Development of guidelines allowing to predict the contribution of the superstructure to the hull girder strength

Abstract

Longitudinal vertical bending of a ship consists of stress and strength transfer between hull and superstructure. This sharing of stress and strength is a very complex phenomenon, namely in the case of multiple deck vessels, which is being studied by naval architects for a long time. On passenger vessels for architectural reasons, structural connections between decks are reduced to the minimum in passenger and public area. In addition, due to design specificity such as recesses in way of life boat, openings in side shell (windows), openings in longitudinal bulkheads, all superstructure tiers are not contributing similarly to the vessel hull girder strength. To take account of this phenomenon, one can introduce bending efficiencies of superstructure tiers. Bending efficiencies are percentages given tier by tier and used as follows: - When calculating hull girder transverse section characteristics, contribution of one given superstructure tier is weighted by its bending efficiency. - When evaluating the hull girder stress in way of a given tier, the stress is reduced by multiplying it by the tier bending efficiency. It is to be noted that bending efficiencies have no real physical meaning but are very useful to take into account the specific global behavior of a vessel but still using usual beam theory to assess vessel strength. Bending efficiency of each superstructure tier is linked with global behavior of vessel. Hence, they cannot be evaluated without a complete finite element model (FEM) of the vessel. But, FEM is very time consuming and thus, very prohibitive namely for small ships. The main purpose of this research is to propose guidelines allowing prediction of the contribution of the superstructure to the hull girder strength. A standard investigation finite element model is made of two superimposed box girders. The effect of different parameters in hull-superstructure interaction were investigated on this standard model. The investigated parameters are: ratio of superstructure length to hull length (rL), ratio of superstructure side openings to total lateral area (rS), location of superstructure side openings and ratio of deck openings to total deck area (rD). Based on these investigations, a new expression for bending efficiency (ν) is developed. This new formula is more accurate for finding hull girder normal stresses. Besides, guidelines were developed for implementation of bending efficiency in order to find the hull girder normal stresses of a passenger vessel.