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

Abstract

Method and system for evacuating liquefied gas from a liquefied gas transport or storage tank 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 inerting step (503) of the tank, a step of sending a flow of an inert gas (504), directed into the enclosure.(Figure 6).

Description

Title of the invention: Method and system for evacuating liquefied gas from a liquefied gas transport or storage tank

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

[0002] Such a tank has a capacity of several thousand or even several tens of thousands of cubic meters of liquefied gas. It has a generally parallelepiped shape, its walls generally resting on internal partitions of a hold of a transport ship. The ship's hold generally comprises several tanks. In order to maintain the liquefied gas in the liquid state, therefore at -163° C (degrees Celsius) for liquefied natural gas, the walls of each tank comprise several thermally insulating and waterproof layers, which line the internal partitions of the ship.

[0003] In order to fill or unload the tank with liquefied natural gas, a so-called unloading tower is generally secured to an upper wall of the tank. This tower extends vertically to the bottom of the tank, i.e. into the tank, close to a lower wall of the tank. The tower generally comprises several masts, comprising loading and / or unloading pipes. The unloading pipes are coupled to unloading pumps which are for example installed at one end of the tower at the bottom of the tank, and which each comprise a suction member driving the liquefied gas into one of the unloading pipes.

[0004] Other pipes are generally integrated into the tower, in particular a pipe allowing the supply of natural gas to the ship's engines. Such a pipe also includes a pump for sucking up the liquefied natural gas from the bottom of the tank.

[0005] In order to protect the pumps present in the tank from sudden movements of the liquefied natural gas within the tank, enclosures generally surround the suction member of each pump. Such an enclosure generally takes the form of a sump arranged in the lower wall of the tank, or of a container whose side walls are arranged vertically around the suction member of the pump.

[0006] When the enclosure is a sump, it also allows the pump present in this sump to suck up a maximum of liquefied gas present in the tank, when the latter must be completely emptied. In addition, a sump guarantees that the suction member of the pump is always immersed in a minimum level of liquid and thus avoids any deterioration of this suction organ during its operation.

[0007] When the tank must be completely emptied, for example for inspection, the pumps in the discharge pipes drain the tank, then the residual liquid present in the tank is vaporized by a circulation of hot gas inside it. The tank then undergoes an inerting operation during which an inert gas is introduced at the bottom of the tank. When the gas mixture present in the tank reaches a predefined proportion making it non-flammable in the open air, the tank is placed under air, so as to make it suitable for a visit by tank maintenance personnel.

[0008] The inventors have however found that following these operations of evacuating the liquefied gas in the tank, evaporated liquefied gas remains trapped in the pump protection enclosures. However, the fumes of this evaporated liquefied gas can be harmful to personnel working near the pumps protected by these enclosures, or during maintenance of these pumps.

[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 insulating tank and a corresponding evacuation system, making it possible to protect the tank maintenance personnel from residual gas emissions.

[0010] To this end, the invention proposes a method for evacuating liquefied gas from a sealed and thermally insulating 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 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 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 stage 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 method being characterized in that it further comprises, following or during the step of inerting the tank, a step of sending a flow of an inert gas, capable of expelling from the enclosure a quantity of liquefied gas vaporized during the heating step.

[0011] The flow of inert gas is directed into the enclosure so as to allow this 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. It is for example fixed to a bottom wall of a hold of a transport ship, while the upper wall of the tank is for example secured to the deck of the transport ship. The walls of the tank each comprise for example on the one hand a primary layer consisting of a primary waterproof membrane intended to be in contact with the liquefied gas and a primary thermal insulation barrier made of one or more insulating materials, and on the other hand a secondary layer consisting of a secondary waterproof membrane in contact with the primary thermal insulation barrier and a secondary thermal insulation barrier made of one or more insulating materials and fixed to an internal partition, a bottom wall or a deck of the ship depending on the wall of the tank in question.

