Internal flow effect on the parametric instability of deepwater drilling risers

This study attempts to explore and distinguish the effects of internal flow induced centrifugal force and Coriolis force on the parametric instability of deepwater drilling risers. The governing equation of a fluid-conveying flexible riser subjected to parametric excitations is established at first, which is then reduced into a system of first-order linear equations by use of the Galerkin method with attention to the Coriolis-related skew-symmetric matrix. The stability charts of a deepwater drilling riser at different internal flow velocities are plotted by applying the Floquet theory, and the effects of the centrifugal force and Coriolis force on the primary instability have been identified, distinguished and exemplified. Owing to the centrifugal force effect which can reduce the effective tension and decrease the natural frequencies of the riser, the simple instability zones are enlarged and shifted in the stability chart, and more modes can be triggered in the chosen excitation domain. On the other hand, the simple resonance zones become narrower ascribed to the Coriolis force's damping effect. The most notable finding is that the Coriolis force creates and/or strengthens the coupling effect of excited modes, which can make the combination resonance much more profound.