Marine LNG Engine

A marine LNG engine is a dual fuel engine that uses natural gas and bunker fuel to convert chemical energy in to mechanical energy. Due to natural gas’ cleaner burning properties, the use of natural gas in merchant ship propulsion plants is becoming an option for companies in order to comply with IMO and MARPOL environmental regulations. The natural gas is stored in liquid state (LNG) and the boil-off gas is routed to and burned in dual fuel engines. Shipping companies have been cautious when choosing a propulsion system for their fleets. The steam turbine system has been the main choice as the prime mover on LNG carriers over the last several decades. The decades old system on steam propelled LNG carriers uses BOG (boil-off gas). LNG carriers are heavily insulated to keep the LNG at around -160 °C – to keep it liquefied. What happens is that even with all the insulation, the LNG containment area is penetrated by heat which allows for naturally generated boil-off gas (BOG).

LNG shipping first became a possibility in 1959 when the Methane Pioneer, a converted WWII freighter safely transported Liquefied Natural Gas into the United Kingdom. After proving that LNG can be safely transported across the ocean, the LNG shipping industry boomed and now employs 200 billion dollars annually in capita. Since the start of the LNG industry in 1964, international trade has increased 50 times over, production capacity has increased 10 times over, and individual ship capacity has increased 5 times over. The LNG tanker design was initially created by Worm’s and Co. This design is now referred to as the Gaz Transport Design. The tanks were initially created to hold 34,000 cubic meters, but the design has transformed into 71,500 cubic meters. Spherical LNG tanks showed up in 1973, when Hoegh built the Norman Lady. Spherical tanks are common among modern LNG vessels. In 1999, Samsung Heavy Ind. created the largest New Membrane-type LNG carrier of its time. She was the largest single hull vessel of her time, with a length of 278.8 meters, and the capability of 20.7 knots.The Artic Princess, delivered in 2006, was the largest LNG tanker ever created. She is 288 meters long, and has a capacity of 147,000 cubic meters.

The natural gas that fuels dual fuel engines is carried on ships as a boiling liquid, and transported at slightly higher than atmospheric pressure. When tank insulation is penetrated by any influx in heat, it will cause the temperature of the liquefied natural gas to rise, which allows for vaporization from liquid to gas. When heat penetrates the tank, the tank’s pressure increases due to boil-off. The insulation of the tanks is designed with the most advanced technology. Even still, the insulation of the tanks is penetrated by heat. The boil-off occurs during the ships voyage. During a storm, the LNG cargo moves and sloshes around in the tanks. The boil-off gas represents 0.1% - 0.25% of the ships capacity per day. Tanks need to be maintained at a steady pressure. If the pressure in tanks is not controlled relief or safety valves are forced to open, venting the boil-off into the atmosphere until pressure is relieved. At this point, it has been proven that on board LNG re-liquefaction is uneconomical for most ships. Instead, the gas produced by this boil-off effect is routed to the ship’s propulsion system and used as fuel for power plants such as steam boilers and dual fuel marine diesel engines. This reduces the use of bunker fuel, reducing fuel costs and equipment maintenance costs.

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Wikipedia