EP3129700B1 - Sealed, heat-insulated vessel housed in a buoyant structure - Google Patents

Description

Technical area

The invention relates to the field of sealed and thermally insulating vessels for storing a liquefied gas at low temperature, and more particularly to devices and methods for detecting leaks in a secondary sealing membrane of such a vessel.

Technological background

In a vessel of a LNG tank, the upper wall of the tank has structures called vapor dome and liquid dome which are in the form of two turrets or chimneys protruding from an outer surface of the upper supporting wall and intended to pass cargo handling equipment for handling a liquid phase and a vapor phase of the liquefied gas contained in the tank.

Due to this geometry, leak detection methods based on the observation of abnormally hot or abnormally cold zones can be defeated, in particular because of the influence of external climatic conditions and because the temperature fields in and in the vicinity of these turrets are too complex.

It is known from the document FR2517802A1 a tank for storing a liquefied gas with leak detection and a corresponding leak detection method by means of a tracer gas.

summary

An idea underlying the invention is to provide devices and methods for detecting leaks in a sealed and thermally insulating tank in and around these protruding structures.

• une paroi étanche interne formant une gaine engagée à travers l'ouverture de la paroi porteuse supérieure et liée de manière étanche avec la membrane d'étanchéité primaire de la paroi de cuve supérieure tout autour de la gaine,
• une paroi étanche externe disposée autour de la gaine à distance de la gaine parallèlement à la gaine, la paroi étanche externe étant liée de manière étanche à la paroi porteuse supérieure autour de l'ouverture,
• une paroi de séparation agencée entre la paroi étanche externe et la paroi étanche interne de la tourelle et divisant l'espace défini entre la paroi étanche externe et la paroi étanche interne de la tourelle en, d'une part, un espace secondaire communiquant à travers l'ouverture de la paroi porteuse supérieure avec la barrière isolante secondaire de la paroi de cuve supérieure disposée autour de l'ouverture et, d'autre part, un espace primaire communiquant à travers l'ouverture de la paroi porteuse supérieure avec la barrière isolante primaire de la paroi de cuve supérieure disposée autour de l'ouverture,

un dispositif d'échappement primaire comportant une soupape de surpression primaire et une conduite d'échappement primaire communiquant directement avec l'espace primaire de la tourelle et traversant la paroi étanche externe de la tourelle pour permettre un échappement de gaz depuis l'espace primaire en réponse à une ouverture de la soupape de surpression primaire,
un dispositif d'échappement secondaire comportant une soupape de surpression secondaire et une conduite d'échappement secondaire communiquant directement avec l'espace secondaire de la tourelle et traversant la paroi étanche externe de la tourelle pour permettre un échappement de gaz depuis l'espace secondaire en réponse à une ouverture de la soupape de surpression secondaire,
l'ouvrage flottant comportant en outre :

• un réservoir de gaz contenant un gaz traceur incondensable ou présentant une température de condensation inférieure à la basse température du gaz liquéfié contenu dans la cuve, le réservoir de gaz étant relié par l'intermédiaire d'une vanne de contrôle à l'un parmi le dispositif d'échappement primaire, notamment la conduite d'échappement primaire, et le dispositif d'échappement secondaire, notamment la conduite d'échappement secondaire, et
• un détecteur de gaz apte à détecter le gaz traceur, le détecteur de gaz étant en communication avec l'autre parmi le dispositif d'échappement primaire, notamment la conduite d'échappement primaire, et le dispositif d'échappement secondaire, notamment la conduite d'échappement secondaire.

According to one embodiment, the invention provides a floating structure comprising a shell including load-bearing walls defining a polyhedral space inside the shell, the floating structure comprising a sealed and thermally insulating tank housed in the polyhedral space for store a liquefied gas at low temperature, wherein an upper carrier wall of the hull has an opening and carries a turret projecting from an outer surface of the upper carrier wall around the opening, the opening and the turret being for passing cargo handling equipment for handling a liquid phase and / or a vapor phase of the liquefied gas contained in the tank,
wherein the vessel has a plurality of vessel walls attached to the carrier walls of the hull,
wherein an upper vessel wall has a multilayer structure attached to an inner surface of the upper carrier wall, the multilayer structure being formed of a primary sealing membrane for contacting the liquefied gas contained in the vessel, a secondary sealing membrane disposed between the primary sealing membrane and the upper supporting wall, a secondary insulating barrier disposed between the secondary sealing membrane and the upper supporting wall and a primary insulating barrier disposed between the secondary sealing membrane and the primary waterproofing membrane,
in which the turret comprises:

• an inner sealed wall forming a sheath engaged through the opening of the upper support wall and sealingly connected with the primary sealing membrane of the upper tank wall all around the sheath,
• an external sealing wall disposed around the sheath at a distance from the sheath parallel to the sheath, the external sealing wall being sealingly connected to the upper bearing wall around the opening,
• a partition wall arranged between the external sealing wall and the internal tight wall of the turret and dividing the space defined between the external sealing wall and the internal tight wall of the turret, on the one hand, a secondary space communicating through the opening of the upper supporting wall with the secondary insulating barrier of the upper tank wall disposed around the opening and, secondly, a primary space communicating through the opening of the upper supporting wall with the insulating barrier primary of the upper tank wall disposed around the opening,

a primary exhaust device comprising a primary pressure relief valve and a primary exhaust pipe communicating directly with the primary space of the turret and passing through the outer wall of the turret to allow a gas escape from the primary space in response to an opening of the primary pressure relief valve,
a secondary exhaust device comprising a secondary pressure relief valve and a secondary exhaust pipe communicating directly with the secondary space of the turret and passing through the outer tight wall of the turret to allow exhaust gas from the secondary space in response to an opening of the secondary pressure relief valve,
the floating structure further comprising:

• a gas tank containing an incondensable tracer gas or having a condensing temperature lower than the low temperature of the liquefied gas contained in the tank, the gas tank being connected by means of a control valve to one of the primary exhaust system, including the primary exhaust pipe, and the secondary exhaust system, including the secondary exhaust pipe, and
• a gas detector capable of detecting the tracer gas, the gas detector being in communication with the other one of the primary exhaust device, in particular the primary exhaust duct, and the secondary exhaust device, in particular the duct secondary exhaust.

