The present invention is concerned with the transportation of liquefied gases and particularly (but not exclusively) liquefied natural gas (LNG). Specifically the invention is concerned with a novel insulation arrangement for a particular type of LNG tank, known in the industry as an I MO type-C LNG tank. Such tanks can be used to transport LNG as a cargo but can additionally be used as a fuel tank for ocean going vessels thus allowing LNG to be used to fuel a ship with consequential environmental benefits.

The invention may also be applied to other tanks for transporting cryogenic liquids such as liquefied propane gas (LPG) or liquefied ethylene gas (LEG). Background

International Maritime Organisation (IMO) Rules stipulate the types of tanks that can be used to convey LNG on ships. A common tank that is used for LNG is a type C tank which is in the form of a sphere or cylinder acting as a pressure vessel to contain the LNG. The rules stipulate a range of technical characteristics a tank must have in order for a tank to be certified as safe to use.

Cryogenic liquids such as LNG are transported / stored at extremely low temperatures i.e. minus 163 degrees C and these temperatures must be maintained to keep the gas in a liquid form.

One technique to insulate such a tank is to apply a sprayed on foam insulation to the outside of the tank. Typically, polyurethane is applied or sprayed onto the exterior of the LNG tank. Such spray foam materials are convenient to apply and cost effective and are very popular with ship constructors owing to the cost effective nature of the insulation that can be applied. It is also very easy to apply.

The combination of LNG type C tanks and spray foam insulation allows LNG to be safely transported either in large volumes as a cargo or as a fuel for a ship. The present invention, as described herein, provides an effective way to insulate a ship LNG fuel tank. However, although existing insulation systems are widely accepted in the industry as being fit for purpose the present inventor has established an improved insulation system and method using a new spray foam technique. Specifically, the new system and method further enhances the advantages of spray foam insulation and prolongs the insulation efficiency. It can also advantageously provide an added fire barrier to an LNG tank.

Summary of the Invention

According to a first aspect of an invention described herein, there is provided a method of insulating a tank for containing liquefied gas, said method comprising the steps of sequentially applying a plurality of layers to the outer surface of the tank, said sequential layers comprising:

(A) a first foam layer applied to the tank;

(B) a second impervious coating applied to the first layer;

wherein each of said layers surrounds a preceding layer. Optionally, after step (B), an additional layer may be applied of:

(C) an outer cladding layer applied to the second layer.

The present invention advantageously provides a multi-layered insulation system in which a foam layer is encapsulated or sealed between the tank itself and a further impervious layer. Specifically a gas tight metallic vapour barrier may be provided to the first layer. This forms a vapour tight barrier around the foam and the tank which has not previously been conceived.

Advantageously, the barrier is a metallic barrier providing the impervious property of the insulation system. The metallic barrier may include a self-adhesive layer to connect it to the foam or may be applied with a separate adhesive.

The combination of a tank, foam insulating layer and impervious vapour barrier has not previously been considering and thus, viewed from another aspect of an invention, there is provided a liquefied natural gas tank comprising a first foam layer surrounding the tank and a second impervious barrier surrounding the first foam layer. Thus, an LNG fuel lank may be provided with a foam insulation and a barrier surrounding the foam which is impervious to vapour and liquids as will be discussed further herein.

Advantageously, the impervious barrier provides multiple advantages.

For example, it prevents damage to the foam layer during use. More importantly the impervious barrier prevents humidity from entering into the open cavities or cells within the foam layer itself which can damage and degrade the foam. Furthermore, the inventor has established that such an impervious layer prevents the emission of CO? (also known as the blowing agent) and the ingress of air into the foam cells which advantageously prevents ageing or degradation of the foam layer. The blowing agent is used to expand the foam and remains in the cells after expansion. This might emigrate from the ceils over time and reduce the thermal properties of the insulation.

Presently, Polyurethane (PU) foams are blown with a combination of C0 ₂ , obtained through the chemical reaction between preciseiy dosed quantities of water in the poiyol blend and the isocyanate component, and with high molecular weight blowing agents such as hydrofluorocarbons (namely HFC-365mfc, HFC-227ea and/or HFG-245fa) and/or other fluorinated- or chlorofluorinated- olefins (namely HCFO-1233zd, HFQ-1336mzz). Other suitable blowing agents may also be used either at the present time or in the future. While the C0 ₂ may readily diffuse out of the foam cells, the high molecular blowing agents will substantially stay inside the foam cells for periods well in excess of those required for a reasonable economic life. These blowing agents are therefore called 'permanent', while C0 ₂ is called 'non-permanent' blowing agent. Ageing of the thermal properties of such PU products is therefore predominantly caused by the inward diffusion of air into the product ceils and outward diffusion of C0 ₂ , if diffusion tight facings do not prevent both.

