RIASSUNTO
ABSTRACT:
Ballast tanks on ships are used in various modes - completely filled with seawater, partially filled, or sometimes completely empty. It is the normal practice to coat the interior of the tanks with a suitable coating and also apply cathodic protection (CP). During service, the interiors of the ballast tanks are exposed to complete immersion in seawater when they are full. When half full, the top half of the tank interior is exposed to moist, salt-laden air while the bottom half is completely immersed. When completely empty, the tank interior only has moist salt-laden air. A project was undertaken to study the performance of some marine coatings under simulated ballast tank conditions by utilizing a large (2000-gallon) mock-up ballast tank. The impact of the type of surface preparation, in particular abrasive blasting (AB) and ultra high pressure waterjetting (UHP-WJ), on coating performance was investigated and the results are reported in the paper.
INTRODUCTION
Ballast tanks constitute an integral and crucial component of all marine vessels. A vessel may have a single ballast tank near its center or multiple ballast tanks typically on either side. A large vessel will often have several ballast tanks including double bottom tanks, wing tanks as well as front and rear tanks. Adding ballast to a vessel lowers its center of gravity, and increases the draft of the vessel. Increased draft may be required for proper propeller immersion. A ballast tank can be filled or emptied in order to adjust the amount of ballast force. Ships designed for carrying large amounts of cargo must take on ballast water for proper stability when travelling with light loads and discharge water when heavily laden with cargo. Thus ballast tanks on ships can be used in various modes - completely filled with seawater, partially filled, or sometimes completely empty. It is the normal practice to coat the interior of the tanks with a suitable coating and also apply cathodic protection (CP) for corrosion control purposes. It is reported1 that ballast tanks often represent more than 50% of the entire coated area of a vessel. During service, the coated interiors of the ballast tanks are exposed to complete immersion in seawater when they are full. When half full, the top half of the tank interior is exposed to moist, salt-laden air while the bottom half is completely immersed. When completely empty, the tank interior only has moist salt-laden air. Since large sections of the tanks are inaccessible when a ship is in service, the quality of the coating as well as the quality of surface preparation and coating application is of utmost importance. Surface preparation by conventional abrasive grit blasting (AB) and/or high pressure (HP) or ultra high pressure (UHP) waterjetting (WJ) inside tanks have unique challenges. Application of coatings inside tanks involves additional factors, which may affect the coating process as well as the performance. Waterjetting creates significant amounts of water vapor (mist). Waterjetting inside tanks amplifies this factor and will require remediation.