FR3161257A1 - System for storing and/or transporting a gas in the liquid state - Google Patents

Abstract

System for storing and/or transporting a gas in the liquid state The present invention relates to a storage and/or transport system (1) comprising a tank (3) which delimits a main space (4) and a first layer (5), the system comprising a venting device (9) and a discharge branch (7), characterized in that the system comprises a safety branch (10) and a safety device (11), the safety device (11) maintaining a seal of the first layer (5) when a pressure prevailing inside the first layer (5) is lower than a first pressure threshold, the safety device (11) fluidically connecting the first layer (5) and the venting device (9) when the pressure prevailing inside the first layer (5) is higher than the first pressure threshold. (Figure 1)

Description

System for storing and/or transporting a gas in liquid state

The present invention relates to the field of floating structures for the storage and/or transport of a cargo of gas in liquid state, and relates more particularly to a system for the storage and/or transport of said gas in liquid state present within such floating structures.

Liquid gas is frequently stored and/or transported by sea in a floating structure, most often within at least one tank housed inside said floating structure. The tank maintains the gas in a liquid state at very low temperatures thanks to an assembly of walls and an insulating layer that ensures both sealing and thermal insulation.

Each tank defines a main space intended to contain the gas in its liquid state. The tank includes at least one thermally insulating layer for the main space, advantageously two thermally insulating layers, the second layer extending around the first. The thermally insulating layer is separated from the main space and/or from another thermally insulating layer by a sealing membrane.

Each insulating layer is subject to its own specific pressure. A pressure increase can occur within the insulating layer, for example, in the event of a leak followed by gas evaporation within that layer. Overpressure of the insulating layer relative to the main space can cause deformation of the membrane and thus result in significant and costly damage to the tank.

To prevent overpressure in the insulating layer, pipes equipped with safety valves and connected to the atmosphere are linked to each insulating layer. Thus, in the event of excessive pressure within an insulating layer, the safety valve opens, allowing the gas to be released into the atmosphere to reduce the pressure in the insulating layer before damage to the tank occurs.

However, in certain situations, such as during maintenance operations or testing of the tank's properties while the floating structure is docked, overpressure can occur, for example, due to operator error, especially when the pressure relief valves are not functional or are absent. Therefore, there is a need for tank protection against overpressure, regardless of the circumstances.

The present invention falls within this context and, as such, proposes a system for storing and/or transporting a gas in a liquid state, comprising at least one tank that delimits a main space intended to contain the gas in the liquid state and at least one first thermally insulating layer extending around the main space and constituting a means of thermally insulating the main space. The storage and/or transport system further comprises at least one venting device and at least one discharge branch connected to the first layer and extending to the venting device. It is characterized in that the storage and/or transport system comprises at least one safety branch connecting the first layer to the venting device, the safety branch being arranged parallel to at least a portion of the discharge branch, and a safety device positioned on the safety branch. The safety device maintains the seal of the first layer when pressure exists within the first layer. is less than a first pressure threshold, the safety device linking the fluidic first layer and the venting device when the pressure inside the first layer is greater than the first pressure threshold.

Thus, in the event of a pressure increase in the first layer, the vent branch allows the first layer to open to the atmosphere by fluidically connecting it to the venting device. The fluid responsible for pressurizing the first layer can then circulate through the vent branch thanks to the pressure differential with the atmosphere.

Thus, in the event of overpressure of the first layer which cannot be resolved by any means which allows this pressure rise to be detected and said pressure to be lowered in response to such a pressure rise, it is the safety device which ensures an evacuation to the atmosphere of the gas causing the pressure rise within the first layer, thus avoiding an overpressure which could cause damage within the tank.

Preferably, the safety device ruptures during a pressure increase within the first layer. In other words, when the pressure inside the first layer exceeds the initial pressure threshold, the safety device establishes a fluid connection between the first layer and the venting system by rupture of the safety device itself. Thus, in the event of a rupture, the safety device is individually replaceable, requiring a much shorter and less expensive operation than repairing a membrane.