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

[0014] The pumping line extends mainly vertically in the tank, its main dimension being oriented vertically, that is to say orthogonally to the lower and upper walls of the tank. The second end of the pumping line is connected to a suction member arranged at the bottom of the tank, that is to say in the lower part of the tank. The second end of the pumping line is preferably located between the lower wall of the tank and a distance therefrom forming, for example, one tenth of the volume of the tank. In this way, the suction member operates even when the tank is almost empty. The pump comprising the suction member also comprises a motor for driving the suction member, which can be offset relative to the suction member.

[0015] The pumping line is for example intended to supply consumers of a ship, such as a ship's engine. Alternatively, it is used to unload the tank, in particular when it is the tank of a LNG carrier type ship. In the latter case, the pumping line is generally integrated into a mast of an unloading tower. Such an unloading tower extends vertically in the tank and has a first end located outside the tank, for example on the deck of the ship, and a second end located at the bottom of the tank. It therefore passes through the upper wall of the tank. The second end of the unloading tower does not touch the lower wall of the tank, the unloading tower being carried at the level of the upper wall of the tank.It consists of several hollow masts arranged vertically and in which loading or unloading pipes are arranged vertically, the masts being secured to each other by means of crosspieces.

[0016] The pipes in the tank are generally grouped at the unloading tower. When the pumping pipe is intended to supply consumers of the ship, the pump drive motor is for example supported by a support structure of the unloading tower, arranged at the second end of the latter and consisting for example of plates and crosspieces forming a base arranged mainly parallel to the lower wall of the tank but at a distance from it. Alternatively, the pump drive motor may be located on the deck of the ship.

[0017] When the pumping line is a tank discharge line, the pump drive motor is for example arranged on the deck of the ship, a drive shaft then connecting the drive motor to the suction member in the mast.

[0018] Alternatively, the pump drive motor, used to supply consumers of the ship or to unload the tank, is grouped with its suction member in the same casing fixed to the support structure.

[0019] The suction member 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 of a container or a deflector carried by the support structure of the unloading tower.

[0020] Thanks to the invention, the vaporized liquefied gas remaining in the enclosure during the inerting step is expelled from this enclosure by a flow of inert gas sent into or towards the enclosure. The vaporized liquefied gas mixed with the inert gas can then be expelled by dry air sent into the tank to make it suitable for receiving personnel. The safety of the personnel working in the tank is therefore ensured, without however renouncing the use of a container or a sump receiving the suction member of a pump. It should be noted that the invention is not limited to transport tanks, the sumps being able to be used in land-based devices integrating liquefied gas tanks, to perfect their unloading.

[0021] The inerting step preferably uses an inert gas produced by an inert gas generator resulting from the combustion of diesel. 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 for sending a flow of inert gas is preferably nitrogen produced by a service inert gas supply system, i.e. supplying nitrogen at five bars of pressure. This inert gas is therefore not loaded with carbon dioxide except for possible traces, unlike the inert gas used for inerting the tank.

[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 nitrogen or only a mixture of nitrogen and carbon dioxide.

[0023] In one embodiment of the invention, the method for evacuating liquefied gas further comprises a step of sending dry air into the tank, capable of expelling from the tank the inert gas(es) resulting from the inerting step and from the step of sending the flow of an inert gas.

[0024] The dry air is sent into the tank only after the tank has been inerted and the vaporized liquefied gas present in the enclosure has been evacuated using the means for sending a flow of inert gas into the enclosure. This dry air is the product of dehumidification of the air by a dry air generator, using a dehydrating product such as activated alumina.

[0025] According to an optional and advantageous characteristic of the method according to the invention, the duration of the step of sending the flow of an inert gas is predefined so as to allow the evacuation from the enclosure of the vaporized liquefied gas remaining within it. This characteristic allows a simple implementation of the invention, because it does not require control of the flow rate of the inert gas. The predefined duration is for example measured by tests prior to the implementation of the method according to the invention.

