Tank Over-Pressurization: Lack of Effective Communications

The National Transportation Safety Board (NTSB) has released a Marine Accident Brief about the over-pressurization of the number 3 port and starboard cargo tanks that occurred on the Marshall Islands-flagged chemical tanker Fairchem Filly in May 2019.  

On May 30, 2019, about 0750 local time, the Fairchem Filly, with a crew of 22, experienced an over-pressurization of the number 3 port and starboard cargo tanks while discharging liquid hexene at Vopak Terminal in Deer Park, Texas. The over-pressurization resulted in damage to the number 3 port cargo tank and the tank top (deck). All cargo was contained on board the double-hulled vessel, with no pollution or injuries reported. Damage to the Fairchem Filly was estimated at $750,000, and the contaminated cargo was an estimated $100,000 loss.

The NTSB determined that the probable cause of the over-pressurization and rupture was the vessel and terminal personnel involved not following policies and procedures related to cargo discharge and nitrogen-blanketing operations. Contributing to the casualty was the lack of effective communication between the vessel and terminal personnel and the decision of the vessel’s PIC to continue discharge operations after being unable to communicate with the terminal.

When a low-pressure alarm was activated on the vessel’s hexene tanks, the chief mate repeatedly attempted to contact terminal personnel via a handheld radio to request nitrogen, but the Vopak PIC (dockman) did not answer. Since the chief officer could not contact the Vopak Person In Charge, he ordered the ship’s pumpman to open the ship’s nitrogen supply valve all the way, after which the pressure in the tank should have risen, but it did not. 

At this point, instead of securing the vessel’s nitrogen valve and stopping the operation until communications could be restored, the chief officer had the pumpman fully open the nitrogen valve, effectively removing all shipboard throttling control of the nitrogen coming on board. With the ship’s throttling ability removed, the combined effect of the nitrogen pressure at the dock, the amount that the terminal valve was open, and the larger four-inch hose (without a flow-reducing orifice installed before the ship’s manifold) resulted in the rapid pressurization of the cargo tanks, exceeding the relief valve capacity and overpressurizing the tanks to 15.8 psi (109 kPa), as recorded on the cargo vapor pressure monitoring system. 

Since the nitrogen hose connection was improperly configured (without a one-inch hose or orifice), the flow rate of nitrogen had to be controlled by the ship or terminal personnel by manually adjusting the dock or ship valve. Therefore, communication between the ship and terminal personnel was critical, says the NTSB. 

The report is available here.