A gas return temperature reduction system and efficient transfer method for equipping an LNG regasification vessel

Abstract

The application discloses a back-gas temperature reducing system and an efficient transfer method for equipping an LNG regasification device ship, and is characterized in that a back-gas temperature reducer is innovatively introduced into a receiving ship, and the volatile gas to be returned to a filling party and the LNG (liquefied natural gas) supplied to the LNG regasification device of the receiving party are transported into the back-gas temperature reducer, the temperature of the volatile gas to be returned to the filling party is reduced by using the cold energy of the liquefied natural gas supplied to the regasification device, the volume of the returned volatile gas is reduced, the density of the returned volatile gas is always kept at a high value, more volatile gas is returned, and thus the cabin pressure of the storage cabin of the receiving party is effectively controlled, and efficient transfer of the liquefied natural gas is ensured.

Description

Technical Field

[0001] This invention relates to the field of marine engineering equipment technology, and in particular to a return gas de-cooling system and an efficient transfer method for ships equipped with LNG regasification units. Background Technology

[0002] Against the backdrop of low-carbon and environmental protection, LNG-FSRUs (Floating Liquefied Natural Gas Storage and Regasification Units), which serve as offshore "alternatives" to onshore natural gas regasification terminals, and LNG-fueled power generation vessels developed from them, have made significant progress since the beginning of the new century.

[0003] LNG-FSRUs and LNG-fueled power generation vessels both use LNG (liquefied natural gas) as their working medium. They regasify their stored LNG, directly exporting the gaseous natural gas or using it as fuel to generate electricity, providing clean and green energy for production and daily life at their deployment sites. Any disruption to the LNG supply will severely impact the stability of their operation, consequently affecting the energy supply of the deployment site. Therefore, achieving efficient and safe LNG resupply is a prerequisite for ensuring the stable operation of LNG-FSRUs and LNG-fueled power generation vessels.

[0004] LNG-FSRUs and LNG-fueled power generation vessels possess significant LNG storage capacity. Currently, LNG is typically transported by ship, with large-scale replenishment achieved through ship-to-ship transfers. During ship-to-ship transfers, as LNG is transferred from the transporter (LNG carrier) to the receiver (LNG-FSRU or LNG power generation vessel), the liquid levels in both the transporter's and receiver's tanks change, leading to changes in their respective tank pressures. However, the LNG transfer speed is constrained by these pressure changes. If the receiver's tank pressure rises due to increased volatile gas levels, the transporter must passively reduce the transfer speed to accommodate the pressure changes, undoubtedly extending the ship-to-ship LNG transfer operation time. Longer transfer times require more effort from ship operators and pose higher potential risks. Summary of the Invention

[0005] In view of this, the present invention provides a return gas desuperheating system and an efficient transfer method for ships equipped with LNG regasification units. By innovatively introducing a return gas desuperheater into the receiving ship, the volatile gas to be returned to the refueling party and the LNG regasification unit supplied to the receiving party are transported to the return gas desuperheater. The cold energy of the liquefied natural gas supplied to the regasification unit is used to reduce the temperature of the volatile gas to be returned to the refueling party, thereby reducing the volume of the returned volatile gas and keeping the density of the returned volatile gas at a high value. This allows for the return of more volatile gas, effectively controlling the tank pressure of the receiving party's storage tank and ensuring efficient transfer of LNG.

[0006] A return gas desuperheating system for ships equipped with LNG regasification units includes a liquid injection pipe, a first liquid delivery pipe, a second liquid delivery pipe, a gas collection pipe, a volatile gas return pipe, a volatile gas temperature regulation mechanism, and a ship's central control system. One end of the liquid injection pipe extends to the lower part of the storage tank of a ship equipped with an LNG regasification unit, and the other end is connected to the liquefied natural gas export equipment of the refueling party. The liquid injection pipe is used to inject the liquefied natural gas of the refueling party into the bottom of the storage tank. One end of the first infusion pipe is connected to the supply pump installed in the storage tank, and the other end is connected to the first inlet of the volatile gas temperature control mechanism. The first outlet of the volatile gas temperature control mechanism is connected to one end of the second infusion pipe, and the other end of the second infusion pipe is connected to the LNG regasification device. One end of the gas gathering pipe is connected to the dome outlet of the storage tank, and the other end is connected to the second inlet of the volatile gas temperature control mechanism. The second outlet of the volatile gas temperature control mechanism is connected to the volatile gas return pipe, which is connected to the refueling party's return gas receiving equipment. The volatile gas temperature control mechanism is used to adjust the temperature of the volatile gas returned by the gas gathering pipe according to the pressure inside the storage tank using the cold energy of the liquefied natural gas transported by the first liquid delivery pipe, so that the volatile gas is returned to the refueling party's return gas receiving equipment at a specific temperature, and the heated liquefied natural gas is transported to the LNG regasification unit through the second liquid delivery pipe.

[0007] In a preferred embodiment, the volatile gas temperature control mechanism includes a pressure sensor for detecting the pressure inside the storage tank, a gas compressor, a temperature-controlled three-way valve, and a return gas desuperheater. The gas compressor's inlet is connected to a gas collection pipe, and its outlet is connected to port a of the temperature-controlled three-way valve via a compressed gas pipe. Port b of the temperature-controlled three-way valve is connected to the volatile gas return pipe via a first gas pipe. Port c of the temperature-controlled three-way valve is connected to the second inlet of the return gas desuperheater via a second gas pipe. The second outlet of the return gas desuperheater is connected to the volatile gas return pipe. The first inlet of the return gas desuperheater is connected to a first liquid supply pipe, and the first outlet is connected to a second liquid supply pipe. A temperature sensor is installed on the compressed gas pipe to detect the temperature of the volatile gas after being pressurized by the gas compressor. The pressure sensor, temperature sensor, and temperature-controlled three-way valve are all electrically connected to the ship's central control system.

[0008] In a preferred embodiment, a one-way valve is provided on the volatile gas return pipe, and the connection node between the first gas pipe and the volatile gas return pipe is located downstream of the one-way valve.

