Design enhancements to improve survivability of a lightweight trawl-resistant bottom mount for workhorse acoustic Doppler current profilers

To protect instrumentation used to measure ocean currents in heavily trawl-fished areas, the Naval Oceanographic Office (NAVOCEANO) has been involved with development (since 1989) and utilization (since 1992) of trawl-resistant bottom mounts (TRBMs). The first-generation TRBMs were very large and equipped with RD Instruments (RDI) Acoustic Doppler Current Profilers (ADCP) of various measurement frequencies (75 to 1,200 kHz) and a Sea-Bird Wave and Tide Recorder. The first-generation IRBMs had high development and construction costs, and deployment logistics were limited to large vessel operations. Additionally, modifications to the overall design were constrained by the use of instrumentation developed for traditional single-point deep-water moorings, which were, at that time, state of the art. The introduction of the RDI Workhorse ADCP in late 1995 provided a stimulus for the development of smaller auxiliary instrumentation and “downsized” bottom-founded platforms. While long-term survivability of these smaller platforms remains a concern, these technological developments have increased opportunities for littoral water surveys in heavily fished seas. One of these new platforms was developed by Mooring Systems, Inc. (MSI) in early 1996, and consists of a low-profile, conical-shaped aluminum and fiberglass structure. In 1997, NAVOCEANO released a specification for a lightweight TRBM (LWTRBM), which resulted in the acquisition of one MSI mooring, modified to meet more rigorous requirements. Seven additional LWTRBMs have been procured and further modified, based upon testing of the original unit to improve data and instrumentation recovery rates