Thus, using a new combination of a foam layer with an impervious encapsulating layer significantly minimizes the ageing of the thermal properties of the insulation layer by both preventing the ingress of ambient air (oxygen) into the cells within the foam and the egress of C0 ₂ from inside the cells of the foam.

The present insulating layer significantly reduces polyurethane foam ageing thus substantially improving the operating life of insulation layers. The term impervious is intended herein to refer to a layer that prevents gas and liquid from passing through the layer in either direction.

The first foam layer maybe applied to the tank surface using various techniques. The foam layer may for example be applied using a spray-on technique using a spray gun. This allows the foam to be easily and quickly applied to large tank surfaces.

An expanded polyurethane foam is particularly convenient to apply in this manner. Specifically the foam layer may be a premix of polyoi and isocyanate.

The foam layer may be a single layer of foam in some applications. However, to maximise the insulating properties the foam layer may be formed of a plurality of individual polyurethane sub-layers, each applied over a preceding layer. The sub-layers may be any suitable thickness. For an LNG application the foam layer may be formed of a plurality of sub-layers, each layer being between 5mm and 35mm in thickness, in such an application, in order to meet the insulating needs for ocean travel the total thickness may be 300mm measured from the tank outer wall Furthermore the individual layers may be applied with dissimilar (different) densities.

Specifically, in an LNG application a sub-layer closer to the tank may have a density greater than 50kg/m ³ and a subsequent layer may have a density greater than 40 kg/m ³ . It has been established by the inventor that such a combination of layers and differing densities optimised the insulation properties and mechanical strength of the insulating layer. Both are of importance in an LNG application.

In another arrangement the densities of the foam layers may be substantially the same. This simplifies the application process and provides a uniform density of foam.

It has further been established that thermal performance can be further enhanced if the first 50mm of foam layer measured from the tank surface has a density of greater than 5Qkg/m ³ and the remaining depth of the foam layer has a density greater than 40kg/m ³ . In order to further enhance the structure integrity of the foam layer the layer (or sub-layers) may be provided with glass fibres within the foam. This may be incorporated into the sprayed polyurethane mix or may be applied as one or more mesh layers. This advantageously acts as a crack preventer,

A polyurethane primer may be applied to the tank before the foam is applied. This improves the bonding strength between the foam and the tank surface.

As described above the impervious layer is impervious to gas and liquid but specifically prevents air from penetrating into and C0 ₂ from diffusing out of the ceils of the foam. The impervious layer may advantageously be an aluminium foil with a polymeric backing layer and an optional self-adhesive coating. Such a combination of aluminium foil and polymeric backing provides the desired impervious quality in combination with flexibility and ease of application to the outer surface of the foam layer. Thus a vapour tight barrier is formed.

Furthermore, the polymeric layer advantageously self-seals if punctured because of the elastic nature of the material. Thus, the integrity of the impervious seal is not compromised if the layer is accidentally punctured. The impervious layer may use a polymeric backing layer such as butyl rubber.

The impervious layer may further be provided with a self-adhesive layer facing the foam side of the impervious layer to provide a bonding surface to hold the layer in position against the foam.

The insulating layer may further be enhanced with the inclusion of an optional fire retardant layer or coating. For example, a third fire retardant layer may be provided between the impervious layer and the outer cladding. This fire retardant layer may be a mineral wool layer such as a stone wool / mineral wool fire retardant layer. Other similar A60 fire retardant or insulating layers may also be used.

The outer layer of the insulating system may advantageously be in the form of an aluminium, galvanised steel or stainless steel layer surrounding the preceding layer. This may for example be installed as a plurality of panels or rings which tessellate to encapsulate the preceding layers. The cladding provides structural strength to the insulating system and protects the inner layers. In effect the outer cladding layer and tank wall create a cavity in which the insulating foam, impervious layer and fire retardant are securely contained and protected.

Advantageously the method described herein may be applied to any type of cryogenic transport tank including, for example, an I MO type-C tank.

Viewed from another aspect of an invention described herein, there is provided an 1MO type- C tank comprising an outer insulation layer, said layer comprising: (A) a first foam layer applied to the tank;

(B) a second impervious coating applied to the first layer.

Optionally, the tank may be provided with a further layer around layer (B), namely:

(C) a third cladding layer surrounding the second layer. Such a tank may further comprise an optional third fire retardant layer between the second impervious layer and the cladding layer as described above.

The term coating is used herein to refer to a layer including, but not limited to, an aluminium foil with a butyl rubber layer attached thereto.

A further aspect of an invention described herein extends to an ocean going vessel (such as a cryogenic transport ship) comprising a tank and insulation system as described herein, each with a metallic impervious layer or coating surrounding the foam.

Brief Description of the Drawings

Embodiment of the invention will now be described by way of example with reference to the following figures.

