WO2019122577A1

WO2019122577A1 - Vessel propelled using liquefied gas

Info

Publication number: WO2019122577A1
Application number: PCT/FR2018/053155
Authority: WO
Inventors: Mickaël HERRY
Original assignee: Gaztransport Et Technigaz
Priority date: 2017-12-22
Filing date: 2018-12-07
Publication date: 2019-06-27
Also published as: FR3075754B1; KR20200096651A; CN111491856A; FR3075754A1; CN111491856B

Abstract

The invention relates to a vessel (1) comprising a propulsion system (5) and a tank (6) for combustible liquefied gas, wherein the tank (6) comprises: - a shaft traversing a top wall of the tank (6) and directed towards a bottom wall of the tank; - a main pump (11), which is a retractable pump arranged in the shaft and is movable, without having to drain the tank, between a position inside the tank and a position outside the tank, the main pump being configured to discharge the liquefied gas from the tank towards the propulsion system; - an auxiliary pump (12), which is a pump fixed on a lower level of the tank relative to the main pump (11), the auxiliary pump being configured to discharge the liquefied gas from the tank towards the propulsion system (5).

Description

Ship propelled with liquefied gas

Technical area

The invention relates to the field of tanks carrying liquefied fuel gas on board a ship or other floating structure to power a propulsion system of the ship or other floating structure, including a vessel propelled by liquefied natural gas. Liquefied natural gas or LNG is mainly composed of methane.

Technological background

Liquefied natural gas can be shipped to constitute the, or at least one of the fuel propelling ships of all types, for example LNG carriers or LNG carriers but also container carriers. This is called an LNG fueled ship or LFS. The invention applies more particularly to floating structures of this category.

When the ship is a tanker, the cargo tanks have a very large capacity, for example of the order of 100,000 to 200,000 m ³ .

When the vessel carries a further cargo and that the vessel is only used for fuel, the capacity is much smaller, for example from 5 000 to 25 000 m ³ according to vessel size and the length of journeys to perform.

Liquefied natural gas, in that it has complete combustion, is much less polluting than conventional fuels such as gasoline or gas oil. Liquefied natural gas has a high mass energy and because of its liquefaction, providing a high compression, a large volume energy. Also, liquefied natural gas is becoming an advantageous alternative for the propulsion of ships.

To discharge a vessel containing LNG, it is known to use a submerged fixed pump as the main pump. This pump is immersed throughout its life in LNG and is permanently fixed in the tank on a mast structure. It is necessary to carry out maintenance of these pumps after a certain number of hours of use, for example 15 000 hours, to check their correct operation.

However, these submerged pumps are very difficult to maintain and maintenance because they require the tank to be completely purged and this makes the tank unusable during this maintenance.

summary

An idea underlying the invention is to facilitate the maintenance of LNG tanks or other liquefied fuel gas.

More particularly, an idea underlying the invention is to propose a vessel propelled by combustion of a combustible liquefied gas, comprising a sealed and insulating tank for liquefied gas, which feeds the liquefied gas propulsion system using a retractable pump. Indeed, such a pump can be retracted out of the tank easily for maintenance without the need to purge or stop the tank.

In addition, an advantage of the invention is to facilitate the unloading of the tank when the level is low, with the aid of an auxiliary pump placed at a lower level of the tank relative to the main pump.

Another idea underlying the invention is to allow the tank to be still usable during maintenance of the retractable pump with the aid of the auxiliary pump.

Another idea underlying the invention is to allow the ship propelled by combustion of a combustible liquefied gas to continue to navigate even when the retractable pump is in maintenance.

According to one embodiment, the invention provides a vessel comprising a propulsion system and a sealed and insulating tank for combustible liquefied gas, wherein the tank comprises:

- A barrel provided with a valve arranged at a lower end of the barrel, the barrel passing through a top wall of the tank and directed towards a bottom wall of the tank;

a main fuel pump which is a retractable pump arranged in the drum and movable without having to empty the tank between a position inside the tank, in which the pump actuates the valve of the drum and a position outside the tank, the main pump in the position inside the tank being configured to discharge the liquefied gas from the tank to the propulsion system ; an auxiliary fuel pump which is a pump fixed inside the tank and which is configured to suck the liquefied gas at a lower level of the tank relative to the main pump, the auxiliary pump being configured to discharge the gas liquefied from the tank to the propulsion system.

