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

-42-(d)(e)Poor quality welds due to moisture inclusion or porosity,often resulting from wind disturbing the shielding envelopeof gas around the weld arc.Improper weld sequence resulting in high residual stress.MAINTENANCEGeneral maintenance of aluminum hulls has reportedly been excellent.h many cases, the aluminum above the water line is unpainted, and onlyrequires an occasional washdown with fresh water. The tendency of unpaintedaluminum to streak and spot often leads to painting for aesthetic reasons,which can lead to problems if the coating breaks dowm locally, therebytending to concentrate any corrosive or electrolytic attack. Hulls aregenerally painted with antifouling paint below the water line. Primerand tributyl tin oxide AF paint or other paints not containing cuprousoxide are generally used. These paint systems have stood up well, andwhen repainting is required, a careful sand washing is employed to removeold paint and barnicles.The operational experience with existing aluminum vessels and deckhouseshas generally been satisfactory. All problems encountered in design~construction and operation to date have been or can he satisfactorilysolved, and the practical experience gained cam be applied to large aluminumhulls such as the bulk carrier under consideration. Undoubtedly, the mostserious challenge to be faced in designing large aluminum ships is to avoidconditions which might lead to stress concentrations and subsequent cracking,since larger hulls will be more highly stressed than those now in operation.IIC.DESIGN CRITERIA FOR HUL~ STRUCTUREThe development of acceptable design criteria for the hull girder andlocal structure of an aluminum bulk carrier represents one of the mostchallenging and important considerations in this study. These criteria arefundamental in developing a technically.feasibledesign, and require athorough evaluation of the empirical and theoretical considerations leadingto the steel scantlings presently required by regulatory bodies.Design criteri’ahave been developed for the primary hull girder structureand secondary structure to the extent necessary to fully demonstrate technicalfeasibility, including the following:1. Hull girder section modulus at midships.2. Hull girder moment of inertia at midships.3* Prhmry hull structure: deck, tank top, shell plating andframing, longitutial floors and girders, center vertical, keel and web frames.h. Secondary hull structure: bulkhead plating and framing,deep tank structure, deckhouses, etc.Additional consideration is given to crack arresting and thermal stresses.