A method and system for realizing simultaneous loading and unloading of ships at an LNG receiving station

Abstract

A method and system for simultaneous ship loading and unloading at an LNG receiving terminal includes: a mooring line connecting the LNG ship loading and unloading pipelines, a set of pneumatically controlled on-off valves, a BOG compressor, an LNG storage tank, a flare control system, and a corresponding LNG pressure control system. LNG unloaded from a first LNG carrier is partially discharged to the LNG storage tank via a normal unloading line, while another portion is discharged to a second LNG carrier via a mooring line, controlled by a regulating valve installed on the loading line. Simultaneously, the ship's cabin pressure matches the LNG tank pressure at a constant value. In an emergency, the gas phase pressure at the top of the LNG tank interlocks and shuts off the XV valve on the mooring line, resuming the unloading process. Furthermore, pressure control is used to adjust the load on the BOG compressor, while a PV control valve installed before the flare releases a certain amount of BOG to the flare, maintaining a stable pressure throughout the BOG system. This control system primarily utilizes pressure sensors, transmitters, and PLC hard wiring to achieve interlocking of the entire control system.

Description

Technical Field

[0001] The present invention relates to the field of LNG receiving stations being provided with loading and unloading docks and being able to simultaneously realize the functions of loading and unloading ships, and in particular to a method and system for realizing simultaneous loading and unloading ships at LNG receiving stations. Background Art

[0002] As a clean and efficient energy source, liquefied natural gas (LNG) has been gaining increasing share in both civil and industrial sectors year by year. The "gas shortage" in the winter of 2017, in particular, severely impacted the domestic LNG supply and demand. With the country opening up to short-distance LNG shipping by sea and along rivers, the LNG transshipment market will no longer be limited to transportation via LNG tankers. The use of small boats for LNG transshipment is gradually coming into the view of the government and businesses.

[0003] In the field of LNG receiving stations, most receiving stations only have unloading terminals, and there are very few LNG receiving stations with loading terminals. For the LNG receiving stations that currently have loading terminals, their loading pipelines are drawn from the low-pressure LNG main pipeline, and a stable loading function is achieved through the flow control loop set in the loading pipeline.

[0004] The above system generally has the following problems in actual engineering cases:

[0005] 1. The loading function is achieved from the low-pressure LNG pipeline through the ship loading pipeline. Since the low-pressure LNG pipeline relies on the in-tank pump for power, it is also necessary to ensure the normal operation of the loading and BOG recondenser. To ensure the pressure of the BOG recondenser, the in-tank pump generally has a high head of about 270m. The low-pressure LNG main pipe pressure needs to be maintained at about 0.8MPag to 1.0MPag, while the loading pressure only needs to be 0.2MPag to 0.3MPag, resulting in a large amount of ineffective energy consumption.

[0006] 2. The storage capacity of small LNG ships ranges widely, from 3,000 cubic meters to 150,000 cubic meters, making it difficult to accurately measure the volume.

[0007] 3. The pressure control of the BOG system is complex and unstable, which leads to prolonged loading and unloading time.

[0008] 4. Using a low-pressure LNG main for loading results in large pressure loss, large amounts of BOG generated, and unstable BOG system pressure control. Summary of the Invention

[0009] The purpose of the present invention is to solve the above technical problems and provide a new ship loading and unloading system, which also has the necessary emergency stop function.

[0010] According to a first embodiment of the present invention, a system for simultaneously loading and unloading ships at an LNG receiving station is provided, the system comprising:

[0011] The first LNG carrier planned to unload LNG, the second LNG carrier planned to load LNG, LNG storage tanks for storing LNG, BOG compressors for regulating the system's BOG pressure, flares for venting BOG, and flare separators for temporarily storing BOG;

[0012] Among them, the unloading liquid phase arm of the first LNG transport ship merges into an unloading main pipe connected to the LNG storage tank. A loading pump is installed in the LNG storage tank. The loading pump is connected to the loading liquid phase arm through the LNG loading pipeline. The loading liquid phase arm is further connected to the second LNG transport ship. The unloading main pipe and the LNG loading pipeline are interconnected through a mooring pipeline.

