## 4 Discussion and Conclusions  
The key principle of preventive maintenance is to conduct regularly performed maintenance to lessen the likelihood of failure of critical systems. It is important to perform while the systems are still working before serious failure happens. The proposed preventive maintenance topic models provide groups of associated maintenance works based on semantic association. If any maintenance works were identified, the person in charge of the maintenance is recommended to perform maintenance works of other items in the topic model to lessen the likelihood of failure of the associated systems. Initially, the frequencies of maintenance keywords were analyzed to understand the relative significance of each maintenance task. The proposed maintenance topic models were not identically aligned to the maintenance keyword frequencies because the topic modeling algorithm takes into account the proximity and co-occurrence of keywords. On the basis of the proximity and co-occurrence of multiple keywords, the proposed maintenance topic models constructed meaningful and useful insights regarding the critical maintenance topics of LNG containment systems based on age categories. The proposed LNG containment topic models provided a relationship between the age and maintenance strategy of LNGCs. The relationships are categorized into the following three phases:  
1. Initial phase less than 10 years after building a new ship  
In the earliest LNGC life phase, condition-maintaining tasks are mainly conducted during dry dock repairs. Examples of the regular maintenance tasks include general gas detecting system inspection, gas compressor inspection, cargo valve gage pressure inspection, and cargo tank visual inspection. Moreover, the initial deficiencies during this initial operation phase are minor, and the deficiencies are generally caused by inappropriate hull design and inadequate operation of LNG transfers. Therefore, condition maintenance and regular inspection topics should be included in the preventive maintenance strategy. 2. Stable period between 10 and 20 years  
In this phase, manufacturing deficiencies are limited; hence, the fundamental maintenance interest is now to ensure the acceptable and operational condition of LNG handling instruments. For this purpose, consumable spare parts, such as O-rings on many valves, bearings on rotating parts, and \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Vessel Age**} & \multicolumn{3}{c}{**Topic Keywords**} & \multicolumn{2}{c}{**Maintenance Topic Model**} \\ \cline{2-5}  & **Topic 1** & **Topic 2** & **Topic 3** & **Summaries** \\ \hline \multirow{3}{*}{Less than 10} & check & valve & tank & \multirow{3}{*}{Topic 1—General inspection} \\  & cargo & pump & spare & & Topic 2—LNG handling equipment \\  & gas & inspection & test & & Topic 3—Tank integrity check \\ \hline \multirow{3}{*}{Between 10 and 20} & test & tank & valve & \multirow{3}{*}{Topic 1—General inspection} \\  & inspection & pump & gas & & Topic 2—LNG handling equipment \\  & valve & cargo & renewed & & Topic 3—Cargo valve check \\ \hline \multirow{3}{*}{More than 15} & check & valve & cargo & Topic 1—LNG tank integrity \\  & check & renewed & pump & inspection \\ \cline{1-1}  & system & gas & test & Topic 2—Valve and line inspection \\ \cline{1-1}  & pipe & inspection & open & Topic 3—LNG handling equipment \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Top ranking keywords and maintenance topic summaries. parts of heaters, are renewed and refitted during the maintenance. Additionally, LNG tank integrity related to cargo tank, insulation material, and inner hull required more attention from the maintenance operators. The identified topics of the proposed maintenance topic models are therefore acceptable to incorporate these tasks. 3. Aging period after 20 years of operation  
Various deficiencies and defects are caused by aging, including tearing, fatigue, and corrosion.