Mechanical analysis of drilling riser based on pipe-in-pipe model

Due to the presence of a drill string inside drilling risers, the mechanical analysis becomes somewhat challenging which has attracted attention from design engineers and drilling operators. The purpose of this study is to investigate the response of a riser with a drill string inside based on the FEM method, and to compare the numerical results with those corresponding to single pipe conditions. Two different riser models were studied, (i) Single Pipe Model: the riser system is assumed to consist of a single pipe, (ii) Pipe-in-Pipe (PIP) model: the riser system comprises an outer riser with an internal drill string. Both the single pipe and the PIP models were established within the framework of the global riser analysis software Riflex. Mechanical analysis of a deep-water riser in the South China Sea was performed in order to compare the riser response behavior obtained by application of the two different models. In the case of static analysis, modest differences were observed, whereas the dynamic analysis demonstrated more significant deviations between results based on the two models. The characteristics of the riser response for the PIP model were further investigated in terms of the displacements, dynamic motion and stress distribution. A definition of an influence coefficient in order to account for the effect of the drill string on the mechanical behavior of the riser was proposed, and the influence of the drill string corresponding to different top tension levels and environmental conditions was discussed