Fatigue Analysis The fatigue damage is calculated from the VIV response, using the SN curves defined in the model. 1. Fatigue Calculation Option : Method used to calculate the fatigue damage Default : Determined by the response frequency option given in Analysis Parameters. Concurrent response frequencies give rain-flow counting, consecutive response frequencies give Rayleigh distributed stress cycles. Rainflow counting : Fatigue calculated in the time domain using rain-flow counting. Only available for concurrent response frequencies. Rayleigh distributed stress cycles : Fatigue calculated in the frequency domain. Only available for consecutive response frequencies, and only for SN curves with a single straight line and for bilinear SN curves. Constant stress amplitude : Fatigue calculated directly from a constant stress amplitude. Not available for combined cross-flow and in-line VIV, or for concurrent response frequencies with multiple response frequencies. Num Cross Section Points : Number of points around the cross-section where the fatigue is calculated, between 1 and 16 Result Print Option : Amount of fatigue results printed Most critical point : Fatigue results are printed only for the most critical point in the cross-section All points : Fatigue results are printed for all the cross-section points Time Series Print Option : Print of tension, y- and z-curvature time series to file No print : No time series are printed Print Y- and Z-curvature : The time series of tension, Y- and Z-curvature are printed to file. Print, skip fatigue calculation : The time series of tension, Y- and Z-curvature are printed to file. The fatigue calculation is skipped Calculated scaled fatigue contributions : If ☑, the fatigue damage contributions, i.e. the calculated fatigue damage scaled by the relative duration, are written to the _vivana.mpf file Relative duration / probability : Relative duration / probability of the current condition [-]. The value must be between 0.0 and 1.0, and greater than zero for the scaled contributions to be written. The format of the time series file is described in Appendix B: Format of Time Series File. 1.1. Time series options Used for rain-flow counting. Time Series Length : Length of the stress time series to be generated for the fatigue calculation. Minimum length is 60. Time Step : Time step used in the stress time series Seed : Seed for generating random phase angles Random Generator : Algorithm for generating the pseudo-random numbers Mersenne twister Legacy If the default length and time step are used, both are calculated by VIVANA. The time step is set to 1/30 of the shortest cross-flow response period with significant response. Both random generators are expected to be acceptable for the moderate amount of random numbers needed in VIVANA. The legacy method should be used if backwards compatibility with previous versions is required. The choice of random generator applies to the generation of stress time series for fatigue calculations using the rain-flow method, and to the output of curvature time series consistent with these stress time series. If Mersenne twister is chosen, the results will not be consistent with previous VIVANA versions. If Legacy is chosen, the results will be consistent with previous VIVANA versions. 1.2. Cross section data Axial Factor : Default stress concentration factor for the axial force contribution My Factor : Default stress concentration factor for bending about the local y-axis Mz Factor : Default stress concentration factor for bending about the local z-axis Cross Section Area : Optional cross-section area. If zero, the value derived from the cross section properties is used. Section Modulus : Optional section modulus. If zero, the value derived from the cross section properties is used. Wall Thickness : Optional wall thickness. If zero, the value derived from the cross section properties is used. The stress concentration factors are included in the reported stress distributions. 1.3. SN Curves The SN curves used in the fatigue calculation are selected from the SN curves defined in the model. Include All SN Curves : If ☑, all SN curves defined in the model are used. If ☐, the SN curves to be used are specified in the table. Sn Curve : Reference to an SN curve defined in the model To specify SN-curve, right click on the model and choose New→SN curve. The SN curve specification is described in SN Curve. The reference thickness and the thickness correction exponent must either both be zero, no thickness correction, or both be positive, thickness correction included. 1.4. Cross section references This input is used to specify lines, segments and elements with stress concentration factors (SCFs) that differ from the default values given in Cross section data. Line : Line identifier All Segments : If ☑, all segments in the specified line are used. If selected, all elements must also be used. Segment : Local segment number in the specified line All Elements : If ☑, all elements in the specified segment are used Element Number : Local element number in the specified segment Element End : End with the smallest or the largest node number Scfa : Stress concentration factor for the axial force contribution Scfy : Stress concentration factor for bending about the local y-axis Scfz : Stress concentration factor for bending about the local z-axis The cross section references affect the extent of the fatigue calculation as follows: Rainflow counting : The fatigue calculation is limited to the specified cross section references. If no cross sections are specified, the fatigue is calculated for all elements. Rayleigh distributed stress cycles : The fatigue is calculated for all elements Constant stress amplitude : The fatigue is calculated for all elements If SCFs are given several times for one element, the last one given is used. For details, see VIV fatigue analysis - input reference. Hydrodynamic Properties Capacity checks