WO2021230751A1

WO2021230751A1 - A boil-off gas reliquefaction system, a method for reliquefaction of boil-off gas in a reliquefaction system and a method for operating a boil-off gas reliquefaction system

Info

Publication number: WO2021230751A1
Application number: PCT/NO2020/050124
Authority: WO
Inventors: Kjetil LEINUM; Jørn IVERSEN; Ingeborg DAHL
Original assignee: Wärtsilä Gas Solutions Norway AS
Priority date: 2020-05-14
Filing date: 2020-05-14
Publication date: 2021-11-18
Also published as: CN115885145A; EP4150273A1; KR20230040948A

Abstract

The disclosure relates to a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the LNG 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) configured for heat exchanging between the BOG and the HTF, the BOG recondenser (14) located downstream the BOG preheater (11), and a liq-uid trim heater (16) located downstream the BOG recondenser (14) configured for heat-ing the HTF. The disclosure further relates to a method for reliquefaction of boil-off gas (BOG) in a reliquefaction system and a method for operating a boil-off gas (BOG) reliquefaction system.

Description

A boil-off gas reliquefaction system, a method for reliquefaction of boil-off gas in a reliquefaction system and a method for operating a boil-off gas reliquefaction system Technical field

The present invention relates to a liquefied natural gas (LNG) boil-off gas (BOG) rel iquefaction system, a method for reliquefaction of BOG and a method for operating a BOG reliquefaction system. More specifically, the disclosure relates to a boil-off gas (BOG) reliquefaction system, a method for reliquefaction of boil-off gas (BOG) in a rel- iquefaction system and a method for operating a boil-off gas (BOG) reliquefaction sys tem as defined in the introductory parts of claim 1, claim 15 and claim 28.

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 LNG to the market in specially de signed storage tanks, often placed aboard a sea-going vessel.

LNG is a mixture of light hydrocarbons with methane as the main component and nitro gen 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 - 161 °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.

Plants for the continuous reliquefaction of this BOG are well known. The installation of a BOG reliquefaction system on liquefied natural gas (LNG) carriers with dual fuel en gines allows ship operators the added flexibility to switch between fuels to take ad- vantage of price differentials between LNG and heavy fuel oil. State-of-the-art propul sion systems are efficient and not all the BOG can be utilized in the engine. Further more, slow speed sailing and ship holding operations often result in an excess of BOG. Instead of burning this gas in a gas combustion unit, it can be reliquefied and returned to the cargo tanks.

The reliquefaction of BOG on LNG carriers results in increased cargo deliveries and al- lows owners and operators to choose the optimal propulsion system and operating pro file. 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 liquefy ing BOG. They have the capability to handle all BOG (100% capacity) or only exces sive 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.

Existing BOG reliquefaction systems are based on reversed nitrogen Brayton cycle re frigeration technology. This means that there is a closed nitrogen cycle in the system for extracting heat from the BOG. Nitrogen gas (N2) is used as refrigerant, with the purpose to control tank pressure by cooling down and reliquefying pressurized BOG. The rel- iquefied BOG is then returned back into the tank.

WO 2009/136793 A1 discloses a gas supply system for dual-fuel or gas engines inte grated with a BOG reliquefaction plant. The two systems for BOG reliquefaction and gas supply are “stand alone” assemblies. Cold duty is removed from LNG by an exter nal heating source and is not utilized.

WO 2011/078689 A1 discloses a gas supply system for dual-fuel or gas engines inte grated with a BOG reliquefaction where available cold duty in the LNG is utilized to cool down and condensate BOG in a device in which BOG or condensate thereof and LNG are heat exchanging with each other. LNG is heated by using available “warm” duty as cooling water from the compressors in the reliquefaction system.

The BOG leaves the LNG tank at temperatures typically between -140 and -110 °C and is preheated before entering a BOG compressor. The temperature at the suction of the compressor is of importance when it comes to the choice of compressor. A higher tem perature on the compressor inlet can be achieved by preheating the BOG. Present sys tems with preheating of the BOG upstream the BOG compressor have incorporated the refrigerant (N2) as a preheating medium. This involves a simultaneous removal of heat from the nitrogen gas. N2 heating is not always available or sufficient, for example if reliquefaction is not running and/or at high fuel gas consumption. Then an additional heat source might be necessary and this is normally done with additional BOG preheat ers in parallel.

Existing BOG reliquefaction systems are not providing an efficient preheating of the BOG and there is a need for a more efficient and simple BOG reliquefaction system. A more efficient and simple preheating of the BOG in the BOG reliquefaction system would provide a simpler system and thus reduce equipment costs. A more efficient pre heating of the BOG would for example make it possible to use a cheaper type of com pressor than commonly used compressor types for cryogenic gas in such installations Further, the overall efficiency of the system would improve compared to prior art solu tions.

Summary of the invention

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.

According to a first aspect, there is provided a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank, a BOG preheater, at least one BOG compressor with a BOG compressor cooler, a BOG recondenser with a refrigeration cycle, wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the LNG cargo tank before the BOG entering the BOG compressor, the sep arate liquid circuit comprising a liquid pump configured to pump the HTF, the BOG preheater configured for heat exchanging between the BOG and the HTF, the BOG re- condenser located downstream the BOG preheater, and a liquid trim heater located downstream the BOG recondenser configured for heating the HTF.

