NTSB issued the investigation report of the towing vessel Mary Lynn in which a fire broke out last May resulting to damages estimated at over $700,000, but hopefully, no injuries or pollution reported.
The investigation contributed the engine room fire to the overpressurization of the fuel day tank (which did not have an independent vent) and the main engine fuel return system which ultimately led to ignition of spraying diesel fuel from a main engine’s fuel system onto an uninsulated engine component.
The Incident
About 0315 on May 18, the Mary Lynn tied up at the Gasconade Street fleet near mile 175 of the Upper Mississippi River in St. Louis after dropping off barges about 3 miles downriver.
The vessel’s crew planned to take on fuel, lube oil, and potable water from a delivery tug and barge about 0600. The chief engineer woke up about 0300, in advance of his 0500 watch, to prepare for the fuel, lube oil, and potable water transfer.
He went to the engine room, conducted a visual inspection of the space, and then began the weekly task of removing residual water from the four fuel storage tanks (no. 2 port and starboard and no. 3 port and starboard) using a dewatering filtration system.
About 0330, while the chief engineer was still operating the dewatering filtration system, the fuel delivery tug and barge arrived, about 2.5 hours earlier than expected. The chief engineer shut off the dewatering system pump and closed the fuel storage suction and return valves for the tank he was dewatering at the time, leaving all fuel return line valves closed. The chief engineer later told investigators that he had thought the no. 2 port and starboard fuel storage tank fuel return line valves were open, but he did not physically check their positions. The chief engineer then shifted his focus to preparing for the fueling operation, completing the bulk cargo transfer checklist (used for fuel) at 0335.
About 0500, the transfer of 25,550 gallons of ultra-low-sulfur diesel fuel to the Mary Lynn was completed. The fuel delivery barge and tug departed the Mary Lynn at 0537, and, about 0540, the captain arrived in the wheelhouse to relieve the pilot for his scheduled watch.
The chief engineer changed the disposable fuel filters (not a part of the main engines) for the fuel transfer pumps and the fuel suction cartridge filters on the fuel supply line for both main engines (the chief engineer said the changeout was usually done based on engine running hours). Once the changeout of the fuel filters was complete, he informed the captain that the engines were ready for use.
Probable cause
The National Transportation Safety Board determines that the probable cause of the engine room fire on board the towing vessel Mary Lynn was the overpressurization of the fuel day tank (which did not have an independent vent) and a main engine fuel return system when the fatigued chief engineer inadvertently left the day tank overflow valves to the storage tanks closed, which ultimately led to ignition of spraying diesel fuel from a main engine’s fuel system onto an uninsulated engine component.
Lessons Learned: Tank Ventilation
Subchapter M regulations for towing vessels require vessels built after 2000 to have vents for each fuel tank. Regulations for vessels ranging from small passenger vessels to cargo ships require that fuel tanks be independently vented from the highest point of the tank to atmosphere on a weather deck.
Tank ventilation is important to ensure a valve line up error does not lead to the overpressurization of or vacuum in a fuel tank. Operators should be aware of their fuel tank ventilation system arrangements.
On vessels without independent fuel day tank ventilation, it is critical to ensure proper valve position during transfer and operation of the fuel system.
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Source: Marine Insight
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