Sensitivity analysis of fatigue life prediction for deepwater steel lazy wave catenary risers

Abstract

Steel lazy wave catenary riser (SLWR) has been an attractive choice for deepwater oil field developments. However, fatigue is a critical issue in assessing the feasibility of applying SLWR to large motion vessels such as floating production storage and offloading (FPSO) or semi-submersibles. In this work, the time-domain fatigue analysis of SLWR was adopted for better representing the structural nonlinearity, fluid load nonlinearity and riser-soil nonlinear interaction. The Palmgren-Miner rule was employed for the fatigue life prediction along the riser length. The main purpose of this analysis is to present sensitivity analyses of SLWR fatigue life under various input parameters, which include the structural damping, the hydrodynamic coefficients along the riser, the seabed stiffness, the vessel motions, etc. The analyses indicated the strong dependence of the riser fatigue life on these parameters. The results can help designers to understand the dynamic behavior of the SLWR and provide guidance for selection of some critical parameters that are used in the fatigue design.