Leak test method for sealed insulated tanks for storing fluids

Abstract

The present invention relates to a method for leak testing of a sealed, insulated tank for storing cryogenic liquefied gas, the tank comprising a primary space (8) and a secondary space (6), wherein a pressure difference is generated between the two spaces (6 and 8) to conduct the test, and wherein at least a portion of the inert gas injected into the primary space (8) is withdrawn via at least one secondary outlet (19, 20 or 23) of the secondary space (6) for reinjection into the primary space (8).

Description

[Technical Field]

[0001] The present invention relates to the field of sealed, insulated tanks comprising a membrane. Specifically, the present invention relates to the field of sealed, insulated tanks for storing and / or transporting cryogenic liquids, such as tanks for transporting liquefied petroleum gas (also known as LPG) having a temperature between -50°C and 0°C, or tanks for transporting liquefied natural gas (LNG) at atmospheric pressure at approximately -162°C. These tanks may be located on land or on floating structures. In the case of floating structures, the tank may be for transporting cryogenic liquefied gas or for containing liquefied gas used as fuel for propelling the floating structure. More specifically, the present invention relates to an apparatus and method for detecting leaks in the secondary membrane of such tanks. [Background technology]

[0002] Tanks on ships for transporting liquefied natural gas generally comprise a carrier structure providing mechanical rigidity, a primary sealing membrane intended to be in contact with the product contained in the tank, and a secondary sealing membrane arranged between the primary sealing membrane and the carrier structure. The secondary sealing membrane is intended to retain the product in the event of a leak in the primary sealing membrane. The space between the primary and secondary membranes is called the primary space, and the space between the secondary sealing membrane and the carrier structure is called the secondary space.

[0003] The secondary sealing membrane becomes invisible and inaccessible after the tank is manufactured, and therefore any defects in the membrane, such as scratches in the membrane, localized dislocations in the membrane, or voids between the two components from which it is made, cannot be directly observed.

[0004] Known references relating to diagnosing the sealing state of a secondary membrane, in particular WO 2020128370 filed in the name of the applicant, describe a method for testing the sealing state of this secondary membrane, in which the presence of a leak in the secondary sealing membrane is detected in particular by applying a higher inert pressure to the primary insulating space compared to the pressure in the secondary insulating space.

[0005] The inert gas used in this manner is typically nitrous oxide, however, although this gas is stored in limited quantities on board ships, it is also often used on board ships whenever there is a need to circulate the inert gas.

[0006] Now, the applicant has found that if a leak in the secondary sealing membrane does exist, the amount of inert gas required for the leak detection operation will become very large, even to the extent that the ship's inert gas supply will be completely depleted during the operation to test the tank's sealing, since once the inert gas is used for each test operation, it will be released into the ambient air.

[0007] At present, there is no method for testing the seal of secondary sealing membranes with reduced amounts of inert gas, or for testing the seal with a neutral balance sheet with respect to inert gas. Summary of the Invention [Problem to be solved by the invention]

[0008] The concept on which the present invention is based is to provide a device and a method for detecting leaks in a sealed, insulated tank filled with a cryogenic liquid that does not have these drawbacks. The object of the present invention is thus to be able to locate abnormal voids in the secondary sealing membrane, even in the case of very high leakage rates. [Means for solving the problem]

[0009] The present invention therefore relates to a method for checking the tightness of a sealed, insulated tank for storing cryogenic liquefied gas, the tank being of the cryogenic type, the tank comprising a carrier structure with an inner shell and an outer shell, a closed space between the inner shell and the outer shell, a primary sealing membrane intended to be in contact with the cryogenic liquefied gas contained in the tank, 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 containing a thermal insulating material, the primary space containing at least one primary gas inlet and the secondary space containing at least one secondary gas outlet, the primary sealing membrane resting directly on the thermal insulating material contained in the primary space, and the secondary sealing membrane resting directly on the thermal insulating material contained in the secondary space, the method comprising the following successive steps for detecting the location of a sealing defect of the secondary sealing membrane in the form of a cold spot occurring on the outer surface of the inner shell: a step called the main step, in which an inert gas is injected into the primary space through the primary gas inlet, and then the gas is released or sucked through the secondary gas outlet of the secondary space, and the pressure in the secondary space is reduced by a pressure difference P1 below the pressure in the primary space by measuring the temperature of the outer surface of the inner shell from a closed space located around the inner shell; and measuring the temperature of the outer surface of the inner shell from the enclosed space under normal operating conditions of the tank, referred to as the subsequent step.

[0010] The present invention is characterized in that at least a portion of the inert gas injected into the primary space is recovered by at least one secondary outlet of the secondary space, and the gas is re-injected into the primary space.

[0011] The applicant therefore proposes a simple, effective and inexpensive system for carrying out leak tests on secondary membranes of tanks for liquefied gases without wasting or reducing the amount of inert gas used, which allows the test method to be carried out at all times regardless of the sealing condition of the secondary sealing membrane.

[0012] In particular, after several trials, the applicant has found a structure that is at once simple, robust and efficient for recycling the inert gas used during the performance of the method according to the invention.

[0013] The expression "cryogenic liquefied gas" is intended to be understood as any substance that is in the vapor state under normal conditions of temperature and pressure and that becomes liquid by lowering its temperature.

[0014] The term "enclosed spaces" is intended to be understood as tank ballast, also called "trunk deck", duct keels, cofferdams, gangways and enclosed bridges.

[0015] The expression "cryogenic tank" refers to a tank in which cryogenic liquefied gas occupies at least 20%, preferably 70%, of the volume of the tank, or a tank that does not contain a significant amount of liquefied gas, or a tank that contains a volume of liquefied gas that is less than 20% of the volume of the tank, but in which case the tank is cooled by spraying or injection of cryogenic liquefied gas, for example liquefied nitrogen or LNG.

