Method for monitoring the conformity of a tank

Abstract

The invention relates to a method for monitoring a tank, the tank (1) comprising a primary space (8) between a primary sealing membrane and a secondary sealing membrane and a secondary space (6) between the secondary sealing membrane and an inner hull, the method being carried out when the tank is at least partially filled with liquefied gas (30), the method comprising the following steps: placing the secondary space at a pressure lower than a threshold pressure P1, the threshold pressure P1 being lower than a pressure of the primary space, measuring a pressure rise rate for the pressure in the secondary space during a period of time (101) by means of a pressure sensor in order to obtain a measurement M1 of the pressure rise rate, determining a state of conformity of the secondary space as a function of the measurement M1.

Description

Method for checking the conformity of a tank

[0001] The invention relates to the field of leak-proof and thermally insulated membrane tanks. In particular, the invention relates to the field of leak-proof and thermally insulated tanks for the storage and / or transport of liquefied gases at low temperatures, such as tanks for transporting Liquefied Natural Gas (LNG) at approximately -162°C at atmospheric pressure. These tanks can be installed on land or on a floating structure. In the case of a floating structure, the tank can be used for transporting liquefied gas or for receiving liquefied gas to serve as fuel for the propulsion of the floating structure. More particularly, the invention relates to a method for checking the conformity of a tank wall. Technological background

[0002] Liquefied natural gas (LNG) tanks typically consist of a supporting structure that provides mechanical rigidity, a primary sealing membrane intended for contact with the product inside the tank, and a secondary sealing membrane positioned between the primary sealing membrane and the supporting structure. The secondary sealing membrane is designed to contain the product in the event of a leak in the primary sealing membrane. The space between the primary and secondary sealing membranes is called the primary space, and the space between the secondary sealing membrane and the supporting structure is called the secondary space.

[0003] The secondary sealing membrane is invisible and inaccessible once the tank is manufactured. Therefore, it is impossible to directly observe any defects in this membrane, such as a scratch, a local dislocation of the membrane, or an air channel between two parts composing the membrane.

[0004] Several methods have therefore been established to control the tightness of the secondary membrane.

[0005] Document FR2946428 describes a method for testing the leak tightness of a tank, specifically for locating leaks in the secondary membrane. In this method, the primary space contains a first non-condensable gas or a gas with a condensation temperature lower than the average temperature of the primary membrane. The method includes a step of injecting a second gas with a condensation temperature higher than the average temperature of the primary membrane into the secondary space, a step of pressurizing the secondary space relative to the primary space, and a step of detecting one or more hot spots on the primary membrane corresponding to a deposit of the second condensed gas in contact with the primary membrane. Thanks to these characteristics, in the event of a non-conformity of the secondary membrane, the second gas escapes into the primary space and comes into contact with the primary membrane near the leak.Because it has a condensation temperature higher than the average temperature of the primary membrane, the second gas condenses and, in doing so, transfers energy in the form of heat, corresponding to its latent heat of phase change, to the primary membrane. A hot spot is thus generated on the primary membrane. Detecting this hot spot from inside the tank allows for the localization of the secondary membrane's non-conformity. However, this method only applies when the tank is empty. For safety reasons, it is also important to be able to detect a leak occurring when the tank is full.

[0006] One idea underlying the invention is to provide control methods for detecting non-conformities in a sealed and thermally insulated tank containing liquefied gas that does not have the aforementioned disadvantages.

[0007] Another idea underlying the invention is to provide a control method to detect leaks present in the secondary sealing membrane and / or in the inner shell.

[0008] According to one embodiment, the invention provides a method for checking the leak-tightness of a secondary space in a sealed and thermally insulated liquefied gas storage tank, the tank comprising a load-bearing structure having an inner shell and an outer shell, a primary sealing membrane intended to be in contact with the liquefied gas contained in the tank, and a secondary sealing membrane arranged between the primary sealing membrane and the inner shell, a primary space between the primary sealing membrane and the secondary sealing membrane and a secondary space between the secondary sealing membrane and the inner shell, the primary space and the secondary space comprising thermally insulating elements,said primary sealing membrane resting directly on the thermally insulating elements contained in the primary space and said secondary sealing membrane resting directly on the thermally insulating elements contained in the secondary space, the process being carried out when the tank is at least partially filled with the liquefied gas, the process comprising the following steps: placing the secondary space at a pressure lower than a threshold pressure P1, the threshold pressure P1 being lower than a pressure in the primary space, measuring a rate of pressure rise in the secondary space over a period of time using a pressure sensor in order to obtain a measurement M1, determining a conformity state of the secondary space as a function of the measurement M1.

