Method and system for refrigerating a cryogenic storage tank

Abstract

The invention relates to a method and to a system for refrigerating a first cryogenic storage tank wherein a first flow of a cryogenic fluid stored inside the first cryogenic storage tank is pumped out of the first cryogenic storage tank, subcooled inside a subcooling heat exchanger and reinjected into the first cryogenic storage tank. In that way the first cryogenic storage tank is refrigerated by the reinjection of the colder first flow of the cryogenic fluid into the storage tank.

Description

[0001] The present invention relates to a method and a system for refrigerating a cryogenic storage tank.

[0002] The invention also relates to the use of the method onboard a sea-going LNG carrier and to a ship comprising a system according to the invention.

[0003] The invention is of particular relevance for the transport of natural gas in liquid form especially in sea-going LNG carriers and is primarily described herein with reference to this application. It is, however, to be understood that it is also applicable to other cryogenic liquids or cryogenic liquid mixtures.State of the Art

[0004] LNG is normally liquefied and transported at -162°C. Before being liquefied, the natural gas is treated to remove components that could freeze at this temperature like heavier hydrocarbon having more than six carbon atoms, benzene and aromatics, carbon dioxide, water etc...

[0005] After removal of these components, a stream of dry and lean natural gas is obtained, mainly comprising the components methane, ethane, propane, butane, along with a few mole percent of nitrogen.

[0006] However, according to the internal knowledge and processing of the applicant, for economic reasons, the above mentioned components that could freeze at -162°C are not completely removed, but merely to such an extent to avoid freezing at -162°C with a reasonable safety margin, but not enough to avoid freezing at significantly colder temperatures, because it was not necessary as -162°C used to be the coldest temperature of the whole LNG transportation chain.

[0007] While natural gas is conveniently stored and transported in liquid state in LNG storage tanks on board of LNG carriers, heat ingress is inevitable although LNG carrier's tanks are especially designed to minimize the heat ingress from the outside environment into the bulk LNG transported within one or more LNG tanks of the ship.

[0008] During its transportation at sea by LNG carriers, due to the unavoidable heat ingress into the LNG storage tanks, a part of the LNG evaporates, known as Boil-Off Gas (BOG). This BOG flow is used as fuel for the motors of the ship. However, it can be that the evaporation is higher than the demand from the motors, resulting in a loss of cargo as not all the BOG can be used as fuel for the motors and the BOG in excess must then be disposed by incineration.

[0009] To avoid this excessive evaporation, more and more LNG carriers are equipped with subcoolers, located outside the LNG storage tanks, for subcooling the LNG. Subcooling means to cool down a liquid to a temperature below its condensation point, at a given pressure

[0010] Subcoolers are cryogenic refrigeration systems using the compression, cooling and expansion of a refrigerant stream to create cold energy, which is then transferred to the LNG stream withdrawn from the storage tank in a cryogenic heat exchanger by indirect heat exchanger with the refrigerant stream.

[0011] Thus, the excessive heat ingress can be compensated by cooling, preferably subcooling, a part of the LNG. This is achieved by pumping a flow of LNG from a storage tank of the ship, (sub-) cooling the LNG stream in a so-called "subcooler", and then re-injecting the (sub-) cooled LNG stream inside the storage tank.

[0012] To compensate for the heat ingress, the LNG shall be subcooled down to ca. - 175°C, a temperature level significantly colder than the -162°C usually found along the whole LNG chain. Locally inside the cryogenic heat exchanger of the subcooler, the wall temperature may even reach a temperature of -178°C. The LNG being only treated to avoid freezing at -162°C, there is a risk of freezing of some components of the LNG because of the significantly colder temperature inside the subcooling heat exchanger.

[0013] WO2021 / 023428 discloses a refrigeration process in which in case of freezing, impurities are removed by decreasing the cooling power by inverting the direction of rotation of at least one of the motors of the refrigeration cycle. Because of the decrease in cooling power, the LNG stored in the storage tank could no longer be subcooled enough to balance the heat ingress, resulting in a loss of LNG due to excessive evaporation.

[0014] WO 2021 / 023457 A1 discloses a refrigeration process in which in case of freezing, impurities are removed by stopping the refrigeration cycle and then no longer providing any cooling power to balance the heat ingress to the storage tank, resulting in a loss of LNG due to excessive evaporation.