[0016] The expression "normal operating conditions of the tank" is intended to be understood as referring to the conditions under which the tank is normally used, in which the pressure in the primary space is usually slightly lower than the pressure in the secondary space, for example by a few mbar (millibar), i.e. 2-7 mbar, or in rarer cases the pressure in the primary space may be higher than the pressure in the secondary space, for example by 0-4 mbar.

[0017] In the following, the invention is exemplified by using a vessel for storing and transporting liquefied gas of the LNGC (Liquefied Natural Gas Carrier) type, which is typically equipped with four sealed insulated tanks for storing liquefied gas, although the invention is not limited to this embodiment.

[0018] Further advantageous features of the present invention are briefly described below.

[0019] According to one embodiment, the entire amount of inert gas injected into the primary space is withdrawn by at least one suction means connected to a secondary outlet of the secondary space before being re-injected into said space.

[0020] According to another embodiment, only a portion of the inert gas injected into the primary space, advantageously a portion corresponding to 20% to 80% of the inert gas injected into the primary space, is withdrawn by at least one suction means connected to a secondary outlet of the secondary space and then re-injected into said space.

[0021] Advantageously, the method according to the invention comprises a step, called a preliminary step, of measuring the temperature of the outer surface of the inner shell from the enclosed space under normal operating conditions of the tank.

[0022] Advantageously, the pressure difference P1 is: - in the range of 500 Pa to 1500 Pa, preferably in the range of 800 Pa to 1200 Pa, for a duration of at least 10 hours, preferably for a duration of at least 12 hours, or It is in the range of -1800 Pa to 3200 Pa, and preferably in the range of 2100 Pa to 2900 Pa.

[0023] Advantageously, the main step is preceded by a step called an intermediate step, in which an inert gas is injected into the primary space through the primary gas inlet, and then the gas is released or sucked through the secondary gas outlet of the secondary space, and the pressure in the secondary space is made lower than the pressure in the primary space by a pressure difference of 500 Pa to 1500 Pa, preferably 800 Pa to 1200 Pa, by measuring the temperature of the outer surface of the inner shell from a closed space located around the inner shell.

[0024] Preferably, the inert gas comprises nitrous oxide.

[0025] Advantageously, each temperature measurement lasts for a maximum of 5 hours, preferably a maximum of 3 hours, to prevent the inner shell from cooling too significantly, usually below the temperature allowed by the material of which it is made.

[0026] Preferably, the pressure difference P1 in the step advantageously called the intermediate step is kept stable during the temperature measurement, so that the pressure P1 and the pressure P1 in the step called the intermediate step are preferably maintained at values ​​within + / - 5% of the respective defined pressure ranges.

[0027] The method according to the present invention may further comprise the steps of holding the pressure in the primary space and the secondary space constant, measuring the flow rate of the inert gas injected into the primary space, measuring the flow rate of the inert gas exiting the secondary space, and measuring any potential voids in the secondary membrane by comparing the flow rate of the inert gas injected into the primary space with the flow rate of the inert gas exiting the secondary space to identify and quantify the flow rate of the inert gas passing through the membrane.

[0028] On ships, tanks have structures known as vapor or liquid domes, for example on their upper walls. These domes may be in the form of two towers intended to allow the passage of cargo handling equipment handling the liquid and vapor phases of the cryogenic liquefied gas contained in the tank. As a result of this geometry, methods for detecting leaks based on observing abnormally hot or cold zones may not work, especially as a result of the influence of external climatic conditions and because the temperature fluctuations in and near these towers can be very complex. Therefore, by adding flow meters in the area of ​​the primary gas inlet and secondary gas outlet to monitor the flow rates of inert gas entering the primary space and leaving the secondary space, and comparing these flow rates, it is possible to detect whether a leak actually exists in the tank.

[0029] According to one embodiment, a pre-step can be performed after the main step, which allows for the location of potential leaks in the gas and liquid domes to be identified if temperature measurements have not detected leaks elsewhere.

[0030] According to one embodiment, the step of measuring the flow rate is performed simultaneously with the main steps, including intermediate steps as needed, which can reduce the overall running time of the leak detection method and reduce the consumption of inert gas over the entire running time of the method.

[0031] According to one embodiment, the step of measuring the flow rate of the inert gas injected into the primary space is performed at the primary gas inlet by a flow meter.

[0032] According to one embodiment, the step of measuring the flow rate of the inert gas leaving the secondary space is carried out at the secondary gas outlet by a flow meter.

[0033] According to one embodiment, only the primary gas inlet and the secondary gas outlet are open, the other gas inlets and other gas outlets are closed.

[0034] According to one embodiment, the primary gas inlet is located on the liquid dome.

[0035] According to one embodiment, the secondary gas outlet is located on the gas dome.

[0036] This allows the inert lines, which are also provided in the dome, to be used in these leak detection methods to inert the primary and secondary spaces.

[0037] According to one embodiment, the inert gas is selected from helium, argon and mixtures thereof, with or without nitrous oxide present in the mixture.

[0038] According to one embodiment, the cryogenic liquefied gas is selected from liquefied natural gas (LNG), liquefied petroleum gas (LPG), liquefied ethane, liquefied propane, liquefied nitrogen, liquefied dioxygen, liquefied argon, liquefied xenon, liquefied neon and liquefied hydrogen.

[0039] Such methods may be used on floating structures located onshore or in deep waters, in particular liquefied natural gas tankers, floating storage and regasification units (FSRUs), floating production storage and offloading units (FPSOs), etc. Such storage facilities may also function as fuel tanks on any type of vessel.

