Thruster command and feedback differences leads to – DP incident

Investigation comments:

The DP system reported a thruster pitch prediction error when a large difference between the command and feedback was detected at 20:35:34. The description for a thruster prediction alarm states that the operator must take action to determine if the alarm is caused by a faulty feedback signal (DP performance nearly unaffected) or a faulty command signal (DP performance affected).

The port azimuth thruster was not stopped and only momentarily deselected from DP. The thruster gave thrust in the wrong direction while it was running and the DP system was unable to maintain position.

The prediction alarm went to normal again at 20:51:04 and the DP system was able to maintain position. No problems were reported after this.

Since the command output from the DP control system was correct, it was concluded that the fault had to be in the output control loop. It was decided to replace items in the command loop with spare parts held on board.

Trials and DP tests were carried out to verify system after replacement of parts. No problems or findings to report.

Considerations from the above event:

• The reason for reselecting the errant thruster must be questioned.
• In this situation stopping the errant thruster should have allowed the DP system to regain control.
• Event box No. 2 (pitch command and feedback mismatch indication) provides the trigger to stop the thruster because it indicates there is a problem with thruster commands.