Challenges in O&G Material Selection: Experiences and Case Histories

The paper gives an overview of recent issues in Oil & Gas material selection, as resistance to high levels of H2S, and quality control during production of Carbon Steel and Clad pipes for pipelines, with reference to real cases. Each topic under discussion is first described showing the relevant issues in terms of materials available on the market, failure mechanisms and possible consequences during the service life. Then the challenges are analyzed by mean of a brief review based on the main and most recent corrosion related conferences. Then, making reference to case histories taken from recent offshore development projects, the Company experience is described, highlighting the technical solutions taken and the main results obtained. The first challenge discussed is the corrosion resistance of pipeline materials for transportation of fluids with high levels of H2S. The carbon steel pipes used for large diameters pipes are commonly manufactured with the TMCP, Thermo-Mechanical Controlled Process method. In severe sour service applications they showed to be susceptible to SSC, Sulfide Stress Cracking, due to hard zones that can originate at surface during pipe manufacturing. New non-destructive tests have been developed for the detection of hard zones during pipe production. Besides, SSC tests have been performed on API 5L X52 and X60 pipes samples at H2S partial pressure higher than 10 bar, which gave surprising results. Clad and lined pipes represent alternative materials to TMCP carbon steel, but they are expensive and only few manufacturers are available, with consequence in terms of long delivery time. Furthermore, these materials are not immune to technical problems, as the precipitation of intermetallic phases in the nickel alloy layer during heat treatment. The paper describes the Company experience made in offshore projects where both TMCP carbon steel and clad pipes have been used for pipelines and buckle arrestors. The corrosion and cracking problems described in the paper are relatively new, as well as the technical solutions adopted. New field experience represents additive information to existing literature.