US3864918A

US3864918A - Powered mobile liquefied gas carriers

Info

Publication number: US3864918A
Application number: US361032A
Authority: US
Inventors: Michael Lorenz
Original assignee: Sulzer AG
Current assignee: Sulzer AG
Priority date: 1972-05-27
Filing date: 1973-05-17
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11
Also published as: PL86303B1; NL155361B; FR2189678B1; IT987915B; SU571203A3; SE390057B; NO137991B; NL7307278A; JPS4961707A; GB1431203A; CH570296A5; CA991532A; NO137991C; ES415237A1; FR2189678A1

Abstract

The gas produced by evaporation from the insulated vessels is separated into two component flows. A first component flow is compressed and while being cooled and liquified is used to transfer heat to the second component flow. The heated second component flow is then used as an energy source for driving the carrier, such as a ship. The reliquified flow is returned to the insulated vessel.

Description

0 United States Patent U 1 [1 I1 3,864,918

Lorenz Feb. 11, 1975 [5 POWERED MOBILE LIQUEFIEDGAS 2,938,359 5/1960 Cobb. Jr. et al. 62/50 x CARRIERS 2,940,268 6/l960 Morrison 62/7 3,229.472 l/l966 Beers 62/50 X [75] In entor: M chael Loren Ham rg, 3,733,838 5/1973 Delahunty 62/54 Germany 3,766,734 l0/l973 Jones 62/5] X [73] Assignee: Sulzer Brothers, Ltd., Winterthur,

Switzerland Primary E.\'aminerMeyer Perlin F} d: M 7 7 Assistant Examiner-Ronald C. Capossela [22] 8y 1 l9 3 Attorney, Agent, or Firm-Kenyon & Kenyon Reilly PP 361,032 Carr 8L Chapin [30] Foreign Application Priority Data May 27, 1972 Germany 2225882 [571 ABSTRACT June 10, i972 Germany 2228382 The gas produced by evaporation from the insulated June 21, 1972 Germany 2230263 vessels is separated into two component flows A first component flow is compressed and while being cooled [52] us. Cl 2 J 6 3436 and liquified is used to transfer heat to the SCond [5]} l t Cl Folk 25/08 component flow. The heated second component flow [58] g i 240 243 is then used as an energy source for driving the carrier, such as a ship. The reliquified flowis returned to 62/52, 54, 60/3946, 651, 671 the insulated vessel [56] 'g';f ::f 21 Claims, 5 Drawing Figures l,808,439 6/1931 Serriades 60/671 CID . 1 POWERED MOBILE LIQUEFIED GAS CARRIERS This invention relates to powered mobile liquefied gas carriers, particularly waterborne vessels such as marine tankers, in which the liquefied gas is contained in at least one insulated vessel at the appropriate low temperature and substantially normal pressure, any gas produced by evaporation being collected and being supplied as an energy source to a combustion operation for driving the carrier. The invention also relates to methods of operating such liquefied gas carriers.

When liquefied natural gas, methane or some similar material with a low boiling point, is transported it is not possible to prevent the constant ingress of heat from the exterior into the vessels despite good insulation and to prevent liquefied gas being evaporated by this heat. Efforts are made to minimise the economical losses resulting from evaporation of the charge. This applies more particularly to the transport of liquefied gases by sea. Gas produced by evaporation during such transport is collected and is used as an energy source for the ships propulsion system.

The result of previous methods are however unsatisfactory for reasons explained below. Despite the complex insulation with which the tanks of the liquefied gas tankers are provided the evaporation losses are still between 0.20 to 0.35 percent of the total load per day, depending on the size of the ship. Given a common standard size of ship which has cargo space for 125,000 m of liquefied natural gas, this means that the average daily loss of gas by evaporation amounts'to approximately 300 m corresponding'to 178,000 m of gas at NTP per day. According to a process which in practice is.used almost exclusively, this amount of gas, representing an unavoidable cargo loss, is supplied as fuel to the main propulsion plant of the ship. To this end, it is necessary to extract the gas from the tanks, to compress it and to heat it at least to ambient temperature. The amount of evaporation stated in the previous numerical example corresponds to an output of 29,700 metric shaft hp when the gas is burnt in a modern ships boiler plant. Since regulations do not permit gas to be used as the sole fuel and the maximum proportion may not exceed 85 to 90 percent of the fuel such a ship must at present be provided with a propulsion plant which has a minimum rating of 33,000 shaft hp.

