JP6943373B2 - Liquefied gas storage tank structure and ships - Google Patents

Description

The present invention relates to a liquefied gas storage tank structure capable of efficiently reducing the amount of boil-off gas generated inside a storage tank for storing liquefied gas, a ship, and a method for reducing boil-off gas.

A liquefied gas carrier that transports liquefied natural gas (LNG) or the like is provided with a storage tank for low-temperature liquefied gas. In this storage tank, the liquefied gas evaporates due to the heat flowing in from the outside, and boil-off gas is generated. ing. When this boil-off gas is generated, the internal pressure of the tank increases, so the boil-off gas is released to the outside of the storage tank to keep the pressure inside the storage tank within a safe range.

However, releasing the generated boil-off gas reduces the amount of liquefied gas stored, resulting in an economic loss. Therefore, as a general method of suppressing the amount of boil-off gas generated, a method of improving the heat insulation performance of the storage tank is adopted. However, it costs a lot of money to improve the quality of all the heat insulating materials that cover the tank. Further, since the thickness of the heat insulating material is limited by the installation location of the tank and the like, there is a limit to the increase in the thickness. Further, in a liquefied gas carrier of low temperature liquefied gas, the size of the ship is limited by the port and the canal, and increasing the thickness of the heat insulating material causes a decrease in the volume of the storage tank. There are also limits in terms of aspects.

In addition to this heat protection measure, as a measure to avoid an increase in internal pressure in the storage tank for low-temperature liquefied gas, the low-temperature liquefied gas in the tank is pumped to the injection nozzle provided in the upper part of the tank, and from this injection nozzle. By injecting the low-temperature liquefied gas toward the low-temperature liquefied gas surface layer and destroying the low-temperature liquefied gas surface layer, the temperature of the liquefied gas in the tank is equalized so that the high-temperature surface layer is not formed on the liquid surface in the tank. As a result, a low-temperature liquefied gas agitator that suppresses an increase in the internal pressure of the tank has been proposed (see, for example, Patent Documents 1 and 2).

Further, a method has been proposed in which a liquefied gas is injected onto the liquid surface to destroy the surface layer of the cryogenic liquefied gas, and a liquefied gas is injected into the liquid space inside the tank to stir the inside (for example, Patent Document 3 and 3. 4).

However, in order to destroy the surface layer of the low-temperature liquefied gas, it is necessary to inject the low-temperature liquefied gas with a certain amount of collision energy. Therefore, it is necessary to increase the pressure of the low-temperature liquefied gas, and there is a problem that the driving force and driving energy of the pump increase. Further, when injecting liquefied gas to agitate the inside, a large amount of energy is required to drive the pump as well.

Japanese Unexamined Patent Publication No. 2003-247697 Jitsukaisho 61-73899 Japanese Patent Application Laid-Open No. 2007-504414 JP-A-59-219599

On the other hand, in the present invention, the part where heat from the outside enters the storage tank is from the wall of the tank, and the liquefied gas whose temperature has increased due to the heat input from this wall has a low specific gravity. Since it is smaller and lighter than the liquefied gas, it rises and collects on the liquid level at the top of the tank. On the other hand, since the temperature of the liquid surface is lower than that of this high-temperature liquid, heat is transferred from the high-temperature liquefied gas rising to the surface layer toward the liquid surface. This heat becomes heat of vaporization and is a factor in generating boil-off gas. It was found that the generation of boil-off gas can be reduced with less energy by concentrating and cooling the high-temperature liquefied gas that collects on the surface layer of the liquid by this natural convection.

The present invention has been made in view of the above circumstances, and an object of the present invention is to cool a high-temperature liquefied gas layer below the surface layer of liquefied gas in a tank for storing liquefied gas of a liquefied gas carrier to lower the temperature thereof. The liquefied gas storage tank structure, the ship, and the boil-off gas can suppress the heat transfer that causes the heat of vaporization and reduce the amount of boil-off gas generated without increasing the thickness of the heat insulating material of the storage tank. Is to provide a method of reducing the amount of gas.

Liquefied gas storage tank structure of the present invention for achieving the above object, the liquefied gas storage tank structure with a storage tank for storing liquefied gas, the surface layer lower hot liquefied gas stored liquefied gas A part or all of the cooling system for cooling the layer is provided inside the storage tank, and at least a part of the cooling device for cooling the high temperature liquefied gas layer puts the liquefied gas in the storage tank at least. When it is fully loaded, it is configured to be arranged at or near the liquid level of the liquefied gas, and the cooling device is configured to float on the liquid surface of the liquefied gas .

