Thursday, March 9, 2017

Buoy Away

 One of the first tasks once we selected a starting point for our measurements was to deploy a drifting buoy. This buoy will produce surface measurements of wind, rain, and heat flux using the instruments mounted at the top of the platform as well as subsurface measurements of current, salinity, and temperature using a series of instruments suspended on chains below it. One of the benefits of this type of instrument is that it produces co-located atmospheric and oceanic measurements, which allows us to look at the conditions in both places simultaneously and see how the atmospheric conditions are forcing the ocean. Below the surface, this buoy has two upward-facing Acoustic Doppler Current Profilers (ADCP) and five Conductivity, Temperature, and Depth (CTD) instruments.

Members of the Sikuliaq crew and science team stabilizing the buoy on deck (Photo credit: Rosalind Echols)

Each of these instruments, chains, and attachment mechanisms was packed and shipped individually, so prior to deployment we spent several hours planning out each of the attachment points and organizing the pieces so that the process of getting everything in the water would be as smooth as possible. Deploying the buoy was a complex operation involving close coordination of quite a few people and tools, including the large starboard crane and numerous tie lines and slings. As with all shipboard operations, safety in these situations is of paramount importance. Everyone involved wears a hard hat and life preserver, and communication is clear and unambiguous (getting the buoy in the water is important, but making sure no one falls into the Pacific Ocean or gets hit in the head with the crane always supersedes that). 
Eric Boget from the science team staging the bottom-most instrument, an ADCP, on deck (Photo credit: Rosalind Echols)

Starting with the deepest instrument, we suspended each instrument from its topmost point using a crane, gradually lowered it into the water until the attachment point was at deck level, secured the attachment point to the ship using tie lines, and then prepared the next section of chain and instrument. Thus, one at a time, we built the subsurface instrument chain from the bottom up until we reached the link that would attach to the bottom of the buoy. Getting the 10-foot tall buoy into the water was unquestionably the most exciting part, but both the Sikuliaq team and the science team have a lot of experience with these processes, so it ultimately went quite well.  (As tricky as this was, deploying buoys and moorings is a fairly standard practice in oceanography, as their use is widespread. This one was relatively easy since it isn’t moored and the deepest instrument is only about 80 meters below the surface, not the full depth of the ocean).

Preparing a CTD for deployment, with stabilizing tie-line at the ready (Photo credit: Rosalind Echols)

 This particular buoy is designed to drift along with subsurface currents, unlike the moored buoys that are used to gather long time series at a specific location. In order to accomplish this drifting, the buoy has a series of plastic vanes or “X-wings” suspended roughly 60 meters below it, that create substantial drag on the subsurface chain. (A longstanding engineering challenging in oceanography is achieving a design for drifters that will allow them to follow a parcel of water; this particular design seems to work pretty well). Right now, it has been drifting along following a group of floats we deployed at about the same time, which is the best situation we could hope for.
X-wings hanging from the crane during deployment (Photo credit: Rosalind Echols)

If you want to check out the data the buoy is sending back, see the website hosted at the Woods Hole Oceanographic Institute (one of the collaborators on this project):

Successfully deployed buoy drifting away! (Photo credit: Rosalind Echols)

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