WO2020226504A1

WO2020226504A1 - A method and a system for heating lng before it enters a storage tank of a ship or other gas

Info

Publication number: WO2020226504A1
Application number: PCT/NO2020/050103
Authority: WO
Inventors: Ann Rigmor Nerheim
Original assignee: Ann Rigmor Nerheim
Priority date: 2019-05-09
Filing date: 2020-04-24
Publication date: 2020-11-12
Also published as: NO20190575A1; NO345279B1

Abstract

The present invention relates to a method and system for heating LNG before the LNG enters the storage tank onboard a ship or other gas consumer vessel. The invention provides a solution for pressure control in storage tanks of LNG or other cryogenic fluids, which maintains stable pressure during gas consumption from the storage tank without the use of pressure build-up unit or LNG pump downstream of the LNG storage tank during gas supply to the engine.

Description

A method and a system for heating LNG before it enters a storage tank of a ship or other gas consuming unit.

Technical field

The present invention relates to a method and system for heating LNG before the LNG enters the storage tank onboard a ship or other gas consumer vessel, as disclosed in the preamble of the independent claims. The invention provides a solution for pressure control in storage tanks of LNG or other cryogenic fluids, which maintains stable pressure during gas consumption from the storage tank without the use of pressure build-up unit or LNG pump downstream of the LNG storage tank during gas supply to the engine. Background art

In marine LNG fuel systems delivered today the philosophy is to bunker as cold LNG as possible into the storage tank onboard the ship or other gas consumer vessel. An example is as described in EP 2212186B1. In these LNG fuel systems, the supply pressure to the engine is maintained mainly in either of two ways, or in a combination of these. The simplest systems are the so-called "gravity fed" systems, where there is no pump (EP2932147B1, US9863370B2). As LNG for engine gas supply is consumed from the liquid phase of the tank, and the volume of the gas cap in the top of the tank increases, the pressure decreases. The pressure in the LNG tank is maintained by using a pressure build-up unit (PBU), which takes LNG from the bottom of the LNG tank, vaporizes it and injects it to the gas cap in the upper part of the LNG tank. The driving force in the system is the differential pressure in the tank. The drawback with this solution is that just after bunkering, the LNG is very cold (typically -164 °C). At this temperature the vapour pressure of the LNG is lower than the minimum pressure required by the gas engine. The PBU can still be used to build pressure in the tank to obtain a supply pressure as required by the engine, but this gas cap may condensate rapidly causing a reduction in pressure. If the ship encounters moderately high seas, sloshing will occur in the tank.

Such sloshing will rapidly condensate gas from the gas phase, resulting in pressure loss. This has been a challenge to "gravity fed" systems, where the PBU is incapable of compensating for the pressure loss fast enough. Even if the PBU in such situations injects additional vapour to the gas phase, this gas will re-condensate almost immediately when it enters the gas cap due to the sloshing and the low vapour pressure of the LNG at the given temperature. This will continue until the sea state changes, or until the LNG has heated to a temperature corresponding to a vapour pressure equal to or higher than the required supply pressure to the engine. The latter would take several days, and in the meantime an alternative fuel system would be necessary for propulsion purposes. To mitigate the challenges described above, many LNG fuel systems involve an LNG pump as described in NO338906B1 and WO2018202313A1. Installation of a pump solves the pressure control challenges, but it also adds costs and increases the risk of leakages and breakdowns, and it complicates the process control system. More pipes and valves are needed, which also makes the already limited space even more complex and cramped. US 20110179810 A1 discloses a method and a system for operating an LNG fueled marine vessel. The marine vessel comprises an LNG storage tank and an LNG fueled power plant. LNG is stored in the LNG storage tank and in connection with a bunkering operation the marine vessel is supplied with LNG by connecting a source of LNG to a bunkering line of the marine vessel and subsequently supplying the marine vessel with LNG through the bunkering line. In order to achieve a faster bunkering operation, the bunkering line is cooled down prior to bunkering operation.

The present invention gives an alternative way of controlling the tank pressure and the gas supply pressure, with a simple and low-cost method and system.

Disclosure of the invention Preferred embodiments of the invention are given in the independent claims, and alternative embodiments are given in the dependent claims.

In the previously described LNG fuel systems, it has always been the goal to bunker as cold as possible LNG, in order to maximise the energy content in the LNG tank after bunkering. The density of LNG decreases with increasing temperature, and the LNG vapour pressure increases with increasing temperature. In today's LNG fuel systems, it is therefore focused on keeping the LNG cold to increase the energy content stored in the LNG tank. This is equivalent to an LNG vapour pressure lower than the supply pressure required by the engine. Without using a PBU or other means for building pressure in the tank, the tank pressure will always be defined by the LNG temperature and the corresponding vapour pressure. Therefore, a pressure build-up unit and/or an LNG pump have been necessary components in LNG fuel systems.

