Special Cases |
 
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Special Cases |
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There are two special cases of the above general procedure for dividing the scatter diagram into blocks and nominating a reference seastate within each block.
(i)In the case of the RAOs mode, if you have only a single block (encompassing the full scatter diagram) with a single reference seastate, you perform one random sea analysis only for each wave direction with non-zero percentage occurrence. The RAOs from these analyses are used for all of the seastates in the scatter diagram, but otherwise the fatigue analysis proceeds exactly as per the Equations found in Computation of Stress Spectra, Stress Ranges and Fatigue Damage.
(ii)You want Flexcom to do a random sea analysis for every seastate in the scatter diagram, for every wave direction with non-zero percentage occurrence. In the case of the RAOs mode, this is equivalent to making every cell of the scatter diagram into both a seastate block and the reference seastate for that block. The fatigue life calculations are slightly different in this case to the general procedure outlined above – but only slightly. The result of doing a random sea analysis for every seastate for every wave direction is that you have immediately the axial force and bending moment spectra that are required to complete the fatigue analysis. There is no requirement in this case to postprocess the random sea results to produce response RAOs. So in effect the fatigue calculations begin at Stress Ranges (Eq.1), the calculation of the moments of the combined stress spectrum. Otherwise the procedure is exactly the same as in the general case. Note that the use of the RAOs mode in this way is essentially the same as running LifeFrequency in the Stress Spectra mode.
•*BLOCK is used to input the fatigue analysis wave scatter diagram, including the definition of blocks and/or reference seastates as appropriate.