According to this structure, the low-temperature liquefied gas surface layer of the liquefied gas in the storage tank is not destroyed by injecting the liquefied gas or gas from above the low-temperature liquefied gas surface layer to the low-temperature liquefied gas surface layer, but naturally. By cooling the high-temperature liquefied gas layer concentrated on the lower side of the liquefied gas surface layer by convection, heat transfer that causes heat of vaporization can be suppressed, and the amount of boil-off gas generated can be reduced with less energy.

At the liquid level of the liquefied gas, the heat of vaporization (heat of vaporization) is taken when the liquefied gas evaporates to become a boil-off gas, so the temperature of the liquefied gas on the liquid surface is higher than the temperature of the liquefied gas below it. It is cold. Therefore, even if the low-temperature liquefied gas surface layer is cooled, the cooling efficiency is poor, so the high-temperature liquefied gas layer below the low-temperature liquefied gas surface layer is cooled. This high-temperature liquefied gas layer is formed in a range of, for example, 30 cm to 50 cm below the liquid surface, but changes greatly depending on the environment of the storage tank. Further, the thickness of this high-temperature liquefied gas layer gradually increases as time passes from the loading of the liquefied gas, and this thickness also changes greatly depending on the environment of the storage tank.

Therefore, in the present invention, the natural convection rising along the tank wall surface is used instead of the method of stirring the liquefied gas in the tank or destroying the low-temperature liquefied gas layer on the surface to change the macro temperature distribution. By concentrating the heat in the tank on the lower part of the surface layer of the liquid surface and efficiently cooling the high-temperature liquefied gas layer, a method is adopted in which the heat in the entire tank is efficiently removed.

In the above liquefied gas storage tank structure, when the cooling device is configured to change the height according to the top and bottom of the liquid level of the liquefied gas stored in the storage tank, this configuration is used. Even if the liquefied gas evaporates and the liquid level drops, the cooling device can be lowered accordingly, so that the high-temperature liquefied gas layer below the surface layer of the liquefied gas is efficiently cooled while maintaining good cooling efficiency. You will be able to.

In the above liquefied gas storage tank structure, if the cooling device is configured to float on the liquid level of the liquefied gas, even if the liquefied gas evaporates and the liquid level drops, this configuration allows it. Along with this, the cooling device is automatically lowered, so that the high-temperature liquefied gas layer below the surface layer of the liquefied gas can be efficiently cooled while maintaining good cooling efficiency.

In the liquefied gas storage tank structure, the cooling system extracts the boil-off gas inside the storage tank and reliquefies it, and returns the liquefied liquefied gas to the inside of the storage tank. When the liquefied gas is configured to be supplied to the cooling device, the liquefied gas carrier or the like reliquefies the boil-off gas discharged from the storage tank, and the reliquefied liquefied gas is used. Since it is often equipped with a reliquefaction system that returns the gas to the storage tank, the boil-off gas can be used to efficiently cool the high-temperature liquefied gas layer below the surface layer of the liquefied gas. Further, in the case of the configuration in which the liquefied gas reliquefied by this reliquefaction device is supplied, it is not necessary to operate the pump in the storage tank, so that it is possible to prevent heat from entering the liquefied gas when the pump is driven.

In the above liquefied gas storage tank structure, when the cooling system is filled with a pump arranged in a lower portion in the storage tank and the liquefied gas in the storage tank, it is located near the liquid level of the liquefied gas. Then, the cooling device provided with a diffusion device for diffusing the liquefied gas supplied from the pump into the high temperature liquefied gas layer as a liquid phase, and a supply pipe connecting the pump and the cooling device are provided. With this configuration, the relatively low temperature liquefied gas at the bottom of the storage tank is sucked up and supplied to a diffuser composed of an upward nozzle or the like, and the high temperature below the surface layer of the liquefied gas is supplied. The liquefied gas layer can be cooled efficiently with a relatively low temperature liquefied gas.

In the above liquefied gas storage tank structure, the pump is also used as a cargo handling pump, a spray pump for cooling the tank of the storage tank, or a pump that supplies liquefied gas as fuel to the main engine. With this configuration, the pump can also be used, so that the equipment as an overall facility related to the liquefied gas storage tank can be simplified.

In the liquefied gas storage tank structure, the cooling system connects the supply pipe to an external cooling system outside the storage tank, and the liquefied gas supplied from the pump is supplied to the storage tank by the external cooling system. If it is configured to be cooled to a temperature lower than the saturation temperature corresponding to the internal pressure of the gas and then supplied to the diffuser, the high temperature liquefied gas layer is cooled by the cooling device outside the storage tank. Since the temperature of the liquefied gas supplied to the vehicle can be lowered, the effect of reducing the boil-off gas is increased, and the amount of the boil-off gas generated can be reduced more efficiently.

