As the demand for modern ships grows to meet greater reliability, fuel efficiency, and economy, the interest in reducing structural weight while enhancing safety has also increased. Ship structural optimization is usually a mixed discrete-continuous design problem constrained by buckling and material strength, and involves the optimization of a large number of variables such as (continuous/discrete) 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. Although 3D finite element analysis are widely used for full ship structure stresses and limit state analysis, their application for structural optimization is still prohibitive due to the computational cost. In this paper a multi-level heuristic based multi-objective optimization is formulated and used to optimize a large and complex thin-wall structure on the basis of weight, and safety. The optimization procedure has three levels: (1) For each stiffened panel, a heuristic based multi-objective optimization method is used to minimize its weight and maximize its safety, and a Pareto front is obtained. The ALPS/ULSAP ultimate limit state criteria, which is parametric formulated, mesh free, computational efficient, and is able to predict six different failure modes for a stiffened panel, are used to formulate the safety objective function. (2) Use the Pareto front of each panel as a gene pool to optimize the hull girder global properties, such as hull girder cross section area moment of inertia and vertical center of gravity (VCG). (3) Minimize the difference between the design variables from the local optimization and the global optimization by iterating 3D finite element analysis of the full ship. An example of optimizing the longitudinal structures of a full cargo hold of a 200,000 ton oil tanker is presented. The numerical results show that the proposed method is very useful to perform ultimate strength based ship structural optimization with multi-objectives, namely minimization of the structural weight and maximization of structural safety.

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