WO2023140735A1

WO2023140735A1 - A liquefied gas system and a method for operating a liquefied gas system

Info

Publication number: WO2023140735A1
Application number: PCT/NO2022/050017
Authority: WO
Inventors: Rune Josef KNOTT
Original assignee: Wärtsilä Gas Solutions Norway AS
Priority date: 2022-01-21
Filing date: 2022-01-21
Publication date: 2023-07-27

Abstract

The disclosure relates to a liquefied gas system, comprising at least one liquefied gas cargo tank (10) for storing liquefied gas, wherein the system comprises a recondensing circuit for recondensing boil-off gas (BOG) which is boiled off in the cargo tank (10), wherein the liquefied gas system comprises at least one liquefied gas fuel tank (28) for storing and feeding liquefied gas fuel to at least one engine (34) via a fuel supply line (117), wherein the fuel supply line (117) is connected to a BOG recondenser (14) in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line (117). The disclosure further relates to a method for operating a liquefied gas system.

Description

A liquefied gas system and a method for operating a liquefied gas system

Technical field

The present disclosure relates to a liquefied gas system and a method for operating a liquefied gas system. More specifically, the disclosure relates to a liquefied gas system and a method for operating a liquefied gas system as defined in the introductory parts of claim 1 and claim 11.

Background art

A common technique for transporting natural gas from its extraction site is to liquefy the natural gas at or near this site, and transport the liquefied natural gas (LNG) to the market in specially designed storage tanks, also called cargo tanks, often placed aboard a sea-going vessel, often called an LNG carrier (LNGC).

LNG is a mixture of light hydrocarbons with methane as the main component and nitrogen as inert and small amounts of ethane, propane, butane and pentane may be present. Depending upon its exact composition, the boiling point of LNG is around -162 °C to -158 °C at atmospheric pressure, and is usually loaded, transported and offloaded at this temperature range. This requires special materials, insulation and handling equipment in order to deal with the low temperature and the boil-off vapour. Due to heat leakage, the cargo (LNG) surface is constantly boiling, generating vaporized natural gas, so-called boil-off gas (BOG) - primarily methane - from the LNG.

The standard practice for LNG carriers has been to use boil-off gas (BOG) as fuel and to supplement with liquid fuels such as heavy fuel oil, thus having dual fuel engines, or optionally supplement with vaporising part of the LNG cargo to top up the fuel need. LNG is recognized as a preferable alternative fuel; since it can substantially reduce harmful emissions to comply with stricter environmental regulations.

Systems for the continuous reliquefaction of BOG are well known. State-of-the-art propulsion systems are efficient and not all the BOG can be utilized in the engine. Furthermore, slow speed sailing and ship holding operations often result in an excess of BOG vs ship total fuel demand. Instead of, or in addition to, burning surplus BOG in a gas combustion unit (GSU), it can be reliquefied and returned to the cargo tanks. Reliquefaction of BOG on LNG carriers results in increased cargo deliveries and allows owners and operators to choose the optimal propulsion system and operating profile. The advantages are for example a flexible fuel system, optimized operating costs, and increased delivered cargo capacity.

Typically, reliquefaction systems are used to control the cargo tank pressure by liquefying BOG. They have the capability to handle all BOG (100% capacity) or only excessive BOG not burned in the engines (partly liquefaction).

The BOG can vary in composition, flow, temperature and pressure. These fluctuations require a system that can handle the process conditions fed to the system.

A problem with the solutions of the prior art is that it is not possible to handle the BOG reliquefaction in an optimal manner when the LNG carriers are fuelled by LNG from dedicated LNG fuel tanks. There is thus a need for an improved system that will provide an optimised BOG reliquefaction capacity when the carriers transport liquefied gas such as LNG or similar cargo and are fuelled by LNG or similar fuels from fuel tanks that are separate from the cargo tanks.

It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above-mentioned problem.