[0040] The present invention further relates to a floating or land-based cryogenic liquefied gas storage facility for carrying out the method for inspecting the tightness of a sealed, insulated tank as briefly described hereinabove, the floating or land-based storage facility comprising: a cryogenic tank comprising a carrier structure having an inner shell and an outer shell, the space between which is called the closed space, a primary sealing membrane intended to be in contact with the cryogenic liquefied gas contained in the tank, and a secondary sealing membrane arranged between the primary sealing membrane and the inner shell, the space between the primary sealing membrane and the secondary sealing membrane being called the primary space and the space between the secondary sealing membrane and the inner shell being called the secondary space, the primary space and the secondary space comprising insulating material, the primary space comprising at least one primary gas inlet and the secondary space comprising at least one secondary gas outlet, the primary sealing membrane resting directly on the insulating material contained in the primary space and the secondary sealing membrane resting directly on the insulating material contained in the secondary space; an inert gas reservoir configured to inject an inert gas into the primary space through the primary gas inlet; an injection device capable of injecting inert gas from an inert gas reservoir via a primary gas inlet, so that the primary space is under a higher pressure than the secondary space; a suction device connected to the secondary gas outlet for generating a lower pressure in the secondary space than in the primary space; - a device for measuring the temperature of the outer surface of the inner shell; a system for displaying temperature measurements in order to identify the location of sealing defects of the secondary sealing membrane in the form of cold spots occurring on the outer surface of the inner shell.

[0041] The invention is characterized in that the suction device comprises at least one suction means, preferably consisting of a pump, which suctions at least part of the inert gas from the secondary space and reinjects it into the primary space, preferably using the injection device described above.

[0042] According to one embodiment of the present invention, the aforementioned suction means form the only suction means of the device, so that all of the inert gas recovered in the secondary space is reinjected into the primary space.

[0043] According to another embodiment of the invention, the suction device further comprises at least one suction system that withdraws inert gas but does not reinject the inert gas into the primary space.

[0044] Very advantageously, the suction system is a Venturi effect suction system comprising a main pipe having an inlet connectable to a pressurized gas source and an outlet towards the outside of the tank, and a suction pipe having an upstream side connectable to an outlet port of the secondary space and a downstream side opening laterally into a converging / diverging section of the main pipe so that gas flow in the main pipe creates a reduced pressure in the suction pipe.

[0045] Of course, the suction system may equally consist of conventional pumps, i.e., pumps that operate from an electrical power supply, but are not Venturi pumps. It may likewise be realized that the suction system includes one or more so-called conventional pumps and one or more Venturi pumps, and these various pumps may be operated together or in other ways, depending on the choice of the operator performing the operation in relation to environmental conditions and / or other factors.

[0046] Advantageously, the source of pressurized gas is a compressed air circuit, such sources being typically found on ships carrying hydrocarbons, i.e., more generally, flammable or explosive substances.

[0047] Preferably, the injection device comprises a compressor capable of injecting inert gas from an inert gas reservoir at a pressure of 3 to 8 bar.

[0048] According to one embodiment, the primary sealing membrane has a thickness of 2.5 mm (millimeters) or less, for example a thickness of 1.5 mm or less.

[0049] According to one embodiment, the secondary sealing membrane has a thickness of 1.5 mm (millimeters) or less, for example a thickness of 1.2 mm or less.

[0050] According to one embodiment, the suction device comprises a Venturi effect suction system including a main pipe having an inlet connectable to a pressurized gas source and an outlet towards the outside of the tank, and a suction pipe having an upstream side connectable to an outlet port of the secondary space and a downstream side opening laterally into a converging / diverging section of the main pipe such that gas flow in the main pipe creates a reduced pressure in the suction pipe.

[0051] According to one embodiment, the suction device includes multiple Venturi effect suction systems, preferably arranged in series to increase suction capacity.

[0052] According to one embodiment, the Venturi suction system is arranged in a stepped fashion.

[0053] According to one embodiment, the temperature measuring device is a photodetector.

[0054] According to one embodiment, the light detector is a camera with an infrared sensor.

[0055] According to one embodiment, the infrared sensor is cooled using cryogenic techniques, in particular using Peltier effect techniques. Nevertheless, other techniques can be envisaged, for example the sensor is enclosed in a chamber or a Dewar flask or is cooled using a device using the Stirling effect. By reducing the temperature of the sensor in this way, thermal noise can be reduced.

[0056] According to one embodiment, the present invention provides a vessel for transporting cryogenic liquefied gas, comprising a floating storage facility as described above.

[0057] According to one embodiment, the present invention further provides a method of loading and unloading such a vessel, wherein cryogenic liquefied gas is transported through an insulated pipeline from a floating or onshore storage facility to the vessel's tanks or from the vessel's tanks to the floating or onshore storage facility.

[0058] According to one embodiment, the present invention further provides a cryogenic liquefied gas transfer system comprising a vessel as described above, an insulated pipeline arranged to connect a tank installed in the vessel's hull to a floating or onshore storage facility, and a pump for drawing a flow of cryogenic liquefied gas through the insulated pipeline from the offshore or onshore storage facility to the vessel's storage facility or from the vessel's storage facility to the offshore or onshore storage facility. [Brief explanation of the drawings]

[0059] The invention will be better understood, and other objects, details, features and advantages will become more clearly apparent, from the following description of some particular embodiments thereof, given by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0060] [Figure 1] 1 is a schematic cutaway view of a marine tank.

[0061] [Figure 2] 1 is a functional diagram of a ship's tank in a cross section along the longitudinal axis of the ship; FIG.

[0062] [Figure 3] 1 is a schematic diagram of the method of the present invention.

[0063] [Figure 4] 1 is a schematic diagram showing a first embodiment of a nitrous oxide flow circuit according to the present invention within a storage facility. FIG.

[0064] [Figure 5] FIG. 2 is a schematic diagram showing a second embodiment of a nitrous oxide flow circuit according to the present invention within a storage facility.

[0065] [Figure 6] FIG. 1 is a schematic diagram showing a third embodiment of a nitrous oxide flow circuit according to the present invention within a storage facility.

[0066] [Figure 7] FIG. 1 is a schematic diagram showing the arrangement of a venturi effect suction system for a secondary space.

[0067] [Figure 8] FIG. 8 is an enlarged cross-sectional view of zone IV in FIG. 7.

