Method and system for discharging liquefied gas from tank for transporting or storing liquefied gas

Abstract

To provide a method for discharging liquefied gas from a tank for transporting or storing the liquefied gas.SOLUTION: A method (500) is for discharging liquefied gas from a watertight, heat-insulating tank having an upper wall, a lower wall, and a pressure-feeding duct that extends from a first end opening to the outside of the tank to a second end located on the bottom side of the tank and is connected to a suction member at least partially housed in an enclosure opening to the tank. The method includes: a step (501) of emptying the tank; a step (502) of heating the tank to evaporate remaining liquefied gas present in the tank; and a step (503) of inactivating the tank by introducing an inert gas into the tank, and further includes a step (504) of directing a flow of the inert gas into the enclosure following or during the step of inactivating the tank.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to the field of ships for transporting liquefied gas, and more particularly to the installation and maintenance of tanks configured to store such liquefied gas, such as liquefied natural gas or liquefied ethane. Such tanks are installed either inside the hold of a liquefied gas carrier or in an onshore facility for temporarily storing liquefied gas.

Background Art

[0002] Such tanks have a capacity for storing thousands of cubic meters, and even tens of thousands of cubic meters, of liquefied gas. Tanks generally have a parallelepiped shape, and their walls are generally located in contact with the inner bulkheads of the hold of the carrier. The hold of the ship generally includes a number of tanks. In order to keep the liquefied gas in a liquid state, i.e., at -163 °C (degrees Celsius) in the case of liquefied natural gas, the walls of each tank are provided with a number of thermally insulating watertight layers lining the inner bulkheads of the ship.

[0003] To fill the tank with liquefied natural gas or to discharge the liquefied natural gas from the tank, generally, a tower called a discharge tower is fixed to the upper wall of the tank. This tower extends vertically into the tank to the bottom side of the tank, i.e., near the lower wall of the tank. The tower generally includes a number of columns with loading and / or discharge ducts. The discharge duct is connected to a discharge pump, which is installed, for example, at one end of the tower at the bottom side of the tank and includes a suction member for sucking the liquefied gas into one of the plurality of discharge ducts, respectively.

[0004] Another duct is generally incorporated in the tower and is, in particular, a duct that enables the supply of natural gas to the engine of the ship. Such a duct also includes a pump that enables the suction of liquefied natural gas from the bottom side of the tank.

[0005] To protect the pumps within the tank against the sudden movement of the liquefied natural gas inside the tank, the suction member of each pump is generally surrounded by an enclosure. Such an enclosure generally takes the form of a sump located within the lower wall of the tank or a container with a lateral wall arranged vertically so as to surround the suction member of the pump.

[0006] When the enclosure is a sump, when the tank has to be completely emptied, it is also possible for the pump in the sump to suck up as much liquefied gas as possible within the tank. Further, the sump ensures that the suction member of the pump is always submerged in the liquid at a minimum level, and thus, any damage to the operating suction member is prevented.

[0007] For example, when it is necessary to completely empty the tank for inspection, the pump in the discharge duct empties the tank, and then the remaining liquid in the tank is evaporated by the circulation of hot gas inside the tank. The tank then undergoes an inerting operation where an inert gas is introduced at the bottom side of the tank. When the gas mixture present in the tank reaches a predefined ratio to become non-flammable outdoors, the tank is vented, thereby making it suitable for access by tank maintenance staff.

[0008] However, the inventors have found that after such an operation to discharge the liquefied gas within the tank, the evaporated liquefied gas remains trapped within the protective enclosure of the pump. The divergence of the evaporated liquefied gas will harm the staff working near the pump protected by the enclosure or the staff during the maintenance of this pump.

Summary of the Invention

Problems to be Solved by the Invention

[0009] The object of the present invention is to provide a discharge method for discharging liquefied gas from a watertight and heat-insulating tank and a corresponding discharge system, so as to protect the maintenance staff of the tank against residual gas emission, and at least improve the above-mentioned disadvantages.

Means for Solving the Problems

[0010] For this purpose, the present invention provides a watertight and heat-insulating tank configured to contain liquefied gas, and at least - a plurality of walls including an upper wall and a lower wall, - a pumping duct for pumping liquefied gas, which extends into the tank from a first end opening outside the tank to a second end located at the bottom side of the tank, and is connected to a suction member of a pump and at least partially accommodated in an enclosure opening into this tank. A discharge method for discharging liquefied gas from a tank comprising the above, In sequence, - a step of emptying the tank, - a step of heating the tank capable of evaporating the remaining liquefied gas present in the tank, - a step of inactivating the tank by introducing an inert gas into the tank. In a discharge method for discharging liquefied gas, comprising The discharge method further includes a step of sending an inert gas flow capable of expelling the amount of liquefied gas evaporated during the heating step from the enclosure, following or during the step of inactivating the tank, and the step of sending this inert gas flow is different from the step of inactivating the tank. A discharge method for discharging liquefied gas is proposed.

[0011] The inert gas flow is directed into the enclosure so that the amount of evaporated liquefied gas trapped in the enclosure can be discharged.

