EP2715259A1

EP2715259A1 - Utilization of lng used for fuel to liquefy lpg boil off

Info

Publication number: EP2715259A1
Application number: EP12794038.5A
Authority: EP
Inventors: Carl Jørgen RUMMELHOFF
Original assignee: Wartsila 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: 2014-04-09
Also published as: JP2014522476A; EP2715259A4; KR20140092241A; CN103608632A; KR101904367B1; WO2012165967A1; JP5932985B2; CN103608632B

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

Utilization of LNG used for fuel to liquefy LPG boil off

The present invention relates to a system and method for utilizing LNG used for fuel to liquefy LPG boil off.

Current ships transporting LPG (Liquefied Petroleum Gas) are primarily using slow speed diesel engines for main propulsion and are using marine fuel oils of various types to fuel the engines. However, new environmental restrictions in what is now known as Emission Control Areas limits the release of sulphur, nitrogen oxides and particulate matters to atmosphere. In future, also strict global emission restrictions are expected. Several options to meet the new requirements have been proposed by the industry such as e.g. exhaust gas cleaning. Another option is to use clean and lean burning fuels as Methane. For large fuel consumption and high sailing range requirements that characterizes the shipping industry, Methane must be stored in its most efficient manner that is in liquid state known as LNG (Liquefied Natural Gas).

LNG fuel systems are generally well known in the art, one exemplary system is disclosed in Norwegian patent application no. 20093272 entitled "A LNG fuel tank system for at least one gas engine used for ship propulsion".

LPG carriers have normally up to four cargo tanks where the cargo tanks share common vapor atmosphere when carrying one type of cargo. The LPG carriers are constructed so that they can carry two different cargoes at the same time. That means that the piping system on deck is duplicated enabling complete segregation of the two cargo systems. An LPG carrier can for instance be loaded with propane in three tanks and butane in the fourth cargo tank. The three tanks loaded with propane have a communicating vapor space, but totally isolated from the butane vapor space. Each of the deck piping systems is connected to a number of reliquefaction units ensuring that the cargo tank pressure does not go above maximum allowable pressure. The reliquefaction units compress the boil off gas to sufficient pressure so that the gas can be condensed against seawater. The condensate is returned back to the cargo tanks. In this manner the cargo tank pressure is maintained low. For a VLGC (Very Large Gas Carrier, typically about 82.000 m³ cargo carrying capacity) fully loaded and sailing at maximum ambient temperature conditions, the fuel oil consumption (to generators) to liquefy the boil off gas is about 1900 kg / day. There are currently no solutions to avoid this fuel oil consumption. Further, due to the fact that LNG being approximately 50% less dens than marine diesel oil LNG will require twice the storage volume to hold the same amount of energy. It is important that an increased fuel storage volume is to the cost of the vessels cargo carrying capacity. Another issue related to transport of LPG in e.g. the North Sea basin where the voyage duration is short and consequently the harbor time is a significant contributor to the total round trip duration, thus reductions in the harbor time is of importance.

Another operational cost issue is service and maintenance of the refrigeration compressors which as any rotating equipment has service intervals based on running time. New built ships have normally a docking interval of five years whilst a refrigeration compressor has approximately 20000 hours service interval. Hence services are deemed required whilst sailing. Thus it would be advantageous to reduce the total running time and consequently stretching the time between services and preferably to have the service intervals to match with the docking intervals. It is therefore an object of the present invention to provide a system and a method for utilizing LNG used for fuel to liquefy LPG boil-off gas (BOG), which addresses at least one of the problems above.

To meet the problems above the present invention discloses 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.