[0068] [Figure 9] 1 is a schematic cutaway view of a cryogenic liquefied gas storage facility for a liquefied natural gas tanker and a terminal for loading / unloading tanks of the cryogenic liquefied gas storage facility. DETAILED DESCRIPTION OF THE INVENTION

[0069] Referring to Figure 1, a cross section of a tank 1 of a liquefied natural gas tanker, manufactured according to membrane tank technology, is shown diagrammatically. A ship may thus be equipped with one or several such tanks. This tank is intended to transport a cryogenic liquefied gas 30. The cryogenic liquefied gas is in a vapor state under normal temperature and pressure conditions and is converted to a liquid state, in particular for its transportation, by lowering the temperature of the gas. This cryogenic liquefied gas may be liquefied natural gas, liquefied petroleum gas, liquefied ethane, liquefied propane, liquefied nitrogen, liquefied dioxygen, liquefied argon, liquefied xenon, liquefied neon or liquefied hydrogen.

[0070] The tank 1 comprises a carrier structure that provides mechanical rigidity. The carrier structure is double-walled, comprising an inner shell 2 and an outer shell 3. The inner shell 2 and the outer shell 3 define an enclosed space 4 having sufficient dimensions to allow a person to move within it.

[0071] The enclosed space 4 is a combination of the ballast, duct keel, cofferdam, gangway and enclosed bridge of the tank 1, also known as the "trunk deck".

[0072] The tank 1 further comprises 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 referred to as the primary space 8, and the space between the secondary sealing membrane 7 and the inner shell 2 is referred to as the secondary space 6.

[0073] The primary space 8 and the secondary space 6 contain insulating material in the form of juxtaposed panels of insulating material. These panels may be foam or porous synthetic resin or another natural or synthetic insulating material. Additionally, these spaces 6, 8 contain filler material, such as glass wool or mineral wool, which may be intended to be inserted between the juxtaposed panels.

[0074] The primary sealing membrane 9 rests directly on the insulating material of the primary space 8 , and the secondary sealing membrane 7 rests directly on the insulating material of the secondary space 6 .

[0075] 2 and 3, the tank has an upper wall 14, which is interrupted in two places by two protruding structures in the form of towers or chimneys. These protruding structures are intended to allow the passage of handling equipment for handling the liquid and vapor phases of the cryogenic liquefied gas for storage in the tank. The first tower is a liquid dome 15, which serves as the introduction point for the various elements of the treatment installation, namely, in the example shown, the filling line 10, the emergency pumping line 11, the unloading line connected to the unloading pump 12, the spray line (not shown), and the supply line connected to the spray pump 13. The second tower is a vapor dome 21, which serves as the introduction point for the vapor recovery pipes. The operation of this installation is further known.

[0076] The primary space 8 includes a primary gas inlet 18 and a primary gas outlet 26. The primary space 8 may further include a second primary gas inlet 22. The secondary space 6 includes a secondary gas inlet 25 and a secondary gas outlet 19. The secondary space 6 may further include a second secondary gas outlet 20 and a third secondary gas outlet 23.

[0077] The tank may further comprise a safety valve 24 in case of overpressure in the primary space 8 and the secondary space 6 .

[0078] The secondary sealing membrane 7 is invisible and inaccessible after the tank is manufactured. The method for checking the sealing of a tank according to the invention makes it possible to detect and locate defects in the secondary sealing membrane 7 and is therefore suitable for most tank technologies.

[0079] With reference to Figure 1 or Figure 3, the method is based on using the temperature gradient created between the primary space 8 and the secondary space 6 of a tank 1 filled with a cryogenic liquefied gas 30 to detect by thermal imaging or thermography the effect on the inner shell 2 of an inert gas passing through a secondary sealing membrane 7. The tank is filled with a cryogenic liquefied gas or cooled by spraying with a cryogenic liquid to at least 20% of its total capacity.

[0080] This figure 3 shows one of the main aspects of the testing method according to the invention, namely how at least part of the inert gas injected into the primary space 8 is re-injected by the action of the suction means 80, which in this example is a conventional pump, by collecting some or all of the inert gas injected into the primary space 8 at the secondary outlets 19, 20 or 23 of the secondary space 6 and conveying it through pipes, thereby recirculating the inert gas into the primary space 8 via one of the primary inlets 18 or 22. In figures 3 to 6, valves 40 are arranged on the pipes in order to adjust the flow rate or stop the flow in the relevant pipes.

[0081] 4-6 show configurations for achieving such inert gas recirculation, it being understood that these embodiments are non-limiting examples that can be further combined.

[0082] 4, an injection device 45 is installed at the primary gas inlet 18 and attached to the inert gas reservoir 16. The inert gas is, for example, nitrous oxide. This injection device 45 makes it possible to inject the inert gas into the primary space 8. A suction device 80 is also installed in the region of the secondary gas outlet 19.

[0083] This suction means 80 has two main functions: firstly, to make it easier to create a pressure difference between the primary space 8 and the secondary space 6, even if the porosity of the secondary sealing membrane 7 is quite significant, and secondly, to recirculate all or part of the nitrous oxide injected into the primary space 8. Naturally, the inert gas recovered by the suction means 80 should not be mixed with other gases or components. To ensure that the inert gas thus recirculated does not contain any auxiliary or undesired gases, a particle filter can be inserted in the recirculation circuit.

[0084] 5 and 6 show an aspiration system 80' having the sole function of sucking inert gas from the secondary space 6, but without the ability or function to reinject this inert gas into the primary space 8. Thus, to the structure shown in FIG. 4, which may be sufficient in itself, may be added a module shown in FIG. 5, in which the aspiration system 80' has a structure in which the inert gas is sucked from the gas dome 21 via the secondary outlets 19, 20 or 23.

