Failures by SOHIC in Sour Hydrocarbon Service
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CASE HISTORY—PEER-REVIEWED
Failures by SOHIC in Sour Hydrocarbon Service Mohammed A. Al-Anezi • Sanyasi Rao
Submitted: 7 April 2011 / Published online: 24 May 2011 ASM International 2011
Abstract This article discusses two failures experienced in spiral-welded pipeline/piping due to stress-oriented hydrogen-induced cracking (SOHIC). The first identified SOHIC failure occurred in 1974, before the term SOHIC was coined. A second failure occurred in 1998 in the piping system handling aggressive sour natural gas liquid (NGL) service. The conditions leading to SOHIC failures and differences between the two failures are discussed. The influence of these failures on company standards is covered. Keywords Stress-oriented hydrogen-induced cracking Sour service H2S Elemental sulfur Spiral-submerged arc welded pipe Longitudinal-submerged arc welded pipe
Introduction Carbon steels installed in wet sour service experience hydrogen-induced cracking (HIC), sulfide stress cracking (SSC), and stress-oriented hydrogen-induced cracking (SOHIC). HIC occurs in the absence of applied or residual stresses, which mostly develops in low-strength carbon steel with yield strength \70 ksi. SSC is prevalent in highstrength carbon steels with high hardness levels exceeding 250 Vickers; however, it requires applied and/or residual M. A. Al-Anezi (&) Saudi Aramco, R-14, Building 9155, Dhahran Heights, Saudi Arabia e-mail: [email protected] S. Rao Saudi Aramco, R-15, Building 9155, Dhahran Heights, Saudi Arabia e-mail: [email protected]
stresses to be present. HIC is dependent on product form and the cleanness of steel as it predominantly attacks wrought products. Cast and forged products are significantly less susceptible to HIC compared to rolled products (plates/coil) that are used for the manufacturing of welded pipes and pressure vessels. The failure mechanism of SOHIC resembles both HIC and SSC. This cracking mechanism is relatively rare and occurs typically in low-strength steels containing high residual stresses. There are 10 reported cases of SOHIC failures as compiled by Pargeter [1]. The authors are of the view that the failure reported by Gajam and El-Amari [2], is due to stepwise cracking (SWC), which is a special type of HIC and not due to SOHIC. SOHIC is characterized by a stacked array of cracks that are linked in the through thickness direction in the rolling plane, approximately perpendicular to the principal stress (residual or applied). This form of cracking is more insidious than HIC due to its through thickness direction of propagation that reduces the load bearing capacity of the component and the crack growth rate is considered much higher. Both cracking mechanisms, including HIC and SOHIC, occur in the base metal or heat affected zone (HAZ) adjacent to the weld. There have been instances of failure due to cracking away from the weld [3]. HIC and SSC are relatively well understood and industry standards are in place to effectively prevent their occurrence. HIC is prevented by using HIC resistant
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