Method Of Cooling Boil Off Gas And An Apparatus Therefor

Abstract

A method of cooling a boil off gas stream from a liquefied cargo (50a) in a floating transportation vessel is described. The method comprises: compressing a boil off gas stream in two or more stages of compression to provide a compressed BOG discharge stream; cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream; cooling the first cooled compressed BOG stream against at least one second coolant stream to provide a second cooled compressed BOG stream; providing a vessel fuel stream from the liquefied cargo; using the vessel fuel stream as a coolant stream to cool either the compressed BOG discharge stream, or the first cooled compressed BOG stream, or both said streams.

Claims

1. A method of cooling a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said method comprising the steps of: compressing a BOG stream from said liquefied cargo in at least two stages of compression comprising a first stage and a final stage to provide a compressed BOG discharge stream, wherein said first stage of compression has a first stage discharge pressure and said final stage of compression has a final stage suction pressure, and at least one intermediate compressed BOG streams is provided between consecutive stages of compression; cooling the compressed BOG discharge stream against one at least one first coolant streams to provide a first cooled compressed BOG stream; cooling the first cooled compressed BOG stream against at least one second coolant stream to provide a second cooled compressed BOG stream; providing a vessel fuel stream from the liquefied cargo; using the vessel fuel stream as a coolant stream to cool one or both of the compressed BOG discharge stream and the first cooled compressed BOG stream.

2. The method according to claim 1 further comprising the step of cooling the second cooled compressed BOG stream against a third coolant stream to provide a third cooled compressed BOG stream.

3. The method according to claim 2 further comprising the steps of: expanding a portion of the third cooled compressed BOG stream to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream; using the first expanded cooled BOG stream as the third coolant stream to provide a first expanded heated BOG stream; and using the first expanded heated BOG stream as the at least one second coolant stream.

4. The method according to claim 2 wherein the first cooled compressed BOG stream is cooled against the second coolant stream.

5. The method according to claim 2 wherein the second cooled compressed BOG stream is cooled against the third coolant stream.

6. The method according to claim 2 wherein of the second coolant stream comprises the first expanded heated BOG stream.

7. The method according to claim 2 further comprising the steps of: providing a gaseous vent stream from the first cooled compressed BOG stream; expanding a portion of the third cooled compressed BOG stream to form a fourth coolant stream; and cooling the gaseous vent stream against the fourth coolant stream to provide a cooled vent stream and a heated fourth coolant stream.

8. The method of claim 7 further comprising the step of using the heated fourth coolant stream as a BOG recycle stream.

9. The method according to claim 7 comprising the further steps of: expanding the cooled vent stream to provide an expanded further cooled vent stream; and passing the expanded further cooled vent stream to a storage tank.

10. The method according to claim 7 comprising the further step of: separating the further cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream.

11. The method according to claim 10 comprising the further steps of: expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream; and passing the expanded cooled vent BOG return stream to a storage tank.

12. The method according to claim 6 wherein the fuel stream comprises one of both of using at least an expanded portion of the third cooled compressed BOG stream and at least an expanded portion of the cooled vent BOG return stream.

13. The method according to claim 7 further comprising the step of using at least an expanded portion of the gaseous vent stream, or at least an expanded portion of the vent discharge stream, or at least a portion of both said streams, as a vessel fuel stream.

14. The method according to claim 1 wherein the liquefied cargo is one of the group comprising: ethane, LPG, a liquefied petrochemical gas such as propylene and ethylene, and ammonia.

15. The method according to claim 1 comprising at least three stages of compression.

16. The method according to claim 1 comprising the step of using the vessel fuel stream as a coolant stream to cool both the compressed BOG discharge stream and the first cooled compressed BOG stream.

17. The method according claim 1 further comprising the step of using the vessel fuel stream as a coolant stream to cool the first cooled compressed BOG stream and then to cool the compressed BOG discharge stream.

18. The method according to claim 1 wherein the step of cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream comprises the steps of: pre-cooling the compressed BOG discharge stream against a pre-cooling coolant stream as a first coolant stream to provide a pre-cooled compressed BOG stream; and cooling the pre-cooled compressed BOG stream against a first refrigerant stream as a first coolant stream to provide the first cooled compressed BOG stream.