According to some embodiments, the separate liquid circuit comprises a first bypass connection from downstream of the BOG preheater for the HTF to bypass the BOG Re- condenser.

According to some embodiments, the separate liquid circuit comprises a second bypass connection leading from upstream of the BOG preheater to the top of the BOG recon denser for circulation in reversed direction through the BOG Recondenser. According to some embodiments, a first part of the HTF is passed to the BOG preheater and the second bypass connection is open for passing a second part of the HTF in re versed direction through the BOG recondenser from top to bottom of the BOG recon denser, and the first bypass connection is open for passing the first part of the HTF from the BOG preheater and the second part of the HTF from the BOG recondenser to the trim heater.

According to some embodiments, the HTF is circulated through the separate liquid cir cuit from the liquid pump, through the BOG preheater, through the BOG recondenser, and through the liquid trim heater.

According to some embodiments, a part of the BOG is led from the BOG compressor cooler to the BOG recondenser for condensing, and from the BOG recondenser said part of the BOG is returned as liquefied gas to the liquefied natural gas (LNG) cargo tank.

According to some embodiments, the refrigeration cycle connected to the BOG recon denser is configured for removing heat from the part of the BOG that is condensed in the BOG recondenser. According to some embodiments, the HTF flowing through the BOG recondenser from bottom to top removes heat from the part of the BOG that is condensed in the BOG Re condenser.

According to some embodiments, the separate liquid circuit comprises a third bypass connection upstream the BOG preheater for a part of the HTF to bypass the BOG pre heater.

According to some embodiments, a part of the HTF is recycled back to upstream of the liquid pump from the BOG preheater.

According to some embodiments, a cooling medium from the BOG compressor cooler is utilized in the liquid trim heater.

According to some embodiments, the at least one BOG compressor is a non-cryogenic compressor type. According to some embodiments, the refrigeration cycle in the BOG recondencer is a reversed Brayton nitrogen cycle.

According to some embodiments, the separate liquid circuit comprises an expansion tank.

According to a second aspect, there is provided a method for reliquefaction of boil-off gas (BOG) in a reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank, a BOG preheater, at least one BOG compressor with a BOG com- pressor cooler, a BOG recondenser with a refrigeration cycle, wherein the method com prises pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit com prising a liquid pump, the BOG preheater, the BOG recondenser, and a liquid trim heater, by the liquid pump into the BOG preheater, preheating BOG entering the BOG preheater from the LNG cargo tank by the HTF before transferring the BOG to the BOG compressor, and transferring cooled HTF leaving the BOG preheater in the separate liq uid circuit to the liquid trim heater.

According to some embodiments, the method comprises transferring the HTF leaving the BOG preheater in the separate liquid circuit via a first bypass connection down- stream of the BOG preheater to the liquid trim heater.

According to some embodiments, the method comprises passing a first part of the HTF to the BOG preheater and passing a second part of the HTF through a second bypass connection in reversed direction through the BOG recondenser from top to bottom of the BOG recondenser, and passing the first part of the HTF from the BOG preheater and the second part of the HTF from the BOG recondenser through the first bypass connec tion to the liquid trim heater.

According to some embodiments, the method comprises transferring the HTF leaving the BOG preheater in the separate liquid circuit through the BOG recondenser before entering the liquid trim heater.

According to some embodiments, the method comprises leading a part of the BOG to the BOG recondenser for condensing, and from the BOG recondenser returning said part of the BOG as liquefied gas to the liquefied natural gas (LNG) cargo tank. According to some embodiments, the method comprises removing heat from the part of the BOG that is condensed in the BOG recondenser in the refrigeration cycle connected to the BOG recondenser. According to some embodiments, the method comprises removing additional heat from the part of the BOG that is condensed in the BOG recondenser by the HTF flowing through the BOG recondenser from bottom to top.

According to some embodiments, a part of the HTF is bypassing the BOG preheater via a third bypass connection upstream of the BOG preheater.

According to some embodiments, the method comprises recycling a part of the HTF flowing out from the BOG preheater to upstream of the liquid pump. According to some embodiments, the method comprises using a cooling medium from the BOG compressor cooler as a heating medium in the liquid trim heater.

According to some embodiments, the at least one BOG compressor is a non-cryogenic compressor type.

According to some embodiments, the refrigeration cycle in the BOG recondencer is a reversed Brayton nitrogen cycle.

According to a third aspect, there is provided a method for operating a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank, a BOG preheater, at least one BOG compressor with a BOG compressor cooler, a BOG recondenser with a refrigeration cycle, wherein the method comprises

-starting the system by pumping a heat transfer fluid (HTF) circulating in a separate liq uid circuit comprising a liquid pump, the BOG preheater, the BOG recondenser, and a liquid trim heater, by the liquid pump into the BOG preheater, preheating BOG entering the BOG preheater from the LNG 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 via a first bypass connection downstream of the BOG preheater to the liquid trim heater, - starting up a normal operation mode of the system by passing a first part of the HTF to the BOG preheater and passing a second part of the HTF through a second bypass con nection in reversed direction through the BOG recondenser from top to bottom of the BOG recondenser, and passing the first part of the HTF from the BOG preheater and the second part of the HTF from the BOG recondenser through the first bypass connection to the liquid trim heater,

- running the system in normal operation mode by circulating the HTF leaving the BOG preheater in the separate liquid circuit through the BOG recondenser from bottom to top before entering the liquid trim heater, leading a part of the BOG leaving the BOG com- pressor cooler to the BOG recondenser for condensing, and from the BOG recondenser returning said part of the BOG as liquefied gas to the LNG cargo tank, removing heat from the part of the BOG that is condensed in the BOG recondenser in the refrigeration cycle connected to the BOG recondenser, and removing additional heat from the part of the BOG that is condensed in the BOG recondenser by the HTF flowing through the BOG recondenser.

