One of the primary fire safety systems on a tanker ship is the inert gas system, or IG system. Inert Gas System lines are installed in all cargo tanks to blanket the top space of the tank with inert gas.
Cleaning cargo tanks at regular intervals is critical to avoid the formation of sludge at the bottom, which reduces cargo space, or, in the case of product tankers or OBO carriers, to avoid the mixing of different cargoes.
Itcis a vital task performed by the ship’s crew and overseen by experienced on-board officers (Master and Chief officer).
The safety of the ship’s crew is paramount, and cleaning a cargo tank necessitates a series of precautions to avoid an accident or casualty.
When cleaning a tank supplied by inert gas via an Inert Gas System generator or an IG plant connected to the ship’s boiler, the following procedures must be followed:
One Tank at a Time
Only one tank should be washed at a time. Prior to carrying out the operation, all crew members involved in the operation are briefed on the hazards and safety aspects in a tool box meeting.
The tank being cleaned must be isolated from the main inert gas system, all other tanks, and any other common venting system.
Ensure Maximum Ventilation
Maximum ventilation output should be focused on the tank well in advance of the cleaning operation.
The ventilation should be set up in such a way that it allows for good air circulation (free flow of air) from one end of the tank to the other. Officers must fill out and follow the enclosed space checklist. In the event of a mishap, rescue from the tank must be planned ahead of time, with all equipment ready at the entry point.
Allow An Atmosphere Test To Be Performed To Determine Acceptable Limits
The washing operation should not begin until the atmosphere tests at various levels of the cargo tank are within acceptable limits and the vapour content in any part of the tank is less than 10% of the lower flammable limit.
Fill The Tank To The Brim With Water
After flushing the tank bottom with water, it should be stripped. Water is also used to flush piping lines, cargo pump equipment, crossovers, and discharge lines.
Washing with clean, cold seawater is possible. Make certain that no recirculating systems are used for tank washing.
Continuously Test The Tank Atmosphere
At regular intervals, the tank atmosphere will be tested. If the vapour level exceeds 50% of the lower flammable limit, the washing operation must be immediately stopped until the vapour level falls below 20% of the lower flammable limit.
Examine the Washing Machine Capacity
If washing machines with capacities greater than 60m3/hour are used for tank cleaning, only one machine may be used on board at any given time.
Take Extreme Caution When Using Portable Machines
If portable washing machines are used, ensure that all hose connections are properly drawn and bonding cables are tested for continuity before inserting them into the tank. They must not be broken until the machines have been removed from the tank.
Draining Washing Water Properly
During the washing process, the tank should be drained. If wash water accumulates in the tank, the operation must be stopped and not resumed until all of the water has been removed.
Avoid Using Chemical Additives
Chemical additives should be avoided.
Deck Openings That Are Not Needed Should Be Closed
During the washing operation, all deck openings other than those required for the operation must be closed.
