A Preliminary Study on the Interaction Between Slug Flow and Vortex Induced Vibration in Fatigue Life of Submarines Pipelines

Flow induced vibrations is a very important subject that should be carefully considered when facilities to transport hydrocarbons from marine sub-sea to shore line are designed. In particular, interaction between internal pipeline flow and external marine currents on horizontal, inclined or vertical (i.e. risers) pipelines need to be considered to properly estimate their fatigue life. Between these two form of excitations a coupling of resonances may appear, thus producing fatigue life predictions different from those calculated when each excitation is considered separately. In this paper a fluid-structural model for analyzing the dynamical behavior of a riser subjected to simultaneous internal slug flow and an external marine current uniform flow was developed. The fluid model was based on the classical wake oscillator model. A numerical model based on the finite difference method was implemented for the structure. Different tests cases were modeled to analyze the riser dynamics for a typical field case, when marine current velocity is constant. Preliminary results show that, in some cases of practical interest, the dynamical behavior of the riser could be influenced by the slug flow and the predicted fatigue life may be different from those predicted when each flow is considered separately. Additionally, results show that study of different slug flow scenarios may be simplified considering a monophasic flow whose density is equivalent to that of the average slug flow density.