The output of NIA_NGN_276 highlighted a range of materials that, through assessment of relevant literature, have been categorised as having a high potential for degradation in hydrogen, rendering the asset assembly unsuitable for use with hydrogen without further mitigation. The assessment of materials utilised an assumption-based approach that classified a material as unsuitable due to a lack of literature, or possible variance in actual operation between literature and physical networks operating parameters.
A number of the materials highlighted are prevalent on existing gas infrastructure. The aim of this project is to undertake follow-up review of the outputs and results from the subsequent network assessments, including recommendations to undertake further evaluation of materials of construction, risk mitigation options and propose a testing plan for certain materials.
Benefits
Repurposing the UK gas networks with hydrogen to support the challenge of the climate change act has the potential to save £46 billion with minimal gas customer disruption verses alternative decarbonisation solutions.
As this is continued research and development from NGN_NIA_276, no quanitifable financial benefits are available at this time.
Learnings
Outcomes
Sensitivity analyses
The 27 assets which failed the analysis in Phase 1 (NIA 276) were identified as priority for further analysis to understand the material and number of components affected. Out of these 27 assets, the top five material types, based on lack of confidence in the underlying materials degradation scores were, by least confidence first: Cast aluminium grades; viton fluorocarbon rubber; polyacetyl resin; carbon steel bolts; and sealants (various). The materials ranked by those which appeared in the top five highest number of failed components, by highest prevalence first, were: martensitic stainless steels; carbon steel plate and forged fittings; cast aluminium; cast carbon steel; and Viton fluorocarbon rubber.
Asset assemblies featuring the largest number of failed components and the least number of options for further refinement are governors (asset series 5 and 6), underground modules (asset series 7) and slamshut valves (asset series 8). The family of sealants is different to all others as these are essentially asset assemblies featuring one component and one material. The underlying feature of sealants is the current lack of materials testing data for hydrogen environments.
Materials performance in hydrogen and associated degradation scores
Opportunities to update the scores for materials failing some of the initial assessments have been identified based on new test results and publications. The identified materials are: certain classes of carbon steels such as forgings, pressure vessel steels and steel castings; spring steels; martensitic stainless steels; cast aluminium grades; brasses other than leaded-brass and Viton fluorocarbon rubber (FKM elastomer).
Additional updating to these materials may be possible in the case of further test data becoming available, particularly where this is at the appropriate pressure tier. No changes are currently possible for: Steel bolt materials of any grade; polyacetyl resin and a range of sealants. A test program is known to be underway on sealants.
Re-assessment of selected assets in workshop with GDNs
To improve understanding of the assembly, function and potential failure mode of selected assets, experts and specialists from across four network operators were invited to attend a workshop. Specific expertise on the design, function, operation, and maintenance of assets was applied to a re-assessment of five assets.
Identified Improvements to the NIA 276 Method
The workshop described above also led to the identification of improvements to the method.
Nomenclature: Improved component nomenclature and standard definitions across asset types would improve the consistency of outputs between assessments.
Under and above ground assets: Asset locations to be modified to subdivide these into ‘below’ and ‘above’ ground assets.
Asset combinations: Consistency of approach between the assessments of ‘asset groupings’ which are made up of combinations of other assets should be agreed.
Conditional consequence: Conditionality is not currently considered, a factor could be included to downgrade the consequence if it is conditional on other simultaneous system failures.
Conditionality of inspection and failure history: The ‘failure history’ and ‘inspection and maintenance’ of the natural gas scoring could be linked.
Groupings of asset model numbers and of components: Grouping of model types as one assessment may lead to incorrect outcomes. Certain model numbers may have different materials or designs even though they come under the same parent number, so separate assessments should be made in these cases.
Pressure tiers and stresses: Where a particular asset can be used at different pressure tiers it is worth dividing up the families according to pressure tier to avoid over-conservatism in the assessment outcome for lower pressures.
Hoop stress definitions: Where components other than pipes have a gas retention function, such as valve bodies, the hoop stress does not need to be calculated or modelled but is assigned to one of three levels depending on the ratio of operating pressure to asset design pressure.
Proposed Updates to the NIA 276 method and worksheet tool
The materials degradation scores should be updated to reflect the re-calculated values based on new data. A Location scoring scheme for above-ground assets should be added with a decision step to choose ‘below’ or ‘above’ ground’ assets. The Location scoring for belowground assets should remain in its current format.
A conditional factor should be added for failure history and ‘inspector and maintenance’ to ensure that inspected assets with known failure histories are not doubly penalised.
Re-issue of the worksheet with appropriate version marking and updates described.
Lessons Learnt
Due to the suitability and applicability of the tool, a new proposed NIA project is looking at updating the tool to be used above 7 bar to act as an initial assessment in the suitability of assets with hydrogen in the above 7 bar network.
