LNG cargo tank testing method and marine structure using the same and marine structure liquid nitrogen supply system

Abstract

Disclosed are a LNG cargo tank testing method, an offshore structure using the same, and an offshore structure liquid nitrogen supply system. The LNG cargo tank testing method according to an embodiment of the present disclosure includes: step (a) performing a first integrity test on a secondary barrier of a LNG cargo tank of an FLNG after the LNG cargo tank is constructed; step (b) performing a cooling operation on the inside of the LNG cargo tank; and step (c) performing a second integrity test on the secondary barrier of the LNG cargo tank, wherein steps (a) to (c) are performed on the LNG cargo tank of the FLNG in a state in which the FLNG is configured on a shore, and step (b) is performed using any one of refrigerants other than LNG.

Description

Technical Field

[0001] This invention relates to a method for testing LNG cargo holds, a marine structure using this method, and a liquid nitrogen supply system for the marine structure. Background Technology

[0002] After the LNG cargo tank of the offshore structure is completed, the secondary barrier wall will undergo its first integrity test on shore. Then, the offshore structure will be moved to offshore, where LNG supplied through the LNG fuel tank or produced directly offshore will be injected into the LNG cargo tank for cooling. Following a cold shock generated by the cooling process, the secondary barrier wall of the LNG cargo tank will undergo a second integrity test.

[0003] Thus, after conducting the first integrity test on the secondary barrier ashore, the marine structure must be moved to nearshore in order to conduct the second integrity test.

[0004] For example, FLNG (Floating Liquefied Natural Gas) facilities in offshore structures that lack self-propelled capabilities have difficulty receiving LNG because they cannot enter LNG ports. Therefore, in the case of FLNG facilities located near the coast, LNG supply is achieved through LNG fuel storage or direct production in the nearshore area.

[0005] However, the problem is that the approval process for conducting LNG cargo tank testing operations after receiving LNG offshore is extremely complex and costly. Furthermore, it is difficult to respond quickly to emergencies in offshore areas.

[0006] In addition, for self-propelled vessels in marine structures, after completing the first integrity test of the secondary barrier, they leave the shipyard under construction and sail to the LNG land terminal to receive LNG refrigerant. Then, they must return to conduct LNG cargo tank tests, which takes a lot of time and is very expensive. Summary of the Invention

[0007] The embodiments of the present invention aim to provide an LNG cargo tank testing method and a marine structure and liquid nitrogen supply system for the marine structure using the method, so as to conduct LNG cargo tank testing at sea, thereby reducing costs and improving operational efficiency.

[0008] According to one aspect of the present invention, an LNG cargo tank testing method is provided, comprising: step (a) after the LNG cargo tank of the FLNG is constructed, performing a first integrity test on the secondary wall of the LNG cargo tank; step (b) performing a cooling operation on the interior of the LNG cargo tank; and step (c) performing a second integrity test on the secondary wall of the LNG cargo tank, wherein steps (a) to (c) are performed on the LNG cargo tank of the FLNG while the FLNG is configured on shore, and step (b) is performed using any refrigerant other than LNG.

[0009] According to another aspect of the present invention, an LNG cargo tank testing method is provided, comprising: step (a) after the LNG cargo tank of a ship is constructed, performing a first integrity test on the secondary wall of the LNG cargo tank; step (b) after connecting to the LNG cargo tank, performing a pipeline cold resistance test on the pipeline used for LNG loading or unloading; step (c) performing a cooling operation on the interior of the LNG cargo tank; and step (d) performing a second integrity test on the secondary wall of the LNG cargo tank, wherein steps (a) to (d) are performed on the LNG cargo tank of the ship while the ship is docked on shore, and steps (b) and (c) are performed using liquid nitrogen.

[0010] The steps of performing the cooling operation may include injecting liquid nitrogen into the interior of the LNG cargo tank at room temperature, and may also include controlling the temperature inside the LNG cargo tank to be maintained at -158℃±5℃ for a specified period of time.