The unavoidable evaporation losses also involve a reduction of the actually available transportation space because the evaporation losses must be allowed for during travelling. Moreover, a certain quantity of liquefied gas must remain in the cargo tanks when the ship travels empty so that the tanks may be constantly maintained at the specified low temperature so that the effective space is still further reduced. Owing to this reduction of the cargo space and because of the high cost of the liquefied gas thus transported, it follows that the solution adopted hitherto of utilising the evaporation losses for heating the ship's boilers or the like is economically unsatisfactory.

Conventional re-liquefying methods and plants have not so far been employed because such known methods and equipment involve high capital costs and call for large quantities of energy for operating them.

According to one aspect of the present invention, in a method of operating a powered mobile liquefied gas carrier in which the liquefied gas is contained in at least one insulated vessel at the appropriate low temperature and substantially normal pressure, gas produced by evaporation is collected and is divided into two part flows, a first part flow being compressed in itself and, while being cooled and liquefied, transferring heat to the second part, the re-liquefied gas of the first part flow being expanded and returned to the vessel and the heated second part flow being supplied as an energy source for driving the carrier.

Accordingly, the basic idea of the invention is to divide the gas produced by evaporation into two part flows of which one is used to preheat the other while requiring only a small amount of energy for compression, heat dissipation being regulated so as to achieve renewed liquefaction of the previously compressed part flow.

By contrast to the previous methods in which the gas produced by evaporation and to be supplied for combustion is first compressed and then heated, additional energy being required for both operations, the invention proposes that the part flow to be used in the propulsion plant is first heated by the energy which is supplied for compressing the part flow that is to be reliquefied and is then compressed for further utilisation. This results in a particularly advantageous energy utilisation, the effective evaporation losses being also substantially reduced.

To cool the first part flow which is to be returned it is possible for the second part flow to be utilised in a lower temperature range of the first part flow. The entire amount of gas produced by evaporation may be divided into the two part flows in a ratio controlled by reliquefaction.

Preferably the first part flow is compressed substantially adiabatically by utilising energy derived from the drive of the carrier but the energy may be derived from an auxiliary source of power.

The second part flow, i.e., the part flow which is to be supplied to the propulsion system of the gas carrier, may be compressed after it has been heated by the first part flow, i.e., the part flow which is to be re-liquefied. The first and second part flows may be conducted in countercurrent for the said transfer of heat. The reliquefied gas may be subjected to after-cooling. In a modified method, in order to improve the efficiency still further, the entire collected gas may be utilised for cooling the first part flow prior to division into the two part flows. The first part flow which is to be supplied to the compressor therefore has a higher temperature than in the first described method. The latter method has the advantage of reducing the amount of equipment required. It is possible to use simpler and less expensive compressors for the part flow which is to be returned to the vessel and it is also possible to utilise heat exchangers which are smaller. In addition to permitting the use of less expensive apparatus, the modified method also leads to better utilisation of energy thus providing an overall economic improvement. The compressor may operate with the part flow to be returned at an inlet temperature which is approximately 40C higher than in the first mentioned method.

Preferably cooling is performed with the collected gas in the low-temperature range and the part flow which is to be supplied for combustion is used for cooling thepart flow which is conducted through the compressor'and is to be returned to the vessel in the higher temperature range. The part flow of the collected gas to be supplied for combustion may be branched off at a temperature level which corresponds substantially to the condensation temperature.

