ssc-367 - Ship Structure Committee

d) Stress SDectrum
Hot-spot stresses combined with the long-term effects of the
environment allow development of the stress spectrum.
Randomnessof ocean environmentmakes both the short and longterm
prediction of sea states quite difficult. The
uncertainties of nature that influence the life-time stress
history of a stationary structure are:
- Use of full scatter diagram ofHs andT
- Variations ofT
- Percentage of occurrence estimates
- Wave directionality
- Interactionof wave and current
For some site-specific stationary structures, a good existing
databasemay allow comprehensivehindcastingstudies to predict both
short- and long-term environment with a reasonable certainty. A
reliability-based full probabilistic fatigue analysis al1ows
selection of the degree of reliability that affects the fatigue
life, such as the environmental loading, size and distribution of
defects, fatigue strength, etc. However, even commonly used
spectralfatigueanalyses,which isdeterministic, (i.e. application
of only probabilisticenvironmentalconditions),the desirablelevel
of uncertainty for the environment can be chosen to be compatible
with the other factors that affect the computed fatigue life.
For oceangoing ships which move through various site-specific
environmentsin a singleroute, the stress history is very difficult
to define. A full probabilisticreliabilityanalysis, orthe useof
conservativeupper bound conditions,is necessary to account for the
many different routes over the the uncertainties regarding the use
of very different routes over the life of the vessel as well as
route changes due to extreme environmental conditions.
9-18