Method and system for discharging liquefied gas from a liquefied gas transport or storage tank

Abstract

The present invention relates to a method for evacuating (500) liquefied gas from a sealed and thermally insulating tank comprising: - an upper wall and a lower wall, - a pumping pipe extending into the tank from a first end opening outside the tank to a second end located at the bottom of the tank and connected to a suction member housed at least partly in an enclosure open to the tank, the evacuation method (500) comprising: - a step of emptying (501) the tank, - a step of heating (502) the tank capable of vaporizing residual liquefied gas present in the tank, - an inerting step (503) by introducing an inert gas into the tank, the evacuation method (500) further comprising, following or during the step of inerting (503) the tank, a step of sending a flow of an inert gas (504), directed into the enclosure.

Description

[0001] The present invention relates to the field of liquefied gas transport vessels and more specifically concerns the installation and maintenance of tanks intended for the storage of such liquefied gas, for example, liquefied natural gas or liquefied ethane. These tanks are installed in the holds of liquefied gas transport vessels, or in land-based facilities for the temporary storage of liquefied gas.

[0002] Such a tank has a capacity of several thousand, or even tens of thousands, of cubic meters of liquefied gas. It is generally rectangular in shape, its walls typically resting against the internal bulkheads of a cargo ship's hold. The ship's hold usually contains several tanks. To maintain the liquefied gas in a liquid state, i.e., at -163°C (degrees Celsius) for liquefied natural gas, the walls of each tank have several thermally insulating and airtight layers lining the ship's internal bulkheads.

[0003] To fill or empty the liquefied natural gas (LNG) tank, a discharge tower is typically attached to the tank's upper wall. This tower extends vertically to the tank's bottom, near a lower wall. The tower generally consists of several masts, each carrying loading and / or discharge pipes. These discharge pipes are connected to discharge pumps, which are typically located at one end of the tower at the tank's bottom. Each pump has a suction device that draws the LNG into one of the discharge pipes.

[0004] Other pipes are usually integrated into the tower, including one that supplies natural gas to the ship's engines. This pipe also includes a pump for drawing liquefied natural gas from the bottom of the tank.

[0005] To protect the pumps inside the tank from sudden movements of the liquefied natural gas (LNG), enclosures are typically placed around the suction side of each pump. Such an enclosure usually takes the form of a sump built into the tank's bottom wall, or a container with sides arranged vertically around the pump's suction side. When the enclosure is a sump, it also allows the pump within the sump to draw the maximum amount of LNG from the tank when it needs to be completely emptied. Furthermore, a sump ensures that the pump's suction side is always submerged in a minimum level of liquid, thus preventing damage to the suction side during operation.

[0006] When the tank needs to be completely emptied, for example for inspection, the pumps in the discharge lines drain the tank. The residual liquid inside is then vaporized by circulating hot gas. The tank then undergoes an inerting process during which an inert gas is introduced to the bottom. When the gas mixture inside the tank reaches a predetermined proportion that renders it non-flammable in open air, the tank is ventilated to make it suitable for inspection by tank maintenance personnel.

[0007] The inventors observed, however, that following these operations to evacuate the liquefied gas from the tank, some evaporated liquefied gas remains trapped in the pump protection enclosures. The emissions from this evaporated liquefied gas can be harmful to personnel working near the pumps protected by these enclosures, or during pump maintenance.

[0008] KR 2021 0125347 A describes a module intended for the maintenance of a tank.

[0009] The present invention aims to remedy at least in part the aforementioned drawback by providing a method for evacuating liquefied gas from a sealed and thermally insulated tank and a corresponding evacuation system, enabling the tank maintenance personnel to be protected from residual gas emissions.

[0010] To this end, the invention proposes a method for evacuating liquefied gas from a sealed and thermally insulated tank intended to contain liquefied gas, the tank comprising at least: a plurality of walls including an upper wall and a lower wall, a liquefied gas pumping line extending into the tank from a first end opening outside the tank to a second end located at the bottom of the tank and connected to a suction device of a pump, housed at least partly in an enclosure open to the tank, the evacuation process comprising in order: a step of emptying the tank, a step of heating the tank capable of vaporizing a residual portion of liquefied gas present in the tank, a step of inerting the tank by introducing an inert gas into it, the evacuation process further comprises, following or during the inerting step of the tank, a step of sending a flow of an inert gas, capable of displacing from the enclosure a quantity of liquefied gas vaporized during the heating step, the step of sending a flow of inert gas being distinct from the inerting step.

[0011] The inert gas flow is directed into the enclosure in such a way as to allow the evacuation of the quantity of vaporized liquefied gas remaining trapped in the enclosure.

