NEW CP MAINTENANCE CONCEPT FOR BURIED STEEL PIPELINES -- Current Density-based CP Criteria, and On-line Surveillance System for CP Rectifiers

摘要

New cathodic protection (CP) criteria based on DC and AC current densities have been developed to assess the increasing risks of AC corrosion and overprotection on cathodically protected buried steel pipelines. Furthermore, in order to perform the risk assessment of the pipelines subject to these risks with respect to the new CP criteria, a new hand-held instrument that enables the simultaneous measurement of DC and AC current densities in the field was developed. The risk of rectifier failure due to lightning surge and the aging of ground beds is also increasing. An on-line surveillance system for rectifiers was then developed in order to immediately detect the failure of rectifiers and allow corrective action when required. New CP maintenance program consisting of 1) CP protection level monitoring using the new criteria and instrument, and 2) remote monitoring of the rectifiers using on-line surveillance system, was conducted on a newly installed pipeline in close proximity to overhead high-voltage AC power transmission lines. AC corrosion risk was successfully detected and eliminated using solid-sate DC decoupling devices. Reliable remote monitoring of the rectifiers was also successfully performed, thus ensuring the adequate supply of CP currents. Finally, a new concept of CP maintenance using these new techniques was established.