The aim of the project is to study experimentally the influence of the semi-elliptical defect on the steel of grade API 5L X52 pipeline and its reparation by welding operation to keep the safety of hydrocarbons transport network. The  tests have been performed on  the specimens with/without semi-elliptical defects by using two ratios a/c and a/t, where the defects issue on the piping systems wall is treated by using the welding operation, which focuses on the filling the defected surface as same thickness of the base wall of the pipeline after removing the defects. The decrease of ultimate stress is mainly evident depending on the size of crack, where they compared to the base metal results. The experimental results showed that the reliability of the welding operation is proved especially in the ratio a/c =1, which is similar to that found in automatically welded. It is proved that the ratio a/t = 0.2 curve exhibits similarity to the base metal curve and their elongation conforms to API standard of steel X52 grade. Clearly, corrosion defects cause degradation of mechanical property in the order of 44% to 50% depending on the ratio of 0 < a/c ≤1 and from 20% to 80% according to the second ration of 0 < a/t ≤0.8. The obtained results proved that the enhanced of mechanical properties by welding reaches to ultimate stress is 559 MPa for the ratio a/t = 0.2 and 531 MPa for the ratio a/t = 0.5. Finally, the experimental results  provided that the impact of semi-elliptical defect has effects on the safety of piping systems where welding became as one of among solutions to repair the piping systems.