WO2022122982A1 - Methods for gassing up and for gas tests in a storage facility for liquefied gas - Google Patents

Abstract

A gassing-up method comprises: introducing a flow of liquefied gas (25) in the liquid phase into a first storage tank (10A) in order to bring about cooling of the first tank and vaporisation of the liquified gas in the liquid phase in the first tank, while the flow of the liquefied gas (25) in the liquid phase is introduced into the first storage tank (10A), directing a liquefied gas flow (26, 126) in the vapour phase, which flow is produced by the vaporisation of the liquefied gas in the liquid phase, from the upper portion of the first tank to the upper portion of the second storage tank (10B), and allowing a flow of inert gas (27) to be discharged from a lower portion of the second tank (10B) so that the liquefied gas (21) in the vapour phase replaces the inert gas (22) at least in the upper portion of the second storage tank (10B).

Description

Description

Title of the invention: (PROCESS FOR GASING AND GAS TESTING IN A LIQUEFIED GAS STORAGE FACILITY

Technical area

The invention relates to the field of liquefied gas storage facilities, in particular facilities on board floating structures, such as LNG carriers or others.

[0002] A liquefied gas storage installation, in particular for storing LNG, can for example be an onshore storage installation, a storage installation placed on a seabed, or an installation on board a floating, coastal or in deep water, including an LNG carrier, a floating storage and regasification unit (FSRU), a floating production and remote storage unit (FPSO) and others.

The liquefied gas can be a combustible gas, in particular liquefied natural gas (LNG) or liquefied petroleum gas (LPG), or other.

Prior technique

[0004] The commissioning of an LNG storage tank after its manufacture or return to service after a major repair involves successive operations known as: drying, inerting, gassing-up. , cooling (in English cool-down) then loading.

[0005] These operations are carried out in particular during gas tests, which are tests carried out before the commissioning or re-commissioning of an LNG carrier to confirm the correct operation at low temperature of the storage tanks and the handling system. of cargo. A more complete description of gas trials and recommended practices can be found in the publication “Guide for planning Gas Trials for LNG Vessels” in SIGTTO Information Papers (Edition 2019), ISBN 13: 978-1-85609-810-6 ( 9781856098106).

[0006] In particular, during operations at sea, it is usual to carry out gas tests synchronously in several storage tanks, namely:

- the gassing of the storage tanks by means of a gas flow in the vapor phase produced by forced vaporization in an LNG vaporization unit,

- then the cooling of the storage tanks by means of a flow of gas in the liquid phase. [0007] All these operations at sea consume LNG previously loaded in another tank and generate boil-off gas, that is to say liquefied gas in the vapor phase, which cannot be accumulated on board. Conventional techniques for treating this boil-off gas consist of reliquefying it, consuming it in a propulsion engine, burning it in a combustion unit, and/or releasing it into the atmosphere.

Summary of the invention

[0008] Certain aspects of the invention are based on the observation that the gas tests, in particular the gassing and cooling operations, produce a large quantity of evaporation gas, which it would be desirable to reduce to facilitate its treatment. , save liquefied gas, and/or reduce gas emissions into the atmosphere.

[0009] One idea at the basis of the invention consists in exploiting the evaporation gas produced during the cooling of a tank to carry out the gassing of another tank, in particular during a gas test involving several tanks in a liquefied gas storage facility.

For this, the invention proposes a gassing method intended to gasse a storage tank in a liquefied gas storage installation, the liquefied gas storage installation preferably being on board a floating structure, the method comprising: placing a liquefied gas storage installation in a preparatory state, the liquefied gas storage installation comprising a plurality of storage tanks and at least one collector connected parallel to an upper portion of each of said storage tanks, a first of said storage tanks in the preparatory state being filled with a liquefied gas in the vapor phase, the liquefied gas in the vapor phase in the first tank being at a temperature higher than a liquid-vapor equilibrium temperature of said liquefied gas, a second of said storage tanks in the preparatory state being filled with an inert gas, introducing a flow of the liquefied gas in liquid phase into the first tank , to cause cooling of the first tank and partial or total vaporization of the liquefied gas in the liquid phase in the first tank, while the flow of the liquefied gas in the liquid phase is introduced into the first tank, conducting a flow of liquefied gas in vapor phase produced by the vaporization of the liquefied gas in the liquid phase from the upper portion of the first storage tank to the upper portion of the second storage tank through said at least one manifold connected to the upper portion of each of said tanks storage, said liquefied gas in vapor phase state less dense than the inert gas, letting out a flow of inert gas from a lower portion of the second storage tank under the pressure of said flow of liquefied gas in the vapor phase so that the liquefied gas in the vapor phase replaces the inert gas at least in the upper portion of the second storage tank.

