ssc-367 - Ship Structure Committee

9.
FATIGUE AVOIDANCE STRATEGY
Most marine structures are designed and analyzed to resist extreme
loadings. Some structures,includingoffshore structures and ships
with special features,are also checked for fatigue. This approach
may be valid for structures in environments not susceptible to
fatigue loadings. A good overall design of marine structures
susceptible to fatigue loading (largeships and tankers, stationary
fixed and floatingstructures,etc.) can be achievedwhen fatigue is
given an equal emphasis to stability, strength and other
considerations during design, long before steel is ordered.
Fatigue design should be both an integral part of an overall design
effort and a part of a strategy covering the entire design life of
the structure. Thus the design, fabrication, inspection and
operationalmaintenanceshould be treated as interactiveparameters
that affect fatigue avoidance strategy.
While most offshore structuressusceptibleto fatigue were properly
analyzed and designed to prevent fatigue failures, ship-shaped
vessels were seldom analyzed and designed for fatigue. The use of
high strength steel in recently constructed vessels proved that an
indirect fatigue design (i.e. member sizing, detailing) is not
sufficient to prevent fatigue failures. As a result, large number
of vessels constructed by reputable firms now incorporate detailed
finite element analysis and design to prevent fatigue failures.
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REVIEW OF FACTORS CONTRIBUTINGTO FAILURE
Mobile vessels and stationary structures differ not only in their
general configuration but also in the nature of applied
environmental loading. A stationary structure’s site-specific
environment usually determines the stress ranges and the number of
stress cycles, and is a major variable affecting fatigue life. The
next most importantvariables are the parameters affecting design
and fabrication quality. While maintenance may not be important
early in design life, it assumes a major role as the structure
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