Gas dome and sealed and thermally insulated tank comprising such gas dome

Abstract

A gas dome (14) for a tank (2) for storing liquefied natural gas has a vapor collection line (18) extending axially along an axis of revolution (X) from a first axial end (50) arranged in a wall (4) of the tank (2) to a second axial end (52) arranged outside the tank (2), the vapor collection line (18) makes it possible to define a vapor circulation path between the interior space of the tank (2) and a vapor collector (20) located outside the tank, characterized in that the vapor collection line (18) comprises three coaxial lines in continuous communication, in which the vapor supply line (54) comprises a vapor inlet (60) near a first axial end (50), the gas dome (14) has a steam inlet opening into the interior space of the tank (2), in which the liquid liquefied natural gas can be sprayed via the spraying means (78) of the gas dome (14) in the intermediate line (56), and the discharge line (58) comprises, at one end, a steam outlet communicating with the steam collector (20).

Description

Technical Field

[0001] The present invention relates to the field of facilities for handling liquefied gases, such as liquefied natural gas (LNG). The invention more particularly relates to such facilities installed in sealed, thermally insulated tanks for storing and / or transporting fluids, preferably liquefied gases, such as LNG. Background Art

[0002] LNG is transported by sea in sealed, insulated storage tanks loaded on transport vessels. The natural gas is kept in liquid form to increase the amount of natural gas that can be carried in the tanks, as one kilogram of liquid natural gas takes up much less volume than one kilogram of gaseous natural gas. These tanks keep the LNG at very low temperatures, more specifically at a temperature below -163°C, at which natural gas is liquid at atmospheric pressure.

[0003] The sealed insulated tank comprises a wall which may have, in sequence along the thickness direction, a secondary insulation barrier against a load bearing structure, a secondary sealing membrane against the secondary insulation barrier, a primary insulation barrier against the secondary sealing membrane and a primary sealing membrane designed to be in contact with the liquefied gas contained in the tank.

[0004] The insulation barrier of an LNG tank is where the heat flows which tend to heat the contents of the tank, which causes the LNG to vaporize.

[0005] Therefore, the liquefied gas exists in the tank in a liquid-vapor two-phase equilibrium state. A portion of the liquefied natural gas is in the vapor phase, thus forming boil-off gas (BOG) in the upper part of the tank, and the liquefied natural gas is mainly in the liquid phase in the lower part of the tank. This stratification is naturally obtained due to the specific density of each phase.

[0006] In an attempt to minimize the emission of such boil-off gases in the storage tanks, it is known to spray the liquefied natural gas in liquid form in the upper part of the tank so that at least a portion of the boil-off gases is recondensed. More specifically, the liquefied natural gas is pumped in liquid phase from the bottom of the tank and then directed to a spraying bar which is arranged against the top wall covering the upper part of the tank and which sprays the liquefied natural gas in liquid phase in the upper part of the tank containing the liquefied natural gas in vapor phase.

[0007] Furthermore, in this context of managing boil-off gases, it is also known to provide a gas dome, i.e. a device for discharging such boil-off gases (e.g. the device in application WO2015155377). Such a gas dome has a vapor collection line defining a vapor flow path between a space inside the tank and a vapor collector (not shown) located outside the tank. The gas produced by the natural evaporation of the liquefied natural gas can be burned or reused to supply gas consuming components. Summary of the invention

[0008] The present invention belongs to this background and aims to provide an alternative to existing gas dome structures by proposing a gas dome for a liquefied natural gas storage tank, which alternative tends to completely condense boil-off gas generated in the tank, the gas dome for the liquefied natural gas storage tank having a vapor collecting line extending axially along a rotation axis X from a first axial end of the vapor collecting line arranged at the wall of the tank to a second axial end of the vapor collecting line arranged outside the tank, the vapor collecting line defining a vapor flow path between a space inside the tank and a vapor collector located outside the tank.

[0009] According to the present invention, the steam collecting pipeline includes three coaxial pipelines which are continuously connected, and the three coaxial pipelines are a steam supply pipeline, an intermediate pipeline and a discharge pipeline. The steam supply pipeline includes a steam inlet near the first axial end, and the steam inlet leads to the space inside the tank. Liquid liquefied natural gas can be sprayed in the intermediate pipeline via the spraying device of the gas dome. The discharge pipeline includes a steam outlet at one end, and the steam outlet is connected to the steam collector.

[0010] The gas dome according to the invention makes it possible to define, within the enclosure delimiting the gas dome, a vapour flow circuit which provides control over the zone in which the liquefied natural gas vapour comes into contact with the sprayed liquid liquefied natural gas. More specifically, it is possible to ensure that the vapour and the liquid liquefied natural gas are co-current during the spraying process, i.e. they flow in the same direction, so that the heat exchange aimed at at least partially recondensing the vapour is as effective as possible.

[0011] Preferably, the liquid LNG sprayed via the spraying device is first cooled before being sprayed onto the vapor.

[0012] Preferably, three coaxial lines of the vapour collecting line are arranged around one another.

[0013] Preferably, three coaxial lines extend concentrically.