[0009] Thanks to these characteristics, the process makes it possible to detect the presence of a local leak in the secondary space. Indeed, in the event of a local leak, the pressure in the depressurized secondary space will not be stable and will increase at a certain rate, a rate that can indicate a non-conformity of the secondary space and therefore a non-conformity of the tank. This rate of pressure increase in the secondary space, and thus this non-conformity, can be due, for example, to a leak in the secondary sealing membrane or to a leak in the piping or the valves located on the piping.

[0010] This process, which involves measuring the rate of pressure rise in the secondary space, makes it possible to detect leaks in the secondary space even when the leakage rate is very low.

[0011] In order to calculate the rate of pressure rise, the pressure sensor takes several pressure measurements over a period of time.

[0012] According to some embodiments, such a process may include one or more of the following characteristics.

[0013] According to one embodiment, determining the conformity state of the secondary space further includes the following step: - determining the secondary space as non-compliant if the measurement M1 is greater than or equal to a maximum threshold speed.

[0014] The process allows for the identification of non-conformities, preventing further damage to the tank. Consequently, corrective measures can be taken to bring the tank into compliance, thereby extending its lifespan.

[0015] According to one embodiment, the process further includes the step: determining the secondary space as compliant if the measurement M1 is less than the maximum threshold speed.

[0016] Indeed, if the pressure rise rate is zero or very low, the secondary space can be considered to meet this criterion.

[0017] According to one embodiment, the time period is between 0.5 and 3 hours, preferably less than or equal to 1 hour.

[0018] In one embodiment, the threshold pressure P1 is less than 95 kPa, and optionally greater than 80 kPa. For example, the threshold pressure P1 is between 85 and 94 kPa or 90 and 92 kPa.

[0019] According to one embodiment, the pressure below the threshold pressure P1 is between 80 and 94 kPa, preferably the pressure below the threshold pressure P1 is 91.3 kPa.

[0020] This pressure is lower than the primary space pressure, which can be at atmospheric pressure, i.e., approximately 101.3 kPa, or slightly suppressed, i.e., 101.9 kPa. For example, when the secondary space pressure is set at 91.3 kPa or between 80 and 94 kPa, it represents a slight negative pressure. Thus, in the event of a non-compliance issue, such as a leak in the secondary sealing membrane, it will be possible to quickly restore the initial pressure in the secondary space so that the inner shell of the tank is not damaged by contact with the low temperatures of the liquefied gas inside the tank. The initial pressure can be restored in the secondary space via a gas inlet line opening into the secondary space. This gas line is also known as the secondary manifold.

[0021] According to one embodiment, the process further comprises the step of placing the secondary space at a pressure lower than a threshold pressure P1, which includes the substeps: - activating a suction device connected to a gas outlet line from the secondary space, and

[0022] -stop the suction device when the secondary space has reached a pressure lower than the threshold pressure P1. At the end of this step, the secondary space has a pressure lower than the pressure of the primary space.

[0023] According to one embodiment, the process further comprising, while the secondary space is at said pressure below the threshold pressure P1, a step of monitoring the temperature of the inner shell, the temperature monitoring step comprising, from a confined space located between the inner shell and the outer shell, the substeps:- measuring a temperature of a point of the inner shell in order to obtain at least one temperature measurement T1 of the point of the inner shell, and- determining the state of conformity of the secondary space as a function of the temperature measurement T1.

[0024] According to one embodiment, the substep determining the conformity state of the secondary space includes: - determining the secondary space as non-compliant if the temperature measurement T1 is below a minimum threshold temperature T min .

[0025] According to one embodiment, the substep "determining the conformity state of the secondary space" further comprises the step: - determining the secondary space as compliant if the measurement M1 is greater than or equal to the maximum threshold speed and the temperature measurement T1 is greater than the minimum threshold temperature T min .

[0026] According to one embodiment, the minimum threshold temperature T min corresponds to a temperature higher than the limit temperature of a grade of steel used for the inner hull at the measured point of the inner hull.

[0027] Thus, the process does not generally measure any cold spots that might be visible on the inner shell of the tank, but rather allows for a precise measurement of whether the measured temperature approaches the steel grade limit at the measured point, in order to detect any non-conformity that could damage the tank. This aspect of the process is particularly advantageous because it takes into account the properties of the material on which the temperature is measured.

[0028] According to one embodiment, the minimum threshold temperature T min corresponds to a temperature within a range between 5°C and 30°C above a limit temperature of a grade of steel used for the inner hull at the measured point.

[0029] According to one embodiment, the minimum threshold temperature T min corresponds to 10°C above a limit temperature of a grade of steel used for the inner hull at the measured point.

[0030] According to one embodiment, the minimum threshold temperature T min is defined from a comparison between the location of the measured point and a database listing steel grades according to their location on the inner hull.

