ABSTRACT

This paper presents the CFD investigation of the vortex induced motion (VIM) of a caisson model during the installation phase. The simulations were performed by solving the unsteady 3D Navier-Stokes equation. The Dynamic Fluid-Body Interaction (DFBI) tool is utilized to simulate the motions. Typical draft conditions of a caisson during installation process are considered. The present methodology is verified through the free decay and the VIM of a cylinder simulations in two-dimension. Free decay simulations show that the natural frequency decrease with the increasing draft of the caisson. Maximum response amplitudes occur in the range of 10<Vr<13 for both the draft Dp=0.1m and Dp=0.12m. The resonance occurs when the crossflow response frequency fy is very close to the natural frequency fn. The inline/crossflow response frequency increases as the draft increases at the same reduced velocity Vr. The response frequency in the inline direction is double of that in the crossflow direction. The oscillating lift force dominates the crossflow oscillatory motion. Eight-shaped trajectories are observed the maximum displacements in the crossflow direction increase as the drafts increase.

INTRODUCTION

Flow-induced vibration typically arises as a result of vortex shedding. The vibration is called vortex induced motion (VIM) for large-volume floating platforms, and vortex-induced vibration (VIV) for the long slender cylindrical structures such as risers. Vortex shedding of the caisson generates periodically alternating forces and motions. The motion responses extremely large in the occurrence of a resonance, when the vortex shedding frequency is close to the natural frequency. VIM is an important issue during the caisson installation process, as it impacts the integrity of the mooring system and pose a threat to the construction safety. It is challenging to predict the VIM and capture its physical behavior accurately.

Numerical and experimental studies have been carried out widely on VIMs of floating structures. A number of experiments were conducted to evaluate the impact of key parameters on VIMs, such as different arrangement of a pair of cylinders(Assi 2009), inclined angle of cylinders(Franzini et al. 2009), and a varying aspect ratio of a cylinder (Rahman and Thiagarajan, 2015). Gonçalves et al.(2013) investigated the vortex-induced vibration of circular cylinders with very low aspect ratio and small mass ratio. Blevins et al.(2009) measured vortex-induced vibration of an elastically supported cylinder in water with different flow velocity, damping, mass ratio, combined inline and transverse motions, Reynolds numbers, and strake configurations. Gonçalves et al.(2018) carried out VIM model tests with regular and irregular waves collinear to the current conditions to study the behavior of large-volume semi-submersible platforms.

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