3.2. Add maintenance policy module
Ships are operated under the policy of performing planned maintenance (1.5 months) after a certain operational operation (4.5 months). In Section 3.1, the unit of model was number of failures. The unit for planning maintenance is the number of ships. Although their scale is different, they are easily integrated in system dynamics. The part marked in red in Fig. 5 is the maintenance policy module. Maintenance criterion determines the number of operating days of a ship. It may be set to a different date depending on the policy (In the case of Condition based maintenance (CBM), the maintenance time is determined based on the ship status. In CBM, this variable determines the number of flexible operating days). After 137 days or 138 days, the ship in the Battle field moves to the Repair shop. After 45 days of movement, the ship moves to the Battle field through Return to field. Fig.6(a) shows the relationship of ship movement. Although the two modules have different units, they interact and work at the same time. Fig.6(b) shows the change of failures according to the operation and maintenance of the ship. Failures accumulated from 137 to 138 days are repaired for 45 days. The total life of the ship is 31 years. Since operation and maintenance are repeated twice a year, there are exactly 62 peaks in Fig. 6(b). Through the merging of two modules with different units, it can be seen that the system dynamics supports multi-scale.