[0009] In a preferred embodiment, the volatile gas temperature control mechanism includes a pressure sensor for detecting the pressure inside the storage tank, a gas compressor, a temperature-controlled three-way valve, and a return gas desuperheater. The gas compressor's inlet is connected to a gas collecting pipe, and its outlet is connected to a compressed gas pipe. The compressed gas pipe is connected to a first gas pipe and a second gas pipe. The first gas pipe is connected to port a of the temperature-controlled three-way valve, and port b of the temperature-controlled three-way valve is connected to the volatile gas return pipe. A temperature sensor for detecting the final output volatile gas temperature is installed on the volatile gas return pipe. The second gas pipe is connected to the second inlet of the return gas desuperheater, and the second outlet of the return gas desuperheater is connected to port c of the temperature-controlled three-way valve through a desuperheating gas pipe. The pressure sensor, temperature sensor, and temperature-controlled three-way valve are all electrically connected to the ship's central control system.

[0010] In a preferred embodiment, the gas compressor is a centrifugal, screw, or piston-type oil-free compressor.

[0011] In a preferred embodiment, the return gas desuperheater is a shell-and-tube, printed circuit board, or finned heat exchanger.

[0012] In a preferred embodiment, the supply pump is fixed on a base located at the bottom of the storage compartment or detachably fixed in a pump well within the storage compartment.

[0013] An efficient transfer method for the above-described system specifically includes the following steps: S1 connects the refueling unit's liquefied natural gas export equipment to the liquid injection pipe and the return gas receiving equipment to the volatile gas return pipeline; S2, liquefied natural gas is injected into the bottom of the storage tank through the liquid injection pipe. During the continuous injection of liquefied natural gas, the pressure sensor detects the pressure inside the storage tank in real time. When the pressure inside the tank reaches the high pressure set value, the ship's central control system controls the start of the gas compressor. The volatile gas in the tank is transported to the gas compressor through the gas collection pipe, and the pressurized volatile gas is discharged to the compressed gas pipe. S3, the temperature sensor detects the temperature of the volatile gas in the compressed gas pipe in real time. When the temperature of the volatile gas is lower than the set value, the ship's central control system controls the temperature control three-way valve to connect its a port to its b port. The volatile gas is then returned to the return gas receiving equipment through the first gas pipe and the volatile gas return pipe. When the temperature of the volatile gas is higher than the set value, the ship's central control system controls the temperature control three-way valve to connect its a port to its c port. The volatile gas flows into the return gas desuperheater through the second gas pipe. At the same time, the first liquid delivery pipe delivers the liquefied natural gas in the storage tank to the return gas desuperheater. The volatile gas and the liquefied natural gas exchange heat in the return gas desuperheater. The cooled volatile gas is returned to the return gas receiving equipment through the volatile gas return pipe, while the heated liquefied natural gas is delivered to the LNG regasification unit through the second liquid delivery pipe. S4. During the transshipment process, if the pressure sensor detects that the pressure inside the storage tank drops to the low-pressure set value, the ship's central control system will stop the gas compressor and close the gas collection pipe to stop the return transport of volatile gas. S5. After the liquefied natural gas (LNG) barge operation is completed, the ship's central control system stops the gas compressor and closes the gas gathering pipe and liquid injection pipe. The liquid injection pipe and volatile gas return pipe are disconnected from the LNG export equipment and return gas receiving equipment of the refueling party, and the barge operation ends.

[0014] An efficient transfer method for the above-described system specifically includes the following steps: S1 connects the refueling unit's liquefied natural gas export equipment to the liquid injection pipe and the return gas receiving equipment to the volatile gas return pipeline; S2, liquefied natural gas (LNG) is injected into the bottom of the storage tank through the liquid injection pipe. During the continuous injection of LNG, the pressure sensor monitors the pressure inside the storage tank in real time. When the pressure inside the tank reaches the high pressure set value, the ship's central control system controls the start of the gas compressor. The volatile gas in the tank is transported to the gas compressor through the gas collection pipe. The pressurized volatile gas flows into the compressed gas pipe. Part of the volatile gas flows into the a port of the temperature-controlled three-way valve through the first gas pipe, and the other part flows into the return gas desuperheater through the second gas pipe. The volatile gas and LNG exchange heat in the return gas desuperheater. The cooled volatile gas flows into the c port of the temperature-controlled three-way valve through the desuperheating gas pipe. The two parts of volatile gas are mixed in the temperature-controlled three-way valve and discharged from the b port of the temperature-controlled three-way valve and returned to the return gas receiving equipment through the volatile gas return pipe. The heated LNG is transported to the LNG regasification unit through the second liquid delivery pipe. S3, the temperature sensor detects the temperature of the volatile gas in the volatile gas return pipe in real time. The ship's central control system controls the opening degree of each valve port of the temperature control three-way valve according to the temperature deviation between the volatile gas temperature and the set value. When the volatile gas temperature is lower than the set value, the flow ratio from port a to port b of the temperature control three-way valve increases with the increase of the temperature deviation; when the volatile gas temperature is higher than the set value, the flow ratio from port c to port b of the temperature control three-way valve increases with the increase of the temperature deviation. S4. During the transshipment process, if the pressure sensor detects that the pressure inside the storage tank drops to the low-pressure set value, the ship's central control system will stop the gas compressor and close the gas collection pipe to stop the return transport of volatile gas. S5. After the liquefied natural gas (LNG) barge operation is completed, the ship's central control system stops the gas compressor and closes the gas gathering pipe and liquid injection pipe. The liquid injection pipe and volatile gas return pipe are disconnected from the LNG export equipment and return gas receiving equipment of the refueling party, and the barge operation ends.

[0015] The beneficial effects of this invention are: 1. This invention innovatively introduces a return gas desuperheater into the receiving vessel. By transporting the volatile gas to be returned to the refueling party and the LNG regasification unit supplied to the receiving party into the return gas desuperheater, the cold energy of the liquefied natural gas supplied to the regasification unit is used to reduce the temperature of the volatile gas to be returned to the refueling party, thereby reducing the volume of the returned volatile gas. This ensures that the density of the returned volatile gas is always maintained at a high value, thus ensuring that more mass of volatile gas is returned at a fixed volume flow rate. This effectively controls the tank pressure of the receiving party's storage tank, ensuring that the barge operation can always maintain a high injection rate and effectively shorten the barge operation time.

