CN103608632A

CN103608632A - Utilization of LNG used for fuel to liquefy LPG boil off

Info

Publication number: CN103608632A
Application number: CN201280026200.3A
Authority: CN
Inventors: 卡尔·约根·鲁梅尔霍夫
Original assignee: Hamworthy Oil and Gas Systems AS
Current assignee: Wartsila Oil and Gas Systems AS
Priority date: 2011-05-30
Filing date: 2012-05-30
Publication date: 2014-02-26
Anticipated expiration: 2032-05-30
Also published as: CN103608632B; JP2014522476A; JP5932985B2; KR101904367B1; WO2012165967A1; KR20140092241A; EP2715259A4; EP2715259A1

Abstract

The present invention relates to a method and a system for liquefying LPG boil off gas (BOG), the system comprising a LNG fuel supply system, wherein the LNG fuel system comprises at least one LNG fuel tank 23, a LNG fuel line 5 and a second LNG fuel line 13; and a LPG cargo system, wherein the LPG cargo system comprises at least one LPG cargo tank 20, a BOG line 1, at least one reliquefaction unit 100 and a condensate line 3; wherein the system further comprises: at least one vaporizer 15, 22 provided on the LNG fuel line 5 between the LNG fuel tank 23 and the second LNG fuel line 13, wherein the at least one vaporizer 15, 22 is in thermal exchange with the LPG cargo system.

Description

Utilize the LNG that is used for fuel with liquefaction LPG boil-off gas

The present invention relates to for utilizing the system and method with liquefaction LPG boil-off gas (boil off) for the LNG of fuel.

The shipowner of existing transportation LPG (liquefied petroleum gas) will be used for promoting mainly into and come for engine feed fuels with polytype bunker fuel oil with slow speed diesel engine.Yet new environmental limitations (being now called emission control district) has limited the release to environment of sulphur, nitrogen oxide and particle.In the future, also expect strict global emission limit set.One's own profession had proposed to meet several selections of new demand, for example waste gas purification already.Another kind of selection is to use clean poor (1ean) combustion fuel, as methane.For being the large fuel consumption of shipping industry feature and high tradding limit demand, methane must store in its most effective mode (liquid condition that is called LNG (liquefied natural gas)).

LNG fuel system is generally known in this area, and title discloses a kind of example system in No. 20093272nd, the Norwegian patent applications of " A LNG fuel tank system at least one gas engine used for ship propulsion ".

LPG carrying ship has four product tank of as many as conventionally, and wherein said product tank is total common vapor atmosphere when the goods of one type of delivery.Structure LPG carrying ship makes them can deliver two kinds of different goods simultaneously.This means that the pipe-line system on deck is paired, can complete the isolation of two cargo systems.LPG carrying ship can for example be mounted with propane and be mounted with butane in the 4th product tank in three tanks.Three tanks that are mounted with propane have the vapor space communicating, but completely isolated with butane vapor space.Each deck pipe system is connected with a plurality of liquefaction unit again, has guaranteed that product tank pressure is no more than maximum allowable pressure.Liquefaction unit is compressed to enough pressure by boil-off gas again, so that gas can rely on seawater condensing.Condensate turns back to product tank.In this mode, make product tank pressure remain lower.For VLGC fully loaded under maximum environmental temperature condition and that navigate by water, (huge gas carrier, cargo capacity is generally about 82.000m ³), the fuel oil consumption (to generator) of liquefaction boil-off gas is about 1900kg/ days.The solution of now also not avoiding this fuel oil to consume.In addition, because LNG is than little approximately 50% the fact of the density of marine diesel oil, so LNG will need the reservoir volume of twice to maintain the energy of same amount.Importantly, the fuel storage volume of increase be take shipment ability as cost.Another problem relates to is for example transporting LPG in basin, the North Sea, and here hours underway is short, thereby the layover time is total round trip duration significant contribution person, and it is very important therefore reducing the layover time.

Another operation cost problem is the service and repair of refrigeration compressor, and it is because any slewing all has the service intervals based on running time.Newly-built ship has the interval that lies up of 5 years conventionally, and meanwhile, refrigeration compressor has the service intervals of approximately 20000 hours.Therefore, think and when navigation, need service.Therefore, advantageously reduce total run time, thereby extend the time between service and preferably there is the service intervals of mating with the interval that lies up.

