Measuring the Effects of Hurricanes on Major Ocean Currents and Implications for Ocean Energy Devices

Abstract

Harnessing energy from ocean currents has several advantages to tidal flows, one feature of particular importance being a unidirectional and fairly consistent water velocity. This feature simplifies installation, maintenance, and placement of multiple devices in a relatively small geographical area. The generators need only work in one direction, improving the rotor efficiencies and simplifying the structure, which in tidal applications must be able to change rotational direction or rotate during daily flow reversals. When a design depends upon this flow characteristic, however, it is very important that the flow does not reverse at any time, or at least reverse at a velocity which could cause the mooring or device to malfunction. The Southeastern National Marine Renewable Center (SNMREC), based at Florida Atlantic University in Boca Raton, Florida, is tasked with the development of an offshore test facility to support marine renewable energy developers interested in ocean currents. A data collection program to assess the resource, the Florida Current, has been conducted since 2008, using multiple Acoustic Doppler Current Profilers (ADCPs) deployed at strategic locations off the coast of southeastern Florida. A pair of CODAR surface current measurement radars was added in early 2012, and provides continuous measurement of the waters directly above the bottom mounted ADCP units as well as onshore meteorological measurements. Three dimensional current flow, especially from more than one ADCP, is often difficult to visualize using conventional static two-dimensional time series plots, and is also challenging to describe to a non-scientific audience. MATLAB code was developed to display a time series of ADCP data as depth-referenced velocity vectors, which were then animated through time to provide a clear visualization of temporal flow changes. The resulting animations provide an innovative method of conveying the concept of water flow in the oceans, specifically the Florida Current, as well as the significant variability over various time scales and affects of storms and other forcing events. When multiple time-synchronized ADCP data sets are displayed for a specific area, the interactions and similarities within the current are striking. During the passage of Hurricane Sandy in 2012, although the storm's closest point of approach to the Florida coast was over 300 nautical miles, ADCP data using this visualization tool indicated a significant change in the Florida Current's flow speed and direction within the upper 150 meters of the water column. Subsequent measurements during Hurricane Irma in 2017 using both CODAR and an ADCP deployed two days before its approach verified this phenomenon and provided more detailed information. This paper describes the equipment and deployment methods, collected data, MATLAB code, and preliminary data analysis of the flow changes and the implications of ocean current energy harnessing systems installed in regions where hurricanes are common.