Parametric Study on the Axial Vibrations of Riser Suspended and Moored by Chains (RSAA) Configurations

Abstract

Recently, in order to minimize the influence of the vertical motions in the risers and consequently allow the utilization of FPSOs in deep waters, a new riser configuration called RSAA (riser suspended and moored by chains - in Portuguese), composed of a rigid vertical riser, flexible structures and mooring line segments (top and bottom) was proposed. This configuration presents solutions to the most critical points in a riser design: the top tensions are dissociated from the bending moments at the top region, and the curvatures at the TDP are reduced by utilization of floaters. Feasibility analyses have shown that the vertical riser is the most critical part of the proposed system due to the FPSO high level of vertical motions. These motions are transmitted by the top chains, leading to high levels of axial stress variation due to dynamic tension. Faced with this, a parametric study is vital in order to understand the system’s behavior as well as to establish the main parameters which influence its structural behavior. Analytical methods may require some slight simplifications of the problem to be applicable, but they generally lead to compact formulas that do explain which parameters influence the results and why and how it does so. This work proposes an analytical model to determine axial stress and tension variations at the vertical riser, considering some simplifying hypotheses, like the flexible structures and mooring line segments at the bottom will be replaced by a mass and a spring. Neglecting some nonlinearities but considering the coupling between axial and transversal vibrations, a random dynamic analysis in the frequency domain can be performed to evaluate the maximum stresses and tensions levels with considerably lower computational costs.