FR3101407A1 - Refrigerant fluid intended for a refrigerant circuit of a natural gas treatment system - Google Patents

Abstract

Title: Refrigerant fluid intended for a refrigerant fluid circuit of a natural gas processing system Refrigerant fluid intended to circulate in a refrigerating fluid circuit (4) of a natural gas treatment system (1), the refrigerating fluid being configured to exchange heat with a liquefied natural gas stored in at least one tank (3) d 'a floating structure (15), the coolant comprising 25 to 35 mol% of nitrogen or 35 to 50 mol% of argon or 40 to 50 mol% of a mixture of dinitrogen and argon and, 35 to 55% mol of methane, said coolant having a proportion of methane and dinitrogen and/or argon of between 70 and 85 mol% of the coolant, the remainder being a mixture of hydrocarbons composed of at least ethane, propane and / or butane and/or ethylene and/or propylene. Figure of the abstract: Figure 1

Description

Refrigerant fluid intended for a refrigerant fluid circuit of a natural gas treatment system

The present invention relates to the field of floating structures, at least one engine of which is powered by natural gas and which also make it possible to contain or transport liquefied natural gas. It relates more particularly to a coolant intended for a natural gas treatment system used as fuel for the engine(s) of the floating structure.

In order to more easily transport and/or store gas, such as natural gas, over long distances, the gas is generally liquefied by cooling it to cryogenic temperatures, for example -163°C at atmospheric pressure, in order to obtain liquefied natural gas, commonly known by the acronym "LNG", or "LNG" for "Liquefied Natural Gas". This liquefied natural gas is then loaded into specialized storage tanks in the floating structure.

However, such tanks are never perfectly thermally insulated so that natural evaporation of the gas is inevitable, this phenomenon being called BOG, an acronym for Boil-Off Gas. The storage tanks of the floating structure thus include both natural gas in liquid form and natural gas in gaseous form, the natural gas in the gaseous state forming the top of the tank.

In a known manner, at least part of the natural gas present in the tank in gaseous form can be used to power an engine provided to meet the operating energy needs of the floating structure, in particular for its propulsion and/or its production of electricity for on-board equipment. To this end, it is in particular known to circulate the natural gas in the gaseous state through at least one natural gas processing system, so as to allow it to be heated, said system comprising a heat exchanger used as a superheater and a compressor, both placed upstream of the engine.

It is also known to configure the natural gas treatment system so that it can allow the condensation of the withdrawn part of the natural gas in the gaseous state. Condensation of the natural gas may in particular be required when the quantity of natural gas evaporated in the tank is too large in relation to the operating energy requirements of the floating structure, the natural gas treatment system then makes it possible to condense the evaporated natural gas present in the tank in order to return it there in the liquid state. Such a liquefaction system can in particular be implemented when the floating structure is stopped and the consumption of gaseous natural gas by its engine(s) is zero, or almost zero.

Such natural gas processing systems involve the heating and/or the condensation of the natural gas by heat exchange with a refrigerant fluid circulating through at least one dedicated refrigerant circuit.

This coolant, due to its use in floating structures intended for the transport or storage of natural gas, is subject to numerous constraints. In particular, it is essential that the refrigerant has a state change temperature of between -200 and 15°C at atmospheric pressure. Also, it is essential that the refrigerant fluid is not corrosive or toxic. The refrigerant must also be free of oxidizing compounds and compounds prohibited by environmental regulations, such as chlorofluorocarbons, or "CFCs", perfluorocarbons, or "PFCs", and hydrofluorocarbons, or "HCFCs".

The present invention falls within this context and aims to propose a new refrigerant fluid optimized to operate at the cryogenic temperatures of change of state of natural gas, in particular of a natural gas essentially comprising methane, being particularly configured to allow a natural gas subcooling and also heating thereof. In other words, the refrigerant must be able to change from a two-phase state to a gaseous state so as to cool the LNG in the liquid state below its liquefaction temperature at atmospheric pressure, i.e. -163°C. . This same coolant is also configured to change from a gaseous state to a liquid state, thus causing the BOG to heat up so that the latter is heated to supply a consumer on the floating structure.

Another object of the present invention is to provide a low-cost coolant whose compounds are available in large quantities on the floating structure, while optimizing the thermal efficiency of said coolant within the natural gas processing system. .

The present invention relates to a refrigerant fluid intended to circulate in a refrigerant fluid circuit and configured to exchange heat with a liquefied natural gas stored in at least one tank of a floating structure, the refrigerant fluid comprising:

• 25 to 35 mol % of dinitrogen or 35 to 50 mol % of argon or 35 to 50 mol % of a mixture of dinitrogen and argon and,
• 35-55 mol% methane;

said refrigerant having a proportion of methane and nitrogen and/or argon of between 70 and 85 mol% of the refrigerant, the remainder comprising a mixture of hydrocarbons composed of at least ethane and/or propane and/or butane and/or ethylene and/or propylene.

Advantageously, all of the remainder mentioned above comprises at least ethane and/or ethylene and/or propane and/or propylene and/or butane.

It is understood that throughout this document, the total composition of the refrigerant includes 100 mol%.

The refrigerant fluid according to the present invention is adapted to the various constraints set out above, in particular the constraints relating to the heating of the gas to deliver it to the consumer and that relating to the temperature of the gas sampled in the liquid state in the tank.

In particular, the various compounds that can be used in this refrigerant have, at atmospheric pressure, the following boiling temperatures:

• Argon (Ar), dinitrogen (N ₂ ): [-200°C to -180°C];
• Methane (C1): [-180°C to -125°C];
• Ethane (C2), Ethylene (C2H4): [-125°C to -75°C]
• Propane (C3), Propylene (C3H6), butane (C4): [-75°C to 15°C].

Advantageously, methane, ethane and/or ethylene, propane and/or propylene and butane are available in large quantities within the floating structure and are in particular stored in the form of liquefied natural gas in the tank. .

According to one characteristic of the invention, the mixture of hydrocarbons can comprise 15 to 30% ethane and/or ethylene and propane and/or propylene or ethane and/or ethylene and butane. According to one example, this remainder of 15% to 30% may thus comprise ethane and propane or ethylene and propane. According to another example, this remainder of 15% to 30% can comprise ethane and butane or ethylene and butane.

According to one characteristic of the invention, the refrigerant can comprise 5 to 19 mol% of ethane and/or ethylene, the remainder being supplemented by propane and/or propylene and/or butane.

