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The latest Liquified Natural Gas (LNG) vessels that will ship shale gas from the US are yet another illustration of the choices that confront the LNG industry. The vessels that are scheduled to start sailing in the next two years are for the Cameron LNG project and have been ordered by Mitsui from Mitsubishi Heavy Industries (MHI) to transport shale gas.
Since the introduction of LNG shipping in the 1950s, incidentally enough off the Louisiana coast, the LNG ship has had to wade through trade-offs between efficiency and economy. So much so that even today the LNG ship is not a standard design, unlike a modern tanker or even a container vessel.
For instance, shippers have swayed between choosing membrane-type tanks and self-supporting spherical tanks. The membrane tanks use almost the entire cargo hold space available and can carry more gas for a given volume whereas the spherical tanks carry less fuel since they don’t occupy the entire hold space. The spherical tanks, however, have better load bearing features and can withstand sloshing stresses to a higher degree. They therefore provide that extra comfort level in an industry where safety is often the prime mover.
In yet another trade-off that seeks the best of both worlds, the Cameron LNG project ships will have apple-shaped tanks. In these the upper half of the tanks bulges like in an apple, leading to 16% more cargo-carrying capacity than in spherical tanks without increasing the ship’s width.
Another crucial question is what to do with the boil-off gas: LNG is stored under atmospheric pressure but at -163°C. Modern insulation has brought down the boil-off rates down to 0.25% of volume per day but in a 20-day voyage between the US and the rest of the world that can amount to 5%, which is a substantial amount of gas.
For long, the LNG carriers simply burned the boil-off gas in a dual-fuel boiler to run steam turbines that had become outmoded in other ships. Modern merchant ships had transitioned to the more efficient diesel engines and can use dirty bunker fuel – heavy fuel oil. The LNG carriers stuck to steam turbines since engines using gas were still a novelty in shipping that is much slower to using new technology than the rest of the world. Out in the sea, robustness has a greater claim to achieving the optimum.
In the last ten years, however, efficient dual-fuel reciprocating engines have been introduced. In these, the fuel mix has more boil off gas during cargo voyages but switches to a higher percentage of heavy fuel oil during ballast condition.
At around the same time of dual-fuel engines, reliquefaction plants were introduced on board. Though expensive, it was thought that high-value LNG should be traded rather than burned at sea. This was a conundrum – the USP of the fuel is that it is among the cleanest fossil fuels, yet ships transporting them were burning sulfurous heavy fuel oil that had a high residual content. But some shippers felt that with a reliquefaction plant, the LNG ship can go back to being the regular merchant ship with diesel engines burning heavy oil.
Meanwhile, invoking the principle that a ship transporting clean LNG should use clean fuel, GE and Rolls-Royce proposed gas turbine-based systems – LMS by GE and Marine Trent by R-R. These compact power packs needed an electric propulsion since the power density of gas turbines is so high that a gas turbine system producing say 30 MW power would be enough to propel the ship and supply its power requirements too.
But, once gas prices dropped and with it the price of LNG, onboard reliquefaction has once again become less attractive. The Mitsubishi Cameron LNG carriers feature a hybrid propulsion system dubbed "STaGE" (Steam Turbine and Gas Engines), which as its name implies combines a steam turbine and engines that can be fired by gas. STaGE's components consist of the "Ultra Steam Turbine plant" (UST), a highly efficient reheating steam type marine turbine developed independently by MHI, a dual-fuel diesel engine capable of operating on both gas and oil, and an electric propulsion motor.