[0013] The unloading main pipe is provided with a first flow meter for controlling the total unloading flow of the first LNG carrier. The over-berthing pipeline is provided with a shut-off valve for cutting off the LNG transport to the second LNG carrier and a check valve for preventing LNG backflow in the pipeline. The LNG loading pipeline is provided with a loading flow regulating valve (which may include a flow regulating valve for controlling the flow to a small LNG carrier and a flow regulating valve for controlling the flow to a large LNG carrier arranged in parallel, i.e., split-range regulation). A second flow meter is provided on the LNG loading pipeline to control the loading flow in conjunction with the loading flow regulating valve and the first flow meter.

[0014] The LNG storage tank is equipped with a tank gas phase pipeline. The unloading gas phase arm of the first LNG transport ship is connected to the unloading gas phase pipeline. The loading gas phase arm of the second LNG transport ship is connected to the loading gas phase pipeline. The tank gas phase pipeline, the unloading gas phase pipeline, and the loading gas phase pipeline are all connected to the BOG main pipe. A branch pipe is separated from the BOG main pipe to connect to the BOG compressor. The BOG main pipe is also connected to the inlet of the flare separator tank, and the outlet of the flare separator tank is connected to the flare.

[0015] Furthermore, the LNG loading pipeline may be further provided with a loading pump outlet regulating shut-off valve. The loading pump is preferably a low-lift (eg 130-180 m LNG, preferably 160 m LNG) pump provided in the LNG storage tank and used only for loading.

[0016] Furthermore, a gas phase pressure gauge is provided on the top of the LNG storage tank for monitoring the gas phase pressure in the entire system and for interlocking control with the shut-off valve on the mooring pipeline.

[0017] Furthermore, a stop valve is provided on the mooring pipeline in parallel with the mooring pipeline for ensuring cold circulation.

[0018] Furthermore, a first pressure control valve is provided on the unloading gas phase pipeline, a second pressure control valve is provided on the loading gas phase pipeline, and a third pressure control valve is provided on the BOG input pipeline of the flare separator tank.

[0019] The gas phase pressure gauge on the top of the LNG storage tank is interlocked with the second pressure control valve, BOG compressor and third pressure control valve.

[0020] Furthermore, the first flow meter and the gas phase pressure gauge on the top of the LNG storage tank are connected to the central control room (control unit, computing unit, such as a computer).

[0021] A third flow regulating valve may be provided on the unloading main pipe to control the flow of unloading to the LNG storage tank.

[0022] Traditional LNG receiving stations are equipped with emergency shutoff valves on the unloading and loading gas phase lines, as well as on the BOG compressor inlet. When the pressure of the entire BOG system exceeds the set value, the central control room receives a pressure alarm and closes the emergency shutoff valves in the operating system, isolating it from the BOG main pipe. The present invention adds a shutoff valve to the mooring pipeline, forming a pressure interlock circuit with the gas phase pressure control device at the top of the LNG storage tank. This valve is specifically designed to shut down the mooring operation in the event of BOG system overpressure, restoring the entire unloading process to a single operation to the LNG storage tank.

[0023] The system is suitable for LNG receiving stations that are planned to be built or have already been built for loading or bunkering terminals.

[0024] Existing LNG receiving stations with ship loading capabilities rely primarily on low-pressure LNG mains for loading, significantly wasting the efficiency of high-lift (approximately 270m LNG) in-tank pumps. Due to the high pressure of the low-pressure LNG mains and the low pressure in the ship's hold, the BOG flash gas volume caused by the pressure drop is large. The power generated by the ineffective head provided by the in-tank pumps further increases the BOG evaporation rate, resulting in a certain amount of energy waste. In equipment load accounting, the compressor load needs to be recalculated. Otherwise, when loading and unloading the ship simultaneously, a large amount of BOG gas will have to be burned through the torch, resulting in a waste of resources.

[0025] The fluctuation of BOG volume will also affect the pressure stability of the entire BOG system from another perspective. The pressure fluctuation affects the stable operation of loading and unloading operations, and further affects the loading and unloading speed.