Effects and features of the second and third aspects 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 and third aspects.

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 in tended to be limiting. It should be noted that, as used in the specification and the ap pended 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 preheater" means a heat exchanger, which could be any type of heat ex changer, as e.g., shell & tube, plate/shell or plate/plate heat exchanger.

The term “BOG recondenser” means a heat exchanger, which could be any type of heat exchanger, as e.g., brazed plate/fm 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/shell or plate/plate heat exchanger.

Brief description 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.

Fig. 1 shows an overall process flow diagram of a BOG reliquefaction system according to the present invention with the refrigeration cycle in the BOG recondenser. Fig. 2 shows a process flow diagram of the BOG reliquefaction system where the HTF is circulated through the BOG preheater only.

Fig. 3 shows a process flow diagram of the BOG reliquefaction system with the opti mizing function start-up mode.

Fig. 4 shows a process flow diagram of the BOG reliquefaction system with the opti mizing function on, where the HTF circulates both through the BOG preheater and the BOG recondenser. Fig. 5 shows a process flow diagram of the BOG reliquefaction system where the HTF bypasses the BOG preheater.

Fig. 6 shows a process flow diagram of the BOG reliquefaction system where the HTF is recycled from the BOG preheater back to the liquid pump.

Like parts in the figures have been given like reference numerals. Detailed Description of the Invention

The present disclosure will now be described with reference to the accompanying draw ings, in which preferred example embodiments of the disclosure are shown. The disclo- sure 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.

With the present invention, it is possible to increase the reliquefaction capacity by reus- ing the “cold” obtained from preheating the BOG. The cooled HTF is used to increase the recondensing capacity of the BOG recondenser.

The first aspect of this disclosure is illustrated in Fig. 1, which shows an overall layout of a BOG reliquefaction system according to the present invention, showing a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 with a BOG com pressor cooler 13, a BOG recondenser 14 with a refrigeration cycle wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the LNG cargo tank 10 be- fore 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 configured for heat exchanging between the BOG and the HTF, the BOG recondenser 14 located down stream of the BOG preheater 11, and a liquid trim heater 16 located downstream of the BOG recondenser 14 configured for heating the HTF.

The HTF can bypass the BOG recondenser 14 via a first bypass connection bl from downstream the BOG preheater. This embodiment occurs in situations when the BOG recondenser 14 is not needed for condensing excess BOG back to the LNG cargo tank 10. In this case it is not required/needed that the HTF provides an optimizing function for the BOG recondenser 14 . With a bypass of the BOG recondenser it is ensured that the BOG recondenser 14 is not supplied with excess cold creating imbalance in the BOG recondenser 14. When the cold HTF is not heated up by the BOG recondenser 14, the liquid trim heater 16 will heat up the fluid to typically about + 40 °C. The HTF will always act as a preheating medium in the BOG preheater 11. Valves 20 and 21 are piston valves, which has an on/off function. Valves 18, 19, 22, 23 and 24 are diaphragm valves, which have the possibility to control and regulate the amount of flow through the valves not only by on/off.

Fig. 2 illustrates the BOG reliquefaction system where the HTF is circulated through the BOG preheater 11 only and not through the BOG recondenser 14. This mode can be used when there is no need for the optimizing function with HTF circulation through the BOG recondenser 14. This is the first mode to use after the LNG tank 10 has been filled. The HTF bypasses the BOG recondenser 14 via the first bypass connection b 1. A valve 18 in a line 103 in the separate liquid circuit is 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 for HTF bypass of the BOG recondenser 14. The separate liquid circuit can comprise a second bypass connection b2 leading from upstream of the BOG preheater 11 via line 108 and line 105 to the top of the BOG re condenser 14 for circulation in reversed direction, i.e., from the top to the bottom, through the BOG Recondenser 14. For the start-up of the optimizing function, a first part of the HTF is passed to the BOG preheater 11 and the second bypass connection b2 is open for passing a second part of the HTF in reversed direction through the BOG recondenser 14 from top to bottom of the BOG recondenser 14, and the first bypass connection bl is open for passing the first part of the HTF from the BOG preheater 11 and the second part of the HTF from the BOG recondenser to the trim heater 16.

When referring to “a first part” or “a second part” or “a part” of HTF, the person skilled in the art will understand how to divide the HTF flows properly for the different embod iments, and the exact amounts/ratios of flows will vary due to operational conditions as understood by the skilled person. Each part can be in the range of 0-100 % of the total HTF flow.

When the optimizing function is required to be started, a flow of warm HTF can be pumped via line 108 and line 105 through the BOG recondenser 14 from the top of the BOG recondenser 14, being cooled by the refrigerant in the refrigeration circle in the BOG recondenser 14. The outlet temperature at the bottom will be the same as the tem- perature of cold HTF entering the BOG recondenser 14 from the bottom of the BOG re condenser 14 via line 103 during normal operation with the optimizing function. In this way, the pipes will have a temperature of about -90°C when the optimizing function is started up and the HTF starts flowing into the BOG recondenser 14 from the bottom via line 103, thus preventing the pipes from heating the HTF and the HTF from entering the BOG recondenser 14 too warm.

