Offshore pipelines and risers constructed from carbon steel have reduced levels of performance when operating in a sour environment. Characterizing material properties i.e. fracture toughness and fatigue crack growth rates, in aggressive corroding environment require experimental data. KISCC was suggested as the fracture toughness in the engineering critical assessment (ECA) to derive the acceptance criteria of the root flaw location in the inner surface of the pipeline. It was well known this methodology is over conservative. In this paper, the procedure for the Single Edge Notch Tensile (SENT) specimen fracture toughness testing of the pipeline girth weld in a sour (H2S containing) environment was proposed and the test results were discussed. Based on the fracture toughness tested in sour service environment, the ECA procedure was suggested and used successfully in practice. Because the conservatism was reduced, then less repair, faster installation and therefore less cost can be expected.
For the pipeline transporting sour gas or crude oil containing H2S, the internal surface will work in sour environment. When the pipeline is highly stressed for long periods of time, stress corrosion cracking (SCC) failures may occur. The necessary conditions for SCC are the presence of the crack, a sufficiently high hydrogen concentration, tensile stresses and stress intensity factor (KI) larger then the threshold value at the crack tip. The latter condition is called KISCC, - the threshold stress intensity factor in the presence of hydride, below which no crack growth occurs. At KI values above KISCC the SCC growth rate depends on many factors, i.e. temperature, chemistry of the environment, microstructure and the stress intensity factor at the crack tip. The SCC growth law is similar to that described by fatigue crack extension, but there is no single unified theory to explain SCC.
