Experimental and numerical study on vortex-induced motions of a deep-draft semi-submersible

Abstract

An experimental study on vortex-induced motions (VIM) of a deep-draft semi-submersible (DDS) was carried out in a towing tank, with the aim to investigate the VIM effects on the overall hydrodynamics of the structure. In order to study the fluid physics associated with VIM of the DDS, a comprehensive numerical simulation was conducted to examine the characteristics of vortex shedding processes and their interactions due to multiple cylindrical columns. The experimental measurements were obtained for horizontal plane motions including transverse, in-line and yaw motions as well as drag and lift forces on the structure. Spectral analysis was further carried out based on the recorded force time history. These data were subsequently used to validate the numerical model. Detailed numerical results on the vortex flow characteristics revealed that during the “lock-in”, the vortex shedding processes of the upstream columns enhance the vortex shedding processes of the downstream columns leading to the rapid increase of the magnitude of VIM. In addition to the experimental measurements, for the two uniform flow incidences (0° and 45°) investigated, comprehensive numerical data of the parametric study on the VIM characteristics at a wide range of current strength will also serve as quality benchmarks for future study and provide guidance for practical design

Hydrodynamics Around a Deep-Draft Semi-Submersible With Various Corner Shapes

“…Three different corner shapes (sharp, chamfered and rounded) of a typical deep-draft semi-submersible under 45 degree flow incidence have been numerically investigated after present and previous rigorous validations [21][22][23] against the experimental data. The hydrodynamic loads on different members of the structure, such as four columns and pontoons, are compared in order to quantify the corner shape effects.…”

“…The details of mesh and time step sensitivity studies have been presented in the previous studies [22,23]. In the present study, the results for all cases were obtained by averaging after more than fifty vortex shedding cycles.…”

Vortex shedding characteristics of multi-column structure with or without horizontal connections

“…In the present study, the non-dimensional time step (non-dimensional time step = ∆tU/L, where ∆t is the time step, U is the inlet velocity and L is the width of the column) is set as 0.008 to ensure the Courant number less than 1 (only in some tiny flow areas closed to the wall, the Courant number may reach to 1.5). Based on the recent study of modelling vortexinduced motions of a deep-draft semi-submersible [26,27], the selected time step is considered to be fine enough for the current simulations' requirement.…”

“…Additionally, based on the recent study of modelling vortex-induced motions of a deep-draft semisubmersible [26,27], the selected mesh is considered to be fine enough for the current simulations' requirement.…”

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

“…It was further stated that VIY occurred when vortex shedding frequency about the columns approached the natural frequency of yaw of the semi-submersible. Liang et al (2017) carried out a comprehensive numerical study complimented by experimental measurements on the VIM of a deep draft semi-submersibles to examine the characteristics of vortex shedding processes and their interactions due to multiple cylindrical columns. Gonçalves et al (2018) conducted experiments on deep draft semi-submersibles models to investigate the effects of the column shape and the surface roughness.…”