A new numerical tool, called DEEPLINES TM, has been developed by IFP and Principia RD to perform dynamic analyses of Floating Production Systems. This Finite-Element program may either deal with flexible risers, rigid risers, umbilicals and mooring lines individually or take into account the coupled behaviour of these connections and their floating support. For the analysis of rigid risers, the implementation of elastoplastic material laws is presented in this paper. As an illustration, the case of reeled pipes is examined through a preliminary calculation of residual stresses due to the reeling process. Catenary configurations are then subsequently calculated using the results of the previous calculations, and comparisons are made between the response of classical and reeled pipes. Results show that according to the loading amplitude, residual stresses have an initial influence on the structure behaviour, but this influence may vanish after several loading cycles. Situations are then shown where shakedown states are obtained, so that the behaviour of the pipe would remain fully elastic during the continuation of the loading cycles. This work therefore provides a design tool for a dynamic riser analysis in the elastoplastic range, which could be used to evaluate the behaviour of risers submitted to accidental loads (during installation, for instance). Results also show the limited influence of the reeling process for Steel Catenary Risers applications of reeled pipes, provided the fatigue behaviour is taken into account.
Dynamic riser analysis is frequently carried out using a geometric non-linear Finite-Element formulation, but with linear material laws only. This paper is therefore devoted to the implementation of non linear material laws in the software DEEPLINES TM, and its consequence on the analysis of rigid risers. It points out the possibility, in some cases, to accept plastic deformations occurring during the first cycles of the pipe lifetime.
