GB2562994A

GB2562994A - Multi-layered insulation system for liquefied gas carriers

Info

Publication number: GB2562994A
Application number: GB1705427.1A
Authority: GB
Inventors: Sjølie Strand Kjetil; M Jonas Jørn; Jung Changeun; Hwang Youngho; Ivar Nikolaisen Per; Foss Stein
Original assignee: Lnt Marine Pte Ltd
Current assignee: Lnt Marine Pte Ltd
Priority date: 2017-04-04
Filing date: 2017-04-04
Publication date: 2018-12-05
Also published as: GB201705427D0; WO2018185190A1

Abstract

An insulation for a liquefied transport vessel 1, the vessel comprising an outer hull 2 and at least one liquefied gas tank 3 contained within the hull, the hull and tank defining an insulating void 5 between the inner surface of the hull and the outer surface of the tank wherein the void is an integral layer of the insulation apparatus, the apparatus comprising an inert gas cooling arrangement 7 arranged in use to reduce the temperature of an inert gas. The method discloses the void and the step of controlling the temperature of the inert gas and in the void below 0 degrees C. Additionally, a control system comprising an inert gas temperature control arrangement that controls the temperature of gas within the void to be below 0 degrees C is disclosed. Furthermore, a cooling system for the vessel comprises a gas heat exchanging apparatus 7 that receives a first input of boiled-off gas 8 from the tank on the vessel, and a second input of a supply of inert gas 9, wherein the heat exchanging apparatus is arranged to cool the inert gas by using the boiled-off gas and to output the inert gas at a reduced temperature.

Description

Multi-layered Insulation System for Liquefied Gas Carriers

The present invention is particularly concerned, but not exclusively, with an insulation system and method for insulating ocean going vessels that are configured to carry liquefied gases. Such vessels are known as liquefied gas carriers (LGC’s).

Liquefied gases may include, but are not limited to, liquefied natural gas (LNG), liquefied ethylene gas (LEG) and liquefied petroleum gas (LPG).

Examples of such ocean-going vessels (ships or vessels) include (but not limited to) ships classified by the International Maritime Organization (IMO) as IMO Type A, IMO Type B and IMO Type C. Each of these ships or vessels complies with design criteria according to IMO requirements and each is configured to safely carry liquefied gas (such as LNG) in tanks within the vessel.

Although the discussion herein refers primarily to LNG transport it will be recognized that an invention described herein may equally be used with other types of vessels or storage systems (such as fuel tanks) containing other liquefied gases. These may include, for example, LPG, LEG, LNG or LH2.

Gases are transported in liquid form to increase the volume of gas that can be carried at one time. In the case of liquefied natural gas, the volume in a liquid state is 1/600^th of the volume at ambient temperature.

In order to maintain the gas in a liquid state, it is necessary to cool the gas to very low temperatures. Specifically, for LNG the gas has to be maintained at a temperature in the region of minus 163 degrees C. Maintaining the gas at this temperature during transport is achieved using cooling systems on the vessel to cool the gas in combination with insulation systems which insulate the tank or tanks from ambient outside conditions.

This is achieved in conventional systems in a variety of ways.

For example, in one system the tank containing the liquefied gas is heavily insulated on its outer surface. This prevents heat from outside of the tank reaching the tank containing the liquefied gas (the primary tank) and heating the has causing unwanted gas boil-off. These insulation systems have been found to be effective in insulating the primary tanks of an LNG vessel.

In another arrangement, the inner surface of the hull of the vessel is also insulated. In such an arrangement of the insulation layer is applied to the inside of the hull, again to prevent heat from ambient conditions reaching the primary tank. These arrangements have also been found to be effective at insulating LNG carrying vessels.

However, the inventors of the present invention have formulated an unconventional approach to insulating an LNG (or the like) tank which provides superior thermal insulation per unit of thickness. Being able to provide enhanced insulation with a thinner insulation system advantageously allows for larger primary tanks within a given vessel. This greatly enhances the volume of liquefied gas a given vessel can contain and carry and thus greatly improves the efficiencies of liquefied gas transportation. It also enhances the viability of transporting LNG in smaller vessels and in smaller volumes.

Summary of the Invention

Aspects of the invention are set out in the accompany claims.