[0011] After the cooling operation is carried out, the process may further include filling the LNG cargo tank with liquid nitrogen on shore, conducting pumping tests on the pumps inside the LNG cargo tank, and then using a loading arm to unload the liquid nitrogen from the LNG cargo tank onto other ships.

[0012] After the second integrity test, the process may further include filling the LNG cargo tank with LNG, testing the pumps inside the LNG cargo tank, and unloading the LNG from the LNG cargo tank onto other vessels using a loading arm. In the case of FLNG, this can be done at sea, while in the case of a self-propelled vessel, it can be done at a land terminal.

[0013] According to another aspect of the present invention, the method includes: step (a) performing an integrity test on the main wall of the LNG cargo tank; step (b) performing a first integrity test on the secondary wall of the LNG cargo tank; step (c) performing a cold resistance test on the pipeline connected to the LNG cargo tank; step (d) performing a cooling operation on the interior of the LNG cargo tank; step (e) performing a heating operation to raise the interior temperature of the LNG cargo tank to room temperature; step (f) draining the fluid inside the LNG cargo tank, injecting dry gas into the LNG cargo tank, and then visually inspecting the interior of the LNG cargo tank; and step (g) performing a second integrity test on the secondary wall of the LNG cargo tank, wherein steps (a) to (g) can be performed on the LNG cargo tank of the marine structure while the ship or other marine structure is placed on land.

[0014] Implementing steps (b) and (g) may include filling nitrogen gas into the lower and upper insulation layers of the secondary wall, applying different pressures, measuring the pressure change between the lower and upper insulation layers after a set time, and determining whether the secondary wall is undamaged based on the measured values.

[0015] Step (d) may include injecting refrigerant into the LNG cargo tank to control the internal temperature of the LNG cargo tank to be maintained at -158°C ± 5°C for a set time.

[0016] The main screen wall integrity test may include applying an ammonia reactive substance to the welded parts of the main screen wall, filling the space between the main screen wall and the secondary screen wall with ammonia gas, and determining whether the main screen wall is undamaged based on whether the ammonia reactive substance changes color. The pipe cold resistance test may include allowing the refrigerant to flow into the pipe and measuring whether the contraction and expansion displacement of the pipe are within the allowable range.

[0017] According to another aspect of the invention, a marine structure for applying LNG cargo tank testing methods can be provided.

[0018] According to another aspect of the present invention, a liquid nitrogen supply system for a marine structure is provided, comprising a buffer tank for supplying liquid nitrogen to the LNG cargo tank for conducting testing operations on a marine structure placed on land; and a control unit for adjusting the amount of liquid nitrogen injected into the LNG cargo tank so that the internal temperature of the LNG cargo tank is maintained within a set temperature range for a set time.

[0019] It may also include tank trucks for receiving liquefied nitrogen from the liquid nitrogen production unit, moving it to a dock equipped with buffer tanks, and supplying liquefied nitrogen to the buffer tanks.

[0020] It may also include a liquid nitrogen injection pipeline connecting the buffer tank and the LNG cargo tank, an injection unit that injects the liquid nitrogen supplied through the liquid nitrogen injection pipeline into the interior of the LNG cargo tank to cool the LNG cargo tank, a control valve disposed in the liquid nitrogen injection pipeline, and a temperature sensor for measuring the temperature inside the LNG cargo tank. The control unit controls the opening and closing degree of the control valve according to the measured temperature value to adjust the amount of liquid nitrogen injected into the interior of the LNG cargo tank by the injection unit.

[0021] The control unit can adjust the amount of liquid nitrogen injected into the LNG cargo tank to maintain the internal temperature of the LNG cargo tank at -158℃±5℃.

[0022] The LNG cargo tank testing method and the marine structures and liquid nitrogen supply system for marine structures using this method, as described in this invention, enable onshore testing of LNG cargo tanks, thereby saving costs and improving work efficiency. In particular, large-scale testing operations that could previously only be performed in nearshore areas can now be carried out onshore, making the operations simpler, more stable, and more efficient, and allowing for earlier completion of the project.

