Development of API-X65 Carbon Steel for Severe Sour Service and Hard Spot Detection with Advanced NDT Technology
Proceedings Publication Date
Dr. Seong Ung Koh
Seong Ung Koh, Juseung Lee, Hyosin Kim, Jinyou Kim, Kyungmin Noh, Joonsik Park, Angeire Gonzalez, Ramgopal Thodla, Narasi Sridhar
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Carbon steel pipelines have been widely used for transportation of oil and gas in variety of environments containing different levels of hydrogen sulfide (H2S), which possibly causes sulfide stress cracking (SSC) depending on its partial pressure, operating condition and material’s resistance to SSC. However, hard spots/layers/zones have drawn attention of major oil and gas companies since a giant project has been delayed due to SSC failure of carbon steel pipelines caused by hard spots and high pressure of H2S in 2013.
In this study, a newly developed TMCP (thermo-mechanically controlled process) X65 grade carbon steel plate has been developed for severe sour service with high partial pressure of H2S up to 16 bars. The steel plate has the microstructure of soft ferrite on surface while it comprises acicular ferrite as major microstructure in the middle. Four point bent beam test (FPBT) in reference to NACE TM0316 was performed at 90% actual yield strength (AcYS) in NACE solution A with 16 bar H2S. The developed steel plate was tested along with conventional API-X65MS, API-X80MS, quench & tempered steels, Type 316L stainless steel (SS) and Alloy 625 for comparison. In this test, the developed carbon steel plate, 316L SS and Alloy 625 did not fail, which implies carbon steel pipes with soft ferrite on surface can also be applied to severe sour environment with cost effectiveness.
In addition, an advanced NDT hard spot detection system has been developed using eddy current technology (ECT) with hardness resolution less than 20 Hv, lateral resolution of less than 20mm and maximum scanning speed of 1m/sec. The results of the ECT analysis of hard spots are also presented.

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