According to the invention, the refrigerant can comprise up to 15% mol of propane and/or propylene or butane.

The refrigerant fluid according to the present invention can thus be defined according to three distinct embodiments according to the inert gas or the mixture of inert gases that it comprises. In particular, the refrigerant is produced according to a first embodiment when it comprises dinitrogen as the only inert gas, according to a second embodiment when it comprises argon as the sole inert gas, and according to a third embodiment when it comprises a mixture of dinitrogen and argon.

According to the first embodiment of the invention, the refrigerant fluid may comprise at least dinitrogen, methane, ethane and/or ethylene and propane and/or propylene, the refrigerant fluid having a ratio of methane and nitrogen between 1:1 and 9:5. In other words, the coolant may comprise at least as much methane as nitrogen.

Alternatively, the coolant may comprise at least dinitrogen, methane, ethane and/or ethylene and butane, the coolant having a ratio of methane and dinitrogen of between 4:5 and 7:5 .

According to a characteristic of the first embodiment, the refrigerant fluid may have a molar mass of 24 g/mol±2 g/mol.

According to the second embodiment, the refrigerant fluid may comprise argon, methane, ethane and/or ethylene, butane and/or propane and/or propylene, the refrigerant fluid having a ratio of methane and argon between 3:5 and 6:5. Preferably, the coolant according to this embodiment can comprise as much methane as argon.

According to a characteristic of the second embodiment, the refrigerant fluid may have a molar mass of 31 g/mol±3 g/mol.

According to the third embodiment, the refrigerant fluid may comprise at least dinitrogen and argon, methane, ethane and/or ethylene, as well as butane and/or propane and/or propylene, the refrigerant having a ratio of methane and argon and dinitrogen of between 1:2 and 13:10.

Additionally, the refrigerant fluid according to the third embodiment may have a molar mass of 30 g/mol±3 g/mol.

The present invention also relates to a coolant circuit configured to heat exchange with the liquefied natural gas stored in at least the tank of the floating structure, the coolant circuit containing, in a closed circuit, the coolant as described above.

The refrigerant circuit according to the invention is thus used to allow heat exchanges at cryogenic temperatures between the refrigerant and at least the liquefied natural gas, the latter possibly being in the gaseous state and/or in the liquid state. By "cryogenic" is meant a temperature below -40°C, or even below -90°C, and preferably below -160°C.

The refrigerant as described above is thus particularly suitable for optimizing the efficiency of such heat exchanges, at cryogenic temperatures, between the refrigerant circuit and the natural gas.

According to the present invention, the refrigerant circuit comprises at least:

• a compressor configured to compress the refrigerant fluid,
• a first heat exchanger, used as a condenser, traversed by the refrigerant fluid and configured to be traversed by the natural gas,
• a means for expanding the refrigerant fluid,
• a second heat exchanger, used as a vaporizer, traversed by the refrigerant fluid and configured to be traversed by natural gas.

The refrigerant circuit comprises at least a first portion, which extends between the compressor and an inlet of the expansion means, at the level of which the refrigerant fluid circulates at high pressure, and a second portion, between said expansion means and an inlet of the compressor, at the level of which the refrigerant fluid circulates at low pressure. By way of example, the refrigerant fluid may have a pressure of between 18 and 36 bars in the first portion and a pressure of between 1.2 and 2.5 bars in the second portion. The first portion can then consist of a cold reception zone, while the second portion can, for example, be intended for cold transmission.

Thus, when the refrigerant circulates through the various components of the refrigerant circuit, it undergoes, by exchange of calories, a succession of changes of state and temperatures.

Specifically, the first portion of the refrigerant circuit includes at least a first pass of the first heat exchanger and the second portion includes at least a first pass of the second heat exchanger.

When the compressor compresses the refrigerant fluid, its temperature increases and the refrigerant fluid can transmit its calories from the first pass of the first heat exchanger to an adjacent pass, for example a second pass of the first heat exchanger in which the natural gas circulates. liquefied in the gaseous state coming from the tank.

The refrigerant fluid is then expanded in the expansion means, which lowers its pressure. By way of example, the expansion means can be a Joule-Thomson valve. This coolant then circulates in the second portion of the coolant circuit, and more particularly in the second pass of the second heat exchanger where it can absorb calories from another pass, for example from a second pass of the second heat exchanger of heat in which circulates the liquefied natural gas in the liquid state coming from the tank, then the refrigerant fluid is returned to the compressor.

The invention also relates to a system for processing natural gas used as fuel for an engine of the floating structure, the processing system comprising the refrigerant fluid circuit as described above and the processing system being configured to cooperate with at least one motor and at least the tank of the floating structure. The floating structure can be an LNG carrier but it can also be a ship whose tank is a tank to contain the LNG and supply the ship's engine(s).

As previously explained, the motor can, by way of example, be a propulsion motor for the floating structure and/or a motor for at least one piece of on-board equipment.

By "processing" is meant the heating and/or cooling, respectively intended to cause the evaporation or condensation of the natural gas. The natural gas treatment system thus comprises at least the refrigerant fluid circuit, forming a closed circuit of the refrigerant fluid, and at least a plurality of lines and/or installation(s) ensuring the circulation or the treatment of the natural gas , in particular between the tank and the engine, and the heat exchanges between said natural gas and the refrigerant fluid circulating in the refrigerant circuit.

In order to ensure the supply of at least the engine with natural gas, the processing system according to the invention comprises at least one line for drawing off the natural gas in the gaseous state present in the tank, the first heat exchanger being the seat of a heat exchange between this natural gas in the gaseous state and the refrigerant. The sampling line opens onto the gaseous headspace of the tank. In particular, the natural gas in the gaseous state circulates in the second pass of the first heat exchanger, this natural gas then having a temperature greater than -160°C, more particularly between -120°C and 45°C.

The sampling line takes the evaporated natural gas from the top of the vessel, i.e. near an upper wall of the vessel. Such sampling of natural gas in the gaseous state can be carried out continuously or selectively, the sampling line then comprising at least one valve for controlling the sampling of the natural gas in the gaseous state.

The natural gas in the gaseous state withdrawn can then be sent to the second pass of the first heat exchanger, so as to exchange heat with the refrigerant fluid circulating in the first pass of the first heat exchanger, then to at least the engine of the floating structure. The pressure and temperature of the natural gas in the gaseous state is thus made compatible with the needs of the engine(s) of the floating structure. At least the sampling line and the second pass of the first heat exchanger are thus included in a circuit for supplying at least the engine with natural gas as fuel, said supply circuit being included in the processing system .