[0023] The effects of this invention are not limited to those mentioned above, and those skilled in the art can easily understand other unmentioned effects from the scope of the claims. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the process of testing an LNG cargo tank on land for a marine structure, as described in an embodiment of the present invention.

[0025] Figure 2 Is Figure 1 The diagram shows a flowchart illustrating the process of testing LNG cargo tanks on marine structures following LNG cargo tank testing.

[0026] Figure 3 This is a block diagram of a liquid nitrogen supply system according to an embodiment of the present invention. Detailed Implementation

[0027] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The embodiments described below are merely illustrative examples, intended to better convey the spirit of the invention to those skilled in the art. The present invention is not limited to the embodiments described below, and can be embodied in other forms. To more clearly illustrate the invention, parts unrelated to the description are omitted from the drawings. For convenience, the width, length, thickness, etc., of each component are exaggerated on the drawings. Throughout this specification, the same reference numerals represent the same components.

[0028] Figure 1This is a flowchart illustrating the onshore testing process of an LNG cargo tank for a marine structure, as described in an embodiment of the present invention.

[0029] Figure 1 The tests shown for the LNG cargo tank were conducted on the marine structure while it was in a state of being placed on land.

[0030] The marine structures involved in the embodiments of the present invention include: marine structures with LNG cargo tanks such as floating liquefied natural gas facilities (FLNG), floating storage regasification units (FSRU), and floating oil storage units (FSU) that do not have self-propulsion capabilities, as well as ships with self-propulsion capabilities such as LNG transport routes.

[0031] In embodiments of this invention, the term "onshore" refers to land, including a dock representing a shipyard. That is, using "onshore" implies a broader scope than a dock. LNG cargo tank testing in a dock refers to conducting LNG cargo tank testing within a shipyard where the marine structure is constructed. "Onshore" LNG cargo tank testing includes conducting LNG cargo tank testing in a dock (i.e., a shipyard) and other onshore locations.

[0032] In the various embodiments of the invention described below, although the entire process of testing the LNG cargo tank of the marine structure is carried out on shore in the description, it can also be carried out in a "shore" dock (i.e., shipyard).

[0033] Reference Figure 1 First, a principal bulkhead integrity test (PBGT) (S201) is conducted on shore for the LNG cargo tank of the marine structure. This process (S201) may include applying an ammonia-reactive substance to the welded areas of the principal bulkhead, filling the space between the principal bulkhead and the secondary bulkhead with ammonia gas, and determining whether the principal bulkhead is undamaged based on whether the ammonia-reactive substance changes color. For example, if a groove appears at the welded area of ​​the principal bulkhead, and the ammonia-reactive substance applied to the welded area reacts with the ammonia gas filled between the principal bulkhead and the secondary bulkhead, changing color, then the principal bulkhead can be considered damaged.

[0034] Then, the first secondary barrier tightening test (SBTT) (S202) of the LNG cargo tank of the marine structure is conducted on shore. This process (S202) involves filling the lower insulation layer (IS; Insulation Space) and the upper insulation layer (IBS; Inter Barrier Space) of the secondary barrier with nitrogen gas, applying different pressures, and measuring the pressure change between the lower and upper insulation layers after a set time. The measured pressure change value is used to determine whether the secondary barrier is undamaged. At this time, the pressures of the lower and upper insulation layers of the secondary barrier can be measured separately using pressure gauges (not shown) installed in these spaces. For example, if the measured pressures of the lower and upper insulation layers of the secondary barrier are the same, the secondary barrier can be considered damaged.

[0035] Next, a cold-resistance test (S203) is conducted on the pipeline connected to the LNG cargo tank of the marine structure for LNG loading or unloading. This process (S203) may include allowing refrigerant to flow into the pipeline and measuring the amount of pipeline contraction and expansion displacement using a displacement measuring instrument (not shown). The refrigerant here may include liquid nitrogen. The pipeline may be a pipeline connected to the fill pipe and discharge pipe of the pump tower for LNG loading or unloading, and the cold-resistance test may be conducted with the pipeline installed on the ship's deck. If the amount of pipeline contraction and expansion displacement measured using the displacement measuring instrument is within the allowable range, it can be considered normal.