[0016] 2. By monitoring the pressure inside the storage tank in real time and selecting to start or stop the volatile gas return process according to the pressure inside the tank, the present invention can keep the tank pressure of the injector and the injectee within a controllable range.

[0017] 3. This invention utilizes a return gas desuperheater to achieve heat exchange between the returned volatile gas and the liquefied natural gas supplied to the regasification unit, which initially increases the temperature of the liquefied natural gas, helps to reduce the heat energy required for subsequent liquefied natural gas regasification, reduces system energy consumption, and improves the efficiency of the entire system. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of the present invention in Embodiment 1.

[0020] Figure 2 This is a schematic diagram of the structure of the present invention in Embodiment 2.

[0021] The meanings of the labels in the diagram are as follows: Storage compartment 1, transfer pump 2, pressure sensor 3, gas compressor 4, return gas desuperheater 5, temperature control three-way valve 6, temperature sensor 7, first remote control valve 8, second remote control valve 9, third remote control valve 10, check valve 11, central control system 12; Gas collecting pipe V1, compressed gas pipe V2, first gas pipe V3, second gas pipe V4, volatile gas return pipe R1, liquid injection pipe L1, first liquid delivery pipe F1, second liquid delivery pipe F2, liquid interface C1, gas interface C2, and desuperheating gas pipe V5. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0023] The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The singular forms “a,” “the,” and “the” as used in this disclosure and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0024] It should be understood that although the terms first, second, third, etc., may be used in this disclosure to describe various information, such information should not be limited to these terms and should not be construed as indicating or implying relative importance. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this disclosure, first information may also be referred to as second information, and similarly, second information may also be referred to as first information.

[0025] In the description of this invention, unless otherwise specified and limited, it should be noted that the terms "installation", "connection" and "linking" should be interpreted broadly. For example, they can refer to mechanical or electrical connections, or internal connections between two components. They can be direct connections or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0026] To better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0027] The present invention provides a return gas de-cooling system for ships equipped with LNG regasification units, including a liquid injection pipe L1, a first liquid delivery pipe F1, a second liquid delivery pipe F2, a gas collection pipe V1, a volatile gas return pipe R1, a volatile gas temperature regulation mechanism, and a ship central control system 12.

[0028] One end of the liquid injection pipe L1 extends to the lower part of the storage tank 1 of the ship equipped with the LNG regasification unit, and the other end is connected to the liquefied natural gas export equipment of the refueling party. The liquid injection pipe L1 is used to inject the liquefied natural gas of the refueling party into the bottom of the storage tank 1. A third remote control valve 10 is installed on the liquid injection pipe L1.

[0029] Storage tank 1 is located inside the ship's structure and is used to store liquefied natural gas (LNG). The LNG within storage tank 1 will evaporate, forming volatile gases. To reduce the generation of volatile gases, storage tank 1 is lined with insulating material to minimize the ingress of external heat. Storage tank 1 can be any type of enclosure system as defined in the "Code for Construction and Equipment of Ships Carrying Liquefied Gases in Bulk," including integral, freestanding (Type A, Type B, and Type C), and membrane enclosures.

[0030] The top of storage compartment 1 may be provided with a protruding dome, which can be used as a location for pipes to pass through the compartment and for sensor installation.

[0031] One end of the first infusion pipe F1 is connected to the supply pump 2 installed in the storage tank 1, and the other end is connected to the first inlet of the volatile gas temperature control mechanism. The first outlet of the volatile gas temperature control mechanism is connected to one end of the second infusion pipe F2, and the other end of the second infusion pipe F2 is connected to the LNG regasification device.

[0032] One end of the gas collecting pipe V1 is connected to the dome outlet of the storage chamber 1, and the other end is connected to the second inlet of the volatile gas temperature control mechanism. The second outlet of the volatile gas temperature control mechanism is connected to the volatile gas return pipe R1, and the volatile gas return pipe R1 is connected to the return gas receiving equipment of the refueling party.

[0033] The volatile gas temperature control mechanism is used to adjust the temperature of the volatile gas returned by the gas gathering pipe V1 according to the pressure inside the storage tank 1 and the cold energy of the liquefied natural gas transported by the first liquid delivery pipe 1, so that the volatile gas is returned to the return gas receiving equipment of the refueling party at a specific temperature, and the heated liquefied natural gas is transported to the LNG regasification unit through the second liquid delivery pipe F2.

[0034] Example 1: In this example, the return gas de-cooling system for ships equipped with LNG regasification units of the present invention includes a liquid injection pipe L1, a first liquid delivery pipe F1, a second liquid delivery pipe F2, a gas collection pipe V1, a volatile gas return pipe R1, a volatile gas temperature regulation mechanism, and a ship central control system 12.

[0035] like Figure 1 As shown, the volatile gas temperature control mechanism includes a pressure sensor 3 for detecting the pressure inside the storage compartment 1, a gas compressor 4, a temperature-controlled three-way valve 6, and a return gas desuperheater 5.

[0036] A gas collection pipe V1 is arranged on the protruding dome of the storage compartment 1. One end of the pipe is connected to the upper space inside the storage compartment 1 to collect volatile gases inside the storage compartment 1, and the other end is connected to the inlet of the gas compressor 4. A first remote control valve 8 is installed on the gas collection pipe V1.

[0037] The outlet of gas compressor 4 is connected to port a of thermostatic three-way valve 6 via compressed gas pipe V2. Gas compressor 4 pressurizes excess volatile gas in the compartment via compressed gas pipe V2 and delivers it to thermostatic three-way valve 6, which then returns it to the refueling party. This gas compressor 4 can be a centrifugal, screw, or reciprocating oil-free compressor to prevent oil from entering the cargo hold of the refueling party with the returned volatile gas and thus contaminating the cargo.

[0038] A temperature sensor 7 is installed on the compressed gas pipe V2 to detect the temperature of the volatile gas after being pressurized by the gas compressor 4. The temperature sensor 7 transmits the temperature signal it detects to the ship's central control system 12 in real time, so that the ship's central control system 12 can control the connection state of the temperature control three-way valve 6 according to the temperature of the pressurized volatile gas.

