Incident:
On completion of a FO transfer, the ship crew decided to gauge the tank on its final sounding. When the crew unplugged the sounding pipe for fuel oil tank #1, they observed a strong inflow of air into the tank through the sounding pipe. They right away raised alarm, grabbing immediate attention from the other ship staffs. The inflow of air into the sounding pipe was so high that it created an unpleasant sound. Everyone obviously concluded, that the tank was under vacuum and decided to react quickly before something worse could happen.
Wait!
We are sure that at this moment, you would have guessed the reason for such a high vacuum. And the reason you guessed could be a clogged air vent head mesh.
But this was not the case here.
Keep reading – you will come to know the surprising reason for such high vacuum.
A decision to inspect the opening of FO vent came out as a usual first measure remedial action. The inspection was done, including the spark arresting wire mesh, but no defects were found. The vent head/floater/seals/mesh were found to be in good condition. So, the complete vent head was removed to investigate the reason.
And finally, here is the reason to your surprise.
The first elbow, approximately 1.5 meters below the deck level, was discovered with a blockage, consisting of large rust scales and hardened dust. This prompted the ship staff to check the FO tank #2, which was also in a similar condition but to a lesser extent.
The capacity of FO transfer pump is 2 x 50 m3/hr, which is significant when considering the size of the tank. However with the amount of vacuum it created, it is obvious that the vent pipe was blocked and there was no air relief at all.
The blocked vent pipe could have easily caused a serious tank implosion with spilling fuel oil and further pollution, if it had not been discovered.
Causes:
- Since the external pipe was in a good condition, a thorough inspection of the interiors was not done.
- The rust and scale has been collected in the foot of the upper vertical section of the pipe. The accumulation happened over several years, which did not have a proper way to be washed away into a FO tank, where it would likely sit at the bottom of the tank.
Actions taken:
- A thorough inspection was carried out and the blockages were cleared from the pipes.
- The Planned maintenance system of the vessel included the inspection of both exterior and interiors of the pipe i.e. an internal inspection job was created.
Now this dust, upon clearing it would fall inside the fuel tanks. Thus, obviously one can expect a filter to get clogged or other sediments come out from the tank drains.
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Source: IMCA & Marine Safety Forum
