PROBABILISTIC APPROACH TO PIPE CRACK INITIATION FAILURE AND CRACK PROPAGATION FAILURE IN H_2S ENVIRONMENTS

摘要

A low alloy steel's propensity for sulfide stress cracking in H_2S frequently is evaluated based on passing or failing NACE TM0177 Method-A and Method-D tests performed at a single (usually maximum) value of material hardness. Here we explore a probabilistic assessment of cracking tendency based on characterization of the mean Method-A threshold stress and the mean Method-D stress intensity factor (K_(ISSC)) as continuous functions of hardness. For pressurized pipe in H_2S, the sensitivity to cracking failure is proposed to depend not only on the mean threshold-vs.-hardness and the mean K_(ISSC)-vs.-hardness, but also significantly on multiple factors which contribute to the scatter and standard deviation of cracking performance. The model characterization of these factors enables assessment of the risk of failure due to a combination of material, environment, and applied load. This concept is applied to the pressure performance limit of pipe for cracking failure in H_2S, and this is used to rank the importance of different factors which contribute to the probability of pipe failure in a given H_2S environment. This approach identifies significant risk factors which usually are ignored in material testing at a single hardness. The ranking of relevant factors can be used to guide testing. A case is made for evaluating the risk of environmental cracking failure not at a single hardness point but rather as a function of hardness varied over applicable ranges.