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Dispatch 12: A (Not So) Sinking Feeling

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Joey Wenig

October 2, 2014


We barely needed to deviate from our course this morning to pick up ITP 77 en route from station BGOS-A to station TU-1. (A brief aside on these station names: don’t read too much into them. Early on, I tried to understand the significance of one station carrying the prefix CB, while another was BGOS or TU or something else entirely. Often, they refer to the institution or project that selected them, but there’s no real relevance, as far as I can tell, to the nomenclature. Indeed, Chief Scientist Bill Williams told me that he once named stations random letters, expecting to be asked about it by other scientists. No one ever questioned him.)  Anyways, ITP 77 had the same programming bug as ITP 79, and not only that, but the shelf waters the ITP was traveling over were shallow enough that the cable was actually dragging over the seafloor creating a risk that the instruments might be damaged or even lost during a deep profile. Luckily, when we found the ITP, the profiler was near the surface and unharmed. Getting these ITPs back safely is very important. The programming bug prevented them from relaying any of the oceanographic data they were collecting, but that data (nearly 700 profiles) was still stored on the ITP. Considering the investment that goes into deploying these devices, recovering that stored data is imperative.

Following the ITP recovery was a mooring going in at TU-1. This one belongs to a physical oceanographer at TUMSAT (Tokyo) named Koji Shimada, who collaborated with the WHOI team to put it down for him. Originally deployed in 2012 and recovered for refurbishing in 2013, it’s half the length of the ones we’ve dropped so far at 2068 meters and has a single MMP (salinity, temperature, depth, current meter) ranging over 2000 meters.

The deployment began just after ‘dinner’, i.e., the midday meal I call lunch. All was going well, and the quick-release line that unclips the sphere from the crane was pulled with time to spare for everyone to get warmed up and change clothes before supper. It was immediately obvious that something had gone wrong, however, when, instead of disappearing quickly into the dark water, the yellow sphere stayed visible just under the surface. It turned out that some confusion about the location chosen for this deployment lead to a bottom depth that was too shallow by about 30 meters; a small distance relative to the overall length of the array but nonetheless unacceptable. (This is a business of precision: the nylon-jacketed wire rope of the array stretches under tension, so John Kemp and the WHOI mooring crew actually take it out to an airstrip in Massachusetts beforehand where they simulate the force the rope will experience in the water to make sure the length is right.) There were two options for remedying the situation: either the mooring needed to be released and recovered, which meant losing an anchor (for which there is a single spare on board) and, more importantly, a significant amount of time; or somehow a line needed to be attached to the mooring so that the whole thing could be towed a couple of miles into the right water depth. Because the top float seemed to be just barely beneath the surface, the second option seemed feasible. But before giving it a go, everyone broke for supper.

Now, as I mentioned, the array stretches under the full weight of the 3800-pound anchor while it’s being lowered into the ocean. When the anchor hits bottom, the only tension on the rope is the 2500 pounds of flotation provided by the sphere at the top (ignoring the weight of the instruments and the rope itself). The decrease in tension means that the rope actually contracts once fully deployed, and therefore it’s likely that while we were eating supper the sphere was moving precious feet downwards. It’s also possible that the anchor settled a few feet into the silt on the bottom during that time. Either way, after eating John Kemp tried his best to hook a line onto the submerged float, but to no avail: the mooring lay just out of reach. So, it would need to be ‘popped’, as they say: fully recovered. This, however, would be tomorrow’s job; in the meantime, a pair of CTD/rosette casts was done at TU-1 and a plan was made for the ship to head south during the night to take additional XCTD measurements in the relatively ice-free waters.



Last updated: September 14, 2017
 


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