Coupled thermo-chemo-mechanical phase field-based modelling of hydrogen-assisted cracking in girth welds

TL;DR

This paper introduces a comprehensive computational framework combining thermo-mechanical and phase field models to predict hydrogen-assisted cracking in girth welds of pipelines, accounting for complex interactions between residual stresses, hydrogen transport, and fracture.

Contribution

It presents a novel integrated modeling approach that captures the interplay of residual stresses, hydrogen trapping, and fracture in welds, specifically applied to complex girth welds in pipelines.

Findings

Good agreement with laboratory fracture experiments

Identifies critical failure pressures for various defect scenarios

Highlights the impact of weld microstructure and defects on integrity

Abstract

A new computational framework is presented to predict the structural integrity of welds in hydrogen transmission pipelines. The framework combines: (i) a thermo-mechanical weld process model, and (ii) a coupled deformation-diffusion-fracture phase field-based model that accounts for plasticity and hydrogen trapping, considering multiple trap types, with stationary and evolving trap densities. This enables capturing, for the first time, the interplay between residual stresses, trap creation, hydrogen transport, and fracture. The computational framework is particularised and applied to the study of weld integrity in X80 pipeline steel. The focus is on girth welds, as they are more complex due to their multi-pass nature. The weld process model enables identifying the dimensions and characteristics of the three weld regions: base metal, heat-affected zone, and weld metal, and these are treated distinctively. This is followed by virtual fracture experiments, which reveal a very good agreement with laboratory studies. Then, weld pipeline integrity is assessed, estimating critical failure pressures for a wide range of scenarios. Of particular interest is to assess the structural integrity implications of welding defects present in existing natural gas pipelines under consideration for hydrogen transport: pores, lack of penetration, imperfections, lack of fusion, root contraction, and undercutting. The results obtained in hydrogen-containing environments reveal an important role of the weld microstructure and the detrimental effect of weld defects that are likely to be present in existing natural gas pipelines, as they are considered safe in gas pipeline standards.