Viewed from a first aspect of an invention described herein there is provided an insulation apparatus for a liquefied gas transport vessel, the vessel comprising an outer hull and at least one liquefied gas tank contained within the hull, the hull and tank defining a void between the inner surface of the hull and the outer surface of the tank, the apparatus comprising an inert gas cooling arrangement arranged in use to reduce the temperature of an inert gas and to communicate the inert gas into the void.

Thus, according to an invention the void or space between the tank containing the liquefied gas and the hull is cooled by means of cooling a gas that is introduced into that space. In effect the void, which previously has not been integrated into the insulation arrangement in this way, can act as a functional part of the insulation of the vessel. Providing a cooling layer around the tank and between the tank and hull reduces the temperature differential between the tank and the void i.e. the void is cold like the tank and so thermal losses to the void are reduced.

Put another way the ΔΤ (T being temperature) between the tank containing the liquefied gas and the hull is reduced.

Such an arrangement according to an invention described herein provides some surprising and significant technical advantages as discussed below.

According to the inventions, this reduction in thermal loss from the tank (the primary tank) significantly reduces the boil-off of gas i.e. where heating of the tank causes the liquefied gas to change state into gas. Reducing the boil-off enhances the efficiency of the vessel since less energy is needed to cool the gas.

Furthermore, using the void as a composite part or layer of the insulation system improves the thermal insulation of the system as a whole. Conventionally the void is not part of the insulation. However, in accordance with the present invention the void becomes an integral layer of the insulation system. Specifically, a composite insulation system utilising a conventional insulation layer (or layers) in combination with a gas layer or gas ‘curtain’ around the tank is provided according to the invention.

Improving the thermal efficiency in this way allows the thickness of the thermal insulation around the tank (or on the hull - as discussed below) to be reduced. A consequential reduction in the thickness of insulation allows for a corresponding increase in the size of the tank. It will be immediately recognized that increasing the size and volume of a tank for a given vessel provides significant cost and efficiency advantages.

Conventionally, it has not been contemplated to incorporate the void in a vessel into the insulation system. It has never been contemplated to use the void space in the present way, to cool an inert gas and to use it as a layer of the insulation system.

The void space gas may be cooled in any convenient way. For example, a heat exchanger may be used which cools a supply of inert gas which can be communicated into the void space. Such a heat exchanger may be powered by the conventional ancillary power supply of the vessel.

Advantageously the heat exchanger may be configured to receive boil-off gas from the liquefied gas tank as a supply of coolant. Boil-off of the liquefied gas occurs during the transportation of liquefied gases by virtue of increases in temperature of the liquefied gas. As the gas is vented to atmosphere it expands. The release of extremely cold gas from the liquefied tank in combination with the expansion of the gas as it is released can be advantageously used in a heat exchanger to provide the necessary cooling for the inert gas supply.

Thus, the boil-off gas from the liquefied gas tank can advantageously be used to cool the inert gas which is communicated into the void. This increases efficiency further because no additional cooling equipment is required.

Furthermore, the system advantageously automatically compensates for increases in temperature in the liquefied gas tank. If the amount of boil-off cargo gas increases, so too does the volume of inert gas that can be cooled and introduced into the void. This in turn cools the void and reduces the temperature differential between the tank and the hull (or insulation around the hull) which cools the tank. Boil-off gas is then reduced. This closed-loop effect is significant because it automatically compensates for any increase in the liquefied gas temperature and provides a highly efficient insulation system.

The inert gas may be any suitable gas which is not prone to explosion and which will not corrode or chemically damage components within the void including the insulation, tank and any convection apparatus (discussed below). For example, argon may be used. Advantageously in one arrangement nitrogen may be used which is the major component of atmospheric air and therefore conveniently available at all times during a vessel’s voyage. This may for example be supplied by nitrogen generators on board the ship.

The boil-off gas (BOG) may be communicated to the heat exchanger (or cooling apparatus) by means of a conduit. For example, a conduit may be arranged to intersect with the venting conduits which conventionally either vent BOG to atmosphere or in some instances supply BOG to the engines of the vessel. This line can be intersected with a suitable valve arrangement to allow for selective use of the BOG for inert gas cooling. Such a valve arrangement may be controlled by an apparatus controller described further below.

Similarly a suitable conduit may be provided between the cooling arrangement and the void. This may be by means of a single inlet into the void or by means of a manifold allowing the cooled inert gas to be introduced at a plurality of points along the vessel. This may advantageously further improve efficiency of cooling (and specifically the speed with which the void can be cooled).

