According to the NEB Report of the recent inquiry on pipeline stress corrosion cracking (NEB Order No. MH-2-95), six of the Canadian failures to that time had been leaks resulting from circumferentially-oriented cracks. A review has been made of the five CEPA leaks; two leaks from non-CEPA companies; and a case of non-leaking circumferential SCC, also from CEPA. All of the Canadian cases occurred in regions of west and central Alberta.
Circumferential stress corrosion cracking is a rare cause of pipeline leakage. Canadian cases have occurred in very specific conditions that exist in only a small proportion of the regions where pipelines operate. All cases of circumferential SCC, to date, have occurred under either polyethylene tape or a polyethylene backed shrink sleeve. The SCC appears to have been of the neutral-pH form and initiated and grew in response to high axial stresses generated by soil creep and/or localized pipe bending on slopes of 10° or greater. In this sense, circumferential SCC is a manifestation of a geotechnical instability in an area of SCC susceptibility. It would be prudent for operators of susceptible pipelines to consider the potential risks of such failures and to manage those risks accordingly.
Management of circumferential SCC may be largely achieved by effective management of geotechnical concerns on slopes. Geotechnical programs directed toward preventing soil movement or pipe loading could have benefit with respect to preventing circumferential SCC in susceptible areas. Other management approaches, involving in-line inspection tools, may also be found to have benefit.
Given that circumferential SCC (C-SCC) is driven by geotechnical instability, a significant research effort directed specifically toward C-SCC is not considered warranted at this time. Rather, geotechnical research will augment SCC research in progress to provide insight and capabilities to manage the concern.
To assist pipeline operators, CEPA will issue, in 1998, a revision of the SCC Recommended Practices providing guidance to manage this concern.