KR100834273B1 - Membrane type lng cargo tank, lng carrier with it, and lng carrying method - Google Patents

Abstract

A membrane type LNG cargo tank, an LNG carrier with the same, and an LNG carrying method are provided to remarkably reduce installation and operation cost of the tank using a liquid pump. A membrane type LNG cargo tank allows increase in pressure therein to absorb introduction heat quantity due to increase in sensible heat of LNG and natural gas in the tank by increase in saturation temperature, thereby remarkably reducing generation of an evaporation gas. When the pressure in the LNG cargo tank is 0.7bar, the saturation temperature is increased by about 6°C in comparison with an initial state of 0.06bar. When the LNG cargo tank having an insulating wall normally contains LNG, the pressure in the tank is gradually increased to generate an evaporation gas to 0.7bar from 0.06bar. A vapor pressure in the tank can be increased to 0.25 to 2bar.

Description

Membrane type LN storage tanks, LNK carriers equipped with tanks, and LNK transportation methods {MEMBRANE TYPE LNG CARGO TANK, LNG CARRIER WITH IT, AND LNG CARRYING METHOD}

1 is a view showing the concept of the absorption of heat intake in the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

2 is a view schematically showing an LNG storage tank for an LNG carrier according to a preferred embodiment of the present invention.

3 is a view schematically showing a configuration for treating the boil-off gas at the unloading terminal using the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

Figure 4 is a schematic diagram showing the waste of the boil-off gas of the LNG carrier ship based on the concept that the pressure of the conventional LNG storage tank is maintained at about the same state.

<Explanation of symbols for main parts of the drawings>

1 LNG storage tank for LNG carrier 2 LNG storage tank for cargo terminal

3a, 3b: compressor 4: recondenser

5: carburetor P: high pressure pump

11 LNG pump 13 LNG spray

21: injection nozzle for boil-off gas 23: compressor for boil-off gas

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a somewhat high pressure tank near normal pressure for carrying liquefied gas in a cryogenic state, and to a membrane type LNG storage tank allowing a change in pressure and temperature in the tank during transportation, an LNG carrier equipped with the tank, and an LNG transportation. It is about a method.

In general, natural gas (NG) is produced in the form of liquefied liquefied natural gas (Liquefied Natural Gas, hereinafter referred to as LNG) at the production site, and then transported over a long distance to the destination by LNG carriers. Thereafter, the gas is regasified and supplied to a consumer through an LNG Floating Storage and Regasification Unit (FSRU) or an offshore loading terminal.

When LNG is transported by an LNG Regasification Vessel (RV), LNG is regasified from the LNG Regasification Vessel itself and supplied directly to the consumer without having to go through an LNG floating storage or regasification unit or a land unloading terminal. .

The liquefaction temperature of natural gas is about -163 ° C at ambient pressure, so LNG is evaporated even if its temperature is slightly higher than -163 ° C at normal pressure. In the case of a conventional LNG carrier, for example, the LNG storage tank of the LNG carrier is insulated, but since the external heat is continuously transmitted to the LNG, LNG is transported while the LNG carrier is transporting the LNG. The gas is continuously vaporized in the LNG storage tank to generate a boil-off gas in the LNG storage tank.

When the boil-off gas is generated in the LNG storage tank in this way, the pressure of the LNG storage tank rises and becomes dangerous.

Conventionally, in order to maintain the pressure of the LNG storage tank in a safe state, the boil-off gas generated in the LNG storage tank was used as a fuel for propulsion of the LNG carrier. That is, in the case of a LNG carrier that carries LNG in a low temperature liquid state of the related art, the basic concept is that the LNG temperature in the tank is maintained at about the same temperature and the same pressure at around -163 ° C. As a result, the generated BOG was discharged to the outside and treated.

A steam turbine propulsion method driven by steam generated by burning an evaporated gas generated in an LNG storage tank in a boiler has a problem of low propulsion efficiency.

In addition, the dual fuel diesel electric propulsion system, which compresses the boil-off gas generated in the LNG storage tank and uses it as a fuel for a diesel engine, is more efficient than the steam turbine propulsion method, but the medium speed engine and the electric The propulsion device is complex and there are many difficulties in the maintenance of the equipment. In addition, since this method requires the supply of boil-off gas as fuel, a gas compression method having a large installation cost and an operating cost is inevitably applied.

In this way, the method of using the boil-off gas as the propulsion fuel does not in any case fall short of the efficiency of the two-stroke low speed diesel engine used for the general ship.