[0031] According to one embodiment, the minimum threshold temperature T min is calculated to correspond to a temperature strictly higher than the limit temperature of the corresponding steel grade.

[0032] According to one embodiment, the inner shell temperature monitoring step further includes measuring several points on the inner shell located at a distance from each other, in order to obtain at least one temperature measurement T1 from each point on the inner shell, and determining the conformity state of the secondary space based on the temperature measurements T1 by comparing them to the minimum threshold temperatures Tmin corresponding to each of the measured points.

[0033] According to one embodiment, if the temperature T1 is lower than the minimum threshold temperature T min Then, the depression in the secondary space is stopped and the initial pressure in the secondary space is restored. Thus, the process includes an additional safety measure to limit damage to the inner shell that could be caused by low temperatures.

[0034] According to one embodiment, the tank comprises metallic anchoring devices fixed to the inner shell for anchoring the thermally insulating elements of the secondary space to the inner shell, and wherein the minimum threshold temperature T min is defined locally according to the positions of the metallic anchoring devices.

[0035] Some tanks have cold zones caused by their very structure. Examples of such tanks include those using the NO96 technology marketed by GTT and described, for instance, in publications FR2264712 and FR2549575. For example, the metallic anchoring devices create thermal bridges that cool the inner shell. These thermal bridges could be considered cold zones if the process did not have a minimum threshold temperature T min defined locally. Thanks to this process it is possible to control the conformity of the secondary space in a strict manner, without being disturbed by any structural cold zones of the tank.

[0036] According to one embodiment, the metallic anchoring devices are chosen from: metallic couplers or metal angle beams.

[0037] According to one embodiment, the step of monitoring the temperature of the inner shell is carried out several times in parallel with the step of placing the secondary space at a pressure lower than the threshold pressure P1 and the step of measuring the rate of rise of the pressure.

[0038] Therefore, additional control points are put in place to improve the safety of the control process.

[0039] According to one embodiment, the pressure below the threshold pressure P1 is a first pressure, the process further comprising the step: - if the tank is not considered non-compliant at the end of the time period, place the secondary space at a second pressure lower than the first pressure, and - carry out the step of monitoring the temperature of the inner shell while the secondary space is at a pressure lower than the first pressure.

[0040] In other words, the monitoring of the internal shell temperature is renewed while the pressure decreases from the first pressure to the second pressure and / or to the second pressure and / or during the step of measuring the rate of pressure rise.

[0041] Thanks to this process, the risk of damaging the tank is significantly reduced. Indeed, the tank was not deemed non-compliant at the end of the testing period. Therefore, it can be concluded that there is no significant leak of liquefied gas that could damage the tank's inner shell. Consequently, the secondary space can be placed at a lower pressure without significant risk of damaging the inner shell.

[0042] According to one embodiment, placing the secondary space at a second pressure lower than the first pressure includes a maximum threshold pressure decrease rate of 30 kPa / h.

[0043] According to one embodiment, the step of placing the secondary space at a second pressure lower than the first pressure is carried out in 3 hours.

[0044] According to one embodiment, the process further includes the step: - carrying out the step of monitoring the temperature of the inner shell at the expiry of a period of 3 hours and / or 17 hours from the start of a decrease in pressure towards the second pressure.

[0045] Therefore, additional control points are put in place to improve the safety of the control process.

[0046] According to one embodiment, the process further includes the step: - carrying out the step of monitoring the temperature of the internal shell at regular intervals, for example every 6 or 12 hours, from the start of a decrease in pressure towards the second pressure.

[0047] According to one embodiment, the second pressure is less than 60 kPa, and optionally greater than 30 kPa, between 40 and 50 kPa, for example 47.5 kPa.

[0048] According to one embodiment, the process further includes the step of measuring the evolution of the pressure in the primary space during the time period.

[0049] Thus, it is possible to determine the origin of a potential leak more precisely. Indeed, if during the specified period the measurement M1 exceeds the maximum threshold velocity and the pressure in the primary space decreases, then it is highly likely that the leak originates from the secondary sealing membrane. If during the specified period the measurement M1 exceeds the maximum threshold velocity and the pressure in the primary space remains constant, then it is highly likely that the leak originates from the piping or the valves located on the piping.

[0050] According to one embodiment, the time period is a first time period and the measurement M1 is a first measurement, the process further includes the step: - measuring a second rate of pressure rise in the secondary space during a second time period using a pressure sensor in order to obtain a measurement M2, said second time period being situated after the secondary space has been placed at the second pressure, and - determining a state of conformity of the secondary space as a function of the measurement M2.

