Extension of Non-Sour Severity Boundary for Carbon Steel Pipeline

Abstract Currently the material selection of carbon steel pipeline for oil and gas field is restricted according to sulfide stress cracking (SSC) severity diagram in ISO15156-2 standard. Nevertheless, there is undefined zone between non-sour and sour region in the mentioned specification and subjected to user's experience. The re-evaluation of undefined zone for expanding non-sour services region become much more interesting for cost reduction strategies on exploration and production oil and gas industries. Therefore, the objective of study is the evaluation of a new region between non-sour and sour region to extend non-sour services zone. According to the lack of adequate data to revise the boundary, the experiment was performed on base metal and welded specimens in accordance with the requirement of the international standard ISO15156-2. As reported in the literature, there are two main parameters that affect cracking resistance which are pH and H2S partial pressure. Therefore, the pipeline cracking resistance in this study was investigated based on these two parameters on a normalized pipe and a quench and temper (Q&T) pipe of carbon steel grade API 5L X52. The results showed that the hardness in welding zone including heat affected zone (HAZ) for a normalized pipe is significantly higher than the base metal, while this phenomenal is not observed for a Q&T pipe. Due to the fact that the microstructure of a normalized pipe is not homogeneous as a Q&T pipe causing the change of microstructure after welding and rising the hardness in HAZ and weldment. In conclusion, the non-sour service zone (region 0, sweet service) can be extended from maximum H2S partial pressure of 0.0035 bar to 0.03 bar with minimum pH value of 3.5 to 2.8 in SSC diagram of ISO15156-2. The pipeline to be used in the extended region shall be Q&T pipeline and maximum material grade of API 5L X52 for offshore application. The updated non-sour and sour region for carbon steel API 5L X52 will be highlighted.