WO2019185968A1

WO2019185968A1 - A fuel tank arrangement in a marine vessel and a method of switching between inert and air atmosphere in a tank connection space of an lng-fuel tank

Info

Publication number: WO2019185968A1
Application number: PCT/FI2018/050223
Authority: WO
Inventors: Emanuele D'URSO; Tomi PRITTINEN; Rasmus NYBO
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2018-03-26
Filing date: 2018-03-26
Publication date: 2019-10-03
Anticipated expiration: 2020-09-26

Abstract

The present invention relates to a fuel tank arrangement of a marine vessel comprising an LNG- fuel tank (12), a tank connection space (26) provided in communication with the LNG- fuel tank (12), the tank connection space (26) being provided with a ventilation inlet line (34) having a first fire damper valve (34'), a ventilation outlet line (36) having a second fire damper valve (36') and at least one blower (38) arranged in one of the ventilation inlet line (34) and the ventilation outlet line (36), and a vent mast (32) arranged in communication with both the LNG- fuel tank (12) and the tank connection space (26), wherein the tank connection space (26) is provided with an inert gas inlet (40) for introducing inert gas from an inert gas source (46) into the tank connection space (26).

Description

A fuel tank arrangement in a marine vessel and a method of switching between inert and air atmosphere in a tank connection space of an LNG- fuel tank

Technical field

[001 ] The present invention relates to a fuel tank arrangement in a marine vessel for storing LNG- fuel and a method of switching between inert and air atmosphere in a tank connection space of an LNG- fuel tank. More particularly, the present invention relates to such an LNG- fuel tank arrangement that the tank comprises at least one shell, a heat insulation and a tank connection space arranged at an end or at a side of the LNG- fuel tank, the tank connection space being, most of the time, provided with inert atmosphere.

Background art

[002] The use of LNG (Liquefied Natural Gas) as fuel for marine applications is increas ing since it is an efficient way of cutting emissions. Within the next few decades, natural gas (NG) is expected to become the world's fastest growing major energy source. The driving forces behind this development are the depleting known oil reserves, increasing environmental awareness and the continuous tightening of emission restrictions. All ma jor emissions can be significantly reduced to truly form an environmentally sound solu tion; the reduction in C0₂, in particular, is difficult to achieve with conventional oil-based fuels. NG consists of methane (CH₄) with minor concentrations of heavier hydrocarbons such as ethane and propane. In normal ambient conditions NG is a gas, but it can be liquefied by cooling it down to -162°C. In liquid form the specific volume is reduced sig nificantly, which allows a reasonable size of storage tanks relative to energy content. The burning process of NG is clean. Its high hydrogen-to-coal ratio (the highest among the fossil fuels) means lower C0₂ emissions compared with oil-based fuels. When NG is liquefied, all sulphur is removed, which means zero SOx emissions. The clean burning properties of NG also significantly reduce NOx and particle emissions compared with oil- based fuels. Particularly in cruise vessels, ferries and so called ro-pax vessels, where passengers are on board, the absence of soot emissions and visible smoke in the ex haust gases of ship's engines is a very important feature.

[003] LNG is not only an environmentally sound solution, but also economically inter esting at today's oil prices. The most feasible way of storing NG in ships is in liquid form. In existing ship installations, LNG is stored in cylindrical, heat insulated single- or double- walled, stainless steel tanks. [004] Non-pressurized prior art LNG tanks have normally only a single wall or shell cov ered with a heat insulation of, for instance, polyurethane. Pressurized prior art LNG tanks have an inner wall or shell of stainless steel and an outer wall or shell spaced at a dis tance from the inner shell. The inner and outer shells define an insulation space there- between. The LNG tank is provided, for emptying the tank, with at least one pipe of stain less steel connected at its first end to the LNG tank and at its second end to a tank connection space arranged at a side or at an end of the tank, either as an extension of the tank or as a separate chamber at a short distance from the tank. The tank connection space is normally a gas tight enclosure containing all tank connections, fittings, flanges and tank valves. It is constructed of cryogenic temperature resistant materials, it has a bilge well with a high level indicator and a low temperature sensor. The tank connection space (TCS) is not normally accessible, it may not be entered by personnel unless checked for sufficient oxygen and absence of explosive atmosphere. For safety reasons the TCS is provided with permanent gas detection, fixed fire detection and mechanical forced ventilation, which changes air 30 times an hour.

