WO2015067840A1

WO2015067840A1 - Method and arrangement for pressure build-up in a gas tank containing liquefied gas fuel

Info

Publication number: WO2015067840A1
Application number: PCT/FI2013/051061
Authority: WO
Inventors: Sören KARLSSON
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2013-11-11
Filing date: 2013-11-11
Publication date: 2015-05-14
Anticipated expiration: 2016-05-11

Abstract

A fuel storage and distribution system for a sea-going vessel comprises a gas tank for storing liquefied gas fuel. A heating element is provided for heating said gas fuel while it is inside said gas tank. Said heating element comprises an upper part and a lower part, of which said upper part is located within an upper portion of said gas tank and said lower part is located within a lower portion of said gas tank.

Description

Method and arrangement for pressure build-up in a gas tank containing liquefied gas fuel

TECHNICAL FIELD

The invention concerns in general the technology of using liquefied gas fuel, for example in sea-going vessels. In particular the invention concerns a way in which a suitable overpressure is maintained in a gas tank to ensure that liquefied fuel flows to the fuel distribution and delivery systems in an appropriate way.

BACKGROUND OF THE INVENTION Natural gas, or in general mixtures of hydrocarbons that are volatile enough to make the mixture appear in gaseous form in room temperature, constitutes an advantageous alternative to fuel oil as the fuel of internal combustion engines. In sea-going vessels that use natural gas as fuel, the natural gas is typically stored onboard in liquid form, giving rise to the commonly used acronym LNG (Liquefied Natural Gas). Natural gas can be kept in liquid form by maintaining its temperature below a boiling point, which is approximately -162 degrees centigrade (-260 degrees Fahrenheit). Natural gas can be also stored for use as fuel by keeping it compressed to a sufficiently high pressure, in which case the acronym CNG (Compressed Natural Gas) is used. This description refers mainly to LNG because liquefying is considered more economical than compressing at the time of writing this text.

Fig. 1 illustrates schematically the architecture of a known system onboard an LNG-fuelled vessel. An LNG bunkering station 101 is located on the deck and used to fill up the system with LNG. The LNG fuel storage system comprises one or more thermally insulated gas tanks 102 for storing the LNG in liquid form, and the so-called tank room 103 where the LNG is controllably evaporated and its distribution to the engine(s) is arranged. Evaporation means a phase change from liquid to gaseous phase, for which reason all subsequent stages should leave the L for liquefied out of the acronym and use only NG (Natural Gas) instead.

The engine or engines of the vessel are located in an engine room (not shown in fig. 1 ). Each engine has its respective engine-specific fuel input subsystem, which in the case of gaseous fuel is in some sources referred to as the GVU (Gas Valve Unit). The tank room 103 of fig. 1 comprises two evaporators, of which the first evaporator 104 is the so-called PBU (Pressure Build-Up) evaporator used to maintain a sufficient pressure inside the gas tank 102. Hydrostatic pressure at the inlet of a main supply line 105 inside the gas tank 102 is the driving force that makes the LNG flow into the second evaporator 106, which is the MGE or Main Gas Evaporator from which the fuel is distributed in gaseous form towards the engines. A mixture of glycol and water is used to transfer heat from an external source (not shown) to the evaporators. In order to ensure that evaporated gas flows to the GVU(s) and further to the engine(s) at sufficiently high pressure, the PBU system maintains the internal pressure of the gas tank 102 at or close to a predetermined value, which is typically between 5 and 10 bars.

Fig. 2 illustrates schematically some parts of a PBU system in more detail. Maritime classification regulations stipulate that two barriers must always sepa- rate gas fuel from safe areas. The gas tank 102 has a double wall structure, in which the space 201 between the walls is utilized for thermal insulation. The walls 202 of the tank room 103 constitute a second barrier for all piping and installations inside the tank room 103. All gas pipes that go between the gas tank 102 and the tank room 103 must have a double wall structure such as shown schematically in gas pipe 203.