[0026] According to a second embodiment of the present invention, a control method for achieving simultaneous loading and unloading of ships at an LNG receiving station is provided, the method comprising:

[0027] (1) After the first LNG carrier arrives at the unloading terminal and the second LNG carrier (either a small LNG carrier or a large LNG carrier) arrives at the loading terminal, the LNG in the hold of the LNG carrier docked at the unloading terminal is pumped by the transfer pump in the first LNG carrier through the unloading liquid phase arm to the unloading main pipe, and then sent to the LNG loading pipeline via the mooring pipeline. The first flow meter installed on the unloading main pipe controls the opening of the flow regulating valve on the LNG loading pipeline according to the flow set by the LNG receiving station engineer, so that the LNG flow entering the second LNG carrier is stabilized at the set value, and the remaining LNG is delivered to the LNG storage tank;

[0028] (2) The evaporation gas (e.g., about 35 kPag) displaced after LNG enters the LNG loading hold at the LNG loading terminal reaches the loading gas pipeline through the loading gas phase arm, is reduced in pressure by the pressure regulating valve installed on the loading gas pipeline, and is then transported to the BOG main pipe of the LNG receiving station to maintain the operation of the entire BOG system within the normal pressure range (e.g., 15-24 kPag). The gas phase pressure gauge installed on the top of the LNG storage tank converts the detected pressure signal into an electrical signal and controls the opening of the second pressure control valve on the loading gas pipeline through a hard line. When the pressure on the loading gas pipeline is higher than the gas phase pressure of the LNG storage tank, the opening of the second pressure control valve is closed. Conversely, the opening of the second pressure control valve is opened to keep the set pressure of the loading gas pipeline consistent with the pressure of the LNG storage tank (e.g., 15-24 kPag). The pressure of the second LNG carrier cabin and the pressure of the LNG storage tank are slightly higher than the pressure of the first LNG carrier cabin, to ensure that the BOG gas can be continuously returned to the LNG unloading cabin without pressurization, thereby balancing the pressure of the LNG unloading;

[0029] (3) The gas phase pressure gauge installed on the top of the LNG storage tank converts the BOG system pressure signal into a digital signal, transmits it to the logic unit of the central control room, and compares it with the preset pressure value. If the pressure is higher than this pressure value, the signal is transmitted to the BOG compressor via a hard line, and the load of the BOG compressor is directly increased. On the contrary, if the pressure is lower than the set pressure value, the load of the BOG compressor is reduced to keep the BOG system pressure within a stable range. If the pressure of the BOG system is significantly higher than the high pressure alarm setting value, on the one hand, the BOG gas is released to the torch for combustion through the third pressure control valve, and on the other hand, the cut-off valve of the mooring pipeline can be urgently closed after confirmation by the operator in the central control room, the loading operation is stopped, and the unloading condition of the traditional LNG receiving station is restored;

[0030] (4) When there is no LNG unloading, the loading pump installed in the LNG storage tank is used for loading. The LNG from the loading pump is transported to the loading main pipe and then to the loading dock. It is transported to the LNG loading cabin by the loading liquid phase arm installed at the loading dock. At this time, the check valve installed on the mooring pipeline is opened to prevent the loaded LNG from returning to the unloading main pipe. When the LNG receiving station is operating normally and there is no unloading or loading operation, the cold preservation cycle of the entire mooring pipeline is realized through the shut-off valve, which reduces the use frequency of the shut-off valve and extends the service life of the entire mooring pipeline and the shut-off valve.

[0031] Since the pressure of the low-pressure LNG pipeline needs to take into account the operating pressure of the recondenser, the pressure is relatively high, but the pressure required in the cabin is relatively low. The BOG flash vaporization caused by the pressure drop is significantly high. On the other hand, excessively high operating pressure will cause ineffective pressure loss, increasing the energy consumption of the entire LNG receiving station. To this end, a separate loading pump is installed in the LNG storage tank, and a loading pipeline is installed that is different from the low-pressure LNG system. The loading pump has a low head of about 130-180m LNG, preferably 160m LNG, and only needs to take into account the head of various LNG ships. This will significantly reduce this part of the pressure loss and greatly reduce the BOG generated during the loading process. A separate low-head (about 160m LNG) loading pump is installed in the LNG storage tank, and a separate loading pipeline is different from the low-pressure LNG main pipe, which is used for loading only without unloading.

[0032] A pneumatic shut-off valve and check valve with a high-pressure interlock are installed on the mooring pipeline. The loading function can also be carried out by unloading and berthing. The check valve prevents backflow in the mooring pipeline, and the shut-off valve ensures cold circulation, reducing energy consumption for the entire loading system and equipment operation. The shut-off valve's automatic opening and closing function is controlled by the gas phase pressure control device on the top of the LNG storage tank. In emergency conditions, such as when the BOG system pressure rises to 26kPag, the LNG storage tank's pressure alarm activates the interlock of the XV valve. After confirmation by the operator, the XV valve automatically closes, stopping the loading operation and resuming normal unloading operations. That is, the route from the LNG unloading hold to the LNG storage tank is interrupted without berthing.

