Failure Analysis Of The Cathodic Protection Of Zibo-Qingdao Natural Gas Pipeline

The combination protection of cathode protection and anti-corrosion coating is widely used in the protection for modern buried long-distance pipeline.But with the increase of pipeline operation time,the anti-corrosion coating will be aging,and the protection level of cathode protection will decrease gradually.Lots of domestic old pipelines appear the problem of “under protection”,and the corrosion risk is becoming bigger.In the purpose of reducing the risk of pipeline corrosion leakage,ensuring the safe operation of pipeline,prolong its service life,the under protection problem of old pipeline need to be researched urgently.The total length of Zi-Qing natural gas pipeline is 187.8 km.It was completed and put into operation in December 30,2002,and has been running for 14 years.After years of operation,the external anticorrosive coating of part of the natural gas pipeline was damaged because of ageing.Corrosion on the outer wall of the pipeline becomes aggravated,and the cathode system part of the area was under protection.The 112 km pipe section between Linzi Station and Fangzi Station is particularly serious.This research takes the pipe section as study object.The reasons for under protection of some pipe sections are found out with the method of cause analysis and field test.Through detail analysis on these reasons,control scheme is obtained,which is designed and verified in field.Main contents of this study are as follows:(1)Based on the systematic summerization of cathode protection principles,and cathode protection methods,and testing of main cathode protection parameters,theoretical knowledge and detection methods of anti-corrosion coating are studied.Detail analysis of each cathode protection parameter and anti-corrosion coating testing methods lays a theoretical foundation for the research of Zi-Qing natural gas pipeline under protection problem.(2)Current potential and current-cut-off potential for Zi-Qing natural gas pipeline between Fangzi Sation and Linzi Station are tested.The testing results show that the currentcut-off potential between 062+631# pile and 076+195# pile didn't reach the value of-0.85 V.It can be determined that this pipe section is under potential protection.Through the detection of anti-corrosion coating along the pipe section,some leakage points were exposed.(3)Through the research of Zi-Qing gas pipeline anticorrosion layer,and AC and DC stray current interference,and cathodic protection station maximum protection distance,it can be concluded that: insufficient protection distance of Linzi Cathodic Station Linzi Cathodic Station is the main cause of "the under protection problem” of Zi-Qing natural gas pipeline.(4)In order to solve the under protection problem of Zi-Qing gas pipeline cathodic protection potential,this paper puts forward three kinds of implementation scheme.Through the comparison of these implementation schemes,the one adding a cathodic protection station between Linzi Station and Fangzi Station is chosen.This implementation scheme is put into detailed design and field velidaty,and the under protection problem of Zi-Qing natural gas pipeline is solved effectively.