Method for leak testing of a sealed and heat-insulated tank for storing a fluid

Description

[0001] The invention belongs to the field of sealed and thermally insulating membrane tanks. In particular, the invention relates to the field of sealed and thermally insulating tanks for the storage and / or transport of liquid at low temperature, such as tanks for the transport of Liquefied Petroleum Gas (also called LPG) having for example a temperature between -50°C and 0°C, or for the transport of 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 intended for the transport of liquefied gas at low temperature or for receiving liquefied gas at low temperature serving as fuel for the propulsion of the floating structure. More particularly, the invention relates to devices and methods for detecting leaks in a secondary membrane of such a tank.

[0002] LNG vessel tanks typically include a supporting structure that provides 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 supporting 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 sealing membrane and the secondary membrane 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 waterproofing membrane is invisible and inaccessible once the tank is manufactured. Therefore, it is impossible to directly observe any defects in this membrane, such as scratches, local dislocation of the membrane, or an air channel between two parts of the membrane.

[0004] To diagnose the tightness of the secondary membrane, document WO2020128370, filed in the name of the applicant, is known in particular, which describes a method for checking the tightness of this secondary membrane. In this method, the presence of leaks at the level of the secondary sealing membrane is detected, in particular by imposing an overpressure of inert gas in the primary insulation space relative to the secondary insulation space.

[0005] The inert gas used in such a process is typically nitrogen. However, this gas is stored on the ship in limited quantities and has multiple applications on the ship, whenever inert gas circulation is required.

[0006] However, the applicant noted that in cases where leaks from the secondary sealing membrane are significant, the quantity of inert gas required for the leak detection operation is very high, to the point of exhausting the ship's entire stock of inert gas during the tank leak testing operation. Indeed, the inert gas, once used for each testing operation, is released into the surrounding air.

[0007] Currently, there is no method for testing the leaktightness of the secondary waterproofing membrane in which the quantity of inert gas used is reduced, or even in which a leaktightness inspection is carried out with a neutral balance regarding the inert gas.

[0008] An idea underlying the invention is to provide a device and method for detecting leaks in a sealed and thermally insulating tank filled with liquid at low temperature which does not have these drawbacks. Thus, one aim of the invention is to be able to locate abnormal porosities in the secondary sealed membrane, even when the leak rate is very high.

[0009] Thus, the present invention relates to a method for controlling the leaktightness of a sealed and thermally insulating tank for storing a low-temperature liquefied gas, the tank being at low temperature, the tank comprising a supporting structure having an inner shell and an outer shell, a confined space between the inner shell and the outer shell, a primary sealing membrane intended to be in contact with the low-temperature 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 insulating materials, the primary space comprising at least one primary gas inlet and the secondary space comprising at least one secondary gas outlet,said primary sealing membrane resting directly on the insulating materials contained in the primary space and said secondary sealing membrane resting directly on the insulating materials 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 on the outer surface of the inner shell: , a so-called main step in which the secondary space is put at a pressure lower than the pressure of the primary space, with a pressure differential P1, by injecting an inert gas into the primary space via the primary gas inlet and creating an exhaust or suction at the secondary gas outlet of the secondary space, then measuring the temperature of an external surface of the inner shell from the confined space located around the inner shell; a so-called subsequent step in which the temperature of the external surface of the inner shell is measured from the confined space under normal operating conditions of the tank.

[0010] The invention is characterized in that at least part of the inert gas injected into the primary space is recovered via at least one secondary outlet from the secondary space to be reinjected into the primary space.

[0011] Thus, the applicant proposes a simple, effective and inexpensive system for carrying out a leak test of the secondary membrane of a liquefied gas tank, without loss of inert gas or by losing a reduced quantity of the latter so that the control method can always be implemented, whatever the leak conditions of the secondary sealing membrane.

[0012] Indeed, after several tests, the applicant sought an architecture that was simple, robust and efficient, to recirculate the inert gas used during the implementation of the method according to the invention.

[0013] The term "low-temperature liquefied gas" means any body which is in the vapor state under normal conditions of pressure and temperature and which has been placed in the liquid state by lowering its temperature.

[0014] “Confined space” means ballast tanks, tubular keels, cofferdams or cofferdams, gangways and the closing deck of the tank also called “trunk deck”.

[0015] The term "low temperature tank" means a tank in which the low temperature liquefied gas occupies at least 20% of the volume of the tank, preferably 70%, or a tank not containing a load of liquefied gas, i.e. a volume less than 20% of the volume of the tank, but then, in this case, the tank is cooled using spraying, or projection, of a low temperature liquefied gas, such as for example liquid nitrogen or LNG.

[0016] The term "normal tank operating conditions" means the conditions under which the tank is normally used. In this state, the pressure of the primary space is typically slightly lower than that of the secondary space, for example by a few mbar (millibars), i.e. 2 to 7 mbar, or in rarer cases, the pressure of the primary space may be higher than the pressure of the secondary space, for example by 0 to 4 mbar.

[0017] In the following, the present invention is illustrated, without limitation to this embodiment, with a liquefied gas storage and transport vessel of the LNGC (“Liquefied Natural Gas Carrier”) type which conventionally comprises four sealed and thermally insulating tanks for storing liquefied gas.

[0018] Other advantageous characteristics of the invention are presented briefly below:

[0019] According to one embodiment, all of the inert gas injected into the primary space is reinjected into the latter, after its recovery by means of at least one suction means connected to a secondary outlet of the secondary space.

