A simplified fluid-structure interaction model for underwater shock loading of naval vessels

At the TNO Center for Maritime Engineering several tools are available for the analysis of naval vessels loaded by underwater shock. These tools have been developed in close cooperation with the Royal Netherlands Navy, and range from high level codes (very global models) based on standard pulse shapes, via medium level codes using a beam model and a gas bubble loading model to analyze the whipping behavior to low level codes utilizing detailed FEM-models of both naval vessel and the surrounding water. Another example of such a code combining both detailed FEM-models and measurement data of full-scale shock trials is the Optimal State Estimation Method, see ref. [1] and [2], “The Optimal State Estimation Method, A Tool To Integrate Full Scale Shock Trial Measurement Data And Numerical Models” as presented at the 70th Shock and Vibration Symposium, New Mexico, USA. The tools mentioned above will be briefly discussed. Based on this survey, the need was felt to develop an intermediate level tool which is able to analyze a full three-dimensional model of a naval vessel loaded by underwater shock but with reasonable CPU times such that in the design stage several concepts can be analyzed with reasonable accuracy. To achieve this, the surrounding water is excluded from the FEM-model, and the underwater shock loading is described at the wet hull of the vessel directly. This paper discusses the development of this so-called Simplified Interaction Tool (SIT). Aspects covered are incoming shock wave, reflections, hull and bulk cavitation, reloading and loading caused by the gas bubble expansion. Responses obtained with the SIT method will be compared with measurements from the full-scale shock trials of the RNlN’s M-frigate. The effects of structural modifications on the shock levels and on the attenuation of shock as found for higher deck levels will be briefly discussed.