Accelerated aging tests are the conventional way to evaluate long-term degradation of polymers, in particular for offshore flexible risers. In this article, a multiscale model has been developed combining diffusion, chemical kinetic reactions, structure–property relationships, and composite models to provide faster and less labor extensive property predictions. A general methodology is presented and applied to predict the density and crystallinity evolution. Results are compared with experimental ageing of polyamide 11 in deoxygenated water at 120 °C. For both density and degree of crystallinity the modeled trend is close to the experimental test results. Accurate prediction of the morphological parameters during degradation allows extension of the multiscale model for the prediction of mechanical properties. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42630.