Global and local strength analysis in equivalent quasistatic head waves, for a tanker ship structure, based on full length and 2-3 cargo holds 3D-FEM models

Abstract

The main objective of the thesis is to provide the specific knowledge concerning the methods for global and local ship hull structure strength analysis, under equivalent quasi-static head wave loads. The numerical results have to stress out the adequacy of structural models, with different complexity levels, developed for ship hull strength assessment.

For the analysis has been selected a chemical tanker ship with double hull, granted by the Ship Design Group, Galati, during the internship. The following ship data are required for the strengths analysis: hull offset lines, structural characteristics over several transversal sections, material characteristics, mass distribution over the ship length. The ship hull offset lines is based on the chemical tanker ship and the transversal sections structure scantlings are according to the Bureau Veritas Classification Society Hull Rules. The mass distribution is based on the full 3D-CAD/FEM model developed in the study (for hull steel mass group) and the prototype ship onboard masses groups. The ship hull model has been developed using three different modelling levels: a 3D-FEM model full extended over the ship length; a 3D-FEM model extended on the two cargo holds compartments from the ship central part (coarse and fine mesh size) and a 1D-Equivalent Beam model (as reference for the 3D-FEM Models). The global - local ship hull strength analysis based on 3D-FEM model full extended over the ship length, one sided, includes the following steps: the 3D-CAD of the ship hull offset lines, the 3DCAD/FEM mesh (coarse) of the ship hull structure, the boundary conditions, the gravity loads from structure and other onboard masses, cargo and cargo tanks structure considered as local pressure applied on the double bottom shell in the cargo holds area, the equivalent quasi-static head wave pressure loads applied on the hull external shell, using an iterative procedure for the free floating and trim conditions equilibrium, implemented with user subroutines in the FEM solver. The results are obtained from the 3D-FEM model with post-processing user subroutines, as follows: normal, tangential and vonMises stresses. The strength assessment includes the safety factor in reference to the yield stress limit criteria. The 3D- FEM model has been developed with Solid Works Cosmos/M 2007 program, based on a 3D-CAD model developed with AutoCAD, with iterative analysis and post processing user subroutines developed for Cosmos/M solver, at Galati Naval Architecture Faculty. The global ship strengths analysis based on 1D-equivalent ship girder model, under equivalent quasi-static head waves, is carried on with an iterative algorithm for free floating and trim equilibrium conditions, using an in-house made code from Galati Naval Architecture Faculty. The global - local ship hull strength analysis based on 3D-FEM model extended on two cargo holds compartments (central ship part), with coarse mesh size, for cargo holds structural strength analysis, was performed with Solid Works Cosmos/M 2007, having the same local loads as the 3DFEM Full extended model and at both model ends the global displacement and rotation are taken from the previous 1D-equivalent girder model. In order to apply the external pressure on the ship hull, from the equivalent quasi-static wave, and the global displacements and rotations at both model ends, there were used specific user subroutines implemented in Solid Works Cosmos/M 2007 Solver. The 3D-FEM Model extended on two cargo hold compartments has been refined, in order to include some selected structural details (hot-spots domains). The boundary condition and the globallocal loads are the same as for the previous coarse mesh size model. The strength assessment includes also hot-spot factor evaluation for the deck and bottom structural panels. Comparison between deformations and stress values based on the four structural models, having different complexity levels, was performed. The numerical FEM analysis provides reliable data for the ship strength assessment, with a good concordance between the structural models developed in this study, taking into account the specific models sensitivity.