On the other hand, there is also a way to re-liquefy the boil-off gas generated in the LNG storage tank to return to the LNG storage tank. However, this method of reliquefaction of the boil-off gas has a problem that the LNG carrier to install a boil-off gas reliquefaction device of a complex system.

In addition, when the amount of evaporation gas generated in the propulsion system that can be used as fuel or can be processed by the boil-off gas reliquefaction apparatus is generated, the excess boil-off gas must be incinerated and treated in a gas combustor, so that the treatment of the excess boil-off gas There is a problem in that additional equipment such as a gas burner is added.

For example, as shown in Figure 4, when looking at the LNG carriers based on the basic concept of maintaining the pressure of the conventional LNG storage tank in almost the same state, after the LNG is shipped (3-5 days after shipment) LNG As the storage tank is a little hot, as indicated by the solid upper line, a considerable amount of excess BOG occurs compared to the BOG generation (NBOG, natural BOG) in operation. It is more than fuel consumption of fuel diesel electric propulsion system. Therefore, the BOG corresponding to the hatched portion representing the difference with the lower dotted line showing the amount of BOG used in the boiler or engine is inevitably burned out through the gas combustion unit (GCU). In addition, even when LNG carriers pass through the canal (e.g., 5-6 days in FIG. 4), there is no BOG consumption in the boiler or engine (when waiting for the canal), or less (when passing the canal), so that the BOG more than the engine needs is burned. I have to throw it away. In addition, even when the LNG carrier is waiting to enter or enter the port while the consumption of the BOG occurs or a small amount, there is no choice but to burn the excess BOG.

The amount of BOG ll ride like this when converted to the amount reaches to 1500-2000 tons per year of LNG carriers at 150,000m ³ capacity is equivalent to 600 million won. Furthermore, the burning of BOG also causes environmental pollution.

On the other hand, unlike the low-pressure tank as described above, without forming a thermal insulation wall in the LNG storage tank to maintain a high pressure of about 200 bar (gauge pressure) in the LNG storage tank to suppress the generation of boil-off gas in the LNG storage tank The technique is disclosed in Korean Patent Publication No. KR2001-0014021, KR2001-0014033, KR2001-0083920, KR2001-0082235, KR2004-0015294 and the like. However, in order for the LNG storage tank to receive the boil-off gas at a high pressure of about 200 bar, the thickness of the LNG storage tank must be considerably thick. There is a problem in that separate equipment such as a high pressure compressor for maintaining is added. Unlike this technology, there is a technology known as a pressure tank, which stores a highly volatile liquid in a tank of high temperature at room temperature, so that there is no problem in treating the BOG, but there is a limitation that the size of the tank cannot be increased, and the manufacturing cost thereof There is an increasing problem.

As described above, the LNG tank of the LNG carrier conventionally is a way to maintain the pressure of the cryogenic liquid at a pressure near the normal pressure during transport and to allow the generation of BOG, the consumption of BOG is large or separate reliquefaction There was a problem that the device must be mounted. In addition, unlike the tank for transporting the cryogenic liquid at atmospheric pressure and low pressure, the method of transporting the tank to a tank capable of withstanding high pressure at a somewhat high temperature such as a pressure tank does not have a treatment of BOG, but the size of the tank is limited and manufactured. There is a problem that the cost is high.

Accordingly, the present invention is to solve such a problem of the prior art, and relates to a somewhat high pressure tank near normal pressure for carrying a liquefied gas in a cryogenic state, and it is possible to manufacture a large capacity tank without increasing the manufacturing cost of the tank. An object of the present invention is to provide a membrane type LNG storage tank and a method of transporting LNG using the same or a method of treating boil-off gas using the same, which can reduce BOG waste.

In order to achieve the above object, the present invention relates to a somewhat high pressure tank near normal pressure for carrying a liquefied gas in a cryogenic state, characterized in that it allows a certain pressure change in the tank during transportation.

According to one embodiment of the present invention, a storage tank of an LNG carrier carrying liquefied natural gas in a cryogenic state, the steam pressure in the tank is regulated within a pressure range near normal pressure, and the steam pressure in the tank and An LNG storage tank for an LNG carrier is provided, which allows an increase in the temperature of the LNG. The vapor pressure in the tank is characterized in that more than 0.25 to 2 bar, preferably more than 0.25 to 0.7 bar, more preferably 0.7 bar. In addition, in order to make the temperature distribution inside the LNG tank uniform, the LNG of the lower portion of the LNG storage tank and the boil-off gas of the upper portion of the LNG storage tank are mixed. Since evaporation of LNG tends to occur more locally at high temperatures in the LNG storage tank, it is desirable to maintain a uniform temperature of LNG or BOG in the LNG storage tank. In addition, from the viewpoint, the boil-off gas in the upper portion of the LNG storage tank has a smaller heat capacity than the LNG in the bottom of the tank, which may cause a sudden increase in pressure due to a rise in temperature due to external inflow heat. As a result, a sudden increase in pressure of the LNG tank can be suppressed.

