Thruster command and feedback differences
- DP Event
- Published on 2 November 2017
- Generated on 11 September 2026
- DPE 04/17
- 2 minute read
Incident
It was concluded that the fault had to be in the output control loop, and it was decided to replace items with spares held on board.
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
- 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.
Latest DP incidents
-
Holding position, losing references
While approaching a turret-moored, weather-vaning offshore installation (FPSO), in preparation for cargo operations, a DP equipment class 2 vessel experienced significant rolling of up to 15–20° due to a long-period swell close to the vessel’s natural roll period, resulting in a resonance effect
DPE 02/26
1 September 2026
Observation
-
Know your equipment, or at least where to find the answers
During an update in the Follow Target Mode, the crew noticed an anomaly with joystick control.
DPE 02/26
1 September 2026
Observation
-
A chain of small failures leads to a major consequence
While conducting cargo operations on DP a vessel experienced a partial blackout caused by an unexpected shutdown of generator 3
DPE 02/26
1 September 2026
Incident
-
New equipment, unexpected consequences
During the operation, a loss of DGNSS correction signals was experienced across multiple vessel DGNSS units.
DPE 02/26
1 September 2026
Incident
-
DP drill scenario – Loss of seawater cooling pump redundancy
Exercise scenario – Loss of seawater cooling pump redundancy
DPE 02/26
1 September 2026
Drill Scenario
The case studies and observations above 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 technical crew.
IMCA makes every effort to ensure both the accuracy and reliability of the information, but it is not liable for any guidance and/or recommendation and/or statement herein contained.
Any queries should be directed to DP team at IMCA. Share your DP incidents with IMCA online. Sign-up to receive DP event bulletins straight to your email.