[0020] According to another embodiment, only a portion of the inert gas injected into the primary space, advantageously representing between 20% and 80% of the inert gas injected into the primary space, is reinjected into the latter, after its recovery by means of at least one suction means connected to a secondary outlet of the secondary space.

[0021] Advantageously, the method according to the invention comprises a so-called preliminary step in which the temperature of the outer surface of the inner shell is measured from the confined space under normal operating conditions of the tank.

[0022] Advantageously, the pressure differential P1 is understood: between 500 Pa and 1500 Pa, preferably between 800 Pa and 1200 Pa, for a duration of at least 10 hours, preferably at least 12 hours, or between 1800 Pa and 3200 Pa, preferably between 2100 Pa and 2900 Pa.

[0023] Advantageously, the so-called main step is preceded by a so-called intermediate step in which the secondary space is put at a pressure lower than the pressure of the primary space, with a pressure differential of between 500 Pa and 1500 Pa, preferably between 800 Pa and 1200 Pa, by injecting an inert gas into the primary space via the primary gas inlet and creating an exhaust or suction at the secondary gas outlet of the secondary space, then measuring the temperature of an outer surface of the inner shell from the confined space located around the inner shell.

[0024] Preferably, the inert gas consists of nitrogen.

[0025] Advantageously, the duration of each temperature measurement is at most five hours, preferably at most three hours. This duration is conventionally intended to avoid excessive cooling of the internal shell below the temperatures tolerated by the material making up the internal shell.

[0026] Preferably, P1, advantageously the pressure differential of the so-called intermediate stage, is kept stable during the temperature measurement. Thus, the pressures P1 and that of the so-called intermediate stage are ideally maintained within a value of + / - 5% in the respective defined pressure ranges.

[0027] The method according to the invention may further comprise a step of stabilizing the pressures in the primary space and the secondary space, a step of measuring the flow rate of inert gas injected into the primary space, a step of measuring the flow rate of inert gas leaving the secondary space, and a step of comparing the flow rate of inert gas injected into the primary space with the flow rate of inert gas leaving the secondary space so as to identify and quantify the flow rate of inert gas passing through the membrane and thus determine any porosity of the secondary membrane.

[0028] In a ship, the tank, for example on its upper wall, has structures called vapor dome and liquid dome. These can take the form of two turrets intended to pass cargo handling equipment to handle a liquid phase and a vapor phase of the low-temperature liquefied gas contained in the tank. Because of this geometry, leak detection methods based on the observation of abnormally hot or abnormally cold areas can be frustrated, in particular because of the influence of external climatic conditions and because the temperature fields in and around these turrets can be very complex.By monitoring the flow rates of inert gas entering the primary space and leaving the secondary space by adding flow meters at the primary gas inlet and the secondary gas outlet and comparing the values ​​of these flow rates, it is then possible to detect whether there is actually a leak in the tank.

[0029] According to one embodiment, the preceding steps can be performed after the main steps. This makes it possible to locate a possible leak from the gas dome and the liquid dome when no leak has been detected elsewhere by the temperature measurements.

[0030] According to one embodiment, the flow measurement steps are carried out simultaneously with the main step including the intermediate step, if applicable. This makes it possible to shorten the total duration of the leak detection process and to reduce the consumption of inert gas throughout the duration of the process.

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

[0032] According to one embodiment, the measurement of the flow rate of 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 the other gas outlets being 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 use of inerting lines also present in the domes for these leak detection processes to inert the primary and secondary spaces.

[0037] According to one embodiment, the inert gas is chosen from helium, argon and their mixture, with or without nitrogen present in the mixture.

[0038] According to one embodiment, the low temperature liquefied gas is selected from Liquefied Natural Gas (LNG), Liquefied Petroleum Gas (LPG), liquid ethane, liquid propane, liquid nitrogen, liquid oxygen, liquid argon, liquid xenon, liquid neon and liquid hydrogen.

[0039] Such a process can be used in a floating, coastal or deep-water structure, including an LNG carrier, a floating storage and regasification unit (FSRU), a floating production and remote storage unit (FPSO) and others. Such a storage facility can also serve as a fuel tank in any type of vessel.

[0040] The present invention also relates to a storage or land-based installation for a low-temperature liquefied gas for implementing the method for checking the tightness of a sealed and thermally insulating tank as briefly stated above, comprising: a low-temperature tank comprising a supporting structure having an inner shell and an outer shell, the space between the inner shell and the outer shell being called a confined space, a primary sealing membrane intended to be in contact with the low-temperature 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 is called a primary space and the space between the secondary sealing membrane and the inner shell is called a secondary space, the primary space and the secondary space comprising insulating materials, the primary space comprising at least one primary gas inlet, the secondary space comprising at least one secondary gas outlet,said primary sealing membrane resting directly on the insulating materials contained in the primary space and said secondary sealing membrane resting directly on the insulating materials contained in the secondary space, an inert gas reservoir arranged to inject inert gas into the primary space via the primary gas inlet, an injection device capable of injecting inert gas from the inert gas reservoir via the primary gas inlet, and thus pressurizing the primary space relative to the secondary space, a suction device connected to the secondary gas outlet to create a pressure in the secondary space lower than the pressure 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 locate a leak in the secondary sealing membrane in the form of a cold spot on the outer surface of the inner shell.

[0041] The invention is characterized in that the suction device comprises at least one suction means, consisting of a pump, sucking at least part of the inert gas into the secondary space to reinject it into the primary space, preferably via the aforementioned injection device.

[0042] According to one embodiment of the invention, the above-mentioned suction means forms the only suction means of said 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 also comprises at least one suction system recovering the inert gas without reinjecting it into the primary space.

