Efeito da deformação plástica acumulada no desempenho à fadiga de aço super duplex empregado em tubos umbilicais para produção de petróleo

Abstract

The umbilical cable is today an important technology used to control equipment on the seabed in the production of offshore oil in deep and ultra-deep waters. The umbilical is composed of several components, including steel tube for transporting fluids for hydraulic control of equipment in the bed. Such tubes are susceptible to fatigue during their life in umbilical operation. Fatigue strength is influenced by the accumulated plastic strain (APS) undergone by the tubes during the entire manufacturing process until installation/storage on spools. This study investigated the way in which two different conditions of application of APS (repeated flexion and alternating flexion) influence the fatigue strength of the tubes. For this, fatigue specimens were machined from super duplex umbilical tube, which received 5% APS by repeated cyclic bending and alternating cyclic bending. Residual stresses and strains that were accumulated in the specimens were measured by computer simulation. Microstructural and microhardness analyzes were performed on the specimens after fatigue fracture. Although the microhardness of the group subjected to alternating bending showed a slightly higher hardness, ANOVA showed that this difference was not significant, as there were no relevant changes in the microstructure of the two groups of specimens after fatigue fracture. However, the group subjected to alternating plastic strain experienced lower fatigue strength. Therefore, the way in wich the tubes are deformed during manufacture and storage influences their fatigue performance.