Cargo stripping features for dual-purpose cryogenic tanks on ships or floating storage units for LNG and liquid nitrogen

Abstract

An apparatus and method of storing and transporting, in a dual-use cryogenic storage tank, a cryogenic liquid having a liquefaction temperature. A first pump empties the tank of a first portion of the cryogenic liquid, thereby leaving a second portion of the cryogenic liquid in the cryogenic storage tank. A second portion of the cryogenic liquid is focused at a location on a bottom of the cryogenic storage tank. Using a second pump located at the location, the cryogenic storage tank is emptied of the second portion of the cryogenic liquid, whereby a residual portion of the cryogenic liquid is left therein. Using a focused heating structure, heat may be delivered to the location to raise the temperature of the residual portion above the liquefaction temperature, thereby vaporizing all of the residual portion.

Claims

1. A carrier for storing and transporting cryogenic liquids, comprising: a tank configured to store and transport a cryogenic liquid having a liquefaction temperature; a first pump configured to fill the tank with the cryogenic liquid and empty the tank of a first portion of the cryogenic liquid, thereby leaving a second portion of the cryogenic liquid in the tank; a tank structure that focuses the second portion of the cryogenic liquid at a location on a bottom of the tank; a second pump located at the location and configured to empty the tank of the second portion of the cryogenic liquid except for a residual portion of the cryogenic liquid that is left therein; and a focused heating structure configured to deliver heat to the location on the bottom of the tank; wherein the heat is configured to raise a temperature of the residual portion above the liquefaction temperature, thereby vaporizing all of the residual portion.

2. The carrier of claim 1, wherein the tank structure comprises baffles surrounding the second pump, the baffles being attached to the bottom of the tank.

3. The carrier of claim 2, further comprising a baffle top that encloses the second pump within the baffles, the baffle top, and the bottom of the tank.

4. The carrier of claim 1, wherein the tank structure comprises a pump well at the bottom of the tank, the pump well comprising an indented portion of the bottom of the tank into which the second pump is situated.

5. The carrier of claim 4, further comprising a pump well top that covers the pump well and encloses the second pump in the pump well.

6. The carrier of claim 1, wherein the focused heating structure comprises a gas injection line having an outlet adjacent the second pump, the gas injection line being configured to introduce a gas at the location at the bottom of the tank, the gas having a temperature above the liquefaction temperature.

7. The carrier of claim 6, further comprising a first pump line connected to the first pump and configured to transport the cryogenic liquid in or out of the tank, wherein the gas injection line is disposed within the first pump line.

8. The carrier of claim 1, wherein the focused heating structure comprises a heating element disposed underneath the location on the bottom of the tank, wherein the heating element is configured to heat the residual portion of the cryogenic liquid above the liquefaction temperature.

9. A method of transporting liquefied cryogenic liquids in a carrier, comprising: in a dual-use cryogenic storage tank, storing and transporting a cryogenic liquid having a liquefaction temperature; using a first pump to empty the cryogenic storage tank of a first portion of the cryogenic liquid, thereby leaving a second portion of the cryogenic liquid in the cryogenic storage tank; focusing the second portion of the cryogenic liquid at a location on a bottom of the cryogenic storage tank; and using a second pump located at the location, emptying the cryogenic storage tank of the second portion of the cryogenic liquid except for, a residual portion of the cryogenic liquid that is left therein.

10. The method of claim 9, wherein the second portion of the cryogenic liquid is focused using baffles surrounding the second pump, the baffles being attached to the bottom of the cryogenic storage tank.

11. The method of claim 10, wherein a baffle top that encloses the second pump between the baffles, the baffle top, and the bottom of the cryogenic storage tank.

12. The method of claim 9, wherein the second portion of the cryogenic liquid is focused using a pump well at the bottom of the cryogenic storage tank, the pump well comprising an indented portion of the bottom of the cryogenic storage tank into which the second pump is situated.

13. The method of claim 12, wherein a pump well top covers the pump well and encloses the second pump in the pump well.

14. The method of claim 9, wherein the second portion of the cryogenic liquid is focused using a slanted tank bottom that slants downwardly from opposite sides of the cryogenic storage tank.

15. The method of claim 9, further comprising: delivering heat only to the location using a focused heating structure; and using the focused heating structure, raising a temperature of the residual portion above the liquefaction temperature, thereby vaporizing all of the residual portion.

16. The method of claim 15, wherein the focused heating structure comprises a gas injection line having an outlet adjacent the second pump, the method further comprising: using the gas injection line, introducing a gas at the location at the bottom of the cryogenic storage tank, the gas having a temperature above the liquefaction temperature.

