Hybrid Riser Resistance to In-Service Fatigue Loading

Abstract

Riser systems are subjected to complex loading spectra. A wide range of amplitude loading are induced by naturally complex sea states. The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (riser pipe and its girth welds or threaded connections). The present paper reports numerical calculations and a comparison of the actual fatigue loading experienced by different riser systems, Steel Catenary Risers (SCR), Lazy Wave Riser (LWR), Hybrid Risers (HR). For the Hybrid riser system, a good fatigue resistance, due to the vessel and wave first motion decoupling, is obtained. On the other hand, the phenomenon of cross flow vibration induced by Vortex Induced Vibrations (VIV), could significantly affect the hybrid riser fatigue performance. In this case, a sensitivity analysis has been performed to evaluate the influence of different parameters, on the applied fatigue loading, like: riser tension, hydro diameter (i.e. external pipe diameter including coating), riser wall thickness, current velocity. The HR variable amplitude loading spectrum, derived from in-service conditions, referred to Gulf of Mexico scenario, has been calculated and applied in laboratory tests on girth welds of X65, 10.75″ OD, 25.4 mm WT riser. Furthermore, the fatigue performance of these tests has been compared to analogous tests on samples subjected to constant and variable amplitude loading available from previous work on SCR system[1].