[005] Most of the above mentioned measures are taken to prevent the risk of explosion or combustion of the NG if natural gas from the LNG- tank happens to leak in the tank connection space. However, the blower/s required to perform the continuous ventilation of the TCS causes noise and spends a considerable amount of energy.

[006] Thus, an object of the present invention is to design such an LNG- fuel tank ar rangement for a marine vessel that at least one of the above mentioned problems is solved.

[007] Another object of the present invention is to design an LNG- fuel tank arrange ment for a marine vessel wherein the use of continuous mechanical ventilation is avoided.

[008] A further object of the present invention is to design such an LNG- fuel tank ar rangement that the tank connection space thereof is provided with inert atmosphere whenever the TCS is closed, i.e. not occupied by service and/or maintenance personnel.

Disclosure of the Invention

[009] At least one object of the present invention is substantially met by a fuel tank arrangement in a marine vessel for storing LNG- fuel, the arrangement comprising an LNG- fuel tank, a tank connection space provided in communication with the LNG- fuel tank, the tank connection space being provided with a ventilation inlet having a first fire damper valve, a ventilation outlet having a second fire damper valve and at least one blower arranged in one of the ventilation inlet and the ventilation outlet, and a vent mast arranged in communication with both the LNG- fuel tank along a line and the tank con nection space along a discharge line, wherein the tank connection space is provided with an inert gas inlet for introducing inert gas from an inert gas source into the tank connec- tion space.

[0010] At least one object of the present invention is substantially also met by a method of switching between inert and air atmosphere in a tank connection space of an LNG- fuel tank, the tank connection space being provided with a ventilation inlet having a first fire damper valve, a ventilation outlet having a second fire damper valve and at least one blower arranged in one of the ventilation inlet and the ventilation outlet, and a vent mast arranged in communication with both the LNG- fuel tank and the tank connec tion space, the method comprising initial steps of

• providing the tank connection space with an inert gas inlet, the inert gas inlet being connected to an inert gas source,

• providing a first pressure regulating valve in communication with the inert gas inlet and the inert gas source,

• providing the tank connection space with a gas outlet, the gas outlet being connected to a vent mast,

• providing a second pressure regulating valve in communication with the gas outlet and the vent mast,

• providing an oxygen analyser in connection with the tank connection space or in the discharge line between the second pressure regulating valve and the vent mast,

and further steps of

• when switching from air atmosphere to inert atmosphere, i. opening the first pressure regulating valve to allow inert gas enter the tank connection space,

ii. opening the second pressure regulating valve to allow gas to be discharged from the tank connection space, iii. determining oxygen concentration of the gas in the tank connec tion space or of the gas discharged from the tank connection space by the oxygen analyser,

iv. when the oxygen concentration reaches a predetermined value, closing at least one of the first pressure regulation valve and the second pressure regulation valve, • or

• when switching from inert atmosphere to air atmosphere, i. opening at least one first closing valve in the ventilation inlet, ii. opening at least one second closing valve in the ventilation outlet, and

iii. starting the blower.

[001 1] Other characteristic features of the present invention become apparent in the appended dependent claims.

[0012] The fuel tank arrangement of the present invention offers at least some of the following advantages:

• no continuous ventilation of the tank connection space, whereby

• continuous noise related to TCS ventilation is avoided,

• the amount of energy needed for TCS ventilation is reduced signifi cantly, and

• risk of combustion or explosion of NG in the tank connection space is reduced, or, in fact, eliminated, and

• condensation and ice build-up on cold equipment in TCS due to humid ventilation air is avoided.

Brief Description of Drawings

[0013] In the following, the present invention will be described in more detail with refer ence to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates schematically a side view of a marine vessel having an LNG- fuel tank of the present invention on the deck thereof,

Figure 2 illustrates schematically a longitudinal cross-section of an LNG- fuel tank in ac cordance with a first preferred embodiment of the present invention.

Detailed Description of Drawings

[0014] Figure 1 illustrates schematically and in a very simplified manner a marine vessel 10 with an LNG- fuel tank 12 in accordance with a first preferred embodiment of the present invention provided on the deck thereof. Naturally, the LNG- fuel tank may also be positioned below the deck. The Figure shows also the internal combustion engine 14 receiving fuel from the LNG- fuel tank 12 and the drive means 16 coupled to both the engine and the propeller 18. The drive means may here comprise either a mechanical gear or a generator - electric drive combination.