[0012] The lower wall of the tank is the wall that supports the weight of the liquefied gas in the vertical direction. While the lower wall of the tank is attached to, for example, the bottom wall of the hold of a transport ship, the upper wall of the tank is fixed to, for example, the deck of the transport ship. Each of the walls of the tank comprises, on the one hand, a primary layer consisting of a primary watertight membrane configured to contact the liquefied gas and a primary heat insulation barrier composed of one or more insulating materials, and on the other hand, a secondary layer consisting of a secondary watertight membrane in contact with the primary heat insulation barrier and a secondary heat insulation barrier composed of one or more insulating materials and attached to the inner bulkhead, bottom wall or deck of the ship according to the relevant wall of the tank.

[0013] Alternatively, the walls of the tank are similarly arranged in an onshore liquefied gas storage facility.

[0014] The pumping duct mainly extends vertically within the tank, and its main dimension is directed in the vertical direction, that is, in a direction orthogonal to the lower and upper walls of the tank. The second end of the pumping duct is connected to a suction member located at the bottom side of the tank, that is, the lower part of the tank. The second end of the pumping duct is preferably located between the lower wall of the tank and a distance, for example, forming 1 / 10 of the volume of the tank, from this lower wall. Thus, the suction member operates even when the tank is almost empty. The pump provided with the suction member also comprises a motor for driving this suction member, which may be offset from the suction member.

[0015] The pressure duct is configured, for example, to supply a consumer of a ship, such as the engine of a ship. Alternatively, especially in the case of the tank of an LNG tanker type ship, the pressure duct may be used for discharging from the tank. In such a case, the pressure duct is usually incorporated in the support column of the discharging tower. Such a discharging 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 side of the tank. Therefore, the discharging tower penetrates the upper wall of the tank. Since the second end of the discharging tower is supported at the level of the upper wall of the tank, it does not contact the lower wall of the tank. The discharging tower includes a number of hollow columns arranged vertically. In this case, the loading or discharging duct is arranged vertically, and the columns are fixed to each other by crossbars.

[0016] The ducts in the tank are generally grouped together at the level of the discharging tower. When the pressure duct is configured to supply a consumer of the ship, the pump drive motor is supported, for example, by the support structure of the discharging tower. This support structure is arranged at the second end of the discharging tower and mainly consists of a plate and a crossbar that are parallel to the lower wall of the tank but are arranged at a distance from this lower wall to form a base. Alternatively, the pump drive motor may be located on the deck of the ship.

[0017] When the pressure duct is a duct for discharging from the tank, the pump drive motor is located on the deck of the ship. For example, the drive shaft connects the drive motor to a suction member provided on the column.

[0018] Alternatively, the pump drive motor used for supplying a consumer of the ship or for discharging from the tank is grouped together with its suction member in the same casing attached to the support structure.

[0019] The suction member is at least partially housed within an enclosure that takes the form of, for example, a sump disposed within the lower wall of the tank or a container or deflector supported by the support structure of the gantry.

[0020] According to the present invention, the evaporated liquefied gas remaining within the enclosure during the inactivation step is expelled from the enclosure by an inert gas stream sent into or towards the enclosure. The evaporated liquefied gas mixed with the inert gas is then expelled by dry air sent into the tank, thereby enabling the tank to be made ready to receive personnel. This ensures the safety of personnel working within the tank without the need to use a container or sump that houses the pump suction member. Incidentally, the present invention is not limited to transport tanks, since a sump may be used to complete the offloading from a liquefied gas tank in an onshore installation incorporating a liquefied gas tank.

[0021] The inactivation step preferably uses an inert gas generated by an inert gas generator resulting from the combustion of diesel fuel. The inert gas thus generated contains approximately 80% - 90% nitrogen and 20% - 10% carbon dioxide and is not as expensive as pure nitrogen. Preferably, the inert gas has a composition of 85% nitrogen and 15% carbon dioxide. The inert gas used by means of the means for sending the inert gas stream is preferably nitrogen generated by an inspection inert gas supply system, i.e., a system that supplies nitrogen at a pressure of 5 bar. Thus, the inert gas, unlike the inert gas used to inactivate the tank, contains no carbon dioxide, except for a negligible amount that may be considered.

[0022] Alternatively, the same inert gas is used to inactivate and send an inert gas stream into the enclosure, said inert gas being either simply nitrogen or a mixture of nitrogen and carbon dioxide.

[0023] In one embodiment of the present invention, a method for discharging liquefied gas includes the step of sending dry air into a tank that can expel or inactivate an inert gas from the tank and expel the gas resulting from the steps of inactivating and sending an inert gas stream.

[0024] The dry air is sent into the tank only after the tank has been inactivated and the evaporated liquefied gas present in the enclosure has been discharged by means of sending an inert gas stream into the enclosure. The dry air is generated by dehumidifying air using a dehydration product, for example, a dry air generator using activated alumina.