According to a characteristic of the invention, the compressor of the refrigerant circuit can be configured to compress the natural gas stored in at least the tank.

In particular, the compressor installed in the refrigerant circuit is configured to compress the natural gas in the gaseous state used as fuel for at least the engine of the floating structure, for example the propulsion engine of the floating structure.

In other words, depending on the needs of the floating structure, the compressor of the refrigerant circuit, hereinafter called the first compressor, can be used to compress the liquefied natural gas in the gaseous state or the refrigerant, depending on whether It is used to circulate and compress the coolant or to supply the engine with liquefied natural gas. Such an arrangement is notably implemented for security purposes. Thus, in the event of failure of a second compressor of the treatment system, solely intended to compress the natural gas sent to the engine of the floating structure and separate from the first compressor of the refrigerant circuit, the first compressor can be used in order to ensure the compression of natural gas instead of the second compressor. The first compressor therefore ensures the redundancy of the second compressor in the event of failure of the latter.

To this end, the first compressor and/or the second compressor can be preceded and/or succeeded by at least one valve. Such valves allow on the one hand the selective supply of the first compressor with natural gas or with refrigerant, and on the other hand the isolation of the said first compressor with respect to the engine, when the first compressor is operating for the refrigerant circuit. , or with respect to the refrigerant circuit, when the first compressor operates to supply at least the engine with natural gas as fuel.

Due to this safety system, the first compressor is subject to particular constraints. In particular, the first compressor must be capable of compressing the natural gas from a pressure of the order of atmospheric pressure to a pressure of approximately 13 bars. By way of example, in order to be able to compress the natural gas or the refrigerant fluid, the compressor preferably has a compression ratio of at least 13±20% and a flow rate of 5000 m ³ /h±10%. In such a context, the first compressor is particularly suitable for ensuring the compression of the natural gas and can, by way of example, be similar to the second compressor. Advantageously, the refrigerant fluid according to the invention is particularly optimized in order to reduce the power necessary for the first compressor to compress said refrigerant fluid.

The processing system may include a subcooling plant and/or a natural gas condensing plant.

By "subcooling installation" is meant an installation configured to cool natural gas in the liquid state to a temperature below -160°C, such a phenomenon being in particular implemented by heat exchange with the refrigerant fluid circulating at the within the refrigerant circuit, whose composition is suitable for this purpose.

According to the invention, the treatment system can comprise at least one line for withdrawing the natural gas in the liquid state present in the tank, the second heat exchanger being the seat of a heat exchange between this natural gas at the liquid state and the refrigerant.

The sampling line can be configured to supply natural gas to the processing system subcooling plant. By way of example, the natural gas in the liquid state withdrawn from the tank is then sent to the second heat exchanger, more particularly to the second pass of the second exchanger, so as to exchange heat with the refrigerant fluid circulating in the second portion of the circuit, in particular in the first pass of the first heat exchanger.

The sampling line extends at least partially into the tank but in such a way that it is in contact with the liquid part of the natural gas stored in the tank. In particular, the sampling line can comprise at least one pump. Specifically, said pump is at least partially submerged in liquefied natural gas.

By "condensation installation" of the treatment system, we mean an installation configured to ensure, by heat exchange, the transition to the liquid state of natural gas initially in the gaseous state, for example the BOG resulting from natural evaporation natural gas in the tank. In particular, the condensation installation can be configured to ensure the condensation of natural gas in the gaseous state previously withdrawn to be used as fuel for at least the engine of the floating structure. In other words, it is natural gas in the gaseous state circulating in the supply circuit of at least the engine of the treatment system.

According to the invention, the treatment system can comprise a natural gas return line through which a flow of excess natural gas passes, the treatment system comprising a third heat exchanger which is the seat of a heat exchange between this natural gas surplus and the natural gas in the liquid state coming from the tank .

In particular, according to the invention, the natural gas in the liquid state coming from the tank comes from the second heat exchanger. In other words, the liquid natural gas coming from the tank circulates successively in the second heat exchanger then in the third heat exchanger in which it exchanges calories with the excess natural gas.

The return line can also be part of the condensation installation in that it participates in the condensation function by bringing the condensed natural gas back to the tank.

"Surplus natural gas" means a portion of the gaseous natural gas withdrawn by the sampling line and compressed, with a view to supplying at least the engine with natural gas in the gaseous state as fuel, but not used by said engine. For example, excess natural gas has a pressure of less than or equal to 13 bars.

For example, the return line can take excess natural gas between the second compressor, or the first compressor, and the engine of the floating structure.

In particular, the treatment system's return line can bring the excess natural gas to the third heat exchanger so that it transmits its calories, that is to say it captures the cold, liquid natural gas also circulating.

In particular, this liquid natural gas may have been subcooled beforehand by heat exchange with the refrigerant fluid, so that the natural gas is sent to a first pass of the third heat exchanger, while the excess natural gas is brought up to a second pass from the third heat exchanger through the return line.

Thus, the liquefied natural gas treatment system stored in at least the tank of the floating structure can combine the refrigerant circuit with the sub-cooling installation and/or the natural gas condensation installation.

The invention also relates to a floating structure comprising at least one tank intended for the transport or storage of liquefied natural gas, the floating structure comprising at least one engine of the floating structure and at least one treatment system as explained above. , the engine being configured to be powered by natural gas in the gaseous state flowing at least in part in the treatment system.

The present invention also relates to a system for loading or unloading a liquefied natural gas which combines at least one means on land and at least the floating structure for transporting liquefied natural gas comprising at least the tank.

The invention finally relates to a method for loading or unloading a liquefied natural gas from the tank of the floating structure for transporting liquefied natural gas as described above.

Other characteristics, details and advantages of the invention will emerge more clearly on reading the description which follows on the one hand, and several examples of embodiment given by way of indication and not limitation with reference to the appended schematic drawings on the other. part, on which:

schematically represents a system for processing liquefied natural gas stored in a tank of a floating structure for transporting or storing said natural gas;

represents an alternative of the natural gas treatment system illustrated in FIG. 1;

represents the natural gas processing system during a first mode of operation;

represents the natural gas processing system implementing an emergency mode of operation, alternative to the first mode of operation illustrated in FIG. 3;

represents the natural gas processing system during a second mode of operation;

is a cutaway schematic representation of the tank of a floating structure and a terminal for loading and/or unloading this tank.