17. The method of claim 16, further comprising: transporting the cryogenic liquid in or out of the cryogenic storage tank using a first pump line connected to the first pump; wherein the gas injection line is disposed within the first pump line.

18. The method of claim 15, wherein the focused heating structure comprises a heating element disposed underneath the location on the bottom of the cryogenic storage tank, the method further comprising: using the heating element, heating the residual portion of the cryogenic liquid above the liquefaction temperature.

19. The method of claim 15, wherein the cryogenic liquid is a first cryogenic liquid, the method further comprising: after the residual portion is vaporized, cooling the cryogenic storage tank to a temperature that is at or below a liquefaction temperature of a second cryogenic liquid, wherein a composition of the second cryogenic liquid is different from a composition of the first cryogenic liquid; and filling the cryogenic storage tank with the second cryogenic liquid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below.

(2) FIG. 1 is a simplified diagram of a method of LNG liquefaction and regasification according to known principles.

(3) FIG. 2 is a top plan view of a cargo ship capable of carrying LNG and liquid nitrogen (LIN) according to aspects of the disclosure.

(4) FIG. 3A is a cutaway perspective view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(5) FIG. 3B is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(6) FIG. 3C is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(7) FIG. 3D is a cutaway perspective view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(8) FIG. 3E is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(9) FIG. 3F is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(10) FIG. 3G is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(11) FIG. 3H is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(12) FIG. 3I is a cutaway view of a loading/discharge line shown in FIG. 3H.

(13) FIG. 3J is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(14) FIG. 3K is a cutaway side elevational view of a storage tank on the cargo ship of FIG. 2 according to aspects of the disclosure.

(15) FIG. 4A is a top plan view of a cargo ship capable of carrying LNG and liquid nitrogen (LIN) according to other aspects of the disclosure.

(16) FIG. 4B is a cutaway side elevational view of storage tanks on the cargo ship of FIG. 4A according to aspects of the disclosure.

(17) FIG. 5 is a method for transporting liquefied cryogenic liquids in a carrier according to aspects of the disclosure.

(18) It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure.

DETAILED DESCRIPTION

(19) To promote an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. For the sake clarity, some features not relevant to the present disclosure may not be shown in the drawings.

(20) At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

(21) As one of ordinary skill would appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name only. The figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. When referring to the figures described herein, the same reference numerals may be referenced in multiple figures for the sake of simplicity. In the following description and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus, should be interpreted to mean “including, but not limited to.”

(22) The articles “the,” “a” and “an” are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements.

(23) As used herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure.

(24) FIG. 2 is a top plan view of a cargo ship or carrier 200 having one or more dual-use storage tanks 202 according to disclosed aspects. The storage tanks are designed to carry both LNG and liquid nitrogen (LIN). Cryogenic loading/discharge lines 204 are used to fill and empty the storage tanks 202. Cryogenic loading/discharge lines are connected to piping (not shown) for loading and unloading LNG and LIN. A stripping line 206, smaller than the loading/discharge lines, is used to remove from the storage tanks the LNG or LIN that the loading/discharge lines are unable to remove. LNG extracted from storage tank 202 using stripping line 206 may be offloaded from the LNG cargo ship, or may be collected on a separate on-deck tank for use as fuel. LIN extracted from the storage tank using stripping line 206 may be stored and used as inert gas for purging the storage tank.

(25) FIGS. 3A-3K depict cross-sections of the storage tank 202, and details thereof, according to aspects of the disclosure. In each of the aspects disclosed in FIGS. 3A-3K, the storage tank and/or the drain systems have been designed to enable complete removal of cryogenic liquids, such as LNG or LIN. In FIG. 3A, raised corrugated and perforated baffles 310 are disposed at either side of the centerline of the storage tank 202. The baffles have a mild concave camber. A stripping pump 312 is situated between the baffles 310 and is connected to stripping line 206. Loading/discharge pumps 314 are disposed outside of baffles 310 and are connected to loading/discharge lines 204. Alternatively, stripping pump 312 and/or loading/discharge pumps 314 are disposed outside the storage tank 202. In an evacuation operation, the cargo ship naturally tilts so that liquid in the storage tank flows toward the loading/discharge pumps and the stripping pump. The loading/discharge pumps remove most of the cryogenic liquid from storage tank 202 through loading/discharge lines 204. Stripping pump 312 then evacuates the remnant cryogenic liquid through stripping line 206. Baffles 310 force the remnant cryogenic liquid to be concentrated within the baffles, where the stripping pump can readily access the remnant liquid. Using the baffles, more of the remnant liquid can be evacuated by the stripping pump than with known cryogenic tank designs that do not use baffles. Because less of the cryogenic remnant liquid remains in the storage tank, the storage tank may be heated for a shorter period of time, and the vaporization/cooling portion of the tank evacuation process may be significantly shortened.