The tank is designed for storing gas in its liquid state and therefore offers various properties that optimize such storage, such as maintaining very low temperatures, for example -163°C, and complete leak-proofness. The main compartment is delimited by a membrane, which also serves as the boundary for the first thermally insulating layer. It is this membrane that is most at risk of damage due to potential overpressure in the first layer. Alternatively, an intermediate layer can extend between the main compartment and the first layer, in which case the membrane separating the intermediate and first layers is the one most likely to be damaged by overpressure in the first layer.

The venting device can, for example, be a vent or any other element capable of releasing a fluid into the atmosphere. Such a venting device advantageously includes an exhaust opening positioned at a height to avoid releasing potentially harmful fluid near operators present within the floating structure housing the storage and/or transport system.

The safety branch and the safety device ensure a pressure reduction in the first layer, even if the vent branch malfunctions and is unable to lower the pressure in the first layer. The safety device may be a disc positioned across the safety branch to ensure the first layer remains sealed. This disc may be designed to rupture if the initial pressure threshold is exceeded, thus allowing fluid to circulate within the safety branch and reducing the pressure in the first layer. Like the vent branch, the safety branch is connected to the venting device to release the fluid responsible for the pressure increase in the first layer via differential pressure. Therefore, even if constraints prevent the vent branch from functioning, such as during maintenance or testing, it is still possible to establish a fluid connection to the atmosphere through the safety branch.

In this configuration, when the pressure inside the first layer exceeds the initial pressure threshold, the safety device ruptures irreversibly to prevent further pressure buildup. However, as previously mentioned, the safety device can be easily replaced without having to replace the safety branch or any other component of the storage and/or transport system.

According to one feature of the invention, the first pressure threshold is 45 mbar +/- 9 mbar. Beyond this threshold value, the pressure prevailing in the first layer risks causing significant damage to the membrane separating the main space and the first layer.

According to one feature of the invention, the discharge branch includes at least one safety valve configured to connect the first layer and the venting device fluidically when the pressure inside the first layer exceeds a second pressure threshold, the second pressure threshold being lower than the first pressure threshold. The safety valve ensures the first layer remains sealed against the discharge branch. The safety valve can open if the pressure measured within the first layer exceeds the second pressure threshold, and then close once the pressure in the first layer falls below the second pressure threshold. The safety valve thus constitutes the primary means of pressure regulation within the first layer. However, during maintenance or testing, this safety valve may be removed or rendered inoperative. It is particularly in this type of configuration that the safety device performs its function.

The second pressure threshold, that is, the pressure threshold that triggers the opening of the safety valve, is lower than the first pressure threshold, at which point the safety device is activated. This allows the safety valve to operate as a priority.

In cases where the safety device is a disc designed to break, this avoids recurring replacement of the safety device.

According to one feature of the invention, the second pressure threshold is between 10 mbar and 35 mbar. This range of values depends on the type of tank used, as the structural characteristics vary according to the type of tank considered. The value of the second pressure threshold is nevertheless always lower than the value of the first pressure threshold.

According to one feature of the invention, the safety branch is connected to the discharge branch between the safety valve and the venting device. Connecting the safety branch and the discharge branch reduces the amount of piping connected to the venting device. This configuration ensures the compactness of the storage and/or transport system while guaranteeing a connection between the first layer and the venting device, regardless of the safety valve configuration.

According to one feature of the invention, the storage and/or transport system comprises a second thermally insulating layer extending around the first layer. Advantageously, the thermal insulation layers and the membranes ensuring the tank's watertightness are doubled to reinforce the tank's structure and limit the risk of malfunctions. If the tank comprises multiple thermally insulating layers, the pressure within each layer must be taken into account to prevent any risk of tank overpressure.

As previously mentioned, the purpose of the safety branch and safety device is to fluidly connect a thermally insulating layer of the tank to the atmosphere. This layer may extend directly around the main space of the tank or be separated from it by an intermediate layer. Thus, when the tank of the storage and/or transport system comprises two thermally insulating layers, the safety branch and safety device can be configured to connect either the first layer to the atmosphere or the second layer to the atmosphere.

