## 3 Results
### Frequency Analysis  
Single-word frequencies were analyzed to examine the distribution of maintenance items. Histograms were used to represent the probabilistic priority of each maintenance item. Table 2 summarizes the most frequently appearing maintenance keywords in the dock specifications. Three key maintenance items, \"check\", \"valve\", and \"tank,\" were the most frequently appearing words in the dock specifications for LNGCs aged less than 10, between 10 and 20, and more than 20 years, respectively. The lower priority word \"check\" as a root form of \"checking\" or \"checked\" appeared more in the less than 10-year LNGC group, whereas the higher priority word \"tank\" was evident in the aging LNGC category. The term \"renewed\" appeared only for vessels aged more than 20 years.  
A similar tendency was observed in the histogram statistics, shown in Figure 4. The histograms additionally demonstrate the frequencies of the top-used maintenance keywords by rank. For example, the relative rank of the term \"valve\" was third, first, and second in the increasing age categories; the same term continuously increased as the vessel age increased. The pattern showed that older LNGCs require more LNG cargo valve-related maintenance tasks. A similar maintenance trend was also seen for the \"tank\"-related maintenance tasks. Maintenance tasks related to \"renewed\" started to appear for LNGCs aged more than 10 years, and the frequency of \"renewed\" tasks significantly increased for older LNGCs (i.e., vessels aged more than 20 years).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Frequency** & \multicolumn{3}{c}{**Vessel Age**} \\ \cline{2-4}
**Rank** & **Less than 10** & **Between 10 and 20** & **More than 20** \\ \hline
1 & check & valve & tank \\
2 & cargo & tank & valve \\
3 & valve & check & cargo \\
4 & tank & cargo & check \\
5 & test & pump & gas \\
6 & system & gas & pump \\
7 & spare & test & renewed \\ \hline \hline \end{tabular}
\end{table}
Table 2: Top-ranking maintenance items based on single-word frequencies.  
Additionally, the relative significance among maintenance keyword frequencies was analyzed using word clouds (see Figure 5). The maintenance word cloud is an intuitive visual representation of maintenance patterns, mainly used to illustrate key maintenance metadata. The most frequent maintenance keyword is located in the center of each word cloud, and a larger font indicates a higher maintenance frequency. Word clouds are utilized as graphical representations of maintenance keyword frequency in dock specification. The larger the maintenance keyword in the word clouds the more common the word was in the dock specifications.  
Figure 4: Histogram statistics to illustrate relative maintenance item frequencies: vessels aged (**a**) less than 10 years; (**b**) between 10 and 20 years; and (**c**) more than 20 years.  
Figure 5: Word clouds using single-word frequencies of preprocessing dock specification texture data from the collected documents. Maintenance item word cloud of (**a**) vessels aged less than 10 years, (**b**) vessels aged between 10 and 20 years, and (**c**) vessels aged more than 20 years.