Analyzing the simulation results of the DT model is the same as
analyzing reality. Real variables are composed of complex causal
relationships. In the DT model, many loops are created by the connection
of modules. In the case of Fig. 8, there are about 230,000 loops. Humans
cannot intuitively know these complex relationships. So, in reality,
sometimes unexpected results can come out. This is due to the influence
of variables that were not considered. Assume that the Naval operation
period is relatively longer than the maintenance time. The longer the
equipment run time, the more failures. As maintenance costs increase,
total life cycle cost increases. This sequence may seem like a natural
result. However, the simulation results show that the total life cycle
cost is reduced. See section 5.1 for more details. In the DT model, it
is possible to simulate latent effects or potential problems that occur
due to the simultaneous influence of multiple modules and variables.
By checking the simulation results, the relationship between cause and
effect can be established. Examples are the relationship between the
operating period of the ship and the total life cost, or the
relationship between the number of mechanics and the failure probability
of the ship. In this study, linear regression is simply used, but a
high-precision method such as multi-regression may be applied if
necessary (section 5.2).
It is meaningful to perform optimization by integrating all the
variables, but computation time constraints may occur. In this study, a
rough optimization is performed using the equation calculation
established by linear regression (section 5.2).
Section 5.2 examines maintenance policies with the DT model. As shown in
Table1, a total of 4 models are composed. In the current Naval policy,
ships start maintenance 1.5 months after 4.5 months naval operation.
However, to simulate advanced policies such as condition-based
maintenance, it is assumed that normal failure means the condition of
the ship. Normal failure limits the performance and worsens the
condition of the ship. On the other hand, the repair period is not fixed
at 1.5 months, but consider the situation in which the maintenance
period can be terminated as soon as the repair is completed.