[0014] The gas dome extending on the top wall of the LNG storage tank has a given height, which is referred to as the gas dome height.

[0015] Preferably, the height of the three coaxial pipelines is less than the gas dome height.

[0016] Preferably, the height of the discharge line is greater than the corresponding heights of the steam supply line and the intermediate line.

[0017] According to one feature of the invention, the heat exchange member is arranged in the intermediate line between the first axial end of the vapor collecting line and the liquid liquefied natural gas spraying device. The arrangement of the vapor collecting line in three interconnected coaxial lines and the precise position of the heat exchange member in the intermediate line mean that the heat exchange member is arranged downstream of the liquid liquefied natural gas spraying device with respect to the flow direction of the liquefied natural gas vapor. The heat exchange member is then advantageously arranged to maximize the contact surface between the liquefied natural gas vapor phase and the liquefied natural gas liquid phase and to optimize the heat exchange between these two phases.

[0018] Preferably, the heat exchange means also enables mass exchange between the two phases.

[0019] According to one feature of the invention, the heat exchange member is a lining section formed by grids stacked crosswise with each other. The lining section is formed in particular by a stack of grids formed by corrugated strips, respectively. These grids can be arranged relative to each other to perform two functions simultaneously. First, when the liquid liquefied natural gas is sprayed in the intermediate pipeline, heat exchange is carried out between the different phases of the liquefied natural gas mixed upstream of the heat exchange member, thereby increasing the heat exchange surface. The second function of these grids is to ensure that the boil-off gas flows with very low pressure losses (a few millibars) when the liquid liquefied natural gas spraying device is inactive and only the boil-off gas flows completely towards the discharge pipeline.

[0020] Such a lining is called “structured.” Alternatively, a “random” lining may be used, ie a lining formed from solid elements of different shapes and assembled randomly.

[0021] According to one feature of the invention, the supply line and the intermediate line are delimited by a common cylindrical wall configured to leave a passage for the vapor from the supply line to the intermediate line.

[0022] According to one feature of the invention, an opening enabling the steam to flow from the supply line to the intermediate line is formed in the common cylindrical wall near the second axial end of the steam collecting line, opposite the first axial end where the steam inlet is arranged.

[0023] According to one feature of the invention, the gas dome comprises a discharge outlet opening opposite the opening enabling the passage of steam between the supply line and the intermediate line.

[0024] According to the different features of the present invention, taken alone or in combination, the following features can be provided:

[0025] The liquefied natural gas spraying device comprises an annular ring arranged in the intermediate pipeline, said annular ring being connected to the liquefied natural gas inlet via a connecting pipe extending at least partially into the intermediate pipeline;

[0026] connecting conduits connecting the liquid liquefied natural gas inlet to the annular ring such that the annular ring is disposed between the vapor passage from the supply line to the intermediate line and the first axial end of the vapor collecting line;

[0027] the annular ring including a spray hole oriented toward a first axial end of the vapor collection line;

[0028] Therefore, any one of these features ensures that liquid LNG is completely sprayed in the intermediate pipeline and does not partially escape through the vapor channel formed between the supply pipeline and the intermediate pipeline, thereby ensuring efficiency of heat exchange between gas and liquid phases of the LNG.

[0029] According to another series of features of the present invention, taken individually or in combination, the following features can be provided:

[0030] The three coaxial lines of the vapour collecting line are arranged so that the supply line surrounds an intermediate line which surrounds a discharge line forming a central line;

[0031] a steam inlet arranged at one end of the steam supply line being radially offset relative to the axis of revolution X of the steam collecting line;

[0032] The gas dome comprises a liquefied natural gas recovery tank arranged in the vicinity of the first axial end of the vapor collecting line, said recovery tank comprising overflow means for overflowing the liquefied natural gas in liquid form into the storage tank;

[0033] The recovery tank is arranged to axially face the discharge line and be at a distance from the discharge line so that vapor can pass between the intermediate line and the discharge line.

[0034] The invention also relates to a sealed thermally insulated tank for storing liquefied natural gas, the sealed thermally insulated tank comprising at least one wall, the at least one wall having at least one thermal insulation barrier and at least one sealing membrane in the thickness direction, the at least one wall being penetrated by a gas dome as described above. The tank involved may be, for example, a self-supporting tank and is particularly referred to as a type A tank, a type B tank or a type C tank.

[0035] According to one feature of the present invention, the sealed tank comprises at least one wall, the at least one wall having two thermal insulation barriers and two sealing films. More specifically, the sealed tank can be constructed so that at least one tank wall comprises, from the outer face to the inner face, a secondary thermal insulation barrier, a secondary sealing film, a primary thermal insulation barrier and a primary sealing film intended to be in contact with the liquid stored in the tank.

[0036] The invention also relates to a transport and / or storage unit comprising at least one sealed and thermally insulated tank as described above.

[0037] The transport and / or storage unit comprising at least one sealed and thermally insulated tank as described above may in particular comprise a floating structure or an onshore structure, which may be a ship, a barge, a liquefaction or reliquefaction unit, a gasification unit or a gravity platform.