Therefore, an object of the present invention is to provide for utilizing the system and method with liquefaction LPG boil-off gas (BOG) for the LNG of fuel its at least one in managing to overcome the above problems.

In order to deal with above problem, the invention discloses the system for LPG boil-off gas (BOG) that liquefy, described system comprises LNG fuel system and LPG cargo system, and wherein said LNG fuel system comprises at least one LNG fuel tank 23, LNG burning line 5 and the 2nd LNG burning line 13; Wherein said LPG cargo system comprises at least one LPG product tank 20, BOG pipeline 1, at least one liquefaction unit 100 and condensate line 3 again; Wherein said system also comprises: be provided at least one

vaporizer

15,22 on the LNG burning line 5 between LNG fuel tank 23 and the 2nd LNG burning line 13, wherein at least one

vaporizer

15,22 and LPG cargo system carry out heat exchange.

The invention also discloses the method for LPG boil-off gas (BOG) that liquefy in the system that comprises LNG fuel system and LPG cargo system, wherein said LNG fuel system comprises at least one LNG fuel tank 23, a LNG burning line 5 and the 2nd LNG burning line 13, and wherein said LPG cargo system comprises at least one LPG product tank 20, BOG pipeline 1, at least one liquefaction unit 100 and condensate line 3 again; Wherein said method comprises: at least one

vaporizer

22,15 on the LNG burning line being positioned between LNG fuel tank and the 2nd LNG burning line 13 is provided; By making stream from the LNG of LNG fuel tank vaporization and condensation BOG; And the BOG of condensation is transported to LPG product tank with fixed course.Conventionally, the number of

vaporizer

22,15 is corresponding to the number of freight simultaneously.These normally two.

With reference to accompanying drawing, by dependent patent claim and detailed description hereinafter, should understand other favourable embodiments of the present invention, wherein:

Fig. 1 is the general introduction figure of the liquefaction unit again of current techniques;

Fig. 2 is according to the general introduction figure of system of the present invention embodiment;

Fig. 3 is the general introduction figure according to another embodiment of system of the present invention;

LPG is interpreted as a series of different cultivars or the product as the oil gas of liquid goods storage and transportation.In multiple oil gas, propane and butane are main examples; Propane generally includes the ethane of any concentration of 0 volume % to 5 volume %; Butane content in propane is any concentration of 0 volume % to 20 volume %.This mixture being mainly comprised of propane (being generally 70 volume % to 98 volume %) is called commercial propane, and hereinafter referred to propane.

Butane can be any mixture of the possible fraction of normal butane and iso-butane and unsaturated hydrocarbons, and hereinafter referred to as butane.

Except propane and butane, LPG should minimumly comprise following kind:

Ammonia,

Butadiene,

Butane-propane mixture (any mixture),

Butylene,

Ether,

Propylene,

Vinyl chloride.

The LPG that stores at the temperature lower than environment and transport is a certain amount of steam of sustained release naturally.The normal method that maintains pressure in product tank is to extract this steam, and liquefaction also turns back to product tank by it with condensate.The liquefaction unit refrigeration unit of described steam that is interpreted as hereinafter liquefying again, prefix " again " indicates liquefaction from the steam of liquid gas.

LPG is being greater than under atmospheric pressure or at the temperature lower than environment or its combination is transported with liquid form.The present invention relates to the LPG carrying ship of conveying liquified goods LPG at the temperature lower than environment, be called full refrigeration-type LPG carrying ship; Be greater than the LPG carrying ship of conveying liquified goods LPG at atmospheric pressure and the temperature lower than environment.The latter is called half refrigeration-type/half adding pressure type.

Cargo type is any one in above-mentioned LPG kind or product.

Condensate is interpreted as the boil-off gas that liquefies, and wherein boil-off gas is because constant heat leakage is to product tank and the steam giving off from goods.

Warm goods is the LPG loading at the temperature higher than LPG saturation temperature under current product tank pressure.

current techniques

KR20100102872 disclose be used in the storage of complicated ship or Floating Production unload a ship on time receive the method for LPG and LNG stream.The invention that KR20100102872 describes has solved in compression and before liquefying, how to have utilized the low temperature of LNG boil-off gas to come condensation LPG boil-off gas to reduce the LPG complexity of liquefaction system again.

