Merchant ships are categorised on the basis of the cargo they carry. The two basic pool of cargo ships are- dry ships, comprising of container ships, general cargo ships, bulk carrier ships etc., and tanker ships or wet ships such as oil tanker, chemical tanker, gas carriers etc., which carry cargo of hazardous nature in liquid, vapour or solid form.
The cargo carried in later type of the ship is much more hazardous and reactive than that carried by dry ship. It’s therefore very important to carry such cargo in cargo tanks of approved types and as per the requirements of different safety codes such as- IGC, IBC, IMDG etc.
The atmosphere inside cargo spaces of tanker ships can be one or combination of the following-inflammable, toxic, reactive with air, water, other cargo, or human skin, oxygen deficient etc. All the stated factors largely depend on:
- Type of Cargo to be carried.
- Variety of cargo to be carried at the same time.
- System and machinery fitted to maintain the required atmosphere condition.
Importance of Tank Atmosphere Evaluation
A cargo tank of a tanker ship is the most important and highly dangerous space onboard ships as it carries cargo worth millions of dollars and has high risk of explosion, spillage, fire, health hazard etc.
It is important to evaluate the atmosphere of the cargo tanks because of the following reasons-
- Before entering for inspection as it is an enclosed space.
- Before entering for repair work
- Before taking a ship to dry dock or shipyard
- Before gas-freeing operation
- Before “Inerting” the tank
- Before doing purging operation
- Before loading new cargo in the tank
Data for Tank Atmosphere Evaluation
An evaluation of the cargo spaces in tanker ships is the only way to get the correct statistics about the composition of the tank atmosphere to carry out the above mentioned operations. Following are the data required to be taken from the tank to evaluate the atmosphere inside it-
- Nature of Constituent gases
- Oxygen Content
- Hydrocarbon content/ Flammability
Different equipment for atmosphere evaluation:
- Combustible Gas Detector
- Toxic Gas Detector (Dragger gas detector)
It is very important to take measurement of the tank atmosphere at various points to ensure that there are no harmful gases trapped inside. Gases such as hydrocarbons have a tendency to form pockets of inflammable gas which remains trapped within layers of tank even when the rest of the tank atmosphere is showing normal.