A new methodology for the deterministic decomposition of multidirectional irregular surface gravity waves accurate up to second order in wave steepness has been developed. Based on measured timeseries of wave properties the short-crested ocean waves can be decomposed into a set of free-wave components of definite amplitudes, frequencies, directions of propagation, and initial phases. Using the decomposed free-wave components, accurate prediction of wave kinematics, pressure, and elevation can be made, which might be useful to the design of offshore structures, the generation of multidirectional waves in wave basins, and the comparison of different measurements from field and laboratory measurements.
One of the main reasons for studying ocean waves is to understand the interaction between waves and ocean structures, commonly, found in a variety of offshore, coastal, naval, and environment applications. For example, to predict the wave loads on slender offshore structures using the Morison equation, the wave kinematics should be known. Furthermore, wave directionality may play an important role in reducing the overdesign of ocean structures because the estimated forces on a structure can be 25 to 30% lower than when the waves are assumed to be unidirectional [Dean, 1977; Aage, 1990]. Because of the complicated, physical nature of ocean waves, unidirectional and linear wave assumptions are commonly made to simplify wave modeling. The unidirectional wave assumption is due to insufficient short-crested wave measurements, both for model and full scale, and because the long-crestedness usually leads to conservative estimation of extreme wave load, which is commonly accepted in the offshore codes [Aage, 1990]. The short-crested wave theories have not. been tested enough to be implemented in offshore practice. The linear wave assumption is used because of simplicity and statistical applicability [Borgman, 1990].
