*Article* **Numerical Investigation on Hydrodynamic Characteristics of Immersed Buoyant Platform**

**Jinjiang Yao, Xingwei Zhen \*, Yi Huang and Wenhua Wang**

School of Naval Architecture and Ocean Engineering, Dalian University of Technology, Dalian 116024, China; yjj294980436@163.com (J.Y.); huangyi@dlut.edu.cn (Y.H.); wangwenhua0411@163.com (W.W.) **\*** Correspondence: zhenxingwei@dlut.edu.cn

**Abstract:** The Next Generation Subsea Production System (NextGen SPS) is considered as a competitive alternative system used for offshore petroleum production in ultra-deep sea based on the artificial seabed technology. The Immersed Buoyant Platform (IBP), which is located at a constant depth below the free surface of the water to minimize wave loading, provides a buoyant stable platform for supporting the well completion equipment. Therefore, the hydrodynamic characteristics of IBP in the currents play an essential role in determining the global responses of NextGen SPS. In this paper, aiming at acquiring an optimum structural form of IBP, the hydrodynamic characteristics of the flow past the cylindrical IBP with different height-to-diameter ratios are systematically investigated by use of the large eddy simulation (LES) approach. The simulations with fifteen different height-to-diameter ratios (*H*/*D*) are investigated. The Reynolds numbers are ranged from 0.94 <sup>×</sup> 106 to 3.45 <sup>×</sup> 106. It can be verified that the separated fluid reattaches on the surface of the cylinder when the aspect ratio is between 0.1 and 0.4. Due to the specific shape ratio and obvious 3D effect of the cylindrical IBP, no significant vortex shedding has been clearly observed when the aspect ratio is between 0.1 and 0.4. In the case of 0.4 ≤ *H*/*D* ≤ 5.0, a series of regular and alternating vortex street shedding appear behind the circular cylinder. The simulation results also show that the recirculation region length behind the cylindrical IBP can be significantly reduced with the decreasing aspect ratio. It can be concluded that the cylindrical IBP performs the best hydrodynamic characteristics when the aspect ratio is between 0.3 and 0.4. The research findings will be of great significance to providing valuable reference and foundation to determine the optimum form of underwater structures, such as the buoyancy cans of the hybrid riser system.

**Keywords:** next generation subsea production system; Immersed Buoyant Platform; hydrodynamic characteristics; ultra-deep sea