The present invention describes a method and a system to obtain a "self-sustaining" operating tank pressure situation during bunkering, so that there is no need for using a pressure build-up unit or an LNG pump to maintain the operating pressure in the tank during voyage. The present invention is a novel LNG fuel system processing philosophy, which takes advantage of the thermodynamics of the LNG. The invention describes a method and system which is completely different from existing LNG tank pressure control philosophies. The invention is to heat the LNG during bunkering using a heat exchange unit (HEX). When the LNG thereafter enters the storage tank, it will have a temperature as high as corresponding to a vapour pressure equal to, or higher than, the minimum supply pressure of the engine. The HEX can be a permanent unit installed onshore at the location of the LNG supplier. The HEX unit can also be a mobile unit, e.g. for use when bunkering smaller boats with LNG from LNG trucks. A mobile unit can be located onshore or on the ship being bunkered. Another alternative is to install a HEX in or along the bunkering pipe. The amount of heat to be added can be calculated based on the LNG composition, LNG temperature To from the LNG supplier, and the specifications of the

HEX unit. For example, the LNG would be heated 20-30 °C, from -164°C to ca -134°C. The LNG could also be heated less than 20°C or more than 30°C. During bunkering, the tank pressure is monitored continuously. The heat input rate through the HEX can be regulated to meet the required LNG temperature and corresponding vapour pressure in the tank, which is equal to or higher than the required supply pressure to the engine. At bunkering completion, the LNG temperature and the corresponding LNG vapour pressure in the tank will correspond to the desired tank operating pressure. As LNG is withdrawn from the bottom of the tank for supply to the engine, the pressure in the tank will be sustained by natural evaporation from the LNG. Hence, PBU or LNG pump is not required during voyage. The invention thus represents a simpler and far less costly installation onboard the ship or other gas consumer. Because of the reduced amount of piping, valves and equipment like pump and sensors, the risk of leakages is reduced, and the space allocated by the LNG fuel system is reduced. The system can be combined with a re-condensation unit for condensation of part of the gas cap in cases where the tank pressure increases above a maximum pressure limit. Such a situation could occur if the gas consumption is low compared to the heat inflow to the tank with time. Brief description of drawings

In the following, the invention will be described in more detail with reference to the accompanying exemplary drawings, in which

Figure 1 illustrates a configuration of the invention with a permanent heat exchanger unit located onshore. Figure 2 shows an alternative where the heat exchanger is a mobile unit that can be located onshore or onboard the ship.

Figure 3 illustrates a permanent heat exchanger unit located onboard the ship. Figure 4 shows an alternative to Figure 3, with the heat exchanger built around a length of the bunkering line onboard the ship.

Figures 5, 6 and 7 are alternative configurations to Figure 4, with the heat exchanger unit built around a length of one or several of the bottom and top bunkering lines of the LNG tank onboard the ship.

Figures 8, 9, 10 and 11 show the configurations of Figures 1-7 with the addition of one or more bypass lines, so that the whole, part of, or no part of the bunkering flow can be routed through the bypass instead of through the heat exchanger unit.

In Figure 12 the invention is shown applied on a fuel system containing more than one LNG storage tank as one of several possible configurations.

Figure 13 shows an embodiment in which a re-condensation unit has been included.

Figure 14 shows a simplified drawing of a typical LNG fuel processing system used today. For simplicity, valves are not shown.

Figure 15 shows the corresponding and simple processing system that would be required if the present invention is used during bunkering. Valves are not shown, since this would be obvious to a person skilled in the art.

Detailed description of drawings

Figure 1 Shows one of several embodiments of the invention. The system 1 contains a ship or a gas consumer 2, including an LNG tank 3. An LNG plant or LNG supplier delivers LNG into a bunkering line 6 located onshore or on a quay 4. A heat exchange unit 5 permanently installed onshore or on the quay 4 heats the LNG by transferring heat from a heating fluid without mixing with the LNG flowing in the bunkering pipe. Typically, 5 will consist of a shell and tube heat exchanger or a shell and plate heat exchanger or the like. The heat exchanger 5 can be one unit or a combination of several units. The heating fluid can be delivered from an onshore facility, from the ship, or it can be sea water fed directly to the heat exchanger unit. The output line 7 from the heat exchanger unit 5 delivers the heated LNG to the bunker line 8 of the ship or gas consumer 2. The LNG tank 3 is bunkered from line 8 through line 9 to the top of the LNG tank or through line 10 to the bottom of the tank. The complete set of valves and sensors are not shown in the figures, since these are evident for a person skilled in the art. During bunkering, top or bottom bunkering of the tank is regulated using valves 11a and lib. Tank level measurements and tank pressure on the receiving ship is monitored on the control panel of the ship, and the data is continuously sent to the bunkering operation control station from a transducer 12 by wireless communication 13. The heat exchange rate data, including temperature of LNG and heating fluid at input and output of the unit, flowrate and gas sensor data are measured and sent from the heat exchange unit 5 using a transducer 14 and wireless communication 15 to the bunkering operation control station 16. The LNG pump of the LNG supplier facility is not shown in the figure, since this pump in most cases will be located upstream of the heat exchanger unit 5. Alternatively, the LNG pump can be installed in or downstream of the heat exchanger unit. Gas supply to the engine is mainly supplied from the liquid phase of the LNG tank through line 20 and a valve 20a. The LNG from line 20 is vaporized in a Main Vaporizer and the gas heated in a Gas Heater to the temperature required by the gas engine. This is a well-known processing stage, as described in prior art and therefore not shown in detail in the present invention. Simplified re-gasification process system drawings are given in Figures 14 and 15. In given situations, gas can also be supplied to the engine from the gas phase of the LNG tank through a pipe 20b and valve 20c, and heated by the same Main Vaporizer and Gas Heater.

