CTOA: a tool for risk prevention in pipelines transporting dangerous gases

This paper presents the Crack Tip Opening Angle (CTOA) as an effective fracture mechanics criterion for preventing risks associated with pipelines transporting hazardous gases such as hydrogen and ammonia. CTOA is used as a global parameter to characterize resistance to ductile crack propagation and to predict crack arrest conditions in pressurized pipelines. Unlike initiation-based fracture parameters, CTOA directly describes steady-state crack growth and is therefore well suited for assessing long-running cracks that may lead to catastrophic failures. The paper reviews the fracture behavior of pipeline steels, distinguishing between crack initiation and propagation, and highlights the advantages and limitations of the CTOA approach. Several CTOA measurement techniques are discussed, including direct optical methods, indirect load — displacement curve analysis, and fracture surface microtopography. The influence of material properties, specimen geometry, thickness, loading mode, and plastic constraint on CTOA values is examined, with particular emphasis on the significant scatter observed in experimental data. CTOA is implemented in finite element simulations using a node-release technique to model ductile crack propagation and arrest under internal pressure. The predicted arrest pressure and crack length are compared with established approaches such as the Battelle Two-Curves Method, showing good agreement. Once crack arrest is determined, the resulting breach size is used to calculate gas outflow rates. Finally, gas dispersion modeling, combined with CTOA-based fracture analysis, enables the determination of safety distances for toxic or explosive gas releases. A case study using ammonia and the ALOHA dispersion software demonstrates how CTOA can be integrated into a risk-based framework to ensure that lethal risk remains below regulatory thresholds over the pipeline’s service life.

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