The High Performance Oceanographic Mooring (HIPOM)

Abstract

In 1985 and 1986 the Wean Structures and Moorings ((36&M) Laboratory of the Woods Hole Oceanographic Institution (WHOI) designed, deployed and recovered a High Performance Oceanographic Mooring in the Gulf Stream. This paper describes the need for this new type of mooring; the design features for substantially increasing the reliability and the performance of subsurface moorings set in strong current regimes; and the special instrumentation developed to monitor the mooring response. The pper then reviews the data acquired during the two month deployment and compares the HIPOM performance against the performance of previous moorings. Finally, recommendations for the design of future high current subsurface mrings conclude the paper. B h m -Traditionally surface moorings have been used to obtain long term measurements of currents over the entire water depth.In this approach, contamination of the measurements due to wave action is a major drawback. Furthermore, maintaining any kind of surface mooring in strong current regimes r m i n s a risky proposition. The alternate approach is to expand the capability of subsurface moorings by bringing the upper buoy closer to the surface. This solution becomes impractical when strong surface and near surface currents prevail. Current measurements, however, can now be made remotely with the help of Doppler Acoustic Profiling Current Meters. A rigid subsurface m r i n g with such an instrument at its top and a n w e r of current meters inserted in the mooring line could provide good current measurements from the surface to the bottom. The intermediate mooring, using clusters of glass balls and long lengths of wire rope has been extensively used over the years as a reliable platform for long term measurements of currents in the deep ocean (5) . As the scientific interest expanded and included measurements of m c h stronger current regimes (Kuroshio and Gulf Stream studies), this basic design was altered and m r e efficient mrings using syntactic foam spheres and Kevlar lines were introduced. One such mooring (GUSTO experiment, Bryden, 1983) deployed in the Gulf Stream, provided a fascinating 12 month record of currents and instrument depth excursions. Large values of instrument dip (up to 275 meters) andinclination (15-22 degrees) were observed, indicating a need for further irprovenlent. This need was recognized by the Office of Naval Research who sponsored in 1985 an experimental study to design and denonstrate a new, improved, deep sea subsurface mooring. To this end the well documented GUSTO mooring was to be used as a standard for comparison. First the proposed improvements of the new design would be quantified by comparing the HIPOM predicted response to the GLTSTO measured response. Next , the HIPOM mooring would be instrumented and deployed at the GUSTO site. Its performance could then be again compared with the GUSTO mooring performance. Actual improvements could thus be clearly demnstrated and quantif ied.