[0026] According to another optional and advantageous characteristic of the method according to the invention, the step of sending the flow of an inert gas uses a pipe with a section strictly smaller than that of an inerting pipe used during the inerting step. The pipe has for example a diameter of DN 8 (the abbreviation DN designates the nominal internal diameter in millimeters) while the inerting pipe has a diameter of DN 15. This characteristic makes it possible not to modify the inerting pipe used during the inerting of the tank. The pipe is for example connected to the service inert gas supply system, only when it is desired to open the tank. As a result, this embodiment of the invention is inexpensive and easy to implement.

[0027] The invention also relates to a system for discharging liquefied gas from a sealed and thermally insulating 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 member, housed at the bottom of the tank at least partly in an enclosure open to the tank, - a liquefied gas 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 the suction member, the evacuation system further comprising: - means of emptying the tank, - means for heating the tank, capable of vaporizing a residual portion of liquefied gas present in the tank after use of the draining means, - means for inerting the tank, capable of introducing an inert gas into the tank, after use of the heating means, the evacuation system being characterized in that it 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.

[0028] These means for sending a flow of inert gas are implemented during or after the implementation of the inerting means and make it possible to direct the flow of inert gas into the enclosure.

[0029] The liquefied gas evacuation system according to the invention comprises 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 sending a flow of an inert gas comprise a pipe with a cross-section strictly smaller than that of an inerting pipe coupled to inert gas supply means, the inerting pipe opening into the tank and forming part of the inerting means, the pipe comprising a first end located outside the tank and a second end arranged so as to direct the flow of inert gas into the enclosure. As explained in relation to the method according to the invention, the use of such a pipe makes it possible not to modify the inerting means of the tank, and in particular not to modify the inerting pipe supply. The pipe being of smaller diameter (DN8 instead of DN15 for example), it is easy to install.

[0031] The pipe comprises for example a flexible portion at its first end, capable of being coupled or decoupled from an inert gas supply system. The inert gas supply system is, in this embodiment of the invention, a service inert gas supply system, i.e. supplying nitrogen at five bars of pressure, while the inert gas used by the inerting means comes from a combustion of diesel, as explained above.

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

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

[0034] In an exemplary embodiment related to this embodiment, the pumping pipe is a discharge pipe from the tank integrated into the mast and the emptying means are capable of actuating the pump and driving the liquefied gas into the pumping pipe. The enclosure is then, for example, a container secured to the support structure located at the lower end of the mast and surrounding the suction member. Alternatively, the enclosure is a sump located under the mast in the thickness of the lower wall.

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

[0036] In this other exemplary embodiment, the pipe comprises 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 member, - 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 send a flow of inert gas into the enclosure is therefore brought to the bottom of the tank by attachments to a mast of the unloading tower, preferably chosen as the one closest to the enclosure housing the suction member. The pipe must make a detour at the bottom of the tank before arriving above the enclosure in which the suction member is protected, using the support structure of the unloading tower as support.

[0038] According to an exemplary embodiment of the invention, the enclosure may be located outside a perimeter delimited by the masts of the unloading tower. This is for example the case when the pumping pipe, whether it is a pipe intended for fuel supply or an unloading pipe, is located outside this perimeter.

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

[0040] Alternatively, the enclosure is a sump, arranged in 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 according to the invention and the method according to the invention.

[0042] Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several exemplary embodiments given for informational and non-limiting purposes with reference to the attached schematic drawings on the other hand, in which:

[0043] [Fig-1] represents a ship according to the invention, equipped with gas transport tanks liquefied and evacuation systems according to the invention of the liquefied gas present in these tanks, in an embodiment according to the invention,

[0044] [Fig.2] represents a tower for unloading a tank transported by the ship of [Fig.l], integrating means of a liquefied gas evacuation system according to the invention, in a first exemplary embodiment of the invention,

[0045] [Fig.3] represents a second exemplary embodiment of the invention, in which means of a liquefied gas discharge system according to the invention also use elements of the unloading tower shown [Fig.2],

[0046] [Fig.4] is a sectional view in a vertical plane of a sump protecting a member suction of a pump connected to a pumping pipe, the sump being associated with the means of the liquefied gas evacuation system mentioned in relation to [Fig.3],

[0047] [Fig.5] is a sectional view in a vertical plane of a container protecting a suction member of a pump connected to a pumping pipe, the container being associated with means of a liquefied gas evacuation system according to the invention in a third exemplary embodiment of the invention, these means also using elements of the unloading tower shown [Fig.2], and

[0048] [Fig.6] represents steps of a liquefied gas evacuation process according to the invention, in the embodiment of the invention mentioned [Fig.l].

