Page 44: of Maritime Reporter Magazine (March 2001)

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This came as a shock to the system to an industry that had been used to doing major corrosion repair work around the time of the second special survey.

Suddenly, owners found that they were footing hefty repair bills on very young ships. But why were double-hulled tankers rusting away so quick- ly? The answer was provided only recently, thanks to work done by Sigma Coatings, for whom Jansen acts as a consultant. "Research done by Rodney Towers at Sigma sug- gests that the corrosion problems caused on board double-hulled tankers can be attributed to the ther- mos-flask effect of the design. Crude oil is usually loaded in locations where temperatures can reach as high as 45 to 50 degrees Celsius. It is then shipped to locations that are much cooler. If it is carried on board single-hulled ships, the oil cools down comparatively quickly under the influence of the ambient sea temperature, reaching sea tempera- ture between 36 and 48 hours after being loaded, depending on the volume of oil and other factors.

On board double-hulled vessels, cooling down times are much longer, taking days rather than hours," Jansen explains. Oil in double-hulled tanks takes an average of 20 days to reach sea tempera- ture, he adds.

The significance of the longer cooling down peri- od is found in the lifecycle of corrosion inducing bacteria. The bacteria survive longer in the warmer oil in double-hulled ships, lengthening the electro- chemical corrosion process that naturally occurs at the bottom of cargo tanks on board oil carriers.

Not only do the bottom of tanks on board double- hulled vessels rust more rapidly, under-deck plating and ullage areas are more prone to corrosion as well. The reason for this is a different one, howev- er. Here, it is the choice of construction material that is significant. To offset the increase in weight due to the extra steel required for double-hulled newbuilds, shipyards incorporated higher levels of high-tensile steel into their designs. "High-tensile

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The teams at Cascade General have been honing this kind of precision planning to a science over the last 50 years. It's what allows us to complete repairs in record time—even accomplish projects previously considered impossible. It's why when you think of the most capable shipyard in the western hemisphere that consistently delivers pit stop performance, you'll think

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Maritime Reporter

First published in 1881 Maritime Reporter is the world's largest audited circulation publication serving the global maritime industry.