Thanks to these characteristics, it is possible to detect a leakage gap between the primary space and the secondary space of the turret and / or between the primary insulating barrier and the secondary insulating barrier of the upper tank wall. In addition, the use of primary and secondary exhaust devices for injection and tracer gas detection makes the implementation of leak detection particularly simple.

According to embodiments, such a floating structure may include one or more of the following features.

The exhaust devices can be designed in different ways. According to one embodiment, the primary or secondary exhaust device further comprises a primary or secondary control line which communicates directly with the primary or secondary space of the turret and which passes through the outer tight wall of the turret to control the primary or secondary pressure relief valve as a function of the pressure in the primary or secondary space, and wherein the gas reservoir is in direct communication with the primary or secondary control line.

Alternatively, the gas tank can be connected directly to the primary or secondary exhaust pipe.

According to one embodiment, the primary or secondary exhaust device further comprises a primary or secondary control line which communicates directly with the primary or secondary space of the turret and which passes through the outer tight wall of the turret to control the primary or secondary overpressure valve as a function of the pressure in the primary or secondary space, and wherein the gas detector is in direct communication with the primary or secondary control line.

Alternatively, the gas detector can be connected directly to the primary or secondary exhaust pipe.

According to one embodiment, the turret is a steam dome of the tank, the sheath engaged through the opening of the upper bearing wall being a collecting pipe connected to a main steam collector of the floating structure.

Thanks to these characteristics, it is possible to detect a leakage gap between the primary space and the secondary space of the steam dome and / or between the primary insulating barrier and the secondary insulating barrier of the upper tank wall in the vicinity of the steam dome.

The steam dome can be designed in different ways. Preferably in this case, the separation wall of the turret forms a primary sampling pipe extending parallel to the collecting duct in the space defined between the external waterproof wall and the internal tight wall of the turret and having an inner end. opening into the primary insulating barrier of the upper tank wall and an outer end opening directly into the primary exhaust device, the primary space of the turret having the inner space of the primary bleed pipe.

According to another embodiment, the turret is a liquid dome of the vessel further comprising a top wall disposed on an upper end of the outer wall of the liquid dome and having an opening aligned with a central zone of the opening of the upper carrier wall, the sheath formed by the inner sealed wall of the liquid dome being a primary sealing membrane having an upper edge sealingly attached to an edge of the apex wall all around the aperture of the apex wall.

Thanks to these characteristics, it is possible to detect a leakage fault between the primary space and the secondary space of the liquid dome and / or between the primary insulating barrier and the secondary insulating barrier of the upper tank wall in the vicinity of the liquid dome.

The liquid dome can be designed in different ways. Preferably in this case, the partition wall comprises a secondary sealing membrane extending all around the sheath between the outer sealing wall and the sheath and having an inner end sealingly connected to the secondary sealing membrane of the upper vessel wall all around the sheath and an outer end sealingly connected to the apex wall all around the opening of the top wall of the liquid dome.

According to one embodiment, the tank comprises the aforementioned provisions on both the liquid dome and the steam dome to be able to detect leaks in these two areas of the tank.

According to one embodiment, the wall of the liquid dome comprises a multilayer structure fixed on an inner surface of the outer wall, the multilayer structure being formed of the primary sealing membrane of the liquid dome, the secondary sealing membrane of the dome liquid, a secondary insulating barrier of the liquid dome disposed between the secondary sealing membrane and the outer sealing wall and a primary insulating barrier disposed between the secondary sealing membrane and the primary sealing membrane of the liquid dome.

Preferably in this case, the floating structure further comprises a connecting plate disposed between the outer end of the secondary sealing membrane of the liquid dome and the top wall, the connecting plate comprising a main branch extending parallel to the external sealing wall between the external sealing wall and the sheath formed by the internal sealed wall of the liquid dome, the main branch having an upper end attached to the top plate and a lower end extending by a bent flange to the interior of the liquid dome with respect to the main panel, the outer end of the secondary sealing membrane being sealingly attached to the rim,
wherein the secondary insulating barrier of the liquid dome has a trim fibrous material disposed between the main branch of the connecting plate and the outer watertight wall, and
wherein the secondary exhaust duct opens into the fibrous liner.

Thanks to these characteristics, the pressure drop induced by the fibrous lining in the zone of the secondary space where the tracer gas is injected or removed is relatively low, which facilitates the circulation of the tracer gas, especially around the liquid dome.

According to a corresponding embodiment, the primary exhaust pipe passes through the main branch of the connecting plate and opens into the primary insulating barrier between the main branch of the connecting plate and the primary sealing membrane of the liquid dome.

Preferably, the floating structure further comprises a nitrogen distribution system including a nitrogen gas reservoir and a distribution network, the distribution network comprising a primary distribution pipe extending from an upper deck of the a structure floating through the primary space of the liquid dome and through the primary isolation barrier of a transverse wall of the vessel to a bottom zone of the vessel, and a secondary distribution pipe extending from the upper bridge of the structure floating through the secondary space of the liquid dome and through the secondary insulation barrier of the transverse wall of the tank to the bottom zone of the tank.

Advantageously, the nitrogen distribution system further comprises pressure regulating means for regulating the pressure prevailing in the primary insulation barrier and the secondary insulation barrier of the walls of the tank by means of the primary distribution pipes and secondary.

Thanks to such pressure regulating means, it is possible to avoid damaging the sealing barriers by the effect of accidental overpressures.

According to one embodiment, these pressure regulating means are used to generate a pressure difference between the zone where the tracer gas is injected and the zone where the tracer gas is sought so as to show more quickly the leaks or faults. sealing.

Such a tank can be used to store all kinds of liquefied gases at atmospheric pressure, for example butane, propane, ethane, ethylene, methane and others. According to one embodiment, the liquefied gas contained in the tank is liquefied natural gas (LNG), namely a gas with a high methane content, stored at a temperature of about -162 ° C.

