LNG ships are specially designed ships, easily recognizable from their large spherical tanks. They are the most expensive and long living cargo ships because of their advanced design and construction and their high quality control systems.
They are double-hulled featuring two layers of steel, each approximately 3/4 inch thick. The space between the steel hulls is approximately 10 feet wide on the sides and the bottom.
The cargo tanks are located inside the inner hull and are capable to maintain the LNG at atmospheric pressure and at extreme low temperature. Cargo tanks are made of very thick aluminum or nickel steel, heavily insulated with expanded foam. (If LNG was poured into a mild steel tank, the steel would shatter like glass). LNG tanks remain clean and spotless unlike oil tanks which remain black with residues.
As LNG warms, it expands and "boils off". The boiled off gas can either be refrigerated again by ship's equipment and re-injected back into the tanks or used as fuel in the ship's boilers if the LNG ship is a steamship.
As liquefied natural gas is a volatile cargo, gas detectors and safety alarms are located between the steel hulls to continuously monitor for cargo leaks.
The LNG fleet has an exceptional record of safety because of the well-designed and maintained ships and the efficient risk management procedures.
International regulations enforce safety and security standards for the safe design, construction and operation of LNG ships. Among other numerous issues, these regulations specify hull and cargo tank thickness, safety and alarm systems, inspections and operating procedures. The crew must be certified for LNG operations. The regulations are so strict that three new large vessels had to be abandoned after they failed their insulation trials and two other large vessels built "on spec" without a nominated trade in the 70s, had to be laid up for 20 years, until they could be employed.