الفهرس 
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Abstract
A ship is an assembly of various types of structural components. All these structural components provide the basic strength and support to the ship’s shell structure. These components broadly fall under either longitudinal or transverse components which are prefabricated and subsequently aligned then welded in position. Welding is one of the most widely used hot-work processes used in the shipbuilding industry. Analysis of welded structures are still remaining a challenge for the designer to produce desired output results. During welding, distortions and residual stresses are induced by material thermal expansion and contraction during heating and cooling as well as material plastic deformation at elevated temperatures. The mechanical response of the material depends on temperature changes during welding. Due to these imperfections, the fabricated structures may not attain their design load that is supposed to be carried and will require extra manhours to remove them. Nonlinear finite element method (NLFEM) has been applied to simulate two different types of welding process (Arc welding and hybrid laser arc welding (HLAW)) used in shipbuilding industry, giving the three-dimensional distribution of residual stresses and out-of-plane distortion. For this purpose, three-dimensional coupled thermo-elastic-plastic finite element model has been developed with ABAQUS using an additional numerical subroutine of DFLUX written in the FORTRAN programming language to simulate the volumetric heat flux distributions of the welding process. The numerical results are validated with experimental measurements that have been done in BAM, Germany. The weld induced imperfections can have severe effects on the ultimate strength of panels and hence resulting in decrease in load carrying capacity of ship as well as lead to an increase in manhours for fitting, flame straightening and rework. Therefore, in process control of welding imperfections is more desirable than post welding rectification from the point of manufacturing efficiency. The effect of geometrical properties such as frame spacing, slenderness ratio β, aspect ratio a/b and welding sequence on the generation of welding induced imperfections (deflection and residual stresses) have been investigated in this work in order to address a rational structural design procedure, as these parameters are of great importance from a structural design perspective. iv The effect of weld-induced imperfections on structural designs is of great importance as it is important for predicting the behavior of stiffened and unstiffened plate elements under different loading conditions. Many of the current studies take traditional initial imperfections as a mixture of geometric distortion and residual stress. This work specifically considers the three-dimensional finite element method (FEM) results of the imperfections caused by welding as initial state for the ultimate strength analysis. A non-linear finite element analysis (NLFEA) was performed under uniaxial compression to predict the ultimate strength behavior of the specified structure to make an appropriate selection of design parameters. A case study of double hull oil tanker has been specified to select the optimum frame spacing through investigating the effect of five different frame spacing on the structure weight, weld induced imperfections and ultimate load carrying capacity. from this study, the rational design of the ship structural components that achieves the minimum weight, minimum welding induced imperfections, and maximum load-carrying capacity has been attained through choosing the optimum frame spacing of the stiffened panels, the optimum slenderness ratio and aspect ratio of the plate panels, and the optimum geometrical properties of the fabricated T-Girders with the optimum welding sequence. Furthermore, this study will improve the structural appearance, mitigating time due to structural assembly, and hence reduce reworks in shipyard.