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Abstract The aim of the thesis is to develop a three-dimensional (3D) numerical model using finite element for simulation, based on continuity equation, energy equation, momentum equation, K-ε algorithm and heat transfer equations, computational package Common Solution software (COMSOL) is used to determine cooling rate, time of the cooling and temperature distribution, to predict the effect of natural gas velocity on hot tapping process. Numerical modeling predicts the safe zone for applying hot tapping process and the un-safe zone for applying hot tapping process, for all ranges of accepted natural gas velocities up to 20 m/s, using different commercial pipe diameters employed in natural gas transmission pipeline in Egypt. Experiments were carried out on a pipe of 32-inch transmits national natural gas in Egypt in-service, during a hot tapping process for execution of 6-inch new branch. Measurements were obtained and recorded from the experiments under 2 m/s gas velocity, 46 bar gas pressure and 16 ˚C gas temperature. To verify the proposed numerical model, the predicted results are compared with those obtained from experimental measurements for a wide range of working conditions. The predicted results are presented using natural gas velocity ranged from 0.4 m/s to 20 m/s to determine the risk of welding zone and heat affected zone (HAZ) crack and also the burn through risk; and to precise the safe range of gas velocity during hot tapping process for various commercial pipe diameters (ranged from 26-inch to 34-inch). To determine the influence of natural gas velocity on hot tapping process various numerical iterations for wide velocity ranges of iii natural gas with heat input changed from 6 to 9 kJ/mm, fair agreement was obtained from this comparison with maximum deviation between numerical model results and experimental work is 0.96%. In conclusions, numerical results were used to determine the case in which the possibilities of the two most significant problems can be avoided when welding during in-service pipelines. The first problem is concerned with the gas flow where the flowing gas creates a large heat loss through the pipe-wall due to convection heat transfer, resulting in acceleration of cooling rate for the welding operation. The influence of welding cooling rates causes a great hardness levels within the welding zone and in the HAZ which cause formation of cracks. The second problem is related to the localized heating and loss of material strength during the welding process, since the pipe-wall is reduced in thickness and cause explosion due to the high internal pressure if this reduction in strength is too great and if the inner surface temperature of the pipeline reaches 987 ˚C [1]. Results are used to obtain a sufficient map for in-service welding on pipeline depending on the change of natural gas velocities as a main factor and hence affects the cooling rates. |