Method for Operating A Liquefied Gas Tank and Liquefied Gas Tank for Receiving LNG and Boil-Off Gas

Abstract

The invention relates to a method for operating a liquefied gas storage tank (1) having a tank volume (2) for receiving LNG and boil-off gas (BOG), wherein a gaseous BOG stream and a liquid LNG stream are fed to the tank volume, and wherein the BOG stream is introduced into the LNG stream, and wherein subsequently the resulting BOG-LNG mixture is introduced into the tank volume. The invention also relates to a corresponding liquefied gas storage tank.

Claims

1. A method for operating a liquefied gas storage tank (1) having a tank volume (2) for receiving liquefied natural gas (LNG) (10) and boil-off gas (BOG) (12), wherein a gaseous BOG stream and a liquid LNG stream are supplied to the tank volume, the method comprising the steps of: introducing the BOG stream into the LNG stream; and subsequently introducing the resulting BOG-LNG mixture into the tank volume.

2. The method according to claim 1, wherein at least a part of the BOG for the BOG stream and/or at least a part of the LNG for the LNG stream is supplied to the liquefied gas storage tank proceeding from outside the tank volume, in particular from at least one connected reservoir (50) of a Floating Storage Unit (FSU) or an LNG carrier or a regas barge.

3. The method according to claim 1, wherein at least a part of the BOG for the BOG stream and/or at least a part of the LNG for the LNG stream is withdrawn from the tank volume.

4. The method according to claim 1, wherein BOG from the tank volume and from at least one connected reservoir (50) is supplied to the BOG stream, and/or wherein LNG from the tank volume and from at least one connected reservoir (50) is supplied to the LNG stream.

5. The method according to claim 1, wherein the BOG-LNG mixture is introduced, in particular sprayed, into gaseous BOG (12) above an LNG filling level (8) of the tank volume.

6. The method according to claim 1, wherein the BOG-LNG mixture is introduced into liquid LNG (10) below an LNG filling level of the tank volume.

7. A liquefied gas storage tank (1) with a tank volume (2) for receiving liquefied natural gas (LNG) (10) and boil-off gas (BOG) (10), the liquefied gas storage tank comprising: a BOG conduit (14, 114, 214) for conducting gaseous BOG, an LNG conduit (16, 116, 216) for conducting liquid LNG, wherein the BOG conduit and the LNG conduit open into a common BOG-LNG conduit (24, 224, 324) that is formed to conduct a BOG-LNG mixture and that opens into the tank volume.

8. The liquefied gas storage tank according to claim 7, wherein the BOG conduit and/or the LNG conduit, in particular at their end away from the tank volume, includes a fluid interface (15; 17) to at least one connectable reservoir (50), in particular to a Floating Storage Unit (FSU) or an LNG carrier or a regas barge, and is equipped to supply BOG or LNG from a connected reservoir to the tank volume.

9. The liquefied gas storage tank according to claim 7, wherein the BOG conduit is equipped to withdraw gaseous BOG from the tank volume above an LNG filling level (8) and/or wherein the LNG conduit is equipped to withdraw liquid LNG from the tank volume below an LNG filling level.

10. The liquefied gas storage tank according to claim 7, wherein the orifice (26, 326) of the common BOG-LNG conduit into the tank volume is arranged above an, in particular predetermined, LNG filling level of the tank volume.

11. The liquefied gas storage tank according to claim 10, wherein the orifice includes a spray nozzle (30) for spraying the BOG-LNG mixture.

12. The liquefied gas storage tank according to claim 7, wherein the orifice of the common BOG-LNG conduit into the tank volume is arranged below an, in particular predetermined, LNG filling level of the tank volume.

13. The liquefied gas storage tank according to claim 7, wherein the LNG conduit includes an interface to a consumer network (36) that is equipped to feed LNG from the tank volume into the consumer network.

14. The liquefied gas storage tank according to claim 7, wherein the BOG conduit includes an interface to a consumer network (52) that is equipped to feed BOG from the tank volume into the consumer network.

15. A Floating Storage Unit (FSU) or LNG carrier or regas barge or LNG-operated vehicle or LNG-operated power plant with at least one liquefied gas storage tank (1) according to claim 7.

