ssc-367 - Ship Structure Committee

judgement errors) and facilities carrying out of a cost-effective
spectral fatigue analysis.
Some studies show that full-scale service stress data match the
predicted design stresses reasonablywell. However, it should also
be noted that full-scale service stress data may substantially
differ from those predicted during design. This may be especially
true for ships and both the short-term and the long-term service
stress data require a careful scrutiny. Evaluation of full-scale
service stress data on three different ship types (a high-speed
containership, a bulk carrier and a VLCC) by Dalzell et al
(Reference6.11) shows that short-termwave-induced bending moment
do not reasonably fit the Rayleigh distribution. The combined
dynamic stress distributionsfor two of the three ship types did not
fit the Rayleigh or the exponential distributions. Dalzell et al
recommend that additionalresponse calculationsare carried out for
different ship types utilizing Rayleigh and broad-band
distributions. Comparison of response calculations with
experimentaland/orfull-scaleresultsshould indicatethe magnitude
of error and advisabilityof corrective measures.
6.2.4 Decompose into Stress Record
To obtain a stress histogram from the response statistics, the
stress response spectrum for each wave spectrum in the scatter
diagram can be decomposed into a finite Fourierseries. In orderto
produce a realistic stress record, the number of frequencies
required will be on the order of 100. Each component will have an
amplitude defined by the differential stress energy in the
neighborhood of the frequency. Each component will be given a
random phase. By summingthe componentsateach time step, a stress
value is obtained. The stress value is then accumulated into the
stress histogram, accordingto the probabilityof occurrenceof the
particularwave spectrum. The stress histogram can then be used to
evaluate the fatigue life at the hot spot.
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