A numerical model had been developed using three dimensional finite element method to predict temperature history of pipe wall thickness by using a FORTRAN computer program. The developed numerical model concerned with the effect of power and velocity of moving heat source, temperature dependent material properties, and transient heat transfer and phase change transformation. Also the effect of heat flux and flow rate on cooling time was investigated numerically. An experimental setup had been designed, manufactured and instrumented to investigate the thermal history of an in-service welding process considering air as the flowing fluid inside test pipe. The burn-through was investigated experimentally under the influence of gas flow rate, heat flux and the depth of defect presented in pipeline wall. It has been concluded that the burn-through do not occur for the studied parameter range. The results showed that as the flow rate increases from (24 - 30 lpm) the cooling time decrease by a rate of (11.7 %) and when the flow rate increase from (24-60 lpm) the cooling time decrease by a rate of (41.1 %) without affecting peak temperature with pipe wall thickness of (6mm). Also it has been concluded that as the heat flux increases from (1110- 1370 kJ/mm) the peak temperature increases with a rate of (22.11 %) and cooling time increase with (25.4 %) when the heat flux increases from (1110- 1659 kJ/mm) the peak temperature increases with a rate of (46.86%) and cooling time increase with (50.7 %) with pipe thickness of (6mm).

Welding of pipeline in active operation (called in-service welding) is an advancedtechnique employed to repair pipelines to safe transmission of petroleum or gas. A thermalexperimental study is accomplished on welding process of gas pipeline in this work. Anexperimental setup had been designed, manufactured and instrumented to estimate the methodfor repairing locally pipelines defects (such as cracks and holes wall surface) under the influenceof internal pressure. Also the thermal history of welding process was investigated experimentallyto three cases which are : traditional welding process with no fluid flows inside the test pipe, anin-service welding process considering air as the flowing fluid inside test pipe, and an in-servicewelding process considering the liquefied petroleum gas (LPG) as flowing fluid inside test pipe.Results show that direct deposition of welding metal on a through hole under the influence ofinternal pressure succeeded for flow rate less than (12lpm) and failed for flow equal or higherthan (12lpm) in which a sleeve repair is considered. The calculations are followed by a separatethermal stress analysis based on the thermal history.