Computational modelling of cathodic protection systems for pipelines in multi-layer soil

Computational modelling of cathodic protection (CP) systems involving thin multi-layer media represents a real challenge in terms of accuracy and efficiency required in the numerical calculation.In the case of CP for transmission pipelines, these long metallic structures are usually buried a distance H (approximately a metre or so) below ground level and extend horizontally typically more than ten thousand times H.A number of impressed current anode beds are distributed along the pipeline, providing protection against corrosion of the structure.In addition, the vertically stratified nature of the soil needs to be considered in the model, in order to obtain more accurate representation of the environment.This is particularly relevant when considering the effect of different types of rocks, soil porosity, or water saturation, at different depths.This type of scenario requires three dimensional modelling involving a thin multi layered electrolyte, with a typical aspect ratio (lateral extension to thickness) of the order 1E4 to 1E6.The paper presents an efficient and accurate computational approach based on the Boundary Element Method for simulating the level of protection against corrosion of the pipeline as well as current densities and electric potential in different points of the soil.The resulting modelling approach is then applied to assessing real case scenarios.The simulation approach considers the non linear electrode kinetics on the metal surfaces in the form of polarisation data and also the internal resistance of the pipeline and other electrical connections involved in the CP system.Example applications are presented showing how the model can be used to predict the "signatures" associated with different defect types in the pipe coating.