Description

[0038] Advantageous embodiments of the various aspects of the invention are subject-matter of the sub-claims. Further features, advantages and possible applications of the invention can be taken from the following description in conjunction with the Figures, wherein similar components in different Figures can be designated with the same reference numeral. In the drawings, partly in a strongly schematic representation,

[0039] FIG. 1 shows a sectional view of a liquefied gas storage tank according to an embodiment of the invention with an externally supplied BOG-LNG conduit;

[0040] FIG. 2 shows a sectional view of a liquefied gas storage tank according to another embodiment of the invention with a BOG-LNG conduit supplied from the tank volume;

[0041] FIG. 3 shows a sectional view of a liquefied gas storage tank according to another embodiment of the invention, in which the BOG conduit and the LNG conduit are arranged completely within the tank shell;

[0042] FIG. 4 shows a sectional view of a liquefied gas storage tank according to another embodiment of the invention, wherein the BOG-LNG mixture is injected below the LNG filling level;

[0043] FIG. 5 shows a sectional view of a liquefied gas storage tank according to another embodiment of the invention, to which a coupleable transport tank and customer networks are connected;

[0044] FIG. 6 shows a sectional view of a liquefied gas storage tank that in particular differs from the one according to FIG. 5 in that the BOG-LNG mixture is injected below the LNG filling level; and

[0045] FIG. 7 shows a sectional view of a liquefied gas storage tank to which beside a coupleable transport tank and customer networks a firmly installed storage tank also is connected.

[0046] FIG. 1 shows a liquefied gas storage tank 1 with a tank volume 2, in which the invention is applied to a filling assembly 3.

[0047] The liquefied gas storage tank 1 is delimited by a tank shell 4 that is thermally insulated with respect to the surroundings of the liquefied gas storage tank 1 by means of a heat insulation layer 6. The liquefied gas storage tank 1 is filled with liquid LNG 10 up to an LNG filling level 8; above the LNG filling level 8 the liquefied gas storage tank 1 is filled with gaseous BOG 12.

[0048] The liquefied gas storage tank 1 includes a BOG conduit 14 for conducting gaseous BOG, via which BOG can be supplied to the tank volume 2 from an external source. The liquefied gas storage tank 1 also includes an LNG conduit 16 for conducting liquid LNG, via which LNG can be supplied to the tank volume 2 from an external source. In this exemplary embodiment, a conduit compressor 18 is mounted in the BOG conduit 14 and a conduit pump 20 is mounted in the LNG conduit 16 for a sufficient conveyance or compression.

[0049] Downstream of the conduit compressor 18 and of the conduit pump 20 there is provided a joining point 22 at which a pipe conduit element of the BOG conduit 14 and a pipe conduit element of the LNG conduit 16 open into a common BOG-LNG conduit 24. In the exemplary embodiment, the common BOG-LNG conduit 24 is configured as a pipe conduit element and extends from the joining point 22 up to an orifice point 26 within the tank volume 2, wherein the pipe conduit element traverses the tank shell 4 at a breakthrough point 28.

[0050] At the orifice point 26 the BOG-LNG conduit 24 ends with a spray nozzle 30, wherein the orifice point 26 is arranged above the LNG filling level 8.

[0051] Below the LNG filling level 8 an unloading pump 32 is provided for conveying liquid LNG 10 through an unloading conduit 34 towards an interface of an LNG consumer network 36.

[0052] The liquefied gas storage tank 1 according to this embodiment hence can be loaded from a non-illustrated, external source of LNG and/or BOG, for example an LNG carrier, and independently be unloaded again in the direction of an LNG consumer network 36.

[0053] By joining a BOG stream in the BOG conduit 14 and an LNG stream in the LNG conduit 16 at the joining point 22 according to the invention, the gaseous BOG can already be condensed into the anyway liquid LNG before entry into the tank volume 2 of the liquefied gas storage tank 1.

[0054] In this exemplary embodiment, the necessary pressure is provided by the compressor 18; it would also be conceivable, however, that the BOG from the external source already is provided with sufficiently high pressure. Based on the existing conditions, the supplied LNG is sub-cooled and thus can liquefy the BOG pressed in.