Different chemical bodies can be used as tracer gas, particularly depending on the nature and temperature of the liquefied gas stored. According to one embodiment, particularly suitable for an LNG tank, the tracer gas is selected from argon, helium and mixtures thereof.

According to one embodiment, the tracer gas reservoir and / or the gas detector are removably attached to the primary or secondary exhaust device. Thanks to these characteristics, it is possible to dismantle the tracer gas reservoir and / or the gas detector from the access of the exhaust system to which it is attached, for example a pipe or a flange, so as to release this access of the exhaust device for another use outside the phases where the leak detection is implemented.

• injecter le gaz traceur à travers l'un des dispositifs d'échappement primaire et secondaire dans l'espace primaire ou secondaire de la tourelle sans dépasser la pression d'ouverture de la soupape de surpression primaire ou secondaire,
• détecter le gaz traceur à travers l'autre des dispositifs d'échappement primaire et secondaire dans l'espace primaire ou secondaire de la tourelle, et
• diagnostiquer une fuite dans la barrière d'étanchéité secondaire de la paroi supérieure de cuve et/ou dans la paroi de séparation de la tourelle en réponse à la détection du gaz traceur.

The invention also provides a method of operating a floating structure mentioned above, comprising:

• injecting the tracer gas through one of the primary and secondary exhaust devices in the primary or secondary space of the turret without exceeding the opening pressure of the primary or secondary pressure relief valve,
• detecting the tracer gas through the other primary and secondary exhaust devices in the primary or secondary space of the turret, and
• diagnose a leak in the secondary sealing barrier of the vessel top wall and / or in the separation wall of the turret in response to detection of the tracer gas.

According to one embodiment, the tracer gas is injected into the secondary space through the secondary exhaust device and detected in the primary space through the primary exhaust device, the method further comprising:
maintaining a higher total pressure in the secondary space than in the primary space by injecting nitrogen gas into the secondary space, without exceeding the opening pressure of the secondary pressure relief valve.

Reverse operation is also possible, where the tracer gas is injected into the primary space through the primary exhaust and detected in the secondary space through the secondary exhaust. The pressure levels can be reversed in this case.

In one embodiment, the step of diagnosing a leak includes one of the measures selected from the group consisting of recording the presence of a leak, measuring a quantity or a concentration of the tracer gas to determine a flow rate of the leak, and measuring a time delay between the injection and the tracer gas detection to determine a position of the leak.

Such a tank can be part of a land storage facility, for example to store LNG or be installed in a floating structure, coastal or deep water, including a LNG tank, a floating storage and regasification unit (FSRU) , a floating production and remote storage unit (FPSO) and others.

According to one embodiment, the invention also provides a method of loading or unloading such a floating structure, in which liquefied gas is conveyed through isolated pipes to or from a floating or land storage facility to or from the waterproof and thermally insulating tank.

According to one embodiment, the invention also provides a transfer system for liquefied gas at low temperature, the system comprising a floating structure above, insulated pipes arranged to connect the sealed tank and thermally insulating to a floating storage facility or terrestrial and a pump for driving a flow of cold liquid product through the insulated pipelines from or to the floating or land storage facility to or from the vessel vessel.

Some aspects of the invention start from the idea of limiting the distance to be traveled by the tracer gas between the injection point and the detection point, so that the detection of leaks in the turret zone can be implemented. relatively quickly and with a relatively small amount of tracer gas relative to the volume of all the walls of the tank. Certain aspects of the invention start from the idea of proposing a test method that can be executed in sea with the tank in cold, so as to avoid immobilizing the floating work in dry dock.

Brief description of the figures

• La figure 1 est un schéma fonctionnel d'une cuve de navire méthanier vue en coupe selon un axe longitudinal du navire.
• La figure 2 est un diagramme fonctionnel d'un dôme liquide de la cuve de la figure 1, vu de dessus.
• La figure 3 est une vue en perspective isométrique écorchée d'une paroi transversale délimitant un côté avant du dôme liquide de la figure 2.
• La figure 4 est une vue agrandie de la zone IV de la figure 1 selon un mode de réalisation
• La figure 5 est un diagramme fonctionnel d'un dôme vapeur de la cuve de la figure 1, vu de dessus.
• La figure 6 est une vue agrandie de la zone VI de la figure 1 selon un mode de réalisation
• La figure 7 est une représentation schématique écorchée d'une cuve de navire méthanier et d'un terminal de chargement/déchargement de cette cuve.

The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly in the course of the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes. with reference to the accompanying drawings.

• The figure 1 is a block diagram of a LNG vessel tank seen in section along a longitudinal axis of the vessel.
• The figure 2 is a functional diagram of a liquid dome of the tank of the figure 1 , seen from above.
• The figure 3 is a cut isometric perspective view of a transverse wall delimiting a front side of the liquid dome of the figure 2 .
• The figure 4 is an enlarged view of area IV of the figure 1 according to one embodiment
• The figure 5 is a functional diagram of a steam dome of the tank of the figure 1 , seen from above.
• The figure 6 is an enlarged view of zone VI of the figure 1 according to one embodiment
• The figure 7 is a cutaway schematic representation of a tank of LNG tanker and a loading / unloading terminal of this tank.

Detailed description of embodiments

With reference to the figure 1 , schematically shows a longitudinal section of the hull 2 of a LNG tank in which is disposed a sealed and insulating tank 1 made according to the technology of the membrane tanks.

The vessel 1 is disposed between two transverse partitions 3 generally called "cofferdam" which divide the inner space of the shell into a plurality of polyhedral compartments for each to accommodate a respective tank. A vessel may thus have one or more similar vessels, as sketched on the right of the figure 1 .

The shell 2 is a double wall defining a ballast space shown at number 4 for the lower part of the tank. The tank 1 is built on the inner wall 5 of the shell 2 which serves as a load-bearing wall. The upper wall 6 of the tank 1 is similarly carried by an upper supporting wall 7 forming part of the shell 2.

