METHOD FOR HANDLING A CARGO OF LIQUEFIED GAS AND STORAGE FACILITY

Abstract

The invention relates to a method for handling a cargo of liquefied gas in a liquefied gas storage facility (1) comprising: - a sealed and thermally insulating liquefied gas storage tank (2); - a loading / unloading tower (3) comprising at least two vertical masts (7, 8, 9); and - at least one unloading pump (6); the method comprising: - a step of cooling the vertical masts (7, 8, 9) during which a flow of liquefied gas is conducted selectively towards each of the two vertical masts (7, 8, 9); and / or - a step of cooling the unloading pump (6) during which a flow of liquefied gas is conducted selectively towards said unloading pump (6).

Description

Technical field The invention relates to the field of storage of a cargo of liquefied gas, such as liquefied natural gas (LNG). It relates more particularly to a method of handling a cargo of liquefied gas and to a liquefied gas storage facility. Technological background In the state of the art, ships are known that are equipped with a storage facility for a liquefied gas, such as LNG, comprising a sealed and thermally insulating tank for storing the liquefied gas and a loading / unloading tower suspended from a ceiling wall of the tank. The loading / unloading tower of the tank comprises a tripod-type structure, i.e. comprising three vertical masts connected to each other by crosspieces and each defining a cargo unloading and / or discharge line and / or an emergency well allowing an emergency unloading pump and an unloading line to be lowered into the tank. The loading / unloading tower supports, at its lower part, a cargo unloading pump which is associated with an unloading line. Such a storage facility is described in particular in document FR2785034. It is known to carry out a cooling step of the tank prior to loading the cargo into the ship's tank. This step aims to reduce the temperature inside the tank, in particular to avoid excessive vaporization of the liquefied gas during loading and to limit the intensity of thermal stresses in certain components of the installation. This cooling step is carried out by spraying and vaporizing, in the upper part of the tank, liquefied gas supplied by the loading terminal. The heat necessary for the vaporization of the liquefied gas is released by the gas contained in the tank and by its walls. The gas produced is extracted from the tank and returned in vapor form to the loading terminal. The operation continues until the average temperature inside the tanks is below a threshold temperature. The duration of the aforementioned stage of cooling the tank is relatively long, in the order of 10 to 20 hours, which means that the ship is immobilized for a long time when loading. In addition, the quantity of liquefied gas required to cool the tank is significant. Summary An idea underlying the invention is to propose a method for handling a cargo of liquefied gas making it possible to improve the efficiency of the cooling step, in particular by reducing its duration and / or by reducing the quantity of liquefied gas required for its implementation. According to one embodiment, the invention provides a method for handling a cargo of liquefied gas in a liquefied gas storage facility comprising: - a sealed and thermally insulating tank for storing liquefied gas; and - at least one unloading pump which is arranged near a bottom wall of the tank and which is associated with a line which passes through a wall of the tank and which allows the unloading of the tank; the process comprising: - a step of cooling the unloading pump during which a flow of liquefied gas is conducted selectively towards said unloading pump and said flow of liquefied gas is brought into contact with said unloading pump so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said unloading pump. According to an alternative or complementary embodiment, the invention provides a method for handling a cargo of liquefied gas in a liquefied gas storage facility comprising: - a sealed and thermally insulating tank for storing liquefied gas; - a loading / unloading tower suspended from a ceiling wall of the tank and comprising a plurality of lines which pass through the ceiling wall of the tank and each allow the unloading and / or the discharging of the tank; said loading / unloading tower comprising at least two vertical masts which are fixed to each other by crosspieces and which are fixed to the ceiling wall of the tank; said at least two masts being hollow and each defining one of the lines; and - at least one unloading pump which is fixed to a lower end of the loading / unloading tower and which is associated with one of the lines; the process comprising: - a step of cooling the vertical masts during which a flow of liquefied gas is conducted selectively towards each of the two vertical masts and each of the flows of liquefied gas is brought into contact with one of the two vertical masts so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts. Thus, the methods according to the aforementioned embodiments make it possible to locally cool the equipment of the installation most sensitive to thermal stresses, such as the unloading pump and the vertical masts of the loading / unloading tower, so that the cooling of the entire atmosphere inside the tank is no longer necessary. Such methods therefore make it possible to reduce the duration of the cooling step and / or to reduce the quantity of liquefied gas necessary for this cooling step. Therefore, at the end of the cooling operation of the vertical masts, respectively at the end of the cooling operation of the unloading pump, it is not necessarily the entire tank that is cooled to the point of satisfying a defined cooling criterion. On the contrary, a predefined localized cooling criterion can be satisfied by the vertical masts, respectively by the unloading pump, even though the majority of the material components of the tank, in particular the majority of the walls of the tank, or even 90% of their surface area, remain at a temperature well above the temperature obtained in the vertical masts, respectively in the unloading pump.Therefore, by conditioning the termination of the cooling operation on the satisfaction of such a localized cooling criterion, the duration of time and / or the quantity of energy consumed by the cooling operation until its termination can be considerably limited. For example, the duration of the cooling operation of the vertical masts, respectively the unloading pump, can be less than 2 hours. The cold energy applied during this operation can be between 100 and 5000 megajoules for the unloading pump and between 500 and 15000 megajoules for the vertical masts, therefore a total of between 100 and 20000 megajoules for the selective cooling of both equipment. According