[0051] According to one embodiment, the process further includes the step: - determining the secondary space as compliant if the measurement M2 is less than the maximum threshold speed.

[0052] According to one embodiment, the second time period is between 6 and 14 hours, preferably 10 hours.

[0053] According to one embodiment, the maximum threshold velocity is less than or equal to 0.03 kPa / h.

[0054] For example, with a maximum threshold velocity of 0.03 kPa / h, if the pressure in the secondary space increases by 0.01 kPa over a one-hour period, the secondary space is considered compliant. Conversely, if the pressure in the secondary space increases by, for example, 0.05 kPa over a one-hour period, then the secondary space is not considered compliant.

[0055] For example, with a maximum threshold velocity of 0.03 kPa / h, if the secondary space pressure increases by 0.1 kPa over a 10-hour period, the secondary space is considered compliant because the increase is less than 0.3 kPa / 10 hours. Conversely, if the secondary space pressure increases by, for example, 0.5 kPa over a 10-hour period, then the secondary space is not considered compliant because the increase is greater than 0.3 kPa / 10 hours.

[0056] According to one embodiment, the time period begins after a delay of between 1 and 8 hours, preferably between 2 and 6 hours, for example 4 hours, after the secondary space has been placed at a pressure lower than the threshold pressure P1, after the secondary space has been placed at the second pressure lower than the first pressure.

[0057] This time lag allows the time period to begin when the secondary space pressure is, or should be, stabilized. This significantly reduces false positives that could incorrectly indicate a non-conformity in the tank's secondary space. For example, after reducing the secondary space pressure and stopping the suction, the secondary space pressure may increase slightly. This phenomenon can be explained by the desorption or degassing of insulating materials present in the secondary space.

[0058] In one embodiment, the process further includes the step of measuring the temperature of the internal space, particularly near the secondary membrane. In one embodiment, the step of measuring the temperature of the internal space is carried out after the secondary space has reached the second pressure. A particular embodiment proposes measuring temperature variations over a period of time; if the temperature variation is less than 0.3 °C, then the tank is not considered non-compliant; if the temperature variation is more than 0.3 °C, then the tank is considered non-compliant.

[0059] According to one embodiment, the pressure in the secondary space is measured by a high-precision pressure sensor such as a piezoelectric diaphragm pressure sensor.

[0060] According to one embodiment, the secondary sealing membrane comprises flat metal struts with raised edges welded together in a watertight manner, the metal struts being located on an external surface of the thermally insulating elements contained in the secondary space.

[0061] According to one embodiment, the primary sealing membrane comprises flat metal struts with raised edges welded together in a watertight manner, the metal struts being located on an external surface of the thermally insulating elements contained in the primary space.

[0062] According to one embodiment, the primary sealing membrane does not include any corrugation.

[0063] In one embodiment, the thermally insulating elements contained in the secondary or primary space comprise insulating boxes containing thermally insulating material or insulating panels comprising polymer foam sandwiched between two rigid sheets, for example, plywood. The insulating material is, for example, perlite or mineral wool.

[0064] In one embodiment, the liquefied gas is LNG, namely a mixture with a high methane content stored at a temperature of about -162°C at atmospheric pressure.

[0065] In one embodiment, the process is carried out when the tank is filled with the liquefied gas. In another embodiment, the process is carried out when the liquefied gas occupies at least 20% of the tank's volume, preferably at least 50%, and even more preferably at least 70% of the tank's volume. In another embodiment, the process is carried out when the tank is cooled by spraying a low-temperature liquefied gas, such as liquid nitrogen or LNG, for example, when the internal temperature of the tank is below -130°C, for example -150°C.

[0066] The tank can be made using different techniques, including as an integrated membrane tank or a self-supporting tank.

[0067] Such a tank, in which the process is carried out, can be part of an onshore storage facility, for example for storing LNG, or be installed in a floating structure, whether coastal or deep-water, including an LNG carrier, a floating storage and regasification unit (FSRU), a floating production and storage unit (FPSO), and others. Such a tank can also serve as a fuel tank in any type of vessel. Brief description of the figures

[0068] The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent from the following description of several particular embodiments of the invention, given solely by way of illustration and not limitation, with reference to the accompanying drawings.

[0069] The diagram represents a cross-sectional view of a sealed and thermally insulated tank integrated into the hull of a ship and containing a liquefied gas.

[0070] Laillustre a cutaway, perspective, partial view of an angle of the sealed and thermally insulating tank of the.

[0071] Laillustrates a thermogram of a tank wall, according to one embodiment.

[0072] Laillustre a partial cross-sectional view of the double hull of a ship suitable for constructing the watertight and thermally insulating tank of the, showing grades of steel.

