Design waves are widely used to assess stress in vessels. Compared to a full spectral analysis, this method reduces the number of required structural calculations and is expected to provide similar results. Moreover in a rule context, it allows loading a structural model with a simplified hydrodynamic loading, via explicit formulae for extreme hydrodynamic loads and associated load combination factors (LCF).

In a broad sense, an equivalent design wave (EDW) is a wave on which a selected response is equal to a target value (generally the spectral extreme response). If general practice is to define the EDW as a regular wave, it is possible to define it more realistically; there are also different ways to define heading and period of the EDW which can lead to significantly different results. In this paper the EDW are not restricted to regular waves: response conditioned waves (RCW) and directional response conditioned waves (DRCW) are also investigated. Those waves include more physics and are shown to overcome some of the inconsistencies of the regular wave (Load Combination Factor above 1 for instance).

The different methods are then applied to an FSRU; hydro-structure calculations are carried out, the resulting stress is calculated in more than 2000 relevant elements. The way to select a few design waves among a lot of individual loads from the hydro-dynamic calculation (motion, acceleration, internal loads, pressure) is discussed and finally, the stress computed by the different design waves is compared to the direct spectral calculation.

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