The present invention also discloses a method of liquefying LPG boil off gas (BOG) in a system comprising a LNG fuel supply system, wherein the LNG fuel system comprises at least one LNG fuel tank 23, a first 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 method comprises: providing at least one vaporizer 22, 1 positioned on the first LNG fuel line 5 between the LNG fuel tank and the second LNG fuel line 13; condensing BOG by vaporizing LNG flowing from the LNG fuel tank; and routing condensed BOG to the LPG cargo tank. Typically, the number of vaporizers 22, 15 corresponds to the number of cargoes that can be transported at the same time. This is normally two. Other favorable embodiments of the present invention are to be understood by the dependent patent claims and the detailed description herein after, with reference to the amended figures, wherein:

Fig. 1 is a schematic overview of a reliquefaction unit of prior art;

Fig. 2 is a schematic overview of one embodiment of a system according to the present invention;

Fig. 3 is a schematic overview of another embodiment of a system according to the present invention;

LPG is to be understood as a range of different grades or products of petroleum gases stored and transported as liquid cargo. Among the various petroleum gases Propane and Butane being the principal examples; Propane typically including any concentration of Ethane from 0 % by volume up to 5 % by volume, and Butane content in Propane including any concentration from 0 % by volume up to 20 % by volume. This mixture consisting of mainly Propane, between typically 70 - 98 volume % is known as commercial Propane and hereinafter called Propane.

Butane can be any mixture of normal-Butane and iso-Butane with possible fractions of unsaturated hydrocarbons and hereinafter called Butane. In addition to Propane and Butane LPG should as a minimum include the following grades:

Ammonia,

Butadiene,

Butane - Propane mixture (any mixture),

Butylenes,

Diethyl ether,

Propylene,

Vinyl chloride. LPG stored and transported at temperatures below ambient will naturally continuously release a certain amount of vapor. The normal procedure of maintaining the pressure in the cargo tanks is to extract this vapor, liquefy and return it back to the cargo tanks as condensate. A reliquefaction unit is hereinafter to be understood as a refrigeration unit liquefying said vapor, and the prefix "re" points to liquefaction of vapor from liquefied gases.

LPG' s are transported in liquid form either at pressures greater than atmospheric or at temperatures below ambient, or a combination of both. This invention relates to LPG carriers transporting liquefied cargoes, LPG, at temperatures below ambient, known as fully refrigerated LPG carriers; and LPG carriers transporting liquefied cargoes, LPG, at pressures greater than atmospheric and temperatures below ambient. The latter is known as semi-refri gerated/semi -pressuri sed.

A cargo type is any of the LPG grades or products mentioned above.

Condensate shall be understood as liquefied boil off gas, where boil off gas is vapor emitting from the cargo due to a constant heat leakage into the cargo tanks.

Warm cargo is LPG loaded at a temperature above the LPG saturation temperature at a current cargo tank pressure.

Prior art

KR 20100102872 discusses a method to be used on complex ships or Floating

Production Storage and Offloading vessels that simultaneously receives a stream of LPG and LNG. The invention described in KR 20100102872 deals with how to utilize the low temperature of the LNG boil off gas prior to compression and liquefaction to condense the LPG boil off gas in order to reduce the complexity of the LPG

reliquefaction system.

KR 20100102872 uses the sensible heat of the LNG boil off gas in order to condense the LPG boil off gas opposed to this invention that are using the combination of latent heat and sensible heat of LNG to condense LPG boil off gas in combination with a LNG fuel gas system.

US 5860294 describes a method for condensation of gaseous hydrocarbons from a mixture of inert gas and gaseous hydrocarbons, particularly as found in storage tanks of an LPG / LEG ship (LEG = Liquefied Ethylene Gas). US 5860294 describes firstly a reliquefaction system identical to reliquefaction unit 100 in this invention but with a downstream second condenser system with purpose to recover gaseous hydrocarbons from a gas mixture that will not condense in the normal cargo condenser. In order for the second condenser system in US 5860294 to function, the main cargo condenser must be in operation and part condensation must occur in this exchanger.

In this invention the second condenser system (22) is located upstream the cargo reliquefaction system and is not dependent on operation of the main cargo condenser.

KR 20010077227 describes a method for reliquefaction of LNG boil off gas where the LNG boil off gas is liquefied against a portion of a LNG stream routed to a vaporizer prior to send out to e.g. a national natural gas distribution grid. Boil off gas condensate is mixed with the LNG export line. No condensate is returned back to the storage tank. Liquid LNG is here used to control the pressure in the LNG storage tank by heat exchange / absorption of boil off gas into LNG prior to vaporization of the combined LNG and condensate stream.

EP 1990272 is in principle similar to KR 20010077227 but the condensate is returned back to the storage tank and the vaporized product is sent to a gas engine as fuel.

