CA1225605A

CA1225605A - Insulation of vessels having curved surfaces

Info

Publication number: CA1225605A
Application number: CA000453786A
Authority: CA
Inventors: Hiroshi Tonokowa; Emil Ekker; Hubert S. Smith
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1983-05-09
Filing date: 1984-05-08
Publication date: 1987-08-18
Also published as: AU558074B2; PL146941B1; JPH0723760B2; FI81545C; AU2962484A; FI850076A0; KR840008835A; BR8406894A; PL247579A1; EP0142552A1; EP0142552B1; IT8420858A1; FI850076L; FI81545B; WO1984004514A1; ES8505011A1; EP0142552A4; JPS60500821A; IT1173596B; KR910001696B1

Abstract

ABSTRACT OF THE DISCLOSURE

Cryogenic vessels provided with improved thermal insulation by spirally generating two layers of thermal insulation thereon; permits thicker insula-tion without requirement of increased space and heavier equipment.

31,843-F

Description

l~Z5~5 INSULATION OF VESSELS
EAVING CURVED SURFACES

In the transportation of cryogenic liguids such as liquefied natural gas, vessels that are utilized must have a relatively high degree of thermal insulation to prevent loss of the liquefied cargo during transpor-tation. A variety of ships have been designed for thetransportation of liquefied natural gas. One of the , more desirable designs for such vessels employs spherical tanks which are equatorially supported. Ships generally utilize a plurality o~ such spherical tanks having an e~uatorial flange, wherein the flange is supported by a generally cylindrical skirt which is affixed to the hull of the ship. The use of spherical tanks provide a minimal surface to volume ratio thereby minimizing the surface which must be provided with suitable thermal insulation to prevent undesirable vaporization of the liquefied cargo. Such spherical tanks are useful not only for shipboard applications but for land based storage also. Tanks for cryogenic liquids have been employed which have a variety of configurations such as rectangular, cylindrical and the like. Cryogenic tanks have been insulated by a variety of methods. One 31,843-F -1-i~S605 method that has found some favor is the so-called panel insulating method wherein the cryogenic tank has applied thereto a plurality of panels of insulating material and the adjacent panels are bonded to each other by means of a suitable adhesive, for example a foamed in place urethane foam. The panel insulating technigue is labor intensive and for many applications is therefore undesirable. When employed with tanks of rectangular configuration, the panel system provides minimal difficulty in fitting. When the panel system is applied to non-rectangular tanks such as those of cylindrical or spherical configuration, the amount of effort in the cutting and fitting of the tanks is generally found to be undesirably time consuming.
Vessels having rotational symmetry such as those of cylindrical or spherical configuration beneficially may be insulated using the so-called spiral generation process. The term "spiral generation" refers to the preparation of structures by the progressive deposition of an insulating foam or like construction material by a material depositing means which travels about a - predetermined path to deposit successive loops or turns of the insulating material on the cylindrical or spherical vessel, wherein the progressive loops are adhered to adjacent loops. In effect, when the spiral generation techni~ue is employed, a strip of insulating material is wound about the vessel to be insulated and the adjacent turns of the strip are effectively affixed to each o~her. Spiral generation tech~i~ues and variations thereof are set forth in the following U.S. Patents:
3,206,899; 3,337,384; 3,443,276; 3,507,735; 3,874,983;
3,879,254; 3,902,943; 3,919,034; 3,923,573; 3,9~4,039;
4,017,346; 4,050,607; 4,098,635; and 4,175,998.

31,843-F -2-, iZZ5605 ~ .S. 4,017,346 discloses an apparatus and a method for the application of foam thermoplastic insulation to a cryogenic vessel of rotational symmetry. A composite strip or log is applied to the vessel, the log having a vapor barrier layer disposed remote from the vessel, the vapor barrier layer generally being a flexible aluminum sheet. The thermoplastic foam of the log is heat bonded to adjacent loops as is the aluminum skin bonded to its adjacent turns by means of heat sealing or crimping or like connecting means. Insulation of a vessel in accordance with the present invention is readily accomplished employing the method and apparatus as set forth in U.S. Patent 4,017,346.

