CN115289394A

CN115289394A - Floating body device

Info

Publication number: CN115289394A
Application number: CN202211119558.2A
Authority: CN
Inventors: 中山彻
Original assignee: Nikkei Global Co ltd
Current assignee: Nikkei Global Co ltd
Priority date: 2018-11-01
Filing date: 2018-11-01
Publication date: 2022-11-04
Anticipated expiration: 2038-11-01
Also published as: KR102645626B1; KR20210082429A; WO2020090085A1; CN112585395B; CN112585395A; CN115289394B

Abstract

The invention provides a technology for dealing with surplus boil-off gas generated when outputting liquefied natural gas to a carrier. When liquefied natural gas is delivered from a floating body facility (1) disposed on water and provided with a liquefaction device (2) for liquefying the natural gas to a transport ship (3), the reception amount of the natural gas to the liquefaction device (2) is reduced before the delivery of the liquefied natural gas is started in a reception amount reduction step. In the delivery step, liquefied natural gas is delivered from the storage tank (110) to the carrier (3). In the reliquefaction process, when the liquefied natural gas is delivered, the boil-off gas generated on the side of the carrier (3) is received by the liquefaction device (2) and reliquefied.

Description

Floating body device

The application is a divisional application of an application with the application number of 201880096798.0, which is proposed by 11/1 in 2018.

Technical Field

The present invention relates to a technique for delivering liquefied natural gas from a floating body facility placed on water to a carrier.

Background

As a facility for liquefying Natural Gas (NG) produced from a Gas field at the bottom of a water, there is known a Floating facility called FLNG (Floating LNG) in which a Floating body part such as a hull is disposed on an ocean near the Gas field and an NG liquefaction device is provided on the Floating body part.

Liquefied Natural Gas (LNG) discharged from the liquefaction apparatus is stored in a storage tank provided in the floating body section, and then is transported to a consumption site via a transport ship such as an LNG tanker.

Part of the LNG delivered from the floating body equipment to the carrier is gasified by heat input from the carrier side or the like to generate Boil-Off Gas (BOG). The BOG generated by the carrier is returned to the floating body facility side, and then used as fuel gas or re-liquefied by a liquefaction device into LNG. In addition, the surplus BOG exceeding the throughput of the liquefaction plant may be burned in a flare stack.

However, the float device may be installed at a position visible from a residential coastal region, and it is also required to reduce the possibility of flame growth of the flare stack as much as possible. In addition, from the viewpoint of the influence on the environment, it is also preferable to suppress the combustion of BOG in the flare stack.

Here, patent document 1 describes a receiving facility provided with a gas engine that drives a generator using BOG generated on the storage tank side as fuel when unloading LNG from a transport ship (LNG tanker) to a storage tank provided on the ground.

However, patent document 1 does not describe a technique for responding to an increase in the amount of BOG generated without providing any special device in the floating body device isolated from the surroundings.

Documents of the prior art

Patent literature

Patent document 1: international publication No. 2015/12890

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made under such a background, and provides a technique for coping with surplus BOG generated when outputting liquefied natural gas to a carrier.

Means for solving the problems

The method for delivering liquefied natural gas according to the present invention is a method for delivering liquefied natural gas from a floating body facility disposed on water to a transport ship,

the floating body device is provided with:

a liquefaction device that liquefies natural gas received from a water side;

a storage tank that stores liquefied natural gas liquefied by the liquefaction apparatus,

comprises the following steps:

a receiving amount reducing step of reducing a receiving amount of the natural gas to the liquefaction apparatus before starting the output of the liquefied natural gas;

an export step of exporting the liquefied natural gas from the storage tank to a transport ship;

and a reliquefaction step of receiving the boil-off gas generated by the transportation ship and reliquefying the boil-off gas at the liquefaction device when the liquefied natural gas is delivered.

The method for outputting liquefied natural gas may have the following features.

(a) In the receiving amount reducing step, the receiving of the natural gas is reduced within a range corresponding to ± 10% of the amount of the liquefied natural gas obtained by reliquefying the boil-off gas.

(b) In the output step, the output flow rate of the liquefied natural gas is adjusted to 3000 to 7500m ³ A value in the range of/h.

(c) The export process includes a BOG storage process of returning to the storage tank an amount of boil-off gas larger than a volume of a gas phase space in the storage tank, which increases with the export of the liquefied gas, out of the boil-off gas received from the carrier. The pressure in the storage tank is increased during the period in which the BOG storage step is performed, as compared with other periods.