to other advantageous embodiments, such a method may have one or more of the following characteristics. According to one embodiment, the step of cooling the unloading pump is implemented until the unloading pump reaches a temperature between -120 and -140°C. According to one embodiment, the step of cooling the unloading pump is implemented on an unloading pump having a temperature greater than -120°C, for example greater than -50°C, or even greater than -10°C, and for example less than 40°C. According to one embodiment, the temperature of the unloading pump is measured and the step of cooling the unloading pump is stopped when the temperature of the unloading pump reaches a threshold temperature. According to one embodiment, the temperature of the unloading pump is measured by means of a temperature sensor disposed in the unloading pump, preferably in a stator of the pump, or in contact with a surface of the pump housing. According to one embodiment, the average temperature inside the tank, i.e. the average temperature of the entire gas phase stored in the tank, remains above -120°C, for example above -50°C, or even above -10°C, and for example below 40°C during the step of cooling the unloading pump. According to one embodiment, the average temperature of the walls of the tank remains above -120°C, for example above -50°C, or even above -10°C, and for example below 40°C during the step of cooling the unloading pump. According to one embodiment, the flow of liquefied gas is selectively conducted towards said unloading pump with a flow rate of between 0.05 and 10 m3 / h, for example between 0.1 and 1 m3 / h, The volume of the sealed tank is, for example, between 1000 and 60000 m3. According to one embodiment, during the step of cooling the unloading pump, the flow of liquefied gas is sprayed against the unloading pump. According to one embodiment, the liquefied gas stream is sprayed against the unloading pump via one or more spray nozzles. According to one embodiment, the step of cooling the unloading pump is implemented by means of a device for cooling the unloading pump which is associated with the unloading pump and which is arranged to conduct the flow of liquefied gas towards the unloading pump and bring it into contact with said unloading pump so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool the unloading pump. According to one embodiment, the step of cooling the pump is carried out prior to a step of loading the liquefied gas into the tank during which liquefied gas is conducted from a loading terminal to the tank. According to one embodiment, the cooling step is carried out before each step of loading the liquefied gas into the tank. According to one embodiment, during the step of loading the liquefied gas into the tank, liquefied gas is conducted from the unloading terminal to the tank through a line passing through a wall of the tank. According to one embodiment, the flow of liquefied gas which is conducted towards said unloading pump, during the step of cooling the unloading pump, is previously sucked into the tank via a drain pump arranged near the bottom wall of the tank. According to one embodiment, the flow of liquefied gas which is conducted towards the unloading pump, during the step of cooling the unloading pump, is previously extracted from a loading terminal. According to one embodiment, the installation further comprises a loading / unloading tower suspended from a ceiling wall of the tank and comprising a plurality of lines which pass through the ceiling wall of the tank and each allow the unloading and / or the unloading of the tank; said loading / unloading tower comprising at least two vertical masts which are fixed to each other by crosspieces and which are fixed to the ceiling wall of the tank; said at least two vertical masts being hollow and each defining one of the lines; said unloading pump being fixed to a lower end of the loading / charging tower, the method further comprising: - a step of cooling the vertical masts during which a flow of liquefied gas is conducted selectively towards each of the two vertical masts and each of the flows of liquefied gas is brought into contact with one of the two vertical masts so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts. According to one embodiment, the loading / unloading tower comprises three vertical masts. According to one embodiment, the step of cooling the vertical masts is carried out until the vertical masts reach a final temperature of between -120 and -140°C. According to one embodiment, the step of cooling the vertical masts is implemented on vertical masts having an initial temperature greater than -120°C, for example greater than -50°C, or even greater than -10°C, and for example less than 40°C. According to one embodiment, the average temperature inside the tank, i.e. the average temperature of the entire gas phase stored in the tank, remains above -120°C during the step of cooling the vertical masts. According to one embodiment, the average temperature of the walls of the tank remains above -120°C, for example above -50°C, or even above -10°C, and for example below 40°C during the step of cooling the vertical masts. According to one embodiment, the flow of liquefied gas is conducted selectively towards each of the vertical masts with a respective flow rate of between 1 and 15 m° / h. According to one embodiment, the average temperature inside the tank remains above -120°C during the stage of cooling the vertical masts. According to one embodiment, during the step of cooling the vertical masts, the liquefied gas flows are sprayed against each of the vertical masts. According to one embodiment, the flow rates of liquefied gas sprayed on each of the vertical masts are equal. According to one embodiment, the liquefied gas streams are each sprayed against one of the vertical masts via one or more spray nozzles. According to one embodiment, the step of cooling the vertical masts is implemented by means of a device for cooling the vertical masts which is associated with said vertical masts and which is arranged to conduct a flow of liquefied gas towards each of the vertical masts and to bring it into contact with the respective vertical mast so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts. According to one embodiment, the step of cooling the vertical masts is carried out prior to a step of loading the liquefied gas into the tank during which liquefied gas is conducted from a loading terminal to the tank. According to one embodiment, the liquefied gas flows which are conducted towards the vertical masts, during the step of cooling the vertical masts, are previously sucked into the tank via a drain pump. According to one embodiment, the liquefied gas flows which are conducted towards the vertical masts, during the step of cooling the vertical masts, are