In the above liquefied gas storage tank structure, if the diffuser is configured to be used also as a spray nozzle for cooling the tank, this configuration makes the equipment as an overall facility related to the liquefied gas storage tank. Can be simplified.

When the cooling device is composed of a heat exchanger that exchanges heat between the external liquefied gas and the internal heat medium in the liquefied gas storage tank structure, the heat medium of the heat exchanger By adjusting the temperature and the flow rate of the above, the temperature of the liquefied gas in the storage tank can be finely adjusted and controlled, and the cooling effect can be easily controlled. Therefore, the amount of boil-off gas generated can be reduced more efficiently. Further, since it is not necessary to operate the pump in the storage tank, it is possible to eliminate heat input to the liquefied gas when the pump is driven.

Further, in the above-mentioned liquefied gas storage tank structure, if the storage tank is configured so that the horizontal cross-sectional area inside the tank decreases as the horizontal cross-sectional area inside the tank increases in the upper quarter, the temperature rises due to heat convection. Since the surface area of the liquid surface, which becomes relatively high, becomes small and the cooling range covered by the cooling device becomes small, the amount of boil-off gas generated can be reduced more efficiently. As the storage tank having this shape, a spherical tank is generally used, but other non-spherical tanks and the like are also available.

The ship of the present invention is characterized by having the above-mentioned liquefied gas storage tank structure, and can similarly exhibit the effects that the above-mentioned liquefied gas storage tank structure can exert. In addition to liquefied gas carriers such as LNG tankers and LPG tankers, this vessel includes liquefied gas fuel vessels that use liquefied gas as fuel.

According to the liquefied gas storage tank structure, the ship, and the boil-off gas reduction method of the present invention, the high temperature liquefied gas layer below the surface layer of the liquefied gas of the tank for storing the liquefied gas of the liquefied gas carrier is cooled to reduce the temperature. By reducing the amount, heat transfer that causes heat of vaporization can be suppressed, and the amount of boil-off gas generated can be reduced without increasing the thickness of the heat insulating material of the storage tank.

It is a side sectional view which shows typically the structure of the liquefied gas storage tank structure of 1st Embodiment which concerns on this invention, and is the figure which the distribution pipe is above the liquid level. It is a figure for explanation which shows typically the structure of the cooling apparatus of the part AA of FIG. It is a side sectional view which shows typically the structure of the liquefied gas storage tank structure of 1st Embodiment which concerns on this invention, and is the figure which the distribution pipe is below the liquid level. It is a side sectional view schematically showing the structure of the liquefied gas storage tank structure of the 2nd Embodiment which concerns on this invention. It is a side sectional view schematically showing the structure of the liquefied gas storage tank structure of the 3rd Embodiment which concerns on this invention. It is a side sectional view schematically showing the structure of the liquefied gas storage tank structure of the 4th Embodiment which concerns on this invention. It is a side sectional view schematically showing the relationship between the low temperature liquefied gas surface layer, the high temperature liquefied gas layer, and the nozzle in the liquefied gas storage tank. It is a schematic explanatory view which shows the structure which provided the nozzle to the distribution pipe rotatably in the horizontal direction, and is the figure which the distribution pipe is above the liquid level. It is a schematic explanatory view which shows the structure which provided the nozzle to the distribution pipe rotatably in the horizontal direction, and is the figure which the distribution pipe is below the liquid level.

Hereinafter, the liquefied gas storage tank structure, the ship, and the method for reducing boil-off gas according to the embodiment of the present invention will be described with reference to the drawings.

First, the liquefied gas storage tank structure of the first to fourth embodiments according to the present invention will be described. As shown in FIGS. 1 to 6, a cooling system (20A to 20D, hereinafter (Use 20 as a generic term) It is configured to include a part or all of 20. The storage tank 11 is mounted or installed on the ship 1 via the tank support member 2, and a tank cover 13 is provided around the storage tank 11.

The cooling system 20 is a cooling system for cooling the high-temperature liquefied gas layer Lb on the lower side of the surface layer of the liquefied gas L stored in the storage tank 11. When the storage tank 11 is filled with the liquefied gas L, the cooling device 21 for cooling the high-temperature liquefied gas layer Lb of the cooling system 20 is arranged at the liquid level S of the liquefied gas L or in the vicinity of the liquid level. ..

