Evaluation of test methodologies for assessing SCC-CO2 in tensile armor of flexible pipes

Abstract

A flexible pipe is constructed using multiple helicoidal metallic and polymeric layers, which give the pipe the necessary strength, and an outer polymeric layer that prevents contact of the transported fluid with seawater. This configuration allows greater flexibility than rigid risers with the same pressure capacity. One of the layers of the flexible pipe is the tensile armor, which is composed of two or four layers of metallic wires that provide axial resistance when the pipe is subjected to internal or external pressure. They are coiled in opposite directions to generate torsional resistance. The National Petroleum Agency, ANP, reported the occurrence of stress corrosion cracking in a flexible gas injection pipeline in its second year of operation, despite having 20 years of lifespan. The failure occurred due to the entry of water and CO₂ into the annular space. The main objective of this work is to assess laboratory methodologies for SCC-CO₂ evaluation in a carbon steel wire of a flexible pipeline, at CO₂ pressure of 10,6 barg, 40 °C and V/S of 1 mL/cm² in synthetic seawater. This condition is known to provoke SCC-CO₂ failures. The methodologies were: slow strain rate tests, four-point bend test, constant load test and alternating load test, which is the new methodology proposed in this paper. We have observed that SCC-CO₂ occurs by successive localized ruptures of FeCO₃ layer, which is addressed in the alternating load test.