FIG. 1 represents a treatment system 1 for a natural gas used as fuel for an engine 2 of a floating structure for transporting and/or storing said natural gas. The processing system 1 is configured to cooperate with at least the engine 2 and at least one tank 3 for storing said natural gas in liquefied form of the floating structure, the processing system 1 thus ensuring the supply of at least the engine 2 with natural gas coming from the tank 3. To this end, the treatment system 1 comprises at least one refrigerant circuit 4 and a circuit 5 for supplying fuel to at least the engine 2.

By way of example, the engine 2 can be a propulsion engine for the floating structure and/or a power engine for at least one on-board equipment. The processing system 1, in particular the fuel supply circuit 5, is used to heat the natural gas in the gaseous state which comes from the tank and to raise its pressure so as to put said natural gas under conditions of pressure and temperature compatible with the needs of the engine 2.

Also, the treatment system 1 can comprise at least one condensation installation 6 and/or a natural gas sub-cooling installation 14 arranged within the treatment system 1 in order to carry out at least one heat exchange, for example with the refrigerant circuit 4.

Within the treatment system 1, the condensing plant 6 and the subcooling plant 14 can be used independently or in combination with each other. They ensure, according to the natural gas needs in the gaseous state of the engine 2, the treatment of at least a part of the natural gas withdrawn from the tank 3, in particular in order to ensure the condensation of gaseous natural gas or the sub- cooling of a liquid portion of natural gas. These different installations and their modes of operation of the floating structure requiring their implementation will be further detailed below.

Within the treatment system 1, the refrigerant circuit 4 consists of a unit capable of transferring thermal energy at cryogenic temperatures close to the storage temperature of natural gas when the latter is liquefied. In particular, in the present invention, the liquefied natural gas concerned essentially comprises methane and has a temperature of change of state, from the gaseous state to the liquid state, of approximately -163°C.

The refrigerant circuit 4 is a closed circuit within which a refrigerant fluid circulates. The refrigerant circuit 4 successively comprises at least one compressor 7, called first compressor 71, a first heat exchanger 8, expansion means 9, for example a Joule-Thomson valve, and a second heat exchanger 10.

Specifically, the refrigerant fluid circuit 4 comprises at least a first portion 151 which extends between an outlet of the first compressor 71 and the expansion means 9 and in which the refrigerant fluid circulates at high pressure, between 18 and 36 bars. The first portion 151 of the refrigerant circuit 4 constitutes a cold reception zone. To this end, it comprises at least a first pass 81 of the first heat exchanger 8, arranged between the compressor 7 and the expansion means 9.

The refrigerant circuit 4 comprises at least a second portion 152, between an outlet of the expansion means 9 and the first compressor 71, within which the refrigerant circulates at low pressure, for example at a pressure of the order from 1.2 to 2.5 bar. The second portion 152 of the refrigerant circuit is intended for cold transmission. It comprises at least a first pass 101 of the second heat exchanger 10 arranged between the expansion means 9 and the compressor 7.

Thus, the refrigerant circulating within the refrigerant circuit 4 is first compressed by the compressor 7, then circulates in the first pass 81 of the first heat exchanger 8 which functions as a condenser of the refrigerant fluid and in which the refrigerant releases calories. The refrigerant is then expanded in the expansion means 9 before being sent to the first pass 101 of the second heat exchanger 10, operating as a vaporizer, where it captures calories, to then return to the compressor 7.

The composition of the refrigerant fluid according to the invention is particularly suitable for use at cryogenic temperature with a view to heat exchange with natural gas essentially comprising methane, that is to say the liquefaction temperature of which is of the order -163°C. The refrigerant fluid thus has a temperature of change of state from the gaseous state to the liquid state of between -160 and 20°C, when the refrigerant fluid is subjected to a pressure of between 18 and 36 bars. The refrigerant has a temperature of change of state from the liquid state to the gaseous state between -183 and -50 °C, when the refrigerant is subjected to a pressure of the order of 1.2 to 2.5 bars. The refrigerant fluid according to the present invention is also non-corrosive and non-toxic.

The refrigerant fluid according to the invention is configured to allow at least one heat exchange with the natural gas circulating in the treatment system 1. In particular, the refrigerant fluid is configured to heat the natural gas circulating in the fuel supply circuit 5 at least the engine so as to bring said natural gas to a temperature compatible with the engine. In particular, the first heat exchanger 8 is the seat of a heat exchange between the refrigerant fluid and this natural gas intended to supply at least the engine 2. Advantageously, the processing system 1 can be configured to put in implements at least a second heat exchange, for example between the refrigerant fluid leaving the first heat exchanger, cooled, and natural gas in the liquid state circulating in the natural gas sub-cooling installation 14, at temperatures of the order of -170°C. The refrigerant fluid is thus particularly suitable for the treatment system 1 in that it does not freeze at temperatures below −160° C. at atmospheric pressure.

The refrigerant circulating in the refrigerant circuit 4 comprises at least one inert gas, more particularly dinitrogen and/or argon, the function of which is to ensure the change of state of the natural gas at cryogenic temperatures higher lower than the liquefaction temperature of natural gas at atmospheric pressure.

The refrigerant fluid also comprises a mixture of hydrocarbons from methane, ethane and/or ethylene and/or propane and or propylene and/or butane. Methane and ethane and/or ethylene are particularly suitable for heat exchanges carried out with natural gas in the gaseous state, for example with the BOG, from the acronym for "Boil-off gas", that is to say the natural gas in the gaseous state resulting from the natural evaporation of the natural gas in the tank 3 which has a temperature between -140 and -90°C.

Advantageously, the coolant can be produced at low cost since some of the compounds of said coolant are available in large quantities within the floating structure, the various hydrocarbons making up in particular the natural gas stored in the tank 3.

In order to optimize the thermal efficiency of the heat exchanges carried out between the refrigerant fluid and the supply circuit and/or the sub-cooling installation 14, the refrigerant fluid comprises at least 70-85% mol of methane and dinitrogen and/or argon. It can be made according to three distinct types of compositions:

• according to a first embodiment, the refrigerant fluid comprises 25 to 35 mol% of dinitrogen and a mixture of hydrocarbons chosen from minus ethane and/or propane and/or butane and/or ethylene and/or propylene,
• according to a second embodiment, the refrigerant fluid comprises 35 to 50% mol of argon and a suitable mixture of hydrocarbons, this mixture possibly being distinct from or identical to that of the first embodiment,
• or according to a third embodiment, the refrigerant fluid comprises 35 to 50 mol% of a mixture of nitrogen and argon as well as a mixture of hydrocarbons, this mixture possibly being distinct from or identical to that of the first embodiment or of the second embodiment.