[0025] According to an optional and advantageous feature of the method according to the present invention, the duration of the step of sending the inert gas stream is pre-specified so that the evaporated liquefied gas remaining in the enclosure can be discharged from the enclosure. This feature eliminates the need to control the flow rate of the inert gas, making it possible to easily implement the present invention. The pre-specified time is measured, for example, by tests before the implementation of the method according to the present invention.

[0026] According to another optional and advantageous feature of the method according to the present invention, a tube having a cross-sectional area strictly smaller than the cross-sectional area of the inactivation duct used during the inactivation step is used in the step of sending the inert gas stream. For example, while the tube has a diameter of DN8 (the abbreviation DN means the nominal inner diameter in millimeters), the inactivation duct has a diameter of DN15. This feature means that there is no need to change the inactivation duct used during the inactivation of the tank. For example, the tube is connected only to the inspection inert gas supply system when the tank is open. As a result, this embodiment of the present invention is inexpensive and easy to implement.

[0027] The present invention also relates to a discharge system for discharging liquefied gas from a hermetically sealed and thermally insulated tank configured to contain the liquefied gas, the system comprising a tank which comprises at least - a plurality of walls including an upper wall and a lower wall, - a pump provided at the bottom side of the tank and having a suction member at least partially received in an enclosure opening into the tank, - a pumping duct for pumping the liquefied gas, which extends into the tank from a first end opening outside the tank to a second end located at the bottom side of the tank and is connected to the suction member, and the discharge system further comprises - means for emptying the tank, - means for heating the tank, which can evaporate the remainder of the liquefied gas present in the tank after the emptying means has been operated, - means for inerting the tank, which can introduce an inert gas into the tank after the heating means has been operated, In a discharge system for discharging liquefied gas, further comprising means for sending an inert gas stream capable of expelling the amount of liquefied gas evaporated by the heating means from the enclosure, the means for sending the inert gas stream being different from the means for inerting the tank,

[0028] The means for sending the inert gas stream is operated during or after the operation of the means for inerting and enables the inert gas stream to be directed into the enclosure.

[0029] The liquefied gas discharge system according to the present invention comprises means for implementing the liquefied gas discharge method according to the present invention. In other words, in the liquefied gas discharge method according to the present invention, the liquefied gas discharge system according to the present invention is used, and the liquefied gas discharge system according to the present invention has advantages similar to those of the liquefied gas discharge method according to the present invention.

[0030] In one embodiment of the present invention, the means for sending an inert gas stream comprises a tube having a cross-sectional area that is strictly smaller than the cross-sectional area of the inerting duct connected to the means for supplying the inert gas. The inerting duct opens into the tank and forms part of the means for inerting. The tube comprises a first end located outside the tank and a second end arranged to direct the inert gas stream into the enclosure. As explained in connection with the method according to the present invention, the use of such a tube means that the means for inerting the tank remains unchanged, in particular the inlet of the inerting duct remains unchanged. Since the tube has a smaller diameter (e.g., DN8 instead of DN15), the tube is easy to install.

[0031] For example, the tube comprises, at its first end, a flexible portion adapted to be connected or disconnected to the inert gas supply system. In this embodiment of the present invention, while the inert gas supply system is an inert gas supply system for inspection, i.e., a system that supplies nitrogen at a pressure of 5 bar, the inert gas used by the means for inerting is, as described above, generated from diesel fuel combustion.

[0032] For example, the second end of the tube is located below the plane defining the opening provided in the enclosure, and the opening is located opposite the upper wall of the tank. Alternatively, although the second end of the tube is arranged above the plane, the inert gas sent under pressure through the tube is arranged to pass through the opening and enter the enclosure. For example, the second end of the tube forms part of the end portion of the tube arranged vertically above the opening of the enclosure.

[0033] In one embodiment of the present invention, the means for sending an inert gas stream comprises means for attaching the tube to the support column of the lifting tower. The lifting tower comprises a plurality of support columns, extends into the tank, and has a support structure at an end close to the lower wall of the tank for interconnecting the support columns of the lifting tower.

[0034] In an example of an embodiment related to this embodiment, the pumping duct is a duct for discharging from a tank, incorporated within a support column, and the means for opening can operate a pump to suck liquefied gas into the pumping duct. In this case, the enclosure is, for example, a container surrounding a suction member fixed to a support structure located at the lower end of the support column. Alternatively, the enclosure is a sump arranged within the thickness of the lower wall below the support column.

[0035] In another embodiment related to this embodiment, the pumping duct is a duct configured to supply fuel to at least one consumer of a ship.

[0036] In a further example of this embodiment, for example, the pipe is - a first portion extending along the support column, - a second portion extending along the support structure and connecting one end of the support column and the suction member to each other, - a third portion arranged vertically from one end of the second portion, wherein the said end of the second portion is located within the vertical extension length of the enclosure, and includes.

[0037] Therefore, the pipe used to send an inert gas flow into the enclosure is brought to the bottom side of the tank by attaching it to a support column preferably selected as the support column closest to the enclosure housing the suction member, provided in the discharging tower. The pipe must change direction at the bottom side of the tank before reaching the upper side of the enclosure. In this case, the suction member is protected using the support structure of the discharging tower as a support.