By introducing the warm (typically about +40°C) HTF from the top of the BOG recon denser 14 via line 105, heat is introduced to the BOG recondenser 14. This additional heat can be used as a heat source when reliquefaction of the BOG is not executed and warm BOG is not entering the BOG recondenser 14, ensuring that the BOG recondenser 14 is kept at stable operation.

The additional heat from the HTF introduced in the top of the BOG recondenser 14 via line 105 could also be used to heat up the entire BOG recondenser 14 after a trip, when the entire BOG recondenser 14 is cooled down.

Fig.3 illustrates the BOG reliquefaction system with the optimizing function start-up mode of the separate liquid circuit. Valve 20 is closed, ensuring that the upper right cor- ner of the circuit is closed. The liquid pump 15 drives HTF through both the optimizing function start-up connection via line 108, and the BOG preheater 11 via line 101. The valve 20 in line 105 is in a closed position and valve 21 in line 108 is in an open posi tion for transferring a part of the HTF flow from the pump 15 via line 108 and line 105 to the BOG recondenser 14 and via line 103 out of the BOG recondenser 14 with valve 18 in open position and via line 104 with valve 19 in open position. Valve 18 is used to control the part of reverse flow through the BOG recondenser in the start-up of the opti mizing function. The valve 21 is used to allow the part of the flow into line 108, the rest of the HTF flow from the pump 15 entering the BOG preheater 11 via line 101. Fig. 4 illustrates the BOG reliquefaction system in a normal operation mode, where the HTF is 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), and bypasses and/or recycling are not used. 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 posi- tion 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 be fore entering the liquid trim heater 16 via line 106. The normal operation mode is the mode where the optimizing function is in use. The optimizing function may also be referred to as an optimizer function or optimizer/opti mizing mode or simply optimizer. In the optimizing function mode, the HTF enters the BOG Recondenser 14 from the bottom, also referred to as upstream, and exits from the top, also referred to as downstream, of the BOG recondenser 14. The HTF may flow through the BOG recondenser 14 in one or several connected flow channels.

At normal operation, the separate liquid circuit comprises the following equipment in order of flow the normal direction: liquid pump 15, BOG preheater 11, BOG reconden ser 14, liquid trim heater 16, and optionally an expansion tank 17.

At normal operation, the liquid pump 15 pumps the HTF at typically +40 °C to the BOG preheater 11, where the HTF heats up the BOG. Simultaneously, the HTF is cooled down to typically -90 °C. The HTF is sent into the BOG Recondenser 14 from the bot tom of (upstream of) the BOG recondenser 14. In the BOG recondenser 14, the HTF acts as a cooling medium for BOG and exits the BOG Recondenser 14 from the top (downstream) at approximately +35 °C. The HTF might bypass the BOG recondenser 14 via bypass connection bl also in the normal operation mode with the optimizing function. The valves 18 and 19 may control the amount of the HTF bypassing the BOG recondenser 14 to control the temperature out of the top of the BOG recondenser 14 and thereby the HTF temperature profile such that the temperature differences across the BOG recondenser 14 do not exceed allowa- ble limits.

The liquid trim heater 16 is located in the line 106, to which line 105 connected to the BOG Recondenser 14, and line 104 bypassing the BOG recondenser are both con nected. The trim heater 16 is located upstream of the liquid pump 15. The liquid trim heater 16 uses a heating medium such as for example water to heat up the HTF that is sent back into the liquid pump 15. The heating medium flowing into the liquid trim heater 16 is led through the BOG compressor cooler 13 connected to the BOG compres sor 12 for cooling of the BOG. There may be one or several steps of BOG compressors and coolers in series performing the required compressing of the BOG for the gas con- sumers in the ship. The BOG pressure could be e.g. 7 - 12 bar for gas- or dual fuel en gines, or it could be precompression in case the engines use high compression gas up to 300 bar, for which a separate compression system would be needed. The heating me dium flow into the liquid trim heater 16 may be connected to one or several of the com pressor coolers, depending on reaching a suitable heating capacity for the liquid trim heater 16. One or more compression stages can follow the connection to the reliquefac- tion system.

During the normal operation mode as described above, the separate liquid circuit with the HTF serves two different functions at once: Preheating the BOG upstream the BOG compressor 12 to about -30 °C, enabling the possibility to use a non-cryogenic BOG compressor. By non-cryogenic BOG compres sor is meant a compressor that does not have to withstand temperatures lower than - 150 °C. This provides a reduction of the investment cost for the system. With a liquid heat transfer fluid circuit serving as the purpose as preheater, a typical sys tem can handle BOG flows between about 1750 kg/h up to about 5000 kg/h. The flow of the heat transfer fluid can be adjusted, depending on the BOG flow to keep the tem perature of the BOG flow at the outlet of the preheater to about - 30 °C. In existing systems using N2 for preheating, and when N2 preheating is not available or sufficient, another heat source is necessary, which is normally done with additional BOG preheaters in parallel. The system according to the present invention has only one BOG preheater, which simplifies the system and makes the transitions between modes much easier. One gas/liquid and one liquid/liquid exchanger will also be more compact compared to at least one gas/gas and one gas/liquid.

