Thermal Insulation Structure

This invention relates generally to a thermal insulation structure of a double-walled type, and more particularly to such a structure where the cavity between the walls is hermetically sealed.

In considering the environment to which an oil well casing would be exposed in the Prudhoe Bay area of Alaska, certain new requirements were developed. In this vicinity there exists a layer of permafrost; i.e., a layer of subsurface soil which may extend down 2,000 feet and is permanently frozen. If a well is drilled utilizing the same approach followed in temperate climates the oil (which is approximately 180.degree. F) passing through the well casing will cause the permafrost about the well casing to melt. The melting of the permafrost causes it to subside, exerting downward drag on the well casing which may cause failure. Moreover, if the flow of oil is terminated, the soil surrounding the casing will eventually refreeze which may produce forces sufficient to cause the casing to collapse.

In a preferred form of the invention, a double-walled thermal insulating structure is provided with the cavity between the walls hermetically sealed. The outer surface of the inner wall is covered with a reflective film. The cavity further contains a polyurethane foam which has been blown in an atmosphere of a fluorinated hydrocarbon such as trichloromonofluoromethane, CCl.sub.3 F, FREON-11. The cavity also contains a gaseous fluorinated hydrocarbon such as difluorodichloromethane, CCl.sub.2 F.sub.2, FREON-12.

FIG. 1 is a schematic cross-section of one embodiment of the thermal insulating structure of this invention; and

FIG. 2 is a detail of a portion of FIG. 1 showing another embodiment of the invention.

Referring to FIG. 1, a double-walled, heat insulating structure is illustrated having inner wall 10 and outer wall 12 connected by end walls 14. Inner and outer walls 10 and 12 are fabricated of a material having sufficient structural strength for conveying liquids. End walls 14 are designed to have a low heat conductivity by utilization of a suitable material such as stainless steel, and hermetically seal the cavity between walls 10 and 12. Although not shown in FIG. 1, end walls 14 may also be designed to have a long heat transfer path by any of the well-known techniques of the prior art such as that shown in U.S. Pat. No. 3,275,345.

Plug 16 is provided in outer wall 12 for a purpose to be described later.

In accordance with the invention, a composite heat insulating material is contained between the double walls of the conduit. Immediately adjacent to inner wall 10 is a layer of reflective material 18. Material 18 may be an aluminized plastic film such as Mylar. For some purposes it may be found that sufficient heat insulation characteristics are achieved without reflective material 18. Filling the remaining space in the cavity is a polyurethane foam 20. Foam 20 may be of an opened-cell or closed-cell type and is blown in a fluorinated hydrocarbon atmosphere; e.g., trichlorofluoromethane, CCl.sub.3 F, FREON-11.

In some cases it may be desired to alternate layers of reflective material 18 with layers of foam 20, as shown in FIG. 2.

Film 18 and foam 20 are placed in the cavity between the double walls prior to the time end walls 14 are secured to inner and outer walls 10 and 12.

The space between the double walls is then evacuated to a pressure below 0.01 millimeters of mercury by removing plug 16. This ensures the removal of air and water vapor which would cause deterioration of the insulating qualities of foam 20 (also called aging of the foam). It is then refilled with a fluorinated hydrocarbon; e.g., difluorodichloromethane, CCl.sub.2 F.sub.2, FREON-12. The FREON-12 has the advantage of a low thermal conductivity (about one-third that of air) while not causing aging of the foam. The pressure of the FREON-12 is a critical factor in forming the composite of the insulation of this invention. If the pressure is raised high enough to reach the dew point of the gas the resulting condensate will tend to circulate within the double-walled cavity, condensing on the cooler wall and evaporating on the warmer wall. Thus the temperature to which the inner and outer walls of the casing will be exposed must be considered. A pressure is consequently chosen which will preclude condensation within the operating temperature range of the structure while still being maximized. The FREON-12 impedes any leakage of air into the cavity, and prevents the aging of foam 20. Polyurethane foam of the type described herein which has not aged has a thermal conductivity, K = 0.117 Btu/hr ft.sup.2.degree. F/in. at 75.degree. F mean temperature, while aged foam has a thermal conductivity, K - 0.20 Btu/hr ft.sup.2.degree. F/in. at the same mean temperature. Thus preventing aging of the foam preserves the insulation capabilities of the structure.

Preventing aging of the foam is also necessary prior to fabrication of the heat insulating structure. Thus unless the foam will be used within one or two days it should be kept in a hermetically sealed container which has been evacuated and backfilled with a fluorinated hydrocarbon as described above.

As is well known, heat may be transfered by conduction, convection, or radiation. As indicated above conduction is retarded through the use of low thermal conductivity foam and gas. The presence of the foam also prevents heat transfer by convection which would otherwise occur by circulation of the gas between the hot and cold walls of the structure. Finally the reflective film reduces heat transfer by radiation.

In the application of this heat insulating structure to a well casing, conduit, etc., the particular dimensions employed will vary. In order to provide some basis for comparison, however, for a 40 foot long well casing, with the inner steel pipe having an outside diameter of 51/2 inches, and the outer steel pipe having an inside diameter of 8-11/16 inches, a heat loss of about 1190 Btu/hr. was calculated. This assumed the inside pipe contained oil at 180.degree. F, and the outside was exposed to air and kept at 20.degree. F.

While a particular embodiment of a heat insulating structure, and the process for making it, has been shown and described, it will be obvious that changes and modifications can be made without departing from the spirit of the invention and the scope of the appended claims.