[0015] Figure 2 illustrates schematically the basic construction of the LNG- fuel tank 12 in accordance with a preferred embodiment of the present invention. The fuel tank 12 is, as an example, formed of an inner shell 20, an outer shell 22 and a heat insulation 24 therebetween. At an end of the fuel tank 12 a so called tank connection space 26 is arranged. Naturally, the tank connection space may as well be located at a side of the LNG- fuel tank, and not necessarily as an extension of the shell of the tank but also at a distance from the shell of the tank, i.e. as a separate chamber at a side or at an end of the LNG- fuel tank. The tank connection space 26 is, preferably, but not necessarily, provided with heat insulation 28.

[0016] The tank connection space 26 houses normally an emergency pressure relief valve 30, which opens a vent connection along a line 30’ from the top of the LNG- fuel tank 12 to the vent mast 32 in case pressure in the tank 12 exceeds a predetermined value. The tank connection space 26 also houses a cryogenic pump for providing the engine with the fuel it needs, an evaporator for evaporating the liquid fuel to gaseous state and a fuel valve unit for controlling the gas feed to the engine, just to mention a few components found in the TCS. In addition to the above mentioned standard equipment, which are not shown in the drawings, the TCS 26 further includes a standard ventilation equipment including an air or ventilation inlet line 34 with a first fire damper valve 34’ and a ventilation outlet line 36 with a second fire damper valve 36’ leading from the TCS to the vent mast 32, and a blower 38, which is positioned in either the ventilation inlet line 34 or the ventilation outlet line 36 for ventilating the tank connection space 26. The first and the second fire damper valves 34’ and 36’ are in normal operating conditions always open and are automatically closed only if fire is detected in the tank connection space.

[0017] In accordance with the present invention the tank connection space 26 is provided with a first closing valve 34” in the ventilation inlet line 34 and a second closing valve 36” in the ventilation outlet line 36 so that the tank connection space 26 may be closed from outside atmosphere for inerting thereof. The tank connection space 26 further comprises an inlet 40 for an inert gas and a gas outlet 42 for discharging gas from the tank connec tion space 26. The inert gas inlet 40 is connected via an inert gas line 44 and a first pressure regulating valve 44’ therein to a source 46 of inert gas arranged, preferably but not necessarily, outside the tank connection space 26. The gas outlet 42 is connected via a discharge line 48 and a second pressure regulating valve 48’ therein to the vent mast 32. The discharge line 48 is provided with an oxygen analyser 50 for monitoring the oxygen concentration of the gas discharged from the TCS 26. The oxygen analyser may also be located in connection with the tank connection space 26, i.e. upstream of the second pressure regulating valve 48’ or the discharge line 48. And finally, the tank con nection space 26 comprises also a pressure relief valve 52’, which connects the interior of the tank connection space 26 via a pressure relief line 52 to the vent mast 32. The relief valve 52’ is set to open when the pressure in the tank connection space exceeds, for instance due to a raised temperature, the maximal allowed TCS- pressure pO. The maximal allowed pressure pO in the TCS is usually between 0.1 and 0.5 barg (gauge pressure) or 1.1 - 1.5 bar absolute pressure, preferably between 0.2 - 0.4 barg.

[0018] In accordance with a first preferred operating scheme of the present invention the first pressure regulating valve 44’ is a pilot operated valve that receives its control signal from the pressure of the tank connection space 26 such that the first pressure regulating valve 44’ opens when pressure in the TCS goes below upper limit pressure p1 , i.e. the first pressure regulating valve 44’ allows inert gas enter the TCS 26, when the pressure in the TCS is below p1 . In accordance with the same operating scheme the second pres sure regulating valve 48’ is also a pilot operated valve that receives its control signal from the pressure of the tank connection space 26. The second pressure regulating valve 48’ opens, i.e. bleeds gas from the TCS 26 to the vent mast 32, when the pressure in the TCS is above a predetermined lower limit pressure p2. Thus p1 >p2.

