The effect of plate stiffeners of double bottom during ship grounding with large contact surface was analyzed in this paper, by numerical simulation technique. It is always taken for granted that the effect of the stiffeners on the plate can be neglected, but they should not be dealt with in such a simple way. The objective ship is a double bottom tanker, and four different finite element models are made. Five numerical simulations are designed for each model, with different indentations from 10 percent to 50 percent of the double bottom height. With the simulation results, two sets of comparisons are made. The purpose of the comparison is to demonstrate how much role the stiffeners play in the structural performance during the grounding, and whether the smeared thickness method can predict the role of the stiffeners adequately. It is found that the vertical and horizontal resistances of the cases with stiffeners are all larger than those cases without stiffeners quite a while, which can prove that the stiffeners do have obvious influences on the resistance of the double bottom structure. The difference for horizontal resistance is more obvious than that for vertical resistance. Besides, the value of resistance in the model with stiffeners can be nearly double of that in the corresponding case in the model without stiffeners, especially for those cases with small indentation, which means the stiffeners play an important role in the structure deformation in mild grounding course. Furthermore, it is also indicated that the effect of smeared thickness can not predict the role of the stiffeners adequately, due to the perpendicular disposal of the stiffeners on the plate is of crucial importance in the structural deformation mode. A conclusion is drawn that the influence of the stiffeners on the plate should be taken into consideration and the deformation mechanism of the stiffeners should be included in the simplified analytical method in future.

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