## 2 Design of a Structural Health Monitoring System for a Jacket Offshore Platform
### Assessment Subsystem
#### 2.3.1 Safety Monitoring and Early Warning Evaluation Using Static Measurements  
Safety monitoring and early warning evaluation of the offshore platform are based on static measurements. This evaluation mainly checks whether each index is within a defined safety threshold by measuring the platform displacement and inclination and the pile foundation load. Under the action of various factors, an offshore platform will experience a certain degree of displacement and inclination. Within the safety threshold, this will not affect the service performance of the structure. However, once the displacements or loads exceed certain ranges, this will threaten the safety of the platform. Platform Displacement Monitoring  
Measurement of the displacement and its direction is realized by positions on the top of offshore platform using the GPS or Beidou system. As shown in Table 1, the risk to the offshore platform can be divided into three levels according to the UC value (the ratio of the maximum allowable stress under combined working conditions of member compression, tension, and bending). It is necessary to determine the relationship between the displacement of the observation platform and the UC range of its members. Many scholars have examined this according to structural collapse theory and historical data, and it will not be repeated here. For specific criteria, please refer to the relevant literature [27]. Platform Tilt Monitoring  
In the relevant technical specifications for jacket platforms, the platform inclination is generally required to be controlled within 0.5% [28]. The index and the early warning condition (\(\beta_{\text{max}}\)) is calculated according to the following equation. \[\beta_{\text{max}}=\left|\theta_{\text{t}}-\theta_{\text{b}}\right|_{\text{max }}<0.5\% \tag{1}\]  
where \(\theta_{\text{t}}\) and \(\theta_{\text{b}}\) are the inclination of the top and bottom of a guide leg as measured by its inclination sensors, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Failure Form** & **Consequence** & **Risk Level** & **Warning Color** \\ \hline First member UC exceeds 1.05 & Local strength reduction & Class A & Blue \\ Local member UC exceeds 1.05 & Local failure & Class B & Orange \\ First node invalid & Overall structural failure & Class C & Red \\ \hline UC: Maximum allowable stress ratio of member under combined working conditions of compression, tension, and bending. & & & \\ \end{tabular}
\end{table} 
Table 1: Classification of structural failure modes. The pile foundation load is mainly composed of the initial load and the variation of the load. The initial load and the dead weight of the steel piles can be regarded as fixed values, and the change in the load on the pile foundation can be obtained through monitoring [29]. Under normal sea conditions, the jacket structure can be regarded as a linear system, as shown in Figure 2. In this paper, the load change at the top of the jacket leg is denoted by \(\Delta\mathbf{F}\) and the load transfer function of the platform structure is \(\boldsymbol{C}\), the change in the pile foundation load \(\Delta\boldsymbol{P}\) can be expressed as  
\[\Delta\boldsymbol{P}=\boldsymbol{C}\times\Delta\boldsymbol{F}. \tag{2}\]  
If the initial load is \(\boldsymbol{P}_{0}\) and the dead weight of the steel-pipe pile is \(\boldsymbol{W}\), the pile foundation load \(\boldsymbol{P}\) is  
\[\boldsymbol{P}=\boldsymbol{P}_{0}+\Delta\boldsymbol{P}+\boldsymbol{W}= \boldsymbol{P}_{0}+\boldsymbol{C}\times\Delta\boldsymbol{F}+\boldsymbol{W} \tag{3}\]  
It is also necessary to determine the change in the load at the top of the conduit leg \(\Delta\boldsymbol{F}\) to obtain the change in the load at the pile foundation \(\Delta\boldsymbol{P}\).