Finite element analysis of combined static loadings on offshore pipe riser repaired with fibre-reinforced composite laminates

The stress-strain response of an offshore pipe riser subjected to combined internal pressure, tension and bending is studied. Finite element analysis was used to study three conditions of pipe riser - uncorroded, corroded and corroded and repaired with designated laminate orientation of carbon/epoxy or E-glass/epoxy fibre-reinforced composite. The behaviour of the pipe riser (grade API 5L X60 steel) was studied using Ramberg-Osgood model. Two composites laminate systems, a pre-cured prepreg (grade AS4 3501-6 carbon/epoxy) and a wet-layup filament-wounded composite (grade Gevetex LY556/HT917/DY063 E-glass/epoxy), were characterised. Design conditions were determined via a limit analysis known as the double-elastic slope method. The results showed that under combined hoop, tensile and bending loads, the riser tends to approach failure at a much lower strain compared with each of these loads being applied individually. The limiting design factor of the composite repair system was due to excessive tensile strain experienced in a bent riser while the compressive stress caused by reversing the bending load occurred well within the linear-elastic region. With respect to the types of composite repair system, carbon fibre displayed a much better strength rehabilitation over glass fibre. In the aspect of laminate orientation, off-axis plies [90 degrees/ +/- 30 degrees](s) and [90 degrees/ +/- 45 degrees/0 degrees](s) laminates were found capable of restoring the strength of the corroded riser and provide superior reinforcement in both hoop and axial directions.