[0038] In an example of an embodiment of the present invention, the enclosure may be located outside the outer periphery defined by the support columns of the discharging tower. This is, for example, a case where the pumping duct is located outside the said outer periphery, whether it is a duct configured to supply fuel or a discharging duct.

[0039] For example, the enclosure is a container fixed to a support structure, and this container is held above the lower wall of the tank.

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

[0041] The present invention also relates to a ship for transporting liquefied gas, comprising a liquefied gas discharge system according to the present invention. The ship according to the present invention has advantages similar to those of the system and the method according to the present invention.

[0042] Another feature and advantage of the present invention should become apparent from the following description, on the one hand, and from a number of examples of embodiments shown in connection with the accompanying schematic drawings without limitation, on the other hand.

Brief Description of the Drawings

[0043]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0044] According to an embodiment of the present invention, the ship 1 according to the present invention shown in FIG. 1 is a ship for transporting liquefied gas G, which is liquefied natural gas in this embodiment of the present invention. The liquefied gas G is transported in a tank 16 installed in the cargo hold of the ship 1. The cargo hold includes a lower partition wall 12 substantially parallel to the deck 18 of the ship 1 and an inner lateral partition wall 14 that divides the cargo hold into four compartments.

[0045] At least one compartment houses a watertight and heat-insulating tank 16. This tank 16 includes an upper wall 162 fixed to the deck 18 of the ship 1 (partially shown in FIG. 2), a lower wall 164 fixed to the lower partition wall 12 of the ship 1 (partially shown in FIG. 2), and a lateral wall fixed to the inner lateral partition wall 14. The walls of the tank 16 line the partition walls of each compartment of the cargo hold with a watertight and heat-insulating layer, whereby the tank 16 forms a substantially parallelepiped container capable of containing 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) that allows the crew to descend into the tank.

[0046] The upper wall 162 of the tank 16 is passed through by a lifting tower 2 having a first end accessible from the outside of the tank 16 on the deck of the ship and a second end located on the bottom side of the tank 16 but not in contact with the lower wall 164 of the tank 16. This lifting tower 2 is supported by the deck 18 of the ship and extends vertically into the tank 16. To limit a certain amount of movement of the lifting tower 2, which is several tens of meters in length, a holding device 15 fixed to the lower wall 164 of the tank 16 holds the lifting tower 2 vertically, that is, perpendicular to the upper wall 162 and the lower wall 164 of the tank 16. Also, as a precautionary note, the holding device 15 serves to support the lifting tower 2 having a length that changes according to thermal fluctuations in the tank 16, especially when the ship is exposed to the rolling phenomenon.

[0047] FIG. 2 shows a more detailed view of the lifting tower 2 that can be seen as arranged longitudinally along the vertical axis (Oz) of the orthonormal reference system (O, x, y, z). The terms "upper", "high", "lower", and "low" in the present application are referred to based on the direction of the axis Oz directed upward, that is, toward the upper wall 164 of the tank 16.

[0048] The lifting tower 2 includes a plurality of hollow struts 21, 22, 23 that penetrate the upper wall 162 of the tank 16 and extend vertically into the tank 16. Thus, while the first end of at least one of these struts 21, 22, 23 is outside the tank 16, the second end of at least one of the struts 21, 22, 23 is arranged at the bottom side of the tank 16 without contacting the lower wall 164 of the tank 16. The second ends of at least two struts 21, 22, 23 are fixed to each other by a support structure 24 formed of a plate and a crossbar, which is arranged at a distance from the lower wall 164 of the tank 16. Another crossbar represented by a thick black line connects the struts to each other along the length of the lifting tower 2. The support structure 24 is connected to the lower wall 164 by the holding device 15.

[0049] In this embodiment of the present invention, the support column 23 houses a pressure feed duct 25 which is a duct for discharging liquefied gas G. This pressure feed duct 25 extends in the vertical direction from a first end portion located outside the tank to a second end portion connected to a discharge pump provided with a suction member 8 at the bottom side of the tank. The discharge pump is supported by a support structure 24. The discharge pump is positioned between the second end portion of the support column 23 and the lower wall 164 in the vertical direction without contacting the lower wall 164.

[0050] To protect the discharge pump from the influence of waves, a container 30, which is preferably cylindrical and has an open upper portion, surrounds the discharge pump, particularly its suction member 8. The container 30 is attached to the support structure 24 and does not contact the lower wall 164. The container 30 forms a corner. In this case, even if the liquefied gas is being evaporated, without a specific discharge system, such as the discharge system S1 for the liquefied gas present in the tank 16, which forms the first example of the embodiment of the present invention, the liquefied gas will remain trapped.

[0051] The discharge system S1 includes means for emptying the tank 16, which includes the pressure feed duct 25 and the discharge pump, and means for operating the discharge pump to suck out the liquefied gas G from the tank until the tank only contains the remaining liquefied gas. The emptying means may include another pressure feed device (for example, another discharge duct in another support column connected to another discharge pump) that is operated in parallel with the discharge pump, or another pressure feed device (for example, a pump called a dehydration pump) that is operated to assist the operation of the discharge pump when the discharge from the tank 16 is approximately carried out. Also, when the tank 16 is emptied, vapor phase gas is returned to equalize the pressure inside the tank 16.

