Ship-to-ship liquefied natural gas unloading safety control system and method

Abstract

The invention provides a safety control system for ship-to-ship liquefied natural gas unloading. The system comprises a liquefied transport ship, a floating storage ship, a dynamic control module and a land terminal processing mechanism. The land terminal processing mechanism comprises a gas discharge unit and a liquid storage unit; the dynamic control module is composed of an unloading regulation and control unit, a temperature regulation and control unit and a pressure regulation and control unit. The output end of the transport ship is connected with the floating storage ship through a first liquid channel; the connecting output end of the floating storage ship is connected with the liquid storage unit through a second liquid channel; the floating storage ship is connected with the transport ship through a first gaseous loop; the floating storage ship is connected with the gas discharge unit through a second gaseous loop; the dynamic control module controls the first liquid channel, the second liquid channel, the first gaseous loop and the second gaseous loop to work through a remote network. The potential safety hazard caused by improper unloading rate control, temperature difference over-limit and insufficient pressure regulation of the liquefied natural gas in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of liquefied natural gas loading, unloading, storage and transportation, and in particular to a ship-to-ship liquefied natural gas unloading and safety dynamic control system and method thereof. Background Art

[0002] As a clean and efficient energy source, liquefied natural gas (LNG) occupies an important position in the global energy structure. The transportation and storage of LNG usually need to maintain an extremely low temperature (-162℃) to maintain its liquid state, which places high technical requirements on related loading and unloading and storage and transportation equipment.

[0003] Ship-to-ship (STS) operation is an important LNG transshipment method, which is widely used in offshore floating terminals, small receiving stations and emergency gas supply scenarios. STS operation realizes the transshipment of LNG through the hose connection between the LNG carrier and the FSU or FSRU, but in the existing technology, the LNG unloading operation still has the following technical difficulties:

[0004] 1) Insufficient minimum unloading rate setting: During the unloading process, if the flow rate is too low, it may cause "plug flow", affecting the stable flow of LNG in the pipeline and causing unsafe operation of the unloading system;

[0005] 2) Limitation of the maximum unloading rate: In the prior art, the unloading rate is mostly determined by a single factor (such as pipeline flow limit or evaporation gas processing capacity), and fails to comprehensively consider the dynamic adjustment of multiple factors, resulting in low system efficiency;

[0006] 3) Insufficient temperature difference control: If there is a large temperature difference between the cargo tanks of the transport ship and the floating storage vessel, it may cause excessive thermal stress on the pipelines and equipment, increasing the risk of failure. At the same time, the amount of boil-off gas (BOG) generated under the high temperature difference will increase dramatically, increasing the burden on the system;

[0007] 4) Low operational adaptability: Existing LNG unloading operations mostly use a single hose connection method, which has poor efficiency and stability in high-flow scenarios and fails to fully utilize the advantages of the unloading arm in high-intensity and high-flow scenarios.

[0008] At present, the research on LNG unloading operation mainly focuses on the optimization of single operating equipment or static analysis of fluid characteristics, lacks dynamic control of unloading rate and multi-scenario adaptability design, and cannot meet the operation requirements under variable working conditions. Among them:

[0009] There are already some invention patents to solve the problems of insufficient temperature difference control, complex calculation of unloading rate and poor adaptability of connecting equipment in the prior art. For example:

[0010] CN107514541A discloses a floating storage and regasification unit for continuously and uninterruptedly supplying natural gas fuel to port natural gas users and power plants. The unit includes at least one medium-pressure zone storage tank provided on a hull, and a low-pressure zone buffer tank, a submersible pump box, and an LNG regasification device that are sequentially connected to the medium-pressure zone storage tank. Limited by industry standards, the liquid outlet of the low-cost medium-pressure zone storage tank with a carbon steel outer layer is provided at the top of the tank body, and the continuous flow of LNG is achieved through the working air pressure difference between the medium-pressure zone storage tank and the low-pressure zone buffer tank. However, this patent still has the problem that the sizes and weights of the medium-pressure zone storage tank and the low-pressure zone buffer tank need to be further optimized to improve the storage and transportation efficiency.