[0015] WO 2021 / 023458 A1 discloses a method for defrosting a cryogenic heat exchanger by opening a by-pass valve bypassing said cryogenic heat exchanger. This could lead to disturbance of the thermal load and temperature gradient inside the cryogenic heat exchanger, which could result in differential thermal dilatation between the different channels and thus a catastrophic failure of this equipment because of the thermally induced mechanical stress.

[0016] It is therefore an object of the present invention to provide a method and a system to overcome the above-mentioned disadvantages of the prior art.Summary of the present invention

[0017] This object is solved by a method for refrigerating a first cryogenic storage tank according to claim 1, a system for refrigerating a first cryogenic storage tank according to claim 8, to the use of the method onboard a sea-going LNG carrier according to claim 12 and to a ship comprising a system according to claim 13.

[0018] According to the present invention there is provided a method for refrigerating a first cryogenic storage tank wherein a first flow of a cryogenic fluid stored inside the first cryogenic storage tank is pumped out of the first cryogenic storage tank, subcooled inside a subcooling heat exchanger and reinjected into the first cryogenic storage tank. In that way the first cryogenic storage tank is refrigerated by the reinjection of the colder first flow of the cryogenic fluid into the storage tank.

[0019] The first flow of the cryogenic fluid is made colder by subcooling it inside the subcooling heat exchanger by indirect heat-transfer with a gaseous refrigerant circulating inside a closed cryogenic refrigeration loop, the closed cryogenic refrigeration loop comprising at least one expansion turbine to expand the gaseous refrigerant from a high pressure to a low pressure and the subcooling heat-exchanger to indirectly transfer heat from the first flow of the cryogenic fluid to the gaseous refrigerant, the subcooling heat exchanger being located downstream the at least one expansion turbine.

[0020] The gaseous refrigerant circulating inside the closed cryogenic refrigeration loop is always in gaseous state in any section of the cryogenic refrigeration loop. There is no refrigerant in liquid phase in any point of the cryogenic refrigeration loop, thus allowing to simplify operation by avoiding to control any liquid level in gas / liquid separator, which can be unreliable when on moving environment like a ship at sea.

[0021] The subcooling heat exchanger being located downstream the expansion turbine is with reference to the circulation of the gaseous refrigerant inside the cryogenic refrigeration loop, i.e. the gaseous refrigerant circulating inside the closed refrigeration loop is first expanded from a high pressure to a low pressure inside the expansion turbine, and then the low pressure gaseous refrigerant indirectly exchanges heat with the first flow of cryogenic fluid pumped from the first storage tank inside the subcooling heat exchanger.

[0022] In case of freezing of at least one component of the first flow of the cryogenic fluid inside the subcooling heat exchanger, the method further comprises the steps of: Detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid pumped from the first cryogenic storage tank inside the subcooling heat-exchanger, Defrosting the subcooling heat exchanger by partially opening a control valve located on a bypass line bypassing a part of the gaseous refrigerant around only the at least one expansion turbine, and Simultaneously increasing the first flow of the cryogenic fluid pumped from the first cryogenic storage tank.

[0023] Opening a control valve located on a bypass line bypassing a part of the gaseous refrigerant around the expansion turbine allows to decrease the amount of refrigeration performed by isentropic expansion of the gaseous refrigerant through the turbine as less gaseous refrigerant is expanded through the expansion turbine, and to isenthalpically expand the part of the gaseous refrigerant bypassed through the bypass valve.

[0024] During an isenthalpic expansion through a valve the temperature of the gaseous refrigerant decreases by 0.4°C per bar of pressure difference, instead of 0.8 °C per bar pressure difference during an isentropic expansion, then when both the bypassed part and the remaining part expanded through the expansion turbine are reunited, the overall temperature of the gaseous refrigerant is less decreased thus increasing the temperature of the low pressure gaseous refrigerant entering the subcooling heat exchanger when compared with the isentropic expansion of all the gaseous refrigerant through the expansion turbine, hence reheating the part of the subcooling heat exchanger where components of the cryogenic fluid have frozen.

[0025] At the same time, that is to say simultaneously, increasing the first flow of the cryogenic fluid pumped from the first storage tank allows to compensate the increase of temperature of the first flow of cryogenic fluid resulting from the increase of temperature of the gaseous refrigerant because the smaller mass enthalpy drop of the first flow of the cryogenic fluid being subcooled in the subcooling heat-exchanger is compensated by a higher mass flow of the cryogenic liquid passing through the subcooling heat-exchanger. In that way there is no decrease of the refrigeration power of the closed cryogenic refrigeration loop.