According to one feature of the invention, the safety branch and the safety device are respectively a first safety branch and a first safety device, the storage and/or transport system comprising a second safety branch and a second safety device, the second safety branch fluidly connecting the second layer to the first layer, the second safety device maintaining the seal between the first and second layers when a pressure differential measured between the first and second layers is less than a differential threshold, and fluidly connecting the first and second layers when the pressure differential measured between the first and second layers is greater than the differential threshold. Like the first layer, the second layer includes its own safety device in case of overpressure of the second layer relative to the first layer, in order to prevent damage to the membrane separating the two thermally insulating layers.

Unlike the first safety device, which activates when the pressure is too high relative to atmospheric pressure, the second safety device activates when there is too great a pressure difference between the pressure in the first layer and the pressure in the second layer, particularly when the pressure in the second layer is too high compared to the pressure in the first layer. In this configuration, the second safety device activates, fluidly connecting the two insulating layers to equalize the pressure between them.

It is also possible that the two safety devices may be triggered simultaneously or successively in the event of simultaneous or successive exceeding of the first pressure threshold and the differential threshold.

Just like the first safety device, the second safety device can be a rupture disc that breaks in case of overpressure and can be replaced individually.

According to one feature of the invention, the differential threshold is 45 mbar +/- 9 mbar. In other words, the second safety device is triggered when the pressure in the second insulating layer is 45 mbar +/- 9 mbar higher than the pressure in the first insulating layer.

According to one feature of the invention, the second safety branch is connected at a connection point located on the first safety branch, between the first layer and the first safety device. This configuration allows for fluid connection both between the two thermally insulating layers and between the first thermally insulating layer and the venting device, without either of the safety devices interfering with such connections.

According to one feature of the invention, the first safety branch includes a control element disposed between the first layer and the point of connection with the second safety branch.

According to one feature of the invention, the second safety branch comprises a control device disposed between the second layer and the second safety device.

The control element and the control device can, for example, be valves capable of switching between an open and a closed position. The control element and the control device are useful when replacing safety devices, for example, when they break. They allow the safety device to be isolated from the first and/or second layer. The positioning of the control element and the control device is advantageous because these two elements alone are capable of isolating the two safety devices.

According to one feature of the invention, the safety device comprises at least one element capable of breaking upon exceeding the first pressure threshold or the differential threshold; preferably, said element is a seal. In such a case, the safety device comprises a first flange and a second flange connected to each other by assembly means, the seal being interposed between the first and second flanges, the first and second flanges jointly defining a channel that at least partially accommodates the seal. When the seal breaks, the channel is then fully open, and fluid can flow through it for the purpose of pressure reduction.

The invention also covers a method for securing at least one tank of a floating structure implemented by a system for storing and/or transporting a gas in a liquid state as described above, carried out during a maintenance operation of said storage and/or transport system, characterized in that the securing method includes a step of maintaining a seal between the first layer and the venting device when a pressure prevailing inside the first layer is less than a first pressure threshold, and in that the securing method includes a step of establishing fluidic contact between the first layer and the venting device when the pressure prevailing inside the first layer is greater than the first pressure threshold.

Other features and advantages of the invention will become apparent from the following description on the one hand, and from several illustrative and non-limiting examples of embodiments given with reference to the attached schematic drawings on the other hand, in which:

FIG. 1 schematically represents a system for storing and/or transporting a gas in its liquid state, comprising at least one tank equipped with a thermally insulating layer.

FIG. 2 schematically represents the storage and/or transport system comprising at least one tank equipped with two thermally insulating layers.

There FIG. 1 This illustrates a storage and/or transport system 1 containing a cargo of gas in a liquid state 2 and which can be disposed of within a floating structure suitable for storing and/or transporting said cargo. This floating structure can, for example, be a liquid gas transport vessel, such as an LNG carrier, tasked with delivering the cargo to a destination point, for example, a port. Alternatively, the storage and/or transport system 1 can be integrated within an onshore tank.

To do this, the storage and/or transport system 1 includes at least one sealed and thermally insulated tank 3, said tank 3 allowing the storage of the gas in the liquid state 2. In figures 1 and 2, only one tank 3 is shown, but the storage and/or transport system 1 may include a plurality of tanks 3 in order to increase the storage capacity of the floating structure.

Tank 3 illustrated on the FIG. 1 comprises a main space 4, a first thermally insulating layer 5 and a membrane 6 separating the main space 4 and the first layer 5. The main space 4 contains the gas in liquid state 2 stored and/or transported and the first layer 5 extends around the main space 4.

