The problem of skips due to road crossings, road casings, and areas that are impossible to do a surface survey due to paved surfaces over the pipe. It was hoped that the government regulatoy agencies would approve of the use of guided long wave ultrasonic inspection to eliminate the skips due to road crossings but that has not happened. Some pipeline operators have dealed with the ECDA skips by running smart pigs. Smart Pigs as a means of eliminating ECDA skips is a unique solution since ECDA was designed for use on non-piggable pipelines. A new technology is robo-pigs (self propelled robot inspection tools). Since government regulatory agencies desire 100% inspection of HCA (High Consequence Areas), the ECDA skips become an issue in reaching compliance.
In paved areas, it may be too much trouble to drill holes in the paved street so that ECDA survey can be done. In addition, ECDA requires more digs to prove that the pipeline has no significant corrosion damage than a smart pig survey. As expensive as digs are to do in city streets, it doesn't take too many ECDA digs to justify upgrading the pipeline to run a smart pig to reduce the number of inspection digs required to prove pipeline integrity.
After San Bruno, manufacturing and construction defects that may grow and fail in service is a major issue. ECDA is not able to find such defects but smart pigs do have the ability to find some manufacturing and construction defects before a failure occurs. This issue will be covered in soon to be published blog articles.
ECDA can find external corrosion damage but not internal corrosion activity. A smart pig finds both internal and external corrosion damage. For pipelines with an internal corrosion risk, ECDA is simply not an acceptable technique. There is internal corrosion direct accessment (ICDA) but one has to have complete information on where all of the low points are on the natural gas pipeline where liquids can become trapped resulting in internal corosion activity. For some pipeline operators, a GeoPig tool may be the only way of knowing were all of the liquid trap locations may be located. It may be cheaper to upgrade the pipeline for the running of smart pigs rather than attempting to prove pipeline integrity using internal corrosion direct accessment.
If the pipeline has a SCC (stress corrosion cracking) risk, ECDA per current NACE Interational SCCDA (Stress Corrosion Cracking Direct Assessment) recommended practices is successful in finding areas of classic (high pH) SCC but not near neutral SCC. The only effective way of finding near neutral SCC colonies is at a minimum a TFI smart pig or hydrotesting with a EMAT smart pig being more effective per some pipeline operators experience in locating and sizing SCC defects. I plan on posting a more detailed blog article on this mattter soon.
Shielded corrosion cells are best found by smart pigs than ECDA.
I will probably add more items to this blog posting in the future.
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