Analytical Methodology for Wave Fatigue Analysis on Riser Suspended and Moored by Chains (RSAA) Configurations

Abstract

As oil exploration moves into fields located in deep water, the need for developing new concepts of riser systems aiming at reducing the influence of environmental loads consequently increases. Therefore, a new riser configuration called RSAA (riser suspended and moored by chains - in Portuguese) is proposed. This structure consists on a rigid vertical riser connected on its top to a floating unit and on its bottom to flexible and mooring lines segments. This new configuration improves the structural response for some of the most critical points in riser design: top tensions and bending moments at the touch down zone. Feasibility analyses have shown that the vertical riser is the most critical part of the proposed system due to the high level of vertical motions and dynamic tension. Therefore, 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 that evaluates the vertical motions and axial tension along the vertical riser subjected to wave loads applied to the floating unit. This procedure could enable parametric studies and the evaluation of the stress amplitudes and, consequently, the fatigue damage along the vertical riser of the RSAA configurations. Furthermore, this work aims to show that this methodology is a fast and effective pre-design tool by comparing it with a nonlinear time domain finite element method analysis.