[0049] According to one embodiment of the invention, a ship 1 according to the invention shown [Fig.l] is a ship for transporting liquefied gas G, which is in this embodiment of the invention, liquefied natural gas. The liquefied gas G is transported in tanks 16 installed in the hold of the ship 1. The hold comprises a lower partition 12, substantially parallel to the deck 18 of the ship 1, and internal lateral partitions 14 which divide the hold into four compartments.

[0050] A sealed and thermally insulating tank 16 is arranged in at least one compartment. This tank 16 comprises an upper wall 162 (partially shown in [Fig.2]) secured to the deck 18 of the ship 1, a lower wall 164 (partially shown in [Fig.2]) secured to the lower partition 12 of the ship 1, and side walls secured to the internal side partitions 14. The walls of the tank 16 line the partitions of each compartment of the hold with sealed and thermally insulating layers, so that the tank 16 forms a substantially parallelepiped container capable of receiving the liquefied gas G and keeping it at a temperature less than or equal to -163°C, at atmospheric pressure. The upper wall 162 also includes a cover (not shown) allowing personnel in particular to descend into the tank.

[0051] The upper wall 162 of the tank 16 is crossed by an unloading tower 2, comprising a first end accessible from the outside of 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 unloading tower 2 is carried by the deck 18 of the ship and extends vertically in the tank 16. To limit certain movements of the unloading tower 2, the length of which is several tens of meters, a holding device 15 secured to the lower wall 164 of the tank 16, holds the unloading tower 2 in a vertical direction, that is to say perpendicular to the upper 162 and lower 164 walls of the tank 16.It should also be noted that the holding device 15 participates in supporting the unloading tower 2, the length of which varies according to the thermal variations in the tank 16, in particular when the ship is exposed to rolling phenomena.

[0052] [Fig.2] shows in more detail the unloading tower 2, which can be seen to is arranged along its length along a vertical axis (Oz) of an orthonormal reference frame (O, x, y, z). The terms “upper”, “high”, “lower”, “lower” in this application refer to the orientation of this axis Oz, directed upwards, that is to say towards the upper wall 164 of the tank 16.

[0053] The unloading tower 2 comprises several hollow masts 21, 22, 23 extending vertically in the tank 16 by crossing the upper wall 162 thereof. A first 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 arranged 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 secured to each other by a support structure 24 formed of plates and crosspieces and arranged at a distance from the lower wall 164 of the tank 16. Other crosspieces 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 holding device 15.

[0054] In this embodiment of the invention, the mast 23 houses a pumping pipe 25 which is a liquefied gas G unloading pipe. The pumping pipe 25 extends vertically from a first end located outside the tank to a second end connected at the bottom of the tank to an unloading pump comprising a suction member 8. The unloading 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 the latter.

[0055] In order to protect the unloading pump from the effects of waves, a container 30, preferably cylindrical, open at the top, surrounds the unloading 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 evaporated, can remain confined without a specific evacuation system such as an evacuation system SI for the liquefied gas present in the tank 16, and which forms a first example of embodiment of the invention.

[0056] The evacuation system SI comprises means for emptying the tank 16, these emptying means comprising the pumping pipe 25 and the unloading pump, as well as means for actuating the unloading pump to suck the liquefied gas G out of the tank, until the latter no longer contains more than a residual portion of liquefied gas. The emptying means possibly comprise other pumping devices actuated in parallel with the unloading pump (for example another unloading pipe in another mast and connected to another unloading pump) or actuated to complete its action when the tank 16 is almost unloaded (for example a so-called dewatering pump). A return of gas in the vapor phase is also operated to balance the pressure in the tank 16 as it is emptied.