[0019] In accordance with a second preferred operating scheme, the first pressure reg ulation valve 44’ receives its control signal from the pressure of the tank connection space 26 such that it is adjusted or instructed to close when an upper pressure limit p1 is reached, in other words, it remains open below pressure of p1. The second pressure regulating valve 48’ receives its control signal from the pressure of the tank connection space 26 such that it is adjusted or instructed to open at a pressure of px and remain open until pressure has reduced to p2, whereby px>p2. Pressures p1 and px may be equal or different, the only thing that matters is that p1 and px are greater than p2.

[0020] In accordance with a first alternate further feature of the second preferred oper ating scheme the opening of the second pressure regulating valve 48’ at pressure of px is used to instruct the first pressure regulating valve 44’ to close such that the first pres sure regulating valve 44’ remains closed until the second pressure regulating valve 48’ closes at a pressure of p2. The closing of the second pressure regulating valve 48’ returns the control of the first pressure regulating valve 44’ to the TCS pressure, whereby the first pressure regulating valve 44’ opens and pressure in the TCS increases until the second pressure regulating valve 48’ receives its control signal from the TCS pressure at px, opens and takes over the control of the first pressure regulating valve 44’ closing

[0021 ] In accordance with a second alternate further feature of the second preferred op erating scheme the closing of the first pressure regulating valve 44’ at a pressure of p1 is used to instruct the second pressure regulating valve 48’ to open, to take over the control and to keep the first pressure regulating valve 44’ closed until the second pressure regulating valve 48’ closes at a pressure p2. Thereafter, the control of the first pressure regulating valve 44’ is given to the TCS pressure, so that the first pressure regulating valve 44’ opens allowing the pressure in the TCS to increase and keeps the second pressure regulating valve 48’ closed until pressure of p1 is reached.

[0022] As the maximal TCS pressure pO is somewhere between 0.1 and 0.5 barg, the pressures p2, p1 or px being used when setting the first and the second pressure regu lating valves in operative condition are quite low, however it always applies that p2<p1 and p2<px.

[0023] Further, the oxygen concentration has an effect on the functioning of the first and the second pressure regulating valves 44’ and 48’ as will be discussed later on.

[0024] When either taking a new tank connection space 26 into use or making the TCS inert after an inspection, i.e. switching the TCS from air atmosphere to inert atmosphere, the ventilation inlet line 34 and the ventilation outlet line 36 are closed by means of the first and second closing valves 34” and 36” and the first and the second pressure regu lating valves 44’ and 48’ are activated, i.e. the valves 44’ and 48’ receive their control signal from at least the pilot pressure of the TCS. The inerting of the TCS may be per formed by two basically different ways, i.e. by continuous purging or by using pressuriza tion cycles.

[0025] The first way includes keeping both the first and the second pressure regulating valves 44’ and 48’ open, i.e. continuously purging the TCS until the oxygen concentration of the gas in the tank connection space 26, i.e. upstream of the discharge line 48, or of the gas discharged from the tank connection space 26 downstream of the second pres sure regulation valve 48’ in the discharge line 48 is, determined by the oxygen analyser 50, reduced below maximum allowable oxygen concentration. Here, the first preferred operation scheme is used such that the activation of the pressure regulating valves 44’ and 48’ means that as long as the oxygen concentration in the TCS, i.e. in the analyser 50, is high both valves 44’ and 48’ remain open, and only after the maximum allowable oxygen concentration is reached either the first pressure regulation valve 44’ or the sec ond pressure regulating valve 48’ closes, i.e. after having received control signal from the oxygen analyser 50. In the former instance, the second pressure regulating valve 48’ closes, too, as soon as the pressure in the TCS is reduced below p2, and in the latter instance, the first pressure regulating valve 44’ closes when the pressure in the TCS reaches p1. Thus, in the former instance the TCS pressure after inerting is p2 and in the latter instance p1. Naturally, the oxygen concentration may, optionally, be followed such that the first or the second pressure regulating valve 44’ or 48’ is manually (in place of automatic control) closed. Thereby both valves 44’ and 48’ are inactivated, i.e. set to a stand-by mode from which they would be re-activated by increase in the oxygen concen tration or reduction of pressure in the TCS 26. Thus, the pressure control of the tank connection space 26 is left for the pressure relief valve 52.

