With the advent of new naval ship types, interest in reducing the structural weight and lowering vertical center of gravity (VCG) while meeting military safety requirements has increased dramatically in recent years. Ship structural optimization is usually a mixed discrete continuous design problem constrained by buckling of structural elements and material strength, and involves the optimization of a large number of variables (continuous/discrete) such as plate thickness, scantlings of stiffeners and frames, and the (discrete) number of stiffeners and frames. Further complication arises when the structure is constrained by buckling and strength under compression and subject to practical design rules. Traditionally, safety measures and VCG are formulated as a set of constraints of the weight optimization problem. Inclusion of safety measures and VCG, not only as design constraints, but also as design objectives exposes multiple design solutions (non-dominated designs), which facilitate the important design decision making of the objective tradeoff for a given investment. This paper presents a method to optimize hull girder cross section scantlings using three objectives: structural weight, vertical center of gravity and structural safety. To ensure that structural safety meets US military standards, Naval Vessel Rules (ABS) and SPECTRA are used for the limit state assessment and design load respectively. The structural response and working stress are computed directly using 3D finite element analysis. An example of optimizing a cross section of a 150 meter frigate is presented. The numerical results show that the proposed method is very useful for performing ultimate strength based ship structural optimization with multi-objectives, namely minimization of the structural weight, maximization of structural safety and lowering vertical center of gravity. The hull girder ultimate strength is obtained by nonlinear progressive hull girder strength analysis tools, such as ALPS/HULL and ProColl. The results show that the hull girder ultimate strength usually improves as a result of optimizing local panel scantlings.

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