The Storm Current Response of Gulf of Mexico Hurricanes

Abstract

ABSTRACT A four-layered 3-D numerical model (Gordon, 1991) was used to hindcast time histories of the storm current response of nineteen Gulf of Mexico (GOM) hurricanes. Current speed and direction and mixed layer depths were derived over the GOM. Joint occurrence statistics for winds, waves, and currents were generated from these data by linear regression for water depths greater than 150 m. The results provide a basis for wind, wave and current joint event design criteria. 1.0 INTRODUCTION The traditional analysis of forces on fixed offshore structures focused on wave loads. As industry's analytic techniques have matured and the design emphasis shifted to newer generation deep water structures, the forces imposed by winds and currents have become of increasing concern. This concern, combined with the recognition that peak winds, waves, and currents are not necessarily coincident, has led to the development of next generation GOM design criteria which incorporate the joint occurrence of winds, waves, and currents. This requirement in turn mandated the development of a robust storm current model capable of predicting the magnitudes and directions of hurricane storm currents. To this end, a 3-D, layered numerical current model (SCORS; Gordon, 1991) which incorporates both density stratification and bottom interaction was developed. Although techniques for hindcasting storm winds and waves are well established, storm current hindcasting techniques are less well developed. For inner shelf depths, reliable single layer models exist (Bunpapong et al., 1985). For deeper waters (off the continental shelf) reliable predictions can be made using multilayer models which do not account for topographic interaction (Price, 1981). The SCORS model combines the capabilities of both model types into one analytic tool. For traditional platform design, the important current related parameters are both the uppermost layer (mixed layer) current speed in-line with the wave direction, and the mixed layer depth at the time of peak wave response. To fully understand the storm current driving mechanism, it is important to examine the spatial and temporal variations in the current response to the passage of the storm. The mixed layer storm response is known to be asymmetric with higher currents to the east of the storm track than to the west of the track (northern hemisphere). The opposite holds in the southern hemisphere. This results from a near resonant coupling between the wind stress and inertial period on the east side of the track (Price, 1981). This study involved simulations of the storm current response of 19 historical Gulf of Mexico (GOM) hurricanes, ranging in severity from Class 1 to Class 5 (Figure 1). Although the simulations covered all water depths, only the results of analysis of the data for a series of grid locations in water depths greater than 150 m are presented. These data are then combined with wave hindcast data derived from the same wind fields to develop empirical relationships for the joint occurrence of winds, waves, and currents.