Measurements of Current Blockage by the Bullwinkle Platform

Abstract

Although most offshore structures are open space frames, they still disturb the currents that flow through them. This flow blockage has important consequences both for calculating forces on the structures and for choosing sites for current meters. Full-scale measurements of current blockage were made from the Bullwinkle platform in the Gulf of Mexico using two acoustic Doppler current profilers (ADCPs) that were modified so that all eight beams were in one horizontal plane. The current field was mapped using an objective analysis scheme that was based on the radial current components given by the ADCP beams. This objective analysis technique may also be useful in the interpretation of current measurements made by high-frequency radar systems, which also measure current components along radial lines from their antennas. The analysis of the ADCP measurements at Bullwinkle showed that the average current speed inside the outline of the platform was about 0.8 times the upstream speed. This result agrees with the predictions of actuator disk theory. The wake of the structure extended at least 100 m behind the structure past the end of the ADCP beams. Great care must be taken in measuring currents near offshore structures.