TWI Industrial Member Report Summary 621/1997
B W Kenzie and M R Kirby
Background
In-service inspection is widely carried out on safety critical and environmentally sensitive components such as pipelines. Depending on the service conditions, several degradation mechanisms can exist for welded pipe which may cause leaks or failures in-service. A degradation mechanism of particular concern to pipe integrity is external SCC. SCC is initially detected by 'pigging', using an ultrasonic or 'elastic wave' pig to inspect a long run of buried pipeline from the inside. Where isolated areas of SCC are detected, the pipe external surface is exposed by excavating and removal of protective coatings. Magnetic particle inspection (MPI) is generally used to confirm and highlight areas of SCC on the external surface of ferritic pipe.
The task of ultrasonic inspection is to quantify the extent of cracking, particularly the through-wall height. Ultrasonic inspection procedures need to be carefully tailored to detect the flaws of concern and determine their severity. Existing procedures are often based on those used during fabrication, which may not be entirely appropriate for detection and sizing of external SCC.
Objectives
To evaluate the performance of different ultrasonic inspection techniques for detection and sizing of external SCC in ferritic steel line pipe.
To provide guidelines on the optimum ultrasonic inspection procedure for detection and sizing of external SCC in ferritic steel line pipe.