ssc-367 - Ship Structure Committee

energy about the central direction by using a ‘spreading function”
is to represent the nature more realistically. Considering the
uncertainties and complexity of implementation,wave spreading is
not generally incorporated into design. While it is a valid
parameter that can be used to more accuratelydetermine the fatigue
lives ofan as-designedor as-builtstructure (seeSection 6.1.4 for
definition of spreading function), it is often unconservative to
neglect it when dynamics are significant.
It is also necessary to assess the significance of short-term
density functionsdevelopedfrom statisticalparameters. The joint
probability of significant wave height and characteristic period
(i.e., each sea state) is used to develop short-term probability
density function of the stress range. This function is often
idealized by a Rayleigh distribution and can be further improved.
This improvement,incorporationof a rainflow correction factor, is
discussed by Wirsching (Reference 4.9). Fatigue damage is then
typically computed for each sea state by using the S-N curve and the
Miner-Palmgren cumulative damage formulation. An alternative to
this approach is based on weighting and sunrningthe probability
density functions to obtain a long-term probability density
function. Total damage can then be computed based on either
numerical integrationor the use of Weibull shape parameter and a
closed form solution. Chen (Reference 4.10) offers a short-term
closed form method that facilitates spectral fatigue analysis.
Spectral fatigue analysis is discussed further in Sections 5, 6 and
7.
Mobile and Stationary Vessels
Both conventional single-hull and twin-hull mobile and stationary
vessels differ from fixed structures in the characteristics of
applied environmental forces and the response of the structure to
these forces. Thus, fatigue analysisof these vessels differs from
that of fixed structures primarily in generation of applied forces
and determination of stresses. Those vessels going from port-to-
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