# Numerical Analysis of Local Scour of the Offshore Wind Turbines in Taiwan

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## Abstract

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## 1. Introduction

## 2. Numerical Model

#### 2.1. Fluid Solver

#### 2.2. Rheological Model

## 3. Model Validation

## 4. Monopile Wind Turbine

## 5. Tripod and Jack-Up (4-Leg) Wind Turbines

^{10}Pa·s. The wave maker creates random waves at the left boundary. Four numerical cases are carried out using the numerical set-up in Figure 16 and Figure 17. Two cases are provided without the current, and two are supplied with the current. The diameter of the vertical and horizontal structures are 3.0 m and 2.0 m, respectively. The sponge layer is used to absorb the reflective wave. The right boundary condition is modified with a Hydrostatic Outflow condition to mimic the open boundary condition. A wave gauge system including three gauges estimates the wave and velocity along the domain.

#### 5.1. Case 5-1: Tripod Wind Turbine under Random Waves

#### 5.2. Case 5-2: Tripod Wind Turbine under Random Waves and Current

#### 5.3. Case 5-3: Jack-Up (4-Leg) Wind Turbine under Random Waves

#### 5.4. Case 5-4: Jack-Up (4-Leg) Wind Turbine under Random Wave—Current

## 6. Discussions

## 7. Conclusions

- Waves, including regular and irregular waves, do not increase the scour depth compared with currents only.
- The backfilling phenomenon of the scour hole explains the disappearance and reappearance of the local scour in the wave conditions.
- In the case of random wave–current coupling, the results present a signal of scour evolution. However, the scour depth is shallow at $0.033\le S/D\le 0.046$.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Comparison between numerical results and analytical solution of the regular wave. The wave solution is the second-order Stokes wave with a wave height of 6 m, wave period of 8 s, and water depth of 30 m; (

**a**) Gauge location at x = 25 m (near the wave maker); (

**b**) Gauge location at x = 105 m (0.83 L).

**Figure 2.**(

**a**) Comparison between numerical results and analytical solution of the random wave. The theoretical JONSWAP spectrum (red line) was plotted based on Equation (8). The input wave parameters were taken from buoy data in the Taichung Sea, with a significant wave height of 6 m, peak wave period of 8 s, and water depth of 30 m. The black and blue lines were the JONSWAP spectrum analyzed from the predicted wave at Gauge 1 and 2, respectively. (

**b**) Predicted wave at Gauge 1 (x = 25 m—near the wave maker). (

**c**) Predicted wave at Gauge 2 (x = 105 m).

**Figure 4.**The local scour generation just upstream of the monopile wind turbine under a constant current U = 1.0 m/s.

**Figure 5.**A local scour upstream of the monopile wind turbine (top view) at t = 100.1 s. The scour depth is 1.0 m.

**Figure 7.**The mud bed change and the local scour generation around the wind turbine under the regular wave.

**Figure 8.**The change of the mud bed and the generation of local scour around the wind turbine under the regular wave and current coupling.

**Figure 11.**The mud bed change and local scour generation around the wind turbine under irregular waves.

**Figure 13.**The mud bed change and the local scour generation around the wind turbine under irregular waves and current coupling.

**Figure 14.**In the case of random waves, water-free surface, horizontal velocity u, and vertical velocity w, the significant wave height Hs = 6.0 m, peak period Tp = 8.0 s.

**Figure 15.**In the case of random wave and current, water-free surface, horizontal velocity u, and vertical velocity w, respectively. The significant wave height Hs = 6.0 m, peak period Tp = 8.0 s, and current velocity U = 2.6 m/s.

**Figure 19.**In this case, the maximum and minimum velocities near the random bottom wave affect the tripod wind turbine.

**Figure 21.**The maximum and minimum velocities near the bottom in the random wave–current affecting the tripod wind turbine.

**Figure 23.**The maximum and minimum velocities near the bottom in the case of random waves affecting the jack-up wind turbine.

**Figure 25.**The maximum and minimum velocities near the bottom in the random wave–current affecting jack-up wind turbine.

**Figure 26.**The 3D and the top view of the random wave–current case affecting the jack-up wind turbine.

Condition No. | Wave Type | Hydrodynamic Conditions |
---|---|---|

1 | Constant ocean current | U = 1.0 m/s |

2 | Regular wave (100-year return period storm wave conditions) | Wave height H = 3.0 m Wave period T = 5.0 s |

3 | Regular wave—current (100-year return period storm wave conditions) | Wave height H = 3.0 m Wave period T = 5.0 s Current velocity U = 1.0 m/s |

4 | Irregular wave (100-year return period storm wave conditions) | Significant wave height Hs = 3.0 m Peak period Tp = 5.0 s |

Case No. | Wave Type | Wind Turbine Type | Significant Wave (Hs) | Peak Period (Tp) | Current Velocity (U) |
---|---|---|---|---|---|

5-1 | Irregular wave | Tripod | 6.0 m | 8.0 s | - |

5-2 | Irregular wave—current | Tripod | 2.6 m/s | ||

5-3 | Irregular wave | Jack-up (4-leg) | - | ||

5-4 | Irregular wave—current | Jack-up (4-leg) | 2.6 m/s |

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**MDPI and ACS Style**

Vuong, T.-H.-N.; Wu, T.-R.; Huang, Y.-X.; Hsu, T.-W.
Numerical Analysis of Local Scour of the Offshore Wind Turbines in Taiwan. *J. Mar. Sci. Eng.* **2023**, *11*, 936.
https://doi.org/10.3390/jmse11050936

**AMA Style**

Vuong T-H-N, Wu T-R, Huang Y-X, Hsu T-W.
Numerical Analysis of Local Scour of the Offshore Wind Turbines in Taiwan. *Journal of Marine Science and Engineering*. 2023; 11(5):936.
https://doi.org/10.3390/jmse11050936

**Chicago/Turabian Style**

Vuong, Thi-Hong-Nhi, Tso-Ren Wu, Yi-Xuan Huang, and Tai-Wen Hsu.
2023. "Numerical Analysis of Local Scour of the Offshore Wind Turbines in Taiwan" *Journal of Marine Science and Engineering* 11, no. 5: 936.
https://doi.org/10.3390/jmse11050936