[0033] The unloading pipeline flow control loop converts the flow signal into an electrical signal through the first flow control device installed on the unloading main pipe, and transmits it to the operator through a hard line for comparative operation. The signal is then transmitted to the operator of the loading pipeline flow control loop, and the actuator adjusts the valve group opening to automatically adjust the loading flow. The remaining LNG is directly unloaded into the LNG storage tank.

[0034] The LNG tank pressure control circuit controls overpressure in stages. The pressure control circuit installed in the loading pipeline can directly control the opening of the PV valve at the loading pump outlet, stabilizing the pressure in the loading main.

[0035] In order to adapt to the ship types with different volume ranges, the loading flow regulating valve is a two-stage controlled regulating valve. The large valve is used to fill large ships, and the small valve is used to fill small LNG ships, which expands the scope of loading.

[0036] The present invention has the following beneficial effects:

[0037] 1. Due to the use of a relatively small lift LNG loading pump (for example, 130-180m LNG, preferably 160m LNG), the amount of BOG generated is reduced, and the energy consumption of the loading process is reduced.

[0038] 2. Since a regulating valve group is set on the loading pipeline, the large valve is suitable for large ships, and the small valve is suitable for small LNG ships. It is flexible and adaptable, and the applicable tank volume range is increased.

[0039] 3. Effectively control the BOG system pressure stability of the entire system and reduce the flash volume of the BOG system.

[0040] 4. Based on the pressure control of the BOG system in traditional LNG receiving stations, an interlocking circuit of the pressure control shut-off valve pipe section is added to stabilize the pressure of the BOG system using multiple methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a schematic diagram of a system for realizing simultaneous loading and unloading of ships at an LNG receiving station according to the present invention.

[0042] Figure 2 This is a schematic diagram of the system when loading and unloading a ship are performed simultaneously.

[0043] Figure 3 This is a schematic diagram of the system when only loading is done.

[0044] Reference numerals:

[0045] First LNG carrier 1, second LNG carrier 2, LNG storage tank 3, BOG compressor 4, flare 5, flare separator 6, loading pump 7,

[0046] Unloading liquid phase arm l1, unloading main pipe L1, LNG loading pipeline L2, mooring pipeline L3, loading liquid phase arm l2, storage tank gas phase pipeline L4, unloading gas phase arm l3, unloading gas phase pipeline L5, loading gas phase arm l4, loading gas phase pipeline L6, BOG main pipe L7, branch pipe L8,

[0047] PV-01 is the first pressure control valve for the unloading gas pipeline, XV-01 is the shut-off valve for the mooring pipeline, GB-01 is the shut-off valve for the small-size pipeline, CH-01 is the check valve on the mooring pipeline, PV-02 is the second pressure control valve for the loading gas pipeline, PV-03 is the third pressure control valve on the BOG input pipeline of the flare separator, FV-02 is the flow regulating valve, FV-03 is the flow regulating valve, FV-01 is the flow regulating valve, HV-01 is the regulating shut-off valve for the loading pump outlet, the first flow meter FIC-01, the second flow meter FIC-02, the gas pressure gauge PIC-01. DETAILED DESCRIPTION

[0048] Below, we will combine the Figure 1 、 2 , 3 and Examples further illustrate the present invention in detail.

[0049] like Figure 1-3 As shown, the present invention provides a system for simultaneously loading and unloading ships at an LNG receiving station, the system comprising: a first LNG carrier 1 for unloading LNG, a second LNG carrier 2 for loading LNG, an LNG storage tank 3 for storing LNG, a BOG compressor 4 for regulating the BOG pressure of the system, a flare 5 for venting BOG, and a flare liquid separator 6 for temporarily storing BOG;

[0050] The unloading liquid phase arm l1 of the first LNG carrier 1 merges into a discharge main pipe L1 connected to the LNG storage tank 3. The LNG storage tank 3 is provided with a loading pump 7. The loading pump 7 is connected to the loading liquid phase arm 12 through the LNG loading pipeline L2. The loading liquid phase arm l2 is further connected to the second LNG carrier 2. The unloading main pipe L1 and the LNG loading pipeline L2 are connected to each other through the mooring pipeline L3.

