Page 121 - Marine Structural Design

Chapter 5 Ship Hull Scantling Design by Analysis 97

Fatigue Check
Only cyclic loads are relevant in the fatigue analysis. Static loads should therefore be
subtracted from the total design loads. Environmental loads for fatigue analysis may differ
from those for the yielding and buckling analysis. Either a stochastic or a simplified fatigue
analysis can be performed. When a simplified fatigue analysis is applied, the stress range
corresponding to a return period equal to the design life is calculated and the fatigue life is
then computed based on S-N curves. If the stochastic fatigue analysis is used, stress ranges
corresponding to each sea state in the wave scatter diagram are calculated by FE analysis, and
the fatigue life of each stress range is found using S-N curves. The cumulative fatigue damage
is then computed using the Miner-Palmgren Hypothesis:

where,
n, = Number of cycles in the ith stress range interval between stress range doiand
ACi+l
Ni = Number of cycles to failure at stress range (Ani + Aoi+,)/2 and can be read from
S-N curves
D = Allowable cumulative damage varies for different structural members, which
should normally be less than 1.
However a significant safety factor is usually employed with the Miner-Palmgren Hypothesis,
and D is often less than 0.1, 0.3 or 0.6 depending on the type of structure, the strength
significance of the member, availability for inspection, etc. Reference is made to Part 111 of
this book for more information on fatigue assessment.

5.5 References

1. ABS (2002), “Rules for Building and Classing Steel Vessels”, American Bureau of
Shipping.
2. Liu, D, Spencer, J, Itoh, T, Kawachi, S and Shigematsu, K (1992), “Dynamic Load
Approach in Tanker Design”, SNAME Transactions, Vol. 100.
3. Lloyd’s Register (1 996), “Structural Design Assessment Procedure, Direct Calculation-
Guidance Notes”.
4. Ziliotto, F. et al. (1991), ”Comparison of Different Finite Element Analysis of
Transverse Frame of a 350,000 TDW Tanker”, Marine Structures, Vol. 4, No. 3.
5. Zhao, CT, Bai, Y and Shin Y. (2001), “Extreme Response and Fatigue Damages for
FPSO Structural Analysis”, Proc. of ISOPE’2001