The invention described herein may also advantageously include a control arrangement which may selectively control the supply of inert gas into the void. Such a control arrangement may be arranged to activate and deactivate the cooling apparatus and valves in the conduits between the liquefied gas tank and the void. Thus, the supply of coolant to the void can be selectively controlled in response to manual control or automatic control. In an automatic mode of operation the control arrangement may be configured to control the introduction of coolant gas in response to temperature indications from the tank and/or the void. A feedback control arrangement can thus be provided.

For example, the control arrangement may be arranged to receive a temperature indication from the void and to control the flow of cooled inert gas in response to that temperature indication.

The control arrangement may also be more sophisticated and may be arranged to receive a temperature indication of ambient conditions around the vessel and to control flow of cooled inert gas in response to the ambient temperature conditions. This control may be in combination with temperature indications from the tank. Thus, effective control of the tank temperature may be achieved.

Still further, the control arrangement may be arranged to receive weather conditions for the route of the vessel. Temperature data for the route can be used by the control arrangement to begin to change the temperature of the void in advance of entering regions of warmer waters or ambient conditions for example. This predictive control arrangement allows the tank or tanks to be maintained at optimal temperatures as the vessel moves between regions of differing temperatures which is common on long voyages for LNG carrying vessels.

In a further mode of operation the control arrangement may advantageously be operable between a first state in which the amount of inert gas is controlled within the void and a second inspection state in which inert gas is expelled from the void.

Allowing for the ventilation of the void permits operatives to enter the void to inspect and or repair components within the void including the insulation layers. This may conveniently be achieved by means of suitable venting ports allowing the void to discharge the cooled inert gas to atmosphere.

The insulation of the tank may be configured in a number of different ways.

In one arrangement only the tank may be insulated and the void into which cooled inert gas is communicated may be disposed between the inner surface of the hull and an outer surface of the insulation surrounding the gas tank or tanks.

In another arrangement the hull may be insulated and the void into which inert gas is communicated may be disposed between the inner surface of the insulation on the hull and the outer surface of the tank.

In yet another arrangement both the hull and the tank may be insulated and the void may be disposed between the outer surface of the insulation surrounding the tank (the primary insulation layer) and the inner surface of the insulation arranged on the hull (the secondary insulation layer). In such an arrangement an impervious layer may additionally be applied to the inner surface of the secondary insulation layer. This provides the secondary barrier against egress of liquefied gas should the primary tank fail.

In each case the invention may be used to cool and control the atmosphere within the void to reduce the thermal temperature differential and thermal conductivity. This greatly improves efficiency and reduces the required thickness of the insulation layers using the cooled inert gas as an integral layer within the arrangement.

It will also be recognized that the invention described herein may also advantageously be retro-fitted to existing liquefied gas carrying vessels. This may be conveniently achieved using the BOG of the existing vessel in combination with a heat exchanger and inert gas supply.

The insert gas may be communicated into the tank by any suitable means such as by fans, air pumps or the like. Advantageously the inert gas within the void may be circulated or moved within the void to further enhance cooling. For example, the inert gas may be caused to circulate around the tank or tanks and pass over the surfaces of the tank. In effect this creates a ‘wind-chill factor’ and further enhances cooling of the tank. This may be described as ‘forced convection’.

Forced convection may be achieved by means of a plurality of fans within the void or by controlling the position and speed with which the inert gas is introduced into the void. Suitably arranging the inlets (or nozzles) of inert gas into the void and the speed of inlet can create a gaseous flow path around the tanks enhancing the cooling effect to the cold inert gas through gaseous flow as opposed to static cooled gas.

Nitrogen may re-circulated by means of suitable conduits to the heat exchange(s). Thus, the nitrogen can be recycled and re-cooled to maintained the desired temperature and conditions within the void.

The forced convection system may advantageously be in communication with the cooling system control arrangement so as to provide a further input to control the temperature with the void. If it is established for example that a cooling target is achieved (through temperature indicators) the supply of coolant may be suppressed. Similarly if further cooling is required the forced convection system may be increased (increased gas flow) and further coolant gas may be provided. Thus, a complex coolant control system can be provided.

Viewed from another aspect there is provided a method of insulating a vessel for transporting a liquefied gas, the vessel comprising a tank for containing a liquefied gas and a hull surrounding the tank, the vessel further comprising a void between the hull and the inner surface of the tank, the method comprising the step of reducing the temperature of an inert gas and introducing the gas into the void.

