Page 376 - Marine Structural Design

352 Part Ill Fatigue and Fracture

where C is a calibrated constant and g is a calibrated exponent. The long-term wave height
distribution is represented by the sum of two Weibull distribution: one for normal condition Ho,
the other for hurricane condition HI: :

ACT = CHOg for normal condition (1 8.15)
ACT = CH,g for hurricane condition (1 8.16)
Based on the methodology described in Chapter 19, the cumulative fatigue damage may be
easily derived for normal condition and hurricane condition respectively. The formulae for the
cumulative fatigue damage based on the deterministic method may be found form the
commentary on fatigue in API RP 2A - WSD.

-
...........
fi -------'---
& ..................................... .^ .......
HI ......................................................................................................

I Log N

Figure 18.1 Stress Distribution Illustration

18.3.4 Long Term Stress Distribution - Spectral Approach
A spectral approach requires a more comprehensive description of the environmental data and
loads, and a more detailed knowledge of these phenomena. Using the spectral approach, the
dynamic effects and irregularity of the waves may be more properly accounted for.
This approach involves the following steps:
Selection of major wave directions. The same considerations as discussed for the
deterministic approach apply,
For each wave direction, select a number of sea states and the associated duration, which
adequately describe the long-term distribution of the wave,
For each sea state, calculate the short-term distribution of stress ranges using a spectral
method,