Threshold Geometrical Dimensions of Stage II Cracks Versus Required Resolution of Crack-Detection Techniques

Environmental-assisted cracks in pipeline steels usually undergo the following three sequential stages prior to the failure: • Stage 1 – crack initiation and early stage crack growth, in which cracks initiate at imperfections but grow slowly depth-wise with time. Crack length may be seen to increase either because of merging with new small cracks in the vicinity of an existing crack or faster crack growth at the crack tip. Some cracks pose little threat to pipeline steel integrity if they remain dormant. • Stage 2 – Increased crack growth rate where crack growth can be dictated by mechanical driving forces and crack growth rate increases with time. • Stage 3 – The final stage of crack growth where crack growth rate is very high. Typical crack management programs mitigate cracks prior to entering Stage III. It is of great importance that pipeline steels with Stage II cracks are detected, monitored, and managed to ensure operational pipeline integrity. Although a range of crack in-line inspection and detection techniques with varied detection limits are available, it is not clear how their detection limits match the threshold geometrical dimensions of Stage 2-cracks. This investigation is aimed to define critical geometrical dimensions of cracks that are considered to be Stage 2 cracks. The determination of critical geometrical dimensions of Stage 2 cracks was made with a consideration of a wide range of situations including pipeline operating conditions, susceptible environments for crack growth, metallurgical, fabrication and construction conditions of pipeline steels. A comparison of the threshold geometrical dimensions of Stage 2 cracks with the crack detection limits of modern crack inspection and detection techniques are made at the end of the paper.