The methods according to the invention may be relatively easily and simply controlled. The ratio between the part flows fluctuates only within narrow limits in normal operation. A three-way valve, which is controllable by the condensation pressure and is disposed at the point of division into the branch ducts may be used to provide the main control.

According to another aspect of the present invention, a mobile liquefied gas carrier has means for utilising evaporated liquefied gas to drive the carrier, an insulated vessel to contain liquefied gas, a gas delivery duct leading from the vessel and dividing into branch ducts, the first branch duct leading through a compressor, one side of a heat exchanger means and an expansion element and back to the insulating vessel, and the second branch duct leading through the other side of the heat exchanger means to the drive means.

The invention may be carried into practice in various ways but two systems of operating a liquefied gas carrying ship and their mode of operation in accordance with the invention, together with a previously proposed system, will now be described with reference to the accompanying drawings, in which:

FIG. 1 shows a ship in simplified form;

FIG. 2 is a diagram of a previously proposed system for dealing with evaporated gas;

FIG. 3 is a diagram of a system operating in accordance with the present invention;

FIG. 4 is a graph in which the pressure and enthalpy of the gas/liquid as it passes through the system shown in FIG. 3 are plotted; and

FIG. 5 is a diagram of a modified system.

FIG. 1 shows in simplified form a ship having a number of

insulated vessels

12, 14 which, in this case, are of spherical shape and contain liquefied natural gas. Other shapes are possible and are commonly used. The insulation of the insulated vessels is constructed in the usual way so that the evaporation loss resulting from the action of the heat of water and air on the vessels is reduced to the lowest level which is economically justifiable. The gas nevertheless generated due to evaporation of the liquefied gas is collected by a duct 18 which communicates through

connections

18a, 18b with the insulating vessels l2, 14, the gas being supplied to the ships propulsion apparatus 16. A re-liquefying device 20 is provided into which the duct 18 leads and from which a duct 24 extends to a device in the ships propulsion apparatus 16 to deliver a part flow of the evaporated gas for combustion in the apparatus 16. In this way, the gas is burnt to yield thermal energy. A duct 22 also extends from the device 20 to the

vessels

12, 14 in order to return re-liquefied gas via connections 22a, 2212 into the vessels. Where there is a number of vessels it is not necessary for all the vessels to be connected to the return duct 22. Since only part of the gas produced as a result of evaporation is again liquefied it is sufficient to provide connections for a corresponding maximum re-liquefaction flow.

To illustrate the advantages of the invention, FIG. 2 shows a previously proposed system by means of which gas produced by the evaporation of liquefied gas was utilised for combustion. In this device the collected gas is supplied through a duct 100 to a compressor 102 the output of which is connected through a duct 104 to a heat exchanger 106. The gas discharged from the heat exchanger is fed through a duct 108 into a combustion device. At the inlet to the compressor 102 the gas has a temperature of approximately -l50C and a pressure p 1 atm abs. At the output of the compressor t -l25C, p 1.7 atm abs. After leaving the heat exchanger, t= +20C, p 1.7 atm abs.

The heat exchanger 106 is operated with a glycolwater mixture which must be correspondingly preheated. To this end, a heat exchanger 112 is provided which is supplied with steam via a duct 110. The exhaust steam from the heat exchanger 112 is discharged via a duct 114. The glycol-water mixture heated by the steam passes from the heat exchanger 112 via a duct 116 to the heat exchanger 106. Convection in this case is insufficient to ensure circulation and for this reason a pump is provided for the glycol-water mixture circulation. The partially evaporated glycol-water mixture passes from the heat exchanger 106 via a duct 119 into a glycol-water mixture storage tank 118. A by-pass connection 117 is provided between the duct 116 and the duct 119. The storage tank 118 communicates with the inlet of the pump 120 through a duct 121. The system is provided with valves for regulation purposes, these valves being controlled by devices designated by the letters TC in dependence on the temperatures which prevail in the various parts of the system.

Pressure control means 103 are provided for the compressor 102. A level indicator LI is also provided for monitoring purposeson the tank 118 and is adapted to deliver a signal for controlling the system when the level approaches a maximum or minimum.