19. The method according to claim 16 wherein the pre-cooling coolant stream is one or more of the group selected from: seawater stream, an air stream, more particularly an ambient air stream and/or a refrigerant stream.

20. The method according to claim 18 wherein the first refrigerant stream is one or more of the group selected from propane and propylene.

21. The method according to claim 1 wherein stages of compression are the compression stages of a multi-stage compressor.

22. An apparatus to cool a boil off (BOG) gas stream from a liquefied cargo, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, in a floating transportation vessel comprising a plurality of components, said apparatus comprising at least: a compression system to compress a BOG stream from a liquefied cargo, said compression system comprising at least two stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein at least one intermediate compressed BOG streams is provided between consecutive stages of compression, one or more first heat exchangers to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; at least one second heat exchanger to further cool the first cooled compressed BOG stream against a mixed phase coolant stream to be separated in the one or more second heat exchangers, to provide a second cooled compressed BOG stream; at least one vessel fuel heat exchanger to cool one or both of the compressed BOG discharge stream and the first cooled compressed BOG stream, using a vessel fuel stream as a coolant stream.

23. The apparatus as claimed in claim 22 further comprising at least one third heat exchanger to further cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream.

24-25. (canceled)

26. A method of designing an apparatus for cooling a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel comprising a plurality of components, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, comprising the steps of: selecting a compression system to compress a BOG stream from a liquefied cargo, said compression system comprising at least two stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein at least one intermediate compressed BOG stream is provided between consecutive stages of compression, selecting one or more first heat exchangers to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; selecting one or more second heat exchangers to further cool the first cooled compressed BOG stream against a mixed phase coolant stream to be separated in the one or more second heat exchangers to provide a second cooled compressed BOG stream; selecting one or more third heat exchangers to further cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream; and selecting one or more vessel fuel heat exchangers to cool one or both of the compressed BOG discharge stream and the first cooled compressed BOG stream, or both said streams, using a vessel fuel stream as a coolant stream.

27-29. (canceled)

Description

[0125] Embodiments of the invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

[0126] FIG. 1 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, in a floating transportation vessel according to one embodiment of the invention;

[0127] FIG. 2 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied cargo, in a floating transportation vessel according to another embodiment of the invention; and

[0128] FIG. 3 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied, in a floating transportation vessel according to another embodiment of the invention.

[0129] Floating re-liquefaction systems draw the vapor, also known as boil off gas, from one or more storage tanks and pass the boil off gas to a compressor in which it is compressed such that the compressed vapor can be cooled and condensed against one or more coolants as the heat sink/refrigerant. For instance, seawater may be used to pre-cool, typically de-superheat, the compressed vapour in an open cycle pre-cooling circuit. The pre-cooled compressed vapour can then be further cooled against a refrigerant in a closed cycle refrigerant circuit.

[0130] Those lighter components of the compressed vapor which cannot be condensed against the refrigerant are usually vented to the atmosphere or recycled to the storage tanks in vapor form. Typically, the liquefied cargo is kept in the storage tank under one or both of reduced temperature (versus ambient) and increased pressure (versus atmospheric).

[0131] The methods and apparatus disclosed herein seek to provide improved methods and apparatus of re-liquefying BOG, and/or using the cargo as a vessel fuel, and/or using BOG as a vessel fuel.

[0132] An embodiment of the method and apparatus according to the present invention is disclosed in FIG. 1. FIG. 1 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to one embodiment of the invention. The cargo can be any one of the gases discussed herein: ethane is selected as a representative.

[0133] Liquefied ethane cargo is stored in a tank 50a which may be insulated and/or pressurized in order to maintain the ethane in a liquefied state. Vaporization of the ethane in the tank, for instance due to imperfect thermal insulation, will result in the formation of ethane gas in the overhead space of the tank 50a, and such gas is commonly termed boil off gas (BOG). In order to prevent the build-up of this gas, it is removed from the tank 50a as a boil off gas stream 01a. All the components are compressed, and as many of the components as possible of the removed boil off gas are normally cooled to condense them before it is returned to the tank 50a.

