CRACK GROWTH IN PIPES UNDER SERVICE CONTORTIONS

At the Asgard field a leak on a 10", 13Cr production pipeline was discovered in December 2000 during pressure testing. The cause was a crack at an anode pad fillet weld (pads are connectors for the cathodic protection system). Later, a similar leak occurred on another Asgard flowline. During pigging inspection (AUT) several smaller crack indications were found at similar locations. Propagation of such cracks will depend on loading and environmental conditions. To investigate this further, a test programme was carried out using 13Cr pipe materials. Both small scale tests and full scale pipes were used. Specimens were prepared with small initial fatigue cracks at the pad weld. The propagation of the cracks was then recorded under various environmental and loading conditions. The loading was selected to cover a crack growth rate range of similar to 10(-6) to 10(-3) mm/cycle for various crack depths and for two loading frequencies. Tests were conducted under cathodic protection (hydrogen in the material measured) and for temperatures up to 140 degrees C and pressures up to 30bar. The crack growth was recorded by the potential drop method (ACPD). For the full scale pipe tests, specially developed equipment was used for simultaneous measuring at up to 24 individual locations. The results showed that low loading frequency (0.1 Hz) enhances the growth rates; elevated temperature gave equal or lower propagation rates than at 25 degrees C and a pressure of 30bar did not influence the results. A few cracks were also initiated during the corrosion fatigue tests and exhibited high growth rates; possibly due to the so-called "small crack" effect and possibly in synergy with the influence of hydrogen.