The Marine Accident Investigation Branch (MAIB) of the UK has issued a safety alert following the failure of a capacitor and explosion in the aft harmonic filter room aboard Cunard Line’s QM2 in September.
The QM2’s alarm logs were found to contain early warnings about the impending failure some 36 minutes before the accident, according to the MAIB. However, as the vessel’s alarm system regularly logged more than one alarm every minute, this information was not seen and was hence not acted upon. A current detection warning system was also found to be non-functional, despite the ship having a history of harmonic filter capacitor failures.
The safety alert made the following recommendations:
*Protection systems for critical equipment must be “fail safe” and should be thoroughly tested at regular intervals. In particular, the harmonic filters with current imbalance protection systems should be thoroughly checked by competent personnel.
*Awareness of the damaging effects of harmonic distortion needs to improve throughout the marine industry.
*Regular monitoring of electrical networks should be undertaken to provide early warning of deterioration.
*Procedures need to be established to manage distortion levels in the event that power conversion equipment and harmonic mitigation equipment fails.
*Machinery alarms should be regularly reviewed and prioritized so that the risk of the watch keeper being overloaded with too many alarms and not notice those from critical systems can be minimized.
Following the Sept. 23, 4:25 am explosion, all four of the podded propulsion motors of t he QM2 shut down and a few seconds later the ship suffered an electric blackout. Thick black smoke was also seen coming from the aft main switchboard room. Fortunately, according to the MAIB, the QM2 was clear of navigational hazards and no one was injured.
The crew was able to restore some electric power and by 5:23 the ship was underway powered by two propulsion motors.
The MAIB said that subsequent inspection of the aft harmonic filter showed that one of its capacitors had failed catastrophically and another had developed a severe bulge.
The QM2 has had a history of harmonic filter capacitor failures, at an average rate of one per year, according to the MAIB. Although the exact cause of the failure could not be determined, it was concluded that capacitor degradation was probably caused by a combination of transient high voltage spikes due to frequent switching operations and occasional network overvoltage fluctuations. The deterioration had not been detected because there were no internal fuses or pressure relief devices, and continued until it failed.
Although the aft harmonic filter circuit breaker disconnected it from the rest of the electrical network to isolate the electrical fault, the disruption probably led to the loss of propulsion and the blackout, the MAIB concluded.
The only protection against the catastrophic failure of the capacitors was a current imbalance detection system. It consisted of a current transformer, which was connected to the capacitor circuit. After the accident, the transformer’s windings were found to have failed. The MAIB stated that there had not been any alarms on this part of the system for years, and it was likely that the imbalance detection system had not work for some time.