[0044] Very advantageously, the suction system is a Venturi effect suction system comprising a main pipe having an inlet capable of being connected to a source of pressurized gas and an outlet to the outside of the tank, a suction pipe having an upstream side capable of being connected to the outlet port of the secondary space and a downstream side opening laterally into a convergent-divergent portion of the main pipe so that a flow of gas in the main pipe produces a vacuum in the suction pipe.

[0045] Of course, the suction system may also consist of a conventional pump, in other words one that is not a Venturi pump, which operates with an electrical power source. It is also possible to provide for the suction system to comprise one or more so-called conventional pumps and one or more Venturi pumps, these different pumps being capable of being implemented together or not depending on the choice of the operators carrying out the operations with regard to the environmental conditions and / or other factors.

[0046] Advantageously, the source of pressurized gas is a compressed air circuit. Such a source is typically present in a ship transporting hydrocarbons or, more generally, flammable or explosive materials.

[0047] Preferably, the injection device comprises a compressor capable of injecting the inert gas from the inert gas tank at a pressure of between 3 and 8 bars.

[0048] According to one embodiment, the thickness of the primary waterproofing membrane is less than or equal to 2.5 mm (millimeter), for example said thickness is less than or equal to 1.5 mm.

[0049] According to one embodiment, the thickness of the secondary waterproofing membrane is less than or equal to 1.5 mm (millimeter), for example said thickness is less than or equal to 1.2 mm.

[0050] According to one embodiment, the suction device comprises a Venturi effect suction system comprising a main pipe having an inlet capable of being connected to a source of pressurized gas and an outlet towards the outside of the tank, a suction pipe having an upstream side capable of being connected to the outlet port of the secondary space and a downstream side opening laterally into a convergent-divergent portion of the main pipe so that a flow of gas in the main pipe produces a vacuum in the suction pipe.

[0051] According to one embodiment, the suction device comprises a plurality of Venturi effect suction systems, these systems preferably being arranged in series in order to increase the suction capacity.

[0052] According to one embodiment, the Venturi effect suction systems are arranged in stages.

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

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

[0055] In one embodiment, the infrared sensor is cooled by cryogenic techniques, in particular using Peltier technology. However, other techniques can be considered in which, for example, the sensor is enclosed in an enclosure or enclosed in a Dewar vessel, or cooled by a Stirling effect device. This lowering of the sensor temperature reduces thermal noise.

[0056] According to one embodiment, the invention provides a vessel for transporting low-temperature liquefied gas, comprising an aforementioned floating storage facility.

[0057] According to one embodiment, the invention also provides a method of loading or unloading such a vessel, in which a low-temperature liquefied gas is conveyed through insulated pipes from or to a floating or land-based storage facility to or from the vessel's tank.

[0058] According to one embodiment, the invention also provides a transfer system for low temperature liquefied gas, the system comprising the aforementioned vessel, insulated pipes arranged to connect the tank installed in the hull of the vessel to a floating or land-based storage facility and a pump for driving a flow of low temperature liquefied gas through the insulated pipes from or to the floating or land-based storage facility to or from the storage facility of the vessel.

[0059] The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes, with reference to the accompanying drawings. [ Fig.1 ] There [ Fig.1 ] is a cutaway schematic representation of a ship's tank. [ Fig.2 ] There [ Fig.2 ] is a functional diagram of a ship's tank seen in section along a longitudinal axis of the ship. [ Fig.3 ] There [ Fig.3 ] is a diagram of the principle of the method of the invention. [ Fig.4 ] There [ Fig.4 ] is a schematic representation of a first embodiment of the nitrogen circulation circuit according to the invention in a storage installation. [ Fig.5 ] There [ Fig.5 ] is a schematic representation of a second embodiment of the nitrogen circulation circuit according to the invention in a storage installation. [ Fig.6 ] There [ Fig.6 ] is a schematic representation of a third embodiment of the nitrogen circulation circuit according to the invention in a storage installation. [ Fig.7 ] There [ Fig.7 ] is a schematic representation of the arrangement of the Venturi suction system in relation to the secondary space. [ Fig.8 ] There [ Fig.8 ] an enlarged cross-sectional representation of zone IV of the [ Fig.7 ]. [ Fig.9 ] There [ Fig.9 ] is a schematic cutaway representation of a low-temperature liquefied gas storage facility for an LNG carrier and a loading / unloading terminal for a tank of the low-temperature liquefied gas storage facility.

[0060] With reference to the [ Fig.1 ], a cross-section of a tank 1 of an LNG carrier ship made using membrane tank technology is shown schematically. A ship may thus have one or more similar tanks. This tank is intended for the transport of low-temperature liquefied gas 30. A low-temperature liquefied gas is in the vapor state under normal pressure and temperature conditions and is placed in the liquid state by lowering its temperature, particularly for its transport. This low-temperature liquefied gas may be Liquefied Natural Gas, Liquefied Petroleum Gas, liquid ethane, liquid propane, liquid nitrogen, liquid oxygen, liquid argon, liquid xenon, liquid neon or even liquid hydrogen.

[0061] The tank 1 comprises a supporting structure providing mechanical rigidity. The supporting structure is a double wall comprising an inner shell 2 and an outer shell 3. The inner shell 2 and the outer shell 3 delimit a confined space 4 whose dimensions are sufficient for human beings to move therein.

[0062] Confined space 4 includes the ballast tanks, tubular keels, cofferdams, gangways and the closing deck of tank 1, also called the "trunk deck".