The explanation above of the previously proposed system indicates that on the one hand it calls for a substantial amount of equipment and on the other hand prepares the gas resulting from evaporation practically only for combustion to which end additional substantial amounts of energy are required.

A re-liquifying device 20 constructed and operating in accordance with the invention is shown in detail in FIG. 3. In this device, the duct 18 leads to a controllable three-way valve 26 in which the entire incoming flow of gas is divided into two part flows. This division is performed at a defined, controlled ratio. One part flow is fed by the valve 26 through a duct 28 to the inlet of a compressor 30 and compressed. The outlet of the compressor is connected through a duct 32 to a condenser 34 which, together with a condensate collector and after-,cooler 36, ,forms an integral structural unit operating as a heat exchange means. The gas which is heated and compressed by the compressor 30 is liquefied in the unit 34, 36 after giving up heat to the gas which is to be burnt. To this end, the compressed gas is passed into heat exchange relation with the other gas part flow to cool and reliquify the compressed part flow while heating the other part flow. After being cooled, the liquefied gas which is received by the collector may be returned by a duct 22 and an expansion valve 62 to the

vessels

12, 14. The second, larger part flow flows from the valve 26 through a duct 40 to the gas duct system of the collector and aftercooler 36, illustrated in simplified form as a cooling coil, in counter-flow to the compressed gas flow so as to cool and reliquify the compressed gas flow. A duct 44 with which the duct 40 communicates through a by-pass 46 containing a valve, supplies the gas to a cooling coil 48 in the condenser 34. From there the gas which has been substantially heated passes through a duct 50 to the inlet of a com pressor 52 and is appropriately compressed therein for combustion. The gas then passes through a duct 24 to the combustion means of the propulsion system. The

compressors

30, 52 are also used for drawing the gas from the vessels l2, l4.

The system shown in FIG. 3 is provided with appropriate means for controlling the process in the individual sections. Pressure-dependent regulating devices are designated with the letters PC in FIG. 3 while regulating devices which depend on the filling level are designated with the letters LC. Pressure regulating means 54 are disposed between the duct 18 and the compressor 52 to ensure that the pressure in the vessels l2, 14 remains constant. Pressure-dependent rotational speed regulating means 56 are provided for the compressor 30. To divide the gas which flows through the duct 18 into the part flows the valve 26 is controlled by the condensation pressure (compression pressure) in the duct 32 by means of a device 58.

As regards the collector 36 it is important that it is always filled to a minimum level and does not exceed a maximum level. A level control system 60 which controls an expansion valve 62 in the return duct 22 is provided to control this state.

The invention is further explained by reference to the graph shown in FIG. 4. The enthalpy i is plotted on the abscissa and the logarithm of the pressure log p is plotted on the ordinate. The curves labelled rh and "'1 show the changes in pressure in enthalpy in the first part flow passing at a rate ril from the valve 26 along the duct and the second part flow passing at a rate rh from the valve 26 along the line 40, the referenced points on the curves corresponding to the similarly referenced parts of the system shown in FIG. 3.

The following relationships must be observed in considering the graph:

Q effective for liquefaction ,5 I [2 (in I) Table 1 Point T in K p in mm abs 1' in kcal/kg l 123 1.0 127.7 2 339 40 232.4 3 186.5 40 79.5 4 I33 40 19.8 5 112.5 1.06 19.8 2 a 300 0.95 218.8 3 a 350 1.7

Gas proportion in the condensate x 0.155

Ai =i i 232.4 19.8 212.6 kcal/kg Ai =.i i, 218.8 127.7 91.1 kcal/kg m lm total 91.1/212.6 91.1 130.3 30

The numerical example confirms that approximately one third of the gas yielded by evaporation may be reliquefied with apparatus of the same order of magnitude as that employed hitherto. The graph of FIG. 4 also shows that pressure and temperature are initially increased for the part flow that is to be liquefied. The temperature is then reduced at constant pressure. liquefaction occurring at a defined point which depends on p and T. After further cooling the gas is expanded combined with further temperature reduction.