In a typical application of thermal insulation to a vessel having rotational symmetry by the spiral generation method, a log of about 20 centimeters square cross section may be employed. In general, the appli-cation o~ insulation by the spiral generation technique becomes easier as the cross section of the insulating strip is reduced. Generally as the size of the cross section configuration of the strip increases, the mass of equipment required to deposit such a strip also increases. In most instances it is very desirable that the size of the apparatus employed to deposit a strip of insulating material in the spiral generation process be maintained at a minimum. This is particularly true in the case of shipboard installations where generally a tan~ such as a spherical tank is installed at least partially within the hull of a ship and the insulation is applied to the tank after such installation. Oftentimes a weather shield is provided for such a tank when a portion of the tank protrudes above the hull of the ship. Advantageously, the mass and volume of the 31,843-F -3-. ~

~ ~5605 spiral generation equipment employed to apply insulation is maintained at a minimum volume and minimal weight.
Ideally the spiral generation equipment is of relatively low volume and low weight which facilitates moving the equipment from one location to another and requires minimal space about the tank for its operation during the application of insulating material.

A very critical element in cryogenic insulation is the vapor barrier which as disclosed in U.S. Patent 1~ No. 4,017,346 comprises an external aluminum skin, enclosing the vessel and the insulation material. In the event that water vapor has ready access to the insulation area of the cryogenic vessel, a build-up of ice can occur and result in a substantial and significant loss of the insulating value. The insulating system set forth in U.S. Patent No. 4,017,346 was very satis-factory for use in shipping liquefied natural gas when the value of li~uefied natural gas was relatively low.
The single layer of insulation and vapor barrier provided adeguate insulation and protection of the cryogenic vessel even when liquefied natural gas which vaporized was conducted from the cryogenic tank to provide fuel for the ship. As the value of liquefied natural gas increased relative to heavy fuel oils such as Bunker C, it became desirable to have increased thermal insulation on vessels employed to transport and/or store liguefied natural gas.

The present invention provides an improved cryogenic vessel having thermal insulation disposed thereabout which has improved vapor barrier characteristics.

31, a43-F -4--~2Sti05 The present invention also provides an improved method for the spiral generation application of thermal insulation to cryogenic vessels.
More particularly, the invention provides for an improved method for the application of thermal insulation to cryogenic vesselsby the spiral generation technique which permits the application of thermal insulation having a relatively high resistance to the flow of heat utilizing equipment having minimal weight and volume.
The present invention particularly resides in an improved insulated cryogenic vessel comprising at least a first containment vessel having an exterior curved surface, a first layer of a foam insulation disposed adjacent the exterior surface and covering at least a substantial portion thereof;
the insulation being in the form of a plurality of turns of a composite strip-like material, having a continuous gas barrier layer affixed thereto remote from said containment vessel, adjacent turns of the first insulation being heat bonded to adjacent turns of the first insulation, characterized by a second layer of foam insulation being disposed over and unadhered to the first layer of foam insulation and external thereto and spaced from the continuous gas barrier layer affixed to said first layer of insulation; the second layer of insulation being in the form of a plurality of turns of strip-like material, adjacent turns of the second layer of insulation being heat bonded to adjacent turns of insulation of the second layer of insulation; the first layer of foam insulation being a laminated strip-like material, the first layer of foam insulation having j 12256~5 affixed thereto a first reinforcing scrim and which in turn has affixed thereto said gas barrier layer, with a second reinforcing scrim located within the laminated strip-like material, the laminated strip-like material defining a plurality of longitudinally extending slots opening toward the exterior surface of the containment vessel, the slots terminating at a location generally adjacent said second reinforcing scrim.
The present invention also resides in a method for the insulation of at least a first cryogenic vessel having an outer surface wherein a thermally insulating material is a flexibilized thermoplastic foam insulation having a density of from about 1.5 to 2.5 pounds per cubic foot and having a strip-like configuration having a metal gas barrier layer affixed thereto, depositing the foam with the gas barrier layer remote from the outer surface of the vessel about the periphery of the vessel in the form of a plurality of closed loops, heat bonding adjacent loops and gas barrier layers together to thereby envelop at least a portion of the vessel within a first insulating layer, subsequently depositing at least a second insulating layer about the first insulating layer, the second insulating layer having a metal gas barrier layer disposed remote from the vessel, the layers being in the form of a plurality of closed loops to thereby envelop at least a portion of the first insulating layer, the second heat insulating layers being deposited and heat bonded in a like manner as the first layer.
Further features and advantages of the present invention will become more apparent from the following specification taken ~` ` ?
I ~ .