As described above the insulation may be arranged on the tank, the hull or on both the tank and the hull. The void, into which a controlled cooled inert gas is introduced may thus be between the tank and hull or the various configurations of insulation layers on the hull or the tank.

As also described above with reference to the apparatus, the method may advantageously be performed by means of one or more heat exchangers arranged to cool an inert gas before it is introduced into the void.

The method may advantageously comprise the steps of reducing the temperature of the inert gas by means of a heat exchanger, said heat exchanger arranged to receive a coolant input and an inert gas input.

Advantageously the coolant input may be arranged to receive boil-off gas from the liquefied gas tank.

The vessel may further comprise:

(i) a conduit arranged between an outlet port of the gas tank and the gas cooling arrangement; and (ii) a conduit between the gas cooling arrangement and an inlet port of the void.

The method according to an invention described herein may then further comprise the steps of:

(i) causing boil-off gas to flow from the outlet of the gas tank to the gas cooling arrangement to effect cooling; and (ii) causing cooled inert gas to flow from the gas cooling arrangement to at least one inlet port of the void.

Advantageously, the method may further comprise the step of selectively controlling the supply of inert gas into the void from the inert gas cooling arrangement. Furthermore, the control arrangement may be controlled in response to a temperature indication from the void and arranged to control the flow of cooled inert gas in response to the temperature indication. Such control may, for example, be by means of gas valves.

The control arrangement according to the method may further be arranged to to receive a temperature indication of ambient conditions around the vessel and to control the flow of cooled inert gas in response to the ambient temperature conditions and optionally in combination with the temperature indications from the tank.

Still further the control arrangement and method may further be operable to control the atmosphere in the void between:

(i) a first operational state in which the amount of inert gas is controlled within the void; and (ii) a second inspection mode in which inert gas is expelled from the void.

This may, for example, be achieved by means of a plurality of venting ports arranged in use to ventilate the void to atmosphere. The method may additionally include a step of optionally venting the void to atmosphere through activation of the venting ports. Forced ventilation may be used to speed up the change between the two modes.

The method may further comprise the step of causing the inert gas to be circulated within the void. This can advantageously improve the cooling effect of the inert gas by movement of the gas with respect to the tank.

The method may also comprise selective activation of one or more of a plurality of fans disposed within the void or arranged to communicate gas flow into the void. Thus the cooling of the tank can be selectively controlled. For example, temperature indications may indicate temperature differentials around the tank and the gas can then be caused to circulate in response to the temperature indications.

Viewed from yet another aspect there is provided an inert gas control system for a liquefied gas transport vessel, the vessel comprising a hull and a gas tank within the hull and a space between the hull and tank, wherein the system comprises a gas temperature control arrangement arranged in use to selectively control the temperature of gas within the space so as to be below 0 degrees C in a first mode of operation and above 0 degree C in a second mode of operation.

Accordingly, the void can be selectively used as (a) an integral part of a multi-layer insulation system in a first mode (where insulation is required) and (b) an inspection space in which an operative can enter the void to inspect equipment and surfaces within the void. This advantageously allows an insulation system to be used which can also be safely monitored for any damage or malfunction.

Viewed from a still further aspect there is provided a liquefied gas transport vessel comprising a gas tank and a hull surrounding the tank, the hull and tank defining a space there-between, the vessel further comprising an inert gas cooling arrangement, wherein the inert gas cooling arrangement is configured to cool an inert gas and to communicate the cooled inert gas into the space.

Viewed from yet another aspect there is provided a cooling system for a liquefied gas transport vessel, the system comprising a gas heat exchanging apparatus arranged to receive:

(i) a first input of boiled-off gas from a gas tank on the vessel; and (ii) a second input of a supply of inert gas, wherein the heat exchanging apparatus is arranged in use to cool the inert gas by means of the boiled-off gas and to output the inert gas at a reduced temperature.

Thus, the boiling-off of the liquefied gas can advantageously provide the cooling required to cool the inert gas which can then be communicated to the void around the tank.

The pressure of the inert gas may also be advantageously controlled within the void. Increasing the pressure of the inert gas within the void allows for forced convection of the inert gas (by means of fans for example) to increase the cooling effect of the gas against the tank. Higher density gas allows for greater thermal convection. Tank cooling can thereby be enhanced.