[0011] CN102654239B discloses a regasification system for a liquefied natural gas receiving terminal. The system includes a floating regasification ship module and an onshore module. The floating regasification ship module includes a floating regasification ship, and at least 1 storage tank is provided on the floating regasification ship, and a backflow pump and an in-tank pump are provided in the storage tank. This system overcomes the limitations of conventional large onshore LNG receiving terminals, such as long construction periods, being greatly affected by the surrounding environment and land resources, and poor flexibility. However, this patent still has the problem that the structure and layout of the floating regasification ship module need to be further optimized to improve the efficiency and reliability of the regasification facilities.

[0012] Therefore, there is an urgent need for a ship-to-ship liquefied natural gas unloading and safety dynamic control method that can comprehensively and dynamically adjust the unloading rate, optimize the temperature difference control, and adapt to a variety of connecting devices to improve the safety, economy, and efficiency of operations. Summary of the Invention

[0013] In view of the technical problems existing in the prior art, the present invention provides a ship-to-ship liquefied natural gas unloading and safety dynamic control system and method. Through scientific and reasonable unloading rate control and parameter optimization, the present invention solves the problems of insufficient temperature difference control, complex unloading rate calculation, and poor adaptability of connecting devices existing in the prior art; the present invention improves the safety and efficiency of liquefied natural gas unloading operations, and is particularly suitable for liquefied natural gas transfer operations between liquefied natural gas carriers and floating storage devices.

[0014] In order to solve the problems existing in the prior art, the present invention adopts the following technical solutions:

[0015] A safety control system for ship-to-ship liquefied natural gas unloading, the system includes a liquefied gas carrier, a floating storage liquefied gas ship, a dynamic control module and a land terminal processing mechanism; the land terminal processing mechanism includes a gas discharge unit and a liquid storage unit; the dynamic control module is composed of an unloading regulation unit, a temperature regulation unit and a pressure regulation unit; the output end of the carrier is connected to the floating storage ship through a first liquid channel; the connection output end of the floating storage ship is connected to the land liquid storage unit through a second liquid channel; the floating storage ship is connected to the carrier through a first gaseous circuit; and the floating storage ship is connected to the land gas discharge unit through a second gaseous circuit; the dynamic control module controls the operation of the first liquid channel, the second liquid channel, the first gaseous circuit and the second gaseous circuit respectively;

[0016] Wherein:

[0017] The unloading regulation unit is used to judge whether the natural gas liquid unloading rate of the first liquid channel and the second liquid channel is within the safe unloading area range; if the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area; then start the temperature regulation unit to control the temperature difference between the floating storage ship and the carrier and increase the unloading rate; if the natural gas liquid unloading rate is greater than the upper limit of the safe unloading area; then start the pressure regulation unit to dynamically control the gaseous natural gas to be input into the carrier and the gas discharge unit respectively through the first gaseous circuit and the second gaseous circuit and reduce the unloading rate.

[0018] Furthermore, the lower limit rate of the safe unloading area is obtained by calculating according to the following steps:

[0019] Calculate the thermal resistance by the following formula:

[0020]

[0021] Where, D o and D i , are the outer diameter and inner diameter of the unloading channel respectively, and k is the thermal conductivity.

[0022] Calculate the heat flux per unit length of the liquid channel by the following formula:

[0023]

[0024] Where, T ∞ is the ambient temperature, T LNG is the temperature of the carrier, and the R is the thermal resistance value calculated according to the channel parameters;

[0025] Calculate the gasification mass flow per unit length of the liquid channel by the following formula:

[0026]

[0027] Among them, L v is the latent heat of vaporization of the transport ship, and the q is the heat flux per unit length;

[0028] The lower limit rate of the unloading area is obtained by the following calculation formula:

[0029]

[0030] Among them, ρ is the density of liquefied natural gas, with the unit of kg / m 3 , and n is the safety factor.

