Study of Internal Pressure Effect on Gas Transmission Pipeline Safety Using Failure Assessment Diagrams

The use of steel pipes is one of the most important methods for gas transmission in energy sector. Different types of cracks (axial, circumferential and a combination of them) may cause catastrophic results in a pipeline. The safety of such cracks is investigated by applying failure assessment diagrams. If the point corresponded to each crack, is located under assessment curve, the crack is safe, otherwise the crack is unsafe. In this research, the API X65 steel pipe (commonly used in Iran's energy transmission lines) with axial crack of fixed 8 millimeter depth and fixed crack length of 150 millimeter in internal pressure ranges of 4, 5, 6, 7, 8 and 8.5 MPa is studied. The reference stress applied to the pipe is calculated by standard equations of BS 7910. Moreover, ABAQUS simulation is used to calculate the energy release rate and the stress intensity factor at the crack tip. In the failure assessment diagrams, the horizontal and vertical factors are calculated accordingly, by means of dividing the reference stress by the yield stress and the stress intensity factor by the fracture toughness for different points. By comparing the locations of these points with the failure assessment diagrams, the pipe safety is investigated. The results indicated that, the crack was safe in pressure ranges of 4, 5, 6, 7 MPa and also in critical area in pressure 8 MPa. However, the crack was unsafe in pressure 8.5 MPa and above.