Stress corrosion cracking of oil and gas pipelines: New insights on crack growth behaviour gained from full-scale and small scale tests

A great deal of research has been onduted in the past two decades on pipeline stress corrosion racking (SCC) in near-neutral pH environment. Findings on the effects of operating onditions and the soil environment have been very useful in preventing and mitigating failures. The effets of dynamic loading on rack growth were most evidently demonstrated in full-scale tests carried out on pipes of various grades and dimensions. The initial SCC data suggested a correlation of crack growth rate with the rate of energy input into the crak, as expressed by a trend line between the growth rate and the time rate of J. In the past few years, the fous of research at CANMET-MTL has shifted to crack growth under static loading onditions. Three grades of line pipe steel were tested, namely X-65, X-80 and X-100. Some of the tests lasted for as long as 320 days. It was found that when the applied stress was sufficiently high, albeit well below the yield stress level, craks can initiate and grow under near-neutral conditions. The incubation time is quite long and the growth rates, in the initial stage at least, are very low. Growth rates generally on the order of 10-⁹ mm/s were found for all three steels. The new test results stimulated our renewed attention to the overall SCC behaviour ourring on oil and gas pipelines. It is clear that the overall growth proess consists of slow initiation and growth under relatively stable loading and relatively fast growth when there is dynamic loading, suh as repeated cyclic loads. Crack growth modeling needs to take these various load conditions into aount.