Numerical modelling of offshore risers conveying slug flow under the ALE–ANCF framework

Abstract

A nonuniform distribution of internal flow along the offshore riser can lead to transient in-plane excitation. To predict the global riser response due to slug flow by nonlinear time domain analyses, a finite element method is developed in this study by combining the absolute nodal coordinate formula (ANCF) and arbitrary Lagrange-Eulerian (ALE) description. This proposed method has the capacity to effectively simulate both the nonuniform gravity distribution of slug flow and geometric nonlinearity of the riser. The nodal position and slope vectors described in the global coordinate system avoid the complex coordinate transformation of the elements while the internal fluid is effectively integrated into the framework. After the accuracy of the proposed program is verified, a kind of steep wave riser (SWR) with internal flow is taken as an example to discuss the effect of slug flow. The results show that slug flow can result in large local deformations and lead to variable pattern changes in the bending moment.