The World's Biggest Marionette

June 5th @ 11:45am – Yesterday afternoon, down at 2.6 km depth below the ship, ROPOS began a marathon and very delicate operation: laying cable. You’ll have to use your imagination for this… Picture ROPOS underwater with a large spool of cable – in this case 25 km long – hanging underneath it. The cable and moterized spool (called ROCLS for ‘Remotely Operated Cable Laying System) weighs something like 7,000 pounds (3,200 kg), which is too heavy for ROPOS to lift on its own. So ROPOS and ROCLS are dangling above the seafloor by the 2.6 km long tether cable from the crane on the ship.

Now, the ship moves very slowly along a set course, adjusting for winds, wave, and ocean swell, towing ROPOS and ROCLS along far below as the cable is slowly unwound from ROCLS, much like a marionette moving across a stage. Move too quickly or unwind too slowly and the cable snaps. Move too slowly or unwind too quickly and the cable won’t lie in a straight line and you won’t get the full desired length. There is also the constant possibility of the cable becoming stuck, or of slack loops gumming up the works, just to make thing that much more stressful.

Now, do this continuously for over 30 hours.

Sound crazy? It is, but it gets the job done, and the ROPOS team and crew on the Thompson have got this down to a fine art as one of the few ROV-deployed cable laying systems in the world. They’ll put the cable right where you want it.

WHY are we doing this? The goal is to lay three 25 km cables all spreading out from a central point. At the centre of this triangle, and each of the ends will be positioned a bottom pressure recorder (BPR) which will be able to detect miniscule changes in the ocean surface height (it will be able to ignore wave action and ocean swell due to its depth in the water column). With three stations and one in the middle, we hope to be able to detect the size, direction, and speed of travel of tsunamis with a great deal of precision and accuracy.

So, while the data loggers loathe this – there isn’t much to do on a logging shift except note how many rows of cable have been unwound and laid, it is a fantastic piece of engineering that we can do this at all.

Blog post and photos by Scott Doehler 
Marine Educator