Examples of actual defects in high pressure pipelines and probabilistic assessment of residual life for different types of pipeline steels

During recent internal inspection of an operated high pressure pipeline of the diameter DN700, distinct, quite large indications of cracks were found in one of the pipe sections. A part of the section was taken from the ground for a comprehensive experimental programme containing analyses of the defect type, dimensions and character and also fatigue test at variable internal pressure with the maximum value corresponding to real operation. The analysed defects mostly occurred during pipe manufacture followed by stress corrosion cracking mechanism in some cases. An existence of fatigue crack growth (FCG) during fatigue pressure test in some cases was demonstrated. Within another experimental programme, an extensive measurement of FCG rates in different high pressure pipeline steels was performed. The evaluated data enabled to carry out serious statistical evaluation. Probabilistic assessment of residual life of pipes made of different steels, containing semi elliptical external surface axial crack was carried out using ALIAS HIDA software developed within the European Framework Programme project.