[0134] The boil off gas stream Ola can be passed to a compression system 60, such as the two stage compressor shown in FIG. 1 which comprises a first compression stage 65 and a second compression stage 75. The two-stage compression system 60 produces a compressed BOG discharge stream 06a which can be passed to a pre-cooling heat exchanger 100, in which the compressed BOG discharge stream 06a is cooled against a seawater stream 102. The pre-cooling heat exchanger 100 produces a pre-cooled compressed BOG stream 07a and a warmed seawater stream 104. The pre-cooling heat exchanger 100 can de-superheat the compressed BOG discharge stream 06a.

[0135] The pre-cooled compressed BOG stream 07a can be passed to a refrigerant heat exchanger 250, in which the pre-cooled compressed BOG stream 07a is cooled against a refrigerant stream 252. The refrigerant should be capable of condensing liquefied cargo at the discharge pressure of the compression system 60. The refrigerant may be propane or propylene. The refrigerant stream 252 can be part of a refrigerant circuit (not shown) comprising the refrigerant heat exchanger 250, a refrigerant compressor and a refrigerant cooler. The refrigerant circuit may be a closed refrigerant system. Such refrigerant circuits, also called refrigerant packs, are well known.

[0136] The refrigerant heat exchanger 250 produces a cooled compressed BOG stream 8a and a heated refrigerant stream 254. The cooled compressed BOG stream 8a is an at least partially condensed stream comprising those components of the boil off gas capable, at the discharge pressure of the second stage of compression 75, of ‘re-liquefaction’, i.e. condensation, against the refrigerant.

[0137] The ‘non-condensed’ components which are incapable of re-liquefaction against the refrigerant in this system, and which may comprise both non-condensable’ components and ‘in-condensable’ components as discussed herein, may be removed from the refrigerant heat exchanger 250, or an associated accumulator (not shown) located downstream of the refrigerant heat exchanger 250 as a vent stream 49, which is a vapor stream.

[0138] The cooled compressed BOG stream 8a can be passed to a further heat exchanger 80, to provide a cooled return fluid stream 18, which is typically a fully condensed stream.

[0139] The cooled return fluid stream 18 may then be passed to a return pressure reduction device 22, such as an expander or Joule-Thomson valve, to provide an expanded cooled return fluid stream 24. Typically, the return pressure reduction device 22 will reduce the pressure of the cooled return fluid stream 18 from at or near the pressure of the compressed BOG discharge stream 06a to a pressure close to that of the liquid cargo and BOG in the tank 50a, such as a pressure just above that of the BOG in the tank which is sufficient to ensure an adequate flow of the expanded cooled return fluid stream 24 to the tank 50a. The pressure of the expanded cooled return fluid stream 24 is below that of the discharge pressure of the first stage 65 of compression.

[0140] Returning to compression system 60, the first stage 65 of compression provides a first intermediate compressed BOG stream 02a, which is passed to further heat exchanger 80. The first intermediate compressed BOG stream 02a can be heat exchanged against an expanded portion 8b of the cooled compressed BOG stream 8a in the further heat exchanger 80 to provide a cooled first intermediate compressed BOG stream 03a, which can then be passed to the suction of the second stage 75 of compression. The second stage 75 compresses the cooled first intermediate compressed BOG stream 03a to provide the compressed BOG discharge stream 06a.

[0141] FIG. 1 also shows a vessel fuel stream 40 provided from the tank 50a, optionally using an internal low pressure pump 48 and an external high pressure pump 49.

[0142] The vessel fuel stream 40 can be provided to a first vessel fuel heat exchanger 42 via line 40a. The first vessel fuel heat exchanger 42 located in the path of the cooled compressed BOG stream 8a prior to the further heat exchanger 80. Optionally, the first vessel fuel heat exchanger 42 could be located after the further heat exchanger 80 to cool the stream 18 rather than the stream 8a.

[0143] The first warmer vessel fuel stream 40b could be provided to the second vessel fuel heat exchanger 44 to cool the compressed BOG discharge stream 07a such that the vessel fuel stream 40 passes serially through the first and second vessel fuel heat exchangers 42, 44.

[0144] The first and second vessel fuel heat exchangers 42, 44 provide a warmer vessel fuel gas stream 40c.