KR20100102872 contrary to the present invention used the sensible heat of LNG boil-off gas with condensation LPG boil-off gas, and described the present invention and LNG fuel gas system are used in combination the latent heat of LNG and the combination of sensible heat with condensation LPG boil-off gas.

US5860294 has described the method from the gaseous hydrocarbon of the mixture of inert gas and gaseous hydrocarbon (particularly as find at the holding vessel of LPG/LEG ship (LEG=liquefied ethylene gas)) for condensation.

First US5860294 has described the again liquefaction system identical with liquefaction unit again 100 in the present invention, but it has downstream the second condenser system, and object is to be recovered in the gaseous hydrocarbon in uncondensable admixture of gas in normal goods condenser.

For the second condenser in US5860294 is played a role, main goods condenser must move and must take part of the condensate in this interchanger.

In the present invention, the second condenser (22) is positioned at the goods upstream of liquefaction system again, and it does not rely on the operation of main goods condenser.

KR20010077227 has described the method for the LNG boil-off gas that liquefies again, and described LNG boil-off gas outputs to for example national Distribution of Natural network after being liquefied by means of the part LNG stream that is transported to vaporizer therein.With LNG export pipeline, steam gas condensate is mixed.Do not have condensate to turn back in holding vessel.

Liquid LNG is here for controlling the pressure of LNG holding vessel before the LNG making to combine and condensate flow vaporization to heat exchange/absorption of LNG by boil-off gas.

Similar with KR20010077227 in EP1990272 principle, but condensate turns back in holding vessel, and vaporized product is transferred into gas engine as fuel.

US3306660 has described for storing the method and system of multicomponent cryogen, has caught therein larger volatile component that natural evaporation is fallen, and makes its condensation and draws and get back in holding vessel again.The cryogenic liquid pumping out is for condensation evaporation gas.

WO2011062505 has described the technique of retrieving from the boil-off gas of LNG holding vessel, and LNG is transported to the drainer again that absorbs therein LNG boil-off gas therein.

US2795937 has described for storing the technology and equipment with conveying liquified gas, and a tank holds liquefied natural gas (LNG) therein, and a tank holds the liquid of higher.LNG is as the fuel on ship, and it was vaporized and was heated by heat exchange in the tank liquid at higher liquid before entering combustion engine.US2795937 has instructed this heat exchange to prevent any boil-off gas from liquid surface, does not therefore describe and evaporates liquefaction system again.

US3864918 has described and has caught therein from the boil-off gas of LNG product tank and be divided into the method for two component stream.First-class compressed, cooling and liquefaction.Second is as the fuel that drives ship.

US2006/0053806 has described for supply boil-off gas on LNG cargo ship peculiar to vessel and as fuel pressure, has controlled the system of product tank.

Fig. 1 shows the typical current techniques that is connected with at least one product tank 20, and liquefaction unit 100 is for your guidance again.Not shown Container Loading pipeline.Boil-off gas from product tank 20 flow to goods compressor 40 via boil-off gas pipeline 1, and this goods compressor is the first order of compound compressor normally, therein by steam compressed to middle pressure.The steam of any amount of not processing by the liquefaction unit again shown in Fig. 1 flow to operating unit in parallel via boil-off gas pipeline 51, not shown.

From goods compressor 40, via pipeline 46 boil-off gas out, enter and make therein the economizer (economizer) 43 of boil-off gas close to its saturation temperature.Then boil-off gas flow to goods compressor 41 via pipeline 47 from economizer 43, and wherein boil-off gas is compressed into corresponding to obtaining the bubble point pressure of temperature in goods condenser 42.Goods compressor 41 is the second level of compound compressor normally.Once in a while need to be more than the compression stage of two, and they are connected with 41 conventionally.

Then compressed boil-off gas enters goods condenser 42 to rely on seawater or usually above any cooling medium condensation of ocean temperature via pipeline 48.Seawater is up to now for the most frequently used heat dump of goods condenser 42 (heat snk), but the mixture of water and low-temperature receiver is also fine.Low-temperature receiver can be the glycol of any appropriate.

