The Magazine for Underwater Professionals
Voyage data recorder could provide investigators with crucial evidence as they seek to understand the sequence of events that led to the cargo ship's sinking
Technology and vehicles developed and operated by Woods Hole Oceanographic Institution (WHOI), USA, scientists and engineers have been instrumental in assisting the US National Transportation Safety Board (NTSB) in locating the voyage data recorder (VDR) of the ill-fated cargo ship El Faro. The vessel sank in the infamous Bermuda Triangle last October after sailing directly into Category 4 Hurricane Joaquin, killing all 33 crew on board. Last transmissions from the freighter suggest that it lost power before taking on water, though the exact cause of sinking is unknown. The VDR could contain vital clues about the last manoeuvres of the vessel, recordings of the final conversations between captain and crew, and why the ship ultimately sank.
The voyage data recorder was located in April, in 15,000 feet (4600 metres) of water, about 41 miles (66 kilometres) northeast of Acklins and Crooked Islands, Bahamas, by an investigative team made up of specialists from the NTSB, the US Coast Guard, WHOI and Tote Services, the Florida, USA-based owner and operator of El Faro.
“The search employed a suite of WHOI vehicles working together in a ‘nested’ fashion to map the search zone, identify targets of interest on the seafloor, and ultimately enable the NTSB to locate the VDR,” explains Andy Bowen, the WHOI expedition leader on board the oceanographic research vessel Atlantis, which is owned by the US Navy and operated by WHOI.
In October and November of 2015, the NTSB conducted a search mission to locate the sunken vessel and conduct an initial survey of the debris field. Althought the VDR was not located at that time, data collected during that mission were used by investigators to plot “high probability” search zones for the latest mission. Within one of the identified search zones, the team used two underwater vehicles – the autonomous underwater vehicle Sentry, a one-of-a-kind vehicle designed and built by WHOI with funding from the US National Science Foundation (NSF), and the towed, fibre optically-controlled ‘Alvin Observation Vehicle’ (previously called Camper) to collect video imagery around the site.
Sentry is one of three unique vehicles in the federally-funded National Deep Submergence Facility, which is operated at WHOI and includes the human occupied submersible Alvin and the remotely operated vehicle Jason. Sentry is capable of operating in depths of 6000 metres. It carries a variety of scientific sensors and uses sidescan and other sonar to create detailed maps of the seafloor and is capable of sophisticated autonomous routines for conducting the types of surveys needed for this search. Its unique hovering capabilities enabled by thrusters mounted on its articulating fins allow it to operate over the large range of altitudes and survey speeds required for the various survey modes. These characteristics have made Sentry particularly well-suited to such a challenging and precise survey.
The Alvin Observation Vehicle (AOV) was designed to aid the Alvin submersible and was adapted for use on the NTSB mission. The vehicle is suspended from the ship using a fibre optic cable and is equipped with control and telemetry systems and two high-definition colour video and electronic still cameras transmitting imagery in real-time to its operators at the shipboard control station. Two thrusters enable crew to position the vehicle over targets of interest selected from maps generated by Sentry.
"Sentry and AOV were designed for oceanographic research with funds from the National Science Foundation, but they’re also well-suited for the task of ocean search and recovery,” says Adam Soule, a geologist and chief scientist for Deep Submergence at WHOI. “It’s gratifying that they can be applied to help solve this national need.”
The system of operating autonomous vehicles in tandem with human occupied or remotely operated vehicles was developed during research expeditions, and often to locate hydrothermal vent sites on the seafloor. The technique uses different vehicles and sensors to methodically search a wide area in ever narrowing patterns, with increasingly higher-resolution imaging, to ultimately home in on the target.
As an example, a debris field could be defined by an AUV like Sentry, conducting a multibeam sonar bathymetric survey that maps the shape of the seafloor at resolutions of one metre or less to map where larger fragments of debris are located. Subsequently, Sentry can conduct a smaller scale sidescan sonar survey to resolve the physical characteristics of the seabed (e.g., roughness, reflectivity, presence/absence of debris) at tens of centimetres or less. Finally, photographic surveys with still cameras on Sentry or video cameras on AOV can be conducted to identify specific targets identified from mapping surveys.
The final survey stage of digital photography is generally the most time intensive as the vehicle must be very near to the seafloor and thus can image only approximately 30 square-metres at a time. In addition, the vehicle must travel at slow speeds and low altitudes to ensure crisp imagery. Performing such work within a debris field such as is associated with El Faro is challenging.
“These techniques have been honed during scientific expeditions over many years supported by federal agencies,” says Soule. “This is a tremendous example of the unanticipated use of basic research for society’s benefit. It also shows how multiple federal agencies – the NTSB, Navy, Coast Guard, NSF – and civilian academic institutions like WHOI can cooperate effectively to achieve these results.”