Abstract
Ballast tanks are vulnerable to corrosion, this is due to the severe environmental conditions, when compared to other parts of the vessel. Normally durable epoxy coatings are applied to the ballast tanks to prevent corrosion.
The complicated structure of ballast tanks makes it extremely difficult for the coating applicator to achieve perfect coating, this due to their complex structural geometry and awkward areas to coat properly. This is especially the case in the region of welding seams in the ballast tanks, and the whole process tends to slow down production and is a draw back to an otherwise high quality coating system
One of the serious coating failures with ballast tank coating systems is cracking of the coating system in the versinity of the welding seams on stiffeners. The theory on how this occurs is thought to be thermal stress this is because either side of the divisional bulkheads which separates the crude oil tanks from the ballast tanks, experiences periodic temperature changes caused by the cargo tanks filled with hot crude oil on one side, while the adjacent or opposite side of the bulkhead which forms the ballast tank is filled with cold seawater. The coating material therefore expands and contracts during operation. We can assume one of the most probable reasons for the coating cracking is due to thermal stress concentration during unloading crude oil from cargo tanks.
In this study, various experiments, such as thermal cycling test and other mechanical tests, were carried out to verify the reason of cracking. Also, on the basis of thermo-mechanical properties of coating materials FEM analysis was carried out to understand the reason for cracking, dependent on coating thickness. Cracking-probability can be estimated and predicted quantitatively through the results of thermal stress analysis. It can be used as a parameter in selecting the most suitable coating materials, which have better cracking-resistance.