[0026] In addition to that, the increase of the first flow of cryogenic fluid circulating inside the cryogenic heat exchanger also increases the pressure drop within the channels of the subcooling heat exchanger obstructed by the frozen components, thus resulting in an additional mechanical force exerted by the first flow of the cryogenic fluid on the component frozen inside the cryogenic fluid path of the subcooling heat exchanger, thus contributing to the mechanical removal of the frozen components from the heat exchanger.

[0027] In a preferred embodiment, to melt any frozen components of the first flow of the cryogenic fluid that could have been solidified inside the subcooling heat exchanger, the opening of the bypass valve is adjusted to increase the temperature of the gaseous refrigerant entering the subcooling heat-exchanger above the freezing point of the first flow of the cryogenic fluid pumped from the first cryogenic storage tank.

[0028] In another preferred embodiment, the temperature increase of the gaseous refrigerant entering the subcooling heat exchanger is within the range from 0.1 °C to 10 °C, preferably from 0.1 to 5 °C, more preferably from 0.1 to 1 °C above the freezing point of the first flow of the cryogenic fluid pumped from the first cryogenic storage tank. The freezing point of the cryogenic fluid can be determined by in-situ direct determination of the composition of the cryogenic fluid, for example by chromatography of a gas sample of the cryogenic fluid, or by an indirect measurement of one or more physical parameters of the cryogenic fluid like thermal conductivity of sound celerity, which are correlated to the freezing point.

[0029] Because of the temperature increase of the gaseous refrigerant entering the subcooling heat exchanger the increase of the first flow of the cryogenic fluid pumped from the at least one cryogenic storage tank can be adjusted to compensate the heat ingress to the at least one cryogenic storage tank If the first pump installed inside the first cryogenic storage tank has reached its maximum flow capacity, a second flow of the cryogenic fluid can be pumped out a second cryogenic storage tank, and mixed with the first flow of the cryogenic fluid pumped out of the first cryogenic storage tank and then subcooling both the first and the second flows of the cryogenic fluid into the subcooling heat-exchanger.

[0030] In a further embodiment, the occurrence of freezing of at least one component of the cryogenic fluid inside the subcooling heat exchanger is detected by measuring a pressure drop across the cryogenic fluid path of the subcooling heat exchanger.

[0031] Optionally, the at least one expansion turbine of the closed cryogenic refrigeration loop drives at least a compressor for compressing the gaseous refrigerant of the closed cryogenic refrigeration loop through a direct mechanical connection between the at least one expansion turbine and the at least one compressor, the at least one compressor being driven only by the at least one expansion turbine. Preferably, the expansion turbine can be directly connected to the highest-pressure compressor. The expansion turbine being partly bypassed, the mechanical energy recovered by the turbine and transferred to the compressor is decreased, and as there is no motor connected between the compressor and the turbine, the pressure ratio of the compressor decreases accordingly.

[0032] According to a second aspect, the present invention relates to a system for refrigerating at a first cryogenic storage tank comprising: A first pump immerged into a cryogenic fluid stored into the first cryogenic storage tank for pumping a first flow of the cryogenic fluid out of the first cryogenic storage tank; A subcooling heat exchanger fluidically connected to the first pump immerged into the cryogenic fluid stored into the first cryogenic storage tank for subcooling the first flow of the cryogenic fluid pumped out of first cryogenic storage tank by indirect heat exchange with a gaseous refrigerant circulating inside a closed cryogenic refrigeration loop, the closed cryogenic refrigeration loop comprising at least one expansion turbine for expanding the gaseous refrigerant from a high pressure to a low pressure, the subcooling heat exchanger being located downstream of the expansion turbine; Means for detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid pumped from the first cryogenic storage tank inside the subcooling heat-exchanger, A control valve located on a bypass line, the bypass line bypassing only the at least one expansion turbine; Command and control computer means for opening of the control valve located on the bypass line of the at least one expansion turbine and for simultaneously increasing the first flow of the cryogenic fluid pumped out of the first cryogenic storage tank by the first pump located inside the first cryogenic storage tank if the freezing of at least one component of the first flow of the cryogenic fluid inside the subcooling heat-exchanger is detected.

[0033] In case the first pump immerged into the cryogenic fluid stored inside the first cryogenic storage tank can not pump enough cryogenic fluid to be subcooled for compensating the heat ingress toward the storage tank, the system can optionally comprise a second pump immerged into the cryogenic fluid stored into a second cryogenic storage tank for pumping a second flow of the cryogenic fluid out of the second cryogenic storage tank, the first flow and the second flow of the cryogenic fluid being mixed together before being subcooled together inside a subcooling heat-exchanger by indirect heat transfer with a gaseous refrigerant circulating inside the closed cryogenic refrigeration loop.