In particular, the coolant, independently of its embodiment, may comprise 5 to 19% mol of ethane and/or ethylene. Additionally, the coolant may include up to 15% propane or butane, the latter favoring the heat exchanges carried out with the BOG, i.e. at higher temperatures.

When the refrigerant fluid is produced according to the first embodiment, the latter may comprise at least dinitrogen, methane, ethane and/or ethylene and propane and/or propylene, the refrigerant fluid then having , preferably, a ratio of methane and dinitrogen of between 1:1 and 9:5.

Alternatively, the refrigerant fluid according to the first embodiment may comprise at least dinitrogen, methane, ethane and/or ethylene and butane, the refrigerant fluid then preferably having a ratio of methane and dinitrogen comprised between 4:5 and 7:5.

Advantageously, such coolants have a molar mass of 24 g/mol±2 g/mol.

When the refrigerant fluid is produced according to the second embodiment, it may comprise at least argon, methane, ethane and/or ethylene, butane and/or propane and/or propylene, the refrigerant fluid then preferably having a ratio of methane and argon of between 3:5 and 6:5.

Additionally, said coolant may have a molar mass of the order of 31 g/mol ± 3 g/mol.

When the refrigerant fluid is produced according to the third embodiment, it may comprise at least dinitrogen and argon, methane, ethane and/or ethylene, as well as butane and/or propane and/or propylene, the refrigerant fluid then preferably has a ratio of methane and argon and dinitrogen of between 1:2 and 13:10. Additionally, said coolant may have a molar mass of the order of 30 g/mol ± 3 g/mol.

Table 1 illustrates, without limitation, various examples of the composition of the refrigerant, examples 1 to 5 being representative of the first embodiment of the refrigerant, and examples 6 and 7 being representative of the second embodiment and the third embodiment respectively.

1
2
3 4 5 6 7 Components (%mol) N2 30 30 30 32 32 0 20 Ar 0 0 0 0 0 46 30 Methane (C1) 45 45 45 44 40 35 35 Ethane (C2) 10 0 10 12 10 9 8 Ethylene (C2H4) 0 10 0 0 0 0 0 Propane (C3) 15 15 0 0 9 0 7 Propylene (C3H6) 0 0 15 0 0 0 0 Butane (C4) 0 0 0 12 9 10 0

Due to its composition, the refrigerant circulating in the refrigerant circuit 4 is intended to be in the gaseous state or in a two-phase gas-liquid state. By way of example, the refrigerant fluid is essentially in the gaseous state in the compressor 7 and within the first portion 151 of the refrigerant fluid circuit 4, while it is in the two-phase state at the outlet of the means of expansion 9 and in particular between the expansion means 9 and the second heat exchanger 10. As a result, within the refrigerant fluid circuit 4, the refrigerant fluid can be brought to cryogenic temperatures, in particular down to lower temperatures at -170°C, for example, at the outlet of the expansion means 9, but also be raised to temperatures of around 45°C, for example at the outlet of the compressor 7.

In the example illustrated and as previously explained, the first heat exchanger 8 allows an exchange of calories between the refrigerant fluid circulating at a temperature between 20 and 45° C. in the first pass 81 of the first heat exchanger 8, and natural gas in the gaseous state which circulates through a second pass 82 of the first heat exchanger 8, included in the supply circuit 5, having a lower temperature, for example between -140 and -90°C. The refrigerant circulating in the first pass 81, hotter than the natural gas, then transmits calories to the natural gas and receives the cold, thus ensuring the heating, and more precisely the heating, of the said natural gas circulating in the supply circuit fuel from engine 2.

In the second heat exchanger 10, the refrigerant, entering the first pass 101 at a temperature between -180 and -168 ° C, can heat exchange with natural gas in the liquid state which circulates in a second pass 102 of the second heat exchanger 10, in particular included in the natural gas sub-cooling installation 14 and having a temperature of the order of -160°C. The refrigerant, colder than liquid natural gas, transmits cold to the latter. The refrigerant passes, for example, from a temperature of about -178°C after the expansion means 9 to a temperature of the order of -172°C at the outlet of the second heat exchanger 10.

Alternatively, and as illustrated in Figure 2, the treatment system 1 can be made to include a combined heat exchanger 11 which replaces the first heat exchanger 8 and the second heat exchanger 10, the heat exchanger combined heat 11 comprising a first pass 111, a second pass 112, a third pass 113 and a fourth pass 114 respectively taking up the characteristics previously explained relating to the second pass of the first heat exchanger, to the first pass of the first heat exchanger, to the first pass of the second heat exchanger and to the second pass of the second heat exchanger described in FIG. 1. Advantageously, in such a treatment system 1, the second pass 112 and the third pass of the combined heat exchanger 11 mutually exchange calories so that the refrigerant fluid circulating in the second pass 112 of the combined heat exchanger 11 transmits calories to the cooler fluid circulating in the third pass 113 of this same heat exchanger, in the second portion 152 of the refrigerant circuit. The second pass 112 and the third pass 113 thus form an internal exchanger integrated into the combined heat exchanger 11.

Thus, and as illustrated in the embodiments of the treatment system 1 shown in Figures 1 and 2, the refrigerant circuit 4 is integrated centrally in the treatment system 1 of the natural gas transported and stored in the tank 3 of the floating structure so as to allow heat exchange between the coolant flowing in the coolant circuit 4 on the one hand and the natural gas from the tank 3 flowing in the fuel supply circuit 5 and / or in the sub-cooling installation 14 on the other hand.

The refrigerant leaving the first compressor 71 is thus engaged in at least two heat exchanges, the first taking place within the first portion 151 and inducing its cooling, upstream of the expansion means 9, so as to cause the heating of the gas. natural gas circulating in the treatment system 1, and the second taking place in the second portion 152 of the refrigerant fluid circuit and inducing the heating of the refrigerant fluid in order to allow the cooling, and more particularly the sub-cooling, of the natural gas circulating in the subcooling installation 14.