The vessel 1 has a generally polyhedral geometry and all the walls of the vessel consist of a multilayer structure known in the art of membrane tanks. It suffices to recall that this multilayer structure comprises successively a secondary insulating barrier 10, a secondary waterproof membrane 11, a primary insulating barrier 12 and a primary waterproof membrane 13 which directly contains the LNG stored in the tank 1. This multilayer structure can be realized according to various techniques, for example techniques marketed by the applicant company under the name Mark III®.

The figure 1 shows that the upper supporting wall 7 is interrupted at two locations where the vessel wall forms a protruding structure in the form of turret or chimney. The first turret is a liquid dome 15 which serves as a point of penetration for various LNG handling equipment, namely in the example shown a filling line 16, an emergency pumping line 17, unloading lines connected to discharge pumps 18, a spray line 20 and a feed line connected to a spraying pump 19. The second turret is a steam dome 21 which serves as a point of entry for a vapor collection pipe 22. The operation of this equipment is known elsewhere.

With reference to Figures 2 to 4 , we will now describe more precisely the characteristics of the liquid dome 15. The elements similar or identical to those of the figure 1 carry the same reference number increased by 100.

As better visible on the figure 2 , the liquid dome has a square section formed of four similar vertical walls in which there is each time the aforementioned multilayer structure, namely carrier wall 103, secondary insulating barrier 110, secondary waterproof membrane 111, primary insulating barrier 112 and primary waterproof membrane 113.

Since the waterproofing membranes 111 and 113 are relatively fragile elements that are not designed to withstand high tearing forces, the liquid dome is equipped with a primary exhaust device 25 to protect the primary waterproof membrane 113 against the overpressures and a secondary exhaust device 35 to protect the secondary waterproof membrane 111 against overpressures.

More specifically, the primary exhaust device 25 comprises an exhaust pipe I opening on one side inside the primary insulating barrier 112 of the liquid dome and on the other side in an exhaust mast towards the atmosphere 30 disposed outside on the deck of the ship. A pressure relief valve 27 is arranged on the pipe I, which is closed by default. The valve 27 opens under the control of a valve driver 26 when the total pressure in the primary insulating barrier 112 exceeds a predefined level, for example 30 mbar or 3kPa. The valve driver 26 is connected to the pressure of the primary insulating barrier 112 by a control line N. Thus, the gas phase present in the primary insulating barrier 112 escapes automatically to the exhaust mast 30 when its pressure exceeds the predefined level.

In the same way, the secondary exhaust device 35 has an exhaust pipe K opening on one side inside the secondary insulating barrier 110 of the liquid dome and on the other side in an exhaust line towards the atmosphere 40. A pressure relief valve 37 is arranged on the pipe K, which is closed by default. The valve 37 opens under the control of a valve driver 36 when the total pressure in the secondary insulating barrier 110 exceeds a predefined level, for example 30 mbar or 3kPa. The valve driver 36 is connected to the pressure of the secondary insulating barrier 110 by a control line M. Thus, the gas phase present in the secondary insulating barrier 110 escapes automatically to the exhaust line 40 when its pressure exceeds the predefined level. The opening pressures of the valves 27 and 37 may be equal or different.

In order to detect leaks or leaks in the secondary waterproof membrane 111 at the level of the liquid dome 15, a device for injecting and detecting a tracer gas is used in the liquid dome 15. This device comprises a tracer gas tank 41 which is connected to the control line M via a valve 42 so as to be able to transfer tracer gas to the secondary insulating barrier 110 when the valve 42 is open. The tracer gas is for example argon or helium or any other gas or gas mixture that is not likely to liquefy during operation.

This device also comprises a gas detector 43 capable of detecting the tracer gas and connected to the exhaust pipe I, so as to detect the presence of the tracer gas in the gas phase present in the primary insulating barrier 112.

The fundamental principle of the detection is as follows: since the secondary waterproof membrane 111 is supposed to isolate in gas-tight manner the secondary insulating barrier 110 of the primary insulating barrier 112, a positive detection of the tracer gas in the primary insulating barrier 112 while this gas has been injected into the secondary insulating barrier 110 necessarily betrays the existence of a leak.

Alternatively, the tank 41 can be connected to the pipe K and / or the detector 43 can be connected to the pipe N without changing the operating principle.

The figure 2 also shows nitrogen feed lines that enter the tank at the liquid dome to allow control of the total pressure in the secondary 110 and primary 112. These supply lines come from a tank of gaseous nitrogen symbolized at number 45. They include a secondary nitrogen line V opening into the secondary insulating barrier 110 at the bottom of the tank and a primary nitrogen line 44 which branches out into a plurality of nitrogen distribution lines A, B, C, D, E, F, G, H, J, L all opening in the primary insulating barrier 112 at the bottom of the tank.

The figure 3 shows further details on the possible route of the gaseous dinitrogen supply lines in the vessel wall. It shows in particular that these lines open at the bottom of the tank at a considerable distance from the liquid dome and the steam dome. Gaseous dinitrogen supply lines can be used in particular to render inert the vessel walls and to adjust the total pressure therein by means of a pressure control system which is known elsewhere.

• Injecter le gaz traceur dans l'espace secondaire 110 du dôme liquide
• Régler la pression à un niveau légèrement plus élevé dans l'espace secondaire 110 que dans l'espace primaire 112, de préférence sans atteindre la pression d'ouverture de la soupape d'échappement secondaire 37 pour éviter d'endommager la membrane secondaire 111. Ces deux étapes peuvent aussi être réalisées simultanément ou dans un autre ordre.
• Détecter la présence du gaz traceur dans l'espace primaire 112.

This pressure control system can be used to improve the operation of leak detection. According to a corresponding embodiment, the leak detection can be implemented in the following manner:

• Inject the tracer gas into the secondary space 110 of the liquid dome
• Set the pressure at a slightly higher level in the secondary space 110 than in the primary space 112, preferably without reaching the opening pressure of the secondary exhaust valve 37 to avoid damaging the secondary diaphragm 111. These two steps can also be performed simultaneously or in another order.
• Detect the presence of the tracer gas in the primary space 112.