In addition, according to another embodiment of the present invention, there is provided a method for transporting liquefied natural gas in a cryogenic state having the above characteristics and an LNG carrier provided with the tank.

In particular, according to another embodiment of the present invention, the present invention relates to a somewhat high-pressure membrane-type LNG tank near normal pressure that carries a liquefied gas in a cryogenic state, characterized in that it allows a certain degree of pressure change in the tank during transportation. . The membrane tank described in the present invention means a membrane tank defined in the IGC Code (2000) regarding the cargo hold of the LNG tank. Specifically, Membrane tanks are non-self-supporting tanks, which means that a heat insulating wall is formed on the hull and a thin membrane is formed thereon. This includes semi-membrane tanks.

In the following, the tanks described in GTT NO 96, Mark III, Korean Patent Nos. 499710 and 644217 are examples of membrane type tanks.

Such membrane-type tanks can be designed to withstand up to 0.7 bar (gauge pressure) by reinforcement of the tank, but are generally designed to not exceed 0.25 bar. All conventional membrane-type tanks are in compliance with this regulation and managed so that the vapor pressure inside the tank is 0.25 bar or less, and the temperature and pressure of LNG are almost constant during operation. In contrast, the present invention is characterized in that the management to allow a rise in the tank internal pressure and the temperature of the LNG at a pressure of more than 0.25 bar, preferably more than 0.25 bar 2 bar or less, more preferably more than 0.25 bar 0.7 bar or less. . In addition, the boil-off gas treatment method using the LNG storage tank of the present invention is characterized in that to maintain a uniform temperature distribution inside the LNG storage tank.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

The LNG storage tank of the present invention can be applied to LNG storage tanks such as LNG carriers, LNG floating storage and regasification apparatus (FSRU), land unloading terminal, LNG regasification vessel (RV). As the LNG transport tanks allow for an increase in pressure and temperature while solving the BOG problem, the waste of BOG can be reduced, and LNG can be stored for a long time in the various LNG tanks in consideration of LNG demand at the destination. It also has the advantage of greater flexibility in transportation and storage.

In the present embodiment, the LNG storage tank to be applied to the LNG carrier will be described by way of example.

1 illustrates a concept of absorbing heat intake in an LNG storage tank for an LNG carrier according to the present invention. In the related art, inflow heat from the outside is maintained by maintaining a pressure in the LNG storage tank for an LNG carrier within a predetermined range. While most of these contribute to the generation of boil-off gas and process all of the boil-off gas generated in the LNG carrier, the present invention allows the pressure in the LNG storage tank for LNG carriers to rise, thereby increasing the saturation temperature following the pressure rise. And by the increase in the sensible heat of natural gas (hereinafter referred to as NG) most of the incoming heat is absorbed, so that the generation of boil-off gas is greatly reduced. For example, when the pressure of the LNG storage tank for LNG carriers is 0.7 bar, the saturation temperature rises about 6 ° C compared to the initial 0.06 bar.

2 schematically shows an LNG storage tank for an LNG carrier according to a preferred embodiment of the present invention. In the case of LNG storage tank (1) for LNG carriers with heat insulation walls, when the LNG is normally loaded, the internal pressure is about 0.06 bar (gauge pressure) at the start, and the evaporation gas is generated during the operation of the LNG carrier. Pressure increases gradually. For example, after loading LNG at the LNG producing site, the pressure inside the LNG storage tank 1 for LNG carriers becomes 0.06 bar, and when the LNG carrier starts and operates for about 15 to 20 days, when it arrives at the destination, the LNG carrier The pressure inside the LNG storage tank 1 can rise to 0.7 bar.

