Following an exhaustive inspection and analysis of the damaged product tanker Castor, that suffered heavy weather damage in December 2000, the Cyprus Department of Merchant Shipping and ABS have jointly announced preliminary findings that point to hyper-accelerated corrosion as the probable principal cause behind the structural failure. A formal report into the casualty will not be issued by the Cypriot authorities and by ABS until the conclusion of detailed laboratory testing of steel samples cut from the damaged section of the tanker.
The 600 tons of steel, primarily in the deck plating and underdeck longitudinals, that was renewed on the Castor at Special Survey in late 1997 has provided the key to understanding what transpired in the interim. The critical element, according to the preliminary findings, is the presence, and absence of coatings. "The original steel had been coated," explained ABS Chief Surveyor Gus Bourneuf. "This coating had begun to break down with age. At the fourth special survey, the new steel was not coated. There were no sacrificial anodes in the tanks so the uncoated steel in the underdeck area acted as the anode with the partially corroded, original steel providing the principal point of attack."
According to an independent corrosion expert, brought in by ABS to analyze the condition of the ship, three other elements are considered likely to have contributed to the rapid deterioration. The vessel had been engaged in the gasoline trades, the most corrosive of all oil products. The critical Number 4 tanks were used for ballast purposes, introducing salt water into the chemical equation. And the vessel had been trading into hot areas, such as West Africa, greatly raising the ambient temperatures in the ullage spaces and creating a fertile environment for the corrosive action.
"Given the scantlings applicable to this size of ship, the loss of nearly 5mm of the new steel in just over three years of trading represents a very high percentage loss of section," said Bourneuf. "The loss of section of the uncoated steel that was not replaced was clearly greater. It is reasonable to conclude that this loss of strength in this critical area of the vessel may have contributed to the buckling, and subsequent cracking of the deck plating in the severe weather conditions encountered on passage."