Sinking of fishing vessel probably caused by fuel leak igniting, says NTSB

Although it was hard to establish the exact circumstances behind the sinking of fishing vessel Ariel off the Alaskan coast in 2019, the US National Transportation Safety Board has concluded that the probable cause of the fire was the ignition of fuel leaking from the generator fuel supply line in the engine room.

Fluctuating revolutions per minute of the diesel engine generator were observed by the captain just after the fire was discovered. The NTSB said that this was probably the result of fuel starvation. This suggested that the fuel line to the generator’s engine was breached.

At about 18:30 local time on August 26th 2019 fishing vessel Ariel was transiting Sheep Bay, Prince William Sound, Alaska, when a fire broke out in the vessel’s engine room. The four crewmembers aboard the Ariel attempted to fight the fire, but they were unsuccessful and abandoned ship into the vessel’s skiff. The Ariel continued to burn and subsequently sank. The crew was rescued by Good Samaritan vessels and returned to port uninjured. About 500 gallons of diesel fuel was aboard the vessel when it sank. The Ariel, valued at an estimated $600,000, was a total loss.

The Ariel was not salvaged, and investigators therefore could not inspect the machinery or hull to determine an exact cause of the fire.

However, observations by the crew during the accident provided some indication of the location and potential source of the fire. The engineer stated that when he first opened the engine room hatch to investigate smoke, he saw flames in the vicinity of the generator located on the port side aft in the engine room. The long delay in the heat-sensor activation of the Halon fixed fire-extinguishing system, located on the forward bulkhead of the space, further indicated that the fire started aft in the space. It was therefore unlikely that the hydraulic systems for steering, throttle control, and deck machinery – whose main components were located forward in the space near the Halon system – were the source of the fire.

Investigators could not determine why the captain could not reduce the main propulsion engine speed prior to shutting the engine down, but based on the information above, it is unlikely that this was causal to the fire.

The fluctuating rpm of the diesel engine generator observed by the captain just after the fire was discovered was probably the result of fuel starvation, which suggested that the fuel line to the generator’s engine was breached. The fuel hoses that ran between the manifold, fuel filter, and generator met USCG material specifications. However, the NTSB hypothesized that, over time, a hose could have become worn from contact, its connections could have loosened through vibration, or it could have otherwise failed, allowing fuel to leak into the engine room.

Leaking fuel or fuel vapour could then have come into contact with a hot surface, igniting the fire.

The vessel was fitted with manually operated fuel oil shut-offs in the lazarette and engine room, but there were no remote emergency shut-offs. The crew was unable to close the manual shutoff valves before abandoning the vessel.

The fuel still in the tanks would have continued to feed the fire once the hoses melted.

Based on crewmember accounts, the Halon fixed fire-extinguishing system on the Ariel activated and appeared to reduce the fire, but the fire rekindled and eventually consumed the vessel.

The NTSB noted that, to work effectively, fixed fire-extinguishing systems required a minimum concentration of firefighting agent to either halt the chemical reaction producing the fire, displace oxygen feeding the fire, or effect a combination of both. The fixed-open louvers on the inlets to the Ariel’s engine room ventilation ducts prevented the space from being sealed off. This meant that the discharged Halon was allowed to escape, and new air was introduced to the fire.

The NTSB said that the engineer’s attempt to seal off the ventilation inlet louvers using available materials was commendable, but was probably too late to prevent the spread of the fire.

The NTSB determined that the probable cause of the fire was the ignition of fuel leaking from the generator fuel supply line in the engine room. The fluctuating rpm of the diesel engine generator observed by the captain just after the fire was discovered was likely the result of fuel starvation, which suggests that the fuel line to the generator’s engine was breached.

https://www.ntsb.gov/investigations/AccidentReports/Reports/MAB2013.pdf