[0011] Thanks to these characteristics, it is possible to carry out the gassing and cooling of several storage tanks in a sequential or partially sequential manner, to produce less boil-off gas than in the conventional synchronous procedure.

[0012] According to advantageous embodiments, such a method may have one or more of the following characteristics.

[0013] The connection making it possible to conduct the flow of liquefied gas in the vapor phase from the first storage tank to the second storage tank can be made in different ways.

According to one embodiment, said at least one manifold comprises a maintenance manifold, the maintenance manifold being connected parallel to the upper portion of each of said storage tanks via a first respective isolation valve , the flow of liquefied gas in the vapor phase being led from the upper portion of the first tank to the maintenance manifold through the first isolation valve associated with the first tank and/or from the maintenance manifold to the upper portion of the second tank through the first isolation valve associated with the second tank.

[0015] Preferably, this maintenance collector is not insulated. In particular, it may be a manifold connected to an inert gas production unit and usually used during tank inerting.

According to one embodiment, said at least one manifold also comprises a steam manifold, the steam manifold being insulated, the steam manifold being connected parallel to the upper portion of each of said storage tanks via a second respective isolation valve, the vapor manifold being connected in series with the maintenance manifold, the flow of liquefied gas in the vapor phase being led successively through the first isolation valve associated with the first vessel, the manifold maintenance, the steam manifold and the second isolation valve associated with the second vessel or successively through the second isolation valve associated with the first vessel, the steam manifold, the maintenance manifold and the first isolation valve associated with the second tank.

The flow of liquefied gas in the vapor phase from the first storage tank to the second storage tank can be achieved in different ways.

According to one embodiment, the flow of liquefied gas in the vapor phase flows from the upper portion of the first storage tank to the upper portion of the second storage tank by natural convection. Thanks to these characteristics, the flow is achieved passively without additional energy expenditure.

According to one embodiment, the liquefied gas storage installation further comprises a gas heater device having an inlet connected to one of the maintenance manifold and the vapor manifold and an outlet connected to the another among the maintenance collector and the vapor collector, the flow of liquefied gas in the vapor phase being further conducted through the gas heater device to be reheated before reaching the upper portion of the second tank. Thanks to these characteristics, it is possible to recover relatively cold boil-off gas, in particular the boil-off gas obtained in the first tank when the cooling operation of the first tank is in an advanced state.

[0020]According to one embodiment, the liquefied gas storage installation further comprises a gas heater device having an inlet connected to one of the maintenance collector and the vapor collector and an outlet connected to the another of the maintenance collector and the vapor collector, and, during a first flow period, the flow of liquefied gas in the vapor phase flows from the upper portion of the first storage tank to the upper portion of the second storage vessel by natural convection, and during a second flow period, the flow of liquefied gas in the vapor phase is further led through the gas heater device to be reheated before reaching the upper portion of the second tank.

According to one embodiment, the method further comprises the steps of: monitoring a temperature of the liquefied gas in the vapor phase leaving the first vessel during the first flow period, and switching the flow of liquefied gas in the vapor phase to the heater device when the temperature of the liquefied gas in the vapor phase satisfies a predetermined criterion.

[0022] Thanks to these characteristics, a natural flow can be achieved at the start of the cooling operation of the first tank until a predetermined criterion is reached, for example a temperature threshold below which the gas liquefied in phase vapor becomes too dense to perform the gassing operation. The flow is then switched to the heater to continue the gassing operation of the second storage tank.

The flow of liquefied gas in the liquid phase can be produced in several ways, for example outside or inside the liquefied gas storage installation. According to embodiments, the flow of liquefied gas in the liquid phase is conducted from a land terminal or from a supply ship to which the liquefied gas storage facility is connected.

According to one embodiment, the liquefied gas storage installation comprises a third storage tank and a spray manifold connected parallel to each of said storage tanks, the third storage tank in the preparatory state being partially or completely filled with the liquid phase liquefied gas, and the flow of the liquid phase liquefied gas is pumped into the third tank and led to the first tank through the spray manifold.

According to one embodiment, the flow of liquefied gas in the liquid phase is sprayed into the first storage tank by a spray device.

The inert gas can be evacuated in different ways. According to one embodiment, the liquefied gas storage installation comprises a liquid collector connected parallel to a lower portion of each of said storage tanks and a degassing mast connected to the liquid collector, and the flow of inert gas leaving the second tank is led through the liquid collector to the degassing mast.