According to a first aspect there is provided a liquefied gas system, comprising at least one liquefied gas cargo tank for storing liquefied gas, wherein the system comprises a recondensing circuit for recondensing boil-off gas which is boiled off in the cargo tank, wherein the liquefied gas system comprises at least one liquefied gas fuel tank for storing and feeding liquefied gas fuel to at least one engine via a fuel supply line, wherein the fuel supply line is connected to a BOG recondenser in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line.

According to some embodiments, the recondensing circuit comprises at least one BOG compressor with a BOG compressor cooler.

According to some embodiments, the recondensing circuit comprises a BOG preheater. According to some embodiments, the recondensing circuit comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid for preheating of BOG from the liquefied gas cargo tank.

According to some embodiments, the separate liquid circuit comprises a liquid pump, the BOG preheater, the BOG recondenser, and a liquid trim heater.

According to some embodiments, the fuel supply line is a separate flowline in the BOG recondenser.

According to some embodiments, the system further comprises a return line from the BOG recondenser for returning reliquefied BOG to the liquefied gas cargo tank.

According to some embodiments, a refrigeration cycle is connected to the BOG recondenser configured for removing heat from the BOG that is condensed in the BOG recondenser.

According to some embodiments, the liquefied gas in the liquefied gas cargo tank is liquefied natural gas or liquefied biogas or liquefied synthetic methane.

According to some embodiments, the liquefied gas in the liquefied gas fuel tank is selected from liquefied natural gas, liquefied biogas, liquefied synthetic methane, or any combination thereof.

According to a second aspect there is provided a method for operating a liquefied gas system, comprising at least one liquefied gas cargo tank for storing liquefied gas, wherein the method comprises recondensing boil-off gas which is boiled off in the cargo tank in a recondensing circuit, wherein the method comprises passing liquefied gas fuel from at least one liquefied gas fuel tank to at least one engine via a fuel supply line, the fuel supply line being connected to a BOG recondenser in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line.

According to some embodiments, the method comprises passing the BOG from the liquefied gas cargo tank through at least one BOG compressor with a BOG compressor cooler to the BOG recondenser.

According to some embodiments, the method comprises passing the BOG from the liquefied gas cargo tank through a BOG preheater. According to some embodiments, the method comprises preheating BOG from the liquefied gas cargo tank by a liquid heat transfer fluid circulating in a separate liquid circuit.

According to some embodiments, the method comprises pumping the HTF by a liquid pump into the BOG preheater, preheating BOG entering the BOG preheater from the liquefied gas cargo tank by the HTF before transferring the BOG to the BOG compressor, and transferring cooled HTF leaving the BOG preheater in the separate liquid circuit to a liquid trim heater.

According to some embodiments, the method comprises passing liquefied gas fuel from said at least one liquefied gas fuel tank to the BOG recondenser via the liquefied gas fuel line through a separate flowline in the BOG recondenser.

According to some embodiments, the method comprises returning reliquefied BOG via a return line from the BOG recondenser to the liquefied gas cargo tank.

According to some embodiments, the method comprises removing heat from the BOG condensed in the BOG recondenser in a refrigeration cycle connected to the BOG recondenser.

Effects and features of the second aspect are to a large extent analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.

The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure. Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

The term "BOG recondenser" means a heat exchanger, which could be any type of heat exchanger, as e.g., brazed plate/fin heat exchanger.

The term "BOG preheater" means a heat exchanger, which could be any type of heat exchanger, as e.g., shell & tube, plate/shel I or plate/plate heat exchanger.

The term "liquid trim heater" means a heat exchanger, which could be any type of heat exchanger as e.g., shell & tube, plate/she II or plate/plate heat exchanger.

The term "forcing vaporiser" means a heat exchanger that is used for vaporising LNG liquid to provide gas for supply to the engines, which could be any type of heat exchanger as e.g., shell & tube, p late/shel I or plate/plate heat exchanger.

Brief descriptions of the drawings

The above objects, as well as additional objects, features and advantages of the present disclosure will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings.

Figure 1 shows a detailed process flow diagram of a liquefied gas system according to an embodiment of the present disclosure.

