Propagação de trinca por fadiga em risers rígidos

Abstract

Risers are subject to severe and cyclic environmental loading throughout their service life. In this context, the concepts of Linear Elastic Fracture Mechanics (LEMM) can be a valuable tool to ensure their safe operation and reduce the conservatism of their designs. This paper presents two approaches, one analytical and one numerical, to analyze fatigue crack propagation in three rigid riser geometries. Initially, the global model of each riser was developed in Orcaflex software with loads originating from different sea states. These analyses provided remote stresses applied to local three-dimensional models. The propagation of a surface crack at the first weld at the top of the risers was evaluated by the arbitrary and incremental growth technique in FRANC3D software and by the empirical expressions of BS7910, 2013. Elastoplastic analyses were performed and limited the propagation to a/t<0.8. Stress Intensity Factors (SIF) were calculated with the numerical method of the M-integral and generalized expressions were fitted. For a/t<0.5, the SIFs of the analytical and numerical approach are in agreement, but for a/t>0.5, the SIF of BS7910, 2013 proved conservative. The fatigue life calculated using the numerical crack propagation approach was superior to those calculated using the BS7910, 2013 equations and the classical methodology of S-N curves. Loading sequence effects of 500 (five hundred) historical variable amplitude block loadings, with application of Willenborg's model, were analyzed during propagation and showed to be non-significant