Learnings

Outcomes

This project has identified Hydrogen Embrittlement (HE) as a new deterioration mechanism for AGI piping converted to H2/Blend duty. The implications of hydrogen blending vary with the concentration of hydrogen. Relatively low concentrations of hydrogen, 5%–15% by volume, appear to be feasible with very few modifications required to existing pipeline systems or AGIs, provided the carbon steel grade complies with the codes and standards which exist for hydrogen duty. Where this does not occur then qualification test in hydrogen environment is required. 

HE can occur at all concentrations of hydrogen, but the likelihood decreases considerably at lower levels. At lower levels the extent of frequency of NDT for detection of HE would be reduced; however, this is also dependent upon existing flaws/cracks in the existing AGI piping.

There are existing NDT techniques for the detection of HE deterioration in service that are well established and used now extensively in industry. These can be applied to the above ground sections of AGI Piping. Underground sections of AGI piping are more difficult to inspect for HE. There are robot crawler systems available that can be fitted with the correct type of NDT sensors, but there is no published data on their application for detection of HE. The use of these NDT robot crawlers is a recommended area for further development.

Engineering Critical Assessments using fracture mechanics will be required for each AGI site to define acceptable flaw sizes and identify critical locations where NDT for HE needs to be applied. 

There are several existing deterioration mechanisms that apply for both the existing Natural Gas service and for hydrogen/blend duty. However, the likelihood and consequence of failure for hydrogen are greater than for methane duty. Leaks from screwed and compression fittings are an example; these are non-preferred for hydrogen duty in existing industry guidance – based upon the RBI Safety scoring, these are best eliminated by re-design.

It is recommended that the current 6 yearly inspection interval for AGI Piping is reduced to 2 years for hydrogen/blend duties due to the higher consequences of failure and lack of inspection history on the new service. Collecting and analysing inspection history is key to ongoing improvement of integrity monitoring for H2/Blend service. 

Currently AGI Piping is inspected against a set of Management Procedures applied across all sites. For H2/Blend duty, each site will have different inspection requirements and possibly different examination intervals; it is recommended that individual Written Schemes of Examination are developed for each AGI Piping system. 

Inspection and assessment of AGI piping for HE requires competency in specialist NDT (PAUT/TOFD or AUBT UT) which exists now in certain NDT contract companies. The assessment of NDT results requires expertise in Fracture mechanics.

The ongoing integrity monitoring of AGIs for H2/Blend duty is going to generate much more data than existing inspections; this is best achieved using a central software system that collects all relevant data in one location. There are existing commercial packages available now. These would be recommended rather than National Gas developing a bespoke solution.

The outputs of this project mark a significant step towards the safe management and transportation of hydrogen and hydrogen blends on the National Transmission System. Managing the increased integrity risks posed by the introduction of hydrogen into repurposed pipelines is a crucial aspect of repurposing the NTS for hydrogen transport. The outputs of this project help National Gas Transmission to ensure that the excellent safety record of the system can be maintained and provide a clear recommendation of next steps to prepare AGIs and associated integrity management practices for the introduction of hydrogen into the NTS.

Lessons Learnt

The first work package and the Co-creation workshop has regular meetings between the National Gas stakeholders and the project partner team from TÜV Rheinland. This enabled the areas to be explored in work package 2 to be discussed and agreed by all parties.

The first part of the project required a large amount of information from National Gas to TÜV Rheinland for the RBI assessments. National Gas nominated a single point of contact for the transfer of this information which worked very well.

Currently, there is limited industry data available on the likelihood of hydrogen embrittlement occurring in service for the lower H2 blend percentages for carbon steel piping. This has resulted in a conservative approach in the RBI assessments, stating that HE is credible for these lower blend percentages. As more industry data becomes available, this could result in a less conservative approach.