[0039] The temperature-controlled three-way valve 6 has three connectors, namely interface a, interface b and interface c. Interface a is connected to compressed gas pipe V2, interface b is connected to the first gas pipe V3, and interface c is connected to the second gas pipe V4. The gas flow direction in the temperature-controlled three-way valve 6 is selected according to the temperature of the volatile gas after being pressurized by the gas compressor 4.

[0040] The first gas pipe V3 is connected to the volatile gas return pipe R1.

[0041] The second gas pipe V4 is connected to the second inlet X1 (i.e., the hot-side inlet) of the return gas desuperheater 5, and the second outlet X2 (i.e., the hot-side outlet) of the return gas desuperheater 5 is connected to the volatile gas return pipe R1. A one-way valve 11 is installed on the volatile gas return pipe R1. The connection point between the first gas pipe V3 and the volatile gas return pipe R1 is located downstream of the one-way valve 11. The one-way valve 11 is used to restrict the unidirectional flow of internal gas. The volatile gas return pipe R1 has a gas interface C2, which is used to connect to the return gas facility of the refueling party, typically a hose or a gas phase loading / unloading arm.

[0042] The first inlet Y1 (i.e., the cold side inlet) of the return gas desuperheater 5 is connected to the first infusion pipe F1, and the second outlet Y2 (i.e., the cold side outlet) of the return gas desuperheater 5 is connected to the second infusion pipe F2.

[0043] The return gas desuperheater 5 is used to exchange heat between the pressurized return volatile gas and liquefied natural gas, thereby reducing the temperature of the return volatile gas. The return gas desuperheater 5 can be a shell-and-tube type, a printed circuit board type, or a finned type heat exchanger, used to exchange heat between the return volatile gas and liquefied natural gas.

[0044] One end of the first liquid delivery pipe F1 is connected to the first inlet Y1 of the return gas desuperheater 5, and the other end extends into the storage tank 1 and is connected to the supply pump 2. The supply pump 2 can be a submersible pump or a deep well pump, installed at the bottom of the storage tank 1. It can be fixedly installed on the base or detachably installed in the pump well. Its inlet opening is located inside the storage tank 1. An inlet filter screen is installed at the inlet of the supply pump 2, and the outlet is connected to the first liquid delivery pipe F1, which is used to transport the liquefied natural gas in the storage tank 1 to the regasification unit. The first liquid delivery pipe F1 extends from the top of the storage tank 1, and its exit point must be airtight. A second remote control valve 9 is installed on it. The storage tank 1 is also equipped with a liquid injection pipe L1. One end of the liquid injection pipe L1 is located at the bottom of the storage tank 1, and the other end is connected to the liquid interface C1. A third remote control valve 10 is installed on the liquid injection pipe L1. The liquid interface C1 is used to connect to the injection facilities of the injector, which is usually a hose or a liquid phase loading and unloading arm.

[0045] The aforementioned first remote control valve 8, second remote control valve 9, and third remote control valve 10 can be hydraulically, electrically, or compressed gas driven, and can be remotely operated via the ship's central control system 12, or opened and closed locally using a handwheel; the first remote control valve 8, second remote control valve 9, and third remote control valve 10 are also equipped with valve position indicators for remotely displaying the open and closed status of the valves; the pressure sensor 3 can transmit the pressure measurement value to the ship's central control system 12 for display, facilitating operators to monitor pressure changes in the storage tank 1, and can also serve as a local display function.

[0046] When the temperature of the volatile gas detected by the temperature sensor 7 is lower than the set value, the ship's central control system 12 controls the temperature control three-way valve 6 to connect its a port to its b port. The volatile gas, after being pressurized by the gas compressor 4, is returned to the refueling party's return gas receiving equipment through the first gas pipe V3 and the volatile gas return pipe R1. When the temperature of the volatile gas detected by the temperature sensor 7 is higher than the set value, the ship's central control system 12 controls the temperature control three-way valve 6 to connect its a port to its c port. The volatile gas, after being pressurized by the gas compressor 4, flows into the return gas desuperheater 5 through the second gas pipe V4. At the same time, the first liquid delivery pipe F1 delivers the liquefied natural gas in the storage tank 1 to the return gas desuperheater 5. The volatile gas and the liquefied natural gas exchange heat in the return gas desuperheater 5. The cooled volatile gas is returned to the refueling party's return gas receiving equipment through the volatile gas return pipe R1, while the heated liquefied natural gas is delivered to the LNG regasification unit through the second liquid delivery pipe F2.

[0047] This embodiment also provides an efficient transfer method for the aforementioned system. This method employs the aforementioned return gas desuperheating system and is suitable for the transfer of liquefied natural gas. The ship equipped with an LNG regasification unit, as the recipient of the LNG transfer, is defined in this invention as the "receiver," which can be an LNG-FSRU or an LNG-fueled power generation vessel. The party providing LNG bunkering services is defined as the "bundler," which can be an LNG carrier or an LNG bunkering vessel with ship-to-ship delivery capabilities.

[0048] Specifically, the efficient transfer method of the present invention includes the following steps: S1 connects the liquefied natural gas (LNG) export equipment of the refueling unit to the liquid injection pipe L1 and the return gas receiving equipment to the volatile gas return pipe R1. The pipelines at the connection points are inerted and pre-cooled. After the operation requirements are met, the next step of LNG refueling is prepared.

[0049] S2, open the third remote control valve 10, connect the liquid injection pipe L1, and inject liquefied natural gas from the refueling party into the bottom of the storage tank 1 through the liquid injection pipe L1.

[0050] As liquefied natural gas (LNG) is continuously injected, the LNG level in storage tank 1 rises continuously, compressing the upper gas space. Furthermore, with the injection of LNG, external heat is also drawn into storage tank 1, causing the LNG to evaporate and produce volatile gases. The compression of the gas space and the generation of volatile gases cause the pressure inside storage tank 1 to rise continuously. Pressure sensor 3 monitors the pressure inside storage tank 1 in real time and transmits the detected pressure signal to the ship's central control system 12.

[0051] When the pressure inside the cabin reaches the high pressure set value, the ship's central control system 12 controls the start of the gas compressor 4. The volatile gas inside the cabin is transported to the gas compressor 4 through the gas collection pipe V1, and the pressurized volatile gas is discharged to the compressed gas pipe V2.

