Gas Carriers and their types
Introduction
This article is in continuation of an article dealing with different types of gas carriers, their specifications, advantages and disadvantages. The article will describe few types of LPG ship. It will also describe the design and LNG ships and their types.
Fully Refrigerated LPG Ships
These types of ships are used when large quantity of cargo is to be transported for a longer duration of time. They are fully refrigerated but non-pressurized vessels. The tanks have a special self-supporting prismatic shape for better utilization of hold spaces.
The tanks are designed for a working pressure of 0.28 kg / cm2 and a minimum temperature of -50 degree Celsius. The ships comprises of a high pumping and refrigeration plant capacity especially for the large cargo capacity and the extended voyages.
The tanks are so constructed that they extend to the full width of the ship. The ships are double bottomed with ballasting facility. The tanks have a centerline bulkhead to divide them into two parts, which are fitted with equalizing valves.
Ethylene Carriers
These are special types of vessels for transporting ethylene gas. They are fully refrigerated ships that carry ethylene at its atmospheric pressure boiling point of -104 degree Celsius. They have a cargo capacity in the range of 5000-15000 cubic meters and are fitted with a re-liquefaction plant. They are also double bottomed.
LNG carriers
LNG carriers are specialized vessels that are used for transporting liquefied natural gas.They carry LNG at its atmospheric pressure boiling point of -162 degree Celsius. These carriers can also carry LPG cargo, though a separate re-liquefaction plant is needed.They have a cargo carrying capacity in the range of 80 -135000 cubic meters.
LNG is liquefied to – 162 degree Celsius with the help of a refrigerated system on shore. The cargo is loaded in the liquefied form. Liquefying natural gas on board is not at all economical and thus there are not re-liquefaction plants on board. The tanks are made fully insulated. Any leakage in the cargo can lead to instant fire and explosion in presence of a fire source. All these ships are double hulled.
These ships are classified on the basis of the types of storage tanks.There are mainly two types : Membrane type and Moss Type
Membrane Type
The basis construction of a membrane system consists of insulations called barriers. The barriers are divided into two types - primary and secondary barriers. Generally all the ships have double membrane. The insulation is directly fitted into the inner hull. The primary barrier consists of thin metal membrane with thickness less than 1 mm.
Moss Type
They are Special spherical tanks produced with aluminum and nickel steel. A sphere is welded to a steel skirt that is connected to the hull of the ship. This sphere can expand and contract according the capacity of the cargo. In this type, the insulation is fitted on the outer shell of the sphere and there is no secondary membrane.
Both these types of tanks act as giant thermoses where the liquid can be kept cold in order to transport it safely. Also, many modern LNG ships are fitted with onboard re-liquefaction plants. For propulsion, LNG carriers can be fitter with either steam turbines or dual diesel engines.
References
Tanker Safety Training (Liquified Gas) by Seamanship International
Image Credits
https://www.ships-info.info/design/Trinity_Arrow.jpg
https://img.nauticexpo.com/images_ne/photo-g/cargo-ship-lpg-carrier-194361.jpg
