The Ultimate Guide to Cargo Operations for Tankers

• Start the IG plant and open the deck main isolation valve
• Check that the inert gas supplied should have oxygen less than 8%
• Maintain the Inert gas velocity as per the method used for purging (dilution or displacement)
• Keep checking the gas readings of the tank to ensure that the hydrocarbon (HC) concentration is reduced to 2% or less and the oxygen content of the tanks is less than 8%
• Upon achieving the above readings, shut the gas freeing lid of 1 Port and Starboard
• Close the Inert gas inlet to the tanks and open them for the next set of tanks to be purged
121. l Marine Insight
• This procedure can be followed to purge the remaining tanks For Entry in the tank, ensure gas readings of the tank show HC concentration reduced to 2% or less and the oxygen content is less than 8%.” Cargo Operations for Tanker I 122. Gas Detector

A cargo oil tank should never be entered in inert condition. Gas-freeing, as the name suggests, is to remove all hydrocarbons (HC) and inert gases from the tank to make it fit for human entry i.e. oxygen content to be 21%. A cargo tank which has hydrocarbon level-up to 2 % or less can only be gas freed. Fresh air is introduced into the tank using inert gas blowers through the inert gas mainline itself. The safest way togas free an oil tank fitted with an inert gas system is to use a fan and vent the tank. Before entry, an O meter must be used, and the tank must also be checked with an explosimeter. These meters must also be used while in the tank.
123. l Marine Insight The sketch shows a diagrammatic view of an explosimeter. The combustion chamber is cleared of any residual gases by the aspirator bulb in fresh air. The meter is then switched on and the detector element is allowed to heat up.
Gas Freeing
Cargo Operations for Tanker I 124.
Explosimeter

With the check switch is closed, the zero adjustments of the meter are checked. Precautions for gas freeing operation
• Chief officer to ensure only personnel designated for the Gas freeing operations should be allowed on the main deck
• The proposed duration for the gas freeing operation to be decided well ahead of the operation in a pre-operation meeting
• If there are small crafts alongside the vicinity of the tanker, their personnel should also be notified about the gas freeing operation and hazards related to the same
• The atmosphere near the operational area should
125. l Marine Insight be continuously monitored for explosive hydrocarbon vapors
• All crew involved in the gas freeing activities must wear appropriate protective gears and clothing Cargo Operations for Tanker I 126. Tank Entry

freeing operation is being drawn into the accommodation, the air conditioning and mechanical ventilating systems should be stopped immediately
• Many tanks have common venting arrangement for all cargo tanks. In such cases, each tank should be isolated to prevent the transfer of gas to or from other tanks
• When the ship is underway, it may become necessary to slightly alter the course for preventing hydrocarbon vapors from accumulating in the accommodation areas
• During the gas freeing operation, the intakes of central air conditioning or mechanical ventilating systems should be monitored and adjusted, if required, to prevent the entry of harmful gases
• Ensure to maintain Positive pressure in accommodation
• If it is suspected that harmful gases from gas
127. l Marine Insight

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