When this is described in relation to temperature, LNG generally contains various impurities and is lower than the boiling point of pure methane liquid. Pure methane has a boiling point of -161 ° C at 0.06 bar, while LNG carried in LNG transport actually contains a few impurities such as nitrogen and ethane, resulting in a boiling point of around -163 ° C. In terms of pure methane, the LNG temperature in the tank is around -161 ° C at 0.06 bar after LNG shipment. The LNG temperature is -159 ° C when the vapor pressure in the tank is controlled to 0.25 bar in consideration of the transport distance and BOG consumption. When the steam pressure in the tank is controlled to 0.7 bar, the LNG temperature rises to around -155 ° C, and when the steam pressure in the tank is controlled to 2 bar, the LNG temperature rises to around -146 ° C.

LNG storage tank for LNG carrier of the present invention is designed in consideration of the pressure rise caused by the generation of the boil-off gas while having a heat insulating wall, that is, it is designed to withstand the pressure rise caused by the generation of boil-off gas. . Therefore, the boil-off gas generated inside the LNG storage tank 1 for LNG carriers is accumulated in the LNG storage tank 1 for LNG carriers during the operation period of the LNG carrier.

For example, the LNG storage tank 1 for LNG carriers according to the embodiment of the present invention is preferably designed to withstand a pressure of more than 0.25 to 2 bar (gauge pressure) with a heat insulating wall, more preferably. It is designed to withstand a pressure of 0.6 to 1.5 bar (gauge pressure). Given the distance of LNG transport and the current IGC Code, it is desirable to be designed to withstand pressures greater than 0.25 bar to 0.7 bar, in particular 0.7 bar. However, if the pressure is too low, the distance for transporting the LNG is too short, it is not preferable, if too high there is a problem that the production of the tank is not easy.

In addition, the LNG storage tank (1) for LNG carriers according to the present invention is designed to increase the thickness of the initial design, or to add a reinforcement only without adding a reinforcement to the existing LNG storage tanks for general LNG carriers appropriate reinforcement It is economical in terms of production cost because it can be fully realized.

On the other hand, there are various known LNG storage tanks for LNG carriers according to the prior art having a heat insulation (heat dissipation) wall as described below. Therefore, in FIG. 1, illustration of the heat insulation wall is omitted.

First, the LNG storage tank installed inside the LNG carrier can be divided into an independent tank type and a membrane type. The details are as follows.

In Table 1, aka GTT NO 96-2 type and GTT Mark III type were renamed Gaz Transport (GT) and Technigaz (TGZ) in 1995 as GTT (Gaztransport & Technigaz), respectively, GT type was GTT NO 96 -2 type, TGZ type is renamed GTT Mark III type.

GT and TGZ tank structures described above are described in US Pat.

Korean Patent Nos. 499710 and 0644217 disclose a heat insulating wall as another concept.

LNG storage tanks for LNG carriers having various types of insulating walls have been previously disclosed, which are intended to suppress possible vaporization of LNG.

As described above, it is possible to apply the present invention to LNG storage tanks for LNG carriers having various types of thermal insulation functions. Most LNG storage tanks for LNG carriers are designed to withstand a pressure of 0.25 bar or less, and when the pressure reaches or exceeds a pressure of 0.2 bar or less, for example, 0.1 bar, a portion of the boil-off gas is reached. Or burn everything to the GCU. In addition, the LNG storage tank is provided with a safety valve (safety valve) is discharged to the outside air through the safety valve (normal opening and closing pressure of 0.25 bar) when the above control fails.

In addition, the LNG storage tank according to the present invention is configured to reduce the pressure of the LNG storage tank by reducing the local rise in temperature and pressure, thereby reducing the relatively low temperature LNG at the bottom of the LNG storage tank for LNG carriers. LNG carrier for LNG carriers by injecting into the upper part of the LNG storage tank for relatively high temperature LNG carriers and injecting relatively high temperature boil-off gas of the upper part of the LNG carrier tank for LNG carriers to the lower part of the LNG storage tank for LNG carriers that are relatively low temperature. Maintain a uniform temperature distribution in the storage tank.

In FIG. 2, a pump 11 for LNG and an injection nozzle 21 for boil-off gas are provided at a lower portion of the LNG storage tank 1 for an LNG carrier, and a LNG storage tank 1 for an LNG carrier is provided at an upper portion of the LNG storage tank 1 for an LNG carrier. A spray 13 and a compressor 23 for the boil-off gas are provided. Here, the LNG pump 11 and the boil-off compressor 23 can be freely installed at the upper and lower parts. The relatively low temperature LNG in the lower portion of the LNG storage tank 1 for LNG carriers is supplied to the upper LNG spray 13 by the LNG pump 11 so that the LNG storage tank 1 for the LNG carrier is relatively hot. And the relatively high temperature boil-off gas of the upper portion of the LNG storage tank 1 for LNG carriers is supplied to the lower boil-off gas injection nozzle 21 by the boil-off compressor 23 for relatively low temperature. The amount of generation of the boil-off gas can be reduced by spraying the lower portion of the LNG storage tank 1 for LNG carriers and maintaining the temperature distribution of the LNG storage tank 1 for LNG carriers uniformly.