According to one embodiment, the invention also provides a method for carrying out gas tests in a liquefied gas storage installation on board a floating structure, said gas tests comprising: putting the second storage tank under gas by the aforementioned method and, after the second storage tank is gassed, introducing a flow of liquefied gas in the liquid phase into the second tank, to cause cooling of the second storage tank.

In the same way as before, the gassing of another storage tank can be carried out at the same time as the cooling operation of the second storage tank. Thus a quantity of boil-off gas produced during the cooling operation of the second storage tank is in turn exploited, which reduces the overall production of boil-off gas compared to the conventional synchronous procedure. According to one embodiment, the invention also provides a liquefied gas storage installation, the liquefied gas storage installation being preferably on board a floating structure and comprising: a plurality of storage tanks, a collector maintenance connected parallel to an upper portion of each of said storage tanks via a respective first isolation valve, a steam collector connected parallel to the upper portion of each of said storage tanks via a respective second isolation valve, the vapor collector being insulated, a liquid collector connected parallel to a lower portion of each of said storage tanks, the liquid collector being insulated, and a degassing mast connected to the liquid collector, the first isolation valves being switchable to selectively place the maintenance manifold in communication with the upper portion of a pr first of said storage tanks to conduct a flow of liquefied gas in the vapor phase from the first storage tank to the maintenance manifold through the first isolation valve associated with the first storage tank.

According to advantageous embodiments, such a liquefied gas storage facility may have one or more of the following characteristics.

[0031]According to one embodiment, each of said storage tanks comprises a filling line connected to the liquid collector and a vapor line opening into the upper portion of the storage tank and connected parallel to the maintenance collector by the first valve associated with the storage tank and the vapor collector by the second isolation valve associated with the storage tank.

According to one embodiment, the steam manifold is connected in series with the maintenance manifold, the second isolation valves being switchable to selectively put the steam manifold in communication with the upper portion of a second of said tanks tank to conduct the flow of liquefied gas in the vapor phase from the first storage tank to the second storage tank successively through the first isolation valve associated with the first storage tank, the maintenance manifold, the manifold steam and the second isolation valve associated with the second storage tank. According to one embodiment, the liquefied gas storage installation further comprises a spray manifold connected parallel to each of said storage tanks, and a spray device arranged in the upper portion of each of said tanks and connected to the spray collector.

According to one embodiment, the liquefied gas is liquefied natural gas.

According to one embodiment, the floating structure is a vessel for transporting a liquefied gas. Such a vessel for the transport of a liquefied gas may comprise a double hull and storage tanks arranged in the double hull. According to one embodiment, the storage tanks are made with a membrane technique and the double shell comprises an internal shell forming the bearing wall of the storage tanks.

According to one embodiment, the invention also provides a test system for carrying out a gas test, the system comprising an aforementioned liquefied gas storage installation, insulated pipes arranged so as to connect the liquid collector or the spray manifold at a land terminal and a pump for driving a flow of liquid phase liquefied gas through the insulated pipes from the land terminal to the liquid manifold or the spray manifold.

Brief description of figures

The invention will be better understood, and other aims, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely by way of illustration. and non-limiting, with reference to the accompanying drawings.

[0038] [Fig.1] Figure 1 is a diagram partially representing a liquefied gas storage and handling system in which methods according to the invention can be implemented.

[0039] [Fig.2] Figure 2 is a view similar to Figure 1, illustrating the liquefied gas storage and handling system in a state preceding a gassing operation of a tank.

[0040] [Fig.3] Figure 3 is a view similar to Figure 1, illustrating the liquefied gas storage and handling system in a first phase of the operation of gassing a tank.

[0041] [Fig.4] Figure 4 is a graph illustrating the time evolution of the state of a tank during a gassing operation. [0042] [Fig.5] Figure 5 is a view similar to Figure 1, illustrating the system for storing and handling liquefied gas in a second phase of the operation of gassing a tank.

[0043] [Fig.6] Figure 6 is a view similar to Figure 1, illustrating the liquefied gas storage and handling system in a third phase of the gas filling operation of a tank.

[0044] [Fig.7 Figure 7 is a timing diagram illustrating a test procedure implemented in a liquefied gas storage and handling system.

[0045] [Fig.8] Figure 8 is a cutaway schematic representation of an LNG carrier connected to a loading/unloading terminal.

Description of embodiments

[0046] In Figure 1, there is shown schematically an LNG storage facility capable of being embarked on a floating structure, for example on an LNG carrier.

[0047] Three storage tanks 10A, 10B and 10C are shown for illustrative purposes, but this number could be higher or lower. The tanks can be arranged successively in the length of the ship's hull or can be arranged differently. The storage tanks have sealed and insulating walls which can be manufactured by different techniques, for example by a double membrane technique or the like.