[0012] The lower wall of the tank is the one that vertically supports the weight of the liquefied gas. For example, it is attached to the bottom wall of a cargo ship's hold, while the upper wall of the tank is attached to the ship's deck. Each tank wall comprises, for example, a primary layer consisting of a primary airtight membrane in contact with the liquefied gas and a primary thermal insulation barrier made of one or more insulating materials, and a secondary layer consisting of a secondary airtight membrane in contact with the primary thermal insulation barrier and a secondary thermal insulation barrier made of one or more insulating materials, attached to an internal bulkhead, a bottom wall, or the ship's deck, depending on the tank wall in question.

[0013] Alternatively, the tank walls are arranged in a similar way in a land-based liquefied gas storage device.

[0014] The pumping line extends primarily vertically within the tank, its main dimension being oriented vertically, that is, perpendicular to the upper and lower walls of the tank. The other end of the pumping line is connected to a suction device located at the bottom of the tank, i.e., in the lower part of the tank. The other end of the pumping line is preferably situated between the lower wall of the tank and a distance from it equal to, for example, one-tenth of the tank's volume. In this way, the suction device operates even when the tank is nearly empty. The pump incorporating the suction device also includes a drive motor for the suction device, which can be located remotely from the suction device.

[0015] The pumping pipeline is used, for example, to supply power to loads on a ship, such as the ship's engine. Alternatively, it is used to discharge the tank, particularly in the case of an LNG carrier. In this latter case, the pumping pipeline is generally integrated into the mast of a discharge tower. Such a discharge tower extends vertically into the tank and has one end located outside the tank, for example, on the ship's deck, and the other end located at the bottom of the tank. It therefore passes through the upper wall of the tank. The second end of the discharge tower does not touch the lower wall of the tank, as the discharge tower is positioned at the level of the upper wall of the tank.It comprises several hollow masts arranged vertically and in which loading or unloading pipes are arranged vertically, the masts being joined together by means of crossbeams.

[0016] The pipes within the tank are generally grouped at the discharge tower. When the pumping line is intended to supply the ship's loads, the pump drive motor is, for example, supported by a support structure for the discharge tower, located at its far end and consisting, for example, of plates and crossbeams forming a base positioned primarily parallel to, but at a distance from, the lower wall of the tank. Alternatively, the pump drive motor may be located on the ship's deck.

[0017] When the pumping line is a tank discharge line, the pump drive motor is, for example, located on the ship's deck, with a drive shaft then connecting the drive motor to the suction device in the mast.

[0018] Alternatively, the pump drive motor, used to supply consumers on the ship or to discharge the tank, is grouped with its suction unit in the same housing fixed to the support structure.

[0019] The suction device is housed at least partly in an enclosure taking for example the form of a sump arranged in the lower wall of the tank, or a container or deflector carried by the support structure of the unloading tower.

[0020] Thanks to the invention, the vaporized liquefied gas remaining in the containment during the inerting stage is expelled from the containment by a flow of inert gas directed into or towards the containment. The vaporized liquefied gas mixed with the inert gas can then be expelled by dry air directed into the tank to make it suitable for personnel. The safety of personnel working inside the tank is thus ensured, without foregoing the use of a container or sump housing the suction inlet of a pump. It should be noted that the invention is not limited to transport tanks; sumps can be used in land-based systems incorporating liquefied gas tanks to facilitate their unloading.

[0021] The inerting stage preferably uses an inert gas produced by an inert gas generator resulting from the combustion of diesel fuel. The inert gas thus produced comprises approximately 80% to 90% nitrogen and 20% to 10% carbon dioxide, and is less expensive than pure nitrogen. Preferably, the inert gas has a composition of 85% nitrogen and 15% carbon dioxide. The inert gas used by the means of delivering an inert gas stream is preferably nitrogen produced by a service inert gas supply system, i.e., one supplying nitrogen at a pressure of five bar. This inert gas is therefore not charged with carbon dioxide, except for possible traces, unlike the inert gas used for tank inerting.

[0022] Alternatively, the same inert gas is used for inerting and sending a flow of inert gas into the enclosure, this same inert gas then being only dinitrogen or only a mixture of dinitrogen and carbon dioxide.

[0023] In one embodiment of the invention, the liquefied gas evacuation process further includes a step of sending dry air into the tank, capable of driving out of the tank the inert gas or gases resulting from the inerting step and the step of sending the flow of an inert gas.

[0024] Dry air is introduced into the tank only after the tank has been inerted and the vaporized liquefied gas present in the enclosure has been removed using means for supplying a flow of inert gas into the enclosure. This dry air is the product of air dehumidification by a dry air generator, using a desiccant such as activated alumina.

[0025] According to an optional and advantageous feature of the process according to the invention, the duration of the inert gas flow delivery step is predefined to allow the vaporized liquefied gas remaining within the chamber to be evacuated from it. This feature simplifies implementation of the invention, as it does not require monitoring the flow rate of the inert gas. The predefined duration is, for example, measured by tests conducted prior to implementing the process according to the invention.

