The Theoretic Studies On The Propulsion Shafting Alignment Of Ultra-large Vessels

The alignment of the propulsion shaft is the vital part of the design and installation of the shafting. A misalignment of the shaft is highly harmful since it may affect the vibration behavior and the strength of the shafting, or even more destroy the main engine and the gearbox. Early in the 1960's, the foreign scholars did some researches on the theory and the testing analysis of the shaft alignment and got some achievements, which can meet the need of the design of the ship at that time. But with the appearance of the newly Very Large Crude Carrier (VLCC) and Ultra-Large Crude Carrier (ULCC), the hull becomes relatively more flexible when compared with the rigidity of the shafting, it made the alignment highly difficult and complex. This the traditional alignment method can't meet the requirement of the shafting alignment any more. As a result, the new calculation method, testing measure and installation checking criterion of the shafting alignment are worth studying now.After studying lots of the literatures about the design, manufacture, installation of the propulsion shaft home and abroad, comprehensively considering the hull deflection, propeller dynamic force, bearing rigidity, oil film rigidity, environment temperature, crankshaft model of the main engine and so on, the elastic alignment calculation model of the shafting of the huge ship is established. Meanwhile, the relative calculation methods, such as the finite elements method (FEM), the transition matrix method and the three bending moment method of the elastic alignment are discussed in detail. At last, a general shaft alignment calculation software, which satisfies all the needs of the huge ship's shaft alignment is developed.The main factors that effect the calculation model of the elastic alignment have been further discussed in this paper. ①Considering the feature of the stern-tube bearing of the huge ship, and on the base of the hydrodynamic theory of lubrication, a non-linear model of the stern-tube bearing is built, then an FEM approach of computing the pressure distribution of the oil film and the relationship of the main oil-film characteristic parameters are presented. ②According to the traditional calculation method of propeller's dynamic force, the surface element method is applied to calculate the propeller's dynamic force, associating with the calculationmethod of the aviation and space-flight propeller. The numerical result is closer to the experimental result. So it is more precise, convenient and can be used in the dynamic force calculation of the elastic alignment.(3)The optimum method of the elastic alignment is analyzed, then the calculation model of elastic alignment is established and the genetic optimum arithmetic is used in the elastic alignment. According to the calculating characteristic of elastic alignment, the genetic arithmetic and the optimum genetic model are improved, considering best choose technology, non-linearity rank choose mechanism and real number coding method. Compared with the standard genetic arithmetic, the improved optimum calculation is better in quality and efficiency.?According to the feature of the main propulsion shafting of the huge ship, an simplified calculation model, namely the equivalent shaft of crankshaft alignment calculation of the large low-speed main engine, is proposed firstly. Then the numerical result is gained by using this model and compared with the finite element numerical calculation result, the error is in the allowable range, which proves the correctness and feasibility of the approach. It meets the requirement of the elastic alignment and can be used expediently.(§)Compared all the checking methods used in the shaft installation, the advantages of strain-gage method are described. It can be used to check the shaft state constantly with the change of the rigidities. On the basis of the measured strain and the Riccati matrix method, the strain of the shaft and the bearing reaction can be checked, so as to satisfy the requirement of the shaft alignment by the iterative method.(6)Based on the previous studies, the elastic alignment software is developed. Then the elastic alignment calculation of one actual VLCC is carried out by using the software. At last some tentative checking rules of the elastic alignment are proposed in this paper.