Warm condensate (the condensation evaporation gas from goods condenser 42 obtaining) leaves goods condenser 42, via pipeline 49, flow to economizer 43.Pipeline 50 is by pipeline 49 branches, and the fraction fluid level control valve 44 of flowing through wherein, for the major part of warm condensate provides the inter-stage needing cooling and excessively cold.Conventionally, liquid receiver is arranged on pipeline 49, not shown.The flow through coil pipe 52 of economizer 43 inner sides and leave coil pipe 52 with supercooled state of the place of waiting to turn back to the warm condensate You50 of the residue branch of one or more product tank 20.Excessively cold condensate flow back into product tank via pipeline 3 now.Condensate is produced the amount that valve 45 regulates the condensate flow that turns back to one or more product tank 20.Condensate from the liquefaction unit again of other parallel runnings is connected in pipeline 3, not shown.

Fig. 2 has illustrated two system LNG fuel gas supply systems different, that separate (on dotted line) and LPG cargo systems up to now.LNG fuel gas supply system disposes the LNG fuel tank 23 of one or more connection.Fuel gas supply system also disposes the valve 27 that controlled pressure increases the amount of LNG to be vaporized in heat exchanger 24, to guarantee in fuel tank 23 constant vapour pressure more or less, thereby guarantees fuel gas supply pressure enough in burning line 5.According to known principle, make LNG be supplied to vaporizer 25, wherein make LNG vaporization and gained steam flow to superheater 26, steam is heated to preferred fuel gas temperature therein, then via pipeline 13, is transferred to sustainer.Fig. 2 also illustrates vapor line 8 and valve 28, if need the pressure decreased in fuel tank 23, its permission is transported to superheater 26 by the LNG of evaporation.Yet, in most of the cases, in vapor line 8, there is no air-flow.

Engine fuel gas injection pressure depends on that it is Otto (Otto) engine or Diesel engine, and Otto engine only needs the medium gas injection pressure of 4 bar, and Diesel engine needs the fuel gas injection pressure of 300 to 350 bar.Need to the dedicated pump for boost in pressure except high pressure replacement scheme, this significant difference of pressure has limited impact to the present invention.Not shown other pumping system, because it is generally known for those skilled in the art.

For the Diesel engine of the higher fuel gas pressure of needs, as mentioned above, need the other LNG petrolift being connected with burning line 5 with before entering vaporizer 25 by LNG boost in pressure required pressure extremely.In this embodiment, vapor line 8 is sent to home or is alternatively sent to user, in most of the cases in vapor line 8, there is no air-flow.

As disclosed with reference to Fig. 1 above, the LPG cargo system of Fig. 2 disposes liquefaction unit 100 and at least one LPG product tank 20 again, makes boil-off gas flow to the BOG pipeline 1 of liquefaction unit 100 again and make condensate via valve 45, turn back to the condensate line 3 of product tank 20.

Consider normal navigation, the typical VLGC (huge gas carrier) under the maximum environmental temperature condition of delivery propane is about 425kW to the heat leakage in goods can system.The LPG evaporation rate average out to approximately 3300 of gained is to 3800kg/ hour.In addition, the propelling power supply that the upper typical case of VLGC installs is 14MW, and the typical energy consumption under maximum continuous rating is 7500kJ/kWh, and the LNG needing under this condition amount is about 2120kg/ hour.Therefore, for all aspects of putting into practice, the refrigeration potentiality of LNG have been eliminated the needs that load term of voyage moved again to liquefaction unit, in particular for the environment temperature lower than design maximum and/or the maximum needs that continue the speed under specified.In fact, under these conditions, can get rid of Operation for liquefaction more completely.

Therefore, according to of the present invention for utilizing the system with liquefaction LPG boil-off gas for the LNG of fuel, comprise at least one

vaporizer

15,22 on the LNG burning line 5 being provided between LNG fuel tank 23 and the 2nd LNG burning line 13, wherein said at least one

vaporizer

15,22 and LPG cargo system carry out heat exchange, effectively LNG fuel system are incorporated in LPG evaporated gas system.

The special-purpose air blast being arranged on the boil-off gas line that is directly supplied to vaporizer 22 is obvious selection, but when goods compressor is installed, take other equipment as cost.Therefore, this solution may not be preferred, so not shown.Or steam to flowing freely also of vaporizer 22 is fine, yet, for this combination, should consider loopback pump (return pump).

