HYDROELASTICITY OF VERY LARGE CONTAINER SHIPS ANALYSED BY COUPLING OF 1D STRUCTURAL MODEL AND 3D HYDRODYNAMIC MODEL

摘要

Some new results on the hydroelasticity of very large container ships, achieved after presentation on the RINA conference on Design & Operation of Container Ships in 2008 [0], realized within EU FP7 project TULCS (Tools for Ultra Large Container Ships), are presented. An advanced thin-walled girder theory based on the modified Timoshenko beam theory for flexural vibration and analogically developed torsional beam theory, is used for formulation of the beam finite element for analysis of coupled horizontal and torsional ship hull vibrations. The model includes bending, shear, torsional and warping stiffness, and rotary inertia. Special attention is paid to the contribution of transverse bulkheads on the open hull stiffness, as well as to the reduced stiffness of the short engine room structure. Also, distortion of transverse bulkheads at transition from open to closed ship cross-section is analysed. Another problem related to the ship hydroelasticity is proper definition of restoring stiffness of rigid and elastic modes. Two definitions are considered: consistent one, which includes hydrostatic and gravity properties, and unified one with geometric stiffness as structural contribution via calm water stress field. The finite element formulation of the restoring stiffness is presented. Hydrodynamic model, based on the modal superposition method, is shortly described for zero ship speed as the simplest case. The problem is solved by the BV software HYDROSTAR for real ship speed. Beam sectional displacements are transferred to the wetted surface in order to determine modal hydrodynamic forces: damping, inertia and wave excitation. 1D finite element model is verified by comparing dry natural frequencies and modes with those of 3D FEM analysis performed by NASTRAN for an 11400 TEU container ship. Complete hydroelastic response for the same vessel is determined by coupled 1D structural model and 3D hydrodynamic model as well as for 3D structural and hydrodynamic model. The obtained results agree very well. In addition fatigue of structural elements exposed to high stress concentration is considered.