Advanced Global Nonlinear Dynamic Simulations of Flexible Riser Systems

Large-scale simulation of flexible riser systems with detailed models has been called an industry "quest". Advanced commercial finite element analysis packages can model the complex geometry and multi-layer interaction of flexible risers; however, the massive computational requirements of these models for spans of just a few meters makes them impractical for larger scales. These models are limited to local analyses; post-processing tensions and curvatures from the usual riser global analysis software where the flexible riser is modeled with line elements. The accuracy of armour stresses from this global/local analysis procedure is impacted by a series of approximations in the global/local procedures including the over-simplified global analysis model, the boundary conditions of the local model, and the computational requirements for the local model preventing a time-consistent treatment of global analysis inputs. The resulting armour stress predictions are often over conservative impacting fatigue life predictions and field deployment. This paper expands on recent published papers by the authors (2013 OTC and 2013 ISOPE) on applications of nonlinear dynamic substructuring to computationally efficient detailed flexible pipe simulations and presents a large-scale nonlinear dynamic simulation of a flexible riser system. The substructure finite element models are 8-layer shell element detailed models including geometrically nonlinear effects and multi-layer interaction via friction hysteresis. Armour stress time-histories are directly recovered from the large-scale analysis using stress transformation matrices completely by-passing the local analysis. This advancement allows for a more detailed and realistic large-scale nonlinear simulation of the flexible riser system resulting in the direct recovery of detailed armour stress time-histories without having to resort to a secondary local analysis of a detailed sub-model. The methodology is capable of efficiently incorporating multi-layered pipe models in a fully geometrically nonlinear dynamics setting including armour layer friction hysteresis. This significantly expands the current industry practice of global/local analysis with simplified line model representations. This powerful and computationally efficient method is implemented in a stand-alone computer program.