Detailed

The present disclosure will now be described with reference to the accompanying drawings. The disclosure may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person.

The system and method according to the present invention provides an optimised boil-off gas reliquefaction capacity on a liquefied gas carrier where a recondensing circuit is used in conjunction with a fuel flow of liquefied gas that can be LNG, liquefied biogas, liquefied synthetic methane or any combination thereof that will be used as fuel in at least one engine to provide the required reliquefaction capacity. Both the fuel flow and recondensing circuit will each provide a portion of the reliquefaction capacity to achieve the required reliquefaction capacity.

The liquefied gas system and method according to the present invention can be in operation when the ship is running on LNG fuel, liquefied biogas fuel, liquefied synthetic methane fuel or any possible combination thereof from a dedicated fuel system, while the BOG is being handled by a recondensing circuit integrated with the fuel system both in a traditional manner or a segregated manner, also called a closed loop. When the system and method according to the invention is operated in a closed loop, the cargo system is segregated from the fuel supply system and the BOG is reliquefied by a recondensing circuit integrated with both the cargo system and the fuel system. When the system and method according to the invention is operating in a closed loop, no BOG is released to the engines while the ship is running on LNG, liquefied biogas, liquefied synthetic methane or any possible combination thereof. In the traditional manner, a portion of the BOG can be released to the engines, and the portion of the BOG not released to the engines is reliquefied and routed back to the cargo tanks.

The present invention increases the reliquefaction rate and reduces the external heat required to do the vaporizing of the outgoing liquefied gas fuel.

The first aspect of this disclosure shows a liquefied gas system, comprising at least one liquefied gas cargo tank 10 for storing liquefied gas, wherein the system comprises a recondensing circuit for recondensing boil-off gas BOG which is boiled off in the cargo tank 10, wherein the liquefied gas system comprises at least one liquefied gas fuel tank 28 for storing and feeding liquefied gas fuel to at least one engine 34 via a fuel supply line 117, wherein the fuel supply line 117 is connected to a BOG recondenser 14 in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line 117. The recondensing circuit can comprise at least one BOG compressor 12 with a BOG compressor cooler 13. The recondensing circuit can further comprise a BOG preheater 11. The recondensing circuit can further comprise a separate liquid circuit with circulation of a liquid phase heat transfer fluid HTF for preheating of BOG from the liquefied gas cargo tank 10. The separate liquid circuit can comprise a liquid pump 15, the BOG preheater 11, the BOG recondenser 14, and a liquid trim heater 16.

The fuel supply line 117 is a separate flowline in the BOG recondenser 14.

The system can further comprise a return line 112 from the BOG recondenser 14 for returning reliquefied BOG to the liquefied gas cargo tank 10.

A refrigeration cycle can be connected to the BOG recondenser 14 configured for removing heat from the BOG that is condensed in the BOG recondenser 14.

The liquefied gas in the liquefied gas cargo tank is liquefied natural gas LNG or liquefied biogas or liquefied synthetic methane.

The liquefied gas in the liquefied gas fuel tank is selected from liquefied natural gas LNG, liquefied biogas, liquefied synthetic methane, or any combination thereof.

There will not be any differences in the system when handling LNG, bio gas or synthetic methane. The only difference between these fuels is that LNG have larger variations in the methane concentration typically between 88 - 93 mole % and biogas is practically pure methane with some nitrogen (normally less than 1 mole % which is the same for LNG). Synthetic methane is practically pure methane.

Fig. 1 shows a detailed layout of a liquefied gas system according to the present invention. Figure 1 includes several optional components that may be present in addition to the mandatory components of the system according to the invention. Figure 1 shows a liquefied gas cargo tank 10 with BOG line 100 to a BOG preheater 11, line 102 to a BOG compressor 12 with a BOG compressor cooler 13, line 111 to a BOG recondenser 14, said components being part of a BOG recondensing circuit.