Conversely reducing the pressure in the tank to low pressures or close to a vacuum reduces the convection of heat flux i.e. thermal convection within the void is reduced and less heat is transferred to the tank.

Selectively controlling the pressure in this way allows the temperature within the void to be closely controlled. This thereby reduces boil-off rate within the tank.

Figures

Aspects of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a cross-section of in IMO Type A vessel incorporating an invention described herein;

Figure 2 shows a cross-section of in IMO Type B vessel incorporating an invention described herein; and

Figure 3 shows a cross-section of in IMO Type C vessel incorporating an invention described herein.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood however that drawings and detailed description attached hereto are not intended to limit the invention to the particular form disclosed but rather the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention.

Detailed Description

The present invention provides a number of surprising advantages over conventional liquefied gas carrying vessels, such as liquefied natural gas carriers (LNGCs).

According to an invention described herein there is provided a cryogenic liquid storage tank insulation system in which an inert gas contained within the void (between a tank and the hull or insulation) is controlled so as to reduce the difference in temperature (the Δ T) between the hull and the tank by means of the intermediate temperature of the inert gas. This effect leads to a reduction in Boil-Off Gas (BOG) and a reduction in Boil-off Rate (BOR) from the cargo tanks during the operation of the vessel.

Advantageously, the inert gas is cooled to a temperature at or approximate to the temperature of the liquefied gas within the tank. This not only minimizes BOG and BOR but also maximizes the thermal insulation provided by the void and minimizes thermal losses from the primary tank.

Referring to the figures, Figure 1 shows a cross—section of an IMO Type A vessel. In such a vessel an insulation layer is provided on the inner surface of the hull and the tank is contained within the hull.

Figure 1 shows a vessel 1, the vessel comprising an outer hull 2 which provides the structural strength for the vessel 1. Contained within the hull is a primary storage tank 3 which is arranged to receive a liquefied gas, such as in one example, a liquefied natural gas.

The tank 3 is supported and spaced from the hull 2 by means of supports 4 which are located around and along the periphery of the tank and couple the tank to the hull of the vessel. Additionally, the supports 4 define a void or space 5 between the hull 2 and the tank 3. The supports may be arranged along the entire length of the vessel providing structural support to the tank with respect to the hull.

The tank 3 is arranged to contain a gas that has been converted into a liquid form. In the case where the gas is a liquefied natural gas (LNG) the gas must be cooled to a temperature of approximately minus 163 degree C. This is conventionally achieved by compressors remote from the vessel which compress the gas before it is conveyed into the tanks.

A gas carrier vessel with IMO Type-A tanks is provided with an insulation layer 6 which is positioned on the inner surface of the hull 2. This insulation protects the hull from the low temperature of the primary tank 3. If the hull temperature reduces too much there is the potential for brittle fracture of the hull. Consequently the hull is insulated as shown in figure 1.

The insulation of the hull may be achieved in a number of ways as will be recognized by the skilled person. For example the hull may be insulated using a layer of polyurethane foam or the like attached to the inner surface of the hull. More elaborate and complex insulation layers may equally be applied to the hull.

Referring again to figure 1 the vessel 1 comprises a hull 2 containing a tank 3. Between the hull and tank a space or void 5 is provided which thermally de-couples the tank from the hull. In use the hull is exposed to ambient sea conditions which are significantly higher than the temperature needed to maintain a gas in a liquefied state.

According to the invention the void is incorporated into the insulation system of the vessel. Specifically, the void around the tank is provided with a super-cooled inert gas as described below.

The void around the tank or tanks defines a space into which air is conventionally introduced. According to the invention a heat exchanger is used to cool an inert gas before it is introduced into the void. Thus, the void can not only be filled with an inert gas (preventing any exposure to an explosion in the void) but also filled with a medium that enhances the thermal insulation between the tank and the hull. In effect, the void becomes an integral part of an insulation system described herein.

According to an invention described herein there is provided a heat exchanger 7.

The heat exchanger 7 is provided with a first input 8 which is arranged to communicate boil-off gas from the tank 3 to the heat exchanger. The heat exchanger is also provided with a second input 9 which is source of inert gas. In one embodiment the inert has supply may be nitrogen gas. This may, for example, be obtained from atmospheric air. The operation of a heat exchanger that can exchange heat between a cold gas supply and an ambient gas supply will be understood by a person skilled in the art.