[0026] According to another optional and advantageous feature of the method according to the invention, the step of supplying the inert gas stream uses a pipe with a cross-section strictly smaller than that of the inerting line used during the inerting step. For example, the pipe has a diameter of DN 8 (the abbreviation DN designates the nominal internal diameter in millimeters), while the inerting line has a diameter of DN 15. This feature makes it possible to avoid modifying the inerting line used during the tank inerting process. The pipe is, for example, connected to the service inert gas supply system only when it is necessary to open the tank. Therefore, this embodiment of the invention is inexpensive and easy to implement.

[0027] The invention also relates to a liquefied gas evacuation system from a sealed and thermally insulated tank intended to contain liquefied gas, the system comprising the tank, the latter comprising at least: a plurality of walls including an upper wall and a lower wall, a pump comprising a suction device, housed at the bottom of the tank at least partly in an enclosure open to the tank, a liquefied gas pumping line extending into the tank from a first end opening outside the tank to a second end located at the bottom of the tank and connected to the suction device, the evacuation system further comprising: means for emptying the tank, means for heating the tank, capable of vaporizing a residual portion of liquefied gas present in the tank after the emptying means have been implemented, means for inerting the tank, capable of introducing an inert gas into it, after the heating means have been implemented, the evacuation system further comprises means for sending a flow of an inert gas, capable of expelling from the enclosure a quantity of liquefied gas vaporized by the heating means, the means for sending a flow of inert gas being distinct from the means for inerting the tank.

[0028] These means of sending a flow of inert gas are implemented during or after the implementation of the inerting means and allow the flow of inert gas to be directed within the enclosure.

[0029] The liquefied gas evacuation system according to the invention includes means for implementing the liquefied gas evacuation method according to the invention. In other words, the liquefied gas evacuation method according to the invention uses the liquefied gas evacuation system according to the invention, which has advantages similar to those of the liquefied gas evacuation method according to the invention.

[0030] In one embodiment of the invention, the means for delivering a flow of inert gas comprise a pipe with a cross-section strictly smaller than that of an inerting line coupled to inert gas inlet means. The inerting line opens into the tank and forms part of the inerting means. The pipe has a first end located outside the tank and a second end positioned to direct the flow of inert gas into the tank. As explained in relation to the method according to the invention, the use of such a pipe avoids modifying the tank's inerting means, and in particular avoids modifying the inlet of the inerting line. Since the pipe has a smaller diameter (DN8 instead of DN15, for example), it is easy to install.

[0031] The pipe, for example, includes a flexible section at its first end, suitable for being coupled or decoupled to an inert gas supply system. In this embodiment of the invention, the inert gas supply system is a service inert gas supply system, i.e., providing nitrogen at five bar pressure, while the inert gas used by the inerting means is produced from diesel combustion, as explained above.

[0032] The second end of the pipe is, for example, located below a plane defining an opening in the enclosure, this opening being situated opposite the upper wall of the tank. Alternatively, the second end of the pipe is positioned above this plane, but in such a way that the inert gas supplied under pressure by this pipe passes through the opening and enters the enclosure. For example, the second end of the pipe is part of a section of pipe positioned vertically above the opening in the enclosure.

[0033] In one embodiment of the invention, the means for sending a flow of an inert gas include means for fixing the pipe to a mast of a discharge tower comprising several masts and extending into the tank, presenting at its proximal end to the lower wall of the tank, a support structure connecting the masts of the discharge tower together.

[0034] In an example of this embodiment, the pumping line is a discharge line for the tank integrated into the mast, and the discharge means are capable of actuating the pump and drawing the liquefied gas into the pumping line. The containment structure is then, for example, a vessel attached to the support structure located at the lower end of the mast and surrounding the suction device. Alternatively, the containment structure is a sump located below the mast within the thickness of its lower wall.

[0035] In another example of an embodiment related to this mode of embodiment, the pumping line is a line intended for supplying fuel to at least one consumer of a ship.

[0036] In this other example of implementation, the pipe includes, for example: a first portion running along the mast, a second portion extending along the support structure, the latter connecting one end of the mast and the suction device, and a third portion arranged vertically from one end of the second portion, said end of the second portion being located in the vertical extension of the enclosure.

[0037] The pipe used to deliver a flow of inert gas into the containment is brought to the bottom of the tank by means of attachments to a mast of the discharge tower, preferably chosen to be the one closest to the containment housing the suction device. The pipe must make a detour at the bottom of the tank before reaching the top of the containment protecting the suction device, using the support structure of the discharge tower for support.

[0038] According to one embodiment of the invention, the enclosure can be located outside a perimeter delimited by the masts of the discharge tower. This is the case, for example, when the pumping line, whether it is a fuel supply line or a discharge line, is located outside this perimeter.

[0039] The enclosure is, for example, a container, attached to the support structure, the container being held above the lower wall of the tank.

[0040] Alternatively, the enclosure is a sump, fitted into a thickness of the lower wall of the tank.

[0041] The invention also relates to a liquefied gas transport vessel comprising a liquefied gas evacuation system according to the invention. The vessel according to the invention has advantages similar to those of the system and method according to the invention.

