Naval Architect: October 2019
In 1964, the world’s first purpose-built LNG carrier entered service, paving the way for commercial LNG carriers. The Methane Princess, a 27,000m3 vessel, featured nine Conch independent cargo tanks insulated with a balsa wood insulation system attached to the inner hull. Although the vessel was a bold testament of the advancements in ship technology, other more popular cargo containment designs evolved to suit ever larger LNG carriers.
Today’s global fleet is composed of two main containment systems: Moss and membrane. Moss-type tanks make up 33% of the global LNG fleet while membrane-type containment systems account for 67%, according to data published by IHS Markit.
Traditionally, LNG has been transported around the world by large ocean-going LNG carriers, but heightened demand has prompted a rise in smaller terminals, more LNG ports and new trading patterns. This means that the distances between exporter and importer are shrinking rather than increasing in length, as was once expected.
With that, opportunities for new types of containment systems have emerged. One company on course to fill that market gap is Singapore-headquartered LNT Marine, with the introduction of its new patented system based on IMO independent type A tank – the LNT A-Box.
“There is a growing need for local and regional distribution of LNG, which requires a wide range of different ship sizes, as opposed to before where it used to only be large ships sailing from A to B”, says Kjetil Sjølie Strand, CEO of LNT Marine.
Historic solutions and new tech
By the end of 2017, there were 28 LNG vessels with a capacity of less than 25,000m3, 464 vessels with over 90,000m3 of hold volume, and only 19 vessels with a capacity between 25,000m3-90,000m3. Moss and membrane tanks dominate the large vessel segment whereas type C tanks are the preferred choice for small ships. None of these options, however, have proven to be very efficient nor adaptable for mid-sized LNG carriers.
LNT Marine, created following a merger between LNG New Technologies and MGI Thermo, began development on the LNT A-Box around 10 years ago. The prismatic containment system is based on similar design principles as the Methane Princess’ Conch tanks but are arranged in a new patented-protected configuration. Classified under the IMO IGC Code as an independent type A tank, the self-supporting LNT A-Box is situated within an insulated cargo hold with a full liquid tight secondary barrier. It does not form a part of the ship’s hull, but instead depends on bulkheads and internal structures for strength.
“This structure also acts as swash bulkheads,” explains Strand. “That means you don’t have any issues with sloshing in this type of tank and no loading limitations.” Membrane tanks, on the other hand, are prone to sloshing because they lack any internal sub-divisions to break up the liquid movements.
Another advantage of the self-supporting LNT A-BOX is that it does not have any impact on the insulation during normal operations. Therefore, the insulation doesn’t need to be designed for dynamic loads from the cargo. “You can select relatively low-density foam for the insulation system, which is giving better thermal performance than if we had to use a foam with higher compressive strength, and this in the end translates into low boil-off rates. So, for the same thickness of insulation we can have lower boil-off rates than our key competitors,” says Strand.
Type A tanks are based on classical ship structural design and construction methods, making them the simplest to design and build in comparison to other IMO IGC Code tanks. “It’s relatively straightforward for most shipbuilders to build a type A tank,” says Strand. “Thus, it differs from other containment systems in regards to construction friendlessness.”
LNT Marine recently tested this claim with the construction of Saga LNG Shipping’s newbuild, Saga Dawn. Ordered from China Merchants Heavy Industry, the 45,000m3 capacity vessel is the first ship to feature the LNT A-Box and marks the first time the yard has constructed an LNG carrier. The ABS-classed vessel successfully underwent gas trials in June and at the time of writing, was about to be delivered to Saga LNG Shipping.
