The cargo ship was enroute and during routine engine rounds, the oiler noticed some abnormal noise from the main engine. The oiler reported to duty engineer who came down from the control room to investigate the noise from the engine.
The aft exhaust valve cage assembly from one of the units blew out of the engine, damaging the rocker arm assembly and destroying the rocker cover. The assembly struck the lifting beam above the main engine and was deflected directly aft over the turbo-charger, narrowly missing the duty engineer, who had been inspecting a tappet knocking noise at a nearby cylinder head.
Location of impact above cylinder
The assembly landed on the bottom plates near CPP pumps. Jacket cooling water was gushing out of the cylinder head of the damaged unit, restricting direct access to the main engine controls. The duty engineer ran from the engine room to the engine control room and informed the bridge to use the emergency stop to shut down the main engine. The main engine was stopped within twenty seconds.
Sequence of Events:
- the exhaust valve cage assembly was blown out of the cylinder head, shearing one holding-down stud and stripping the thread out of the nut of the second holding-down stud;
- the rocker arm assembly was damaged beyond repair;
- the rocker cover was destroyed;
- the rocker gear bridge piece casting had a section of the holding-down base broken off and was beyond repair;
- the lifting beam gantry above the main engine had a significant dent;
- the alarm light and fluorescent light above the main engine were destroyed.
The immediate assumption was that the unit had hydraulically locked due to ingress of water into the cylinder liner and preparations were started to remove the unit for inspection and repair.
The company’s subsequent investigation established that the incident could be directly attributed to sea water entering the combustion space of the unit via the scavenge air system, hydraulically locking the engine cylinder, due to the failure of a sea water cooling tube on the charge air cooler, the failure of the scavenge air/water drains and the failure to check the drains’ condition during normal watchkeeping rounds.
Rocker cover fragments
The exact time of the failure of the sea water cooling could not be ascertained, but it is believed that it could have been leaking over a period of time. The company is of the opinion that a slug of water had been trapped behind the built-up sludge at the charge air cooler. This slug of water/sludge travelled along the charge air manifold, blocking the drain cocks and entered the cylinder. The volume of water entering the cylinder was greater than what could have been consumed in the combustion cycle. The quantity of water would have been minimal, as there was only 0.2mm bump clearance from the piston crown to the cylinder head. A full hydraulic lock with complete flooding of the unit did not occur, as in such a case greater damage to the piston, connecting rod and crankshaft would have resulted. The second engineer was in attendance, investigating an unusual noise coming from another unit as part of his normal watchkeeping duties. The failure occurred before the analysis of that problem had been completed.
Position of engineer when incident happened
The company has instigated the following preventative actions as a result:
- removal of current scavenge air drain cocks and replacement with ball valves with a bigger bore;
- fitting of drain cock clearing devices at each drain cock;
- further development of previous engine room rounds check sheet to provide regular and accurate inspection of plant and equipment against specific performance criteria;
- more thorough checking during watchkeeping rounds in accordance with engine room practice.
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