Influence of shot peening on the fatigue life of steel armours in flexible pipes

This study reports on the impact of residual stresses imposed by shot peening on the surface of steel wires in flexible pipes on their fatigue life in corrosive environment, which is relevant when they are used as risers and jumpers in the oil and gas industry. Shot peening was performed on high strength carbon steel samples, which were further subjected to annealing either before or after the shot peening treatment.

Shot peening was applied using different process parameters and a non-peened sample is used as reference. In order to simulate the same stress state as the wires experience during the production, both as received samples and samples after rolling are investigated. The corrosion fatigue behavior was evaluated by applying the 4 point bending test under corrosive environment compared to fatigue testing in air.

Residual stress analysis was performed by Xray diffraction on various treated samples. The results indicate significant influence of residual stresses in the surface on the fatigue life of steel armours. Corrosion fatigue testing of shot peened and non-peened samples in the as-received conditions revealed longest fatigue life for samples shot peened after annealing.

Moreover, results suggest a large reduction in compressive residual stresses caused by plastic deformation of the samples applied by hand rolling of the samples.