A modified re-liquifying device 20 constructed and operating in accordance with the present invention is illustrated in FIG. 5. The duct 18 leads to the three-way valve 26 from which a line 28 leads to a heat exchanger unit 65. The heat exchanger unit 65 comprises three parts, namely an outlet cooler 66, a condenser 67 and an inlet cooler 68. All parts of the heat exchanger unit are preferably combined in one structure. The duct 28 extends through the outlet cooler 66 as a cooling duct 69 which leads into the cooling duct 70 of the condenser. A cooling duct 72 in the inlet cooler 68 communicates with the cooling duct 70 via a three-way valve 71. The stream of collected evaporated gas is subdivided into two part flows by the three-way valve 71. One part flow passes from the valve 71 through the cooling duct 72 of the inlet cooler and a duct 73 to a compressor 74 where the gas of the part flow which has been heated in the meantime is compressed for combustion.

A part flow which is smaller than the part flow intended for combustion passes from the valve 71 through a duct 40 into a compressor 75 in which the gas which has already been heated in the heat exchanger unit above the original temperature of the

vessels

12, 14 is compressed while being heated, substantially'adiabatically. The compressed gas which now has a temperature substantially higher than the original temperature of the vessels then passes through a duct 76 into the heat exchanger unit 65. In this unit the gas from the compressor 75 gives up heat by counter-flow to the gas which passes to combustion and to the undivided stream of the collected gas. After pre-cooling in the inlet cooler 68, the gas is liquefied in the condenser 67 and is further cooled in the outlet cooler 66. The liquefied cooled gas passes through an expansion valve 77 into the duct 22 for return to the

vessels

12 and 14.

To start the system a part flow is first branched off at the valve 26 and passed via a duct 78 into the duct 40 and thence to the compressor 75. The duct 78 passes through a heating device 79 which may be operated, for example, with sea water and which replaces preheating in the zones 66 and 67 during the starting phase. The device or system is changed over after starting up so that none of the collected gas is branched off at the valve 26 and the division is performed at the valve 71.

The system incorporates various controllers which are shown as P or LC respectively in the drawing. LC refers to a level controller disposed on the outlet cooler to ensure that a defined liquid level is always present in the outlet cooler 66. The controller LC therefore communicates with a valve 77 which also has a volumetric control function for expansion. Control devices are also provided at the

compressors

74, 75. For example the controller on the compressor 74 may communicate with the valve 26 and establish a relationship between the gas supplied from the vessels and the gas which is to be delivered for combustion. The pressure at the outlet from the compressor 75 may be related to the dividing ratio at the valve 71.

In the system shown in H6. the entire mass flow from the

vessels

12 and 14 gives up part of its refrigeration energy for condensation and final cooling of the part flow which is branched off from the total flow after emerging from the condenser. It is essential that the entire mass flow is utilised for cooling the return part flow over a substantial part of the negative temperature range.

What we claim is:

l. A method of operating a powered mobile liquefied gas carrier in which liquefied gas is contained in at least one insulated vessel at the appropriate low temperature and substantially normal pressure, in which method gas produced by evaporation is collected and is divided into first and second part flows, the first part flow being compressed in itself and, while being cooled and liquefied, transferring heat to the second part flow, the reliquefied gas of the first part flow being expanded and returned to the vessel and the heated second part flow being supplied as an energy source for driving the carrier.

2. A method as claimed in claim 1 in which the second part flow is additionally used for cooling the first part flow, which is to be returned, in a lower temperature range of the first part flow.

3. A method as claimed in claim 1 in which the whole of the gas produced by evaporation is divided into the two part flows at a ratio controlled by the reliquefaction of the first part flow.

4. A method as claimed in claim 1 in which the first part flow is compressed substantially adiabatically by utilising energy derived from the drive of the carrier.

