This past week we have been tied to the dock doing various repairs and getting ready for the next cruise. The crew sanding and painting in the wet lab, the mechanics welding in the galley, the marine techs opening and cleaning all hardware in electronic racks, and the engineers installing a flow through system pump in the engine room.
But last Tuesday was not an ordinary workday. After lunch Jonha, the marine technician, and I climbed through a hatch below the main deck and down a 16 foot narrow tube into a chamber the size of a Volkswagen Beetle.
After making sure we had all the tools we needed, we established radio contact with Chip, the engineer, and told him to close the door and start pressurizing. Air started coming through the vents and our ears started popping as if we were scuba diving. After 30 minutes of swallowing to equalize we reached seven psi. Only then we began to remove the bolts from a thick steel plate located at the bottom of the ship. Sixteen bolts later, I lift up the plate to see the ocean beneath me as still as if it was a bowl of water.
The objective of all this was to replace the two plates with Acoustic Doppler Current Profiler (ADCP) transducers that emit high frequency pulses of sound that scatter off moving particles in the ocean. From the change in transmitted and received frequency we can determine current velocity.
These transducers look like one-inch thick steel 19th century top hats with sensitive instruments inside. Securing them into their exact position in this small space was a challenge to say the least. It took us three hours to install them both, check for leaks, repeat, add more washers and check again.
The simple physics behind the reason why we didn’t sink the ship by opening a hole in the hull is that the pressure of the water at that depth is less or equivalent to the pressure of air in our chamber, so ideally there is not fluid movement either way.
On one occasion the waterline started rising so we radioed the engineer above to increase the pressure and the water started going down. What we found out later is that every time we made a seal between the hull and the transducer the air pressure jumped up to 9 psi, so the engineers had to immediately turn off the intake and open the valves to release air. Big thanks to Chip and Don for standing by for three hours monitoring the gauges. Overall, it was an amazing experience.