The recondensing circuit further comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the liquefied gas cargo tank 10 before the BOG entering the BOG compressor 12. The separate liquid circuit comprising a liquid pump 15 configured to pump the HTF, the BOG preheater 11, the BOG recondenser 14, a liquid trim heater 16 located downstream of the BOG recondenser 14 configured for heating the HTF, and optionally an expansion tank 17.

Figure 1 shows a BOG line 100 from the liquefied gas cargo tank 10 to the BOG preheater 11, a line 102 from the preheater 11 through the at least one BOG compressor 12 with at least one BOG compressor cooler 13. After compression, the BOG is brought to the BOG recondenser 14 via line 111 for reliquefaction, with a valve 33 in closed position. The BOG can also be passed to the at least one engine 34 with valve 33 open. From the BOG recondenser 14, BOG can be returned to the liquefied gas cargo tank 10 via return line 112 comprising a valve 24.

The HTF can bypass the BOG recondenser 14 via a first bypass connection bl. Line 101 goes from the liquid pump 15 to the preheater 11, and line 103 from the preheater 11. A valve 18 in line 103 is then in a closed position, a valve 19 in a line 104 is in an open position and a valve 20 in line 105 is in a closed position. Line 106 connects to the liquid trim heater 16 and line 107 from the liquid trim heater to the liquid pump 15, with an optional expansion tank 17. The separate liquid circuit can comprise a second bypass connection b2 via a line 108 and a valve 21 that will then be in an open position and a line 105 to the BOG recondenser 14. The separate liquid circuit can comprise a third bypass connection b3 for a part of the HTF to bypass the BOG preheater 11 in a line 109 with a valve 22 that will be in an open position for bypass. The HTF can also be recycled to the liquid pump 15 in line 110 with a valve 23 then in an open position.

In normal operation mode, HTF can be circulated through the separate liquid circuit from the liquid pump 15, through the BOG preheater 11, through the BOG recondenser 14, and through the liquid trim heater 16 without bypasses. The valve 18 in line 103 in the separate liquid circuit is in an open position, the valve 19 in line 104 is in a closed position and the valve 20 in line 105 is in an open position for circulation of the HTF through the BOG recondenser 14 after leaving the BOG preheater 11 in line 103 and before entering the liquid trim heater 16 via line 106. During normal operation mode, the separate liquid circuit can serve two functions; preheating the BOG in the BOG preheater 11 and additional heat removal in the BOG recondenser 14.

Valves 20 and 21 may be piston valves, which has an on/off function. Valves 18, 19, 22, 23 and 24 may be diaphragm valves, which have the possibility to control and regulate the amount of flow through the valves not only by on/off. Figure 1 further shows a liquefied gas fuel tank 28, with at least one pump 31 for pumping the liquefied gas, connected to the BOG recondenser 14 via fuel line 117 for passing liquefied gas fuel from the liquefied gas fuel tank 28 through a separate flowline inside the BOG recondenser 14 for cooling and reliquefying BOG from the liquefied gas cargo tank 10 flowing through the BOG recondenser 14, the BOG heating and vaporising the liquefied gas fuel flowing in the fuel line 117 to at least one engine 34. BOG can be returned to the cargo tank 10 via return line 112. The flowline 17 from the fuel tank 28 to the BOG recondenser 14 can comprise a valve, not shown in figure 1. The engine 34 can also be a fuel cell. In case the engine 34 is a fuel cell, hydrogen from the liquefied gas fuel is separated from carbon before use in the fuel cell.

Figure 1 further shows the fuel tank 28 connected to a forcing vaporiser or a fuel gas supply system (FGSS) 29 via line 114 and a valve 30. Line 116 out from the forcing vaporiser or FGSS 29 connects to line 117 to the at least one engine 34.

Figure 1 further shows a pump 32 in the cargo tank 10 for pumping liquefied gas from the cargo tank 10 through line 115 to the forcing vaporiser or FGSS 29 and via line 116 and line 117 to the at least one engine 34.

Figure 1 further shows the option of routing liquefied gas from line 115 to line 118 to the fuel tank 28.