Thanks to these characteristics, the vessel makes it possible to use a retractable pump as main pump and thus after reaching the number of hours of use to allow an easy extraction for maintenance. The retractable pump also has the advantage of not requiring the purge of the tank to extract it unlike the auxiliary pump.

In order to maximize the operating efficiency of such a tank, it is desirable to optimize the useful volume of cargo that can be unloaded from the tank. However, the use of a pump sucking the liquid up the tank requires to maintain a certain liquid height at the bottom of the tank, otherwise the suction member of the pump enters into communication with the gas phase which defuses and / or degrades the pump. Given the swelling of the cargo by the swell, the necessary liquid height can hardly be minimized.

Thus, the auxiliary pump is placed at a lower level of the tank relative to the main pump so as to continue unloading the tank before the main pump is defused or damaged by the suction of the gas phase. Indeed, the retractable pump is inserted into a shaft attached to the mast structure, the barrel comprising a valve which closes its lower end. The retractable pump when placed at the lower end pushes by gravity the valve to open the barrel and thus allow the pump to be able to suck the liquefied gas. So by design, it is not possible to place a retractable pump at the bottom of the tank because it is necessary to leave a clearance for the opening of the valve and for its thickness. The invention therefore allows using the auxiliary pump which is a pump attached to the inside of the tank and which is not in a drum, to suck up the liquefied gas remaining in the tank below the tank. lower end of the barrel. According to embodiments, such a tank may comprise one or more of the following characteristics.

According to one embodiment, the tank comprises a mast structure comprising a plurality of pillars interconnected by a bottom plate, and a foot extending between the bottom wall of the tank and the bottom plate, and wherein the shaft is formed in one of the pillars of the mast structure.

Thanks to these features, the mast structure securely secures the barrel to the tank and thus allow the retractable pump to have secure access to the interior of the tank.

It should be noted that in view of the very low temperature of the liquefied gas, a phenomenon of thermal contraction is observed on the entire structure of the mast including the barrel, which tends to raise the whole in the and thus move the retractable pump a little further, emphasizing the need for an auxiliary pump at a lower level to draw the liquid to the bottom of the tank.

According to one embodiment, the mast structure comprises a trellis system connecting the pillars two by two.

According to one embodiment, the barrel has a sectional dimension of the order of

600mm.

According to one embodiment, the auxiliary pump is attached to the bottom plate of the mast structure.

According to one embodiment, the auxiliary pump is fixed to the foot of the mast structure.

Thus, the attachment of the auxiliary pump to the mast structure stabilizes the pump and fix the auxiliary pump to the same element as the main pump which avoids adding additional elements inside the tank.

According to one embodiment, the auxiliary pump is fixed to the mast structure with the aid of a fixing arm. Thanks to these characteristics, the attachment arm allows to shift the auxiliary pump of the mast structure and thus move the suction member of the auxiliary pump of the foot of the mast structure.

According to one embodiment, the valve is configured to be open when the main pump is positioned at the lower end of the barrel, a suction member of the main pump protruding from the lower end of the barrel, and to be closed by closure means in the absence of the main pump at the low end, the lower end of the barrel being sealed when the valve is in the closed position.

In this way, when the main pump is removed from the lower end of the barrel for example a maintenance, the valve goes into the closed position and thus tightens the barrel liquefied gas preventing any communication of the liquefied gas with the outside of the tank.

According to one embodiment, the closure means may be a return means, for example a spring.

According to one embodiment, the valve comprises a seal on its circumference which when the valve is in the closed position, the seal abuts against the barrel.

Thus, thanks to the seal, the tightness of the barrel when the valve is in the closed position is improved.

According to one embodiment, the tank comprises a sump, and the auxiliary pump comprises a suction member, the suction member being placed in the sump.

Thanks to these characteristics, the suction element of the auxiliary pump is placed at the lowest point of the tank in a sump. Thus it is possible to optimize the unloading of the tank, the auxiliary pump can thus suck the liquefied gas into the sump.

Preferably such a sump is disposed at a sufficient distance from the foot of the mast structure not to cause a structural problem that could destabilize the mast structure. When the drum is too close to the foot, it is not possible to place a sump at the right of the pump shaft, a fixing arm can act as an intermediary for attaching the auxiliary pump to the right of a sump far from the mast base.

