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

-86-REPAIRSObtaining proper repairs to hull damage, or minor structural modificationsto an aluminum ship will be more difficult than with a steel ships since thenumber of repair yards with qualified aluminum welders is relatively limitedyas is availability of required materials in the alloys, tempers and thichessesrequired. The lack of qualified welders is a particularly important factor,since the use of improperly trained welders can lead to significant problems.Downtime while awaiting arrival of necessa~ materials and skilled personnelto effect repairs could be a significant economic factor, though the effectsof this factor will diminish as aluminum gains wider acceptance.SPECIAL SURVEYSAt this time, the Regulatory Bodies have no special policy relative toadditional surveys for aluminum hulled vessels. However, based upon thelarge size of the aluminum bulk carrier being considered, as well as theproblems with cracking of aluminum ship structures in the past, it wouldappear advisable to schedule additional structural surveys, at least forthe prototype vessels. In order to be effective, these surveys shouldinclude close examination of internal structures, particularly in way ofwelded connections. Since this would entail gas freeing tanks and cleaningof all surfaces, it would be advisable to spot check in a limited numberof tanks, and check others only if problems are uncovered. Additional itemsto be checked would include those noted in the previous discussion of hullmaintenance, as well as a careful examination of shell and deck plates forsigns of cracking or corrosion. It would be desirable to periodically re-Xray selected plate seams and butts in critical locations to ensure thatinternal fatigue cracks are not developing. It wouldalsobe desirableto remove selected pieces of equipment from their foundations to check thecondition of the interface, the insulating material and the bolts.HULL INSURANCEThe cost of hull insurance for an aluminum bulk carrier will undoubtedlybe higher than that of an equivalent steel hull, due to its higher replacementand repair cost and the greater risk of loss by fire. ‘Therelativeincrease is difficult to predict, since it is dependent upon the degree offire protection provided, types of cargo to be carried, risk of fire asaffected by type of machinexy and equipment installed and other factors.III.COMPARATIVE SHIP DESIGN AND EVALUATIONIn this Phase of the study, equivalent hypothetical aluminum and steelbulk carriers-are developed wh~;h are essentially identical to the M.V.CHALLENGER. This includes the following tasks:oSelection of principal dimensions.ooDesign ofDesign ofmidship section.typical bulkhead.ooLight Ship Weight Estimate.Stability and Trim.