This incident occured during bunkering of fresh water. A blocked air pipe caused an overpressure in the tank which led to permanent deformations of the tank.
The air vent pipes of the two adjacent fresh water tanks were connected to a common horizontal section before it was led all the way up to the top of the deck. The internal surface of the
vent pipe was corroded and scales were formed and dropped. These scales got collected and settled during a long period of time, and in the end blocked the horizontal section of the air pipe.
The best way to avoid a similar situation is to keep all air pipes
- as short as possible,
- directly lead to the top of the deck vertically,
- without any bends where horizontal sections are joining or shared with other tanks.
Precautions can be made during maintenance by flushing air vent pipes at regular intervals.
Source: DNV
Officer of the Watch 
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