Optimizing function by additional heat removal in the BOG Recondenser 14 contrib uting to a higher cooling capability for the reliquefaction process, thus increasing the ef ficiency of the system. Compared with a traditional nitrogen loop based preheating and recondensing system for the BOG, the system according to the invention is capable of providing a more efficient and simple system. By transferring the excess “cold” from the BOG preheater 11 into the BOG recondenser 14 by the HTF, the needed capacity from the refrigeration cycle there is reduced. Thus, using the HTF to optimize the rel iquefaction may increase the reliquefaction capacity by as much as about 67%, com- pared to operation without the HTF in the BOG Recondenser 14. Once started, the optimizing function will most often be used as long as the reliquefac tion system is running. And if necessary, with some of the HTF going in the BOG Re condenser bypass. Thus, the mode with the optimizing function in use is on often during the whole trip of the LNG carrier ship. However, the optimizing function may be stopped if the reliquefaction system is set in stand-by or stopped completely. This can happen if e.g. the pressure in the LNG tank is dropping because the fuel consumption by main engine and other consumers is more than the BOG generation (practically all LNG vessels have engines that use the boil-off gas for ship’s power). The running time of the reliquefaction system can vary between ships.

The separate liquid HTF circuit with its technical details as disclosed herein ensures a flexibility not made possible by the conventional preheater systems in existing BOG rel iquefaction plants. After exiting the BOG compressor cooler 13, a part of the BOG can be led via a line 111 to the BOG recondenser 14 for condensing, and from the BOG recondenser 14 said part of the BOG is returned as liquefied gas to the LNG cargo tank 10 via a line 112 with a valve 24 in an open position. The BOG that is led to the BOG recondenser 14 can be in the range 0 - 100 % of the total amount of BOG exiting the BOG compressor cooler 13. The remaining part of the BOG exiting the BOG compressor cooler 13 that is not led to the BOG recondenser 14 for recondensing, is led out of the reliquefaction sys tem to fuel consumers.

The refrigeration cycle connected to the BOG recondenser 14 is configured for remov- ing heat from the part of the BOG that is condensed in the BOG recondenser 14.

When the optimizing function is running, the HTF flowing through the BOG reconden ser 14 from bottom to top, removes heat from the part of the BOG that is condensed in the BOG Recondenser 14.

The separate liquid circuit can comprise a third bypass connection b3 upstream the BOG preheater 11 for a part of the HTF to bypass the BOG preheater 11. The HTF can bypass the BOG preheater 11 to control the HTF temperature into the BOG recondenser 14. Fig. 5 illustrates the BOG reliquefaction system where the HTF bypasses the BOG preheater 11. A valve 22 parallel to the BOG preheater 11 in a line 109 is in an open po sition for HTF bypass of the BOG preheater 11. As illustrated in Fig. 5, at the same time as the third bypass connection b3 is open for a part of the HTF to bypass the BOG pre heater 11, the bypass connection bl may be open for controlling flows and/or tempera tures in the system components. The bypass connection bl may also be closed when the bypass connection b3 is open.

The HTF can be recycled from downstream the BOG preheater 11 to control tempera ture on HTF to the BOG recondenser 14. By recycling a larger amount of cold HTF back to the liquid pump 15, and thus ensuring a larger HTF flow through the BOG pre heater 11, the flow out of the BOG preheater 11 is kept at the desired temperature. Fig. 6 illustrates the BOG reliquefaction system where the HTF is recycled. A valve 23 downstream the BOG preheater 11 in line 110 is in an open position for HTF recycling back to the liquid pump 15. As illustrated in Fig. 6, at the same time as part of the HTF is recycled from downstream the BOG preheater 11 back to the liquid pump 15, the by pass connection bl may be open for controlling flows and/or temperatures in the system components. The bypass connection bl may also be closed during said recycling of the HTF.

The bypass of the BOG preheater 11 via bypass connection b3, the recycling of the HTF back to the liquid pump 15 and the bypass of the BOG recondenser via bypass connecti- tion bl may also all be in use at the same time.

Valve 22 and/or valve 23 may be used to control the HTF temperature to the BOG re condenser 14 by respectively controlling the amount of HTF bypassing the BOG pre heater 11 and the amount of HTF being recycled from downstream the BOG preheater 11 back to the liquid pump 15.

The heat transfer fluid in the separate liquid circuit in the BOG reliquefaction system according to the present invention 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. Exam ples of suitable heat transfer fluids are heat transfer fluids based on for example hydro carbons or silicone. Example of a composition of a suitable heat transfer fluid is a mix ture of methyl cyclohexane and trimethyl pentane, such as in the commercially available

®

Ther inol® VLT Heat Transfer Fluid produced by Eastman. Therminol VLT heat transfer fluid is a synthetic liquid-phase fluid for use in extremely low-temperature ap plications, 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 Cal- therm UBT (silicone) for uses between - 100 to +260°C produced by Cal die, 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 com pressors 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. 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.

The heated-up cooling medium from the BOG compression leaving the BOG compres- sor cooler 13 can be re-used as a heating medium for the liquid trim heater 16. Option ally, separate supply to the BOG compressor cooler 13 and the liquid trim heater 16 is also possible. 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 separate liquid circuit can comprise an expansion tank 17 connected to the HTF cir culation. The expansion tank 17 can be any type of expansion tank and the purpose of the expansion tank 17 is to allow the HTF to expand.

