External corrosion of oil and gas pipelines: A review of failure mechanisms and predictive preventions

The submarine oil and gas pipelines steel buried in sea mud are always influenced by microorganisms and cathodic protection potential. In this paper, the single and combined effect of sulfate-reducing bacteria (SRB) and cathodic potential on stress corrosion cracking (SCC) behavior of X70 pipeline steel in sea-mud simulated solution was investigated by slow strain rate tests and fractographic observation. Whether in sterile or SRB-inoculated solution, the SCC susceptibility at − 850 mVSCE showed a decline compared with that at open circuit potential. The reason is that the lower number of corrosion pits and the slighter hydrogen evolution at − 850 mVSCE inhibited both of the cracks nucleation and propagation. The SCC susceptibility of X70 steel increased drastically as the cathodic potential shifted from − 850 mVSCE to − 1200 mVSCE in sterile or SRB-inoculated solution. SRB assisted pitting corrosion and the promotion effect on hydrogen permeation make SRB plays a positive role in promoting SCC susceptibility. The SCC susceptibility can be elevated in the co-existence of SRB and cathodic potential but the combined action between them became limited as the potential shifted negatively, which was attributed to the fact that alkalization of solution caused by cathodic potential restricts the growth of SRB.

Effect of sulfate-reducing bacteria and cathodic potential on stress corrosion cracking of X70 steel in sea-mud simulated solution

Field experiment of stress corrosion cracking behavior of high strength pipeline steels in typical soil environments

Mechanical analysis at different scales of gas pipelines

Stress Corrosion Cracking (SCC) is often identified as one of the predominant failure causes of high-pressure gas transmission pipelines. Long-term operation of pipelines causes degradation of mechanical properties of pipeline steel, including a significant decrease in brittle fracture resistance and resistance to SCC. For oil and gas pipeline steels, there are regulatory requirements for impact toughness, disregarding steel condition, namely as-received or serviced. However, SCC resistance of pipeline steels, being very important for structural integrity, is not regulated. In the present work a new method to evaluate in-service degradation of pipeline steels, taking into account increasing susceptibility of operated metal to SCC, was developed. To ensure a safe operation of gas pipelines, it is suggested that the minimum allowable value of SCC resistance characteristics, in particular, the threshold of J-integral based stress intensity factor for SCC Jscc, should be defined and regulated. It is quite difficult to input these characteristics into regulatory requirements, therefore the developed method is based on modification of regulated limit values of impact toughness for as-received and serviced metal conditions separately, taking into account increasing susceptibility of serviced steel to SCC. The method was applied to evaluate in-service degradation of API 5L X52 ferrite-pearlite pipeline steels. The developed method considers different SCC susceptibility of as-received and serviced pipeline steels and shows there are limitations in regulating the same limit value of impact toughness for as-received and serviced pipeline steels.

Assessment of in-service degradation of gas pipeline steel taking into account susceptibility to stress corrosion cracking

With the development of the natural gas industry, gas transmission pipelines have been developed rapidly in terms of safety, economy and efficiency. Our recent studies have shown that an important factor of main pipelines serviceability loss under their long-term service is the in-bulk metal degradation of the pipe wall. This leads to the loss of the initial mechanical properties, primarily, resistance to brittle fracture, which were set in engineering calculations at the pipeline design stage. At the same time stress corrosion cracking has been identified as one of the predominant failures in pipeline steels in humid environments, which causes rupture of high-pressure gas transmission pipes as well as serious economic losses and disasters. In the present work the low-carbon pipeline steels with different strength levels from the point of view of their susceptibility to stress corrosion cracking in the as-received state and after in-laboratory accelerated degradation under environmental conditions similar to those of an acidic soil were investigated. The main objectives of this study were to determine whether the development of higher strength materials led to greater susceptibility to stress corrosion cracking and whether degraded pipeline steels became more susceptible to stress corrosion cracking than in the as-received state. The procedure of accelerated degradation of pipeline steels was developed and introduced in laboratory under the combined action of axial loading and hydrogen charging. It proved to be reliable and useful to performed laboratory simulation of in-service degradation of pipeline steels with different strength. The in-laboratory degraded 17H1S and X60 pipeline steels tested in the NS4 solution saturated with CO 2 under open circuit potential revealed the susceptibility to stress corrosion cracking, reflected in the degradation of mechanical properties, and at the same time the degraded X60 steel showed higher resistance to stress corrosion cracking than the degraded 17H1S steel. Fractographic observation confirmed the pipeline steels hydrogen embrittlement caused by the permeated hydrogen.

Volodymyr Myroslavovych Tsependa

Papers

Stress corrosion cracking of gas pipeline steels of different strength