Despite all the care and expertise that is used in manufacturing the system, all gas pipes to and from the gas tank 102 are inherently more vulnerable to mechanical failure than the gas tank 102 itself. A break or leak in the gas pipe at the point illustrated with arrow 204 would quickly cause extremely cold lique- fied gas to flood at least the inside of the outer wall of the gas pipe 203, possibly also the space 201 between the walls of the gas tank 102, and possibly even the tank room 103. Prior art solutions are known from the patent publications WO 2012/032219 A1 and WO 2013/128063 A1 that partly tackle the problem; the first-mentioned introduces a baffle connection from the outer wall of the gas pipe 203 to the inner wall of the gas tank 102, while the other publication suggests continuing the outer wall of the gas pipe 203 as far as the body of the shut-off valve 205. Nevertheless it would be even safer if it could be ensured that even a mechanical breakdown would not cause the cold, liquefied gas fuel to flood undesired locations. SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

According to an aspect of a present invention, there is provided a fuel storage and distribution system for a sea-going vessel, in which fuel storage and distribution system the risk of liquefied gas unexpectedly occurring at inappropriate locations is small . According to another aspect of a present invention there is provided a fuel storage and distribution system that has advantageous features from the manufacturing point of view. According to yet another aspect of the present invention there is provided a method for building up pressure inside the gas tank of a gas-fuelled sea-going vessel, which method utilizes the advantageous characteristics mentioned above.

Advantageous objectives of the invention are achieved by building up the pressure of the gas fuel by heating it while it is still inside the gas tank. This way no liquefied gas fuel needs to be led out of the gas tank for pressure buildup evaporation, which consequently eliminates a possible flooding risk. A heating element, such as a heat exchanger, inside the gas tank can be used for this purpose: when a heating medium circulates through a heat exchanger, it donates heat to the gas fuel by condensing and/or cooling down while it is in- side the heat exchanger. Also other types of heating elements can be used.

During use the gas tank will be partly filled with liquefied gas, while a gaseous phase of the same substance fills the remaining upper portion of the gas tank. The effect of a heating element will be different depending on whether it donates heat to the gaseous or liquid phase of the gas inside the gas tank. Con- trolling of the heating may be easier if the construction of the heating element ensures that it always heats both phases, irrespective of the surface level of the liquid phase. This can be achieved by making the heating element comprise an upper part and a lower part, located in the upper and lower portions of the gas tank respectively. A channel that only contains heating medium is inherently safer in case of mechanical failure than a pipe that contains liquefied gas, because the amount of heating medium that circulates in the pressure build-up system is very much smaller than the amount of stored liquefied gas. This means that the pressure build-up circulation may safely contain pipe sections outside the gas tank that are below the surface level of the liquefied gas in the gas tank. If bottom-level lead-throughs in the gas tank wall are to be avoided, the channel for the heating medium can be formed so that it penetrates the gas tank wall always at a relatively high level. Reheating the circulated heating medium may take place in an evaporator or heat exchanger that is located close to the gas tank, for example in the tank room or an associated space. Various solutions are available for use as the heat source that reheats the heating medium, each with their own advantages. A control system is preferably arranged that monitors the pressure and surface level inside the gas tank and controls the operation of the pressure build-up system so that the pressure remains within the desired range.

The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 illustrates a prior art LNG fuel distribution architecture,

fig. 2 illustrates an exemplary pressure build-up system according to prior art,

fig. 3 illustrates schematically a pressure build-up system according to an embodiment of the invention, fig. 4 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 5 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 6 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 7 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 8 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 9 illustrates schematically a pressure build-up system according to another embodiment of the invention,

fig. 10 illustrates schematically a pressure build-up system according to another embodiment of the invention, and