[0026] The second way includes utilizing pressurisation cycles which requires that the operation of the valves are set in a manner different from the first operating scheme. Thus, in accordance with the second preferred operating scheme and the first alternate further feature thereof, when taking the TCS into use or inerting the TCS after service or maintenance, the first pressure regulating valve 44’ used for introducing inert gas into the tank connection space 26 opens as the pressure in the TCS is atmospheric pressure plthe second pressure regulating valve 48’ remains closed until the pressure in the TCS exceeds the predetermined pressure p1 p1 causing opening of the second pressure reg ulating valve 48’ and, as a function thereof, closing of the first pressure regulating valve 44’, whereby the pressure is allowed to decrease below the second predetermined value p2, which causes the closing of the second pressure regulating valve 48’ and, as a func tion thereof, opening of the first pressure regulating valve 44’. The operation is continued until the oxygen concentration of the gas in the tank connection space 26, i.e. upstream of the discharge line 48, or of the gas discharged from the tank connection space 26 downstream of the second pressure regulation valve 48’ in the discharge line 48 is, de termined by the oxygen analyser 50, reduced below maximum allowable oxygen concen tration, i.e. such an oxygen concentration is reached that combustion of NG is not any more possible irrespective of the concentration of the fuel. When the desired oxygen concentration is reached at least either the first or the second pressure regulating valve 48’ or 48’ is closed. Thereafter the pressure in the TCS is either p2 or p1 , respectively, and both valves 44’ and 48’ may be inactivated, i.e. set to a stand-by mode from which they would be re-activated by increase in the oxygen concentration or reduction of pres- sure in the TCS. Thus, the pressure control of the tank connection space 26 is left for the pressure relief valve 52. [0027] Further, in accordance with the second preferred operating scheme and the sec ond alternate further feature thereof, when taking the TCS into use or inerting the TCS after service or maintenance, the first pressure regulating valve 44’ used for introducing inert gas into the tank connection space 26 opens as the pressure in the TCS is atmos- pheric pressure and thus below p1 and the second pressure regulating valve 48’ remains closed until the pressure in the TCS exceeds the predetermined pressure p1 causing closing of the first pressure regulating valve 44’ and, as a function thereof, opening of the second pressure regulating valve 48’. The first pressure regulating valve 44’ remains closed, whereby the TCS pressure is allowed to decrease below the second predeter- mined value p2, which causes the closing of the second pressure regulating valve 48’ and, as a function thereof, opening of the first pressure regulating valve 44’. The opera tion is continued until the oxygen concentration of the gas in the tank connection space 26, i.e. upstream of the discharge line 48, or of the gas discharged from the tank connec tion space 26 downstream of the second pressure regulation valve 48’ in the discharge line 48 is, determined by the oxygen analyser 50, reduced below maximum allowable oxygen concentration, i.e. such an oxygen concentration is reached that combustion of NG is not any more possible irrespective of the concentration of the fuel. When the de sired oxygen concentration is reached at least either the first or the second pressure regulating valve 48’ or 48’ is closed. Thereafter the pressure in the TCS is either p2 or p1 , respectively, and both valves 44’ and 48’ may be inactivated, i.e. set to a stand-by mode from which they would be re-activated by increase in the oxygen concentration or reduction of pressure in the TCS. Thus, the pressure control of the tank connection space 26 is left for the pressure relief valve 52.

[0028] In normal operating conditions, i.e. when the ventilation inlet and outlet are closed and the inert atmosphere in the tank connection space 26 is controlled by the first and the second pressure regulating valves 44’ and 48’, the operation of the first fire damper valve 34’ and first closing valve 34” in the ventilation inlet line 34 and the second fire damper valve 36’ and the second closing valve 36” in the ventilation outlet line 36 should be checked regularly. In order to minimize the wasting of the inert gas both the inlet line 34 and the outlet line 36 are thus provided with the first and second closing valves 34” and 36”. The checking of the functionality of the fire damper and the closing valves is performed such that first the fire damper valve 34’ or 36’ is closed and the closing valve 34” or 36” is opened, and the inlet line 34 or the outlet 36 leading away from the closing valve 34” or 36”, i.e. in a direction opposite to the TCS 26, is monitored to see if any leak from the TCS may be noticed. If not, the (inner) fire damper valve 34’ or 36’ appears to be in good condition, whereafter the closing valve 34” or 36” is closed and the fire damper valve 34’ or 36’ opened. Next, again the inlet line 34 or the outlet line 36 leading away from the closing valve 34” or 36”, i.e. in a direction opposite to the TCS 26, is monitored to see if any leak from the TCS 26 may be noticed. If not, the closing valve 34” or 36” is also in good condition, and may be closed. Naturally, if any leakage is detected through any one of the fire damper and closing valves 34’, 34”, 36’ or 36”, or any other problems in their operation is found out, the malfunctioning valve needs to be replaced or main tained.

