ssc - 419 supplemental commercial design guidance for fatigue ship ...

Executive SummaryThis report was prepared for the interagency Ship Structure Committee Current U.S. government shipacquisition directives emphasize the use of commercial practices wherever possible. The objective of thisproject, in compliance with those directives, was to evaluate commercial methods for analyzing the fatigueloadings on ships over their operational life. The scope of the project included documenting currentcommercial approaches and practices for the structural design of a ship hull girder for environmental loads. Asa minimum, the scope included service life, operating time and area, speed and headings, wave height andwhipping probabilities, S-N curves, allowable stress range criteria, hull girder strength, and construction and inserviceinspection requirements. It further required that the current commercial design practices for fatigue beapplied to 5 past and 5 current Navy Hulls. At the project kick-off meeting the Ship Structure CommitteeProject Technical Committee SR-1403 agreed upon the 10 U.S. Navy and Canadian Navy ships to be analyzed.Current methods of fatigue analysis of ship structure for commercial and naval ships were reviewed todevelop background for the study. This review included primary and secondary structural loads, shipoperational environments, methods of computing hull response to the loads, commercial and naval structuraldetails, and the nominal strength of the hull girder. Structural inspection requirements were reviewed.There are considerable differences between the documented methods used for fatigue analysis ofcommercial ships and of military ships. The commercial methods best documented are those of the AmericanBureau of Shipping (ABS). Specific procedures have been developed and calibrated for three types of ships:containerships, tankers, and bulk carriers. These ABS simplified fatigue analysis procedures have beenincorporated into the ABS computer program SafeHull, implementing their classification rules for these typesof ships. The ABS philosophy towards fatigue is that the fatigue strength of welded joints and details in highlystressed areas is to be based upon at least 20 years of operation of the ship. Fatigue considerations will increasescantlings above minimum rule requirements, but will not be used to reduce scantlings. Through analysis of anumber of ships, ABS developed lifetime fatigue loading spectra for the hull structure that are characterized bya Weibull distribution function [see Glossary for explanation of Weibull distribution]. These fatigue loadingspectra are used with the fatigue S-N curves [see Glossary for explanation of S-N curve.] for welded structuraldetails developed by the U.K. Department of Energy (DEN) (UK DEN, 1990), and interpreted for ship structureby ABS. Other ship classification societies have also developed their own procedures for incorporating fatigueanalysis into ship structural design.The U.S. Navy has developed fatigue analysis procedures using a fatigue loading spectrum computed forthe assumed operating conditions of each individual ship, using generalized wave response functions fromexperimental data. The fatigue strength of structural details is obtained from U.S. Navy experimental datasupplemented by data developed by the American Association of State Highway Transportation Officials(AASHTO) (AASHTO, 1996).The Canadian Navy fatigue design procedure is based on the procedures of the U.K. Navy. The procedureuses an exponential frequency distribution function of a maximum lifetime hull girder bending momentdeveloped from static balance of the ship on an 8-meter high wave. Data on fatigue strength of structural detailsis taken from a British standard that is similar to the U.K. DEN fatigue data (Maddox, 1991).The differences in the above methods for fatigue analysis are based mostly on historical development andpreferences of analysts who developed the methodologies, and not on structural or hydrodynamic differencesbetween commercial and naval ships. Therefore, a methodology developed for a commercial ship should be3