According to one embodiment, the tank comprises a level sensor measuring the filling level of the tank, the sensor being preferably an optical sensor, or a radar system or a thermal sensor.

Thus, it is possible to monitor the level of filling of the tank to be able to deactivate the pumps before any problem of defusing.

According to one embodiment, the vessel comprises a control unit configured to, when the level sensor indicates a filling level of the tank greater than a predefined threshold, discharge the liquefied gas contained in the tank using the pump main to the propulsion system,

and wherein the control unit is configured to, when the level sensor indicates a tank fill level below the preset threshold, discharge the liquefied gas contained in the tank using the auxiliary pump to the tank system. propulsion.

Thanks to these characteristics, it is possible to automatically control the activation of the pumps according to a certain level of filling allowing greater flexibility of the pumps with regard to their service life and maintenance phases.

Thus, the auxiliary pump takes over from the main pump before it is defused for lack of liquid. Depending on the design of the tank (eg in the presence of lower chamfers, corrugated plates) and the retractable pump (flapper), it may be necessary to switch to the auxiliary pump at different filling levels.

According to one embodiment, the predefined threshold is between 2 and 10% of the total volume of the tank, preferably between 2 and 5% of the total volume of the tank, preferably equal to 3.7% of the total volume. of the tank.

In the case of a prismatic tank, because of the lower chamfers, the threshold of 3.7% by volume substantially corresponds to a threshold of 5% in height. According to one embodiment, the predefined threshold is between 200 and 1000mm of the tank height, preferably between 200 and 500mm of the tank height, preferably equal to 350 mm of the tank top.

If the risk of defusing the main pump is used as a criterion to set the filling threshold for which the auxiliary pump takes over, it leads to a procedure that minimizes the use of the auxiliary pump and therefore its maintenance. This criterion is not the only one possible. In particular, it could lead to overuse of the main pump.

In another embodiment, the threshold is defined in order to optimize the maintenance cycles of the pumps. In fact, by adapting the predefined threshold to a certain quantity, it is possible to quantify the number of hours of use of each of the main and auxiliary pumps and consequently to predict the maintenances according to a predefined cycle.

For example, this threshold can be chosen to obtain a utilization rate of the main pump that requires two maintenance over a period of five years and a rate of use of the auxiliary pump that requires a single maintenance after five years.

According to one embodiment, the threshold thus predefined is between 20 and 35% of the total volume of the tank.

According to one embodiment, the invention also provides a method of operating a aforesaid vessel wherein the method comprises the steps of:

when the filling level of the tank is higher than a predefined threshold, discharging the liquefied gas contained in the tank using the main pump to the propulsion system,

- When the tank level is below the predefined threshold, discharge the liquefied gas contained in the tank using the auxiliary pump to the propulsion system.

Thanks to these characteristics, the method makes it possible to easily discharge the tank taking into account its level of filling, moreover by means of the predefined threshold, it is possible to avoid any damage of the pumps. According to one embodiment, the method comprises the step of stopping the main pump when the level of the tank is below the predefined threshold.

Thus, the retractable pump is stopped before reaching a critical threshold where it may be damaged by lack of fluid to suck.

According to one embodiment, the method comprises the steps of: when starting the unloading of liquefied gas, determining whether the filling level of the tank is greater than a starting threshold greater than said predefined threshold, between 1000 mm and 2000 mm of height of the tank, preferably equal to 1500 mm of height of the tank, and

start the main pump if this test is positive, otherwise start the auxiliary pump.

Indeed, to start the main pump without damaging it may be necessary that it is sufficiently immersed in the liquefied gas to be uniformly cold in order to operate without cavitation.

According to one embodiment, the invention also provides a method of loading such a vessel, wherein a liquefied fuel gas is conveyed through isolated pipes from a floating or land storage facility to the vessel vessel.

According to one embodiment, the invention further provides a transfer system for a combustible liquefied gas, the system comprising the abovementioned vessel, insulated pipes arranged to connect the vessel installed in the hull of the vessel to a storage facility. floating or terrestrial and a pump for driving a combustible liquefied gas stream through the isolated pipelines from the floating or land storage facility to the vessel vessel.