(26) FIG. 3B shows tank 202 according to another aspect of the disclosure, in which a perforated top 318 is disposed on the baffles 310 to form a box-type structure around the stripping pump 312. The box-type structure further enhances performance of the stripping pump 312 by further concentrating the remnant cryogenic liquid around the stripping pump.

(27) Instead of using baffles to concentrate the remnant cryogenic liquid to be adjacent the stripping pump, the shape of the storage tank itself may be modified to produce a similar effect. FIG. 3C shows a storage tank 320 in which the tank bottom 322 is downwardly slanted toward the stripping pump 312. The stripping pump is located at the lowest portion 324 of the tank bottom. Remnant cryogenic liquid will naturally collect adjacent the stripping pump, thereby easing the process of removing the remnant cryogenic liquid. The downwardly slanted tank bottom 322 may prevent the loading/discharge pumps 314 from being situated as low in the storage tank as previously disclosed aspects, however. FIG. 3D shows tank 202 according to another aspect of the disclosure, in which a pump gutter or pump well 330 is made at the bottom of the storage tank. Stripping pump 312 is placed within the pump well 330 to remove as much remnant cryogenic liquid as possible. Loading/discharge pumps 314 are located as close to the bottom of the storage tank as possible to minimize the amount of remnant cryogenic liquid in the storage tank. FIG. 3E depicts a tank 202 with a variation of FIG. 3D in which a perforated top 332 is placed on the pump well 330 to create a box-like structure which, like top 318 in FIG. 3B, further concentrates the remnant cryogenic liquid around stripping pump 312.

(28) Aspects of the disclosure as described above concentrate remnant cryogenic liquid at a specific location—adjacent the stripping pump—on the floor of a cryogenic storage tank. Not only does this enable more of the remnant cryogenic liquid to be evacuated from the tank using the stripping pump, but the remnant liquid that cannot be evacuated by the stripping pump or the loading/discharge pumps is still concentrated adjacent the stripping pump. This liquid, termed herein the “residual liquid”, can only be removed through vaporization, but because of its localized concentration only a small portion of the storage tank needs to be heated to vaporize it. FIGS. 3F and 3G show tanks 202 constructed similar to FIGS. 3D and 3E, respectively, with the addition of an insulated warm gas injection line 340 having an outlet within the pump well 330 and adjacent the stripping pump 312. Warm gas, such as nitrogen, may be pumped into the pump well 330 at a temperature higher than the liquefaction temperature of the cryogenic liquid being evacuated from the storage tank, after operation of the stripping pump 312. Residual liquid, all of which is concentrated in the pump well 330, is vaporized and can then be removed from the storage tank 202. It can be seen that only the pump well 330, and the portion of the storage tank directly adjacent thereto, is warmed by the warm gas injected therein. Consequently, the temperature of the storage tank has not warmed appreciably, and therefore the time required to cool the storage tank for use with other cryogenic liquids, such as LIN, has been significantly reduced when compared to known storage tank designs. The aspects depicted in FIGS. 3F and 3G may also be implemented with the baffle structures disclosed in FIGS. 3A and 3B, or with the slanted floor disclosed in FIG. 3C.

(29) Other methods of localized storage tank heating may be implemented. FIGS. 3H and 3I show a tank 202 according to another aspect of the disclosure in which a warm gas injection line 350 is inserted into one or more of the loading/discharge lines 204 and run to an outlet 352 adjacent stripping pump 312. Warm gas, such as nitrogen or other gas, is pumped out of the outlet 352 only when liquids are not being evacuated from or discharged into the storage tank 202, and preferably only after stripping pump 312 has removed as much remnant cryogenic liquid as possible.

