| Because the structure design of a ship is one of the most important procedures in the ship designing and the midship is one of the most important parts of the hull, proper structure design of the midship is the problem that all engineers must face. In this article Equivalent Thickness Method and Genetic Algorithm (GA) are applied in the optimal designing of the midship of several kinds of conventional oil tankers, based on the requirements of <> (1998) and its Notes No.31.3. Significant weight savings and remarkable design improvement are achieved compared with the current design practices.Equivalent Thickness Method has been applied extensively in the field of ship structure design for its relatively simple principle and easy computation. In this article it is employed in the direct calculation of the cross-sectional longitudinal member designing, and make the structure weight lighter, which satisfy the requirement of longitudinal strength, local strength, shearing strength, buckling strength and building engineering. Genetic Algorithm has proved to be a modern optimize method in the past 20 more years, especially successful in the complex global optimum problems. It is introduced to solve the mixed variable problems in optimizing the cross-sectional transverse members. Computing examples of three different ships in this paper show that the Genetic Algorithm has better global search ability and faster computing speed in nonlinear multi-variable and multi-constraint optimization design of midship structure. Finally the three dimensional finite element model is created and calculated by the advanced CAM software NASTRAN to verify the strength of the structures designed by the methods mentioned above.Furthermore, the author also writes a program based on the principles above, which is powerful and has convenient user interface. In the period of initial design, it will take less time to obtain optimized midship structure by using the program developed in this paper.
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