# Dropped Object Impact Analysis Considering Frequency and Consequence for LNG-FPSO Topside Module
## 5 Numerical Simulation and Test Evaluation
### Drop Impact Analysis of the Cone-Type Object  
The drop impact analysis was performed using ANSYS/LS-DYNA software, which is able to calculate valid results for nonlinear impact phenomena. Figure 18 shows finite element (FE) models for the dropped object and specimen. The shape and size of the finite element model were identically generated to those of the dropped object and specimen used during the experiment. The FE model was generated using a four-node solid element consisting of 4517 elements and 5564 nodes. Figure 16: Specim strain variation during the experiment (drop height of 3 m). Figure 17: Specim strain variation during the experiment (drop height of 5 m). Figure 18: Finite element model for drop impact analysis. The dropped object was assumed to have a rigid body and the average values of the mechanical properties of the specimen used are shown in Table 5 based on the results of the self-performed tensile test. A free fall condition (gravity acceleration of 9.81 m/s\({}^{2}\)) for the dropped object was applied as a loading condition of the drop impact analysis and a fixed support condition (six degrees of freedom of nodes on the four edge faces are set to zero) was applied as a boundary condition of the specimen. Based on these analytical conditions, drop impact analysis was performed and the analytical results of the vertical deformation and strain of the specimen according to the drop height were derived and compared to the experimental results. Figure 19 shows analytical and experimental results for vertical displacement in the case of a drop height of 3 m. Considering the symmetry of the analysis model, the analytical result is a plot of vertical displacement for the center line (A-A' and B-B') of the specimen and the experimental result (EXP) is a plot of the average values between the A-A' line and B-B' line shown in Figure 14.  
As illustrated in Figure 19, the analytical and experimental results show some differences between the 300 and 900 mm positions, but the overall pattern of change is similar. In particular, the maximum vertical displacements at the impact point which is intimately related to the value of the impact energy are \(-\)79.6 mm (experimental value) and \(-\)75.5 mm (analytical value). This is an error of approximately 5.2%, but is acceptable in the research field of nonlinear collision analysis. The drop impact analysis method as proposed can be used for relatively accurate calculation of drop impact energy considering the angle of collision of the suction drum performed in this study. Figure 20 compares the analytical results to the experimental results when the drop height is 5 m. As in the case of a drop height of 3 m, the overall pattern of deformation in the analytical and experimental results is similar. In addition, the maximum vertical displacement value during the experiment is \(-\)98.4 mm and the  
Figure 19: Vertical displacement comparison (drop height of 3 m). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \(s\) & \(E\) & \(\sigma_{y}\) & \(\sigma_{t}\) & \(\rho\) \\  & **(mm/s)** & **(GPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(kg/m\({}^{3}\))** \\ \hline \(1\) & 0.06 & 226.9 & 410.1 & 555.6 & 0.355 & 7851 \\ \hline \(2\) & 100 & 202.2 & 452.7 & 565.6 & 0.373 & 7847 \\ \hline \(3\) & 0.06 & 204.9 & 412.2 & 553.2 & 0.370 & 7854 \\ \hline \(4\) & 100 & 216 with an error rate of approximately 3.3%. This value is also acceptable in the research field of nonlinear collision analysis. In Figure 20, it can be observed that the kink occurred at approximately 900 mm in the FEA curve. This is considered to be the point where a relatively large deflection occurs in very small area. To analyze local large deflection phenomena, the mesh control in that area is important. However, the simulation time cost for fine meshes increases significantly. In this study, the mesh size selected can yield analysis results of vertical displacements that agree well with the experimental values.