Yesterday we returned to OBS9 and rescued it. It is amazing that it worked. We could communicate clearly with OBS9, but it never dropped its weight and surfaced. We were not sure why – maybe stuck in the mud, or having a net draped over it, or simply the burn commands to release the weight not working. We tried for about 6 hours to send commands to release the weights, and finally moved on to retrieve OBS14 with the plan to return to rescue OBS9 in daylight.
OBS9 was at 1335 m water depth. By acoustic sounding we knew where it was on the bottom within a few meters. The ResTech (resident research technician) Meghan devised a plan to drop a cable with grappling hook and dredge basket at the site of the OBS, forming a circle of wire around the OBS of about 200 m radius, and then closing the loop and hopefully jarring the OBS loose from whatever held it down.
If the weight was still on the OBS we would be out of luck – if we jarred it loose it still wouldn’t float up. Meghan fashioned a plan on the whiteboard, with a list of things that needed to be constructed from whatever we already had on the ship. Arriving the morning of Feb 1, Meghan, crew, and the Scripps engineers were ready. The grappling hook and dredge basket were lowered. The lowering rate was about 45 meters/minute, then slowed to about 5 m/minute when the basket approached the seafloor (to prevent it from slamming in to the OBS or the seafloor). The ship then steamed slowly and carefully around the predetermined circle, lowering cable as it went. When the circle was completed, the cable was reeled in very slowly, at about 10 m/minute. The whole time Meghan carefully monitored the cable tension, it was usually around 2000 lbs, but spiked up to 4100 lbs as it apparently hooked or dragged on something. We hoped that it was the OBS.
From the sounding John could see that the OBS had moved slightly. As the cable was reeled in, John confirmed that the OBS was lifting off of the seafloor surface, tracking the dredge basket on the cable. Wow. This clearly showed that the cable had reached the OBS and was somehow pulling it up. The depth of the dredge, tension on the cable, rate of cable retrieval, and depth of the OBS were continuously and carefully monitored. Against the odds, the cable was pulling up the OBS. They were coming up together, not independently. We could only guess how they might be connected, and we knew that it was possible for the OBS to come loose at any time and plummet back to the seafloor. Meghan, Mark, Martin, and Ernie assembled on the fantail (back lower deck of the ship) to try to grab the OBS as it neared the surface. The edge of the fantail does not have a guardrail, is open to the ocean, and bobs up and down with the swell.
It takes steady nerves and good balance to work there, leaning over with ropes and poles and hooks for the OBS, but nothing for yourself. The team crouched and leaned over, watching for the OBS. From my vantage point (a deck above) the first thing visible was the radio antenna. Ernie hooked the OBS with pole and attached a rope, then Mark, then Martin. As each line was attached it was tied down. Steve was farther back on deck with one of the lines on a winch, which was used to help bring up the package. As the OBS appeared, we saw that it had been hooked by the grappling hook, just like a fish. Amazing. What followed was a careful act of getting the OBS onto the fantail without it breaking off of the hook (just like landing a big fish, a lot of times they get away right by the boat), and then recovering the dredge from below the grappling hook. It worked.[youtube]http://www.youtube.com/watch?v=82xX9ASBO7A[/youtube]