As explained above, in the natural gas treatment system 1 the fuel supply circuit 5, the sub-cooling installation 14 and the condensation installation 6 are configured in order to ensure the treatment of at least a portion of natural gas withdrawn from the tank 3, the implementation of these various installations being dependent on the fuel requirements, that is to say natural gas in the gaseous state, of the engine 2 of the floating structure. Figures 3 to 5 illustrate different modes of operation of the treatment system 1 that can be implemented according to the needs of the floating structure. These different modes of operation will be described with reference to the processing system 1 as illustrated in Figure 1, it is nevertheless understood that these can be transposed to the processing system 1 produced according to the alternative configuration previously described with reference to Figure 2, i.e. wherein the processing system includes the combined heat exchanger.

FIG. 3 particularly illustrates the mode of operation of the treatment system 1 when it allows the supply of the engine 2 of the floating structure with natural gas in the gaseous state coming from the top of the tank 3. Such a mode of operation can be implemented when the needs of the engine 2 of the floating structure are substantially equal to the quantity of BOG naturally produced within the tank 3.

In order to supply the engine 2, the BOG is sampled by a sampling line 51 from the fuel supply circuit of the engine and is conveyed from the tank 3 to at least the engine 2. The extraction of the natural gas in the state gas can be controlled, by way of example, by a pressure sensor, not shown, arranged in the tank 3 in order to measure the BOG pressure in the tank 3 and to detect an overrun of a predetermined pressure threshold above beyond which it is necessary to evacuate gaseous natural gas, for example in order to prevent any damage to the tank 3.

In the processing system 1, the sampling line 51 of the supply circuit 5 brings the natural gas in the gaseous state, or BOG, sampled at a temperature between -140° C. and -90° C., for example the order of -120° C., to the second pass 82 of the first heat exchanger 8. As explained above, due to its temperature, the natural gas in the gaseous state which circulates in the second pass 82 of the first heat exchanger 8 captures the calories of the refrigerant fluid circulating in the first pass 81 of the first heat exchanger 8. The gaseous natural gas is thus heated and leaves the first heat exchanger 8 at a temperature between 20 and 45° C., to be brought to a second compressor 72 of the treatment system 1, separate from the first compressor 71 included in the refrigerant circuit. The passage of the gaseous natural gas through the first heat exchanger and the second compressor 72 successively thus makes it possible to bring the natural gas to a temperature compatible with its use as fuel for at least the engine of the floating structure.

The second compressor 72 has, for example, a compression ratio of 13 and a flow rate of approximately 5000 m ³ /h. The natural gas in the gaseous state, once compressed, has a temperature of the order of 43° C. and can be sent to at least the motor 2 of the floating structure, for example the propulsion motor 2 or the motor 2 of on-board equipment, via at least one supply line 53 of the supply circuit 5.

The composition of the refrigerant fluid is thus particularly suitable for ensuring the heating of the natural gas circulating in the fuel supply circuit 5 of at least the engine. As previously explained, the refrigerant fluid leaving the first exchanger 8 then passes through the expansion means 9 and then is brought into the second heat exchanger 10.

In the example illustrated, this second heat exchanger is used as a vaporizer of the refrigerant fluid and is the site of an exchange of calories between the refrigerant fluid, circulating in the first pass 101 of the second heat exchanger, and the natural gas taken in the liquid state in the tank 3, circulating in the second pass 102 of the second heat exchanger 10, so as to allow the cooling of at least a portion of the natural gas.

Advantageously, the calories given up by the coolant in the first heat exchanger 8, in other words the cryogenic energy generated by the coolant circuit as a result of the exchange of calories implemented in the first heat exchanger 8, can be used to sub-cool a portion of natural gas withdrawn in the liquid state from tank 3 to a temperature which may be of the order of -170°C. In such a mode of operation, the processing system 1 can thus simultaneously ensure the supply of at least the engine 2 with gaseous natural gas as fuel via the supply circuit 5 and the operation of the installation. subcooling 14.

The sub-cooling installation 14 of the processing system 1 comprises a sampling line 61 for the natural gas in the liquid state. Unlike the sampling line 51, the sampling line 61 is submerged so as to sample the natural gas in the liquid state. For this purpose, the sampling line 61 may include a pump 54, for example a submerged pump. The sampling of natural gas in the liquid state can be controlled by at least one sampling valve 64 arranged on the sampling pipe 61, upstream of the second heat exchanger 10.

The sampling pipe 61 brings the natural gas in the liquid state to the level of the second heat exchanger 10, in the second pass 102 of said second heat exchanger 10. By way of example, at the inlet of the second heat exchanger 10, natural gas in the liquid state can have, depending on its composition, a temperature less than or equal to -159°C.

As previously explained, this natural gas in the liquid state circulating in the second pass 102 of the second heat exchanger 10, hotter than the refrigerant fluid circulating in the first pass 101 of the second heat exchanger 10, transmits calories to the fluid refrigerant which has a temperature between -180°C and -168°C, for example of the order of -178°C. The refrigerant, colder than the natural gas in the liquid state, is thus heated while the natural gas is subcooled, the latter leaving the second heat exchanger 10 at a temperature between -165 and -172 ° C . The subcooled natural gas is then reinjected via a reinjection pipe 63 into the natural gas storage tank 3, particularly in a lower portion of the tank 3, thus forming a cold storage layer 145 which can be reused later.

FIG. 4 represents an alternative mode of operation, particularly a backup system of the supply circuit 5 of the processing system 1, which can be implemented in the event of failure of the second compressor 72 of the supply circuit 5. Indeed, within the treatment system 1, the compressor 7 of the refrigerant fluid circuit 4, or first compressor 71, can be configured to compress the gaseous natural gas withdrawn by the withdrawal line 51 and intended to supply at least the engine 2 with gas natural in a gaseous state, as a fuel. Such redundancy of the compressors 7 supplying at least the motor 2 of the floating structure is put in place for safety purposes, so that in the event of failure of the second compressor 72, the supply of the motor 2 with natural gas compressed gas can still be made.

Due to this architecture, the first compressor 71 can be similar to the second compressor 72, that is to say it is subject to different constraints related to its natural gas compression function in addition to its fluid compression function. refrigerant. Specifically, the first compressor 71 has, for example, a compression ratio of 13 and a flow rate of approximately 5000 m ³ /h.

As previously explained, with a view to the implementation of this emergency system, the composition of the refrigerant fluid is particularly adapted and optimized in order to minimize the consumption of the first compressor 71, configured to compress the refrigerant fluid or the natural gas.