Membrane 6 ensures the sealing of the main space 4, while the first layer 5 ensures thermal insulation of the gas cargo in liquid state 2. The latter can thus be kept at a temperature low enough to be maintained in a liquid state.

The pressure within the first layer 5 is constantly monitored. Indeed, overpressure within the first layer can cause significant damage, particularly to the membrane 6, the repairs to which can be substantial and costly.

To prevent such overpressure within the first layer 5, the storage and/or transport system 1 includes a discharge branch 7 equipped with a safety valve 8, and a venting device 9. The discharge branch 7 extends between the first layer 5 and the venting device 9. The venting device 9 can, for example, be a vent or any other means of access to the atmosphere. Advantageously, the venting device 9 is positioned at a height relative to a deck of the floating structure so that the occupants of the latter are not exposed to a potentially toxic fluid released into the atmosphere from the first layer 5.

Thus, in the event of a pressure increase in the first layer 5, a fluidic connection can be opened between this layer and the atmosphere to lower the pressure in the first layer 5 and thereby prevent irreversible damage to the tank 3. The safety valve 8 is kept closed to ensure the seal of the first layer 5, but can open temporarily if the pressure within the first layer 5 becomes too high. Damage resulting from overpressure in the first layer 5 is thus prevented. The pressure within the first layer 5 can, for example, be measured in real time by a pressure sensor (not shown).

However, in certain situations, the safety valve 8 may not be functional, preventing the potential opening of the discharge branch 7 and thus a connection between the first layer 5 and the venting device 9. This can occur, for example, during certain maintenance operations when the floating structure is stationary, rendering the safety valve inoperative and thus preventing any venting to the atmosphere during this maneuver. Although such maintenance is closely monitored, and alternative measures are implemented to prevent overpressure in the first layer 5, human error can still occur and generate overpressure within the first layer 5, leading to significant damage within the tank 3 because the pressure within the first layer 5 cannot be reduced via the discharge branch 7 and the safety valve 8.

To overcome such a situation, the storage and/or transport system 1 includes a safety branch 10 equipped with a safety device 11. The safety branch 10, like the evacuation branch 7, allows the first layer 5 to be connected to the atmosphere via the venting device 9. The safety device 11 is arranged across the safety branch 10 and ensures the sealing of the first layer 5 as long as the pressure within the first layer 5 is below a first pressure threshold. If the pressure within the first layer 5 exceeds the first pressure threshold, the safety device 11 is configured to activate, thereby creating a fluid connection between the first layer 5 and the atmosphere via the venting device 9, and thus ensuring a decrease in the pressure within the first layer 5. The tank 3 therefore remains protected from potential overpressure events, even when the safety valve 8 is not functioning. Advantageously, the fluid connection thus created between the first layer 5 and the atmosphere is achieved by the rupture of the safety device 11.

In a manner not illustrated, the safety device 11 may be in the form of a rupture disc comprising an element such as a liner capable of rupturing upon exceeding the first pressure threshold. According to this example, the safety device 11 comprises a first flange and a second flange connected to each other by fastening means, and the liner is interposed between the first and second flanges. The first and second flanges jointly define a channel that at least partially accommodates the liner. When the liner ruptures, the channel is then fully opened, allowing fluid to flow through it in order to reduce the pressure.

When the safety device 11 breaks, it can subsequently be easily and affordably replaced, as it is only necessary to replace the safety device 11 with another unbroken safety device 11, without having to replace any other element of the storage and/or transport system 1.

On the FIG. 1 , the first layer 5 extends directly around the main space 4. In a way not illustrated in the figure, an intermediate layer can be arranged between the main space 4 and the first layer 5.

To facilitate the replacement of the safety device 11, the safety branch 10 includes a control element 12 located between the first layer 5 and the safety device 11. The control element 12 may, for example, be in the form of a valve. The control element 12 is predominantly in the open position to fluidly connect the first layer 5 to the safety device 11 in case the latter ruptures due to high pressure in the first layer 5. When the safety device 11 needs to be replaced, the control element 12 is closed to isolate the safety device 11 from the first layer 5 and allow for the replacement of the ruptured safety device 11. It should be noted that the discharge branch 7 includes a control element 13 located between the first layer 5 and the safety valve 8, which is in the open position during normal operation of the storage and/or transport system 1.