[0036] Then, cooling operations are performed onshore on the interior of the LNG cargo tank of the marine structure (S204). This process (S204) may include the use of refrigerants other than LNG; similar to the cold-resistance testing of the pipeline, liquid nitrogen may be used. Upon contact with the ambient temperature LNG cargo tank, most of the liquid nitrogen will immediately vaporize, creating convection within the LNG cargo tank. This process (S204) may include injecting liquid nitrogen into the ambient temperature LNG cargo tank while simultaneously controlling the internal temperature of the LNG cargo tank to maintain it at -158°C ± 5°C.

[0037] Next, a preheating (S205) operation is carried out on shore to raise the internal temperature of the LNG cargo tank of the marine structure to ambient temperature. This process (S205) may include using a heater (180, reference) Figure 3 This raises the temperature inside the LNG cargo tank, and the fluid inside the LNG cargo tank is heated by a heater (180) and then re-injected into the LNG cargo tank.

[0038] Then, the fluid inside the LNG cargo tank of the marine structure is drained on shore, and dry air is injected into the LNG cargo tank, followed by a visual inspection of the interior of the LNG cargo tank (S206). When the temperature inside the LNG cargo tank is raised to room temperature by the heater (180), most of the gaseous nitrogen remains inside the LNG cargo tank. Therefore, workers must enter the LNG cargo tank for a visual inspection, the nitrogen inside the LNG cargo tank is purged, and room temperature dry air (oxygen concentration of 20%) is injected into the LNG cargo tank.

[0039] Next, a second secondary barrier integrity test (S207) is conducted on shore on the LNG cargo tank of the marine structure. This process (S207) may include filling the lower and upper insulation layers of the secondary barrier with nitrogen gas and applying different pressures to each layer. After a set time, the pressure change between the lower and upper insulation layers is measured, and the secondary barrier is determined to be undamaged based on the measured pressure change.

[0040] When the LNG cargo tank is of Mark-III type, unlike the main wall, it is not welded but adhesively bonded. As mentioned above, the reason for conducting the first and second secondary wall integrity tests is that after the first secondary wall integrity test during the construction of the LNG cargo tank, liquefied gas is injected into the LNG cargo tank for cooling operations, causing the LNG cargo tank to experience a rapid temperature change (hereinafter referred to as cold shock). At this time, the secondary wall material inside the LNG cargo tank will shrink and stretch, so it is necessary to conduct a second secondary wall integrity test to confirm the integrity of the secondary wall.

[0041] In other words, the first secondary barrier integrity test (S202) is conducted after the LNG cargo tank construction is completed, and the second secondary barrier integrity test (S207) is conducted under conditions where the LNG cargo tank is subjected to cold shock. In an embodiment of the present invention, as described above, during process S204, liquid nitrogen is injected to cool the interior of the LNG cargo tank, but the internal temperature is maintained at -158°C ± 5°C for a set time (e.g., 22 hours) while cold shock is applied.

[0042] Thus, through embodiments of the present invention, various tests on LNG cargo tanks of marine structures can be conducted onshore, thereby reducing costs and improving work efficiency. In particular, by carrying out large-scale testing operations onshore that previously required near-shore conditions, the simplicity, stability, and efficiency of the work are improved, and engineering tasks can be completed ahead of schedule.

[0043] Furthermore, the testing process (S201-S207) of the LNG cargo tank of the marine structure can also be carried out in a dry dock (shipyard). That is to say, the above process (S201-S207) can be carried out in a shipyard where the marine structure has already been built on land. In this case, the movement of the marine structure is minimized, thereby reducing costs and improving the efficiency of the testing work.

[0044] like Figure 1 As shown, after onshore testing of the LNG cargo tanks, non-self-propelled marine structures (such as FLNG) can be tested at sea, while self-propelled marine structures (such as LNG carriers) can conduct the remaining LNG cargo tank tests at the onshore terminal. The following explanation will use FLNG as an example of a non-self-propelled marine structure and LNG carriers as an example of a self-propelled marine structure.