With the present invention, it is possible to increase the BOG reliquefaction capacity on a liquefied gas carrier when the carrier is operating on liquefied gas fuel that can be LNG, liquefied biogas, liquefied synthetic methane or any combination thereof in place of BOG and is operating in closed loop, wherein closed loop means that no BOG is released to the engines of the carrier. Figure 1 shows a valve 33 on line 102 upstream the point where line 102 is connected to fuel line 117 from the BOG condenser 14 to the engine 34. Valve 33 will be closed when no BOG is to be released to the engine(s).

The increase in capacity is achieved by utilising outgoing liquefied gas from the at least one liquefied gas fuel tank 28 to cool and reliquefy BOG flow from the at least one liquefied gas cargo tank 10 and at the same time heat and vaporise the outgoing liquefied gas fuel from the at least one liquefied gas fuel tank 28, in the BOG recondenser 14. More specifically, said increase in capacity is achieved by implementing a separate passage/flowline in the BOG recondenser 14 in a recondensing circuit where the liquefied gas fuel gas stream from the liquefied gas fuel tank 28 is routed via the fuel line 117 from the liquefied gas fuel tank 28 through a separate passage in the recondenser 14. The liquefied gas fuel gas de-superheat the BOG from the liquefied gas cargo tank 10 that is flowing through the recondenser 14 via line 111 and sub-cool it while the BOG is condensed (reliquefied) in the recondenser 14. The outgoing liquefied gas from the liquefied gas fuel tank 28 heat and vaporise while sub-cooling and de-superheating the BOG. With the system and method according to the invention, it is possible to handle 100 % of the BOG flow in a closed loop system.

The system disclosed herein may comprise a liquid boil-off gas (LBOG) separator (not shown in figure 1) for collecting the reliquefied BOG from the BOG recondenser 14. The separator separates gas components that has not been liquefied and releases them from the separator to exit the system, while the reliquefied BOG is returned to the cargo tank 10.

The reliquefied BOG (LBOG) exits the recondenser 14 and is sent to the at least one cargo tank 10 via return line 112. Before being sent to the cargo tank, said BOG may be sent to the LBOG separator. The separator removes accumulated Nitrogen (N2). The N2 together with some entrained methane are sent to the carrier's gas combustion units (GCU) (not shown in figure 1) for combustion and release to atmosphere.

The LBOG will be discharged from the bottom of the LBOG separator and sent back to the at least one cargo tank 10 driven by the differential pressure between the LBOG separator and cargo tank(s) via an expansion valve.

The capacity control of the reliquefaction system according to the invention is done by varying the liquefied gas flow through the separate passage in the BOG recondenser 14 by by-passing more or less liquefied gas past the BOG recondenser 14. The liquefied gas that is not used in the BOG recondenser 14 is routed to the forcing vaporiser or a fuel gas supply system (FGSS) 29.

Depending on the ship speed, the capacity of the system for optimising/increasing BOG reliquefaction will vary because the outgoing liquefied gas fuel flow will vary accordingly. The liquefied gas system according to the invention should then handle the balance between the total BOG flow and the amount that is reliquefied in the system. Thus, at low speed, the system according to the invention will have low capacity and at high speed, it will have high capacity. The liquefied gas system according to the invention receives BOG from the BOG compressor 12 at high pressure ranging from 2 barg to 16 barg (200 - 1600 kPag) and temperature ranging from -50 °C to +50 °C subject the pressure. The recondenser 14 will de-superheat and reliquefy the BOG by utilising the outgoing liquefied gas from the liquefied gas fuel tank 28 at approximately -160 °C (subject to pressure and composition) where the liquefied gas fuel will be heated, vaporised and super-heated. Liquefied gas that is not used for the de-superheating and sub-cooling the BOG will be sent to the downstream forcing vaporiser or FGSS 29. The forcing vaporiser or FGSS conditions the vaporised liquefied gas correctly for consumption in the engines.

Barg is the unit commonly used for the measurement of gauge pressure. The SI unit is Pag. Gauge pressure is measured against the ambient pressure and is equal to absolute pressure minus atmospheric pressure.