[0052] In this embodiment, storage tank 1 is a membrane-type cargo tank with a design pressure of 0.7 barg. When the pressure rises to the set value (0.45 barg), an alarm is triggered by the central control system 12, alerting the operator. At this time, the operator remotely opens the first remote control valve 8, or the ship's central control system 12 controls the opening of the first remote control valve 8, connecting the gas space in storage tank 1 to the inlet of gas compressor 4 through the gas collection pipe V1. This allows the gas compressor 4 to be remotely started, extracting and pressurizing the volatile gas in storage tank 1. The pressure of the returned gas, after being pressurized by gas compressor 4, can be increased to 1.96 bara to overcome the pressure loss in the pipeline returning to the injection point.

[0053] The volatile gas, after being pressurized by the gas compressor 4, is discharged into the compressed gas pipe V2.

[0054] S3, temperature sensor 7 detects the temperature of the volatile gas in the compressed gas pipe V2 in real time.

[0055] When the temperature of the volatile gas is lower than the set value (-100℃), the ship's central control system 12 controls the temperature control three-way valve 6 to connect its a port to its b port, and the returned volatile gas is returned to the return gas receiving equipment through the first gas pipe V3 and the volatile gas return pipe R1. When the temperature of the volatile gas is higher than the set value (-100℃), the ship's central control system 12 controls the temperature control three-way valve 6 to connect its a port to its c port. The volatile gas flows into the return gas desuperheater 5 through the second gas pipe V4. At the same time, since the ship (i.e., the ship equipped with the LNG regasification unit) needs to provide fuel for its power generation system and needs to operate continuously, the supply pump 2 continuously supplies LNG through the first liquid transfer pipe F1 during the entire LNG transfer process. The second remote control valve 9 is always in the open state. The first liquid transfer pipe F1 transports the LNG in the storage tank 1 to the return gas desuperheater 5. The volatile gas and the LNG exchange heat in the return gas desuperheater 5. The cooled volatile gas is returned to the return gas receiving equipment through the volatile gas return pipe R1, while the heated LNG is transported to the LNG regasification unit through the second liquid transfer pipe F2.

[0056] After heat exchange, the temperature of the liquefied natural gas increases, reducing the amount of heat required for subsequent regasification.

[0057] S4. During the transfer process, if the pressure sensor 3 detects that the pressure inside the storage tank 1 drops to the low pressure set value (e.g., 0.1 barg), the ship's central control system 12 controls the gas compressor 4 to stop and closes the gas collection pipe V1 (closes the first remote control valve 8) to stop the return of volatile gas.

[0058] S5. After the liquefied natural gas (LNG) transshipment operation is completed, the ship's central control system 12 controls the gas compressor 4 to stop and closes the gas gathering pipe V1 and the liquid injection pipe L1 (i.e., closes the first remote control valve 8 and the third remote control valve 10). The liquid injection pipe L1 and the volatile gas return pipe R1 are disconnected from the LNG export equipment and return gas receiving equipment of the refueling party, and the transshipment operation ends.

[0059] Example 2: In this example, the return gas de-cooling system for ships equipped with LNG regasification units of the present invention includes a liquid injection pipe L1, a first liquid delivery pipe F1, a second liquid delivery pipe F2, a gas collection pipe V1, a volatile gas return pipe R1, a volatile gas temperature regulation mechanism, and a ship central control system 12.

[0060] like Figure 2 As shown, the volatile gas temperature control mechanism includes a pressure sensor 3 for detecting the pressure inside the storage compartment 1, a gas compressor 4, a temperature-controlled three-way valve 6, and a return gas desuperheater 5.

[0061] A gas collection pipe V1 is arranged on the protruding dome of the storage compartment 1. One end of the pipe is connected to the upper space inside the storage compartment 1 to collect volatile gases inside the storage compartment 1, and the other end is connected to the inlet of the gas compressor 4. A first remote control valve 8 is installed on the gas collection pipe V1.

[0062] The outlet of the gas compressor 4 is connected to the compressed gas pipe V2, which is connected to the first gas pipe V3 and the second gas pipe V4. The gas compressor 4 can be a centrifugal, screw, or reciprocating oil-free compressor to prevent oil from entering the cargo hold of the injection party with the returned volatile gas and thus contaminating the cargo.

[0063] The first gas pipe V3 is connected to port a of the temperature-controlled three-way valve 6 to deliver a portion of the volatile gas compressed by the gas compressor 4 to the temperature-controlled three-way valve 6.

[0064] In this embodiment, the temperature-controlled three-way valve 6 is installed downstream of the return gas desuperheater 5. The temperature-controlled three-way valve 6 has three connectors, namely interface a, interface b and interface c, wherein interface a is connected to the first gas pipe V3, interface b is connected to the volatile gas return pipe R1, and interface c is connected to the desuperheating gas pipe V5.

[0065] The volatile gas return pipe R1 has a gas interface C2, which is used to connect to the refueling party's return gas facility, usually a hose or a gas phase loading and unloading arm.

[0066] The second gas pipe V4 is connected to the second inlet X1 (i.e., the hot-side inlet) of the return gas desuperheater 5, and the second outlet X2 (i.e., the hot-side outlet) of the return gas desuperheater 5 is connected to the desuperheating gas pipe V5. The second gas pipe V4 is used to transport a portion of the volatile gas compressed by the gas compressor 4 to the return gas desuperheater 5, where the temperature of this portion of volatile gas is regulated using the cold energy of liquefied natural gas.

[0067] The first inlet Y1 (i.e., the cold side inlet) of the return gas desuperheater 5 is connected to the first infusion pipe F1, and the second outlet Y2 (i.e., the cold side outlet) of the return gas desuperheater 5 is connected to the second infusion pipe F2.

[0068] The return gas desuperheater 5 is used to exchange heat between a portion of the pressurized return volatile gas transported by the second gas pipe V4 and liquefied natural gas, thereby reducing the temperature of the return volatile gas. The return gas desuperheater 5 can be a shell-and-tube type, a printed circuit board type, or a finned type heat exchanger, used to exchange heat between the return volatile gas and liquefied natural gas.

