The present invention relates generally to a method and a system for conditioning of LNG in fuel system, such system also supplying natural gas to a consumer for the purpose of heating or power generation. The method and system according to the present invention are particularly suitable for use on board a vessel adapted for storage and transportation of liquefied natural gas (LNG) for the purpose of utilising a part or the total of the LNG to fuel the vessel' s engine(s) and/or other consumers.

In the marine industry, a variety of gas fuelled systems exist. These systems include both LNG cargo ships, where part of the cargo is consumed as fuel, and other ships were gas is stored onboard in tanks for fuel consumption purposes only. LNG carrier solutions often use atmospheric LNG cargo tanks (max 0,7 barg), but solutions using pressure tanks (so- called C -tanks) are also used. In cargo ships atmospheric tanks have been dominating in LNG carriers. In passenger ships and other ships pressure tanks (C-tanks) have been the dominant solution so far, but use of atmospheric tanks may increase in the future, especially for long-endurance vessels.

In LNG carriers a common solution is to run the gas engines for propulsion on boil-off gas (BOG) from the cargo. However, the amount of BOG is often too low to maintain supply of gas to the engines, and a system for forced BOG is therefore installed. Such forced BOG is typically achieved by using a submerged cargo pump and a heat exchanger. Liquid LNG is withdrawn from the bottom of the tank by the LNG cargo pump and fed to a heat exchanger, where the LNG is heated from approximately -160°C to approximately -130°C before the fluid is returned to the upper layer of the LNG in the cargo tank. In this way, the upper layer of the LNG in the cargo tank increases in temperature, and hence gains a higher saturation pressure. A higher saturation pressure gives a higher boil-off rate and therefore an increased mass of gas for consumption by the engines. At the same time as this method aims at increasing the temperature in the upper LNG layer, it is also a goal to keep the rest of the LNG at lowest possible temperature to maximise cargo capacity and holding time.

Present invention applies mainly to solutions based on pressure tanks (C-tanks), but can easily be adjusted to fit atmospheric tank solutions. According to the present invention, a combination of an external LNG pump and a heat exchanger is used to heat the whole content of the LNG tank, while at the same time controlling the pressure in the tank and supplying gas to the engines. The aim of the process is to increase the temperature of the LNG as fast as possible, in a controlled way, while obtaining a homogenous temperature throughout the bulk of the LNG. The heating of the LNG continues until a temperature corresponding to a given saturation pressure is achieved, which is equal to or higher than the minimum pressure required by the gas engine (plus pressure loss due to piping, valves etc.). As soon as this temperature and saturation pressure is reached, the pump is turned off, and the gas fuel supply pressure is maintained by the increased saturation pressure of the liquid and gravity feed mode. Hence, this resulting in saving running hours of the pump, reducing the complexity of the system, and avoiding risk of cavitation of the pump at low liquid level in the tank (or during violent sloshing movements). The usage of natural gas as energy source in ships for propulsion is advantageous due to its efficient burning and low emissions. Usually gas is stored in liquefied form because of less space is required for the storage that way. However, the gas engine(s) requires the fuel to be fed in gaseous phase. To supply gas to the engine(s), LNG is withdrawn from one or more LNG storage tanks by gravity-forces and/or a LNG pump and/or the tank pressure. The LNG thereafter flows into at least one vaporizer, where heat is added to the LNG to transfer the natural gas to a gaseous phase. The natural gas thereafter flows to one or more heat exchangers (super heaters) where the gas is heated to the temperature required by the gas engine(s). Because of the withdrawal of LNG for consumption purposes, the pressure in the LNG tank decreases. In order to maintain a pressure in the LNG tank above the minimum pressure required by the gas engine, some LNG is withdrawn from the LNG tank to be vaporized and heated in one or more heat exchangers called pressure build-up units (PBU), before the natural gas from the pressure build-up units is returned to the top of the tank. Hence, adding mass to the gas cap, thereby increasing the pressure in the LNG tank. All or part of the output from the PBUs may also be returned to the bottom of the LNG tank, where it adds heat to the LNG.

