Pipeline transportation safety advisory letter 617-02/19
Management of stress corrosion cracking on susceptible pipelines
Place du Centre
200 Promenade du Portage
26 June 2019
Letter addressed to Senior Vice President and Chief Operations Officer, Enbridge Inc.
Subject: Pipeline safety advisory: Management of stress corrosion cracking on susceptible pipelines
On 09 October 2018, at approximately 1724 Pacific Daylight Time, a sweet natural gas pipeline rupture and ignition occurred on Westcoast Energy Inc.’s (Westcoast) nominal pipe size (NPS) 36 L2 pipelineFootnote1 about 13 km northeast of Prince George, British Columbia. There were no injuries (TSB occurrence number P18H0088).
Immediately adjacent to the NPS 36 L2 pipeline is a Plateau Pipeline Ltd. (Pembina) NPS 12 oil pipeline and another Westcoast NPS 30 natural gas pipeline. Prior to the occurrence, the NPS 36 L2 pipeline was operating within its maximum operating pressure approved by the National Energy Board (NEB).
Approximately 37 m of the ruptured pipeline was taken to a laboratory for failure analysis. Based on the metallurgical analysis, it was determined that:
- Numerous indications consistent with colonies of stress corrosion cracking (SCC) were identified on the outside surface of the pipe segments.
- The rupture originated at stress corrosion cracks on the outside surface of the pipe, which exceeded the critical crack size.
- The cracking mode was consistent with near-neutral pH SCC.
Near-neutral pH SCC on pipelines occurs as a result of the combination of a susceptible metallic material, an applied or residual tensile stress, and a suitable environment. This mechanism initiates on the external surface of susceptible pipe and can grow in both depth and length (circumferentially or longitudinally).
Over the past 20 years, following the NEB’s Report of the Inquiry: Stress Corrosion Cracking on Canadian Oil and Gas PipelinesFootnote2and ensuing recommendations, many measures have been put in place to help address SCC. These measures included the development and publication of industry recommended practices and the update of federal regulations and pipeline standards. New requirements to develop and implement management systems and integrity management programs were put in place, resulting in improvements to the management of near-neutral pH SCC by Canadian pipeline operators.
One of the factors associated with near-neutral pH SCC on pipelines is the use of polyethylene tape coating. This type of coating has a tendency to separate (disbond) from the pipeline, allowing moisture present in the soil to contact the pipe surface while shielding it from the cathodic protection current.
Based on company records, there are approximately 500 km of pipe coated with polyethylene tape in Westcoast’s NPS 36 Transmission South system. Given the presence of SCC-susceptible pipe, Westcoast may wish to review its SCC management practices, including in-line inspection intervals, to ensure that the risks associated with polyethylene tape-coated pipe are mitigated.
Original signed by
Vice President, Systems Operations - National Energy Board
Vice President, Performance - Canadian Energy Pipeline Association