202111 Negative pressure in cargo tanks

29 Jan 2021 MARS 2021

Negative pressure in cargo tanks

In the early morning hours a tanker docked to commence unloading. Contrary to port procedures and best practices, there was a negative pressure in the cargo tanks of about 270 mm/wg. This was corrected by the addition of inert gas from the inert gas system (IGS) before unloading commenced. Unloading then proceeded without incident and once empty the vessel departed. The company investigation found, among others, that; During the vessel’s approach to the berth, which lasted for about 2.5 hours, a constant rain was falling. This accelerated the cooling of the cargo and reduction of its volume, and hence the pressure inside the cargo tanks.

Although the IGS alarm sounded at the time of the low pressure, the crew were otherwise engaged in the tie-up procedures, and did not appreciate the rate of the pressure drop.

Lessons learned

  • On a vessel you can sometimes be faced with conflicting priorities. By keeping your situational awareness you will be in a better position to react accordingly.
  • This incident shows the importance of investigating close calls of all kinds. Find out why something happened in order to introduce corrective action so it doesn’t happen again