[0057] The evacuation system SI further comprises means for heating the tank 16, capable of vaporizing the residual portion of liquefied gas G present in the tank 16 after implementation of the emptying means. These heating means comprise means for circulating heated gas in the tank 16, this gas in vapor phase being for example recovered in the upper part of the tank 16 then compressed. The residual portion of liquefied gas G is entirely vaporized when the thermal insulation of the tank 16 reaches 5°C.

[0058] The evacuation system SI further comprises means for inerting the tank, making it possible to introduce into it 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 pipe, a first end of which, located outside the tank 16, is connected to the inert gas generator and a second end of which is located at the bottom of the tank. The inert gas introduced at the bottom of the tank, thanks to its density, makes it possible to raise the natural gas vaporized by the heating means, into an upper part of the tank where it is sucked in and extracted from the tank.

[0059] The evacuation system SI further comprises 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, to expel the vaporized gas remaining confined in the container 30.

[0060] These means for sending a flow of inert gas comprise a pipe 4 shown [Fig.2], this pipe 4 being of a smaller diameter than that of the inerting pipe, and a service nitrogen supply system. The pipe 4 has a first end located outside the tank 16 and a second end 42 opening into the container 30. The pipe 4 is fixed along the mast 23 with the exception of two end portions of the pipe 4, which are a first end portion located outside the tank 16 and comprising a flexible portion 41 and a second end portion vertically extending the part of the pipe 4 fixed to the mast 23 into the container 30.

[0061] The flexible portion 41 of the pipe 4 allows it to be connected to the service nitrogen supply system, this nitrogen being produced by an inert gas generator present on the ship, from the ambient air. This nitrogen supply system provides nitrogen at a pressure of 5 bars and can be connected to various equipment depending on requirements. The pipe 4 therefore allows, when connected to this nitrogen supply system, to act as a device connected to a compressed air pipe otherwise called a blower, with sufficient pressure to expel the vaporized gas remaining confined in the container 30. It should be noted that the inerting means could not, even positioned above the container 30, expel the vaporized liquefied gas confined inside this container 30. Indeed, the inerting means diffuse the inert gas with insufficient pressure to succeed in expelling the vaporized gas.

[0062] The evacuation system SI finally comprises means for venting the tank 16. These means comprise a dry air conduit in the upper part of the tank 16, making it possible to introduce into this upper part, dry air produced by a dry air generator present on the ship. This generator uses the ambient air which it dehumidifies. The dry air introduced in the upper part expels the other gases present in the tank 16 after implementation of the inerting means and sending a flow of inert gas into the container 30, pushing them into the lower part of the tank 16 where they are extracted until an oxygen content greater than 20% is obtained.

[0063] The evacuation system SI according to the invention thus allows personnel to enter the tank without breathing in natural gas fumes in the vapor phase which would have remained confined in the container 30.

[0064] As now described in relation to [Fig. 3], according to a second embodiment, the tank 16 may also comprise a pumping pipe 25b which is used to supply consumers of the ship 1 with fuel.

[0065] The pumping pipe 25b has a first end located outside the tank and extends vertically near the unloading tower 2 to an end portion located at the bottom of the tank. This end portion has a first horizontal part fixed to the mast 21 as well as to the support structure 24, and a second vertical part connected to the first horizontal part and to a second end of the pumping pipe 25b, this second end being connected to a feed pump comprising a suction member 8b. The feed pump is supported by the support structure 24, as more particularly visible [Fig.4],

[0066] In order to protect it from the effects of waves, this feed pump is housed in a sump 7 arranged in the lower wall 164 of the tank 16. As visible [Fig.4], the lower wall 164 comprises: - a primary waterproof metallic membrane 1640, intended to be in contact with the liquefied gas G in the tank 16, - a primary insulation layer 1641 comprising insulating materials, arranged under the waterproof primary membrane 1640,

[0067] - a secondary waterproof membrane 1642 metallic or composite, arranged under the primary insulation layer 1641, and - a secondary insulation layer 1643, arranged under the secondary waterproof membrane 1642 and resting on the lower bulkhead 12 of the ship's hold 1.