The refrigeration cycle connected to the BOG recondenser 14 is configured for removing heat from the BOG that is condensed in the BOG recondenser 14.

The refrigeration cycle in the BOG recondenser 14 can be a nitrogen cycle. Then the refrigerant will be N₂. A reversed Brayton type of nitrogen cycle may be used for cooling the BOG recondenser. However, other refrigeration cycles might also be used within the scope of the invention. The overall process flow diagram in Fig. 1 shows the details of the basic reversed Brayton cycle with opposite N₂ flows in the BOG recondenser 14. The cycle is illustrated with one compressor stage with a compressor 25 and an aftercooler 26, but more than one compressor stage may be used. In the refrigeration cycle, N2 flows in a line 113 in cycle, passing through the compressor 25 and aftercooler 26, then flowing through the BOG recondenser 14, and after exiting the BOG recondenser 14 flowing through an expander 27 before flowing through the BOG recondenser 14 again from the opposite direction.

The heat transfer fluid in the separate liquid circuit in the system disclosed herein is a liquid phase heat transfer fluid suitable for heat transfer down to low temperatures. HTF can be a synthetic liquid-phase fluid. Examples of suitable heat transfer fluids are low- temperature synthetic heat transfer fluids. Examples of suitable heat transfer fluids are heat transfer fluids based on for example hydrocarbons or silicone. Example of a composition of a suitable heat transfer fluid is a mixture of methyl cyclohexane and trimethyl pentane, such as in the commercially available Therminol® VLT Heat Transfer Fluid produced by Eastman. Therminol VLT heat transfer fluid is a synthetic liquid-phase fluid for use in extremely low-temperature applications, such as in single-fluid heating and cooling systems between -115°C and +175°C. Examples of other suitable commercially available heat transfer fluids are Caltherm UBT (silicone) for uses between - 100 to +260°C produced by Caldic, Dynalene MW (hydrocarbons) for uses between -112 to +163°C produced by Dynalene, and Fragoltherm X-T9-A (silicone) for uses between -112 to +200°C produced by Fragol.

Compression of the BOG is done in at least one BOG compressor 12 with at least one BOG compressor cooler 13 with a cooling medium. There may be several BOG compressors and coolers arranged in series. Preferably, the compression of the BOG is done in multiple stages with inter- and after-cooling with a cooling medium. If a preheater 11 is used, the at least one BOG compressor 12 can be a non-cryogenic compressor type. Examples of suitable compressors are positive displacement compressors as for example screw compressors, or dynamic compressors as for example centrifugal compressors. If the pre-heater 11 is omitted in the system, the compressor must be cryogenic.

The heated-up cooling medium from the BOG compression leaving the BOG compressor cooler 13 can be re-used as a heating medium for the liquid trim heater 16. Optionally, separate supply to the BOG compressor cooler 13 and the liquid trim heater 16 is also possible (not shown in figure 1). The cooling medium is usually water, water-glycol mixture or similar. The system according to the present invention could be used without the liquid trim heater 16 if the BOG also could be heated with another heat exchanger downstream the BOG preheater 11 when the heat added to the HTF in the BOG recondenser 14 is not sufficient. However, the preferred solution will be the liquid trim heater 16.

The BOG leaving the liquefied gas cargo tank 10 is typically at a temperature of about -140 to about -110 °C when entering the BOG preheater 11 via line 100. The BOG leaves the BOG preheater 11 at a temperature of about -30 °C via line 102 and then enters the at least one BOG compressor 12 with at least one BOG compressor cooler 13. After compression, the BOG leaves the at least one BOG compressor 12 with at least one BOG compressor cooler 13 at a temperature of approximately + 40 °C, and is then brought to the BOG recondenser 14 via line 111 for reliquefaction. The BOG can also be passed to the at least one engine 34. If the BOG shall be passed to the engine(s), valve 33 is open. If all BOG shall be reliquefied, valve 33 is closed. The BOG is cooled and liquefied in the BOG Recondenser 14, entering at temperatures typically around +40 °, and returning to the liquefied gas cargo tank 10 via a return line 112 with a valve 24 in an open position at a temperature of at least about -163 °C to be liquid without pressurization.

