Computation of Stress Spectra |
 
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Computation of Stress Spectra |
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In the RAOs mode of operation, the first step in the fatigue analysis is to extract the required hot spot RAO or RAOs from the Flexcom output (for each combination of seastate and direction) and to transform and/or combine these as required. The appropriate RAO file will depend on what seastate is being analysed and what block this corresponds to in the scatter diagram.
If you specify that combined stresses are to be considered at a particular location, then LifeFrequency scans the Flexcom RAO output for axial force RAOs and RAOs of bending about both local axes. The axial force RAOs are transformed to axial stress and the two bending RAOs are transformed to bending stress. The three RAOs are then combined to produce combined stress RAOs. The combining of the RAOs uses complex arithmetic to include the effect of relative phasing between the stress components.
Note that LifeFrequency produces fatigue life estimates for eight points (denoted ‘stress points’) around the section circumference, and so bending stress RAOs are factored as appropriate depending on the actual ‘stress point’ under consideration. Each one of the eight points corresponds to an integer multiple of 45°, measured in degrees anti-clockwise from the local element cross-section y-axis. This methodology is consistent with the Location Parameter Input which is accepted convention in Flexcom post-processing.
Once LifeFrequency has read and evaluated the RAO data it requires, the next step in the fatigue analysis is to use this data to produce a hot spot stress spectrum according to the formula:
where 
is the output stress spectrum, 
is the stress RAO or transfer function, 
is the particular seastate spectrum, 
denotes frequency, 
and denotes the magnitude of the complex quantity 
. Note that the seastate spectrum 
may be either Pierson-Moskowitz or Jonswap; you specify which when inputting the seastate scatter data.
The availability of this option requires that LifeFrequency incorporates an algorithm to select values for the three parameters usually used to define a Jonswap spectrum (peak frequency, γ and α) for each seastate, since these are defined only in terms of Hs and Tz or Tp. The procedure is not described in detail here – instead the interested reader is referred to Jonswap Wave.
•*FATIGUE DATA is used to assign fatigue data to hot spot sets, including an option to nominate the type of stress to be used in the fatigue calculations. You can choose between bending stresses only, axial stresses only or combined bending and axial stresses, which is the default.