[0031] Furthermore, the upper limit rate of the safe unloading area is calculated according to the following formula:

[0032] Q max = 3600·v max ·A

[0033] Among them, Q max is the maximum unloading rate, with the unit of m 3 / h; v max : the maximum flow velocity of the channel, with the unit of m / s, A: the cross-sectional area of the channel, m 2 ; 3600: the time conversion coefficient, converting seconds to hours; among them:

[0034] The calculation formula for the cross-sectional area of the hose is as follows:

[0035]

[0036] Among them, D is the inner diameter of the hose.

[0037] Furthermore, the temperature control unit controls to adjust the temperature difference between the floating storage ship and the transport ship within 2.5°C to 3.5°C by using the binary phase diagram method.

[0038] Furthermore, the pressure control unit regulates the pressure difference between the first gaseous circuit and the second gaseous circuit within the range of 1 to 10 kPa.

[0039] Furthermore, the first liquid channel, the second liquid channel, the first gaseous circuit, and the second gaseous circuit are all composed of metal hoses with connectors; the inner layer of the metal hose is made of low-temperature-resistant stainless steel alloy; the outer layer of the metal hose is coated with a flexible composite material; the connectors of the metal hose are made of ultra-low-temperature nickel alloy materials; the diameter range of the metal hose is 6 to 10 inches, and the length range of the metal hose is 20 to 50 meters.

[0040] The present invention also adopts the following technical solutions, including the following steps:

[0041] S1. The dynamic control module performs a precooling cycle on the first liquid channel and the second liquid channel; meanwhile, the dynamic control module adjusts the unloading rates of the first liquid channel and the second liquid channel to within the maximum unloading rate range;

[0042] S2. The unloading control unit determines whether the natural gas liquid unloading rates of the first liquid channel and the second liquid channel are within the safe unloading area range;

[0043] If the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area; that is:

[0044]

[0045] Then start the temperature control module to control the temperature difference between the floating storage ship and the transport ship within 2.5°C to 3.5°C;

[0046] If the natural gas liquid unloading rate is greater than the upper limit rate of the safe unloading area; that is:

[0047] Q max =3600·v max

[0048] Then start the pressure control module to dynamically control the gaseous natural gas to be input into the transport ship and the gas discharge unit through the first gaseous circuit and the second gaseous circuit respectively; and the pressure control module regulates the pressure difference between the first gaseous circuit and the second gaseous circuit within the range of 1 to 10 kPa;

[0049] S3. After the unloading operation is completed, the dynamic control module closes the first liquid channel and the first gas channel.

[0050] Beneficial effects

[0051] 1. The present invention avoids the problems of "piston flow", gas-liquid separation and system overpressure by dynamically adjusting the minimum unloading rate and the maximum unloading rate; the minimum unloading rate is achieved by the principle of the binary phase diagram of liquefied natural gas, combined with the pressure regulation function of the onshore terminal and the floating storage and regasification unit or the floating storage unit, to achieve precise control of the temperature difference. Preferably, the temperature difference is controlled within 3°C, thereby reducing the thermal stress of the equipment and the generation amount of evaporation gas during the unloading process, and improving the overall stability of the system.

[0052] 2. The present invention improves the versatility and adaptability of various operation scenarios such as ship-to-ship transfer of liquefied natural gas (LNG), onshore terminal reception and pressure regulation through two operation modes of hose connection and unloading arm connection. The present invention supports a multi-pipe parallel unloading mode to meet different flow requirements and improve operation flexibility.