[0069] The volatile gas that exchanges heat with liquefied natural gas in the return gas desuperheater 5 is discharged from the second outlet X2 (i.e., the hot side outlet) of the return gas desuperheater 5. This part of the volatile gas is transmitted to the temperature-controlled three-way valve 6 through the desuperheating gas pipe V5. The temperature-regulated volatile gas and the unregulated volatile gas are mixed in the temperature-controlled three-way valve 6, and then discharged from the b port of the temperature-controlled three-way valve 6. It is then returned to the return gas receiving equipment of the refueling party through the volatile gas return pipe R1.

[0070] A temperature sensor 7 is installed on the volatile gas return pipe R1 to detect the temperature of the final output volatile gas. The temperature sensor 7 transmits the detected temperature signal to the ship's central control system 12 in real time. The ship's central control system 12 compares the temperature value with the set value to control the flow ratio of the "a→b" and "c→b" of the temperature control three-way valve 6, thereby controlling the proportion of gas passing through the return gas desuperheater 5, so as to keep the temperature of the gas downstream of the temperature control three-way valve always at the set temperature value.

[0071] One end of the first liquid delivery pipe F1 is connected to the first inlet Y1 of the return gas desuperheater 5, and the other end extends into the storage tank 1 and is connected to the supply pump 2. The supply pump 2 can be a submersible pump or a deep well pump, installed at the bottom of the storage tank 1. It can be fixedly installed on the base or detachably installed in the pump well. Its inlet opening is located inside the storage tank 1. An inlet filter screen is installed at the inlet of the supply pump 2, and the outlet is connected to the first liquid delivery pipe F1, which is used to transport the liquefied natural gas in the storage tank 1 to the regasification unit. The first liquid delivery pipe F1 extends from the top of the storage tank 1, and its exit point must be airtight. A second remote control valve 9 is installed on it. The storage tank 1 is also equipped with a liquid injection pipe L1. One end of the liquid injection pipe L1 is located at the bottom of the storage tank 1, and the other end is connected to the liquid interface C1. A third remote control valve 10 is installed on the liquid injection pipe L1. The liquid interface C1 is used to connect to the injection facilities of the injector, which is usually a hose or a liquid phase loading and unloading arm.

[0072] The aforementioned first remote control valve 8, second remote control valve 9, and third remote control valve 10 can be hydraulically, electrically, or compressed gas driven, and can be remotely operated via the ship's central control system 12, or opened and closed locally using a handwheel; the first remote control valve 8, second remote control valve 9, and third remote control valve 10 are also equipped with valve position indicators for remotely displaying the open and closed status of the valves; the pressure sensor 3 can transmit the pressure measurement value to the ship's central control system 12 for display, facilitating operators to monitor pressure changes in the storage tank 1, and can also serve as a local display function.

[0073] When the temperature sensor 7 detects that the temperature of the volatile gas transported in the volatile gas return pipe R1 is lower than the set value, the ship's central control system 12 controls the flow ratio from port a to port b of the temperature control three-way valve 6 based on the temperature deviation between the temperature value detected by the temperature sensor 7 and the design value. The flow ratio from port a to port b of the temperature control three-way valve 6 increases as the temperature deviation increases. When the temperature sensor 7 detects that the temperature of the volatile gas transported in the volatile gas return pipe R1 is higher than the set value, the ship's central control system 12 controls the flow ratio from port C to port B of the temperature control three-way valve 6 based on the temperature deviation between the temperature value detected by the temperature sensor 7 and the design value. The flow ratio from port C to port B of the temperature control three-way valve 6 increases as the temperature deviation increases.

[0074] This embodiment utilizes the efficient transfer method of the system described above, specifically including the following steps: S1 connects the liquefied natural gas (LNG) export equipment of the refueling unit to the liquid injection pipe L1 and the return gas receiving equipment to the volatile gas return pipe R1. The pipelines at the connection points are inerted and pre-cooled. After the operation requirements are met, the next step of LNG refueling is prepared.

[0075] S2, open the third remote control valve 10, connect the liquid injection pipe L1, and inject liquefied natural gas into the bottom of storage tank 1 through the liquid injection pipe L1.

[0076] As liquefied natural gas (LNG) is continuously injected, the LNG level in storage tank 1 rises continuously, compressing the upper gas space. Furthermore, with the injection of LNG, external heat is also drawn into storage tank 1, causing the LNG to evaporate and produce volatile gases. The compression of the gas space and the generation of volatile gases cause the pressure inside storage tank 1 to rise continuously. Pressure sensor 3 monitors the pressure inside storage tank 1 in real time and transmits the detected pressure signal to the ship's central control system 12.

[0077] When the pressure inside the cabin reaches the high pressure set value, the ship's central control system 12 controls the start of the gas compressor 4. The volatile gas inside the cabin is transported to the gas compressor 4 through the gas collection pipe V1, and the pressurized volatile gas is discharged to the compressed gas pipe V2.

[0078] In this embodiment, storage tank 1 is a membrane-type cargo tank with a design pressure of 0.7 barg. When the pressure rises to the set value (0.45 barg), an alarm is triggered by the central control system 12, alerting the operator. At this time, the operator remotely opens the first remote control valve 8, or the ship's central control system 12 controls the opening of the first remote control valve 8, connecting the gas space in storage tank 1 to the inlet of gas compressor 4 through the gas collection pipe V1. This allows the gas compressor 4 to be remotely started, extracting and pressurizing the volatile gas in storage tank 1. The pressure of the returned gas, after being pressurized by gas compressor 4, can be increased to 1.96 bara to overcome the pressure loss in the pipeline returning to the injection point.

[0079] After being pressurized by the gas compressor 4, the return volatile gas is discharged to the compressed gas pipe V2. The compressed gas pipe V2 then delivers the volatile gas to the a port of the temperature control three-way valve and the return gas desuperheater 5 via the first gas pipe V3 and the second gas pipe V4, respectively.

[0080] The volatile gas entering the return gas desuperheater 5 exchanges heat with liquefied natural gas. After cooling, this part of the volatile gas is transmitted to the temperature-controlled three-way valve 6 through the desuperheating gas pipe V5. It mixes with another part of the volatile gas entering the temperature-controlled three-way valve 6 from the first gas pipe V3. Then it is discharged from the b port of the temperature-controlled three-way valve 6 and returned to the return gas receiving equipment of the refueling party through the volatile gas return pipe R1.

