# The Multistate Reliability Assessment of Ship Hull Girders
## 2 Materials  
This study is based on the analysis of historical data regarding the corrosion damage of structural parts of longitudinal girder plates, i.e., steel plates between the ballast tanks and the fuel tank. The measured data represent general corrosion and are expressed as percentages of damages. They were systematized based on thickness measurement data collected during regular special surveys after five, ten, fifteen, twenty, and twenty-five years, while the corrosion damage data collected during intermediate surveys were included in the results of special surveys. Previous research on fuel tanks has shown that besides inner bottom plating and longitudinal watertight girder plates, the upper plates of longitudinal girders have significant thickness damages due to corrosion [10; 12; 17]. Thus, two specific thickness measurement locations on the girder were considered, the upper and lower part of the girder plating, presented as A\({}_{5}\) and A\({}_{6}\) in Figure 1.  
This study employed a database gathered over the last two decades, encompassing the following data:  
* A total of 25 aging bulk carriers between five and twenty-five years of age;
* The period investigated, between 2005 and 2017;
* The analysis results of 38 varied special surveys. Table 1 shows a basic database of the considered vessels, number of inspections, number of fuel oil tanks, number of thickness measurements, and specific sections. The methodology for selecting measurement positions was created in such a way as to objectively assess the condition of each structure plate of fuel oil tanks and is described in detail in [10]. Table 1 presents the average diminution (due to corrosion) values of the total number of measured values of the thickness of the girder steel plates at different time intervals, from five to twenty-five years. The measurements were carried out in accordance with the rules of the classification societies, which prescribe a minimum scope of measurements, precisely define the measurement locations, and additionally require measurements in corro \begin{table}
\begin{tabular}{c c c c c c} \hline
**The Age of** & **The Number** & **The Number** & **The Number** & **The Number** & **The Mean Values of Plate** \\
**Ships** & **of Ship** & **of Tanks** & **of Measured** & **of Sections** & **Thickness Diminution Caused** \\
**(Years)** & **Surveys** & **of Tanks** & **Points** & **of Corrosion (\%)** & **by Corrosion (\%)** \\ \hline
0–5 & 4 & 9 & 180 & 45 & Lower & Upper \\
5–10 & 4 & 10 & 200 & 55 & 0.6\% & 1.4\% \\
10–15 & 7 & 19 & 380 & 100 & 1.1\% & 6.7\% \\
15–20 & 13 & 43 & 800 & 220 & 4.8\% & 9.9\% \\
20–25 & 10 & 29 & 360 & 150 & 9.6\% & 23.0\% \\ Total: & 38 & 110 & 1920 & 570 & & \\ \hline \end{tabular}
\end{table} 
Table 1: Dataset used to study longitudinal steel girders. Figure 1: The fuel oil tank structure arrangement areas (A1–A10). Legend: A1—Bottom plate; A2—Inner bottom plate; A3—Inside girder (water tight), lower; A4—Inside girder (water tight), upper; A5—Side (water tight) girder, lower; A6—Side (water tight) girder, upper; A7—Floor after (water tight), lower; A8—Floor after (water tight), upper; A9—Floor fore (water tight), lower; A10—Floor fore (water tight), upper. the consistency of the data in all structural areas of the ships and an objective assessment of the corrosion wear of the steel structure. The data above enabled the reliability analysis of longitudinal watertight girders' steel plates in upper and lower areas along with further assessment of fuel tank exploitation. The example of a corroded girder plate after twenty years of exploitation presented in Figure 2b shows corroded surfaces that had to be replaced due to excessive corrosion. The rules and regulations of the classification society under which the ships were classified were used. For the present research, considering different classification societies and their requirements, we adopted their most common rule stating that excessive corrosion occurs when diminution due to corrosion is greater than 20%.