Val Schmidt and the Lost AUV
CCOM Researcher Val Schmidt was on Little Cayman Island in the Caribbean assisting University of Delaware Associate Professor Art Trembanis with AUV operations. There was a bit excitement when an AUV was lost during a deployment. Val tells the story…
Things got exciting late last week when an inexperienced operator planned a constant altitude (i.e., seafloor following) AUV mission without have a good idea of the depth in the area. The mission started in 20 m of water and was small―just 200x200 m. It should have taken only about half an hour to complete but the AUV never returned. Attempts to range to the vehicle with the acoustic modem produced no returns. The operators were forced to return to shore without the AUV and no idea what had happened.
Art Trembanis was teaching an undergraduate course there for UDel’s J-term, focused on research diving (biology) but he and a colleague had an AUV and glider there to demonstrate other ocean science methods. Art had returned home that very day (last Thursday) and called me when he heard what had happened and we started to "work the problem." (In case you don't get the reference, this is a famous line by Gene Kranz who was the NASA flight director when three astronauts were in peril during the Apollo 13 mission. :0) ) We used Dropbox to quickly pass data, maps, sketches and contact information around and Skype for conference calls between folks on Little Cayman, Art in Delaware and myself at CCOM. It was a stressful go and we considered every resource we could bring to bear.
The team on site returned to the survey area later that night with the acoustic modem and a hydrophone to listen for the AUV's emergency pinger. Although the primary battery would have been depleted long before, an emergency backup battery keeps the modem powered for several days and the emergency pinger operates for months. Thankfully, by maneuvering into deeper water they were able to identify the chirp of the emergency beacon and get several ranges to the vehicle with the modem. The closest range returned was 185 m, the furthest was 350 m. We breathed a sigh of relief to know the vehicle had not flooded and was relatively nearby. But clearly the vehicle was not going to be recovered by divers.
There is a longer story here about a scramble to get assets of all kinds―from around the island, back at Delaware and here at CCOM, folks pitching in rope, moorings, hand-carrying several hundred pounds of ROV gear on airplanes, charter flights, etc. etc. I'll leave that out for now other than to say nothing would have been possible without lots of folks pitching in. Indeed, when I arrived here several days later, I found everyone on the island knew what had happened and was following every update. My part was a relatively small one that I wanted to tell you about.
Back at CCOM, I took the ranges measured that first night along with several more measured the next morning and, using the same method taught in the Geodesy course "Virtual GPS" assignment, solved the non-linear least-squares problem for the position of the vehicle. The math is identical.
On Saturday, the team here on site set a mooring in shallower waters and prepared a depressor weight for the recovery effort. Art arrived on Sunday with a small ROV and the day was spent readying things to go. On Monday, as I set out for Little Cayman, a recovery effort was launched, focused on the position I had calculated and backed up by the audible loudness of the emergency acoustic beacon.
It is hard to illustrate what the team was up against. In the area the AUV was lost the seafloor drops off a precipice. The operators were unable to acoustically range to the vehicle the first day because the vehicle was well over the cliff. The scale in the image below is difficult to judge, but the distance to the wall in this image is 20 m or more and the field of view looking up the wall is well over 60m (the water is quite clear here).
The seafloor is limestone karst, scalloped from erosion and other processes and pock-marked by holes and caverns. None-the-less, after 45 minutes of searching, the AUV was found, lodged in a crevice under an overhang and adjacent to a several meter wide crack whose depth into the wall was further than our light could shine. The vehicle was at 154 m of water depth.
Maybe you can see it in this image. For scale, the AUV is about 2.6 m long. When you see the video and what Carter (Art's grad student) noticed that turned out to be the vehicle, well it's just frightening. I'm still amazed that we found it.
Here is the AUV during the 30 minute pull to the surface by the ROV.
|Photo Credit: Matt Oliver. Gavia AUV retrieval with Outland ROV.|
|Photo Credit: Matt Oliver (University of Delaware). Art Trembanis (University of Delaware) and his Teledyne Gavia AUV.|
We feel extraordinarily lucky to have the vehicle back, but we didn't guess where to go look (although given the terrain it was still very lucky we found it). It was the product of the work of lots of folks all working the problem, thinking about different possible failure scenarios, talking through different methods for localizing the vehicle, considering the availability of various equipment, working through methods of recovering it and preparing for unexpected problems (bad weather, getting the ROV stuck too, etc.). It is one of the events that, because it went so quickly looks easy, but only because of all the preparatory work that went into it.
In the end, the position estimate I calculated turned out to be just 12 m off. (Alas, I didn't correct the range measurements for refraction :0) )
|Aerial imagery with 2 position estimates for the lost AUV. The actual location was within 12 m of position marked with the red dot.|
I thought you'd enjoy the story and hearing about how sometimes the things you learn are handy for unexpected reasons.