Study for Preventing Hull Cracks Caused by Resonance of the Diesel Engine Exhaust Pipe

Recently, quality concerns have emerged in a specific ship, causing cracks in the engine exhaust pipe and hull. Since the vibration of the diesel engine could transfer to the hull through exhaust pipes, resilient mounts are applied generally similar to the engine mount. When the base line of the ship could be inclined according to the sea state, excessive load could be concentrated on the exhaust pipe connecting part on the hull. However, the structural analysis results revealed that the stress on the hull where the crack occurred in the exhaust pipe was not as high as that at other points when such ship motion occurred, determining that this was not the cause of the crack. In order to define the root cause of the crack on the hull, vibration measurement was performed and it could be revealed that the exhaust pipe connecting to the hull experienced significant vibrations at the particular rotational speed. Moreover, vibration analysis results indicated that the impeller blade pass frequency coincided with the natural frequencies of one of the exhaust pipes at that rotational speed, resulting in resonance. Therefore, by increasing the elastic mount stiffness, the natural frequency was changed far from the impeller blade passing frequency in the operation rpm range, and the exhaust pipe vibration was confirmed to be greatly reduced as the resonance phenomenon disappeared.