Advance Warning of Loop Current from Single-Site SeaSonde on Genesis Oil Platform in Gulf of Mexico Donald Barrick and Cort Cooper codar ocean Sensors, Ltd. Energy Technology Company, ChevronTexaco Mountain View, ca 94043 San Ramon



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Advance Warning of Loop Current from Single-Site SeaSonde on Genesis Oil Platform in Gulf of Mexico
Donald Barrick and Cort Cooper

CODAR Ocean Sensors, Ltd. Energy Technology Company, ChevronTexaco Mountain View, CA 94043 San Ramon, CA 94583

Abstract
A single SeaSonde HF radar was deployed and operates on Chevron's Genesis deep-water platform in the Gulf of Mexico. Because a single radar obtains a map of surface currents directed toward or away from the radar (a radial map), normally two radars that view the same area are needed to produce a field of horizontal total current vectors. Offshore rig location -- the rationale for which is recovery of oil -- rarely allows this luxury. The timely detection of intense loop current features and their movements while distant from the rig allows for effective response, with concommitant safety and savings of resources.
The two major limitations to offshore CODAR operation in the past have been: (i) severe distortion of antenna patterns caused by the ferromagnetic and conducting metals of the platform; and (ii) the logistical difficulties of high-quality modem access to the system from shore. Placing the antenna at locations to minimize distortions is usually not possible because of other operational constaints. Distortion -- if uncorrected -- causes biases in bearing estimation. Measurement of the patterns with a transponder on a boat circling the rig proved a successful calibration that overcame the first limitation. The rapid evolution of internet connectivity mitigated the second problem for the deployment beginning July, 2003.
Radial current maps clearly revealed the presence of strong loop current features to a distance of 80 km, when operating at 13 MHz. To evaluate quantitatively the system accuracy for this purpose, radial currents from the SeaSonde were compared to the radially directed ADCP current on the Atlantis platform 72 km to the SE. Although the surface current seen by the SeaSonde contains other contributions to flow besides the geostrophic loop feature (i.e., inertial oscillations and short-term weather events), low-pass filtering removed these higher-frequency "noiselike" spectral components, resulting in quite good agreement with the ADCP signal 40 m down. Three months data were compared to reach this conclusion. Total SeaSonde velocity -- possible near the Genesis radar location -- are compared to the deep-bin ADCP signal from the rig to reveal a greater difference between the more energetic surface signals and the weaker flows at depth. Reasons for this difference are explored.

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