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Application of sour linepipes has expanded toward severe sour environment regions including higher H2S partial pressure conditions. In 2013, actual sour gas pipeline failure occurred due to SSC (Newbury et al., 2018). One of the possible root cause of SSC was assumed to be a formation of hard spots in asurface region of steel. Fairchild et al. investigated and proposed three hard zone formation mechanisms including carbon contamination, dual phase microstructure and heat transfer variation in a recent paper (Fairchild et al., 2019; Newbury et al., 2019).
UOE linepipe has been long used for high-strength and severe sour application, however, recently hard-zone issue has been often discussed regarding sulfide stress cracking (SSC) in severe sour environment containing H2S gas. One of the possible root cause of SSC was assumed by formation of hard-zones in the steel pipe inner surface. Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining sufficient tensile properties. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure can be obtained, resulting in stable low surface hardness even in the inner surface of Grade X65 pipe. SSC propagation behaviors were investigated using Grade X65 samples with two different artificial hard-zones. One is the hard-zones with shallower depthless than 0.5 mm made by the laser heat treatment. The other is the hard-zones with depth of approx. 1.2 mm made by the accelerated cooling with intentionally high surface cooling rate. Using these samples, four-point bend (4PB) SSC tests were conducted under 1 bar H2S condition. As the result, when the hard-zone depth was less than 0.5 mm, crack arrested regardless of the hard-zone width or length. On the other hand, when the hard-zone depth was approx. 1.2 mm, crack propagated. It was found that the deeper the hard-zone, the easier the crack propagates. In the mill trial tests of Grades X60 and X65 UOE pipes, it was found that lowering the surface hardness to less than 240HV0.5 led to no SSC appearance in 1 bar H2S condition.
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This paper describes experimental work investigating the influence of steel surface roughness on the adhesion performance of fusion bonded epoxy (FBE) pipeline coatings. The paper begins with a summary of the standards and methods that can be used to measure surface roughness. Several parameters are used to characterize the roughness of a blast cleaned steel including profile peak height and peak count. Tortuosity and rugosity indicate the proportional increase in steel surface area developed by roughening the surface. Normal pipeline coating industry practice is to specify and control a single roughness parameter termed “surface profile”. It is measured with replica tape and corresponds to the maximum peak-to-valley height.In the experimental work steel panels were abrasive blast cleaned with various steel shot and grit abrasives and the roughness characteristics of the blast cleaned surface were measured with stylus profilometers conventional replica tape and 3D imaging of replica tape.A FBE pipeline coating was applied to the prepared steel panels. The adhesion performance of the FBE coating was evaluated using the following test methods.<ul><li>Hot water immersion adhesion rating per CSA Z245.20 section 12.14 </li><li>Pull-off adhesion strength after hot water soak exposure per ASTM 4541 </li><li>Cathodic disbondment radius at 65 and 80 °C per CSA Z245.20 section 12.8 </li><li>Time before blisters were observed in Atlas Cell per NACE TM0174 modified </li><li>Average blister diameter in Atlas Cell </li><li>Pull-off adhesion strength after Atlas Cell exposure per ASTM 4541 </li></ul>The experimental data were analyzed using statistical techniques to investigate the relationship between the measured surface roughness and the adhesion test results. The adhesion results were found to be positively and linearly correlated with substrate tortuosity and rugosity. Profile peak height and peak count were found to contribute to tortuosity.