Explaining the high-temperature liquefied gas layer Lb, as shown in FIG. 7, when the liquefied gas L is stored inside the heat-insulating storage tank 11, the liquefied gas L in the liquid phase and the liquefied gas L evaporate to boil off. The gas G is stored. Then, since the temperature of the liquefied gas L near the wall surface rises due to the heat transferred from the wall surface of the storage tank 11 to the liquefied gas L inside, the specific gravity becomes lighter and natural convection that rises along the wall surface is generated, and this natural convection occurs. The relatively high temperature liquefied gas L that has reached the liquid level S forms a high temperature liquefied gas layer Lb on the surface layer portion of the liquid level S.

However, at the liquid level S, when the liquefied gas L evaporates to become the boil-off gas G, the heat of vaporization (heat of vaporization) is taken away. The low temperature liquefied gas surface layer La, which is lower than the temperature of the above, is formed on the high temperature liquefied gas layer Lb.

The thickness ta of the low-temperature liquefied gas surface layer La depends on the state of the storage tank 11 and the state of the stored liquefied gas L, but is, for example, about 30 cm to 50 cm, and the high-temperature liquefied gas layer Lb is usually about 30 cm to 50 cm. It is formed directly below the surface layer La of the low-temperature liquefied gas. The amount of evaporation of the boil-off gas G depends on the temperature difference between the high-temperature liquefied gas layer Lb and the liquid level S. Further, the thickness Rb of the high-temperature liquefied gas layer Lb gradually increases as time passes from the loading of the liquefied gas L, but this thickness is greatly affected by the environment of the storage tank 11 and changes.

However, the thickness ta and Rb of the low-temperature liquefied gas surface layer La and the high-temperature liquefied gas layer Lb can be determined based on several factors from measurements and numerical experiments on the actual machine, so they are applied to the actual machine. At that time, the thicknesses ta and Rb of the low-temperature liquefied gas surface layer La and the high-temperature liquefied gas layer Lb can be assumed.

Then, as shown in FIGS. 1 to 3, in the liquefied gas storage tank structure 10A of the first embodiment, the cooling system 20A is a lower portion in the storage tank 11 in addition to the cooling device 21A. It has a pump 22 arranged in the above and a supply pipe 23A connecting the pump 22 and the cooling device 21A.

Further, the cooling device 21A has a nozzle (diffusing device) 21Aa for diffusing the liquefied gas L supplied from the pump 22 into the high temperature liquefied gas layer Lb as a liquid phase, and a nozzle for liquefied gas L from the supply pipe 23A. It is configured to include a distribution pipe 21Ab that distributes to 21Aa. Further, at least a part of the cooling device 21A (here, the nozzle portion of the nozzle 21Aa) is located below the liquid level S of the liquefied gas L when the storage tank 11 is filled with the liquefied gas L. It is configured as follows.

That is, inside the storage tank 11, a supply pipe 23A for supplying the liquefied gas L from the lower part to the surface layer is installed, and as shown in FIG. 2, the supply pipe 23A has a circular shape in the vicinity of the liquid surface layer. The liquefied gas L is uniformly supplied to a plurality of nozzles 21Aa provided in the distribution pipe 21Ab by connecting to the distribution pipe 21Ab extending to the area. Further, a pump 22 for sucking up the liquefied gas L is connected to the lower part of the supply pipe 23A.

In this configuration, the liquefied gas L in the storage tank 11 is naturally convected, so that the liquefied gas L having a high temperature is collected in the liquid surface layer portion and the liquefied gas L is collected in the lower portion of the storage tank 11. Therefore, the temperature of the liquid surface layer portion is lowered by sucking up the low-temperature liquefied gas L by the pump 22 and ejecting it from the nozzle 21Aa to the liquid surface layer portion. The nozzle 21Aa is arranged so that its nozzle (discharge port) is immersed in the liquid.

Then, in the storage tank 11 of the liquefied gas L that can hold the liquefied gas L without boiling, the position where the injection port (discharge port) of the nozzle 21Aa is immersed in the liquefied gas L under the liquid surface S when the liquefied gas L is loaded. The liquefied gas L is supplied as the liquid phase toward the high temperature liquefied gas layer Lb below the low temperature liquefied gas surface layer La of the surface layer portion, and the high temperature liquefied gas layer Lb inside the storage tank 11 is supplied. By cooling the gas, the amount of boil-off gas G generated is reduced.

The nozzle 21Aa is configured such that its injection port is arranged slightly below the liquid level S of the liquefied gas L, but the purpose is not to stir the high temperature liquefied gas layer Lb with the liquefied gas L, and the temperature is low. Since the purpose is to mix the liquefied gas L with the high-temperature liquefied gas layer Lb and cool it, the jet output of the nozzle 21Aa does not need to be strong, and the liquefied gas L can be permeated into the high-temperature liquefied gas layer Lb and diffused. It suffices if there is a jet output of.