[0038] According to one feature of the invention, the transport and / or storage unit comprises pumping means for pumping liquid LNG from a tank, cooling means for cooling the pumped liquid LNG, and guiding means for guiding the cooled liquid LNG, connecting the cooling means to the liquid LNG spraying means of the gas dome.

[0039] The invention also relates to a method for loading or unloading a vessel forming a floating structure as described above, during which method fluid is conducted via insulated pipes from shore or a floating storage facility to a tank on the vessel, or vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Other characteristics, details and advantages of the invention are more clearly elucidated in the following description and in a number of exemplary embodiments provided by way of non-limiting examples with reference to the accompanying drawings, in which:

[0041] [ Figure 1 ] is a side view of a transport vessel comprising a plurality of tanks, at least one of the tanks being equipped with a gas dome according to an aspect of the present invention;

[0042] [ Figure 2 ]yes Figure 1 A cross-sectional view of a gas dome is schematically shown in FIG.

[0043] First of all, it should be noted that the accompanying drawings show the present invention in detail to enable the present invention to be implemented, and these drawings can naturally be used to better define the present invention when appropriate. It should also be noted that in all the drawings, similar elements and / or elements performing the same function are indicated using the same number. DETAILED DESCRIPTION

[0044] Figure 1 There is shown a floating structure, in this case in the form of a transport vessel 1, such as a methane tanker, comprising a plurality of tanks 2 for transporting or storing liquefied gas.

[0045] Each tank 2 is sealed and thermally insulated and has a generally rectangular parallelepiped shape with a plurality of tank walls including a top wall 4 .

[0046] Each tank wall includes, from the outer face to the inner face, a secondary insulation barrier, a secondary sealing film, a primary insulation barrier and a primary sealing film intended to be in contact with the liquid stored in the tank. The secondary insulation barrier and the secondary sealing film form a secondary space 6, and the primary insulation barrier and the primary sealing film form a primary space 8.

[0047] The tank 2 defines an inner space intended to be filled with a liquefied gas. The liquefied gas may in particular be liquefied natural gas (LNG), i.e. a gaseous mixture comprising mainly methane, one or more other hydrocarbons (such as ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane) and a small amount of nitrogen. The liquefied gas will be described below as liquefied natural gas, but the liquefied gas may also be ethane or liquefied petroleum gas (LPG), i.e. a mixture of hydrocarbons produced by refining oil (mainly comprising propane and butane), without departing from the context of the present invention. The liquefied gas may also be ammonia, hydrogen or carbon dioxide transported in a liquid state in the tank 2.

[0048] The liquefied gas exists in the tank 2 in a liquid-vapor two-phase equilibrium state. More specifically, Figure 1 As shown schematically, the liquefied natural gas present in the tank is mainly in the liquid phase, and this liquid part of the liquefied natural gas is located in the lower part 10 of the tank 2. A part of the liquefied natural gas is in the vapor phase, so that in the upper part 12 of the tank 2, boiled-off gas (BOG) is formed between the liquid / vapor boundary and the top wall 4 of the tank mentioned above.

[0049] At least one sealed, insulated tank 2 of the vessel 1 is equipped with a gas dome 14 associated with the top wall 4 and having the effect of enabling a portion of the boil-off gas present in the upper part 12 of the tank 2 to pass through the top wall.

[0050] The gas dome 14 comprises in particular a shaft which participates in delimiting therein a vapour collecting line 18 intended to conduct boil-off gases from the space inside the tank 2 to a vapour collector 20 situated outside the tank 2 .

[0051] The vapor collector 20 (the shape and position of which on the vessel are given here only by way of non-limiting illustration of the invention) enables the vapor to flow to the vessel's venting mast, burners, propulsion equipment and / or liquefaction / recondensation equipment, in which the gas in the vapor phase is reliquefied / recondensed and then returned to the tank in the liquid phase.

[0052] Furthermore, the vessel 1 comprises a facility for injecting liquefied natural gas in liquid form into the gas dome in order to participate in the recondensation of boil-off gas in the gas dome. The facility comprises in particular: a pumping device 22 for pumping the liquid liquefied natural gas from the lower part 10 of the tank; a cooling device 24 for cooling the pumped liquid liquefied natural gas; and a guide device 26 connecting the cooling device to the gas dome in order to introduce the cooled liquid liquefied natural gas into the gas dome.

[0053] The cooling device 24 may comprise in particular a heat exchanger in which a coolant fluid flows in order to recover heat from and cool the liquefied natural gas leaving the tank.

[0054] The installation for injecting liquefied natural gas in the liquid state into a gas dome as described above may comprise in particular a control unit intended to control the operation of the pumping device 22 in order to shut off or allow the liquefied natural gas to reach the gas dome in the liquid state. The control via the control unit will be described in more detail below in parallel with the description of the operation of the gas dome according to the invention.

[0055] As will be described in particular below, the gas dome 14 according to the present invention is intended, on the one hand, to discharge the liquefied natural gas in the gas phase to the vapor collector 20, and, on the other hand, to recondense a portion of the liquefied natural gas in the gas phase by exchanging heat and mass with the cooled liquid liquefied natural gas sprayed in the gas dome, so that the recondensed liquefied natural gas can be directly reinjected into the tanks associated with the gas dome.