In order to allow the use of the first compressor 71 in the refrigerant circuit 4 or in the fuel supply circuit 5 of at least the engine 2, the inlet of the first compressor 71 is connected on the one hand to the first pass 101 of the second heat exchanger 10, included in the refrigerant circuit 4, and on the other hand at an outlet of the second pass 82 of the first heat exchanger 8, in which the natural gas circulates. The connection between the output of the second pass 82 of the first heat exchanger 8 and the first compressor 71 is made via an AC line 52.

In order to control the circulation of the natural gas leaving the second pass 82 of the first heat exchanger 8 in the direction of the first compressor 71 and/or the second compressor 72, the first compressor 71 like the second compressor 72 are also preceded by at least an isolation valve 12 intended to control. Specifically, at least one first isolation valve 121 may be arranged upstream and/or downstream of the first compressor 71, for example in the AC line 52. Additionally, the refrigerant fluid circuit 4 may comprise at least one shut-off valve 45, also arranged upstream of the first compressor 71.

In other words and as shown in Figure 3, by default, that is to say when the second compressor 72 is operational and used to supply at least the engine 2 with natural gas, the first valve of isolation 121 is closed and the shut-off valve 45 is opened in order to isolate the first compressor 71 from the circulation of natural gas in the supply circuit 5 of the treatment system 1 and to use it in the refrigerant fluid circuit 4 to compress the refrigerant fluid.

Conversely and as illustrated in Figure 4, when the second compressor 72 is faulty and the backup system is implemented, at least one second isolation valve 122, arranged upstream of the second compressor 72, is closed, the first isolation valve 121 is open and the stop valve 45 is closed. The natural gas leaving the second pass 82 of the first heat exchanger 8 is then sucked in by the first compressor 71 and it is possible to supply at least the motor 2.

Figure 5 shows the treatment system 1 when the condensation plant 6 is implemented. This second mode of operation can in particular be used when too large a quantity of BOG is produced in relation to the needs of motor 2. This is particularly the case, for example, when the speed of movement of the floating structure is low. . In such a case, part of the natural gas in the gaseous state, or BOG, which has been sent into the fuel supply circuit 5 and which is not used by the engine 2 of the floating structure, is then withdrawn and sent to the condensation installation 6 with a view to its liquefaction and then its reinjection into the tank 3. This portion of natural gas sent to the condensation installation 6 will be qualified, hereinafter, as surplus natural gas.

The excess natural gas is taken from the supply circuit 5 of the processing system 1 by a return line 62 of the condensation installation 6. In particular, the return line 62 can carry out the puncture of the excess natural gas between the second compressor 72 and at least the engine 2 of the floating structure, the excess natural gas then being compressed and having a temperature of between 20 and 45° C. and a pressure less than or equal to 13 bars. This puncture can be carried out selectively, and can, for example, be controlled by a puncture valve 65 arranged on the return line 62.

Also, the condensation installation comprises a third heat exchanger 13 which is configured to ensure an exchange of calories between the excess natural gas, flowing in a second pass 132 of the third heat exchanger 13, and natural gas in the state liquid, circulating in a first pass 131 of the third heat exchanger 13, coming from the tank 3.

Advantageously, the liquid natural gas circulating in the first pass 131 of the third heat exchanger 13 can be the natural gas from the second heat exchanger 10, that is to say the natural gas circulating in the sub-cooling installation 14, subcooled by heat exchange with the refrigerant fluid in the second heat exchanger 10. This subcooled natural gas may in particular exit from the second heat exchanger 10 at a temperature of between -165 and -172°C.

According to an alternative not shown, the natural gas in the liquid state circulating in the first pass 131 of the third heat exchanger can be taken from the tank 3 to be directly brought to the third heat exchanger 13.

The excess natural gas flow is thus brought to the second pass 132 of the third heat exchanger 13 of the treatment system 1 in order to be cooled and reliquefied before being reinjected into the tank 3. The liquefaction of the excess natural gas, having a temperature of the order of 43° C. is ensured by heat exchange with liquid natural gas or subcooled natural gas by the subcooling installation 14. This subcooled natural gas, colder than the gas excess natural gas circulating in the second pass 132 of the third heat exchanger 13, then captures the calories of the latter, thus cooling the excess natural gas and causing its condensation. At the outlet of the third heat exchanger 13, the natural gas in the liquid state circulating in the first pass 131 of the third heat exchanger has a temperature of approximately -152° C., while the condensed natural gas, circulating in the second pass 132 of this same third heat exchanger 13, has a temperature of about -158°C.

The outputs of the passes of the third heat exchanger 13 are connected to the reinjection pipe 63 in order to allow the mixing and then the reintroduction of the natural gas in the liquid state and the recondensed natural gas, also liquid, into the tank 3. This reinjection step can be carried out selectively, for example by means of a reinjection valve 66 provided on the reinjection pipe 63. Similar to the reinjection of the subcooled natural gas described above, the reinjection pipe 63 can be extend into tank 3 so as to deliver the liquid natural gas near the bottom of tank 3.

When the treatment system 1 operates according to this second mode of operation, that is to say when it implements the condensation of the excess natural gas, the refrigerant circuit 4 is indirectly involved with the condensation installation 14 , the coolant circuit 4 being, as previously explained, used to sub-cool the liquid natural gas withdrawn by the withdrawal pipe 61 and used to capture the calories of the excess natural gas. The liquefaction is thus more efficient because the temperature of the liquid natural gas which passes through the first pass 131 of the third exchanger 13 is particularly low, and in particular lower than the temperature of the natural gas stored in the liquid state in the tank 3.

The refrigerant circuit 4, the fuel supply circuit 5, the sub-cooling installation 14 and the condensation installation 6 thus cooperate directly or indirectly with each other within the processing system 1, d on the one hand in order to ensure the temperature setting of the natural gas in the gaseous state, intended to supply at least the engine 2 of the floating structure, on the other hand in order to ensure the sub-cooling and the storage of liquid natural gas and finally to ensure the condensation and reinjection of gaseous natural gas when it is present in excess quantity.

Finally, FIG. 6 is a cutaway representation of the floating structure 15 which shows the natural gas storage tank 3 mounted in a double hull 16 of the floating structure 15 formed by a set of at least one sealing membrane primary, a secondary sealing membrane, arranged between the primary sealing membrane and the double hull 16 of the floating structure 15, and two insulating barriers, respectively arranged between the primary sealing membrane and the secondary sealing membrane and between the secondary sealing membrane and the double shell 16.