Thanks to the slight difference in pressure, the tracer gas transfer can be accelerated, which reduces the duration of the leak detection test. For example, the pressure in the primary barrier is set to 10mbar (100kPa) relative and the pressure in the secondary barrier is set to 17mbar (170kPa) relative, a difference of 70kPa. This difference can be higher, for example up to 250kPa, to accelerate the implementation of the test. The complete duration of the test can thus be less than 4 hours per dome, and preferably of the order of 60 minutes.

In a mode not shown, the positions of the gas detector 43 and the tracer gas tank 41 are exchanged and the pressure differences are reversed.

• L'existence d'une fuite, si le gaz traceur est détecté en quantité non négiigeabie,
• le débit de la ou des fuites par intégration de la quantité de gaz traceur au cours du temps,
• la localisation sur le dôme liquide de la ou des fuites, par mesure de l'instant de première détection du gaz traceur par rapport au temps de trajet dans les barrières 'isolantes.

The gas detector may be a commercially available gas analyzer operating according to any suitable technology, for example by mass spectrometry or the like. To refine the diagnosis of leaks, it is preferable to measure over time the concentration of tracer gas present in the primary space 112. Thus, the duration and the amount of tracer gas make it possible to obtain information on:

• The existence of a leak, if the tracer gas is detected in a non-negligible quantity,
• the flow rate of the leak (s) by integrating the quantity of tracer gas over time,
• the location on the liquid dome of the leak or leaks, by measuring the time of first detection of the tracer gas with respect to the travel time in the insulating barriers.

Since the vessel wall in the liquid dome has a relatively small volume relative to the entire vessel, for example about 2 m ³ , the leak test can be carried out with a relatively small volume of gas tracer, for example about 3 m ³ of argon.

The figure 4 shows further details of implementation of the liquid dome 15 in one embodiment using the Mark III® technology. For the sake of brevity only one pipe has been shown to illustrate the pipe K or M of the secondary exhaust device 35 and a single pipe has been shown to illustrate the pipe N or I of the primary exhaust device 25. In addition these pipes have been represented in a single plan. However, these pipes are four in a real implementation and it is not necessary that they are in the same plane, which shows the figure 2 . In addition, it is possible to provide one or more additional tracer gas injection points to improve the speed of execution of the test, especially for a large liquid dome. These additional injection points can be evenly distributed around the periphery of the liquid dome 15.

In the liquid dome 15 of the figure 4 , the supporting structure comprises a vertical supporting wall 103 called surbau which rises above the deck 107 of the ship and a horizontal wall 46 at the top of the carrier wall 103. The horizontal wall 46 extends all around the liquid dome and It supports a tank cover 47. The cover 47 essentially consists of a metal cover wall 48 and a thermal insulation 49 which is inserted into the top of the liquid dome.

The horizontal wall 46 carries an L-shaped metal plate 48 which is welded to the inner face of the wall 46 and extends downwards. Prefabricated panels are attached to the carrier wall 103 to form the primary heat-insulating barrier, the secondary sealing barrier and the secondary heat-insulating barrier.

In the stopping zone of the secondary membrane 111, a flexible sealed composite ply 50 sealingly connects the waterproof ply of the prefabricated panel to the bent rim 51 of the plate 48. The bonding of the ply 50 to the flange 51 is carried out at using a suitable adhesive, for example polyurethane type.

A glass wool pad 52 is inserted between the metal plate 48 and the carrier wall 103 to extend the secondary insulating barrier 110, which is essentially made of insulating foam panels. A layer of putty 53, for example epoxy resin, is crushed between the underside of the flange 51 and the last insulating foam panel to precisely fix and position the panel. A second layer of putty 54, for example also epoxy resin, is carried by the upper face of the flange 51 and is crushed between the flange 51 and a wooden beam 55 which is arranged horizontally along the plate 48. The beam 55 may be bolted to the plate 20. Other insulating foam blocks 56 are disposed between the top of the beam 55 and the horizontal wall 46 of the supporting structure to extend the primary insulation barrier.

The end of the primary sealing barrier 113 is sealingly attached to the supporting structure by welding on a profiled section 57 with a U-section which is carried by the end of the horizontal wall 46.

In this embodiment, the glass wool packing 52 which runs all the way around the liquid dome constitutes a preferred passage zone for the tracer gas which has passed a leak, due to the low pressure drop. Thus, it is possible to detect leaks at any location around the liquid dome even with a single or a small number of gas detection points.

The methods described above for performing a leak detection in a liquid dome can be carried out analogously in a steam dome, as will be described now with reference to figures 5 and 6 .

As better visible on the figure 5 , the vapor dome has a circular section in which there is found, at least functionally, the aforementioned multilayer structure, namely carrier wall 203, secondary insulating barrier 210, secondary waterproof membrane 211, primary insulating barrier 212 and primary waterproof membrane 213.

Since the waterproofing membranes 211 and 213 are relatively fragile elements that are not designed to withstand high tearing forces, the steam dome is equipped with a primary exhaust device 125 to protect the primary waterproof membrane 213 against the overpressures and a secondary exhaust device 135 to protect the secondary waterproof membrane 211 against overpressures.

More specifically, the primary exhaust device 125 comprises an exhaust pipe Q opening on one side inside the primary insulating barrier 212 of the steam dome and on the other side in the exhaust mast towards the atmosphere 30 of the figure 2 as symbolized by the arrow 130 of the figure 5 . A pressure relief valve 127 is arranged on line Q, which is closed by default. The valve 127 opens under the control of a valve driver 126 when the total pressure in the primary insulating barrier 212 exceeds a predefined level, for example 30 mbar or 3kPa. The valve driver 126 is connected to the pressure of the primary insulating barrier 212 by a control line R. Thus, the gas phase present in the primary insulating barrier 212 escapes automatically to the exhaust mast 30 when its pressure exceeds the predefined level.

In the same way, the secondary exhaust device 135 has an exhaust pipe S opening on one side inside the secondary insulating barrier 210 of the steam dome and on the other side in an exhaust line to the atmosphere 140. A relief valve 137 is arranged on the pipe S, which is closed by default. The valve 137 opens under the control of a valve driver 136 when the total pressure in the secondary insulating barrier 210 exceeds a predefined level, for example 30 mbar or 3kPa. The valve pilot 136 is connected to the pressure of the secondary insulating barrier 210 by a control line T. Thus, the gas phase present in the secondary insulating barrier 210 escapes automatically to the exhaust line 140 when its pressure exceeds the predefined level. The opening pressures of the valves 127 and 137 may be equal or different.

