Corrosion Analysis of Offshore Oil and Gas Pipelines in Africa: Nonlinear Finite Element Assessment of Burst Pressure

Abstract

Corrosion-induced metal loss reduces the burst capacity of ageing offshore pipelines, yet its effect on pipelines in African offshore fields remains poorly quantified. This study presents a nonlinear finite element analysis (FEA) of an API 5L X70 steel pipeline (outside diameter 600 mm, wall thickness 30 mm) containing a rectangular corrosion defect (depth-to-wall-thickness ratio 0.667, length-to-width ratio 2) under internal pressure, using design data from the ultra-deepwater Medgaz pipeline as a representative case. A quarter-symmetry three-dimensional model was developed in ANSYS Workbench with multilinear isotropic hardening and large-strain, large-displacement formulations. Internal pressure was increased incrementally from 10 to 45 MPa, and failure was defined as the von Mises stress along a radial path through the defect exceeding the ultimate tensile strength of 535 MPa. The predicted burst pressure was 30 MPa, at which the maximum von Mises stress reached 548.02 MPa. Stress, strain and deformation concentrated at the centre of the defect, with hoop stress the dominant stress component. A parametric study examined the influence of defect depth, length and width on hoop stress. The FEA burst pressure was 20.1% lower than the DNV-RP-F101 estimate of 37.55 MPa, indicating a conservative numerical prediction for this deep defect. The results provide a numerical basis for remaining-strength assessment of corroded offshore pipelines.