Dredge 16 was about 500 meters below the ship, and once more headed down the bottom. Suddenly, we received a call over the radio, “All Stop! All Stop!” Instead of the requested two knots forward, the ship was backing up over the dredge, putting the cable in danger of being caught in the props.  The ship was brought to a stop, and in a little while we tried to move forward again.  The same thing happened.  It turned out that part of the Dynamic Positioning system had failed, and was misdirecting the ships computer.

The Dynamic Positioning System (DPS) is a computer system that takes information from the GPS, and data from wind and current sensors.  It allows the user to input Latitude/Longitude coordinates and the ship will automatically stay in that one location over the seafloor.  The DPS works by controlling the ships two main electric propulsion motors, rudders, and a bow thruster (capable of propelling the ship at 1.5 knots by itself) mounted on the starboard pontoon.  Using different combinations of the three electric motors, and varying the rudder angles, the DPS software can move the ship in any direction.  For example, when docking, the Kilo Moana stops parallel its intended docking spot, and then, using the DPS, moves laterally into position.

The Dynamic Position System is a very important part of shipboard operations.  We use it whenever we need to maintain a position, like when we are running the CTD or taking a core sample.  We even use it for dredging, unfortunately the part of the DPS that allows a course to be plotted and then followed, failed.  As a result, out dredge profile was different from the usual one.  Because the ship was stopped while the DPS was looked at, the dredge (which is usually trailing behind the ship) swung into position directly under the ship.  By this time, we were already close to the planned start of the dredge, and not wanting to raise the dredge and start the run all over again, we lowered the dredge to the bottom and then paid out wire as the ship moved forward.  Once we were far enough away, the ship stopped, holding position, and we reeled in the dredge.  This was different from out usual approach of ‘flying’ the dredge to the ground (which has a much lower chance of the dredge wire getting snagged).  It seemed to work though; the dredge was full when we pulled it up.

This was also the first dredge where the dredge predicting software was actively used. In addition to providing bathymetry, the opportunity was used to try and calibrate the program’s drag coefficients to better computer model’s dynamics.

Blake English onboard the R/V Kilo Moana.