The transportation of natural gas to the end-user is a tedious and expensive task. Providing the fact that 70% of the Earth’s surface is covered with water, it becomes clear that one of the best and most viable modes of transport is by tank ships, also known as LNG carriers. Based on the physical characteristics of natural gas, the LNG carriers must be equipped with special facilities that liquefy and maintain the temperature of the gas in order to transfer it safely by water.
Natural gas contains 98% methane. When liquefied, it occupies 0.17% of the volume of the same amount in the gaseous state. This equals approximately one-six hundredth of the gas volume. As you can imagine, reducing the volume 600 times is a great advantage, both for storage and transportation. What is even more, the liquefied gas is also colorless, odorless, non-toxic, and non-corrosive.
The gas liquefaction process involves purification by separating specific components such as carbon dioxide, helium, hydrogen sulfide, dust, and water, which would otherwise cause difficulties in converting the natural gas into liquid form. The next step is condensation at a pressure that is close to the atmospheric one.
The cooling temperature is approximately -162 ° C. The energy density of liquefied natural gas is 2.4 times higher than that of compressed gas and is equal to 60% of that of diesel fuel.
Liquefied gas production requires heat exchange between the gas and the refrigerant cold enough to cause condensation. Its liquefaction takes place in surface heat exchangers in contact with the refrigerant at a temperature lower than the dew temperature for the gas.
Liquefied natural gas is stored in cylindrical tanks at a temperature of 160 ° C. The internal pressure in the tanks is maintained by the vapors formed during the spontaneous evaporation of the gas from the top layer. It is necessary to maintain an approximate atmospheric pressure by controlled extraction of vapors, after which the latter are compressed and sent for liquefaction in heat exchangers and returned by the pumps back to the tanks.
So, after becoming more familiar with natural gas and the way it is being liquefied and stored, let’s now learn a bit more about the type of ships that are used for transportation of this natural fuel, and namely the LNG carrier.
What is LNG carrier
Natural gas is transported in specially designed double-walled vessels, protecting the cargo from damage or leaks, with good insulating properties in order to maintain the temperature in the required range. This kind of vessel is known as an LNG carrier. The gas transportation ships usually have 4 to 6 tanks located near the longitudinal axis. The double walls surrounding the gas tanks can be used as ballast tanks or simply cavities providing the advantages already listed. There are usually 3 submersible pumps in each tank.
The 2 large pumps are used for loading and unloading operations, while the third, called an injection pump, is designed to pump liquefied gas for use on the LNG carrier or for cooling and recirculation to maintain its temperature. All equipment such as pumps, measuring instruments, and other equipment in the tanks is mounted on the pump tower – a structure attached to the wall of the tank, extending from the bottom to the top.
There is also a backup line in the tank, with a spring-loaded valve that can be opened by weight or pressure. This is the emergency pumping equipment. In the event of an accident and failure of both main pumps, the load can be unloaded from the emergency pump.
All main pumps use a single line that goes around the deck and has branches to both sides of the ship. This pipeline reaches collectors used for loading and unloading of products. Also in the upper part of the tanks where the gas phase is contained, a common gas pipeline passes parallel to the loading and unloading pipeline.
The conventional types of LNG carriers are membrane tankers and Moss tankers. On the other hand, the unconventional ones are the enlarged Q-Max and Q-Flex, owned by Nakilat – Qatar Gas Transport Company. They are specially developed by order of the company and are currently one of the largest LNG tankers in the world. At the moment, the total number of LNG carriers in operation is about 550.