[0051] The unloading main pipe L1 is equipped with a first flowmeter FIC-01 for controlling the total unloading flow rate of the first LNG carrier. The mooring pipeline L3 is equipped with a shut-off valve XV-01 for cutting off the LNG transport to the second LNG carrier and a check valve CH-01 for preventing LNG backflow in the pipeline. The LNG loading pipeline L2 is equipped with a loading flow regulating valve (which may include a flow regulating valve FV-02 for controlling the flow rate to the small second LNG carrier and a flow regulating valve FV-03 for controlling the flow rate to the large second LNG carrier, arranged in parallel). A second flowmeter FIC-02 is installed on the LNG loading pipeline L2, which is interlocked with the loading flow regulating valves (FV-02, FV-03) and the first flowmeter FIC-01 for control.

[0052] The LNG storage tank 3 is provided with a storage tank gas phase pipeline L4, the unloading gas phase arm l3 of the first LNG transport ship is connected to the unloading gas phase pipeline L5, and the loading gas phase arm l4 of the second LNG transport ship is connected to the loading gas phase pipeline L6. The storage tank gas phase pipeline L4, the unloading gas phase pipeline L5, and the loading gas phase pipeline L6 are all connected to the BOG main pipe L7. The BOG main pipe L7 branches out a branch pipe L8 connected to the BOG compressor 4. The BOG main pipe L7 is also connected to the inlet of the flare separator tank 6, and the outlet of the flare separator tank 6 is connected to the flare 5.

[0053] In one embodiment, the LNG loading pipeline may be further provided with a loading pump outlet regulating shut-off valve HV-01. The loading pump 7 is preferably a low-lift (e.g., 130-180 m LNG, preferably 160 m LNG) pump provided in the LNG storage tank 3 and used only for loading.

[0054] In another embodiment, a gas phase pressure gauge PIC-01 is provided on the top of the LNG storage tank for monitoring the gas phase pressure in the entire system and interlocking control with the shut-off valve XV-01 on the mooring pipeline.

[0055] In one embodiment, a stop valve GB-01 is provided on the mooring pipeline in parallel with the mooring pipeline for ensuring cold circulation.

[0056] In another embodiment, a first pressure control valve PV-01 is provided on the unloading gas phase pipeline, a second pressure control valve PV-02 is provided on the loading gas phase pipeline, and a third pressure control valve PV-03 is provided on the BOG input pipeline of the flare separator tank.

[0057] The gas phase pressure gauge PIC-01 on the top of the LNG storage tank is interlocked with the second pressure control valve PV-02, the BOG compressor and the third pressure control valve PV-03.

[0058] The first flow meter FIC-01 and the gas phase pressure gauge PIC-01 on the top of the LNG storage tank are connected to the central control room (control unit, such as a computer).

[0059] A third flow regulating valve FV-01 may be provided on the unloading main pipe L1 to control the flow of unloading to the LNG storage tank.

[0060] An orifice flow meter (not shown) may be provided on the discharge main pipe.

[0061] Example

[0062] (1) One LNG carrier (as the first LNG carrier, 270,000 m 3 LNG) arrived at the unloading terminal, and the second LNG carrier (40,000m 3 ) After arriving at the loading dock, the total unloading flow (12000m3 ) The LNG receiving station operator sets the return loading flow (3200m 3 ), through the transfer pump in the first LNG transport, the liquid phase arm of the unloading is transported to the unloading main pipe, and then sent to the LNG loading pipeline through the mooring pipeline. The first flow meter FIC-01 installed on the unloading main pipe controls the opening of the flow regulating valve FV-02 on the LNG loading pipeline through the flow set by the LNG receiving station engineer, so that the LNG flow entering the second LNG transport ship is stable at the set value (3200m 3 ), the remaining LNG is transported to LNG storage tanks;

[0063] (2) The evaporated gas (35kPag) replaced after LNG enters the second LNG carrier cabin at the LNG loading terminal is transported to the BOG main of the LNG receiving station through the loading gas phase pipeline and the pressure regulating valve to maintain the operation of the entire BOG system within the normal pressure range (15-24kPag). The first flow meter FIC-01 installed on the top of the LNG storage tank converts the detected pressure signal into an electrical signal and controls the opening of the second pressure control valve PV-02 on the loading gas phase pipeline through a hard line. When the pressure on the loading gas phase pipeline is higher than the gas phase pressure of the LNG storage tank, the opening of the PV-02 valve is closed. On the contrary, the opening of the PV-02 valve is opened to keep the set pressure of the loading gas phase pipeline consistent with the pressure of the LNG storage tank (15-24kPagkPag). The pressure of the second LNG carrier cabin and the pressure of the LNG storage tank are slightly higher than the pressure of the first LNG carrier cabin to ensure that the BOG gas can be continuously returned to the LNG unloading cabin without pressurization, thereby balancing the pressure of the LNG unloading;