iZ256~)S

in connection with the drawing wherein Figure 1 is a fractional view of a vessel being insulated in accordance with the present invention;
Figures 2, 3 and 4 depict cross sectional configurations of insulation applied in accordance with the present invention.
In Figure 1, there is schematically depicted a fractional view of a vessel 10 being insulated in accordance with the present invention. The vessel 10 has a generally spherical configuration having an external surface 13. Adjacent the external surface 13 of the vessel 11 is a first layer of insulation 14 which is formed of a plurality of closed loops 15 of insulating material which are joined to each other. A second layer of insulating material 17 is disposed external to the layer 14 of insulating material. The layer 17 comprises a plurality of closed loops 18 of insulating material, the adjacent portions of the loops 18 being affixed to each other.
A spiral generation depositing apparatus 20 is rotatably mounted adjacent the tank 11 and spaced therefrom. The spiral generation apparatus 20 comprises an arcuate boom 21 having an -6a-i22560S

upper or axial end 22 and a lo~er or equatorial end 23.
The ends 22 and 23 are supported by means of rollers 24 and 25 respectively. The upper end 22 of the boom 21 is rotatably fixed to an axially disposed pivot 27 affixed to a dome 12 of the tank 11. An insulation depositing head 28 is supported by the boom 21 and is adapted to move along the boom 21 between the ends 22 and 23. A strip or log of insulating material 31 is engaged by the depositing heat 28 and is effectively wound around the vessel 11 to form the second insulating layer 17. Both of the insulating layers 14 and 17 are deposited by the depositing apparatus 20 which rotates about the vessel 11 as the insulation is deposited.

In Figure 2 there is schematically depicted a cross sectional view of a portion of insulation 40 in accordance with the present invention. The insulation 40 has a first or inner side 41 and a second or external side 42. The insulation 40 comprises a first or inner layer 43 and a second or outer layer 44. The inner layer 43 comprises a plurality of loops or turns of a synthetic resinous thermoplastic foam insulating material 45 each of which comprises a first or inner log portion 46 and a second or outer log portion 47. Adjacent portions of the turns 45 are joined together at joints 48. Advantageously, such joints 48 are formed between adjacent turns by heating the thermoplastic foam of log portions 46 and 47 to a temperature sufficiently high so that, in effect, the foam melts and the adjacent turns are heat bonded together. The inner surface 41 of the insulation 40 has disposed thereon and affixed thereto a reinforcing scrim 49. Advantageously, the reinforcing scrim 49 is an open weave glass cloth which provides mechanical reinforcement for the log portions 31,843-F -7-i2256~5 46. Each of the turns 45 defines slots 51 which extend from the inner face ~1 partially inwardly and in a direction generally normal to the log portions 46. A
reinforcing scrim 53 is disposed between log portions 46 and 47 and heat bonded therebetween. The scrim 53 advantageously is an open weave glass cloth and provides mechanical reinforcement for the turns 45. The log portions 47 remote from the inner face 41 have affixed thereon an optional glass reinforcing scrim 54 which, in turn, is adhered to a vapor or gas barrier layer 55, advantageously of aluminum. The vapor barrier layer 55 is wider than the turns 45 and is heat sealed by means of appropriate hot melt adhesives to adjacent portions of vapor barrier material 55 on adjacent turns 45.
Each of the turns 45 adjacent its edges defines a recess 57 adjacent the inner or cold side 41 and recesses 58 adjacent the vapor barrier layer 55.
Advantageously, the adjacent recesses 57 form a groove on the inner face 41 to provide clearance for a heat sealing platen when the joints or fused portions 48 are formed. The grooves 58 facilitate the sealing of adjacent portions of the vapor barrier layer 55 and folding of the sealed portions so that the sealed portions lie generally parallel to the major portions of the vapor barrier layer 55. The outer insulation layer 44 comprises a plurality of loops or turns 61.
Each of the turns 61 has a body portion 62 of a synthetic resinous thermoplastic foam insulating material and an inner face 6~, having affixed thereto an optional reinforcing scrim 64 such as an open weave glass cloth.
The body portion 62 has an external or outer face 65 having adhered thereto a reinforcing scrim 66 which in turn has adhered thereto a gas or vapor barrier layer 67 generally e~uivalent to the vapor barrier layer 55.