[0045] Figure 2 Is Figure 1 The flowchart shown is a process for testing LNG cargo tanks on marine structures following LNG cargo tank testing.

[0046] Reference Figure 2 The process involves filling the LNG cargo tank with LNG and conducting a pumping test on the pumps inside the LNG cargo tank (S208). For example, for an FLNG without self-propulsion capability, this process is performed with the FLNG in a near-shore state (S208); for an LNG carrier with self-propulsion capability, this process is performed with the LNG carrier in a shore terminal state (S208).

[0047] Additionally, taking FLNG as an example, after producing LNG offshore, the produced LNG can be filled into the LNG cargo tank, and the pumps (not shown) installed inside the LNG cargo tank can be tested immediately. Alternatively, as another example, FLNG can receive LNG using the LNG fuel tank method, and then test the pumps installed inside the LNG cargo tank.

[0048] At this time, the pump can be an LNG unloading pump installed on a pump tower, and the test can be carried out with the LNG cargo tank filled with LNG.

[0049] Next, a test will be conducted to unload LNG from the LNG cargo tank onto other LNG vessels using a loading arm (S209). This process (S209) will also be conducted using FLNG in offshore areas and LNG carriers at land terminals as examples.

[0050] In the LNG cargo tank testing process (S208-S209) of the marine structure, LNG was used for testing. However, in other examples, it is not limited to this, and the refrigerant used in the cooling operation process (S204) can also be used instead of LNG for testing.

[0051] In this scenario, the cooling process (S204) is performed, followed by pumping and loading arm tests on shore, and then the preheating operation (S205). Additionally, the second secondary barrier integrity test is conducted (S207), followed by pumping and loading arm tests on shore. At this time, refrigerant is filled into the LNG cargo tank on shore, pumping tests are performed on the pumps inside the LNG cargo tank, and the refrigerant inside the LNG cargo tank is unloaded onto other LNG vessels for testing using a loading arm. This allows pumping and loading arm tests to be performed on shore without moving marine structures, including FLNG, to nearshore areas.

[0052] Figure 3 The diagram shown is a block diagram of a liquid nitrogen supply system according to an embodiment of the present invention.

[0053] Reference Figure 3 The liquid nitrogen supply system involved in this embodiment of the invention supplies nitrogen onshore. Figure 1 The liquid nitrogen shown is used in the test operation of the LNG cargo tank 101 of the marine structure 100 located on shore.

[0054] This liquid nitrogen supply system may include: a tank truck 110 that receives liquid nitrogen from a liquid nitrogen production unit 105 and transfers it to a buffer tank 120, and supplies liquid nitrogen to the buffer tank 120; a buffer tank 120 that supplies liquid nitrogen to an LNG cargo tank 101; and a control unit 130 that adjusts the amount of liquid nitrogen injected into the LNG cargo tank 101 in order to maintain the internal temperature of the LNG cargo tank 101 within a set temperature range for a set time.

[0055] Additionally, the liquid nitrogen supply system may include: a buffer tank 120; a liquid nitrogen injection pipe 140 connected to the LNG cargo tank 101; an injection unit 150 that injects liquid nitrogen received through the liquid nitrogen injection pipe 140 into the LNG cargo tank 101; a control valve 160 disposed on the liquid nitrogen injection pipe 140; and a temperature sensor 170 for measuring the internal temperature of the LNG cargo tank 101.

[0056] In addition, the liquid nitrogen supply system is equipped with one or more safety valves (193) and safety systems (193) on the liquid nitrogen injection pipeline 140 to deal with emergency situations and abnormal conditions.

[0057] The following will provide a detailed explanation of each component.

[0058] Liquid nitrogen production unit 105 can be produced by a company specializing in gases. The liquid nitrogen produced by liquid nitrogen production unit 105 is transported to the location of the onshore marine structure 100 while stored in tank truck 110.

