The Effect of H2S Consumption on SSC Susceptibility of High Strength Wire Grades for Flexible Pipes

Abstract Flexible pipes are widely used in offshore installations, especially in Brazil to produce and export oil and gas. In order to guarantee safety operation in pre-salt, layer that occurs at a depth of 5 to 7 km and under a layer of salt up to 2 km thick, without overestimating the field corrosiveness, many efforts are dedicated to study the actual H2S concentration in the pipe annulus. The data presented in the literature points to considerably lower H2S concentrations than those predicted based on traditional annulus permeation models. The consequences of these studies have direct impact on flexible pipe design, suggesting the possibility to use lighter, lower cost pipes with good performance structures. However, to correctly specify the armor wires it is crucial to evaluate the corrosion resistance on annulus condition, especially the stress corrosion cracking. The present study goal is divided in three parts: 1 - to compare experimental data with simulated data obtained by a commercial software to estimate pH and the concentration of H2S dissolved in different flow conditions (mL of H2S/min) ; 2 - to evaluate the H2S consumption due to the confinement condition considering different flows and 3 – to evaluate the Sulfide Stress Cracking (SSC) by 4 points bend test of high resistance wires exposed to confinement conditions by using an experimental setup that despite the fully packed test vessel guarantees the fluid homogeneity throughout the test. All experiments were carried out at 30 °C, 0.1 MPa using a gas mixture with 1% H2S in CO2. The ratio of liquid phase/steel surface area (0.7 mL/cm2) was also controlled. The lowest flow rate studied was 10-6 mL/min/cm2. For the three flow conditions studied, without confinement, the saturation concentration of H2S in solution was approximately 24 ppm but less than 1 ppm was found with confinement. The pH also changed from 4.9 to 5.6 and 6.1, to experiments without and with confinement, respectively. After three months test no HIC or SSC evidences were found on wire samples tested under confinement conditions equivalent to 10-6 mL of H2S/min/cm2.