[0029] When the tank connection space itself or any piece of equipment inside the tank connection space needs service or maintenance the inert atmosphere in the TCS has to be switched to air atmosphere, whereby both pressure regulating valves 44’ and 48’ are inactivated, the fire damper and closing valves 34’, 34”, 36’ or 36” are opened and blower 38 started, i.e. standard ventilation is switched on for flushing the nitrogen out and filling the tank connection space with air.

[0030] In view of the above description is should be noted that the inert gas used for inerting the tank connection space 26 is preferably nitrogen, though also argon may be used. The inert gas source 46 is either a generator separating the inert gas from air, or a pressurized container carrying the inert gas. In case the generator is used, it is preferable to store the inert gas in a buffer tank for later use. With regard to the above discussed preferred embodiments, running schemes and their variations it has to be understood that they are just exemplary ones and other embodiments, running schemes and varia tions may also be used without departing from the spirit of the present invention. In a similar manner pressures p1 , p2, px or pO do not necessarily refer to the same pressure values in each and every example, but they may change. Thus, as mentioned already earlier, the only thing that matters is that p2<p1 <p0 and p2<px<p0 in each exemplary embodiment, running scheme or variation. Further, it should be noted that the first and the second pressure regulation valves 44’ and 48’ may be positioned either inside or outside the tank connection space. And finally it should also be understood that though Figure 2 discusses a tank and a tank connection space with an inner and an outer shell the present invention is as well applicable to LNG- tanks having only an inner shell with a heat insulation thereon.

[0031] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments of the pre sent invention, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its fea- tures, and several other applications included within the scope of the invention, as de fined in the appended claims. It should be understood that the tank arrangement com prises several features which are not shown in figures for the sake of clarity, for example, all such equipment present in each tank arrangement that concern fuel handling has been left out, as the present invention is not related fuel handling but features concerning in erting the tank connection space. The details mentioned in connection with any embodi ment above may be used in connection with any other embodiment when such combina tion is technically feasible.

Claims

1 . A fuel tank arrangement in a marine vessel for storing LNG- fuel, the arrangement comprising an LNG- fuel tank (12), a tank connection space (26) provided in communi cation with the LNG- fuel tank (12), the tank connection space (26) being provided with a ventilation inlet line (34) having a first fire damper valve (34’), a ventilation outlet line (36) having a second fire damper valve (36’) and at least one blower (38) arranged in one of the ventilation inlet line (34) and the ventilation outlet line (36), and a vent mast (32) arranged in communication with both the LNG- fuel tank (12) along a line (30’) and the tank connection space (26) along a discharge line (48), characterized in that the tank connection space (26) is provided with an inert gas inlet (40) for introducing inert gas from an inert gas source (46) into the tank connection space (26).

2. The fuel tank arrangement as recited in claim 1 , characterized in a first pressure regulating valve (44’) arranged in flow communication with the inert gas inlet (40).

3. The fuel tank arrangement as recited in claim 1 or 2, characterized in that the tank connection space (26) is provided with a gas outlet (42) arranged in flow communi cation with the vent mast (32)

4. The fuel tank arrangement as recited in claim 3, characterized in that a second pressure regulating valve (48’) is arranged in flow communication with the gas outlet (42) for maintaining a desired pressure in the tank connection space (26).

5. The fuel tank arrangement as recited in claim 4, characterized in an oxygen an alyser (50) provided either in connection with the tank connection space (26) or down stream of the second pressure regulating valve (48’), between the second pressure reg ulating valve (48’) and the vent mast (32).

6. The fuel tank arrangement as recited in any one of the preceding claims, characterized in a first closing valve (34”) in the ventilation inlet line (34) and a second closing valve (36”) in the ventilation outlet line (36).

