# Influence of Central Platform on Hydrodynamic Performance of Semi-Submerged Multi-Buoy Wave Energy Converter

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Methods

#### 2.2. Model of the WEC

^{2}. The maximum grid size of the central platform and buoys are 0.6 and 0.2 m, respectively. For the sea state parameters setting of the time domain calculation, which refer to the sea conditions of the sea trial experiment, the wave height is 1 m, the period is 3 s, and the direction is pointed by the bow to the stern. The hinge damping of the buoys and central platform is set to 3000 (N*m/(°/s)), the additional damping parameter is set to 3500 (N/(m/s)), and the additional damping is intended to simulate the damping force of the hydraulic system of the device. The simulation time is set as 120 s, and the time step is 0.1 s.

## 3. Results of the Numerical Analysis

#### 3.1. Numerical Results at Different Aspect Ratio of the Damping Layer

#### 3.1.1. Hydrodynamic Performance of the Central Platform

#### 3.1.2. Wave Energy Capture Width Ratio of the Device

^{2})/32πH

^{2}T,

^{3}; the gravity g is 9.81 m/s

^{2}; H and T are the height of wave and period of wave, respectively.

#### 3.2. Numerical Results at Different Areas of the Damping Layer

#### 3.2.1. Hydrodynamic Performance of the Central Platform

^{2}. As we considered, the wave energy collection efficiency is better when the aspect ratio of the damping layer is 0.45. Hence, in this simulation when the S varies, B remains at 0.45. Additionally, the values of S in each computation is presented in Table 3. The other parameters use the initial parameters in this simulation.

^{2}, the central platform has the smallest amplitude of oscillation, and the difference in oscillation amplitude of the central platform is obvious under different areas of the damping layer, which means the area of the damping layer has great effect on the platform oscillation. Additionally, it can be seen that when the area of the damping layer is 320 m

^{2}, the oscillation of the central platform is not transient, which is due to large area and low aspect ratio of the damping layer, resulting in excessive platform roll. In addition, the curve is based on the mass point of the central platform, and the excessive platform roll would result in volatility of the oscillation of the central platform.

#### 3.2.2. Wave Energy Capture Width Ratio of the Device

^{2}and the minimum value occurs when the area is 108 m

^{2}, which is about 6.8 kW. For Buoy 2, the maximum value is 12 kW when the area is 320 m

^{2}and the minimum value is 6.3 kW when the area is 108 m

^{2}. Figure 13 is the curve of capture width ratio with the area of the damping layer, which shows the capture width ratio of the device generally increases as the area of the damping layer increases. In order to better show its regularity, one set of simulation data was added in the process of drawing the curve, which is S = 268.8 m

^{2}(L*W is 11*24.4). When the area of damping layer is smaller than 222 m

^{2}, the capture width ratio of Buoy 1 is better than Buoy 2, and the difference is narrowed as the area increases. However, when the area of the damping layer is further increased, the wave energy capture ratio of Buoy 1 reduced significantly while Buoy 2 increased gradually and finally stabilized.

#### 3.3. Numerical Results at Different Heights of the Damping Layer

#### 3.3.1. Hydrodynamic Performance of the Central Platform

^{2}, respectively. Hence, in this simulation, the aspect ratio and area of the damping layer are 0.45 and 320 m

^{2}, respectively.

#### 3.3.2. Wave Energy Capture Width Ratio of the Device

#### 3.4. Numerical Results at Different Drafts of the Damping Layer

#### 3.4.1. Hydrodynamic Performance of the Central Platform

^{2}, and 3.5 m, respectively.

#### 3.4.2. Wave Energy Capture Width Ratio of the Device

## 4. Discussion

## 5. Conclusions

- (1)
- There exists a relationship between the hydrodynamic performance of the WEC and the geometry of central platform.
- (2)
- For a certain wave condition, there exists an optimal geometry of the central platform. At the wave condition mentioned in the paper, when the aspect ratio of the damping layer is 0.45, the area of the damping layer is 320 m
^{2}and the height of the damping layer is 3.5 m, the wave energy capture width ratio of the WEC is better, so that more wave energy can be extracted from the ocean. - (3)
- It is found that increasing the draft of the central platform is conducive to improving the wave energy capture width ratio of the WEC, and in this paper, the wave energy capture width ratio of the WEC is the largest when the draft of the central platform is 4.6 m.
- (4)
- Further related research should be carried out in a physical prototype test and focused on fluid analysis of the central platform, to find out the influence of different shaped platforms on wave distribution.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 4.**Four hydrodynamic parameter curves: (

**a**) wave force, (

**b**) diffraction force, (

**c**) radiation damping, (

**d**) added mass.

**Figure 9.**Four hydrodynamic parameter curves. (

**a**) Wave force, (

**b**) diffraction force, (

**c**) radiation damping, (

**d**) added mass.

**Figure 14.**Four hydrodynamic parameter curves. (

**a**) Wave force, (

**b**) diffraction force, (

**c**) radiation damping, (

**d**) added mass.

**Figure 19.**Four hydrodynamic parameter curves. (

**a**) Wave force, (

**b**) diffraction force, (

**c**) radiation damping, (

**d**) added mass.

Designation | Parameters |
---|---|

Molded length | 18 m |

Molded width | 10 m |

Molded height | 2.5 m |

Diameter of buoy | 3.2 m |

Height of buoy | 2 m |

Draft depth of the device | 5.1 m |

B | L/W (m) |
---|---|

1 | 13.4/13.4 |

1.25 | 15/12 |

1.8 | 18/10 |

2.22 | 20/9 |

2.69 | 22/8.18 |

S (m^{2}) | LW (m) |
---|---|

108 | 7 × 5.56 |

180 | 9 × 20 |

222 | 10 × 22.2 |

320 | 12 × 26.67 |

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**MDPI and ACS Style**

Hu, Y.; Yang, S.; He, H.; Chen, H.
Influence of Central Platform on Hydrodynamic Performance of Semi-Submerged Multi-Buoy Wave Energy Converter. *J. Mar. Sci. Eng.* **2020**, *8*, 12.
https://doi.org/10.3390/jmse8010012

**AMA Style**

Hu Y, Yang S, He H, Chen H.
Influence of Central Platform on Hydrodynamic Performance of Semi-Submerged Multi-Buoy Wave Energy Converter. *Journal of Marine Science and Engineering*. 2020; 8(1):12.
https://doi.org/10.3390/jmse8010012

**Chicago/Turabian Style**

Hu, Yuan, Shaohui Yang, Hongzhou He, and Hu Chen.
2020. "Influence of Central Platform on Hydrodynamic Performance of Semi-Submerged Multi-Buoy Wave Energy Converter" *Journal of Marine Science and Engineering* 8, no. 1: 12.
https://doi.org/10.3390/jmse8010012