Therefore, the discharge pressure of the pump 22 may be lower than the supply pressure to the nozzle for stirring in the prior art, so that the energy for driving the pump 22 is small, and accordingly, when the pump 22 is driven, The transfer of heat generated by the pump 22 to the liquefied gas L can be reduced.

The ejection direction of the nozzle 21Aa is for supplying the liquefied gas L to the high-temperature liquefied gas layer Lb. Therefore, as shown in FIG. 1, when the distribution pipe 21Ab is located above the liquid level S, the nozzle 21Aa is ejected. , The nozzle port is below the liquid level S, and the injection direction of the nozzle 21Aa is toward the high temperature liquefied gas layer Lb, preferably downward, or even if it is inclined, it is downward below the horizontal. It is composed of.

Alternatively, as shown in FIG. 3, when the distribution pipe 21Ab is located below the liquid level S, the nozzle of the nozzle 21Aa is below the liquid level S, and the injection direction of the nozzle is liquefied at a high temperature. It is preferably configured so as to be directed toward the gas layer Lb, or upward above the horizontal even if it is inclined.

Further, as shown in FIGS. 8 and 9, the nozzle 21Aa is rotatably provided in the distribution pipe 21Ab in the horizontal direction, and as shown in FIG. 8, the distribution pipe 21Ab is above the liquid level S. On the other hand, as shown in FIG. 9, when the distribution pipe 21Ab is below the liquid level S and the nozzle 21Aa is below the liquid level S, the buoyancy of the nozzle itself It is preferable to configure it so that it faces upward.

In this configuration, even if both the nozzle 21Aa and the distribution pipe 21Ab are not moved up and down in response to the vertical change of the liquid level S, the nozzle 21Aa automatically corresponds to the vertical movement of the liquid level S. The position of the nozzle and the direction of the nozzle can be made to follow, so that the ejection direction of the nozzle 21Aa can be directed toward the high temperature liquefied gas layer Lb.

The pump 22 may also be used as a cargo handling pump, a spray pump for cooling the storage tank 11, or a pump that supplies liquefied gas as fuel to the main engine. The combined use of the pump 22 can simplify the equipment as an overall facility related to the storage tank 11.

Further, the nozzle 21Aa may also be used as a part of the spray nozzle for cooling the tank. The combined use of the nozzle 21Aa can simplify the equipment as an overall facility related to the storage tank 11. In this case, since the spray nozzles are usually provided in a plurality of stages in the vertical direction in order to cool the entire empty storage tank 11, the uppermost spray nozzle and the nozzle 21Aa are shared, and the lower side is used. The spray nozzle is dedicated.

According to the liquefied gas storage tank structure 10A of the first embodiment described above, the liquefied gas L at the lower part of the storage tank 11 is sucked up by the pump 22 and supplied to the nozzle 21Aa to supply the liquefied gas L. The high-temperature liquefied gas layer Lb below the low-temperature liquefied gas surface layer La on the liquid level S can be thermally efficiently cooled by the relatively low-temperature liquefied gas L. Further, as the equipment, a cooling device 21A having a nozzle 21Aa for supplying the low-temperature liquefied gas L and a distribution pipe 21Ab and a supply pipe 23A may be provided near the liquid level S of the storage tank 11. It is possible to reduce the amount of boil-off gas generated without changing the heat insulation performance of the storage tank 11.

Next, the liquefied gas storage tank structure 10B of the second embodiment will be described. As shown in FIG. 4, in the liquefied gas storage tank structure 10B, the cooling system 20B extracts the boil-off gas G inside the storage tank 11 by the degassing pipe 32, reliquefies it by the reliquefaction device 31, and liquefies it. The reliquefied liquefied gas L from the reliquefaction system 30 that returns the liquefied gas to the inside of the storage tank 11 by the return pipe 33 is configured to be supplied to the cooling device 21B by the cooling pipe 34. The cooling device 21B has a nozzle 21Ba and a distribution pipe 21Bb, and is configured in the same manner as the cooling device 21A of the liquefied gas storage tank structure 10A of the first embodiment.

That is, in the liquefied gas storage tank structure 10A of the first embodiment, the liquefied gas L for cooling the high temperature liquefied gas layer Lb is the liquefied gas L in the lower part of the storage tank 11 sucked up by the pump 22. In the liquefied gas storage tank structure 10B of the embodiment, the liquefied gas L is obtained by reliquefying the boil-off gas G. Other than that, it is the same.