previously extracted from a loading terminal. According to one embodiment, the method further comprises a step of cooling the unloading pump during which a flow of liquefied gas is conducted selectively towards said unloading pump and said flow of liquefied gas is brought into contact with said unloading pump so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said unloading pump. According to one embodiment, the liquefied gas is selected from liquefied natural gas (LNG), liquefied petroleum gas (LPG) and ethane. According to one embodiment, the invention provides a liquefied gas storage facility comprising: - a sealed and thermally insulating tank for storing liquefied gas; - at least one unloading pump which is arranged near a bottom wall of the tank and which is associated with a line which passes through a wall of the tank and which allows the unloading of the tank; and - a device for cooling the unloading pump which is associated with the unloading pump and which is arranged to conduct a flow of liquefied gas towards the unloading pump and bring it into contact with said unloading pump so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool the unloading pump. According to an alternative embodiment, the invention provides a liquefied gas storage facility comprising: - a sealed and thermally insulating tank for storing liquefied gas; - a loading / unloading tower suspended from a ceiling wall of the tank and comprising a plurality of lines which pass through the ceiling wall of the tank and each allow the unloading and / or the discharging of the tank; said loading / unloading tower comprising at least two vertical masts which are fixed to each other by crosspieces and which are fixed to the ceiling wall of the tank; said at least two masts being hollow and each defining one of the lines; and - at least one unloading pump which is fixed to a lower end of the loading / unloading tower and which is associated with one of the lines; and - a device for cooling the vertical masts which is associated with said vertical masts and which is arranged to bring a flow of liquefied gas into contact with each of said vertical masts so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts. According to other advantageous embodiments, such an installation may have one or more of the following characteristics. According to one embodiment, the device for cooling the unloading pump comprises a supply line for the flow of liquefied gas leading to one or more spray nozzles directed towards the unloading pump. According to one embodiment, the or each spray nozzle of the cooling device of the unloading pump is located in line with the unloading pump. According to one embodiment, the or each spray nozzle of the cooling device of the unloading pump is arranged in an area of ​​the tank close to the intersection between the bottom wall and the rear wall of the tank. According to one embodiment, the or each spray nozzle of the cooling device of the unloading pump is located at a distance of less than 5 meters from the unloading pump. According to one embodiment, the supply line of the cooling device of the unloading pump is connected to a drain pump positioned in the tank and / or is capable of being connected to the loading terminal. According to one embodiment, the device for cooling the vertical masts comprises a supply pipe for the flow of liquefied gas leading to one or more spray nozzles directed towards each of the vertical masts. According to one embodiment, the or each spray nozzle of the device for cooling the vertical masts is located at a distance of less than 5 meters from its respective vertical mast. According to one embodiment, the or each spray nozzle of the cooling device of the unloading pump is arranged in an area of ​​the tank close to the rear wall of the tank. According to one embodiment, the supply line of the vertical mast cooling device is connected to a drain pump positioned in the tank or is capable of being connected to the loading terminal. According to one embodiment, the installation comprises a temperature sensor arranged to measure a temperature of the pump or a temperature of the fluid contained in the interior space of the tank at a height located between a lowest point of the pump and a highest point of the pump. Thanks to these characteristics, a relevant temperature measurement for determining the actual state of the pump can be obtained, given that either the temperature of a component of the pump or the temperature of a fluid environment in which the pump is immersed, including the temperature of a gaseous atmosphere in which the pump is immersed. Several arrangements of the temperature sensor may be suitable for obtaining reliable temperature information. A first possibility is to arrange the temperature sensor, that is to say at least the sensitive part of this sensor, in direct contact with a component of the pump, in particular on or in a component of the pump. According to embodiments, the temperature sensor is arranged on an element chosen from the group comprising an outer casing of the pump, a portion of the discharge line adjacent to the outlet of the pump, and a fixing flange providing the connection between the discharge line and the outlet of the pump. According to embodiments, the temperature sensor is arranged in the pump, preferably in a stator of the pump when the pump is in the form of a centrifugal pump or other rotating machine. Such a temperature sensor may in particular be arranged so as to measure the temperature in an internal channel of the pump intended to discharge the fluid in operation. According to one embodiment, the temperature sensor is disposed on the loading / unloading tower to measure the temperature of a fluid contained in the interior space of the tank. According to one embodiment, the temperature sensor is disposed on a surface of the loading / unloading tower facing the unloading pump. Several technologies are available for producing the temperature sensor, particularly depending on the temperature ranges to be reached in service. In embodiments suitable for LNG, the temperature sensor is a thermocouple or a platinum resistance thermometer. In one embodiment, the installation further comprises an information system functionally connected to the temperature sensor to acquire a temperature measurement signal from the temperature sensor, the information system further comprising a human-machine interface configured to communicate the temperature measurement provided by the temperature sensor to personnel responsible for operating the tank. According to one embodiment, the installation comprises an information system which is arranged to control the cooling device of the unloading pump as a function of the temperature measurement signal from the unloading pump. According to one embodiment, the