This week, I went with my lab (The Robotic Systems Laboratory at Santa Clara University) on a week-long ROV and autonomous boat deployment in South Lake Tahoe to implement the equipment we've been developing and survey the bottom of Emerald Bay, on the Southwest side of the lake. We met up with several geologists who are experts on the area and who could guide us to particular features of interest.
We brought two ROVs: Nautilus (the red ROV to the left) and Triton (the yellow ROV to the right)
Two of SCU's ROVs: Nautilus (right) and Triton (left)
Day 1 was used to prepare equipment and do a basic "shake down" test of Nautilus, which had not scene water since a major overhaul done several weeks before the trip. Wake-up was at 5am, as weather forecasts reported high winds and snow later in the day. Our base camp was an extremely comfortable rented cabin with a dock in the Tahoe Keys, a channel system attached to the south part of the lake. After some technical delays, we were finally able to do a basic test with Nautilus, but the storm was already starting to move in, so we stayed in the cove where our cabin was located. Within an hour of arriving back, the storm started.
Bad weather just after arriving back from first Nautilus test
Nautilus covered with snow
The weather on day 2 was much better. We had planned to use a larger research vessel which would drive down from the North part of the lake, but because of accumulated snow and ice from the night before, and high wind warnings for later in the day, the captain of the vessel scrubbed for the day. We met up with the scientists and, with our own skiff and a rented pontoon boat, we made our way to the site.
One of SCU's autonomous boats, SWATH (Small Waterplane Area Twin Hull), had been mapping the area using multi-beam sonar, so the geologist already had an idea of where they wanted to look. For this trip, we would also be bringing a fleet of 6 autonomous kayaks which would act as 'body guards' for SWATH to keep curious boats from getting to close as it mapped a new area of the bay that has a lot of boat traffic. Towing these down the channel was not a trivial task, to say the least.
Leaving base camp towing autonomous kayaks used to protect mapping vehicle from other boats during operation.
Scientists describing geological features that we'd be searching for during the deployment
The deployment went well, although there were certainly some challenges we faced. Nautilus has been a work-in-progress for some time, and its vertical thruster was not spinning anywhere near full power which limited our vertical control authority. Probably due to escaping air bubbles caught somewhere on the ROV when we were setting the ballast (a task done by adding diving weights until the ROV is just barely positively buoyant), we ended being slightly heavy and when we reached the bottom, we did not have enough power to thrust upward. Instead the ROV just turned up a bunch of silt. To retrieve Nautilus, we ended up (very carefully) having to pull it back up by its tether. We finally moved to a dive site where there was a sloping rock face. Even without very good vertical control authority, we could keep ROV slightly negatively buoyant and control the ROV as it slid from the top of the face toward the bottom. We were just getting to some interesting features when I noticed a line of blurry-ness at the bottom of the screen that rocked back and forth with rotational thrusts.. it appeared that the camera bottle was flooding with water. We immediately aborted and headed for the surface. Sure enough, about a cup of water had made its way into the camera bottle. Luckily, the leak was slow, and because the camera points at a downward angle, the water had accumulated against the container's lens but had not reached the electronics. Finding the leak and repairing the bottle would require tools we left at base camp, so we called it a day. The 45 min boat ride back was beautiful.
Departing Emerald Bay with Nautilus
Getting some rest after a day on the water
Day 3 was to be the most interesting and eventful deployment. This time, we would bringing our (more capable) ROV, Triton to the site, and we'd be deploying from a hired research vessel with a heated cabin. Being able to stay comfortable while driving the ROV has more of an effect on performance then one would expect. We also brought Nautilus and SWATH with our smaller boat.
SCU's deployment boat carrying Nautilus and towing SWATH. Note SWATH's below-the-water-pontoon design which keeps it stable even in choppy water.
Once again, we consulted with the scientists on what the best place to deploy would be. Having the mapping data from SWATH proved to be invaluable.
Scientists consulting a map made earlier in the day using SWATH, an autonomous vehicle equipped with multi-beam sonar.
In addition to the SWATH data, we were also equipped with an acoustic tracking system which could report the position of the ROV relative to the boat by bouncing signals off an attached ROV beacon.
Unlike Nautilus, Triton is equipped with two "vertrans" thrusters which allow it to crab from side to side. Triton is also equipped with an on board compass and depth (pressure) sensor. We were now able confidently navigate along the lake floor, and it was amazing to see everything. It was surprising how many man-made objects we came across- old coke bottles, anchor chains, sunglasses- you name it. Being able to explore with the ROV made it possible to really appreciate how much there is going on underneath the waves.
Students and scientists looking at monitor as Triton is flown over the lake floor
Flying Trition over rock features
Both Triton an Nautilus use an approximately 1" thick tether that is cumbersome and fragile. We had brought a 700-foot tether that would allow us to look at some really interesting features at great depth, but when we plugged it into our system, the system detected a ground fault, so we had to use our shorter, 150-foot tether. Whenever the ROV was deployed, we had people on deck doing tether management.
Managing Triton's tether as it is flown below the ship
Eventually it was time to bring Triton back up. The boat's on-board generator had failed and we had switched to a separate stand-alone unit. The system was working well, but the time it took to make the switch didn't leave us with very much margin before we'd have to head home to beat the weather. Using the acoustic tracker, we navigated Triton to just off the ships stern, and surfaced.
Triton surfacing after the last deployment of the day
Lifting Triton out of the water to set it back on deck
Photo-op with Triton after the last deployment