On the Dynamic Response of Flexible Risers Caused by Internal Slug Flow

Abstract

Slug flow through flexible risers is a frequent phenomenon which occurs during production of a mixture of oil and gas. The dynamic nature of the slug pattern induces time varying forces, which leads to structural vibrations of the riser. These vibrations can produce large deflections and stresses, which can leave it to fail by fatigue, excessive bending or local buckling. In this work the influence from slug flow on the structural dynamic response of a lazy wave flexible riser is analyzed using a computational tool consisting of one program for calculation of slug flow dynamics, and another program for structural dynamic response. Both programs apply a time integration method, and since slug flow will lead to dynamic motion response of the riser, and riser motion dynamics will influence slug flow dynamics, the two codes need to exchange information during the integration process. Information exchange is established by making a federation based on High Level Architecture (HLA). The federation is composed of SLUGIT and RISANANL. SLUGGIT is a two-phase flow code written in C++ which simulates dynamic slug flow through pipes and riser using a Lagrangian tracking model. RISANANL is a FORTRAN program for static and dynamic structural analysis of slender marine structures based on a finite element formulation. Using the HLA standard these two programs can carry out synchronized time integration and exchange information for each time step. In this work the structural analysis code accomplishes the dynamic response using a linear finite element (FE) formulation. Hence, forces from centripetal acceleration of the internal flow, relative velocity between the riser and surrounding water, and varying gravity of the pipe and content will be accounted for in the dynamic analysis. Displacements, stresses, internal pressure, and outlet flow rates of liquid and gas will be accounted for. The results encourage us to carry out a fully non-linear finite element analysis, in order to have a better understanding of the dynamic behaviour of flexible risers undergoing an unsteady internal two-phase flow.

Dynamic behavior of pipes conveying gas–liquid two-phase flow

“…Seyed and Patel (1992) deduced an analytical model for calculating the pressure and internal slug flow induced forces on flexible risers. Ortega et al (2012) reported a computational method to analyze the interaction between an internal slug flow and the dynamic response of flexible risers. Extending previous work, Ortega et al (2013) studied the interaction between an internal slug flow plus an external regular wave and the dynamic response of a flexible riser.…”

Two-Way Coupled Fluid-Structure Interaction of Gas-Liquid Slug Flow in a Flexible Riser: Small-Scale Experiments and Simulations

“…This way of analysis performs numerical solution for both pipe flow and structural response, where relevant forces and geometry changes are communicated between the physical phenomena in order to include the influence of the flow dynamics in the structural response and vise-versa. Ortega et al (2012) studied the influence of slug flow in the dynamic response of a lazy wave riser using a feedback connection between a structural dynamic software and a multiphase flow software by means of high level architecture. Ortega et al (2017) used an improved structural model in order to predict the influence of waves and slugs in the movement of a catenary riser.…”

Non-linear time domain analysis of cross-flow vortex-induced vibrations

“…Hence, the VIV response can be stationary in some parts (due to current) while in other parts it may be intermittent (due to relative oscillatory flow). In addition, internal slug flow may also cause riser vibrations [9], which will interact with the VIV response. To capture all these effects, a non-linear time domain analysis is required.…”