fig. 1 1 illustrates schematically a control architecture of a pressure build-up system according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 3 is a simplified schematic partial cross section of certain parts of a fuel storage and distribution system for a sea-going vessel. The application envi- ronment being a sea-going vessel has some important consequences: first and foremost, we must then assume that strict maritime classification requirements apply. An example of such maritime classification requirements is constituted by the classification regulations of the Germanischer Lloyd Aktiengesellschaft, which comprise a volume VI, Part 3 (Machinery Installations), Chapter 1 : Guidelines for the Use of Gas as Fuel for Ships, published in 2010. Another consequence concerns the assumed dimensions of the system: as an example, a gas tank sufficiently large to supply the engine(s) of a sea-going vessel with fuel over a reasonable period typically has a net volume of at least dozens of cubic metres, and possibly several hundreds of cubic metres. For example at the time of writing this description the standard gas tank sizes of the commercially available Wartsila LNGPac series range from 3.5 to 5.0 metres in diameter and 16.7 to 23.5 metres in length. Yet another consequence of the maritime application environment is the likely occurrence of unexpected movements: sea-going vessels may encounter high seas, causing the whole vessel to sway and shake in a variety of ways and directions. The left portion of fig. 3 shows a part of a gas tank 301 for storing liquefied gas fuel. Assuming that the gas fuel is natural gas, the vapour pressure of which is 1 bar at about -160 degrees centigrade and 10 bar at about -120 degrees centigrade, we may conclude that the temperature of the liquefied gas fuel is be- low -120 degrees centigrade and may be even in the order of -160 degrees centigrade. The gas tank has a thermally insulating double wall structure that as such conforms to prior art. We assume that the pressure inside the gas tank 301 should be maintained within a predetermined range; typically the pressure value to be aimed at is somewhere between 5 and 10 bars. Leading gas fuel out of the gas tank 301 for burning in the engine(s) of the sea-going vessel tends to decrease the pressure inside the gas tank 301 . The pressure build-up system that is used to maintain the pressure within said predetermined range comprises a heating element 302 for heating the gas fuel while it is inside the gas tank 301 . In particular, the heating element 302 of fig. 3 comprises a heat exchanger that comprises a channel 303 for circulating heating medium through the heat exchanger. Mechanisms for transferring heat from the heating medium to the gas fuel may comprise condensation of initially gaseous heating medium inside the channel 303 and/or conduction of heat from the warmer heating medium through the structures of the heat exchanger to the colder gas fuel.

In equilibrium state the system consisting of the liquid and gaseous phases of the gas fuel would have a constant temperature, and the pressure of the gaseous phase would equal the vapour pressure at that temperature. Donating heat to the gaseous phase causes superheating of the gaseous phase, which in turn causes a corresponding increase in pressure, because no essential changes in volume are possible. Heating the gaseous phase is illustrated in fig. 3 with schematic heat rays. Donating heat to the liquid phase causes an increase in temperature of the liquefied gas and may cause local boiling, which in turn increases pressure in a similar way. Heating the liquid phase is illustrat- ed in fig. 3 with schematic bubbles, although it should be noted that the liquid phase is not necessarily heated enough to make it boil.

In addition the donation of heat both to the gaseous and the liquid phase causes convective flows, in which warmer portions of the gaseous or liquid medium float upwards inside the gas tank 301 . The significance of such con- vective flows may depend on other movements of the sea-going vessel; for example in high seas the natural sloshing of the liquid phase may be far more significant than any heat-induced convective flows.

If forces that would cause significant internal flows inside the gas tank 301 are small, for example when the sea-going vessel is at port or cruises along a route through protected waters, and if no preventive action is taken, temperature stratification may develop. This means that a considerable mass of relatively cold LNG remains at the bottom of the tank, while only a topmost layer and the gaseous phase above it are warmer. A pressure reading of the gaseous phase increases relatively fast as a function of donated heat, but the pres- sure is prone to instantaneous collapsing if for example a larger movement of the sea-going vessel suddenly causes the liquid phase to splash so that the temperature differences even out. Temperature stratification may thus be considered detrimental to the aim of maintaining good control of the pressure.

In order to ensure that at least some heating of both the gaseous and the liquid phase takes place irrespective of how full the gas tank 301 is, and also in order to fight temperature stratification by donating heat at various heights inside the gas tank, it may be advisable to have the heat exchanger comprise an upper part and a lower part. The upper part is located within an upper portion of the gas tank 301 and the lower part is correspondingly located in the lower portion of the gas tank 301 . The terms "upper" and "lower" refer to the position and orientation in which the gas tank 301 would appear during normal use on board the sea-going vessel. In the embodiment of fig. 3 the lower part consists of a pipe 304 extending from the upper part to a lead-through 305 located in a wall (here: the bottom wall) of the lower portion of the gas tank 301 . Although there is a lead-through 305 located within the lower portion of the gas tank 301 , it does not constitute a similar flooding risk as in the prior art arrangement of fig. 2. A mechanical failure of the channel would only cause some heating medium to leak out (or, if the mechanical failure was inside the gas tank, some heating medium to mix with some gas fuel inside and/or around the channel depending on the pressure differences). While the amount of liquefied gas fuel in the gas tank 301 may be hundreds of cubic metres, the amount of heating medium must necessarily be several orders of magnitude smaller. Its temperature may, however, be between -160 and -120 degrees centigrade when it has circulated through the whole heating element 302 in- side the gas tank. In order to protect adjacent structures and materials, as well as crew members in the vicinity, against the hazards of extremely cold substance leaking out of a possible break, it may be advisable to cover the heating medium pipes to and from the gas tank with double walls.