invention provides a vessel for transporting a fluid, the vessel comprising a aforementioned installation. According to one embodiment, the invention provides a method of loading or unloading a vessel, in which a fluid is conveyed through insulated pipes from or to a floating or land-based loading / unloading terminal to or from the tank. According to one embodiment, the invention provides a transfer system for a fluid, the system comprising a vessel, insulated pipes arranged to connect [the tank installed in the hull of the vessel to a floating or land-based loading / unloading terminal and a pump for driving a fluid through the insulated pipes from or to the floating or land-based loading terminal to or from the tank of the vessel. Brief description of the figures The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes, with reference to the accompanying drawings. - Figure 1 is a sectional view of a liquefied gas storage facility according to a first embodiment. - Figure 2 is a sectional view of a gas storage facility according to a second embodiment - Figure 3 is a schematic sectional view of an installation according to another embodiment of the invention. - Figure 4 is an enlarged schematic view of zone IV of Figure 3. - Figure 5 is a cutaway schematic representation of a LNG carrier vessel comprising a liquefied gas storage facility and a loading / unloading terminal for this facility. Detailed description of embodiments In Figure 1, a liquefied gas storage facility 1 according to a first embodiment is shown. Such a facility 1 can be installed on land or on a floating structure. In the case of a floating structure, the facility 1 can be intended for a liquefied natural gas transport vessel, such as an LNG carrier, but can also be intended for any vessel whose powertrain, generator sets, steam generators or any other consuming unit are supplied with gas. The installation 1 comprises a sealed and thermally insulating tank 2 for storing liquefied gas. According to one embodiment, the tank 2 is a membrane tank for storing liquefied gas at atmospheric pressure. Such a membrane tank may in particular comprise a multi-layer structure comprising, from the outside to the inside of the tank 2, a secondary thermally insulating barrier comprising insulating elements resting against a supporting structure, a secondary sealing membrane, a primary thermally insulating barrier comprising insulating elements resting against the secondary sealing membrane and a primary sealing membrane intended to be in contact with the liquefied gas contained in the tank 2. By way of example, such membrane tanks are described in particular in patent applications WO14057221, FR2691520 and FR2877638. The liquefied gas to be stored by the facility 1 may in particular be liquefied natural gas (LNG), i.e. a gas mixture comprising mainly methane and one or more other hydrocarbons, such as ethane, propane, n-butane, i-butane, n-pentane, i-pentane, neopentane, and nitrogen in small proportions. The liquefied natural gas is stored at atmospheric pressure at a temperature of approximately -162°C. The liquefied gas may also be ethane or liquefied petroleum gas (LPG), i.e. a mixture of hydrocarbons from oil refining comprising mainly propane and butane. The installation 1 also comprises a loading / unloading tower 3, which in particular allows the cargo to be loaded into the tank 2 before its transport and the cargo to be unloaded after its transport. When the installation 1 is embarked on a ship, such as an LNG carrier, the loading / unloading tower 3 is for example installed in the vicinity of the rear wall 4 of the tank 2, because during unloading, the LNG carrier tilts backwards, which makes it possible to optimize the quantity of LNG likely to be unloaded by the loading / unloading tower 3. The loading / unloading tower 3 is suspended from a ceiling wall of the tank 2 and extends substantially over the entire height of the tank 2. In one embodiment, not illustrated, the loading / unloading tower 3 is suspended from the ceiling wall in an area of ​​the tank 2 designated by the term liquid dome. Such a liquid dome is notably described in application FR2991430. The loading / unloading tower 3 supports, at its lower end, one or more cargo unloading pumps 6. The unloading pump 6 is arranged close to the bottom wall of the tank. The purpose of this proximity is to allow the pump to remove the majority of the cargo stored in the tank, with the exception of a possible liquid heel representing less than 10% of the volume of the tank. The unloading pump 10 is a centrifugal pump capable of sucking a flow of liquid through an inlet located at the bottom of the unloading pump and discharging this flow into an unloading line, which is connected to an outlet located at the top of the unloading pump. The unloading pump 6 has a high nominal flow rate to ensure rapid handling of a large cargo, for example 500m“h. A single unloading pump 6 is shown in Figure 1. The loading tower 3 has a tripod structure, i.e. it has three vertical masts 7, 8, 9 which are each fixed to each other by crosspieces 10. Each of the vertical masts 7, 8, 9 is hollow and passes through the ceiling wall 5 of the tank 2. Each of the vertical masts 7, 8, 9 thus defines either a loading and / or unloading line for loading or unloading liquefied gas or an emergency well for lowering an emergency pump and an unloading line in the event of failure of the other unloading pumps 6. In a particular embodiment, two of the vertical masts 7, 8 define a tank unloading line and are, for this purpose, each associated with an unloading pump 6 fixed to the lower end of the loading / unloading tower 3 while the third vertical mast 9 defines a relief well. In such an embodiment, the loading / unloading tower 3 carries one or more loading lines which do not constitute one of the vertical masts 7, 8, 9 of the tripod structure. Furthermore, the installation 1 comprises a device for cooling the vertical masts. Such a device for cooling the vertical masts 11 is arranged to conduct a flow of liquefied gas to each of the vertical masts 7, 8, 9 in such a way that said flow of liquefied gas vaporizes upon contact with the vertical masts 7, 8, 9 and the latent heat of vaporization of the flow of liquefied gas cools the vertical masts 7, 8, 9. Such a device for cooling the vertical masts 11 makes it possible to cool the vertical masts 7, 8, 9 homogeneously prior to loading the tank 2 with liquefied gas.Indeed, if all the vertical masts 7, 8, 9 are not cooled uniformly before loading the tank 2 with liquefied gas, the loading / unloading tower 3 would tend to bend under the effect of the temperature differences resulting from the passage of the liquefied gas inside the loading line, which would exert significant forces on the ceiling wall 5 likely to damage it. In the