Keywords: ship structure design, genetic algorithm, design under uncertainty, Monte Carlo simulation, mid-ship section design, optimization design.

Ship structural design starts with the selection of elements of the mid-ship section. This initial design phase is based on beam ship theory. The main hypothesis is that the ship behavior is a beam under global load (light weight and cargo load) combined with the hydrostatic load. This is the case of calm water structural design. To take into account the wave action, the classification societies indicate expressions to be added as an equivalent wave load to the main structure (bending moment and shear force). More complex methods can also be used to calculate the wave load on the structure. This method, based on a linear theory, includes the evaluation of the wave characteristics as a wave spectrum. The spectrum is in many cases defined by the mean wave period and the wave height.

Once the total load is determined the structural design search for the main longitudinal elements geometry (plate thickness and longitudinal stringers and girders) to resist the forces with an established and accepted safety level proceeds.

Although a deterministic design procedure has been used, this paper proposes an optimization procedure considering uncertainty in some variables and parameters. The case study used to highlight the procedure is a double hull tanker structural mid-ship section design. The optimization model with constraints searches for a safe and lighter mid-ship section. The loads and some structural elements are treated considering uncertainty.

The mathematical model is developed using the Excel software and programs written in the Fortran language. The optimization procedure taking into account uncertainty is handling by the Mode Frontier software. This software use the genetic algorithm method to solve the mathematical model and Monte Carlo techniques.

The paper presents the mathematical model developed for the case study and the solution found. Conclusions and future work are also highlighted at the end of the paper.

The method presented here in shows potential to handle optimization procedures with multi-objectives. Also, uncertainty was included and well considered by the approach using the Monte Carlo technique.

The case study shows the possibility of establishing a design procedure with many optimization steps. Also the design flowchart is included and understandable using the methodology suggested by the Mode Frontier software.

A complete ship design process should be testedion, to fully apply the optimization and design arrangement tools. The authors are going further and started a research topic to include more elements in the double hull tanker design.

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