[0042] 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 ] represents a vessel according to the invention, equipped with liquefied gas transport tanks and evacuation systems according to the invention for the liquefied gas present in these tanks, in an embodiment according to the invention, [ fig 2 ] represents a tank unloading tower transported by the ship of the figure 1 , incorporating means of a liquefied gas evacuation system according to the invention, in a first embodiment of the invention, [ fig 3 ] represents a second embodiment of the invention, in which means of a liquefied gas evacuation system according to the invention also use elements of the discharge tower shown figure 2 , [ fig 4 [ ] is a cross-sectional view in a vertical plane of a sump protecting a suction device of a pump connected to a pumping line, the sump being associated with the means of the liquefied gas evacuation system mentioned in relation to the figure 3 , [ fig 5 [ ] is a cross-sectional view in a vertical plane of a container protecting a suction member of a pump connected to a pumping line, the container being associated with means of a liquefied gas evacuation system according to the invention in a third embodiment of the invention, these means also using elements of the discharge tower shown figure 2 , And [ fig 6 ] represents steps in a liquefied gas evacuation process according to the invention, in the embodiment of the invention mentioned figure 1 .

[0043] According to one embodiment of the invention, a ship 1 according to the invention represented figure 1 is a liquefied gas transport vessel G, which in this embodiment of the invention is liquefied natural gas. The liquefied gas G is transported in tanks 16 installed in the hold of the vessel 1. The hold has a lower bulkhead 12, substantially parallel to the deck 18 of the vessel 1, and internal side bulkheads 14 which divide the hold into four compartments.

[0044] A sealed and thermally insulated tank 16 is fitted into at least one compartment. This tank 16 has an upper wall 162 (partially shown in the figure 2 ) attached to deck 18 of ship 1, a lower wall 164 (partially shown in the figure 2 ) attached to the lower bulkhead 12 of the vessel 1, and the side walls attached to the internal side bulkheads 14. The walls of the tank 16 line the bulkheads of each compartment of the hold with watertight and thermally insulating layers, so that the tank 16 forms a substantially parallelepiped-shaped container suitable for receiving the liquefied gas G and maintaining it at a temperature of -163°C or lower, at atmospheric pressure. The upper wall 162 also includes a cover (not shown) allowing personnel to descend into the tank.

[0045] The upper wall 162 of the tank 16 is traversed by a discharge tower 2, having a first end accessible from outside the tank 16 on the deck of the ship and a second end located at the bottom of the tank 16, without however touching the lower wall 164 of the tank 16. Indeed, the discharge tower 2 is supported by the deck 18 of the ship and extends vertically into the tank 16. To limit certain movements of the discharge tower 2, which is several tens of meters long, a support device 15 attached to the lower wall 164 of the tank 16, holds the discharge tower 2 in a vertical direction, that is to say perpendicular to the upper wall 162 and lower wall 164 of the tank 16.It should also be noted that the support device 15 contributes to the support of the unloading tower 2, the length of which varies according to thermal variations in the tank 16, particularly when the ship is exposed to rolling phenomena.

[0046] There figure 2 The diagram shows in more detail the unloading tower 2, which is oriented lengthwise along a vertical axis (Oz) of an orthonormal coordinate system (O, x, y, z). The terms "upper", "top", "lower", and "bottom" in this application refer to the orientation of this Oz axis, directed upwards, that is, towards the upper wall 164 of the tank 16.

[0047] The unloading tower 2 has several hollow masts 21, 22, 23 extending vertically into the tank 16 through its upper wall 162. One end of at least one of the masts 21, 22, 23 is therefore external to the tank 16, while a second end of at least one of the masts 21, 22, 23 is located at the bottom of the tank 16, without touching the lower wall 164 of the tank 16. The second ends of at least two masts 21, 22, 23 are joined together by a support structure 24 made up of plates and crossbeams and located at a distance from the lower wall 164 of the tank 16. Other crossbeams, represented by thick black lines, connect the masts to each other along the length of the unloading tower 2. The support structure 24 is connected to the lower wall 164 by the retaining device 15.

[0048] In this embodiment of the invention, the mast 23 houses a pumping line 25 which is a liquefied gas discharge line G. The pumping line 25 extends vertically from a first end located outside the tank to a second end connected at the bottom of the tank to a discharge pump having a suction member 8. The discharge pump is supported by the support structure 24. It is vertically located between the second end of the mast 23 and the lower wall 164, without touching it.

[0049] To protect the discharge pump from the effects of waves, a container 30, preferably cylindrical, open at the top, surrounds the discharge pump and in particular its suction member 8. The container 30 is fixed to the support structure 24 and does not touch the lower wall 164. This container 30 forms a recess in which the liquefied gas, even if evaporated, can remain confined without a specific evacuation system such as an evacuation system S1 for the liquefied gas present in the tank 16, and which forms a first example of an embodiment of the invention.