Brief description of the figures

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

- Figure 1 shows a cutaway side view of a vessel propelled by liquefied natural gas. - Figure 2 is a schematic diagram of the interior of a tank with a mast structure and a retractable pump.

- Figure 3 is a schematic diagram of the interior of a tank with a mast structure, a retractable pump and a fixed immersed pump located in a sump.

- Figure 4 is a top view of a mast structure having a shaft for retractable pump and an auxiliary pump attached to its foot.

- Figure 5 is a graph illustrating the use of pumps according to the level of filling of the tank.

FIG. 6 represents a schematic view of a vessel equipped with a liquefied natural gas insulated and sealed tank and liquefied gas fueled power generation equipment for the propulsion of the ship as well as a loading terminal. of this tank.

Detailed description of embodiments

FIG. 1 represents a ship 1 comprising a hull 4 and capable of supporting on its deck 3 a cargo 2. This ship is propelled by means of a propulsion system 5 which uses liquefied natural gas as fuel.

In order to store a sufficient quantity of liquefied gas to operate, the vessel 1 comprises a sealed and insulating tank 6 able to carry a certain quantity of liquefied gas. The sealed and insulating tank 6 may be for example a membrane tank, or a self-supporting tank.

In order to feed the propulsion system 5 with liquefied gas, it is necessary to arrange in the tank 6 at least one main pump 1 1, which makes it possible, with the aid of a suction device, to transfer liquefied gas to the propulsion system. 5.

The ship 1 presented in this embodiment also comprises an auxiliary pump 12 in the tank 6, this auxiliary pump 12 is in particular provided in case of maintenance of the main pump 1 1, in case of failure of the main pump 1 1 or for replace it when it can not effectively suck the liquefied gas at the bottom of the tank 6. The main pumps 1 1 and 12 auxiliary are here designed to suck the liquefied gas from the bottom to the top of the tank 6.

FIG. 2 shows a schematic diagram of the interior of a tank 6 according to one embodiment of the present invention. The tank 6 contains in particular a bottom wall 8 which is supported by the hull 4 of the ship 1 and a top wall 7 through which the liquefied gas is transmitted to the propulsion system 5.

The tank 6 further comprises a barrel 9 provided with a valve 10 arranged at a lower end of the barrel 9, the barrel passing through the top wall 7 of the tank 6 and directed towards the bottom wall 8 of the tank 6. C it is in this shaft 9 that the main fuel pump 11 is arranged. The main fuel pump 1 1 here is a retractable pump 1 1 which can be moved without having to empty the tank 6 between a position inside the tank 6, in which the main pump 1 1 actuates the valve 10 of the drum 9 and a position outside the tank 6 to perform the maintenance of the pump.

Indeed, during its descent into the barrel 9 to the lower end of the barrel 9, the retractable pump 11 is supported by its weight on the valve 10 and thus passes the valve 10 in the open position. When the valve 10 is open, the suction member of the retractable pump 1 1 protrudes from the barrel 9 and can thus suck the liquefied gas into the interior space of the tank 6.

The valve 10 is provided with a closing means, for example a spring system, which allows it when the retractable pump 1 1 is removed from the lower end of the barrel 9, to go into the closed position by the restoring force. In the closed position, the valve 10 obstructs the lower end of the barrel 9 sealing the barrel 9 to the liquefied gas avoiding any rise of the liquefied gas inside the barrel 9 during removal of the retractable pump 1 January.

The valve 10 also comprises a seal on its circumference which when the valve 10 is in the closed position abuts against the barrel 9. The valve 10 then becomes completely sealed to the liquefied gas. The valve 10 is shown in the closed position in FIG. 2 and in the open position in FIG. As shown in FIG. 2, the tank 6 comprises a mast structure 13 comprising a plurality of pillars 14 interconnected by fixing beams 19. A bottom plate 16 connects each of the pillars 14. 15 extends between the bottom wall of the tank 6 and the bottom plate 16, the foot 15 serving as support for the mast structure 13. One of the pillars 14 is designed to contain the drum 9.

