Insulation of pipe-in-pipe systems
A pipe-in-pipe section comprises an inner pipe spaced within an outer pipe to define an annulus between the inner and outer pipes. The annulus contains a solid insulating material, which may be a microporous aerogel, and an inert gas such as krypton at near-atmospheric pressure.
1. A method of filling an initially air-filled annulus of a pipe-in-pipe section with a noble gas comprises:
pressurising the air in the air-filled annulus from an initial pressure to an elevated pressure that is above the initial pressure, the elevated pressure being above 5 bars;
flushing the pressurised annulus with the noble gas; and
after flushing with the noble gas, reducing the pressure in the annulus to an absolute pressure of less than 1.5 bars.
2. The method of claim 1 , wherein the noble gas in the annulus is at an absolute pressure of greater than 0.5 bar.
3. The method of claim 1 , wherein the noble gas in the annulus is substantially at atmospheric pressure.
4. The method of claim 1 , wherein pressure in the annulus is reduced to between 0.5 and 1.5 bars after flushing with the noble gas.
5. The method of claim 1 , wherein the noble gas is krypton.
6. The method of claim 1 , comprising flushing the pressurised annulus until the noble gas is at least a majority, by concentration, of all gases in the annulus.
7. The method of claim 1 , comprising placing a solid insulating material into the annulus before filling the remainder of the annulus with the noble gas.
8. The method of claim 7 , wherein the insulating material is a microporous material.
9. The method of claim 8 , wherein the microporous material has an average pore diameter of less than 100 nm.
10. The method of claim 7 , wherein the insulating material is silica-based.
11. The method of claim 7 , wherein the insulating material is an aerogel.
12. The method of claim 7 , comprising placing the insulating material as an annular layer in continuous contact with the inner pipe.
13. The method of claim 7 , comprising spacing the insulating material from the outer pipe by an annular gap.