(d) The method includes a tank pressure reduction step of reducing the pressure in the tank in order to reduce the vapor pressure and temperature of the liquefied natural gas output from the tank before starting the output of the liquefied natural gas.

Effects of the invention

The present invention reduces the amount of natural gas received from the underwater side to the liquefaction device before the start of the delivery of liquefied natural gas to the carrier, and therefore, the processing capacity of the liquefaction device increases, and the amount of surplus BOG that cannot be processed by the liquefaction device can be suppressed.

Drawings

Fig. 1 is a side view of the FLNG of the present embodiment.

FIG. 2 is a top view of the FLNG.

Fig. 3 is a block diagram of a liquefaction device and the like provided in the FLNG.

Fig. 4 is an explanatory diagram showing a flow of processing performed by the FLNG in accordance with LNG sendout.

Detailed Description

First, a configuration example of an FLNG (floating liquefied natural gas production storage/discharge apparatus) 1 to which the LNG (liquefied natural gas) export method of this example is applied will be described with reference to fig. 1 to 3. Fig. 1 and 2 are schematic configuration diagrams of the FLNG1 viewed from the side surface side and the top surface side, respectively.

As shown in fig. 2, the FLNG1 of the present example is configured such that the liquefaction device 2, the common energy source unit 202, and the living unit 13 are disposed on the hull 11 having a planar shape longer in the ship length direction than in the ship width direction.

A turret support 123 is provided on the bow of the hull 11 so as to project in the lateral direction toward the front of the bow, and a turret (turret) 12 for bolting the hull 11 is provided on the turret support 123.

A plurality of tethers 122 for tethering the hull 11 are provided in a manner to extend from the turret 12 to the water bottom. Further, a riser 121 for underwater transportation of NG produced from a gas field at the bottom of the water is connected to the turret 12. NG received from the riser 121 is supplied to the liquefaction apparatus 2 provided on the hull 11.

When the bow on which the turret 12 is provided is one end side of the hull 11, the liquefaction plant 2, the common energy source unit 202, and the living unit 13 are arranged in this order from one end side to the other end side in the ship length direction on the hull 11.

Here, the liquefaction device 2 is arranged on the hull 11 in a state of being divided into a plurality of units by using a set of facilities included in each of the units 21 to 26 described later with reference to fig. 3. The components are division units each configured by incorporating a part or all of the device groups included in each of the units 21 to 26 in a common configuration. In each module, a plurality of equipment groups such as static equipment such as a column tank and a heat exchanger, dynamic equipment such as a pump, and connection piping for connecting between the static equipment and the dynamic equipment and between piping on the pipe frame portion 201 side are arranged. Here, the pipe frame portion 201 is a structural body that holds piping groups through which various fluids treated in the liquefaction apparatus 2 flow.

The hull 11 of the FLNG1 is provided with a turbine or a generator for power generation, a power source of the turbine, a boiler for generating steam serving as a heat source of each distillation column, a heating system for heating a heat medium such as warm water or hot oil, and the like. In the FLNG1 of the present example, the utility device groups are located in a centralized area. Hereinafter, the group of the utility devices arranged so as to be concentrated in this area will be referred to as a utility section 202.

The hull 11 is provided with a living unit 13 in which an operator living through the FLNG1 operation and the like resides. For example, the living portion 13 is formed of a multi-story reinforced concrete building or a steel-reinforced concrete building.

Further, a flare stack 14 for burning gas discharged from the FLNG1 is disposed on the starboard side bow of the hull 11 farthest from the living portion 13.

As shown in fig. 2, on the port side of the hull 11 opposite to the starboard side on which the flare stack 14 is provided, for example, in the center region in the ship length direction of the hull 11, an export facility 15 for exporting LNG from a tank 110 (not shown in fig. 1 and 2) formed in the hull 11 to the LNG tanker (carrier) 3 is provided. The side of the region where the export facility 15 is provided is a docking position where the LNG tanker 3 is docked with the FLNG1.

Next, an outline of the liquefaction apparatus 2 will be described with reference to fig. 3. NG received from the water side via the riser 121 separates liquid contained in NG by the gas-liquid separation portion 21. Next, acidic gas (carbon dioxide or hydrogen sulfide), moisture, and mercury are removed by the pretreatment unit 22.