[0145] Optionally, the second vessel fuel heat exchanger 44 could be located before the pre-cooling heat exchanger 100 to cool the compressed BOG discharge stream 06a rather than the pre-cooled compressed BOG stream 07a.

[0146] Alternatively, the vessel fuel stream 40 could be provided to the line 40b (not shown) such that the vessel fuel stream 40 passes in parallel through the first and second vessel fuel heat exchangers 42, 44.

[0147] The warmer vessel fuel gas streams 40c can be combined where separate, and then passed towards one or more engines (not shown), generally one or more engines in one or more engine compartments, optionally in combination with one or more other-fuel engines that are powering the floating transportation vessel of the cargo tank 50a and on which the arrangement shown in FIG. 1 resides.

[0148] The cooling arrangement shown in FIG. 1 using a vessel fuel stream 40 to provide additional cooling to reduce the compressor power requirement of the compression system, as the vessel fuel stream 40 is cooler than the refrigerant stream 252 and/or the sea water stream 102. Reduction in the compressor power requirement increases the overall efficiency of the BOG recovery operation. The requirement for and external heat source for the vessel fuel stream is reduced or eliminated. For the system under consideration a power reduction of about 20% is achievable. Additionally or alternatively, for the system under consideration an increase in cooling of 5-10% is achievable.

[0149] A second embodiment of the method and apparatus according to the present invention is disclosed in FIG. 2. Where appropriate, identical stream and component names, and the same reference numerals as those in FIG. 1 have been used for corresponding streams and components in the remaining Figures.

[0150] FIG. 2 shows a liquefied cargo storage tank 50 in a floating transportation vessel, such as an LPG carrier. In order to cool, particularly re-liquefy, evaporated cargo from the storage tank 50, a boil off gas stream 01, comprising evaporated cargo, is passed to a compression system 60 having two or more stages of compression. The boil off gas stream 01 may have a pressure (the “BOG pressure”) in the range of from above 0 to 500 kPa gauge. The compression system 60 may be a multi-stage compressor comprising two or more stages. By “multi-stage compressor” it is meant that each compression stage in the compressor is driven by the same drive shaft. Alternatively, the compression system 60 may comprise independently driven compressors for each of the stages of compression. When the compression system 60 is a multi-stage compressor, it is typically a reciprocating compressor.

[0151] The embodiment of FIG. 2 shows a compression system 60 having a first stage 65 and a second stage 70 and a third and final stage 75, although the method and apparatus described herein is also applicable to compressors having two stages or more than three stages. The first stage 65 and final stage 75 of compression provide low and high pressure streams respectively at their discharge.

[0152] The compression system 60 compresses the boil off gas stream 01 to provide a compressed BOG discharge stream 06. The compressed BOG discharge stream 06 may have a pressure (the “final stage pressure”) in the range of from 1.5 to 3.2 MPa or above, eg. up to 6 MPa.

[0153] The compressed BOG discharge stream 06 is cooled in one or more first heat exchangers 200, 300 against one or more first coolant streams 202, 302 to provide first cooled compressed BOG stream 08. In the embodiment of FIG. 2, the compressed BOG discharge stream 06 can be passed to a pre-cooling heat exchanger 200 as one of the one or more first heat exchangers. The compressed BOG discharge stream 06 is pre-cooled against a pre-cooling coolant stream as one of the one of more first coolant streams. The pre-cooling coolant stream 202 may be an air or a water stream, such as an ambient air or seawater stream. The pre-cooling heat exchanger 200 may be a shell and tube heat exchanger or a plate heat exchanger. The pre-cooling heat exchanger may de-superheat the compressed BOG discharge stream 06. The pre-cooling heat exchanger 200 provides a pre-cooled compressed BOG stream 07 and heated pre-cooling coolant stream 204. Typically, the seawater used as the pre-cooling coolant would have a temperature of +36° C. or below, more typically +32° C. or below.

[0154] The pre-cooling heat exchange/exchanger 200 is optional in the method and apparatus disclosed herein. It is advantageous because it reduces the cooling duty of the subsequent cooling steps. However, is it not an essential aspect, such that in an alternative embodiment, the compressed BOG discharge stream 06 can be passed directly to the discharge heat exchanger 300 via line 06′, such that the equipment shown by numeral 210 may be omitted. In such circumstances, the cooling capacity of the discharge heat exchanger 300 would have to be increased to compensate for the absence of pre-cooling.

