The aim of this study is to try to establish new criteria for the design of offshore structures against extreme forces caused by breaking waves. The objective is to study shock forces and drag and inertia wave forces by means of several series of laboratory experiments and extensive field measurements. Ina series of laboratory experiments dispersion properties of water waves are used to generate a non-steady situation where one wave train overtakes another resulting in-the generation of an extreme wave. Collisions between wave solutions are studied in detail qualitatively and quantitatively, and results are obtained for characteristic wave properties: form, attenuation, dispersion, initiation and type of breaking, total pressure head and shock pressures. Results are also included for cases where currents are superimposed upon the mechanism for the generation of extreme waves.
This deterministic approach in the laboratory will later be followed by extensive field measurements on a concrete gravity platform in the North Sea. Shock pressures near mean water level together with spectral and directional wave properties will be recorded in the field.
The laboratory experiments showed that three distinct types of breaking waves could be generated by wave-wave interaction in deep water, namely plunging breakers, deep water bores and spilling breakers. Field data on extreme waves obtained from wave rider buoys was analysed with the zero downcross method which provided a wave height parameter relevant to the evaluation both of shock pressures and of the operation and stability of smaller vessels.
Crest front steepness of the waves was used in the analysis as the total wave steepness s = H/L is not sufficient for asymmetric waves. The magnitude of shock pressures depends on the type and the wave form of the breaking waves.
One of the main subjects of importance in marine technology is wave action. Wave action does not only consist of a very regular symmetric and steady design wave passing a fixed or floating structure. For an offshore engineer wave action should also include impacts against structures from very asymmetric non-steady extreme waves and breaking waves.
The generation of extreme waves and breaking waves in deep water is therefore of practical importance and research efforts should be directed into this field for three reasons.
The breaking of waves is the principal loss mechanism regulating the growth of the sea state toward spectral equilibrium under steady winds. (PHILLIPS 1966)
Breaking waves are the most efficient mechanism for transfer of momentum from wind to mean surface flow in the form of ocean currents. (LONGUET-HIGGINS 1969)
In advanced sea states breaking waves exert by far the largest individual loads on floating and fixed structures with all, the following implications both for design and operation.