The BOG leaving the LNG 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 com pression, 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 fuel consumers and/or to the BOG recondenser 14 via line 111 for reliquefaction. The BOG is cooled and liquefied in the BOG Recondenser 14, entering at temperatures typi- cally around +40 °, and returning to the LNG cargo tank 10 via a 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 refrigeration cycle in the BOG recondencer 14 can be a nitrogen cycle. Then the re frigerant 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 after cooler 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 BOG reliquefaction system could be installed for vessels, offshore installations, or onshore facilities such as LNG terminals.

The second aspect of this disclosure shows a method for reliquefaction of boil-off gas (BOG) in a reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 with a BOG com- pressor cooler 13, a BOG recondenser 14 with a refrigeration cycle, wherein the method comprises pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid pump 15, the BOG preheater 11, the BOG recondenser 14, and a liquid trim heater 16, by the liquid pump 15 into the BOG preheater 11, preheating BOG entering the BOG preheater 11 from the LNG 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 the liquid trim heater 16.

An embodiment of the method illustrated in Fig. 2 comprises pumping the HTF circu lating in the separate liquid circuit by the liquid pump 15 via a line 101 into the BOG preheater 11, preheating BOG entering the BOG preheater 11 from the LNG cargo tank 10 via a line 100 by the HTF in the BOG preheater 11 before transferring the BOG to the BOG compressor 12 via a line 102, transferring cooled HTF leaving the BOG pre heater 11 directly to a liquid trim heater 16 via a line 103 with a valve 18 in closed posi tion, a line 104 with a valve 19 in open position and via a line 106.

An embodiment of the method illustrated in Fig. 3 comprises passing a first part of the HTF to the BOG preheater 11 and passing a second part of the HTF through a second bypass connection b2 in reversed direction through the BOG recondenser 14 from top to bottom of the BOG recondenser 14, and passing the first part of the HTF from the BOG preheater 11 and the second part of the HTF from the BOG recondenser 14 through the first bypass connection bl to the trim heater 16. This is the start-up of the optimizer function of the separate liquid circuit by passing the HTF via a line 108 with a valve 21 in open position to the top of the BOG recondenser 14 via line 105 and valve 20 in closed position, returning the HTF leaving the BOG recondenser 14 from the bottom via line 103 with valve 18 in open position to the liquid trim heater 16 via line 104 with valve 19 in open position.

Fig. 4 illustrates an embodiment of the method transferring the HTF leaving the BOG preheater 11 in the separate liquid circuit through the BOG recondenser 14 from the bot tom (upstream) entering via the line 103 with the valve 18 in open position and valve 19 in line 104 in closed position, and leaving the BOG recondenser 14 from the top (down- stream) via a line 105 with a valve 20 in open position before entering the liquid trim heater 16 via a line 106 and leaving the liquid trim heater 16 via a line 107. As de scribed above for the system, this is the normal operation mode with the optimizing function in use. After the BOG is leaving the BOG compressor cooler 13, a part of the BOG can flow via a line 111 to the BOG recondenser 14 for condensing, and from the BOG reconden ser 14 returning said part of the BOG as liquefied gas to the LNG cargo tank 10 via a line 112 with a valve 24 in an open position. The refrigeration cycle connected to the BOG recondenser 14 removes heat from the part of the BOG that is condensed in the BOG recondenser 14.

When the optimizing function is in use, additional heat from the part of the BOG that is condensed in the BOG recondenser 14 is removed by the HTF flowing through the BOG recondenser 14 from bottom to top.

Fig. 5 illustrates an embodiment of the method where a third bypass connection b3 is open for a part of the HTF to bypass the BOG preheater 11. HTF flows via a line 109 with a valve 22 in open position for bypass of the BOG preheater 11. As illustrated in Fig. 5, at the same time as the third bypass connection b3 is open for a part of the HTF to bypass the BOG preheater 11, the bypass connection bl may be open for controlling flows and/or temperatures in the system components. The bypass connection bl may also be closed when the bypass connection b3 is open.

Fig. 6 illustrates an embodiment of the method comprising passing the HTF via a line 110 and a valve 23 in open position after the BOG preheater 11 for recycling the HTF to upstream of the liquid pump 15. As illustrated in Fig. 6, at the same time as a part of the HTF is recycled from downstream the BOG preheater 11 back to the liquid pump 15, the bypass connection bl may be open for controlling flows and/or temperatures in the system components. The bypass connection bl may also be closed during said recycling of the HTF.

As described above for the reliquefaction system, the heat transfer fluid in the separate liquid circuit in the method for reliquefaction of according to the present invention is a liquid phase heat transfer fluid suitable for heat transfer down to low temperatures. HTF can be a synthetic liquid-phase fluid as specified above for the system according to the invention.

As described above for the reliquefaction system, in an embodiment of the method ac cording to the invention, compression of the BOG is done in at least one BOG compres sor 12 with at least one BOG compressor cooler 13 and a cooling medium. Preferably, the compression of the BOG is done in multiple stages with inter- and after-cooling with a cooling medium. The at least one BOG compressor 12 can be a non-cryogenic compressor type as specified above for the system according to the invention.

As described above for the reliquefaction system, in an embodiment of the method ac- cording to the invention, 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 liq uid trim heater 16. Optionally, separate supply to the BOG compressor cooler 13 and the liquid trim heater 16 is also possible. The cooling medium is usually water, water- glycol mixture or similar.

