# Horizontal Loading Performance of Offshore Wind Turbine Pile Foundation Based on DPP-BOTDA
## 3 Results and Discussion  
### Distributed Strain Data  
In this experiment, the load direction is the connection direction of FUT1 and FUT3, and the action point is located on the FUT1 side. The test pile is simplified to a fixed cantilever beam structure. So, the strain values of FUT1 and FUT3 are antisymmetric according to the force state of the pile. Based on this property, the BFS values caused by temperature can be calculated. The calculation equation is as follows:  
\[\Delta(BFS_{T})=\frac{1}{2}[\Delta(BFS_{FUT1})+\Delta(BFS_{FUT2})], \tag{1}\]  
\[\varepsilon_{FUT1}=[\Delta(BFS_{FUT1})-\Delta(BFS_{T})]\times 1000/C_{s}, \tag{2}\]  
\[\varepsilon_{FUT3}=[\Delta(BFS_{FUT3})-\Delta(BFS_{T})]\times 1000/C_{s}, \tag{3}\]  
where \(\Delta(BFS_{T})\) is the BFS change value induced by temperature; \(\Delta(BFS_{FUT1})\) and \(\Delta(BFS_{FUT3})\) are the actual measured BFS change values of FUT1 and FUT3, respectively; \(\varepsilon_{FUT1}\) and \(\varepsilon_{FUT3}\) are the FUT1 and FUT3 strain values after removing temperature, respectively; and \(C_{s}\) is the strain coefficient--here, \(C_{s}\) is 0.0482 MHz/\(\mu\epsilon\).  
The temperature-compensated strain values can be calculated by Equations (1)-(3). At the corresponding elevation, the strain values of FUT1 and FUT3 are shown in Figure 6. The FUT1 side is subjected to tensile stress and the FUT3 side is subjected to compressive stress. Above sea level, as the load increases, the strain values on both sides of FUT1 and FUT3 show large errors. The reason is that the received light is different on both sides during the test (12:50 p.m.-07:00 a.m.), which in turn affects the temperature and ultimately causes the different changes in the Brillouin frequency shift. However, this error does not affect the determination of the bearing capacity of the pile. The maximum strain value at the load of 900 kN was 1436 \(\mu\epsilon\), and the position was at \(-\)19.63 m. This position is in the fine sand layer. At the elevation of \(-\)35.0 m to \(-\)43.3 m, the strain values hardly changed with the load and were near 0 \(\mu\epsilon\). The reason is that, at the elevation of \(-\)30.5 m, the test pile was the tube filled with reinforced concrete. Then, the bending stiffness was dramatically enhanced, so the strain values were always small with the load increasing.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Class of Loading** & **Load (kN)** & **Data Acquisition Time** \\ \hline
0 & 0 & 12:50 \\ load1 & 60 & 13:30 \\ load2 & 120 & 14:20 \\ load3 & 180 & 15:10 \\ load4 & 240 & 16:10 \\ load5 & 300 & 18:10 \\ load6 & 360 & 19:10 \\ load7 & 420 & 20:10 \\ load8 & 480 & 20:50 \\ load9  
From the distributed strain values of the pile, the load and maximum strain (\(F\)-\(\epsilon_{\rm max}\)) curve could be obtained, as shown in Figure 7. A linear fit was performed on the data in the figure, and the fitted results are shown in the solid red line. The R-square is 0.9986. The fitted results show that the maximum strain value of the pile was linearly related to the load, and the strain coefficient under load was 1.631 \(\mu\)\(\epsilon\)/kN. The maximum stress was 295.8 MPa, which does not exceed the yield stress of Q345 steel. The stress ratio of 0.86 indicates that the pile was still in the elastic stage. Therefore, it can be concluded that the horizontal bearing capacity of this pile exceeds 900 kN, which satisfies the standard of horizontal load capacity of offshore wind turbines.