The natural gas from which the impurities have been removed is separated by distillation in the distillation unit 23 into methane and heavy hydrocarbons, which are liquid hydrocarbon components having 2 or more carbon atoms. In the pretreatment unit 22, a heavy hydrocarbon having 2 or more carbon atoms separated from methane is sequentially subjected to distillation separation to obtain ethane, propane, butane, and the like. The heavy hydrocarbons thus separated may be a lighter fraction having not more than 4 carbon atoms, and the fraction may be reinjected to LNG.

The methane separated in the distillation unit 23 is cooled and liquefied in the liquefaction unit 24 to become Liquefied Natural Gas (LNG). The liquefaction unit 24 is provided with a Main ultra low temperature Heat Exchanger (MCHE: main Cryogenic Heat Exchanger) for liquefying methane using a Main refrigerant (a mixed refrigerant composed of methane, ethane, propane, butane, nitrogen, or the like, or a nitrogen refrigerant). The main ultra low temperature heat exchanger (MCHE) is a heat exchanger such as a spiral coil type or a cold box type.

The LNG liquefied in the liquefaction unit 24 has a higher pressure and a higher temperature than the conditions stored in the tank 110. Therefore, the LNG pressure is reduced to the pressure of the storage tank 110, and a part of the LNG is vaporized (end flash), and temperature adjustment and light-weight component adjustment are performed. The LNG whose temperature, pressure, and composition have been adjusted by the end flash is transferred to, for example, a storage tank 110 provided in the hull 11.

The tail end flash gas and the gas vaporized from the LNG in the tank 110 are pressurized as BOG by the pressure increasing unit 26 including a compressor and the like. A part of the BOG is used as a fuel gas, and the remainder is returned to the inlet side of the liquefaction section 24 and reliquefied. Further, the line that receives BOG from the tank 110 to the booster section 26 may also be used as a line that delivers the gas carried back from the LNG tanker 3 to the tank 110. In addition, instead of the example shown in fig. 3, the pressure of the end flash gas may be increased in the end flash section 25, and the end flash gas and the BOG increased in pressure by the pressure increasing section 26 may be joined to obtain a fuel gas.

In addition, excess BOG that exceeds the capacity of the liquefaction section 24 is sometimes burned in the flare stack 14.

In the FLNG1 having the above configuration, the LNG stored in the tank 110 is delivered to the LNG tanker 3.

At the time of LNG sendout, after the LNG tanker 3 is brought into contact with the ship-side contact position and the sendout facility 15 is connected to the LNG tanker 3, sendout of LNG to the LNG tanker 3 is started using the LNG pump 111 provided in the tank 110.

In a normal state where LNG is not delivered, BOG is generated in the tank 110 by using, as main heat sources, (i) natural heat input to the tank 110, (ii) natural heat input to a pipe for discharging LNG to the tank 110, and (iii) heat input that is operated by a pump (not shown) for transporting LNG in the FLNG1.

In addition, at the time of LNG sendout, (iv) heat input in accordance with the work of the transfer pump 111 of the tank 110, (v) natural heat input to the sendout piping of LNG and the loading arm constituting the sendout facility 15, and (vi) heat input to the wall surface of the tank on the LNG tanker 3 side in accordance with cooling until the liquid temperature of LNG does not become low temperature, and the like are main heat sources, a large amount of BOG is generated in the tank on the LNG tanker 3 side. On the other hand, in the tank on the LNG tanker 3 side, since LNG received from the FLNG1 flows in and the liquid level thereof rises, BOG in the tank is pushed outside.

The BOG generated in the tank on the LNG tanker 3 side and the BOG pushed out from the tank are merged, pressurized by a compressor, not shown, provided in the LNG tanker 3, and then sent to the FLNG1 side. In addition, a part of the BOG is sometimes used as fuel for the LNG tanker 3.

As shown in fig. 3, the BOG from the LNG tanker 3 side is carried back to the FLNG1 via the export equipment 15 already described, and is merged with the BOG generated on the FLNG1 side.

Therefore, at the time of LNG export, the BOG generated on the LNG tanker 3 side is transported back to the FLNG1, used as a fuel gas together with the BOG generated in the FLNG1, or reliquefied. On the other hand, since a large amount of BOG is returned from the LNG tanker 3 as compared with the BOG generated in the FLNG1, the amount of gas used and the amount of re-liquefaction in the liquefaction unit 24 are increased, and the total amount may not be handled.

