Review of ASME B31.8S on Reinspection Intervals for Pipelines Carrying Hydrogen or Hydrogen/Natural Gas Blends
The objective of Task 2 was to review ASME B31.8S-2018 Table 5.6.1‑1 (Integrity Assessment Intervals) and other risk assessment tools and propose a technically sound basis for determining Predicted Failure Pressure (Pf) and Reinspection Interval (RI) for pipelines carrying hydrogen or hydrogen-blended gas. Additional work included an assessment of the limitations and possible improvements of Figure 7.2.1‑1 (Timing for Scheduled Responses).
Terminology related to ambient-temperature hydrogen effects on pipelines can be confusing. To alleviate this, Chapter 6 of this report provides a description of the effects of hydrogen.
There is an ongoing effort by the Hydrogen Task Group of ASME B31.8 (Gas Transmission and Distribution Piping Systems) to move content from ASME B31.12 (Hydrogen Piping and Pipelines) into new hydrogen chapters of ASME B31.8 and ASME B31.8S (Managing System Integrity of Gas Pipelines). To help this process, a review of the potential threats to pipeline integrity described in ASME B31.8S Paragraph 2.2 (Integrity Threat Classification) was performed.
The following conclusions and recommendations were formulated based on the results from this Task 2 review.
Review of ASME B31.8S-2018 Table 5.6.1-1 Integrity Assessment Intervals and ASME B31.8S‑2018 Figure 7.2.1-1 Response Time
ASME B31.8S-2018 uses the terminology for Integrity Assessment Interval and Response Time (RT) interchangeably, which is incorrect. RT should refer to the time to repair or mitigate a present condition. RT should be shorter than the (re-)assessment interval (RI), which in turn should be shorter than the remaining life (RL) to reach a future critical condition.
The Maximum Allowable Operating Pressure (MAOP) coefficients and Specified Minimum Yield Strength (SMYS) coefficients in ASME B31.8S-2018 are not consistently chosen. For In-Line Inspections (ILI), ASME B31.8S-2018 requires RTs of 10, 15, and 20 years to reach a Pf of 1.00 x SMYS for MAOPs up to 30%, between 30% and 50%, and above 50% of SMYS, respectively. The other SMYS‑coefficients are also inconsistent: 0.50, 0.66, 0.70, 0.83, and 0.90 x SMYS.
A more technically sound methodology would be to: first calculate the RL to reach a Pf of “SMYS‑coefficient x SMYS,” then apply a Safety Factor to the RL to set an RI, and finally, set an RT for expedited mitigation of conditions with a short RI. This methodology is adopted in API 570‑2023, as described in this report.
ASME B31.8S-2018 Figure 7.2.1-1 specifies the required RT for external and internal corrosion. However, the curves in the figure are independent of the pipe wall thickness and the corrosion rate (CR), which is a shortcoming for the threat of corrosion metal loss because the mechanism reduces wall thickness with time. Example calculations using the equations from ASME B31G‑2023 show that the RL for different wall thicknesses and diameters varies greatly. The RL can be much shorter than the RT for all but the thickest pipes.
It is recommended to (1) revise ASME B31.8S Table 5.6.1‑1 and Figure 7.2.1‑1 with consistent application of Pf and factors related to SMYS, and (2) introduce the concepts of RL and RI into the Code while maintaining the concept of RT.
Description of Hydrogen Effects
The terminology used for ambient-temperature hydrogen effects can be confusing. These effects can occur in the base metal, weld fusion metal, or weld heat-affected zone (HAZ). Some are named after their mechanism, and others after the source of hydrogen. This report describes and distinguishes each of the following terms:
- Hydrogen ingress reversible; no cracks
- Hydrogen embrittlement (HE) reversible; embrittles steel; no cracks
- Hydrogen-assisted cracking (HAC) irreversible; H2 gas micro-fissures the steel
- Hydrogen-induced cracking (HIC) irreversible; no external stress; cracks grow
- Hydrogen stress cracking (HSC) irreversible; with external stress; cracks grow
- Stress-oriented HIC (SOHIC) irreversible; HSC parallel to external stress
- Sulfide stress cracking (SSC) irreversible; source of H is wet H2S corrosion
- Hydrogen-enhanced cracking (HEC) H enhances SCC or fatigue crack growth rate
It is recommended to use consistent terminology when describing hydrogen effects.
Review of Potential Threats to Integrity
An analysis of the potential threats to pipeline integrity listed in ASME B31.8S-2018 showed four generally applicable and six hydrogen-applicable threats missing.
Integrity threats applicable to any gas transmission pipeline – add to ASME B31.8S Paragraph 2.2:
- Y1 “Time-Dependent – Fatigue cracking”
- Y4 “(b) Resident (1) Manufacturing-related defects (a) Defective pipe seam – Low‑toughness seam”
- Y5 “(b) Resident (1) Manufacturing-related defects (a) Defective pipe seam – Defect in or near an electric resistance welded (ERW) seam”
- Y7 “(b) Resident (2) Welding/fabrication-related – At in‑service/repair welds – (Delayed) Hydrogen Stress Cracking”
Integrity threats applicable to gas pipelines in hydrogen and blend service – add to future ASME B31.8S chapter on hydrogen:
- Y2 “Time-Dependent – Hydrogen-Induced Cracking (HIC)”
- Y3 “Time-Dependent – Fittings and sharp transitions”
- Y6 “(b) Resident (2) Welding/fabrication-related – Near girth welds, and near in‑service/repair welds – Arc burn/strike on pipe body”
- Y8 “(b) Resident (2) Welding/fabrication related – At welded sleeves and fittings – Type B sleeves and Puddle welds”
- Y9 “(b) Resident (2) Equipment – Other material/equipment failure – Disbonded coating caused by hydrogen”
- Y10 “(c) Random or Time Independent (3) Weather-related and outside force – At locations that may experience an electrical arc or fire – Fire Damage”
It is recommended to add these missing integrity threats to future versions of ASME B31.8S.