In accordance with one (or more) embodiments of the present invention the Figures show the following:

Figure 1 shows a tank insulated according to the invention with the plurality of layers cut- away.

Figure 2 shows the insulation layer according to the invention and each of the sub-layers.

Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field.

As used in this specification, the words "comprises", "comprising", and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean "including, but not limited to".

The invention is further described with reference to the following examples, it will be appreciated that the invention as claimed is not intended to be limited in any way by these examples.

Detailed Description

Figure 1 shows an LNG tank 1 incorporating an insulation system as described herein.

Such a tank 1 comprises an inner tank body 2 which in figure 1 is generally cylindrical but other shapes may equally benefit from the insulation system described herein. The tank body comprises supports 3 (only 1 being shown) which are the means of supporting the tank on the ship's deck/structure.

The insulation layer comprises 4 main layers each with different properties and thickness:

- The first layer is a spray foam polyurethane layer 5.

The second layer 6 is an impervious layer of aluminium foil with a polymeric backing - The third (optional) layer 7 is a fire retardanf layer of mineral/ glass wool; and

- The outer layer 8 is a metal cladding which may (as shown in figure 1 ) be in the form of a plurality of panels each abutting one another to encapsulate the insulation system. As shown in figure 1 each layer surrounds the preceding layer with the tank 2 housed within the insulation system.

In an arrangement with no layer 7 the outer layer 8 may be applied directly onto layer 8, Figure 2 shows a cross-section of the insulation system shown in figure 1 .

Like reference numerals refer to the same layers. As shown the main thickness of the insulation system is the polyurethane layer. A polyurethane primer may be applied to the outer surface of the tank 2 to assist in bonding the polyurethane foam layer 5 to the tank 2.

Application of the insulation layer will now be described. The invention is not limited to the specific embodiment described below which provides an IMO type-C tank with an average thermal conductivity K-Value of approx. 0.085W/m2 K and has an overall insulation thickness 300mm of PU foam and 50mm of Rockwool fire retardant wool. This represents just one example of the application of the invention.

Stage 1 - Foam Application

The foam applied is a premix of polyoi and isocyanate manufactured by Tagos Sri These components are brought together and sprayed to the tank surface through a conventional spray gun. A polyurethane primer may be applied to the tank surface prior to the foam application to improve the bonding strength between the foam and the tank surface.

The foam is sprayed to the tank surface in several layers to build up the total specified insulation thickness of approximately 300mm. The thickness for each layer is 5 to 35 mm.

The first 50mm of the foam has a density of 60 kg/m ³ and the remaining 250 mm a density of 40 kg/m ³ . The foam density variation will be obtained by utilizing different polyoi formulations having the correct specification of free-rise and installed applied foam density. Alternatively, to simplify installation a single density could be used for all foam layers.

Stage 2 - Application of the impervious layer

After application of spray foam, a aluminium foil with a butyl rubber backing is applied all over the foam's outer surface to form a vapor tight barrier. This may be using a self-adhesive layer on the metal or using a separate adhesive. The vapor barrier will self-close or seal in the event of an accidental puncture by virtue of the elasticity of the backing.

Stage 3 and 4 - Optional fire retardant layer and cladding The metal cladding may be selected from a range of materials and thicknesses, in this specific embodiment the cladding is 0.6mm thick stainless steel (SS 316L).

Metal strapping band to be applied in a sufficient pattern circumferential and longitudinal around the tank to make a fixation arrangement for the metal cladding. The metal cladding is fixed with overlapping joints to the strapping band by use of pop rivets or screws. Each joint between cladding sheets are salad with sealant. Before the cladding is applied an insulation layer similar to an A60 fire insulation layer may be applied all over the impervious layer surface and fastened by conventional steel strapping bands. A suitable material is manufactured by Rockwooi. The thickness may be between 3- to 80mm.

The 0.6mm SS 316L cladding is then applied outside the Rockwooi insulation fixed to the strapping band outside the Rockwooi. The metal cladding and the fire blankets provide integrated fire and mechanical protection.

Optionally a Glass Reinforced Polyester (GRP) to be applied on the outer surface of the stainless steel cladding. The thickness of the GRP may be >2mm and this protects the stainless steel from the effects of seawater in cases where the tank is located on the deck of a ship.

Each of the layers is applied so as to surround the preceding layer. The insulation system maintains the LNG at the required low temperature, inside the tank 2, Advantageously a fuel or cargo tank on an ocean going vessel incorporating this system exhibits improved operational life and reliability owing to the fact that the insulating foam layer does not degrade as is the case with conventional insulation systems.

Although the invention has been described by way of example, it should be appreciated that variations and modifications may be made without departing from the scope of the invention as defined in the claims. Furthermore, where known equivalents exist to specific features, such equivalents are incorporated as if specifically referred in this specification.