An overheated hydrodynamic fluid coupling melted three fusible plugs, releasing hot oil into a tugboat’s machinery space and starting a fire that caused extensive damage, the Transportation Safety Board of Canada (TSB) has determined.
At 0200 on Sept. 15, 2017, the bulk carrier AM Tubarao finished loading its cargo and the tugs Vachon and Brochu were tasked to guide it out of the harbor at Port-Cartier, Quebec, stern first into the channel.
The tugs guided the bulker out of its berth by their towlines, Vachon by the bow and Brochu by the stern. The pilot on AM Tubarao noticed that the stern of the bulker had stopped moving. At the pilot’s request, the master of Brochu removed the lock on the tug’s propulsion control lever, and the vessel pulled on AM Tubarao at maximum power.
The 99-foot Brochu is equipped with two main engines, each one driving a cycloidal propeller with a hydrodynamic fluid coupling. The maximum continuous operating temperature of the couplings should not exceed 185 to 194 degrees Fahrenheit, and each is equipped with a thermal control element that triggers an aural and visual alarm when the coupling reaches 257 degrees. When this occurs, the drive must be shut down immediately so that the temperature will not continue to rise. Each coupling is also equipped with three fusible plugs that melt when the coupling reaches 284 degrees. This allows the oil to drain, stopping all power transmission and preventing excessive pressure buildup inside the coupling.
At 0433, the high-temperature alarm activated in Brochu’s wheelhouse, indicating that the oil in the starboard fluid coupling had reached 257 degrees. The bridge team was not aware of the significance of this alarm and Brochu continued to pull at maximum power, causing the internal temperature of the coupling to increase further.
A diagram provided by the Transportation Safety Board of Canada shows the stages of maneuvering as Brochu and the tug Vachon escorted AM Tubarao out of the harbor.
Pat Rossi illustration
“At approximately 0434, Brochu’s starboard fluid coupling oil reached 140 degrees Celsius (284 degrees Fahrenheit) and the three fusible plugs melted,” the TSB report stated. “Once the plugs melted, the hot oil released into the tug’s machinery space. With no oil in the coupling, the starboard cycloidal propeller disengaged and the overspeed protection system shut the engine down.
“The hot oil vaporized and ignited, either when it made contact with a hot surface such as the engine exhaust manifold, or owing to a spark that may have come from the nearby generator,” investigators wrote.
With smoke in the wheelhouse, engine room and accommodation space, the tug’s crew detached from the bulk carrier and returned to the berth, activating the fuel shutoff valves and closing the engine room ventilation dampers.
The vessel owner’s fire department entered the tug’s engine room at about 0540 and, with the help of a thermographic camera, determined that the flames had not spread beyond the space and were extinguished.
The TSB determined there were unsafe conditions when the engine was kept at maximum power after the high-temperature alarm sounded, and that crewmembers were not provided with specific training on what to do in such cases. Investigators also noted that the coupling was not monitored manually or by a continuous temperature monitoring system, and that the machinery compartment’s gas-tight devices were not maintained in accordance with regulations. This caused smoke to enter the wheelhouse and accommodation space.
No injuries or pollution were reported, and a monetary estimate of damage was not available. Brochu was owned by ArcelorMittal Mines Canada at the time of the incident. After the fire, it was sold to Ocean Group and renamed Ocean Brochu.