US 3306660 describes a method and system for storage of multi component cryogenic fluids where the more volatile components naturally boiling off are captured, condensed and reintroduced back to the storage tank. Pumped out cryogenic liquid is used to condense the boil off gas.

WO 201 1062505 describes a process to recover boil off gas from LNG storage tanks where LNG is routed to a re-condensing tank where it absorbs LNG boil off gas.

US 2795937 describes a process and apparatus for storage and transportation of liquefied gases where one tank holds liquefied natural gas (LNG) and one tank holds a liquid of higher boiling temperature. The LNG is used as fuel onboard the vessel and vaporized and heated prior to entering the combustion engine by heat exchange in the tank liquid of the higher boiling temperature liquid. US 2795937 teaches that this heat exchange prevents any boil off from the liquid surface and thus no boil off

reliquefaction systems are described.

US 3864918 describes a method where boil off gas from a LNG cargo tank is captured and separated into two component flows. The first flow is compressed, cooled and liquefied. The second flow is used as fuel to drive the vessel.

US 2006/0053806 describes a system for supplying boil off gas as fuel on a marine LNG carrier and pressure control of the cargo tank. Figure 1 shows for reference a typical prior art reliquefaction unit 100 connected to at least one cargo tank(s) 20. Cargo loading lines are not shown. Boil off gas from the cargo tank 20 flows via a boil off gas line 1 to a cargo compressor 40, the cargo compressor typically being the first stage of a multistage compressor, in which the vapor is compressed to an intermediate pressure. Any amount of vapor not being handled by the reliquefaction unit shown in figure 1 flows via a boil off gas line 5 1 to parallel operating units, not shown. Boil off gas exiting the cargo compressor 40 via a line 46 enters an economizer 43 in which the boil off gas is brought close to its saturation temperature. The boil off gas then flows via a line 47 from the economizer 43 to a cargo compressor 41 , wherein the boil off gas is compressed to bubble point pressure corresponding to an obtainable temperature in a cargo condenser 42. The cargo compressor 41 is typically the second stage of a multistage compressor. Occasionally more than two compression stages are needed, and they will typically be in series with 41.

The compressed boil off gas then enters the cargo condenser 42 via a line 48 to be condensed against seawater or any cooling medium typically above seawater temperatures. Sea-water is by far the most common used heat sink for the cargo condenser 42 but a mixture of water and an anti freeze agent is also possible. Anti freeze agents can be any suitable glycol.

Warm condensate, the resulting condensed boil off gas from the cargo condenser 42, leaving the cargo condenser 42 flows via a line 49 to an economizer 43. A line 50 branches off from the line 49 in which a small portion flows via a level control valve 44 providing required inter stage cooling and sub cooling of the main portion of warm condensate. Normally a liquid receiver is installed on line 49, not shown. The remaining warm condensate to be returned to the cargo tank(s) 20 flows further from where 50 branches off via a coil 52 inside the economizer 43 and leaves the coil 52 at a sub cooled state. The now sub cooled condensate flows via line 3 back to the cargo tanks. A condensate production valve 45 regulates the amount of condensate flow back to the cargo tank(s) 20. Condensate from other parallel running reliquefaction units connects into line 3, not shown.

Fig. 2 illustrates two hitherto distinct, separate, systems, an LNG fuel gas supply system (over the stippled line) and a LPG cargo system. The LNG fuel gas supply system is provided with one or more connected LNG fuel tanks 23. The fuel gas supply system is further provided with a valve 27 controlling the amount of LNG to be vaporized in a pressure build-up heat exchanger 24 to ensure more or less constant vapor pressure in fuel tank 23, thus ensuring a sufficient fuel gas supply pressure in fuel line 5.

According to well known principles, LNG is supplied to a vaporizer 25, wherein the LNG is vaporized and the resulting vapor flows to a super heater 26, where the vapor is heated to preferred fuel gas temperature, before being transferred to the main engines via line 13. Also shown in fig. 2 is a vapor line 8 and valve 28, allowing routing of LNG boil-off to the super heater 26 if pressure reduction in the fuel tank 23 is required.

