Investigating the Collapse Pressure of Corroded Pipelines of Various API 5L X Grades With Multiple Interacting Defects Using Finite Element Analysis

The collapse pressure of pipelines is becoming an important factor as oil and gas operations are increasing conducted in the deep sea, where the ambient external pressure is high enough to be a design consideration. Meanwhile, pipeline defects caused by corrosion is a perennial problem in the oil and gas operations. This study investigates the effect of interacting defects on pipelines with different steel grades. The steel grades chosen for simulation are API 5L X42, X52, X65 and X80. Longitudinally and circumferentially aligned interacting defects were considered in the study. Longitudinally aligned defects are positioned parallel to the longitudinal axis but separated by an island of the pipeline's entire wall thickness. Meanwhile, circumferentially aligned defects completely overlap longitudinally in the circumferentially aligned position. Past studies have shown that interaction between defects will reduce the collapse pressure of the pipeline. The length, outer diameter, and wall thickness of the X65 pipeline specimen are 5000 mm, 273 mm, and 12 mm, respectively. A pressure load was applied on the external surface of the pipeline model and a fixed support boundary condition was applied to the ends of the model. A parametric study considering normalized circumferential and longitudinal defect distance was conducted. It was found that the X52 pipe had the strongest defect interaction effect. However, the X52 pipe sample also had a slightly higher collapse pressure than the X65 sample, with the X42 sample being the weakest. The X80 grade pipe had the highest collapse pressure and weakest defect interaction effect of all the samples. In conclusion, the interaction between defects becomes weaker with stronger steel grade, in general. The X80 grade was the strongest grade when measuring collapse pressure (34.45 – 34.7 MPa), followed by X52 (32.1 – 32.5 MPa), X65 (31.4 – 31.7 MPa) and X42 (26.2 – 26.5 MPa) being the weakest.