embodiment shown, the device for cooling the vertical masts 11 comprises a supply pipe 12 passing through the ceiling wall 5 and a spray bar 13 which is arranged inside the tank 2 and connected to the supply pipe 12. The spray bar 13 is placed in line with the vertical masts 7, 8, 9 and comprises spray nozzles 14 which are each directed towards one of the vertical masts 7, 8, 9 and arranged near the upper end of the vertical masts 7, 8, 9, typically at a distance of less than 5 meters [, for example of the order of a few tens of centimeters to 1 meter. According to one embodiment, the supply pipe 12 is connected to a drain pump 15 which is arranged near the bottom of the tank 2. The drain pump 15 can in particular be fixed to the lower end of the loading / unloading tower 3. The drain pump 15 makes it possible to pump the remaining liquefied gas heel in the tank 2. Thus, thanks to such an arrangement, the vertical masts 7, 8, 9 can be cooled at sea, before the ship docks at the loading terminal, which allows the duration of the operations to be significantly reduced. Alternatively or additionally, the supply pipe 12 is capable of being connected to the loading terminal so as to allow the vertical masts 7, 8, 9 to be cooled by means of a flow of liquefied gas coming from the loading terminal. In other embodiments not shown, the spray ramp 13 of the device for cooling the vertical masts 11 is replaced by other means making it possible to use the latent heat of vaporization of the flow of liquefied gas to cool the vertical masts 7, 8, 9. Thus, by way of example, the device for cooling the vertical masts 11 may in particular comprise one or more heat exchangers comprising a cooling circuit integrated into the vertical masts 7, 8, 9 such that the flow of liquefied gas vaporizes in said cooling circuit and thus cools the vertical masts 7, 8, 9. In relation to Figure 2, a liquefied gas storage installation 1 according to a second embodiment is shown. This installation 1 differs from the installation 1 disclosed in relation to Figure 1 in that it does not include a device for cooling the vertical masts 11 but a cooling device 16 for the unloading pump 6. Note that, according to an advantageous embodiment, the embodiments of Figures 1 and 2 are combined and the installation 1 includes both a device for cooling the vertical masts 11 and a device for cooling the unloading pump 16. The device for cooling the unloading pump 16 makes it possible to gradually cool the temperature of the unloading pump 6 before loading the tank 2. In fact, the unloading pumps 6 include components sensitive to thermal shocks so that if they are not gradually cooled before loading the tank 2 with liquefied gas, they risk being damaged. In the embodiment shown, the cooling device of the unloading pump 16 comprises a supply pipe 17 passing through the ceiling wall 5 and a spray bar 18 which is arranged inside the tank 2. The spray bar 18 is arranged close to the bottom wall 4 of the tank 2 and in line with the unloading pump(s) 6. The spray bar spray 18 comprises one or more spray nozzles 19 which are each directed towards the unloading pump 6. Each spray nozzle 19 is arranged close to the unloading pump 6, i.e. typically at a distance of less than 5 meters and for example of the order of tens of centimeters to 1 meter. As in the embodiment of Figure 1, the supply line 17 is connected to the drain pump 15 which is arranged near the bottom of the tank 2 and / or is capable of being connected to the loading terminal. Furthermore, in other embodiments not shown, the spray ramp 18 of the cooling device of the unloading pump 16 is replaced by other means making it possible to use the latent heat of vaporization of the flow of liquefied gas to cool the unloading pump 6. By way of example, the cooling device of the unloading pump 16 may in particular comprise a heat exchanger comprising a cooling circuit which is associated with the body of the unloading pump 6 such that the flow of liquefied gas vaporizes in said cooling circuit and thus cools the body of the unloading pump 6. We will describe below the steps of cooling the unloading pump 6 and of cooling the vertical masts 7, 8, 9 of the loading / unloading tower 3 which are carried out prior to the loading of liquefied gas from the loading terminal to the tank 2 through a loading line carried by the loading / unloading tower 3. Advantageously, in one embodiment, the step of cooling the vertical masts and the step of cooling the unloading pump 6 are implemented simultaneously. Before such cold sets, the average temperature of the vapor phase inside tank 2 is generally above -120°C. To cool the vertical masts 7, 8, 9, a flow of liquefied gas is either sucked into the tank 2, at the level of the heel of liquefied gas remaining in the tank 2, by the drain pump 15; or extracted from the loading terminal, then is sprayed against each of the vertical masts 7, 8, 9. In order to ensure temperature homogeneity between the vertical masts 7, 8, 9, the flow rates of the flows of liquefied gas sprayed against each of the vertical masts 7, 8, 9 are substantially equal. For example, the flow rate of liquefied gas sprayed against each vertical mast 7, 8, 9 is of the order of a few m° / h or tens of m* / h, and is for example between 1 and 15 mŸh. The step of cooling the vertical masts 7, 8, 9 is implemented until the upper end of the vertical masts 7, 8, 9 has a final temperature of between -120 and -140°C, for example of the order of -130°C. The duration of the cooling step can in particular be estimated based on one or more measurements of the initial temperature inside the tank 2 and the flow rates of the liquefied gas stream. To cool the unloading pump 6, a flow of liquefied gas is either sucked into the tank 2, at the level of the remaining heel of liquefied gas, by the drain pump 15 or extracted from the loading terminal, then is sprayed against the unloading pump 6. In order to ensure gradual cooling of the unloading pump 6, the flow rate of the flow of liquefied gas sprayed against the unloading pump 6 is of the order of a few tenths of a m / h or a few m* / hm, and for example between 0.05 and 10 m° / h, preferably 0.1 and 1 m° / h. The step of cooling the unloading pump 6 is implemented until the body of the unloading pump 6 has a final temperature of between -120 and -140°C, for example of the order of -130°C. According to one embodiment, the duration of this step can be determined based on one or more measurements of the initial temperature inside the tank and the flow rate of the liquefied gas stream. In one embodiment, the temperature of the unloading pump 6 is measured and the step of cooling the unloading pump 6 is terminated when the temperature of the unloading pump reaches a threshold. An installation for measuring the temperature of an unloading pump