Will the "Regular Wave Concept" Yield Meaningful Motion Predictions for Offshore Structures?

It has long been recognized that ocean waves, and therefore their induced motions and forces on structures, are random phenomena whose exact time cannot be predicted. Today, however, a spectral analysis technique is available that can be used to predict accurately important statistical properties, such as the significant and maximum values of waves, motions, and forces. For these predictions the wave energy distribution over the frequency and the motion, or force responses to regular waves over a wide range of frequencies, should be known.In design procedures for offshore structures very often the study of motions and forces is made on the basis of one single regular wave whose properties are believed to be equivalent to those of an irregular sea (regular wave concept).
To investigate the merits of the regular wave concept, the motions of a semi submersible and two supply vessels (one conventional and one twin-hull type), as predicted by the statistical methods and the regular wave concept, are compared. The comparison shows that in some cases the regular wave concept underestimates, while in other cases it overestimates, the motions that may be expected. - It may be concluded that the regular wave concept does not always lead to a conservative motion prediction. In considering the effect of the wave direction on the motions of a semi submersible, both methods give the same tendency in a Beaufort 8 North Sea condition. However, the differences between the motions in various wave directions are considerably smaller when predicted by the statistical methods.
Finally, a motion optimization study reveals that the regular wave concept yields erroneous results.

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