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The stress concentration factors can be generated
by the program using either the Wordsworth Smedley Equations
or the Lloyds Equations as presented in a paper by P.
Fisher and P. Smedley, 1991, or input manually.
The analysis can consider up to 16 wave approach directions
to the structure. In each wave direction the reference
wave height is stepped through the structure in equal
increments using the program WAVE to generate the member
loadings, which are input to the structural analysis
program STAAD.Pro to generate the member end forces.
The maximum number of wave positions within the wave
length is ten. The cyclic stress range is determined
from quadratic curve fitting through the 2 maxima and
the 3 minima points. Alternatively, three wave positions
at phase 0, 90 & 180 degrees apart may be used to generate
the member end force data. For this condition the fatigue
program will fit a sinusoidal function through the data
to determine the cyclic stress range for the wave height.
This method is not so accurate as the curve fitting
approach, especially for those members in the wave zone,
and members that cut the wave surface profile.
A number of waves can be passed through the structure
to obtain the stress range versus wave height relationship
for each wave direction, or the cyclic stress range
versus wave height relationship can be obtained from
a single reference wave height in each wave direction.
For the single reference wave height method it is assumed
to be that the stress range, S, is equal to the constant,
a, times the wave height, H, raised to the power constant,
b. From the stress range, Sr, associated with the reference
wave height, Hr, the constant, a, is computed by assuming
a value for the power constant, b, is of the order of
1.4 for jacket structures. This operation uniquely defines
the cyclic stress range relationship to be used for
the fatigue damage calculation. The program will evaluate
the fatigue lives at up to 16 points around the tubular
and output the minimum life of the chord, stub, and
brace.
The wave exceedance curve data is log-linear based
on a one-year return period, i.e. the wave height will
only be exceeded once in one year.
The program includes the D. En S-N curves B, C, D,
E, F, F2, G, W, and T curves and the facility to declare
a user defined S-N curve. The S-N curve is log-bi-linear,
and is defined by the S-N curve constant LOG10(A), the
allowable cycles at the intersection of the first and
second parts of the S-N curve at NO cycles, (normally
107 cycles), and slope of each part.
The fatigue damage calculation is based on Miners
cumulative damage rule. Using Miners rule, and the above
relationships for wave exceedance, S-N curve, and cyclic
stress range versus wave height, a closed form solution
is obtained for fatigue damage.
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