Scavenging space explosion and Turbocharge damage due to cracked piston

What happened

The vessel was drifting during the night waiting for first daylight to board pilot. The main Engine was started once a while just enough to maintain position in a drifting area.
After the Main Engine was started after a while, the Duty Engineer noticed a high exhaust temperature on the outlet of one of the cylinders and the High Exhaust Temperature and Deviation Alarms activated. He informed the Duty Officer and Chief Engineer immediately. Upon investigation, during replacing fuel injector the oil noticed on top of the piston. As pistons were oil-cooled it was obviously that piston crown cracked. The engine crew conducted a piston pull and replaced a cracked piston crown with a spare one. At 10:00 Main Engine was ready to start again. At the same time observed that 3000L of LO missing from the ME sump tank. ER crew checked bilges, LO drain tank but missing oil was not found. Main Engine sump tank was refilled, Engine turned on air and started successfully. It was operating well at Dead Slow and Slow Ahead, but when it speeded up to Half two sudden explosions occurred internally. The Engine Room filled with white smoke, Fire Alarm activated and ER crew evacuated the area.
The Firefighters party was deployed to the Engine Room as per Fire Fighting plan to evaluate the situation. No open fire or high-temperature sources where observed. As no visual damage was found it was decided the cause of the explosions was ME safety valves, probably because of excessive cylinder oil lubrication supplied when preparing ME for starting. The Main Engine was started again and the vessel proceeded to the Pilot station. When Main Engine reached Slow speed it was noticed that it is unable to reach Half and Turbo Charger RPM was low. It appeared that internal explosion caused the turbocharger damage.
The vessel was directed to the safe anchorage area as she was unsafe for navigation to enter the port.

Root cause

The crew started an investigation of the explosions. It was observed a huge amount of oil in the Scavenging Air Receiver. The Receiver was not checked during the initial investigation of missing oil from the crankcase. The cause of explosions became clear at that point:

1. During increased speed operation of the M/E, the oil in the Receiver vaporized in the scavenging space and formed a combustible atmosphere.
2. The combustible atmosphere transferred into the cylinder via the scavenging ports.
3. The atmosphere self-ignited due to the high temperature when one of the pistons was at Bottom Dead Center. It was the first explosion.
4. The combustion wave in the cylinder propagated to the scavenging space filled with combustible atmosphere.
5. A second explosion occurred inside the scavenging space.

The Turbo Charger inlet grating was damaged by the explosion and pieces of broken piston ring entered the turbine. Impeller blades and nozzle ring where completely destroyed. The cause of this unfavorable sequence of damages was an old piston crown. It was reported by the crew unfit for service during last piston overhaul due to burnout of the crown top. But the Ship Manager insisted to dismantle the crow, turn it 90 deg and mount again in order to turn burn out areas from injection pattern. Obviously, this did not work and led to severe damages, vessel offhire for 7 days and huge spending on new Turbo Charger rotor, bearings, nozzle ring and service engineer.

Lesson learned and corrective actions

1. Never use parts beyond their service life stated by Maker.
2. Carry out regular maintenance and discard any part which already over or near its service life or limits.
3. Always conduct a full investigation of any malfunction. If the crew did observe missing 3000L of oil in the Scavenging Space before starting the engine, the Turbo Charged could be safe.