[0053] 3. By dynamically adjusting the unloading rate, the present invention effectively reduces the incidence of the liquefied natural gas rollover effect, decreases the generation amount of the evaporation gas during the unloading process, preferably reduces the BOG emission amount by 10% - 15%, and improves the economy and safety of the liquefied natural gas transportation. In addition, it can prevent the overpressure of the liquid cargo tank and the gas-liquid separation problem, and ensure the safety and stability of the whole process of the unloading operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 FIG. is a schematic structural diagram of a ship-to-ship liquefied natural gas unloading safety control system of the present invention;

[0055] Figure 2 FIG. is a schematic diagram of the channel connection in a ship-to-ship liquefied natural gas unloading safety control system of the present invention;

[0056] Figure 3 FIG. is a schematic diagram of dividing the safe unloading area in a ship-to-ship liquefied natural gas unloading safety control system of the present invention.

[0057] DESCRIPTION OF THE REFERENCE NUMERALS:

[0058] 100 - transport ship, 101 - first liquid channel, 102 - first gas circuit;

[0059] 200 - floating storage ship, 201 - second liquid channel, 202 - second gas circuit;

[0060] 300 - dynamic control module, 301 - unloading regulation unit, 302 - temperature regulation unit, 303 - pressure regulation unit, 304 - safe unloading area

[0061] 400 - land terminal processing mechanism, 401 - gas emission unit, 402 - liquid storage unit DETAILED DESCRIPTION OF THE EMBODIMENTS

[0062] The following will make a detailed description of the present invention in conjunction with the attached Figure 1 ~:

[0063] As Figure 1As shown in the figure, the present invention provides a safety control system for ship-to-ship liquefied natural gas unloading. The system includes a liquefied gas carrier 100, a floating storage vessel 200, a dynamic control module 300, and a land terminal processing mechanism 400. The land terminal processing mechanism 400 includes a gas discharge unit 401 and a liquid storage unit 402. The dynamic control module 300 is composed of a unloading regulation unit 301, a temperature regulation unit 302, and a pressure regulation unit 303. The output end of the carrier 100 is connected to the floating storage vessel 200 through a first liquid channel 101. The connection output end of the floating storage vessel 200 is connected to the liquid storage unit 402 through a second liquid channel 201. The floating storage vessel 200 is connected to the carrier 100 through a first gas circuit 102. And the floating storage vessel 200 is connected to the gas discharge unit 401 through a second gas circuit 202. In the present invention, the first liquid channel 101, the second liquid channel 201, the first gas circuit 102, and the second gas circuit 202 are all composed of metal hoses with connectors. The inner layer of the metal hose is made of low-temperature resistant stainless steel alloy. The outer layer of the metal hose is coated with a flexible composite material. The connectors of the metal hoses are made of cryogenic nickel alloy materials. The diameter range of the metal hose is 6 - 10 inches, and the length range of the metal hose is 20 - 50 meters. In the present invention, both the first liquid channel 101 and the second liquid channel 201 are composed of three metal pipes, as Figure 2 shown.

[0064] Among them: The dynamic control module 300 controls the operation of the first liquid channel 101, the second liquid channel 201, the first gas circuit 102, and the second gas circuit 202 respectively through a remote network.

[0065] The unloading regulation unit 301 is used to judge whether the natural gas liquid unloading rate of the first liquid channel 101 and the second liquid channel 201 is within the range of the safe unloading area 304, as Figure 3 shown.

[0066] The pressure regulation unit 303 is used to judge whether the natural gas liquid unloading rate of the first gas channel 102 and the second gas channel 202 is within the range of the safe unloading area 304, as Figure 3 shown.

[0067] If the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area, then start the temperature regulation unit to control the temperature difference between the floating storage vessel and the carrier. Among them: The lower limit rate of the safe unloading area is obtained by calculating according to the following steps:

[0068] Calculate the thermal resistance by using the following formula:

[0069]

[0070] Among them, D o and D i are the outer diameter and inner diameter of the unloading hose respectively, and k is the thermal conductivity of the hose material.