5. A method as claimed in claim 1 in which the second part flow is compressed after having been heated by the first part flow.

6. A method as claimed in claim 1 in which the first and second part flows are conducted in countercurrent for the said transfer of heat.

7. A method as claimed in claim 1 in which the reliquefied gas is subjected to final cooling.

8. A method as claimemd in claim 1 in which the entire collected gas is utilised for cooling the first part flow prior to division into two part flows.

9. A method as claimed in claim 3 in which cooling is performed with the entire collected gas in a low temperature range and the second part flow is utilised for cooling the first part flow conducted through the compressor and returned to the vessel in a temperature range above the low temperature range.

10. A method as claimed in claim 9 in which the second part flow is branched off from the collected gas at a temperature level which corresponds substantially to the temperature of condensation.

11. A method as claimed in claim 9 in which the entire gas flow is divided into the two part flows after passing through a final cooling zone and condensation zone for the first part flow.

12. A mobile liquefied gas carrier having means for utilising evaporated liquefied gas to drive the carrier, an insulated vessel to contain liquefied gas, a gas delivery duct leading from the vessel and dividing into branch ducts, the first branch duct leading through a compressor, one side of a heat exchanger means and an expansion element and back to the insulating vessel, and the second branch duct leading through the other side of the heat exchanger means to the drive means.

13. A gas carrier as claimed in claim 12 in which the heat exchanger means includes a condenser for the flow in the first branch duct and a condensate receiver through both of which the gas delivery duct passes.

14. A gas carrier as claimed in claim 12 in which the heat exchanger means comprises an inlet cooler, a condenser and an outlet cooler which are connected in series in the first branch duct and are combined into one structural unit.

15. A gas carrier as claimed in claim 12 in which the Heat, exchanger means includes a condenser for the flow in the first branch duct and a condensate receiver through both of which the second duct passes.

16. A gas carrier as claimed in claim 15 in which the second branch duct extends from the heat exchange means via a compressor to the drive means.

17. A method of operating a propulsion system for a powered mobile liquified gas carrier having at least one insulated vessel containing liquified gas, said method comprising collecting gas generated due to evaporation in the vessel,

dividing the collected gas into two part flows,

compressing one of said part flows,

passing the compressed part flow into heat exchange relation with the other of said part flows to cool and re-liquify the compressed part flow while heat ing said other part flow,

returning said re-liquified part flow to one of the vessels, and

passing said heated part flow to the propulsion system for driving the carrier.

18. A mobile liquid gas carrier having a propulsion apparatus for driving the carrier,

at least one vessel for containing liquified natural gas,

a gas delivery duct communicating with said vessel to collect evaporated gas from said vessel,

a re-liquifying device connected to said duct to receive the evaporated gas for re-liquifying a part flow of the evaporated gas,

a first duct extending from said device to said vessel to return the part flow of re-liquified gas from said device to said vessel, and

a second duct extending from said device to said propulsion apparatus to deliver a second part flow of evaporated gas from said device to said propulsion apparatus for combustion therein.

19. A mobile liquid gas carrier as set forth in claim 18 wherein said re-liquifying device includes a controllable three-way valve connected to said gas delivery duct; a heat exchanger unit having an outlet cooler, condenser and inlet cooler; a first cooling duct in said outlet cooler connected to said valve to receive the evaporated gas; a second cooling duct in said condenser connected to said first cooling duct to receive the evaporated gas therefrom; a second three-way valve connected to said second cooling duct; 21 third cooling duct in said inlet cooler connected to said secand valve to receive a part flow of evaporated gas therefrom; and a compressor connected to said second valve to receive and compress a second part flow of the evaporated gas therefrom, said compressor being connected to said heat exchanger unit to deliver the compressed second part flow thereto for counterflow over said cooling ducts.

