine.

8. The vessel according to claim 1, characterized in that the first liquefied gas is liquefied natural gas or liquefied petroleum gas, and the second liquefied gas is liquefied carbon dioxide.

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