Computational predictions of weld structural integrity in hydrogen transport pipelines

TL;DR

The paper addresses the risk of hydrogen embrittlement in seam welds of gas pipelines by coupling weld-process modelling with deformation-diffusion-fracture simulations to predict critical fracture pressures. It develops a multi-physics phase-field framework implemented in Abaqus to simulate diffusion, elastic-plastic deformation, and fracture with hydrogen degradation. Validations against crack-growth resistance curves for X52 and X80 indicate the approach can reproduce hydrogen-assisted fracture behavior, and the framework is applied to two seam-weld geometries to map safe hydrogen pressures. Results show that welding residual stresses, microstructural heterogeneity, porosity, and pre-existing defects can reduce the maximum admissible pressure to as low as 15 MPa, providing a mechanistic basis for assessing the hydrogen viability of existing pipelines.

Abstract

We combine welding process modelling with deformation-diffusion-fracture (embrittlement) simulations to predict failures in hydrogen transport pipelines. The focus is on the structural integrity of seam welds, as these are often the locations most susceptible to damage in gas transport infrastructure. Finite element analyses are conducted to showcase the ability of the model to predict cracking in pipeline steels exposed to hydrogen-containing environments. The validated model is then employed to quantify critical H$_2$ fracture pressures. The coupled, phase field-based simulations conducted provide insight into the role of existing defects, microstructural heterogeneity, and residual stresses. We find that under a combination of deleterious yet realistic conditions, the critical pressure at which fracture takes place can be as low as 15 MPa. These results bring new mechanistic insight into the viability of using the existing natural gas pipeline network to transport hydrogen, and the computational framework presented enables mapping the conditions under which this can be achieved safely.

Figures (17)

• Figure 1: Schematic representation of the finite element modelling framework, aimed at predicting hydrogen-assisted failures of welded joints in pipelines subjected to an internal hydrogen pressure $p$. The first stage of the modelling framework handles the simulation of the welding process, as illustrated here with one weld bead at initial melting temperature $T_\mathrm{melt}$. The second stage uses a coupled, multi-physics phase field formulation to represent the nucleation and growth of cracks, as assisted by hydrogen.
• Figure 2: Temperature-dependence of the material properties of carbon steel SA-516 Grade 70, as per ASME, Section II, Part D ASME_BPVC_IID. The normalised mechanical properties are presented in (a), where $E$ and $\sigma_y$ denote Young's modulus and yield strength at room temperature, respectively, and $\tilde{\Box}$ represents the corresponding temperature-dependent parameter. The temperature dependence of the thermal properties (specific heat $c$, conductivity $k$, coefficient of thermal expansion $\alpha$) is given in (b).
• Figure 3: Hydrogen-dependent critical fracture energy for API steels: X52 ronevichMaterialsCompatibilityConcerns2021sanmarchiMaterialsEvaluationHydrogen2021, X80 shangDifferentEffectsPure2021sanmarchiFractureFatigueCommercial2011sanmarchiFractureResistanceFatigue2012sanmarchiHYDROGENCOMPATIBILITYSTRUCTURAL2021. The fitted curves are obtained using \ref{['eq:hydrogen-Gc']} with fitting parameters listed in \ref{['tab:database']}. For convenience, the corresponding critical stress intensity factor $K_{Ic}$ is added on the right and it is related to $J_{Ic}$, as $K_{Ic} = \sqrt{E'J_{Ic}}$ with $E' = E/(1-\nu^2)$ for plane strain conditions. Also the corresponding hydrogen pressure $p$ is added on top; lattice hydrogen concentration $C$ and hydrogen pressure $p$ are related via Sievert's law $C = S\sqrt{p}$, with the solubility of steel taken to be $S = 0.077\;\mathrm{wppm}\;\mathrm{MPa}^{-0.5}$martinHydrogenEmbrittlementFerritic2020.
• Figure 4: Schematic representation of the boundary value problem considered, a pipeline containing a longitudinal weld. Two weld configurations were employed based on images of ex-service welds from natural gas pipelines: (a) a two-pass weld of dimensions 17 x 14 mm, and (b) a three-pass weld of dimensions 21 x 13 mm. The microstructural heterogeneity of the second weld was characterised using a Vickers Hardness (VH) mapping approach. The associated chemical boundary conditions, including the internal exposure to a hydrogen concentration $C^*$, are also displayed.
• Figure 5: Two-pass weld welding process modelling: (a) schematic representation of the temperature boundary conditions for a two-pass weld; and (b) thermal loading history and corresponding accumulated plastic strain at point 'X' near the weld. Only the first 150 seconds are shown but the simulation is continued until reaching room temperature ($20\degree$C).