Multiphase flow-induced dynamic loads in rigid tie-in spool are mainly generated by fluid density variation, slugging, and direction change at bends. When the frequencies of the induced dynamic loads are close to the natural frequencies of the rigid tie-in spool, resonance will occur. This may cause significant strength and fatigue issues, and should therefore be included in the rigid tie-in spool system design.

In this paper, a procedure for assessing structural response of a tie-in spool using fluid-structure interaction (FSI) method is introduced to investigate the internal multiphase flow-induced vibration and fatigue damage due to fluid density change, slugging, and direction change at bends. A numerical example to demonstrate the procedure are also included.

In the procedure, the rigid tie-in spool structural mode shapes and natural frequencies are first determined by FEA. Then, the inlet slug frequencies are selected based on the natural frequencies of the tie-in spool. Finally, coupled CFD and structure analysis are carried out to calculate the stress and stress range at the critical locations of the rigid tie-in spool for strength and fatigue evaluation. In the coupled CFD and structure analysis, fluid-induced loads due to fluid density variation, slugging, and flow direction change at bends are all included.

For the numerical example, a typical tie-in spool is selected. Star CCM is used as CFD software for multiphase flow-induced load analysis. ABAQUS is used as structure software for mode shape and FSI analysis for strength and fatigue evaluation.

The proposed procedure together with the numerical example provides an in-depth understanding of the resonance effect on strength and fatigue for a typical rigid tie-in spool. It also demonstrates the importance of applying the FSI method to the rigid tie-in spool system design when slugging and resonance are expected.

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