[0063] 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 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.

[0064] The primary space 8 and the secondary space 6 comprise insulating materials in the form of juxtaposed panels made of thermally insulating material. These panels may be made of expanded or cellular synthetic resin or another thermally insulating material, natural or synthetic. In addition, these spaces 6, 8 comprise a filling material such as glass wool or mineral wool. This filling material may be intended to be inserted between the juxtaposed panels.

[0065] The primary waterproofing membrane 9 rests directly on the insulating materials of the primary space 8, and the secondary waterproofing membrane 7 rests directly on the insulating materials of the secondary space 6.

[0066] In reference to the figures 2 And 3, the tank has an upper wall 14 which is interrupted in two locations by two projecting structures in the form of a turret or chimney. They are intended to pass cargo handling equipment to handle a liquid phase and a vapor phase of the low-temperature liquefied gas for storage in the tank. The first turret is a liquid dome 15 which serves as a penetration point for the various handling equipment, namely in the example shown a filling line 10, an emergency pumping line 11, unloading lines linked to unloading pumps 12, a spray line (not shown) and a feed line linked to a spray pump 13. The second turret is a vapor dome 21 which serves as a penetration point for a vapor collecting pipe. The operation of this equipment is known elsewhere.

[0067] The primary space 8 comprises a primary gas inlet 18 and a primary gas outlet 26. It may further comprise a second primary gas inlet 22. The secondary space 6 comprises a secondary gas inlet 25 and a secondary gas outlet 19. The secondary space 6 may further comprise a second secondary gas outlet 20 and a third secondary gas outlet 23.

[0068] The tank may further include a safety valve 24 in the event of overpressure in the primary 8 and secondary 6 spaces.

[0069] The secondary sealing membrane 7 is invisible and inaccessible once the tank is manufactured. The method for checking the tightness of a tank according to the invention makes it possible to detect and locate defects in the secondary sealing membrane 7 and is suitable for most tank technologies.

[0070] Referring to the [ Fig.1 ] Or 3, the method is based on the use of a thermal gradient generated between the primary 8 and secondary 6 spaces of a tank 1 loaded with low-temperature liquefied gas 30 to detect by thermal imaging or thermography the impact on the inner shell 2 of an inert gas passing through the secondary sealing membrane 7. The tank is loaded to at least 20% of its total capacity with low-temperature liquefied gas or is cooled by spraying a cryogenic liquid.

[0071] On this [ Fig.3 ] one of the main aspects of the control method according to the invention is illustrated, namely the reinjection of at least part of the inert gas introduced into the primary space 8 by means of a suction means 80, in this case a conventional pump, which recovers at a secondary outlet 19, 20 or 23 of the secondary space 6 part or all of this injected inert gas to cause it, via a pipe, to circulate again in the primary space 8, via one of the primary inlets 18 or 22. On the figures 3 à 6 , valves 40 are arranged on the conduits to regulate the flow or stop the circulation in the conduit in question.

[0072] THE figures 4 à 6 present arrangements for carrying out this recirculation of the inert gas, it being understood that these embodiments are non-limiting examples which can also be combined.

[0073] So, on the [ Fig.4 ], an injection device 45 is installed at the primary gas inlet 18 and connected to the inert gas tank 16. The inert gas is, for example, nitrogen. This injection device 45 will allow the inert gas to be injected into the primary space 8. A suction device 80 is also installed at the secondary gas outlet 19.

[0074] This suction means 80 has two main functions, firstly to facilitate the creation of a pressure differential between the primary space 8 and the secondary space 6 even if the porosity of the secondary sealing membrane 7 is very high, and to recirculate all or part of the nitrogen injected into the primary space 8. Of course, the inert gas recovered by the suction means 80 must not be mixed with another gas or constituent. In order to ensure that this recirculation of inert gas does not include any additional or parasitic gas, it is possible to provide for the insertion of particle filters in the recirculation circuit.

[0075] On the figures 5 And 6is visible a suction system 80' which only presents the function of suction of the inert gas at the level of the secondary space 6, but not the possibility or the function of reinjection of this inert gas into the primary space 8. Thus, one can add to the architecture represented on the [ Fig.4 ], which can be sufficient in itself, the architecture module represented on the [ Fig.5 ] in which the suction system 80' sucks the inert gas at the level of the dome gas 21, via a secondary outlet 19, 20 or 23.

[0076] More precisely, in the architecture of the [ Fig.4 ], the inert gas is injected into the primary space 8 at the level of the liquid dome 15 and the recovery of the inert gas is also carried out at the level of the liquid dome 15, via a secondary outlet 19, 20 or 23. A valve 40 makes it possible to adjust the flow rate sucked in by the suction means 80. The suction means 80 is connected to the inert gas supply circuit, from the reservoir 16, downstream (as shown in this [ Fig.4 ]) or upstream of the injection device 45, preferably upstream of said device 45 so as to use the suction power of the latter 45.

[0077] On the [ Fig.6 ] is shown an architecture in which a suction means 80 is used, recirculating the inert gas in the primary space 8, for example via the primary inlet 18, and a suction system 80' which conventionally discharges the recovered inert gas into the surrounding air. Of course, the architecture of the [ Fig.6 ] must be associated for example with an injection device 45 to initiate the injection of inert gas into the primary space 8.

