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Vortex-induced vibration (VIV) of a long riser in sheared current is often considered as an energy balance problem: Excitation forces in the power-in region add an equal amount of energy to the system as is dissipated by damping forces outside this region and by structural damping. A riser may have different excitation and damping regions depending on the actual oscillation frequency, cross-section properties and local flow velocity. A damping model must hence be able to handle higher and lower flow velocities than the excitation velocity range. In this paper the fluid damping models proposed by Venugopal [ 1] are compared with the experiments conducted by Gopalkrishnan [2] and Vikestad [3]. The results show that the models are conservative at high and low reduced velocities.

Most floating fish cages consist of a floating cage collar, mooring system, and net cage hanging freely down from the floater. Any current moving through the net will deform the net. This deformation depends on the current velocity, the nets ability to attract forces, and the gravity forces from weights at the lower end of the net. The forces on the net will vary with the deformation. This paper shows that for this kind of cage, a reduced velocity Vred = U·(ρ/2G)1/2, where G is the equivalent weight per area of the net, will be sufficient to estimate the drag force and deformation of the net. The present work is analytical, combined with computer simulations. The reduced velocity parameter could be used to determine the needed bottom weights. The quick estimation of the drag force on the net could be used as an input to mooring design.

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