Vortex-induced motions of multiple cylindrical offshore structures

Abstract

Vortex-induced motions (VIM) has been receiving continuous attention in the field of offshore exploration and exploitation as an increasing number of deep-draft floating structures have been operating in different regions around world. Deep-draft floating structures are well known for their favourable vertical motions behaviour compared with other types of floating offshore structures. However, the increases in the structure’s draft can also lead to more severe VIM, which may lead to potential damage particularly causing fatigue to the mooring and riser systems. This research is to carry out an in-depth study on the fundamental fluid physics and the associated hydrodynamic characteristics of a multi-column structure, i.e. the deep-draft semi-submersible (DDS). A comprehensive set of numerical simulations has been conducted and experimentally validated in physical models. Good correlation has been demonstrated among the vortex shedding patterns, the fluctuation forces on the structures, and the VIM trajectory in the present work. The “lock-in” phenomenon was found to have the most striking effect on the vortex shedding processes, the force and the VIM trajectories. Analysis of the drag and lift force coefficients on and the work done by different members of the DDS revealed that the portside and starboard side columns are the key structure members responsible for amplifying the VIM responses while the pontoons are acting to restrain VIM responses. Additionally, based on the analysis of flow over a stationary structure, it is revealed that adding the pontoons into the structures can significantly alter the flow patterns around the structure. The vortex street tends to be more tidy and structured. Hence, the vortex shedding period and the lift force coefficient for the overall structure are increased.