The main pump 1 1 when it is at the lower end of the barrel 9 can not be glued to the bottom wall 8 of the tank 6. Indeed, it is necessary to leave a space for the opening of the valve 10 and for its thickness. In addition, the bottom wall 8 is often composed of corrugated plates, it is therefore necessary to place the main pump 1 1 away from the peaks of the corrugations. Since the liquefied gas is very cold, a phenomenon of thermal contraction acts especially on the drum 9 which further moves its lower end away from the bottom wall 8 of the tank 6.

Finally, the shaft 9 being very close to the foot 15 of the mast structure 13, it is not easy to place the main pump 1 1 in a sump 18 for structural reasons. All these constraints mean that the main pump 1 1 is sufficiently far from the bottom wall 8 of the tank 6 so as to make it impossible to suction the liquefied gas from a certain filling threshold of the tank 6.

FIG. 3 shows a schematic diagram of the interior of a tank 6 according to another embodiment of the present invention.

In this configuration, an auxiliary pump 12 which is a pump fixed inside the tank 6 has been placed at a lower level of the tank 6 relative to the main pump 1 1. In particular, the auxiliary pump 12 has been placed in a sump 18, that is to say a recess in the bottom wall 8 of the tank 6. In this way, the auxiliary pump 12 can suck the liquefied gas to a very low level. It is thus possible to replace the main pump 1 1 so as to suck all the liquefied gas contained in the tank 6.

In addition, the auxiliary pump 12 is a fixed pump that is not in a drum 9 and is not subject to as many constraints as the retractable pump 1 1 which allows it to optimize its placement at a very low level in the tank 6. However, the fixed pump is difficult to maintain, so it is desirable to limit its use to space its maintenance phases. For example, the duration of use of the auxiliary pump 12 is provided in alternative operation with the main pump over a period of 5 years corresponding to the period between two maintenance phases of the auxiliary pump 12.

FIG. 3 also shows the level sensor 21 measuring the filling level of the tank 6. This sensor 21 is situated inside the tank 6. The sensor 21 can be an optical type sensor, a sensor radar type, a thermal type sensor or any other type of sensor to raise the level where the liquefied gas is. The sensor 21 transmits the filling level information to a control unit 20 located outside the tank 6. The control unit 20 receives the level information given by the sensor 21 and sends commands to activate or deactivate one or the other of the main pump 1 1 and the auxiliary pump 12.

In another embodiment, the sensor 21 transmits the fill level information to a display device to an operator. The activation or deactivation of the pumps can also be done by the operator manually according to the information visible on the display device.

Figure 4 shows a top view of the mast structure 13 where it is possible to see more clearly the position of the auxiliary pump 12 and sump 18 in one embodiment.

Indeed, in this embodiment, the sump 18 is remote from the foot 15 of the mast structure 13 to respect the structural balance of the entire vessel 6. As a result, the auxiliary pump 12 is connected to the foot 15 of the mast structure 13 by a fixing arm 17 for deporting the attachment of this pump.

To increase the stability of the mast structure 13, it is composed of three pillars 14 forming a triangle around the foot 15, one of the three pillars 13 containing the barrel 9. As shown in this figure, the pillars 13 are assembled with each other by means of fixing beams 19.

In addition, it is also possible to stiffen the structure to add lattices between the pillars 13.

FIG. 5 represents a graph of a filling curve of the tank 6 as a function of pump use. The use of a pump sucking up the liquid towards the top of the tank makes it necessary to maintain a certain height of liquid at the bottom of the tank 6, otherwise the suction member of the pump enters into communication with the gas phase. which can degrade the pump. In the embodiment shown, the main pump 1 1 is a retractable pump which has the advantage of being easily removed for now unlike the auxiliary pump 12 which is fixed.

This is why it seems more interesting to use a maximum of the main pump 1 1 to avoid using the fixed pump 12 and thus push back its maintenance date. However, as explained above, the retractable pump 1 1 can no longer suck liquefied gas beyond a certain threshold without risking damage. The auxiliary pump 12 on the other hand thanks to its placement in the sump 18 and its design can suck the liquefied gas to empty the tank 6.

Thus, when the filling level of the tank is higher than this threshold, the liquefied gas contained in the tank 6 is discharged by means of the main pump 1 1 to the propulsion system 5. And when the level of the tank 6 is below this threshold, the main pump 1 1 is deactivated and the liquefied gas contained in the tank 6 is discharged with the aid of the auxiliary pump 12 to the propulsion system 5.

