Fatigue analysis in the frequency domain

[Dimis414]

Assuming that a TLP platform is examined and the wave forces acting on the tendons of the mooring lines are presented in RAOs (as a function of wave frequency). Using Jonswap spectra for North sea (i.e peakness factor equal to 3.3) we can obtain Hs (significant wave height); Tp (peak period) and the acting forces on the tendons for the specific location. These forces are instant forces from waves but not cyclic loading. The question is how to calculate fatigue in the tendons from these RAOs values, and how to produce S-N curves from RAOs values.

 

[JBA]

If this is a TLP (Tension Leg Platform) retained by tendons there are two planes of wave loading, vertical due to buoyancy and horizontal due to fluid drag on the submerged portion of the floating structure and its anchoring tendons will always be in tension due to the structure buoyancy.
Only in the case of breaking waves would any instant horizontal forces as opposed to cyclical be expected; and, the vertical component due to structure buoyancy would be expected to be cyclical, except for the case where the wave crest would suddenly encounter the bottom of some significant horizontal face on the structure.

Please clarify your "instant forces but not cyclical" statement.