Abstract
Historically, C-Mn steels have been extensively used in line pipe applications for sour service oil and gas environments, i.e. in the presence of hydrogen sulfide (H2S). In the past few decades, the emergence of the Thermo-Mechanically Control Process (TMCP) manufacturing method has further optimized the benefits of increased fabricability, weldability, and large cost benefits over alternate materials for large diameter pipe manufacturing techniques. However, implementation of C-Mn steels in sour service does require increased focus on steel cleanliness and hardness control to avoid susceptibility to hydrogen damage mechanisms such as hydrogen induced cracking (HIC) and sulfide stress cracking (SSC). These additional requirements have been addressed in industry standards such as NACE MR-0175/ISO 15156. Despite these industry practices, recent pipeline projects have experienced failures related to SSC and have found increased surface hardness due to Local Hard Zones (LHZs) in delivered and installed pipe. The LHZ phenomena appears to be a relatively new concern to the industry, and has led to concern over use of TMCP steels after the Kashagan pipeline failure in 2013. ExxonMobil has developed a qualification program based on the requirements of NACE MR-0175/ISO 15156 and new insights regarding the root cause of LHZs. This work reports details on this qualification approach implemented for X65 grade line pipe from recent qualification activities with TMCP plate and line pipe suppliers.
