Abstract
Offshore risers are structures of vital importance in extraction of oil and gas from the seabed. Being subjected to changing ambient excitation from the flow of the fluid surrounding them and conditions at the ends, their motions can change considerably with time. The most dangerous situation occurs when the frequency with which vortices are shed behind the body of the riser synchronises with one of its transverse natural frequencies. Bishop and Hassan [1] were the first to experimentally observe this lock-in phenomenon for a cylinder oscillating in cross-flow due to vortex-induced vibration. Based on some of their findings, the van der Pol equation was proposed to model shedding of vortices because of its properties as a self-excited oscillator. The wake oscillator models based on this equation that describe the change of the lift coefficient have been coupled with oscillators representing rigid or flexible structures [2]. In this work the structure is a flexible one with nonlinear properties. Its response is compared to the response of the linear structure oscillating under excitation from a wake oscillator that was developed by Facchinetti et al. [3].
Published Version
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