However, under most circumstances there is no flow in the vapor line 8.

An engines fuel gas injection pressure depends on whether it is a Otto or Diesel engine, Otto engines requires only moderate gas injection pressures of 4 - bars whilst diesel engines requires fuel gas injection pressures of 300 - 350 bars. This significant difference in pressure has limited influence on the invention apart from that the high pressure alternative requires a dedicated pump for pressure lift. The additional pump system is not shown since it is generally known for persons skilled in the art.

For diesel engines requiring higher fuel gas pressure, as described above, an additional LNG fuel pump connected to fuel line 5 is required in order to lift the LNG pressure to necessary pressure before entering the vaporizer 25. In this embodiment, the vapor line 8 is routed to safe location or alternatively to a utility consumer, under most circumstances there is no flow in the vapor line 8.

The LPG cargo system of fig. 2 , as disclosed above with reference to fig. 1 , is provided with a reliquefaction unit 100 and at least one LPG cargo tank 20, a BOG line 1 flowing boil of gas to the reliquefaction unit 100 and a condensate line 3 returning condensate via valve 45 back to the cargo tank 20.

Considering a normal voyage, a typical VLGC (Very Large Gas Carrier) at maximum ambient temperature conditions carrying Propane has a heat leakage of about 425 kW into the cargo tank system. The resulting LPG boil off rate averaging about 3300 - 3800 kg/hr. Further, a typical installed propulsion power on a VLGC is 14MW having a typical energy consumption of 7500 kJ/kWh at maximum continuous rating, the amount of LNG required at this condition is about 2120 kg/hr. Thus, the refrigerant potential of the LNG will for all practical terms eliminate the need for running the reliquefaction units during the laden voyage and particularly for ambient temperatures below maximum design and/or speeds at maximum continuous rating. In fact, at these conditions reliquefaction operations can be eliminated completely.

A system for utilizing LNG used for fuel to liquefy LPG boil off according to the present invention therefore includes 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, efficiently integrating the LNG fuel system into the LPG boil off gas system. A dedicated blower installed on the boil off gas line feeding directly to vaporizer 22 is an obvious choice, but to the cost of additional equipment when there is already a cargo compressor installed. Thus this solution is likely not to be preferable, and hence not shown. Alternatively free flow of vapor to the vaporizer 22 is also possible, however, for this combination a return pump should be considered.

According to the first embodiment of the present invention the least one vaporizer 22 is a BOG condenser adapted to condense BOG by vaporizing LNG flowing by pressure from fuel tank 23 via line 5. The condensate is routed back to cargo tank 20 via a line 2. Seen the other way around, the LNG is vaporized against the boil off gas, and dependent on amount of boil off and fuel consumption the LNG can be partly vaporized, fully vaporized or fully vaporized and super heated. The resulting product leaves 22 and enters the vaporizer 25 where any residual LNG is vaporized. The vapor from the vaporizer 25 flows to the super heater 26 where the vapor is heated to preferred fuel gas temperature. In the case the amount of boil off and the fuel consumption of LNG is such that the vapor leaving the at least one vaporizer 22 is superheated both the vaporizer 25 and the super heater could be omitted. In the case the amount of boil off and the fuel consumption of LNG is such that the vapor leaving the at least one vaporizer 22 is fully vaporized the vaporizer 25 could be omitted.

In the above described embodiments all LNG flowing from the fuel tank 23 is passed through the vaporizer 22. However, under some circumstances, e.g. not sufficient boil of gas or no boil off gas, it might be advantageous to control the flow of LNG through the vaporizer. According to one embodiment of the present invention two valves 29, 30 is provided in front of, and in parallel with vaporizer 22, respectively, so that valve 29 being open and valve 30 being closed when there are no BOG in the BOG line 1 , and vice versa. The valves 29, 30 adjust the amount of LNG flowing to the vaporizer 22, 15 to the amount of available BOG present in the BOG line 1 and current LNG

