Safety requirements for the Second high-pressure and large-diameter West-East Gas Pipeline

The Second West-East Gas Pipeline (WEGP) is planed be constructed trans-China within recent years. The Pipeline is about 7,000 kilometers with 0.9, there is much possibility for the happening of fracture initiation in the weld zone. When Ms is higher than 1.2, the limit load will not increase with the increase of weld strength. Although the outside diameter 1219mm, operation pressure 12MPa, and steel grade API-X80 [1], all of which is the first time in China. Both safety and reliability of the Pipeline are the most important issues which should be thought over by the gas company. In this paper, the method of limit state analysis is used to study the effect of strength, toughness and allowable flaw size on the safety and reliability of the Pipeline. The calculation results suggest that the overmatched weld has the advantages of improving limit load and maximum allowable defect sizes of the pipelines. When Ms (Ms is the ratio of yield strength of weld sigma (SW) over that of base metal sigma (SB), i.e. M-S = sigma (SW) /sigma (SB)) is lower than Second WEGP will transmit sweet gas, there is still possibility that sour gas releases to the Pipeline in case of operation accident. As to the high-grade pipeline, in order to reduce the sensitivity of cold crack, hydrogen induced crack (HIC) and stress induced corrosion crack (SCC), etc., However, chemical composition and hardness of both base and weld metal can influence the resistance to HIC and SCC. It can be concluded that the best range of strength mismatched ratios Ms is 0.9 1.2 for the Second WEGP. The critical defect sizes proposal is 10.0mm of maximum allowable defect length and 0.95mm of maximum allowable defect depth at the weld mismatch ratio 0.9. For evenmatched and overmatched weld, the critical defect sizes are much bigger than that of the undermatched. According to the full-scale burst test results, the more practical proposal for the minimum average arrest toughness is 190J, but the more conservative or safer proposal is 220J for the Second WEGP. As a result, the research work will give guidance to the technical requirements of the specification that will be drawn up for the Pipeline in a near future.