[0052] The discharge system S1 also includes means for heating the tank 16, which can evaporate the remaining liquefied gas G present in the tank 16 after the emptying means has been operated. This heating means includes means for circulating the heated gas within the tank 16, and the gas in the vapor phase is recovered, for example, from the upper part of the tank 16 and then compressed. The remaining liquefied gas G is completely evaporated when the insulation of the tank 16 reaches 5°C.

[0053] The discharge system S1 also includes means for inerting the tank, which enables an inert gas to be introduced from an inert gas generator into the tank. The inert gas is composed of, for example, 80% nitrogen and 20% carbon dioxide and is introduced into the tank via an inerting duct. The first end of the inerting duct is located outside the tank 16 and is connected to the inert gas generator, and the second end of the inerting duct is located at the bottom side of the tank. The inert gas introduced at the bottom side of the tank can, due to its density, cause the natural gas evaporated by the heating means to rise to the upper part of the tank, where the natural gas is sucked up and withdrawn from the tank.

[0054] The discharge system S1 also includes means for sending an inert gas flow towards the container 30, which enables the evaporated gas trapped within the container 30 to be expelled after the operation for inerting the tank 16 has been carried out or at the end of such an operation.

[0055] This means for sending the inert gas flow includes the pipe 4 shown in Figure 2, which has a diameter smaller than that of the inerting duct, and a discharge system for supplying nitrogen for inspection. The pipe 4 includes a first end located outside the tank 16 and a second end 42 opening into the container 30. Although the pipe 4 is attached along the support 23, except for the two end portions of the pipe 4, namely, the first end portion located outside the tank 16 and having a flexible portion 41, and the second end portion of the pipe 4 attached to the support 23 and extending vertically into the container 30.

[0056] The flexible portion 41 of the pipe 4 enables the pipe 4 to be connected to the nitrogen supply system for inspection, and the nitrogen is generated from ambient air by an inert gas generator provided on the ship. The nitrogen supply system supplies nitrogen at a pressure of 5 bar and may be connected to various individual devices as required. Therefore, when the pipe 4 is connected to the nitrogen supply system, it can act at a pressure sufficient to expel the evaporated gas confined within the container 30 as a device connected to the compressed air line, also called a blower. Incidentally, the means for inactivation may not be able to expel the evaporated liquefied gas confined inside this container 30 even when positioned above the container 30. The means for inactivation diffuses the inert gas at a pressure insufficient to expel the evaporated gas.

[0057] Finally, the discharge system S1 includes means for ventilating the tank 16. This means includes a dry air duct in the upper part of the tank 16, whereby it is possible to introduce the dry air generated by a dry air generator provided on the ship into the upper part. The generator uses ambient air and dehumidifies this ambient air. The dry air introduced into the upper part expels another gas present in the tank 16 after the means for inactivation and the means for sending an inert gas flow into the container 30 are operated, and pushes it into the lower part of the tank 16, where the gas is withdrawn until an oxygen content higher than 20% is obtained.

[0058] Therefore, with the discharge system S1 according to the present invention, it becomes possible for the staff to enter the tank without inhaling the fumes of the vapor-phase natural gas that has hitherto remained confined within the container 30.

[0059] As described below in relation to FIG. 3, according to the second embodiment, the tank 16 may include a pressure feed duct 25b used to supply fuel to a consumer provided on the ship 1.

[0060] The pressure - feeding duct 25b has a first end, which is located outside the tank, near the unloading tower 2, and extends vertically at the end portion located at the bottom side of the tank. This end portion includes a first horizontal portion attached to the support column 21 and the support structure 24, and a second vertical portion connected to the first horizontal portion and the second end of the pressure - feeding duct 25b. The second end is connected to a supply pump provided with a suction member 8b. As shown in FIG. 4, the supply pump is supported by the support structure 24.

[0061] To protect the supply pump against the influence of waves, the supply pump is housed in a sump 7 arranged within the lower wall 164 of the tank 16. As shown in FIG. 4, the lower wall 164 - includes a primary metal watertight membrane 1640 configured to contact the liquefied gas G within the tank 16; - has a primary insulation layer 1641 provided with an insulating material, which is arranged below the primary watertight membrane 1640; - has a secondary watertight membrane 1642 made of metal or composite material, which is arranged below the primary insulation layer 1641; - has a secondary insulation layer 1643, which is arranged below the secondary watertight membrane 1642 and is placed on the lower bulkhead 12 of the hold of the ship 1. and is provided with these components.

[0062] The sump 7 consists of a cylindrical metal enclosure, which is open towards the tank 16 and is welded to the primary metal watertight membrane 1640 at the upper part, thereby sealing the tank 16. The sump 7 has a bottom side parallel to the lower bulkhead 12, for example, arranged within the secondary insulation layer 1643. Since the supply pump located within the sump 7 is supported by the support structure 24, the supply pump does not contact the bottom side of the sump 7.

