Modelling of excitation and response induced by impact : an investigation for slamming of mono- and multi-hulled vessels

Abstract

Waves, winds and currents can cause specific environmental effects that a marine structure has to withstand. Amongst these, wave action is the fundamental source of load on the marine structure. In order to ensure safety, operability, economy and design-life duration of a marine structure, theoretical estimates of wave loads and structural response play an increasingly important role in the overall design process. The interaction between a structure and a fluid medium is of great concern in numerous engineering problems, e.g., slamming of ships in rough seas, vibration of water retaining structures under earthquake loading etc. All these dynamic problems include the interaction, which takes place between the structure and surrounding fluid. It is of practical importance to estimate the effect of the induced fluid loading on the dynamic state of the vibrating structure. If the vibration takes place in a relatively low-density fluid, such as air, in comparison with the structural material, in most situations, the loading will have a comparatively small influence on the vibration. However, when the vibrating structure is in contact with a fluid which has a comparable density, such as water, the fluid loading which depends on the structural surface motions will significantly alter the dynamic state of the structure from that of the in vacuo vibration. In other words, the equations of structural and fluid motions are inexorably linked. Therefore, development, improvement and application of numerical techniques for analyzing such an interaction become one of the most important activities of naval architecture researchers. The following document is about the interaction mentioned above and particularly studied on the slamming issue and its main characteristic, transient excitation and response. A dry analysis is presented on simple beams, idealized SWATH ship as a preamble to a future wet deck slamming analysis and plates (unstiffened and stiffened). As the basis of subsequent harmonic and transient analyses, modal characteristics of each system is studied and in conjunction with the results obtained from these, responses on frequency and time domain are calculated in this document. In the following part of the thesis beams and plates are analysed under transient excitation, since this is the basis for modelling the excitation and response induced by slamming. Results are produced and compared both using theoretically established convolution method and ANSYS (transient analysis with full and mode superposition methods). Realistic stiffened plates and their equivalent flat plates are also studied and analysed in the subsequent sections. Difficulties encountered during the structural modelling (finite element modelling) are briefly outlined, with particular emphasis to the importance of the selection of appropriate finite elements.

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