[0056] Now refer to Figure 2 Describe the gas dome in more detail.

[0057] The gas dome structure 14 comprises a shaft 28 of cylindrical shape about the axis of revolution X of the gas dome, the shaft extending mainly in a longitudinal direction oriented orthogonally to the tank top wall 4. The shaft 28 passes through an opening 30 formed in the tank top wall 4. The shaft 28 is welded to a flange extending radially outwardly relative to the axis of revolution X of the gas dome 14 and the flange is welded to the outer face 34 of the tank.

[0058] The upper end of the shaft 28, i.e. the end arranged outside the tank at a distance from the top wall, is sealingly closed by a removable cover 36. The cover 36 is fastened to an annular assembly flange 38 by means of a plurality of fastening members. A gasket (not shown) is compressed between the cover 36 and the assembly flange 38, thereby forming a seal between the outside and the inside of the gas dome 14.

[0059] The gas dome 14 has two successive stages inside the shaft 28 along the axis of revolution X, specifically a first stage 40 and a second fluid distribution stage 42, the first stage being formed by the above-mentioned vapor collecting line 18, into which the vapor collecting line opens, and the second fluid distribution stage being configured to be connected to the vapor collector 20 and the guide device 26, which introduces the liquefied natural gas pumped and cooled from the tank in the liquid state into the gas dome.

[0060] Thus, the second stage 42 is axially delimited on the one hand by the vapour collecting line 18 and on the other hand by the cover and comprises in particular a radial connection end piece 44 for connection to the vapour collector. An opening 45 enables the vapour to flow from the first stage 40 to the second stage 42, so that any boil-off gas which is not recondensed in the vapour collecting line 18 as will be described below passes through the opening to enter the vapour collector via the radial connection end piece 44.

[0061] In the example shown, the second stage 42 is also configured to receive two separate supply pipes 46, 48, within which the liquefied natural gas flows in the liquid state and which form the above-mentioned guide means 26. In the example shown, both supply pipes are connected to cooling means for cooling the pumped liquefied natural gas as mentioned above, but it should be noted that the liquefied natural gas flowing in the liquid state in the first supply pipe 46 may come from a different source than the liquefied natural gas flowing in the liquid state in the second supply pipe 48 without departing from the context of the present invention.

[0062] Furthermore, the first supply pipe 46 as will be described below is an essential element of the present invention, since it enables liquid LNG to be sprayed within the vapor collecting line 18 onto the path of the boil-off gas.

[0063] The gas dome 14 according to the invention can be provided without the second supply pipe 48. The function of this second supply pipe is to pass through the gas dome from one axial end to the other axial end (if necessary by pressing against the wall defining the vapour flow line, in order to exchange heat with the vapours), to open outside the gas dome and to enable liquid liquefied natural gas to be sprayed on the upper part of the tank, thus contributing to the general cooling of the cargo stored in the tank 2.

[0064] The steam collecting line 18 forming the first stage 40 extends axially around the axis of rotation X of the gas dome 14 mainly from a first axial end 50 of the steam collecting line 18 arranged at the top wall 4 of the tank 2 to a second axial end 52 of the steam collecting line arranged outside the tank, and the steam collecting line defines a steam flow path between the space inside the tank and the steam collector 20. In this case, the steam collecting line defines a steam flow path between the space inside the tank and the steam collector 20 via the second stage of the gas dome mentioned above.

[0065] The vapor collection line 18 is particularly configured to optimize the heat exchange and separation of the gas / liquid phases within the gas dome between the boil-off gas recovered in the upper part of the tank and the liquid LNG recovered in the lower part of the tank and sprayed onto the boil-off gas. It is worth noting that the liquid LNG and the LNG in vapor form mix when they flow in parallel, i.e., in the same direction.

[0066] For this reason, Figure 2 As shown, the vapor collection line 18 includes three coaxial lines in continuous communication, including a vapor supply line 54, an intermediate line 56, and a discharge line 58. The three lines are considered to be in continuous communication because the three lines are in fluid communication in pairs, with the intermediate line 56 being interposed between the vapor supply line 54 and the discharge line 58.

[0067] More specifically, the vapor supply line 54 is constituted by a line through which boil-off gas from the inside of the tank can enter the gas dome and in which the boil-off gas flows in a first direction S1 (specifically, a direction from the first axial end 50 of the vapor collecting line to the second axial end 52 of the vapor collecting line). The intermediate line 56 in fluid communication with the supply line is constituted by a line in which the boil-off gas flows in a second direction S2 (parallel to and opposite to the first direction, i.e., a direction from the second axial end 52 of the vapor collecting line to the first axial end 50 of the vapor collecting line). The discharge line 58 is supplied with the boil-off gas present in the intermediate line and again guides the boil-off gas in a direction similar to the first direction (i.e., a direction from the first axial end of the vapor collecting line to the second axial end of the vapor collecting line).