7. The fuel tank arrangement as recited in any one of the preceding claims, charac terized in a pressure relief valve (52’) providing a flow communication from the tank con nection space (26) to the vent mast (32), the pressure relief valve (52’) being set to open when pressure in the tank connection space exceeds maximal allowed pressure pO.

8. The fuel tank arrangement as recited in any one of the preceding claims, characterized in that the gas source (46) is one of an inert gas generator or a pressurized container.

9. The fuel tank arrangement as recited in any one of the preceding claims, charac- terized in that the inert gas is one of nitrogen and argon.

10. The fuel tank arrangement as recited in any one of the preceding claims 2 - 9, characterized in that the first pressure regulation valve (44’) is set to open when pres sure in the tank connection space (26) is reduced below a predetermined pressure p1 and set to close when pressure in the tank connection space (26) exceeds a predeter- mined pressure p1 .

1 1 . The fuel tank arrangement as recited in any one of the preceding claims 4 - 10, characterized in that the second pressure regulation valve (48’) is set to open when pressure in the tank connection space exceeds a predetermined pressure p2 and set to close when pressure in the tank connection space is reduced below a predetermined pressure p2.

12. The fuel tank arrangement as recited in any one of the preceding claims 5 - 1 1 , characterized in that either the first pressure regulation valve (44’) or the second pres sure regulating valve (48’) is set to close when the oxygen analyser (50) indicates oxygen concentration below a predetermined value.

13. A method of switching between inert and air atmosphere in a tank connection space (26) of an LNG- fuel tank (12), the tank connection space (26) being provided with a ventilation inlet line (34) having a first fire damper valve (34’), a ventilation outlet line (36) having a second fire damper valve (36’) and at least one blower (38) arranged in one of the ventilation inlet line (34) and the ventilation outlet line (36), and a vent mast (32) arranged in communication with both the LNG- fuel tank (12) and the tank connection space (26), the method comprising initial steps of

a. providing the tank connection space (26) with an inert gas inlet (40), the inert gas inlet (40) being connected to an inert gas source (46), b. providing a first pressure regulating valve (44’) in communication with the inert gas inlet (40) and the inert gas source (46),

c. providing the tank connection space (26) with a gas outlet (42), the gas outlet (42) being connected to a vent mast (32),

d. providing a second pressure regulating valve (48’) in communication with the gas outlet (42) and the vent mast (32) in a discharge line (48), e. providing an oxygen analyser (50) in connection with the tank connection space (26) or in the discharge line (48) between the second pressure reg ulating valve (48’) and the vent mast (32),

and further steps of

f. when switching from air atmosphere to inert atmosphere,

i. opening the first pressure regulating valve (44’) to allow inert gas enter the tank connection space (26),

ii. opening the second pressure regulating valve (48’) to allow gas to be discharged from the tank connection space (26),

iii. determining oxygen concentration of the gas in the tank connec tion space (26) or of the gas discharged from the tank connection space (26) by the oxygen analyser (50),

iv. when the oxygen concentration reaches a predetermined value, closing at least one of the first pressure regulation valve (44’) and the second pressure regulation valve (48’), or

g. when switching from inert atmosphere to air atmosphere,

i. opening at least one first closing valve (34”) in the ventilation inlet (34),

ii. opening the at least one second closing valve (36”) in the ventila tion outlet (36), and

iii. starting the blower (38).

14. The switching method as recited in claim 13, characterized by dividing step f.i. further into sub steps of

1 . allowing inert gas enter the tank connection space (26) until gas pressure in the tank connection space reaches an up per predetermined value p1 , and

2. closing the first pressure regulating valve (44’), and in dividing step f.ii. into further sub steps of

1. allowing gas to be discharged from the tank connection space (26) until the pressure is reduced to a predetermined lower value p2, and

2. closing the second pressure regulating valve (48’), and repeating the steps f.i. and f.ii. until the oxygen concentration reaches a predetermined value.

15. The switching method as recited in claim 13, characterized by keeping both the first and the second pressure regulation valves (44’; 48’) open until, in step f.iv., at least one of the valves (44’; 48’) is closed.

16. The switching method as recited in claim 13, characterized by, after step e), providing the ventilation inlet line (34) with a first closing valve (34”) and the ventilation outlet line (36) with a second closing valve (36”).