engleski

The contribution of the engine room structure to the hull stiffness of large container ships

Very large container ships are rather flexible due to their large deck openings. Hydroelastic stress analysis is therefore required as a base for reliable structural design. In the early design stage, the coupling of the beam model with a 3D hydrodynamic model is rational and preferable. The calculation is performed utilizing the modal superposition method, so natural hull modes have to be determined in an appropriate way. Consequently, the advanced thin-walled girder theory, which takes the influence of shear on both bending and torsion into account, is applied to calculate the hull flexural and torsional stiffness properties. A characteristic of very large container ships is the quite short engine room, whose closed structure behaves as an open hold structure with a shear centre outside the cross-section, very close to that of the open section. As a result, torsionally induced horizontal bending is negligible, while the distortion of the cross-sections appears as a new problem. The task is solved by an energy balance approach that enables the use of effective stiffness. Hence, the effect of interior decks is taken into account by increasing the torsional stiffness of the open cross-section within the engine room domain. The procedure is checked by the 3D FEM analysis of a ship-like pontoon. Such a modified beam model of the engine room structure can be included in the general beam model of a ship hull for the need of hydroelastic analysis, where only a few first dry natural frequencies and mode shapes are required. For practical use in the preliminary design of ship structures, the simplicity of the beam model presents an advantage over 3D FEM models.

Container ship; Engine room structure; Torsion; Thin-walled girder; Analytical solution; FEM; Hydroelasticity

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