[0048] A cargo handling system that connects all the tanks is partially shown. Each storage tank 10A-C comprises in particular:

- A filling line 9 connected to a liquid collector 1.

- a steam pipe 6 opening into the upper part of the storage tank and connected in parallel to a maintenance manifold 4 by a first isolation valve 11 and to a steam manifold 2 by a second isolation valve 12.

- one or more spray booms 5 opening into the upper part of the storage tank and connected to a spray manifold 3.

- a spray pump 7 connected by a pumping line 8 to the spray manifold 3.

Other elements not shown may be present, for example one or more unloading pumps in each storage tank. The aforementioned manifolds, for example the liquid manifold 1 to which the filling lines 9 of all the tanks are connected and the spray manifold 3 to which the spray ramps 5 of all the tanks are connected, can be connected to d other fluid circuits. For example, as sketched by links 16 and 17, liquid manifold 1 and spray manifold 3 are connected to a transshipment circuit to send and receive fluids to and from a shore terminal or another vessel.

To limit the number of pipes crossing the wall of the tank, a single steam pipe 6 has been provided here to connect the maintenance collector 4 and the steam collector 2 in parallel to the interior space of the tank. Alternatively, two separate steam lines could be provided.

The spray collector 3, the liquid collector 1 and the vapor collector 2 are intended to conduct cold fluids and are therefore preferably insulated. Conversely, the maintenance manifold 4 is not or only slightly insulated because its usual function is to conduct inert gas from an inert gas production unit (not shown) for the inerting operations of the tanks and pipes.

[0053] In Figure 1, there is also shown a bent connection 19 connecting one end of the maintenance manifold 4 to the steam manifold 2, in order to form a particularly long circulation path, the use of which will be explained with reference to FIG. 3. This elbow connection 19 is optional.

A gas heater 14 is connected via isolation valves to the maintenance manifold 4, for example at the outlet of the gas heater 14, and to the vapor manifold 2, for example at the inlet of the gas heater 14. Similarly, a vaporization unit 15 is connected via isolation valves to the spray manifold 3, for example at the inlet of the vaporization unit 15, and to the vapor manifold 2, for example at the outlet. of the vaporization unit 15.

[0055] Other elements not shown can be connected to the various manifolds. For example, the vapor collector 2 can be connected to equipment consuming gases in the vapor phase, as outlined by the connection 18, for example to a combustion unit or a propulsion engine.

With reference to Figures 2 to 6, we will now describe a simultaneous cooling operation of the tank 10A and gas supply of the tank 10B. For this, the installation has been put beforehand in a preparatory state, by operations of inerting the tank 10B and putting the tank 10A under gas. These operations may have been carried out by any appropriate method. [0057] By convention, the bold lines in the figures represent pipes or circuits suitable for conveying the fluids used in the installation. The arrows in the figures represent gas or liquid flows circulating in the pipe located under the arrow, for a horizontal arrow, or in the pipe located to the left of the arrow, for a vertical arrow.

In the preparatory state shown in Figure 2, the tank 10A is therefore filled with natural gas in the vapor phase 21 at room temperature and the tank 10B is filled with an inert gas 22 at room temperature, for example dinitrogen or a gas rich in carbon dioxide from oil combustion.

Another storage tank which is partially filled with liquefied gas in the liquid phase in the preparatory state is shown in broken lines in Figure 2. The spray manifold 3 is also connected to this other tank.

Figure 3 shows a first phase of the simultaneous operation of cooling tank 10A and gassing tank 10B.

A flow of LNG 25 is introduced into tank 10A via spray collector 3 and sprayed by spray boom 5 to cool tank 10A. The LNG vaporizes by yielding its latent energy of vaporization in the tank 10A, which creates a surplus of natural gas in the vapor phase. This surplus of natural gas in the vapor phase must be evacuated from the tank 10A as it is produced to avoid an increase in the pressure in the tank 10A. For this, a circulation path is created in order to conduct a flow of natural gas in the vapor phase 26 from the tank 10A to the tank 10B to effect the gassing of the tank 10B.

[0062] As visible in Figure 3, the LNG flow 25 can be pumped into the other tank and led to tank 10A via spray collector 3.