is illustrated in relation to Figures 3 and 4. In Figure 3, there is shown for illustrative purposes an unloading pump 110 which is attached to a loading / unloading tower 107 in proximity to the bottom wall 108, for example at a distance of approximately 1 m above the bottom wall 108. The unloading pump 110 is a centrifugal pump capable of sucking a flow of liquid through an inlet 114 located at the bottom of the unloading pump 110 and to discharge this flow into an unloading line 111, which is connected to an outlet located at the top of the unloading pump 110 at a fixing flange 112. The unloading line 111 rises all along the loading / unloading tower 107, to a liquid collection circuit not shown. To measure the temperature of the unloading pump 110, a temperature sensor 120 is arranged in, on or in the immediate vicinity of the unloading pump 110. The temperature sensor 120 is functionally connected to an information system 121, for example by a wired or wireless link 122, to allow the exploitation of the temperature measurements by the personnel responsible for operating the tank and / or by automatons controlling the operation of the tank, and in particular by automatons controlling the cooling device of the unloading pump. Figure 4 illustrates several possible positions of the temperature sensor 200, namely a sensor 201 located inside the pump, a sensor 202 located on the outer surface of the pump casing, a sensor 203 located on the fixing flange 212 and a sensor 204 located on the loading / unloading tower 207 at the same height as the pump, namely at a level located between the highest point (here the fixing flange 212) and the lowest point (here the inlet 214) of the pump 210. Because it is immersed in the same surrounding fluid as the pump casing 210 and in close proximity to it, the sensor 204 located on the loading / unloading tower 207 can provide a temperature measurement that relatively reliably reflects the actual state of the pump 210. In particular, in the case where the surrounding fluid is a vapor phase having a stratified temperature field, the position of the sensor 204 at substantially the same height as the pump 940 ensures that a reliable OT TT measurement is obtained. As indicated in number 123, the information system 121 may also have connections with other sensors according to known technology, in particular temperature sensors measuring the temperature, for example in the walls of the tank at different heights, as shown diagrammatically in numbers 124. In the case of a ship, the information system 121 belongs to the tank management system on board the ship. The information system 121 includes human-machine interfaces not shown, for example screens, dials, printers or the like, located in a tank operating room or in a supervision room of the ship to inform an operator supervising the tank about the status of the tank. The information system 121 formats the temperature data and transmits it to the human-machine interface. The same temperature sensor arrangement can be used with other pumps in the tank, for example a bilge pump or a circulation pump for spraying liquid into the tank. Thanks to the reliable temperature measurement provided by the sensor 120, it is possible to make decisions regarding the control of the tank by ensuring each time that the temperature state of the pump 110 is compatible with the planned operation. The decision concerns in particular the stopping of the cooling step of the unloading pump. Thanks to the temperature sensor 120, the operator can know that the pump 110 has actually reached the planned temperature threshold, for example -130°C, in order to stop its cooling and trigger the filling of the tank. The stopping of the cooling of the unloading pump can also be carried out by a programmed automaton. In contrast, in the absence of the temperature sensor 120, the temperature state of the pump 110 would have to be estimated from other available measurements, for example wall temperature measurements, which is not satisfactory when the unloading pump is specifically cooled during a dedicated cool-down. Referring to Figure 5, a cutaway view of an LNG carrier ship 70 shows a sealed and insulated tank 71 of generally prismatic shape mounted in the double hull 72 of the ship. The wall of the tank 71 comprises a primary sealed barrier intended to be in contact with the LNG contained in the tank, a secondary sealed barrier arranged between the primary sealed barrier and the double hull 72 of the ship, and two insulating barriers arranged respectively between the primary sealed barrier and the secondary sealed barrier and between the secondary sealed barrier and the double hull 72. In a manner known per se, loading / unloading pipes 73 arranged on the upper deck of the ship can be connected, by means of suitable connectors, to a terminal of maritime or port loading or unloading to transfer a cargo of LNG from or to tank 71. Figure 5 represents an example of a maritime terminal comprising a loading and / or unloading station 75, an underwater pipeline 76 and an onshore installation 77. The loading and / or unloading station 75 is a fixed offshore installation comprising a mobile arm 74 and a tower 78 which supports the mobile arm 74. The mobile arm 74 carries a bundle of insulated flexible pipes 79 which can be connected to the loading / unloading pipes 73. The orientable mobile arm 74 adapts to all sizes of LNG carriers. A connecting pipe, not shown, extends inside the tower 78. The loading and / or unloading station 75 allows the loading and unloading of the LNG carrier 70 from or to the onshore installation 77. The latter comprises liquefied gas storage tanks 80 and connecting pipes 81 connected by the underwater pipe 76 to the loading and / or unloading station 75.The subsea pipeline 76 allows the transfer of liquefied gas between the loading or unloading station 75 and the onshore installation 77 over a long distance, for example 5 km, which makes it possible to keep the LNG carrier 70 at a great distance from the coast during loading and unloading operations. To generate the pressure necessary for the transfer of the liquefied gas, pumps on board the ship 70 and / or pumps equipping the onshore installation 77 and / or pumps equipping the loading and unloading station 75 are used. Although the invention has been described in connection with several particular embodiments, it is quite obvious that it is in no way limited thereto and that it includes all technical equivalents of the means described as well as their combinations if these fall within the scope of the invention. The use of the verb "comprise", "comprise" or "include" and its conjugated forms does not exclude the presence of other elements or other steps than those set out in a claim. In the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim.

