Dispatch 18: How to Deploy an ITP     Print

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Joey Wenig

October 8, 2014

Why not start off this dispatch with some good news? Today, Bill Williams told me that the Louis is the only ship in the western Arctic Ocean right now—all of the other icebreakers have headed for port or more temperate waters. That’s reassuring, because Bill also said that we are currently stuck in between two big pressure systems up here at the top of the world, one high and one low, and are therefore expecting gale-force winds in the next couple of days. At least rough seas won’t be an issue: according to satellite imagery, we’ll be slogging through thick, multi-year ice from now until we leave the Beaufort Sea at the end of the cruise.

Okay, okay, none of that is actually good news. Fingers are crossed that the forecast winds won’t materialize during our next ice station on Saturday (I guess none of us relish the idea of being turned into human icicles and blown across the north pole to Greenland). And we’re already pressed for time, so having to slow down to plow through meters of ice isn’t exactly ideal. I’m not going to dwell on what lies ahead, though, but instead on the ITP deployment system John Kemp and Don Peters (not onboard ship) came up with on a beer napkin in Bermuda twelve years ago.

An ITP, unlike a mooring, has to be deployed out on the ice and therefore without the use of the heavy machinery available onboard ship.  If you think back to an earlier dispatch (#10) when I talked about ITP design, you’ll recall that there’s 800 meters of steel cable dangling beneath the surface float, with a profiling instrument (the actual Ice Tethered Profiler) hanging somewhere along the way. (‘ITP’, depending on context, can refer to the whole system or just the profiling instrument. I’ll try and stick to ITP for system and profiler for profiling instrument, even though that might not be technically true to the acronym. If you’re the kind of person who gets annoyed when somebody says ‘Derek Jeter had four RBIs’, then please forgive me.) When Rick Krishfield and John Toole designed the ITP, they asked Mr. Kemp and Mr. Peters to come up with a deployment system that was easily transportable (by sled, helicopter sling, or in the hold of a twin Otter aircraft), and simple enough so the people deploying it could do so wearing heavy gloves most of the time. 

This is the system they came up with: A folding tripod is erected over the hole drilled in the ice. The tripod is about 3 meters high, and a block and tackle are hung at the top of it. A custom-made winch is placed next to the hole. The winch already has the entire steel cable wound on it, and is powered by gravity—it’s only made to let the cable out, not reel it back in again (we aren’t ice-fishing for whales, after all). The cable is strung off the winch, through the block and tackle at the top of the tripod and down into the hole. The profiler gets attached to the bottom of the cable, so it goes into the water early. Then, with someone manning the speed control (amount of friction) on the winch, all of the cable is slowly let out. Things get a little tricky at the top of the cable (the part that goes in last). A yale grip is tied onto the cable and made fast to the top of the tripod. The yale grip looks like a jellyfish: it has several strands of flattened rope dangling from a loop. The strands of rope are wound around the cable one by one so that they overlie each other, and taped off at the ends. The loop (head of the jellyfish) is anchored and when tension is applied, the braided strands grip the cable like a finger trap. This takes up the weight of the profiler and all of the cable that has been reeled out so far so that the balance of the cable on the winch can be unwound and hooked up to the surface package. The ITP we were deploying on Sunday had a couple of extra instruments going in with it—SAMI-pH and SAMI-pCO₂ sensors and a MicroCAT. These are affixed to the cable just underneath the surface float. John Kemp had to slip some cable through the yale grip to make room for them. He did this by feel—exactly like gently loosening a finger trap so you could pull out some of your finger and then quickly pulling it tight again, if the finger was actually thousands of pounds of steel cable and expensive instruments. Once that’s done, a line is attached to a special bail (steel loop) near the top of the ITP cable and run through a second block on the tripod. This line is called a slip rope. It takes the weight off of the yale grip, and is used to gently lower the remaining cable through the ice. The surface buoy is pulled and pushed upright on top of the hole, and it’s done! (Please don’t try this at home.)

I’ll end this dispatch with some real good news: today, the final mooring of this year’s cruise was successfully deployed at station BGOS-B.