consumption. This embodiment ensures that the resulting product leaving the vaporizer 22, 15 is fully vaporized, and the vapour is transferred to the super heater 26 where the vapour is heated to a preferred fuel gas temperature. Regulation is by known principles and not shown. Any excess LNG not needed in the vaporizer 22, 15 is transferred directly to the vaporizer 25, where the excess LNG is vaporized before being transferred to the super heater 26. Valves 29, 30 can equally well be replaced by a three-way valve, not shown. The valve arrangement 29, 30 thus represent any alternative valve arrangement providing same functionality. The piping system around the cargo compressor 40, 41 in the reliquef action 100 unit is arranged such that it operates in a single stage compression modus. The arrangement of the cargo compressor 40, 41 in a single stage compression modus is obvious for the person skilled in the art and the proposed is one of a few possible solutions. In single stage compression modus, the volumetric capacity is high and normally only one compressor is required to be in operation and hence offering overall reduction in total compressor running time. Boil off gas from the LPG cargo tank 20 then flows in the line 1 directly to the cargo compressor 40, 41 where the boil off gas is moderately lifted in pressure. At a convenient location on either line 48 or 49, the boil off gas is branched off via line 61 and flows to the vaporizer 22 where the boil of gas is condensed and returned back to the cargo tank 20 via line 2. Valves 71 and 72 regulates where the branching of boil off gas shall be made.

In yet another embodiment of the present invention the at least one vaporizer 15 is a condensate sub cooler provided on the condensate line 3, between the reliquefaction unit 100 and the cargo tank 20, the condensate sub cooler 1 being adapted to sub cool LPG condensate by vaporizing LNG from the fuel tank 23. The condensate sub cooler could be used alone in embodiments as described above with reference to the BOG condenser 22.

In a preferred embodiment of the present invention both vaporizer 15 and vaporizer 22 is provided increasing operational flexibility and utilization of excessive refrigerant capacity in the LNG when available, as will be disclosed in further detail below. In this embodiment valves 16 and 17 is provided to enable bypass or pass-through of vaporizer 15.

According to an exemplary embodiment of the present invention boil off gas emitted from the LPG cargo tank 20 Rows in the line 1 directly to cargo compressor 40, 41 where the boil off gas is moderately lifted in pressure. The piping system around the cargo compressor 40, 41 in the reliquefaction 100 unit is arranged such that it operates in a single stage compression modus. Moderately compressed boil off gas from the cargo compressor 40, 41 flows via line 2 to the vaporizer 22 where the boil of gas is condensed and returned back to the cargo tank 20. In vaporizer 22 BOG is condensed by vaporizing LNG flowing by pressure from fuel tank 23 via line 5. The condensed BOG is routed back to cargo tank 20 via a line 2. Seen the other way around, the LNG is vaporized against the boil off gas, and dependent on amount of boil off present and current fuel consumption the LNG can be partly vaporized, fully vaporized or fully vaporized and super heated. The resulting product leaves the vaporizer 22 and enters the vaporizer 25 where any residual LNG is vaporized. The vapor from the vaporizer 25 is transferred via line 12 to the super heater 26 where the vapor is heated to a preferred fuel gas temperature. In the case the amount of boil off gas and the fuel consumption of LNG is such that the vapor leaving the at least one vaporizer 22 is superheated both the vaporizer 25 and the super heater could be omitted. In the case the amount of boil off and the fuel consumption of LNG is such that the vapor leaving the at least one vaporizer 22 is fully vaporized the vaporizer 25 could be omitted.

In the above described embodiments all LNG flowing from the fuel tank 23 is passed through the vaporizer 22. However, under some circumstances, e.g. not sufficient boil of gas or no boil off gas, it might be advantageous to control the flow of LNG through the vaporizer. According to one embodiment of the present invention two valves 29, 30 is provided in front of, and in parallel with vaporizer 22, respectively, so that valve 29 is opened and valve 30 is closed when there are no BOG in the BOG line 1 , and vice versa. The two valves 29, 30 can also adjusts the amount of LNG flowing to the vaporizer 22, 15 to the amount of available BOG present in the BOG line 1 and current LNG fuel consumption. The LNG flowing to the vaporizer 22, 15 is fully vaporized, and the vapor is transferred to the super heater 26 where the vapor is heated to a preferred fuel gas temperature. Any excess LNG not needed in the vaporizer 22, 1 is transferred directly to the vaporizer 25, where the excess LNG is vaporized before being transferred to the super heater 26. The two valves 29, 30 can be replaced by a three way valve providing the same functionality.