[0055] By joining at an early stage already at the joining point 22, the length of the pipe conduit elements required altogether can be reduced distinctly; moreover, there is only required one breakthrough point 28 through the tank shell 4, instead of at least two of such apertures in the case of a separate conduit. This simplifies the manufacture of the liquefied gas storage tank 1, as compared with a conventional design.

[0056] By spraying the BOG-LNG mixture by means of the spray nozzle 30 into the BOG 12 present already in the tank volume 2, gaseous BOG 12 additionally is liquefied and thusin relative termsthe pressure in the tank volume 2 is reduced.

[0057] FIG. 2 shows a liquefied gas storage tank 1, in which the invention is applied to a recirculation circuit 38.

[0058] The liquefied gas storage tank 1 of this embodiment includes a conduit pump 120 for conveying LNG into the LNG conduit 116. The liquefied gas storage tank 1 also includes a BOG conduit 114 in which a conduit compressor 118 is arranged.

[0059] The LNG conduit 116 and the BOG conduit 114 converge in the joining point 22 and open into the common BOG-LNG conduit 24.

[0060] To provide for filling of the liquefied gas storage tank 1 with BOG or LNG (filling assembly 103) a BOG loading conduit 40 is provided, which opens above the LNG filling level within the tank volume 2, and an LNG loading conduit 42 which opens below the LNG filling level.

[0061] To provide for draining of the liquefied gas storage tank an unloading conduit 44 branches off from the LNG conduit 116, which can be connected to an LNG consumer network 36 via a standard interface. At the separation point 46 it can be switched via a suitably switchable, non-illustrated valve whether the LNG is conducted in the direction of the joining point 22 and/or in the direction of the consumer network 36.

[0062] In the liquefied gas storage tank 1 according to this embodiment the quantity of the BOG 12 present in gaseous form can be reduced by means of the recirculation circuit 38 independent of loading and/or unloading operations and hence the pressure in the tank volume 2 can be reduced.

[0063] For this purpose, gaseous BOG 12 is sucked off from the upper part of the tank volume (above the LNG filling level) by means of the conduit 114 and the associated compressor 118 and liquefied by pressing it into the sub-cooled LNG conveyed into the conduit 116 by means of the pump 120. This is accomplished in the common conduit 24 from the joining point 22.

[0064] Due to the subsequent injection (spray nozzle 30 is not shown here) a part of the BOG 12 not sucked off additionally is liquefied in the tank volume, which additionally reduces the pressure in the tank volume 2. This method can be carried out as long as the liquid LNG 10, which is heated by feeding in the relatively warmer gaseous BOG 12, still is sub-cooledbased on the pressure conditions prevailing in the tank. Cooling by an externally supplied, relatively cold LNG therefore only becomes necessary at a distinctly later time.

[0065] FIG. 3 shows a liquefied gas storage tank 1 in which the BOG conduit and the LNG conduit of the recirculation circuit 38 are arranged completely within the tank shell 4. This liquefied gas storage tank also includes a filling assembly 103 and an interface to the LNG consumer network 36 that is configured analogous to FIG. 1.

[0066] To form the recirculation circuit 38, an LNG conduit 216 and a rudimentary BOG conduit 214 are formed within the tank shell 4. The two conduits 216 and 214 both open into a Venturi nozzle 48 that is arranged above the LNG filling level 8. The joining point 222 is arranged at the inlets of the Venturi nozzle 48; the orifice point 26 is arranged at the common BOG-LNG conduit 224, which proceeds from the downstream end of the Venturi nozzle 48.

[0067] The recirculation circuit 38 hence is designed such that no aperture of the tank shell 4 is necessary therefor and it also requires only relatively short pipe conduit elements.

[0068] For pressure regulation in this embodiment, liquid LNG 10 is conveyed to the Venturi nozzle 48 by means of the pump 120, which in the nozzle 48 can entrain gaseous BOG 12 that is under the pressure of the tank volume 2. In the representation of FIG. 3 a very short BOG conduit 214 is depicted, which might however also be omitted as long as the Venturi nozzle 48 has a suitable inlet.

[0069] FIG. 4 shows a liquefied gas storage tank 1 in which the filling assembly 3 of FIG. 1 and the recirculation circuit 38 of FIG. 2 substantially are combined with each other, so that in the embodiment described here an integrated line assembly is formed with a recirculation circuit 338 and a filling assembly 303.

