ABS Says Large Ship Hull Deflections Impact the Shaft Alignment

Monday, June 07, 2004
Larger ships, driven by the latest generation of very high-powered diesel engines, are particularly susceptible to alignment problems on the bearings supporting the propulsion shafting as well as the main engine bearings. Hull girder deflection is one of the major reasons for the increased number of propulsion shafting and the main engine bearing failures and damages that are being reported. Hull deflections are quite pronounced on large vessels where discrepancy between the flexibility of the hull and the shafting results in high sensitivity in the alignment dependent upon the vessel’s loading condition. Responding to industry need, ABS experts have investigated the hull deflections effect on the shaft alignment, conducting measurements on a large number of vessels and using the information to develop a state of the art alignment optimization software. ABS has also released Guidance Notes on Propulsion Shafting Alignment to provide detailed information on propulsion shafting alignment design procedure and to improve the design review process and survey. ABS research indicates that the alignment related damages are mostly attributable to inadequate analysis and design of the alignment, inadequate construction practices and lack of proper guidance. Ship hull structures have become more flexible with scantling optimization and with an increase in ship’s length. As the powering requirements increased with the increase in the ship’s size, shafting diameters become larger and the shafts stiffer. This is particularly true for VLCCs, ULCCs, large bulk carriers and containerships. As a result, the propulsion shafting alignments are increasingly more sensitive to disturbances affecting vertical offset of the bearings. These disturbances primarily result from hull deflections and temperature change. As the alignment analysis is the first step in the alignment process, it is of paramount importance to clearly define the criteria and determine a robust alignment that will have a sufficiently low sensitivity to disturbances affecting the propulsion shafting and the main drive. Accounting for hull girder deflections is one of the most important issues in that process. However, hull deflections are not of constant magnitude, but rather a function of different vessel loading conditions as well as the sea conditions affecting the vessel on passage. To define satisfactory alignment for all expected operating conditions, design engineers need to optimize the bearing offset to satisfy all expected disturbances. Advanced knowledge of hull deflections is of critical importance in this process. ABS conducts hull deflection measurements to obtain information on expected behaviors of the hull structure under different sea conditions and inputs the data into the shaft alignment optimization software. The ABS Guidance Notes on Propulsion Shafting Alignment offer technical guidance based upon the class society’s extensive experience and data collection that has allowed its naval architects and engineers to analyze shaft alignment and evaluate tail shaft bearing conditions using deflection data derived from the finite element modeling of the hull structure with ABS’ proprietary ABS Shaft Alignment software. The new guidance notes outline the analysis and design process for proper shaft alignment, indicating plan review procedures as well as the ABS survey and inspection criteria of shaft alignment.
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