[0068] 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 waterproof membrane 1640 to ensure the sealing of the tank 16. It has a bottom, for example arranged in the secondary insulation layer 1643, parallel to the lower partition 12. The feed pump arranged in the sump 7 being supported by the support structure 24, it does not touch the bottom of the sump 7.

[0069] So that vaporized liquefied gas does not remain confined in the sump 7 during complete evacuation of the liquefied gas in the tank 16, a system S2 for evacuating the liquefied gas present in the tank 16, shown in Figures 3 and 4, and forming a second exemplary embodiment of the invention, comprises means similar to those of the system S1 for evacuating the liquefied gas described previously.

[0070] In particular, the liquefied gas evacuation system S2 comprises the same means for emptying, heating, inerting and venting the tank 16 as those of the liquefied gas evacuation system SI. It should be noted that the liquefied gas evacuation system S2 coexists, in this embodiment of the invention, with the liquefied gas evacuation system SI.

[0071] The liquefied gas evacuation system S2 also comprises means for sending a flow of inert gas directed into the sump 7 and which can be implemented during or after inerting of the tank 16 by the inerting means.

[0072] These means for sending a flow of inert gas comprise a pipe 4b of smaller diameter than that of the inerting pipe, as well as the service nitrogen supply system to which the pipe 4b is connectable. The pipe 4b comprises a first end located outside the tank 16 and a second end 42b opening into the sump 7. The pipe 4b comprises a vertical main portion 40 fixed along the mast 21, a first end portion located outside the tank 16 and comprising a flexible portion for its connection to the service nitrogen supply system and a second end portion extending at the bottom of the tank the main portion 40 fixed to the mast 21 into the sump 7.

[0073] The second end portion of the pipe 4b comprises a horizontal or substantially horizontal portion 44 fixed to the support structure 24, this horizontal portion 44 connecting a lower end of the main portion 40 to an upper end of a last vertical portion 48 of the pipe 4b, this upper end being arranged above the sump 7. The last vertical portion 48 of the pipe 4b is arranged vertically so that its lower end corresponding to the second end 42b of the pipe 4b is in the sump 7, as visible [Fig.4].

[0074] The attachments 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.

[0075] The pipe 4b allows, when connected to this nitrogen supply system, to act as a blower, with sufficient pressure to expel the vaporized gas remaining confined in the sump 7. This connection, aimed at sending a flow of inert gas in order to expel the vaporized liquefied gas, is carried out at the end of inerting or after inerting of the tank 16.

[0076] [Fig.5] now illustrates a liquefied gas evacuation system S3 forming a third exemplary embodiment of the invention and corresponding to a variant of the liquefied gas evacuation system S2 previously described.

[0077] The liquefied gas evacuation system S3 differs from the liquefied gas evacuation system S2 mainly by the fact that the feed pump for the consumers of the ship 1, comprising the suction member 8b, is protected from the effects of the waves not by a sump but by a container 30c, similar to the container 30. The other elements, identical to those of the liquefied gas evacuation system S2, are referenced identically.

[0078] In this variant embodiment, the pumping line allowing the fuel to be supplied to the consumers of the ship 1 and connected at the bottom of the tank to the pump supply, is a pumping line 25c identical to the pumping line 25b of the liquefied gas evacuation system S2, except that it descends less deeply into the tank 16 since the container 30c is not arranged in the lower wall 164 of the tank 16 but above it.

[0079] 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.

