S1.1 This Supplement gives additional requirements and qualifications for the transmission of Hydrogen, including Natural Gas/Hydrogen blended mixtures (subsequently referred to as NG/H blends), and for the repurposing of Natural Gas (NG) pipelines to Hydrogen service. For the purposes of this document any NG/H blend above 10% MOL is considered to be an equivalence to 100% hydrogen. For blends below 10% MOL there is no evidence to confirm that blends containing up to 10 mol.% hydrogen do not cause material degradation, but it is considered that the risk is low.
The clause numbers in this Supplement are as in IGEM/TD/1 Edition 6 (preceded by ‘S’), refer to the clause numbers in the main document and specify the requirements and qualifications which are in addition to those in the main document. Where there is no numbered clause in this supplement corresponding to a clause in the main document, the requirements of the main document apply in full.
Repurposing in accordance with the requirements of this Supplement are only be considered for pipelines which have full maintenance records, and which have been audited in accordance with the requirements of clause 12.4.2.1.
The methodology for repurposing from NG to Hydrogen service follows a general approach that demonstrates compliance with the requirements for an equivalent new pipeline (material, design factor, Building Proximity Distances (BPDs), etc.). If there are  deviations and/or infringements, a case-specific Risk Assessment and/or Structural Reliability Analysis is required to demonstrate compliance.
S1.2 The term Hydrogen service is used to indicate a pipeline transporting Hydrogen or a NG/H blend. The requirements for  Hydrogen service apply equally to NG/H blends, unless the text indicates that the requirement depends upon the concentration of Hydrogen in the blend.
S1.12 This Supplement was published in November 2021.

S2.1 This Supplement covers the design, construction, inspection, testing, operation and maintenance of steel pipelines and certain associated installations in Hydrogen service, and the repurposing of NG pipelines to Hydrogen service, at maximum operating pressure (MOP) exceeding 7 bar and not exceeding 137.9 bar.

S2.3 Where required reference is to be made to IGEM/TD/13 Edition 2 and the Hydrogen supplement to it.

S2.8 NG/H blends are considered to be equivalent to 100 mol % Hydrogen with respect to limits on design stresses, the potential effect on the material properties, and damage and defect categories and acceptance levels, unless an additional technical evaluation is carried out to qualify the materials (see clause S5.8). NG/H blends containing in excess of 10 mol % Hydrogen are considered to be equivalent to 100 mol % Hydrogen with respect to all other requirements.

S2.9 The design factor of pipelines in Hydrogen service is limited to 0.5 (as compared to 0.72/0.8 in NG service). A material performance reduction factor, as defined in ASME B31.12, is applied to reduce the allowable design factor for grades higher than L360 (X52) (see Table 1). Materials may be qualified to operate in Hydrogen service at higher design factors through testing (see clause S5.8).
Note: The material performance factors defined in ASME B31.12 are based on limiting the hoop stress, not derating the tensile properties. The design factors in Table S1 are based on the material performance factors for pressures not exceeding 137.9 barg (2,000 psig).