Failure pressure estimation for cracked aged and unaged pipelines using finite element method

Abstract

Natural gas plays a key role in a world of restricted carbon technology. Pipelines typically transport it, so it is important to keep these structures safe. One strategy to reduce the risk in working pipelines is adequately estimating the failure pressure. Some corrosion mechanisms or external mechanical damage can cause longitudinal cracks on the pipe surface. The geometry and size of these cracks can vary depending on many factors. Additionally, the mechanical properties of the pipe material (steel) tend to deteriorate as the pipe ages; this is especially true for older pipes. For this reason, the present research jointly studies the effect of longitudinal cracking of the pipe and the changes in the mechanical properties (mainly yield strength and ultimate tensile strength) derived from the aging process of the steel. To obtain the aged material, an accelerated isothermal aging procedure is proposed, and afterward, the stress-strain curves of the API 5 L Gr. B steel in aged and non-aged conditions are computed. Then, a Finite Element Analysis (FEA) study is conducted to estimate the failure pressure for 24 different-sized crack defects. Failure estimations are also done using prediction models (Ln-Sec and modified Ln-Sec). Finally, the results of both estimation methods are compared. An actual mean burst capacity loss (BCL) of 5.4 % (1.05 MPa loss) is found due to the aging of the material. The results suggest that taking into account the changes in the mechanical properties for the estimation of pipeline failure pressure can increase the pipeline's reliability.