The exact nature of the arrangement that is used to reheat (and possibly re- evaporate) the heating medium outside the gas tank 301 is not defined in the embodiment of fig. 3. Shown schematically is an external heat exchanger or evaporator 306 that is coupled to the channel 303 for heating and/or evaporating the heating medium while it circulates through the external heat exchanger or evaporator 306. The external heat exchanger or evaporator 306 could also be called a re-boiler. Input and output valves 307 and 308 of the heating element, controlled by respective remotely controlled actuators 309 and 310, can be used to control the flow of the heating medium through the channel 303, which in turn affects the power at which heat is donated to the gas fuel.

According to one embodiment of the invention, the device referred to as 306 in fig. 3 is an evaporator or re-boiler. Heating medium is evaporated, which causes it to rise upwards and flow through the input valve 307 to the heating element 302. Condensation of the initially gaseous heating medium within the upper part of the heat exchanger donates heat to the gaseous phase of the gas fuel inside the gas tank 301 . The condensed heating medium flows downwards in the pipe 304 under the effect of the Earth's gravity, donating residual heat on the way by conduction to the surrounding liquid phase of the gas fuel. Eventually the heating medium flows out of the heating element through the output valve 308 and enters the evaporator or re-boiler 306, in which the cycle starts anew. Fig. 4 illustrates a slightly different embodiment that aims at avoiding lead- throughs in the lower portion of the gas tank 301 to the largest extent possible. The two lead-throughs 401 and 402 that are needed for allowing heating medium to flow through the channel 303 to and from the heating element inside the gas tank 301 are located in a wall (here: the side wall) of the upper portion of the gas tank 301 .

In the embodiment of fig. 4 the heat exchanger that acts as a heating element inside the gas tank 301 has nevertheless a lower part that is located within the lower portion of the gas tank 301 . Said lower part comprises a pipe section 403 that extends from the upper part - located within the upper portion of the gas tank 301 - towards the bottom of the gas tank 301 . In order to take the channel to the outgoing lead-through 402 said lower part comprises also a pipe section 404 leading upwards. As a result, contrary to the case illustrated in fig. 3, the heating medium inside the channel does not have a continuously descending return path towards the external evaporator 306. Also the flow resistance experienced by the heating medium may be higher than in the case of fig. 3. In order to ensure sufficient flow of the heating medium, the dimensioning of the system should ensure that the surface level of liquid heating medium inside the whole channel is always high enough to allow a well-working siphon effect to draw the returning heating medium out of the heating element. Alternatively or additionally a circulation pump may be used to ensure a sufficient flow of the heating medium.

Above it was already pointed out that changes in the surface level of the liquid phase of the gas fuel may affect the way in which heat transfers from the heat- ing medium to the gas fuel. For most of the time in use, the surface level of the liquid phase will be somewhere between the very top and the very bottom of the gas tank. Therefore it may be advantageous to build the heating element in the form of a two-part heat exchanger, as illustrated in fig. 5. At the upper part of the heat exchanger is an upper heat transfer element 501 , constructed in such a way that it is effective in transferring heat from a heating medium flowing through the channel to the surrounding gaseous phase of the gas fuel. At the lower part of the heat exchanger is a lower heat transfer element 502, which in turn is constructed so that it is effective in transferring heat from a heating medium flowing through the channel to the surrounding liquid phase of the gas fuel.

