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HISTORY:
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 LMG 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.

Technology:
Samsung’s Supreme was a Mark-III type LNG container ship that was the largest of its kind. The Supreme had the newest technology seen in LNG ships. She features tanks completely surrounded by double bottom hull, and a cofferdam between tanks. Each tank stores its cargo at -163 degrees Celsius. This is a standard storage temperature for LNG This is accomplished by 250 mm of insulation, and a 1.2 mm thick stainless steel membrane. Each cargo tank has submerged centrifugal pumps to unload cargo rapidly. This is the standrard unloading method for LNG tanks. The maximum draught for LNG ships is typically 12 meters. This is due to harbor facilities sizes and restrictions. The most common size of LNG vessels ranges between 120,000 and 180,000 cubic meters because of ship dimensions. (Propulsion Trends in LNG Carriers Two-stroke Engines, 2017).

The two common types of LNG carriers are Moss and Membrane. Moss type carriers feature spherical tanks to hold LNG, while Membrane style carriers feature more traditional rectangular style tanks with a stainless-steel membrane. Membrane tankers are more common because they are smaller than moss ships for the same amount of LNG fuel carried, but they create more boil-off gas than Moss style ships.

A study by MEC Intelligence found said that LNG will be the primary fuel source for all merchant ships within 40 years. Many companies have already begun looking at the process of switching their fleets over to LNG propulsion systems.

LNG vessel propulsion systems  typically come equipped with WHRs, because they can lead to reduced emmisions, reduced fuel consumption, and better efficiency. Switching to LNG powered vessels is a complicated task for companies, but combines with modern Waste Heat Reduction systems (WHRs), LNG vessels can be more efficient than diesel or steam propelled vessels.

Heat loss of standard combustion engine:  

Environmental issues:
In general, the benefits behind the use of LNG largely outweigh the potential risks. With constantly increasing emissions controls, an efficient and affordable solution is always in demand. Natural gas in relatively clean, when compared to Diesel and HFO.

Using LNG cuts carbon emissions by approximately 25 percent. It reduces Sulfur Oxides by nearly 100 percent, and it reduces Nitrogen Oxide emission by about 85 percent.

On current, non-LNG vessels, approximately 50 percent of heat created by combustion is ejected back into the environment. This is a significant loss of heat and efficiency. LNG vessels come equipped with WHR (waste heat reduction) systems. The purpose of a WHR system is to eliminate heat sent back into the environment.

There are typically four main areas for heat loss that can be addressed in order to see efficiency increases from an LNG vessel. The primary contributor is heat loss from exhaust gasses. This contributes to approximately 25.5% of the total heat loss. Heat loss from Jacket water contributes to 5.2% of heat lost. Heat lost from lubrication oil contributes to 2.9% heat loss, and heat loss in the air cooler for the turbocharger contributes to 16.9% of lost heat.

The initial expenses of switching to LNG ships is countered over time by the rise in efficiency and a reduction in fuel consumption.

Propulsion:
Steam turbines are exclusively the primary moving source for LNG ships, even though 2-stroke diesel engines are more efficient. This is because the boil-off gas from LNG needs to be utilized.