Aluminium in LNG Applications

Can Aluminium be used for LNG applications? The answer is yes.
MOSS-Rosenburg type tanks (Independent Type B tanks - Spherical) is fabricated of aluminium alloy 5083 annealed.

LNG Vessel - Moss-Rosenburg containment (source:http://www2.gov.bc.ca/assets/gov/taxes/lng.jpg)

The use of aluminium for LNG applications is permitted by the IGC Code (Chapter 6). It is also permitted by IGF Code for use in LNG fuel pipe lines as well as containment systems.

Aluminium alloy 5083 forms part of the approved 5000 series for marine use. It is supplied annealed and has a UTS 275 to 350 N/mm2.

It has a lower melting point of app.660 C when compared with stainless steel. However, this is not a consideration for use in LNG applications.

It has a lower elongation rate and also has toughness, which keeps the material properties retained to a temperature of up to -196C. Hence it does not require an impact test (IGC/IGF Codes)

However, it compromises on the tensile stress it can take when compared with stainless steel. In addition there are difficulties in sourcing the approved type plating and class II type piping as required by the classification societies.
It has poor fatigue strength and it is more difficult to weld than stainless steel.

Why is aluminium difficult to weld?

In simple terms, aluminium is very reactive and it forms oxides instantly. These oxides readily form a protective layer over the weld region. While aluminium has as melting point of 660C, oxides have a melting point close to 2000C. Getting through this oxide layer into the aluminium sheet/pipe would require a large amount of heat. It will consume more welding consumables and generate a larger weld pool which will be hard to control. Oxide removers are to be used extensively during the process.

TIG (https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/GTAW.svg/2000px-GTAW.svg.png)

Structural Transition Joints (STJ)

In joining aluminium with steel/stainless steel, these joints are required. MOSS-Rosenburg tank skirts use these joints, which are formed using an explosion bonding process.

In LNG as a fuel applications and LNG reliquefaction plants, cold boxes and the piping within these units are fabricated using aluminium alloy pipes. The LNG lines of stainless steel are then joined to these aluminium lines using specially fabricated STJ.

With more ferries and inland water vessels going for the option of LNG as fuel, the use of aluminium piping will become more prevalent. The service record and acceptability from various statutory codes makes the application of aluminium piping for LNG applications an economical and suitable alternative to stainless steel.

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