[0073] Laillustre un méthode de contrôle de contrôle de lettre de la espace secondaire dans une cuve le sealed et thermally insulating de stockage d’un gaz liquefié, selon les modes d’incarnations.

[0074] This is a graph representing pressure measurements over a period of time, according to a first scenario of the process.

[0075] This is a graph representing pressure measurements over a period of time, according to a second scenario of the process.

[0076] By convention, the terms "external" and "internal" are used to define the relative position of one element with respect to another, by reference to the inside and outside of the tank.

[0077] With reference to the diagram, a schematic cross-section of a tank 1 of an LNG carrier, constructed using membrane tank technology, is shown. A ship may thus have one or more similar tanks. This tank 1 is intended for the transport and / or storage of liquefied gas at low temperature. Figure 3 illustrates a tank containing liquefied gas.

[0078] A low-temperature liquefied gas is in a vapor state under normal pressure and temperature conditions and is liquefied by lowering its temperature, particularly for transport. Liquefied Natural Gas (LNG) is an example of a low-temperature liquefied gas.

[0079] Tank 1 includes a load-bearing structure providing mechanical rigidity. The load-bearing structure is a double wall comprising an inner shell 2 and an outer shell 3. The inner shell 2 and the outer shell 3 define a confined space 4 whose dimensions are sufficient for human beings to move within it.

[0080] Confined space 4 includes the ballast tanks, tubular keels, cofferdams, walkways and the closing deck of tank 1 also called "trunk deck".

[0081] The load-bearing structure has a generally polyhedral shape. It has two load-bearing walls, forward and aft, 31, here octagonal in shape, of which only the aft load-bearing wall 31 is shown. The forward and aft walls 31 are, for example, cofferdam walls of the ship that extend transversely to the longitudinal direction of the ship. The load-bearing structure also includes an upper load-bearing wall 32, a lower load-bearing wall 33, and lateral load-bearing walls 34, 35, 36, 37, 38, 39.

[0082] The tank 1 further includes a primary sealing membrane 9 intended to be in contact with the product contained in the tank, and a secondary sealing membrane 7 arranged between the primary sealing membrane 9 and the inner shell 2. The secondary sealing membrane 7 is intended to retain the product in the event of a leak in the primary sealing membrane 9. The space between the primary sealing membrane 9 and the secondary sealing membrane 7 is called the primary space 8 and the space between the secondary sealing membrane 7 and the inner shell 2 is called the secondary space 6.

[0083] In the particular embodiment described here, the primary space 8 and the secondary space 6 comprise insulating elements, for example, in the form of juxtaposed insulating boxes filled with thermally insulating material such as perlite. These boxes may be made of plywood. Furthermore, these spaces 6 and 8 also include metal fasteners for, for example, fixing the boxes of the primary space to the secondary waterproof membrane 7 and the boxes of the secondary space to the inner shell 2.

[0084] The primary sealing membrane 9 rests directly on the insulating boxes of the primary space 8, and the secondary sealing membrane 7 rests directly on the insulating boxes of the secondary space 6.

[0085] An example of such a membrane tank on which the process can be carried out is described, for example, in patent application WO2018122498A1 relating to the technology named NO96 developed by the applicant.

[0086] This illustrates more precisely a tank wall in which the process can, for example, be carried out. The tank wall comprises: a secondary thermally insulating barrier 17 positioned in the secondary space 6 and a primary thermally insulating barrier 18 positioned in the primary space 8.

[0087] The primary thermal insulation barrier 18 and the secondary thermal insulation barrier 17 are each made up of heat-insulating elements, and more specifically of parallelepiped-shaped insulating boxes 19 which are juxtaposed in a regular pattern. Various techniques are known for making such heat-insulating elements.

[0088] The secondary 7 and primary 9 waterproof membranes illustrated with las are made up of a series of parallel metal plates called struts 15 with folded edges, which are arranged alternately with elongated weld supports. The struts 15 and the weld supports are made of an alloy with a low coefficient of thermal expansion.

[0089] The insulating boxes 19 located in the secondary space 6 are fixed to the inner shell 2 by metallic anchoring devices called metallic couplers 11.

[0090] In addition, connecting metal beams 12 are located at the corners of the tank walls. These connecting metal beams 12 include, in particular, a first anchoring metal flange 13 and a second anchoring metal flange 14, enabling anchoring to the inner shell 2.

[0091] When the tank contains liquefied gas, the metal couplers 11 and the metal connecting beams 12 create thermal bridges which can generate structural cold zones which may be visible on the inner shell 2, from the confined space 4. An example of the structural cold zones is illustrated with the.