Claims

CLAIMS 1. Method for handling a cargo of liquefied gas in a liquefied gas storage facility (1) comprising: - a sealed and thermally insulating tank (2) for storing liquefied gas; and - at least one unloading pump (6) which is arranged in the tank (2) close to a bottom wall of the tank (2) and which is associated with a line which passes through a wall of the tank (2) and which allows the unloading of the tank (2); the method comprising: - a step of cooling the unloading pump (6), prior to a step of loading the liquefied gas into the tank (2), during which a flow of liquefied gas is conducted selectively towards said unloading pump (6) and said flow of liquefied gas is brought into contact with said unloading pump (6) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said unloading pump (6); - the step of loading liquefied gas into the tank (2) during which liquefied gas is conducted from a loading terminal to the tank (2).

2. Method according to claim 1, wherein the step of cooling the unloading pump (6) is carried out until the unloading pump (6) reaches a temperature between -120 and -140°C.

3. Method according to claim 1 or 2, in which the temperature of the unloading pump (6) is measured and the step of cooling the unloading pump is stopped when the temperature of the unloading pump reaches a threshold temperature.

4. Method according to claim 3, wherein the temperature of the unloading pump is measured by means of a temperature sensor (201, 202, 203) arranged in the unloading pump.

5. Method according to any one of claims 1 to 4, in which the flow of liquefied gas is selectively conducted towards said unloading pump (6) with a flow rate of between 0.05 and 10 m3 / h, for example between 0.1 and 1 m3 / h.

6. Method according to any one of claims 1 to 5, wherein, during the step of cooling the unloading pump (6), the flow of liquefied gas is sprayed against the unloading pump (6).

7. The method of claim 6, wherein the gas flow liquefied is sprayed against the unloading pump (6) via one or more spray nozzles.

8. Method according to any one of claims 1 to 7, in which the flow of liquefied gas which is conducted towards said unloading pump (6), during the step of cooling the unloading pump (6), is previously sucked into the tank (2) via a drain pump (15) arranged near the bottom wall of the tank (2).

9. A method according to any one of claims 1 to 8, wherein the installation (1) further comprises a loading / unloading tower (3) suspended from a ceiling wall (5) of the tank (2) and comprising a plurality of lines which pass through the ceiling wall (5) of the tank (2) and each allow the loading and / or unloading of the tank (2); said loading / unloading tower (3) comprising at least two vertical masts (7, 8, 9) which are fixed to each other by crosspieces and which are fixed to the ceiling wall (5) of the tank (2); said at least two vertical masts (7, 8, 9) being hollow and each defining one of the lines; said unloading pump (6) being fixed to a lower end of the loading / unloading tower (3), the method further comprising: - a step of cooling the vertical masts (7, 8, 9) during which a flow of liquefied gas is conducted selectively towards each of the two vertical masts (7, 8, 9) and each of the flows of liquefied gas is brought into contact with one of the two vertical masts (7, 8, 9) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts (7, 8, 9).