[0050] The evacuation system S1 includes means for emptying the tank 16. These means comprise the pumping line 25 and the discharge pump, as well as means for actuating the discharge pump to draw the liquefied gas G out of the tank until it contains only a residual portion of liquefied gas. The emptying means may optionally include other pumping devices operated in parallel with the discharge pump (for example, another discharge line in a different mast and connected to another discharge pump) or operated to supplement its action when the tank 16 is almost empty (for example, a so-called dewatering pump). A return of gas in the vapor phase is also carried out to equalize the pressure in the tank 16 as it is being emptied.

[0051] The evacuation system S1 further includes means for heating tank 16, capable of vaporizing the residual portion of liquefied gas G present in tank 16 after the emptying means have been activated. These heating means include means for circulating the heated gas within tank 16, this gas being, for example, recovered in the upper part of tank 16 and then compressed. The residual portion of liquefied gas G is completely vaporized when the thermal insulation of tank 16 reaches 5°C.

[0052] The S1 evacuation system also includes tank inerting means, allowing the introduction of an inert gas from an inert gas generator. This inert gas is composed, for example, of 80% nitrogen and 20% carbon dioxide, and is introduced into the tank via an inerting line. One end of this line, located outside the tank 16, is connected to the inert gas generator, and the other end is located at the bottom of the tank. The inert gas introduced at the bottom of the tank, due to its density, forces the natural gas vaporized by the heating equipment to rise to an upper part of the tank, where it is drawn in and extracted.

[0053] The evacuation system S1 further includes means for sending a flow of an inert gas directed into the container 30, allowing, after an inerting operation of the tank 16 has been carried out or at the end of such an operation, the vaporized gas remaining confined in the container 30 to be expelled.

[0054] These means of sending a flow of inert gas include a pipe 4 shown figure 2 This pipe 4 is of a smaller diameter than the inerting line, and a service nitrogen supply system is included. Pipe 4 has a first end located outside the tank 16 and a second end 42 opening into the container 30. Pipe 4 is fixed along the mast 23 except for two end portions of pipe 4, which are a first end portion located outside the tank 16 and comprising a flexible section 41, and a second end portion extending vertically from the part of pipe 4 fixed to the mast 23 into the container 30.

[0055] The flexible section 41 of pipe 4 allows it to be connected to the service nitrogen supply system. This nitrogen is produced by an inert gas generator on board the vessel, using ambient air. This nitrogen supply system provides nitrogen at a pressure of 5 bar and can be connected to various equipment as needed. Therefore, when connected to this nitrogen supply system, pipe 4 acts as a device connected to a compressed air line, also known as a blow gun, with sufficient pressure to expel the vaporized gas remaining confined in container 30. It should be noted that the inerting equipment, even if positioned above container 30, could not expel the vaporized liquefied gas confined within it. This is because the inerting equipment disperses the inert gas at insufficient pressure to expel the vaporized gas.

[0056] The S1 evacuation system also includes means for venting tank 16. These means comprise a dry air duct in the upper part of tank 16, allowing dry air produced by a dry air generator on board to be introduced into this upper part. This generator uses ambient air, which it dehumidifies. The dry air introduced at the top displaces the other gases present in tank 16 after the inerting means and the delivery of an inert gas flow into container 30 have been activated. These gases are then forced into the lower part of tank 16 where they are extracted until an oxygen content exceeds 20%.

[0057] The S1 evacuation system according to the invention thus allows personnel to enter the tank without breathing natural gas vapors that would have remained confined in the container 30.

[0058] As described now in relation to the figure 3 , according to a second embodiment, the tank 16 may also include a pumping line 25b which is used to supply consumers of the ship 1 with fuel.

[0059] The pumping pipe 25b has a first end located outside the tank and extends vertically near the discharge tower 2 to an end portion located at the bottom of the tank. This end portion has a first horizontal section fixed to the mast 21 and to the support structure 24, and a second vertical section connected to the first horizontal section and to a second end of the pumping pipe 25b, this second end being connected to a feed pump having a suction member 8b. The feed pump is supported by the support structure 24, as more particularly visible figure 4 .

[0060] To protect it from the effects of waves, this feed pump is housed in a sump 7 built into the lower wall 164 of the tank 16. As visible figure 4 The lower wall 164 comprises: a primary watertight membrane 1640 metallic, intended to be in contact with the liquefied gas G in the tank 16, a primary insulation layer 1641 comprising insulating materials, disposed under the primary watertight membrane 1640, a secondary watertight membrane 1642 metallic or composite, disposed under the primary insulation layer 1641, and a secondary insulation layer 1643, disposed under the secondary watertight membrane 1642 and resting on the lower bulkhead 12 of the ship's hold 1.

[0061] The sump 7 is formed of a cylindrical metal casing open towards the tank 16 and welded on its upper part to the primary metal sealing membrane 1640 to ensure the sealing of the tank 16. It has a bottom, for example disposed in the secondary insulation layer 1643, parallel to the lower partition 12. The feed pump disposed in the sump 7 being supported by the support structure 24, it does not touch the bottom of the sump 7.

