Variability Of Wave Spectral Shapes Associated With Hurricanes

Abstract

ABSTRACT An analysis of the variability of ocean wave spectral shapes associated with hurricanes is presented. Various mathematical representations, including the Pierson-Moskowitz, Bretschneider, JONSWAP and the six-parameter formulation, are evaluated in an attempt to develop a suitable representation of the wave spectra during all stages of the hurricanes for both deep and shallow water. Wave spectra from hurricanes Celia, Edith, Camille, and Eloise are considered in the study. These results indicate that the six-parameter formulation represents the spectra over the full frequency range for all stages of the hurricane. Mean spectral shapes of the various stages of hurricanes Camille and Eloise are generated using the six-parameter formulation. INTRODUCTION In applying the modern probabilistic approach for a rational design of offshore structures, a highly desirable objective is to estimate the safety of a particular design under the loads expected to occur during hurricanes. An accurate representation of sea wave spectra associated with sea conditions arising from hurricanes in the location in question is required in order to achieve this objective. One serious problem involved in the design practice of offshore structures is the variability of the shapes of wave spectra during the stages of growth and decay of the storm and variability of spectra with respect to geographic location. Since the responses of offshore structures may not necessarily be a maximum when the hurricane is at its maximum intensity, it is necessary to characterize the sea (wave spectra) at the location of the structure over the entire period of the storm. To date, several different approaches for generating wave information for the estimation of wave induced loads necessary for the design of offshore structures have been presented. These include a series of studies on hurricane wave models developed by Bretschneider [1], [2], [3], [4], and hindcasting hurricane models developed by Cardone [5], Goldman [6] and others. One approach to provide wave information for offshore structure design is to develop a family of spectra from analysis of measured spectra. Several wave spectra obtained from measurements during hurricanes are available. These measurements include those made during the Ocean Data Gathering Program [7], [8], [9] and those made by the national Oceanic and Atmospheric Administration Buoy Program [10]. All measured spectra are of partially developed seas because of the short duration of strong wind. The results of various analyses indicate that most of the spectra have a sharp peak in the frequency range between 0.40 and 0.70 radian per sec. However, the shapes are quite different from those for fully developed seas which have often been used in design practice of offshore structures. Even with the two parameter wave spectral representation, which covers spectra for partially-developed seas, it is not possible to represent the spectral shape satisfactorily over the entire range of the frequencies. Also, the mean JONSWAP spectrum which contains wind speed and fetch effects fails to represent the spectral shape over the entire frequency range.