The pressure in the LNG tank is the sum of the saturation pressure of the LNG in the tank at the given temperature, and the pressure added by the pressure build-up units (PBUs). If sloshing or other movements of the liquid in the LNG tank occurs, the gas pressure may be reduced down to the saturation pressure of the LNG at the given temperature. At low LNG temperatures, the saturation pressure is lower than the minimum pressure required by the engine. This may lead to power reduction of the engine. To avoid such situations, a LNG pump may be used. A pump secures sufficient supply of gas to the engine(s) independent of low saturation pressure of the LNG in the tank. The capacity of the pump may be higher than that required for supply to the engines, and a return line from the pump to the LNG storage tank is therefore installed. The LNG from the pump return line may be sprayed through the gas phase, thus reducing the tank pressure, and/or it can be returned to the bottom of the LNG tank thereby adding heat to the LNG. Heating the LNG increases the saturation pressure of the LNG. The pump can also be used to feed the PBUs and to increase the pressure in the LNG tank and/or increase the temperature of the LNG in the tank.

The pressure and heat and mass flow in the system is controlled and regulated by various sensors, valves and regulators. The type and number of valves and regulators and sensors may vary from project to project, depending on the number of tanks, PBUs, pumps etc., as easily worked out by someone skilled in process system design.

An object of the present invention is to provide a system and method for conditioning of LNG in fuel systems, such a system also supplying natural gas to a consumer which minimize and/or alleviate one or more of the disadvantages of prior art, or to provide a useful alternative.

It is also an object of the present invention to provide a system and a method for conditioning of LNG in fuel systems, such a system also supplying natural gas to a consumer which minimizes the risk of damage, spillage of gas etc. These objects are achieved with a system and a method for supplying natural gas according to the following independent claims, with additional embodiments set forth in the dependent claims.

The present invention relates to a system for conditioning of LNG in fuel systems, such system also supplying natural gas to one or more consumers, where the system comprises at least one LNG tank, a pump and a pressure build-up unit, where the pump is arranged to be in fluid communication with the at least one LNG tank through an outlet conduit, a pump inlet conduit and at least one pump return conduit, while the pressure build-up unit is arranged to be in fluid communication with the LNG tank through at least one inlet conduit, an outlet conduit, and/or a gravity feed conduit and a conduit. The pump and the pressure build-up unit are also in fluid communication with each other, through a pump outlet conduit and a pressure build-up conduit.

The pump may be arranged as a free-standing unit externally the at least one LNG tank or may be installed in the LNG tank.

According to one aspect of the present invention, the system for supplying natural gas may further comprise a vaporizer and/or a super heater (heat exchanger).

The vaporizer may be in fluid communication with the at least one LNG tank through an outlet conduit and vaporizer inlet conduit. The vaporizer may further be in fluid

communication with the LNG tank through a pump through a pump outlet conduit and a vaporizer inlet conduit.

The vaporizer may also be in fluid communication with the LNG tank through a gravity feed conduit.

In one aspect of the present invention, the super heater (heat exchanger) may be in fluid communication with the vaporizer through a conduit.

The inlet conduit of the LNG tank may be divided into two sub-conduits, a first sub- conduit may be arranged in such a way that the vapour is delivered to a top of the at least one LNG tank, and a second sub-conduit may be arranged to extend to the bottom of the LNG tank.

The pump return conduit may be divided into two sub-conduits, a first sub-conduit may be arranged in such a way that the fluid is delivered to a top of the at least one LNG tank, and a second sub-conduit may be arranged to extend to the bottom of the LNG tank.

According to one aspect of the present invention, one or more of the pressure build-up unit, vaporizer and super heater (heat exchangers), may be combined in a common unit.

According to one aspect of the present invention, the system may comprise one or more additional pumps and/or pressure build-up units.

According to one aspect of the present invention, the system may also comprise one or more additional vaporizer(s) and/or super heater(s) (heat exchangers). The inlet conduit of the LNG tank and the pump return conduit may in one embodiment be connected in a common conduit.

According to the present invention a method for conditioning of LNG in fuel systems, such system also supplying natural gas to one or more consumers, comprises the following steps: storing LNG in at least one LNG tank, transferring an amount of LNG from the at least one LNG tank to a pump when a pressure in the at least one LNG tank fall below a certain pressure, using the pump to rise the pressure of the LNG, transferring wholly or in part the raised pressure LNG to a pressure build-up unit to heat and evaporate the LNG to the at least one LNG tank.

According to one aspect, the method may further comprise the steps of returning the vapour to a top of the LNG tank, thereby regulating the pressure of the LNG in the at least one LNG tank, and/or returning the vapour to a bottom of the LNG tank, thereby heating up the LNG in the at least one LNG tank.

According to one other aspect of the present invention, the method may further comprise the step of transferring wholly or in part the raised pressure LNG to a vaporizer.