[0085] More specifically, in the structure of Fig. 4, the inert gas is injected into the primary space 8 at the liquid dome 15, and the withdrawal of the inert gas is carried out in the same way at the liquid dome 15 via the secondary outlets 19, 20 or 23. The valve 40 allows for the regulation of the flow aspirated by the suction means 80, which is connected to a circuit conveying the inert gas from the reservoir 16 downstream (as shown in this Fig. 4) or upstream of the injection device 45, preferably upstream of said device 45, in order to use the suction power of said device 45.

[0086] Figure 6 shows an arrangement using suction means 80 to recirculate the inert gas into the primary space 8, for example via the primary inlet 18, and a suction system 80' to exhaust the recovered inert gas into the ambient air in a conventional manner. Of course, to initiate the injection of the inert gas into the primary space 8, the arrangement of Figure 6 must be combined with, for example, an injection device 45.

[0087] As a non-limiting example, in such a configuration using suction means 80 and suction system 80', 3 45 injection devices with an injection flow rate of 1000 cubic meters per hour (cubic meters per hour), each 50 m 3 It is possible to have pumps 80 and 80' with a discharge flow rate of 50 m / hr. 3 Therefore, the injection capacity of the injection device is set to 50 m / h to achieve a constant injection volume of 45 m / s. 3 / hour. Of course, this 50% inert gas recirculation or reinjection rate can be selected to be different by selecting a higher or lower power pump(s) 80 and / or adjusting the flow rate using the circuit valve 40.

[0088] If the suction means 80 only performs the suction and therefore the re-injection of inert gas into the primary space 8, the operation of the injection device 45 can be shut off or stopped after a few minutes so that the suction means 80 only circulates the inert gas, constantly recirculating this gas for the time it takes to perform the inspection task. Naturally, in such a solution the suction means 80 must consist of a pump or series of pumps powerful enough to be able alone to perform the role of injecting and recirculating the inert gas.

[0089] The suction device 80' may be, for example, a Venturi-type system. To this end, the suction device 80' is connected to the ship's compressed air system 71. The operation of the Venturi system will be described with reference to Figures 7 and 8. When the valves 72 and 75 are open, a compressed air flow is guided to the inlet side of the converging / diverging section, as indicated by arrow 84, and as a result of the Venturi effect, a pressure reduction occurs in the lateral pipeline 81 of the Venturi effect suction system 80', which is connected to the sampling pipeline 70, which is connected to the secondary gas outlet 19 of the secondary space 6. Subsequently, a certain amount of inert gas contained in the secondary space 6 is sucked in, as indicated by arrow 82. The sucked in inert gas flow and the compressed air flow mix at the outlet side of the converging / diverging section, as indicated by arrow 85, and flow into the pipeline 76, which opens outside the tank.

[0090] Preferably, a valve 40 is also provided in the sampling pipeline 70, which is opened only after a steady flow of compressed air has been established in the main pipe 83 at an appropriate velocity, thereby preventing a backflow of air towards the secondary space 6 during the start-up phase of compressed air flow generation. Similarly, the valve in question can be fully or partially closed or the flow rate / suction rate controlled before shutting off the compressed air flow.

[0091] Before carrying out the main steps, i.e. the steps that create the pressure difference P1 (called the main step) and the steps that are called intermediate steps, the heating of the enclosed space 4 can be reduced for at least 3 hours before the thermographic detection, since the heating may mask potential cold spots on the inner shell 2.

[0092] When the tank 1 is then filled with cryogenic liquefied gas, the gas pressure in the tank 1 is ensured to be higher than 50 mbarg. Then, without using a suction device, the pressure in the secondary space 6 is reduced to a value between 1 mbarg and 5 mbarg while keeping all other gas inlets and outlets in the secondary space 6 closed and only the secondary gas outlet 19 in the open position. The secondary gas outlet 19 is preferably located on the liquid dome 15. However, this process can be accelerated by connecting a suction device 80, 80' to the second secondary gas outlet 20 of the secondary space in the open position. The second secondary gas outlet 20 is preferably located on the liquid dome 15. If this configuration is not sufficient, a suction device 80, 80' can be connected to the third secondary gas outlet 23 of the secondary space, which is also in the open position. The third secondary gas outlet 23 is preferably located on the liquid dome 15. If necessary, a suction device 80 can be used for each gas outlet in the secondary space.

[0093] Subsequently, or in parallel with the above steps, the injection device 45 is activated to inject nitrous oxide from the gas source 71 into the primary space 8 via the primary gas inlet 18 until the pressure in the primary space 8 reaches a value 21-29 mbarg higher than the pressure in the secondary space 6. All other gas inlets and outlets in the primary space 8 are closed, leaving only the primary gas inlet 18 in an open position. If this configuration is not sufficient, the injection device 45 may also be connected to a second primary gas inlet 22 in the primary space 8. This second primary gas inlet 22 may be located in the gas dome area. If necessary, pressure stabilization is awaited, which may take 30-60 minutes.

[0094] The pressure in the primary and secondary spaces is controlled within an allowable pressure range by safety valves (not shown) in the primary and secondary spaces.

[0095] Measurement of the temperature on the outer surface of the inner shell 2 can then be carried out by thermography from the enclosed space 4 .

[0096] After the measurements have been carried out, the pressure in the space is returned to its value under normal operating conditions and heating of the enclosed space 4 .

[0097] Using this method, an inert gas is passed through the primary space 6 to cool it. The cooled inert gas then passes through the secondary sealing membrane 7 if the secondary sealing membrane 7 has abnormal voids. The cooled inert gas then generates cold spots on the inner shell 2. A thermal camera is then used to detect potential cold spots on the outer surface of the inner shell 2.

[0098] To measure the temperature of the outer surface of the inner shell 2 from the enclosed space 4, a photodetector such as a thermographic camera with an infrared sensor can be used, depending on the temperature measurement required. The photodetector records the different infrared rays transmitted by the observed surfaces, which vary as a function of their temperature. This type of camera uses a container cooled by cryogenic techniques, and the sensor can be enclosed in a vacuum chamber. Reducing or controlling the temperature of the sensor in this way proves advantageous in reducing thermal noise to a level below that of the signal of the imaged scene.