[0071] The heat flux per unit length is calculated by the following formula:

[0072]

[0073] Among them, T ∞ is the ambient temperature, T LNG is the temperature of LNG, and the R is the thermal resistance value calculated according to the hose parameters;

[0074] The gasification mass flow per unit length is calculated by the following formula:

[0075]

[0076] Among them, L v is the latent heat of vaporization of LNG, and the q is the heat flux per unit length;

[0077] The lower limit rate of the unloading area is calculated by the following formula:

[0078]

[0079] Among them, ρ is the density of liquefied natural gas (LNG), with the unit of kg / m 3 , and n is the safety factor.

[0080] If the unloading rate of the natural gas liquid is greater than the upper limit of the safe unloading area; then the pressure control unit is activated to dynamically control the gaseous natural gas to be input into the transportation ship and the gas discharge unit through the first gaseous circuit and the second gaseous circuit respectively and reduce the unloading rate, where:

[0081] The upper limit rate of the safe unloading area is calculated by the following formula:

[0082] Q max = 3600·v max ·A

[0083] Among them, Q max is the maximum unloading rate, with the unit of m 3 / h; v max : the maximum flow velocity of the channel, with the unit of m / s, A: the cross-sectional area of the channel, m 2 ; 3600: the time conversion coefficient to convert seconds to hours; where:

[0084] The cross-sectional area formula of the hose is as follows:

[0085]

[0086] Among them, D: inner diameter of the hose.

[0087] The present invention realizes the safe unloading of liquefied natural gas from ship to ship by the following steps

[0088] S1. The dynamic control module performs a precooling cycle on the first liquid channel and the second liquid channel; meanwhile, the dynamic control module adjusts the unloading rates of the first liquid channel and the second liquid channel to within the maximum unloading rate range;

[0089] S2. The unloading control unit determines whether the natural gas liquid unloading rates of the first liquid channel and the second liquid channel are within the safe unloading area range;

[0090] If the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area; that is:

[0091]

[0092] Then start the temperature control module to control the temperature difference between the floating storage ship and the transport ship within 2.5°C to 3.5°C;

[0093] If the natural gas liquid unloading rate is greater than the upper limit rate of the safe unloading area; that is:

[0094] Q max = 3600·v max ·

[0095] Then start the pressure control module to dynamically control the gaseous natural gas to be input into the transport ship and the gas discharge unit through the first gaseous circuit and the second gaseous circuit respectively; and the pressure control module regulates the pressure difference between the first gaseous circuit and the second gaseous circuit within the range of 1 - 5 kPa;

[0096] S3. After the unloading operation is completed, the dynamic control module closes the first liquid channel and the first gas channel. Specific implementation method 1:

[0098] This embodiment is applicable to the operation scenario of unloading natural gas from a liquefied natural gas (LNG) carrier to a floating storage unit (FSU) or a floating storage and regasification unit (FSRU). The dynamic control module performs a pre-cooling cycle on the first liquid channel and the second liquid channel; meanwhile, the dynamic control module adjusts the unloading rates of the first liquid channel and the second liquid channel to within the maximum unloading rate range; ensuring the safety and efficiency of the natural gas unloading operation. The liquefied natural gas is transferred from the carrier to the cargo tank of the floating storage vessel through 3 eight-inch metal hoses (20 meters in length); the floating storage vessel also supplies liquefied gas to the onshore terminal through 3 eight-inch metal hoses (20 meters in length). The weather spaces of the carrier and the floating storage vessel are connected by 1 hose, namely the first gaseous circuit; the gaseous treatment of the floating storage vessel and the onshore terminal is connected by 1 hose, namely the second gaseous circuit. The carrier in this embodiment is a spherical tank with a pressure range of 5 kPa < P ship < 30 kPa; the cargo tank of the floating storage vessel is a spherical tank with 5 kPa < P FSRU / FSU < 25 kPa. The pressure control range of the gas-liquid cargo tank is adjusted through the first gaseous circuit and the second gaseous circuit to maintain the dynamic balance of the pressure during the unloading process, keeping the pressure difference between the carrier and the floating storage vessel at 10 kPa. Specific Embodiment 2

[0100] Since the floating storage vessel temporarily stops supplying gas to the onshore terminal (e.g., during maintenance or overhaul) to ensure its gas supply capacity can be restored within 6 hours to meet the terminal demand.