[0062] To prevent vaporized liquefied gas from remaining confined in sump 7 during complete evacuation of liquefied gas from tank 16, an evacuation system S2 for the liquefied gas present in tank 16, shown figures 3 And 4 , and forming a second example of an embodiment of the invention, includes means similar to those of the liquefied gas evacuation system S1 described previously.

[0063] In particular, the liquefied gas evacuation system S2 includes the same means for draining, heating, inerting, and venting the tank 16 as those of the liquefied gas evacuation system S1. It should be noted that, in this embodiment of the invention, the liquefied gas evacuation system S2 coexists with the liquefied gas evacuation system S1.

[0064] The S2 liquefied gas evacuation system also includes means for sending a directed flow of inert gas into the sump 7 and which can be implemented during or after inerting of the tank 16 by the inerting means.

[0065] These means for delivering a flow of inert gas include a pipe 4b with a smaller diameter than the inerting line, as well as the service nitrogen supply system to which the pipe 4b can be connected. The pipe 4b has a first end located outside the tank 16 and a second end 42b opening into the sump 7. The pipe 4b has a main vertical section 40 fixed along the mast 21, a first end section located outside the tank 16 and including a flexible section for its connection to the service nitrogen supply system, and a second end section extending from the main section 40 fixed to the mast 21 to the bottom of the tank into the sump 7.

[0066] The second end portion of pipe 4b comprises a horizontal, or substantially horizontal, portion 44 fixed to the support structure 24. This horizontal portion 44 connects a lower end of the main portion 40 to an upper end of a final vertical portion 48 of pipe 4b, this upper end being disposed above the sump 7. The final vertical portion 48 of pipe 4b is arranged vertically such that its lower end, corresponding to the second end 42b of pipe 4b, is in the sump 7, as visible figure 4 .

[0067] The fixings of the pipe 4b to the mast 21 and to the support structure 24 use flanges 5 welded or screwed to the mast 21, or respectively to the support structure 24.

[0068] Pipe 4b, when connected to this nitrogen supply system, acts like a blow gun, with sufficient pressure to expel the vaporized gas remaining confined in sump 7. This connection, intended to send a flow of inert gas in order to expel the vaporized liquefied gas, is made at the end of inerting or after the inerting of tank 16.

[0069] There figure 5 now illustrates a liquefied gas evacuation system S3 forming a third embodiment of the invention and corresponding to a variant of the liquefied gas evacuation system S2 previously described.

[0070] The S3 liquefied gas evacuation system differs from the S2 liquefied gas evacuation system mainly in that the supply pump for the consumers of vessel 1, comprising the suction member 8b, is protected from the effects of waves not by a sump but by a container 30c, similar to the container 30. The other elements, identical to those of the S2 liquefied gas evacuation system, are referenced in the same way.

[0071] In this embodiment variant, the pumping line which supplies the consumers of ship 1 with fuel and which is connected at the bottom of the tank to the supply pump, is a pumping line 25c identical to the pumping line 25b of the liquefied gas evacuation system S2, except that it goes less deep into the tank 16 since the container 30c is not located in the lower wall 164 of the tank 16 but above it.

[0072] The container 30c takes the form of a cylindrical bucket open at the top, which surrounds the feed pump and in particular its suction member 8b. The container 30c is fixed to the support structure 24 and does not touch the lower wall 164.

[0073] To evacuate vaporized liquefied gas that might remain confined in container 30c after or at the end of the inerting of tank 16, the liquefied gas evacuation system S3 includes a pipe 4c connected to the service nitrogen supply system at one end, and whose other end 42c is located in container 30c. The arrangement of pipe 4c and its attachment to the discharge tower 2 is identical to that of pipe 4b, except that pipe 4c extends less deeply into the bottom of the tank since container 30c is located at a shallower depth than the sump 7. Consequently, pipe 4c extends, for example, less deeply along the mast 21 and is attached horizontally to the support structure 24 higher than the horizontal portion 44 of pipe 4b of the liquefied gas evacuation system S2.

[0074] Pipe 4c, when connected to the service nitrogen supply system, can also act as a blow gun, with sufficient pressure to expel any remaining vaporized gas confined in container 30c. The inert gas flow is introduced to expel the vaporized liquefied gas at the end of the inerting process or after the tank 16 has been inerted.

[0075] Alternatively, the liquefied gas evacuation systems S1, S2 or S3 according to the invention can, depending on the configuration of the tank, be considered individually; in such a case there is no coexistence of several evacuation systems.

[0076] We now describe in relation to the figure 6 , steps of an evacuation process 500 according to the invention, suitable for evacuating the liquefied gas G from the tank 16, and implemented by the liquefied gas evacuation system S1, S2 or S3 according to the invention, in this embodiment of the invention.

[0077] A first step 501 of the evacuation process 500 is the emptying of the tank 16. For this, the tank 16 discharge pumps are activated, and optionally a dewatering pump at the end of the emptying process. These pumps and their associated pipes form part of the means for emptying the evacuation system S1, S2, or S3 in this embodiment of the invention.

