May 16, 2023
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Gas tanker ships are specialised vessels designed to carry gaseous substances. However, contrary to the name, the contents of a gas tanker ship are carried in liquified form.
So, gas tanker ships are all those vessels designed and constructed to stow, store, and transport gaseous petroleum or chemical substances in liquified form from one place to another.
From the viewpoint of fuel transportation, gas carriers occupy an equally important stake at par with the oil tankers deemed to carry petroleum-based substances in liquid form.
So, what makes them different from conventional oil tankers?
The contents of the oil tanker are substances that are originally in liquefied form like crude oil, petrol, diesel, vegetable oils, kerosene etc. On the other hand, gas carriers carry substances originally in a gaseous state but are transported in a liquified form through special means. We shall see more about these special means later on.
Oil tankers are of various types based on size, kind, and the kind of content they carry. Based on the contents they carry, oil tankers are broadly designated as crude oil and refined oil carriers.
While crude oil tankers carry petroleum substances in crude, unrefined or naturally extracted form, refined oil carriers mainly transport refined or processed petroleum products for ready usages like petrol or diesel. B
Based on the size of the vessels, oil tankers are classified into various types, such as small-size tankers, Very Large Crude Oil Carriers (VLCCs), Ultra Large Crude Oil Carriers (ULCCs), and so on.
However, to understand the design and construction of gas tankers better, it is essential to know more about the types of contents they carry and the nature of their chemistry first.
Liquified gases are substances that exist as gases in ambient or atmospheric conditions but, under controlled or specially designed conditions, can be converted into gas. They are rich in hydrocarbons and have a high calorific value, indispensable to qualify a substance as a fuel. Though we do not go much into the extensive chemistry of liquified gases, it is imperative to know about vapour pressure.
Vapour pressure is the equilibrium pressure exerted by a substance in its gaseous form above its liquid in a closed system at a given temperature. Equivalently, it is also the measure of the tendency of a liquid substance to get evaporated into gaseous form and is directly proportional to the temperature.
The temperature at which the vapour pressure of a substance becomes equal to the atmospheric pressure is called the boiling point of a substance. For example, in a closed container half filled with water at room temperature, the vapour pressure of water is the pressure exerted by the vapours of water in the above space on the liquid surface below. As the temperature increases, this vapour pressure increases proportionally. At 100 degrees centigrade, water starts converting into water vapours; hence, this is the boiling point of water.
At this point, the vapour pressure of water becomes equal to the atmospheric pressure. Thus, simply, it can be that a gaseous substance exists in liquid form only below a certain temperature and vapour pressure. As per IMO (International Maritime Organization) regulations, the substances that qualify as liquified gases are to have a vapour pressure exceeding a minimum of 2.8 bars at a temperature of 37.8 degrees centigrade for all practical purposes.
So, only in those situations can the given substance be originally existent in a gaseous form and entail special means to maintain it in a liquid form. Otherwise, it shall pass for a liquid substance itself! For example, the vapour pressure of propane is around 13 bars, whereas ammonia’s is around 14.7 bars. Thus, they tend to exist in gaseous form and only require special means to be stored as a liquid of a finite volume.
LNG, or Liquified Natural Gas, is a principal gaseous substance that occupies a significant part of the fuel economy. LNG is a universal gas used for many purposes, from domestic to industrial to commercial. It is mainly tapped from underground reserves, oil fields and sometimes below seabeds.
So, before transportation by sea, it needs to be liquified, and this is done through various means such as cascade process or refrigerant process, the details of which we do not discuss here. The principal content for LNG carriers is methane, which can vary from 70 to almost 99%. As per reports, LNG carriers carry almost 80 million tonnes of cargo each year, escalating due to rising global demand. There are more than 200 LNG carriers registered as of now.
They comprise the majority of the gas tanker ship fleet. Over 800 vessels are presently registered as LPG carriers or Liquified Petroleum Gas. These are mainly propane, butane, butadiene, propylene, and anhydrous ammonia. Almost 45 million tonnes of LPG gas are transported by sea every year.
These vessels carry liquid gases like methane, ethane, LPG, gasoline, and crude oil.
They are mainly ammonia, ethylene, and vinyl chloride.
Ammonia: This is a major agricultural component and is primarily used as fertiliser. It is also often used as a refrigerant for explosives production and other industrial purposes.
Ethylene: Ethylene has many uses, mainly in the synthetic industry for producing plastic goods, ethyl alcohol and antifreeze products. According to reports, about 3 million tonnes are transported yearly by sea.
Propylene: Like ethylene, this also has extensive usage in the plastic industry and for industrial consumption. Roughly 1.5 million tonnes of propylene occupy the maritime transportation sector related to a liquified gas carriage.