In order to detect leaks or leaks in the secondary waterproof membrane 211 at the vapor dome 21, a device for injecting and detecting a tracer gas is used. This device comprises a tracer gas reservoir 141 which is connected to the exhaust pipe S via a valve 142, so as to transfer tracer gas to the secondary insulating barrier 210 when the valve 142 is open. The tracer gas is for example Argon or Helium or any other gas or mixture of gases that is not likely to liquefy during operation.

This device also comprises a gas detector 143 capable of detecting the tracer gas and connected to the exhaust pipe Q, so as to detect the presence of the tracer gas in the gas phase present in the primary insulating barrier 212.

For the rest, the operation of the leak detection in the steam dome is identical to the case of the liquid dome described above.

The figure 6 shows further details of implementation of steam dome 221 in one embodiment using Mark III® technology. Elements similar or identical to those of figure 1 carry the same reference number increased by 200.

For the sake of brevity only one pipe has been shown to illustrate the S or T pipe of the secondary exhaust device 135. In addition, this pipe has been shown in a same plane as the pipes R and Q of the primary exhaust device 125 However, these channels Q, R, S, T are indeed four in a real implementation and it is not necessary that they are in the same plane, which shows the figure 5 . In addition, it is possible to provide one or more additional tracer gas injection points to improve the speed of execution of the test, especially for a large steam dome. These additional injection points can be distributed evenly around the periphery of the steam dome 221.

In the steam dome 221 of the figure 6 , the upper bearing wall 207 has a circular opening 31 around which is welded a barrel 32 which extends outside the upper bearing wall 207. A metal vapor collecting pipe 222 is anchored inside the barrel 32 and is intended to extract the vapors produced by the evaporation of the fluid in the tank. For this purpose, the collecting duct 222 passes through the vessel wall in the center of the circular opening 31 as well as the sealed membranes 211 and 213 and the insulating barriers 210 and 212 to open into the vessel. This collecting duct 222 is connected in particular to a steam collector outside the tank which extracts this steam and transmits it for example to the propulsion device of the ship to power the propulsion of the ship or a liquefaction device to then reintroduce the fluid into the tank.

The primary sealing barrier 213 is sealingly connected to the collecting duct 222. Likewise, the secondary sealing barrier 211 is sealingly connected to the collecting duct 222 except in two passages 58 and 59 allowing the fluid present between the two sealing barriers to flow to the sampling lines 60 and 61. The absence of the secondary waterproof membrane at this location is symbolized by the broken lines in the passages 58 and 59. In this way, the space between the secondary watertight barrier 211 and the primary watertight barrier 213 forms a sealed primary space connected to the two exhaust pipes 60 and 61.

Moreover, the shaft 32 is sealingly connected to the upper supporting wall 7 and to the collecting pipe 222. The collecting pipe comprises an insulating layer 62 distributed uniformly over its outer surface which has a diameter smaller than the circular opening 31. In this way, the spacing between the insulating layer 62 and the circular aperture 31 allows the flow of gas between the secondary insulating barrier 210 and an intermediate space 64 between the mat 32 and the insulating layer 62, as indicated by arrow 99. The intermediate space and the secondary insulating barrier 210 thus form a secondary sealing space.

The two sampling lines 60 and 61 extend parallel to the collecting duct 222 in the insulating layer 62 from the outside of the barrel 32 to the primary sealing space. The pipe 61 opens into the pipe Q of the figure 5 and allows a passage between the primary sealing space and the pressure relief valve not shown. The pipe 60 opens into the pipe R of the figure 5 and allows a passage between the primary space and the valve driver not shown. Two other pipes represented by the reference S, T are welded to the shaft 32 and open inside the shaft 32 in the secondary sealed space to also allow the management of fluids and the pressure measurement in the secondary sealed space.

It will be noted that, in the barrel 32 of the steam dome 221, the structure of the tank wall is not rigorously a multilayer structure such as it exists in the walls of the tank, since the primary space is here limited to the section of passage of two sampling lines 60 and 61 which pass entirely within the secondary space. However, the structure remains that of a primary space and a secondary space supposed to be isolated from each other by a gas-tight separation, so that the leak detection tests described above retain all their meaning in this slightly different structure.

The aforementioned leak detection methods can be implemented in the steam dome 221 of the figure 6 by injecting tracer gas through line S or T and detecting tracer gas through line Q or R. Arrows 63 of the figure 6 schematically illustrate the path of the tracer gas in the intermediate space 64 between the S or T pipe which injects the tracer gas and the secondary insulating barrier 210 of the tank top wall where it can penetrate.

Further details of setting up a steam dome can be found in the publication FR-A-2984454 .

The technique described above for producing a leak detection device in a protruding part of the tank wall can be used in different types of tanks, for example to form the liquid dome or the steam dome of an LNG tank in a land installation or in a floating structure such as a LNG tank or other.

With reference to the figure 7 , a cutaway view of a LNG tanker 70 shows a sealed and insulated tank 71 of generally prismatic shape mounted in the double hull 72 of the ship. The wall of the tank 71 comprises a primary sealed barrier intended to be in contact with the LNG contained in the tank, a secondary sealed barrier arranged between the primary waterproof barrier and the double hull 72 of the ship, and two insulating barriers arranged respectively between the primary watertight barrier and the secondary watertight barrier and between the secondary watertight barrier and the double hull 72.

In a manner known per se, loading / unloading lines 73 arranged on the upper deck of the ship can be connected, by means of appropriate connectors, to a marine or port terminal to transfer a cargo of LNG from or to the tank 71.

