CFD Analysis of Unsteady Flows Around a New Cell-Truss Spar and the Corresponding Vortex-Induced Motions

Abstract

A new configuration of Spar platform called the Cell-Truss Spar has recently been put forward by State Key Laboratory of Ocean Engineering (SKLOE) of Shanghai Jiao Tong University. Since the Cell-Truss Spar is a new design concept that has several physical characteristics which are different from those of the present Classic, Truss and Cell Spars, many aspects of its performances in various sea conditions should be carefully studied. For any type of Spars, Vortex-Induced Motions (VIM) under current flow is an important consideration since it not only affects the motion performance of the Spar, but also reacts on the fatigue analysis and Spar mooring design. This paper mainly discusses the unsteady flows around the new Cell-Truss Spar and the corresponding vortex-induced motion performances of the Spar in uniform currents. A CFD model of the Cell-Truss Spar upper hull (without strake designs for VIM mitigation since it’s still at the concept design stage at the present time) with a scale ratio of 1:100 is created and numerical simulations at different current conditions are performed. The software FLUENT is chosen as the computational fluid dynamic tool to simulate the flow fields around the Cell-Truss Spar and the resulting vortex-induced motions. Both 2D and 3D simulations are carried out. Dynamic meshes and user defined functions are used in the fluid-structure interaction for solving the equations of motion. The SST k-ω method is used as the turbulence model in the 2D simulation and the Detached Eddy Simulation (DES) is used in the 3D simulation. In the 2D simulation, the unsteady flows around the Cell-Truss Spar hard tank at different Reynolds numbers are calculated and the corresponding vortex shedding features and other fluid parameters are obtained and analyzed. Moreover, in order to set up a baseline for the comparison and analysis of the Cell-Truss Spar, a typical Truss Spar hard tank with the same diameter and draft is also modeled and calculated. Comparisons with several classical experimental and numerical results are conducted to validate the numerical method. The Cell-Truss Spar hull VIM responses are then simulated. For the 3D simulation, the fluid domain is made of a hybrid mesh comprising of millions of wedgy and hexahedral elements. Since 3D simulation is very time-consuming, only the results of the nonlinear flow field features for the currents passing the stationary Spar are presented in this paper, and more advanced studies related to this subject including both numerical and experimental investigations would be carried out successively.