51316-7845-Adequate CP Design Can Prevent Costly Premature Failure of Seachest Waster-Sleeves of Marine Vessels

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Product Number: 51316-7845-SG

ISBN: 7845 2016 CP

Author: Moavin Islam

Publication Date: 2016

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Adequate CP Design Can Prevent Costly Premature Failure ofSeachest Waster-Sleeves of Marine VesselsA seachest is a rectangular recess near the bottom of a marine vessel from which piping systems draw raw water for cooling or other uses. Its size can vary from a few sq. ft. for a small inland tug up to several hundred sq. ft. for a larger 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 thick carbon steel plate is recommended as additional sacrificial protection for the non-ferrous components with a projected service life of 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 $5000 to $25000 while underwater installations can be as much as $200000. Hence early failure of these and other seachest components can seriously impact the extended life cycle goals for these vessels. 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 inch was not sufficient to survive the 12-year service life and even doubling the thickness only had marginal effect. Engineering calculations by Corrpro Engineers indicated that the current CP design (number of Zn anodes) was inadequate and a re-designed CP system with a larger number of anodes would possibly solve the problem.Key Words: Seachest Waster-Sleeve corrosion CP design