Experimental validation of the influence of lamination defects in electrical resistance seam welded pipelines

Experimental results from seven strain gaged instrumented hydrostatic tests on ex-service tubes from a 350 mm diameter, 6.3 mm thickness electrical resistance seam welded oil pipeline are presented. Lamination defects had been detected in these sectors by means of transversal magnetic flow in-line inspection and the FAST ultrasonic technique. Five tubes failed in the weld, these yielded the lowest burst pressures. The largest laminations found in the base material reduced the burst pressure by less than 25%. Defects propagated in the weld reduced the burst strength by up to 50%. Fracture mechanics modelling of the most representative defect shows that an inclined lamination defect with a through-thickness depth of up to 3 mm is not critical; test results show that this prediction is conservative. These results show that the location of the defect is more important than its size, the main reason being the low toughness of the weld. The results of this study confirm the validity of the restrictions of Section 8 of API RP 579.