A Case Study on the Axial-Torsional Behavior of Flexible Risers

Flexible risers are complex structures composed of several concentric polymeric and steel armor layers which withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. Determining the axial and torsional stiffness values of such structures is an important task for the global structural analysis, since it provides a probable value that can be used in this analysis to predict the load distribution along the line and permitting, thus, to estimate the expected life of the structure. Although such stiffness values may be provided by the manufacturer, it is quite desirable that they can be estimated by analytical models instead. However, any analytical model proposed for such a task must be checked with well-conducted experimental results in order to be considered as an acceptable analysis tool. The aim of this work is to present the main results involving axial-torsional tests in a 2.5" flexible riser, carried out at the Technological Research Institute of Sa˜o Paulo (IPT). Besides presenting full data concerning the internal structure of the riser, this paper describes the experimental procedures used to perform the tests and the main obtained results (e.g., Force × Displacement and Twisting moment × Displacement curves). Tests involving internal pressure were also performed and the obtained results are also presented in this work. Comparisons between analytically calculated values of the axial and torsional stiffnesses with those obtained experimentally are made and discussed. A brief discussion about the validity of some hypotheses which are usually assumed by analytical models found in the technical literature is made at the end of the work.