[0078] As a non-limiting example, for such an architecture using a suction means 80 and a suction system 80', there may be an injection device 45 of 100 m 3 < / hour (cubic meter per hour), pumps 80 and 80' each with a flow rate of 50 m 3 < / hour. In this case, the flow reinjected by the pump 80 being 50 m 3 < / hour, the capacity of the injection device can be regulated or lowered to 50 m 3 < / hour to have a constant injection flow rate 45. Of course, this ratio of 50% recycling or reinjection of inert gas can be chosen differently, by choosing more or less powerful pump(s) 80 and / or by a flow regulation set using the valves 40 of the circuit.

[0079] In the case where the suction means 80 alone carries out the suction and therefore the reinjection of inert gas into the primary space 8, after a few moments, the injection device 45 can be cut or stopped so that only the suction means 80 carries out the circulation of inert gas, by permanent recirculation of the latter for the time required to carry out the control operations. Of course, in such a solution, the suction means 80 must consist of a pump, or a series of pumps, sufficiently powerful to take on the function of injecting inert gas and recirculating it alone.

[0080] The 80' suction device can be, for example, a Venturi type system. It will then also be connected to the boat's compressed air system 71. With reference to the figures 7 et 8 , the operation of the Venturi type system will be described. When the valves 72 and 75 are open, a flow of compressed air enters the inlet side of the convergent-divergent, as indicated by the arrows 84 and causes by Venturi effect a depression in the lateral pipe 81 of the Venturi effect suction system 80' which is connected to the sampling pipe 70 connected to the secondary gas outlet 19 of the secondary space 6. This results in the suction of a quantity of inert gas contained in the secondary space 6, as indicated by the arrow 82. The flow of inert gas sucked in and the flow of compressed air mix in the outlet side of the convergent-divergent, as indicated by the arrows 85 and flow into the pipe 76 leading to the outside of the tank.

[0081] Preferably, a valve 40 is also provided on the sampling line 70 which is opened only after establishing a stable compressed air flow at an appropriate speed in the main line 83. This makes it possible to avoid a backflow of air towards the secondary space 6 during the start-up phase of the compressed air flow. In the same way, this valve can be completely or partially closed before interrupting the flow of compressed air or to regulate the suction flow rate / flow.

[0082] Before proceeding with the main steps, namely the pressure differential P1 (so-called main step) and the so-called intermediate step, the heating of the confined space 4 can be reduced at least 3 hours before detection by thermography. Indeed, the heating could mask a possible cold spot on the internal shell 2.

[0083] Then, it is ensured that the gas pressure in the tank 1, if it is filled with a low-temperature liquefied gas, is greater than 50 mbarg. Then the pressure value in the secondary space 6 is reduced to reach a value between 1 mbarg and 5 mbarg without using the suction device and leaving only the secondary gas outlet 19 in the open position, all the other gas inlets and outlets of the secondary space 6 being closed. The secondary gas outlet 19 is preferably located on the liquid dome 15. However, the process can be accelerated by connecting the suction device 80, 80' to a second secondary gas outlet 20 of the secondary space which is then put in the open position. The second secondary gas outlet 20 is preferably located on the liquid dome 15.If this should not be sufficient then the suction device 80, 80' could be connected to a third secondary gas outlet 23 of the secondary space which is then also put in the open position. The third secondary gas outlet 23 is preferably located on the liquid dome 15. Optionally it is possible to use a suction device 80 for each gas outlet of the secondary space.

[0084] Then, or in parallel with the step described above, the injection device 45 is actuated to inject nitrogen from the gas source 71 into the primary space 8 through the primary gas inlet 18 so that the pressure value in the primary space 8 reaches a value between 21 and 29 mbar above the pressure of the secondary space 6. Only the primary gas inlet 18 is left in the open position, all the other gas inlets and outlets of the primary space 8 being closed. If this is not sufficient, the injection device 45 may also be connected to a second primary gas inlet 22 of the primary space 8. This second primary gas inlet 22 may be located at the gas dome. Optionally, the pressure is then waited for to stabilize. This may last between 30 and 60 min.

[0085] The pressures of the primary and secondary spaces are managed within acceptable pressure ranges by safety valves (not shown) in the primary and secondary spaces.

[0086] The temperatures on the outer surface of the inner shell 2 can then be measured by thermography from the confined space 4.

[0087] Once the measurement is carried out, the pressures of the spaces are returned to their normal operating condition values ​​as well as the heating of the confined space 4.

[0088] With this process, the inert gas is cooled as it passes through the primary space 6. Then the cooled inert gas passes through the secondary sealing membrane 7 if it has abnormal porosities. The cooled inert gas will then create a cold spot on the inner shell 2. A thermal camera is then used to detect the potential cold spot on the outer surface of the inner shell 2.

[0089] To measure the temperature of the outer surface of the inner shell 2 from the confined space 4, a photodetector may be used, such as a thermographic camera with an infrared sensor, due to the extent of the temperature to be measured. It records the different infrared radiations emitted by the observed surface, which vary according to their temperature. This type of camera uses a container cooled by cryogenic techniques, the sensor being able to be enclosed in a vacuum chamber. This lowering or regulation of the sensor temperature may be useful for reducing thermal noise to a level below that of the signal from the filmed scene.

[0090] Typically, an infrared sensor thermographic camera capable of detecting wavelengths between 7.5 and 13 µm can be used, with a sensitivity of less than 0.05 K on a blackbody at 303K ± 10K and an accuracy of less than 2K on a blackbody in the range of 253K to 353K.

[0091] The image obtained with the type of cameras previously explained is called a thermogram and consists of an image in which each point of the image is assigned a temperature value observed by the thermographic camera with infrared sensor. To facilitate the visual detection of temperatures and therefore facilitate the location of a cold point on the internal shell 2, a color representing a temperature can be assigned to the points of the thermogram.