[0092] Figure 1 illustrates a thermogram measured on the external surface of the inner shell 2 when the tank is filled with LNG. Rectangle 20 represents a cold zone due to thermal bridges formed by the connecting metal beam 12 of the secondary space 6. The circles 21 represent cold zones due to thermal bridges formed by the metal couplers 11 of the secondary space 6.

[0093] These cold areas are not anomalies and should therefore not be considered as a non-conformity of the tank during the execution of the process.

[0094] Furthermore, as illustrated with the, the inner hull 2 ​​and the outer hull 3 can be made from different grades of steel depending on the location of the inner hull 2 ​​or the outer hull 3. The references "A", "B", "D", "E" correspond respectively to a grade A, B, C and E steel, according to the IGC code ("International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk"), 2016 edition, §6.4 "Requirements for metallic materials".

[0095] For example, the inner shell 2 at the lower load-bearing wall 33 is made of a grade "A" steel alloy, while the inner shell 2 at the upper load-bearing wall 32 is made of a grade "E" steel alloy, which does not have the same physicochemical properties as grade "A", "B", or "D" and cannot withstand the same temperatures.

[0096] Therefore, it is important that the conformity control process can take into account this particularity of the internal shell 2.

[0097] It is illustrated below, with a method for checking the tightness of a secondary space in a sealed and thermally insulated liquefied gas storage tank such as the tank shown in Figures 1 to 4, according to several embodiments.

[0098] The process is carried out when the tank contains liquefied gas such as LNG and when the primary space is substantially at atmospheric pressure, i.e. about 101 kPa or slightly suppressed, for example at 102 kPa.

[0099] In the case of a tank considered compliant, the procedure comprises the following steps. First, place the secondary space at an absolute pressure of 91.3 kPa by activating a vacuum pump connected to a gas outlet line from secondary space 6. Stop the vacuum pump when the secondary space reaches this pressure; this step (step 100) is illustrated in Figure 1. At this stage, no opening or passage intentionally connects secondary space 6 to the outside. The pressure in secondary space 6 must therefore be stable, subject to any potential desorption or degassing of the insulating materials present in secondary space 6. Next, measure the rate of pressure rise in the secondary space over a period of approximately 1 hour (step 101). Optionally, this period can be between 0.5 and 3 hours.The time period begins when the pressure in the secondary space reaches 91.3 kPa or after a pressure stabilization period has elapsed. In other words, we passively observe whether and how the pressure evolves in secondary space 6 using pressure sensors. If the pressure in secondary space 6 remains stable or increases by less than 0.03 kPa / h during the time period, here 1 hour, then decrease the pressure in secondary space 6 to 47.5 kPa by activating the vacuum pump connected to the gas outlet line of secondary space 6, and then stop the vacuum pump when the pressure of 47.5 kPa is reached (step 102). The pressure decrease in secondary space 6 is gradual, with a maximum threshold rate of 30 kPa / h.Next, measure the temperatures of the inner shell 2 from the confined space 4 for the first time three hours from the start of the pressure decrease to a pressure of 47.5 kPa, step 103. Compare the measured temperatures respectively with the minimum threshold temperature T. min which corresponds to a temperature higher than the limit temperature of the steel grade at each of the measured points, for example, a limit temperature of the steel grade increased by 10°C, step 104. If the measured temperatures are higher than the minimum threshold temperature T min, effectuer une deuxième mesure des températures 14 heures après la première mesure des températures, étape 105. Si les températures de la deuxième mesure des températures sont supérieures à la température seuil minimale Tmin, alors effectuer une mesure récurrente des températures toutes les 12 heures à partir de la deuxième mesure des températures, étape 106. Après que la pression dans l’espace secondaire 6 ait atteint 47,5 kPa et en parallèle des étapes 103, 104, 105, 106 précitées, sélectionner une période de temps, par exemple une période de temps de 10 heures, période de temps dans laquelle l’évolution de la pression est la plus stable. Par exemple, la période de temps sélectionnée peut débuter après 4 heures ou 8 heures, après que la pression dans l’espace secondaire 6 a atteint 47,5 kPa. Ensuite, mesurer la vitesse de remontée de la pression dans la période de temps sélectionnée, étape 111. En d’autres termes, on observe passivement si et comment la pression évolue dans l’espace secondaire 6 au moyen de capteurs de pression. Si la pression dans l'espace secondaire 6 reste stable ou augmente de moins de 0,3 kPa / 10h durant la période de temps choisie, alors la cuve n’est pas considérée non conforme.

[0100] In the case of a tank scenario with anomalies, the process includes the following steps. Steps 100 and 101 mentioned above are identical.

[0101] If during the step of measuring the rate of rise of pressure in secondary space 101, the pressure in secondary space 6 increases by more than 0.3 kPa during the selected 10-hour period, step 107, then perform a measurement of the temperatures of the inner hull 2 ​​from the confined space 4, step 108.