[0101] 1) Unloading conditions between the carrier and the floating storage vessel: The carrier is a spherical storage tank type with a capacity of 28,000 m 3 ; the floating storage vessel is a spherical storage tank type with a capacity of 125,000 m 3 ; the pressure range of the liquefied gas tank of the carrier is 5 kPa ≤ P shiip ≤ 30 kPa; the pressure of the cargo tank of the FSU is 5 kPa ≤ P FSU ≤ 25 kPa;

[0102] 2) Onshore terminal conditions: The storage tank volume is 600 m 3 ; the gasification rate required by the downstream is 100 m 3 / h.

[0103] 3) Unloading system:

[0104] The first liquid channel between the transport ship and the floating storage ship is connected by three 8-inch metal hoses, each with a length of 20 meters. The second liquid channel between the floating storage ship and the onshore terminal processing is connected by three 8-inch metal hoses, each with a length of 20 meters. It includes the maximum unloading rate of a single hose and the total unloading rate of multiple hoses in parallel. The specific calculation process can refer to the relevant description in Embodiment 1, where:

[0105] The maximum unloading rate of a single hose is Qmax,pipe = 796.2 m 3 / h

[0106] The total unloading rate of three hoses in parallel is Qmax,total = 2388.6 m 3 / h

[0107] Assume that the onshore terminal storage tank can maintain the gas supply for 6 hours. Then the floating storage ship must reserve liquefied natural gas during this period to meet the subsequent gas supply demand.

[0108] Total demand at the onshore terminal:

[0109] V demand = Q demand × T 供气 = 100 × 6 = 600 m 3

[0110] Reserved liquefied natural gas volume of the floating storage ship:

[0111] Assume that the floating storage ship needs to reserve 3,600 m 3 LNG to meet the continuous demand of the onshore terminal for 36 hours. Then the required unloading time is:

[0112]

[0113] During the unloading process, it is necessary to dynamically adjust the pressure and temperature difference between the transport ship and the floating storage ship. The specific measures are the same as those in Embodiment 1, including the following: the pressure in the cargo tank of the transport ship gradually decreases, while the pressure in the floating storage ship gradually increases; in the initial stage, by reducing the pressure in the floating storage ship (such as reducing it to 5 kPa), the temperature of the natural gas is close to the temperature of the transport ship; in cooperation with the pre-mixing cycle operation, the temperature difference is controlled within 3°C. The operation process of this embodiment is similar to that of Embodiment 1.

Claims

1. A safety control system for ship-to-ship liquefied natural gas unloading, the system comprising a liquefied gas carrier, a floating storage liquefied gas ship, a dynamic control module, and a land terminal processing mechanism; the land terminal processing mechanism includes a gas discharge unit and a liquid storage unit; characterized in that; The dynamic control module is composed of an unloading control unit, a temperature control unit, and a pressure control unit; the output end of the transport ship is connected to the floating storage ship through a first liquid channel; the output end of the floating storage ship is connected to the onshore liquid storage unit through a second liquid channel; the floating storage ship is connected to the transport ship through a first gas circuit; and the floating storage ship is connected to the onshore gas discharge unit through a second gas circuit; the dynamic control module separately controls the operation of the first liquid channel, the second liquid channel, the first gas circuit, and the second gas circuit; where: The unloading control unit is used to determine whether the natural gas liquid unloading rates of the first liquid channel and the second liquid channel are within the safe unloading area range; if the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area, the temperature control unit is activated to control the temperature difference between the floating storage ship and the transport ship and increase the unloading rate; if the natural gas liquid unloading rate is greater than the upper limit of the safe unloading area, the pressure control unit is activated to dynamically control the gaseous natural gas to be respectively input into the transport ship and the gas discharge unit through the first gas circuit and the second gas circuit and reduce the unloading rate.

