Blog Entry: The mother of all tangles...
One of our missions on EN-522 was to rescue and re-anchor one of the NOAA "Prawler" moorings that had come adrift. The Prawler is a unique new type of mooring that has a surface float with a jacketed wire rope suspended beneath it upon which a CTD can travel. The CTD is part of a package that uses a clever ratcheting mechanism to crawl up the wire as the wire heaves up and down with the surface waves. Once it gets near the surface the ratchet is released and the CTD package drops and collects a profile of temperature and salinity, transmitting the data by inductive coupling through the wire to the surface pod, where it is radioed via satellite to the scientists back home.
The wire extends to 700 m depth, where it is connected to synthetic line. There are two types of line used, the first after the wire is heavier than water, then that transitions to line that is lighter than water. Finally, there are glass balls for floatation, an acoustic release, and anchor. The arrangement of heavy and light synthetic causes the line to take on a sideways "S" shape in the water, since much more line is used than the depth of the water. This makes for a sizable "watch circle" that the buoy will occupy at the ocean surface, but an "S-tether" mooring means much less stress on all the line than a taut mooring would have, and the line does not get wrapped around the anchor.
When we recovered the top portion of the mooring from its drift 100km to the south, it was still working and collecting data and we had no problem locating it and pulling in the wire. We found no synthetic at all at the end of the wire, just an empty thimble in the shackle. It looked like the eye-splice had failed.
Several days later we sent the the acoustic signal for the release to let go of its hold on the anchor. This allows the very buoyant hollow glass balls to bring the lower portion of the mooring to the surface. Waiting for the balls to reach the surface is always a bit of a tense time in a mooring recovery, balls might have collapsed and there might not be sufficient buoyancy to bring everything to the surface. We could tell by acoustic ranging on the release that it was coming up, but very slowly. It ended up taking three hours, but the bright yellow hardhats were easy to spot a few hundred meters off the port quarter when they did surface. We grabbed the balls and started reeling in the many kilometers of line. The first section of buoyant line came in very nicely, but we quickly ran into trouble with the heavy line. A lot of it was muddy, so it had been lying on the seafloor in a big tangle. And what we brought up was a series of large tangled knots, as you can see in the photos. It looked to be a hopeless tangle, and prospects did not look good for redeploying the mooring.
We were particularly interested in finding the bitter end of the line, to see how it had failed. But because of the large tangle and the necessity to make several cuts juts to get it aboard, we never found the expected shorter transitional lines that the design called for. WHOI mooring maestro Jeff Lord and crewman Paul Rousell worked diligently over the next few days to untangle the mess. Jeff went over every inch of the line and re-spliced the larger pieces and re-spooled them on to the winch. However, since we could not find the expected termination of the line, we did not know how much of the line we had left. How a line could part in two places is a mystery, but the most likely scenario has the line parting somewhere mid-length, then a short length still attached to the wire but with no tension, was cyclically whipped around by the vertically heaving wire rope till the eye-splice parted.
We decided to go ahead with the redeployment; we had brought another anchor for the purpose. But since we did not know how much line we had, we had to be flexible in the depth of water we chose for the anchor site. As the mooring was deployed we had three different people count the revolutions of the drum and use the minimum and maximum radii to estimate the line length. The estimates came to about 2600m. It was deployed with 3400m of the heavy line so we were missing some 800m of line! These moorings were designed with quite a bit of scope but clearly we had to choose the shallowest spot available. This we could do by choosing where to release the anchor, as we trailed over 6 kilometers of line behind us. Fortunately we had a very good SeaBeam survey of the bottom in the area that we had done last September from the Knorr. Chris Duncombe-Rea searched for the minimum depth in the data set and found a nice rise in the seafloor that would still give a reasonable shape to the array of three moorings. We watched the echosounder anxiously as we steamed for the spot, and the bottom did come up smoothly. In fact we got exactly the same depth at the same spot that the Knorr had measured 6 months previously! We steamed another 300 meters to allow for anchor fall back and let it go. We surveyed the anchor by triangulating on three acoustic distances from nearby sites and hoped for the best.
Fortunately, a few hours later we got word from the folks at NOAA that the Prawler was working nicely. Its very gratifying when such a big mess can be straightened out by smart, hardworking people, the numbers fall into place nicely and a novel contribution to our ocean observing tools can be put right again!
--Ray Schmitt 4/06/13