[0099] Typically, a thermographic camera with an infrared sensor capable of detecting wavelengths between 7.5 and 13 μm can be used, with a sensitivity of less than 0.05 K for a blackbody of 303 K±10 K and an accuracy of less than 2 K for a blackbody in the range of 253 K to 353 K.

[0100] The image obtained by the camera of the type described above is called a thermogram, and consists of an image in which each point in the image is assigned a temperature value observed by the thermographic camera with an infrared sensor. To facilitate visual detection of temperature and thus the location of cold spots on the inner shell 2, a color representing the temperature may be associated with each point in the thermogram.

[0101] On the other hand, cold spots can be obtained that are not due to poor sealing. They can be the result of other phenomena occurring in the secondary space, such as conduction, natural convection, forced convection or radiation. To eliminate these effects and refine the leak detection of the secondary sealing membrane 7, the data obtained using the thermographic camera can be post-processed. Thus, there are two suitable conditions for the temperature gradient indicated by a cold spot to occur:

[0102] |ΔT ulterieur |<|ΔT intermediaire |<|ΔT principal | and |ΔT principal |-|ΔT ulterieur |≧1K

[0103] The terms "ulterieur", "intermediaire", and "principale" in relation to temperature refer to temperature measurements after steps referred to as subsequent, intermediate, and principal steps, respectively.

[0104] ΔT ulterieur denotes the temperature difference between the temperature of the image point measured in a subsequent step and the average temperature of the reference zone of the inner shell measured in a subsequent step,

[0105] ΔT intermediaire denotes the temperature difference between the temperature of the preceding point of the image measured in the intermediate step and the average temperature of the reference zone of the inner shell measured in the subsequent step,

[0106] ΔT principale denotes the temperature difference between the temperature of the preceding point of the image measured in the main temperature measurement step and the average temperature of the reference zone of the inner shell measured in the main step.

[0107] In its entire procedure, the testing method according to the invention comprises four successive steps: 1. A preliminary step of taking temperature measurements under normal operating conditions of tank 1, then 2. An intermediate step of temperature measurement in which the pressure difference between the primary space 8 and the secondary space 6 is in the range of 500 Pa to 1500 Pa, preferentially in the primary space 8, and preferably in the range of 800 Pa to 1200 Pa; 3. The main step of performing a temperature measurement in which the pressure difference between the primary space 8 and the secondary space 6 is equal to P1, then 4. A subsequent step of measuring the temperature of Tank 1 under normal operating conditions.

[0108] It is important to note that only steps 3 and 4 are mandatory, ie the control method according to the invention requires that at least these two steps are carried out in succession.

[0109] The method for testing the sealing of a tank may therefore include a preliminary step aimed at ensuring that no cold spots exist on the inner hull under normal operating conditions of the tank, and may also involve locally verifying the emissivity of the inner hull coating to determine the performance level of the temperature measurement.

[0110] First, heating of the enclosed space 4 is reduced or stopped at least three hours before the inspection. The pressure in the primary space 8 and secondary space 6 is maintained according to the tank's normal operating conditions, i.e., the pressure in the secondary space 6 is higher than the pressure in the primary space 8. The inspection of the inner hull 2 ​​is then completed using a thermal camera. This allows the inner hull 2 ​​to be thermally inspected under the tank's normal operating conditions. The systems in the enclosed space 4 are returned to normal operating conditions at the end of the inspection.

[0111] In order to ensure, among other things, that the tank 1 has not been damaged or its condition has not deteriorated by the primary step, a subsequent step is carried out in which the temperature of the outer surface of the inner shell 2 is measured. This subsequent step is identical in all respects to the preliminary step for operational conditions. If the preliminary step has been carried out, the thermograms obtained under normal operating conditions can be compared to draw conclusions about the state of the tank. Even if the preliminary step has not been carried out, it is possible to ensure that no cold spots are present on the thermogram.

[0112] Finally, the method may further include an intermediate step to determine whether the tank is capable of completing the main step. Thus, the intermediate step can be performed before the main step and after the preliminary step. This step includes measuring the temperature of the outer surface of the inner shell 2 from the enclosed space 4 when the pressure difference between the primary space 8 and the secondary space 6 is in the range of 800 Pa to 1200 Pa and the primary space 8 is under a pressure exceeding the pressure difference compared to the secondary space 6.

[0113] Before performing this intermediate step, heating of the enclosed space 4 can be reduced at least three hours before thermographic detection. This is because, in this example, heating may mask any potential cold spots on the inner shell 2. Next, when the tank 1 is filled with cryogenic liquefied gas, the gas pressure in the tank 1 is ensured to be higher than 50 mbarg. Then, without using the suction device and with only the secondary gas outlet 19 in the open position, the pressure in the secondary space 6 is reduced to a value of 1 mbarg to 5 mbarg. Then, the injection device 45 is activated to inject nitrous oxide from the gas source 71 through the primary gas inlet 18 into the primary space 8 so that the pressure in the primary space 18 reaches a value of 8 to 12 mbarg higher than the pressure in the secondary space 6. All other gas inlets and outlets in the primary space 8 are closed, with only the primary gas inlet 18 in the open position. If necessary, pressure stabilization is awaited, which may take 30 to 60 minutes. The pressure in the primary and secondary spaces is controlled within acceptable pressure ranges by primary and secondary space safety valves (not shown). A thermal inspection can then be carried out on the outer surface of the inner shell 2 by thermography from the enclosed space 4. After the measurements have been carried out, the pressure in the spaces is returned to their values ​​under normal operating conditions and heating of the enclosed space 4.

[0114] In this way, if the thermographic inspection carried out following this intermediate step does not detect any significant cold spots, the tank can proceed to the main steps of the method.

