Abstract
Due to the embrittlement effects of hydrogen on carbon steel, cracking has become a key technical focus for industry practitioners looking at the feasibility of converting existing pipelines to hydrogen operation. A prudent repurposing study should establish what crack size would be acceptable under hydrogen operation and at what rate it would grow through fatigue. This information allows for informed decision making when receiving baseline crack inspection data or considering what plausible crack-like anomalies could be present in the pipeline.
Although the potential repurposing of pipelines to hydrogen is new to the industry, the threat of cracking is not and there is extensive experience in the management of cracking. This experience includes quantifying acceptable crack sizes and the predicted remaining life of a pipeline. This quantification is based upon Engineering Critical Assessments, which rely for their inputs on knowledge of the crack dimensions and material properties (principally fracture toughness). For existing pipelines there is already a large body of work which helps define how to assess fracture toughness when different fracture toughness measurands (e.g. CTOD measurements or J-R curve data) or only Charpy V-notch tests are available. The applicability of these methods to a hydrogen pipeline is questionable.
This paper will explore what fracture toughness means in a hydrogen pipeline by comparing the different test methods and measurands, for example the threshold stress intensity factor (KIH) as defined in ASME B31.12 / ASTM E1681 as compared to fracture toughness measured under a rising load (ASTM E1820 type testing) and a potential “da/dt” threshold for crack growth under a static load. It will also illustrate some of the uncertainties around how to define fracture toughness in a rising load test, together with a discussion of the use of Charpy V-notch data in hydrogen service.
The paper will next illustrate what some of the uncertainties mean in terms of assessing cracks in hydrogen pipelines and demonstrate how the choice of test protocol and measurand can affect the critical crack size and detection capability. It will investigate the validity of the 50 ksi.in1/2 threshold stress intensity factor defined in ASME B31.12 and the implications of assuming this as a minimum fracture toughness value for a hydrogen pipeline.
In summary, the paper intends to provide an overview of quantifying the threat from cracking in H2 while showing how to establish a suitable operation window to meet remaining life expectations.
