Vortex-Induced-Vibration of jack-ups with cylindrical legs in multiple modes

Abstract

A simple mathematical model was developed based on the single-degree-of-freedom analogy and principle of conservation of energy evaluating various modes of Vortex-Induced-Vibration (VIV) of a jack-up with cylindrical legs in steady flow. Mass ratio, damping ratio and mode factor were found to be the important parameters controlling the inline and cross flow VIV and radius of gyration for the yaw VIV. Criteria for the initiation of the three VIV modes were developed for the cases of a single 2D cylinder, four rigidly coupled 2D cylinders in rectangular configuration and a jack-up experiencing uniform flow. The model tests demonstrated that the jack-up with cylindrical legs experienced cross flow and yaw VIV in uniform flows, with amplitude ratios greater than 0.1D. Further, there was considerable overlap of the lock-in ranges and coupling at higher current speeds of the aforementioned modes making the jack-up practically redundant throughout the operating currents. The analysis of the mean inline responses of the model revealed drag amplification due to the VIV. The test results validated the developed VIV model, VIV criteria and the importance of mass ratio in suppressing VIV. The mathematical method will enable practising engineers to consider the effect of VIV in jack-up designs.