2. The safety control system for ship-to-ship liquefied natural gas unloading according to claim 1, wherein: The lower limit rate of the safe unloading area is obtained by calculating according to the following steps: Calculate the thermal resistance by using the following formula: Among them, D o and D i are the outer diameter and inner diameter of the unloading channel respectively, and k is the thermal conductivity. Calculate the heat flow per unit length of the liquid channel by using the following formula: where, T ∞ is the ambient temperature, and T LNG is the temperature of the transport ship, and the aforementioned R is the thermal resistance value calculated according to the channel parameters; Calculate the gasification mass flow per unit length of the liquid channel by using the following formula: Among them, L v is the latent heat of vaporization of the transport ship, and the q is the heat flux per unit length; Calculate the lower limit rate of the unloading area by using the following formula: Where ρ is the density of liquefied natural gas, with the unit of kg / m3, and n is the safety factor.

3. The safety control system for ship-to-ship liquefied natural gas unloading according to claim 1, characterized in that: The upper limit rate of the safe unloading area is obtained by calculating according to the following formula: Q max = 3600·v max ·A Among them, Q max The highest unloading rate, unit: m 3 / h; v max : The maximum flow velocity of the channel, unit: m / s, A: The cross-sectional area of the channel, m 2 ; 3600: The time conversion coefficient, converting seconds to hours; among them: The cross-sectional area calculation formula of the hose is as follows: Where D: the inner diameter of the hose.

4. A safety control system for ship-to-ship liquefied natural gas unloading according to claim 1, characterized in that, The temperature control unit controls and adjusts the temperature difference between the floating storage ship and the transport ship within 2.5°C to 3.5°C by using the binary phase diagram method.

5. The safety control system for ship-to-ship liquefied natural gas unloading according to claim 1, characterized in that, The pressure control unit regulates the pressure difference between the first gas circuit and the second gas circuit within the range of 1 to 10 kPa.

6. A safety control system for ship-to-ship liquefied natural gas unloading according to any one of claims 1-5, characterized in that, The first liquid channel, the second liquid channel, the first gas circuit, and the second gas circuit are all composed of metal hoses with connectors; the inner layer of the metal hose is made of cryogenic-resistant stainless steel alloy; the outer layer of the metal hose is coated with a flexible composite material; the connectors of the metal hoses are made of ultra-low temperature nickel alloy materials; the diameter range of the metal hoses is 6 to 10 inches, and the length range of the metal hoses is 20 to 50 meters.

7. A method for controlling ship-to-ship liquefied natural gas unloading using the system according to claim 1, characterized in that Including the following steps: S1. The dynamic control module performs a precooling cycle on the first liquid channel and the second liquid channel; meanwhile, the dynamic control module adjusts the unloading rates of the first liquid channel and the second liquid channel to within the maximum unloading rate range; S2. The unloading control unit determines whether the natural gas liquid unloading rates of the first liquid channel and the second liquid channel are within the safe unloading area range; If the natural gas liquid unloading rate is less than the lower limit rate of the safe unloading area; that is: Then the temperature control module is activated to control the temperature difference between the floating storage ship and the transport ship within 2.5°C to 3.5°C; If the natural gas liquid unloading rate is greater than the upper limit rate of the safe unloading area; that is: Q max = 3600·v max Then, the starting pressure regulation module dynamically controls the gaseous natural gas to be respectively input into the transportation ship and the gas discharge unit through the first gaseous circuit and the second gaseous circuit; and the pressure regulation module regulates the pressure difference between the first gaseous circuit and the second gaseous circuit within the range of 1 to 10 kPa. S3. After the unloading operation is completed, the dynamic control module closes the first liquid channel and the first gas channel.

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