[0063] When the liquefied gas in the tank 16 is completely discharged, in order to prevent the evaporated liquefied gas from being trapped in the sump 7, the discharge system S2 for discharging the liquefied gas existing in the tank 16, which forms the second example of the embodiment of the present invention shown in FIGS. 3 and 4, comprises means similar to those of the system S1 for discharging the liquefied gas described above.

[0064] In particular, the liquefied gas discharge system S2 comprises the same means as those of the system S1 for emptying, heating, inactivating and venting the tank 16. Incidentally, in this embodiment of the present invention, the liquefied gas discharge system S2 coexists with the liquefied gas discharge system S1.

[0065] The liquefied gas discharge system S2 also comprises means for sending an inert gas flow towards the sump 7, which may be operated while or after the tank 16 has been inactivated by the means for inactivation.

[0066] The means for sending this inert gas flow comprises a pipe 4b having a diameter smaller than the diameter of the inactivation duct and an inspection nitrogen supply system to which this pipe 4b may be connected. The pipe 4b comprises a first end portion located outside the tank 16 and a second end portion 42b opening into the sump 7. The pipe 4b comprises a vertical main portion 40 attached along the support 21, a first end portion located outside the tank 16 and having a flexible portion for connection to the inspection nitrogen supply system, and a second end portion extending the main portion 40 attached to the support 21 into the sump 7 provided at the bottom side of the tank.

[0067] The second end portion of the pipe 4b comprises a horizontal or substantially horizontal portion 44 attached to the support structure 24, this horizontal portion 44 connecting the lower end of the main portion 40 to the upper end of the 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 in the vertical direction such that the lower end corresponding to the second end portion 42b of the pipe 4b is located in the sump 7, as shown in FIG. 4.

[0068] For the attachment of the pipe 4b to the support column 21 and the support structure 24, flanges 5 welded or screwed to the support column 21 or the support structure 24 are respectively used.

[0069] When the pipe 4b is connected to the nitrogen supply system, it is capable of acting as a blower with sufficient pressure to expel the evaporated gas trapped within the sump 7. The connection designed to send an inert gas stream to expel the evaporated liquefied gas is carried out at the end or after the inactivation of the tank 16.

[0070] Subsequently, FIG. 5 shows a liquefied gas discharge system S3 which forms a third example of an embodiment of the present invention and corresponds to a variation of the aforementioned liquefied gas discharge system S2.

[0071] The liquefied gas discharge system S3 is different from the liquefied gas discharge system S2 mainly in that a pump for supplying the consumer of the ship 1, which includes a suction member 8b, is protected against the influence of waves by a container 30c similar to the container 30 but not the sump. Other elements identical to those of this liquefied gas discharge system S2 shall be referred to in the same manner.

[0072] In the embodiment of this variation, the pumping duct connected to the supply pump at the bottom side of the tank for supplying fuel to the consumer of the ship 1 is the same pumping duct 25c as the pumping duct 25b of the liquefied gas discharge system S2, except that the container 30c is arranged above the lower wall 164 of the tank 16 rather than within the lower wall 164 of the tank 16, so it does not descend very deeply into the tank 16.

[0073] The container 30c is in the form of a cylindrical bucket that is open at its upper part and surrounds the supply pump, particularly its suction member 8b. The container 30c is attached to the support structure 24 and does not contact the lower wall 164.

[0074] In order to discharge the evaporated liquefied gas that remains trapped within the container 30c after or at the end of the inactivation of the tank 16, the liquefied gas discharge system S3 includes a pipe 4c that may be connected by one end to an inspection nitrogen supply system, and the other end 42c of this pipe 4c is located within the container 30c. The arrangement of the pipe 4c and the attachment of the pipe 4c to the unloading tower 2 are the same as in the case of the pipe 4b, except that since the container 30c is located at a shallower depth than the sump 7, the pipe 4c does not descend very deeply to the bottom side of the tank. As a result, the pipe 4c, for example, does not descend very deeply along the support column 21 and is horizontally attached to the support structure 24 at a level higher than the horizontal portion 44 of the pipe 4b of the liquefied gas discharge system S2.

[0075] When the pipe 4c is connected to the inspection nitrogen supply system, it may act as a blower at a pressure sufficient to expel the evaporated gas that remains trapped within the container 30c. Sending an inert gas stream to expel the evaporated liquefied gas is carried out at the end of or after the inactivation of the tank 16.

[0076] Alternatively, the liquefied gas discharge systems S1, S2 or S3 according to the present invention may be considered individually according to the configuration of the tank, in which case there is no coexistence of the respective discharge systems.

[0077] Subsequently, in connection with FIG. 6, the steps of the discharge method 500 according to the present invention, which can be implemented by the liquefied gas discharge systems S1, S2 or S3 according to the present invention in this embodiment of the present invention for discharging the liquefied gas G from the tank 16, will be described.