According to the first embodiment of the present invention, at least one vaporizer 22 is BOG condensers, and it is adapted to pass through and makes the LNG flowing out from fuel tank 23 via pipeline 5 by pressure vaporize condensation BOG.Condensate sends back product tank 20 via pipeline 2.See conversely, LNG relies on boil-off gas to be vaporized, and according to the amount of evaporation and fuel consumption, LNG can be by part vaporization, vaporization or vaporization overheated completely completely.Products therefrom leaves 22, enters the vaporizer 25 that makes any remaining LNG vaporization.Steam from vaporizer 25 flows to the superheater 26 that steam is heated to preferred fuel gas temperature.In the situation that the amount of LNG evaporation and fuel consumption makes to leave the steam of at least one vaporizer 22 is overheated, can omit vaporizer 25 and superheater the two.In the situation that the amount of LNG evaporation and fuel consumption is vaporized the steam that leaves at least one vaporizer 22 completely, can omit vaporizer 25.

In the above-described embodiment, all LNG that flow out from fuel tank 23 are by vaporizer 22.For example, yet under certain situation (, there is no enough gas evaporations or there is no boil-off gas), control may be favourable by the stream of the LNG of vaporizer.According to one embodiment of the invention, at vaporizer 22 above and with it, two

valves

29,30 are set in parallel, make when there is no BOG in BOG pipeline 1 respectively, valve 29 is opened and valve 30 cuts out, and vice versa.Valve 29,30 is adjusted to the available BOG of existence in BOG pipeline 1 and the amount that current LNG consumes by the amount that flow to the LNG of vaporizer 22,15.This embodiment has guaranteed that the products therefrom that leaves

vaporizer

22,15 is vaporized completely, makes vapor transfer to steam be heated to the superheater 26 of preferred fuel gas temperature.By known principle, regulate, not shown.Make unwanted any excessive LNG in

vaporizer

22,15 directly be transferred to vaporizer 25, here excessive LNG is transferred to superheater 26 after being vaporized.Valve 29,30 equally well Yong San road is logical alternative, not shown.Therefore, valve arranges that 29,30 representatives provide any alternative valve of same functionality to arrange.

Goods compressor

40,41 in the Unit 100 that liquefy again arranges that pipe-line system makes it in single stage compress mode, move around.With single stage compress mode arrange

goods compressor

40,41 be for those skilled in the art apparent and proposed be one of several possible solutions.In single stage compress mode, capacity is higher and conventionally only need to move a compressor, therefore makes total compressor totally reduce running time.Then, from the boil-off gas of LPG product tank 20, in pipeline 1, be flowed directly to

goods compressor

40,41, in described goods compressor, make the moderate pressure of boil-off gas promote.On

pipeline

48 or 49, facilitate position, boil-off gas is via pipeline 61 branches and flow to vaporizer 22, makes boil-off gas condensation and turn back to product tank 20 via pipeline 2 in described vaporizer.Valve 71 and 72 regulates the position that should carry out boil-off gas branch.

In yet another embodiment of the present invention, at least one vaporizer 15 is condensate subcooler (sub cooler), it is provided in the condensate line 3 between liquefaction unit 100 and product tank 20 again, and condensate subcooler 15 is adapted to pass through the LNG making from fuel tank 23 and vaporizes to make LPG condensate excessively cold.With reference to BOG condenser 22, condensate subcooler can be used alone in embodiment described above.

In a preferred embodiment of the invention, as disclosed more in detail below, provide vaporizer 15 and vaporizer 22 the two, increased operating flexibility and the utilization to refrigerant charge excessive in LNG when available.In this embodiment, provide

valve

16 and 17 can get around or by vaporizer 15.

According to an exemplary of the present invention, the boil-off gas of discharging from LPG product tank 20 is flowed directly to

goods compressor

40,41 in pipeline 1, makes the moderate pressure of boil-off gas promote in described goods

compressor.Goods compressor

40,41 in the Unit 100 that liquefy again arranges that pipe-line system makes it in single stage compress mode, move around.The boil-off gas through moderate compression from

goods compressor

40,41 flow to vaporizer 22 via pipeline 2, makes boil-off gas condensation and turn back to product tank 20 in described vaporizer.

