by Heather Furey
06 June 2018
Well, here we are again – in the middle of the gray raw North Pacific in June; must be OSNAP time!
Amy Bower and I are the only OSNAP people on a ship full of Ocean Observatories Initiative (OOI) folks. But almost everyone on this ship is from WHOI. I will really miss sailing with Bill (Johns) and Stuart (Cunningham) and well, all the rest of the UMiami and SAMS crew, and sometimes the NIOZ crew, as I have done almost each year since 2014. Your shipmates become like family, and it was nice to have the same science crew together as we bounced from ship to ship each year. And I will miss seeing Rockall. This year, though, since the third and final seeding of RAFOS float deployments were completed in 2016, Amy and I are onto something else.
The RV Armstrong is here primarily for the turn-around of the OOI Global Irminger Sea Array off the eastern tip of Greenland (http://oceanobservatories.org/array/global-irminger-sea/). Amy arranged with the National Science Foundation (NSF) for four additional days of shiptime and mooring crew time to deploy four moorings for OSNAP east of Greenland, and also to recover two of the sound source moorings used for tracking the RAFOS floats. We took over this mooring array, instrumented with CTDs and current meters, from the UK as a swap: they took over some moorings in the Iceland Basin closer to their home country. Although this is a turn-around year for these moorings, we are only doing the re-deployment. Johannes Karstensen and Penny Holliday, onboard the RV MSMerian, will recover the moorings that were deployed by Penny onboard the RRS Discovery in 2016. Johannes and Penny will be recovering at about the same time that we are deploying, and we will have to carefully coordinate our timing so that we do not literally get our equipment tangled up together. Stay tuned on that one.
This cross-slope array will measure the properties and transport of the Deep Western Boundary Current which flows along the ocean bottom from north to south. The DWBC is composed of two primary water sources, the Denmark Strait Overflow Water (DSOW) and the Iceland-Scotland Overflow Water (ISOW). The DSOW is ‘newer’; its source is the dense (relatively cold and fresh) water that overflows the sill at the Denmark Strait, at the head of the Irminger Sea. Above it resides the relatively saltier and warmer ISOW, which originally came from the head of the Iceland Sea, in a similar deep overflow, but much farther upstream. Both are principal components of the subpolar overturning circulation.
One thing I like about being at sea is learning new things; things that have nothing to do with sitting in a chair at a desk on a computer. This morning were the first release tests for the surface mooring (SUMO) at the OOI Irminger Sea Array. Talking with Jim Dunn, a salty mooring technician from WHOI, I learned that it is common practice for the OOI team to test releases by lowering them to 1000 meters depth on the CTD frame, and to let them sit for 20 minutes to get cold before doing the test. The release needs to be acclimated and under pressure to make for a good test. Seems like every group has a slightly different depth and duration for wet tests of the releases. Some groups do not do a wet test at all. I’ll plan to stick to the OOI protocol for our own mooring releases.
As well, I got a good show-and-tell from Jim Dunn and Meaghan Donohue, another salty mooring technician from WHOI, about how wire is wound on a reel with respect to the winch. On this ship, we have a Lebus winch, a double barreled winch where the barrels can be moved independently, at independent speeds, and also oriented differently from each other, or skewed, to control the spread of the loops on the reel – so they do not get tangled. The tension is controlled by the number of loops wrapped around the two barrels. Nominally there are six loops wrapped around the winch barrels as the mooring line and instrument are fed over the fantail into the ocean. After one reel of wire is spooled out, the wire is tied off, and the next reel of wire is moved into place. Instruments are put on the mooring either between the two wires, or clamped onto the wire itself.
A more traditional winch, a “normal” winch, is a TSE winch – with a single barrel. The single barrel cannot have reels of wire fed to it, the wire tangles easily. This means that all reels must be carefully fed onto the single barrel winch before deployment. All segments, starting with the mooring bottom, are wrapped onto the barrel in order from bottom to top. Then, at deployment time, the line is payed out from top to bottom. This means that to deploy with a Lebus is generally twice as fast as deploying with a TSE.
The two winches have one upstream difference: the wirerope or rope may be wound on the spools in opposite directions. This winding preference is correctable at sea, but it takes a long, long time to rewind spools of wire on a 3000 meter mooring, with the wire split onto many spools. And at sea, it is best to be prepared. Months before leaving for sea, I notified WHOI’s Rigging Shop that these moorings would be deployed on this ship with a Lebus winch.
Our OSNAP mooring deployments are days off. This morning (Wednesday 06 June 2018) we went out to make sure the OSNAP mooring wire rope was wound on the spools correctly, and it was. Thank you, WHOI Rigging Shop. For the Lebus winch, the top of the wire is on the outside of the spool, and the opposite holds true for the TWE winch. If there are no instruments clamped to a wire segment, this wrap direction is inconsequential. But if the wire segment is ‘marked’, which means an instrument like a microcat will be attached with clamps to the wire, rather than being deployed in-line, the wirerope is taped at the correct depth where the instrument will be attached on the mooring. For this reason, marked reels need to be wound properly with respect to the winch used.
Jim also told me that moorings are deployed anchor first in ice-covered water. Which I did not know and had never thought about. You can’t steam into anchor-over position dragging a mooring cable through the ice, like you do with a mooring deployment in open water. So in ice-covered water, the desired anchor position is where the mooring deployment starts. Which means the exact opposite holds true for the winch type and wire wrapping relationship if you have an ice-deployed mooring.
What amazes me is that the same attention to detail needs to be paid to nearly every system set or tasks performed on a research cruise. I think it will be great sailing with this new group of folks.