Nowadays using large diameter seam welded pipes in order to transfer oil and gas between cities or countries is greatly increased. Because of the high speed and efficiency of submerged arc welding (SAW), this method is widely used in production of spiral (SSAW) and direct seam pipes (LSAW). The quality control of the welded pipes usually consists of a hydrostatic test in which water is pumped into the pipe. Cold bending of large diameter pipes is depended to the local topology. Bend angle and curvature is limited in cold bending. This process is generally performed at the assembly site with special pipe bending machine based on three roll bending. The main goal of this dissertation is to study the effect of welding residual stresses in cold bending process of large diameter pipes. Therefore, a good estimation of welding residual stress fields in the whole length of the pipe is needed.In the first step of the present study, three-dimensional model of submerged arc welding in spiral and direct seam pipes are simulated by using Ansys commercial software. A decouple thermal-mechanical model is developed in welding simulations and Thermo-physical and mechanical properties used for welding simulations are depending on the temperature. In thermal analysis, the weld heat source model based on the Goldak’s double-ellipsoid power density distribution was applied. After the thermal model was analyzed and the temperature history was achieved, the mechanical model was investigated. Finally the welding residual stress distributions were calculated. After welding simulations, the effect of hydrostatic test process in decreasing the residual stresses is evaluated. The welding simulation in spiral pipe is validated using hole drilling measurements performed before and after hydrostatic test in this research. Then residual stresses are determined according to ASTM standard E837. Comparison between residual stress distributions which measured by experimental and the numerical analysis, are shown that obtained results from the FE simulations are in relatively good agreement with the experimental measurements. Also the results of this research show that applying a hydrotest pressure after welding process can reduce welding residual stresses up to 70%.In the second step, residual stress and strain fields which have been obtained in the former step, are generalized to the whole length of LSAW and SSAW pipes. Then the effect the resulted residual stresses of welding process are surveyed on the bending of LSAW and SSAW pipes and geometrical defects such as pipe ovality. Results show that the spiral seam pipes are more sensitive to bending ovality than direct seam pipes. Key words: Spiral pipe, Direct seam pipe, Submerged arc welding, Hole-drilling method, Residual stress, Hydrostatic test, Cold bending.