Abstract
A seachest is a void space or box on the body of a marine vessel below the water line from which piping systems draw raw water for cooling or other uses such as ballasting of the vessel and fire-fighting. The inlet grid or intake of the seachest, which is protected by a grating, can vary in size from under 1 sq. meter for a small inland tug to several hundred sq. meters for a large vessel like an oil tanker. The seachest is normally fabricated from carbon steel like the rest of the vessel, while the adjoining piping and valves are nonferrous. Zinc anodes in combination with coatings are used for the corrosion protection of the ship’s hull as well as the seachest. A waster-sleeve, typically fabricated from 0.375-inch (9.52 mm) thick carbon steel plate, is recommended as additional sacrificial protection for the non-ferrous components with a projected service life of 10-12 years or more. Since the waster sleeves (as well as adjoining non-ferrous components) are electrically continuous to each other, they also receive some protection from the zinc anodes.
Replacement of waster-sleeves is a high cost item. Depending on the size of the waster-sleeves, dry-dock replacement installation can cost anywhere from $5,000 to $25,000 while underwater installations can be as much as $200,000. Hence, early failure of these and other seachest components can seriously impact the extended life cycle goals for these vessels. During the nineties, premature failure of waster-sleeves were observed on a number of ocean-going vessels. Based on shipyard surveys of failed waster sleeves it was found that the standard thickness of 0.375 in (9.52 mm) was not sufficient to survive the desired 12-year service life and even doubling the thickness only had marginal effect. Engineering analysis indicated that the existing CP design utilizing only 2 to 4 pieces of 42-lb (19.1 Kg) Zn anodes was inadequate and a re-designed CP system with a larger number of anodes would possibly solve the problem.
