The design of top-tensioned risers has to face a number of challenges in deepwater. One of the challenges comes from the heave motion of the platform that leads to the fluctuation of the axial tension in the risers, which is called parametric excitation (PE). Vortex-induced vibration (VIV), induced by sea current, is also a notorious challenge, especially for high aspect ratio risers. Usually a marine riser is subjected to parametric excitation and vortex-shedding, simultaneously. However, so far only a limited number of VIV studies has considered axial parametric excitation. Thus, the objective of this paper is to analyze the influence of parametric excitation on the riser VIV. To simulate the fluid-structure interaction, a marine riser is modeled as a Bernoulli-Euler beam with periodical time-varying tension in the axial, while the lifting force generated by vortex-shedding is represented by a distributed wake oscillator model that is coupled with the vibration acceleration of the riser structure. The VIV model is represented by a set of coupled partial differential equations. Such coupling model is solved with a standard centered finite difference method so as to formulate a reduced-order system which is then numerically integrated in the time domain. The dynamic behavior of a TTR system is investigated with several combinations of amplitudes and frequencies in the heave motion. Interesting results are achieved as they are much different from the ones undergoing VIV only. It is revealed that the periodically varying top tension accounts for the artificial transition of vibration modes. In cases where PE frequency is distant away from the vortex shedding frequency, the amplitude of VIV response under dynamic axial tension is smaller than that under static tension, while in the special cases where PE frequency is close to the vortex shedding frequency, the response of VIV is significantly amplified. The sum and difference frequencies of the parametric and vortex-shedding excitation are noticeable in the power spectrum density of riser’s lateral displacement. Under the combined excitations, when the heave motion is mild, vortex shedding dominates riser’s response. However, in a severe sea state with remarkable heave motion amplitudes, the parametric excitation contributes more to the TTR dynamic response, because in this scenario the parametric instability disrupts the generation of vortex-shedding at the spanwise locations of riser.

This content is only available via PDF.
You do not currently have access to this content.