10. Method for handling a cargo of liquefied gas in a liquefied gas storage facility (1) comprising: - a sealed and thermally insulating tank (2) for storing liquefied gas; - a loading / unloading tower (3) suspended from a ceiling wall (5) of the tank (2) and comprising a plurality of lines which pass through the ceiling wall (5) of the tank (2) and each allow the loading and / or unloading of the tank (2); said loading / unloading tower (3) comprising at least two vertical masts (7, 8, 9) which are fixed to each other by crosspieces and which are fixed to the ceiling wall (5) of the tank (2); said at least two vertical masts (7, 8, 9) being hollow and each defining one of the lines; and - at least one unloading pump (6) which is fixed to a lower end of the loading / unloading tower (3) and which is associated with one of the lines; the method comprising: - a step of cooling the vertical masts (7, 8, 9), prior to a step of loading the liquefied gas into the tank (2), during which a flow of liquefied gas is conducted selectively towards each of the two vertical masts (7, 8, 9) and each of the flows of liquefied gas is brought into contact with one of the two vertical masts (7, 8, 9) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts (7, 8, 9), - the step of loading liquefied gas into the tank (2) during which liquefied gas is conducted from a loading terminal to the tank (2).

11. Method according to claim 10, wherein the step of cooling the vertical masts (7, 8, 9) is carried out until the vertical masts (7, 8, 9) reach a final temperature of between -120 and -140°C.

12. Method according to claim 10 or 11, in which the flow of liquefied gas is conducted selectively towards each of the at least two vertical masts (7, 8, 9) with a respective flow rate of between 1 and 15 m3 / h.

13. Method according to any one of claims 10 to 12, in which, during the step of cooling the vertical masts (7, 8, 9), the liquefied gas flows are each sprayed against one of the vertical masts (7, 8, 9).

14. A method according to claim 13, wherein the liquefied gas streams are each sprayed against one of the vertical masts (7, 8, 9) via one or more spray nozzles.

15. Method according to any one of claims 10 to 14, in which the liquefied gas flows which are conducted towards the vertical masts (7, 8, 9), during the step of cooling the vertical masts (7, 8, 9), are previously sucked into the tank (2) via a drain pump (15).

16. Method according to any one of claims 10 to 15, further comprising a step of cooling the unloading pump (6). wherein a flow of liquefied gas is selectively conducted to said unloading pump (6) and said flow of liquefied gas is brought into contact with said unloading pump (6) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said unloading pump (6).

17. Installation (1) for storing liquefied gas comprising: - a sealed and thermally insulating tank (2) for storing liquefied gas; - at least one unloading pump (6) which is arranged in the tank (2) near a bottom wall of the tank (2) and which is associated with a line which passes through a wall of the tank (2) and which allows the unloading of the tank (2); - a device for loading liquefied gas into the tank (2) which is arranged to conduct liquefied gas from a loading terminal to the tank (2); and - a device for cooling the unloading pump (16) which is associated with the unloading pump (6), which is configured to be actuated before the actuation of the loading device and which is arranged to conduct a flow of liquefied gas to the unloading pump (6) and bring it into contact with said unloading pump (6) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool the unloading pump (6).

18. Installation (1) according to claim 17, further comprising a temperature sensor (201, 202, 203) arranged in the unloading pump, preferably in a stator of the pump or in contact with a surface of the pump casing.

19. Installation (1) according to claim 17 or 18, in which the device for cooling the unloading pump (16) comprises a supply pipe (17) for the flow of liquefied gas leading to one or more spray nozzles (19) directed towards the unloading pump (6).

20. Installation (1) for storing liquefied gas comprising: - a sealed and thermally insulating tank (2) for storing liquefied gas; - a loading / unloading tower (3) suspended from a ceiling wall (5) of the tank (2) and comprising a plurality of lines which pass through the ceiling wall (5) of the tank (2) and each allow the loading and / or unloading of the tank (2); said loading / unloading tower (3) comprising at least two vertical masts (7, 8, 9) which are fixed to each other by crosspieces and which are fixed to the ceiling wall (5) of the tank (2); said at least two masts being hollow and each defining one of the lines; and - at least one unloading pump (6) which is fixed to a lower end of the loading / unloading tower (3) and which is associated with one of the lines; and - a device for cooling the vertical masts (11) which is associated with said vertical masts (7, 8, 9) and which is arranged to conduct a flow of liquefied gas towards each of the vertical masts (7, 8, 9) and to bring it into contact with the respective vertical mast (7, 8, 9) so as to use the latent heat of vaporization and / or the sensible heat of the liquefied gas to cool said vertical masts (7, 8, 9).

21. Installation (1) according to claim 20, in which the device for cooling the vertical masts (11) comprises a supply pipe (12) for the flow of liquefied gas leading to one or more spray nozzles (14) directed towards each of the vertical masts (7, 8, 9).

22. Vessel (70) for transporting a fluid, the vessel comprising an installation (1) according to any one of claims 18 to 21.

23. A method of loading or unloading a vessel (70) according to claim 22, wherein a fluid is conveyed through insulated pipes (73, 79, 76, 81) from or to a floating or land-based loading / unloading terminal (77) to or from the tank.

24. A transfer system for a fluid, the system comprising a vessel (70) according to claim 22, insulated pipes (73, 79, 76, 81) arranged to connect the tank (71) installed in the hull of the vessel to a floating or land-based loading terminal (77) and a pump for driving a fluid through the insulated pipes from or to the floating or land-based loading / unloading terminal to or from the vessel tank.

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