A new methodology to evaluate the SSC susceptibility of high strength low alloy carbon steels (HSLA) based on a round notched tensile slow strain rate (NTSSRT) method was introduced and discussed by the authors in CORROSION 20211. This novel method provides an economic and more efficient way of evaluating SSC susceptibility of high strength carbon steels compared to the traditional DCB test2 or other fracture mechanics test methods.
The approach unitizes round notch tensile specimen under slow strain rate tested in situ in sour environment. The post-processing of the tensile test results is done with the use of fracture mechanics based approach where the notch is postulated to behave as a sharp crack. The critical stress intensity factor is calculated using an FEA calibrated correlation function (the eta function) and designated as the KISSC for the materials.
In the present study, the calibrated eta-values were derived by use of finite element modeling to calculate the plastic component of the fracture toughness as measured in terms of the Rice's J-integral for the NTSSRT geometry. The eta values were obtained by use of FEA simulations for an Oil Country Tubular Goods (OCTG) C110 carbon steel. The effect of the notch depth and sample size (gauge diameter) on the eta factors were investigated. A new eta function was developed to capture a range of the notch sizes and test sample gauge radiuses (diameters). The new eta factors were applied to the test results obtained using the NTSSRT sample of C110 material. It was found that the new eta function can capture the fracture toughness of C110 material over a range of H2S concentrations when compared to the standard DCB based test method.
Carbon steels and low alloy steels have been used extensively in the oil and gas industry for many decades. The success of their applications in the energy sector is a balanced result of the materials affordability and operation needs in the fields. Producing H2S containing fluids remains a challenge for well integrity as no compromise is acceptable. In recent years, there have been various investigations in the methods of measured material fracture resistance to reduce the conservatism or at least to define new ways that better fit with the field experience in producing H2S containing fluids. This is because the presence of H2S is a contributing factor that may lead to cracking in susceptible materials and therefore shorten their service duration and also compromise well integrity. Therefore, evaluation of the performance of steels in sour environment is critical prior to the applications of those steels to prevent asset loss and environmental damages.