In another embodiment, as illustrated in Figure 2, the heat exchanger can be a mobile unit 5a. A mobile heat exchanger can be transported around together with an LNG truck, which is often used for bunkering smaller LNG ships or tanks. The principle of the heat exchanger unit is as described above.

An alternative to the configurations shown in Figure 1 and 2 is to install the heat exchanger unit onboard the ship, as shown in Figure 3. A heating unit 5b similar to the one described above, is installed permanently onboard the ship. In this case the most practical solution is to use a heating medium supplied by the ship, but it could also be connected to a heating fluid supplier onshore.

Figure 4 illustrates yet another embodiment, in which the heat exchanger unit 5c is built around a length of the main bunkering pipe 8, either onshore or onboard the ship. The length of bunkering pipe to be included in the heat exchange unit will depend on the dimension of the bunkering pipe, heat exchange coefficients, heating medium temperature, flow rates of LNG and heating medium and can be specified for each individual ship.

The configuration in Figure 5 is similar to Figure 4, but with the heating unit 5d installed around the bottom bunkering line 10. In Figure 6 an alternative is shown, where the heating unit 5e is installed around the top bunkering line 9. Figure 7 shows a combination of the solutions illustrated in Figure 5 and 6, with heating units 5d and 5e around the bottom bunkering line 10 and top bunkering line 9.

In order to control the end temperature of the LNG in the LNG tank onboard the ship the heat exchanger unit(s) can be combined with a bypass line (17, 17a, 17b, 17c, 17d and/or 17e), as illustrated in Figures 8, 9, 10 and 11. With the bypass line(s), the resulting LNG temperature in the LNG tank can be adjusted by allowing the whole, part of it, or no part of the bunkering flow to pass through the bypass instead of through the heat exchanger unit by regulating valves (18, 18a, 18b, 18c, 18d and 18e and 19, 19a, 19b, 19c, 19d and 19e. The need of, and dimension of, the bypass will depend on the heating medium temperature, LNG and heating medium flow rates, and heat exchange unit specifications, but can also be customized. The regulation of the valves can be automatic and integrated in the heat exchange control system 16. The control system 16 can be a permanent installation or a handheld mobile unit.

Figure 12 illustrates that the invention can easily be adapted for ships containing more than one storage tank. The invention can also be combined with a re-condensation unit 22, as illustrated in Figure 13, to be used if the pressure in the tank rises above a given limit. The re-condensed gas can be injected to the supply line or returned into the tank gas or liquid phase. For simplicity, the heat exchanger bypass lines, the gas supply line from the top of the LNG tank (20b) and connected units are not shown in Figures 12 and 13. Flowever, in most cases these components will be included in combination with one or more of the described embodiments. Figure 14 shows a simplified drawing of a typical LNG fuel processing system used today. For simplicity, valves are not shown. An LNG pump is fed through a line 20' from the bottom of the tank. Alternatively, it could be fed from the line 20. The pump may feed both the PBU 33 and the vaporizer/heater 36. The pump 30 ensures sufficient supply pressure to the engine 38.

Figure 15 shows the LNG fuel processing system for a ship where the LNG has been heated during bunkering, as described in this invention. The PBU and pump are not necessary components of the processing system any more, and pipeline 32 could be removed. In one embodiment a gas line 39 can be connected between the output of the vaporizer/heater 36 and the top of the tank. Valves are not shown, since this would be obvious to a person skilled in the art.

As can be understood from the above, the configuration can be custom made for each ship. The configuration can also easily be standardized. The invention is easy to customize to the individual ship or fleet. Only a few of the possible combinations are described here.