As described above for the reliquefaction system, in the method according to the inven- tion, the BOG leaving the LNG 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 com pression, 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 fuel consumers and/or to the BOG recondenser 14 via line 111 for reliquefaction. The BOG is cooled and liquefied in the BOG Recondenser 14, entering at temperatures typi cally around +40 °, and returning to the LNG cargo tank 10 via a line 112 with a valve 24 in an open position at a temperature of at least about -163 °C to be liquid without pressurization.

As described above for the reliquefaction system, in the method according to the inven tion the refrigeration cycle in the BOG recondencer 14 can be a nitrogen cycle. How ever, other refrigeration cycles might also be used within the scope of the invention. Advantageously a reversed Brayton nitrogen cycle may be used.

As described above for the reliquefaction system, in the method according to the inven tion, the separate liquid circuit might comprise an expansion tank 17. The third aspect of this invention relates to a method for operating a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 with a BOG compressor cooler 13, a BOG recondenser 14 with a refrigeration cycle. The method comprises first to start the system by pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid pump 15, the BOG preheater 11, the BOG recondenser 14, and a liquid trim heater 16, by the liquid pump 15 into the BOG preheater 11, preheating BOG entering the BOG preheater 11 from the LNG 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 via a first bypass connection bl downstream of the BOG preheater 11 to the liquid trim heater 16. After the system has been started up, a normal operation mode of the system is started up by passing a first part of the HTF to the BOG preheater 11 and passing a second part of the HTF through a second bypass connection b2 in reversed direction through the BOG recondenser 14 from top to bottom of the BOG recondenser 14, and passing the first part of the HTF from the BOG preheater 11 and the second part of the HTF from the BOG recondenser 14 through the first bypass connection bl to the trim heater 16. When the normal opera- tion mode has been started up, the system is run in normal operation mode by circulat ing the HTF leaving the BOG preheater 11 in the separate liquid circuit through the BOG recondenser 14 from bottom to top before entering the liquid trim heater 16, lead ing a part of the BOG leaving the BOG compressor cooler 13 to the BOG recondenser 14 for condensing, and from the BOG recondenser 14 returning said part of the BOG as liquefied gas to the LNG cargo tank 10, removing heat from the part of the BOG that is condensed in the BOG recondenser 14 in the refrigeration cycle connected to the BOG recondenser 14, and removing additional heat from the part of the BOG that is con densed in the BOG recondenser 14 by the HTF flowing through the BOG recondenser 14.

The person skilled in the art realizes that the present disclosure is not limited to the pre ferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. For example, 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

P a t e n t c l a i m s

1

A boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, c h a r a c t e r i z e d i n that the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) for preheating of BOG from the LNG 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) configured for heat exchanging between the BOG and the HTF, the BOG recondenser (14) located downstream the BOG preheater (11), and a liquid trim heater (16) located downstream the BOG recon denser (14) configured for heating the HTF.

2

The boil-off gas (BOG) reliquefaction system according to claim 1, c h a r a c t e r i z e d i n that the separate liquid circuit comprises a first by pass connection (bl) from downstream of the BOG preheater (11) for the HTF to by- pass the BOG Recondenser (14).

3.

The boil-off gas (BOG) reliquefaction system according to claim 2, c h a r a c t e r i z e d i n that the separate liquid circuit comprises a second bypass connection (b2) leading from upstream of the BOG preheater (11) to the top of the BOG recondenser (14) for circulation in reversed direction through the BOG Recon denser (14).

4. The boil-off gas (BOG) reliquefaction system according to claim 3, c h a r a c t e r i z e d i n that a first part of the HTF is passed to the BOG preheater (11) and the second bypass connection (b2) is open for passing a second part of the HTF in reversed direction through the BOG recondenser (14) from top to bottom of the BOG recondenser (14), and the first bypass connection (bl) is open for passing the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser to the trim heater (16).

5.

The boil-off gas (BOG) reliquefaction system according to claim 1, c h a r a c t e r i z e d i n that the HTF is circulated through the separate liq- uid circuit from the liquid pump (15), through the BOG preheater (11), through the BOG recondenser (14), and through the liquid trim heater (16).

6

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that a part of the BOG is led from the BOG compressor cooler (13) to the BOG recondenser (14) for condensing, and from the BOG recondenser (14) said part of the BOG is returned as liquefied gas to the LNG cargo tank (10).

7.

The boil-off gas (BOG) reliquefaction system according to claim 6, c h a r a c t e r i z e d i n that the refrigeration cycle connected to the BOG recondenser (14) is configured for removing heat from the part of the BOG that is con densed in the BOG recondenser.

8

The boil-off gas (BOG) reliquefaction system according to any one of claims 6-7, c h a r a c t e r i z e d i n that HTF flowing through the BOG re condenser (14) from bottom to top removes heat from the part of the BOG that is con- densed in the BOG Recondenser (14).

9.

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that the separate liquid circuit comprises a third bypass connection (b3) upstream the BOG preheater (11) for a part of the HTF to bypass the BOG preheater (11).

10

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that a part of the HTF is recy cled back to upstream of the liquid pump (15) from the BOG preheater

(11).

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that a cooling medium from the BOG compressor cooler (13) is utilized in the liquid trim heater (16).

