Multi-scale modelling of hydrogen-induced fracture and applications to structural integrity of API steel pipelines

Abstract

The increasing demand for energy and natural resources has spurred a flurry of exploration and production activities of oil and natural gas in more hostile environments, including very deep water offshore hydrocarbon reservoirs. One of the key challenges facing the oil and gas industry is the assurance of more reliable and fail-safe operations of the infrastructure for production and transportation. Currently, structural integrity of submarine risers and flowlines representsa key factor in operational safety of subsea pipelines. Advances in existing technologies favor the use of cathodic protection in C-Mn steel pipelines (for example, API X65 grade steel) as an effective technology to prevent metal corrosion in marine and aggressive environments. However, the electrochemical reactions associated with the cathodicprotection system have strong potential to produce atomic hydrogen at the metal surface which can diffuse and enter into the crystalline steel structure causing hydrogen-induced degradation of mechanical and, particularly, toughness properties, often termed Hydrogen Embrittlement (HE). Degraded pipeline materials by HE often fail prematurely and catastrophically after a few years in service making this a key issuein structural integrity assessments of the pipeline infrastructure. Along with experimental studies, computational techniques and numerical simulations are of highly importance to provide both a wider understanding of the HE phenomenon and engineering tools for prediction of materials susceptibility toward HE. As a step in this direction, the main objective of the present research project is to develop a generic multi-scale structural integrity model based on correlation between simultaneously predominant HE micro-mechanisms and macro-mechanical response of API steel pipelines. This requires a much greater understanding of HE mechanisms, which will be achieved by a combined experimental, modelling and simulation programme. In this work, a coupled hydrogen diffusion and cohesive zone modelling approach for modelling hydrogen-induced crack initiation is introduced. In addition, fracture toughness testing of hydrogen charged and dehydrogenated fracture specimens is conducted to calibrate the Cohesive Zone Model (CZM) parameter and to verify the predictive capability of the proposed methodology. The primary contribution of the project is to shed light on the impacts of hydrogen on material degradation by coupling the hydrogen-based finite element model incorporating cohesive elements, the microstructural characterization and the fracture toughness results, including load-Crack Mouth Opening Displacement (CMOD) curves. Indeed, this work will fill the gap in the existing body of knowledge by the development of a multi-scale structural integrity model for prediction of HE and damage in steel pipelines. (AU)