New Approach to Recent Propulsion Shafting Alignment Taking into Account Variation in Bearing Offsets while in Service

概要

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Recently several main engine bearing damage incidents have been reported. They are generally said to be related to improper shafting alignments that fail to absorb bearing offset changes between different operating conditions. These damages tend to occur on ships with large draft variations from ballast to fully loaded conditions, such as VLCCs. In this paper, the authors propose a new scientific approach to propulsion shafting alignment taking into account variation in bearing offsets while in service, after conducting onboard ship measurements and FE analysis to determine the hull deflection between different conditions. The results show that static hull deflection arising from draft change can be predicted by FEM with satisfactory accuracy, but a significant dynamic deflection component that is considered to be related to ship motions in waves, and difficult to predict by analysis is also present. The new scientific approach includes a high accuracy alignment calculation model, an optimization procedure for longitudinal placement of intermediate bearing to minimize the sensitivity of shafting to bearing offset variations between different operating conditions, and the measurement and numerical estimation of hull deflection induced bearing offset changes as well as a way of taking into account these bearing offsets changes.