[0102] If a cold spot is detected at step 108, i.e., a cold spot not expected due to the structure of the tank, then the tank is considered non-compliant and the process is stopped, step 109. To detect a cold spot, one can proceed by comparing the limit temperature of the corresponding steel grade, as illustrated on the.

[0103] If the temperatures measured during step 108 are higher than the minimum threshold temperature T min , then continue the process with steps 102, 103 and 104 mentioned above.

[0104] If during step 104 the measured temperatures are below the minimum threshold temperature T min , then the tank is not considered compliant and the process is stopped, step 109.

[0105] If during step 104 the measured temperatures are higher than the minimum threshold temperature T min , then continue the process with steps 105 and 106 mentioned above.

[0106] If, during any of the steps 103 to 106, the measured temperatures are higher than the minimum threshold temperature T min but vary abnormally, then proceed with repeated temperature measurements, for example every six and / or twelve hours. If the measured temperatures are below the minimum threshold temperature T min then stop the process, step 109.

[0107] In addition, and optionally or mandatorily, measure in parallel the pressure of the primary space 8.

[0108] If, during step 101 or 111, the pressure in secondary space 6 increases by more than 0.03 kPa over a 1-hour period for step 101, or by more than 0.3 kPa over a 10-hour period for step 111, and if the pressure in primary space remains unchanged, then there is likely a leak in the piping or the valves on the piping. The process is stopped, and steps 110 are taken. Investigations are then carried out to correct the leak and restart the process. After the leak has been corrected, the process can resume at step 100.

[0109] If, during step 101 or 111, the pressure in secondary space 6 increases by more than 0.03 kPa over the 1-hour period for step 101, or by more than 0.3 kPa over the 10-hour period for step 111, and if the pressure in primary space decreases, then there is possibly a leak in the secondary sealing membrane. In other words, over the considered time period, the average flow rate exceeded 0.03 kPa / h. In this case, continue the process with step 108.

[0110] This is illustrated with an example graph obtained according to a first scenario of the process. The y-axis on the left of the graph represents the pressure in kilopascals (kPa). The x-axis represents time expressed in hours.

[0111] Segment 200, containing the two arrows, represents the 10-hour period during which the pressure evolution is considered most stable, and which is used to measure the rate of pressure rise. This average rate of pressure rise over this period is compared to the maximum threshold rate S. max of 0.3 kPa / 10h, or 0.03 kPa / h on average.

[0112] Curve 201 represents the pressure in secondary space 6 according to a first scenario. It illustrates that the secondary space was initially set at approximately 47.3 kPa and that the pressure increased to 47.8 kPa between 8:10 AM and 4:10 PM, i.e., over 8 hours. Such a pressure increase could, for example, be due to the degassing of the insulating elements present in the secondary space. During the chosen 10-hour time period 200, the pressure in the internal space changes from 47.8 kPa at 4:10 PM to 47.84 kPa at 2:10 AM, representing a change of less than 0.3 kPa over the 10-hour period 200. Thus, the tank is considered compliant.

[0113] This is illustrated with an example of a graph obtained according to a second scenario of the process. The y-axis on the right of the graph represents the pressure differential in kilopascals (kPa). Curve 301 represents the pressure in secondary space 6 according to the second scenario. It is shown that the pressure in the secondary space increases between 5 PM and 9 PM and that this increase continues over the chosen 10-hour period, i.e., between 9 PM and 7 AM. At 9 PM, the pressure in the secondary space is 52.6 kPa. This pressure at 9 PM evolves until it reaches 53.43 kPa, representing a pressure increase of 0.83 kPa over the 10-hour period. This rate of pressure rise is greater than the maximum threshold rate S max of 0.3 kPa / 10h. Consequently, in this scenario, the tank is considered non-compliant.

[0114] To carry out the process, the equipment used is chosen, for example, from: pressure sensors, temperature sensors, piping, valves, a vacuum pump, a control device…

[0115] Although the invention has been described in connection with several particular embodiments, it is clearly evident that it is by no means limited to them 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.

[0116] The use of the verb "comporter", "comprendre" or "include" and its conjugated forms does not exclude the presence of other elements or steps than those stated in a claim.

[0117] In claims, any reference sign in parentheses should not be interpreted as a limitation of the claim.

