1 Abstract:

The measured kinematics under the crest of the highest wave in some simple two-dimensional wave groups are presented. The highest wave in each group has the same gross characteristics of height and period but each is embedded in a different wave environment. The measurement technique used, Particle Image Velocimetry, allows accurate velocity measurements to be made up to 95% of the wave height. The measured kinematics show significant differences between wave groups for very high crests, although this is not reflected well in comparisons with standard theories (Wheeler stretching and Dean's Stream function).

3 Introduction
3.1 Loading and Design

Extreme wave loading has long been recognised as producing the single most significant contribution towards the total environmental load upon a typical steel space frame offshore structure. Therefore the ability to model accurately the extreme storm conditions, and from them the peak loads and overturning moments, is of considerable importance in the design of such structures. Current practice in the design of these structures starts by determining certain design criteria. Statistical processes are applied to sources of environmental data collected from near the site or hindcast from the most appropriate and longest running records available. Design criteria covering winds, waves and currents for a (normally) 100 year return period are produced. In the case of waves, this amounts to calculating a most probable maximum wave height and its corresponding period for the given depth. The design wave is therefore assumed to be monochromatic, unidirectional, twodimensional and regular (to allow the higher order theory to be applied). The presence of the structure is ignored in calculating the kinematics. The third stage combines the current and wave kinematics and calculates the local forces by the application of a Morison's-type equation (assuming no interaction between structure and flow).

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