Claims

1. A method of heating LNG before it enters the storage tank of a ship or other gas consuming unit, characterized in that:

the LNG is heated by one or several permanent heat exchange unit(s) (5) installed at the LNG supplier facility or onboard the ship, or

the LNG is heated using one or several mobile heat exchange unit(s) (5a) that can be transported together with e.g. an LNG truck to the site where bunkering is taking place and located onshore or onboard the ship, or

the LNG is heated by one or several heat exchange unit(s) (5b, 5c, 5d, 5e) installed in or around any of the bunkering lines onboard the ship or other gas consumption unit, or

the LNG is heated by a combination of any of said heat exchanger units,

wherein

a bypass line (17) is connected to the one or several of the heat exchanger units, and the flow through the bypass can be regulated by one or more valves (18) and (19) in order to adjust the temperature of the combined flow from the heat exchanger and the bypass line to the desired LNG bunkering temperature in the receiving LNG tank, and

the heating medium is delivered from the on-shore facility, or from the ship, or the heating medium is sea water fed directly to the heat exchange unit or pumped by the heat exchange unit itself.

2. Method according to claim 1, characterized in that the heating medium can be maintained by an arrangement internal of the heat exchanger unit, or it may be supplied from an external source of the heat exchanger.

3. Method according to claim 1, characterized in that the heat exchange unit can be controlled by a bunkering operator connected to the onshore supplier facility, or onboard the ship or other receiving LNG consumer, or from a handheld mobile control unit.

4. Method according to claim 1, c h a r a c t e r i z e d by supplying gas to one or more engines or gas consumers from an LNG tank where the pressure is sustained by the vapour pressure of the LNG at the given temperature, and where vapour gas is not injected from a heat exchange unit downstream the tank in order to maintain or increase the pressure in the LNG storage tank.

5. A system for heating LNG during bunkering to one or several LNG tank(s) onboard a ship or onboard other gas consumption units by using the method of claim 1, c h a r a c t e r i z e d in that: the LNG is heated by one or several permanent heat exchange unit(s) (5) installed at the LNG supplier facility or onboard the ship, or

the LNG is heated by one or several heat exchange unit(s) (5b, 5c, 5d, 5e) installed in or around any of the bunkering lines onboard the ship or other gas consumption unit,

the LNG is heated by a combination of any of said heat exchanger units,

wherein

- the heating medium is delivered from the on-shore facility, or from the ship, or the heating medium is sea water fed directly to the heat exchange unit or pumped by the heat exchange unit itself.

6. System according to claim 5, c h a r a c t e r i z e d in that one or several heat exchange unit(s) are constructed as a shell and tube heat exchanger, shell and plate heat exchanger or the like.

7. System according to claims 5-6, characterized in that the heating medium is supplied from said onshore facility, or from the internal heating/cooling circuit onboard the ship or other gas consumer facility receiving the LNG, or the heating medium is sea water.

8. System according to any of the above claims 5-7, characterized in that several heat exchange units are combined, where one or all of the heat exchange units may have different heating medium from the rest.

9. System according to claim 5, characterized by including one or several bypass(es) with connected valves, so that the whole, part of it, or no part of the bunkering flow can be routed through the bypass instead of through the heat exchanger unit(s), wherein the end temperature of the LNG in the tank onboard the ship after bunkering can be efficiently regulated so as to give the desired vapour pressure at end of bunkering.

10. System according to claim 5, characterized by comprising a control system for monitoring the tank pressure and LNG temperatures and regulating the heat exchange rate to the LNG.

11. System according to claim 5, characterized by comprising a control system consisting of a transducer connected to the ship control system,

sending wireless tank pressure and filling level data to the bunkering station operator, in addition to being monitored on the ship control system, and

a transducer and receiver installed on the heat exchange unit to send heat transfer data to the bunkering operator such as temperature and flow rate, and

a receiver to receive control signals to regulate the heat transfer from the heat exchanger unit to the LNG.

12. System according to claim 5, characterized by a control system with wired or wireless communication, or a combination.

13. System according to claim 5, characterized in that the bunkering operation control station can be a permanent installation, or a handheld unit.

14. System according to claims 5, characterized in that the processing system downstream the tank includes a re-condensation unit.

15. System according to claim 5, characterized in that the LNG fuel system consists of an LNG storage tank and one or more downstream heat exchangers for supply of vapour gas to one or more engines or gas consumers, and where there is no injection of gas from the downstream heat exchangers back into the LNG storage tank.

16. Use of a method according to claims 1-4 or a system according to claims 5-15,

characterized in that the transferred fluid is other fluids or cryogenic fluids other than LNG.

17. Use of a method according to claims 1-4 or a system according to claims 5-15,

characterized in that said method or system is combined with other processing components like pressure build-up unit (PBU) and/or pump onboard the ship or other gas consuming or fluid processing vessel.