Marine Log - August 2007 - (Page 35) LNGSHIPPING APPETITE FOR LNG Korean shipyards dominate swollen order book; new containment system under development W hat’s driving the current boom in LNG ship construc- -161° C that if it “were to contact a large section of the ship’s tion? Put simplistically, gas markets that once relied steel structure, the hull would shatter like glass,” according to on local sources of output are now facing problems of the Nonstop article. falling production and import demands are on the rise—particThe membrane is the inner layer of a gigantic hexagonal ularly in North America and Western Europe. There are major cargo tank. It is supported by a multi-layered insulating strucsources of supply, notably in the Middle East. ture composed of glass fibers, synthetic material and a second According to U.K. consultancy Douglas-Westwood, in 2003, barrier made of aluminum or other metal that will remain leak 93% of gas transportation was by pipeline, but by 2025, that proof even in rough seas and extreme ship movements. will have dropped to 69% and the percentage of gas transported The cargo maintains its ultra cold temperature for its entire as LNG will have risen to 29%. voyage, but is continuously boiling. A small percentage of the Which, of course, is where LNG ships come in. When gas is vapor (or boil-off) is allowed to escape, maintaining the internal cooled to -161°C it becomes a liquid, and is reduced to 1/600th pressure of the tank and the cargo temperature. of its original volume—making it economic to transport in insuIn the Moss ships, a 4 cm thick aluminum shell, surrounded lated carriers. by layers of insulating material keeps the gas from warming. There are currently around 220 LNG ships in the world Both LNG carrier types are double hulled. fleet, and though there are other types of containment systems, According to Douglas-Westwood, around 52% of the current the overwhelming majority of ships use either the Moss LNG fleet uses membrane containment and around 45% spherical containment system, in which the liquse the spherical. uefied gas is transported in insulated, cylinHowever, the story is very different Hull thickness about two meters drical tanks, or a GTT membrane syswhen we look at the 134 LNG ships tem (see the cutaway illustration currently on order. Only a handful Ship’s outer hull below). use the Moss system. The membrane is “just a flexiWhat has proven to be a ble bag, unable to support itself,” real challenge to the Moss sysInner explains Georg-Alexander Martem is the way that Suez membrane of tin, a Germanischer Lloyd Canal dues are calculated. stainless steel expert quoted in a recent issue The dues are based on the of GL’s publication Nonstop. external dimensions of the But the 1.5 mm thick memship, rather than on cargo volFirst insulating layer ume. That has largely limited brane contains a cargo chilled to Ship’s double hull Second layer of stainless steel Second insulating layer Ballast tanks LNG carrier designed with the GTT membrane containment system. Above, is a cross section of the membrane www.marinelog.com AUGUST 2007 MARINE LOG 35 Illustrations courtesy of Germanischer Lloyd http://www.marinelog.com
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