[0059] The buffer tank 120 can be configured as a mobile trailer to receive liquid nitrogen from the tank truck 110 and stably supply liquid nitrogen to the LNG cargo tank 101 of the marine structure 100. Like the tank truck 110, the buffer tank 120 can be transported to the location of the marine structure 100 onshore. Although not illustrated, the buffer tank 120 can be equipped with cryogenic pumps, vaporizers, agitators, and other equipment to deliver liquid nitrogen to the LNG cargo tank 101.

[0060] The control unit 130 controls the opening and closing of the control valve 160 based on the temperature value measured by the temperature sensor 170, thereby adjusting the amount of liquid nitrogen injected into the LNG cargo tank 101 by the injection unit 150. For example, the control unit 130 can control the opening and closing of the control valve 160 to maintain the temperature inside the LNG cargo tank 101 within the range of -158°C ± 5°C. This allows the internal temperature of the LNG cargo tank 101 to be maintained at approximately the LNG temperature level (approximately -163°C), enabling onshore testing of the LNG cargo tank using liquid nitrogen instead of LNG. In other words, by maintaining the liquid nitrogen at the LNG temperature level, the same effect as using LNG for cargo tank testing can be achieved.

[0061] Specifically, during the cooling operation inside LNG cargo tank 101, if liquid nitrogen at approximately -195°C is injected into the LNG cargo tank 101, which is at room temperature, the temperature of the LNG cargo tank 101 will gradually decrease. Once the liquid nitrogen comes into contact with the room-temperature LNG cargo tank 101, most of it will immediately vaporize, causing convection within the LNG cargo tank 101. At this time, when adjusting the rate of temperature reduction of the LNG cargo tank 101 according to the amount of liquid nitrogen injected, if the temperature of the LNG cargo tank 101 becomes too low, it may damage the components of the LNG cargo tank 101. Therefore, the amount of liquid nitrogen injected can be adjusted according to a preset rate.

[0062] After liquid nitrogen is injected onto the inner wall of the LNG cargo tank 101 at room temperature, its temperature rises, and it can immediately convert to a gaseous state. If cooling operations continue inside the LNG cargo tank 101 in this state, the temperature can reach approximately -158°C, at which point the injection unit 150 can stop injecting liquid nitrogen. At this point, the control unit 130 can close the control valve 160 based on the temperature value measured by the temperature sensor 170, thereby stopping the injection of liquid nitrogen through the injection unit 150.

[0063] Furthermore, the control unit 130 can maintain the internal temperature fluctuation range of the LNG cargo tank 101 within -158℃±5℃ for 22 hours according to the preset cargo tank cooling conditions. That is, after the injection of liquid nitrogen into the LNG cargo tank 101 stops, as time goes by, the internal temperature of the LNG cargo tank 101 gradually rises. The control unit 130 opens the control valve 160 based on the temperature value measured by the temperature sensor 170, and gradually injects liquid nitrogen into the LNG cargo tank 101 little by little through the injection unit 150, maintaining the temperature fluctuation range within -158℃±5℃ for 22 hours according to the preset cargo tank cooling conditions.

[0064] The liquid nitrogen injection pipeline 140 is configured to supply liquid nitrogen from the buffer tank 120 to the LNG cargo tank 101 and is capable of withstanding extremely low temperatures of liquefied gas. The liquid nitrogen injection pipeline 140, along with the buffer tank 120, can be moved ashore to the location of the offshore structure 100. The tanker truck 110, buffer tank 120, and liquid nitrogen injection pipeline 140, as described above, can be configured to be mobile, allowing testing operations of the LNG cargo tank 101 of the offshore structure 100 to be conducted at any location ashore, not just at a shipyard, but elsewhere.

[0065] The injection unit 150 may include nozzles for injecting liquid nitrogen supplied through the liquid nitrogen injection pipe 140 into the interior of the LNG cargo tank 101.

