ssc-367 - Ship Structure Committee

thickness effect is necessary and the ship structure plate
thicknesses are not excessive, the correction factor may be
neglected.
The S-N curves reconanendedbyAPI for offshore platforms may be used
in the computation of tubular component fatigue damage. The API X-
curve and the DEn T-curve (identicalto DnV T-curve up to 10 million
cycles for catholically protected areas - see Section 4.2.2)
intersect at about 500,000 cycles and would yield similar lives for
a plate thicknessof 1-1/4 inch (32nmI). Most tubular chord and stub
thicknesses are likely to be greater than 1-1/4 inches and the
applicationof correctedDEn or DnV T-curves to compute fatigue lives
will result in shorter lives and considered to be appropriate.
Consideringthe effectsof plate thickness,weld profile and undercut
on fatigue strength and the S-N curves it may be prudent to reassess
the hot spot stress range concept. Tolloczko et al (Reference 7.8)
recommend modifying the definition of hot spot stress range to
reflect weld toe defects. Then, the S-N curves will reflect only the
size effects.
7.5.2 Fatiaue DamacteComputation
Fatigue lives determined based on S-N curves and Miner’s cumulative
damage rule are uniformlyacceptableto certifyingand classification
agencies. The national and internationalstandards allow the use of
simple cumulative damage rule for the computation of damage. Large
number of test results as well as the in-serviceperformance records
of marine structures indicate adequacy of this approach.
Alternative rules to compute fatigue damage and methods to upgrade
Miner’s rule have been proposed. Although necessary to evaluate
possible benefitsof such alternatives,additionalcomplexity and the
cost should also be considered. Since the S-N curves are developed
based on constant amplitude stress ranges, the effect of variable
7-21