Abstract
Novel experiments and computational models are presented to quantify safe regimes of operation in natural gas pipelines retrofitted for hydrogen transmission. The focus is on characterising the structural integrity of the weld and its most susceptible regions, as these will govern pipeline integrity and limit operation conditions. The use of mechanistic, validated models enables resolving the weld complexity (residual stresses, microstructural heterogeneities) and efficiently assessing pipeline integrity over the vast combination of asset conditions (pre-existing defects, weld and pipe materials) and operational scenarios (H2 purity and pressure) relevant to a natural gas pipeline infrastructure that has been deployed over more than five decades. Thermo-metallurgical weld process modelling reveals an excellent agreement with experiments (hardness maps, residual stress measurements), validating the resulting distribution of residual stress and phase volume fractions/properties. Subsequent phase field-based deformation-diffusion-fracture simulations enable predicting critical failure pressures and arbitrary cracking patterns as a function purely of physical parameters that can be independently measured and are well-established (solubility, diffusivity, toughness). The results reveal that hard, brittle phases in the weld Heat-Affected Zone (HAZ) have a significant impact on the structural integrity of hydrogen transmission pipelines, revealing the inadequacy of assessments based on weld or base metal, and the need to appropriately resolve the microstructural heterogeneity of welds. To facilitate the integration into engineering design and fitness-for-service assessment of the mechanistic insight provided by modelling, virtual Failure Assessment Diagrams (FADs) are built, showing how phase field fracture models can naturally capture the FAD regimes and therefore establish a safety factor due to hydrogen and weld microstructure.