In use an invention described herein operates as follows:

Liquefied gas (such as natural gas) is conveyed to the tank 3. Any liquefied gas that is boiled off from the tank is communicated via conduit 8 to the heat exchanger 7. As the BOG expands it provides a coolant source to the heat exchange. The heat exchanger 7 received a supply 9 of inert gas which is brought into proximity with the cold BOG gas within the heat exchanger causing the inert gas to cool to a temperature proximate to the liquefied gas temperature of minus 163 C. Cooled inert gas is output from the heat exchanger via conduit 10 and introduced into the void 5.

One or more control arrangement(s) 11 may control valves 12, 13 to divert the flow of BOG through the heat exchanger. For example, when the void is at a desired temperature there may be no requirement for the BOG gas to be communicated to the heat exchanger 7. In such as situation valves 12, 13 may be operated to communicate the BOG directly to the output 14 which might either supply the engines with boiled-off gas or alternatively be vented to atmosphere.

The control arrangement(s) 11 may communicate with the valve in any suitable way such as a wired connection 15,16 or wirelessly.

The control arrangement may also be in communication with one or more force convection devices, such as fans or the like, disposed within the void or directing gas into the void. Such convection devices 17, 18 may be in communication with the control arrangement and configured to cause the gas within the void to move with respect to the tank surface. Movement of cooled gas increased the efficiency of heat transfer and thus cools the tank.

The control arrangement may be configured to receive indications of temperatures within the tank (for example from thermocouples) and to control the convection devices in response to the temperature indications received. The control arrangement may also be configured to receive weather data indicating expected temperature conditions for the vessel (and therefore the liquefied gas tank) and to control the introduction of coolant inert gas and forced convection in response to predicted conditions.

Referring to figures 2 and 3 the same concepts apply as described with reference to figure 1.

In figure 2 an IMO type B tank is shown. Here the tank is insulated by insulation layer 19. In this embodiment the void 20 is located between the outer surface of the layer 19 and the inner surface of the hull 21

In figure 3 an IMO type C tank is shown. Here the tank is also insulated as in figure

2.

In both figures 2 and 3 the void is shown into which the cooled inert gas can be introduced. Optionally forced convection also be used to enhance cooling as described herein.

Claims

1. An insulation apparatus for a liquefied gas transport vessel, the vessel comprising an outer hull and at least one liquefied gas tank contained within the hull, the hull and tank defining a void between the inner surface of the hull and the outer surface of the tank, the apparatus comprising an inert gas cooling arrangement arranged in use to reduce the temperature of an inert gas and to communicate the inert gas into the void.

2. An apparatus as claimed in claim 1, wherein the inert has cooling arrangement comprises a heat exchanger arranged to receive a coolant input and an inert gas input.

3. An apparatus as claimed in claim 2, wherein the coolant input is arranged to receive boil-off gas from the liquefied gas tank.

4. An apparatus as claimed in claim 2 or 3, wherein the inert gas input is arranged to receive a supply of gas selected from dry air, nitrogen or argon.

5. An apparatus as claimed in any preceding claim further comprising a conduit arranged between an outlet port of the gas tank and the gas cooling arrangement and conduit between the gas cooling arrangement and an inlet port of the void.

6. An apparatus as claimed in any preceding claim further comprising a control arrangement arranged in use to selectively control the supply of inert gas into the void from the inert gas cooling arrangement.

7. An apparatus as claimed in claim 6, wherein the control arrangement is arranged to receive a temperature indication from the void and to control the flow of cooled inert has in response to the temperature indication.

8. An apparatus as claimed in claim 6 or 7, wherein the control arrangement is further arranged to receive a temperature indication of ambient conditions around the vessel and to control flow of cooled inert gas in response to the ambient temperature conditions and optionally in combination with the temperature indications from the tank.

9. An apparatus as claimed in any of claims 6 to 8, wherein the control arrangement is operable to control the atmosphere in the void between:

a first operational state in which the amount of inert gas is controlled within the void; and a second inspection mode in which inert gas is expelled from the void.

10. An apparatus as claimed in claim 9, wherein the apparatus further comprises a plurality of venting ports arranged in use to ventilate the void to atmosphere.

11. An apparatus as claimed in any preceding claim wherein the inner surface of the hull comprises a secondary barrier and the said void is disposed between the inner surface of the secondary barrier and the outer surface of the tank.

12. An apparatus as claimed in claim 11, wherein the secondary barrier comprises an insulation layer proximate the hull and an impervious layer proximate the void.