[0115] As a result of the geometric shape of the liquid dome 15 and the gas dome 21, the above-mentioned method may not work in some cases, since external climatic conditions and temperature fluctuations in and near these towers may distort the temperature measurements taken by the thermographic camera and / or their incorporation into the post-processing of the temperature measurements may be very complicated. For this reason, the method may be supplemented by measuring the flow of nitrogen through potential leaks in the secondary sealing membrane 7. Preferably, the flow is directed from the primary space to the secondary space.

[0116] The first flow meter is installed in the liquid dome 15. It is installed on the pipeline connecting the nitrogen source 16 and the primary gas inlet 18. The other primary inlet is in the closed position. The primary outlet itself is also in the closed position. Thus, if the secondary sealing membrane 7 has an abnormal void, the only way for the nitrogen to flow is through to the secondary space. This flow meter therefore allows the flow of nitrogen into the primary space to be measured.

[0117] A second flow meter is installed, which is located at the height of the secondary gas outlet 19 in the secondary space 6, which is located on the gas dome 21. The secondary inlet and other secondary outlets are in the closed position. In this way, the flow meter accurately measures the flow of nitrogen passing from the primary space 8 to the secondary space 6 via the abnormal gap in the secondary sealing membrane 7.

[0118] This arrangement ensures that no information is lost as the nitrogen flow passes through the flow meter, the location of which may vary from vessel to vessel.

[0119] Before installing the flow meters, ensure that the pressure in the primary and secondary spaces is normal, i.e., the pressure observed under normal operating conditions. When filled with cryogenic liquefied gas, the vapor pressure in the tank must be maintained above 50 mbarg, preferably above 100 mbarg. The primary and secondary inlets and the primary and secondary outlets, which may affect the measurement of nitrogen flow, are then closed. The flow meters are then installed, with the first flow meter located at the primary gas inlet 18 and the second flow meter located at the secondary gas outlet 19 or the second secondary gas outlet 20. After the flow meters are installed, nitrogen is supplied to the primary space 8, but the valves are used to limit the flow, e.g., up to 12 m. 3 / h (cubic meters per hour). Then, flow meters are used to start measuring the flow rates into the primary space and out of the secondary space. Control and measurement of the flow rates at the primary gas inlet 18 and secondary gas outlet 19 lasts for up to 5 hours, preferably 3 hours.

[0120] At the end of the measurement, the flow rates at the primary gas inlet 18 and the secondary gas outlet 19 are compared. If the measured values ​​are similar, then the secondary sealing membrane 7 has abnormal voids, particularly in the region of the liquid dome and / or gas dome. On the other hand, if the flow rates are significantly different or the flow rate at the secondary gas outlet is zero, then the secondary sealing membrane 7 does not have abnormal voids either.

[0121] The installations described above and using the methods described above may be used, for example, on land-based installations or floating structures such as liquefied natural gas tankers.

[0122] Referring to Figure 9, a cutaway view of a liquefied natural gas tanker 100 shows a storage facility for cryogenic liquefied gas comprising a sealed, insulated tank 1 of generally prismatic shape mounted within the vessel's double hull 101. The tank 1 comprises a primary sealing membrane intended to be in contact with the cryogenic liquefied gas LNG contained in the tank, a secondary sealing membrane arranged between the primary sealing membrane and the vessel's double hull 101, and two insulating barriers arranged between the primary sealing membrane and the secondary sealing membrane and between the secondary sealing membrane and the double hull 101, respectively.

[0123] FIG. 9 shows an example of a marine terminal comprising a loading station 103, submerged pipes 104, and an onshore facility 105. The loading station 103 is a fixed offshore facility comprising a movable arm 106 and a tower 107 supporting the movable arm 106. The movable arm 106 carries a bundle of flexible insulated pipes 108 that can be connected to a loading channel 109. The orientable movable arm 106 is compatible with all gauges of liquefied natural gas tankers. A connecting pipe (not shown) extends inside the tower 107. The loading station 103 allows loading and unloading of the vessel 100 to and from the onshore facility 105. The facility comprises a cryogenic liquefied gas storage tank 110 and a connecting pipe 111 connected to the loading station 103 by a submerged pipe 104. The underwater pipes 104 allow cryogenic liquefied gas to be transported over long distances, for example 5 km, between the loading station 103 and the onshore facility 105, thereby keeping the vessel 100 a long distance from shore during loading and unloading operations.

[0124] To provide the pressure required for the transfer of the cryogenic liquefied gas, pumps on board the ship 100 and / or pumps provided at the land facility 105 and / or pumps provided at the loading station 103 are used.

[0125] Although the present invention has been described with reference to some specific embodiments, it is extremely clear that the invention is in no way limited thereto, but includes all technical equivalents of the described means and combinations thereof, provided that these fall within the scope of the invention.

[0126] Use of the verbs "have", "comprise" or "include" and their conjugations does not exclude the presence of elements or steps other than those stated in a claim.

[0127] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Claims

1. A method for inspecting the tightness of a sealed, insulated tank (1) for storing cryogenic liquefied gas (30), said tank (1) being of the cryogenic type (30), said tank (1) comprising a carrier structure having an inner shell (2) and an outer shell (3), a closed space (4) between said inner shell (2) and said outer shell (3), a primary sealing membrane (9) intended to be in contact with said cryogenic liquefied gas (30) contained in said tank (1), a secondary sealing membrane (7) arranged between said primary sealing membrane (9) and said inner shell (2), a primary space (8) between said primary sealing membrane (9) and said secondary sealing membrane (7) and a secondary sealing membrane (8) in front of said secondary sealing membrane (7). and a secondary space (6) between the primary space (8) and the inner shell (2), wherein the primary space (8) and the secondary space (6) contain thermal insulating material, the primary space (8) contains at least one primary gas inlet (18), the secondary space (6) contains at least one secondary gas outlet (19), the primary sealing membrane rests directly on the thermal insulating material contained in the primary space, and the secondary sealing membrane rests directly on the thermal insulating material contained in the secondary space, the method comprising the following successive steps for detecting the location of a sealing defect of the secondary sealing membrane (7) in the form of a cold spot occurring on the outer surface of the inner shell (2): a step, called the main step, of injecting an inert gas into the primary space through the primary gas inlet (18) and then releasing or sucking the gas through the secondary gas outlet (19) of the secondary space, and then lowering the pressure in the secondary space (6) by a pressure difference P1 below the pressure in the primary space (8) by measuring the temperature of the outer surface of the inner shell (2) from the closed space (4) located around the inner shell (2); measuring the temperature of the outer surface of the inner shell (2) from the enclosed space (4) under normal operating conditions of the tank (1), referred to as the subsequent step; characterised in that during the main step at least a part of the inert gas injected into the primary space (8) is recovered by at least one secondary outlet (19, 20 or 23) of the secondary space (6) and re-injected into the primary space (8), method.

