Low drag subsurface buoy with integral acoustic Doppler current profiler

Abstract

This paper describes investigations by FlotationTechnologies and Mooring Systems Inc. to develop a low drag subsurface mooring apex buoy that contains an RD Instruments Acoustic Doppler Current Profiler current meter. Five different candidate mooring apex buoy systems are compared with regard to their drag, inclination and stability in current and effect on the performance of a typical mooring. The Drag Coefficients of cylindrical and spherical buoy candidates are Reynolds number sensitive in the velocity range of 1 to 3 knots. This leads to calculated and observed hydrodynamic instability due to minute variations in current velocity. An airship shaped streamlined body was found to have the lowest drag, best inclination response and little or no Reynolds number sensitivity in the current regime of interest. KNTRQDUCTIQN:The US Naval Oceanographic office gathers current speed and direction and other data in support of Naval operational and planning requirements throughout the oceans of the world. There are occasionally requirements for accurate measurements in areas of high current velocity in a range of water depths.The ADCP style of current meter mounted in the mooring apex flotation allows a subsurface mooring to profile to very close to the surface while keeping the top of the mooring below the area where maximum velocities occur. The drag of every mooring component that is in the high current zone causes the mooring to lean over and undergo vertical excursions, "dip".The inclination relative to vertical can cause errors in measured velocity. Dip of the mooring apex can cause displacement of the range bins such that the much desired upper water column information may be lost out of range.Several experiments at WHOI, [ 1.21 have shown that reducing the drag of mooring components in the effected area can greatly improve mooring performance.Option 1, A three ball radio float and 19 glass balls on chain above a 2.33 meter long stainless steel frame containing the current meter. This configuration is an old oceanographic standard. The noise of the glass balls and their occasional appearance in the beam pattern would probably make this option non-viable with an upwards looking instrument.Options 2 and 3 are cylinders of syntactic foam with embedded stainless steel frames and ADCPs. NAVOCEANO owns several ADCP buoys similar to Option 3. The two versions show the effect of changes in aspect ratio.Option 4 This option is similar to a commercially available spherical buoy.Option 5 is the low drag airship shaped body. A prototype unit is undergoing fabrication at the time of this writing and will be delivered to NAVOCEANO for testing in early 1995. Criterion for Evaluation:Four criteria were evaluated to illustrate the need for and to predict the performance of the low drag buoy.A. Drag of the buoy with fiame, beacons and ADCP. B. Inclination of the buoy alone in current. C. Hydrodynamic stability in current. D. Expected mooring performance.A. Buov Drag with Frame, ADCP & Beacons.The Drag areas based upon scale drawings ...