[0070] Other than in the embodiments according to FIGS. 1 and 2, the BOG-LNG mixture in the representation of FIG. 4 is pressed into the liquid LNG 10 at the end of the common BOG-LNG conduit below the LNG filling level 8. Due to this configuration, for example the spray nozzle 30 according to FIG. 1 can be saved when the additional liquefaction capacity realized thereby is not needed.

[0071] FIG. 5 shows a liquefied gas storage tank 1 to which a coupleable transport tank 50 (for example of an LNG carrier) and customer networks (LNG customer network 36 and BOG customer network 52) are connected or can be connected.

[0072] Apart from the deviations described below, the embodiment of the liquefied gas storage tank 1 as shown in FIG. 5 substantially represents the same as in FIG. 4, except that the connected external source with the transport tank 50 is shown explicitly with its BOG connecting line 51 (functionally at least in part corresponds to the BOG conduit 14 in FIG. 1) and its LNG connecting line 53 (functionally at least in part corresponds to the LNG conduit 16 in FIG. 1). The BOG connecting line 51 is connected to the BOG conduit 114 according to FIG. 5 by means of a BOG fluid interface 15, and the LNG connecting line 53 is connected to the LNG conduit 116 according to FIG. 5 by means of an LNG fluid interface 17.

[0073] In addition, the embodiment according to FIG. 5 differs from the one according to FIG. 4 by the possibility to feed BOG (both from the tank volume 2 and from the transport tank 50) into a BOG consumer network 52 via a non-illustrated valve and an unloading line 58. Other than in FIG. 4, the BOG-LNG mixture is sprayed in above the LNG filling level 8.

[0074] The embodiment shown in FIG. 6 only differs from the one according to FIG. 5 in that the BOG-LNG mixture is pressed in below the LNG filling level 8 like in FIG. 4.

[0075] FIG. 7 shows a liquefied gas storage tank 1 to which beside a coupleable transport tank 50 and customer networks 36 and 52 a firmly installed storage tank 60 also is connected.

[0076] The storage tank 60 substantially includes only interfaces to the liquefied gas storage tank 1 with respect to BOG and with respect to LNG. Below the LNG filling level (of both tanks) both an LNG pump 54 for conveying LNG from the liquefied gas storage tank 1 into the storage tank 60 and an LNG pump 56 for conveying LNG from the storage tank 60 into the liquefied gas storage tank 1 is provided. Above the LNG filling level both a BOG compressor 62 for conveying BOG from the liquefied gas storage tank 1 into the storage tank 60 and a BOG compressor 64 for conveying BOG from the storage tank 60 into the liquefied gas storage tank 1 is provided. The storage tank 60 hence is provided downstream of or subordinated to the liquefied gas storage tank 1 so to speak as a volume and/or cold capacity expansion.

[0077] The storage tank 60 for example can be used as a reservoir for strongly sub-cooled LNG by being filled with such sub-cooled LNG when it is just available in large quantitiesfor example as a result of an arriving shipload. When in a later operating phase for example a large amount of relatively warm BOG must be taken over from another transport tank 50 of an LNG carrier, the same can be recondensed for a long time in the liquefied gas storage tank 1, because cold LNG can be supplied from the storage tank 60.

[0078] Also independent of the necessity to take over warm BOG from a transport tank 50, the storage tank 60 can serve the longer-term stabilization of the sub-cooling of the LNG in the liquefied gas storage tank 1, in that warmer LNG from the liquefied gas storage tank 1 is pumped into the storage tank 60 and is mixed there with the relatively cold LNG, and at a later time or at the same time colder LNG is pumped from the storage tank into the liquefied gas storage tank 1.

[0079] The embodiments of liquefied gas storage tanks 1 in accordance with the invention as shown in FIGS. 5, 6 and 7 illustrate the typical case of a regas barge (which here accommodates the liquefied gas storage tank 1 in accordance with the invention), which serves as a hold-up tank for BOG 12 and/or LNG 10 after delivery by an LNG carrier (here with the transport tank 50) and before feeding into consumer networks 36 and/or 52. By using a liquefied gas storage tank 1 in accordance with the invention, the duration of the temporary storage in the regas barge, which is not influenced from outside, can be prolonged distinctly with a very simple construction.