An Engineering Tool for Predicting Corrosion-Fatigue Crack Growth Rates for Structural Steels in Sour Environments

Laboratory testing has shown that sour brine environments can reduce the fatigue life of offshore steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air. Thus, in order to ensure safe, reliable, and environmentally-friendly deepwater development, the effect of these sour service environments must be properly accounted for in riser and flowline design. However, to ensure that the environmental effect is fully captured, tests need to be conducted at cyclic loading frequencies representative of those experienced in service (typically 0.1 Hz or less), which makes corrosion-fatigue testing very time-consuming and costly. Consequently, there has been a need for predictive models that can reduce the dependence on extensive long-term testing, while at the same time enable existing data to be interpolated and/or extrapolated over a broad domain of relevant mechanical, environmental, and material variables. In response to this need, a Joint Industry Project (JIP) was organized by Southwest Research Institute^® (SwRI^®) with the objective of developing and validating an analytical model to predict corrosion-fatigue performance of structural steels in sour brine environments. The resulting model is based on the kinetics of hydrogen generation and transport to a fracture process zone (FPZ), where embrittlement occurs in the hydrostatic stress field ahead of the growing crack. The advantage of this kinetic model is that details of the embrittlement process, which are not presently well defined, need not be included since corrosion fatigue crack growth (CFCG) is governed by the rate-controlling process (RCP) in the elemental kinetic steps that supply hydrogen to the FPZ. A general outline of this model is provided here and its validation against independently generated experimental data is demonstrated. The validated model has been implemented in spreadsheet format for convenience as an engineering tool. Due to the fundamental concepts underpinning the model, the engineering tool is shown to be adaptable to predicting CFCG rates in steels exposed to a variety of other environments — including hydrated and dehydrated sour crude oil, moist H₂S gas, sweet brine, and seawater — with and without cathodic polarization. An extension of this Phase 1 model from intermediate to lower CFCG rates is currently underway in Phase 2 of the JIP but will not be discussed in detail in the present paper. The primary objective of this paper is to introduce the engineering tool based on the Phase 1 analytical model and demonstrate its functionality in quantifying CFCG rates over wide ranges of mechanical variables (stress-intensity factor range (ΔK), load ratio (R_σ), and cyclic loading frequency), environmental variables (H₂S partial pressure, pH, temperature, applied potential), and material variables (yield strength).