These colossal vessels transport hazardous cargoes around the world. This documentary reveals the ingenious engineering in some of the biggest vessels afloat. An LNG carrier is a tank ship designed for transporting liquefied natural gas (LNG). As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth. According to a presentation by Golar LNG Partners, in June 2012 there were 72 new builds on order. Today the majority of the new ships under construction are in the size of 120,000--140,000 m3 But there are orders for ships with capacity up to 260,000 m3. As of end of 2011, there are 359 LNG ships engaged in the deepsea movement of LNG. Why is the LNG tanker is flat ? How to counteract the " Free Surface Effect " ? A typical LNG carrier has four to six tanks located along the center-line of the vessel. Surrounding the tanks is a combination of ballast tanks, cofferdams and voids; in effect, this gives the vessel a double-hull type design. Inside each tank there are typically three submerged pumps. There are two main cargo pumps which are used in cargo discharge operations and a much smaller pump which is referred to as the spray pump. The spray pump is used for either pumping out liquid LNG to be used as fuel (via a vaporizer), or for cooling down cargo tanks. It can also be used for "stripping" out the last of the cargo in discharge operations. All of these pumps are contained within what is known as the pump tower which hangs from the top of the tank and runs the entire depth of the tank. The pump tower also contains the tank gauging system and the tank filling line, all of which are located near the bottom of the tank. In membrane-type vessels there is also an empty pipe with a spring-loaded foot valve that can be opened by weight or pressure. This is the emergency pump tower. In the event both main cargo pumps fail the top can be removed from this pipe and an emergency cargo pump lowered down to the bottom of the pipe. The top is replaced on the column and then the pump is allowed to push down on the foot valve and open it. The cargo can then be pumped out. All cargo pumps discharge into a common pipe which runs along the deck of the vessel; it branches off to either side of the vessel to the cargo manifolds, which are used for loading or discharging. A typical cargo cycle starts with the tanks in a "gas free" condition, meaning the tanks are full of air, which allows maintenance on the tank and pumps. Cargo cannot be loaded directly into the tank, as the presence of oxygen would create an explosive atmospheric condition within the tank, and the rapid temperature change caused by loading LNG at −162 °C could damage the tanks. First, the tank must be 'inerted' to eliminate the risk of explosion. An inert gas plant burns diesel in air to produce carbon dioxide (CO2); this is blown into the tanks until the oxygen level is below 4%. Next, the vessel goes into port to "gas-up" and "cool-down", as one still cannot load directly into the tank: The CO2 will freeze and damage the pumps and the cold shock could damage the tank's pump column. Liquid LNG is brought onto the vessel and taken along the spray line to the main vaporiser, which boils off the liquid into gas. This is then warmed up to roughly 20 °C in the gas heaters and then blown into the tanks to displace the "inert gas". This continues until all the CO2 is removed from the tanks. Initially, the IG (inert gas) is vented to atmosphere. Once the hydrocarbon content reaches 5% (lower flamability range of methane) the inert gas is redirected to shore via a pipeline and manifold connection by the HD (high duty) compressors. Shore terminal then burns this vapour to avoid the dangers of having large amounts of hydrocarbons around which may explode. Now the vessel is gassed up and warm. The tanks are still at ambient temperature and are full of methane. The next stage is cool-down. Liquid LNG is sprayed into the tanks via spray heads, which vaporises and starts to cool the tank. The excess gas is again blown ashore to be re-liquified or burned at a flare stack. Once the tanks reach about −140 °C the tanks are ready to load bulk. Bulk loading starts and liquid LNG is pumped from the storage tanks ashore into the vessel tanks. Displaced gas is blown ashore by the HD compressors. The vessel can now proceed to the discharge port. There are four containment systems in use for new build vessels. Two of the designs are of the self-supporting type, while the other two are of the membrane type.

• published: 25 Nov 2014
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