FATIGUE ASSESSMENT
A detailed and accurate assessment of the consequences of flow induced vibrations of the freespan can be performed by predicting the amplitude of freespan vibrations over the freespan life and evaluating the subsequent fatigue damage.
The fatigue calculation is performed by first generating a probability distribution of the incident wave and current velocities over the design life of the freespan.
For waves, this is usually provided by a scatter diagram. For currents, the data are often much more difficult to obtain but can often be estimated from the statistical distributions of the tidal and storm current components.
For a given freespan, the incident reduced velocity can then be calculated for each wave and current combination in the environmental data set. The amplitude of freespan vibration can be predicted by interpolation of experimental measurements of freespan vibrations.4
The prediction of the expected amplitude is complicated by the presence of the oscillatory wave component superimposed on a steady current. This can be resolved by reference to more extensive data or (as followed in this study) more detailed modeling of freespan vibrations with steady and oscillatory flow.
The effect of wave induced oscillations is also included in the analysis. In this case, the wave induced displacement was estimated from the oscillatory hydrodynamic force on the freespan with an allowance for any resonance of the freespan.
For each wave current combination, the oscillatory stresses induced by the applied vortex displacement or wave loading can be predicted from beam column theory. The corresponding fatigue damage at that stress range is calculated from S N curve fatigue data.
If the number of cycles to failure for a particular wave-current combination (i) is given by Ni, then the total fatigue damage for that combination is shown in Equation 3.
The total fatigue damage for all wave current combinations can be calculated with Miner's rule as the sum of all particular combinations (Equation 4).
If the total fatigue damage is greater than 1, then the freespan is deemed to be unacceptable and rectification should be considered. If total fatigue damage is less than 1, the freespan does not require correction, but it would be prudent to monitor the freespan during subsequent surveys to determine if it will grow with time.
It should be noted that spans can reduce with time and the rate of fatigue damage may decline.
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