31,843-F -8-1'~2S~;05 g The body portion 62 has first or inner recess 68. The adjacent recesses 68 form grooves opening toward the inner face 63. Similar recesses 69 are formed adjacent the outer face 65 and form outwardly opening grooves which face the vapor barrier layer 67.

In the insulation 40 as depicted in Figure 2, the slots 51 provide stress relief when the inner face 41 is cooled to cryogenic temperature. The reinforcing scrim 53 provides an effective means of terminating any cracks which might result from shrinkage of the adjacent face 41 o~ log portion 46 and minimize the tendency of cracks to form which would run vertically, that is in the plane of the paper. The scrim 54 adjacent the vapor barrier layer 55 provides added mechanical reinforcement to resist thermal stress on cooling of the inner face 41 of log portion 46 to cryogenic temperatures. The outer insulating layer 44 provides additional thermal insulation and mechanical protection for the vapor barrier layer 55. Accordingly, the insulation 40 effectively provides two concentric vapor barriers which provide a high degree of reliability both from a containment point of view in the event of any leakage of the liquefied gas from the tank and from any water vapor permeation from the space external to the tank. The grooves in the insulation formed by the recesses 57, 58, 68 and 69 provide convenient paths for th~ flow of purge gas when desired.

The thermoplastic foam used in the practice of the present invention may be any one of a variety of foams. ~owever, particularly desirable are styrene polymer foams such as polystyrene foam having a den~ity ge~erally in the range of from 24 to 40 kg/m3 (1.5 to 31,843-F -9-

2.5 lb/ft3), and advantageously for many cryogenic applications, it is desirable to flexibilize the foam.
This is done by a controlled crushing of the foam to render it more easily bendable. Flexibilizing of such foams is well known and set forth in U.S. Patent Nos.

3,159,700 and 3,191,224. Flexibilizing the foam employed for insulation in accordance with the present invention generally facilitates handling of the foam as it is fed to the foam depositing head. Flexibilization also increases the elongation at break of the foam and therefore reduces the tendency of such foam to crack when subjected to cryogenic temperatures.

In Figure 3, there is schematically depicted a cross sectional view of insulation in accordance with the present invention, generally designated by the reference numeral 70. The insulation 70 comprises a first insulating layer 71 and a second insulating layer 72. The insulating layer 71 is disposed adjacent the vessel to be insulated and is of the same construction as the insulating layer 43 of Figure 2. The layer 71 has an inner or cold side 73 and an outer or warm side 74. The second layer 72 is of similar construction to the layer 71 with the exception that an optional scxim beneath the vapor barrier layer 75 of the outer layer 72 has been omitted and the vapor barrier layer is directly adhered by means of a suitable hot melt adhesive 76 to the foam body portion forming the turns which make up the layer 72. The arrangement, as depicted in Figure 3, utilizing two layers of generally equal thickness provides a mechanically desirable insulation and minimizes equipment changes in the preparation of the laminate material forming the loops of each layer.
On completion of the formation of the logs or insulating 31,843-F -10-: .

i2Z56~5 strips which make up the turns of the layer 71, laminates for the second layer 72 are readily prepared by omitting the scrim adjacent the vapor barrier. Thus the materials for layer 71 and 72 may be prepared and deposited without undesirable e~uipment changes or adjustment.

Generally, in the preparation of insulation in accordance with the present invention, it is often desirable to stagger, that is offset, the joints between the turns forming the successive layers. Such an offset or staggering minimizes the possibility of propagation of cracks in the fused joint between adjacent turns from one layer to another.