The choice of the threshold depends mainly on the size of the tank 6, the characteristics of the pumps used but also as a function of the desired use time for each pump to plan the maintenance of the 15,000 hours of use.

Indeed, in a first case, the threshold can be chosen between 2 and 5% of the total volume of the tank. The main pump 1 1 then operates most of the time while the auxiliary pump operates only when the filling level of the tank is low. This case is envisaged when for example the tank 6 is small enough that the main pump 1 1 needs only maintenance every two and a half years or once during the period of 5 years between two major maintenance . Thus the auxiliary pump 12 which is difficult to maintain only needs maintenance every five or even ten years.

In a second case, the threshold can be chosen between 20 and 35% of the total volume of the tank 6. The distribution of operation of the two pumps is then more balanced only in the first case. This second case is envisaged when for example the tank 6 is of a larger capacity and that it is necessary to balance the time of use of each pump to prevent the main pump 1 1 is maintained too recurrently relative to the auxiliary pump 12. Thus the threshold is chosen to cause maximum maintenance every two and a half years for the main pump 1 1.

The curve shown in FIG. 5 represents this discharge in liquefied gas according to this principle. The first part of the curve represents the utilization part of the main pump 1 1 while the second part of the curve which is below the threshold represents the portion of use of the auxiliary pump 12.

The curve of FIG. 5 has been presented with a constant slope, which represents an equal flow rate of the two pumps 11 and 12 and a use of the liquefied gas to supply the propulsion system 5 which is also constant. In another embodiment, it is envisaged to use pumps with different and / or non-constant flow rates, in this way the slope of the curve would not be constant and would vary according to the flow rate of each pump.

Each pump can be equipped with a frequency converter to finely adjust its flow.

In a manner known per se, as shown in FIG. 6, loading / unloading lines can be connected, by means of appropriate connectors, to a marine or port terminal to transfer a cargo of LNG to the vessel 1.

FIG. 6 shows an example of a marine terminal comprising a liquefied natural gas supply station 82, an underwater pipe 83 and an onshore installation 81. The liquefied natural gas supply station 82 is a fixed installation providing shore comprising a movable arm 84 and a tower 85 which supports the movable arm 84. The movable arm 84 carries insulated flexible pipes 80 that can connect to the loading pipes. The swiveling arm 84 adapts to all ship sizes. A connection pipe (not shown) extends inside the tower 85. The liquefied natural gas supply station 82 allows the tank 6 to be filled from the onshore installation 81. This comprises liquefied gas storage tanks 86 and connecting pipes 87 connected by the underwater pipe. 83 at liquefied natural gas supply station 82. Underwater pipe 83 permits the transfer of liquefied gas between liquefied natural gas supply station 82 and onshore installation 81.

In order to generate the pressure necessary for the transfer of the liquefied gas, pumps on board the ship 10 and / or pumps fitted to the shore installation 81 and / or pumps fitted to the loading and unloading station 82 are used.

To load the tank of the ship 10, the marine or port terminal can also be replaced by a supply vessel equipped with an LNG storage tank.

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

The use of the verb "to include", "to understand" or "to include" and its conjugated forms does not exclude the presence of other elements or steps other than those set out in a claim.

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

Claims

A vessel (1) comprising a propulsion system (5) and a sealed and insulating vessel (6) for combustible liquefied gas, wherein the vessel (6) comprises:

- a barrel (9) provided with a valve (10) arranged at a lower end of the barrel (9), the barrel (9) passing through a top wall (7) of the tank (6) and directed towards a wall of bottom (8) of the tank (6);

- A main fuel pump (1 1) which is a retractable pump (1 1) arranged in the barrel (9) and movable without having to drain the tank (6) between a position inside the tank (6) , in which the pump actuates the valve (10) of the shaft (9) and a position outside the vessel (6), the main pump (1 1) in the position inside the vessel being configured to discharging the liquefied gas from the tank (6) to the propulsion system (5);

an auxiliary fuel pump (12) which is a pump fixed inside the tank (6) and which is configured to suck the liquefied gas at a lower level of the tank (6) with respect to the main pump ( 1 1), the auxiliary pump (12) being configured to discharge the liquefied gas from the tank (6) to the propulsion system (5).