[0155] The pre-cooled compressed BOG stream 07 can then be passed to a discharge heat exchanger 300 as another of the one or more first heat exchangers. The discharge heat exchanger 300 cools the pre-cooled compressed BOG stream 07 against a first refrigerant stream 302 as another of the one or more first coolant streams. The discharge heat exchanger 300 provides a first cooled compressed BOG stream 08 and a heated first refrigerant stream 304.

[0156] The first refrigerant stream 302, discharge heat exchanger 300 and heated first refrigerant stream 304 may be part of a first refrigerant system (not shown). Such a first refrigerant system may further comprise a first refrigerant compressor to compress the heated first refrigerant stream 304 to provide a compressed first refrigerant stream, a first refrigerant cooler to cool the first refrigerant to provide a cooled compressed first refrigerant stream and a first refrigerant expansion device to expand the cooled compressed first refrigerant stream to provide the first refrigerant stream 302. The first refrigerant system may be a closed system. The first refrigerant may comprise one or more organic compounds, particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A, as well as one or more inorganic compounds such as ammonia.

[0157] The first cooled compressed BOG stream 08 may be a partially condensed, compressed BOG stream, comprising those components of the boil off gas which can be condensed against the first refrigerant at the discharge pressure of the final stage of compression. Any non-condensed components can be removed either from the discharge heat exchanger 300 as a vent stream (not shown) or from a discharge receiver (not shown) which functions as a gas/liquid separator located downstream of the discharge heat exchanger 300. Discharge heat exchangers suitable for the separation of gaseous and liquid components are shell and tube heat exchangers in which the cooled compressed BOG is located in the shell-side.

[0158] Any discharge receiver may be an accumulator and can operate to maintain a liquid seal in the discharge heat exchanger 300 and/or maintain the discharge pressure at the final stage 75 of compression.

[0159] The discharge heat exchanger 300 may be of a type which could not adequately separate vapor and condensed phases into separate streams, such as a plate and fin type heat exchanger. In such a situation, the discharge receiver will be located downstream of the discharge heat exchanger 300 to separate the non-condensed components as a vent stream.

[0160] The first cooled compressed BOG stream 08 is then second cooled. This can be achieved by passing the first cooled compressed BOG stream 08 to a second heat exchanger 180. The second heat exchanger 180 may be of any type, and an intermediate stage, particularly first stage, economizer for cooling the intermediate BOG streams 02 or 04 as well as the first cooled compressed stream 08 is shown in FIG. 2.

[0161] The cooling of the first cooled compressed BOG stream 08 is against a second coolant stream to provide a second cooled compressed BOG stream 34. Optionally, a portion of the first cooled compressed BOG stream 08 can be used elsewhere prior to passage into the second heat exchanger 180, but in the present embodiment, it is preferred that wholly or substantially all of the first cooled compressed BOG stream 08 passes into the first heat exchanger 180.

[0162] The action of the second coolant, described hereinafter, is to provide a second cooled compressed BOG stream 34. Again, a portion of this stream 34 could be used elsewhere, but preferably wholly or substantially all of the second cool compressed BOG stream 34 passes into a third heat exchanger 195 to further cool the second cooled compressed BOG stream 34 and to provide a third cooled compressed BOG stream 35.

[0163] The third heat exchanger 195 may be of any type, such as an economiser, but is preferably a countercurrent heat exchanger such as a plate and fin heat exchanger known in the art.

[0164] In the present embodiment of the invention, a portion of the third cooled compressed BOG stream 35 is expanded to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream 33a. This action can be carried out through a pressure reduction device 80 such as a Joule-Thomson valve or expander in a manner known in the art.

[0165] The first expanded cooled BOG stream 33a is used as the third coolant in the third heat exchanger 195, which heat exchange provides the third cooled compressed BOG stream 35, and a first expanded heated BOG stream 33b as heated third coolant stream 33b, which can either indirectly, or more preferably directly, be used as the second coolant stream 33b. The first expanded heated BOG stream/second coolant stream 33b is not separated (to separate gas/liquid phases) prior to use as the second coolant stream 33b, to fully utilise all of the remaining cooling effect of the first expanded heated BOG stream after use in the third heat exchanger 195.

