Containment technology on the move

Since transport of Liquefied Natural Gas (LNG) by sea first began in 1964 from Algeria to the U.K. the onboard containment systems have been either an independent tank type – better known as the spherical tank – developed by Norwegian Moss Rosenberg (now Moss Maritime) or the membrane type tanks later developed by Gaz Transport and Technigaz. These companies are now merged into one – Gaz Transport and Technigaz (GTT). For many years very little happened to develop containment systems further as most of the existing vessel and a great deal of the new deliveries were tied up on long-term contracts between exporters and terminals in importing countries. There simply was no need for improvement except in propulsion systems and to adapt to different sizes.

	LNG carrier “Puteri Firus” at speed. The 130,405 cbm vessel was built at Chantier de l’Atlantique in 1997 for Malaysian International Shipping Corporation (MISC). The vessel carry four prismatic insulated tanks carrying a total of 61,813 tonnes.

In the past couple of years export projects developed in less hospitable areas and a rudimentary spot market has injected flexibility into the LNG sea transportation market. The new scene calls for new think-ing, as the LNG is a potentially dangerous cargo. All the existing containment systems have developed to make for safer transport of a seaborne cargo at minus 160� C, a specific gravity of as low as between 0.43 and 0.50 and high flammable.

Moss Rosenberg design
Before the membrane systems were in general use, the market was dominated by the Moss Rosenberg design, built around spherical tanks. But it is more to this containment system than the shape of the tanks.

We called the Moss Rosenberg design an “independent tank type”. The philosophy of the Moss Rosenberg system is that the tanks, normally up to five on a big LNG tanker, is an independent structure from the hull and completely self-supporting. This is important so that deformation due to thermal expansion and contraction is not directly conveyed to the hull – unlike the membrane systems. The Moss Rosenberg design has been further developed by Mitsubishi Heavy Industries in Japan. As a result of development around half of all the LNG tankers today use the spherical system.

A safer system
Mitsubishi says that by using the spherical independent tank systems the whole liquid cargo load in borne by the membrane stress of the tank shell structure and stress concentration is avoided. The tankers are installed on a cylindrical skirt in the hold, which will absorb thermal expansion and contraction by bending a little. As a result the spherical tank system is acknowledged to have the best safety of the existing containment systems. This is mostly due to the independent tank system, but also because the axial symmetry of the spherical tank and cylindrical skirt allows very accurate stress analysis. If cracks develop their progress is extremely slow and any leakage is slight.

The main difference from the spherical tank is that the insulation material is installed on the inner hull and the surface covered with a metallic membrane sheet. Both Gaz Transport and Technigaz use the same concept, which aims to reduce the metallic material exposed to low temperatures. The membrane system prevents leaks, but it has little strength against the cargo load itself. Therefore the ships hull takes the strain via the insulation material. It follows that the insulation structures must act not only as insulation but also to supply strength against the cargo load.

The only substantial difference between the Gaz Transport and the Technigaz systems is the material they use in the membrane. Gaz Transport applies a material with extremely small coefficient of thermal expansion. It is called invar, is 36 per cent nickel steel and is little exposed to either expansion of contraction. Technigaz uses corrugated stainless steel in the membrane. The corrugation is both transverse and longitudinal in order to absorb the expansion and contraction. Balsa wood was used as insulation material at first and later reinforced plastic foam and Triplex. It looks like ideas from the two companies will merge with the use of invar and reinforced plastic foam as insulation material.

The way forward
Both containment systems have proved reliable as more and more safety features are introduced. The size of the LNG carrier is progressively increased to achieve more economy of scale. For many years the biggest was 125,000 cubic metres, but the latest orders are for 138,000 cubic metres and bigger. However, model tests of a four tank, 235,000 cubic metres LNG carrier indicate the development. The latest in containment is the Prism/Pyramid tank concept, which is developed and of part of the tests.

What they are testing is the sloshing effect from the cryogenic cargo on a ship moving in North Atlantic waters. The tests are conducted by Marintek in Norway on behalf of ConocoPhillips to see how the new tank design responded to the strong load pressures as a result of ships movements with three different filling levels to assess the safety aspects.

//Petter Arentz