The US NTSB issued an investigation report on the capsizing of the towing vessel ‘Gracie Claire’, while moored on the Lower Mississippi River in Venice, Louisiana. The report revealed that several factors affecting the stability of the vessel led to its capsizing.
The incident
On 23 August 2017, at 0756, Gracie Claire was moored in Tiger Pass near mile marker 10 on the Lower Mississippi River. While taking on fuel and water, the towboat began to slowly list to starboard. After the wake of a passing crewboat washed onto the Gracie Claire’s stern, the list increased. In a short period of time, water entered an open door to the engine room and flooded the space.
The towboat sank partially, its bow being held above the water by the lines connected to the dock. All three crewmembers escaped to the dock without injury. Approximately 1,100 gallons of diesel fuel were discharged into the waterway. Damage to the Gracie Claire was estimated at $565,000.
Probable cause
NTSB determines that the probable cause of the capsizing and sinking of the Gracie Claire was the towing vessel’s decreased stability and freeboard due to undetected flooding through a hull leak in the rudder compartment, which made the vessel susceptible to the adverse effects of boarding water from the wake of a passing vessel.
Analysis
1. Before the vessel arrived at the Stone Oil dock, the rudder compartment likely had been flooding from the hole in the bottom of the hull, a water ingress that the crew was not aware of. The lost buoyancy from the water in the compartment would have resulted in the towboat having less freeboard, which would have decreased the towboat’s stability, because the main deck edge would be submerged at lesser angles of heel. The lower freeboard would also have made it easier for waves to wash across the main deck.
2. By mooring perpendicular to the seawall and not parallel to the waterway, the Gracie Claire was subjected to a heeling moment caused by the river current.
3. At the time, the equalizing line valves for the fuel oil tank were open, which was intended to balance the level of the tank’s contents on each side of the centerline swash bulkhead. Based on 24 inches of fuel oil above the pipe, investigators calculated that the line would transfer approximately 70 gallons per minute. However, considering a fill rate of 140 to 150 gallons per minute and the vessel’s continued heel to starboard, the captain was filling the tank at a rate that exceeded the capacity of the equalizing line to level the tank.
4. Aware of the increasing starboard list during filling, the captain decided to shift fuel-filling to the port side of the tank to counter the starboard list. He believed that a reduction in the list would occur because the fuel being loaded would accumulate on the port side, or high side, of the tank. However, with the vessel already listing to starboard, once the fuel reached the holes in the swash bulkhead on the port side of the fuel tank, the added fuel would shift by spilling over to the starboard side through the swash bulkhead openings, therefore exacerbating the starboard list. At the same time, the floodwater in the rudder compartment and in the water tank would also shift further to starboard. Had the water in the rudder compartment been discovered, the Gracie Claire’s owner or captain presumably would have taken measures to eliminate the water in the space before taking on fuel and water.
5. While the captain was filling the tank from the port side, the crewboat that passed astern added to the heeling forces on the starboard side. First, the displaced water of the passing vessel pushed the side of the hull below the waterline. Second, as the waves from the crewboat’s wake washed over the bulwark and onto the main deck of the towboat, they added weight on the starboard side of the main deck.
With the starboard deck edge submerged, the Gracie Claire would have rolled more easily. Once the vessel reached a heel angle that allowed water to reach above the 20-inch coaming of the open door leading to the engine room, the vessel down-flooded and rapidly sank.
National Transportation Safety Board Marine Accident Brief: NTSB-Capsizing-and-Sinking-of-Towing-Vessel-Gracie-Claire