[0166] The first expanded heated BOG stream/second coolant stream 33b is passed into the second heat exchanger 180, such that the heat exchange with the first cooled compressed BOG stream 08 provides the second cooled compressed BOG stream 34 and a heated second coolant in the second heat exchanger 180. The heated second coolant may comprise vapour and liquid components, which are conveniently separated in the second heat exchanger 180, and which is discussed hereinafter. The heated second coolant stream, which is a first expanded further heated BOG stream, may be passed to an intermediate compressed BOG stream of the appropriate pressure. In the embodiment of FIG. 2, the heated second coolant stream is combined with the first intermediate compressed BOG stream 02.

[0167] The portion of the third cooled compressed BOG stream 35 which is not used to provide the first expanded cooled BOG stream 33a can be returned as a return stream to the cargo tank 50 via a pressure reduction device 82 as expanded cooled BOG return stream 36 in a manner known in the art.

[0168] FIG. 2 also shows the provision of a vessel fuel stream 40 from the cargo tank 50, to provide a line 40a into a first vessel fuel heat exchanger 42 in the path of the first cooled compressed BOG stream 08 to cool said stream 08 after the discharge heat exchanger 300. The warmer vessel fuel gas stream 40d from the first vessel fuel heat exchanger 42 is provided into a second vessel fuel heat exchanger 44 in the path of the pre-cooled compressed BOG stream 07 to provide cooling to said stream 07 after the pre-cooling heat exchanger 200, (and optionally either before or after any direct stream 06′ not passing through the equipment 210). The warmer vessel fuel stream 40e can then be provided to one or more engines of the floating transportation vessel on which the arrangement shown in FIG. 2 resides in a manner described herein before. Optionally, vessel fuel heat exchanger 42 may alternatively be provided to cool stream 34.

[0169] FIG. 2 also shows optionally using a portion of the third cooled compressed BOG stream 35 to pass along line 35a, through a pressure reduction device 83, to provide an expanded third cooled compressed BOG stream 35b, which can alternatively or additionally be used as vessel fuel. That is, the volume of vessel fuel in line 40a could be partly or wholly provided by the expanded third cooled compressed BOG stream 35b, either for a limited duration or periods, or wholly or substantially over time.

[0170] Optionally, stream 33b is passed to the BOG stream 01. This will reduce the temperature of stream 35 to be more compatible with the requirements of the vessel fuel stream 40.

[0171] Thus, the present embodiment provides that the vessel fuel supplied to the first and/or second vessel fuel heat exchangers 42, 44 is provided either from the third cooled compressed BOG stream 35, or from the cargo tank 50, or from a combination of same, which combination can vary either over time, or over volume, or over both, depending on the fuel demand and/or efficiency desired in and from the overall method and apparatus shown in FIG. 2.

[0172] The skilled man can calculate possible division of the third cooled compressed BOG stream 35 between the line 35a and passage into the pressure reduction devices 80 and 82, which achieves the best overall power/efficiency balance for the floatation transportation vessel requirements. For the system under consideration a power reduction of about 20% is achievable. Additionally or alternatively, for the system under consideration an increase in cooling of about 50% is achievable.

[0173] It is also a particular feature of the present embodiment that no CAPEX change is required in the nature of the first heat exchangers 200, 300 and second heat exchanger 180, such that the operator can continue to use a ‘conventional’ shell and tube economiser as the second heat exchanger 180, and that the present embodiment can be achieved simply with the addition of the one or two vessel fuel heat exchangers 42, 44 and the optional third heat exchanger 195. This allows the overall BOG re-liquefying system to be controlled by existing level controllers in at least the second heat exchanger 180, avoiding potential issues with temperature control that might arise with the use of different BOG compositions and different inter stage pressures.

[0174] Indeed, an improvement of 10-15% in the refrigeration capacity of a BOG re-liquefying method and apparatus for a liquefied cargo is possible for ethane cargoes containing methane (in the liquid phase) above a de minimus level, and even above 0.4 or 0.5 mol % methane. Such methane-containing liquefied ethane cargos may be increasingly common where new or other sources of ethane are being provided, but the desire to purify the ethane (by reducing or eliminating any methane content) prior to transportation is not cost effective, or in some cases, not locally possible.

