As our understanding of stress corrosion cracking in pipelines continues to grow, pipeline integrity programs may also need to evolve. Historically, the industry has had good success with managing SCC, Photo 1, around the threat discriminators detailed in B31.8S.   However, as pipelines and coatings continue to age, we are finding new types of SCC and finding SCC in locations outside of these threat discriminators.

Mears has been working with several pipeline operators as they have enhanced their SCC Management Programs to address new occurrences of SCC.   In addition to the routine use of mag-particle inspections for all excavations, operators’ SCC threat identification and mitigation programs are evolving to assess for SCC in pipelines that historically may have been considered low-risk for SCC.

The historical discriminators for SCC threat identification focused on operating stress (>60% SMYS), coating type (all except FBE), distance from compressor stations (≤20 miles), age (≥10 years) and operating temperature (>100 ˚F).   The first evolution in threat identification was the recognition that temperature could not be used to eliminate the threat of near-neutral SCC.   These historical guidelines continue to be largely substantiated but as ASME STP-PT-011 documents, they are no longer “sharp discriminators” for whether the SCC threat is present.  New “mixed-mode” SCC is being found and SCC failures have been documented in pipelines operating as low as 25% SMYS and segments more than 50 miles from a compressor station.

In concert with the evolving understanding of SCC, the industry’s ability to assess for SCC has been evolving. Once solely assessed by SCCDA, hydrostatic testing and then hydrostatic spike testing, operators have developed EMAT ILI and SCC mitigation procedures to manage this threat.  These procedures enable operators to effectively run and evaluate the EMAT ILI tools they are utilizing and to ensure effective responses to crack indications identified by ILI, Direct Assessment or other excavations.

One of Mears’ clients experienced an SCC rupture on a key pipeline.  The pipeline, operating at 66% SMYS, was 53 years old, Photo 2.  At the time of the rupture, one of our experts was supporting the Operator’s efforts to develop a Stress Corrosion Cracking Management plan to enhance its existing programs for identifying and mitigating SCC.  The rupture accelerated this effort.

The Operator’s existing SCC identification and mitigation program had been limited to a pressure testing program for pipelines with a history of SCC.  The new Stress Corrosion Cracking Management Plan (SCCMP) was developed to comprehensively and consistently address SCC threats in their large system of pipelines.  The new SCCMP included the following sections:

  1. Risk Prioritization of Pipelines
  2. SCC Assessment Methods and Selection
  3. SCC Crack Categorization
  4. Definition for Defining Extent of SCC Identified
  5. Response Requirements
  6. Excavation and Data Collection Requirements
  7. SCC Mitigation
  8. Reassessment Methods and Intervals
  9. Data Integration

Implemented in 2016, the SCCMP was used to determine the inspection and mitigation response to the rupture described above.  Mears worked closely with our client to facilitate the development of the SCCMP and support their efforts to assess the effectiveness of EMAT ILI.

To assess the SCC threat in the 78-mile pipeline segment, 27 hydrostatic spike tests were performed, and the segment was inspected with EMAT ILI.  The hydrostatic spike tests eliminated one SCC colony, but the EMAT ILI run and the SCCMP direct examination requirements resulted in the removal of more than fifty SCC colonies that may have grown to failure during future operations.   The operator’s rigorous efforts provided the confidence to return to full operating pressure and the establishment of a conservative reassessment interval.  The established process ensures that the EMAT ILI re-assessment will be completed before crack growth results in another rupture.

At Mears we are committed to growing and evolving as we support the pipeline industry’s efforts to ensure pipeline integrity.  If we can help you enhance your SCC identification and mitigation, please call us at 1-800-MEARSCP or contact us online at www.MearsCorrosion.com.

Photo 1 Stress Corrosion Cracking

Photo 1 Stress Corrosion Cracking

Line rupture

Photo 2 SCC Rupture On 53 Year Old Pipeline