Electrochemical Corrosion Finite Element Analysis and Burst Pressure Prediction of Externally Corroded Underground Gas Transmission Pipelines

An integrative numerical simulation approach for pipeline integrity analysis is presented in this work, combining a corrosion model, which is the main focus of this paper, with a complementary structural nonlinear stress analysis, using the finite element method (FEM). Potential distributions in the trapped water existing beneath pipeline coating disbondments are modeled in conjunction with reaction kinetics on the corroding exposed steel surface using a moving boundary mesh. Temperature dependencies (25C and 50 C) of reaction kinetics do not greatly affect final corrosion defect geometries after 3- yr simulation periods. Conversely, cathodic protection (CP) levels and pH dependencies within the near-neutral pH range (6.7-8.5) strongly govern depth profiles caused by corrosion, reaching a maximum of 3mm into the pipeline wall. A 0.25 V amplification of CP potential combined with a 0.5mm widening in disbondment opening size reduces defect penetration by almost 30%. Resulting corrosion defect geometries are used for stress examinations and burst pressure evaluations. Furthermore, nonlinear elastic- plastic stress analysis is carried out using shell elements in order to predict the burst pressure of corroded pipes. Corrosion is modeled by reducing the stiffness of a damaged element that has the dimensions of the defect. The predicted burst pressures are in good agreement with those obtained using an experimental-based formula.