Claims

Method for checking the leak-tightness of a secondary space in a sealed and thermally insulated liquefied gas storage tank, the tank (1) comprising a load-bearing structure having an inner shell (2) and an outer shell (3), a primary sealing membrane (9) intended to be in contact with the liquefied gas contained in the tank (1), and a secondary sealing membrane (7) arranged between the primary sealing membrane and the inner shell (2), a primary space (8) between the primary sealing membrane (9) and the secondary sealing membrane (7) and a secondary space (6) between the secondary sealing membrane (7) and the inner shell (2), the primary space (8) and the secondary space (6) comprising thermally insulating elements (19),said primary sealing membrane (9) resting directly on the thermally insulating elements (19) contained in the primary space (8) and said secondary sealing membrane (7) resting directly on the thermally insulating elements (19) contained in the secondary space (6), the process being carried out when the tank (1) is at least partially filled with the liquefied gas (30), the process comprising the following steps: - placing the secondary space (6) at a pressure lower than a threshold pressure P1 (100) by activating a suction device connected to a gas outlet line from the secondary space and then stopping the suction device when the secondary space has reached a pressure lower than the threshold pressure P1, the threshold pressure P1 being lower than a pressure of the primary space (8),then- measure the rate of pressure rise in the secondary space over a period of time (101) using a pressure sensor to obtain a measurement M1,- determine a conformity state of the secondary space as a function of the measurement M1, the method further comprising, while the pressure of the secondary space (6) is below the threshold pressure P1, a temperature monitoring step (103, 104, 108) of the inner shell (2), the temperature monitoring step comprising, from a confined space located between the inner shell (2) and the outer shell (3), the substeps:- measuring a temperature of a point on the inner shell (2) to obtain at least one temperature measurement T1 of the point on the inner shell (2), and- determining the conformity state of the secondary space (6) as a function of the temperature measurement T1. Method according to claim 1, wherein determining the conformity state of the secondary space further comprises the following step: - determining the secondary space as non-conforming if the measurement M1 is greater than or equal to a maximum threshold speed. Method according to claim 2, the method further comprising the step:- determine the secondary space as conforming if the measurement M1 is less than the maximum threshold speed. A method according to any one of claims 1 to 3, wherein the time period is between 0.5 and 3 hours. A method according to any one of claims 1 to 4, wherein the threshold pressure P1 is less than 95 kPa. A method according to any one of claims 1 to 5, wherein the substep determining the conformity state of the secondary space comprises: - determining the secondary space as non-conforming if the temperature measurement T1 is below a minimum threshold temperature T m in. The method according to claim 6, wherein the substep for determining the conformity state of the secondary space further comprises: - determining the secondary space (6) as conforming if the measurement M1 is greater than or equal to the maximum threshold speed and the temperature measurement T1 is greater than the minimum threshold temperature T m in . A method according to any one of claims 6 to 7, wherein the minimum threshold temperature T m in corresponds to a temperature higher than the limit temperature of a grade of steel used for the inner hull (2) at the measured point of the inner hull (2). A method according to any one of claims 6 to 8, wherein the tank (1) comprises metallic anchoring elements fixed to the inner shell (2) for anchoring the thermally insulating elements of the secondary space to the inner shell (2), and wherein the minimum threshold temperature T m inis defined locally according to the positions of the metallic anchoring devices. A method according to any one of claims 1 to 9, wherein the step of monitoring the temperature of the inner shell is carried out several times in parallel with the step of placing the secondary space at a pressure lower than the threshold pressure P1 and the step of measuring the rate of pressure rise. A method according to any one of claims 1 to 10, wherein the pressure below the threshold pressure P1 is a first pressure, the method further comprising the step: - if the tank (1) is not considered non-compliant at the end of the time period, place the secondary space (6) at a second pressure below the first pressure, and - carry out the step of monitoring the temperature of the inner shell (2) while the secondary space (6) is at a pressure below the first pressure. Method according to claim 11, further comprising the step: - carrying out the step of monitoring the temperature of the inner shell (2) at regular intervals, for example every 6 or 12 hours, from the start of a decrease in pressure towards the second pressure. A method according to any one of claims 11 to 12, wherein the second pressure is less than 60 kPa, preferably less than 50 kPa, for example 47.5 kPa. A method according to any one of claims 11 to 13, wherein the time period (101) is a first time period and the measurement M1 is a first measurement, the method further comprising the step: - measuring a second rate of pressure rise in the secondary space during a second time period using a pressure sensor in order to obtain a measurement M2, said second time period being situated after the secondary space (6) has been placed at the second pressure, and - determining a conformity state of the secondary space as a function of the measurement M2. A method according to claim 14, the method further comprising the step: - determining the secondary space as compliant if the measurement M2 is less than the maximum threshold speed x . A method according to claim 15, wherein the second time period is between 6 and 14 hours, preferably 10 hours. A method according to any one of claims 1 to 16, wherein the maximum threshold velocity is less than or equal to 0.03 kPa / h.

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