As the gassing of the tank 10B must be carried out with a less dense gas than the inert gas 22, in order to drive the inert gas 22 towards the bottom of the tank 10B, it is advantageous to use, for the flow of natural gas in the vapor phase 26, a circulation path which employs a long length of the maintenance manifold 4, which is not insulated and can therefore give rise to a certain heat exchange with the ambient atmosphere and thus contribute to reheating the vapor phase natural gas stream 26 before it reaches the vessel 10B. For this, the arrows 26 show a circulation path leaving the tank 10A towards the maintenance manifold 4 via the steam line 6 and the isolation valve 11, traversing the entire length of the maintenance manifold 4 up to the elbow connection 19 and continuing in the steam collector 2 to the steam line 6 of the tank 10B. This circulation path can be configured using isolation valves 11 and 12:

- by closing all the isolation valves 11 except that of the tank 10A

- by closing all the isolation valves 12 except that of tank 10B.

[0065] Other circulation paths can be envisaged. For example, the circulation path described above could be reversed to start with steam manifold 2 and end with maintenance manifold 4. This reverse path can be configured using isolation valves 11 and 12:

- by closing all the isolation valves 12 except that of tank 10A

- by closing all the isolation valves 11 except that of the tank 10B.

Another shorter circulation path is sketched by the arrow 126. In this case, the circulation path leaves the tank 10A towards the maintenance manifold 4 via the steam line 6 and the isolation valve 11 of the tank 10A and enters the tank 10B via the steam line 6 and the isolation valve 11 of the tank 10B.

During the operation of gassing the tank 10B, the flow of natural gas in the vapor phase 26 or 126 expels the inert gas 22 towards the bottom of the tank 10B. To create a pressure differential that permits natural flow of vapor phase natural gas stream 26 or 126 to vessel 10B, the pressure in vessel 10B is maintained at a lower pressure than vessel 10A by allowing a stream of inert gas 27 via the filling pipe 9, the liquid collector 1 and a degassing mast 13 connected to the liquid collector 1 . This is preferably the foremast, furthest from the castle of the ship. The relative pressure in tank 10B during the gassing can be for example about 60 mbar (6 kPa), while the relative pressure in tank OA can be for example between 150 mbar (15 kPa) and 180 mbar (18 kPa) .

Figure 4 is a graph illustrating a change in the thermodynamic state of the tank 10A during the cooling operation. The abscissa axis represents the time expressed in hours. The ordinate axis on the right represents the temperature of the gaseous phase in the tank 10A expressed in degrees Celsius (°C) and the curve 41 represents the evolution of the temperature of the gaseous phase from the ambient temperature (here 30°C ) down to about -130°C. The ordinate axis on the left represents a mass flow rate of evaporation gas expressed in kg/h and the curve 42 represents the evolution of the mass flow rate of evaporation gas leaving the tank 10A, from an initially zero flow rate at the beginning of the operation. The cooling operation can last about 15 hours for a large capacity tank. The quantitative values given in FIG. 4 are purely illustrative. They clearly show the following trend: at the start of the cooling operation, the evaporation gas produced is relatively hot, therefore not very dense, and its production is initially low but increases rapidly. After a certain time, for example approximately 2 hours in FIG. 4, the evaporation gas reaches a temperature that is too cold, for example approximately -25° C. in FIG. 4, so that its density becomes too high to directly carry out the gassing of the tank 10B.

[0070] In the event that this evolution occurs, it may be necessary to complete the first phase described with reference to FIG. 3 and to begin a second phase of the operation by making use of the gas heater 14, as illustrated in Figure 5.

In Figure 5, the flow of LNG 25 and the flow of inert gas 27 continue as before, but the flow of natural gas in the vapor phase 26 is conducted by another circulation path passing through the gas heater 14 , in which it is reheated to an appropriate temperature to continue the gassing of the tank 10B, for example approximately 20°C. The flow of heated natural gas is designated by the arrows 28. In the example of FIG. steam 2, gas heater 14, maintenance manifold 4, isolation valve 11 and steam line 6 from vessel 10B to vessel 10B.

[0072] The switching of the flow of natural gas in the vapor phase 26 to the gas heater 14 to launch the second phase can be manual or automated. The criterion for ending the first phase and starting the second phase can be a criterion of temperature or density of the vapor phase. The achievement of this criterion can be monitored by a human operator or automated by a control system.

Throughout the cooling operation of the tank 10A, if the quantity of evaporation gas produced is excessive compared to the rate of progression of the gas operation of the tank 10B, the excess gas natural vapor can be directed to gas consuming equipment, for example via link 18.

The two phases described above for gassing the storage tank 10B make it possible to use the evaporation gas which is inevitably produced during the cooling operation of the storage tank 10A. The use of two phases is not mandatory for this. For example, if the gassing of tank 10B is complete before the temperature of the evaporation gas in tank 10A has fallen too low, the second phase is not necessary. [0075] Conversely, it can be decided to start the gassing of the tank 10B not from the start of the cooling operation of the tank 10A, where the production of evaporation gas is relatively low, but only after as the boil-off gas flow became more sustained and colder. In this case, it may be decided to use the gas heater 14 directly, in which case the first phase described above does not take place.

