My thanks go to Simon Axcell for providing the information outlined below.

The vessel in question is powered by 4 3800KW engines driving through clutches and gearboxes to two shafts.

During the night the Duty Engineer responded to a L.O. filter differential alarm on the No3 main engine. He changed over to the off line unit of the duplex filter and after witnessing the alarm clear he returned to his bunk.

He was awoken again by a second UMS alarm approx' 50 mins later which was once again the same filter differential alarm.  As soon as he entered the engine room to investigate further he saw smoke eminating from the crankcase relief doors combined with sparks and dense black smoke coming from the vicinity of the Pnuemaflex type air operated clutch between engine and gearbox.  He immediately stopped the engine and called all engineers by hitting the panic button.

Upon investigation into the crankcase, several socket head M24 mm screws were found fractured lying on the grid above the sump.  White metal could be seen splayed out from the Flywheel support bearing and the No7 main bearing.  Debris and white metal fragments were found in the L.O. filters. and Water was seen running down the liners from No 6 A & B banks.

The vessel was due in port the same day, so on arrival it proceeded to a repair berth for further investigation and repairs.

As you can see from the pictures, what greeted us when we removed the split timing sprocket was unexpected to say the least.

Over time, the split sprocket which drove the timing chain, (due to not being correctly tightened down in a previous overhaul) had started fretting causing micro-seizure.  The pitting caused by the micro-seizure caused a stress point on the crank pin which led to a fracture which unfortunately propagated causing the crankshaft to shear. The misalignment of the crank then wiped the flywheel bearing and the No 7 main bearing.  The crank was then so misaligned that the inner and outer elements of the clutch meshed, burning out the clutch pads/rubber support rings which nearly instantly led to metal/metal contact causing a small fire.  White metal and debris from the two damaged main bearings blocked the oil ways in the con-rods of the two end units causing them to overheat and seize also cracking the liners.

Close up of split chain drive sprocket showing evidence of fretting. The M24 cap screws found on the sump grid were responsible for bolting the sprocket to a flange on the crankshaft.  The sprocket was held in place on the fwd end by a two piece strap arrangement.

The photo opposite shows the fracture. Note the flange onto which the timing sprocket was bolted. Close up of fracture

Repair involved removing the entablature, renewal of crankshaft and bearings, replacement of damaged liners and running gear and repair of clutch.