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Tracking Recovery From Deepwater Horizon
MILET System Aids Environmental Monitoring in Gulf of Mexico

Dr. Ian MacDonald

MILET system being recovered after a test deployment in the Gulf of Mexico. (Photo Credit: Henrik S. Mathiesen, MacArtney)
The tragic explosion that sank the Deepwater Horizon rig and caused a catastrophic oil discharge in the Gulf of Mexico set in motion an unprecedented response effort. A major concern throughout the 84-day emergency was finding and tracking the oil across the deep-ocean floor, and this required use of ROVs, AUVs and profiling instruments deployed from surface ships. No expense was spared, and everyone hopes the lessons learned will help prevent other accidents and prepare a more effective response should one occur.

A diverse suite of efforts, systems and instruments are coming online as the post-disaster research begins to produce results. In the aftermath of the disaster, focus has shifted to tracking the recovery of the Gulf ecosystem and improving baseline knowledge of its healthy functions. This important work, which will continue for years to come, has been broadly funded by BP plc (London, England) and the Gulf of Mexico Research Initiative.

However, at a time of reduced research budgets overall, the available resources of ships, submersibles and oceanographic equipment have been substantially scaled back. In this climate, developing cost-effective approaches to marine research and monitoring is crucial.

The industry-class ROVs that were widely utilized during the oil discharge come with very high day rates and mobilization costs and, in many cases, do not feature high-resolution cameras and oceanographic instruments as standard equipment. In response to this, Florida State University has developed the Modular Instrument Lander and Equipment Toolsled (MILET), a cost-effective alternative for deep-ocean surveys that can be deployed from the kinds of coastal ships that are most readily available for academic users.

MILET is essentially a high bandwidth communication and power backbone system to a diverse set of ports on a towed, deep-sea drift platform. The flexible MILET can be used on a wide array of vessels and can carry the latest camera, sonar and sensor technology. MILET is maneuvered as the ship slowly motors along transits covering features of interest. Position in the water column, typically 2 to 4 meters above the seafloor, is controlled by fine-scale adjustment of the winch wire-out. An ultrashort baseline (USBL) transponder and Doppler velocity log (DVL) provide real-time data on location and altitude.

At the heart of the MILET is a custom-made fiber-optical NEXUS telemetry system comprising a MacArtney A/S (Esbjerg, Denmark) NEXUS MK C multiplexer, nicknamed the 'mega mux' due to its numerous connectivity and instrumentation options. The system is interfaced with SubConn connectors, and uses a single fiber-optic cable mounted on an oceanographic winch system to connect topside and subsea multiplexer units. This combination allows for flexible control and versatile application of the MILET and its onboard equipment. The fiber-optic telemetry system and the winch are designed and fabricated by MacArtney.

There are two modes of investigation relevant to tracking oil discharges and their aftermath. Regional surveys seek to measure diverse biological, chemical and geological phenomena to identify hydrocarbon transport pathways and habitat characteristics. Seafloor observatories, on the other hand, undertake detailed, long-term measurements of rates and processes at areas of interest. MILET has a vital role to play in both modes. It can investigate environmental gradients and special features in a site of interest, and can also be used for detailed local mapping of the seafloor and water column.

Deepwater Horizon underscored the lack of basic knowledge of the environment where the accident happened. Among the many unprecedented aspects of the blowout was that it occurred at 1,500 meters depth. Hydrocarbons discharged from the well were transported as diffuse plumes a few-hundred meters above the seafloor and in surface layers that ranged from microns to centimeters in thickness. The deepwater plumes moved primarily to the southwest, while the surface layers were carried by wind and current to the coasts of Louisiana, Mississippi, Alabama and northern Florida. In both directions, impact assessments have been complicated by incomplete prior knowledge.

Nowhere was this more apparent than in the DeSoto Canyon. This prominent submarine canyon extends north- northeast from the Macondo well almost to the Florida Panhandle. At the time of the blowout, the bathymetry for most of the canyon was only mapped to a resolution coarser than 500 meters in the horizontal and 100 meters in the vertical. This limited responders' ability to anticipate how bottom features could affect oil deposition and shape biological habitats. Results from the University of South Florida indicate that some of the surface oil settled out in what they dubbed 'a dirty blizzard.'

DeSoto Canyon is potentially a topographic vector connecting the deep to the coastal zone. When the Gulf of Mexico Research Initiative put out a call for research consortia to investigate potential and actual oil spill impacts in the Gulf of Mexico, 10 major institutions joined under the leadership of Florida State University to form Deep-C with the goal of conducting basic and applied science focused on the canyon's oceanographic features.

Deep-C got an early assist from NOAA's Ocean Exploration program when, in the fall of 2011, the RV Okeanos Explorer was brought into the Gulf. During expeditions lasting several weeks, Okeanos Explorer completed bathymetric charting of DeSoto Canyon and other areas of interest using its onboard Kongsberg (Kongsberg, Norway) EM 302 multibeam echosounder. The effort clearly delineated features like salt domes, erosion channels and natural-gas seeps. This allowed Deep-C investigators to establish the benthic array of study sites at localities in the canyon that represent depth gradients and significant anomalies.

The role of MILET and USBL telemetry is to determine the seafloor characteristics and ecological diversity at the Deep-C study sites. By sampling along kilometer-long transects at sites with depths from 500 to 2,500 meters while logging geotagged images and readings from the instrument suite, MILET provides a repeatable sample of sediment characteristics, mobile fauna and water chemistry. Biannual repeat sampling will make it possible to test for seasonal effects on the biological community and particle fallout. To continue this article please click here.

Dr. Ian MacDonald is a professor of oceanography at Florida State University. In his research, he uses imaging and GIS techniques to investigate the ecology of deep-sea hydrocarbon seeps, primarily in the Gulf of Mexico.

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