ssc-367 - Ship Structure Committee

6.3.1 Stress Statistics
The resulting stress records are then processed to find the stress
statistics. The significantstress can redetermined as four times
the standarddeviation of the stress values. Stress histogramscan
also be derived from the records.
6.3.2 70 Percentile Spectra
Time-domain analyses tend to be computation intensive, and they
often require costly computer runs. Therefore, the number and
extent of time-domain analyses must be kept within reason by
selecting one or a few representative sea spectra for evaluation.
Selecting the representativesea spectrum and the regular waves to
model it will have an effect upon the resulting fatigue life.
Because the fatigue damage is an accumulation over many years of
exposure to mostly mundane sea conditions, the selected
(representative)sea state must be an average or mean condition,
with a slight hedge toward conservatism. A recommendedselectionis
a spectrumalong 70 percentilewave height line, i.e. from a cell in
the scatter diagram below which lie 70% of the scatter diagram
probabilities. Thezero-uncrossing period would be near the median
on the 70 percentile line with a slight offset to the side that is
expected to produce the greater stresses.
6.4 OVERVIEW AND RECOMMENDATIONS
The long term wave environment, as defined by a wave scatter
diagram, is usually based on measurements and hindcasting.
Measurements should be reviewed as to the extent of area covered,
the time lengthof coverage,and the measurementsystem. Typically,
measurements are made for limited time spans. Accelerometers of a
measurement system may have limitations, preventing accurate
description of wave energy content in all frequency ranges and in
all directions.
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