In addition, if the LNG is shipped to the LNG carrier in a supercooled state at the production terminal that produces LNG, it is possible to further reduce the boil-off gas (pressure rise) generated during transportation. After loading under supercooling at the production terminal, the LNG storage tank for LNG carriers may be underpressure (0 bar or less), which can be filled with nitrogen to prevent this.

Referring to the method of processing the boil-off gas using the LNG storage tank for LNG carriers as described above are as follows.

When the LNG carrier operates, the LNG storage tank 1 for LNG carrier according to the present invention does not process the boil-off gas, thereby allowing a pressure increase in the tank thereby allowing most of the heat inflow by the internal temperature rise of the tank. Accumulated by the elevated thermal energy of LNG and NG, and when the LNG carrier arrives at the destination, the boil-off terminal processes the boil-off gas accumulated in the LNG storage tank 1 for LNG carrier.

Figure 3 schematically shows a configuration for processing the boil-off gas at the unloading terminal using the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

A plurality of LNG storage tanks 2, a high pressure compressor 3a, a low pressure compressor 3b, a recondenser 4, a high pressure pump P and a vaporizer 5 are provided in the cargo terminal.

Since the boil-off gas accumulated in the LNG storage tank 1 for LNG carriers is large, it is usually compressed to 70-80 bar by the high-pressure compressor 3a at the unloading terminal and then supplied directly to the consumer. On the other hand, a part of the boil-off gas accumulated in the LNG storage tank (1) for LNG carriers is usually compressed to around 8 bar by the low pressure compressor (3b) and then recondensed through the recondenser (4) and in the vaporizer (5) It may be vaporized again and supplied to the consumer.

When unloading LNG from LNG storage tank for LNG carrier to LNG storage tank for cargo terminal at the loading terminal, the pressure of LNG storage tank for LNG carrier is greater than the pressure of LNG storage tank for cargo terminal. When high pressure LNG is introduced into the gas, additional boil-off gas is generated.To minimize this, the LNG is directly connected to the inlet of the high-pressure pump at the unloading terminal from the LNG storage tank for the LNG carrier. There is this. LNG storage tank for LNG carrier according to the present invention has the advantage that the unloading time is shortened by 10-20% compared to the conventional LNG carrier because the pressure in the LNG tank is high when unloading.

The LNG stored in the LNG storage tank (1) for LNG carriers is not supplied to the LNG storage tank (2) for the loading terminal of the cargo terminal, but is supplied to the recondenser (4) to recondense the boil-off gas and then vaporize in the vaporizer (5). Can be supplied directly to the consumer.

As described above, when a plurality of storage tanks for the cargo terminal 2 are installed in the cargo terminal, the LNG is equalized to the plurality of cargo storage tanks 2 for the cargo terminal from the LNG storage tank 1 for the LNG carrier. When distributed and unloaded, the generation of the boil-off gas is distributed to the plurality of LNG storage tanks 2 of the loading terminal to minimize the influence of the generation of the boil-off gas in the respective LNG storage tanks 2. Since the amount of boil-off gas generated in the storage tank 2 for the unloading terminal is small, it is usually compressed to around 8 bar by the low pressure compressor 3b and then recondensed through the recondenser 4 and vaporized again in the vaporizer 5. And is supplied to the consumer.

In addition, according to the present invention, since the LNG storage tank for LNG carriers is operated above the existing design pressure, to fill the boil-off gas or NG in the LNG storage tanks for LNG carriers to maintain the pressure in the LNG storage tanks for LNG carriers when the LNG unloading The process becomes unnecessary.

In addition, the LNG storage tank for LNG terminal or LNG storage tank for LNG floating storage and regasification unit (FSRU) can be adapted or new LNG terminal LNG so that the storage pressure corresponds to the storage tank pressure for LNG carrier of the present invention. When the LNG storage tank for the storage tank or LNG floating storage and regasification unit (FSRU) is constructed, there is no problem even if the existing unloading method is applied as it does not generate additional boil-off gas during LNG unloading.