[0034] In a preferred embodiment, the means for detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid pumped from the first cryogenic storage tank inside the subcooling heat exchanger comprises a differential pressure transmitter for measuring the differential pressure across the cryogenic fluid path of the subcooling heat exchanger.

[0035] Optionally, the closed cryogenic refrigeration loop comprises at least one centrifugal compressor for compressing the gaseous refrigerant circulating inside the closed cryogenic refrigeration loop, the at least one centrifugal compressor being directly connected and only driven by the at least one expansion turbine of the closed cryogenic refrigeration loop.

[0036] A third aspect for which protection is sought, but which also represents an embodiment of the present invention according to the first aspect, is directed to the use of the method according to claims 1 to 7 onboard a sea-going LNG carrier.

[0037] A fourth aspect for which protection is sought, but which also represents an embodiment of the present invention according to the second aspect, is directed to a ship comprising a system according to any of claims 8 to 11.

[0038] Preferably, the ship is a LNG carrier.

[0039] Regarding further explanations as to the advantages of the method and its embodiments, reference is explicitly made to the statements in connection with the method according to the present invention above.

[0040] Further advantages and preferred embodiments of the invention are disclosed in the following description and figure.

[0041] It is understood by a person skilled in the art that the preceding and the following features are not only disclosed in the detailed combinations as discussed or showed in a figure, but that also other combinations of the features can be used without exceeding the scope of the present invention.

[0042] The invention will now be further described with reference to the accompanying drawing showing a preferred embodiment.Brief description of the drawings

[0043] Figure 1schematically shows a preferred embodiment of a method and a system according to the invention. Detailed description of the drawings

[0044] Figure 1 shows a preferred embodiment of a method and a system according to the invention and relates to the application of the method and the system according to the invention onboard a LNG carrier, but it is appreciated that a person skilled in the art can easily transfer the embodiment to applications involving other cryogenic gases or gas mixtures.

[0045] In this example, the cryogenic fluid is LNG stored at a temperature of for example about -162°C inside a first cryogenic tank 1 of a sea-going vessel (not shown). LNG is typically composed of nitrogen, methane, ethane, propane, butane with ppmv level traces of benzene, carbon dioxide and water. During its transportation, because of the unavoidable heat-ingress inside the first cryogenic storage tank, a part of the LNG evaporates. This evaporation gas from the LNG is known as Boil-Off-Gas (BOG).

[0046] To compensate heat ingress into the first cryogenic storage and to control the generation of BOG only to the amount required to power the ship, a first flow 11 of the cryogenic stored inside the first cryogenic storage tank 1 is pumped out of the first storage tank by a pump 101 immerged into the cryogenic fluid stored inside the first cryogenic storage tank. The first flow of the cryogenic fluid is then passing through a subcooling heat exchanger 3 where it is subcooled by indirect heat transfer with a gaseous refrigerant circulating inside a cryogenic closed refrigeration loop.

[0047] The gaseous refrigerant circulating inside the closed cryogenic refrigeration loop is compressed by the first, second and third compressor 14; 15; 16 and after each compression step the gaseous refrigerant is cooled by first, second and third aftercooler 16; 17; 18 to remove the heat generated by compression from the gaseous refrigerant. After being cooled by the third aftercooler 18, the high-pressure gaseous refrigerant enters the heat exchanger 3 as a second refrigerant stream to be further cooled in heat exchanger 3, and is then expanded by expansion turbine 5 to a low temperature, low-pressure gaseous refrigerant. Then the low-pressure gaseous refrigerant stream exchanges heat in the heat exchanger 3 with both the high-pressure refrigerant stream from the third aftercooler 18 and the cryogenic fluid stream 11 to be subcooled. In any point within the closed cryogenic refrigeration loop the gaseous refrigerant is in gaseous state. Thus, there is no liquid refrigerant circulating inside the closed cryogenic refrigeration loop 4. This is especially advantageous to avoid any fluctuations of any liquid level that could disturb the operation of the closed cryogenic refrigeration loop 4 when used onboard a sea-going vessel like a LNG carrier.

[0048] Nitrogen and methane being the components of the LNG with the lowest boiling temperatures, the concentrations of these two components in the BOG is higher than their concentration in the LNG. As a result, the concentrations of the other components (ethane, propane, etc...) in the LNG are increasing over the duration of the journey of the ship, thus increasing the risk of freezing of at least one component of the first stream of the cryogenic fluid 11 inside the subcooling heat exchanger 3, thus deteriorating the performance of the subcooling heat exchanger 3, and ultimately risking a complete blockage.