In order to reduce space requirements by limiting the piping of the storage and/or transport system 1, the safety branch 10 can be connected to the discharge branch 7 instead of being connected directly to the venting device 9. The connection is implemented so that the safety valve 8 is positioned between the first layer 5 and the connection with the safety branch 10, so that the safety valve 8 and the safety device 11 are both operational without interfering with each other.

It should be noted that the safety valve 8 opens when the pressure inside the first layer 5 exceeds a second pressure threshold, the latter being lower than the first pressure threshold. This is so that, in normal operation of the storage and/or transport system 1, the safety valve 8 ensures the pressure reduction function of the first layer 5 rather than triggering the rupture of the safety device 11. The second pressure threshold can be between 10 mbarg and 35 mbarg depending on the model of the tank 3. The first pressure threshold is fixed at 45 mbarg +/- 9 mbarg.

There FIG. 2 also represents the storage and/or transport system 1 according to the invention, this time comprising a tank 3 with two thermally insulating layers. Only the structural and functional aspects specific to a tank with two thermally insulating layers will be addressed here, and reference will be made to the description of the FIG. 1 for the structural and functional aspects common to both representations in figures 1 and 2.

On the FIG. 2 The tank 3, in addition to the first layer 5, includes a second layer 15 extending around the first layer 5. The second layer 15 provides additional thermal protection, optimizing the safety of the tank 3 compared to a single thermally insulating layer. The first layer 5 and the second layer 15 are separated from each other by an additional membrane 16, which is also at risk of irreversible damage in the event of overpressure within the second layer 15.

To prevent this, the storage and/or transport system 1 includes a vent line 17 on which an additional valve 18 is located. The vent line 17 extends between the second layer 15 and an additional venting device not shown in the FIG. 2 The evacuation line 17 can, however, be fully connected to the venting device 9. The additional valve 18 opens in the event of excessive pressure within the second layer 15. A safety device must also be deployed at the level of the second layer 15 when maintenance operations render the additional valve 18 inoperative.

That is why safety branch 10 and safety device 11 are illustrated on the FIG. 1 here are a first safety branch 10a and a first safety device 11a, while the storage and/or transport system 1 includes a second safety branch 10b and a second safety device 11b arranged on the second safety branch 10b.

The second safety branch 10b extends between the second layer 15 and the first safety branch 10a, at a connection point 14 located between the first layer 5 and the first safety device 11a. This second safety branch 10b thus allows a connection between the first layer 5 and the second layer 15. The second safety device 11b is structurally identical to the first safety device 11a. However, the second safety device 11b is configured to rupture when a pressure differential between the first layer 5 and the second layer 15 exceeds a differential threshold. The rupture of the second safety device 11b thus allows fluid connection between the first layer 5 and the second layer 15, and therefore re-equalizes the pressure of the two layers 5 and 15 with respect to each other. Too high a pressure differential, i.e. a pressure value prevailing in the second layer 15 too high compared to a pressure value prevailing in the first layer 5, risks seriously damaging the additional membrane 16. The differential threshold is for example 45mbarg +/-9mbarg.

Depending on the situation, the two safety devices 11 may rupture simultaneously or successively. For example, assuming that the safety valve 8 and the supplementary valve 18 are not functioning, the second safety device 11b may rupture due to a pressure increase in the second layer 15 when the differential threshold is exceeded. This leads to pressure equalization in layers 5 and 15, and therefore an increase in the pressure within the first layer 5. If this pressure increase in the first layer 5 causes the first pressure threshold to be exceeded, then the first safety device 11a will also rupture, opening the first layer 5 to the atmosphere.

The second safety branch 10b also includes a control device 19, which may be in the form of a valve, allowing the second layer 15 to be isolated from the second safety device 11b in order to replace the latter. Thus, with only two elements—the control element 12 and the control device 19—the two safety devices 11 can be isolated from layers 5 and 15. For this purpose, the control element 12 is precisely positioned on the first safety branch 10a, between the first layer 5 and the connection point 14.

Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without going out of scope of the invention.

The invention, as described above, achieves its intended purpose and provides a storage and/or transport system comprising a tank and a safety device that permanently prevents overpressures that could damage the tank. Variations not described here could be implemented without departing from the scope of the invention, provided that, in accordance with the invention, they include a storage and/or transport system conforming to the invention.

Claims (13)

A storage and/or transport system (1) for a gas in liquid form (2) comprising at least one tank (3) which delimits a main space (4) intended to contain the gas in liquid form (2) and at least one first thermally insulating layer (5) extending around the main space (4) and constituting a means of thermally insulating the main space (4), the storage and/or transport system (1) further comprising at least one venting device (9) and at least one discharge branch (7) connected to the first layer (5) and extending to the venting device (9), characterized in that the storage and/or transport system (1) comprises at least one safety branch (10) connecting the first layer (5) to the venting device (9), the safety branch (10) being arranged parallel to at least a portion of the discharge branch (7), and a safety device (11) positioned on the safety branch (10), the safety device (11) maintaining a seal of the first layer (5) when a pressure prevailing inside the first layer (5) is less than a first pressure threshold, the safety device (11) connecting the first layer (5) and the venting device (9) fluidically when the pressure prevailing inside the first layer (5) is greater than the first pressure threshold. Storage and/or transport system (1) according to claim 1, wherein the first pressure threshold is 45mbarg +/- 9mbarg. Storage and/or transport system (1) according to claim 1 or 2, wherein the discharge branch (7) includes at least one safety valve (8) configured to connect the first layer (5) and the venting device (9) fluidically when the pressure inside the first layer (5) is greater than a second pressure threshold, the second pressure threshold being less than the first pressure threshold. Storage and/or transport system (1) according to claim 3, wherein the second pressure threshold is between 10mbarg and 35mbarg. Storage and/or transport system (1) according to claim 3 or 4, wherein the safety branch (10) is connected to the discharge branch (7) between the safety valve (8) and the venting device (9). Storage and/or transport system (1) according to any one of claims 1 to 5, comprising a second thermally insulating layer (15) extending around the first layer (5). Storage and/or transport system (1) according to claim 6, wherein the safety branch (10) and the safety device (11) are respectively a first safety branch (10a) and a first safety device (11a), the storage and/or transport system (1) comprising a second safety branch (10b) and a second safety device (11b), the second safety branch (10b) fluidly connecting the second layer (15) to the first layer (5), the second safety device (11b) maintaining the seal between the first layer (5) and the second layer (15) when a pressure differential measured between the first layer (5) and the second layer (15) is less than a differential threshold, and fluidly connecting the first layer (5) and the second layer (15) when the pressure differential measured between the first layer (5) and the second layer (15) is greater than the differential threshold. Storage and/or transport system (1) according to claim 7, wherein the differential threshold is 45mbarg +/-9mbarg. Storage and/or transport system (1) according to claim 7 or 8, wherein the second safety branch (10b) is connected at a connection point (14) disposed on the first safety branch (10a), between the first layer (5) and the first safety device (11a). Storage and/or transport system (1) according to claim 9, wherein the first safety branch (10a) includes a control element (12) disposed between the first layer (5) and the connection point (14) with the second safety branch (10b). Storage and/or transport system (1) according to any one of claims 7 to 10, wherein the second safety branch (10b) comprises a control device (19) disposed between the second layer (15) and the second safety device (11b). Storage and/or transport system (1) according to any one of claims 1 to 11, wherein the safety device (11) comprises at least one element capable of breaking in the event of exceeding the first pressure threshold or the differential threshold, preferably said element is a lid. A method for securing at least one tank (3) of a floating structure implemented by a storage and/or transport system (1) of a gas in liquid state (2) according to any one of claims 1 to 12, carried out during a maintenance operation of said storage and/or transport system (1), characterized in that the securing method includes a step of maintaining a seal between the first layer (5) and the venting device (9) when a pressure prevailing inside the first layer (5) is less than a first pressure threshold, and in that the securing method includes a step of establishing fluidic contact between the first layer (5) and the venting device (9) when the pressure prevailing inside the first layer (5) is greater than the first pressure threshold.