Dynamic Response of Flexible Risers due to Unsteady Slug Flow

Abstract

In this work the influence of unsteady slug flow on the dynamic response of flexible risers is analysed using a computational tool consisting of one program for prediction of slug flow development, and another program for calculation of dynamic response of the riser. Both programs apply a time integration method, and since slug flow leads to dynamic response of the riser, and riser dynamics influences slug flow behaviour, the two codes exchange feedback of information during the integration process. Information exchange is established by building a computer federation of programs based on High Level Architecture (HLA). The federation is composed of two standalone in-house computer codes. The first one is a two-phase flow code which simulates unsteady slug flow through pipes and risers using a Lagrangian tracking model. The second one is a program for static and dynamic global riser analysis based on a two dimensional nonlinear finite element formulation. Using the HLA rules, a communication interface was built, so the two programs are able to perform synchronized time integration with mutual feedback of information for each time step. The slug flow – riser dynamics interaction is a complex problem. The riser response depends on the slug flow behaviour, and the slug flow development is influenced by the riser configuration. Displacement response of the riser and the influence of them on the slug flow development have been studied for different flexible riser configurations carrying different liquid and gas flow rates. Results have shown the importance of using complete mathematical models for each phenomena and the importance of a strong interface of communication in order to predict a realistic behaviour of the slug flow - riser dynamics interaction. Analysis on the influence of the slug flow on the effective tension and bending moment of a flexible riser was studied showing increment of axial and bending stresses. Moreover, the simulations have demonstrated the importance to use a full non-linear finite element model for the riser dynamics, because the riser response influence strongly on the slug flow behaviour. Finally, the numerical results were validated against experimental data showing a reasonable agreement.