[0078] A second step 502 of the evacuation process 500, implemented after the emptying step 501, is the heating 502 of the tank 16, until all the residual liquefied gas present in the tank 16 is vaporized. For this, the heating means of the evacuation system S1, S2 or S3 are activated, circulating heated gas in the tank 16, until the walls of the tank 16 reach a temperature of 0°C.

[0079] A third step 503 of the evacuation process 500, implemented after the heating step 502, is the inerting of the tank 16. This is done using the inerting means of the evacuation system S1, S2, or S3, which, in this embodiment of the invention, comprise the inert gas line and the inert gas generator located on the vessel 1. Inert gas, composed of 80% nitrogen and 20% carbon dioxide, arriving at the outlet of the inert gas generator (which receives ambient air and diesel fuel as inlet), is sent through the inert gas line to the bottom of the tank. The liquefied gas vaporized by the heating means then rises to an upper part of the tank 16 where it is drawn in and extracted from the tank.The inerting step 503 is stopped when the gas mixture in tank 16 is no longer flammable on contact with air, for example down to a natural gas content in tank 16, less than 50% compared to a lower limit content at which natural gas ignites on contact with air.

[0080] A fourth step 504 of the evacuation process 500, implemented after or at the end of the inerting step 503, is the sending of a flow of inert gas, here service nitrogen, into the container 30 and into the sump 7 or the container 30c, depending on the chosen embodiment implemented. This step uses the means for sending an inert gas flow of the evacuation system S1, S2, or S3, which in this embodiment of the invention include the pipe 4 and the pipe 4b or 4c, as well as the service nitrogen generator present on the vessel 1. In this fourth step 504, the flexible portion 41 of the pipe 4 is supplied by the service nitrogen generator, which is activated for a predetermined duration, for example, for ten minutes, so that the vaporized liquefied gas remaining confined in the container 30 is expelled from it.Then the flexible portion 41 of pipe 4 is disconnected and the flexible portion of pipe 4b or 4c is connected in turn so as to be supplied by the service nitrogen generator which is also run for about ten minutes, so that the vaporized liquefied gas remaining confined in the sump 7 or the container 30c is expelled.

[0081] Alternatively, this fourth step 504 of the evacuation process 500 is the sending of a flow of inert gas only into the container 30, 30c or into the sump 7, when only one evacuation system S1, S2 or S3 is implemented in the tank 16. In this case, the flexible portion 41 of the pipe 4 is supplied by the service nitrogen generator, which is activated for a predetermined duration, for example, for ten minutes, so that the vaporized liquefied gas remaining confined in the container 30, 30c or the sump 7 is expelled from it. Then the supply to the pipe 4 by the nitrogen generator is shut off. Finally, a fifth and final step 505 of the evacuation process 500, implemented after the inerting step 503 and after the sending of an inert gas flow step 504, is the sending of dry air into the tank 16.This step utilizes the venting means of the exhaust system S1, S2, or S3, which, in this embodiment of the invention, include the dry air duct and the dry air generator located on the vessel. In this dry air delivery step 505, dehumidified air obtained from the outlet of the dry air generator is introduced into the upper part of tank 16 via the dry air duct. This dehumidified air displaces the other gases present in tank 16 at its lower end, where they are extracted until an oxygen content exceeds 20%.

[0082] Tank 16 is then ready to receive maintenance personnel for tank 16, for example for its inspection.

[0083] Of course, the invention is not limited to the examples just described, and many modifications can be made to these examples without departing from the scope of the invention. In particular, the features of the different variants or embodiments of the invention contemplated in this application can be combined to carry out the invention, provided that these variants or examples are not incompatible with each other and without departing from the scope of the attached claims.

Claims

1. An evacuation method (500) for evacuating liquefied gas (G) from a watertight and thermally insulating tank (16) configured to contain liquefied gas (G), the tank (16) comprising at least: - a plurality of walls, including an upper wall (162) and a lower wall (164), - a pumping duct (25, 25b, 25c) for pumping liquefied gas (G) extending into the tank (16) from a first end opening to the outside of the tank (16) to a second end located at the bottom of the tank (16) and connected to a suction member (8, 8b) of a pump, housed at least partly in an enclosure (30, 7, 30c) open onto the tank, the evacuation method (500) comprising in order: - a step (501) of emptying the tank (16), - a step (502) of heating the tank (16) capable of vaporizing a residual portion of liquefied gas present in the tank (16), - a step (503) of inerting the tank (16) by introducing an inert gas therein, the evacuation method (500) being characterized in that it further comprises, following or during the step (503) of inerting the tank (16), a step (504) of sending a flow of an inert gas, capable of expelling from the enclosure (30, 7, 30c) a quantity of liquefied gas vaporized during the heating step (502), the step (504) of sending a flow of inert gas being distinct from the inerting step (503).

