Rain causes negative pressure in cargo tanks

Mooring activities interfered with an inert gas system alarm on board a tanker, causing a negative pressure to develop in the tanks as a result of rain. The Nautical Institute discusses the incident in its latest Mars Report and stresses it is important to maintain situational awareness to better deal with conflicting priorities.

The Nautical Institute gathers reports of maritime accidents and near-misses. It then publishes these so-called Mars Reports (anonymously) to prevent other accidents from happening. A summary of this incident:

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.

Advice from The Nautical Institute

  • 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

Mars Reports

This accident was covered in the Mars Reports, originally published as Mars 202111, that are part of Report Number 340. A selection of this Report has also been published in SWZ|Maritime’s March 2021 issue. The Nautical Institute compiles these reports to help prevent maritime accidents. That is why they are also published on SWZ|Maritime’s website.

More reports are needed to keep the scheme interesting and informative. All reports are read only by the Mars coordinator and are treated in the strictest confidence. To submit a report, please use the Mars report form.

Author: Mariska Buitendijk

Mariska Buitendijk is one of SWZ|Maritime's journalists as well as the magazine's copy editor.