[0080] To evacuate vaporized liquefied gas which would remain confined in the container 30c after or at the end of inerting the tank 16, the liquefied gas evacuation system S3 comprises a pipe 4c connectable to the service nitrogen supply system by one of its ends, and the other end 42c of which is located in the container 30c. The arrangement of the pipe 4c and its attachment to the unloading tower 2 is identical to that of the pipe 4b except for the fact that the pipe 4c descends less deeply to the bottom of the tank since the container 30c is located less deeply than the sump 7. As a result, the pipe 4c descends for example less deeply along the mast 21 and is attached horizontally to the support structure 24 higher than the horizontal portion 44 of the pipe 4b of the liquefied gas evacuation system S2.

[0081] The pipe 4c also allows, when connected to the service nitrogen supply system, to act as a blower, with sufficient pressure to expel the vaporized gas remaining confined in the container 30c. The sending of the flow of inert gas for the purpose of expelling the vaporized liquefied gas is carried out at the end of inerting or after inerting of the tank 16.

[0082] 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.

[0083] We will now describe in relation to [Fig.6], steps of an evacuation method 500 according to the invention, capable of evacuating the liquefied gas G from the tank 16, and implemented by the evacuation system S1, S2 or S3 of liquefied gas according to the invention, in this embodiment of the invention.

[0084] A first step 501 of the evacuation method 500 is the emptying of the tank 16. For this, the unloading pumps of the tank 16 are actuated, as well as possibly a dewatering pump at the end of emptying. These pumps and the associated pipes form part, in this embodiment of the invention, of the emptying means of the evacuation system S1, S2 or S3.

[0085] A second step 502 of the evacuation method 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 SI, S2 or S3 are actuated, circulating heated gas in the tank 16, until the walls of the tank 16 reach a temperature of 0°C. A third step 503 of the evacuation method 500, implemented after the heating step 502, is the inerting of the tank 16. For this, the inerting means of the evacuation system S1, S2 or S3 are used, which comprise, in this embodiment of the invention, the inert gas pipe and the inert gas generator present on the ship 1. Inert gas, composed of 80% nitrogen and 20% carbon dioxide and arriving at the outlet of the inert gas generator receiving ambient air and diesel oil at the inlet, is sent into the inert gas pipe to the bottom of the tank. The liquefied gas vaporized by the heating means then rises into an upper part of the tank 16 where it is sucked and extracted from the tank.The inerting step 503 is stopped when the gas mixture present in the tank 16 is no longer flammable on contact with air, for example up to a natural gas content in the tank 16, less than 50% compared to a lower limit content at which the natural gas ignites on contact with air.

[0086] A fourth step 504 of the evacuation method 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 embodiment variant chosen to be implemented. This step uses the means for sending a flow of inert gas from the evacuation system S1, S2 or S3, which comprise in this embodiment of the invention, the pipe 4 and the pipe 4b or 4c, as well as the service nitrogen generator present on the ship 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 predefined 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 the pipe 4 is disconnected and the flexible portion of the pipe 4b or 4c is connected in turn so as to be supplied by the service nitrogen generator which is also operated for around ten minutes, so that the vaporized liquefied gas remaining confined in the sump 7 or the container 30c is expelled.

[0087] Alternatively, this fourth step 504 of the evacuation method 500 is the sending of a flow of inert gas only into the container 30, 30c or into the sump 7, when a single 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 predefined duration, for example for ten minutes, so that the vaporized liquefied gas remains confined in the container 30, 30c or sump 7 is flushed out of it. Then the supply to pipe 4 is cut off by the nitrogen generator.

[0088] Finally, a fifth and final step 505 of the evacuation method 500, implemented after the inerting step 503 and after the step of sending a flow of inert gas 504, is the sending of dry air into the tank 16. This step uses the venting means of the evacuation system S1, S2 or S3, which comprise, in this embodiment of the invention, the dry air pipe as well as the dry air generator present on the ship. In this step 505 of sending dry air, dehumidified air obtained at the outlet of the dry air generator is introduced into the upper part of the tank 16, via the dry air pipe. This dehumidified air expels the other gases present in the tank 16 in the lower part thereof, where they are extracted until an oxygen content greater than 20% is obtained.

[0089] The tank 16 is then ready to accommodate tank 16 maintenance personnel, for example for its inspection.

