How do I do a spectral fatigue analysis of an offshore frame structure using Sesam?


Spectral fatigue analysis

In a spectral fatigue analysis wave loads are computed for deterministic waves of different periods and heights and from different directions. Any wave theory may be used, i.e. Airy, Stokes 5th, Stream Function, etc. An analysis of the structure subjected to these waves is then performed. This may be a static or a dynamic analysis. In case of a dynamic structural analysis the waves must be repeated for several cycles so as to get passed the transient state (with vibrations caused by the sudden onset of wave forces) and reach the steady state. As a preparatory step for the fatigue analysis transfer functions for the beam forces are created being the force range (maximum minus minimum) divided by the corresponding wave heights. This gives, for each wave direction, the beam forces as functions of the wave frequency (inverse of wave period).

The final step is the fatigue analysis which is performed using the spectral (also termed stochastic) method. I.e. data like wave spectra and wave scatter diagrams combined with wave direction probabilities determine the number of occurrences of all waves (frequency and height) from all directions. The fatigue analysis is based on the Miner’s rule.

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