[0175] FIG. 3 shows a further embodiment of the method and apparatus of the present invention. In common with FIG. 2, FIG. 3 shows a liquefied cargo storage tank 50 from which a boil off gas stream 01, comprising evaporated cargo, is passed to a compression system 60, having three stages of compression being a first stage 65, a second and intermediate stage 70 and a third and final stage 75. The first stage 65 provides a first intermediate compressed BOG stream 02 which passes into the second heat exchanger 180 to provide a cooled first intermediate BOG stream 03 which passes into the intermediate compression stage 70, to provide a second intermediate compressed BOG stream 04 which passes into the suction of the final stage 75 of compression.

[0176] The compression system 60 provides a compressed BOG discharge stream 06 which can be passed into a pre-cooling heat exchanger 200 as one of the one or more first heat exchangers to be cooled against one first coolant being seawater in a seawater stream 202 in a manner previously described, to provide a pre-cooled compressed BOG stream 07.

[0177] The pre-cooled compressed BOG stream 07 can then be passed to a discharge heat exchanger 300 as another of the one or more first heat exchangers in a manner previously described. The discharge heat exchanger 300 provides a first cooled compressed BOG stream 08 and a heated first refrigerant stream 304.

[0178] The first cooled compressed BOG stream 08 can be provided either directly, or optionally after passage through a discharge receiver 305 as shown in FIG. 3.

[0179] Where the cooled compressed BOG stream 08 is not fully condensed, there is a gaseous vent stream 51 also provided, either from the discharge heat exchanger 300 as stream 51a, and/or from the discharge receiver 305 as stream 51b. Whilst FIG. 3 shows the two streams 51a, 51b as separate, such streams may be provided separately or combined or without any distinction, depending upon the nature and construction of the discharge heat exchanger 300 and the discharge receiver 305. The provision of these stream or streams is known in the art.

[0180] The gaseous vent stream 51 may comprise both ‘non-condensable’ components and ‘in-condensable’ components. The in-condensable components are generally considered to be components which cannot practically ever by compressed and condensed within the confines and operating parameters of a particular floating transportation vessel BOG cooling system, and primarily relate to nitrogen.

[0181] In WO2012/143699A, there is shown a method and apparatus for increasing the amount or proportion of condensing of the gaseous vent stream in order to increase the recovery thereof.

[0182] In the present embodiment, the method and apparatus may further comprise, as shown by way of example in FIG. 3, the steps of expanding a portion of the third cooled compressed BOG stream 35 to form a fourth coolant stream 33c, generally by passage of a portion of the third cooled compressed BOG stream 35 through a pressure reduction valve 87 in an amount which allows that portion of the third cooled compressed BOG stream 35 to act as a fourth coolant 33c in a fourth heat exchanger 197, such as a vent heat exchanger.

[0183] The fourth heat exchanger 197 may be of any type, but is preferably a counter-current heat exchanger such as a plate and fin arrangement. As shown in FIG. 3, the gaseous vent stream 51 can be cooled against the fourth coolant stream 33c to provide a cooled vent stream 53 and a heated fourth coolant stream 38.

[0184] Optionally, the heated fourth coolant stream 38 is a BOG recycle stream which can pass into the second heat exchanger 180 such that vapour therefrom can be used as part of the cooled first intermediate BOG stream 03.

[0185] The cooling of the gaseous vent stream 51 in the vent heat exchanger 197 can condense a portion of the components of the boil off gas which could not be condensed in the discharge heat exchanger 300 against the first refrigerant such as propane or propylene. The cooled vent stream 53 is typically an at least partly condensed stream.

[0186] In one possible embodiment shown in FIG. 3, the cooled vent stream 53 can be passed to a vent stream pressure reduction device 61 (dashed line), such as a Joule-Thomson valve or expander, where its pressure is reduced to provide an expanded further cooled vent stream 63 (dashed line). The expanded further cooled vent stream 63 may have a pressure at or slightly above the pressure of the liquefied cargo storage tank 50, so that it can be returned to the tank, for instance by addition to expanded cooled BOG return stream 36 to provide combined expanded cooled BOG return stream 11.

