Structural dynamic response analysis on marine vessels with time-varying mass property and large-scale discontinuous hull structures

Abstract

For some types of special-purpose marine vessels, such as dredgers, the variability of mass in onboard stemming from their operational dynamics causes a distinctive challenge. This variability can swiftly induce unforeseen loads within short time periods during their unloading working conditions, leading to unexpected structural responses and potential fatigue damage. Specifically, in the case of dredgers, the additional loads arising from variations in mass during specialized operations raise significant concerns. What adds to the complexity is that these challenges often escape effective consideration during the design phase. This dynamic interplay of mass variation, occurring within short time intervals, has the potential to compromise structural integrity of the vessel and underscores the need for a more accuracy approach to design and structural analysis in the realm of specialized marine technology. This thesis stands as a pioneering endeavour, proposing a innovate mathematical and numerical model for structural dynamic analysis of variable cross-section hull girder with time-varying mass characteristics subjected to complex operational and sea environmental loads. At its core, the proposed model leverages the modified Euler-Bernoulli beam theory to accommodate variable mass functions and employs a semi-analytical approach for the vibration characteristics analysis in the variable cross-section beam. The loads acting on the hull girder are composed of hydrodynamic loads, engine excitation loads, and propeller excitation loads respectively defined in the dynamic model. Furthermore, an improved Kane’s dynamic equation is established and integrated into the mathematical and numerical model, tailored for time-varying mass systems, serving as the primary dynamic module solver. Dynamic results calculated by the proposed mathematical and numerical model can be transferred into three-dimensional finite element model of the target vessel for the further structural analysis in ANSYS to obtain strength and fatigue assessments. A customized programme, written in FORTRAN language, is developed based on the proposed mathematical and numerical model. In addition, some verification results and user-defined case studies are given in this thesis. The semi-analytical approach for vibration analysis of various cross-section Euler-Bernoulli beam has been verified with FEA results. The varying wet surface and trim characteristics of the ship hull within a short period are also taken into consideration via dividing wet surface into 10 shifting waterlines and load cases under variable mass working conditions. Hydrodynamic results pre-calculated by SESAM are inputted and read by the program for further calculations. Finally, dynamic results including displacement and angular responses of each predefined rigid cross-section in the hull girder are calculated by the programme, which have been used for further FEA to achieve detailed structural assessments. Dynamic response results, including displacement and angular responses of each predefined rigid cross-section in the hull girder, are calculated by the customized program, which has been used for further FEA analysis to achieve detailed stress and deformation structural assessments. The key findings of this research highlight the significant impact of mass variability on the dynamic responses of marine vessels. Structure was found to be more sensitive to oblique wave impacts, necessitating careful design considerations. The study also verified the accuracy of the semi-analytical approach by comparing it with FEA results, demonstrating its efficacy for vibration analysis of variable cross-section beams. Furthermore, the integration of pre-calculated hydrodynamic analysis results from SESAM into the customized program facilitated a comprehensive evaluation of the vessel’s dynamic behaviour under variable mass conditions. These advancements contribute to a more precise and efficient method for assessing and ensuring the structural integrity of specialpurpose marine vessels during their design and operational phases. The proposed mathematical and numerical model can be used in the design stage for marine vessels who have time-varying mass features to evaluate their special structural responses during loading or unloading operations.