In Figure 4, there is schematically depicted a fractional cross-sectional view of an alternate embodiment of insulation in accordance with the present in~ention. The insulation 80 comprises in cooperative combination a first or inner insulating layer 81, a second or intermediate insulating layer 82 and a third or outer insulating layer 83. The inner insulating layer 81 comprises a plurality of loops or turns 85 joined to each other in edge to edge relationship.
Each of t~e turns 85 comprises a first or inner body portion 86 and a second or outer body portion 87. The body portions 86 and 87 are laminated together and have therebetween a reinforcing scrim 88 such as an open weave glass cloth. A generally similar scrim 89 is affixed to the face of inner body portion 86 remote from outer body portion 87. The inner body portion 86 is provided with stress relief slots 91 which extend part-way inwardly into the body 86 from the face having scrim 89. The body portion 87 is provided with stress relief recesses, grooves, or slots 92 which extend 31,843-F -11-i2256VS

part-way into the body portion 87 from the scrim 88.
The recesses 92 may be in the form of relatively long and narrow slots, in the form of relatively short and wide grooves, or in the form of any other cross-sectional shape, i.e., rectangular, hemispherical, ellipsical or the like, to provide the desired stress relief in th~
inner insulating layer 81 and to prevent cracking of the insulation due to temperature stress induced in the insulating layer in a crack barrier zone which passes through the inner body portion 86 in the region of the reinforcing scrim 88. The recesses may extend inwardly as well as outwardly from the reinforcing scrim 88 part-way into the insulation and they may be provided in the inner and outer insulating layers in the two-layered structures, illustrated in Figures 2 and 3.
It will be apparent that stress relief grooves (not shown) may also be provided in the intermediate insulating layer 82 illustrated in Figure 4 such that they extend part-way into the insulating material from one or opposite sides of the reinforcing scrim 107.
Grooves corresponding to groove 91 may also be provided in the outer insulating layer 83 to extend part-way into the insulation from the inner side of the layer, i.e., from the side of the scrim layer 117. Each of the turns 85 has affixed thereto a reinforcing scrim 93 which is disposed on body portion 87 generally parallel to the scrims 88 and 89 and remote from scrim 88. The reinforcing scrim 93 has adhered thereto and to body portion 87 a vapor or gas barrier layer 94. The barrier layer 94 is sealed to adjacent barrier layers on the turns 85 of the first insulating layer 81. The edges of adjacent turns 85 define inwardly extending grooves 96, extending inwardly from the face having scrim 89.
The adjacent turns 85 also define grooves 97 which 31,843-F -12-~w~

`- lZ2560S

extend inwardly from the face having the vapor barrier layer 94. The intermediate layer 82 is of the same construction as the inner layer 81 and comprises a plurality of loops or turns 85a. Each of the turns 85a 5 comprises a first or inner foam body 101 having adjacent turns 85, a reinforcing scrim 102 affixed thereto. The body 101 defines a pair of stress relief slots 103 and 104 which extend inwardly from the scrim 102. A second generally rectangular foam body 106 is laminated to body 101 and a reinforcing scrim 107 is affixed between the bodies 101 and 106. The body 106 has affixed thereto an outer reinforcing scrim 108 which is generally parallel to scrims 107 and 102 and disposed remote therefrom. Affixed to the body 106 immediately adjacent the scrim 108 is a vapor barrier 109. The vapor barrier 109 is affixed and adhered to adjacent vapor barrier 109 of the loops or turns 85a. The turns 85a define therebetween first or inner slots or grooves 110 in the region of a fused joint 111 between the adjacent turns. The groove 110 depend generally inwardly from the scrim 102. Generally, similar grooves 112 are defined between adjacent turns 85a adjacent the scrim 108 and vapor barrier lO9. The third or outer layer 83 comprises a plurality of loops or turns 115 of insulating material. Each of the turns 115 comprises a foam body 116 having a generally rectangular cross-section. The turns 115 have a reinforcing scrim 117 affixed generally adjacent the intermediate insulation layer 82. A vapor barrier layer 118 is affixed remote from and parallel to the reinforcing scrim 117. The vapor barriers 118 are adhered to the body 116 by means of an adhesive layer 119. Adjacent portions of the vapor barriers 118 of the turns 115 are adhered together and folded to lie generally at parallel to the scrim 117. Adjacent turns 31,843-F -13-~256()S

115 define therebetween first grooves 121 which are inwardly facing and which extend inwardly from the scrim 117. The adjacent turns 115 also define outwardly facing grooves 122 which extend inwardly toward the grooves 121 from vapor barrier 118.

