Question 67 OSE02 - Assistant Engineer - OSV

The Correct Answer is D **Explanation for Option D (Correct):** Option D is the required and safest procedure when a large diesel engine has experienced a safety shutdown due to excessive crankcase pressure (often called an oil mist detector trip or an explosion relief valve activation). This type of shutdown indicates a severe internal engine fault, most commonly an overheated bearing, piston seizure, or piston ring blow-by, leading to localized 'hot spots'. These hot spots vaporize the lubricating oil into an explosive oil mist within the crankcase. Opening the crankcase immediately (or soon after shutdown) introduces a fresh supply of oxygen into the enclosed space, which can mix with the superheated oil mist, leading to a violent and potentially lethal secondary explosion (a crankcase explosion). Safety regulations (such as those outlined by classification societies and engine manufacturers, like MAN B&W or Wärtsilä) mandate a cooling-off period—typically a minimum of 20 minutes to 2 hours, depending on the engine size and severity—to allow internal temperatures to drop below the auto-ignition point of the oil mist, thereby ensuring that the environment inside the crankcase is stable and safe for inspection. Only after this mandatory cooling period can the engineer safely open the crankcase, inspect for damage, correct the fault, and prepare for restarting. **Explanation of Incorrect Options:** * **A) Immediately open the crankcase to make the necessary inspections to determine the cause of the high crankcase pressure safety shutdown:** This is extremely dangerous and strictly prohibited. Opening the crankcase immediately introduces oxygen, significantly increasing the risk of a secondary, catastrophic crankcase explosion, which can cause severe injury or death to personnel and major damage to the vessel. * **B) Immediately restart the engine and monitor the crankcase pressure to verify the cause of the shutdown:** Restarting the engine without identifying and correcting the underlying severe fault (the hot spot) will immediately exacerbate the problem, causing further, potentially irrecoverable damage to internal components and significantly increasing the likelihood of an immediate and uncontrolled crankcase explosion. The engine must not be restarted until the cause is found and corrected. * **C) Allow the engine to cool off for two minutes, then restart and monitor the crankcase pressure to verify the cause of the shutdown:** Two minutes is wholly insufficient for a large marine diesel engine to cool down enough to eliminate the threat of an oil mist explosion. Furthermore, restarting the engine before fixing the mechanical fault remains a highly dangerous and damaging action.