Study of the effect of defects on the fatigue behaviour and the fracture toughness for low carbon steel (API 5L grade B) gas transmission pipelines

Abstract

The use of low carbon steel pipes in gas transmission pipelines and network gas pipes has increased. API 5L grade B (American Petroleum Institute) pipes are welded and inspected according to the BGC/PS code (British Gas Standard). Since root undercut and lack of penetration are common defects, the acceptance and nonacceptance of these defects based on the BGC/PS/P2 code of butt welded pipes are evaluated. Pipes are welded with different degrees of root undercut and lack of penetration defects and are X-rayed. The defects are categorized based on both X-ray films and final crack area. Specimens are cut and fatigue precracked at different periods depending on the initial defect size. An equivalent crack length is calculated based on Paris' law for each defect size in the fatigue crack propagation. The crack opening displacement technique, J-integral method based on Begley and Landes and the residual strength method have been used to determine the fracture toughness. The results have been compared to the parent material and the initial defect size to evaluate the restrictions of the BGC/PS/P2 code. The equivalent crack length results showed that the depth of lack of penetration welding defect should be taken into consideration for both API and BGC codes since this depth has a significant effect on the equivalent crack length. The application of the Begley and Landes approach is shown to be suitable since it permits the use of specimens of different initial crack lengths. The technique is applied here for cracks of different shapes. The results are close and can be used as comparative results between different categories and the parent material since the conditions of specimen dimensions are not valid as restricted by Begley and Landes. Critical crack opening displacement is calculated which gives reliable results for different categories. Crack opening displacement technique based on the British Standard requires different specimens with the same initial crack length, which is not satisfied. The residual strength method based on ASTM gives reliable results for the different categories of defects. The depth of lack of penetration welding defect should be taken into consideration for both API and BGC codes. The BGC code should specify an allowable depth for the root undercut welding defects.