The figure 7 represents an example of a marine terminal including a loading and unloading station 75, an underwater pipe 76 and an onshore installation 77. The loading and unloading station 75 is a off-shore fixed installation comprising a movable arm 74 and a tower 78 which supports the movable arm 74. The movable arm 74 carries a bundle of insulated flexible pipes 79 that can be connected to the loading / unloading pipes 73. The movable arm 74 is adjustable. suitable for all models of LNG carriers. A connection pipe (not shown) extends inside the tower 78. The loading and unloading station 75 enables the loading and unloading of the LNG tank 70 from or to the shore facility 77. liquefied gas storage tanks 80 and connecting lines 81 connected by the underwater line 76 to the loading or unloading station 75. The underwater line 76 allows the transfer of the liquefied gas between the loading or unloading station 75 and the onshore installation 77 over a large distance, for example 5 km, which makes it possible to keep the tanker vessel 70 at great distance from the coast during the loading and unloading operations.

In order to generate the pressure necessary for the transfer of the liquefied gas, pumps on board the ship 70 and / or pumps equipping the shore installation 77 and / or pumps equipping the loading and unloading station 75 are used.

Although the invention has been described in connection with several particular embodiments, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention as defined by the claims.

The use of the verb "to include", "to understand" or "to include" and its conjugated forms does not exclude the presence of other elements or steps other than those set out in a claim. The use of the indefinite article "a" or "an" for an element or a step does not exclude, unless otherwise stated, the presence of a plurality of such elements or steps.

In the claims, any reference sign in parentheses can not be interpreted as a limitation of the claim.

Claims (16)

1. A floating structure comprising a hull including bearing walls (3, 5, 7) defining a polyhedral space inside the hull, the floating structure comprising a sealed and thermally insulated tank (1) housed in the polyhedral space to store a low-temperature liquefied gas,
   in which an upper bearing wall (7, 107) of the hull has an opening and bears a turret-like cargo tank dome (15, 21, 221) projecting from an exterior surface of the upper bearing wall around the opening, the opening and the turret-like cargo tank dome being intended for the passage of cargo handling equipment (16, 22, 222) for handling a liquid phase and/or a vapor phase of the liquefied gas contained in the tank,
   in which the tank comprises a plurality of tank walls fixed to the bearing walls of the hull,
   in which an upper tank wall comprises a multilayer structure fixed to an interior surface of the upper bearing wall, the multilayer structure being formed of a primary sealing membrane (13, 113, 213) intended to be in contact with the liquefied gas contained in the tank, a secondary sealing membrane (11, 111, 211) arranged between the primary sealing membrane and the upper bearing wall, a secondary insulating barrier (10, 110, 210) positioned between the secondary sealing membrane and the upper bearing wall, and a primary insulating barrier (12, 112, 212) positioned between the secondary sealing membrane and the primary sealing membrane,
   the floating structure further comprising a primary venting device (25, 125) comprising a primary venting pipe (I, Q),
   a secondary venting device (35, 135) comprising a secondary venting pipe (K, S), a gas reservoir (41, 141) containing a tracer gas that is non-condensable or that has a condensation temperature below the low temperature of the liquefied gas contained in the tank,
   a gas detector (43, 143) able to detect the tracer gas,
   characterized in that the turret-like cargo tank dome comprises:

   an internal fluidtight wall forming a sheath (113, 222) engaged through the opening of the upper bearing wall (7, 107, 207) and connected in a fluidtight manner to the primary sealing membrane (13, 213) of the upper tank wall all around the sheath,

   an external fluidtight wall (103, 32) arranged around the sheath some distance from the sheath parallel to the sheath, the external fluidtight wall being connected in fluidtight manner to the upper bearing wall around the opening,

   a dividing wall (111, 60, 61) arranged between the external fluidtight wall (103, 32) and the internal fluidtight wall (113, 222) of the turret-like cargo tank dome and dividing the space defined between the external fluidtight wall and the internal fluidtight wall of the turret-like cargo tank dome into, on the one hand, a secondary space (110, 62, 64) communicating through the opening of the upper bearing wall with the secondary insulating barrier (10, 210) of the upper tank wall arranged around the opening and, on the other hand, a primary space (112, 60, 61) communicating through the opening of the upper bearing wall with the primary insulating barrier (12, 212) of the upper tank wall positioned around the opening,