Insulation in accordance with the present invention provides considerable flexibility in the choice of materials. Generally, the innermost layer of insulating material can comprise a foam having relatively high elongation and insulating value whereas as one progresses toward the outermost layer of insulation, lower elongation in the foam may be employed depending upon the requirements of the particular insulation installation. The use of multiple vapor or gas barriers lS is a substantial advantage in that from a practical standpoint, it is difficult to insure the integrity of a single vapor barrier. The use of multiple vapor barriers permits the use of subatmospheric pressure adjacent the tank or container being insulated and such atmospheric pressure can be maintained adjacent the vessel employing a pump of minimal capacity.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended laims.
;

31,843-F -14-.,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An improved insulated cryogenic vessel comprising at least a first containment vessel having an exterior curved surface, a first layer of a foam insulation disposed adjacent the exterior surface and covering at least a substantial portion thereof; the insulation being in the form of a plurality of turns of a composite strip-like material, having a continuous gas barrier layer affixed thereto remote from said containment vessel, adjacent turns of the first insulation being heat bonded to adjacent turns of the first insulation, characterized by a second layer of foam insulation being disposed over and unadhered to the first layer of foam insulation and external thereto and spaced from the continuous gas barrier layer affixed to said first layer of insulation; the second layer of insulation being in the form of a plurality of turns of strip-like material, adjacent turns of the second layer of insulation being heat bonded to adjacent turns of insulation of the second layer of insulation; the first layer of foam insulation being a laminated strip-like material, the first layer of foam insulation having affixed thereto a first reinforcing scrim and which in turn has affixed thereto said gas barrier layer, with a second reinforcing scrim located within the laminated strip-like material, the laminated strip-like material defining a plurality of longitudinally extending slots opening toward the exterior surface of the containment vessel, the slots terminating at a location generally adjacent said second reinforcing scrim.

2. The insulated cryogenic vessel of claim 1, characterized in that a second vapor barrier is affixed to said second insulation layer remote from said gas barrier layer.

3. The insulated cryogenic vessel of claim 2, characterized in that a third reinforcing scrim is affixed to the first layer of insulation and is disposed adjacent the exterior curved surface of the containment vessel with the further limitation that adjacent turns of the strip-like material define inwardly facing grooves adjacent the exterior curved surface of the containment vessel and adjacent turns of strip-like material defining outwardly facing-grooves remote from the containment vessel.

4. The insulated cryogenic vessel of claim 1, characterized in that another plurality of longitudinally extending slots extend from said second reinforcing scrim towards said gas barrier.

5. The insulated cryogenic vessel of claim 1, characterized in that the foam insulation of the first layer of foam insulation is a flexibilized thermoplastic foam having a density of from about 1.5 to 2.5 pounds per cubic foot.

6. A method for the insulation of at least a first cryogenic vessel having an outer surface wherein a thermally insulating material is a flexibilized thermoplastic foam insulation having a density of from about 1.5 to 2.5 pounds per cubic foot and having a strip-like configuration having a metal gas barrier layer affixed thereto, depositing the foam with the gas barrier layer remote from the outer surface of the vessel about the periphery of the vessel in the form of a plurality of closed loops, heat bonding adjacent loops and gas barrier layers together to thereby envelop at least a portion of the vessel within a first insulating layer, subsequently depositing at least a second insulating layer about the first insulating layer, the second insulating layer having a metal gas barrier layer disposed remote from the vessel, the layers being in the form of a plurality of closed loops to thereby envelop at least a portion of the first insulating layer, the second heat insulating layers being deposited and heat bonded in a like manner as the first layer.

7. The method of claim 6 including the step of joining adjacent loops together to define inwardly and outwardly facing grooves between adjacent loops.

8. The method of claim 6 including disposing within at least the first insulating layer a reinforcing scrim which is generally parallel to the outer surface of the vessel.

9. The method of claim 6 including the step of providing a plurality of slots in the loops of the first layer extending into the loops along the length of the loops and generally normal to the vessel's outer surface.

10. The method of claim 9 including the step of providing like slots in the second insulating layer.

11. The method of claim 6 including the step of staggering joints between the loops of successive insulation layers.

12. The method of claim 6 including the step of providing a subatmospheric pressure at the outer surface of the vessel.