[0049] If freezing of at least one component of the first flow of the cryogenic fluid inside the cryogenic fluid path 10 of the subcooling heat exchanger 3, the cryogenic fluid path 10 being the channels of the subcooling heat exchanger 3 dedicated to the cryogenic fluid, is detected for example by measuring with a differential pressure transmitter an abnormal increase of the pressure drop of the first flow of cryogenic fluid 11 inside the cryogenic fluid path 10 of the subcooling heat exchanger 3, a signal is send by the differential pressure transmitter to computing means 9 where a command signal will be send simultaneously to a bypass valve 6 located on a bypass line 7 bypassing only the expansion turbine 5 of the closed cryogenic refrigeration loop 4 and to the pump 101 immerged into the first storage tank. As a result, the subcooling heat exchanger is defrosted by opening a bypass valve 6 located on a bypass line 7 bypassing only the expansion turbine 5 of the closed cryogenic refrigeration loop 4 and simultaneously increasing the first flow of the cryogenic fluid 11 pumped by the immerged pump 101 from the first cryogenic storage tank 1.

[0050] If the first pump 101 installed inside the first cryogenic storage tank 1 has reached its maximum flow capacity, a command signal can also be send to a second pump 102 immerged into the second storage tank 2 to pump out a second flow 12 of the cryogenic fluid. This second flow 12 of cryogenic fluid will then be mixed with the first flow 11 of the cryogenic fluid pumped out of the first cryogenic storage tank 1 before subcooling into the subcooling heat-exchanger.

[0051] Optionally, at least one expansion turbine 5 of the closed cryogenic refrigeration loop 4 drives at least a compressor 13; 14; 15 for compressing the gaseous refrigerant of the closed cryogenic refrigeration loop 4 through a direct mechanical connection between the at least one expansion turbine 5 and the at least one compressor 13; 14; 15, the at least one compressor 13; 14; 15 being driven only by the at least one expansion turbine 5. Preferably, the expansion turbine 5 can be directly connected to the highest-pressure compressor 15, the highest-pressure compressor 15 being the compressor compressing the gaseous refrigerant at the highest pressure level. When the expansion turbine 5 is partly bypassed by opening the control valve 6 located on the bypass line 7, the mechanical energy recovered by the turbine 5 and transferred to the compressor 13; 14; 15 is decreased, and as there is no motor connected between the compressor and the turbine, the pressure ratio of the compressor decreases accordingly.List of reference signs

[0052] 1First cryogenic storage tank 2Second cryogenic storage tank 3Subcooling heat exchanger 4Closed cryogenic refrigeration loop 5Expansion turbine 6Control valve 7By-pass line 8Means for detecting the occurrence of freezing 9Command and control computer means 10Cryogenic fluid path of the subcooling heat exchanger 11First flow of the cryogenic fluid 12Second flow of the cryogenic fluid 13, 14, 15Compressors 16, 17, 18Compressors aftercoolers 101Pump immerged in the first storage tank 102Pump immerged in the second storage tank

Claims

1. Method for refrigerating a first cryogenic storage tank (1) wherein a first flow of a cryogenic fluid (11) stored inside the first cryogenic storage tank (1) is pumped out of the first cryogenic storage tank (1), subcooled inside a subcooling heat exchanger (3) and reinjected into the first cryogenic storage tank (1), the first flow of the cryogenic fluid (11) being subcooled inside the subcooling heat exchanger (3) by indirect heat-transfer with a gaseous refrigerant circulating inside a closed cryogenic refrigeration loop (4), the closed cryogenic refrigeration loop (4) comprising at least one expansion turbine (5) to expand the gaseous refrigerant from a high pressure to a low pressure and the subcooling heat-exchanger (3) to indirectly transfer heat from the first flow of the cryogenic fluid (11) to the gaseous refrigerant, the subcooling heat exchanger (3) being located downstream the at least one expansion turbine (5), the method comprising the steps of: - Detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1) inside the subcooling heat-exchanger (3); - Defrosting the subcooling heat exchanger (3) by partially opening a control valve (6) located on a bypass line (7) bypassing a part of the gaseous refrigerant around only the at least one expansion turbine (5), and; - Simultaneously increasing the first flow of the cryogenic fluid (11) pumped out of the first cryogenic storage tank (1).

