Study On Failure Mechanism And Ultimate Load Capacity Of Corroded Submarine Pipeline

Submarine pipelines, which serve as the arteries of the oil and gas industry, have been widely accepted as one of the most economical ways of transporting oil and gas over long distances. These pipelines typically connect an inlet, such as an offshore platform or an onshore compressor station, to an outlet, which can be another offshore platform or an onshore receiver station. Due to the harsh environment that most of these pipelines are located in and the corrosive medium that these pipes are transporting, deterioration due to corrosion is inevitable. Consequently appropriate residual strength evaluation of corroded pipelines is vitally important. Timely repair or replacement can avoid over deterioration and failure which would result in not only large economical loss but also severe environmental pollution.The burst capacity of submarine pipeline with longitudinal corrosion defects is investigated with the intention of the optimizing the cost effectiveness in corrosion allowance limit design and residual strength assessment of corroded pipeline. Based on the elastic-plastic, large-deformation finite element method, the failure behavior and limit load of submarine pipeline with longitudinal corrosion defect subject to internal pressure are studied. The effect of corrosion depth, length, width and location on burst capacity is also discussed. Based on the FEA results, a set of regression equations are proposed for burst capacity prediction of submarine pipeline with longitudinal corrosion defects. The limit loads predicted by the proposed solutions are compared with those predicted by various solutions provided by the assessment codes and literatures. Furthermore, the predicted limit loads are compared with the laboratory test results and it is concluded that solutions proposed in this paper are in extremely good agreement with the experimental test data, which will give more accurate results than the existing methods.Based on the elastic plastic finite element method, the failure behavior and burst capacity of submarine pipeline with interacting corrosion defects i.e. longitudinally and circumferentially aligned double corrosion defects are analyzed. The effect of corrosion depth, longitudinal and circumferential spacing between double corrosion defects on failure behavior and burst capacity are investigated and the corresponding interaction rules for longitudinally and circumferentially aligned corrosion are proposed. Through the projection of the corrosion profile on the longitudinal and circumferential direction, a new method is proposed for pipeline with more general interaction type named as compounded aligned corrosion defects. Furthermore, the evaluation procedure is extended for pipeline with colonies of corrosion defects. Due to the incorporation of the effective length, width and depth in those solutions, the resistance effect of uncorroded region between each corrosion defects can be reasonably taken into account. Therefore, the procedure is proved to be able to yield more accurate predictions than those in current assessment codes after compare with the experimental results.A set of nondimensional solutions for predicting the full plastic bending capacity of intact pipes in the presence of shear force, torsion moment, internal pressure and axial force is presented based on Mises yield criteria. Using Hencky's total strain theory of plasticity, bending capacity of pipes can be determined analytically and numerically assuming an elastic-linear hardening and power law hardening material. Good comparison is observed when bending capacities obtained from analytical solutions are compared with experimental results from full-size tests of steel pipes. It is shown that strain hardening pipes yield higher bending capacity than pipes with assumed elastic-perfectly plastic material, as commonly assumed in current code, which may underestimate the bending capacity of steel pipes, especially for pipeline in the presence of axial tension.Bending capacity of corroded pipes can be determined analytically assuming a full plastic failure mode for the pipe. A set of generalized solutions for bending capacity of the pipeline can be developed if the shape of the corrosion defect is known a priori. The generalized solutions derived in this paper are able to account for the combined action of internal pressure and axial force. For practical purposes, the generalized solutions thus derived are simplified into approximate closed-form equations using three idealized corrosion shapes, namely, constant-depth, elliptical, and parabolic corrosions. Numerical examples indicate that the closed-form approximate solutions provide good comparison with the generalized solutions. The closed-form approximate solutions are subsequently compared to experimental results from full-size tests of pipes with different corrosion depth and width. Parameter study conducted indicates that the shape of the corrosion defect has significant influence on the bending capacity of the corroded pipes. It is further pointed out that the idealization of the corrosion geometry by constant-depth, as commonly assumed in current code related to corroded pipes, will inevitably underestimate the bending capacity of the pipe, especially for the case of deep corrosion defect.