According to the liquefied gas storage tank structure 10B of the second embodiment described above, in the liquefied gas carrier or the like, the boil-off gas G discharged from the storage tank 11 is reliquefied, and the reliquefied liquefied gas L is stored in the storage tank 11. Since the reliquefaction system 30 for returning to the inside is often installed, the high temperature liquefied gas layer Lb below the surface layer of the liquefied gas L can be efficiently cooled by using the boil-off gas G. Further, since the liquefied gas L reliquefied by the reliquefaction system 30 is supplied, it is not necessary to move the pump 22 in the storage tank 11, so that heat input to the liquefied gas L when the pump 22 is driven is prevented. Can be done.

Next, the liquefied gas storage tank structure 10C of the third embodiment will be described. As shown in FIG. 5, in the liquefied gas storage tank structure 10C, the cooling system 20C includes the pump 22, the supply pipe 23C, and the cooling device, as in the liquefied gas storage tank structure 10A of the first embodiment. The cooling device 21C is configured to include 21C, and the cooling device 21C is configured to include a nozzle 21Ca and a distribution pipe 21Cb.

However, in this liquefied gas storage tank structure 10C, the supply pipe 23C is connected to the external cooling system 40 outside the storage tank 11, and the liquefied gas L supplied from the pump 22 is stored by the external cooling system 40. It is configured to be cooled to a temperature lower than the saturation temperature Ts corresponding to the internal pressure Pin of 11 and then supplied to the nozzle (diffusing device) 21Ca. Other points are the same as the liquefied gas storage tank structure 10A of the first embodiment.

That is, in the liquefied gas storage tank structure 10A of the first embodiment, the liquefied gas L for cooling the high temperature liquefied gas layer Lb is the liquefied gas L at the lower part of the storage tank 11 sucked up by the pump 22 as it is in the cooling device 21A. In the liquefied gas storage tank structure 10C of the third embodiment, the liquefied gas L at the lower part of the storage tank 11 sucked up by the pump 22 is supplied to the external cooling system 40 provided outside the storage tank 11. It is supplied via the pipe 23C, and the liquefied gas L is cooled by the external cooling system 40 and then supplied to the cooling device 21C by the supply pipe 23C.

According to the liquefied gas storage tank structure 10C of the third embodiment described above, the liquefied gas L sucked up by the pump 22 is further cooled by the external cooling system 40 and then supplied, so that the liquefied gas L is supplied to the high temperature liquefied gas layer Lb. The temperature of the liquefied gas L to be supplied can be lowered, and the amount of boil-off gas G generated can be reduced more efficiently.

Then, according to the liquefied gas storage tank structures 10A to 10C of the first to third embodiments, the low-temperature liquefaction supplied by arranging the nozzles of the nozzles 21Aa, 21Ba, and 21Ca below the liquid level S Since it is possible to prevent the gas L from exchanging heat with the gas phase portion, it is possible to efficiently cool the gas L.

Next, the liquefied gas storage tank structure 10D of the fourth embodiment will be described. As shown in FIG. 6, in the liquefied gas storage tank structure 10D, the cooling device 21D is a heat exchanger that exchanges heat between the external (surrounding) liquefied gas L and the internal heat medium (refrigerant). Consists of. A low-temperature heat medium is supplied from the cooling device 51 of the cooling system 50 to the heat exchanger 21D via the pipe 52, and the heat medium after heat exchange is returned to the cooling device 51 via the pipe 53.

The heat exchanger 21D is composed of, for example, a cooling pipe, and the cooling pipe is installed at a height such that it is just immersed in the liquid surface S when the liquefied gas L is full, and the heat flowing inside the cooling pipe is installed. The medium may be circulated with the cooling device 51 to exchange heat between the liquefied gas L at the liquid level S and the heat medium, and the medium may be directly cooled by the heat medium.

That is, in this liquefied gas storage tank structure 10D, the high temperature liquefied gas layer is not cooled by supplying the liquefied gas L to the high temperature liquefied gas layer Lb, but by heat exchange with the heat exchanger 21D arranged in the high temperature liquefied gas layer Lb. Cool Lb.

According to the liquefied gas storage tank structure 10D of the fourth embodiment described above, the temperature of the liquefied gas L in the storage tank 11 is finely adjusted and controlled by adjusting the temperature and flow rate of the heat medium of the heat exchanger 21D. This makes it easier to control the cooling effect. Therefore, the amount of boil-off gas G generated can be reduced more efficiently. Further, since it is not necessary to move the pump 22 in the storage tank 11, it is possible to eliminate heat input to the liquefied gas L when the pump 22 is driven.

