Ductile Tearing of Crack-Like Defects During Hydrostatic Testing of Pipes

Hydrostatic testing is a powerful tool in pipeline integrity procedures. However, there is the risk that few non-critical defects grow during the test, eventually reaching a quasi-critical size which can cause failure during future operation of the pipe (pressure reversal). This work evaluates the effect of pipeline hydrostatic testing on the ductile crack growth (ductile tearing) of prior crack-like defects. Testing of six segments of 3m length API X60 steel pipes were carried out, each segment containing one electro-sparking erosion part-wall defect of dimensions (a × 2c) 3 × 60mm, 7 × 140mm, and 10 × 200mm, positioned both externally and internally on the base metal. Additionally, for external part-wall defects 3 × 60mm and 7 × 140mm, crack extension was monitored by ultrasonic technique, so that pipe resistance curves of applied load vs. crack depth, J vs. Δa curves, were obtained for each crack configuration. These experimental curves showed good agreement to analytical solutions for defect-containing cylinders under internal pressure. Laboratory resistance curves, J vs. Δa, were obtained from fracture mechanics C(T) test pieces in TL orientation. These curves were used to determine the initiation of stable extension of the defect J_IC. Finally, results suggest that J_IC is a suitable (conservative) parameter for estimating hydrostatic testing pressure.