A time overlap between the cooling operation of tank 10A and the operation of gassing tank 10B can therefore be total or partial. In the event that the vessel 10A cooling operation is complete before the vessel 10B gassing operation is complete, it is possible to complete the tank 10B gassing operation by any conventional method, as shown in Figure 6.

In Figure 6, the storage tank 10A has been partially filled with LNG in the liquid phase after cooling. To continue the gassing of the tank 10B, a flow of LNG 30 is pumped into the liquid phase 23 stored in the bottom of the tank 10A using the spray pump 7 and led to the unit of vaporization 15, for example via the spray collector 3. The vaporization unit 15 thus produces a flow of vaporized natural gas 29, for example at a temperature of about 20° C., which is led to the storage tank 10B to complete the gassing operation.

The methods described above for simultaneously carrying out the storage tank 10A cooling operation and the storage tank 10B gassing operation can be used to improve a gas test procedure in a LNG carrier or any other LNG storage facility. Such a procedure will now be described with reference to Figure 7.

Figure 7 is a timing diagram illustrating a time sequence of operations that can be used to perform gas tests in an LNG carrier comprising four storage tanks with similar capacities and arranged successively along the length of the ship. Tank A refers to the tank located at the stern of the ship and tank D to the tank located at the front of the ship. The abscissa axis represents the time expressed in hours.

The operations designated by the reference numerals from 101 to 118 are:

101: tank A gassed

102 cooling tank A

103: tank B gassed (partially)

104 partial filling of tank A

105: tank B gassed (completely) 106: cooling of tank B

107: commissioning a free-flowing combustion unit (GCU)

108:commissioning a compressor with low availability (LDC)

109: tank C gassed (completely)

111: final commissioning of the combustion unit

112: liquid phase transfer from tank A to tank B (pump test)

113: cooling of tank C

114: tank D gassed (completely)

115: liquid phase transfer from tank B to tank C

116: cooling of tank D

117: liquid phase transfer from tank C to tank D

118: liquid phase transfer from tank D to tank A

[0081] Steps 101 to 104 are carried out in conjunction with an external source of LNG, namely an onshore terminal or a supply ship to which the LNG carrier is connected. Steps 105 to 118 can be performed at sea, so without incurring the cost of renting a land terminal.

The frames 100 of FIG. 7 underline the fact that the cooling operation of a tank can be carried out each time in partial or total temporal overlap with the gas operation of the following tank, except the cooling of the last tank of course. Thus, the boil-off gas generated by the cooling can be used for gassing by the methods described above. This results in a significant reduction in the total quantity of boil-off gas produced by the gas tests conducted by this procedure, compared to the conventional synchronous procedure.

This reduced quantity of evaporation gas is easier to manage, either by reliquefaction, or by combustion, or by return to the terminal, or by evacuation into the atmosphere and therefore leads to a benefit in all cases. This benefit can be a reduction in operational costs and/or an environmental benefit (reduction of emissions).

The gas tests can be implemented in tanks on board one or more ships. In the case of several vessels, these must be connected in order to be able to exchange fluids during the gas test procedure. Thus, although operations carried out with the cargo handling system of a single ship have been described above, it will be understood that these operations may also be carried out in the cargo handling systems of two or more ships connected together. to each other. In this case, a collector, for example the steam collector, can be understood as the meeting of the steam collectors of the different ships connected to each other.

[0085] Referring to Figure 8, a cutaway view of an LNG carrier 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 leaktight barrier intended to be in contact with the LNG contained in the tank, a secondary leaktight barrier arranged between the primary leaktight barrier and the double hull 72 of the ship, and two insulating barriers arranged respectively between the primary waterproof barrier and the secondary waterproof barrier and between the secondary waterproof 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 appropriate connectors, to a maritime or port terminal to transfer a cargo of LNG from or to the tank. 71 .

[0087] FIG. 8 represents an example of a maritime terminal comprising a loading and unloading station 75, an underwater pipe 76 and an installation on land 77. The loading and unloading station 75 is a fixed offshore installation comprising a movable arm 74 and a tower 78 which supports the movable arm 74. The movable arm 74 carries a bundle of insulated flexible pipes 79 which can be connected to the loading/unloading pipes 73. The adjustable movable arm 74 adapts to all sizes of LNG carriers. A connecting pipe, not shown, extends inside the tower 78. The loading and unloading station 75 allows the loading and unloading of the LNG carrier 70 from or to the shore installation 77. This comprises liquefied gas storage tanks 80 and connecting pipes 81 connected by the underwater pipe 76 to the loading or unloading station 75. The underwater pipe 76 allows the transfer of the liquefied gas between the loading or unloading station 75 and the shore installation 77 over a great 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 fitted to the shore installation 77 and/or pumps fitted to the loading and unloading station are used. 75.

