Why Cargo Tank Atmosphere Evaluation is Essential for Tanker Ships

Tank Atmosphere Evaluation: Merchant vessels are classified based on the type of cargo they transport.

There are two main categories of cargo ships:

  • Dry ships, which include container ships, general cargo ships, and bulk carrier ships.
  • Tanker ships, also known as wet ships, such as oil tankers, chemical tankers, and gas carriers, transport hazardous cargo in liquid, vapour, or solid form.

Compared to dry ships, tanker ships carry significantly more hazardous and reactive cargo. Therefore, tank atmosphere evaluation is crucial to ensure that the tanks used to transport such cargo meet approved standards and comply with various safety regulations like IGC, IBC, IMDG, and others.

The Atmosphere in tanks

The following conditions may exist, alone or in combination, within tanker ship cargo areas:


One of the main risks associated with handling oil and petroleum cargoes is flammability.

This can be divided further into:

  • Non-Volatile Transport: with a flash point of 60° or above as shown by testing using a closed cup
  • Changing Cargo: having a flash point that is lower than 60° according to testing using a closed cup


This factor indicates the degree to which the substance being carried poses a risk to humans who may touch, swallow, or inhale it. Bulk chemical cargoes are typically harmful by nature. Because petroleum products and oil contain HC, which is hazardous to people, the gas produced from them might be regarded as hazardous.


This component is taken into account when it comes to chemical cargo and certain kinds of self-reactions, such as when gaseous or liquid cargo becomes into viscous liquid or solid. The cargo itself is harmed by this process.


Reactivity with air, water, and other cargo: Some chemical cargoes, such isocyanates, have the potential to react with water and produce harmful fumes or to harm the cargo apparatus or storage tank in which they are kept.

Certain cargo, including ethers and aldehydes, combines with air to create an unstable oxygen molecule that could blow up.

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Unknown reactions between chemical cargoes stored together run the risk of causing fires, explosions, the discharge of poisonous vapors, damage to cargo tanks, and equipment failure. As with chemical tankers, it is crucial to transport each kind of cargo separately; in fact, a bulkhead separation is seen as dangerous. To prevent this kind of response, there must be a cofferdam or empty space between two tanks carrying two distinct loads.

Low Oxygen

For the crew’s and ship’s safety, the majority of the chemical and oil cargo is transported in an inert state. By “inerting” the tank, the oxygen content is lowered to a level that is safe for the payload. Unless the tank is prepared with a fresh air supply and monitoring equipment for continuous O2 level checks, the oxygen level will remain dangerously low even after the cargo has been emptied.


Certain cargoes, like sulfuric acid, can cause significant damage to the inside structure of the tank and cargo equipment because they are very corrosive to most metals. They are typically transported in stainless steel tanks, however the tank must be clear of water, chlorides, foreign objects, and leftover cargo material before filling. Nitric acid can be used to passivate the tank, creating a passive layer of chromium oxide that aids in corrosion prevention.

Therefore, the following factors are crucial for tank atmospheric control:

  • Sort of cargo to be transported
  • Variety of cargo to be transported concurrently
  • Equipment and systems installed to sustain the necessary atmospheric conditions
  • State of the coating of the tank, if any
  • Protection kind (passive layer, inerting, etc.) in the tank

Importance of Tank Atmosphere Evaluation

The most significant and hazardous area on board a tanker ship is the cargo tank, which holds goods valued at millions of dollars and is subject to frequent explosions, spills, fires, and other hazards.

For the following reasons, it is crucial to assess the cargo tanks’ atmosphere:

Due to the enclosed nature of the area, please inspect before entering.

  • Before embarking on repair work or bringing a ship into a shipyard or dry dock
  • Before doing the gas-freeing procedure, “inert” the tank.
  • Before carrying out the cleansing process
  • Before adding more cargo to the tank,
  • Before Cargo Tank Washing
  • Before carrying out the chemical Wall test Tankers

Tank Atmosphere Evaluation Data

The only method to obtain accurate data regarding the makeup of the tank atmosphere necessary to perform the aforementioned tasks is to evaluate the cargo areas aboard tanker ships. Since all of the aforementioned activities need for human access into the tank, the crew members in charge of them must be aware of the different compositions present in the tank’s atmosphere and enter in accordance with SOLAS Regulation XI-1/7. The information needed and the actions to be performed from the tank or cargo to assess the environment within are as follows:

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  • Nature of constituent gases
  • Toxicity: There shouldn’t be a noticeable concentration of hazardous gasses in the cargo tank’s environment.
  • Oxygen Content: Prior to entering, the oxygen content should be verified at 21%, and it should remain that way until the work within the tank is completed.
  • Hydrocarbon content/flammability: in all enclosed areas, including cargo tanks, the safe gas concentration shouldn’t exceed 1% of the hazardous material’s lower explosive limit.
  • Reactivity: Verify that the materials used for specific reactive cargoes are not reactive to the MSDS and are authorized under it. For example, hose resistant to acid should be used while carrying sulfuric acid.
  • Status of the inert gas system: it must be isolated. It is advised that cargo tanks next to the area to be accessed be examined for the presence of oxygen, hazardous gases, and/or cargo vapours. If necessary, the inert gas pressure should be decreased to lessen the chance of any inter-tank leaks.

Various tools for assessing the atmosphere:

  • Oxygen Indicator: To determine how much oxygen is in the tank
  • Combustible Gas Detector: This device measures hydrocarbon gas in inert environments and examines the flammability level and composition of the gases inside the tank.
  • Explosimeters: These are contemporary gas detectors that are used to determine the flammability level and composition of the gases inside the tank. An explosimeter uses a catalytic type sensor, commonly referred to as a “Pellistor,” which is a poison-resistant element. Since this element needs oxygen to function properly (at least 11% by volume), explosimeters should not be used to measure hydrocarbon gas in inert atmospheres.
    hazardous Gas Detector (Dragger Gas Detector): to determine the cargo tank’s hazardous level
  • Vapor tracking: H2S is a poisonous, corrosive, and combustible gas that is produced by some cargoes (crude oil, naptha, etc.). The TLV-TWA for H2s is measured with an H2s vapour analyser and should be 5 ppm during eight hours.

To make sure that there are no dangerous gases trapped inside, it is crucial to evaluate the atmosphere of the tank at different times. Hydrocarbons and other gases have the propensity to produce pockets of combustible gas that get trapped inside tank layers, even while the atmosphere outside the tank is acting normally.


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