Comments from the report:
When the vessel was confirmed stopped and on the setpoint, the operator checked the auto track setting to find the cause of the track change. Upon checking, it was verified that the ‘waypoint for radius turn’ had changed to ‘operator’ a pre-set value of 200m; the vessel standard setting is ‘waypoint table’. The operator reselected ‘waypoint table’ and the previous barge track was displayed.
During investigation, the vessel crew were able to recreate the failure. Since the incident occurred the log files have been forwarded to the DP system manufacturer for review. The conclusion is that the issue is specific to move-up mode.
The position setpoint changed dramatically to about 140m, heading setpoint also changed to 14 degrees and thrusters started ramping up. This was due to a dramatic change of barge track
The barge track changed from a waypoint table selected radius of 1828m to ‘Operator’ selected of 200m.
Considerations of the IMCA Marine DP Committee from the above event:
It appears the DP system manufacturer responded well to this event.
The selection of “operator specified turn radius” (radio button selection) should be inhibited and “greyed out” when vessel is in a turn while move-up sub mode is active. This inhibit function was not activated when the dialog box was opened again after the “auto – turn – move up” function had been activated. Four conditions have to be aligned for this to happen. The “move-up” mode was introduced in 2002 and this is the first time this was reported to the original equipment manufacturer. So, the MTBF is 15-years but of course the consequence could be damage to the pipe. A software modification has been made, installed and tested on this specific vessel. The global customer support team have a process in place to go through all software releases which have this function and update the vessels that are affected during next service visit.