Then, in the liquefied gas storage tank structure (10A to 10D, hereinafter collectively referred to as 10) 10 of the first to fourth embodiments described above, the cooling device (21A to 21D, hereinafter collectively referred to as 21) is used. ) With respect to the vertical position of 21, it may be configured in any of the following first to third configurations.

In the first configuration, the cooling device 21 is located at the top position inside the storage tank 11 and at a position 15% lower than the height inside the storage tank 11 in the vertical direction of the storage tank 11. It is placed between and.

With this configuration, when the liquefied gas L is full, the height of the liquid level S of the liquefied gas L inside the storage tank 11 is about 95% of the height inside the tank, so that the cooling device 21 is installed inside the tank. By arranging it at a position higher than 85% of the height, the high temperature liquefied gas layer Lb below the surface layer of the liquefied gas L stored in the storage tank 11 can be efficiently cooled.

In the second configuration, a vertical movement mechanism (not shown) is provided so that the cooling device 21 changes the height of the liquefied gas L stored in the storage tank 11 so as to follow the top and bottom of the liquid level S of the liquefied gas L. Is configured to have. As the vertical movement mechanism, a well-known slide mechanism, a suspension mechanism using a lifting rope (winding machine), or the like can be used.

With this configuration, even if the liquefied gas L evaporates and the liquid level S drops after the storage tank 11 is filled with the liquefied gas, the cooling device 21 can be lowered by using the vertical movement mechanism accordingly. Therefore, the high-temperature liquefied gas layer Lb below the surface layer of the liquefied gas L can be efficiently cooled while maintaining good cooling efficiency.

As a third configuration, the cooling device 21 is configured to float on the liquid surface S of the liquefied gas L. In other words, the cooling device 21 is configured in a float type. With this configuration, even if the liquefied gas L evaporates and the liquid level S drops after the storage tank 11 is filled with the liquefied gas, the cooling device 21 automatically lowers accordingly, which is good. The high-temperature liquefied gas layer Lb below the surface layer of the liquefied gas L can be efficiently cooled while maintaining the cooling efficiency.

Then, in the storage tank 11, it is preferable that the storage tank 11 is configured so that the horizontal cross-sectional area inside the tank decreases as the horizontal cross-sectional area inside the tank goes upward in the upper quarter. With such a configuration, the surface area of the liquid surface where the temperature becomes relatively high due to heat convection becomes small, and the cooling range covered by the cooling device 21 becomes small, so that the boil-off gas G is more efficient. Can be reduced. The storage tank 11 having this shape is generally a spherical tank, but there are other non-spherical tanks and the like.

The ship 1 according to the embodiment of the present invention includes any of the liquefied gas storage tank structures 10A to 10D according to the first to fourth embodiments described above. As a result, the ship 1 can similarly exert the effect that the provided liquefied gas storage tank structure 10 can exert. In addition to liquefied gas carriers such as LNG tankers and LPG tankers, this vessel includes liquefied gas fuel vessels that use liquefied gas as fuel.

The boil-off gas reduction method of the embodiment of the present invention is a boil-off gas reduction method in the storage tank 11 for storing the liquefied gas of the first to fourth embodiments described above. Then, in this boil-off gas reduction method, the liquefied gas L having a temperature lower than the temperature of the liquefied gas L in the high-temperature liquefied gas layer Lb below the surface layer of the stored liquefied gas L remains in the liquid phase, and the high-temperature liquefied gas layer Lb The liquefied gas L of the high temperature liquefied gas layer Lb is cooled by supplying the gas L to the high temperature liquefied gas layer Lb or by removing the heat of the high temperature liquefied gas layer Lb by the heat exchanger 21D arranged directly under the surface of the stored liquefied gas L. This is a method for reducing the amount of boil-off gas G generated inside the storage tank 11.

As described above, according to the liquefied gas storage tank structure, the ship, and the method for reducing boil-off gas according to the embodiment of the present invention, the surface layer of the liquefied gas L of the storage tank 11 for storing the liquefied gas L of the liquefied gas carrier. By cooling the lower high-temperature liquefied gas layer Lb to lower its temperature, heat transfer that causes heat of vaporization is suppressed, and the amount of boil-off gas G generated without increasing the thickness of the heat insulating material of the storage tank 11. Can be reduced.

More specifically, since the temperature of the low temperature liquefied gas surface layer La on the uppermost layer of the liquid surface S of the liquefied gas L is low, the cooling efficiency is poor even if the low temperature liquefied gas surface layer La is cooled. The high temperature liquefied gas layer Lb below the liquefied gas surface layer La is cooled.

