design considerations for aluminum hull structures - Ship Structure ...

-85-systems are grotie~a and shipboard Power SYS~emS us~auy ‘avea~ati‘0the ships hull through ground detection equipment, the shore power cables)when brought aboard provide this return path, and the aluminum hull corrodesrapidly and severely. In order to avoid this problem, aluminum-hulledvessels are usually provided with one-to-one isolating transformers at theshore powe~ connection to effectively avoid creating a return path fo~galvanic corrosion current. It is also important to avoid providing metallicpaths from the hull to ground at pierside by means of accommodation ladders,loading conveyom, etc. Auxiliary anodes may be dropped over the side whenmoored to provide added protection.IIG. OPERATIONAL CHARACTERISTICS OF AN ALUMINUM BULK CARRIERThe operational characteristics of a bulk carrier will be profoundlyaffected by the substitution of aluminum for steel as the hull material,particularly in the areas of hull maintenance, repairs~ special surveys and~suranc e. In the following paragraphs, each of these factors will bebriefly discussed.MAINTENANCEPast experience with aluminum hulls and deckhouses indicates that itis feasible and desirable to keep the topsides and all internal surfacesunpainted~ although antifouling paint will be required below the deep loadline. The unpainted aluminum surfaces mhy eventually develop streaks andblotches and will become progressively darker. “However,for a bulk carrierthis consideration will be of secondary Importance. Topslae ~d internalpainting or coating is not recommended, since any local “breakdownin thesurface of the paint will tend to localize corrosive attack.In general~ it appears that normal topside maintenance will be limitedto an occasional water wash and scrubbing. However, the renew~ of antifoulingpaint will be required periodically, as will bottom scraping. This,coupled with requirements for maintaining equipment, appendages and outfit,will result in essentially the same drydocking cycle for aluminum and steelhulls. It is noted that the removal of paint and marine growth from aluminumsurfaces requires greater care than with steel. Conventional scraping andsandblasting methods must be modified to suit the lower abrasion resistanceof aluminum. Sand washtig has proven successful in removing old paint fromaluminum surfaces.During drydocking~ special attention should be paid to the sacrificialanodes or components of impressed current systems, as well as to the conditionof propellers snd other appendages, sea chestsg overboard discharges, etc.where corrosion could be present. Anodes or waster pieces installed in fueland ballast tanks should be reviewed perfodically~ and all piping and structurein the vicinity of bimetallic joints should be carefully checked for signs ofCorrosianbThe interface between all steel equipment (winches,windlass, etc.) andaluminum foundations should be checked periodically to ensure *hat the isolationmaterial at the faying surfaces is intact and that no corrosion is taking place.Areas subject to chafing, such as in way of chocks and bitts, anchors and hatchcomings, should also be checked and renewed as required.