**1. Introduction**

#### *1.1. Background*

Currently, offshore oil exploration is gradually developing into ultra-deep sea. Therefore, innovative ocean engineering equipment are naturally required to deal with the great challenges brought by extreme depth and severe environment in ultra-deep sea. In this scenario, Zhen et al. [1,2] put forward the NextGen SPS concept, which can be used to conduct as the Subsurface Well Completion (SWC) system for offshore petroleum production in ultra-deep sea. IBP as the key equipment of this system provides a buoyant stable platform, which is located at 300 m below the mean water level to minimize direct wave loading. Thus, the form of the IBP has an critical effect on the hydrodynamic loads that act on the IBP from current. The initial form of the IBP is circular cylinder. However, it is widely believed that the aspect ratio of the circular cylinder has the main impact on the vortex shedding and the forces acting on cylinders. The drag force, lift force and Strouhal number, which define the hydrodynamic characteristics, vary with the aspect ratio.

NextGen SPS [3] is an innovative concept that can be used to implement the extraction and exporting of subsea oil. This system mainly consists of the following four parts:

**Citation:** Yao, J.; Zhen, X.; Huang, Y.; Wang, W. Numerical Investigation on Hydrodynamic Characteristics of Immersed Buoyant Platform. *J. Mar. Sci. Eng.* **2021**, *9*, 168. https:// doi.org/10.3390/jmse9020168

Academic Editor: Yigit Kemal Demirel Received: 31 December 2020 Accepted: 2 February 2021 Published: 6 February 2021

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the IBP that is tethered perpendicularly, SWC equipment, flexible jumpers and rigid risers, as shown in Figure 1. The IBP supplies a buoyant stable platform for carrying the SWC equipment and rigid risers. The IBP [4] is located at a constant depth below the free surface of the water to minimize wave loading and reduce the influence of surface flow field. In addition, the IBP is jointed to the Floating Production Unit (FPU) through flexible jumpers. Flexible jumpers play the role of decoupling the IBP from the motion of FPU with the shape of catenary.

**Figure 1.** Sketch of the NextGen SPS.

The mooring system [5] of NextGen SPS consists of four vertically tensioned mooring ropes, which are fixed in the seabed by suction piles. The mooring system provides the stability of the IBP and reduces the tension forces on rigid risers.

As mentioned above, the NextGen SPS offers specific advantages as follows:

(1) The presence of the IBP takes up the huge riser loads and reduces the dynamic loads and the main dimensions of FPU.

(2) The IBP is located at a constant depth below the free surface of the water to circumvent the surface wave and flow.

(3) The joint production of multiple wells on the seabed can be implemented.

(4) The existence of the flexible jumpers reduces the fatigue damage of the structure.

(5) The SWC is mounted on the IBP, which reduces the requirements of the capability for the SWC.

(6) Under extreme sea conditions, the FPU can be disengaged from the IBP quickly to enhance the security of production.

As we can see, the IBP is the most important part of the NextGen SPS. The structural form of the IBP has great influence on the hydrodynamic performance of the system. With regard to the best hydrodynamic behavior of such novel concept, the IBP has been developed featuring cylindrical shape. However, the height-to-diameter ratio of the cylindrical buoy needs to be ascertained and investigated, because the height-to-diameter ratio of the circular cylinder will affect the distribution of wake field and the forces acting on the cylinder. With this background, there is a strong need to study the hydrodynamic behavior of the circular cylinders with different aspect ratios in order to acquire the optimum structural form of the IBP.