According to one aspect of the present invention, the method may further comprise the step of transferring wholly or in part the vaporized LNG to a super heater (heat exchanger).

As LNG is withdrawn from the LNG tank and sent to the vaporizer (VAP) and super heater (SH) (heat exchangers), the pressure in the tank will be reduced. In order to maintain the pressure in the LNG tank, some LNG is withdrawn from the tank, heated and evaporated in the pressure build-up unit (PBU), before the vapour is returned to the top of the tank. In this way, the pressure in the tank will be increased to a pressure above the LNG saturation pressure at the given temperature of the LNG in the tank. The pressure in the tank is the sum of the LNG saturation pressure at the given temperature and the gas flow to the tank top from the PBU.

The operating pressure in the tank is defined by the required inlet pressure to the at least one consumer and pressure losses in the heat exchangers, piping, valves etc. At low LNG temperatures the saturation pressure in the tank may be lower than the pressure required by the at least one consumer. If the tank contents in such situations are subject to sloshing, a major part of the added pressure created by vapour from the PBU may be condensed rapidly, resulting in a rapid pressure drop down to the saturation pressure. If the saturation pressure is lower than the required tank operating pressure, this will impact the consumer(s) condition. If sloshing continues, the PBU will not be able to mitigate the pressure loss fast enough to maintain the required gas pressure to the consumer.

In situations as described above, supply to the consumer(s) can be secured by using a pump to force LNG to the vaporizer (VAP), thereby maintaining the necessary gas supply and gas pressure to the consumer(s). In addition, the pump can be used to force LNG to the pressure build-up unit (PBU) to produce vapour and increase the tank pressure. At the same time, the PBU can be used to heat the LNG in the LNG tank to a temperature corresponding to a minimum desired saturation pressure. The present invention is in particular relevant for marine fuel gas systems, but is also applicable in other systems, both on land and sea. The invention represents a compact, flexible and robust solution for gas pressure and gas supply control in marine LNG fuel systems.

These and other characteristics of the present invention will be clear from the following description of a preferential form of embodiments, given as non-restrictive examples, with reference to the attached drawings wherein: Figure 1 illustrates an embodiment of a system for conditioning of LNG in fuel systems, the system also supplying natural gas to at least one consumer according to the present invention,

Figure 2 illustrates another embodiment of a system for conditioning of LNG in fuel systems, the system also supplying natural gas to at least one consumer according to the present invention, and

Figure 3 illustrates how two systems according to figure 1 can be assembled to form a common system.

Figure 1 depicts schematically how a system 1 according to the present invention can be utilized with a fuel system for a vessel. The system 1 for conditioning of LNG in the fuel system, the system also supplying natural gas to at least one consumer on board the marine vessel is shown with only one LNG tank 2 for clarity reasons, but it should be understood that the system may comprise more than one LNG tank 2.

The LNG tank 2 is connected to a pump 1 1 through a tank outlet conduit 4 and a pump inlet conduit 6. The LNG tank 2 and the pump 11 may also be in fluid communication through an alternative or additional conduit 17. A pump return conduit 5 is also connected between the LNG tank 2 and the pump 11 , whereby pressurized LNG can be returned to the LNG tank 2.

The pump return conduit 5 is divided into two sub-conduits 5a, 5b, where the first sub- conduit 5a is arranged to be on a top of or in an upper area of the LNG tank 2, while the second sub-conduit 5b is arranged to extend towards the bottom of the LNG tank 2.

The pressure build-up unit (PBU) 12 is connected to the LNG tank 2 through the outlet conduit 4 and gravity feed conduit 9, but LNG may also be transferred to the PBU 12 through an alternative or additional gravity feed conduit 16. The outlet of conduit 16 may be connected to gravity feed conduit 9, or directly to the division point between conduits 7, 8, 9 and 10. Furthermore, an inlet conduit 3 is arranged between the LNG tank 2 and the pressure build-up unit 12. The inlet conduit 3 is divided into two sub-conduits 3a, 3b where the first sub-conduit 3a is arranged to be on a top of or in an upper area of the LNG tank 2, while the second sub-conduit 3b is arranged to extend towards the bottom of the LNG tank 2. The pump 11 and the pressure build-up unit 12 are in fluid communication with each other through a pump outlet conduit 7 and a pressure build up conduit 10. The pump 11 is furthermore connected to a vaporizer (VAP) 13 and a super heater (SH) 14 (heat exchanger) through the pump outlet conduit 7, a vaporizer inlet conduit 8, the vaporizer (VAP) 13 and super heater (SH) 14 being in fluid communication with each other through a conduit 18. The super heater (SH) 14 will then supply the natural gas to at least one end-user or consumer 15, through a conduit 19, this at least one end-user or consumer 15 for instance being one or more gas engines.

