## 3 Results
### Proposed Preventive Maintenance Topic Models of LNGCs  
Topic modeling algorithms were applied to analyze the thematic structure maintenance keywords from the dock specifications. The developed LNG containment topic models were represented using word clouds, and maintenance keywords of higher frequency were highlighted in the LDA topic models. The LDA algorithm determines the key terms with sizes according to probability for the selected LDA topics. Word clouds give a view of the words with the highest probabilities in each topic. Figures 6-8 visualize the first three topics using the word clouds for LNGCs aged less than 10, between 10 and 20, and more than 20 years, respectively. Figure 6: Topic model of LNGCs aged less than 10 years. (**a**) Topic 1: check-cargo-gas-line; (**b**) Topic 2: valve-system-inspection-pump; (**c**) Topic 3: tank-test-spare-renew. Figure 7: Topic model of LNGCs aged between 10 and 20 years. (**a**) Topic 1: test-inspect-valve; (**b**) Topic 2: tank-cargo-pump; (**c**) Topic 3: valve-gas-renew. Initially, the two different tank types were separated analyzed. Since there were no significant differences in terms of key maintenance works and association with relevant maintenance works, the results were combined in single data analysis. For example, both tank types utilize submersible cargo pumps in the cargo tanks, and there was no difference in terms of maintenance-related cargo pumps or support of the cargo pumps. Rather, cargo tank leak inspection or testing received more attention, since MOSS has tank walls several centimeters in thick, whereas a membrane has about a one-millimeter thickness. Unfortunately, results have not demonstrated the difference regarding cargo tank structure including gas leak testing of both tanks types. Figure 6 illustrates the developed LNG containment system maintenance topic of LNGCs aged fewer than 10 years. As illustrated in Table 2 and Figure 5a, \"check\", \"valve\", and \"cargo\" were the three most frequent maintenance keywords in the same vessel age category. The first three LNG containment system maintenance topic models, however, selected topics with the keywords \"check\" \"valve\", and \"tank\" without a topic related to \"cargo.\" As the selected maintenance keyword implies, the main maintenance purpose of LNGCs aged less than 10 years is the general LNG cargo handling system inspection and the regular inspection. Figure 7 depicts the LNG containment system maintenance topic of LNGCs aged between 10 and 20 years. As illustrated in Table 2 and Figure 5b, \"valve\", \"tank\", and \"check\" were the three most frequently appearing maintenance keywords. However, the prior LNG containment system maintenance topic models selected maintenance topics \"test\", \"tank\", and \"valve.\" In the second topic model in Figure 7a, the \"valve\"-related maintenance tasks are clustered with the \"test\" and \"inspect\"-related maintenance tasks. The relative priority of the LNG tank inspection task is higher than that of LNGCs of younger ages. Figure 7c shows that the cargo valves of the vessel age group need to be renewed and refit to ensure reliable LNG cargo handling. Overall, the prior maintenance purpose of LNGCs aged between 10 and 20 years focuses more on LNG tank inspection and cargo valve repair tasks than younger LNGCs. Figure 8 demonstrates the proposed LNG containment system maintenance topic of LNGCs in the oldest age group. As illustrated in Table 2 and Figure 5c, \"tank\", \"valve\", and \"cargo\" were the most frequently appearing maintenance keywords. The developed LNG containment system maintenance topic models reflect the same tendency as the maintenance keyword frequencies. In Figure 8a, LNG tank integrity-related maintenance and inspection tasks get the highest importance for the aging LNGCs. This tendency implies that in aging LNGCs, it is necessary to pay careful attention to maintaining the hull and cargo tank integrity. Figure 8b emphasizes that the LNG cargo valves of aging LNGCs require more serious repair tasks than regular inspection.