Dynamic Optimization Of Ship Grillages

Structural dynamic optimization is an advanced subject in the field of engineering structure design at present, concerned with many different subjects. The dynamic optimization researches have important theoretical significance and practical value. The vibration of marine structures could be reduced by optimizing the dynamic characteristics of these structures. And the dynamic displacement and stress can also be decreased. Dynamic optimum design of structures is an effective method to improve the dynamic capability and ensure it working safety and reliably.This study investigates the dynamic characteristics and response optimization of ship grillage under several constraints considering practical engineering requirements. Besides, according to the metal sandwich structures which have been widely used in marine engineering, a new theoretical method has been proposed in the thesis. Based on this method, an effective dynamic optimum design of corrugated-core grillages has been carried out. The main works presented in the thesis are as follows:(1) In the dynamic characteristics optimization of ship grillage, a novel method is proposed to ingeniously transform the structural layout parameters into substructure length parameters, while the sensitivity of natural frequency to structural layout parameters can be obtained through the sensitivity of element stiffness matrix and mass matrix with respect to length parameters. Besides, the structural reanalysis algorithm is introduced into the process of optimization iteration to reduce the calculation burden. By using this new frequency analysis method and the improved conjugate gradient method, the discrete optimal solutions which satisfy the constraint conditions could be obtained easily.(2) A structural dynamic optimization method which based on the grillage dynamic stiffness (GDS) algorithm and discrete scheme database has been presented. And the equivalent static load method together with the GDS algorithm is further used to obtain dynamic displacement and stress distributions of the grillage system with isolators. With the help of which, good calculation accuracy and higher calculation efficiency are reached compared with the traditional FEM method. A scheme database which consists of all possible T-shape girder design schemes satisfying section design requirements is built and further used to get an optimal grillage design scheme meeting the practical engineering requirements.(3) Aiming at the metal sandwich structures, this thesis presents a novel semi-analytical method based on the superposition of the global displacement field and local displacement field. The discrete geometric nature of the sandwich structures has been taken into account and the bending, global buckling, and free vibration characteristics have been calculated quickly and efficiently. Results from the proposed method agree well with those from detailed finite element analysis and the structural stress fluctuation can be captured with sufficient accuracy. This method lays a good foundation for the dynamic optimization of metal sandwich structures and can be extended to sandwich structures with other kind of cores.(4) The bending, global buckling, and free vibration properties of corrugated-core grillage are analyzed and the structural dynamic characteristics have been optimized using the sequential quadratic programming algorithm. The advantage of the sandwich structures contrasted with the solid plates has been validated by the comparison of the mechanical characteristics of these structures. The effects of some geometrical parameters of the corrugated-core grillage on the mechanical properties are also discussed in the analysis.In this thesis, some novel theoretical methods are proposed and the dynamic optimization of ship grillage structures is carried out efficiently by utilizing these methods. These methods and research findings can be employed to supply new ideas and approaches for improvement of structural dynamic optimization and have reference value for the further research and engineering design.