Developing a Three-Dimensional Model to Estimate Electrical Current Requirement at a Coating Defect Site on Pipeline Segments Installed Using Horizontal Directional Drilling, 20–R8429
Inclusive Dates: 11/19/13 – 03/19/14
Background — Horizontal directional drilling (HDD) is being increasingly used in trenchless installation of coated pipelines. A schematic of an HDD-installed section is presented in Figure 1. The coating on the pipeline is particularly susceptible to damage during the HDD installation process. A cathodic protection (CP) system is often used to mitigate corrosion of the installed pipeline segments. The objective of this project was to develop the capability to estimate performance of the CP system for the HDD pipeline segments. Such a capability can be used to determine whether computer modeling is a feasible approach for confirming effectiveness of a CP system; assess optimal ground bed types, anodes, and configurations to maximize CP effectiveness at HDD locations; evaluate effects of a resistive rock stratum on CP system performance; develop guidelines for incorporating CP system monitoring hardware and configurations into HDD designs to ascertain protection levels throughout the entire HDD length; determine levels of CP protection when the pipe segment is partially in contact with soil and partially exposed to the air in the gap of the HDD borehole; and correlate in-line inspection metal loss data with CP inspection data. The capability can also be used to analyze field scenarios that cannot be adequately addressed by current CP design equations. This project was sought to develop such a capability and respond to a likely request for proposal from a potential client.
Approach — A boundary-element-method (BEM) model was developed to simulate CP systems on coated pipeline segments installed using HDD. The BEM model took advantage of specialized surface elements that explicitly account for the shape of the surface, which speeds up computations by orders of magnitude by greatly reducing the number of unknowns to be determined. A single cylindrical element replaces one or more rings of rectangular elements, which are typically used to discretize pipeline surfaces. Verification of the BEM model used a second model, based on finite element method (FEM)-based computational software. The FEM model provides an independent calculation check for specific conditions where both models are accurate.
Accomplishments — The BEM computation model was successfully developed and validated with the FEM-based model. The BEM model can consider an arbitrary number of objects and subzones in an installed pipeline; can consider bare metal, resistive coatings, and nonconductive coatings; has flexible gridding with automated refinement; allows high-order integration of elements near singularities for improved accuracy; and can consider pipelines that are up to several miles long. The capability developed during this project has positioned SwRI well for conducting modeling work in the field of pipeline corrosion. The validation results indicated that the cylindrical elements in the model were correctly implemented and provide guidance on their applicability to field problems. The validation results suggest that the cylindrical elements provide good estimates when the anode is more than two or three pipe diameters from the pipe segment and suggest that an anode can be represented accurately using cylindrical elements unless it is very close to the pipe segment. Further testing is necessary to determine appropriate separations for a given accuracy level.