Reliability-Based Integrity Assessment of Pipeline Girth Weld Defects

A large number of girth weld defects were found by tri-axial MFL in-line inspection of one natural gas transmission pipeline of PetroChina. In one certain inspection region of 70.9 km, 7392 girth weld defects were reported. These defects were found to be welding defects formed in the construction period through excavation validation. The integrity assessment method in code BS 7910 is used to assess the acceptability of these defects and to make repair decisions. However, the deterministic assessment result is significantly affected by different selected partial safety factors due to the uncertainties of defect sizing and material properties. In this paper, a reliability-based integrity assessment method is proposed. A Monte Carlo simulation method is used to estimate the failure probabilities of girth weld defects. Parameter distributions are discussed and the failure probabilities are calculated using a Matlab program. The assessment result of the reliability-based method is compared with that of deterministic assessment method. As a conclusion, the reliability-based method proposed in this paper can be used in the integrity assessment procedure to provide some important references for making repair decisions.