The untreated BOG is burned in the flare stack 14. However, from the viewpoint of reducing the anxiety associated with the formation of a large flame in the flare stack 14 and suppressing the influence on the environment, it is sometimes necessary to suppress the BOG combustion in the flare stack 14 as much as possible.

As a method of reducing the chance of burning the remaining BOG in the flare stack 14, it is considered that there are a case where reliquefaction of BOG carried back from the LNG tanker 3 is predicted, and the liquefaction unit 24 having a margin in processing capacity is installed, and a case where equipment dedicated to reliquefaction of BOG is installed. However, these solutions are difficult to adopt in many cases because they require equipment investment.

Therefore, the FLNG1 of the present example reduces the handling of BOG carried back from the LNG tanker 3 in the flare stack 14 by performing the turning adjustment. The following describes the contents of processing performed as a BOG surplus countermeasure at the time of LNG sendout with reference to fig. 4.

First, in a normal operation state (process P1), FLNG1 receives NG corresponding to the design flow rate of liquefaction device 2, for example, from riser 121.

In a preparation stage before the LNG sendout starts, the opening degree of the throttle valve 124 provided on the seabed on the upstream side of the riser 121 is adjusted to reduce the NG reception amount. This operation reduces the load on the liquefaction unit 24 to reduce the amount of LNG produced (process P2: receiving amount reduction step).

The LNG export schedule is determined to a schedule, for example, several weeks ago. Therefore, the operation of reducing the NG reception amount is started on the day before the LNG sendout is performed to the half day before, and the load of the liquefaction unit 24 is reduced when the LNG sendout is started.

The processing margin of the liquefaction unit 24 due to the reduction in the receiving amount of NG can be used as the processing margin of the BOG transported back from the LNG tanker 3 at the time of LNG sendout. From this viewpoint, it can be exemplified that the amount of reduction of NG received from the riser 121 is adjusted in accordance with an amount within a range of ± 10% of the amount of LNG corresponding to the reliquefaction of BOG returned from the LNG tanker 3 at the time of LNG sendout.

In the storage period until the start of the delivery, the storage pressure of the LNG stored in the tank 110 may be reduced to reduce the vapor pressure and temperature of the LNG (process P3: tank pressure reduction step). For example, in a normal state, when the pressure in the storage tank 110 is set to be in the range of 0.06 to 0.10barg, the pressure is reduced to about 0.01barg which is lower than 0.06barg before the LNG sendout is started.

As a method for reducing the pressure of the sump 110, the amount of vaporization generated in the sump 110 and the terminal flash unit 25 may be increased within the range of the processing capacities of the pressure increasing unit 26 and the liquefaction unit 24. By reducing the pressure in the tank 110, the vapor pressure of the stored LNG can be reduced, and the liquid temperature of the LN can be reduced to a temperature corresponding to the vapor pressure.

On the other hand, when the LNG tanker 3 that transfers LNG arrives, the LNG tanker 3 is connected to the FLNG1, and the export facility 15 is connected to the LNG tanker 3. Then, in a state where only the above-described process P2 is completed (in a case where the process P3 is not performed), or in a state where the processes P2 and P3 are completed, LNG is unloaded to the LNG tanker 3 (process P4: unloading step).

Here, the LNG export to the standard LNG tanker 3 is often 10000 to 12000m ³ The output flow is set in the range of/h. In contrast, in the LNG sendout method of the present example, the sendout flow rate may be set to 3000 to 7500m ³ 5000m in the range of h ³ H is used as the reference value. Since there is a proportional relationship between the export flow rate of LNG and the flow rate of BOG transported back from the LNG tanker 3, by reducing the export flow rate, the BOG transported back to the FLNG1 can be reduced.

The export time that increases as the export flow rate of LNG decreases may be absorbed by adjusting the voyage schedule of the LNG tanker 3, for example.

In this case, as the process P3 is performed, the vapor pressure of the LNG discharged to the LNG tanker 3 is reduced, and thereby BOG is less likely to be generated on the LNG tanker 3 side than LNG that has not been subjected to the process P3. In particular, the pressure of LNG in the tank on the LNG tanker 3 side is usually 0.06 to 0.10barg, so that the pressure on the FLNG1 side is reduced to less than 0.06barg in the process P3. In this case, the BOG generation amount on the LNG tanker 3 side can be reduced by the amount of the pressure difference between the tank 110 on the FLNG1 side and the tank of the LNG tanker 3.

Further, by lowering the temperature of LNG in the process P3, BOG is less likely to be generated on the LNG tanker 3 side than LNG that has not been subjected to the process P3.