The second aspect of this disclosure shows a method for operating a liquefied gas system, comprising at least one liquefied gas cargo tank 10 for storing liquefied gas, wherein the method comprises recondensing boil-off gas BOG which is boiled off in the cargo tank 10 in a recondensing circuit, wherein the method comprises passing liquefied gas fuel from at least one liquefied gas fuel tank 28 to at least one engine 34 via a fuel supply line 117, the fuel supply line 117 being connected to a BOG recondenser 14 in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line 117.

BOG from the liquefied gas cargo tank 10 can be passed through at least one BOG compressor 12 with a BOG compressor cooler 13 to the BOG recondenser 14. BOG from the liquefied gas cargo tank 10 can also be passed through a BOG preheater 11. BOG from the liquefied gas cargo tank 10 can be preheated by a liquid heat transfer fluid (HTF) circulating in a separate liquid circuit. The HTF can be pumped by a liquid pump 15 into the BOG preheater 11, preheating BOG entering the BOG preheater 11 from the liquefied gas cargo tank 10 by the HTF before transferring the BOG to the BOG compressor 12, and transferring cooled HTF leaving the BOG preheater 11 in the separate liquid circuit to a liquid trim heater 16.

Liquefied gas fuel from said at least one liquefied gas fuel tank 28 can be passed to the BOG recondenser 14 via the liquefied gas fuel line 117 through a separate flowline in the BOG recondenser 14.

Reliquefied BOG can be returned via a return line 112 from the BOG recondenser 14 to the liquefied gas cargo tank 10.

Heat from the BOG condensed in the BOG recondenser 14 can be removed in a refrigeration cycle connected to the BOG recondenser 14.

The liquefied gas in the liquefied gas cargo tank can be LNG or liquefied biogas or liquefied synthetic methane.

The liquefied gas in the liquefied gas fuel tank can be selected from LNG, liquefied biogas, liquefied synthetic methane, or any combination thereof. Liquefied gas fuel from the at least one liquefied gas fuel tank 28 can be passed to a forcing vaporiser or FGSS 29 via a line 114 and a valve 30 and from the forcing vaporiser or FGSS 29 via a line 116 to line 117 to the at least one engine 34, and/or liquefied gas fuel from the at least one liquefied gas fuel tank 28 can be passed to the BOG recondenser 14 via a liquefied gas fuel line 117 through a separate flowline in the BOG recondenser 14 where the liquefied gas fuel cools and reliquefies BOG from the liquefied gas cargo tank 10 flowing through the recondenser 14 and where the BOG heats and vaporises the liquefied gas fuel. Further, liquefied gas can be routed from line 115 to line 118 to the fuel tank 28.

The reliquefied BOG from the BOG recondenser 14 can be sent directly to the cargo tank 10 or be collected in a liquid boil-off gas LBOG separator (not shown in figure 1) before being sent to the cargo tank 10.

Liquefied gas from the liquefied gas cargo tank 10 can be passed through a line 115 entering the forcing vaporiser or FGSS 29 and leaving said forcing vaporiser or FGSS 29 through a line 116 to line 117 to the at least one engine 34.

Heat from the BOG that is condensed in the BOG recondenser 14 can be removed in the refrigeration cycle connected to the BOG recondenser 14 or via the vaporising of the outgoing liquefied gas fuel.

The person skilled in the art realizes that the present disclosure is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A liquefied gas system, comprising at least one liquefied gas cargo tank (10) for storing liquefied gas, wherein the system comprises a recondensing circuit for recondensing boil-off gas (BOG) which is boiled off in the cargo tank (10), c h a r a c t e r i z e d in that the liquefied gas system comprises at least one liquefied gas fuel tank (28) for storing and feeding liquefied gas fuel to at least one engine (34) via a fuel supply line (117), wherein the fuel supply line (117) is connected to a BOG recondenser (14) in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line (117).