[0068] As will be described in detail below, this configuration ensures a flow direction of the boil-off gas from the second axial end to the first axial end of the vapor collecting line in at least one portion of the gas dome 14. By means of this co-current transfer, more specifically in the direction of gravity (i.e., in this case in the direction from the second axial end 52 to the first axial end 50 of the vapor collecting line), this configuration particularly promotes the transfer of mass and heat between the gaseous liquefied natural gas and the liquid liquefied natural gas.

[0069] In the example shown, the three lines are arranged successively in the above-mentioned order from the outside of the gas dome towards the center of the gas dome in a radial direction perpendicular to the axis of revolution X of the gas dome. In other words, the supply line 54 surrounds the intermediate line 56 and the two lines surround the discharge line 58 which thus forms the central line.

[0070] The steam supply line 54 has a steam inlet 60 which is arranged at one end of the steam supply line, arranged at the first axial end 50 of the steam collecting line. The outer position of the steam supply line 54 relative to the other lines 56, 58 means that the steam inlet 60 is radially offset relative to the axis of revolution X of the steam collecting line 18.

[0071] The vapor inlet 60 forms an annular opening surrounding a recovery tank 61 that closes the gas dome 14 at the first axial end 50 .

[0072] In this way, the interior of the gas dome 14 may communicate with the tank 2 only through the vapor inlet 60 , thereby ensuring that all boil-off gas entering the gas dome 14 keeps flowing in different directions S1 , S2 implemented by the arrangement of the three coaxial lines 54 , 56 , 58 .

[0073] The steam supply line 54 is delimited radially on the outside by the wall of the shaft 28 and on the inside by a cylindrical wall 62 that is common to the intermediate line 56 .

[0074] At the second axial end 52, close to the transverse wall 64 of the two stages 40, 42 participating in delimiting the gas dome 14, the cylindrical wall 62 has an opening 66 that enables the boil-off gas to pass through. The opening 66 can in particular be a window extending over the entire circumference of the cylindrical wall, which does not limit the invention. Thus, the boil-off gas can pass from the supply line 54 through the opening 66 to the intermediate line 56 and start flowing in the intermediate line 56 in the second direction S2 mentioned above.

[0075] The opening that enables the steam to flow from the supply line 54 to the intermediate line 56 is formed near the second axial end 52 of the steam collecting line 18, opposite to the first axial end 50 near which the steam inlet 60 is arranged. This arrangement participates in the change of the flow direction of the boil-off gas, which then flows through the intermediate line 56 in the second direction S2, thereby optimizing the displacement length of the boil-off gas in the line in the first direction in the supply line or in the second direction S2 in the intermediate line. As will be mentioned below, this provides a heat exchange area, or if necessary, a transfer area of ​​matter and heat, which is large enough to ensure that the re-condensation rate of the boil-off gas is within an optimal ratio.

[0076] A degassing zone 67 is provided at the second axial end, which has a discharge outlet 68 turned toward the outside of the shaft 28, which opens opposite the vapour passage between the supply line 54 and the intermediate line 56. The discharge outlet 68 comprises a valve which is closed by default and which opens when the pressure inside the tank, and therefore inside the gas dome 14, exceeds a predetermined threshold. In other words, the supply line comprises means capable of managing the overpressure that is liable to occur in the tank, and therefore in the gas dome.

[0077] The discharge line 58 is formed by a cylinder forming a flue centered on the axis of revolution X of the gas dome. The discharge line 58 has a first end 70 that turns toward the first axial end 50 of the vapour collecting line and a second end 72 that turns in the opposite direction toward the second stage 42. The first end 70 of the discharge line 58 extends at a distance from the recovery tank 61 mentioned above, in order to leave a passage for the boil-off gases present in the intermediate line so that they can enter the discharge line 58.

[0078] The discharge line 58 opens into the second stage 42 of the gas dome at a second end 72 of the discharge line for fluid communication with the radial connection end piece 44 for connection to a vapor collector.

[0079] The intermediate line 56 forms the connection between the supply line 54 and the discharge line 58, in which the various components of the gas dome according to the invention are housed in order to optimize the recondensation of the boil-off gases. It is worth noting that the gas dome comprises a liquid liquefied natural gas spraying device 74 configured to spray liquid liquefied natural gas in the intermediate line 56, and the gas dome may also comprise a heat exchange member 76 intended to improve the transfer of mass and heat between the liquid phase and the gas phase, in particular by increasing the exchange surface.

[0080] As mentioned, it is noteworthy that the coaxial lines 54, 56, 58 of the vapor collecting line 18 are configured so that the boil-off gas flows in the intermediate line 56 along the second direction S2, and advantageously according to the invention, the components of the gas dome 14 for optimizing the recondensation of the boil-off gas are housed in the intermediate line 56 and act on the boil-off gas flowing along the second direction S2 (in particular the direction of gravity), which second direction is opposite to the discharge direction of the boil-off gas toward the outside for a portion of the boil-off gas that is not recondensed in the intermediate line 56.