In vaporizer 22, BOG is condensed from the LNG of fuel tank 23 vaporization via pipeline 5 stream by pressure by making.BOG through condensation is transmitted back to product tank 20 via pipeline 2.See conversely, LNG is vaporized by means of boil-off gas, and according to the amount of the evaporation existing and universal fuel consumption, LNG can be vaporized by part, vaporize completely or vaporization is also overheated completely.Products therefrom leaves vaporizer 22 and enters vaporizer 25, makes any remaining LNG vaporization in described vaporizer.Steam from vaporizer 25 is transferred to superheater (super heater) 26 via pipeline 12, makes steam heated to preferred fuel gas temperature in described superheater.In the situation that the amount of LNG evaporation and fuel consumption makes to leave the steam of at least one vaporizer 22 is overheated, can omit vaporizer 25 and superheater the two.In the situation that the amount of LNG boil-off gas and fuel consumption is vaporized the steam that leaves at least one vaporizer 22 completely, can omit vaporizer 25.

In the above-described embodiment, stream passes through vaporizer 22 from all LNG of fuel tank 23.For example, yet under certain situation (, there is no enough gas evaporations or there is no boil-off gas), control may be favourable by the stream of the LNG of vaporizer.According to one embodiment of the invention,, make when there is no BOG in BOG pipeline 1 valve 29 open and valve 30 cuts out above respectively at vaporizer 22 and provide in parallel two

valves

29,30 with it, vice versa.Two

valves

29,30 are also adjusted to the amount that flow to the LNG of

vaporizer

22,15 the available BOG of existence in BOG pipeline 1 and the amount that current LNG consumes.The LNG that flow to

vaporizer

22,15 is vaporized completely, and vapor transfer is to superheater 26, and in described superheater, steam is heated to preferred fuel gas temperature.Make unwanted any excessive LNG in

vaporizer

22,15 directly be transferred to vaporizer 25, in described vaporizer, excessive LNG is transferred to superheater 26 after being vaporized.Two

valves

29,30 can be with providing any three tunnel unifiers of same functionality to replace.

Disclosed exemplary is chiefly directed to the use of load term of voyage system when LNG fuel consumption is higher above.Yet at loading days, boil-off gas speed is its maximum, LNG fuel consumption is low side, and therefore, the amount of the LNG of the BOG that can be used for liquefying is limited.Therefore, liquefaction unit 100 puts into operation again.

Under these conditions, nature should consider as liquefaction unit forecooler 22 in utilize the LNG of the reduction that best loading days can use.Yet, due to for example smog problem of (particularly when loading more warm goods) at place, suction port of compressor, so this is not best solution.

Replace making boil-off gas precooling, make condensate cross the cold flash gas having reduced in product tank 20 and generate, therefore by making flash gas recirculation get back to goods compressor, increased systematic function.

At loading days, all boil-off gas are directly transported to via pipeline 1, make boil-off gas condensation and return to liquefaction unit again via pipeline 3 in described liquefaction unit again.Condensate subcooler 15 is positioned on line 3, uses LNG as cooling agent.After the temperature of condensate being reduced to be preferably not less than-50 ℃, make it turn back to product tank.Condensate temperature depends on the type of goods, for more warm cargo type its can be higher.

As previously mentioned, structure LPG cargo ship makes them can deliver two kinds of different goods simultaneously, and described two kinds of goods can be propane and butane.Then make

vaporizer

22,15 make first LNG at least one vaporizer 22, rely on propane vaporization with series sequence operation, then the LNG of vaporization flow to the pipeline 80 on the unit in parallel that moves butane via pipeline 74.The vaporization LNG that gained is more warm turns back to fuel gas system via pipeline 5.Fig. 3 shows such layout, based on it, can make canned butane and three the canned propane that are loaded with of being loaded with of system.Conventionally, four liquefaction unit 100 are again installed, but at term of voyage common two be standby, particularly when LNG used according to the invention is during as fuel.As can be seen from Figure, the parallel running of vaporizer 22 is also fine.The not shown standby liquefaction unit again of Fig. 3.