2. 2. The method according to claim 1, wherein the total amount of the inert gas injected into the primary space (8) is recovered by at least one suction means (80) connected to a secondary outlet (19, 20 or 23) of the secondary space (6) before being reinjected into the space (8).

3. 2. The method according to claim 1, wherein only a portion of the inert gas injected into the primary space (8) is reinjected into said space (8) after being withdrawn by at least one suction means (80) connected to a secondary outlet of the secondary space (6).

4. A method as described in claim 3, wherein 20% to 80% of the inert gas injected into the primary space (80) is recovered by at least one suction means (80) connected to a secondary outlet of the secondary space (6) and then re-injected into the space (8).

5. 5. The method according to any one of claims 1 to 4, wherein the method comprises a step, called a preliminary step, of measuring the temperature of the outer surface of the inner shell (2) from the enclosed space (4) under the normal operating conditions of the tank (1).

6. The pressure difference P1 in the range of 500 Pa to 1500 Pa for a duration of at least 10 hours; or 6. The method according to any one of claims 1 to 5, wherein the pressure is in the range of 1800 Pa to 3200 Pa.

7. 7. The method according to any one of claims 1 to 6, wherein the main step is preceded by a step called an intermediate step, in which the pressure in the secondary space (6) is reduced by a pressure difference of 500 Pa to 1500 Pa below the pressure in the primary space (8) by injecting an inert gas into the primary space through the primary gas inlet (18), and then releasing or sucking the gas from the secondary gas outlet (19) of the secondary space, and then measuring the temperature of the outer surface of the inner shell (2) from the closed space (4) located around the inner shell (2).

8. 8. The method of any one of claims 1 to 7, wherein the inert gas comprises nitrous oxide.

9. 9. The method of claim 1, wherein the duration of each temperature measurement is up to 5 hours.

10. A method described in any one of claims 1 to 9, wherein P1 is held stable during the temperature measurement.

11. The method described in claim 7, wherein the pressure difference in the step referred to as the intermediate step is held stable during the temperature measurement.

12. A floating or onshore cryogenic liquefied gas (30) storage facility for carrying out the inspection method according to any one of claims 1 to 11, comprising: The carrier structure comprises an inner shell (2) and an outer shell (3), the space between which is called a closed space (4), a primary sealing membrane (9) intended to be in contact with the cryogenic liquefied gas (30) contained in the tank, and a secondary sealing membrane (7) arranged between the primary sealing membrane (9) and the inner shell (2), the space between which is called a primary space (8), and the secondary sealing membrane (7) a cryogenic tank (1) (30), wherein the space between the shell (7) and the inner shell (2) is called the secondary space (6), the primary space (8) and the secondary space (6) contain insulating material, the primary space (8) contains at least one primary gas inlet (18), the secondary space (6) contains at least one secondary gas outlet (19), the primary sealing membrane rests directly on the insulating material contained in the primary space, and the secondary sealing membrane rests directly on the insulating material contained in the secondary space; an inert gas reservoir (16) configured to inject an inert gas into the primary space (8) through the primary gas inlet; an injection device (45) through which the inert gas from the inert gas reservoir (16) can be injected via the primary gas inlet (18), so that the primary space (8) can be placed under a higher pressure than the secondary space (6); a suction device (80, 80') connected to the secondary gas outlet (19) for generating a lower pressure in the secondary space (6) than in the primary space (8); a device for measuring the temperature of the outer surface of the inner shell (2); and a system for displaying temperature measurements in order to identify the location of a sealing defect of the secondary sealing membrane (7) in the form of a cold spot on the outer surface of the inner shell (2), characterized in that the suction device comprises at least one suction means (80) consisting of a pump for suctioning at least a part of the inert gas from the secondary space (6) and reinjecting it into the primary space (8). Floating or onshore cryogenic liquefied gas storage facilities.

13. 13. An installation according to claim 12, wherein said suction means (80) form the only suction means (80) of said apparatus so that all of said inert gas recovered in said secondary space (6) is reinjected into said primary space (8).

14. 13. The installation according to claim 12, wherein the suction device further comprises at least one suction system (80') for withdrawing the inert gas but not reinjecting it into the primary space (8).

15. 15. The installation according to claim 14, wherein the suction system (80′) is a Venturi effect suction system (80′) comprising: a main pipe (83) having an inlet connectable to a pressurized gas source (71) and an outlet towards the outside of the tank (1); and a suction pipe (81) having an upstream side connectable to an outlet port of the secondary space (6) and a downstream side opening laterally into a converging / diverging section of the main pipe (83) so that gas flow in the main pipe (83) creates a reduced pressure in the suction pipe (81).

16. 16. The installation according to claim 15, wherein the pressurized gas source (71) is a compressed air circuit.

17. 17. The installation according to any one of claims 12 to 16, wherein the injection device (45) comprises a compressor capable of injecting the inert gas from the inert gas reservoir (16) at a pressure of 3 to 8 bar.

18. A ship (100) for transporting cryogenic liquefied gas (30), comprising a cryogenic liquefied gas (30) storage facility according to any one of claims 12 to 17.

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