[0081] The spraying device 74 comprises a first supply pipe 46 forming the guide means 26 and delivering the liquid liquefied natural gas into the gas dome 14 at the second stage 42, and a spraying device 78 comprising holes (or, if necessary, spraying nozzles) oriented towards the first axial end 50 of the vapour collecting line.

[0082] In the example shown, the liquefied natural gas spraying device comprises an annular ring 80 which is arranged in the intermediate line 56 and is connected to the first supply pipe 46 via a connecting pipe 82 which passes through the transverse wall 64 and extends at least partially into the intermediate line 56. The annular ring 80 is in particular arranged to straddle the passage in the intermediate line 56 for the boil-off gas.

[0083] The connection pipe 82 connecting the liquid LNG inlet in the gas dome to the annular ring 80 is dimensioned so that the annular ring 80 is arranged between the opening 66 enabling the flow of vapor from the supply line 54 to the intermediate line 56 and the first axial end 50 of the vapor collecting line.

[0084] In combination with the fact that the holes of the spraying device are oriented towards the first axial end 50 of the vapour collecting line, this ensures in particular that no liquid LNG is sprayed in the supply line and that all sprayed liquid LNG is sprayed in the intermediate line 56 in order to optimise heat exchange and mass transfer with the boil-off gas, i.e. the LNG in the gaseous state.

[0085] The orientation of the holes presented (ie facing the first axial end 50 of the vapour collecting line) enables the liquid LNG to be sprayed in the same direction as the flow direction of the boil-off gas in the intermediate line (ie the second direction) to ensure co-current mixing between the two phases of the LNG.

[0086] The heat exchange member 76 is also arranged in the intermediate pipeline between the first axial end 50 of the vapor collecting line and the liquid liquefied natural gas spraying device 78. In other words, the heat exchange member 76 is arranged downstream of the boil-off gas flow relative to the spraying device. Therefore, the boil-off gas in contact with the heat exchange member is mixed with the liquid liquefied natural gas in advance, and the entire exchange surface formed by the heat exchange member across the intermediate pipeline improves the efficiency of the transfer of mass and heat between the two separate phases of the liquefied natural gas.

[0087] More specifically, the heat exchange member 76 is a liner section configured to allow the evaporation gas to pass without significant pressure loss if the liquid liquefied natural gas spraying device 78 is inactive by suitable control means, as will be described in more detail below in the description of the operation of the gas dome according to the invention. For example, the pressure loss may be several millibars. On the contrary, when the liquid liquefied natural gas is sprayed upstream of the heat exchange member 76, the liner section also has the effect of increasing the exchange surface between the gas phase and the liquid phase of the liquefied natural gas. The liner section can be composed in particular of metal plates stacked one on top of another, each of these metal plates having corrugated strips, leaving gaps between two adjacent strips, the metal plates being stacked in such a way that the orientation of the strips changes from one stage of the stacking of the metal plates to another stage. This liner is a structured liner. Alternatively, an unstructured "random" liner can be used.

[0088] The axial dimension of the intermediate line 56 (from the second axial end 52 of the vapor collecting line to the first axial end 50 of the vapor collecting line) is large enough to enable the liquid liquefied natural gas spraying device 78 and the heat exchange member 76 to be arranged in sequence, and to ensure a suitable axial dimension of the heat exchange member 76 (i.e., having a sufficient lining height to enable the boil-off gas to be fully recondensed when necessary).

[0089] As mentioned above, the gas dome 14 comprises, at a first axial end of the vapor collecting line, a liquefied natural gas recovery tank 61, rigidly connected to a cylindrical wall 62 common between the intermediate line 56 and the supply line 54, thereby covering at least a portion of the intermediate line and the discharge line. The recovery tank 61 is arranged axially facing the central discharge line 58 and at a distance therefrom to enable vapor to pass between the intermediate line 56 and the central discharge line 58.

[0090] The recovery tank 61 comprises an overflow device 84 for overflowing the liquefied natural gas in the liquid state into the tank 2. More specifically, the recovery tank comprises two concentric cylindrical walls, the overflow device 84 being formed in this case by a bend between these two cylindrical walls, which can be filled with liquid liquefied natural gas and which communicates with the outside of the recovery tank in the direction of the tank. In this case, the bend extends opposite to the lower portion 86 of the cylindrical wall 62 shared between the supply line 54 and the intermediate line 56, thereby forming a double bend which allows the liquefied natural gas to overflow in the liquid state into the tank if there is too much liquid liquefied natural gas in the recovery tank and which prevents boil-off gases from entering directly into the discharge line 58.

[0091] In the example shown, the second liquid liquefied natural gas supply pipe 48 extends axially inside the gas dome up to the upper part of the tank. The end of the second supply pipe 48 takes the form of a spray ring for spraying liquid liquefied natural gas into the upper part 12 of the tank 2 and which is rigidly connected to the lower portion 86 of the cylindrical wall common between the supply line 54 and the intermediate line 56. Advantageously, this cylindrical wall thus delimits two coaxial lines inside the gas dome and also fastens the liquid liquefied natural gas spray ring associated with the second supply pipe 48.

[0092] A first possible use of the gas dome and the liquefied gas processing facility as a whole will now be described.