   the primary venting device (25, 125) comprising a primary overpressure relief valve (27, 127), the primary venting pipe (I, Q) communicating directly with the primary space of the turret-like cargo tank dome and passing through the external fluidtight wall of the turret-like cargo tank dome in order to allow gas to be vented from the primary space in response to an opening of the primary overpressure relief valve, the secondary venting device (35, 135) comprising a secondary overpressure relief valve (37, 137), the secondary venting pipe (K, S) communicating directly with the secondary space of the turret-like cargo tank dome and passing through the external fluidtight wall of the turret-like cargo tank dome to allow gas to be vented from the secondary space in response to an opening of the secondary overpressure relief valve,
   the gas reservoir being connected via a control valve (42, 142) to one of the venting devices that are the primary venting device and the secondary venting device, and
   the gas detector being in communication with the other of the venting devices that are the primary venting device, notably the primary venting pipe, and the secondary venting device, notably the secondary venting pipe.
2. The floating structure as claimed in claim 1, in which the primary or secondary venting device further comprises a primary or secondary control line (M, N, R, T) which communicates directly with the primary or secondary space of the turret-like cargo tank dome and which passes through the external fluidtight wall of the turret-like cargo tank dome to control the primary or secondary overpressure relief valve according to the pressure prevailing in the primary or secondary space, and in which the gas reservoir (41, 141) is in direct communication with the primary or secondary control line.
3. The floating structure as claimed in claim 1 or 2, in which the primary or secondary venting device further comprises a primary or secondary control line (M, N, R, T) which communicates directly with the primary or secondary space of the turret-like cargo tank dome and which passes through the external fluidtight wall of the turret-like cargo tank dome to control the primary or secondary overpressure relief valve according to the pressure prevailing in the primary or secondary space, and in which the gas detector (43, 143) is in direct communication with the primary or secondary control line.
4. The floating structure as claimed in one of claims 1 to 3, in which the turret-like cargo tank dome is a vapor dome (221) of the tank, the sheath engaged through the opening of the upper bearing wall being a collecting pipe (22, 222) connected to a main vapor manifold of the floating structure,
   and in which the dividing wall of the turret-like cargo tank dome forms a primary withdrawing pipe (60, 61) running parallel to the collecting pipe in the space defined between the external fluidtight wall (32) and the internal fluidtight wall (222) of the turret-like cargo tank dome and having an interior end (58, 59) opening into the primary insulating barrier of the upper tank wall and an exterior end (Q, R) opening directly into the primary venting device (125), the primary space of the turret-like cargo tank dome comprising the interior space of the primary withdrawing pipe.
5. The floating structure as claimed in one of claims 1 to 3, in which the turret-like cargo tank dome is a liquid dome (15) of the tank, further comprising a top wall (46) positioned over an upper end of the external fluidtight wall (103) of the liquid dome and having an opening aligned with a central zone of the opening of the upper bearing wall, the sheath formed by the internal fluidtight wall (113) of the liquid dome being a primary sealing membrane having an upper edge attached in fluidtight manner to an edge (57) of the top wall all around the opening of the top wall, and in which the dividing wall comprises a secondary sealing membrane (111) extending all around the sheath between the external fluidtight wall and the sheath (113) and having an interior end connected in fluidtight manner to the secondary sealing membrane (11) of the upper tank wall all around the sheath and an exterior end (50) connected in fluidtight manner to the top wall (46) all around the opening of the top wall of the liquid dome.
6. The floating structure as claimed in claim 5, in which the wall of the liquid dome (15) comprises a multilayer structure fixed to an interior surface of the external fluidtight wall, the multilayer structure being formed of the primary sealing membrane (113) of the liquid dome, the secondary sealing membrane (111) of the liquid dome, a secondary insulating barrier (110) of the liquid dome positioned between the secondary sealing membrane and the external fluidtight wall and a primary insulating barrier (112) positioned between the secondary sealing membrane and the primary sealing membrane of the liquid dome.
7. The floating structure as claimed in claim 6, further comprising a connecting plate (48) positioned between the exterior end (50) of the secondary sealing membrane of the liquid dome and the top wall (46), the connecting plate comprising a main leg running parallel to the external fluidtight wall between the external fluidtight wall and the sheath formed by the internal fluidtight wall of the liquid dome, the main leg comprising an upper end attached to the top wall (46) and a lower end extended by a flange (51) bent over toward the inside of the liquid dome with respect to the main leg, the exterior end (50) of the secondary sealing membrane being attached in a fluidtight manner to the flange (51),
   in which the secondary insulating barrier of the liquid dome comprises a fibrous filling (52) placed between the main leg of the connecting plate (48) and the external fluidtight wall (103), and
   in which the secondary venting pipe (K, M) opens into the fibrous filling.
8. The floating structure as claimed in claim 7, in which the primary venting pipe (N, I) passes through the main leg of the connecting plate (48) and opens into the primary insulating barrier between the main leg of the connecting plate and the primary sealing membrane (113) of the liquid dome.
9. The floating structure as claimed in one of claims 5 to 8, further comprising a nitrogen distribution system including a gaseous-nitrogen reservoir (45) and a distribution network, the distribution network comprising a primary distribution pipe (44, A-G, L, J) extending from an upper deck of the floating structure through the primary space (112) of the liquid dome and through the primary insulating barrier (12) of a transverse wall of the tank as far as a bottom zone of the tank, and a secondary distribution pipe (V) extending from the upper deck of the floating structure through the secondary space (110) of the liquid dome and through the secondary insulating barrier of the transverse wall of the tank as far as the bottom zone of the tank, and
   in which the nitrogen distribution system further comprises pressure regulating means for regulating the pressure prevailing in the primary insulating barrier and the secondary insulating barrier of the walls of the tank by means of the primary and secondary distribution pipes.
10. The floating structure as claimed in one of claims 1 to 9, in which the tracer gas is selected from argon, helium and mixtures thereof.
11. The floating structure as claimed in one of claims 1 to 10, in which the tracer gas reservoir (41, 141) is fixed removably to the primary or secondary venting device.
12. A method for operating a floating structure as claimed in one of claims 1 to 11, involving:

    injecting the tracer gas through one of the primary and secondary venting devices (25, 125, 35, 135) into the primary or secondary space of the turret-like cargo tank dome (15, 221) of the floating structure as claimed in one of claims 1 to 11 without exceeding the pressure at which the primary or secondary overpressure relief valve opens,

    detecting the tracer gas through the other of the primary and secondary venting devices (25, 125, 35, 135) in the primary or secondary space of the turret-like cargo tank dome and diagnosing a leak in the secondary sealing barrier (11, 211) of the tank upper wall and/or in the turret-like cargo tank dome dividing wall (111, 60, 61) in response to the detection of the tracer gas.
13. The method as claimed in claim 12, in which the tracer gas is injected into the secondary space through the secondary venting device (35, 135) and is detected in the primary space through the primary venting device (25, 125), the method further involving:
    maintaining a higher total pressure in the secondary space (110, 210) than in the primary space (112, 212) by injecting gaseous nitrogen into the secondary space, without exceeding the pressure at which the secondary overpressure relief valve opens.
14. The method as claimed in claim 12 or 13, in which the step of diagnosing a leak comprises one of the measures selected from the group involving logging the presence of a leak, measuring a quantity or concentration of the tracer gas in order to determine a flow rate of the leak, and measuring a time delay between the injection and detection of the tracer gas in order to determine a position of the leak.
15. A method for the loading of, or offloading from, a floating structure (70) as claimed in one of claims 1 to 11, in which liquefied gas is brought through insulated pipelines (73, 79, 76, 81) from or to a floating or on-shore storage facility (77) to or from the sealed and thermally insulated tank (71) of the floating structure as claimed in one of claims 1 to 11.
16. A transfer system for low-temperature liquefied gas, the system comprising a floating structure (70) as claimed in one of claims 1 to 11, insulated pipelines (73, 79, 76, 81) designed to connect the sealed and thermally insulated tank (71) to a floating or on-shore storage facility (77) and a pump for driving a flow of cold liquid product through the insulated pipelines from or to the floating or on-shore storage facility to or from the sealed and thermally insulated tank.

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