Natural frequency analysis of a marine riser considering multiphase internal flow behavior

Abstract Marine risers are subjected to several types of excitation loads, such as motions induced by the floating production system, external hydrodynamics forces due to currents and waves and internal forces due to the internal fluid flow regularly composed by water, oil and gas. When internal flow conditions as pressure, temperature and flow rate fluctuations appear in a riser, they can induce vibration motion to the system. This paper shows a numerical algorithm to evaluate the vibrations of vertical rigid production risers under internal multiphase flow behavior. The numerical algorithm developed computes the natural frequencies of vertical risers model coupled with a hydrodynamic numerical model. The internal flow was composed of water, oil and gas and was treated as a pseudo-single phase (mixture). The numerical algorithm was developed using difference finite method. The results were compared with information available in the literature.