The BOG returned from the LNG tanker 3 can almost entirely be processed in the liquefaction unit 24 with a reduced load by reducing the amount of NG received in the process P1 (process P5-1: reliquefaction process).

At this time, by reducing the load of the liquefaction section 24 in advance, the amount of the end flash gas generated in the end flash section 25 is also reduced. As a result, the total amount of the terminal flash gas and the BOG is reduced, and therefore, the opportunity to perform excessive BOG combustion disposal in the flare stack 14 is further suppressed.

In the case where the returned BOG further remains, the BOG may be conveyed to the storage tank 110. As shown in fig. 3, a part of the BOG transported back from the LNG tanker 3 joins the LNG transferred from the terminal flash 25 toward the storage tank 110. In the storage tank 110, since the gas phase space increases with the LNG sendout, the BOG received in the storage tank 110 is stored in the gas phase space. From this viewpoint, it can be said that the sump 110 is used as an accumulator tank for accumulating the excess BOG.

As described above, the normal set pressure of the FLNG 1-side tank 110 is 0.06 to 0.10barg, and the set pressure is increased while the BOG is being returned from the LNG tanker 3 to the FLNG1.

For example, in the conventional design standard, the design pressure of the sump 110 is set to about 0.25 barg. In this case, the set pressure of sump 110 may be set to a value within a range higher than the set pressure in the normal state and lower than 0.25 barg.

The sump 110 may be designed on the premise of being used as an accumulator tank. In the new design basis in this case, the design pressure of sump 110 may be raised to, for example, 0.69barg. In the sump 110 designed based on the new design criterion, the set pressure of the sump 110 may be set to a value within a range higher than the set pressure in the normal state and lower than 0.69barg.

As described above, by setting the set pressure of the tank 110 higher than normal, BOG of an amount larger than the volume of the gas phase space that increases with the LNG sendout can be received in the tank 110. As a result, the BOG can be stored in the storage tank 110 by the pressure difference compared with the normal operation, and the chance of the excess BOG being disposed of by burning in the flare stack 14 can be reduced (process P5-2.

When the LNG export is completed, the receiving amount of NG is restored to regulate the production amount of LNG. The BOG accumulated in the tank 110 is used as fuel gas, or is sent to the liquefier 24 to be liquefied again, and the pressure in the tank 110 is returned to the normal set pressure (process P6).

By these processes, after the FLNG1 returns to the normal operation, the next LNG discharge timing is waited (process P1).

The LNG sendout method according to the present embodiment has the following effects. Since the receiving amount of NG from the underwater side to the liquefaction device 2 is reduced before the LNG starts to be delivered to the FLNG1, the processing capacity of the liquefaction device 2 (liquefaction unit 24) increases, and the amount of surplus BOG that cannot be processed in the liquefaction device 2 can be suppressed.

Here, compared to FLNG1 to which the LNG sendout method of the present example is applied, for example, a liquefaction device for NG installed on the ground generally has a larger processing amount. Therefore, the excess processing capacity of the liquefaction unit 24 is also large, and the BOG returned when the LNG is unloaded from the storage tank provided on the ground to the LNG tanker 3 can be reliquefied in the liquefaction unit 24 in many cases without reducing the amount of NG received.

From this point of view, the LNG sendout method of the present example can be said to be a technique suitable for solving a problem (surplus of BOG at the time of sendout of LNG) unique to the FLNG1 which is a floating body device.

Further, the configuration example of FLNG1 to which the LNG sendout method of this example is applied may be changed as appropriate.

Instead of the FLNG1 of the external type (external turret) in which the turret 12 is provided outside the body of the hull 11 illustrated in fig. 1 and 2, the FLNG1 of the internal type (internal turret) in which the turret 12 is provided inside the body of the hull 11 may be used. Instead of the end flash section 25 having a structure separate from the tank 110 shown in fig. 3, the end flash section 25 may be integrated with the tank 110. The layout of the liquefaction system 2, the utility section 202, the living section 13, and the like disposed on the hull 11 may be changed as appropriate.

The processing P3 (tank pressure lowering process) and P5-2 (BOG Storage process) described with reference to fig. 4 can be applied to export of LNG from the LNG tanker 3 to FSU (Floating Storage Unit) or FSRU (Floating Storage and retrieval Unit).