Vinyl Chloride: This is a chlorinated breed of liquified gas and is used widely to produce thermoplastic, mainly PVC, a massive market worldwide for all sectors. They have a higher density than other types of liquified gas contents. Approximately 2 million tonnes are transported by sea.
Most of the liquified products on gas tanker ships are of varying degrees of hazard. So, based on the hazard, the codes for the design of gas tanker ships classify them as follows:
Type 1G: They are designated to carry the most hazardous type of cargo. LNG and LPG predominantly fall under this category.
Type 2G and Type 2PG: They have a lesser degree of hazard than the previous type. Most chemical carriers fall under this category.
Type 3G is deemed to carry the least hazardous of all liquified gaseous substances.
This is the most important mode of classification, and a discussion on this gives a detailed insight into the design overview of gas tanker ships. The cargo-carrying capacity of gas tanker ships in terms of volume ranges from 500 to as much as 100000 cubic meters depending on the vessel. Modern LNG carriers can be as large as up to 150000 cubic metres. The largest of the vessels are used for the transport of LNG and LPG in bulk quantities.
LNG and LPG comprise more than 1000 vessels in the global fleet. The crux of design for gas tanker ships, being hazardous carriers, is to accommodate their contents in enclosed large centre tanks. This is in line with the design of modern oil tankers, which also store the cargo in their centre tanks and have the side tanks and bottom tanks of the hull structure for ballast water and other utilities.
However, unlike traditional oil tankers, gas carrier vessels have an intricate system to maintain the cargo at desired pressures. They remain in a liquid state and are staved off by all modes of interference from the ambient air. In simpler words, they should be tightly enclosed and maintained under suitable conditions to the highest level. Moreover, they do not have any kind of venting or ventilation systems at any place to prevent inflammability and other hazards.
This is precisely opposite to that of oil tankers, where there should necessarily be proper venting systems to allow trapped oil vapours to escape and thus prevent inflammability due to the increasingly high inert pressure of the vapours within their enclosures.
All gas tanker ships are designed and constructed based on the International Maritime Organization’s set of rules called the IMO Gas Codes, which take into account all aspects of hazard, safety, and proper storage of the cargo.
They primarily carry LPG, the first liquified gas cargo transported by sea. In a fully pressurised environment and a low temperature maintained at a specific constant level, the cargo remains in liquid form without the risk of being evaporated into gas due to fluctuations. They usually have spherical or cylindrical pressure vessels or domes of a certain capacity that are tanks for storing the cargo. Typically, the capacity of each tank remains within the range of 5000-6000 cubic metres, but some modern designs have larger sizes of up to 10000 cubic metres as well.
These kinds of ships developed after the advent of fully pressurised vessels to overcome the limitation of high costs and availability of suitable materials to maintain the temperatures ideal for liquified gases. These vessels have reliquification plants and other insulation systems and hence do not have strict requirements to be continuously pressurised in a certain state. So, if the liquid converts into gas, the re-liquefication systems can revert the contents into liquid form. Furthermore, the insulation systems help maintain the temperatures at low levels. From a construction point of view, they are similar to fully pressurised vessels and usually have tank capacities in the range of 5000 to 20000 cubic metres.
After the advancement of refrigerating technology towards the latter half of the 20th century, fully refrigerated ships gained popularity. They have very high-end technologies to maintain the temperature of the tanks at ultra-low levels, enabling the carriage of other types of liquified gases, including LNG and LPG, in bulk quantities. Refrigerated vessels are characterised by tanks that increase the carrying capacity of liquid cargo, and modern designs have a cargo-carrying capacity of as much as 100000-150000 cubic metres. They also employ state-of-the-art insulation technologies.
Though the construction of tanks highly varies depending on the vessel and the type of cargo to be carried, the basic design of liquified gas tanks comprises the internal space for cargo, insulations and membrane walls, marginal spaces outside the tank boundary, and associated structural supports that are in turn, connected to the main hull structure.
They are similar in appearance to that traditional oil tankers and even, to some extent, to that bulk carriers. The entire tank walls are constituted of flat surfaces without any kind of curvatures. These are essentially the prismatic tanks that are characteristic of fully refrigerated carriers. The outer edge of the walls is further insulted by suitable materials, and the space or tanks between the tank(s) and the outer shell is often used for ballast purposes.
These are generally spherical or dome-shaped and are used mainly to transport LNG.
These tanks are of fully pressurised or semi-pressurised type. They are usually spherical or cylindrical. They are capable of carrying liquified gases at very low temperatures as required.
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