Multiple Axial Fatigue of Pressure Armors in Flexible Risers

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The design of flexible risers has been challenged by the exploration of oil and gas goes into ever-deep regions as in Mexico Gulf and West Africa. Comparing to the fatigue analysis of the tensile armors which have been extensively investigated in recent years, much less effort has been devoted in the fatigue of pressure armor. The fatigue of the pressure armor is much more complicated than the tensile armors. For the tensile armor, the longitudinal stress along the helix path dominates the fatigue behavior, while for the pressure armor, more stress components will play together to affect its fatigue. If the fatigue of the tensile armor can be characterized as a uni-axial fatigue phenomenon, the fatigue of the pressure armor will be a typical multi-axial problem. The stress components in the pressure armor consist of contribution from the following sources: stress in the hoop direction due to internal/external pressure, stress in the radial direction due to the pressure and contact pressure from the tensile armor layer, stresses caused by the ovalization when the riser is bent, and local stresses due to local bending (nub/valley contact). The friction between the nub and valley interface is reflected in the local stress components as well. A Finite Element (FE) based computer program BFLEX developed by MARINTEK for the stress analysis of flexible risers are capable of calculating the complicate stress components of the pressure armors. In order to perform fatigue damage calculation for the pressure armor, mean stress and stress range must be computed based on these stress components. Mean stress correction becomes very important due to large mean stress experienced by the pressure armor. There are several ways to make use of these stress components to derive the mean stress and stress range. Equivalent stress models and critical plane models are the main models to address the general feature of the multi-axial fatigue. The application of these models on the fatigue of the pressure armor of the flexible risers will be discussed in this paper. The best suited model will be suggested based on the specific stress components in the pressure armor.