The exemplary embodiments disclosed above refer mainly to the usage of the system during laden voyage when the LNG fuel consumption is high. During loading, however, the boil off rate is at its maximum, and the LNG fuel consumption at is low end, hence there is limited amount of LNG available for liquefying the BOG. Hence the reliquefaction unit 100 must be put into operation.

Under these conditions it would be naturally to consider that the reduced amount of LNG available during loading best was utilized in 22 as a pre-cooler to the liquefaction units. However this is not an optimum solution due to e.g. mist problems at inlet to the compressor, particularly when loading warmer cargoes.

Instead of pre-cooling the boil of gas, sub cooling the condensate reduces the flash gas generation in cargo tank 20 and hence increases the system performance by not having to recycle the flash gas back to the cargo compressor.

During loading all the boil off gas is routed via line 1 directly to the reliquefaction units where the boil off gas is condensed and returned via line 3. On line 3, the condensate sub cooler 15 is located using LNG as coolant. The condensate is reduced in temperature preferably not below -50°C before it is returned back to the cargo tanks. Condensate temperature depends on type of cargo and can be higher for the warmer types of cargo.

As previously mentioned, LPG carriers are constructed so that they can carry two different cargoes at the same time, the two cargoes can be propane and butane. The vaporizers 22, 15 will then operate in a series order so that firstly LNG is vaporized against propane in at least one vaporizer 22, then the vaporized LNG flows via line 74 to line 80 on the parallel unit operating on butane . The resulting warmer vaporized LNG returns to the fuel gas system via line 5. Figure 3 shows an arrangement on how this can be with one tank loaded with butane and three tanks loaded with propane. Typically four reliquefaction units 100 will be installed, but normally during voyage two will be in standby, and particularly when using LNG as fuel according to this invention. As can be seen from the diagram parallel operation of the vaporizers 22 is also possible. Figure 3 does not show the standby reliquefaction units.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive and is not intended to limit the invention to the disclosed embodiments. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used advantageously. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

C l a i m s

1. 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), and at least one reliquefaction unit ( 100) provided with a condensate line (3);

c h a r a c t e r i z e d i n t h a t 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),

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

at least one reliquefaction unit ( 100), where the cargo compressor (40, 41 ) being arranged in a single stage modus, is arranged in front of the BOG condenser (22), adapted to sufficiently pressurize BOG flowing from the cargo tank (20) to be conveyed back to the cargo tank (20) as condensate,

wherein the at least one vaporizer (15, 22) is in thermal exchange with the LPG cargo system.

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

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

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

5. Method of liquefying LPG boil off gas (BOG)in a system comprising a LNG fuel supply system, wherein the LNG fuel system comprises at least one LNG fuel tank (23), a first 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 ), and at least one reliquefaction unit (100) provided with a condensate line (3);

c h a r a c t e r i z e d i n t h a t the method comprises:

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

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

- arranging a cargo compressor (40, 41 ) of the at least one reliquefaction unit (100), the cargo compressor being arranged in a single stage modus, in front of the BOG condenser, wherein BOG flowing from the cargo tank (20) is sufficiently pressurized to be conveyed back to the cargo tank (20) as condensate, and.

- routing condensed BOG to the LPG cargo tank.

6. The method according to 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 pumping the condensed BOG to the LPG cargo tank (20).

7. The method according to claim 5, wherein the method further comprising:

- providing at least one vaporizer (25) downstream the at least one vaporizer (15, 22), wherein residual LNG received from the at least one vaporizer (15,22) is vaporized; and -providing at least one super heater (26) downstream the at least one vaporizer (25), wherein vapor received from the at least one vaporizer (25) is heated to a preferred fuel gas temperature.

8. The method according to claim 5, wherein the method further comprising:

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

- operating the vaporizers (22, 15) in series order so that firstly LNG is vaporized against the first cargo in at least one vaporizer (22) and then vaporized LNG flows via line (74) to line (80) on a parallel vaporizer (20) operating on the second cargo, and

- returning the resulting warmer vaporized LNG to the fuel gas system via line (5).

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