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In this paper it is shown that for very long risers in sheared flow there is a surprising outcome — the VIV response amplitude in the power-in region does not depend at all on the amount of damping in the power out region, as long as it is sufficient to prevent waves from reflecting at the boundary and returning to the power-in region. In these cases, the response in the power-in region depends on the wave radiation damping and not on the damping in the power-out regions. Tension plays a major role in the determining the radiation damping and in some cases, but not all, pulling harder will indeed reduce response in the power-in region. Numerical simulations are presented in which a finite element model of a long riser is used to compute the VIV response in a sheared flow for which the power-in region is at one end of the riser. The radiated waves are shown to diminish with distance traveled as expected. When ζoutnout, the product of the number of wavelengths to reach the far termination and the damping ratio in the power-out region is greater than 0.18, it is shown that no significant vibration energy returns to the power-in region and the response in the power-in region is independent of the damping in the power-out region. The numerical simulation is used to illustrate the effect of changing tension on the radiation damping and therefore on the VIV response. The VIV response prediction program SHEAR7 is used to evaluate the effect of increasing tension on a realistic deepwater drilling riser in 3000 m water depth. A 20% increase in tension leads to a 12% reduction in fatigue damage rate. |
