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51313-02613-Role of Hydrogen in Fatigue Crack Growth Rate (FCGR) of X65 Alloys: Modeling Study

Product Number: 51313-02613-SG
ISBN: 02613 2013 CP
Author: Swati Jain
Publication Date: 2013
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Low alloy steels such as X65 are common choices for offshore flowlines and risers in sour service applications. These alloys become susceptible to corrosion fatigue because of large thermal transients and wave motion that lead to fatigue loading and aggressive H2S and water environments that accelerate the corrosion rate. Hydrogen diffusion to the regions with high tensile stress is an accepted mechanism of failure under these conditions. However not much has been quantified in regards to the conditions that lead to build up of hydrogen and its relation to crack growth rate. The variables that are ?K frequency pH and pressure of H2S play a significant role in the fatigue crack growth rate.
A finite element modeling study that uses empirical data as inputs was conducted to understand the mechanism of fatigue crack growth in the presence of hydrogen. The diffusion of hydrogen in the time dependent stress field was modeled at various conditions. The simulation results were compared to the experimental results to validate the model. It was observed that the trends in simulated hydrogen concentration can be directly related to the trends in experimentally observed crack growth rate as different variables were changed. As an extension to the model mathematical relationships between the hydrogen concentrations (at different pH pressure of H2S and ?K) and fatigue crack growth was established. The fatigue crack growth rate at different pressures of H2S followed a power law relationship with the hydrogen concentration build up in the fracture process zone.
 

Low alloy steels such as X65 are common choices for offshore flowlines and risers in sour service applications. These alloys become susceptible to corrosion fatigue because of large thermal transients and wave motion that lead to fatigue loading and aggressive H2S and water environments that accelerate the corrosion rate. Hydrogen diffusion to the regions with high tensile stress is an accepted mechanism of failure under these conditions. However not much has been quantified in regards to the conditions that lead to build up of hydrogen and its relation to crack growth rate. The variables that are ?K frequency pH and pressure of H2S play a significant role in the fatigue crack growth rate.
A finite element modeling study that uses empirical data as inputs was conducted to understand the mechanism of fatigue crack growth in the presence of hydrogen. The diffusion of hydrogen in the time dependent stress field was modeled at various conditions. The simulation results were compared to the experimental results to validate the model. It was observed that the trends in simulated hydrogen concentration can be directly related to the trends in experimentally observed crack growth rate as different variables were changed. As an extension to the model mathematical relationships between the hydrogen concentrations (at different pH pressure of H2S and ?K) and fatigue crack growth was established. The fatigue crack growth rate at different pressures of H2S followed a power law relationship with the hydrogen concentration build up in the fracture process zone.
 

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51312-01477-Environmental Effects on Fatigue Crack Growth Rates in Sour Environments

Product Number: 51312-01477-SG
ISBN: 01477 2012 CP
Author: Ramgopal Thodla
Publication Date: 2012
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