Blackout during a bow-thruster start in a narrow channel
A 2,800 TEU container ship is making 8 knots in a narrow, buoyed channel. Two 1,000 kVA diesel generators are running in parallel on a 450 V, 60 Hz main switchboard; a 600 kW bow thruster is started to check swing. Immediately, switchboard frequency dips, the preferential trip operates, and both generators trip, causing a blackout. The emergency generator starts and reaches rated speed, but its circuit breaker does not close, so emergency lighting remains on battery only. Steering is lost. The channel is about 0.6 nautical mile wide; the vessel has about 4 cables of safe water to the nearest bank and is 1.5 nautical miles from a bridge. The bridge requests immediate power and steering. The engineer on watch must act before the pilot’s next instruction, and the cause is not yet known.
1. What immediate sequence would you follow in the first 10 minutes, and why that order?
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First 10 minutes:
- Acknowledge blackout to bridge; confirm main engine has tripped or is without auxiliaries. The bridge should take way off, prepare anchor, and use manual/emergency steering if available.
- In the engine room, confirm the emergency generator is running. If the breaker does not close, check the emergency board voltage, AVR, breaker closing supply and undervoltage coil; attempt one manual close only after confirming voltage and frequency are correct.
- Start one main generator in black-start condition, close to main switchboard, then restore essential auxiliaries in priority: steering gear, main lube-oil and cooling-water pumps, fuel supply, then main engine starting air.
- Do not reset all trips blindly; check for fire, smoke, or a short circuit first.
- Communicate every step to the bridge; ask for a safe area to anchor if propulsion/steering is not restored quickly. The priority order is control of the ship first, then power, then diagnosis.
2. Analyse the probable cause of the blackout and list the checks needed to confirm it.
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The thruster start is the trigger, but both generators tripping means the protection or load-sharing response failed. Checks:
- Alarm and event log: which trip came first-undervoltage, overcurrent, reverse power, or frequency? A large thruster start causes a voltage and frequency dip; if the preferential trip did not shed enough load before the dip, both generators may trip.
- Generator loading before the event: if one set was near its limit and load sharing was unequal, the step load could overload it. The 600 kW thruster is 30% of the 2,000 kVA installed capacity, but starting current can be several times rated for a short period.
- Governor and AVR response: slow governor response causes frequency dip; AVR fault causes voltage collapse.
- Preferential trip settings and thruster interlock: confirm the thruster is inhibited above a set generator load or that non-essential load is shed first.
- Emergency generator breaker: a failed close may be a control-supply fuse, AVR, or breaker spring/undervoltage coil, not the generator itself. Do not blame the thruster alone; the system should ride through a normal start.
3. What preventive actions and tests would you require after this event?
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Preventive actions:
- Review and test the preferential trip schedule and thruster start interlock; confirm the load limit is set with realistic headroom.
- Check load sharing and governor settings; carry enough generator capacity for thruster starts and ensure the second set shares load before starting.
- Test blackout recovery and emergency generator breaker closure at intervals; include manual closing and emergency steering in drills.
- Record the event, alarm log, settings and repairs; notify the company technical department and consider class notification if protection settings are changed.
- Train the watch team on priority restoration and bridge communication. The key judgement is that a blackout during manoeuvring is a system-design and procedural issue as much as an equipment failure; restoring power without finding why the protection operated invites a repeat.