To supply gas to the engine(s), liquefied natural gas (LNG) is withdrawn from the LNG tank 2 through the gravity feed conduit 16, or conduits 4, 9 and 16, by means of gravity forces, or through the conduits 4, 6, 7 and the pump 11 , and directed to the vaporizer 13, whereafter heat, through the vaporizer 13, is added to the LNG in order to transfer the natural gas to a gaseous phase. Thereafter the gas is fed to the super heater 14 (heat exchanger), where the gas is heated to a temperature required by the one or more gas engines. However, when LNG is withdrawn from the LNG tank 2, the pressure in the LNG tank 2 will decrease. As it is desired to maintain the pressure in the LNG tank 2 at least above a minimum pressure required by the one or more gas engines, some LNG is withdrawn from the LNG tank 2 and is fed to the pressure build-up unit 12 in order to vaporize and heat up the LNG. This can either be done by using the gravity feed conduit 16, or conduits 4, 9 and 16, or by using the pump 1 1 to feed the LNG to the pressure build-up unit 12.

The vaporized and heated LNG will thereafter by returned to the top of the LNG tank 2 through the inlet conduit 3 and sub-conduit 3a, in order to increase the pressure in the LNG tank 2. Alternatively, the vaporized and heated LNG may be returned to the bottom of the LNG tank 2, through the inlet conduit 3 and sub-conduit 3b, thereby adding heat to the LNG.

In order to avoid a situation where the gas pressure in the LNG tank 2 is reduced below the minimum consumer supply pressure, for instance during sloshing or other movements of the liquid in the LNG tank 2, the pump 11 is used. The pump 11 will then secure sufficient supply of gas to the one or more engines independently of low saturation pressure of the LNG in the LNG tank 2. The LNG will then be withdrawn from the LNG tank 2 through the outlet conduit 4 and the pump inlet conduit 6 and fed into the pump 11 , whereafter the LNG is fed to the vaporizer and/or the pressure build-up unit (PBU) and back into the LNG tank 2 through the pump return conduit 5 and sub-conduit 5a, in order to supply gas to the consumer and/or increase the pressure in the LNG tank 2. Alternatively, the LNG may be returned to the bottom of the LNG tank 2, through the pump return conduit 5 and sub- conduit 5b, thereby adding heat to the LNG. Once the LNG has reached the saturation pressure and desired temperature, the pump 11 is shut down, and the gas supply is maintained by the LNG saturation pressure and gravity feed mode. Figure 2 depicts another embodiment of the system according to the present invention which can be utilized with a fuel system for a vessel:

The LNG tank 2 is connected to a pressure build-up unit 12 through a gravity feed conduit 16 and an inlet conduit 3. The inlet conduit 3 is divided into two sub-conduits 3a, 3b, where the sub-conduit 3a is arranged to be on a top of or in an upper area of the LNG tank 2, while the second sub-conduit 3b is arranged to extend towards the bottom of the LNG tank 2.

A pump 11 is submerged in and arranged near the bottom of the LNG tank 2 and is connected to a vaporizer (VAP) 13 through an outlet conduit 20, a pump outlet conduit 7 and a vaporizer inlet conduit 8. The pump 11 is also connected to the LNG tank 2 through a pump return conduit 5, such that LNG can be fed back to the LNG tank 2. The pump return conduit 5 is furthermore divided into two sub-conduits 5a, 5b, the sub-conduit 5a being arranged to be on the top of or in the upper area of the LNG tank 2, and the other sub- conduit 5b being arranged to extend towards the bottom of the LNG tank 2, such as to be able to return back LNG either to top of the LNG tank 2, to bottom of the LNG tank 2 or both to top and bottom of the LNG tank 2.

The pump 11 and the pressure build-up unit 12 are also in fluid communication with each other through the outlet conduit 20, the pump outlet conduit 7 and a pressure build-up conduit 10.

A conduit 18 will set the vaporizer (VAP) 13 and a super heater (SH) (heat exchanger) in fluid communication with each other, whereby a gas can be supplied, from the super heater 14 (heat exchanger) to an end-user or consumer 15 through a conduit 19.

