Effects of rigidity and internal flow velocity on marine riser dynamics

Abstract

It is well known that the internal flow may have a pronounced effect on the dynamics of a pipe conveying the fluid and may even cause dynamic instability. The objective of the present study is to examine the effect of the internal flow and bending rigidity of the pipe on the dynamic behavior of marine risers. A mathematical model for the lateral motion of a marine riser is developed. The model includes a steady flow inside the pipe together with other factors such as currents, wave excitation, rig motions, etc., adopted in most of the previous studies. Since for marine risers, especially those in deep water applications, the elastic rigidity of the pipe is very small in general, a singular perturbation technique is used to solve the equations of the mathematical model. The solution is expressed as the superposition of three components; they are (1) the static displacement due to the current and the mean rig offset, (2) the time dependent component due to the rig sway or surge motion, and (3) the forced vibration which accounts for inertia effects and time-dependent fluid interaction forces. It is found that the internal flow acts to reduce the effect of the top tension. However, its effect on riser dynamics is not significant for the cases studied, in which a relatively high top tension was selected, so that the perturbation technique used would be valid. For the small top tension cases, the effect of internal flow on riser dynamics remains to be investigated.