(30) FIGS. 3J and 3K show tanks 202 according to another aspect of the disclosure in which, instead of inserting a heating medium through the top of the storage tank, a heating system 360 is installed into or under the bottom floor 362 of the storage tank. Specifically, the heating system 360 may be localized and placed directly under the location where the residual liquid has been collected, which in FIGS. 3J and 3K comprise the pump well 330. Heating system 360 may comprise electrical heating elements or, alternatively, a series of pipes built into the bottom floor 362 through which a heating fluid may be directed. The heating fluid may comprise a gas, such as ambient air or nitrogen gas, or may comprise a liquid, such as water or glycol. The heating system provides sufficient heat to vaporize the residual liquid. By heating only the pump well 330 and possibly the adjacent portions of the storage tank, the temperature increase in the tank during the vaporization procedure is minimized. The aspects depicted in FIGS. 3J and 3K may also be implemented with the baffle structures disclosed in FIGS. 3A and 3B, or with the slanted floor disclosed in FIG. 3C.

(31) FIG. 4A is a top plan view of a cargo ship or carrier 400 having one or more storage tanks 402 according to another aspect of the disclosure. In comparison to storage tanks 202 previously described, storage tanks 402 have a significant longitudinal dimension parallel to the length of the cargo ship 400. The storage tanks are designed to carry both LNG and liquid nitrogen (LIN). Cryogenic loading/discharge lines 404 are used to fill and empty the storage tanks 402. Cryogenic loading/discharge lines are connected to piping (not shown) for loading and unloading LNG and LIN. A stripping line 406, smaller than the loading/discharge lines, is used to remove from the storage tanks the LNG or LIN that the loading/discharge lines are unable to remove. LNG extracted from storage tank 402 using stripping line 406 may be offloaded from the LNG cargo ship, or may be collected on a separate on-deck tank for use as fuel. LIN extracted from the storage tank using stripping line 406 may be stored and used as inert gas for purging the storage tank. As shown in FIG. 4B, opposing sides 422 of the bottom 422 of each tank slant toward a central portion 424 of the tank bottom. A stripping pump 412, connected to stripping line 406, is located adjacent the central portion 424. Loading/discharge pumps 414 are connected to the loading discharge lines 404. During an unloading process, loading/discharge pumps 414 evacuate most of the cryogenic liquid, and the stripping pump 412 evacuates the remnant liquid that the loading/discharge pumps cannot evacuate. In this or other aspects, the time when the loading/discharge pumps and the stripping pump are active may overlap. Disclosed methods may be used to heat and vaporize the residual liquid, i.e., liquid that the loading/discharge pumps cannot evacuate.

(32) FIG. 5 is a flowchart of a method 500 for transporting liquefied cryogenic liquids in a carrier according to disclosed aspects. At block 502, a cryogenic liquid is stored and transported in a dual-use cryogenic storage tank. At block 504, a first pump is used to empty the cryogenic storage tank of a first portion of the cryogenic liquid, thereby leaving a second portion of the cryogenic liquid in the cryogenic storage tank. At block 506, the second portion of the cryogenic liquid is focused at a location on a bottom of the cryogenic storage tank. At block 508 a second pump, located at the location, empties the cryogenic storage tank of the second portion of the cryogenic liquid, whereby a residual portion of the cryogenic liquid is left therein. At block 510 a focused heating structure delivers heat only to the location, and not to other parts of the cryogenic storage tank. The delivered heat raises the temperature of the residual portion above the liquefaction temperature of the cryogenic liquid, so that at block 512 all of the residual portion is vaporized.

(33) The steps depicted in FIG. 5 are provided for illustrative purposes only and a particular step may not be required to perform the disclosed methodology. Moreover, FIG. 5 may not illustrate all the steps that may be performed. The claims, and only the claims, define the disclosed system and methodology.

(34) The aspects described herein have several advantages over known technologies. As previously discussed, directing the remnant cryogenic liquid to the stripper pump using baffles, box-like structures, pump wells, or slanted tank bottoms results in more of the remnant liquid being evacuated using the stripper pump. Consequently, there is less residual liquid to be heated and vaporized, and the vaporization process takes less time than known technologies. Additionally, because the residual liquid is concentrated or focused in one place (i.e., between the baffles, within the pump wells, etc.), the means to heat and vaporize the residual liquid (warm gas injection lines, heating elements) may be focused at that place, instead of throughout the storage tank as is done with known storage tanks. The focused heating reduces the temperature of the entire storage tank after vaporization is complete, thereby reducing the time needed to cool the storage tank for the next load of cryogenic liquid. Combined, the disclosed methods of concentrating remnant liquid and the methods of focused heating substantially reduce the time required to prepare a storage tank emptied of, for example, LNG, to be filled with, for example, LIN. Such time reduction may be as much as 30%, or 40%, or 50%, or even 50% of the preparation time required by known technologies.

(35) It should be understood that the numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.