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 the technical equivalents of the means described as well as their combinations if these fall within the scope of the invention.

[0090] The use of the verb "to comprise", "to understand" or "to include" and of its conjugated forms does not exclude the presence of other elements or other steps than those set out in a claim.

[0091] In the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim.

Claims

Claims [Claims 1] A gassing method for gassing a storage tank in a liquefied gas storage facility onboard a floating structure, the method comprising: placing a liquefied gas storage facility in a preparatory state, liquefied gas storage facility comprising a plurality of storage tanks (10A-10C) and at least one manifold (4, 2) connected parallel to an upper portion of each of said storage tanks, a first (1 OA) of said tanks storage in the preparatory state being filled with a liquefied gas in the vapor phase (21), the liquefied gas in the vapor phase in the first tank being at a temperature higher than a liquid-vapor equilibrium temperature of said liquefied gas, a second (10B) of said storage tanks in the preparatory state being filled with an inert gas (22), introducing a flow of the liquefied gas in liquid phase (25) into the first storage tank (1 OA), to cause a cooling of the first tank and partial or total vaporization of the liquefied gas in the liquid phase in the first tank, while the flow of the liquefied gas in the liquid phase (25) is introduced into the first storage tank (1 OA), conducting a flow of liquefied gas in the vapor phase (26, 126) produced by the vaporization of the liquefied gas in the liquid phase from the upper portion of the first tank to the upper portion of the second storage tank (10B) through said at least a collector (4, 2) connected to the upper portion of each of said storage tanks, said liquefied gas in the vapor phase (21) less dense than the inert gas (22), letting out a flow of inert gas (27) d a lower portion of the second tank (10B) under the pressure of said flow of liquefied gas in the vapor phase so that the liquefied gas in the vapor phase (21) replaces the inert gas (22) at least in the upper portion of the second storage tank (10B). [Claims 2] Method according to claim 1, in which said at least one manifold comprises a maintenance manifold (4), the maintenance manifold being connected parallel to the upper portion of each of said storage tanks via a respective first isolation valve (1 1 ), the flow of liquefied gas in the vapor phase (26, 126) being led from the upper portion of the first storage tank to the maintenance manifold (4) through the first valve isolation (11) associated with the first storage tank (1 OA) and / or from the maintenance manifold (4) to the upper portion of the second storage tank (10B) through the first isolation valve (11) associated with the second storage tank. [Claims 3] Process according to claim 2, in which the said at least one manifold also comprises a steam manifold (2), the steam manifold being insulated, the steam manifold (2) being connected parallel to the upper portion of each of said storage tanks via a respective second isolation valve (12), the vapor manifold (2) being connected in series with the maintenance manifold (4), the flow of liquefied gas in the vapor phase ( 26) being led successively through the first isolation valve (11) associated with the first storage tank (10A), the maintenance manifold (4), the steam manifold (2) and the second isolation valve ( 12) associated with the second storage tank (10B) or successively through the second isolation valve (12) associated with the first storage tank, the vapor collector (2), the maintenance collector (4) and the first isolation valve (11) associated with the second storage tank lock. [Claims 4] Method according to one of Claims 1 to 3, in which the flow of liquefied gas in the vapor phase (26, 126) flows from the upper portion of the first storage tank (1 OA) to the upper portion of the second storage tank (10B) by natural convection. [Claims 5] Method according to one of Claims 1 to 3, in which the liquefied gas storage installation further comprises a gas heater device (14) having an inlet connected to one of the maintenance manifold ( 4) and the vapor collector (2) and an outlet connected to the other among the maintenance collector (4) and the vapor collector (2), the flow of liquefied gas in the vapor phase (26, 28) being led further through the gas heater device to be heated before reaching the upper portion of the second storage tank (10B). [Claims 6] Method according to one of Claims 1 to 3, in which the liquefied gas storage installation further comprises a gas heater device (14) having an inlet connected to one of the maintenance manifold and the vapor collector (2) and an outlet connected to the other of the maintenance collector (4) and the vapor collector, in which, during a first flow period, the flow of liquefied gas in the vapor phase (26 ) flows from the top portion of the first storage tank to the top portion of the second storage tank by natural convection, and during a second flow period, the flow of liquefied gas in the vapor phase (26, 28) is further led through the gas heater device (14) to be reheated before reaching the upper portion of the