Technique for detecting corrosion and coating flaw of buried pipeline

The main gas pipeline of southwest oil and gas field is 1810 km long and the newly laid pipeline increases continuously. The active pipeline and newly laid pipeline were all buried underground. The external corrosion prevention was realized by adopting asphaltic coating and impressed current cathodic protection. At present, the crocodile, rupture, exfoliation and aging appear on the asphaltic coating and some pipeline sections coating has effloresced and lost its protection effect. Because the current leakage points increase and the load of cathodic protection is raised, the electric potential of some pipeline sections is difficult to reach the minimum criterion, which causes a serious dissolution of the cathodic coating. The majority of the pipeline's asphaltic coating in Southwest Oil and Gas Field has been used over 20 years. The difficult problem faced at present is to detect the coating flaw and to find out the real polarization potential distribution and corrosion degree. Only the coating flaw is detected and repaired can the cathodic protection potential satisfy the criterion requirement and can the external corrosion be inhibited. Thus, a detection technique which can satisfy engineering use is necessary. In this paper, through optimizing the corrosion detection techniques adopted at worksite, the instrument pig and RD-PCM technique are recommended to be used and the advantages and shortages of other coating flaw detection techniques are introduced in passing.