[0187] In another possible embodiment shown in FIG. 3, the cooled vent stream 53 can be passed to a vent stream separator 150, such as a gas/liquid separator. The vent stream separator 150 provides a vent discharge stream 55 being wholly or substantially the in-condensable components, which is typically a vapour stream, and a cooled vent BOG return stream 57, which is typically a condensed stream, comprising those components of the boil off gas which were condensed in the fourth heat exchanger 197. The pressure of the vent discharge stream 55 may be reduced, for instance to a pressure appropriate for return to the storage tank 50, for storage elsewhere or for venting.

[0188] The cooled vent BOG return stream 57 may be passed through a vent return stream pressure reduction device 58, such as a Joule-Thomson valve or expander, to provide an expanded cooled vent BOG return stream 59. The expanded cooled vent BOG return stream 59 can be passed to the storage tank 50, for instance by addition to the expanded cooled BOG return stream 36.

[0189] That portion of the third cooled compressed BOG stream 35 that is not passed to the pressure reduction devices 80 and 87 to provide the third and fourth coolant streams 33a, 33c, provides a BOG return stream 10, which may be expanded by a pressure reduction valve 82 to at or near the pressure of the storage tank 50 as expanded cooled BOG return stream 36. This can then be returned to the storage tank 50.

[0190] FIG. 3 shows a similar vessel fuel arrangement to FIG. 2, wherein a vessel fuel stream 40 is provided from the cargo tank 50 into a first vessel fuel heat exchanger 42 located in the path of the first-cooled compressed BOG stream 08 (and/or the stream 34 (not shown)), to provide a warmer vessel fuel stream 40c, which passes into a second vessel fuel heat exchanger 44 in the path of the pre-cooled compressed BOG stream 07 after the discharge heat exchanger 200. The second vessel fuel heat exchanger 44 provides cooling to the pre-cooled compressed BOG stream 07, to provide a warmer vessel fuel stream 40h in a manner described herein above.

[0191] FIG. 3 also shows one or more alternative or additional options for providing vessel fuel from the cooling arrangement in FIG. 3, which could be used as an alternative or in addition to the vessel fuel stream 40 to provide the vessel fuel stream 40h that is directed towards the engine or engines of the floating transportation vessel on which the arrangement shown in FIG. 3 resides.

[0192] FIG. 3 shows a first option comprising using a portion of the third cooled compressed BOG stream 35 taken along line 35a, to pass through a pressure reduction device to provide an expanded third cooled BOG stream 35b.

[0193] A second option comprises using a portion of the cooled vent BOG return stream 57 taken along line 57a, and through a pressure reduction device to provide an expanded cooled vent BOG return stream 57b.

[0194] A third option comprises using a portion of the vent stream 51b from the discharge receiver 305 taken along line 51c, through a pressure reduction device to provide an expanded vent stream 51c.

[0195] A fourth option comprises using a portion of, optionally all of, the vent discharge stream 55 from the vent stream separator 150 through a pressure reduction device to provide an expanded vent discharge stream 55b.

[0196] The arrangements shown in FIG. 3 allows each of the expanded streams 51c, 35b, 55b and 57b to be provided either through a combined line 46, or through separate lines (not shown), to provide at least some of the final vessel fuel stream 40h, optionally as an alternative to the vessel fuel stream 40, or in combination therewith, or in one or more varying proportional relationships therein between, depending upon the volume of vessel fuel required, and the volume of each of the streams 35, 51b, 55 and 57 as described hereinabove. The skilled man can calculate optimal flows at each junction of disposition point, and calculate optimal flow rates in order to provide the required final vessel fuel stream 40h. Variance of the use and/or proportions of the vessel fuel streams 40, 46 could be operable using gates or valves 88.

[0197] Optionally, streams 33b, 38 are passed to the boil off gas stream 01. This will reduce the temperature of stream 35 to be more compatible with the requirements of the vessel fuel stream 40.

[0198] The person skilled in the art will understand that the invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, the invention encompasses the combination of one or more of the optional or preferred features disclosed herein.