[0092] However, it is possible to obtain cold spots that are not caused by a leak. They may be due to other phenomena occurring in the secondary space, such as conduction, natural convection, forced convection, or radiation. To eliminate them and refine leak detection in the secondary waterproofing membrane 7, the data obtained with the thermographic camera can be post-processed. Thus, the temperature gradient represented by the cold spot must meet the following two conditions: ΔT ultérieur < ΔT intermédiaire < ΔT principal et ΔT principal − ΔT ultérieur ≥ 1 K

[0093] The terms "subsequent", "intermediate" and "main" in relation to temperature refer to the temperature measurements following the stages referred to respectively as subsequent, intermediate and later.

[0094] ΔT later denotes the temperature difference between the temperature of a point in the image measured at the later stage and the average temperature of a reference area of ​​the inner shell measured at the later stage,

[0095] ΔT intermediate denotes the temperature difference between the temperature of the previous point of the image measured at the intermediate step and the average temperature of a reference area of ​​the inner shell measured at the intermediate step,

[0096] ΔT main denotes the temperature difference between the temperature of the previous point of the image measured at the main temperature measurement stage and the average temperature of a reference area of ​​the inner shell measured at the main stage.

[0097] In its complete process, the control method according to the invention comprises four successive stages, namely: 1. a preliminary step with a temperature measurement under normal operating conditions of the tank 1, then 2. an intermediate step with a temperature measurement in which the pressure differential between the primary space 8 and the secondary space 6 - for the benefit of the primary space 8 - is between 500 Pa and 1500 Pa, preferably between 800 Pa and 1200 Pa, then 3. a main step with a temperature measurement in which the pressure differential between the primary space 8 and the secondary space 6 is equal to P1, then 4. a subsequent step with a temperature measurement under normal operating conditions of the tank 1.

[0098] It is important to note that only steps 3 and 4 are essential, in other words the control method according to the invention must carry out at least these two steps successively.

[0099] The process for checking the tightness of a tank can thus include a preliminary step whose aim is to ensure that there are no cold spots on the internal shell under normal operating conditions of the tank. It can also make it possible to locally check the emissivity of the interior shell paint in order to determine the temperature measurement performance.

[0100] First, the heating of the confined space is reduced or stopped at least 3 hours before the inspection. The pressures of the primary 8 and secondary 6 spaces are maintained in accordance with the normal operating conditions of the tank, for example, the pressure of the secondary 6 space is higher than the pressure of the primary 8 space. Then the inner shell 2 is completely inspected using a thermal imaging camera. This allows the inner shell 2 to be thermally checked under the normal operating conditions of the tank. At the end of the inspection, the system of the confined space 4 is returned to normal operating conditions.

[0101] In order to ensure in particular that the tank 1 has not been damaged or its condition aggravated by the main steps, a subsequent step of measuring the temperature of the outer surface of the inner shell 2 is carried out. This subsequent step is identical in every respect to the preliminary step on the operating conditions. If the preliminary step has been carried out, the thermograms obtained can then be compared to conclude on the state of the tank under normal operating conditions. If the preliminary step has not been carried out, it can then be ensured that there is no cold spot on the thermogram.

[0102] Finally, the method may also include an intermediate step to determine whether the tank is suitable for undergoing the main steps. Thus, the intermediate step may take place before the main steps and after the preliminary step. This step consists of measuring the temperature of the outer surface of the inner shell 2 from the confined space 4 when the pressure difference between the primary space 8 and the secondary space 6 is between 800 Pa and 1200 Pa, the primary space 8 being overpressured by said pressure difference relative to the secondary space 6.

[0103] Before proceeding with this intermediate step, the heating of the confined space 4 may be reduced at least 3 hours before detection by thermography. Indeed, here too, the heating could mask a possible cold spot on the internal shell 2. Then, it is ensured that the gas pressure in the tank 1, if it is filled with a low-temperature liquefied gas, is greater than 50 mbarg. Then the pressure value in the secondary space 6 is reduced to reach a value between 1 mbarg and 5 mbarg by using the suction device and leaving only the secondary gas outlet 19 in the open position. Then, the injection device 45 is actuated to inject nitrogen from the gas source 71 into the primary space 8 through the primary gas inlet 18 so that the pressure value in the primary space 8 reaches a value between 8 and 12 mbar above the pressure of the secondary space 6.Only the primary gas inlet 18 is left in the open position, all other gas inlets and outlets of the primary space 8 being closed. Optionally, the pressure is then waited for to stabilize. This can last between 30 and 60 min. The pressures of the primary and secondary spaces are managed within the acceptable pressure ranges by safety valves (not shown) of the primary and secondary spaces. The thermal inspection of the outer surface of the inner shell 2 can then be carried out by thermography from the confined space 4. Once the measurement has been carried out, the pressures of the spaces are returned to their normal operating condition values ​​as well as the heating of the confined space 4.

[0104] Thus, if the thermographic inspection carried out following this intermediate step does not show any significant cold spots, then the tank can undergo the main steps of the process.

[0105] Due to the geometry of the liquid dome 15 and the gas dome 21, the method described above may possibly fail. Indeed, the external climatic conditions and the temperature fields in and near these turrets may distort the temperature measurements with the thermographic camera and / or be very complex to take into account in the post-processing of the temperature measurements. The method may then be supplemented with a measurement of the nitrogen flow passing through the potential leaks of the secondary sealing membrane 7. Preferably, the flow will be directed from the primary space to the secondary space.

