A Program to Develop Coastal Ocean Dynamics Applications Radar for Offshore Wave, Current, and Wind Monitoring
Authors: Spillane M.W. , Oakley O.H. , Evans M.W. , Lipa B.J. , Barrick D.E. 
Published: October 2008

A project initiated by Gulf Oil to adapt HF radar (called CODAR) for use from offshore platforms is approximately half completed. This compact remote-sensing system measures the wave height directional spectrum, surface-current vectors, and surface-wind vectors around the platform, to a distance of about 50km. Final data products will be available on the platform in real time, for display and archival. The system can be operated from either fixed bases (platforms, gravel islands) or movable bases (semi-submersibles, drill ships). Coastally measured wave and current data from this system at ARSLOE are compared with other information to establish the feasibility and accuracy of the technique.

INTRODUCTION

Nearly three decades ago Crombie (1) observed experimentally that the temporal spectrum of sea echo at HF conveys very accurate velocity information about wave spectral components that are Braggresonant in length with the radar wave. Analyses since that time have established the relationship between higher-order sea echo and the wave height directional spectrum (2,3). Additional Doppler shifts of the echo from its expected positions predicted by the dispersion relation carry information about underlying currents (4). Several definitive experiments (5,6) showed that these mathematical solutions that form the basis for the inversion methods are indeed valid and accurate. Hence, considerable activity ensued in development of ocean-surface remote sensing systems based on these principles.

HF radars are characterized by their long wavelength (in contrast with more familiar microwave radars), and hence the required large antenna sizes if conventional beam forming methods are employed. Narrow-beam antenna arrays vary in size between several hundred meters to 2.5km (6). Because of the problems posed by large antennas for most wave and current measurement applications, NOAA (National Oceanic Atmospheric Administration) began a program to develop and demonstrate compact, HF coastal radars that could make these measurements in near-real time. Such compact HF radar systems became known as CODAR, for Coastal Ocean Dynamics Applications Radar. Based on novel antenna concept. and the known physics of sea scatter, several experiments proved that such systems were feasible and accurate (7,8). The present program, described herein, adapts this technology to operational use from offshore platforms, both fixed and semi-submersible.

THE NEED

Measurement of ocean surface parameters around offshore platforms is of importance both for the design of and safe operation from these structures. This need is exacerbated as operations extend into ever uncharted domains, as for example deeper water, areas of higher waves (such as the North Sea), and the seasonally frozen seas of the Arctic. Operations from semi-submersibles and gravel islands are obviously subject to wave, current, and meteorological constraints. Quantitative data are required, rather unreliable forecasts and "eyeball" mates.