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Friday, October 19, 2018

POINTS OF VIEW - Tanker cargo tank bottoms may be better left uncoated

Ripples have broken the otherwise smooth surface of a tanker industry still very much on its summer holidays. Research by two of Japan’s biggest shipowners has concluded that tanker cargo tank bottoms may be better left uncoated.

Mitsui OSK Lines claims that basic shop primer on the steel forming the cargo tank bottom plates is better than expensive coatings at preventing the pitting corrosion that has so seriously affected many double-hulled tankers which eats into steel at up to 3 or 4 millimetres a year.

Comparative tests on some of MOL’s VLCCs using shop primer show 50 pittings per vessel compared to 2,000 on a VLCC without shop primer. Analysis has shown that the shop primer reacts chemically when the vessel is in loaded codition to become a hard coating of ferrous zinc oxide, resistant to corrosion, says MOL.

Other industry sources are not yet convinced that the protection offered by shop primers is any more than relatively short-lived.

Nippon Yusen Kaisha has used newly-developed steel plate from Nippon Steel in the construction of the cargo tank bottoms of a VLCC newbuilding being delivered from MHI in 2q2004. Tests in a simulated environment have shown that this uncoated plate can slow corrosion to about one fifth the rate of conventional steel, says NYK. The new steel resists corrosion irrespective of the surrounding environment or the quality of maintenance work, claims NYK.

Japan’s shipowners are concerned that the new Unified Requirements currently being prepared by IACS with input from the rest of the tanker industry, which include protective cargo tank coatings on newbuildings, will exclude vessels built with uncoated tank bottoms but with other anti-corrosion features.