When I wrote yesterday about the Arkansas failure the PHMSA website was down. It’s now available again and has a page devoted to the Arkansas incident. Of greatest interest is the Corrective Action Order, which contains a whole lot of interesting background information as well as the actions themselves.
The pipeline was hydrostatically tested in 2006 to establish the MAOP, the operating pressure at the time of failure was a little under 90% of MAOP, and there were in-line inspections in 2010 and 2013 (but results of the latter not yet available). At face value it appears that the pipeline was reasonably well managed and this failure is not analogous to the San Bruno disaster (where the pipe had never been subject to either hydrostatic test or in-line inspection). All of which makes the cause of the failure even more intriguing.
Interesting to note that among numerous other requirements specified by PHMSA there is an interim pressure restriction of not more than 80% of the pressure at the time of the failure, clearly treating that pressure as a de facto hydrostatic test. The principle is very good, but exactly how it is applied when the pressure profile at the time of failure would probably have had a substantial hydraulic gradient (ie. not hydrostatic) is something that ExxonMobil and PHMSA might need to discuss further.