ABSTRACT
Steel pipelines are sometimes subjected to demanding sour environments resulting from the presence of high H2S contents. Pipeline materials, therefore, must be resilient against sulfide stress cracking (SSC) which is caused by H2S. Beginning in the 1980s, thermo-mechanically controlled processed (TMCP) steels have been widely used for the manufacturing of large-diameter sour service pipelines. The failure of the Kashagan pipelines in 2013 raised concern regarding the use of TMCP steels in sour environments. These concerns arise from the potential for local hard zones (LHZs) to be produced on the surface of the line pipe during TMCP processes, ultimately leading to through-wall SSC failures. In the present study, several X60–X65 TMCP steels (both with and without LHZs) have been tested under different Region 3 (R3) conditions in the NACE MR0175/ISO15156-2 pH-H2S partial pressure diagram. It can be concluded that the presence of LHZs increases TMCP steels' sour cracking susceptibility; however, TMCP steels without LHZs pass the SSC tests at even the most severe R3 environments. Traditional HRC or HV10 testing are not able to detect LHZs, and so lower load HV 0.5 or HV 0.1 tests are necessary. For TMCP steels, the current R3 may be further divided into R3-a and R3-b sub-regions. The sour cracking severity of R3-a is less than that of R3-b. Additional actions, like enhanced mill qualification of the TMCP plate, should be considered to ensure that no LHZs exist in steels to be utilized in R3-b environments.
The Kashagan project experienced the failure of two 28″ diameter, ∼100 km pipelines in September 2013.1 It was stated by materials experts that the root cause was sulfide stress cracking (SSC) due to unexpected formation of hard spots in the steel.2 The carbon steel pipelines were replaced with corrosion resistant alloy (CRA) clad pipe, with estimated remediation cost of $ 3.6B.3,4 Fairchild et al5 investigated and summarized three hard zone formation mechanisms. Among the three mechanisms, "carbon contamination" and "dual phase" have been known historically in the steel industry. The third mechanism, formation of "local hard zones" (LHZs) is a newly realized phenomenon, which involves extreme heat transfer variations (notably aggressive cooling) during the TMCP accelerated cooling process. These heat transfer upsets are caused primarily by small variations in steel plate surface oxide thickness and roughness. The resulting LHZs are very thin (less than ∼500 μm) and very difficult to characterize through traditional hardness measurement techniques.