That is, the conventional method of injecting the liquefied gas L or gas onto the low temperature liquefied gas surface layer La from above the low temperature liquefied gas surface layer La of the liquefied gas L in the storage tank 11 to destroy the low temperature liquefied gas surface layer La. However, in the present invention, the high-temperature liquefied gas layer Lb that is naturally formed by natural convection under the surface layer of the liquefied gas L is cooled, and the liquefied gas L in the storage tank 11 is efficiently temperature-equalized with less energy. The amount of boil-off gas G generated is reduced.

Therefore, in the present invention, the storage tank uses natural convection instead of the conventional method of stirring the liquefied gas L in the storage tank 11 or destroying the low-temperature liquefied gas layer La on the surface layer to change the macro temperature distribution. By concentrating the heat in the high temperature liquefied gas layer Lb on the lower side of the liquid surface S and efficiently cooling the high temperature liquefied gas layer Lb, the heat in the entire storage tank 11 is efficiently removed, or The temperature inside the storage tank 11 can be equalized by dispersing heat.

Since the amount of evaporation of the liquefied gas L on the liquid surface S, that is, the amount of the boil-off gas G generated depends on the liquid temperature of the high-temperature liquefied gas layer Lb, the amount of the boil-off gas generated by cooling the high-temperature liquefied gas layer Lb portion. Can be reduced. In particular, in a spherical tank, the area of the liquid surface S is smaller than the surface area of the tank, and heat tends to concentrate on the high-temperature liquefied gas layer Lb, so that efficient cooling is possible. Further, since the area of the liquid level S is small, the equipment for supplying the liquefied gas L can be small.

1 Ship 2 Tank support members 10, 10A, 10B, 10C, 10D Liquefied gas storage tank structure 11 Storage tank 12 Heat shield 13 Tank cover 20, 20A, 20B, 20C, 20D Cooling system 21, 21A, 21B, 21C Cooling device 21D heat exchanger (cooling device)
21Aa, 21Ba, 21Ca nozzle (diffusing device)
21Ab, 21Bb, 21Cb Distribution piping 22 Pumps 23, 23A, 23B, 23C Supply piping 30 Reliquefaction system 31 Reliquefaction device 32 Degassing piping 33 Return piping 34 Cooling piping 40 External cooling system 50 Cooling system 51 Cooling device 52, 53 Piping G Boil-off gas L Liquefied gas La Low temperature liquefied gas Surface layer Lb High temperature liquefied gas layer S Liquid level of liquefied gas

Claims (7)

In a liquefied gas storage tank structure provided with a storage tank for storing liquefied gas,
A cooling device for cooling the high-temperature liquefied gas layer while providing a part or all of a cooling system for cooling the high-temperature liquefied gas layer below the surface layer of the stored liquefied gas inside the storage tank. At least a part of the storage tank is configured to be arranged at or near the liquid level of the liquefied gas when the storage tank is filled with the liquefied gas.
A liquefied gas storage tank structure characterized in that the cooling device floats on the liquid surface of the liquefied gas. When the cooling system is filled with a pump arranged in a lower portion in the storage tank and the storage tank with liquefied gas, it is located near the liquid level of the liquefied gas and is supplied from the pump. A claim characterized by having the cooling device provided with a diffusion device for diffusing the liquefied gas into the high temperature liquefied gas layer as a liquid phase, and a supply pipe connecting the pump and the cooling device. Item 2. The liquefied gas storage tank structure according to Item 1. The second aspect of claim 2, wherein the pump is also used as a cargo handling pump, a spray pump for cooling the tank of the storage tank, or a pump that supplies liquefied gas as fuel to the main engine. Liquefied gas storage tank structure. The cooling system connects the supply pipe to an external cooling system outside the storage tank, and the liquefied gas supplied from the pump is saturated at a saturation temperature corresponding to the pressure inside the storage tank by the external cooling system. The liquefied gas storage tank structure according to claim 2 or 3 , wherein the liquefied gas storage tank structure is configured to be cooled to a lower temperature and then supplied to the diffuser. The liquefied gas storage tank structure according to any one of claims 2 to 4 , wherein the diffusion device is also used as a spray nozzle for cooling the tank. The invention according to any one of claims 1 to 5 , wherein the storage tank is configured such that the horizontal cross-sectional area inside the tank decreases as the horizontal cross-sectional area inside the tank increases in the upper quarter. Liquefied gas storage tank structure. A ship comprising the liquefied gas storage tank structure according to claims 1 to 6.

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