[0090] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. In particular, the characteristics of the different variants or different examples of embodiment of the invention envisaged in this application can be combined to achieve the invention, to the extent that these variants or examples are not incompatible with each other.

Claims

Claims

1. Method for evacuating (500) liquefied gas (G) from a sealed and thermally insulating tank (16) intended 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 pipe (25, 25b, 25c) for liquefied gas (G) extending into the tank (16) from a first end opening outside 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 to the tank, the evacuation method (500) comprising in order: - a step of emptying (501) the tank (16), - a heating step (502) of the tank (16) capable of vaporizing a residual portion of liquefied gas present in the tank (16), - a step of inerting (503) the tank (16) by introducing an inert gas into it,the evacuation method (500) being characterized in that it further comprises, following or during the inerting step (503) of the tank (16), a step of sending a flow of an inert gas (504), capable of expelling from the enclosure (30, 7, 30c) a quantity of liquefied gas vaporized during the heating step (502).,

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

3. Method for evacuating (500) liquefied gas according to claim 1 or 2, in which the duration of the step of sending the flow of an inert gas (504) is predefined so as to allow the evacuation from the enclosure (30, 7, 30c), of the vaporized liquefied gas remaining within it.

4. Cl. Method for evacuating (500) liquefied gas according to any one of claims 1 to 3, in which the step of sending the flow of an inert gas (504) uses a pipe (4, 4b, 4c) of section strictly smaller than that of an inerting pipe used during the inerting step (503).

5. System (SI, S2, S3) for discharging liquefied gas from a sealed and thermally insulating tank (16) intended 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 to the tank (16), - a pumping pipe (25, 25b, 25c) for liquefied gas extending into the tank (16) from a first end opening outside the tank (16) to a second end located at the bottom of the tank (16) and connected to the suction member (8, 8b), the system (SI, 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 implementation of the emptying means, - means for inerting the tank (16), capable of introducing an inert gas into it, after implementation of the heating means, the evacuation system (SI, 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.,

6. Liquefied gas evacuation system (S1, S2, S3) according to claim 5, in which the means for sending a flow of an inert gas comprise a pipe (4, 4b, 4c) of section strictly smaller than that of an inerting pipe coupled to inert gas supply means, the inerting pipe 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. Liquefied gas evacuation system (S1, S2, S3) according to claim 6, wherein the pipe (4, 4b, 4c) comprises a flexible portion (41) at its first end, capable of being coupled or decoupled from an inert gas supply system.

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

9. Liquefied gas evacuation system (S1, S2, S3) according to any one of claims 6 to 8, in which the means for sending a flow of an inert gas comprise means for fixing the pipe (4) to a mast (21, 23) of an unloading tower (2) comprising several masts and extending into the tank (16) while having at its end proximal to the lower wall of the tank (16), a support structure connecting the masts (21, 22, 23) of the unloading tower (2) together.

10. Liquefied gas evacuation system (SI) according to claim 9, in which the pumping line (25) is a discharge line from the tank (16) integrated in the mast (23), and in which the emptying means are capable of actuating the pump, and of driving the liquefied gas (G) into the pumping line (25).

11. A liquefied gas discharge system (S2, S3) according to any one of claims 6 to 9, wherein the pumping line (25b, 25c) is a line intended for supplying fuel to at least one consumer of a ship (1).

12. Liquefied gas evacuation system (SI, S3) according to any one of claims 9 to 11 taken in dependence on claim 9, in which the pipe (4b, 4c) comprises: - a first portion (40) running along the mast (21), - a second portion (44) extending along the support structure (24), the latter 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. Liquefied gas evacuation system (SI, S3) according to any one of claims 9 to 12 taken in the dependency of claim 9, in which the enclosure is a container (30, 30c), integral with the support structure (24), the container (30, 30c) being held above the lower wall (164) of the tank (16).

14. Liquefied gas evacuation system (S2) according to any one of claims 5 to 12, in which the enclosure is a sump (7), arranged in a thickness of the lower wall (164) of the tank (16).

15. 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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