[0064] (3) The gas phase pressure gauge installed on the top of the LNG storage tank converts the BOG system pressure signal into a digital signal, transmits it to the logic unit of the central control room, and compares it with the preset pressure value. If the pressure is higher than this pressure value, the signal is transmitted to the BOG compressor via a hard line, and the load of the BOG compressor is directly increased. On the contrary, if the pressure is lower than the set pressure value, the load of the BOG compressor is reduced to keep the BOG system pressure within a stable range. If the pressure of the BOG system is significantly higher than the high pressure alarm setting value, on the one hand, the BOG gas is released to the torch for combustion through the third pressure control valve PV-03. On the other hand, the cut-off valve of the mooring pipeline can be urgently closed after confirmation by the operator in the central control room, the loading operation is stopped, and the unloading condition of the traditional LNG receiving station is restored.

[0065] (4) When there is no second LNG transport ship, the loading pump (lift of 160m LNG) installed in the LNG storage tank is used for loading. The LNG from the loading pump is transported to the loading main pipe, and then transported to the loading dock. It is transported to the LNG loading cabin by the loading liquid phase arm installed at the loading dock. At this time, the check valve CH-01 installed on the mooring pipeline is opened to prevent the loaded LNG from returning to the unloading main pipe. When the LNG receiving station is operating normally and there is no unloading or loading operation, the cold preservation cycle of the entire mooring pipeline is realized through the shut-off valve GB-01, which reduces the use frequency of the shut-off valve and extends the service life of the entire mooring pipeline and the shut-off valve.

[0066] Consistent with conventional LNG storage tank gas phase pressure control of the BOG compressor load, the BOG system pressure is maintained stable by controlling the compressor load through the LNG storage tank pressure. In addition, when the BOG system pressure further increases, this can be achieved not only by opening the third pressure control valve (flare relief valve) PV-03, but also by using the pressure interlock described in the present invention to close the XV-01 valve of the mooring pipeline, allowing the entire system to resume the operation of unloading from ships to LNG storage tanks at a traditional LNG receiving station, reducing the impact of BOG system instability.

[0067] Advantages of the above system include:

[0068] (1) Greatly reduce the energy consumption of LNG receiving stations, reduce the amount of BOG generated during loading and unloading, and avoid unnecessary economic losses.

[0069] (2) The loading operation is more flexible, and the loading and unloading operations can be carried out simultaneously. At the same time, low-lift loading pumps can also be used for loading.

[0070] (3) Increase the stability of the loading process. Through step-by-step pressure control, ensure that the pressure of the entire BOG system is maintained within a stable pressure range, ensuring the stable operation of the entire loading process.

[0071] (4) Improve the safety of the loading process. When conditions that are not conducive to the simultaneous loading and unloading operations occur, the operation of a certain system can be directly shut down through the automatic control system to restore to a safer operating process.

[0072] (5) Enhance the adaptability of ship types with loading functions, which not only takes into account the flow requirements of small ship loading, but also can use loading pumps to supplement the flow when loading large ships to meet the functional requirements of loading large ships.