second vessel. [Claim 7] A method according to claim 6, further comprising the steps of: monitoring a temperature of the liquefied vapor phase gas exiting the first storage tank (10A) during the first flow period, and switching the gas flow vapor-phase liquefied gas (26) to the heater device (14) when the temperature of the vapor-phase liquefied gas satisfies a predetermined criterion. [Claims 8] Method according to one of Claims 1 to 7, in which the flow of the liquefied gas in the liquid phase (25) is conducted from an onshore terminal (77) to which the liquefied gas storage installation is connected. [Claims 9] Method according to one of Claims 1 to 7, in which the flow of liquefied gas in the liquid phase (25) is conducted from a supply ship to which the liquefied gas storage installation is connected. [Claims 10] Method according to one of Claims 1 to 7, in which the liquefied gas storage installation comprises a third storage tank and a spray manifold (3) connected parallel to each of said storage tanks (10A- 10C), the third storage tank in the preparatory state being partially or completely filled with the liquefied gas in the liquid phase, and in which the flow of the liquefied gas in the liquid phase (25) is pumped into the third tank and leads to the first tank through the spray manifold (3). [Claims 11] Method according to one of Claims 1 to 10, in which the flow of liquefied gas in the liquid phase is sprayed into the first storage tank by a spray device (5). [Claims 12] Method according to one of claims 1 to 11, wherein the liquefied gas storage installation comprises a liquid collector (1) connected parallel to a lower portion of each of said storage tanks (10A-10C) and a degassing mast (13) connected to the liquid collector (1), and in which the flow of inert gas (27) leaving the second tank (10B) is led through the liquid collector (1) to the degassing mast (13). [Claims 13] Method for carrying out gas tests in a liquefied gas storage installation on board a floating structure, said gas tests comprising: gassing (103, 105) the second storage tank by the method according to one of claims 1 to 12 and, after the second storage tank is gassed, introducing (106) a flow of liquefied gas in the liquid phase into the second tank, to cause cooling of the second storage tank. [Claims 14] Installation for storing liquefied gas, the installation for storing liquefied gas being embarked on a floating structure and comprising: a plurality of storage tanks (10A-10C), a maintenance manifold (4) connected parallel to an upper portion of each of said storage tanks via a first isolation valve (11) respectively, a steam collector (2) connected parallel to the upper portion of each of said storage tanks via a second isolation valve (12) respectively, the vapor collector being insulated, a liquid collector (1) connected parallel to a lower portion of each of said storage tanks, the liquid collector being insulated, and a degassing mast (13) connected to the collector liquid, each of said storage tanks (10A-C) comprising a filling line (9) connected to the liquid collector (1) and a vapor line (6) opening into the upper portion of the storage tank and connected parallel to the maintenance manifold (4) by the first isolation valve (11) associated with the storage tank and to the steam manifold (2) by the second isolation valve (12) associated with the storage tank, the first isolation valves (11) being switchable to selectively place the maintenance manifold (4) in communication with the upper portion of a first (10A) of said storage tanks to conduct a flow of liquefied gas in the vapor phase (26) from the first storage tank e to the maintenance manifold (4) through the first isolation valve (11) associated with the first storage tank (10A). [Claims 15] Liquefied gas storage installation according to claim 14, in which the vapor collector (2) is connected in series with the maintenance collector (4), the second isolation valves (12) being switchable to selectively place the vapor collector (2) in communication with the upper portion of a second (10B) of said storage tanks to conduct the flow of liquefied gas in the vapor phase (26 ) from the first storage tank to the second storage tank successively through the first isolation valve (11) associated with the first storage tank, the maintenance manifold (4), the steam manifold (2) and the second isolation valve (12) associated with the second storage tank. [Claims 16] Liquefied gas storage installation according to claim 14 or 15, further comprising a spray manifold (3) connected parallel to each of said storage tanks, and a spray device (5) arranged in the upper portion of each of said tanks and connected to the spray manifold (3). [Claims 17] A liquefied gas storage facility according to any of claims 14 to 16, wherein the floating structure is a vessel (70) for transporting liquefied gas. [Claims 18] A test system for carrying out a gas test, the system comprising a liquefied gas storage installation according to claim 17, insulated pipes (73, 79, 76, 81) arranged to connect the liquid collector (1) or the spray collector (3) to a land terminal (77) and a pump for driving a flow of liquefied gas in the liquid phase through the insulated pipes from the land terminal to the liquid collector (1) or the spray collector (3).