20. A mobile liquid gas carrier as set forth in claim 18 wherein said re-liquifying device includes a controllable three-way valve connected to said gas delivery duct, a compressor connected to said valve to receive and compress a first part flow of the evaporated gas, a heat exchange means connected to said compressor to receive the compressed first part flow from said compressor, said heat exchange means being connected to said valve to receive a second part flow of the evaporated gas for flow therethrough in 'countercurrent to the compressed first part flow to cool and re-liquify the compressed first part flow.

21. A mobile liquid gas carrier as set forth in claim 20 wherein said device further includes a second compressor connected to said heat exchange means to receive and compress the second part flow and wherein said first duct connects to said heat exchange means to receive the cooled first part flow and said second duct connects to said second compressor to receive the

Claims

1. A method of operating a powered mobile liquefied gas carrier in which liquefied gas is contained in at least one insulated vessel at the appropriate low temperature and substantially normal pressure, in which method gas produced by evaporation is collected and is divided into first and second part flows, the first part flow being compressed in itself and, while being cooled and liquefied, transferring heat to the second part flow, the re-liquefied gas of the first part flow being expanded and returned to the vessel and the heated second part flow being supplied as an energy source for driving the carrier.

3. A method as claimed in claim 1 in which the whole of the gas produced by evaporation is divided into the two part flows at a ratio controlled by the re-liquefaction of the first part flow.

7. A method as claimed in claim 1 in which the re-liquefied gas is subjected to final cooling.

15. A gas carrier as claimed in claim 12 in which the heat exchanger means includes a condenser for the flow in the first branch duct and a condensate receiver through both of which the second duct passes.

17. A method of operating a propulsion system for a powered mobile liquified gas carrier having at least one insulated vessel containing liquified gas, said method comprising collecting gas generated due to evaporation in the vessel, dividing the collected gas into two part flows, compressing one of said part flows, passing the compressed part flow into heat exchange relation with the other of said part flows to cool and re-liquify the compressed part flow while heating said other part flow, returning said re-liquified part flow to one of the vessels, and passing said heated part flow to the propulsion system for driving the carrier.

18. A mobile liquid gas carrier having a propulsion apparatus for driving the carrier, at least one vessel for containing liquified natural gas, a gas delivery duct communicating with said vessel to collect evaporated gas from said vessel, a re-liquifying device connected to said duct to receive the evaporated gas for re-liquifying a part flow of the evaporated gas, a first duct extending from said device to said vessel to return the part flow of re-liquified gas from said device to said vessel, and a second duct extending from said device to said propulsion apparatus to deliver a second part flow of evaporated gas from said device to said pRopulsion apparatus for combustion therein.

19. A mobile liquid gas carrier as set forth in claim 18 wherein said re-liquifying device includes a controllable three-way valve connected to said gas delivery duct; a heat exchanger unit having an outlet cooler, condenser and inlet cooler; a first cooling duct in said outlet cooler connected to said valve to receive the evaporated gas; a second cooling duct in said condenser connected to said first cooling duct to receive the evaporated gas therefrom; a second three-way valve connected to said second cooling duct; a third cooling duct in said inlet cooler connected to said second valve to receive a part flow of evaporated gas therefrom; and a compressor connected to said second valve to receive and compress a second part flow of the evaporated gas therefrom, said compressor being connected to said heat exchanger unit to deliver the compressed second part flow thereto for counterflow over said cooling ducts.

20. A mobile liquid gas carrier as set forth in claim 18 wherein said re-liquifying device includes a controllable three-way valve connected to said gas delivery duct, a compressor connected to said valve to receive and compress a first part flow of the evaporated gas, a heat exchange means connected to said compressor to receive the compressed first part flow from said compressor, said heat exchange means being connected to said valve to receive a second part flow of the evaporated gas for flow therethrough in countercurrent to the compressed first part flow to cool and re-liquify the compressed first part flow.

21. A mobile liquid gas carrier as set forth in claim 20 wherein said device further includes a second compressor connected to said heat exchange means to receive and compress the second part flow and wherein said first duct connects to said heat exchange means to receive the cooled first part flow and said second duct connects to said second compressor to receive the heated second part flow.