[0093] The vessel 1 is susceptible to consuming the boil-off gas to supply the boil-off gas via the vapor collector 20 and operate the vessel's engines to propel the vessel or generate electricity on board the vessel.

[0094] When the vessel has to sail at a low speed, the engine requires less fuel gas and therefore consumes less boil-off gas, and therefore most of the boil-off gas in the tank needs to be recondensed. To this end, the pumping device 22 is actuated via a suitable control unit so that liquid LNG is pumped from the tank, then directed to the cooling device 24 and finally sprayed via the spraying device 78 inside the gas dome 14. The two phases of the liquefied natural gas are mixed in parallel in the intermediate pipeline and the passage of this mixture through the heat exchange member 76 (which provides an increased exchange surface inside the intermediate pipeline) causes the boil-off gas to be recondensed, if necessary, completely recondensed. The boil-off gas flows through the supply line 54 in a first direction S1, as shown in FIG. Figure 2 The mixture of boil-off gas and liquid LNG formed in the intermediate pipeline 56 flows through the heat exchange member and the outlet of the heat exchange member 76 in the second direction S2, and the recondensed portion of the boil-off gas and the sprayed liquid LNG fall into the recovery tank, while any remaining portion in vapor form escapes through the discharge pipeline 58, as shown in FIG. Figure 2 As shown in FIG. 1 , the solid arrow is extended by a dotted arrow.

[0095] When the ship needs to sail at full speed, the engine needs a large amount of boil-off gas to run, so there is no need to re-condense the boil-off gas in the tank. In this case, the pumping device 22 is deactivated via a suitable control unit so that no liquid liquefied natural gas is sprayed via the spraying device 78 in the gas dome 14. Only boil-off gas flows in the gas dome, passing through the supply line 54, the evaporation line 56 and the discharge line 58 in sequence. It is worth noting that the heat exchange member 76 is constructed (particularly by taking the form of a liner section) so that the pressure loss when the boil-off gas enters the heat exchange member is minimized.

[0096] This type of gas dome is also particularly useful in floating structures constituting FSRUs (Floating Storage and Regasification Units), a mode of recondensing the maximum amount of boil-off gas being particularly useful when these floating structures are stationary and not exporting gas in the vapor phase to the LNG network.

[0097] It will be appreciated that according to the invention, in particular in this first use case, the injection device for injecting liquid liquefied natural gas can be controlled to operate continuously and achieve complete recondensation of boil-off gas, or, conversely, to be deactivated when it is necessary to allow boil-off gas to escape.

[0098] In another case of use, a tank equipped with a gas dome according to the invention discharges its liquid LNG to another tank (in this case, the other tank is called a receiving tank) in particular via a suitable unloading pump. It is then necessary to return the liquefied natural gas in the gaseous state to compensate for the pressure drop caused by the unloading of the liquid LNG.

[0099] The natural gas then flows in the direction opposite to that mentioned above (from the outside of the tank to the inside of the tank, more specifically, from the radial connection end piece 44 and the second stage 42 of the gas dome through the vapor collecting line 18 to the first stage 40 of the gas dome and the vapor inlet 60 communicating with the inside of the tank).

[0100] In this configuration, the gas dome according to the present invention can partially recondense the vapor using a counterflow. More specifically, the liquefied natural gas is made to flow in the vapor collecting line in a gaseous state in the discharge line 58 from the second axial end 52 of the vapor collecting line to the first axial end 50 of the vapor collecting line, then in the intermediate line 56, this time from the first axial end 50 of the vapor collecting line to the second axial end 52 of the vapor collecting line, and finally in the supply line 54 from the second axial end 52 of the vapor collecting line to the first axial end 50 of the vapor collecting line.

[0101] When the liquefied natural gas passes through the intermediate pipeline in the gaseous state, the spraying device 74 is configured so that the liquid liquefied natural gas is sprayed on the gaseous liquefied natural gas. Since the holes of the spraying device are oriented toward the first axial end 50 of the vapor collecting pipeline, the liquid liquefied natural gas is sprayed countercurrently in this use case. As mentioned above, a part of the gaseous liquefied natural gas that becomes liquid under the action of this spray flow flows into the tank via the recovery tank 61. This partial condensation is particularly useful because it maximizes the amount of gaseous liquefied natural gas from the receiving tank, and after receiving the liquid liquefied natural gas, the liquefied natural gas is returned in the gaseous state to the tank equipped with the gas dome according to the present invention, thereby facilitating the control of the pressure in the receiving tank.

[0102] As described above, the present invention achieves the stated objectives by providing a facility for optimizing the storage properties of liquefied gas in a tank by enabling the boil-off gas formed in the tank to be completely recondensed (if necessary, using the non-recondensed portion of the boil-off gas). This objective is achieved by a specific configuration of a gas dome associated with the tank, the interior of which includes a plurality of coaxial lines, and by integrating a specific component in one of the lines thus arranged.

