Prediction of Ductile Fracture Arrest in Natural Gas Pipelines Using the Crack Tip Opening Angle

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Abstract In this paper, an analytical equation is developed to predict fracture arrest CTOA requirements for gas pipelines pressurized with natural gas that undergo single phase decompression using finite element (FE) analysis. The FE model is briefly reviewed. Dynamic pipe fracture simulations were performed to determine fracture resistance curves (crack velocity as a function of pressure at constant CTOA) for modern materials and geometries. American petroleum Institute (API) standard X80 and X100 steels were examined. Pipe diameters of 914 mm, 1219 mm, and 1422 mm, and diameter to thickness ratios ranging from 50 to 100 were considered. The commercial finite element code ABAQUS 2017x-explicit was used to generate the models and solve the analyses. The constant CTOA model was implemented through a user subroutine in conjunction with shell elements. Backfill effects were modelled using smooth particle hydrodynamics and the flap pressure profile was modelled using experimental data from previous burst tests. A CTOA-modified Two-Curve approach was used to determine the arrest CTOA for a given material and geometry. An equation for specification of the arrest CTOA as a function of steel grade and design parameters (thickness, diameter, and material properties) is presented for a gas that undergoes single-phase decompression. The proposed equation was validated through comparison with experimental data.

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