Abstract
With the rapidly increasing energy demand, the oil/gas production and pipeline activities have been found in remote regions, such as the Arctic and sub-Arctic regions in North America, which are featured with geological hazards and are prone to large ground movement. The soil induced strain, combined with internal pressure, results in a complex stress/strain condition on pipelines, especially at corrosion defects. It has been demonstrated that the presence of corrosion defect constitutes one of the main threats to pipeline safety. The local stress concentration developed at defect further accelerates the localized corrosion. Moreover, the applied cathodic protection (CP) can be shielded, or at least partially shielded, at corrosion defect. To date, there has been no systematic investigation on the synergism of mechanical and electrochemical factors on localized corrosion reaction at defect. The intrinsic science of this problem has remained unknown, and assessing and predictive models that can be used in practice for pipeline integrity management have been lacking. In this research, various macro- and micro-electrochemical measurements, mechanical testing, and numerical simulation were combined to study the synergism of internal pressure, soil strain and local stress concentration on corrosion at defect on X100 high-strength steel pipelines, and develop theoretical concepts and predictive models to provide guidelines and recommendations to industry for an improved integrity management of pipelines.
A mechano-electrochemical (M-E) effect concept, which was built upon the mechanical-electrochemical interaction on metallic corrosion, is proposed to illustrate quantitatively pipeline corrosion under complex stress/strain conditions. Under elastic deformation, the mechanical-electrochemical interaction would not affect pipeline corrosion at a detectable level. However, the plastic formation is able to enhance pipeline corrosion remarkably. Quantitative relationships between the electrochemical potential of steel and the elastic and plastic strains are derived, which guide the mechanistic aspects of the M-E effect of pipeline corrosion.
A finite element (FE) model is developed to quantify the M-E effect of pipeline corrosion through a multi-physical fields coupling simulation that analyses the solid mechanics field in steel, electrochemical reactions at the steel/solution interface and the electric field in both solution and the steel. Simulation results demonstrate that the corrosion at defect is composed of a series of local galvanic cells, where the region with a high stress, such as the defect center, serves as anode and that under the low stress, such as the sides of the defect, as cathode.
While CP is applied on pipelines for corrosion prevention, a potential drop can be developed inside the defect due to both the solution resistance effect and the current dissipation effect. As a consequence, the CP potential is shielded, at least partially, at the defect bottom, reducing the effectiveness of CP for corrosion protection at defects. Empirical equations are derived to enable determination of the potential drop inside defect, and thus the potential and current density distributions in the defect while CP is applied on the pipeline. They are capable of assessing conveniently for industry the CP effectiveness at corrosion defects and the further corrosion scenario on pipelines.
Furthermore, the present industry models, such as ASME B31G, the modified B31G and the DNV model, for prediction of pipeline failure pressure were evaluated. It is found that the industry models do not apply for pipelines made of high-strength steels, such as X100 steel, and contain corrosion defect with complex geometries, and thus do not provide accurate results. A new, FE-based model, named UC model, is developed to enable accurate prediction of failure pressure of pipelines made of various grades of steel in the presence of corrosion defect under synergistic effect of internal pressure and soil strain. The results predicted by UC model has been echoed by the actual experiences in the field.
Finally, a novel FE model is developed, at the first time in this area, to enable assessment and prediction of the time-dependent growth of corrosion defect on pipelines. The synergistic effects of local stress concentration, corrosion reaction and the defect geometry are critical to the defect growth. The presence of the M-E effect results in an accelerating corrosion at the defect center, generating a geometrical flaw and enhancing the local stress level. The developed model can predict the time dependences of local stress, corrosion rate and the geometrical shape of corrosion defect, thus providing a promising alternative for assessing the long-term growth of corrosion defect on pipelines.