[0081] S3, temperature sensor 7 detects the temperature of volatile gas in volatile gas return pipe R1 in real time, and the ship's central control system 12 controls the opening degree of each valve port of the temperature control three-way valve 6 according to the temperature deviation between the volatile gas temperature detected by temperature sensor 7 and the set value.

[0082] When the temperature of the volatile gas detected by the temperature sensor 7 is lower than the set value, the flow ratio from port a to port b of the temperature-controlled three-way valve 6 increases with the increase of the temperature deviation; when the temperature of the volatile gas is higher than the set value, the flow ratio from port c to port b of the temperature-controlled three-way valve 6 increases with the increase of the temperature deviation.

[0083] For example, a temperature setpoint (-100℃) is set through the ship's central control system 12. The difference between the volatile gas temperature detected by the temperature sensor 7 and the setpoint is compared to control the flow ratio of the temperature-controlled three-way valve 6 "a→b" and "c→b", thereby controlling the proportion of gas passing through the return gas desuperheater 5 to keep the downstream gas temperature of the temperature-controlled three-way valve at the setpoint. When the volatile gas temperature detected by the temperature sensor 7 is higher than the setpoint (-100℃), the flow ratio of the temperature-controlled three-way valve 6 "c→b" increases with the increase of temperature deviation. That is, more pressurized return gas is decooled by the return gas desuperheater 5 and mixed with the return volatile gas from "a→b" to keep the temperature of the mixed gas at or close to the setpoint (-100℃). When the temperature of the volatile gas detected by temperature sensor 7 is lower than the set temperature (-100℃), the flow ratio of the temperature-controlled three-way valve 6 "a→b" increases with the increase of the temperature deviation. That is, more pressurized return gas bypasses the return gas desuperheater 5 and mixes with the desuperheated return volatile gas from "c→b", so that the temperature of the mixed gas is maintained at or close to the set temperature (-100℃). This ensures that more mass of volatile gas is returned to the injection point at a certain volumetric flow rate.

[0084] S4. During the transfer process, if the pressure sensor 3 detects that the pressure inside the storage tank 1 drops to the low pressure set value, the ship's central control system 12 controls the gas compressor 4 to stop and closes the gas collection pipe V1 to stop the return of volatile gas. S5. After the liquefied natural gas (LNG) transshipment operation is completed, the ship's central control system 12 controls the gas compressor 4 to stop, and closes the gas gathering pipe V1 and the liquid injection pipe L1. The liquid injection pipe L1 and the volatile gas return pipe R1 are disconnected from the LNG export equipment and return gas receiving equipment of the refueling party, and the transshipment operation ends.

[0085] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A return gas desuperheating system for ships equipped with LNG regasification units, characterized in that, It includes a liquid injection pipe (L1), a first liquid delivery pipe (F1), a second liquid delivery pipe (F2), a gas collection pipe (V1), a volatile gas return pipe (R1), a volatile gas temperature control mechanism, and a ship central control system (12). One end of the liquid injection pipe (L1) extends to the lower part of the storage tank (1) of the ship equipped with the LNG regasification unit, and the other end is connected to the liquefied natural gas export equipment of the refueling party. The liquid injection pipe (L1) is used to inject the liquefied natural gas of the refueling party into the bottom of the storage tank (1). One end of the first infusion pipe (F1) is connected to the supply pump (2) installed in the storage tank (1), and the other end is connected to the first inlet of the volatile gas temperature control mechanism. The first outlet of the volatile gas temperature control mechanism is connected to one end of the second infusion pipe (F2), and the other end of the second infusion pipe (F2) is connected to the LNG regasification device. One end of the gas collecting pipe (V1) is connected to the dome outlet of the storage tank (1), and the other end is connected to the second inlet of the volatile gas temperature regulating mechanism. The second outlet of the volatile gas temperature regulating mechanism is connected to the volatile gas return pipe (R1), and the volatile gas return pipe (R1) is connected to the refueling party's return gas receiving equipment. The volatile gas temperature regulating mechanism is used to adjust the temperature of the volatile gas returned by the gas collecting pipe (V1) according to the pressure inside the storage tank (1) using the cold energy of the liquefied natural gas transported by the first liquid delivery pipe (1) so that the volatile gas is returned to the refueling party's return gas receiving equipment at a specific temperature, and the heated liquefied natural gas is transported to the LNG regasification unit through the second liquid delivery pipe (F2).

2. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 1, characterized in that, The volatile gas temperature control mechanism includes a pressure sensor (3) for detecting the pressure inside the storage compartment (1), a gas compressor (4), a temperature-controlled three-way valve (6), and a return gas desuperheater (5). The inlet of the gas compressor (4) is connected to the gas collecting pipe (V1), and the outlet is connected to the a port of the temperature-controlled three-way valve (6) through the compressed gas pipe (V2). The b port of the temperature-controlled three-way valve (6) is connected to the volatile gas return pipe (R1) through the first gas pipe (V3), and the c port of the temperature-controlled three-way valve (6) is connected to the second gas pipe (V4). The first inlet of the return gas desuperheater (5) is connected to the second inlet of the return gas desuperheater (5), the second outlet of the return gas desuperheater (5) is connected to the volatile gas return pipe (R1), the first inlet of the return gas desuperheater (5) is connected to the first liquid delivery pipe (F1), and the first outlet is connected to the second liquid delivery pipe (F2). A temperature sensor (7) is installed on the compressed gas pipe (V2) to detect the temperature of the volatile gas after being pressurized by the gas compressor (4). The pressure sensor (3), the temperature sensor (7) and the temperature control three-way valve (6) are all electrically connected to the ship's central control system (12).

3. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 2, characterized in that, A one-way valve (11) is provided on the volatile gas return pipe (R1), and the connection node between the first gas pipe (V3) and the volatile gas return pipe (R1) is located downstream of the one-way valve (11).

4. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 1, characterized in that, The volatile gas temperature control mechanism includes a pressure sensor (3) for detecting the pressure inside the storage compartment (1), a gas compressor (4), a temperature-controlled three-way valve (6), and a return gas desuperheater (5). The inlet of the gas compressor (4) is connected to the gas collecting pipe (V1), and the outlet is connected to the compressed gas pipe (V2). The compressed gas pipe (V2) is connected to the first gas pipe (V3) and the second gas pipe (V4) respectively. The first gas pipe (V3) is connected to the a port of the temperature-controlled three-way valve (6). 6) is connected to the volatile gas return pipe (R1). A temperature sensor (7) is installed on the volatile gas return pipe (R1) to detect the temperature of the final output volatile gas. The second gas pipe (V4) is connected to the second inlet of the return gas desuperheater (5). The second outlet of the return gas desuperheater (5) is connected to the c port of the temperature control three-way valve (6) through the desuperheating gas pipe (V5). The pressure sensor (3), temperature sensor (7) and temperature control three-way valve (6) are all electrically connected to the ship's central control system (12).

5. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 2 or 4, characterized in that, The gas compressor (4) is a centrifugal, screw, or piston type oilless compressor.

6. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 2 or 4, characterized in that, The return gas desuperheater (5) is a shell-and-tube, printed circuit board, or finned heat exchanger.

7. The return gas desuperheating system for ships equipped with LNG regasification units according to claim 1, characterized in that, The supply pump (2) is fixed on a base located at the bottom of the storage compartment (1) or can be detachably fixed in the pump well of the storage compartment (1).

8. An efficient transfer method for the system described in claim 2 or 3, characterized in that, Specifically, the following steps are included: S1 connects the liquefied natural gas export equipment of the refueling unit to the liquid injection pipe (L1) and the return gas receiving equipment to the volatile gas return pipe (R1); S2, liquefied natural gas is injected into the bottom of the storage tank (1) through the liquid injection pipe (L1). During the continuous injection of liquefied natural gas, the pressure sensor (3) detects the pressure inside the storage tank (1) in real time. When the pressure inside the tank reaches the high pressure set value, the ship's central control system (12) controls the start of the gas compressor (4). The volatile gas in the tank is transported to the gas compressor (4) through the gas collection pipe (V1). The pressurized volatile gas is discharged to the compressed gas pipe (V2). S3, temperature sensor (7) detects the temperature of volatile gas in compressed gas pipe (V2) in real time. When the temperature of volatile gas is lower than the set value, the ship's central control system (12) controls the temperature control three-way valve (6) to connect its a port to its b port. All volatile gas is returned to the return gas receiving equipment through the first gas pipe (V3) and the volatile gas return pipe (R1). When the temperature of volatile gas is higher than the set value, the ship's central control system (12) controls the temperature control three-way valve (6) to connect its a port to its c port. The volatile gas flows into the return gas desuperheater (5) through the second gas pipe (V4). At the same time, the first liquid delivery pipe (F1) delivers the liquefied natural gas in the storage tank (1) to the return gas desuperheater (5). The volatile gas and liquefied natural gas exchange heat in the return gas desuperheater (5). The cooled volatile gas is returned to the return gas receiving equipment through the volatile gas return pipe (R1), while the heated liquefied natural gas is delivered to the LNG regasification unit through the second liquid delivery pipe (F2). S4. During the transfer process, if the pressure sensor (3) detects that the pressure inside the storage tank (1) drops to the low pressure setting value, the ship's central control system (12) controls the gas compressor (4) to stop and close the gas collection pipe (V1) to stop the return of volatile gas. S5. After the liquefied natural gas (LNG) transfer operation is completed, the ship's central control system (12) controls the gas compressor (4) to stop and close the gas collection pipe (V1) and liquid injection pipe (L1), disconnecting the liquid injection pipe (L1) and volatile gas return pipe (R1) from the LNG transfer equipment and return gas receiving equipment of the refueling party, and the transfer operation ends.

9. A highly efficient transfer method for the system of claim 4, characterized in that, Specifically, the following steps are included: S1 connects the liquefied natural gas export equipment of the refueling unit to the liquid injection pipe (L1) and the return gas receiving equipment to the volatile gas return pipe (R1); S2, liquefied natural gas is injected into the bottom of the storage tank (1) through the liquid injection pipe (L1). During the continuous injection of liquefied natural gas, the pressure sensor (3) detects the pressure inside the storage tank (1) in real time. When the pressure inside the tank reaches the high pressure set value, the ship's central control system (12) controls the start of the gas compressor (4). The volatile gas in the tank is transported to the gas compressor (4) through the gas collecting pipe (V1). The pressurized volatile gas flows into the compressed gas pipe (V2). A portion of the volatile gas flows into the temperature control three-way valve (6) through the first gas pipe (V3). The volatile gas flows through the second gas pipe (V4) into the return gas desuperheater (5), where it exchanges heat with the liquefied natural gas. The cooled volatile gas flows through the desuperheating gas pipe (V5) into the c port of the temperature control three-way valve (6). The two volatile gases mix in the temperature control three-way valve (6) and are discharged from the b port of the temperature control three-way valve (6) and returned to the return gas receiving equipment through the volatile gas return pipe (R1). The heated liquefied natural gas is transported to the LNG regasification unit through the second liquid delivery pipe (F2). S3, temperature sensor (7) detects the temperature of volatile gas in the volatile gas return pipe (R1) in real time. The ship's central control system (12) controls the opening degree of each valve port of the temperature control three-way valve (6) according to the temperature deviation between the volatile gas temperature and the set value. When the volatile gas temperature is lower than the set value, the flow ratio from port a to port b of the temperature control three-way valve (6) increases with the increase of the temperature deviation. When the volatile gas temperature is higher than the set value, the flow ratio from port c to port b of the temperature control three-way valve (6) increases with the increase of the temperature deviation. S4. During the transfer process, if the pressure sensor (3) detects that the pressure inside the storage tank (1) drops to the low pressure setting value, the ship's central control system (12) controls the gas compressor (4) to stop and close the gas collection pipe (V1) to stop the return of volatile gas. S5. After the liquefied natural gas (LNG) transfer operation is completed, the ship's central control system (12) controls the gas compressor (4) to stop and close the gas collection pipe (V1) and liquid injection pipe (L1), disconnecting the liquid injection pipe (L1) and volatile gas return pipe (R1) from the LNG transfer equipment and return gas receiving equipment of the refueling party, and the transfer operation ends.

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