Rosette test

After five and a half days of sailing almost due south, we finally had our first action off the side of the boat! Very exciting. Yesterday morning at 0800* the ship came to a complete stop just north of 10˚N so that we could conduct the “Rosette test”, which is actually a lot less flowery than it sounds. This test was a check of whether the OBS transponders – the acoustic communication units that we use to communicate between our ship and the OBS instruments (on the seafloor) through miles and miles of water  – are working properly. These units are absolutely essential; if they stop working then we’re unlikely to ever see our precious instruments (let alone the data they’re recording) ever again! That would be very sad. 

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As dawn breaks in the background, our OBS transponder rosette sat strapped down at the back of the ship, ready for its big day!

 

 

In order to make sure that everything is working properly, the SIO (Scripps Institute of Oceanography) folks test all of the transponders before we attach them one-by-one to the seismic instruments. To do this efficiently, they use a “rosette”, which seems to be a generic term for a structure onto which you can mount several instruments at once in a little array. When we first loaded the ship, all the OBS transponders were mounted on what looked like a big 7×3 ft box that had slots for 30 transponders (which look like black cylinders that are about 3 ft long and half a foot in diameter). This rosette has lots of hooks on the side and top so that it can be suspended by a crane to lift it into the water. In our case, we attached it to a heavy cable running through an A-frame on the back of the Kilo Moana. Because the KM is a twin hull, there is a nice space between the engines off the back of the vessel so that we can safely deploy and recover packages from the water without them getting near the dangerous engine propellors. 

 

However, when the cable winch lifts the package of the deck, it is liable to swing back and forth like a pendulum in response to the ship’s motion. To control the suspended load, we loop ropes through bales (metal hoops) on either side of the package (the rosette, in today’s example). These ropes, called “tag lines”, are controlled by crew members, each paying out or taking in rope in order to keep the package from swinging or spinning on its way up off the deck and out into the water. This system is deceptively simple: these ropes can be the difference between a smooth deployment and complete disaster! Yesterday the science party was exhaustively drilled on tag line procedures by the ship’s research technicians.

Here are some takeaways to bear in mind, should you ever find yourself controlling such a line: 

  1. NEVER put any part of your body between the line and the ocean. If the rope gets pulled out to sea, you don’t want to be going with it… 
  2. Don’t get your hands anywhere near the cleats through which the line runs. These metal fixtures, riveted to the deck, are what allow puny humans to get a “mechanical advantage” on the heavy swinging package through leverage and friction. If you get your finger caught between the rope and the cleat… let’s just say it will be rather mechanically disadvantageous!
  3. When the package is in the water, the tag line’s job is done, and you’re pulling in your loop of rope to detach from the package, don’t pull too quickly, or the “bitter end”** of the rope can whip around and knot itself to the bale, meaning that the package will now be tethered to the ship, instead of sinking nicely into the water, where you want it. This is bad. In the worst case scenario, the package then will swing under the ship and get dragged into the propellors. Very bad.

Today, one tag line was handled by the OBS tech, and the other by yours truly (watched VERY carefully by the other tech). It’s pretty tricky to keep an eye on absolutely everything that’s going on once the package is in the air: the directions of the tech, the tension in the rope, how it’s running over the cleat, the amount of rope you have left, the height of the package and its distance to the ship and the water, and what the other tag line is doing. But having the SIO techs, Ernie and Mark, directing operations was very reassuring; they’re total pros, and have done this all hundreds and hundreds of times. And they still ostensibly have 20 fingers between them! 

As well as being our first real action of the cruse, today’s deployment had an aura of extra tension because our package was so very valuable. Everyone was very conscious that we had all of our precious eggs in one basket; if we somehow lost this rosette, with all the transducers, we’d have absolutely no way of talking to the OBS instruments (either locating them once we’ve deployed them or telling them to return to us when we recover them in a year). Which would mean the whole trip was over and the research totally scuppered! So everyone was being just that bit more careful, double-checking knots and confirming their preparations. 

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Wider-angle view of the rosette sitting below the A-frame, ready for deployment. The yellow things lashed to the deck around it are the OBS housings.

With the rosette deployed without a hitch (literally, in this case!**) we all repaired to the main computer lab to watch the screens showing how much cable had paid out, and comparing that with our on-board transponder ranging to the rosette. The whole point of the test was to check that our ship could talk to all 30 of the OBS transponders, including being able to “range to them” – i.e. find out how far away they are from us. We work this out from the time elapsed between our ship sending an acoustic signal into the water and the time we receive the instruments’ reply. Multiply this time by the velocity of sound in water (~1500 m/s) and: hey presto! You know how far away they are. For the rosette test, we know they are all just about directly below us (dangling on a cable), so the distance should be equal to the length of cable paid out by the winch man. Happily for us, the numbers all lined up. Even better, each of the little fellas in the rosette all chirped away with the correct responses to our ship’s commands, even when they were suspended four kilometers beneath the waves in pitch black temperatures and pressures of 400 atmospheres. It’s really pretty amazing to press a series of commands on a box in the lab, hear the ship send out a high-pitched squeal, and about 5 seconds later hear a response coming back from an instrument miles and miles beneath the waves, functioning perfectly and executing various commands based on the details of our chirping.

Gratifyingly, the rosette test was a complete success. Once everything was brought back on board smoothly, we proceeded southwards towards the deployment area, which is approaching fast! Well… approaching at exactly 12 knots (~13mph).

More updates soon!

Zach Eilon, on behalf of the Pacific ORCA team

 

* As with everything at sea, this was timed accurately down to the second. They take timing very seriously out here. I arrived at a meeting 15 seconds past the hour the other day, and they had already begun talking a few seconds prior. Every day at precisely noon there is a short whoop as they test the ship’s alarm siren. You can set your watch by it; mine is always about 24 seconds slow. Which, come to think of it, might explain my tardiness to the meeting!

** We keep thinking of new expressions that surely have a nautical etymology.

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