Corrosion degradation impact on cargo hold main frames of single skin bulk carriers

Damage due to corrosion occurs most often in the form of general corrosion or pitting, although other forms of corrosion such as crevice, microbiological, fretting, etc. can also occur. Damage to structure due to corrosion can be expressed by the loss of metal thickness expressed in mm or percentage of wear, as well as by the loss of metal weight. Corrosion appears at different speeds and in different areas depending on numerous external and internal factors. So far, research has shown that cargo holds, and ballast tanks of bulk carriers are the most susceptible to structural damage due to corrosion. In this paper, we analyze the main frames, as part of a cargo hold for transporting bulk cargo. Using the example of ten old single-hull bulk carriers, we consider the amount of steel that has been replaced over time. Applying the acceptance criterion prescribed by the classification societies, measurements were made of the main frames and the defined damaged surface of the structure, which needed to be replaced due to excessive corrosion. In this way, the cumulative amounts of damaged steel over time were calculated and expressed in tons of replaced steel. Observing the changes in ships in exploitation in the period from 10 to 30 years of exploitation, the ships were monitored during several special surveys. Calculating the annual amount of replaced steel and fitting the distribution showed that the three-parameter log-logistic distribution gives the best values of weight replacement.