12

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that the at least one BOG compressor (12) is a non-cryogenic compressor type.

13.

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that the refrigeration cycle in the BOG recondencer (14) is a reversed Brayton nitrogen cycle.

14.

The boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, c h a r a c t e r i z e d i n that the separate liquid circuit comprises an expansion tank (17).

15.

A method for reliquefaction of boil-off gas (BOG) in a reliquefaction system compris ing at least one liquefied natural gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG reconden- ser (14) with a refrigeration cycle, c h a r a c t e r i z e d i n that the method comprises pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid pump (15), the BOG preheater (11), the BOG recon denser (14), and a liquid trim heater (16), by the liquid pump (15) into the BOG pre heater (11), preheating BOG entering the BOG preheater (11) from the LNG cargo tank (10) by the HTF before transferring the BOG to the BOG compressor (12), and transfer ring cooled HTF leaving the BOG preheater (11) in the separate liquid circuit to the liq uid trim heater (16).

16. The method for reliquefaction of boil-off gas (BOG) according to claim 15, c h a r a c t e r i z e d b y transferring the HTF leaving the BOG preheater (11) in the separate liquid circuit via a first bypass connection (bl) down stream of the BOG preheater (11) to the liquid trim heater (16).

17. The method for reliquefaction of boil-off gas (BOG) according to claim 16, c h a r a c t e r i z e d b y passing a first part of the HTF to the BOG preheater (11) and passing a second part of the HTF through a second bypass con nection (b2) in reversed direction through the BOG recondenser (14) from top to bottom of the BOG recondenser (14), and passing the first part of the HTF from the BOG pre- heater (11) and the second part of the HTF from the BOG recondenser (14) through the first bypass connection (bl) to the liquid trim heater (16).

18.

The method for reliquefaction of boil-off gas (BOG) according to claim 15, c h a r a c t e r i z e d b y transferring the HTF leaving the BOG preheater (11) in the separate liquid circuit through the BOG recondenser (14) before entering the liquid trim heater (16).

19. The method for reliquefaction of boil-off gas (BOG) according to any one of claims 15- 18, c h a r a c t e r i z e d b y leading a part of the BOG to the BOG recondenser (14) for condensing, and from the BOG recondenser (14) returning said part of the BOG as liquefied gas to the LNG cargo tank (10). 20.

The method for reliquefaction of boil-off gas (BOG) according to claim 19, c h a r a c t e r i z e d b y removing heat from the part of the BOG that is condensed in the BOG recondenser (14) in the refrigeration cycle con nected to the BOG recondenser (14).

21

The method for reliquefaction of boil-off gas (BOG) according to any one of claims 19-

20, c h a r a c t e r i z e d b y removing additional heat from the part of the BOG that is condensed in the BOG recondenser (14) by the HTF flowing through the BOG recondenser (14).

The method for reliquefaction of boil-off gas (BOG) according to any one of the claims 15-21, c h a r a c t e r i z e d i n that a part of the HTF is by passing the BOG preheater (11) via a third bypass connection (b3) upstream of the BOG preheater (11).

23.

The method for reliquefaction of boil-off gas (BOG) according to any one of the claims 15-22, c h a r a c t e r i z e d b y recycling a part of the HTF flowing out from the BOG preheater (11) to upstream of the liquid pump (15).

24.

The method for reliquefaction of boil-off gas (BOG) according to any one of claims 15-

23, c h a r a c t e r i z e d i n using a cooling medium from the BOG compressor cooler (13) as a heating medium in the liquid trim heater (16).

25.

24, c h a r a c t e r i z e d i n that the at least one BOG com- pressor (12) is a non-cryogenic compressor type.

26.

25, c h a r a c t e r i z e d i n that the refrigeration cycle in the BOG recondencer (14) is a reversed Brayton nitrogen cycle.

27.

26, c h a r a c t e r i z e d i n that the separate liquid circuit comprises an expansion tank (17).

28.

A method for operating a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, c h a r a c t e r i z e d i n that the method comprises -starting the system by pumping a heat transfer fluid (HTF) circulating in a separate liq uid circuit comprising a liquid pump (15), the BOG preheater (11), the BOG reconden ser (14), and a liquid trim heater (16), by the liquid pump (15) into the BOG preheater (11), preheating BOG entering the BOG preheater (11) from the LNG 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 via a first by pass connection (bl) downstream of the BOG preheater (11) to the liquid trim heater (16),

- starting up a normal operation mode of the system by passing a first part of the HTF to the BOG preheater (11) and passing a second part of the HTF through a second bypass connection (b2) in reversed direction through the BOG recondenser (14) from top to bottom of the BOG recondenser (14), and passing the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) through the first bypass connection (bl) to the liquid trim heater (16), - running the system in normal operation mode by circulating the HTF leaving the BOG preheater (11) in the separate liquid circuit through the BOG recondenser (14) from bot tom to top before entering the liquid trim heater (16), leading a part of the BOG leaving the BOG compressor cooler (13) to the BOG recondenser (14) for condensing, and from the BOG recondenser (14) returning said part of the BOG as liquefied gas to the LNG cargo tank (10), removing heat from the part of the BOG that is condensed in the BOG recondenser (14) in the refrigeration cycle connected to the BOG recondenser (14), and removing additional heat from the part of the BOG that is condensed in the BOG recon denser (14) by the HTF flowing through the BOG recondenser (14).