Methodology Proposal for Corrosion-Fatigue Assessment for Flexible Pipes Tensile Armour in Aggressive Environments

Flexible risers are the main way to connect wells and floaters for petroleum and gas applications on the Brazilian coast. The failure of those risers in operation can generate huge irreversible economic and environmental expenses. Statistics indicate that there is a predominance of premature failure associated with the flooding of the annular space. When cyclic loads are present, this condition will introduce corrosion-fatigue of the metallic reinforcements. Therefore, to ensure the operational safety, their design should consider the combined action of fatigue and corrosion phenomena. The experimental test procedure to be applied for generating S-N curves and qualifying the steel materials with respect to corrosion-fatigue face different challenges. One of the important ones is to introduce representative surface defects that reflects the exposure time of the real life corrosion process as this will have a strong influence of the slope parameter of the S-N curves. The aim of this work is to propose a new methodology to obtain S-N design curves allowing for the samples to be pre corroded to obtain realistic surface defects. The pre corrosion procedure is made to ensure that all samples have the same defects caused by localized corrosion and that those defects are representative of those presented in real risers with flooded annulus. Considering the effect of the defects aspect on fatigue evaluation allows a better prediction of the remaining life of pipes operating in flooded annulus conditions. Two S-N curves were compared allowing for proving the potential of the methodology and promising results were obtained, giving more reliable S-N curves. Furthermore, it was possible to conclude that under the assessed conditions the fatigue in air tests of pre-corroded wire during 60 days showed the same performance as the new one (non-pre-corroded) wires tested in corrosion-fatigue.