[0103] Of course, the present invention is not limited to the examples described above, and many adjustments to these examples can be made without departing from the scope of the present invention. As a non-limiting example, the three pipelines can be arranged differently, as long as the three pipelines are arranged continuously along the specified direction, wherein the intermediate pipeline is arranged between the supply pipeline and the discharge pipeline, and the gas flows in the intermediate pipeline along the second direction S2 mentioned above. In a possible alternative arrangement, the supply pipeline can be in the center, with its vapor inlet arranged in the center of the gas dome, and the discharge pipeline can be radially offset to surround the other two pipelines. In this case, the discharge line is connected to the second stage via an annular through hole, and the liquid liquefied natural gas recovery tank has an annular shape, which is engaged with the wall of the axis defining the gas dome.

Claims

1. A gas dome (14) of a liquefied natural gas storage tank (2), the gas dome having a spraying device and a vapor collecting line (18), the vapor collecting line extending axially along the axis of revolution (X) from a first axial end (50) arranged at the wall (4) of the tank (2) to a second axial end (52) arranged outside the tank (2), the vapor collecting line (18) defining a vapor flow path between a space inside the tank (2) and a vapor collector (20) located outside the tank, It is characterized in that The steam collecting pipeline (18) includes three coaxial pipelines connected in series, the three coaxial pipelines being a steam supply pipeline (54), an intermediate pipeline (56) and a discharge pipeline (58); the steam supply pipeline includes a steam inlet (60) near the first axial end (50), the steam inlet leading to the space inside the tank (2); liquid liquefied natural gas can be sprayed in the intermediate pipeline via the spraying device (78) of the gas dome (14); the discharge pipeline includes a steam outlet at one end, the steam outlet being connected to the steam collector (20).

2. Gas dome (14) according to the preceding claim, characterized in that A heat exchange member (76) is arranged in the intermediate line (56) between the first axial end (50) of the vapor collecting line and the spraying device (78) of liquid liquefied natural gas.

3. Gas dome (14) according to the preceding claim, characterized in that The heat exchange member (76) is a liner segment formed by grids stacked crosswise with each other.

4. A gas dome (14) according to any one of the preceding claims, characterised in that The supply line (54) and the intermediate line (56) are bounded by a common cylindrical wall (62) configured to allow a vapor passage from the supply line (54) to the intermediate line (56).

5. Gas dome (14) according to the preceding claim, characterized in that An opening (66) enabling steam to flow from the supply line (54) to the intermediate line (56) is formed in the common cylindrical wall (62) near the second axial end (52) of the steam collecting line (18), the opening being opposite to the first axial end (50) where the steam inlet (60) is arranged.

6. A gas dome (14) according to any one of the preceding claims, characterised in that The spraying device (78) for liquid liquefied natural gas comprises an annular ring (80) which is arranged in the intermediate pipeline (56), the annular ring (80) being connected to the liquid liquefied natural gas inlet via a connecting pipe (82) which at least partially extends into the intermediate pipeline (56).

7. Gas dome (14) according to the preceding claim in combination with claim 4 or 5, characterized in that The connecting pipe (82) connecting the liquid liquefied natural gas inlet to the annular ring (80) allows the annular ring (80) to be arranged between the vapor passage from the supply line (54) to the intermediate line (56) and the first axial end (50) of the vapor collecting line.

8. The gas dome (14) according to claim 6 or 7, characterized in that The annular ring (80) includes spray holes oriented toward the first axial end (50) of the vapor collecting line.

9. A gas dome (14) according to any one of the preceding claims, characterised in that The three coaxial lines of the vapour collecting line (18) are arranged such that the supply line (54) surrounds the intermediate line (56) which surrounds the discharge line (58) forming a central line.

10. Gas dome (14) according to the preceding claim, characterized in that The steam inlet (60) arranged at one end of the steam supply line (54) is radially offset relative to the axis of revolution (X) of the steam collecting line (18).

11. A gas dome (14) according to any one of the preceding claims, characterised in that The gas dome comprises a liquefied natural gas recovery tank (61) arranged in the vicinity of the first axial end (50) of the vapour collecting line, the recovery tank (61) comprising overflow means (84) for overflowing the liquefied natural gas in liquid form into the tank (2).

12. Gas dome (14) according to the preceding claim, characterized in that The recovery tank (61) is arranged to axially face the discharge line (58) and to be spaced apart from the discharge line at a distance such that vapor can pass between the intermediate line (56) and the discharge line (58).

13. A sealed insulated tank (2) for storing liquefied natural gas, comprising at least one wall (4), the at least one wall having at least one insulating barrier and at least one sealing membrane in the thickness direction, the at least one wall being penetrated by a gas dome (14) according to any one of the preceding claims.

14. A transport and / or storage unit comprising at least one sealed and thermally insulated tank (2) according to the preceding claim.

15. Transport and / or storage unit according to the preceding claim, characterized in that The transport and / or storage unit comprises pumping means (22) for pumping liquid liquefied natural gas from the tank, cooling means (24) for cooling the pumped liquid liquefied natural gas, and guiding means (26) for guiding the cooled liquid liquefied natural gas, connecting the cooling means (24) to the spraying means (78) of the liquid liquefied natural gas of the gas dome (14).

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