| Abstract | Due to their long and flexible structures, deep-water risers may experience multi-modal vortex-induced vibrations (VIV) with very high mode number. These multi-modal vibration responses usually depend on the modal parameters of riser vibration systems, such as modal natural frequencies, modal mass, modal damping and modal stiffness. This paper presents a model riser test, which was designed to investigate these modal parameters. In the test, a highly flexible model riser was used. It was made of rubber hose and was 8.5m in length and 0.04m in diameter. The model riser was fixed horizontally on a supporting frame below the tank carriage and was then placed at a water-depth of 1.0m. A shaker system was used to excite the riser in a controlled manner. The excitation forces and the vibration accelerations over the riser length were respectively measured by a load cell on the shaker and sixteen pairs of accelerometers inside the riser. The riser was excited harmonically and randomly by the shaker system and the frequency response functions (FRF) of the riser were obtained by the relationship between the shaker excitation forces and the riser vibration responses. The modal natural frequencies, the modal mass, the modal damping and modal stiffness were analyzed based on these FRFs. Three different models were respectively employed for the analysis. These models include a frequency-dependent model, a location-dependent model and a frequency- and location-independent model. |
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