[0066] Furthermore, a control valve 160 is configured in the liquid nitrogen injection pipeline 140 to adjust the flow rate of liquid nitrogen supplied to the LNG cargo tank 101.

[0067] Temperature sensor 170 measures the temperature inside LNG cargo tank 101 as it changes due to the injection of liquid nitrogen.

[0068] Safety valve 193 and safety device 195 can be used to protect the equipment from overpressure phenomena, emergency situations, and abnormal conditions caused by interruption of liquid nitrogen supply to the supply equipment (liquid nitrogen supply system).

[0069] One or more of safety valves 193 and safety devices 195 may be configured at one or more of the front and rear ends of control valve 160 in the liquid nitrogen injection pipeline 140.

[0070] Safety valve 193 may include a "pressure relief valve (PRV)" to deal with overpressure in the equipment and a "pressure vacuum valve (PVV)" to deal with low pressure (vacuum) in the equipment.

[0071] In addition, safety device 195 may include one or more means such as alarms, automatic supply, and interruption in response to abnormal situations.

[0072] For example, overpressure phenomena may occur during the supply of liquid nitrogen to the cargo hold (main line) using pumps or other conveying means, and in pipelines and equipment where liquid nitrogen cannot be supplied to the cargo hold (main line) in an emergency. Furthermore, overpressure phenomena during normal operation may be caused by surges resulting from sudden valve operations or changes in driving. Additionally, in cases where cargo holds (main line) are unable to supply cargo for an extended period, overpressure may occur outside the operating range of the supply pipelines and corresponding equipment due to the vaporization of liquid nitrogen within the supply equipment.

[0073] In addition, the low pressure (vacuum) phenomenon may occur when the flow rate supplied by the liquid nitrogen supply equipment to the main line exceeds the supply range, or when the pumps and other conveying equipment are operating when there is insufficient liquid nitrogen in the liquid nitrogen supply equipment.

[0074] In embodiments of the present invention, the overpressure and low pressure (vacuum) phenomena can be effectively addressed by means of safety valve 193 and safety device 195.

[0075] Thus, through the liquid nitrogen supply system involved in the embodiments of the present invention, liquid nitrogen can be used to replace LNG and maintain the temperature level of LNG, while being injected into the interior of the LNG cargo tank 101 of the offshore structure 100, thereby enabling effective testing of the LNG cargo tank of the offshore structure 100 on shore.

[0076] The above description is merely of specific embodiments. However, the present invention is not limited to the described embodiments, and those skilled in the art can make various modifications as long as they do not depart from the core technical concept of the present invention as described in the following claims.

Claims

1. A method for testing LNG cargo holds, comprising: After the LNG cargo tank of FLNG is built in step (a), the first integrity test is carried out on the secondary wall of the LNG cargo tank; Step (b) involves cooling the interior of the LNG cargo hold; and Step (c) involves conducting a second integrity test on the secondary barrier of the LNG cargo hold. Steps (a) to (c) are performed on the LNG cargo tank of the FLNG while the FLNG is configured on shore, and step (b) is performed using any refrigerant other than LNG.

2. A method for testing LNG cargo holds, comprising: Step (a) After the LNG cargo tank of the ship is built, the first integrity test is carried out on the secondary wall of the LNG cargo tank; After connecting to the LNG cargo tank in step (b), a cold resistance test is performed on the pipeline used for LNG loading or unloading. Step (c) involves cooling the interior of the LNG cargo hold; and Step (d) involves conducting a second integrity test on the secondary barrier of the LNG cargo hold. Steps (a) to (d) are performed on the LNG cargo tanks of the vessel while it is docked at shore. Steps (b) and (c) are performed using liquid nitrogen.

3. A method for testing LNG cargo holds according to claim 1 or claim 2. The cooling operation includes injecting liquid nitrogen into the interior of the LNG cargo tank at room temperature, and also includes controlling the temperature inside the LNG cargo tank to be maintained at -158℃±5℃ for a specified period of time.

