Energy Transport [LNG Carriers]
- Tankers that transport liquefied natural gas are called LNG carriers. These vessels boast a range of advanced, versatile technologies—tanks made of special material to keep the cargo at an ultra-low temperature, emergency shut down devices to prevent incidents during cargo handling operations, and turbine engines that can run on vaporized gas.
The transport of natural gas started through overland pipelines, but LNG carriers made it possible to transport massive volumes via sea. LNG is the liquid form of natural gas at the cryogenic temperature of minus 161.5°C, which takes up about 1/600th the volume of natural gas in its gaseous state, allowing efficient transport by ships. MOL's first LNG carrier, the Senshu Maru, which was launched in 1984, had a cargo tank capacity of 125,562m3. Today’s mainstream vessels are in the 150,000-170,000m3 class.
Natural gas consists chiefly of methane. When it burns, it emits carbon dioxide (CO2) and nitrogen oxide (NOx), no sulfur oxide (SOx). Further, it gets cleaner when converted into LNG as hydrogen sulfide, carbon dioxide, water content, and so on are eliminated during the liquefaction process. In addition, as natural gas weighs less than air, it does not accumulate in low spots if it leaks and its spontaneous combustion temperature is relatively high. Thus, natural gas has earned high regard not only as a cleaner fuel, but also as a safer one.
LNG carriers load and discharge their cargo through pipelines connected to shoreside facilities. Loading ports have equipment that liquefies natural gas before loading, while receiving terminals have facilities to re-gasify LNG.
LNG’s boiling point is extremely low—minus 161.5°C—so transporting it by sea requires advanced technologies such as tanks made of materials especially engineered to withstand ultra-low temperatures—ferronickel, stainless steel, and aluminum alloys—and the outer layers are covered by thick heat insulation. Cargo tanks are like giant thermos bottles, however, part of the cargo is affected by the outside temperature and naturally vaporizes during transport. Known as "boil-off gas," this can be used as fuel for the vessel. LNG carriers are also equipped with sophisticated technologies such as emergency shutdown devices to ensure safe operation.
Ethane is the second largest component of natural gas after methane. The calorific value by volume is about 1.75 times that of methane. Ethane is gaseous under atmospheric pressure and liquefies at minus 90°C. It is used mainly as feedstock for ethylene, an important base chemical.
Shale gas extraction began to flourish in the U.S. early in the 21st century, leading to higher export volumes of ethane. Ethane, a by-product of gas extraction, has created huge opportunities for dedicated ethane carriers. In the past, mainstream ethane carriers were mid-sized vessels with capacities of 20,000 – 30,000m3, but in 2016, the world’s first Very Large Ethane Carrier (VLEC), with a capacity of 87,000m3, went into service.
Like LNG, ethane is liquefied for transportation. However, unlike LNG carriers which transport methane, International Maritime Organization (IMO) regulations prohibited the use of vaporized ethane as fuel until 2016.Therefore, ethane carriers in general are feature equipment which re-liquefies boil-off gas and sends it back to the cargo tanks.
Mid-sized vessels usually have pressurized tanks, while the MOL-operated VLEC adopts a membrane-type tank.
The major characteristic of LNG carriers is the tank, which stores the cargo at an ultra-low temperature. There are mainly three types of tanks—Moss type, Membrane type, and Self-supporting Prismatic shape IMO type B (SPB).
Moss type (Independent spherical cargo tanks)
The spherical cargo tank is independent from the hull, and each tank uses its own structure to maintain the internal pressure. This type simplifies quality control because it causes relatively less boil-off gas, and construction involves less welding than other styles. Vessels with continuous tank cover over all the spherical tanks were first delivered in 2014, Due to its appearance, this design is nicknamed the sayaendo (peapod) in Japan.
Cargo is maintained at lower temperature to absorb the expansion of the gas by using thin inside tanks and covering them with a wrinkled stainless steel "membrane." It features high visibility toward the front of the vessel because of greater space efficiency in the cargo tanks and minimal protrusions on the deck. Ethane carriers adopt this style.
Self-supporting Prismatic shape IMO type B (SPB)
This self-supporting prismatic shape tank uses aluminum alloy or stainless steel, with heat insulation on the exterior. This type of tank has the advantage of allowing for broader use on deck because it is more easily fitted on the hull in comparison with spherical tanks and has no protruding structures on deck.
Russia’s Yamal Peninsula contains the nation’s largest reserves of natural gas. However, Yamal, which means "end of the earth" in Russian, is an isolated region in the Arctic, about 2,500km from Moscow. It is covered by ice for most of the year, and temperatures reach minus 40°C. Ice-breaking LNG carriers set the stage for transporting mass volumes of LNG from the Yamal Peninsula.
The world's first ice-breaking LNG carrier, operated by MOL, was delivered in 2018. It is capable of sailing on seas covered by ice up to 2.1m thick, equipped with an "ice bow" that can easily break ice under water. The design also adopts measures to prevent deck equipment from freezing and icing up. This vessel can transport LNG all over the world throughout the year. In the summer it navigates through Arctic Ocean toward East Asia, while in the winter it supplies LNG to Europe.