Field testing of an instrument to measure near-surface currents from offshore platforms

We have carried out extensive field testing of an Acoustic Doppler Current Profiler (ADCP) designed to measure surface currents at ranges of 200-300 m by profiling horizontally from its mounting. The range of vertically profiling ADCPs is well documented, but horizontal profiling from offshore platforms presents unique, less well-understood factors influencing profiling range. These include the potential for interference from other sources on the platform, and that the concentration of scatterers (typically zooplankton) is now relatively uniform over the profiling range. A 300 kHz Horizontal ADCP (H-ADCP), specifically designed with large transducers for maximum range, was deployed from an operating drill ship in the Gulf of Mexico with a stated goal of characterizing its profiling range under varying conditions. The maximum range was determined under a number of possible configurations, which included varying the cell size, bandwidth and mounting depth. The H-ADCP presents a unique, simple means for obtaining continuous, real-time horizontal profiles of the near surface currents at significant ranges from an offshore platform. These near surface measurements can not be made with a conventional vertical ADCP due to interference from the platform and its thrusters. We here present a brief review of the measurement technique followed by a description of the tests performed during the trial. We then present the results from systematic testing intended specifically to empirically characterize the performance of this instrument from an offshore platform.