[0078] The first step 501 of the discharge method 500 is to empty the tank 16. To do this, a pump for unloading from the tank 16 and, if necessary, a dehydration pump at the end of the emptying process are activated. In this embodiment of the present invention, the pump and the assigned duct form part of the emptying means of the discharge systems S1, S2 or S3.

[0079] The second step 502, which is carried out after the emptying step 501 of the discharging method 500, is to heat the tank 16 until all the liquefied gas remaining in the tank 16 is evaporated (502). To do this, the heating means of the discharging systems S1, S2 or S3 are activated, and the heated gas is circulated in the tank 16 until the temperature of the wall of the tank 16 reaches 0°C.

[0080] The third step 503, which is carried out after the heating step 502 of the discharging method 500, is to inactivate the tank 16. This is achieved by using the inactivation means of the discharging systems S1, S2 or S3. In this embodiment of the present invention, the inactivation means includes an inert gas duct and an inert gas generator existing on the ship 1. The inert gas composed of 80% nitrogen and 20% carbon dioxide is sent from the inert gas generator, which receives ambient air and diesel fuel as inflow bodies, through the inert gas duct to the bottom side of the tank. Then, the liquefied gas evaporated by the heating means rises to the upper part of the tank 16, is sucked in at this upper part, and is extracted from the tank. The inactivation step 503 stops when the gas mixture existing in the tank 16 reaches a natural gas content in the tank 16 of less than 50% of the lower explosion limit when the natural gas comes into contact with air and is no longer flammable when it comes into contact with air.

[0081] The fourth step 504, which is carried out after or at the end of the inactivation step 503 of the discharge method 500, is, depending on the variant embodiment implemented, to send an inert gas stream, in this case a nitrogen stream for inspection, into the container 30 and the sample 7 or the container 30c. In this step, means for sending an inert gas stream from the discharge systems S1, S2 or S3 are used, which means, in this embodiment of the invention, comprise the pipes 4 and 4b or 4c present on the ship 1 and a nitrogen generator for inspection. In this fourth step 504, the flexible part 41 of the pipe 4 is supplied by a nitrogen generator for inspection which is operated for a predefined period, for example 10 minutes, whereby the evaporated liquefied gas which remains trapped in the container 30 is expelled from the container 30. Subsequently, the flexible part 41 of the pipe 4 is disconnected and the flexible parts of the pipes 4b or 4c are connected in turn, whereby supply to this flexible part may be carried out by a nitrogen generator for inspection which is also operated for about 10 minutes, whereby the evaporated liquefied gas which remains trapped in the sample 7 or the container 30c is expelled.

[0082] Alternatively, this fourth step 504 of the discharge method 500 is to send an inert gas stream only into the containers 30, 30c or the sample 7 when only one of the discharge systems S1, S2 or S3 is being operated in the tank 16. In this case, the flexible part 41 of the pipe 4 is supplied by a nitrogen generator for inspection which is operated for a predefined period, for example 10 minutes, whereby the evaporated liquefied gas which remains trapped in the containers 30, 30c or the sample 7 is expelled from the containers 30, 30c or the sample 7. Subsequently, the nitrogen generator shuts off the supply to the pipe 4.

[0083] Finally, the last fifth step 505, which is carried out after the step 503 of deactivating the discharge method 500 and the step 504 of sending an inert gas stream, is to send dry air into the tank 16. In this step, the discharge system ventilation means S1, S2 or S3 are used, and the discharge system ventilation means S1, S2 or S3 comprise, in this embodiment of the present invention, a dry air duct and a dry air generator existing on the ship. In the step 505 of sending the dry air, the dehumidified air obtained at the outlet of the dry air generator is introduced into the upper part of the tank 16 through the dry air duct. The dehumidified air expels another gas existing in the tank 16 into the lower part of the tank, and in this lower part the gas is withdrawn until an oxygen content exceeding 20% is obtained.

[0084] Next, the tank 16 is in a state of being ready to receive personnel for maintaining the tank 16, for example, inspecting the tank 16.

[0085] Of course, the present invention is not limited to the examples described above, and many adjustments may be made to the examples without departing from the scope of the present invention. In particular, various variations or features of the examples of the embodiments of the present invention contemplated in the present application may be combined to implement the present invention as long as these variations or examples are compatible with each other.

Claims

1. A hermetically sealed and thermally insulated tank (16) configured to contain a liquefied gas (G), 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 the liquefied gas (G), extending from a first end opening outside the tank (16) to a second end located at the bottom side of the tank (16) within the tank (16), and connected to a suction member (8, 8b) of a pump, at least partially housed within an enclosure (30, 7, 30c) opening into the tank; A discharge method (500) for discharging the liquefied gas (G) from the tank (16) comprising In sequence, - a step (501) of emptying the tank (16); - a step (502) of heating the tank (16) capable of evaporating the remaining liquefied gas present in the tank (16); - a step (503) of inerting the tank (16) by introducing an inert gas into the tank (16); In a discharge method (500) for discharging the liquefied gas (G) comprising The discharge method (500) further includes a step (504) of sending an inert gas stream capable of expelling from the enclosure (30, 7, 30c) the amount of liquefied gas evaporated during the heating step (502), following or during the step (503) of inerting the tank (16), the step (504) of sending the inert gas stream being different from the step (503) of inerting. A discharge method (500) for discharging the liquefied gas (G).

