Insulation of pipe-in-pipe systems

Abstract

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.

Claims

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.

Description

(1) In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawing, FIG. 1. FIG. 1 is a cross-sectional view of a subsea PiP pipeline in accordance with the invention.

(2) The subsea PiP pipeline 10 shown in FIG. 1 comprises an inner pipe 12 spaced concentrically within an outer pipe 14 to define an annulus 16 between them.

(3) An annular layer of microporous insulation 18, exemplified here by a superinsulation material such as silica aerogel, surrounds the inner pipe 12 as a layer and so is disposed within the annulus 16. Krypton (Kr) constitutes substantially all of the gas in the annulus 16.

(4) The microporous insulation 18 substantially fills the annulus 16, extending from a contact interface with the inner pipe 12 across a majority of the thickness of the annulus 16 to near the outer pipe 14. However, a small annular gap 20 is maintained between the microporous insulation 18 and the outer pipe 14 to minimise the possibility of thermal conduction into the outer pipe 14.

(5) The thermal conductivity of a gas in a porous material can be calculated by the following formula, courtesy of Kaganer, 1969:

(6) $k_g=\frac{k_{g,0}}{1+2\beta K_n}$ where: K.sub.g,0 is the intrinsic conductivity of the gas at standard conditions; ß is a constant that depends on the gas (its value is 2 for air); and K.sub.n is the Knudsen number, determined by the following equation:

(7) $K_n=\frac{l_g}{D}$ where: I.sub.g is the mean free course of the gas molecules; and D is the pore diameter of the porous material

(8) When the pores are significantly smaller than I.sub.g (thus, K.sub.n>>1), gas molecules within a pore collide mainly with the surrounding wall of the pore: this is known as the ‘ballistic regime’. Gas conductivity then becomes very low due to the Knudsen effect.

(9) In microporous materials, the pore diameter is reduced below a typical value of I.sub.g, being around 100 nm.

(10) The thermal conductivity of the solid microporous material used in the invention is known at a pressure of 1 bar in air. Conductive and radiative heat transfer coefficients of the solid can be measured. K.sub.g may therefore deduced for air.

(11) In addition, I.sub.g, as determined by the formula:

(12) $\ell =\frac{k_{B}T}{\sqrt{2}\pi d^{2}p}$

(13) is very similar for distinct gases. Based on this formula, I.sub.g is similar for krypton, nitrogen and oxygen, namely 360 pm, 364 pm and 346 pm respectively.

(14) K.sub.g can therefore be deduced for krypton.

(15) Consequently, microporous insulation in an atmosphere of krypton at a pressure of around 1 bar has a thermal insulation performance close to that measured for microporous insulation in an atmosphere of air at a deep vacuum pressure of 10 mbars. This has the advantage of achieving excellent thermal insulation performance without the challenges of drawing a deep vacuum and sustaining that pressure differential over many years despite the risk of leakage.

(16) An exemplary method for filling the annulus 16 with krypton is to pressurise the initially air-filled annulus 16 to an elevated pressure of say 10 bars and then to flush the annulus 16 with krypton. When pressure equilibrium is reached, the krypton flushes the air away, especially between the inner pipe 12 and the microporous insulation 18 that surrounds the inner pipe 12.