A pipe 503 connects the upper heat transfer element 501 to the lower heat transfer element 502, so that the same heating medium will flow through both of them. The pipe 503 may be constructed so that it does not transfer heat very effectively from the heating medium flowing through it to its surroundings. The pipe 503 may even have a thermally insulating jacket, and/or it may go at least partly through the space between the inner and outer shells of the gas tank. Fig. 5 shows there to be a bottom lead-through 504 for the return path of the heating medium, in which sense the solution resembles that of fig. 3. However, the return path could equally well take a route like that shown in fig. 4, with similar consequences concerning the need to ensure sufficient flow of the heating medium. The possible controlling difficulties that the changing surface level of the liquid phase causes may be avoided by placing the heating element so that substantial transfer of heat only takes place at the very bottom and at the very top of the gas tank. Also in order to heat the gas fuel while it is inside the gas tank the heating element does not itself need to be inside the gas tank. In the embodiment that is schematically shown in fig. 6 the heating element comprises an upper heat transfer element 601 , a lower heat transfer element 602, and a channel 603 for circulating heating medium through them. The heat transfer elements 601 and 602 are not heat exchangers in the sense that two fluid sub- stances exchanging thermal energy would only be separated by a single barrier. The heat transfer elements 601 and 602 are located between the inner and outer shells of the gas tank 301 , so they could be characterised as constituting heated portions of the inner shell of the gas tank 301 . Good thermal conduction should be ensured between the heat transfer elements 601 and 602 and the inner shell of the gas tank 301 . For example if a vacuum between the inner and outer shells is used as a thermal insulator of the gas tank 301 , the heat transfer elements 601 and 602 must be attached to the inner shell of the gas tank with suitable heat-conducting medium Similar heated portions could be located at various heights in the walls of the gas tank 301 . Maintaining tank pressure would basically be possible even with only one of the heat transfer elements 501 or 502, or 601 or 602 respectively. However, only heating the liquid phase, as with heat transfer elements 502 and 602, means that the pressure inside the gas tank must be maintained by continuous evaporation, which may cause difficulties in controlling the exact pressure lev- el. Only heating the gaseous phase, as with heat transfer elements 501 or 601 , is notoriously prone to causing temperature stratification. For these reasons embodiments that only involve heating one of the phases inside the gas tank may not be practical in use.

Figs. 7, 8, and 10 illustrate the fact that the number of heating elements, as well as the number of heat sources that are used to power the heating elements, are not limited by the invention. In fig. 7 the fuel storage and distribution system comprises a first heating element 701 and a second heating element 702, so that the first and second heating elements 701 and 702 are located at different heights inside the gas tank 301 . Each heating element 701 and 702 has its own external heat exchanger 703 and 704 respectively, but it would also be possible to branch the channels to two or more heating elements from a common external heat exchanger like the one illustrated as 801 in fig. 8. Branches that lead to and from the individual heating elements are most advantageously equipped with independently controllable valves 802, 803, 804, and 805 in order to be able to optimally distribute the heat to be transferred to the different phases of the gas fuel. With suitable branching pipes and suitable controllable valves all embodiments of the invention may comprise as many heating elements driven by as many heat sources as is considered convenient.

Fig. 9 illustrates the fact that one possible implementation of the heating element inside the gas tank 301 is a heat exchanger 901 that forms a continuous element between the top and bottom of the gas tank 301 . Fig. 10 illustrates an embodiment of the invention where two heating elements are used, one of them being a heat exchanger 1001 that forms a continuous element between the top and bottom of the gas tank 301 , while the other is a heat exchanger 1002 that is located in the lower part of the gas tank 301 , so that most of the time it only donates heat to the liquid phase.

Maritime classification requirements stipulate that the fuel storage and distribution system must comprise a tank room. According to the Lloyds regulations, a tank room means a gas-tight space surrounding the parts of the bunker tank containing all tank connections and all tank valves. In an LNG system the tank room can be characterised as the gastight enclosure in which there is located the evaporator that is used to vaporize liquid gas for delivery to at least one gas-fuelled engine of the sea-going vessel. The tank room constitutes a gas- tight enclosure that is either partly limited by an outer wall of the gas tank (if the tank room walls have been welded to the outer shell of the gas tank) or coupled to the gas tank through (double-walled) pipelines. The heat exchanger or evaporator, which is used to re-heat and/or re-evaporate the heating medium while it circulates through the heat exchanger or evaporator, may be located in the tank room.

Fig. 1 1 illustrates schematically an arrangement for controlling the pressure build-up operation. The central element in such controlling is a controller 1 101 , which may be for example a microprocessor. Computer-readable instructions are stored in a non-volatile memory 1 102 and, when executed by the controller 1 101 , cause the implementation of a method according to an embodiment of the invention. The method comprises heating both a gaseous phase portion and a liquid phase portion of the gas fuel while it is inside the gas tank. Said heating is controlled to maintain a pressure inside the gas tank at a predetermined value that is preferably between 5 and 10 bars.

