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.
The following case studies and observations have been compiled from information received by IMCA. All vessel, client, and operational data has been removed from the narrative to ensure anonymity.
Case studies are not intended as guidance on the safe conduct of operations, but rather to assist vessel managers, DP operators and DP technical crew in appropriately determining how to safely conduct their own operations. Any queries should be directed to IMCA at [email protected]. Members and non-members alike are welcome to contact IMCA if they have experienced DP events which can be shared anonymously with the DP industry.
IMCA’s store terms and conditions (https://www.imca-int.com/legal-notices/terms/) apply to all downloads from IMCA’s website, including this document.
IMCA makes every effort to ensure the accuracy and reliability of the data contained in the documents it publishes, but IMCA shall not be liable for any guidance and/or recommendation and/or statement herein contained. The information contained in this document does not fulfil or replace any individual’s or Member's legal, regulatory or other duties or obligations in respect of their operations. Individuals and Members remain solely responsible for the safe, lawful and proper conduct of their operations.