2. Method according to claim 1, wherein the opening of the bypass valve (6) is adjusted to increase the temperature of the gaseous refrigerant entering the subcooling heat exchanger (3) above the freezing point of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1).

3. Method according to claim 2, wherein the temperature increase of the gaseous refrigerant entering the subcooling heat exchanger (3) is within the range from 0.1 °C to 10 °C, preferably from 0.1 to 5 °C, more preferably from 0.1 to 1 °C above the freezing point of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1).

4. Method according to any of the preceding claims, wherein the increase of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1) is adjusted to compensate heat ingress to the first cryogenic storage tank (1).

5. Method according to claim 4, wherein the first flow of the cryogenic fluid (11) is increased by additionally pumping a second flow of the cryogenic fluid (12) from a second cryogenic storage tank (2), and mixing together both the first flow and the second flow (11; 12) of the cryogenic fluid before subcooling the resulting mixture into the subcooling heat-exchanger (3).

6. Method according to any of the preceding claims, wherein the occurrence of freezing of at least one component of the first flow of the cryogenic fluid (11) inside the subcooling heat exchanger (3) is detected by measuring a pressure drop across the cryogenic fluid path (10) of the subcooling heat exchanger (3).

7. Method according to any of the preceding claims, characterized in that the at least on expansion turbine (5) of the closed cryogenic refrigeration loop (4) drives at least a compressor (13; 14; 15) for compressing the gaseous refrigerant of the closed cryogenic refrigeration loop (4) through a direct mechanical connection between the at least one expansion turbine (5) and the at least one compressor (13; 14; 15), the at least one compressor (13; 14; 15) being driven only by the at least one expansion turbine (5).

8. System for refrigerating a first cryogenic storage tank (1) comprising: - A first pump (101) immerged into a cryogenic fluid stored into the first cryogenic storage tank (1) for pumping a first flow of the cryogenic fluid (11) out of the first cryogenic storage tank (1); - A subcooling heat exchanger (3) fluidically connected to the first pump (101) immerged into the cryogenic fluid stored into the first cryogenic storage tank (1) for subcooling the first flow of the cryogenic fluid (11) pumped out of the first cryogenic storage tank (1) by indirect heat exchange with a gaseous refrigerant circulating inside a closed cryogenic refrigeration loop (4), the closed cryogenic refrigeration loop (4) comprising at least one expansion turbine (5) for expanding the gaseous refrigerant from a high pressure to a low pressure, the subcooling heat exchanger (3) being located downstream the expansion turbine (5); - Means (8) for detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1) inside the subcooling heat-exchanger (3), - A control valve (6) located on a bypass line (7), the bypass line (7) bypassing only the at least one expansion turbine (5); - Command and control computer means (9) for opening the control valve (6) located on the bypass line (7) of the at least one expansion turbine (5) and for simultaneously increasing the first flow of the cryogenic fluid (11) pumped out of the first cryogenic storage tank (1) by the first pump (101) located inside the first cryogenic storage tank (1) if the freezing of at least one component of the first flow of the cryogenic fluid (11) inside the subcooling heat-exchanger (3) is detected.

9. System according to claim 8, further comprising a second pump (102) immerged into a cryogenic fluid stored into a second cryogenic storage tank (2) for pumping a second flow of the cryogenic fluid (12) out of the second cryogenic storage tank (2), the first flow and the second flow of the cryogenic fluid (11; 12) being mixed together before being subcooled together inside a subcooling heat-exchanger (3) by indirect heat transfer with a gaseous refrigerant circulating inside the closed cryogenic refrigeration loop (4).

10. System according to claim 8 or 9 characterized in that the means for detecting the occurrence of freezing of at least one component of the first flow of the cryogenic fluid (11) pumped from the first cryogenic storage tank (1) inside the subcooling heat-exchanger (3) comprises a differential pressure transmitter (8) for measuring the differential pressure across the cryogenic fluid path (10) of the subcooling heat exchanger (10).

11. System according to any of claims 8 to 10, characterized in that the closed cryogenic refrigeration loop (4) comprises at least one centrifugal compressor (13; 14; 15) for compressing the gaseous refrigerant circulating inside the closed cryogenic refrigeration loop (4), the at least one centrifugal compressor (13; 14; 15) being directly connected to and only driven by the at least one expansion turbine (5) of the closed cryogenic refrigeration loop (4).

12. Use of the method of any of claims 1 to 7 onboard a LNG carrier13. Ship comprising a system according to any of claims 8 to 11.

14. Ship according to claim 13, the ship being a LNG carrier.

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