[0073] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A system for simultaneously loading and unloading ships at an LNG receiving station, comprising: The first LNG carrier planned to unload LNG, the second LNG carrier planned to load LNG, LNG storage tanks for storing LNG, BOG compressors for regulating the system's BOG pressure, flares for venting BOG, and flare separators for temporarily storing BOG; The unloading liquid phase arm of the first LNG carrier merges into a discharge main pipe connected to the LNG storage tank. The LNG storage tank is equipped with a loading pump, which is connected to the loading liquid phase arm through the LNG loading pipeline. The loading liquid phase arm is further connected to the second LNG carrier. The unloading main pipe and the LNG loading pipeline are connected to each other through a mooring pipeline. The loading pump has a head of 130-180mLNG. The unloading main pipe is provided with a first flow meter for controlling the total unloading flow of the first LNG carrier. The over-berthing pipeline is provided with a shut-off valve for cutting off the LNG transport to the second LNG carrier and a check valve for preventing LNG backflow in the pipeline. The LNG loading pipeline is provided with a loading flow regulating valve. A second flow meter is provided on the LNG loading pipeline to control the loading flow in conjunction with the loading flow regulating valve and the first flow meter. The LNG storage tank is equipped with a tank gas phase pipeline. The unloading gas phase arm of the first LNG carrier is connected to the unloading gas phase pipeline. The loading gas phase arm of the second LNG carrier is connected to the loading gas phase pipeline. The tank gas phase pipeline, the unloading gas phase pipeline, and the loading gas phase pipeline are all connected to the BOG main pipe. A branch pipe is separated from the BOG main pipe to connect to the BOG compressor. The BOG main pipe is also connected to the inlet of the flare separator tank. The outlet of the flare separator tank is connected to the flare. A gas pressure gauge is installed on the top of the LNG storage tank to monitor the gas pressure in the entire system and is interlocked with the shut-off valve on the mooring pipeline. The first pressure control valve is installed on the unloading gas pipeline, the second pressure control valve is installed on the loading gas pipeline, and the third pressure control valve is installed on the BOG input pipeline of the flare separator tank. The gas phase pressure gauge on the top of the LNG storage tank is interlocked with the second pressure control valve, BOG compressor and the third pressure control valve. The first flow meter and the gas phase pressure gauge on the top of the LNG storage tank are connected to the central control room. A stop valve is provided on the mooring pipeline and is connected in parallel with the mooring pipeline to ensure cold circulation.

2. The system according to claim 1, wherein: The LNG loading pipeline is further provided with a loading pump outlet regulating shut-off valve.

3. The system according to claim 1 or 2, characterized in that A third flow regulating valve is provided on the unloading main pipe to control the flow of unloading to the LNG storage tank.

4. The system according to claim 1, wherein: The loading flow regulating valve includes a flow regulating valve arranged in parallel for controlling the flow of loading into a small LNG carrier and a flow regulating valve for controlling the flow of loading into a large LNG carrier.

5. A method for achieving simultaneous loading and unloading of ships at an LNG receiving station using the system according to any one of claims 1 to 4, the method comprising: (1) The LNG in the cabin of the first LNG carrier is delivered to the unloading main pipe by the transfer pump in the LNG unloading ship through the unloading liquid phase arm, and then sent to the LNG loading pipeline through the mooring pipeline. The first flow meter installed on the unloading main pipe controls the opening of the flow regulating valve on the loading bus, so that the LNG flow entering the second LNG carrier is stable at the set value, and the remaining LNG is delivered to the LNG storage tank; (2) The evaporation gas replaced after LNG enters the second LNG carrier cabin reaches the loading gas phase pipeline through the loading gas phase arm, and is transported to the BOG main pipe of the LNG receiving station after being reduced in pressure by the pressure regulating valve installed on the loading gas phase pipeline to maintain the operation of the entire BOG system within the normal pressure range. The gas phase pressure gauge on the top of the LNG storage tank controls the opening of the second pressure control valve on the loading gas phase pipeline. When the pressure on the loading gas phase pipeline is higher than the gas phase pressure of the LNG storage tank, the opening of the second pressure control valve is closed. On the contrary, the opening of the second pressure control valve is opened to keep the set pressure of the loading gas phase pipeline consistent with the pressure of the LNG storage tank. (3) The gas phase pressure gauge installed on the top of the LNG storage tank converts the BOG system pressure signal into a digital signal, transmits it to the logic unit of the central control room, and compares it with the preset pressure value. If the pressure is higher than the preset pressure value, the BOG compressor load is directly increased. Otherwise, the BOG compressor load is reduced. If the pressure of the BOG system is significantly higher than the high pressure alarm setting value, on the one hand, the BOG gas is released to the flare through the third pressure control valve, and on the other hand, the shut-off valve of the mooring pipeline is closed, the loading operation is stopped, and the unloading condition of the traditional LNG receiving station is restored. (4) When there is no LNG unloading, the loading pump is used for loading. The LNG from the loading pump is transported to the loading main pipe and then transported to the LNG loading hold through the loading liquid phase arm. At this time, the check valve on the mooring pipeline is opened.

6. The method according to claim 5, characterized in that The normal pressure range of the BOG system is 15-24kPag.

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