2. Ship (1) according to claim 1, wherein the vessel (6) comprises a mast structure (13) comprising a plurality of pillars (14) interconnected by a bottom plate (16), and a foot ( 15) extending between the bottom wall (8) of the tank (6) and the bottom plate (16), and wherein the shaft (9) is formed in one of the pillars (14) of the mast structure (13).

3. Ship (1) according to claim 2, wherein the auxiliary pump (12) is fixed to the bottom plate (16) of the mast structure (13).

4. Ship (1) according to claim 2, wherein the auxiliary pump (12) is fixed to the foot (15) of the mast structure (13).

5. Ship (1) according to any one of claims 2 to 4, wherein the auxiliary pump (12) is fixed to the mast structure (13) by means of a fixing arm (17).

6. Ship (1) according to any one of the preceding claims, wherein the valve (10) is configured to be open when the main pump (1 1) is positioned at the lower end of the barrel (9), an organ suction of the main pump (1 1) protruding from the lower end of the barrel (9), and to be closed by a closure means in the absence of the main pump (1 1) at the lower end, the lower end of the shaft (9) being tight when the valve (10) is in the closed position.

7. Ship (1) according to any one of the preceding claims, wherein the bottom wall (8) of the vessel (6) comprises a sump (18), and the auxiliary pump (12) comprises a suction member , the suction member being placed in the sump (18).

8. Ship (1) according to any one of the preceding claims, wherein the tank (6) comprises a level sensor (21) measuring the filling level of the tank (6), the sensor being preferably an optical sensor. , a radar type sensor or a thermal sensor.

Vessel (1) according to claim 8, wherein the vessel (1) comprises a control unit (20) configured for, when the level sensor (21) indicates a filling level of the tank (6) greater than In a predefined manner, discharge the liquefied gas contained in the tank (6) by means of the main pump (1 1) to the propulsion system (5), and in which the control unit is configured for, when the sensor of level indicates a filling level of the tank (6) below the predefined threshold, discharging the liquefied gas contained in the tank (6) by means of the auxiliary pump (12) to the propulsion system (5).

10. Ship (1) according to claim 9, wherein the predefined threshold is between 2 and 10% of the total volume of the tank (6), preferably the threshold is between 2 and 5% of the total volume of the tank (6), preferably the threshold is equal to 3.7% of the total volume of the tank (6).

1 1. A method of operating a ship (1) according to any one of claims 1 to 9 wherein the method comprises the steps of: - when the filling level of the tank (6) is greater than one predefined threshold, discharge the liquefied gas contained in the tank (6) with the main pump (1 1) to the propulsion system (5), when the level of the tank is below the predefined threshold, discharge the liquefied gas contained in the tank (6) by means of the auxiliary pump (12) to the propulsion system (5).

12. Operating method according to claim 1 1, wherein the predefined threshold is between 2 and 10% of the total volume of the tank (6), preferably between 2 and 5% of the total volume of the tank (6). ), preferably equal to 3.7% of the total volume of the tank (6).

13. Operating method according to claim 1 1, wherein the predefined threshold is between 20 and 35% of the total volume of the tank (6).

14. Operating method according to any one of claims 1 1 to 13, comprising the step of stopping the main pump (1 1) when the level of the vessel (6) is less than the predefined threshold.

15. Operating method according to any one of claims 1 1 to 14, comprising the steps of: when starting the unloading of liquefied gas, determine if the filling level of the tank (6) is greater than a threshold starting up above said predefined threshold, between 1000 mm and 2000 mm height of the tank, preferably equal to 1500 mm height of the tank (6), and

start the main pump (1 1) if this test is positive, otherwise start the auxiliary pump (12).

A method of loading a ship (1) according to any one of claims 1 to 10, wherein a liquefied fuel gas is fed through isolated pipes (80, 83, 87) from a floating storage facility or Earth (81) to the tank of the ship (6).

17. Transfer system for a combustible liquefied gas, the system comprising a vessel (1) according to any one of claims 1 to 10, insulated pipes (80, 83, 87) arranged to connect the vessel (6). installed in the hull of the vessel at a floating or land storage facility (81) and a pump for driving a flow of combustible liquefied gas through the insulated pipelines from the floating or land storage facility to the vessel vessel.