LNG and LPG Carriers?
Liquefied Gas carriers are specialized vessels that are used to transport Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG) under controlled temperature and pressure. Today there are approximately 1100 such types of ships at sea, through out the world.
Types of Gas carriers
Gas carriers can be divided into two main types:
- LPG carriers – Which are mainly used to carry liquefied gases such as butane, propane, propylene etc
- LNG carriers – Which are specifically designed to carry, liquefied natural gas.
In this type of carriers, all gases are transported in liquid form, mainly because their physical and chemical properties do not allow them to be carried in vapor form. They are carried in manly two ways :
- By keeping their pressure greater than the atmospheric pressure
- By keeping their temperatures below the ambient temperature
Sometimes, both the methods can also be combined and used.
According to the above methods, LPG carriers are divided into three main categories:
- Fully pressurized
- Semi pressurized and refrigerated
- Fully refrigerated
Fully pressurized Ships
This is the very first type of carrier that was used to transport gases. With a cargo capacity of 3500 cubic meter, these ships are designed to work at a pressure of 17.5 kg/cm2. Cargo is carried in cylindrical or spherical steel tanks with an appropriate storage temperature. All these ships are double bottomed.
- Generally the cargo is carried at an ambient temperature and thus ordinary and less expensive steel is used to make the tanks.
- No additional insulation is required.
- No re- liquefaction plant is required.
- Construction and operation is simple
- Unusual tank shape make the under deck space unusable
- Storing of a particular gas in the tanks depends on the thickness of the tank. Thus those gases that require high design pressure require thick-walled tanks, which increase the weight and cost.
- As the tank diameter increases, the wall thickness increases. This increases the weight of the tank and reduces its capacity, which makes the ship uneconomical for use.
Semi-pressurized / Semi- refrigerated
This type of carrier has both refrigeration and pressurizing facilities. They have full refrigeration facility with high design pressure for the cargo tanks. The tanks are cylindrical in shape and thinner than the pressurized vessels.
Semi-pressurized, semi-refrigerated ships have a cargo carrying capacity of 5000 meter cube under temperature and pressure condition of -10 degree Celsius and 8.5 kg/ cm2 respectively. Larger vessels can carry at temperature of -48 degree Celsius.
A re-liquefaction plant on the ship can load the cargo as gas and liquefy it onboard. Heating and cooling facilities are also provided for smooth discharging of the cargo
They generally have 2-6 tanks and are double bottomed.
- More cargo can be carried as compared to pressurized vessels
- The tanks are lighter and cheaper to build
- The overall structure is economical and easy to construct
Tanker Safety Training ( Liquified Gas) by Seamanship International