The pressures and temperatures that prevail at various locations in the gas fuel storage and distribution arrangement can be measured with a number of suitably located pressure (P) and temperature (T) sensors 1 103. Typical action to be taken to physically control the pressure would involve opening and/or closing some valves that control the flows of gaseous and liquid media, for which purpose there are a number of appropriately placed actuators 1 104. It is also possible that the system comprises an additional heater 1 105 that is used to control the temperature of some critical part of the arrangement.

The pressure and temperature sensors 1 103, the actuators 1 104 and the possible additional heater 1 105 may be commonly designated as the physical action devices. An input and output unit (I/O unit) 1 106 serves as an interface between the controller 1 101 and the physical action devices. It exchanges infor- mation in digital form with the controller 1 101 , receives measurement signals in the form of voltages and/or currents from the pressure and temperature sensors 1 103, and transmits commands in the form of voltages and/or currents to the actuators 1 104 and the possible additional heater 1 105. The input and output unit 1 106 also makes the necessary conversions between the digital rep- resentations it uses in communicating with the controller 1 101 and the (typically, but not necessarily) analog voltage and/or current levels it uses in controlling the physical action devices.

A bus connection 1 107 links the controller 1 101 with one or more user interfaces 1 108, which may be located for example in an engine control room and/or on the bridge of the sea-going vessel. A user interface typically comprises one or more displays and some user input means, such as a touch- sensitive display, a keyboard, a joystick, a roller mouse, or the like. The display part of the user interface is used to display to a human user information about the state and operation of the gas fuel storage and distribution arrangement. The input means of the user interface are available for the user to give commands that control the operation of the gas fuel storage and distribution arrangement. A power source arrangement 1 109 derives and distributes the necessary operating voltages for the various electrically operated parts of the control arrangement.

Variations and developments of the entities described above are possible without parting from the scope of the appended claims. For example, although the illustrated examples only feature one gas tank to maintain graphical clarity, the same principles can be readily applied to systems with two or more tanks, by providing appropriate heating element(s) in all gas tanks of the system. All such heating elements may have their own external heat sources for reheat- ing, or two or more such heating elements may share a common external heat source. Heating elements need not be ones where fluid heating medium flows through a channel, but for example electric heaters (heating resistors, and/or radiation heaters) could be used.

Claims

1 . A fuel storage and distribution system for a sea-going vessel, comprising:

- a gas tank for storing liquefied gas fuel, and

- a heating element for heating said gas fuel while it is inside said gas tank; wherein said heating element comprises an upper part and a lower part, of which said upper part is located within an upper portion of said gas tank and said lower part is located within a lower portion of said gas tank.

2. A fuel storage and distribution system according to claim 1 , wherein said heating element comprises a heat exchanger that comprises a channel for cir- culating heating medium through said heat exchanger.

3. A fuel storage and distribution system according to claim 2, wherein said lower part consists of a pipe extending from said upper part to a lead-through located in a wall of the lower portion of said gas tank.

4. A fuel storage and distribution system according to claim 2, wherein all lead-throughs for allowing heating medium to flow to and from said heating element are located in a wall of the upper portion of said gas tank.

5. A fuel storage and distribution system according to any of claims 2 to 4, wherein said upper part comprises an upper heat transfer element, said lower part comprises a lower heat transfer element, and the heating element com- prises a pipe connecting said upper heat transfer element to said lower heat transfer element.

6. A fuel storage and distribution system according to any of claims 1 to 5, comprising in addition to said heating element a second heating element, so that said heating element and said second heating element are located at dif- ferent heights inside said gas tank.

7. A fuel storage and distribution system according to any of claims 2 to 6, comprising:

- a tank room that constitutes a gastight enclosure either partly limited by an outer wall of said gas tank or coupled to said gas tank through pipelines, and - an evaporator coupled to said channel for evaporating said heating medium while it circulates through the evaporator; wherein said evaporator is located in said tank room.

8. A method for building up pressure inside a gas tank configured to store liquefied gas fuel on board a sea-going vessel, comprising heating both a gaseous phase portion and a liquid phase portion of said gas fuel while it is inside said gas tank.

9. A method according to claim 8, wherein said heating is controlled to maintain a pressure inside said gas tank at a predetermined value that is between 5 and 10 bars.