In addition, the conventional operation method was a method of consuming or re-liquefying the entire amount of evaporated gas generated to lower the pressure of the LNG storage tank for LNG carriers, according to the present invention consumes or re-liquefies only a portion of the evaporated gas for LNG carriers Since the pressure of the LNG storage tank can be maintained, it is also possible to apply the present invention to LNG carriers configured to use or reliquefy the boil-off gas as propellant fuel.

According to the present invention, since the pressure constraint on the LNG storage tank is reduced during LNG transportation, flexibility in the presence and selection of the boil-off gas treatment equipment is increased, and thus the application of the present invention is not limited to operating without the boil-off gas treatment equipment.

According to the present invention, in the case of the LNG floating storage and regasification apparatus (FSRU), since the management flexibility of the boil-off gas is increased, the installation of the recondensation apparatus may be unnecessary.

According to the present invention, the LNG regasification vessel (RV) may have all of the advantages of the aforementioned LNG carrier and the LNG floating storage and regasification apparatus (FSRU).

While the invention has been described above with reference to specific embodiments, various modifications, changes or modifications may be made in the art within the spirit and scope of the appended claims, and thus, the foregoing description and drawings It should be construed as illustrating the present invention, not limiting the technical spirit of the present invention.

As described above, according to the present invention, in the case of LNG carriers, the boil-off gas is accumulated in the LNG storage tank for LNG carriers designed to allow the pressure rise due to the generation of boil-off gas during operation, and upon arrival of the destination of the LNG carriers. Since the unloading terminal is configured to process the accumulated boil-off gas, the propulsion system can be arbitrarily selected, and the propulsion system and the LNG storage system are independent, so that the system can be simplified.

In addition, according to the present invention, various equipment (boiler / steam turbine, reliquefaction apparatus, engine using gas, or fuel supply compressor, etc.) required for treating existing boil-off gas in an LNG carrier is not required. Propulsion equipment can also be used with general-purpose engines that are known to have the highest efficiency.

In addition, the LNG carrier can be used very efficiently even if there are existing evaporative gas treatment engines or reliquefaction devices. In particular, even when the amount of boil-off gas generated during transportation of LNG is larger than the consumption, it can be preserved without loss of the boil-off gas, resulting in economical efficiency and efficiency. For example, in the case of an LNG carrier equipped with an engine for treating boil-off gas, as shown in FIG. The BOG over engine consumption generated in the city is often burned out by using the GCU, and in the case of LNG carriers equipped with a reliquefaction device, the excess BOG must be burned out by using the GCU. Application of the technique of the present invention can reduce such waste of BOG.

In addition, when a gas / liquid combined engine is used in an LNG carrier, fuel can be supplied using a liquid pump instead of an evaporative gas compressor, thereby greatly reducing installation and operating costs.

In addition, according to the present invention, in the case of LNG carriers, in addition to reducing the installation cost, since the treatment of evaporation gas is not performed during operation, labor costs such as minimization of gas engineers can be reduced and economical.

Claims (9)

As the membrane type LNG storage tank, the steam pressure in the tank is regulated within a pressure range near atmospheric pressure, and the tank is configured to allow the increase of the steam pressure in the tank and the temperature of the LNG during transportation of the LNG. Membrane type LNG storage tank of LNG carrier, characterized in that designed to have the strength to withstand the rise. The membrane type LNG storage tank of claim 1, wherein the vapor pressure in the tank allows an increase of more than 0.25 to 2 bar. The membrane type LNG storage tank of claim 1, wherein the vapor pressure in the tank allows an increase to greater than 0.25 to 0.7 bar. The membrane type LNG storage tank of claim 1, wherein the vapor pressure in the tank allows an increase to 0.7 bar. The LNG carrier according to any one of claims 1 to 4, wherein the LNG of the LNG storage tank and the boil-off gas of the upper portion of the LNG storage tank are mixed in order to uniformize the temperature distribution inside the LNG tank. Membrane type LNG storage tank. An LNG carrier provided with a tank according to any one of claims 1 to 4. A method of transporting LNG using a membrane type LNG storage tank, in which the vapor pressure in the tank is regulated within a pressure range near normal pressure, and the vaporized gas generated from evaporation of the LNG is accumulated in the tank during transportation of the LNG. And an increase in the steam pressure in the tank and the temperature of the LNG. 8. The method of claim 7, wherein the vapor pressure in the tank allows an increase to greater than 0.25 to 0.7 bar. The method for transporting LNG according to claim 7 or 8, wherein the LNG in the lower portion of the LNG storage tank and the boil-off gas in the upper portion of the LNG storage tank are mixed in order to make the temperature distribution inside the LNG tank uniform.

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