## 3 Application to a Case Study
### Monitoring Object  
The platform of a three-dimensional comprehensive observation tower under construction in the East China Sea is selected as the case study, the design of the SHM system for this platform is described in detail. The water depth at the location of the jacket offshore platform is 70 m, and it is exposed to the water surface. Its total height is 114 m, and this includes three parts: the jacket 13.5 m, wind tower 100 m, and observation platform 0.5 m. The physical dimensions of jacket, observation platform and wind tower are shown in Figure 4, the structural element parameters of the offshore platform are shown in Table 2.  
Figure 4: Physical dimensions of jacket offshore platform: (**a**) jacket and observation platform; (**b**) wind tower. (unit: m).  
Based on the monitoring objectives, inclination sensors, strain sensors, linear polarization resistance probes, electrical resistance probes, and many other sensors are used. Because of limitations of space, only those sensors considered in the present work are shown in Figure 5. The principle of the layout the GPS stations is to set them at the center point of the platform and the open field of vision on the axis. The system is equipped with a total of eight inclination sensors (T1-T8): one at the top and one at the bottom of each of the four main pipes. The strain sensors (numbered S1-1, S1-2, S2-1, S2-2, S3-1, S3-2, S4-1, and S4-2) are divided into two types of location: the midpoints of each guide-leg rod layer, and at the top of each guide leg. Of these, four groups of strain sensors are set at the top of the four guide legs, and each group contains two strain sensors; their wires pass through the centers of the guide-leg sections.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Position**} & \multirow{2}{*}{**Description**} & \multirow{2}{*}{**Geometric Parameter**} & \multicolumn{3}{c}{**Material Parameters**} \\ \cline{3-5}  & & & **Elasticity Modulus** & **Density** & **Poisson’** \\ \hline \multirow{6}{*}{Jacket} & Main Bar & Round steel pipe, diameter is 1.5 m, & & \\  & & thickness is 0.06 m & & \\ \cline{2-5}  & Interlayer transverse bracing & Round steel pipe, diameter is 0.8 m, & & \\  & & thickness is 0.04 m & & \\ \cline{2-5}  & Diagonal bracing & Round steel pipe, diameter is 0.6 m, & & \\  & & thickness is 0.025 m & & \\ \cline{2-5}  & Internal bracing & Round steel pipe, diameter is 0.6 m, & & \\  & & thickness is 0.025 m & & \\ \hline \multirow{2}{*}{Observation platform} & \multirow{2}{*}{beam} & Rectangular steel tube, width is 0.4 m, & 206 GPa & 7850 kg/m\({}^{3}\) & 0.3 \\  & & height is 0.4 m, thickness is 0.02 m & & \\ \cline{2-5}  & deck & thickness is 0.02 m & & \\ \hline \multirow{6}{*}{Wind tower} & \multirow{2}{*}{Structure leg} & Round steel pipe, diameter is 0.5 m, & & \\  & & thickness is 0.02 m & & \\ \cline{2-5}  & Transverse bracing & Round steel pipe, diameter is 0.25 m, & & \\ \cline{1-1}  & Diagonal bracing & Round steel pipe, diameter is 0.2 m, & & \\ \cline{1-1}  & & thickness is 0.0075 m & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Structural element parameters of offshore platform [33].  
Figure 5: Sensor’s placement on the jacket offshore platform.