13. An apparatus as claimed in any preceding claim further comprising an inert gas movement arrangement arranged to cause inert gas within the void to move with respect to the tank surface.

14. An apparatus as claimed in claim 13, wherein the gas movement arrangement is a plurality of fans disposed within the void to cause convection of gas within the void or arranged to communicate one or more inert gas flows into the void.

15. An apparatus as claimed in claim 13 or 14, wherein the gas movement arrangement is selectively controlled in response to a determined temperature within the void.

16. A method of insulating a vessel for transporting a liquefied gas, the vessel comprising a tank for containing a liquefied gas and a hull surrounding the tank, the vessel further comprising a void between the hull and the inner surface of the tank, the method comprising the step of reducing the temperature of an inert gas and introducing the gas into the void.

17. A method as claimed in claim 16, wherein the inert gas temperature is reduced by means of a heat exchanger, said heat exchanger arranged to receive a coolant input and an inert gas input.

18. A method as claimed in claim 17, wherein the coolant input is arranged to receive boil-off gas from the liquefied gas tank.

19. A method as claimed in claim 17 or 18, wherein the inert gas input is arranged to receive a supply of gas selected from dry air, nitrogen or argon.

20. A method as claimed in any of claims 16 to 19, wherein the vessel further comprises:

(i) a conduit arranged between an outlet port of the gas tank and the gas cooling arrangement; and (ii) a conduit between the gas cooling arrangement and an inlet port of the void;

the method further comprising the steps of:

(i) causing boil-off gas to flow from the outlet of the gas tank to the gas cooling arrangement to effect cooling; and (ii) causing cooled inert gas to flow from the gas cooling arrangement to an inlet port of the void.

21. A method as claimed in any of claims 16 to 20, further comprising the step of selectively controlling the supply of inert gas into the void from the inert gas cooling arrangement.

22. A method as claimed in claim 21, wherein the control arrangement is controlled in response to a temperature indication from the void and controls the flow of cooled inert gas in response to the temperature indication.

23. A method as claimed in claim 21 or 22, wherein the control arrangement is further arranged to receive a temperature indication of ambient conditions around the vessel and to control the flow of cooled inert gas in response to the ambient temperature conditions and optionally in combination with the temperature indications from the tank.

24. A method as claimed in any of claims 16 to 23, wherein the control arrangement is operable to control the atmosphere in the void between:

25. A method as claimed in claim 24, wherein the apparatus further comprises a plurality of venting ports arranged in use to ventilate the void to atmosphere and the void is optionally vented to atmosphere through activation of the venting ports.

26. A method as claimed in any of claims 16 to 25 wherein the inner surface of the hull comprises a secondary barrier and the said void is disposed between the inner surface of the secondary barrier and the outer surface of the tank and wherein the method controls the communication of inert gas into the void.

27. A method as claimed in claim 26, wherein the secondary barrier comprises an insulation layer proximate the hull and an impervious layer proximate the void.

28. A method as claimed in any of claims 16 to 27 wherein the inert gas within the void is caused to move with respect to the tank surface.

29. A method as claimed in claim 28, wherein the inert gas is caused to move by selective activation of a plurality of fans disposed within the void or arranged to communicate gas flow into the void.

30. A method as claimed in claim 28 or 29, wherein the gas movement arrangement is selectively controlled in response to a determined temperature within the void.

31. An inert gas control system for a liquefied gas transport vessel, the vessel comprising a hull and a gas tank within the hull and a space between the hull and tank, wherein the system comprises a gas temperature control arrangement

5 arranged in use to control the temperature of gas within the space so as to be below 0 degrees C.

32. A system as claimed in claim 31 wherein the gas is caused to flow within the void.

33. A liquefied gas transport vessel comprising a gas tank and a hull surrounding the tank, the hull and tank defining a space there-between, the vessel further comprising an inert gas cooling arrangement, wherein the inert gas cooling arrangement is configured to cool an inert gas and to communicate the cooled inert

15 gas into the space.

34. A cooling system for a liquefied gas transport vessel, the system comprising a gas heat exchanging apparatus arranged to receive a first input of (i) boiled-off gas from a gas tank on the vessel and a second input of (ii) a supply of inert gas, wherein

20 the heat exchanging apparatus is arranged in use to cool the inert gas by means of the boiled-off gas and to output the inert gas at a reduced temperature.