The effect of underwater explosion on a mine countermeasures vessel: structural response and material design

In the modern geo-strategic scenario, characterized by the presence of sophisticated mines as well as rudimental and cheaper ones, the Italian Navy decided to design a New Generation Mine Countermeasures Vessel, in which the historical capabilities of minesweepers must be enhanced, considering the necessity of holding unmanned systems. In particular, they must survive in case of an underwater explosion event, according to some design criteria and corresponding limit states. Nowadays, the continuous increase of using modern numerical methods, such as Finite Element (FE) methods, allowed by the evolution of the computational capabilities of computers, has strongly influenced the shock design against underwater explosions. However, the complete numerical simulation of the phenomena in issue is even nowadays a matter of research, and it requests the comparison of numerical results with experimental data and theoretical formulations for validation. The main target of the thesis is to provide suitable methodologies for the ship design phase to assess the strength of naval structures in composite material against non-contact underwater explosions, thus reducing the vulnerability of the New Generation Mine Countermeasures Vessel. The experiments realized for this purpose were conducted in the Naval Support and Experimentation Centre’s (CSSN) laboratory in La Spezia and allowed validation of a set of increasingly complex numerical models. At first, the shock phenomenon involving ship structures only is analysed. Some reliable guidelines to perform a dynamic calculation using the finite element method, not yet available in the literature, are provided. Then, the dynamic response of New Generation Mine Countermeasures Vessel composite structure is studied. An experimental and numerical method to characterize the shock transient response of naval E-glass biaxial laminates is presented. This procedure opens for relatively straightforward material characterization, limiting the need for complex experimental shock trials to a few selected laminates. It can be applied in the design of minesweepers’ structures to select the proper stacking sequences of laminates and to design adequate composite materials. Finally, the effectiveness of different numerical approaches to predict the effects of Fluid-Structure Interaction caused by non-contact underwater explosions on minesweepers hull structures is verified. In particular, a numerical methodology, in which the Volume of Fluid approach is pursued, provides the most realistic results when comparing analytical theory, numerical models and experiments. This modelling strategy includes the simulation of the complete phenomenon, in which structures, water and vapor of cavitation are included. Eventually, it can be considered a valuable and cost-effective tool for the design of composite structures subject to non-contact underwater explosions.