[0106] A first flow meter is installed at the liquid dome 15. The flow meter is installed on the pipe connecting the nitrogen source 16 and the primary gas inlet 18. The other primary inlets are put in the closed position. The primary outlets are also in the closed position. Thus, the only possible path for the nitrogen flow is to pass towards the secondary space if the secondary sealing membrane 7 has abnormal porosity. This flow meter will therefore make it possible to measure the flow of nitrogen entering the primary space.

[0107] A second flow meter is installed, it is placed at the secondary gas outlet 19 of the secondary space 6 located on the gas dome 21. The secondary inlets and the other secondary outlets are in the closed position. Thus the flow meter correctly measures the flow of nitrogen passing from the primary space 8 to the secondary space 6 via an abnormal porosity of the secondary sealing membrane 7.

[0108] This arrangement ensures that the nitrogen flow passes through the flow meters and avoids loss of information. It should be noted that the location of the flow meters may vary on each vessel.

[0109] Before installing the flow meters, it is ensured that the pressures in the primary and secondary spaces are normal, i.e. the pressures observed under normal operating conditions. The vapor pressure in the tank, if it is filled with low-temperature liquefied gas, must be maintained above 50 mbarg, preferably above 100 mbarg. Then the primary and secondary inlets, and the primary and secondary outlets that can influence the measurement of the nitrogen flow are put in the closed position. Then the flow meters are installed, the first flow meter is placed at the primary gas inlet 18 and the second flow meter is placed at the secondary gas outlet 19 or at the second secondary gas outlet 20. Once the flow meters are mounted, the primary space 8 is supplied with nitrogen while regulating this supply using a valve, for example up to 12 m 3 < / h (cubic meter per hour).Then we begin measuring the flow rates entering the primary space and leaving the secondary space with the flow meters. The control and measurement of the flow rates at the primary gas inlet 18 and at the secondary gas outlet 19 lasts a maximum of 5 hours, preferably 3 hours.

[0110] At the end of the measurements, the flow rates at the primary gas inlet 18 and at the secondary gas outlet 19 are compared. If the measurements are similar, then the secondary sealing membrane 7 does indeed have abnormal porosity, particularly at the liquid dome and / or gas dome. On the other hand, if the flow rates are significantly different or even the flow rate at the secondary gas outlet is zero, then the secondary sealing membrane 7 does not have abnormal porosity anywhere.

[0111] The installation described above and using the process described above can be used, for example, in a land-based installation or in a floating structure such as a methane tanker or other vessel.

[0112] With reference to the [ Fig.9 ], a cutaway view of a LNG carrier 100 shows a storage facility for low-temperature liquefied gas comprising a sealed and insulated tank 1 of generally prismatic shape mounted in the double hull 101 of the ship. The tank 1 comprises a primary sealing membrane intended to be in contact with the low-temperature liquefied gas LNG contained in the tank such as LNG, a secondary sealing membrane arranged between the primary sealing membrane and the double hull 101 of the ship, and two insulating barriers arranged respectively between the primary sealing membrane and the secondary sealing membrane and between the secondary sealing membrane and the double hull 101.

[0113] The [ Fig.9] represents an example of a maritime terminal comprising a loading and unloading station 103, an underwater pipeline 104 and an onshore installation 105. The loading and unloading station 103 is a fixed offshore installation comprising a mobile arm 106 and a tower 107 which supports the mobile arm 106. The mobile arm 106 carries a bundle of insulated flexible pipes 108 which can be connected to the loading / unloading pipelines 109. The orientable mobile arm 106 adapts to all sizes of LNG carriers. A connecting pipe, not shown, extends inside the tower 107. The loading and unloading station 103 allows the loading and unloading of the ship 100 from or to the onshore installation 105. The latter comprises low-temperature liquefied gas storage tanks 110 and connecting pipes 111 connected by the subsea pipe 104 to the loading and unloading station 103.The subsea pipeline 104 allows the transfer of low-temperature liquefied gas between the loading and unloading station 103 and the shore installation 105 over a long distance, for example 5 km, which makes it possible to keep the vessel 100 at a great distance from the coast during loading and unloading operations.

[0114] To generate the pressure necessary for the transfer of the low-temperature liquefied gas, pumps on board the ship 100 and / or pumps equipping the onshore installation 105 and / or pumps equipping the loading and unloading station 103 are used.

Claims

1. A method for checking the sealing of a sealed and thermally insulating tank (1) for storing a liquefied gas at low temperature (30), the tank (1) being at low temperature (30), the tank (1) comprising a carrier structure which has an inner hull (2) and an outer hull (3), a confined space (4) between the inner hull (2) and the outer hull (3), a primary sealing membrane (9) which is intended to be in contact with the liquefied gas at low temperature (30) contained in the tank (1), and a secondary sealing membrane (7) which is arranged between the primary sealing membrane (9) and the inner hull (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 hull (2), the primary space (8) and the secondary space (6) comprising insulating materials, the primary space (8) comprising at least one primary gas inlet (18) and the secondary space (6) comprising at least one secondary gas outlet (19), said primary sealing membrane resting directly on the insulating materials contained in the primary space and said secondary sealing membrane resting directly on the insulating materials 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 on the outer surface of the inner hull (2): - a step referred to as main step in which the secondary space (6) is brought to a pressure lower than the pressure of the primary space (8), with a pressure differential P1, by injecting an inert gas into the primary space via the primary gas inlet (18) and causing gas to be expelled or drawn from the secondary gas outlet (19) of the secondary space and then by measuring the temperature of an outer surface of the inner hull (2) from the confined space (4) situated around the inner hull (2); - a step referred to as a subsequent step in which the temperature of the outer surface of the inner hull (2) is measured from the confined space (4) under the normal operating conditions of the tank (1); characterized in that, during the main step, at least some 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 is reinjected into the primary space (8).

