Until about noon on July 29, 2014, everything was working normally on the Tacoma, a Jumbo Mark II ferry owned by Washington State Ferries and carrying passengers and vehicles between Seattle and Bainbridge Island.
At 11:59 a.m., the #4 propulsion alternator of the boat’s diesel-electric system was started to provide additional power during a landing in Seattle. After the landing, while the engineers were removing #4 alternator from service, a voltage surge caused an alarm to sound for .92 seconds. The engineering crew doesn’t know why the alarm went off and apparently thought it was insignificant. It wasn’t.
After unloading, the Tacoma departed Seattle bound for Bainbridge Island. All systems appeared to be working normally. They weren’t.
When #4 alternator was activated to assist the landing on Bainbridge, all hell broke loose. An “arc event” in the transformer cubicles melted copper bus work and damaged electrical conductors. Surge arrestors in alternator cabinets were destroyed, and the heat deformed steel supports. Steel switchboard covers were pushed out due to high pressure inside. Smoke came into the engine room. Individual visual and audible alarms, 110 of them, were triggered. The diesel engines running #3 and #4 alternators shut down.
In-port and emergency generators started automatically and power for the steering system and onboard lighting was restored, but a power management system opened and closed a breaker 35 times in four minutes, banging loudly each time.
As vessel speed dropped due to power failures, the captain turned north to avoid grounding, thanks to power from the emergency generator. At the same time, cooling water pumps weren’t working and engines began to overheat.
At 12:58 p.m., the master dropped anchor to prevent the ferry from drifting into the beach.
By 14:19, the ferry has been towed by the Lindsey Foss and the Pacific Knight to the Bainbridge Island terminal where all vehicles and passengers were offloaded.
This timeline and the many other details about the power failure that day are all included in the recently released Board of Inquiry report that can be found here. It’s fascinating reading.
In the end, the board concluded that “the root cause was a feature of the propulsion control system, namely the phase loss relay, which allowed the #4 alternator circuit breaker to connect or close when it should not have been allowed to energize the circuit, causing loss of propulsion.”
After eight months of inspecting and rebuilding the diesel-electric propulsion system, the Tacoma finally returned to service on March 28.