2. The liquefied gas system according to claim 1, wherein the recondensing circuit comprises at least one BOG compressor (12) with a BOG compressor cooler (13).

3. The liquefied gas system according to claim 2, wherein the recondensing circuit comprises a BOG preheater (11).

4. The liquefied gas system according to claim 3, wherein the recondensing circuit comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the liquefied gas cargo tank (10).

5. The liquefied gas system according to claim 4, wherein the separate liquid circuit comprises a liquid pump (15), the BOG preheater (11), the BOG recondenser (14), and a liquid trim heater (16).

6. The liquefied gas system according to any one of the preceding claims, wherein the fuel supply line (117) is a separate flowline in the BOG recondenser (14).

7. The liquefied gas system according to any one of the preceding claims, wherein the system further comprises a return line (112) from the BOG recondenser (14) for returning reliquefied BOG to the liquefied gas cargo tank (10).

8. The liquefied gas system according to any one of the preceding claims, wherein a refrigeration cycle is connected to the BOG recondenser (14) configured for removing heat from the BOG that is condensed in the BOG recondenser (14).

9. The liquefied gas system according to any one of the preceding claims, wherein the liquefied gas in the liquefied gas cargo tank is liquefied natural gas (LNG) or liquefied biogas or liquefied synthetic methane.

10. The liquefied gas system according to any one of the preceding claims, wherein the liquefied gas in the liquefied gas fuel tank is selected from liquefied natural gas (LNG), liquefied biogas, liquefied synthetic methane, or any combination thereof.

11. A method for operating a liquefied gas system, comprising at least one liquefied gas cargo tank (10) for storing liquefied gas, wherein the method comprises recondensing boil-off gas (BOG) which is boiled off in the cargo tank (10) in a recondensing circuit, c h a r a c t e r i z e d in that the method comprises passing liquefied gas fuel from at least one liquefied gas fuel tank (28) to at least one engine (34) via a fuel supply line (117), the fuel supply line (117) being connected to a BOG recondenser (14) in the recondensing circuit for absorbing heat from the recondensing circuit to the fuel conveyed in the fuel supply line (117).

12. The method according to claim 11, comprising passing the BOG from the liquefied gas cargo tank (10) through at least one BOG compressor (12) with a BOG compressor cooler (13) to the BOG recondenser (14).

13. The method according to claim 12, comprising passing the BOG from the liquefied gas cargo tank (10) through a BOG preheater (11).

14. The method according to claim 13, comprising preheating BOG from the liquefied gas cargo tank (10) by a liquid heat transfer fluid (HTF) circulating in a separate liquid circuit.

15. The method according to claim 14, comprising pumping the HTF by a liquid pump (15) into the BOG preheater (11), preheating BOG entering the BOG preheater (11) from the liquefied gas cargo tank (10) by the HTF before transferring the BOG to the BOG compressor (12), and transferring cooled HTF leaving the BOG preheater (11) in the separate liquid circuit to a liquid trim heater (16).

16. The method according to any one of the claims 11 to 15, comprising passing liquefied gas fuel from said at least one liquefied gas fuel tank (28) to the BOG recondenser (14) via the liquefied gas fuel line (117) through a separate flowline in the BOG recondenser (14)

17. The method according to any one of the claims 11 to 16, comprising returning reliquefied BOG via a return line (112) from the BOG recondenser (14) to the liquefied gas cargo tank (10).

18. The method according to any one of the claims 11 to 17, removing heat from the BOG condensed in the BOG recondenser (14) in a refrigeration cycle connected to the BOG recondenser (14).

19. The method according to any one of the claims 11 to 18, wherein the liquefied gas in the liquefied gas cargo tank is liquefied natural gas (LNG) or liquefied biogas or liquefied synthetic methane.

20. The method according to any one of the claims 11 to 19, wherein the liquefied gas in the liquefied gas fuel tank is selected from liquefied natural gas (LNG), liquefied biogas, liquefied synthetic methane, or any combination thereof.