2. The method (500) for evacuating liquefied gas according to claim 1, further comprising a step (505) of sending dry air into the tank (16), capable of expelling from the tank (16) the inert gas or gases resulting from the inerting step (503) and from the step of sending the flow of an inert gas (504).

3. The method (500) for evacuating liquefied gas according to claim 1 or 2, wherein the duration of the step (504) of sending the flow of an inert gas is predefined so as to allow the evacuation out of the enclosure (30, 7, 30c) of the vaporized liquefied gas remaining therein.

4. The method (500) for evacuating liquefied gas according to any one of claims 1 to 3, wherein the step of sending the flow of an inert gas (504) uses a pipe (4, 4b, 4c) with a cross-section strictly smaller than that of an inerting duct used during the inerting step (503).

5. An evacuation system (S1, S2, S3) for evacuating liquefied gas from a watertight and thermally insulating tank (16) configured to contain liquefied gas (G), the system comprising the tank (16), the latter comprising at least: - a plurality of walls, including an upper wall (162) and a lower wall (164), - a pump comprising a suction member (8, 8b) housed at the bottom of the tank (16) at least partly in an enclosure (30, 7, 30c) open onto the tank (16), - a pumping duct (25, 25b, 25c) for pumping liquefied gas extending into the tank (16) from a first end opening to the outside of the tank (16) to a second end located at the bottom of the tank (16) and connected to the suction member (8, 8b), the evacuation system (S1, S2, S3) further comprising: - means for emptying the tank (16), - means for heating the tank (16), capable of vaporizing a residual portion of liquefied gas (G) present in the tank (16) after the emptying means have been operated, - means for inerting the tank (16), capable of introducing an inert gas into it after the heating means have been operated, the evacuation system (S1, S2, S3) being characterized in that it further comprises means for sending a flow of an inert gas, capable of expelling from the enclosure (30, 7, 30c) a quantity of liquefied gas vaporized by the heating means, the means for sending a flow of inert gas being distinct from the means for inerting the tank (16).

6. The system (S1, S2, S3) for evacuating liquefied gas according to claim 5, wherein the means for sending a flow of an inert gas comprise a pipe (4, 4b, 4c) with a cross-section strictly smaller than that of an inerting duct coupled to means for supplying inert gas, the inerting duct opening into the tank (16) and forming part of the inerting means, the pipe (4, 4b, 4c) comprising a first end located outside the tank (16) and a second end (42) arranged so as to direct the flow of inert gas into the enclosure (30, 7, 30c).

7. The system (S1, S2, S3) for evacuating liquefied gas according to claim 6, wherein the pipe (4, 4b, 4c) comprises a flexible portion (41) at its first end, adapted to be coupled or decoupled to an inert gas supply system.

8. The system (S1, S2, S3) for evacuating liquefied gas according to claim 6 or 7, wherein the second end (42, 42b, 42c) of the pipe (4, 4b, 4c) is located below a plane delimiting an opening in the enclosure (30, 7, 30c), said opening being located opposite the upper wall (162) of the tank (16).

9. The system (S1, S2, S3) for evacuating liquefied gas according to any one of claims 6 to 8, wherein the means for sending a flow of an inert gas comprise means for attaching the pipe (4) to a mast (21, 23) of an unloading tower (2) comprising a plurality of masts and extending into the tank (16) and having, at its end proximal to the lower wall of the tank (16), a support structure interconnecting the masts (21, 22, 23) of the unloading tower (2).

10. The system (S1) for evacuating liquefied gas according to claim 9, wherein the pumping duct (25) is a duct for unloading the tank (16) integrated into the mast (23), and wherein the emptying means are capable of actuating the pump and driving the liquefied gas (G) into the pumping duct (25).

11. The system (S2, S3) for evacuating liquefied gas according to any one of claims 6 to 9, wherein the pumping duct (25b, 25c) is a duct configured to supply fuel to at least one consumer of a vessel (1).

12. The system (S1, S3) for evacuating liquefied gas according to any one of claims 9 to 11 when dependent on claim 9, wherein the pipe (4b, 4c) comprises: - a first portion (40) running along the mast (21), - a second portion (44) extending along the support structure (24), connecting one end of the mast (21) and the suction member (8b), - and a third portion (48) arranged vertically from one end of the second portion (44), said end of the second portion (44) being located in the vertical extension of the enclosure (7, 30c).

13. The system (S1, S3) for evacuating liquefied gas according to any one of claims 9 to 12 when dependent on claim 9, wherein the enclosure is a container (30, 30c) secured to the support structure (24), the container (30, 30c) being held above the lower wall (164) of the tank (16).

14. The system (S2) for evacuating liquefied gas according to any one of claims 5 to 12, wherein the enclosure is a sump (7), arranged in a thickness of the lower wall (164) of the tank (16).

15. A vessel (1) for transporting liquefied gas, comprising a gas evacuation system (S1, S2, S3) according to any one of claims 5 to 14.

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