2. The method (500) for discharging the liquefied gas according to Claim 1, further including a step (505) of sending dry air into the tank (16) capable of expelling the inert gas from the tank (16) or capable of expelling the gas resulting from the step (503) of inerting and the step (504) of sending the inert gas stream.

3. The method (500) for discharging liquefied gas according to claim 1 or 2, wherein the duration of the step (504) of sending the inert gas stream is predefined such that the evaporated liquefied gas remaining in the enclosure (30, 7, 30c) can be discharged from the enclosure (30, 7, 30c).

4. The method (500) for discharging liquefied gas according to any one of claims 1 to 3, wherein a pipe (4, 4b, 4c) having a cross-sectional area strictly smaller than the cross-sectional area of the inactivation duct used during the step (503) of inactivation is used in the step (504) of sending the inert gas stream.

5. A discharge system (S1, S2, S3) for discharging liquefied gas from a hermetic and thermally insulated tank (16) configured to contain liquefied gas (G), comprising the tank (16), the tank (16) having at least - a plurality of walls including an upper wall (162) and a lower wall (164); - a pump provided at the bottom side of the tank (16) and having a suction member (8, 8b) at least partially accommodated in an enclosure (30, 7, 30c) opening into the tank (16); - a pumping duct (25, 25b, 25c) for pumping liquefied gas, extending into the tank (16) from a first end opening outside the tank (16) to a second end located at the bottom side of the tank (16) and connected to the suction member (8, 8b); and the discharge system (S1, S2, S3) further comprises - means for emptying the tank (16); - means for heating the tank (16) capable of evaporating the remainder of the liquefied gas (G) present in the tank (16) after the emptying means is operated; - means for inactivating the tank (16) capable of introducing an inert gas into the tank (16) after the heating means is operated; in a discharge system (S1, S2, S3) for discharging liquefied gas. The discharge system (S1, S2, S3) for liquefied gas further comprises means for sending an inert gas stream capable of expelling the amount of liquefied gas evaporated by the heating means from the enclosure (30, 7, 30c), and the means for sending the inert gas stream is different from the means for inerting the tank (16).

6. The means for sending the inert gas stream comprises a pipe (4, 4b, 4c) having a cross-sectional area strictly smaller than the cross-sectional area of the inerting duct connected to the means for supplying the inert gas. The inerting duct opens into the tank (16) and forms part of the means for inerting. The pipe (4, 4b, 4c) comprises a first end located outside the tank (16) and a second end (42) arranged to direct the inert gas stream into the enclosure (30, 7, 30c). The system (S1, S2, S3) for discharging liquefied gas according to claim 5.

7. The pipe (4, 4b, 4c) comprises a flexible portion (41) adapted at the first end to be connected or disconnected to / from the inert gas supply system. The system (S1, S2, S3) for discharging liquefied gas according to claim 6.

8. The second end (42, 42b, 42c) of the pipe (4, 4b, 4c) is located below the plane defining the opening provided in the enclosure (30, 7, 30c), and the opening is located opposite the upper wall (162) of the tank (16). The system (S1, S2, S3) for discharging liquefied gas according to claim 6 or 7.

9. The means for sending the inert gas stream comprises means for attaching the pipe (4) to the struts (21, 23) of the lifting tower (2). The lifting tower (2) comprises a plurality of struts and extends into the tank (16). At an end close to the lower wall of the tank (16), it has a support structure interconnecting the struts (21, 22, 23) of the lifting tower (2). The system (S1, S2, S3) for discharging liquefied gas according to any one of claims 6 to 8.

10. The pressure duct (25) is a duct incorporated in the support column (23) for discharging the cargo from the tank (16), and the means for emptying is to operate the pump to suck the liquefied gas (G) into the pressure duct (25). The system (S1) for discharging liquefied gas according to claim 9.

11. The pressure ducts (25b, 25c) are ducts configured to supply fuel to at least one consumer of the ship (1). The system (S2, S3) for discharging liquefied gas according to any one of claims 6 to 9.

12. The pipes (4b, 4c) are - a first portion (40) extending along the support column (21); - a second portion (44) extending along the support structure (24) and connecting one end of the support column (21) and the suction member (8b) to each other; - a third portion (48) arranged vertically from one end of the second portion (44), and the end of the second portion (44) is located within the vertical extension length of the enclosure (7, 30c). The third portion (48); The system (S1, S3) for discharging liquefied gas according to any one of claims 9 to 11, which cites claim 9.

13. The enclosure is a container (30, 30c) fixed to the support structure (24), and the container (30, 30c) is held above the lower wall (164) of the tank (16). The system (S1, S3) for discharging liquefied gas according to any one of claims 9 to 12, which cites claim 9.

14. The enclosure is a sample (7) arranged within the thickness of the lower wall (164) of the tank (16). The system (S2) for discharging liquefied gas according to any one of claims 5 to 12.

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

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