Pipes 17 for loading and / or unloading arranged on the upper deck of the floating structure 15 can be connected, by means of appropriate connectors, to a maritime or port terminal 18 in order to transfer the cargo of natural gas in the state liquid from or to the tank 3.

It is understood on reading the foregoing that the present invention proposes a refrigerant fluid intended to circulate in a refrigerant fluid circuit of a natural gas processing system, the refrigerant fluid having a composition particularly suitable for heat exchange at temperatures cryogenic with said liquefied natural gas stored and withdrawn from at least one tank of a floating structure. The coolant according to the invention thus aims to optimize any heat exchange carried out with the natural gas but also to minimize the consumption of a compressor of the treatment system configured to compress said coolant or the natural gas coming from the tank. The present invention also relates to the refrigerant circuit comprising said refrigerant as well as the processing system integrating said refrigerant circuit.

The invention cannot however be limited to the means and configurations described and illustrated here, and it also extends to any equivalent means or configuration and to any technical combination operating such means. In particular, the number of heat exchangers may be modified, the various heat exchangers being able in particular to be grouped together in order to reduce their number, insofar as the treatment system, in fine, fulfills the same functions as those described in this document.

Claims (22)

Cooling fluid intended to circulate in a cooling fluid circuit (4) and configured to exchange heat with a liquefied natural gas stored in at least one tank (3) of a floating structure (15), the cooling fluid comprising:

• 25 to 35 mol% of dinitrogen or 35 to 50 mol% of argon or 35 to 50 mol% of a mixture of dinitrogen and argon and,
• 35 to 55% mol of methane,

said refrigerant having a proportion of methane and dinitrogen and/or argon of between 70 and 85 mol% of the refrigerant, the remainder comprising a mixture of hydrocarbons composed of at least ethane and/or propane and/or butane and/or ethylene and/or propylene. Refrigerant fluid according to the preceding claim, in which the mixture of hydrocarbons comprises 15 to 30% of ethane and/or ethylene and propane and/or propylene or ethane and/or ethylene and butane. Refrigerant fluid according to Claim 1 or 2, comprising 5 to 19 mol% of ethane and/or ethylene. Refrigerant fluid according to one of Claims 1 to 3, comprising up to 15 mol% of propane and/or propylene or butane. Refrigerant fluid according to any one of the preceding claims, comprising at least dinitrogen, methane, ethane and/or ethylene and propane and/or propylene, the refrigerant fluid having a ratio of methane and dinitrogen between 1:1 and 9:5. Refrigerant fluid according to any one of Claims 1 to 4, comprising at least dinitrogen, methane, ethane and/or ethylene and butane, the refrigerating fluid having a ratio of methane and dinitrogen of between 4:5 and 7:5. Refrigerant fluid according to Claim 5 or 6, having a molar mass of 24 g/mol ±2 g/mol. Refrigerant fluid according to any one of Claims 1 to 4, comprising at least argon, methane, ethane and/or ethylene, butane and/or propane and/or propylene, the refrigerant having a ratio of methane and argon between 3:5 and 6:5. Refrigerant fluid according to the preceding claim, having a molar mass of 31 g/mol ± 3 g/mol. Refrigerant fluid according to any one of Claims 1 to 4, comprising at least dinitrogen and argon, methane, ethane and/or ethylene, as well as butane and/or propane and/ or propylene, the refrigerant fluid having a ratio of methane and argon and dinitrogen of between 1:2 and 13:10. Refrigerant fluid according to the preceding claim, having a molar mass of 30 g/mol ± 3 g/mol. Cooling fluid circuit (4) configured to exchange heat with the liquefied natural gas stored in at least the tank (3) of the floating structure (15), the cooling fluid circuit (4) containing, in a closed circuit, the A refrigerant according to any preceding claim. Refrigerant circuit (4) according to the preceding claim, comprising at least:

• a compressor (7, 71) configured to compress the refrigerant fluid,
• a first heat exchanger (8), used as a condenser, through which the refrigerant fluid passes and configured to be passed through by natural gas,
• a means of expansion (9) of the refrigerant fluid,
• a second heat exchanger (10), used as a vaporizer, traversed by the refrigerant fluid and configured to be traversed by natural gas.

Treatment system (1) for the natural gas used as fuel for an engine (2) of a floating structure (15), the treatment system (1) comprising the refrigerant fluid circuit (4) according to claim 13 and the treatment system (1) being configured to cooperate with at least one motor (2) and at least the tank (3) of the floating structure (15). Treatment system (1) according to the preceding claim, comprising at least one sampling line (51) of the natural gas in the gaseous state present in the tank (3), the first heat exchanger (8) being the seat of a heat exchange between this natural gas in the gaseous state and the refrigerant fluid. Treatment system (1) according to one of Claims 14 to 15, comprising at least one sampling line (61) for the natural gas in the liquid state present in the tank (3), the second heat exchanger (10) being the seat of a heat exchange between this natural gas in the liquid state and the refrigerant. Treatment system (1) according to any one of Claims 14 to 16, in which the compressor (7) of the refrigerant circuit (4) is configured to compress the natural gas stored in at least the tank (3). Processing system (1) according to any one of claims 14 to 17, comprising at least one natural gas return line (62) through which a flow of excess natural gas passes, the processing system (1) comprising a third exchanger of heat (13) which is the seat of a thermal exchange between this excess natural gas and the natural gas in the liquid state coming from the tank (3) Treatment system (1) according to the preceding claim, in which the natural gas in the liquid state coming from the tank (3) comes from the second heat exchanger (10). Floating structure (15) comprising at least one tank (3) intended for transporting or storing liquefied natural gas, the floating structure (15) comprising at least one motor (2) of the floating structure (15), at least the tank (3) containing natural gas and at least one treatment system (1) according to any one of claims 14 to 19, the engine (2) being configured to be supplied with fuel by the natural gas in the state gas circulating at least in part in said treatment system (1). System for loading or unloading a liquefied natural gas which combines at least one means on land and at least the floating structure (15) for transporting liquefied natural gas according to claim 20 comprising at least the tank (3). Method for loading or unloading a liquefied natural gas from the tank (3) of the floating structure (15) according to claim 20 for transporting liquefied natural gas, in which pipes (17) for loading and/or unloading arranged on an upper deck of the floating structure (15) can be connected, by means of appropriate connectors, to a maritime or port terminal (18) in order to transfer the cargo of natural gas in the liquid state from or to the tank (3).