ssc-367 - Ship Structure Committee

6. FATIGUE STRESS HISTORY MODELS
Creation of the fatigue stress historymodel requires determination
of the fatigueenvironmentand applicationof the environmentto the
structure to produce stresses. The environment can be applied to
the structure by either a spectral analysis or by a time-domain
analysis. The spectral analysis derives the stress range and an
average N number of cycles from the statistical properties of the
stress response spectrum. A true time-domain analysis sorts the
stress ranges and accumulatesthe stress range counts as the stress
time history is being generated. For practical reasons a hybrid
time-domain method is often used to generate stress history.
6.1
DETERMINATIONOF FATIGUE ENVIRONMENTS
To evaluate the fatigue life of a fixed structure or a floating
vessel a representativefatigue environmentmust be modeled. For a
fixed structurethe fatigueenvironmentwill be the typicalwave and
wind conditions for the surrounding area. For a ship the fatigue
environment will be the typical environmental conditions along
various routes.
6.1.1 Data Sources
The types of environmentaldata range from actual wave and/or wind
records to recreated (hindcast)data. The wave and wind recordsmay
be raw recordings (not generally available) or condensed summary
reports produced by government agencies or environmental
consultants. Hindcastdata are generatedby various computermodels
using environmental information available for the area or nearby
areas.
6-1