Deploying uncrewed vessels in the search for lost shipwrecks

In the summers of 2021 and 2022, an iXblue DriX USV was deployed to try and locate the missing wreck of the sunken French fishing trawler Ravenel. The vessel disappeared in January 1962 with 15 people on board and was never found, despite extensive research campaigns.

In April 2021, the French minister of the sea announced the launch of a major research campaign, requesting for the DriX USV to be used to map the seafloor off the coast of the Saint-Pierre-and-Miquelon Island. A dedicated crew was first sent by iXblue (now Exail) to operate the DriX and perform the survey in the Summer of 2021, and then back in 2022. A Kongsberg EM2040-04 multibeam echosounder system (MBES) was installed in the DriX gondola for seabed mapping.

Conducted in coordination with local authorities, the survey mission also included archeologists from the DRASSM (Departement des Recherches Archéologiques Subaquatiques et Sous-Marines) who’s aim was to analyze the data collected by the DriX USV and inspect potential targets using ROVs.

During the two campaigns. DriX was used in autonomous mode under the supervision of a remote operator. The USV being monitored either from iXblue’s remote control center in La Ciotat (South of France) using 4G or Satcom or from a support vessel for operations very close to the shore.

The uncertainty surrounding the exact time of the sinking led to the definition of a large research area: from outside the port of Saint Pierre to the New Foundland coast for the 2021 campaign (450km2), and the South of the Island of Saint-Pierre and Langlade for 2022 (400km2).

The area environmental conditions are known to be very challenging even during Summer. Typically, sea conditions with tidal current up to 2 knots, wind gust up to 50knts, waves of 2.5m and Atlantic swell up to 3.5m are to be expected. The presence of dense fog is another challenge in terms of situational awareness and obstacles management.

Below is a presentation of statistics of wind (average wind speed in knots and gusts) per month for Saint-Pierre and Miquelon.

Below is an example of heave recorded by DriX Phins C7 Inertial Navigation System (INS) during one of the acquisition days.

These difficult conditions did not prevent DriX from completing the full planned survey area within the allocated time frame. The map below shows the navigation lines performed by DriX during the mission of 2021 (left) and 2022 (right). This represents more than 11,000km.

St Pierre et Miquelon DriX 5 trackline

Despite the rough weather conditions, owing to the DriX unparalleled stability and EM2040 data acquisition performance, the quality of the acquired multibeam data led to a resolution of the bathymetric surveys up to 5 times better than previous survey in the same area. The optimised data gathering environment allowed to record very clean data. Over the 11,000km of survey line acquired, the post-processing phase did not require the use of any bathymetric automatic data filter or any manual despiking. This led to a tremendous gain in processing time and fast tracked the data delivery to the DRASSM archaeology team.

Finally, the very low noise acoustic signature of the DriX gondola allows to maintain high frequency recording and an optimised swath even in deep waters. Therefore, the 400kHz MBES frequency was used from 5m to 250m depths, the deepest section of the survey area.

The survey was conducted between 7 and 8 knots, which provides the best compromise between sounding density vs. area coverage efficiency for a given time objective.

In 2022, DriX’s new “auto-line” feature was used for the first time.  The auto-line is the ability for the DriX to decode the bathymetry provided by the Kongsberg multibeam system and to perform navigation autonomously based on the swath width. This new development allowed around 50% gain in time for line planning and 30% in operation thanks to the swath optimization and overlapping management.

This new software development allowed for tremendous gain to be achieved in the 2022 campaign, resulting in covering an area close in size to the one surveyed in 2021, but in much less time. This allowed for the teams to cover an additional survey area in the remaining timeframe of the mission.

Another challenge was the operational coordination between the organisations involved. iXblue was conducting the survey in Saint-Pierre-and-Miquelon while the DRASSM was performing target identification and the ROV Inspections. The DRASSM operation was fully dependent on the bathymetry interpreted from the DriX measurement. As the ROV Inspection started before the end of the survey project, the bathymetric results needed to be delivered for review within 2 days after acquisition.

The data was thus downloaded directly from the DriX computer to the control centre where the data was checked, processed and delivered to the DRASSM so that analysis and target picking could be performed. This really high detailed bathymetric mapping of the area allowed the DRASSM to identify two high potential targets, both in French waters.

If the identification confirmed that the two targets were indeed shipwrecks, visual inspection confirmed that they were not the Ravenel. Both wrecks were laying on the seabed at around 110m depths. The first one, around 40m long, the second one (image below), approximately 24m long. Both are wooden made.

With over 11,000km covered, completing the full scope of work, those operations showed that using Uncrewed Surface Vessels such as the DriX USV represent a very efficient mean of surveying, providing highly reliable and high-quality survey in very challenging survey conditions. Unfortunately for this mission, the completion of the search area inspection did not allow to provide information regarding the sinking of the Ravenel. The Mystery still stands …