Research On Dynamic Alignment Of Marine Propulsion Shafting Considering Supporting System Characteristics

As one of the most important marine power train and equipments, marine propulsion shafting has to undertake complex dynamic stresses and loads. Therefore, dynamic shafting alignment plays a great role in the normal operation of both propulsion shafting and ship. Among the dynamic influences of shafting alignment, oil film characteristics of the bearings affecting the work state of shafting directly are shown to be the fundamental one. Ship hull deformations caused by ship loading, wave loads and environment temperature influence shafting alignment significantly. In addition, the phenomenon that the crankshaft deflection at the output end of main engine exceeds standard during shafting operation explains the great impact of dynamic excitation and loads on shafting alignment. In the thesis, oil film characteristics of the bearings and ship hull deformations are studied, calculated and applied to rational shafting alignment calculation, so that dynamic alignment of marine propulsion shafting considering supporting system characteristics is realized. Furthermore, crankshaft deflections are simulated and taken as a checking parameter to evaluate and optimize shafting alignment results, and meanwhile, as theoretical alignment parameter to supervise shafting alignment construction.In the thesis, the research on rational shafting alignment is carried out firstly to prepare for dynamic shafting alignment. According to the theory of Finite Element Method, basic essentials are designed and constructed, such as equivalent physical model of the shafting, external loads, bearing offsets, system stiffness matrix, system balance equations and etc. Then an accurate program for rational shafting alignment calculation is accomplished by Visual C++ and Matlab language. Compared with some authoritative alignment calculation cases, the validity and applicability of the calculation program are proved. Thereby, the calculation program lays a steady foundation for dynamic shafting alignment research.Focusing on the dynamic impacts of the bearings'oil film on shafting alignment, a dynamic shafting alignment method considering oil film characteristics is proposed. Based on the fundamental of journal bearing with hydrodynamic lubrication, the oil film features are simulated. The oil film pressure distribution and dynamic parameters of aft stern tube bearing are solved respectively with Finite Difference Method and Small Disturbance Method. Due to the behavior of oil film, the improved physical model of the shafting including oil film is constructed for dynamic alignment calculation. By establishing the relationship between the element stiffness matrix of oil film and system stiffness matrix of the shafting, the dynamic shafting alignment taking the bearings'oil film features into consideration is fulfilled. Taking 76,000 DWT product oil tanker as an example, the dynamic shafting alignment calculations considering oil film characteristics of the bearings are carried out, and the results reveal the active regulation function of oil film to shafting alignment state.In order to optimize the shafting alignment state with the effect of dynamic ship hull deformations, a dynamic shafting alignment method considering ship hull deformations is advanced. Based on the study of the ship hull deformations caused by ship loadings, wave loads and environment temperature, the finite element models of the whole ship are simulated, with ship gravity, buoyancy and environment temperature differences in some extreme conditions, as well as the elastic constraints of the seawater. The ship hull deformations are calculated accurately and the deformations of the double bottom are converted reasonably to bearing offsets by data fitting techniques. Thus the dynamic shafting alignment considering ship hull deformations is achieved. Specially, the method is applied for 76,000 DWT product oil tanker to analyze and conclude the changing regularities of ship hull deformations and shafting alignment under different ship loading, wave loads and environment temperature. Consequently, the measurements to optimize shafting alignment are worked out.Based on the above research, a numerical simulation method of main engine crankshaft deflections is presented to check and optimize shafting alignment state, and moreover, to guide shafting alignment construction. Both rational shafting alignment parameters and dynamic shafting alignment results considering supporting system characteristics (including oil film characteristics and ship hull deformations) are regarded as boundary conditions. The shafting finite element models are then constructed to simulate crankshaft deflections. By analyzing the influences of shafting alignment states on crankshaft deflections, the theoretical crankshaft deflections are adopted to evaluate and optimize shafting alignment state. Besides, the distribution of the crankshaft centerline and the relative position of the main bearings can be obtained according to the calculated crankshaft deflections, which will contribute to shafting alignment construction. Still taking 76,000 DWT product oil tanker as an instance, the numerical simulation method practiced at the design stage of shafting alignment is demonstrated to be feasible and scientific by comparing calculated values to measured data.