4. A method for testing LNG cargo holds according to claim 1 or claim 2, Following the cooling operation, the process further includes filling the LNG cargo tank with liquid nitrogen on shore and conducting pumping tests on the pumps inside the LNG cargo tank. Then, the test process of unloading liquid nitrogen from the LNG cargo tank onto other ships using a loading arm.

5. A method for testing LNG cargo holds according to claim 1 or claim 2, Following the second integrity test, the process further includes filling the LNG cargo tank with LNG and conducting a pumping test on the pumps inside the LNG cargo tank. The test process of using a loading arm to unload LNG from the LNG cargo tank onto other vessels. In the case of the FLNG, it is carried out at sea; in the case of a self-propelled vessel, it is carried out at a land terminal.

6. A method for testing LNG cargo holds, comprising: Step (a) Conduct an integrity test on the main wall of the LNG cargo hold; Step (b) involves conducting a first integrity test on the secondary wall of the LNG cargo hold; Step (c) Perform a cold resistance test on the pipeline connected to the LNG cargo tank; Step (d) involves cooling the interior of the LNG cargo hold; Step (e) involves a heating operation to raise the internal temperature of the LNG cargo hold to room temperature. Step (f) drain the fluid inside the LNG cargo tank, inject dry gas into the LNG cargo tank, and then conduct a visual inspection of the inside of the LNG cargo tank. and Step (g) involves conducting a second integrity test on the secondary barrier of the LNG cargo hold. Steps (a) to (g) are performed on the LNG cargo tank of the marine structure while it is in a state of being placed on land.

7. A method for testing an LNG cargo hold according to claim 6: Steps (b) and (g) involve filling the lower and upper insulation layers of the secondary screen wall with nitrogen gas and applying different pressures, respectively. After a set period of time, the pressure change between the lower insulation layer and the upper insulation layer is measured. The process of determining whether the secondary screen wall is undamaged based on the measured values.

8. The LNG cargo hold testing method according to claim 6: Step (d) includes injecting refrigerant into the LNG cargo tank to control the internal temperature of the LNG cargo tank to be maintained at -158°C ± 5°C for a set time.

9. A method for testing an LNG cargo hold according to claim 8: The main screen wall integrity test includes applying an ammonia reactive substance to the welded areas of the main screen wall, filling the space between the main screen wall and the secondary screen wall with ammonia gas, and determining whether the main screen wall is undamaged based on whether the ammonia reactive substance changes color. The pipeline cold resistance test includes the process of allowing the refrigerant to flow into the pipeline and measuring whether the contraction and expansion displacement of the pipeline are within the allowable range.

10. A marine structure employing the LNG cargo tank testing method according to any one of claims 6 to 9.

11. A liquid nitrogen supply system for a marine structure, comprising: A buffer tank for supplying liquid nitrogen to the LNG cargo tank for testing operations of an LNG cargo tank placed on land; and A control unit that adjusts the amount of liquid nitrogen injected into the LNG cargo tank to maintain the internal temperature of the LNG cargo tank within a set temperature range for a set time. The liquid nitrogen supply system for the marine structure also includes a liquid nitrogen injection pipeline connecting the buffer tank and the LNG cargo tank. Liquid nitrogen, supplied through the liquid nitrogen injection pipeline, is injected into the interior of the LNG cargo tank to cool the injection section of the LNG cargo tank. The control valve configured in the liquid nitrogen injection pipeline, and Temperature sensor used to measure the internal temperature of the LNG cargo tank. The control unit controls the opening and closing degree of the control valve according to the measured temperature value, so as to adjust the amount of liquid nitrogen injected into the LNG cargo tank by the injection unit.

12. A liquid nitrogen supply system for a marine structure according to claim 11: It also includes tank trucks for receiving liquefied nitrogen from the liquid nitrogen production unit, moving it to a dock equipped with buffer tanks, and supplying liquefied nitrogen to the buffer tanks.

13. A liquid nitrogen supply system for a marine structure according to claim 11: The control unit adjusts the amount of liquid nitrogen injected into the LNG cargo tank to maintain the internal temperature of the LNG cargo tank at -158℃±5℃.

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