To supply gas to the engine(s), according to this embodiment, liquefied natural gas (LNG) is withdrawn from the LNG tank 2 through conduit 20, or through the gravity feed conduit 16 by means of gravity forces and directed to the vaporizer 13, whereafter heat is added to the LNG in order to transfer the natural gas to a gaseous phase. Thereafter the gas is fed to the super heater 14 (heat exchanger), where the gas is heated to a temperature required by the one or more gas engines.

However, when LNG is withdrawn from the LNG tank 2, the pressure in the LNG tank 2 will decrease. As it is desired to maintain the pressure in the LNG tank 2 at least above a minimum pressure required by the one or more gas engines, some LNG is withdrawn from the LNG tank 2 and is fed to the pressure build-up unit 12 in order to vaporize and heat up the LNG. This can either be done by using the gravity feed conduit 16 or by using the pump 11 to feed the LNG to the pressure build-up unit 12. In this case the LNG will be fed through the outlet conduit 20, the pump outlet conduit 7 and the pressure build-up conduit 10.

The vaporized and heated LNG will thereafter by returned to the top of the LNG tank 2 through the inlet conduit 3 and sub-conduit 3a, in order to increase the pressure in the LNG tank 2. Alternatively, the vaporized and heated LNG may be returned to the bottom of the LNG tank 2, through the inlet conduit 3 and sub-conduit 3b, thereby adding heat to the LNG. It should be understood that the LNG may be returned to both the top and the bottom of the LNG tank 2, where both sub-conduits 3a, 3b are used.

In order to avoid a situation where the gas pressure in the LNG tank 2 is reduced below the minimum consumer supply pressure, for instance during sloshing or other movements of the liquid in the LNG tank 2, the pump 11 is used. The pump 11 will then secure sufficient supply of gas to the one or more engines independently of low saturation pressure of the LNG in the LNG tank 2. The LNG will then be withdrawn from the LNG tank 2 through the outlet conduit 4 and the pump inlet conduit 6 and fed into the pump 11 , whereafter the LNG is fed to the vaporizer (VAP) and/or pressure build-up unit (PBU) and back into the LNG tank 2 through the pump return conduit 5 and sub-conduit 5a, in order to supply gas to the consumer and/or increase the pressure in the LNG tank 2. Alternatively, the LNG may be returned to the bottom of the LNG tank 2, through the pump return conduit 5 and sub-conduit 5b, thereby adding heat to the LNG.

Once the LNG has reached the saturation pressure and desired temperature, the pump 11 is shut down, and the gas supply is maintained by gravity feed mode only.

Figure 3 shows how two systems as described according to figure 1 can be connected to form a common system. Each of the systems may then function independently of each other, i.e. to supply their own end-user or consumer 15 with gas, or to cooperate with each other or to be a back-up system for each other, thereby also providing a redundancy in the common system.

The common system comprises a first system 1A and a second system IB, where the first and second systems 1A, IB are described according to figure 1. A common system may also be a combination of systems according to figure 2, or a combination of systems according to both figure 1 and 2.

As can be seen, the gravity feed conduits 16 of the first and second system 1A, IB are connected through a connecting conduit 16 A. Furthermore, the division point between the conduits 7, 8, 9 and 10 in the first and second system 1A, IB are also connected through another connecting conduit 20A. A conduit 18A is connected to the conduit lines 18 of the first and second system 1A, IB, whereby LNG from the second system IB, after being transferred to gas in the vaporizer 13, can be fed to the conduit 18 in the first system 1A. Similarly, gas heated in the super heater 14 (heat exchanger) in the second system IB, can be fed from a conduit 19 in the second system IB, to a conduit 19 in the first system 1A, thereby feeding heated gas to an end-user or consumer 15. The connecting conduit 20A may also be connected to either the conduit 18A or conduit 19, in order to be able to feed the gas either after the vaporizer 13 or the super heater 14 (heat exchanger).

The common system is run in the same way as described for the system according to figure 1 and/or figure 2, i.e. both the first and second system 1A, IB when they are run independently of each other. The first and second system 1A, IB may also cooperate with other, whereby gas from the second system IB can be transferred to the first system 1A through one or more of the connecting conduit 16 A, connecting conduit 20A, conduit 18A and/or conduit 19, depending on where the gas should be transferred to in the first system 1A.

The common system may comprise one or several of the described connection pipes, and in other combinations depending on the customer requirements.

The present invention has now been explained with reference to embodiments, but a person skilled in the art will understand that changes and modifications will be able to be made to these embodiments which lie within the scope of the invention as defined in the following claims.