That is, in the process P3, LNG is transferred while setting the pressure in the tank on the LNG tanker 3 side to a pressure lower than 0.06 to 0.10barg, and the LNG is transferred to the FSU/FSRU, and the export facility on the LNG tanker 3 side is connected, and then the LNG is exported. In this example, the BOG generation in the FSU/FSRU can be suppressed by outputting LNG having a low vapor pressure and a low temperature.

In the case where the process P5-2 is carried out, the set pressure on the FSU/FSRU side may be set to a value within a range of from 0.25barg to higher than the set pressure in the normal state. In addition, the pressure of the sump designed based on the above-described new design criteria may be set to a pressure higher than the set pressure in the normal state and lower than 0.69barg. In these cases, the BOG is accumulated in the tank at a pressure difference higher than that during normal operation, and thus the chance of disposal of excess BOG by combustion in the flare stack can be reduced.

Description of the symbols

1 FLNG

110. Storage tank

121. Riser (iser)

124. Throttle valve

13. Living department

14. Torch chimney (flare stack)

15. Output device

2. Liquefaction device

24. Liquefaction section

3 LNG tanker.

Claims

1. A floating body facility which is disposed on water and outputs liquefied natural gas to a carrier, the floating body facility being characterized in that:

the floating body equipment receives natural gas through a throttle valve which is provided for receiving the natural gas from the water side and can adjust the receiving amount of the natural gas, and the floating body equipment comprises:

a liquefaction device having a liquefaction unit configured to liquefy the natural gas received via the throttle valve;

a storage tank for storing the liquefied natural gas liquefied by the liquefaction device; and

an output device for outputting the liquefied natural gas stored in the storage tank to the transport ship,

the liquefaction device is also provided with a pressure rising part,

the pressure raising unit is connected to the carrier through a first pipeline and connected to the liquefaction unit through a second pipeline, and the pressure raising unit can receive the boil-off gas generated on the carrier side through the first pipeline, raise the pressure of the boil-off gas, and send the boil-off gas to the liquefaction unit through the second pipeline to reliquefy the boil-off gas.

2. The float device of claim 1, wherein:

the following steps are carried out in the floating body device: a receiving amount reducing step of reducing the receiving amount of the natural gas to the liquefaction device by the throttle valve before starting the output of the liquefied natural gas; an export process of exporting the liquefied natural gas from the storage tank to the carrier through the export facility; and a reliquefaction step of receiving the boil-off gas generated on the transporting ship side in a liquefaction unit of the liquefaction apparatus via the pressure increasing unit and reliquefying the boil-off gas when the liquefied natural gas is outputted.

3. The float device of claim 1, wherein:

and a third line connecting the pressure increasing part and the storage tank,

the boil-off gas evaporated from the liquefied natural gas in the tank is received by the pressure increasing unit via the third pipeline, is pressurized, and is then sent to the liquefaction unit via the second pipeline to be reliquefied.

4. The float device of claim 1, wherein:

in the received amount reducing step, the reception of the natural gas is reduced within a range corresponding to ± 10% of the amount of the liquefied natural gas obtained by reliquefying the boil-off gas.

5. The float device of claim 1, wherein:

in the output step, the output flow rate of the liquefied natural gas is adjusted to 3000 to 7500m ³ A value in the range of/h.

6. The float device of claim 1, wherein:

the first line is configured to be able to receive the boil-off gas in the sump,

in the floating body facility, a BOG storage step is performed so that, when the discharge step is performed, a larger amount of boil-off gas than the volume of the gas phase space in the tank, which increases with the discharge of the liquefied gas, out of the boil-off gas received from the carrier side is returned to the tank.

7. The float device of claim 6, wherein:

further comprising a terminal flash section for regulating pressure in the sump,

during the period in which the BOG storage step is performed, the pressure in the storage tank is increased by the end flash unit as compared with other periods.

8. The float device of claim 1, wherein:

in the floating body facility, a tank pressure reducing step is performed in which the pressure in the tank is reduced by the end flash unit before the liquefied natural gas is discharged, in order to reduce the vapor pressure and temperature of the liquefied natural gas discharged from the tank.

9. The float device of claim 7 or 8, wherein:

further comprising a fourth line connecting the end flash section and the pressure increasing section,

the end flash gas vaporized in the end flash section is received by the pressure increasing section via the fourth line to be pressure-increased, and then is sent to the liquefaction section via the second line to be reliquefied.

10. The float device of claim 9, wherein: the terminal flash section is integral with the sump.