# Dropped Object Impact Analysis Considering Frequency and Consequence for LNG-FPSO Topside Module
## 4 Dropped Object Frequency and Consequence Analyses  
Research on dropped objects on an offshore plant is divided into the topside and subsea areas, as shown in Figure 5. In this study, topside frequency and consequence analyses were performed simultaneously. Frequency analysis was divided into the crane area and monorail areas. This study focused on heavy objects falling on board. The frequency and consequence analysis results for an on board dropped object can be used to create an exceedance plot. The DAL focused on items exceeding the critical criteria among the maintenance equipment items derived from the exceedance curve, and the influence of the dropped objects on the equipment was analyzed extensively. Figure 6 shows the detailed QRA procedure for topside dropped object analysis of offshore structures. Material handling was investigated based on topside and equipment layouts of offshore structures, and results were derived from crane operation and material handling procedures. Based on the results of the material handling study, QRA engineers constructed an accidental load scenario, identified key service areas where equipment protection was required, and then categorized the areas where objects were likely to fall. QRA engineers identified the equipment and devices that required protection in the specified drop zones and evaluated the impact of dropped objects on said equipment. The six major parameters required for heavy-object drops are weight, position, impact energy, impact orientation, impact type, and stiffener position, as shown in Figure 6. In this study, impact energy was considered to be sufficient for accidental load design because it yielded different results depending on the object impact orientation. \begin{table}
\begin{tabular}{c c} \hline \hline
**Symbol** & **Description** \\ \hline  & Equipment Removal Route \\ \hline  & Cart Wheel Road \\ \hline  & Equipment Footprint \\ \hline  & Escape \\ \hline  & Maintenance \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material handling symbol descriptions. Figure 4: Material handling route: process deck (**left**) and upper deck (**right**). The impact locations were determined, and frequency and consequence analyses were performed according to service areas, moderate areas, and non-critical areas after estimating key variables. The frequency and consequence analytical processes are described in detail in the case of heavy objects. The impacted structure energy absorption was evaluated according to the analytical results. Service and operational constraints were added to the final design, depending on whether the estimated energy could be absorbed. If this was not satisfied, the first six design variables were reselected, and the design process was repeated. The probability of a dropped object is defined as the probability that equipment will fail because of a heavy object drop [26]. The probability of a dropped object is expressed by the following formula:  
\[P_{FDI}=\sum_{I}N_{i}P_{D},\sum_{I}P_{H_{ij}}P_{F_{ij}} \tag{1}\]  
\(P_{FDI}\) = Probability of equipment failure because of a dropped object impact  
\(N_{i}\) = Number of lifts per load category \(i\)  
\(P_{Di}\) = Probability of a load dropped from a crane for load category \(i\)  
\(P_{Hij}\) = Probability of equipment j being hit by a falling load in category \(i\)  
given that the load is dropped  
\(P_{Fij}\) = Probability of failure of equipment \(j\) given impact by load category \(i\)  
The predicted impact energy (\(E\)) due to a dropped object was modeled as potential energy using Equation (2). Table 3 lists the numerical modeling specifications. This table shows the service items, expected impact energy, weight, height, annual lifting frequency, drop probability, and drop impact probability.