An evaluation is given of hindcast spectra obtained from the Spectral Ocean Wave Model (SOWM) being used by the U.S. Navy at Fleet Numerical Weather Central (FNWC) as a source of wave data for the design of floating structures. One year of hindcast spectra at a location close to Station "India' (59°N, 19°W) in the North Atlantic is compared with a 12-year sample of measurements at that location. The data are evaluated for trends, general characteristics, spectral properties and their effect on the resulting responses of typical vessels. Effects on directionality, including the dominant and secondary direction, are discussed. Results obtained for the one location for one-year hindcast are extremely encouraging and point to the immense potential of this data source. Correlation between measured and hindcast parameters, spectral families and resulting responses are shown to indicate excellent agreement.
The dynamics of many types of marine structures are determined to a large extent by their responses to the environment in which they operate. Wind, waves, current and ice are four of the major environmental factors which individually and interactively con tribute to the excitation forces and hence the responses imposed on the system. The definition of the excitation function is therefore of critical importance and a prerequisite for a prediction of the behavior of the structure in a realistic environment. Each of the above four categories is of a complex nature and consists internally of several physical phenomena. The waves, however, seem to be the major influence on the behavior of marine vehicles. Ever since the probabilistic approach has been developed by St. Denis and Pierson (1)*, the complex problem has been conveniently separated into two components, i.e., the waves and the transfer function. While the latter has received rather extensive treatment over the past 20 years, the wave description has been left to the oceanographers studying basic principles such as generation of waves, the energy balance in the waves, growth of waves with wind, etc. Understanding of the mechanism of wave generation has led to formulating the shape of idealized wave spectra rather than spectra obtained from actual measurements at various ocean locations. Thus, partially developed seas or cross seas, as created by local wind sea superimposed on swell or several swells, are not represented by such formulations adequately, yet these conditions are very common.
The availability of wave data in an adequate format and in a sufficiently large quantity has become a major function in the design of marine structures.
Of the three major sources of wave data, observational, measured, and forecast/hindcast, the observational data is limited in its applicability in most cases, while the reliably measured data is extremely scarce and costly to obtain. Hindcast wave data if generated by an acceptable and proven model and based on a realistic wind field input, could provide an irreplaceable source of data for design purposes as well as operation.
