In a new Marine Safety Advisory, RMI explored four case studies establishing poor implementation of procedures, inadequate information,and lack of monitoring as key causes.

-Case A

About two hours after bunkering began while the vessel was anchored, oil overflowed from the No. 1 starboard tank air vent.

The crew contained the oil spill onboard the vessel. The filling valve was closed when the tank was around 93% full.

However, an investigation revealed that this butterfly valve was not sealing completely because the Viton seat was slightly damaged and dislodged.

The bunkering rate was high, so even though the high and high-high alarms activated at 96% and 98%, respectively, there was not enough time to prevent the spill.

The root cause was a lack of monitoring, as the remote level gauge system in the Cargo Control Room was left unattended. Additionally, the No.1 starboard tank level was not physically sounded after the filling valve was shut to confirm no level changes occurred while other tanks were filling.

-Case B

While an anchored vessel was bunkering, oil overflowed from the No. 4 port tank air vent spilling around 1,500 liters. Most of this spill was contained onboard and cleaned up, but around 10 liters went overboard.

The investigation concluded that previously agreed procedures and checklists were not implemented, and the personnel were not at their assigned positions or duties. There was no agreed pumping rate between the vessel and barge, nor was there a request to reduce the delivery speed during the final top up. The lack of proper tank level monitoring and communication resulted in the spill.

-Case C

A vessel at anchor was bunkering when oil overflowed from the first tank being filled from its air vent into the save-all tray. Two tanks were scheduled to be filled and a change in tanks was about to take place. Due to heavy rain, around 110 liters of an oil and water mixture went overboard.

The investigation concluded that there was a lack of:

  • proper level monitoring of the first tank being filled; and
  • establishing proper communication with the barge.

Part of the corrective action was to revise the company forms and procedures, and to include the tank level sounding intervals.

-Case D

A vessel was bunkering at anchorage, when oil overflowed from the No. 2 starboard tank air vent to the save-all tray and onto the main deck. A small quantity fell overboard.

The investigation revealed that during topping up, instead of the intended No. 1 starboard tank, the filling valve of the No. 2 starboard tank (already 85% full) was opened in error. The investigation concluded that this error of failing to cross-check the valves and improper tank level monitoring contributed to the spill.

From the lessons learned, it was proposed that tank valves which were already full (or not to be filled), should be marked with warning or caution signs to prevent any wrong operation.



These cases stress the importance of holding pre-bunkering meetings, planning and execution, that addresses at least the following:

  • Opening and closing of tank filling valves should be cross checked.
  • In addition to regular level monitoring of all tanks, the bunkering rate and bunker manifold pressure should also be monitored.
  • Good communication with the barge should be established prior to the start of, and during bunkering, which will allow the bunker rate and pressure to be adjusted as required.
  • Fuel tank filling valves testing and maintenance should be included in the Planned Maintenance Systems onboard.

In regard to the above, RMI informed that it is working with MPA Singapore to improve onboard operations to prevent any oil spills during bunkering.

Attention is drawn to MPA Singapore Port Marine Circular No 13 of 2019, Guidelines For Preventing Pollution During Bunkering Operations for further guidance.