2. The method as claimed in claim 1, wherein the entirety of the inert gas injected into the primary space (8) is reinjected into said space (8) after having been recovered by at least one suction means (80) connected to a secondary outlet (19, 20 or 23) of the secondary space (6).

3. The method as claimed in claim 1, wherein just some of the inert gas injected into the primary space (8), advantageously representing between 20% and 80% of the inert gas injected into the primary space (80), is reinjected into said space (8) after having been recovered by at least one suction means (80) connected to a secondary outlet of the secondary space (6).

4. The method as claimed in any one of the preceding claims, wherein said method comprises a step referred to as preliminary step in which the temperature of the outer surface of the inner hull (2) is measured from the confined space (4) under the normal operating conditions of the tank (1).

5. The method as claimed in any one of the preceding claims, wherein the pressure differential P1 is: - comprised between 500 Pa and 1500 Pa, preferably comprised between 800 Pa and 1200 Pa for a duration of at least 10 hours, preferably at least 12 hours, or - comprised between 1800 Pa and 3200 Pa, preferably comprised between 2100 Pa and 2900 Pa.

6. The method as claimed in any one of the preceding claims, wherein the step referred to as the main step is preceded by a step referred to as an intermediate step in which the secondary space (6) is brought to a pressure lower than the pressure of the primary space (8), with a pressure differential of between 500 Pa and 1500 Pa, preferably between 800 Pa and 1200 Pa, by injecting an inert gas into the primary space via the primary gas inlet (18) and causing gas to be expelled or drawn from the secondary gas outlet (19) of the secondary space and then by measuring the temperature of an outer surface of the inner hull (2) from the confined space (4) situated around the inner hull (2).

7. The method as claimed in any one of the preceding claims, wherein the inert gas consists of nitrous oxide.

8. The method as claimed in any one of the preceding claims, wherein the duration of each temperature measurement is at the most five hours, preferably a maximum of three hours.

9. The method as claimed in any one of the preceding claims, wherein P1, advantageously the pressure differential of the step referred to as the intermediate step, is kept stable while the temperature is being measured.

10. A floating or onshore storage installation for a liquefied gas at low temperature (30) for implementing the checking method as claimed in any one of the preceding claims, comprising: - a tank (1) at low temperature (30) comprising a carrier structure which has an inner hull (2) and an outer hull (3), the space between the inner hull (2) and the outer hull (3) being referred to as the confined space (4), a primary sealing membrane (9) which is intended to be in contact with the liquefied gas at low temperature (30) contained in the tank, and a secondary sealing membrane (7) which is arranged between the primary sealing membrane (9) and the inner hull (2), 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 hull (2) is referred to as the secondary space (6), the primary space (8) and the secondary space (6) comprising insulating materials, the primary space (8) comprising at least one primary gas inlet (18), the secondary space (6) comprising at least one secondary gas outlet (19), said primary sealing membrane resting directly on the insulating materials contained in the primary space and said secondary sealing membrane resting directly on the insulating materials contained in the secondary space, - an inert gas reservoir (16) which is arranged in order to inject inert gas into the primary space (8) through the primary gas inlet, - an injection device (45) which is capable of injecting inert gas of the inert gas reservoir (16) via the primary gas inlet (18), and thus placing the primary space (8) under pressure relative to the secondary space (6), - a suction device (80, 80') which is connected to the secondary gas outlet (19) in order to generate a lower pressure in the secondary space (6) than the pressure in the primary space (8), - a device for measuring the temperature of the outer surface of the inner hull (2), - a system for displaying the temperature measurements in order to locate a sealing defect of the secondary sealing membrane (7) in the form of a cold spot on the outer surface of the inner hull (2), characterized in that the suction device comprises at least one suction means (80) consisting of a pump that draws at least some of the inert gas out of the secondary space (6) and reinjects it into the primary space (8), preferably via the aforesaid injection device (45).

11. The installation as claimed in claim 10, wherein the aforesaid suction means (80) forms the only suction means (80) of said device such that all of the inert gas recovered in the secondary space (6) is reinjected into the primary space (8).

12. The installation as claimed in claim 10, wherein the suction device also comprises at least one suction system (80') recovering the inert gas and not reinjecting it into the primary space (8).

13. The installation as claimed in claim 12, wherein the suction system (80') is a Venturi effect suction system (80') which comprises a main pipe (83) which has an inlet which is capable of being connected to a pressurized gas source (71), and an outlet toward the outer side of the tank (1), a suction pipe (81) having an upstream side which is capable of being connected to the outlet port of the secondary space (6) and a downstream side which opens laterally in a convergent / divergent section of the main pipe (83) so that a gas flow in the main pipe (83) produces a reduced pressure in the suction pipe (81).

14. The installation as claimed in claim 13, wherein the pressurized gas source (71) is a compressed air circuit.

15. The installation as claimed in any one of claims 10 to 14, wherein the injection device (45) comprises a compressor which is capable of injecting the inert gas from the inert gas reservoir (16) at a pressure between 3 and 8 bar.

16. A vessel (100) for transporting a liquefied gas at low temperature (30), the vessel comprising an installation for storing a liquefied gas at low temperature (30) as claimed in any one of claims 10 to 15.