PSI - Issue 28

Alla V. Balueva et al. / Procedia Structural Integrity 28 (2020) 873–885 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Introduction The present work is motivated by a long-standing problem facing petroleum and natural gas suppliers. Many countries depend on an extensive network of pipelines to transport crude oil and natural gas. These pipelines are often built with some variation of steel alloy, such as High Strength Steel (HSS). The manufacturing process for these pipelines inevitably creates microscopic imperfections on both the surface and within the metal. The products being transported through the pipe, either petroleum or natural gas, carry hydrogen along with them through the pipeline. The phenomenon of Hydrogen Embrittlement (HE) takes place when the hydrogen present diffuses into the metal pipeline. When this takes place, two important things also happen. The diffused hydrogen collects in the microscopic voids of the pipeline and creates an applied load outward to the crack (Figure 1). Further, the diffused hydrogen interferes with the molecular lattice structure of the metal, increasing the susceptibility of crack growth (e.g., Toribio and Kharin, 1998a; Nykyforchyn and Student, 2006). This process is called Hydrogen Induced Cracking (HIC). Over time, these two combined forces result in fracture formation (e.g., Toribio and Kharin, 1998b; Balueva and Goldstein, 1992), which eventually leads to structural failure of the pipeline.

Fig. 1. Mechanism of Hydrogen Crack Formation in Metal.

When pipelines fail in this way, it presents a tremendous health and safety risk to workers in close proximity. The loss of product and necessary repair are highly costly to suppliers, and the ecological impact of such events are devastating and long-lasting. For these reasons and many others, engineers and industry leaders need a way to estimate the safe service lifetime of a given stretch of pipe, in order to mitigate the risk of such events. In order for a safe duration for a pipeline’s service life to be established, an analytical model of the HE/HIC process must be made available. To that end, much research has been conducted, both theoretical and experimental, in order better explain these physical phenomenon. 1.1. Recent Work in Hydrogen Embrittlement In this section, we present a brief summary of some of the most recent developments in research on Hydrogen Embrittlement (HE). Herein, we discuss a number of improvements to existing theory are highlighted and new methods of modeling are discussed as well. Several theories as to the underlying mechanisms governing HE are present in current research. No consensus has been reached, and a generalized theory that covers all types of Hydrogen-induced fractures does not yet exist. Different theories are favored depending on the kind of fracture achieved. This is discussed in more details as follows: 1a. Hydrogen-Enhanced Decohesion (HEDE) HEDE is a theoretical mechanism that assumes a truly brittle failure as the root cause of crack nucleation. Dissolved hydrogen is believed to lower the cohesive strength of the metal lattice at the molecular level (Zhang et al., 2017),