EFFECTS OF RIGIDITY AND INTERNAL FLOW ON MARINE RISER DYNAMICS

A mathematical model for the lateral motion of a marine riser is developed to examine the effect of the internal flow and bending rigidity of the pipe on the dynamic behaviour of the riser. The model includes a steady flow inside the pipe together with other factors such as currents, wave excitation, rig motions, etc. A singular perturbation technique is used to solve the equations of the mathematical model for the risers with a relatively small elastic rigidity. The solution is expressed as the superposition of three components; they are (1) the static displacement due to the current and 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 when the top tension of the riser is relatively high. For small top tension cases, the perturbation technique is no longer valid and the effect of internal flow on riser dynamics remains to be investigated.

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