Although illustrate in detail and described the present invention in accompanying drawing and above description, such illustrating and describe should be thought illustrative or exemplary, rather than restrictive, and is not intended to limit the invention to disclosed embodiment.The fact that only has of some feature of quoting from inequality dependent claims does not show that the combination of these features can not advantageously be used.Reference numeral in claim should not be construed as restriction the present invention.

Claims

1. A system for liquefying LPG boil-off gas (BOG), said system comprising an LNG fuel supply system and an LPG cargo system, wherein said LNG fuel system comprises at least one LNG fuel tank (23), LNG fuel pipeline (5 ) and a second LNG fuel pipeline (13), wherein the LPG cargo system comprises at least one LPG cargo tank (20), a BOG pipeline (1) and at least one reliquefaction unit (100) configured with a condensate pipeline (3);

It is characterized in that the system also includes:

at least one vaporizer (15, 22) provided on said LNG fuel line (5) between said LNG fuel tank (23) and said second LNG fuel line (13),

wherein said at least one vaporizer is a BOG condenser (22) adapted to condense BOG by vaporizing LNG from said LNG fuel tank (23);

at least one reliquefaction unit (100), wherein a cargo compressor (40, 41) arranged in a single-stage manner is arranged before said BOG condenser (22), adapted to make the BOG flowing from said cargo tank (20) sufficiently pressurized to be transported back to the cargo tank (20) as condensate,

Wherein said at least one vaporizer (15, 22) is in heat exchange with said LPG cargo system.

2. A system according to claim 1, wherein a return pump is located after the BOG condenser, adapted to transport condensed BOG to the cargo tank (20).

3. The system according to claim 1, wherein said at least one vaporizer is a condensate subcooler (15) adapted to subcool LPG condensate by vaporizing LNG from said fuel tank (23), The condensate subcooler (15) is positioned on the condensate line (3) between the reliquefaction unit (100) and the cargo tank (20).

4. The system according to claim 1, wherein said system further comprises two valves (29, 29, 30) such that the amount of LNG flowing through said at least one vaporizer (15, 22) is adjusted to the amount of available BOG present in said BOG line (1) and current LNG fuel consumption.

5. A method of liquefying LPG boil-off gas (BOG) in a system comprising an LNG fuel supply system and an LPG cargo system, wherein the LNG fuel system comprises at least one LNG fuel tank (23), a first LNG fuel pipeline (5 ) and a second LNG fuel pipeline (13), wherein the LPG cargo system comprises at least one LPG cargo tank (20), a BOG pipeline (1) and at least one reliquefaction unit (100) configured with a condensate pipeline (3);

It is characterized in that the method comprises:

- providing at least one vaporizer (22, 15) positioned on said first LNG fuel line (5) between said LNG fuel tank and said second LNG fuel line (13);

- condensing BOG by vaporizing the LNG flowing from said LNG fuel tank;

- Arranging the cargo compressor (40, 41) of said at least one reliquefaction unit (100), said cargo compressor being arranged in a single stage before said BOG condenser, wherein the flow from said cargo tank (20 ) of BOG sufficiently pressurized to be transported back to the cargo tank (20) as condensate; and

- Delivery of condensed BOG to said LPG cargo tank.

6. The method of claim 5, wherein the LNG flow from the LNG fuel tank is (23) free flow, the method further comprising providing a return pump after the BOG condenser, the return pump The condensed BOG is pumped to the LPG cargo tank (20).

7. The method of claim 5, wherein the method further comprises:

- providing at least one vaporizer (25) downstream of said at least one vaporizer (15, 22), wherein residual LNG received from said at least one vaporizer (15, 22) is vaporized; and

- providing at least one superheater (26) downstream of said at least one vaporizer (25), wherein vapor received from said at least one vaporizer (25) is heated to a preferred fuel gas temperature.

8. The method of claim 5, wherein the method further comprises:

- providing at least two cargo tanks (20) for simultaneously carrying first and second different cargoes,

- operating said vaporizers (22, 15) in series order such that firstly LNG is vaporized in at least one vaporizer (22) by means of said first cargo and then vaporized LNG flows via line (74) to said second cargo The pipeline (80) on the parallel vaporizer (20) running on the above, and

- Returning the resulting warm vaporized LNG to the fuel gas system via line (5).

9. The method of claim 8, wherein the first cargo is propane and the second cargo is butane.