## 2 Theory and Methods
### Method for Research Based on FRAM  
In this study, FRAM was adopted because sequential causes and causality could be explained in a complex and conceptual manner, as FRAM is suitable for accident cause analysis and risk assessment in complex organizational-technical, human-technical, and organizational-human relationships from the perspective of preventing the recurrence of port contact accidents of mega container ships, such as the MV Milano Bridge. FRAM supports the systematic analysis of variability in functional performance and nonlinear interactions between functions that are essential in accident analysis, such as the analysis of marine accidents with variably linked complex causes. It is an optimal research method with excellent expression of phenomena that can overcome the limitations of existing thought analysis techniques, such as the human performance enhancement system (HPIP), human performance investigation process (HPES), HFACS, Technique for the retrospective analysis of cognitive errors (TRACEr), human error analysis and reduction (HEAR), and the casualty investigation code IMO [25]. Moreover, FRAM provides useful information and explains situations to manage performance variability by analyzing and modeling the functions and interactions of a system operating under normal conditions. Furthermore, it analyzes performance variability and ripple effects. Therefore, the following four steps were followed to analyze the MV Milano Bridge crane contact accident at Busan New Port using the FRAM. Step 1 involved identifying and describing the system functions related to the FRAM modeling target, which was the MV Milano Bridge crane contact accident at the Busan New Port. All functions that signified the interactions between various systems related to the accident were examined, and the interconnections between the functions that must be performed to achieve the actual purpose were identified. Moreover, the related functions were determined by categorizing the functions into six aspects based on the accident results, as shown in Figure 2[30]. Step 2 involved estimating the potential variability of each function, which was then evaluated by categorizing them into six aspects. For each function, the elements of precondition, resource, control, time, and input, excluding the outcome, were selected to minimize variability, which may occur because of the precision and application timing differences of each element. [PERSON] presented a variability that occurred due to the timing and precision difference of each element, as shown in Figure 3[30]. Step 3 involved the analysis of the potential variability of each function identified as the cause of the MV Milano Bridge Busan New Port-crane contact accident using FRAM. The analyzed individual functional resonance accidents had subsequent variability based on the connection of each function. The consequences of variability were explained using connections between the six aspects of each function. Step 4 involved the analysis of each function identified as the cause of the MV Milano Bridge Busan New Port-crane contact accident using the FRAM. Furthermore, the variability of the resultant function was analyzed to determine effective improvement plans. The variability of the function and ripple paths was identified by analyzing the MV Milano Bridge Busan New Port-crane contact accident using the FRAM model. Based on this, the causal relationship between the causes and effects of marine accidents can be understood. When conducting a risk assessment, technical, managerial, and organizational improvement plans can be developed from a logical point of view to develop future improvement plans by predicting the likelihood of multiple potential accidents based on variability and ripple paths. The preconditions for configuring the FRAM model were classified by function and are listed in Table 1. To analyze the Milan Bridge accident using FRAM, the system and its characterized functions were identified as the first step.