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

Structural Design of the Ship Hull Girderspectrum that makes no assumption on shape of the distribution function. The fatigue loadingspectrum can also be easily augmented with loading cycles from slam-induced hull girderwhipping.If a fatigue loading spectrum is developed, then the hull girder bending moments bywhich the spectrum are characterized can be converted to stress at various structural detailsthrough analysis of varying levels of sophistication, although the approach must assume a linearrelationship between bending moments and stress. The resulting stress spectrum is used with theS-N data for the structural detail being investigated to determine the fatigue life. The fatiguedamage computation in this case must be deterministic, assuming fixed values of parameters,such as characterization of the S-N curve. Typically, S-N data with a high probability ofexceedance, such as a lower 95 percent bound, is used to characterize the fatigue data.When different loads are acting simultaneously, they can be combined by either assumingsome phasing between them, or a “stress RAO” approach can be used. In the latter, the RAOsfor load effects such as vertical and horizontal bending are converted to stress and then used withthe wave encounter spectrum to obtain a stress spectrum.The trade-off (other than reduced computational time) between the use of a “simplified”fatigue approach based on a Weibull loading spectrum and a direct analysis using a fatiguespectrum is then between a reliability based approach and a deterministic approach to fatiguelife. As implemented in SafeHull, however, the probability of exceedance is not directlycalculated.ABS provides general guidance for performing a spectral analysis in the SafeHull LoadCriteria for Tanker Structures, Commentary on Load Criteria (ABS 1999). Response is to becomputed at 15 degree increments of heading for response in regular waves at a speed equal to75 percent of maximum ship speed. A sea spectrum appropriate for the anticipated operatingconditions for the ship is used to determine the response spectrum. In most cases, the default is20 years operation in the North Atlantic, but alternative sea conditions and service lives can beused.2.3.6 ABS Benchmarking of the Fatigue Design ProcedureThe SafeHull fatigue analysis was compared to the service experience of six differenttankers that had service experience of 5 to 19 years (ABS, 1992). The number of ships examinedwas limited because of a lack of well-documented damage history of ships in service. In general,the actual time for crack initiation of the ships in service was less than the predicted fatigue lifewhen the life was 11 years or less. However, when the predicted service life was 30 years ormore, there were no reported instances of cracking. Table 2.1 summarizes that comparison. Theratio of the actual stress range for a structural detail, f R , is compared to the computed allowablestress range, P S . A value higher than 1.0 implies a greater than 5% probability of cracking in 20years.2-18